Faraday Discussions Cite this: Faraday Discuss., 2014, 176, 429
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Energy-related catalytic and other materials: general discussion Joachim Maier, Galen Stucky, Rudolf Holze, Gang Chen, Jiafang Xie, Shi-Gang Sun, Guoxiong Wang, Beien Zhu, Yiren Zhong, Pengfei Liu, Lee Cronin, Gang Fu, Shihe Yang, Mingquan Yu, Clare Grey, Andrew Mount, Wee Shong Chin, Fuping Pan, Zhonghua Li, Zhongqun Tian, Dehui Deng, Nanfeng Zheng, Ram Seshadri, Yujiang Song, Xile Hu and Yimin Chao
DOI: 10.1039/c5fd90003d
Joachim Maier opened the discussion of the paper by Ram Seshadri: The Debye temperature is not well-dened in complex crystals unless referring to extremely low temperatures. To what conditions do the DFT-calculations refer? Ram Seshadri replied: We use the quasi-harmonic approximation with 0 K DFT for the computation, and this ts to the very low-temperature heat capacity as one of the experimental conrmations. Lee Cronin asked: Can you use optimisation algorithms to search the state space? Can you use a population based algorithm? Could you make a proxy and minimise the proxies? Ram Seshadri responded: All of these are in principle possible. The impediments at this time are a lack of expertise on the part of my research group, and also, the lack of experimental databases for some of the machine-learning strategies to be applied to. Andrew Mount asked: One opportunity to enhance data-driven discovery in general is to include additional functional relevance to applications to inform better discovery. What are the opportunities for incorporating e.g. phosphorescence data to inform better phosphor discovery? Ram Seshadri answered: I completely agree that other information could be added to our data-mining and searching strategies. The issue is the skills required for many of the machine-learning type approaches that one could employ: skills that us materials chemists may not possess. My group is in the process of mining a lot of experimental data such as Stokes’ shis, and perhaps something may emerge. This journal is © The Royal Society of Chemistry 2014 Faraday Discuss., 2014, 176, 429–445 | 429
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Clare Grey asked: The nitrides, oxyuorides and YAGs all show high efficiencies and yet their Debye temperatures are quite different. What else can you learn about the structural motifs to gain insight into what controls high efficiency? It seems like you are looking for structural motifs that are essentially the opposite of those that people look for in trying to identify materials with negative thermal expansion. You have essentially achieved 100% efficiency. So at one level, this seems like the end of the story! What is le then is the prediction of wavelengths for emission. You made the point that its difficult to predict luminescent properties from DFT methods because of the problems with accurate calculations of band gaps and electronic properties in materials involving 4f electrons. It is, however, straightforward to predict crystal eld splittings with relatively simple computational/semi-empirical approaches. Could you include some of these approaches into your screening methods? Ram Seshadri answered: This is absolutely important. We nd a highly connected structure to be indicative of rigidity and good phosphor performance, and in fact, the garnet structure is absolutely great as an example of a highly connected structure. Likewise, the point about materials (and structures) displaying negative thermal expansion, like the canonical material ZrW2O8 can be expected, as you say, to be terrible phosphor hosts from the perspective of rigidity. At this stage, we are trying to think about factors such as the crystal eld splitting and their computation. Frankly, parametrized approaches using experimental data may work as well and more robustly. Clare Grey said: Are there other parameters you could pick out to put into the optimisation processes (looking at your slide showing performance of different materials)? The parameters you are looking for are inverse to those that people look for for negative thermal expansion materials. Ram Seshadri responded: The parameters that we would like to add are absolute band positions, i.e. the position of the valence band maximum and the conduction band minimum with respect to the vacuum level. Unfortunately, these are not easy to compute. On the point regarding materials displaying negative thermal expansion (NTE), the point is very well taken. Phosphor hosts are emphatically not like NTE materials. Gang Fu commented: Can you design some new structures for energy materials on the basis of your calculations? Ram Seshadri replied: On the basis of what I showed, no; we cannot design a new structure, only look at existing structures but potentially with new compositions. Galen Stucky commented: You calculated the band gaps to help dene the optical performance parameter space. Do you have a way of dening the excited states as part of the performance parameter search space? Ram Seshadri answered: We use calculations of the band gaps only insofar as we wish to ensure that the f-to-d transition on the activator ion (Eu2+ or Ce3+) lies 430 | Faraday Discuss., 2014, 176, 429–445 This journal is © The Royal Society of Chemistry 2014
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within the optical window of the band gap. Calculating the crystal-eld broadening of the d states is not something that we have so far attempted. Nanfeng Zheng asked: Is it possible to apply the developed strategy to prepare efficient white-light phosphors using rare earth elements? The production of rare earth compounds from rare earth ores has caused serious environmental problems in China. Ram Seshadri replied: I think the question is whether the strategies could be used in a manner that avoids rare-earth elements, and the answer is very much so. The strategy does not care about the precise nature of elements. Shihe Yang enquired: It seems to me that structural rigidity is more relevant to organic molecules because they may have many different conformations. Indeed it is well known that rigid organic molecules emit light more efficiently, of course in the relative sense, because of less nonradiative pathways. But when you talk about rigidity of inorganic materials, what are you really referring to? Can you elaborate more on the differences between inorganic and organic light emitting materials with respect to your thesis? Ram Seshadri replied: This is correct, that rigid molecules make better emitters than “oppy” molecules with many degrees of torsional freedom. The point that we are making with crystal structures is that such “oppiness” may not be readily recognizable simply by looking at the crystal structures, and this is where the (computable) proxy that we employ, the Debye temperature, becomes very useful. Zhonghua Li remarked: Can you obtain monochromatic light, then enhance the efficiency? Ram Seshadri responded: I am not sure that I fully understand the question. The goal of white lighting is not to get monochromatic light, but to ll out the spectrum, so monochromatic light is somewhat pointless. Galen Stucky asked: Do we have perfect white light LEDs? How might the performance be improved for other desired applications? Ram Seshadri replied: We are really close to perfect white light, and in fact, we have learned some interesting things in recent years. One of these is that a little bit of near-UV in white light is actually a good thing, since so many everyday objects (like white shirts) have had optical brightening agents added to them. The best solid-state white light devices (in terms of color) are made today using nearUV diodes and red/green/blue phosphors, and these have color rendition indices close to 100. Galen Stucky opened the discussion of the paper by Yimin Chao: Could you give more details on the doping and functionalisation for generating p and n type particles? What are the resultant carrier densities and doping levels?
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Yimin Chao responded: The functionalization is a nucleophilic substitution reaction with chloride termination on the particle surface and there is potential for controlling doping on both the particle and the ligand. However in this work we have not actively doped this material and therefore have not measured the carrier densities or doping levels. In the paper we suggest that charged surface states may be responsible for the electrical resistivity observed but it is difficult to be sure. There is scope for both doping the particles and doping the ligands. The ligands can be doped using typical oxidation processes as used by conductive polymers, although realistically they can only be p-doped, as n doped organics tend to be less stable (difficult using phenyl acetylene). On the other hand the particles can be both n and p doped by using a starting material of known carrier concentration. With regards to producing both p and n doped materials, it has been shown with Bi2Te3 - PEDOT:PSS nanocomposites that n type materials can be achieved without the complication of n doping the organic component (please see B. Zhang, J. Sun, H. E. Katz, F. Fang and R. L. Opila, ACS Appl. Mater. Interfaces, 2010, 2, 3170–3178). Galen Stucky said: Does ZT ¼ 0.6 because the thermal conductivity is low or because the power factor is high? What is the value of the experimental thermal conductivity? Yimin Chao responded: ZT is high mainly due to an extremely low thermal conductivity although the power factor is reasonable in the region of 105 V m1 K1, which is 10 times lower than that of the best conductive polymer and 100 times lower than bismuth telluride (please see O. Bubnova and X. Crispin, Energy Environ. Sci., 2012, 5, 9345–9362 for values). The experimental value of thermal conductivity here in the paper is 0.1 W m1 K1. Shihe Yang opened the discussion of the paper by Yimin Chao: One question concerns the use of organic molecules for thermoelectric devices. What are the parameters you seek to enhance in the thermoelectic gure of merit? Second, it seems that much work is focused on reducing phonon conductivity. How much room is there in improving the thermal power Q by nanostructure engineering? There had been much discussion in this direction early on, but what is the current situation? Yimin Chao responded: By the introduction of organics we are aiming to reduce the thermal conductivity. As such, organic chains on their own display very low thermal conductivities. Additionally these types of materials are more versatile with regards to low-temperature processing techniques such as printing. So essentially we have, as workers before us, focused on reducing the phonon conduction even though our approach is different. Using nanostructuring engineering, it has been possible to match the thermopower of bulk silicon. There were signicant advancements in the ZT early on when compared to bulk silicon. These materials show a signicant decrease in the lattice themal conductivity. At high temperatures these materials look promising but little improvement using bulk nanostructuring has been made since publications by Bux et al. in 2009. With regards to low temperature performance, a more signicant advancement using nanostructuring lies with the production of 432 | Faraday Discuss., 2014, 176, 429–445 This journal is © The Royal Society of Chemistry 2014
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nanoporous ribbons or nanowires that do offer promise for good thermopower and a ZT close to 0.4 (Tang et al. 2010) and 1 (Hochbaum et al. and Boukai et al. 2008) at RT respectively. However, in terms of industrial scale production, bulk nanostructuring of silicon is still very attractive, especially if ZT can be improved to match SiGe alloys. Yiren Zhong opened the discussion of the paper by Xile Hu: I have noticed the CV scans of nano-Ni(OH)2 and NiO are quite different; that is the CV of Ni(OH)2 has a pair of peaks, why does this happen? Xile Hu replied: The peak you refer to is the Ni(II) to Ni(III) peak and its position and shape are different for different catalysts. A more active catalyst tends to have higher intensity for this peak. Mingquan Yu asked: In your experiment, iron impurities were removed on purpose. As far as I know, iron-doping usually enhance the activity of the catalyst a lot. So, could you please explain the synergetic effect between iron and nickle? Xile Hu replied: This is still an open question. In the paper we listed two possibilities such as the synergetic effect of Ni-O-Fe units, or doping of Fe leading to electronic structure change. Professor Maier mentioned the possibility of defect formation by Ni vacancy. It will be an important research topic for the near future. Joachim Maier added: Could the Fe-effect be a formation of point defects such as nickel vacancies (doping effect) which would be extremely basic? Cf. R. Merkle and J. Maier, Top. Catal, 2006, 38, 141. Xile Hu responded: This is a very interesting idea. We will look into it. Guoxiong Wang enquired: Have you investigated the effect of the amount of iron content on OER activity? Xile Hu responded: We have done it only partially. We should do more; thank you for the suggestion. Pengfei Liu asked: According to the CV curve of Ni(OH)2 nanoparticles with Fe impurity, the anodic peak around 1.39V vs. RHE corresponds to the phase change Ni(OH)2 to NiOOH. My question is: does the anodic peak always mean that the metal element valence state changes; does the anodic peak have something to do with the OER performance. If do, how does it inuence this? Xile Hu replied: The position and shape of this redox peak are indicative of the phase of the Ni(OH)2 species. A certain phase is more active than the other. Our paper has a discussion of this, and has cited several papers on this topic. Pengfei Liu spoke: As we know, the HER is easier in acid electrolyte and the OER is easier in basic electrolyte. Some HER catalysts, for example, suldes, carbides, nitrides, phosphides and some carbon materials are active in acid This journal is © The Royal Society of Chemistry 2014 Faraday Discuss., 2014, 176, 429–445 | 433
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electrolyte but less active and more unstable in basic electrolyte. Some OER catalysts like perovskites, amorphous metal oxides and carbon materials are also less active and more unstable in acid electrolyte. My question is that, since we should use the same electrolyte for water electrolysis, should we develop active and stable HER catalysts in basic electrolyte or develop the active and stable OER catalysts in acid electrolyte? Which is more important: perhaps both? Xile Hu answered: You are absolutely right. In general HER is easier in acid and OER is easier in base. Now it is time to combine them. It appears that HER in base is more promising than OER in acid for most non-precious catalysts due to the instability of many oxides in acid. Watch out, we will soon publish a paper on photoelectrochemical HER in base for the rst time. Dehui Deng asked: Why does the performance of the material with Fe impurity increase aer 30 scans (looking at a plot of current density vs. potential)? Can you run for longer scans? Xile Hu replied: During the rst 30 scans, the catalyst undergoes a transformation to form the active phase, likely layered NiOOH species. During this process, the activity increases. Further scans do not give higher activity, as once the phase is formed, it is stable and its activity is constant. Gang Fu enquired: Where is the iron: on the Ni(OH)2 or on the carbon? What is the valence of the iron? Xile Hu replied: Iron should be close to nickel in the catalyst layer. The oxidation state of iron is likely to be Fe(III). Fuping Pan addressed Xile Hu: Much research is dedicated to develop low-cost yet efficient catalysts to replace noble metals for energy-related reactions, such as ORR and OER. How far can the non-precious catalyst go? Can they replace completely precious metal-based catalysts in practical application? Xile Hu answered: For ORR and OER under certain conditions, non-precious catalysts can do better than precious metals. For instance, in alkaline conditions, such catalysts exist and they can be further improved. Also even if non-precious catalysts have lower performance under certain conditions, for example, in acidic solutions, as long as the performance per cost is higher, they can be used. Guoxiong Wang asked: For OER a conductive material is not necessary, why? It is possible to enhance the OER activity by adding some electroactive material? Xile Hu replied: Actually for OER you also need highly conductive catalysts. Oen the catalysts are thin and porous, so you don't notice the problem in preliminary tests. Adding electroactive materials is a good idea. Jiafang Xie enquired: I'm interested in the role that Fe played in your system for its abnormal enhancement of the peak current in the CV sweep. I noticed that with Fe impurities, the potential of the peak current in CV is different to that of 434 | Faraday Discuss., 2014, 176, 429–445 This journal is © The Royal Society of Chemistry 2014
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without Fe impurities. As far as I know, this is likely to imply an different reaction in which Fe and/or your catalyst participate. As a similar example, copper quickly loses its ability to catalyze electrochemical carbon dioxide reduction to hydrocarbons when there is trace Fe impurity; the current is even larger, but only H2 can be detected. The explanation is that Fe atoms occupy the active sites on the surface of the copper and H is highly adsorbed on Fe. So, did you try to detect the product and make a comparison? I think this would be helpful. Xile Hu responded: Very good suggestion! We will look into it. Jiafang Xie then asked: I’m very interested in the role that Fe plays in your system. In your paper, you showed a CV of a comparison of with and without the removal of Fe. For a more complete comparison, experiments of adding Fe into the electrolyte or just doping Fe onto your catalyst should be conducted; I wonder if you have done these? And if you have, what happened? There result would help us determine the role of Fe impurities in your system. Xile Hu replied: Again very good suggestions! We have not done this and we will do it. Jiafang Xie added: This is a small question about the details of your experiment. In your work, when the nal suspension was obtained, it was allowed to rest for 3 h before further centrifugation. I wonder what is the meaning of this step. Is it of special purpose? Xile Hu answered: This is just to separate the solid from the solution. You can probably do it by a centrifuge, which would be faster. Shi-Gang Sun opened the discussion of the paper by Gang Fu: In the theoretical modeling, what are the effects of iron oxide and nickel oxide that may change the structure. Also, what is the effect of substrates of different single crystal planes? Gang Fu responded: Our DFT calculations suggested that the OH groups at Fe3+-OH-Pt react readily with CO adsorbed nearby to yield directly CO2 and produce simultaneously coordinatively unsaturated Fe sites for O2 activation. Ni2+ plays a key role in stabilizing the interface against dehydration through the strong interaction between Ni(OH)x and Fe(OH)x. We have explored different Pt planes, including at (111) surface and stepped (332) and (322) surfaces, and found that CO oxidation is a structure sensitive reaction, and the performance on different exposed surfaces depends on the bond strength of Fe(III)–OH. Beien Zhu asked: There are two well known mechanisms for CO oxidation. One is the Eley–Rideal mechanism which concerns CO and O2 forming OCOO; another is the Langmuir–Hinshelwood mechanism which involves the dissociation of O2. In your calculations, did you consider both mechanisms? Gang Fu answered: We only considered the Langmuir–Hinshelwood mechanism because there were still open Pt sites which can strongly adsorb CO. In our mechanism, CO oxidation is triggered by CO oxidative coupling with This journal is © The Royal Society of Chemistry 2014 Faraday Discuss., 2014, 176, 429–445 | 435
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interfacial OH groups at Fe-OH-Pt, simultaneously creating coordinatively unsaturated ferrous sites (Pt–,–Fe) for the following O2 activation. We also showed that CO can promote O–O bond cleavage via an [OC–O–O(H)Fe] transition state. Beien Zhu asked: In your work, how do you prevent the supported Fe nanoparticles from aggregating? Gang Fu responded: Despite exhibiting a high catalytic activity in CO oxidation, the Fe(OH)x/Pt catalyst did not work steadily for a long time, mainly due to the easy dehydration of Fe(OH)x sub-monolayers into three-dimensional iron oxyhydroxide or iron oxide. To prevent the dehydration-induced loss of interfacial Fe(III)-OH-Pt sites, it was undertaken to dope the overgrowth Fe(OH)x submonolayer with Ni2+ to form Pt/FeNi(OH)x hybrid nanoparticles. The resulting catalysts can maintain high catalytic activity for more than 28 h. Yiren Zhong enquired: Is the Fe(OH)x on the Pt surface amorphous or crystallized? Gang Fu answered: Fe(OH)x was deposited on the sub 5-nm Pt nanoparticles such that there is no long range periodicity in its structure, i.e. it is amorphous. However, the local structure of Fe(OH)x was found to be well-dened because XANES showed that the structure of Fe(OH)x is similar to goethite FeOOH. Computationally, we constructed Fe(OH)x models by cutting the Fe6O18 moiety from the goethite FeOOH crystal, and periodic density functional theory was performed. Yiren Zhong asked: According to your experience, could you give us a comment or a guess on which state of catalyst has more efficient catalytic effect, amorphous or crystal? Gang Fu replied: The efficiency of catalysts depends critically on how many active sites they contain rather than their crystal states. Fundamentally, researchers preferred to use model systems, such as single crystals or well-dened nano catalysts to explore the catalytic mechanisms. For example, we have demonstrated here that well-dened hybrid nanoparticles are an ideal model system to study interfacial effects. Based on our understanding of the interfacial effects, an alloy-assisted strategy was further developed to produce a more practical Pt-based nanocatalyst. Although the catalyst did not have easily characterized catalytic interfaces, it contained much more abundant Fe-OH–Pt catalytic interfaces and displayed a much higher catalytic performance than the welldened systems. So, efficient catalysts might not have well-dened structures for characterization. They could be either amorphous or crystalline. But they have to contain abundant catalytically active sites. Fuping Pan enquired: Can the water absorb and occupy the active sites? What is the accurate role of water?
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Gang Fu responded: The adsorption of water on the active sites is relatively weak, which cannot compete with the adsorption of CO and O2. Therefore, water is unable to block the active sites. However, water is necessary for low-temperature CO oxidation because the Pt/Fe(OH)x catalyst did not work steadily in dry streams. Based on our DFT calculations, we proposed that the water can promote O2 activation and recover the interfacial OH. Zhonghua Li asked: Why did you choose one layer Fe(III)–OH instead of multiple layers? Gang Fu answered: Experimentally, we used a wet-chemical method to fabricate Fe-OH–Pt interfaces by atomically thick Fe-OH partially covering the surface of monodisperse Pt nanocrystals. In order to match the structure characterized by experiment, we built catalyst models with a single layer but without multiple layers of Fe(OH)x deposited on different Pt surfaces, such as Pt(111), Pt(332) and Pt(322). Shihe Yang enquired: Presumably, the charge of the active center should change during the reaction. Does proton-coupled electron transfer have a role to play here? This could lower the activation energy. The fact that water is needed seems to speak in favor of this mechanism. Gang Fu replied: CO oxidation involves a two-electron redox reaction. In our mechanism, CO oxidation began with the adsorbed CO and interfacial –OH moieties, which can be viewed as a two-electron reductive elimination. During this process, the Fe(OH)x species serve as oxidants, which will be reduced by two units. For the rst CO oxidation, only proton transfers take place. Proton-coupled electron transfer plays a key role in forming the OOH species, in which the electron donor is Fe2+, while the proton donor is adsorbed H2O. The role the water plays has been emphasized in our paper. On one hand, the water can stabilize O2 adsorption by forming OOH species; on the other hand, the O–O bond breaking might also be facilitated by the coexistence of water. Gang Chen asked: How large are your iron nanoparticles? Can you estimate what percentage of atoms at the interface are of interest in your study? If so, you can estimate how much signal in your XANES spectra is from the interfacial Fe atoms. Gang Fu responded: Experimentally, we fabricated Fe-OH–Pt interfaces by partially covering the surface of mono disperse Pt nanoparticles (which are 200 cm1) we measured should be attributed to at a xed vq E potential, which is not a Stark-tuning rate. We suggest that both electron-donation/ back–donation and dipole–dipole interactions may contribute to such large slope. vnCO The fact that the on THH Pt NCs is much larger than that on bulk vq E single-crystal electrodes could be attributed to preferential CO adsorption on (100) sites on THH Pt NCs at low coverage, as illustrated in the paper. Although the overall CO coverages are the same, the local adsorbed CO density on the (100) terraces of (310) and (210) subfacets of THH Pt NCs is much larger than that on (100) terraces of (310) or (210) Pt single crystal planes, resulting in stronger dipole–dipole interactions that induce a big blue-shi of CO band position. At a xed coverage, we have determined that the Stark-tuning rate
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Mingquan Yu opened a general discussion by addressing Yujiang Song, Xile Hu and Shi-Gang Sun: When we conduct electrochemical tests, catalysts usually exhibit a redox wave. However, the redox wave would get smaller-and-smaller, even seem to disappear with slowing scan rate. So I’d like to ask what happens when the scan rate gets slower? Yujiang Song replied: I guess you are talking about cyclic voltammetry (CV) curves. When you use a slower scan rate, the current density should get smaller. The current density is proportional to the square root of scan rate. Xile Hu continued: In electrochemistry, scan rate dependent voltammetry will give information about the inuence of mass transport as well as capacitive behavior of the test molecule. You have to analyze it case-by-case. Shi-Gang Sun concluded: The relationship between redox current and scan rate depends on the types of electrochemical reactions and their reversibility. The electrooxidation reactions of most of small organic molecules are usually kineticcontrolled, but not diffusion-controlled (that is, mass transfer rate is larger than intrinsic electrochemical kinetics). So, as the potential scan rate decreases, the oxidation current will gradually decline and trend to a stable value, but not zero. As for adsorbed species reaction, e.g., hydrogen adsorption on Pt, the current can really trend to zero as scan rate decreases. Mingquan Yu addressed Yujiang Song, Xile Hu and Shi-Gang Sun: Has it become necessary for us to set standard electrochemical test conditions to compare the activity of different catalysts? As different groups oen choose to use different test conditions, it is hard for us to compare the activity, so I presume setting a standard electrochemical test condition is important and urgent. Yujiang Song replied: It is necessary to benchmark how to evaluate the electrocatalysts toward oxygen reduction reaction. I agree with you that it is critical and urgent to set up a standard for electrocatalyst examination. Xile Hu followed up these remarks: I agree. In fact, standard testing conditions already exist. It is only because many active researchers in the eld do not have a background in electrochemistry that we see papers reporting results under nonstandard conditions. This situation will improve soon. Shi-Gang Sun concurred: I also agree that a standard electrochemical test is very important in order to compare the results reported by different groups worldwide. Unfortunately, such standards have not been established yet due to the diversity of reactants, catalyst materials, and electrolyte. This requires that academic societies (e.g., IUPAC, ISE or ECS) promulgate the relevant standards. Joachim Maier addressed Gang Fu and Shi-Gang Sun: Do you know why, in catalysis, people do not concentrate more on point defects in the surfaces? Otherwise it is like dealing with catalysis on a water droplet and ignoring H+ and OH ions in the surface layer.
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Gang Fu answered: In the catalysis, the catalytic sites should be considered as “defect sites. The “defect” sites change their molecular structures during catalysis. Shi-Gang Sun added: We agree that point defects are also important in catalysis. For example, Ertl and co-workers observed by STM that the decomposition of NO exclusively occurs on line defects (i.e., edge sites) of Ru(0001) surface (Ertl et al., Science, 1996, 273, 1688–1690). However, it is difficult to control and monitor point defects on surfaces, especially on nanocrystals. So, it is challenging to study the role played by point defects during catalytic reactions at present. Zhongqun Tian continued the general discussion. The speakers have very nicely shown that catalytic reaction sites can be more well-dened with new synthesis methods, in situ characterization and theoretical calculation. What could be the next generation surface for energy chemistry with the synergetic effects, rationally designed paths and higher stability? Xile Hu spoke rst: With increasing understanding of catalysis at a molecular level, researchers will be able to develop next generations of catalysts that are highly active per atom, multifunctional due to synergetic effects, and with high stability once the decomposition pathways are known and inhibited. Yujiang Song continued: This is a very broad and challenging question! My answer will only focus on non-noble metal electrocatalysts (NNMEs) for the oxygen reduction reaction (ORR). In an alkaline medium, NNMEs are close to replacing platinum based electrocatalysts. If we follow the three rules for the rational design and synthesis of NNMEs (high surface area, high density of ORR active sites, and highly ordered active sites), the next step would be the creation of NNMEs with highly ordered active sites at a high density. In an acidic medium, it is not clear what is the exact nature of the ORR active sites of NNMEs even at the nano-scale. In this scenario, the rst task should be the identication of the ORR active sites and then to follow the three rules to develop the next generation of NNMEs for acidic conditions. Gang Fu added: In our opinion, the next generation surfaces should be multicomponent and multifunctional, which will enable simultaneous activation of multiple reactants having different properties, and conversion of them into the products with high selectivity. We have demonstrated that noble metal (NM)– transition metal hydroxide interface systems have a synergy effect to boost CO oxidation because CO can chemisorb on the NM, while transition metals usually have a relatively high oxygen affinity. It should be pointed out that CO oxidation is just a 2e redox process. For more complicated cases, a smart, sophisticated design is required. Generally speaking, to design a multicomponent catalyst, we should understand the functions of each component, and then combine them in a reasonable way to make the active center, and then prevent the loss of activity during the reaction process. To accomplish these steps, enormous efforts should be made to develop new synthesis methods, new characterization techniques, and new theoretical methods as well.
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Shi-Gang Sun concluded: The next generation surface for catalysis should have high-density of active sites and optimal surface atomic arrangement for a certain reaction. This needs to understand the surface structure–catalytic function relationships at the atomic level by a combination of experiment and theory. Another approach is to construct atom–nano–micro multiscale architectures with synergetic effects. In short, we need to engineer rationally multiscale structured catalysts from the atomic to nano and to micro structure, as well as with a local to long-range environment of active sites. Lee Cronin addressed Yujiang Song, Xile Hu, Gang Fu and Shi-Gang Sun: How do we more intelligently discover novel spaces in chemistry and exploit the interplay between theory and experiment in the understanding, design, and prediction of new catalysts? Yujiang Song answered: It is necessary to have experimentalists and theorists to communicate more efficiently, thus building up theoretical models that are close to real reaction systems. I tend to claim that only theoretical models that have been proven in real reactions are reliable. Otherwise, theoretical models should be re-constructed and optimized. Xile Hu noted: There is no smarter method in research than giving intelligent people the opportunity to do what they think is interesting. Novel discoveries will occur along the way. To further exploit the interplay between experiments and theories, we need better communication and to work on the same goal. Gang Fu continued: Rational design should be based on our understanding of the surface structure and reaction mechanism under experimental conditions. However, both of them are not fully understood due to the compositional and/or structural inhomogeneity of conventional catalysts. Thus, we should develop welldened systems (e.g. nanoparticles), which can decouple complicated factors, and allow the application of advance characterization techniques to established structure–activity relationships. To gain deeper insight, theoretical calculations should be performed to explain the experimental results, and further elucidate the intrinsic factors in catalysis. We expect that fruitful interaction between theory and experiment could promote the development of novel catalysts. Shi-Gang Sun added: In our opinion, understanding the surface structure– catalytic function relationships of nanoparticles is of crucial importance in the design and predication of new catalysts. This requires the development of nanocrystal-based model nanocatalysts with a well-ordered and tunable surface structure, which could bridge the huge gap between bulk single-crystal model catalysts and practical nanocatalysts in terms of size and structural complexity, and provide catalytic reaction principles more related to practical nanocatalysts. Theoretical methods can provide new insight into catalytic reaction mechanism and surface processes. On basis of this knowledge and by combination of experiment and theory, we may efficiently predict and design new catalysts.
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Wee Shong Chin communicated: While Professor Chao has discussed the use of phenylacetylene cross-linked to Si NPs, could he comment on the relative amount and the effect of this on the resultant material properties? Yimin Chao communicated in reply: In the paper we have not mentioned crosslinking so I assume you mean be coverage and overlap. The relative surface coverage is approximately 70% of the available surface. The effect the coverage has on the thermoelectric properties of the material has not been studied, as the coverage is difficult to control, but at a lower surface coverage we would expect a higher concentration of surface oxide and as a result a lower electrical conductivity. This is a problem with many semiconductor organic nanocomposites. Additionally the level of overlap between the chains would be expected to have a positive effect on the materials’ performance, as it would give better electron transfer between chains.
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Energy-related catalytic and other materials: general discussion Joachim Maier, Galen Stucky, Rudolf Holze, Gang Chen, Jiafang Xie, Shi-Gang Sun, Guoxiong Wang, Beien Zhu, Yiren Zhong, Pengfei Liu, Lee Cronin, Gang Fu, Shihe Yang, Mingquan Yu, Clare Grey, Andrew Mount, Wee Shong Chin, Fuping Pan, Zhonghua Li, Zhongqun Tian, Dehui Deng, Nanfeng Zheng, Ram Seshadri, Yujiang Song, Xile Hu and Yimin Chao
DOI: 10.1039/c5fd90003d
Joachim Maier opened the discussion of the paper by Ram Seshadri: The Debye temperature is not well-dened in complex crystals unless referring to extremely low temperatures. To what conditions do the DFT-calculations refer? Ram Seshadri replied: We use the quasi-harmonic approximation with 0 K DFT for the computation, and this ts to the very low-temperature heat capacity as one of the experimental conrmations. Lee Cronin asked: Can you use optimisation algorithms to search the state space? Can you use a population based algorithm? Could you make a proxy and minimise the proxies? Ram Seshadri responded: All of these are in principle possible. The impediments at this time are a lack of expertise on the part of my research group, and also, the lack of experimental databases for some of the machine-learning strategies to be applied to. Andrew Mount asked: One opportunity to enhance data-driven discovery in general is to include additional functional relevance to applications to inform better discovery. What are the opportunities for incorporating e.g. phosphorescence data to inform better phosphor discovery? Ram Seshadri answered: I completely agree that other information could be added to our data-mining and searching strategies. The issue is the skills required for many of the machine-learning type approaches that one could employ: skills that us materials chemists may not possess. My group is in the process of mining a lot of experimental data such as Stokes’ shis, and perhaps something may emerge. This journal is © The Royal Society of Chemistry 2014 Faraday Discuss., 2014, 176, 429–445 | 429
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Clare Grey asked: The nitrides, oxyuorides and YAGs all show high efficiencies and yet their Debye temperatures are quite different. What else can you learn about the structural motifs to gain insight into what controls high efficiency? It seems like you are looking for structural motifs that are essentially the opposite of those that people look for in trying to identify materials with negative thermal expansion. You have essentially achieved 100% efficiency. So at one level, this seems like the end of the story! What is le then is the prediction of wavelengths for emission. You made the point that its difficult to predict luminescent properties from DFT methods because of the problems with accurate calculations of band gaps and electronic properties in materials involving 4f electrons. It is, however, straightforward to predict crystal eld splittings with relatively simple computational/semi-empirical approaches. Could you include some of these approaches into your screening methods? Ram Seshadri answered: This is absolutely important. We nd a highly connected structure to be indicative of rigidity and good phosphor performance, and in fact, the garnet structure is absolutely great as an example of a highly connected structure. Likewise, the point about materials (and structures) displaying negative thermal expansion, like the canonical material ZrW2O8 can be expected, as you say, to be terrible phosphor hosts from the perspective of rigidity. At this stage, we are trying to think about factors such as the crystal eld splitting and their computation. Frankly, parametrized approaches using experimental data may work as well and more robustly. Clare Grey said: Are there other parameters you could pick out to put into the optimisation processes (looking at your slide showing performance of different materials)? The parameters you are looking for are inverse to those that people look for for negative thermal expansion materials. Ram Seshadri responded: The parameters that we would like to add are absolute band positions, i.e. the position of the valence band maximum and the conduction band minimum with respect to the vacuum level. Unfortunately, these are not easy to compute. On the point regarding materials displaying negative thermal expansion (NTE), the point is very well taken. Phosphor hosts are emphatically not like NTE materials. Gang Fu commented: Can you design some new structures for energy materials on the basis of your calculations? Ram Seshadri replied: On the basis of what I showed, no; we cannot design a new structure, only look at existing structures but potentially with new compositions. Galen Stucky commented: You calculated the band gaps to help dene the optical performance parameter space. Do you have a way of dening the excited states as part of the performance parameter search space? Ram Seshadri answered: We use calculations of the band gaps only insofar as we wish to ensure that the f-to-d transition on the activator ion (Eu2+ or Ce3+) lies 430 | Faraday Discuss., 2014, 176, 429–445 This journal is © The Royal Society of Chemistry 2014
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within the optical window of the band gap. Calculating the crystal-eld broadening of the d states is not something that we have so far attempted. Nanfeng Zheng asked: Is it possible to apply the developed strategy to prepare efficient white-light phosphors using rare earth elements? The production of rare earth compounds from rare earth ores has caused serious environmental problems in China. Ram Seshadri replied: I think the question is whether the strategies could be used in a manner that avoids rare-earth elements, and the answer is very much so. The strategy does not care about the precise nature of elements. Shihe Yang enquired: It seems to me that structural rigidity is more relevant to organic molecules because they may have many different conformations. Indeed it is well known that rigid organic molecules emit light more efficiently, of course in the relative sense, because of less nonradiative pathways. But when you talk about rigidity of inorganic materials, what are you really referring to? Can you elaborate more on the differences between inorganic and organic light emitting materials with respect to your thesis? Ram Seshadri replied: This is correct, that rigid molecules make better emitters than “oppy” molecules with many degrees of torsional freedom. The point that we are making with crystal structures is that such “oppiness” may not be readily recognizable simply by looking at the crystal structures, and this is where the (computable) proxy that we employ, the Debye temperature, becomes very useful. Zhonghua Li remarked: Can you obtain monochromatic light, then enhance the efficiency? Ram Seshadri responded: I am not sure that I fully understand the question. The goal of white lighting is not to get monochromatic light, but to ll out the spectrum, so monochromatic light is somewhat pointless. Galen Stucky asked: Do we have perfect white light LEDs? How might the performance be improved for other desired applications? Ram Seshadri replied: We are really close to perfect white light, and in fact, we have learned some interesting things in recent years. One of these is that a little bit of near-UV in white light is actually a good thing, since so many everyday objects (like white shirts) have had optical brightening agents added to them. The best solid-state white light devices (in terms of color) are made today using nearUV diodes and red/green/blue phosphors, and these have color rendition indices close to 100. Galen Stucky opened the discussion of the paper by Yimin Chao: Could you give more details on the doping and functionalisation for generating p and n type particles? What are the resultant carrier densities and doping levels?
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Yimin Chao responded: The functionalization is a nucleophilic substitution reaction with chloride termination on the particle surface and there is potential for controlling doping on both the particle and the ligand. However in this work we have not actively doped this material and therefore have not measured the carrier densities or doping levels. In the paper we suggest that charged surface states may be responsible for the electrical resistivity observed but it is difficult to be sure. There is scope for both doping the particles and doping the ligands. The ligands can be doped using typical oxidation processes as used by conductive polymers, although realistically they can only be p-doped, as n doped organics tend to be less stable (difficult using phenyl acetylene). On the other hand the particles can be both n and p doped by using a starting material of known carrier concentration. With regards to producing both p and n doped materials, it has been shown with Bi2Te3 - PEDOT:PSS nanocomposites that n type materials can be achieved without the complication of n doping the organic component (please see B. Zhang, J. Sun, H. E. Katz, F. Fang and R. L. Opila, ACS Appl. Mater. Interfaces, 2010, 2, 3170–3178). Galen Stucky said: Does ZT ¼ 0.6 because the thermal conductivity is low or because the power factor is high? What is the value of the experimental thermal conductivity? Yimin Chao responded: ZT is high mainly due to an extremely low thermal conductivity although the power factor is reasonable in the region of 105 V m1 K1, which is 10 times lower than that of the best conductive polymer and 100 times lower than bismuth telluride (please see O. Bubnova and X. Crispin, Energy Environ. Sci., 2012, 5, 9345–9362 for values). The experimental value of thermal conductivity here in the paper is 0.1 W m1 K1. Shihe Yang opened the discussion of the paper by Yimin Chao: One question concerns the use of organic molecules for thermoelectric devices. What are the parameters you seek to enhance in the thermoelectic gure of merit? Second, it seems that much work is focused on reducing phonon conductivity. How much room is there in improving the thermal power Q by nanostructure engineering? There had been much discussion in this direction early on, but what is the current situation? Yimin Chao responded: By the introduction of organics we are aiming to reduce the thermal conductivity. As such, organic chains on their own display very low thermal conductivities. Additionally these types of materials are more versatile with regards to low-temperature processing techniques such as printing. So essentially we have, as workers before us, focused on reducing the phonon conduction even though our approach is different. Using nanostructuring engineering, it has been possible to match the thermopower of bulk silicon. There were signicant advancements in the ZT early on when compared to bulk silicon. These materials show a signicant decrease in the lattice themal conductivity. At high temperatures these materials look promising but little improvement using bulk nanostructuring has been made since publications by Bux et al. in 2009. With regards to low temperature performance, a more signicant advancement using nanostructuring lies with the production of 432 | Faraday Discuss., 2014, 176, 429–445 This journal is © The Royal Society of Chemistry 2014
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nanoporous ribbons or nanowires that do offer promise for good thermopower and a ZT close to 0.4 (Tang et al. 2010) and 1 (Hochbaum et al. and Boukai et al. 2008) at RT respectively. However, in terms of industrial scale production, bulk nanostructuring of silicon is still very attractive, especially if ZT can be improved to match SiGe alloys. Yiren Zhong opened the discussion of the paper by Xile Hu: I have noticed the CV scans of nano-Ni(OH)2 and NiO are quite different; that is the CV of Ni(OH)2 has a pair of peaks, why does this happen? Xile Hu replied: The peak you refer to is the Ni(II) to Ni(III) peak and its position and shape are different for different catalysts. A more active catalyst tends to have higher intensity for this peak. Mingquan Yu asked: In your experiment, iron impurities were removed on purpose. As far as I know, iron-doping usually enhance the activity of the catalyst a lot. So, could you please explain the synergetic effect between iron and nickle? Xile Hu replied: This is still an open question. In the paper we listed two possibilities such as the synergetic effect of Ni-O-Fe units, or doping of Fe leading to electronic structure change. Professor Maier mentioned the possibility of defect formation by Ni vacancy. It will be an important research topic for the near future. Joachim Maier added: Could the Fe-effect be a formation of point defects such as nickel vacancies (doping effect) which would be extremely basic? Cf. R. Merkle and J. Maier, Top. Catal, 2006, 38, 141. Xile Hu responded: This is a very interesting idea. We will look into it. Guoxiong Wang enquired: Have you investigated the effect of the amount of iron content on OER activity? Xile Hu responded: We have done it only partially. We should do more; thank you for the suggestion. Pengfei Liu asked: According to the CV curve of Ni(OH)2 nanoparticles with Fe impurity, the anodic peak around 1.39V vs. RHE corresponds to the phase change Ni(OH)2 to NiOOH. My question is: does the anodic peak always mean that the metal element valence state changes; does the anodic peak have something to do with the OER performance. If do, how does it inuence this? Xile Hu replied: The position and shape of this redox peak are indicative of the phase of the Ni(OH)2 species. A certain phase is more active than the other. Our paper has a discussion of this, and has cited several papers on this topic. Pengfei Liu spoke: As we know, the HER is easier in acid electrolyte and the OER is easier in basic electrolyte. Some HER catalysts, for example, suldes, carbides, nitrides, phosphides and some carbon materials are active in acid This journal is © The Royal Society of Chemistry 2014 Faraday Discuss., 2014, 176, 429–445 | 433
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electrolyte but less active and more unstable in basic electrolyte. Some OER catalysts like perovskites, amorphous metal oxides and carbon materials are also less active and more unstable in acid electrolyte. My question is that, since we should use the same electrolyte for water electrolysis, should we develop active and stable HER catalysts in basic electrolyte or develop the active and stable OER catalysts in acid electrolyte? Which is more important: perhaps both? Xile Hu answered: You are absolutely right. In general HER is easier in acid and OER is easier in base. Now it is time to combine them. It appears that HER in base is more promising than OER in acid for most non-precious catalysts due to the instability of many oxides in acid. Watch out, we will soon publish a paper on photoelectrochemical HER in base for the rst time. Dehui Deng asked: Why does the performance of the material with Fe impurity increase aer 30 scans (looking at a plot of current density vs. potential)? Can you run for longer scans? Xile Hu replied: During the rst 30 scans, the catalyst undergoes a transformation to form the active phase, likely layered NiOOH species. During this process, the activity increases. Further scans do not give higher activity, as once the phase is formed, it is stable and its activity is constant. Gang Fu enquired: Where is the iron: on the Ni(OH)2 or on the carbon? What is the valence of the iron? Xile Hu replied: Iron should be close to nickel in the catalyst layer. The oxidation state of iron is likely to be Fe(III). Fuping Pan addressed Xile Hu: Much research is dedicated to develop low-cost yet efficient catalysts to replace noble metals for energy-related reactions, such as ORR and OER. How far can the non-precious catalyst go? Can they replace completely precious metal-based catalysts in practical application? Xile Hu answered: For ORR and OER under certain conditions, non-precious catalysts can do better than precious metals. For instance, in alkaline conditions, such catalysts exist and they can be further improved. Also even if non-precious catalysts have lower performance under certain conditions, for example, in acidic solutions, as long as the performance per cost is higher, they can be used. Guoxiong Wang asked: For OER a conductive material is not necessary, why? It is possible to enhance the OER activity by adding some electroactive material? Xile Hu replied: Actually for OER you also need highly conductive catalysts. Oen the catalysts are thin and porous, so you don't notice the problem in preliminary tests. Adding electroactive materials is a good idea. Jiafang Xie enquired: I'm interested in the role that Fe played in your system for its abnormal enhancement of the peak current in the CV sweep. I noticed that with Fe impurities, the potential of the peak current in CV is different to that of 434 | Faraday Discuss., 2014, 176, 429–445 This journal is © The Royal Society of Chemistry 2014
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without Fe impurities. As far as I know, this is likely to imply an different reaction in which Fe and/or your catalyst participate. As a similar example, copper quickly loses its ability to catalyze electrochemical carbon dioxide reduction to hydrocarbons when there is trace Fe impurity; the current is even larger, but only H2 can be detected. The explanation is that Fe atoms occupy the active sites on the surface of the copper and H is highly adsorbed on Fe. So, did you try to detect the product and make a comparison? I think this would be helpful. Xile Hu responded: Very good suggestion! We will look into it. Jiafang Xie then asked: I’m very interested in the role that Fe plays in your system. In your paper, you showed a CV of a comparison of with and without the removal of Fe. For a more complete comparison, experiments of adding Fe into the electrolyte or just doping Fe onto your catalyst should be conducted; I wonder if you have done these? And if you have, what happened? There result would help us determine the role of Fe impurities in your system. Xile Hu replied: Again very good suggestions! We have not done this and we will do it. Jiafang Xie added: This is a small question about the details of your experiment. In your work, when the nal suspension was obtained, it was allowed to rest for 3 h before further centrifugation. I wonder what is the meaning of this step. Is it of special purpose? Xile Hu answered: This is just to separate the solid from the solution. You can probably do it by a centrifuge, which would be faster. Shi-Gang Sun opened the discussion of the paper by Gang Fu: In the theoretical modeling, what are the effects of iron oxide and nickel oxide that may change the structure. Also, what is the effect of substrates of different single crystal planes? Gang Fu responded: Our DFT calculations suggested that the OH groups at Fe3+-OH-Pt react readily with CO adsorbed nearby to yield directly CO2 and produce simultaneously coordinatively unsaturated Fe sites for O2 activation. Ni2+ plays a key role in stabilizing the interface against dehydration through the strong interaction between Ni(OH)x and Fe(OH)x. We have explored different Pt planes, including at (111) surface and stepped (332) and (322) surfaces, and found that CO oxidation is a structure sensitive reaction, and the performance on different exposed surfaces depends on the bond strength of Fe(III)–OH. Beien Zhu asked: There are two well known mechanisms for CO oxidation. One is the Eley–Rideal mechanism which concerns CO and O2 forming OCOO; another is the Langmuir–Hinshelwood mechanism which involves the dissociation of O2. In your calculations, did you consider both mechanisms? Gang Fu answered: We only considered the Langmuir–Hinshelwood mechanism because there were still open Pt sites which can strongly adsorb CO. In our mechanism, CO oxidation is triggered by CO oxidative coupling with This journal is © The Royal Society of Chemistry 2014 Faraday Discuss., 2014, 176, 429–445 | 435
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interfacial OH groups at Fe-OH-Pt, simultaneously creating coordinatively unsaturated ferrous sites (Pt–,–Fe) for the following O2 activation. We also showed that CO can promote O–O bond cleavage via an [OC–O–O(H)Fe] transition state. Beien Zhu asked: In your work, how do you prevent the supported Fe nanoparticles from aggregating? Gang Fu responded: Despite exhibiting a high catalytic activity in CO oxidation, the Fe(OH)x/Pt catalyst did not work steadily for a long time, mainly due to the easy dehydration of Fe(OH)x sub-monolayers into three-dimensional iron oxyhydroxide or iron oxide. To prevent the dehydration-induced loss of interfacial Fe(III)-OH-Pt sites, it was undertaken to dope the overgrowth Fe(OH)x submonolayer with Ni2+ to form Pt/FeNi(OH)x hybrid nanoparticles. The resulting catalysts can maintain high catalytic activity for more than 28 h. Yiren Zhong enquired: Is the Fe(OH)x on the Pt surface amorphous or crystallized? Gang Fu answered: Fe(OH)x was deposited on the sub 5-nm Pt nanoparticles such that there is no long range periodicity in its structure, i.e. it is amorphous. However, the local structure of Fe(OH)x was found to be well-dened because XANES showed that the structure of Fe(OH)x is similar to goethite FeOOH. Computationally, we constructed Fe(OH)x models by cutting the Fe6O18 moiety from the goethite FeOOH crystal, and periodic density functional theory was performed. Yiren Zhong asked: According to your experience, could you give us a comment or a guess on which state of catalyst has more efficient catalytic effect, amorphous or crystal? Gang Fu replied: The efficiency of catalysts depends critically on how many active sites they contain rather than their crystal states. Fundamentally, researchers preferred to use model systems, such as single crystals or well-dened nano catalysts to explore the catalytic mechanisms. For example, we have demonstrated here that well-dened hybrid nanoparticles are an ideal model system to study interfacial effects. Based on our understanding of the interfacial effects, an alloy-assisted strategy was further developed to produce a more practical Pt-based nanocatalyst. Although the catalyst did not have easily characterized catalytic interfaces, it contained much more abundant Fe-OH–Pt catalytic interfaces and displayed a much higher catalytic performance than the welldened systems. So, efficient catalysts might not have well-dened structures for characterization. They could be either amorphous or crystalline. But they have to contain abundant catalytically active sites. Fuping Pan enquired: Can the water absorb and occupy the active sites? What is the accurate role of water?
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Gang Fu responded: The adsorption of water on the active sites is relatively weak, which cannot compete with the adsorption of CO and O2. Therefore, water is unable to block the active sites. However, water is necessary for low-temperature CO oxidation because the Pt/Fe(OH)x catalyst did not work steadily in dry streams. Based on our DFT calculations, we proposed that the water can promote O2 activation and recover the interfacial OH. Zhonghua Li asked: Why did you choose one layer Fe(III)–OH instead of multiple layers? Gang Fu answered: Experimentally, we used a wet-chemical method to fabricate Fe-OH–Pt interfaces by atomically thick Fe-OH partially covering the surface of monodisperse Pt nanocrystals. In order to match the structure characterized by experiment, we built catalyst models with a single layer but without multiple layers of Fe(OH)x deposited on different Pt surfaces, such as Pt(111), Pt(332) and Pt(322). Shihe Yang enquired: Presumably, the charge of the active center should change during the reaction. Does proton-coupled electron transfer have a role to play here? This could lower the activation energy. The fact that water is needed seems to speak in favor of this mechanism. Gang Fu replied: CO oxidation involves a two-electron redox reaction. In our mechanism, CO oxidation began with the adsorbed CO and interfacial –OH moieties, which can be viewed as a two-electron reductive elimination. During this process, the Fe(OH)x species serve as oxidants, which will be reduced by two units. For the rst CO oxidation, only proton transfers take place. Proton-coupled electron transfer plays a key role in forming the OOH species, in which the electron donor is Fe2+, while the proton donor is adsorbed H2O. The role the water plays has been emphasized in our paper. On one hand, the water can stabilize O2 adsorption by forming OOH species; on the other hand, the O–O bond breaking might also be facilitated by the coexistence of water. Gang Chen asked: How large are your iron nanoparticles? Can you estimate what percentage of atoms at the interface are of interest in your study? If so, you can estimate how much signal in your XANES spectra is from the interfacial Fe atoms. Gang Fu responded: Experimentally, we fabricated Fe-OH–Pt interfaces by partially covering the surface of mono disperse Pt nanoparticles (which are 200 cm1) we measured should be attributed to at a xed vq E potential, which is not a Stark-tuning rate. We suggest that both electron-donation/ back–donation and dipole–dipole interactions may contribute to such large slope. vnCO The fact that the on THH Pt NCs is much larger than that on bulk vq E single-crystal electrodes could be attributed to preferential CO adsorption on (100) sites on THH Pt NCs at low coverage, as illustrated in the paper. Although the overall CO coverages are the same, the local adsorbed CO density on the (100) terraces of (310) and (210) subfacets of THH Pt NCs is much larger than that on (100) terraces of (310) or (210) Pt single crystal planes, resulting in stronger dipole–dipole interactions that induce a big blue-shi of CO band position. At a xed coverage, we have determined that the Stark-tuning rate
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Mingquan Yu opened a general discussion by addressing Yujiang Song, Xile Hu and Shi-Gang Sun: When we conduct electrochemical tests, catalysts usually exhibit a redox wave. However, the redox wave would get smaller-and-smaller, even seem to disappear with slowing scan rate. So I’d like to ask what happens when the scan rate gets slower? Yujiang Song replied: I guess you are talking about cyclic voltammetry (CV) curves. When you use a slower scan rate, the current density should get smaller. The current density is proportional to the square root of scan rate. Xile Hu continued: In electrochemistry, scan rate dependent voltammetry will give information about the inuence of mass transport as well as capacitive behavior of the test molecule. You have to analyze it case-by-case. Shi-Gang Sun concluded: The relationship between redox current and scan rate depends on the types of electrochemical reactions and their reversibility. The electrooxidation reactions of most of small organic molecules are usually kineticcontrolled, but not diffusion-controlled (that is, mass transfer rate is larger than intrinsic electrochemical kinetics). So, as the potential scan rate decreases, the oxidation current will gradually decline and trend to a stable value, but not zero. As for adsorbed species reaction, e.g., hydrogen adsorption on Pt, the current can really trend to zero as scan rate decreases. Mingquan Yu addressed Yujiang Song, Xile Hu and Shi-Gang Sun: Has it become necessary for us to set standard electrochemical test conditions to compare the activity of different catalysts? As different groups oen choose to use different test conditions, it is hard for us to compare the activity, so I presume setting a standard electrochemical test condition is important and urgent. Yujiang Song replied: It is necessary to benchmark how to evaluate the electrocatalysts toward oxygen reduction reaction. I agree with you that it is critical and urgent to set up a standard for electrocatalyst examination. Xile Hu followed up these remarks: I agree. In fact, standard testing conditions already exist. It is only because many active researchers in the eld do not have a background in electrochemistry that we see papers reporting results under nonstandard conditions. This situation will improve soon. Shi-Gang Sun concurred: I also agree that a standard electrochemical test is very important in order to compare the results reported by different groups worldwide. Unfortunately, such standards have not been established yet due to the diversity of reactants, catalyst materials, and electrolyte. This requires that academic societies (e.g., IUPAC, ISE or ECS) promulgate the relevant standards. Joachim Maier addressed Gang Fu and Shi-Gang Sun: Do you know why, in catalysis, people do not concentrate more on point defects in the surfaces? Otherwise it is like dealing with catalysis on a water droplet and ignoring H+ and OH ions in the surface layer.
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Gang Fu answered: In the catalysis, the catalytic sites should be considered as “defect sites. The “defect” sites change their molecular structures during catalysis. Shi-Gang Sun added: We agree that point defects are also important in catalysis. For example, Ertl and co-workers observed by STM that the decomposition of NO exclusively occurs on line defects (i.e., edge sites) of Ru(0001) surface (Ertl et al., Science, 1996, 273, 1688–1690). However, it is difficult to control and monitor point defects on surfaces, especially on nanocrystals. So, it is challenging to study the role played by point defects during catalytic reactions at present. Zhongqun Tian continued the general discussion. The speakers have very nicely shown that catalytic reaction sites can be more well-dened with new synthesis methods, in situ characterization and theoretical calculation. What could be the next generation surface for energy chemistry with the synergetic effects, rationally designed paths and higher stability? Xile Hu spoke rst: With increasing understanding of catalysis at a molecular level, researchers will be able to develop next generations of catalysts that are highly active per atom, multifunctional due to synergetic effects, and with high stability once the decomposition pathways are known and inhibited. Yujiang Song continued: This is a very broad and challenging question! My answer will only focus on non-noble metal electrocatalysts (NNMEs) for the oxygen reduction reaction (ORR). In an alkaline medium, NNMEs are close to replacing platinum based electrocatalysts. If we follow the three rules for the rational design and synthesis of NNMEs (high surface area, high density of ORR active sites, and highly ordered active sites), the next step would be the creation of NNMEs with highly ordered active sites at a high density. In an acidic medium, it is not clear what is the exact nature of the ORR active sites of NNMEs even at the nano-scale. In this scenario, the rst task should be the identication of the ORR active sites and then to follow the three rules to develop the next generation of NNMEs for acidic conditions. Gang Fu added: In our opinion, the next generation surfaces should be multicomponent and multifunctional, which will enable simultaneous activation of multiple reactants having different properties, and conversion of them into the products with high selectivity. We have demonstrated that noble metal (NM)– transition metal hydroxide interface systems have a synergy effect to boost CO oxidation because CO can chemisorb on the NM, while transition metals usually have a relatively high oxygen affinity. It should be pointed out that CO oxidation is just a 2e redox process. For more complicated cases, a smart, sophisticated design is required. Generally speaking, to design a multicomponent catalyst, we should understand the functions of each component, and then combine them in a reasonable way to make the active center, and then prevent the loss of activity during the reaction process. To accomplish these steps, enormous efforts should be made to develop new synthesis methods, new characterization techniques, and new theoretical methods as well.
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Shi-Gang Sun concluded: The next generation surface for catalysis should have high-density of active sites and optimal surface atomic arrangement for a certain reaction. This needs to understand the surface structure–catalytic function relationships at the atomic level by a combination of experiment and theory. Another approach is to construct atom–nano–micro multiscale architectures with synergetic effects. In short, we need to engineer rationally multiscale structured catalysts from the atomic to nano and to micro structure, as well as with a local to long-range environment of active sites. Lee Cronin addressed Yujiang Song, Xile Hu, Gang Fu and Shi-Gang Sun: How do we more intelligently discover novel spaces in chemistry and exploit the interplay between theory and experiment in the understanding, design, and prediction of new catalysts? Yujiang Song answered: It is necessary to have experimentalists and theorists to communicate more efficiently, thus building up theoretical models that are close to real reaction systems. I tend to claim that only theoretical models that have been proven in real reactions are reliable. Otherwise, theoretical models should be re-constructed and optimized. Xile Hu noted: There is no smarter method in research than giving intelligent people the opportunity to do what they think is interesting. Novel discoveries will occur along the way. To further exploit the interplay between experiments and theories, we need better communication and to work on the same goal. Gang Fu continued: Rational design should be based on our understanding of the surface structure and reaction mechanism under experimental conditions. However, both of them are not fully understood due to the compositional and/or structural inhomogeneity of conventional catalysts. Thus, we should develop welldened systems (e.g. nanoparticles), which can decouple complicated factors, and allow the application of advance characterization techniques to established structure–activity relationships. To gain deeper insight, theoretical calculations should be performed to explain the experimental results, and further elucidate the intrinsic factors in catalysis. We expect that fruitful interaction between theory and experiment could promote the development of novel catalysts. Shi-Gang Sun added: In our opinion, understanding the surface structure– catalytic function relationships of nanoparticles is of crucial importance in the design and predication of new catalysts. This requires the development of nanocrystal-based model nanocatalysts with a well-ordered and tunable surface structure, which could bridge the huge gap between bulk single-crystal model catalysts and practical nanocatalysts in terms of size and structural complexity, and provide catalytic reaction principles more related to practical nanocatalysts. Theoretical methods can provide new insight into catalytic reaction mechanism and surface processes. On basis of this knowledge and by combination of experiment and theory, we may efficiently predict and design new catalysts.
444 | Faraday Discuss., 2014, 176, 429–445 This journal is © The Royal Society of Chemistry 2014
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Discussions
Faraday Discussions
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Wee Shong Chin communicated: While Professor Chao has discussed the use of phenylacetylene cross-linked to Si NPs, could he comment on the relative amount and the effect of this on the resultant material properties? Yimin Chao communicated in reply: In the paper we have not mentioned crosslinking so I assume you mean be coverage and overlap. The relative surface coverage is approximately 70% of the available surface. The effect the coverage has on the thermoelectric properties of the material has not been studied, as the coverage is difficult to control, but at a lower surface coverage we would expect a higher concentration of surface oxide and as a result a lower electrical conductivity. This is a problem with many semiconductor organic nanocomposites. Additionally the level of overlap between the chains would be expected to have a positive effect on the materials’ performance, as it would give better electron transfer between chains.
This journal is © The Royal Society of Chemistry 2014 Faraday Discuss., 2014, 176, 429–445 | 445
