Formation of supported rhodium clusters from mononuclear rhodium complexes controlled by the support and ligands on rhodium.

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Cite this: Phys. Chem. Chem. Phys., 2014, 16, 1262

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Formation of supported rhodium clusters from mononuclear rhodium complexes controlled by the support and ligands on rhodium† Pedro Serna,‡ab Dicle Yardimci,‡a Joseph D. Kistlera and Bruce C. Gates*a Extremely small supported rhodium clusters were prepared from rhodium complexes on the surfaces of solids with contrasting electron-donor properties. The samples were characterized by infrared and extended X-ray absorption fine structure spectroscopies to determine the changes occurring in the rhodium species resulting from treatments in hydrogen. Rhodium cluster formation occurred in the presence of H2, and the first steps are controlled by the electron-donor properties of the support—which acts as a ligand—and the other ligands bonded to the rhodium. The cluster formation begins at a lower temperature when the support is zeolite HY than when it is the better electron-donor MgO, provided that the other ligands on rhodium are ethene. In contrast, when these other ligands are CO, the pattern is reversed. The

Received 19th July 2013, Accepted 25th November 2013

choice of ligands including the support also allows regulation of the stoichiometry of the surface

DOI: 10.1039/c3cp53057d

combination of MgO as the support and ethene as a ligand allows restriction of the rhodium cluster size to

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the smallest possible—and these were formed in high yields. The data presented here are among the first characterizing the first steps of metal cluster formation.

transformations in H2 and the stability of the structures formed in the presence of other reactants. The

1. Introduction Transition metals are essential components in many catalysts used for the production of fuels, chemicals, pharmaceuticals, and polymers. Many of these metals are expensive, and it is economical to apply them as highly dispersed species with a large fraction of the atoms exposed and accessible to reactants. Thus, commonly used metal catalysts are dispersed on high-area porous solids such as metal oxides, zeolites, and carbon.1–3 The most highly dispersed supported catalytic species consist of only one or several metal atoms—and then an unrivalled degree of structural uniformity of the sites can be achieved, opening the door to incisive structural determinations4,5 and new opportunities for design of catalysts that are highly selective.6–8 Supported clusters or crystallites of metal, especially those that are noble, readily undergo deactivation by migration and aggregation to form more stable bulk structures, whereby a large fraction of the metal atoms become part of the bulk and no longer accessible to reactants.9–11 The well-known mechanisms a

Department of Chemical Engineering and Materials Science, University of California Davis, Davis, CA, USA. E-mail: [email protected] b Instituto de Tecnologia Quimica (UPV-CSIC), Universidad Politećnica de Valencia, Valencia, Spain † Electronic supplementary information (ESI) available. See DOI: 10.1039/ c3cp53057d ‡ Both authors contributed equally.

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of sintering by Brownian motion12 and Ostwald ripening13 typically account for the steps following the initiation of aggregation of supported metal species that are initially present in the most highly dispersed state, but relatively little is understood about the first steps of aggregation. Our goals were to better understand the initial steps of metal cluster formation by investigating the reactions of small, essentially molecular metal species on supports. The samples incorporate metal atoms bonded both to the support and to ligands such as those that may be present during catalysis. A specific goal of the research was to learn how to limit the growth of the metal species to extremely small clusters. Thus, we investigated the formation of the smallest clusters of rhodium (dimers) formed from singlemetal-atom (mononuclear) rhodium complexes on supports. A zeolite was selected as a support because it is crystalline and highly uniform—good for structure determination—and because it bonds to the rhodium as a weak electron-donating ligand.14,15 The other support, MgO, also has the advantage of a high degree of crystallinity (but it is less uniform structurally than the zeolite) and, in contrast to the zeolite, it is a good electron donor.15 The rhodium-containing precursor Rh(C2H4)2(acac) (acac is acetylacetonate) was chosen because it bonds to the supports with retention of the p-bonded ethene ligands, which are highly reactive with H2 (to form ethyl ligands or gas-phase ethane)16 in reactions that trigger cluster formation.10,17 For comparison, CO was also investigated as a ligand on rhodium because it is

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less reactive than ethene and expected to hinder H2 activation15 and, thus, the initiation of cluster formation. Changes in the structure of the supported rhodium species in H2 were tracked at various temperatures by infrared (IR), extended X-ray absorption fine structure (EXAFS), and X-ray absorption near edge structure (XANES) spectroscopies, providing a basis for elucidation of the roles of the support and the ligands in the reactivity of the supported metal species and their susceptibility to cluster formation. These techniques complement each other well, with the IR spectra providing information about changes in the ligands bonded to rhodium,18 EXAFS spectra indicating the formation of Rh–Rh bonds,16 and XANES spectra—by virtue of the presence or absence of isosbestic points—determining whether the transformations are stoichiometrically simple.19

2. Experimental methods 2.1

isolation of the initially formed supported mononuclear complexes to simplify the surface chemistry.20,21 2.3

Infrared spectroscopy

A Bruker IFS 66v/S spectrometer with a spectral resolution of 2 cm 1 was used to collect transmission IR spectra of MgO- and zeolite-supported rhodium samples. In the glovebox, each sample (typically, 50 mg), handled with exclusion of air and moisture, was pressed into a thin wafer and loaded into the cell (In situ Research Institute, South Bend, IN). The cell was connected to a vacuum system with a base pressure of 1.33 10 3 mbar that allowed recording of spectra while reactant gases (H2 and/or CO) flowed through the cell at a temperature in the range of 298–423 K. Spectra were collected in the mid-IR region with a deuterated triglycine sulphate detector. Each reported spectrum is the average of 64 scans. In some experiments, mass spectra of the gasphase effluents from the flow system were analyzed with a Pfeiffer Vacuum Omnistar GSD 301 mass spectrometer.

Materials and sample handling

Sample synthesis and handling were carried out with the exclusion of air and moisture by use of standard Schlenk techniques and a glovebox purged with argon that was circulated through traps containing supported copper and zeolite 4A for removal of O2 and moisture, respectively. Prior to adsorption of the precursor (Rh(C2H4)2(acac) or Rh(CO)2(acac)) on the support (MgO or zeolite HY), the support was pretreated. MgO (EM Science, BET surface area 79 m2 g 1) was treated with deionized water to form a paste, which was dried in air at 393 K. The solid was ground and calcined by treatment in flowing O2 at atmospheric pressure as the temperature was raised from room temperature to 973 K at a rate of 3 K min 1, followed by a 2 h soak. The calcination was followed by evacuation at 1.33 10 3 mbar for 14 h at 973 K and cooling to room temperature. Zeolite HY support (Zeolyst International, Si/Al = 30 (atomic)) was calcined by treatment in flowing O2 at atmospheric pressure as the temperature was raised from room temperature to 773 K at a rate of 3 K min 1, followed by a 2 h soak. The calcination was followed by evacuation at 1.33 10 3 mbar for 14 h at 773 K and cooling to room temperature. Helium (Airgas, 99.999%), H2 (Praxair, 99.999%), and CO (Airgas, 10%, balanced with helium) were purified by passage through traps containing supported copper and zeolite 4A to remove traces of O2 and moisture, respectively. 2.2

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Synthesis of supported species

The supported samples were prepared by contacting of Rh(C2H4)2(acac) (Strem, 99%) or Rh(CO)2(acac) (Strem, 99%) with the calcined MgO or zeolite HY powder in a slurry with n-pentane (Fisher, 99%) for 24 h. The solvent was removed by evacuation for 24 h. The resultant samples, which contained 0.4 wt% and 1.0 wt% Rh on MgO and zeolite HY, respectively, were stored in the glovebox. The rhodium loadings were selected to be high enough for satisfactory signal-to-noise ratios in the EXAFS spectra while still giving high degrees of site


2.4

X-ray absorption spectroscopy

X-ray absorption spectra were recorded at X-ray beamline 4-1 at the Stanford Synchrotron Radiation Lightsource. The storage ring electron energy was 3.0 GeV. The cryogenic double-crystal silicon (Si(220)) monochromator was detuned by 15–20% at the Rh K edge to minimize the effects of higher harmonics in the X-ray beam. Powder samples were pressed into self-supporting wafers in an N2-filled glovebox located at the synchrotron. Each wafer was loaded into a flow-through cell. Spectra of the samples were collected in transmission mode, and a reference rhodium foil was scanned simultaneously with the samples. The mass of each sample wafer was approximately 0.7 g, calculated to give an optimal X-ray absorbance of approximately 2.5 at the Rh K edge (23 220 eV). Each sample at 298 K was scanned in the presence of flowing helium for 1 h and then as it was treated in flowing H2 at a temperature in the range of 298–423 K. The flow rates of helium and of H2 were 50 mL(NTP) min 1. XANES and EXAFS spectra were recorded at intervals of 2 and 15 min, respectively. Details of the fitting of the EXAFS data are provided in the ESI.†

3. Results 3.1 Structural characterization of supported rhodium complexes IR and EXAFS spectra (Fig. S1 and S2, and Table S1, all in ESI†) show that chemisorption of Rh(C2H4)2(acac) took place with removal of the acac group, as Rh(C2H4)2 groups became bonded to each support; these spectra demonstrate the presence of p-bonded ethene ligands on the rhodium. The EXAFS spectra show that each Rh atom was bonded to each support by approximately two Rh–O bonds, consistent with earlier reports.22,23 Each rhodium complex on the zeolite was anchored by reaction with an acidic Al–OH site, as expected;22 the EXAFS Rh–Al coordination number was found to be approximately 1 (Table S1, ESI†),

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and when the sites were probed with CO, sharp IR bands appeared at 2053 and 2118 cm 1, assigned to nearly uniform electrondeficient rhodium gem-dicarbonyl species interacting with the acidic zeolite centres, as depicted below. The bonding at the Al sites (cation exchange sites) is as expected for these cationic complexes (with rhodium formally in the +1 oxidation state).22


3.2 Reaction of CO with supported rhodium diethene complexes Synthesis of zeolite- and MgO-supported rhodium carbonyl complexes was carried out by adsorption of Rh(CO)2(acac) on each support or, alternatively, by treatment of each of the supported rhodium diethene samples with a pulse of CO (350 CO molecules per Rh atom) at 298 K and 1 bar, which led to the immediate replacement of the p-bonded ethene ligands with CO and the formation of rhodium gem-dicarbonyl complexes, as shown by the IR and EXAFS spectra (Fig. S3 and Table S1, ESI,† respectively), consistent with previous results.22,23 The samples formed by either of these methods were equivalent to each other, as indicated by the IR spectra. The frequencies of the nCO bands (2000 and 2074 cm 1, characterizing the complex on MgO; and 2053 and 2118 cm 1, characterizing that on the zeolite) demonstrate significant differences in the electron-donor character of the two supports as ligands,15 with the basic MgO acting as the much stronger electron-donating ligand. The nCO bands of the zeolite-supported sample are markedly sharper than those of the MgO-supported sample, indicating the higher degree of uniformity of the species on the zeolite support, corresponding to the higher degree of uniformity of bonding sites on this crystalline solid.15 3.3 Reaction of supported rhodium complexes with H2 at 298 K Each of the four samples was treated with H2, a reducing agent known to induce aggregation of noble metals on supports.10,17 Treatments were performed as the temperature was ramped up to allow determination of the conditions of onset of Rh–Rh bond formation; spectra were obtained with samples in cells that operated as flow reactors. At the lowest temperature of our experiments (298 K), the zeolite-supported Rh(C2H4)2 complexes were converted into small rhodium clusters within 1.5 h of the start of treatment in H2. These clusters incorporated, on average, approximately 3 Rh atoms each, as shown by the EXAFS data—the Rh–Rh coordination number was approximately 2 (Table 1), in agreement with previous results.24 Consistent with the formation of clusters, the EXAFS contributions characterizing the rhodium–support and rhodium–ligand interactions (the Rh–O and Rh–C contributions in the spectra, Table 1) decreased only slightly from the initial values, an indication that the newly formed supported rhodium species were still low in nuclearity (at least on average), presumably having retained their essentially molecular nature. EXAFS spectra of the zeolite-supported Rh(C2H4)2 samples recorded during the H2 treatments, characterizing the contributions of the various Rh–backscatterer contributions as a function of the time on stream (Fig. 1A), indicate moreover


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that the reaction of the ethene ligands with H2 preceded the formation of Rh–Rh bonds, as the contribution arising from the former, at approximately 1.6 Å (phase shifts are not corrected in the plots) decayed significantly within the first 5 min after contact of the sample with H2, a period during which no Rh–Rh contributions were detected. IR spectra of the sample in flowing H2 at 298 K and 1 bar confirm that the initial p-bonded ethene ligands were converted either to s-bonded ethyl ligands or to gas-phase ethane, the latter detected in the downstream effluent from the flow system by mass spectrometry, within less than 2 min (Fig. 2A). The data show disappearance of the bands ascribed to the CH2 moiety, at 3087, 3061, and 3016 cm 1, with the concomitant appearance of new bands at 2960, 2935, and 2875 cm 1, indicating the formation of CH3 groups (in ethyl ligands). XANES spectra recorded during rhodium cluster formation on the zeolite (Fig. S6, ESI†) do not include isosbestic points and therefore indicate that the new surface species did not arise from a stoichiometrically simple reaction.19 This result instead indicates a mixture of surface species formed during the H2 treatment, presumably including unreduced rhodium complexes as well as rhodium clusters. In support of the inference that the supported species contained a mixture rather than just trirhodium clusters (which would also account for the observed Rh–Rh coordination number of approximately 2), we emphasize that molecular trirhodium clusters are rare,25 and so we doubt that any were present at all; tetrarhodium and hexarhodium clusters are common26 and considered more likely. In contrast to the Rh(C2H4)2 complexes on the zeolite, the isostructural rhodium complexes on MgO were found to be much more stable, as only a minor fraction of the initial mononuclear species underwent cluster formation under the conditions mentioned above (flowing H2 at 298 K and 1 bar) and longer exposure times, as shown by a Rh–Rh coordination number of approximately 0.3 after a 2 h treatment according to our EXAFS data (Table 1). Moreover, the IR spectra of the MgO-supported Rh(C2H4)2 sample after the treatment show only a minor diminution of the bands assigned to p-bonded ethene ligands, at 3059 and 2999 cm 1, indicating that only a small fraction of the initial ethene ligands underwent (partial) hydrogenation (Fig. 2B). The samples incorporating Rh(CO)2 rather than Rh(C2H4)2 complexes were found to be stable on each support in the presence of flowing H2 at 298 K, as indicated by the EXAFS and IR data (Table 1 and Fig. 3). 3.4 Reaction of supported rhodium complexes with H2 at temperatures >298 K In attempts to identify further contrasts in the behaviour of the four samples in the presence of H2, we carried out experiments similar to those described above, but at higher temperatures, in the range of 353 to 423 K. At 353 K, the Rh(C2H4)2 complexes on each support were converted into rhodium clusters. The XANES and EXAFS spectra characterizing the zeolite-supported sample indicate the formation of clusters with a Rh–Rh coordination number of


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Table 1

PCCP EXAFS fit parametersa characterizing supported rhodium species at the Rh K edge; the notationb is stated in the footnotes

Sample

Initial form of rhodium in sample

Support

Treatment gas/T (K)/time (h)

1

Rh(C2H4)2

MgO

H2/298/1

Rh–Rh Rh–Cethene Rh–Osupport Rh–Mg

0.5 3.7 2.2 1.0

2.68 2.09 2.18 2.83

6.50 0.88 3.23 0.21

5.7 1.1 8.7 4.4

14.2

2

Rh(C2H4)2

Zeolite HY

H2/298/1.5

Rh–Rh Rh–Cethene Rh–Osupport Rh–Al

2.1 0.7 1.4 1.1

2.70 2.11 2.17 3.02

13.1 1.27 5.62 6.10

6.7 7.1 6.3 0.5

5.1

3

Rh(CO)2

MgO

H2/298/1

Rh–Rh Rh–Osupport Rh–CCOterminal Rh–OCOterminal Rh–Mg

— 2.0 1.9 1.9 1.1

— 2.15 2.08 3.05 3.15

— 3.42 1.22 4.36 0.01

— 3.4 4.5 7.0 1.3

8.8

4

Rh(C2H4)2

MgO

H2/353/1

Rh–Rh Rh–Cethyl Rh–Osupport Rh–Mg

1.0 1.0 1.2 0.9

2.72 2.07 2.14 2.80

7.66 0.10 3.27 4.00

3.8 4.9 11 8.2

5.2

5

Rh(C2H4)2

Zeolite HY

H2/353/1

Rh–Rh Rh–Osupport Rh–Al

2.8 0.5 0.5

2.69 2.20 3.00

10.5 6.78 2.11

3.8 2.6 7.0

2.4

6

Rh(CO)2

MgO

H2/393/1

Rh–Rh Rh–CCOterminal Rh–OCOterminal Rh–Osupport

1.5 0.5 0.5 1.0

2.72 1.93 2.98 2.09

8.68 10.7 13.1 6.41

5.2 7.0 3.7 7.8

8.4

7

Rh(CO)2

Zeolite HY

H2/423/1

Rh–Rh Rh–Osupport Rh–CCOterminal Rh–OCOterminal Rh–Al

— 2.1 1.9 1.9 1.0

— 2.12 1.83 3.02 3.20

— 2.65 4.86 3.12 0.63

— 3.1 8.0 6.0 8.0

4.6

8

Rh(C2H4)2

MgO

Sample 4 + CO/298/Pulse

Rh–Rh Rh–CCOterminal Rh–OCOterminal Rh–CCObridging Rh–OCObridging

1.0 1.8 1.8 2.1 2.5

2.73 1.83 3.04 1.97 3.22

4.74 1.44 9.31 0.35 3.73

1.0 8.0 7.5 7.9 8.0

5.2

9

Rh(C2H4)2

Zeolite HY

Sample 5 + CO/298/Pulse

Rh–Rh Rh–CCOterminal Rh–OCOterminal Rh–Osupport Rh–Al

0.3 1.5 1.5 1.9 0.5

2.73 1.84 3.06 2.17 3.22

1.62 4.33 8.55 12.0 0.04

3.8 7.8 7.4 7.0 3.5

11.4

Shell

N

R (Å)

103 Ds2 (Å2)

DE0 (eV)

Goodness of fitc

a Sample 1 (Dk = 3.5–12.5 Å 1, DR = 0.8–3.3 Å). Sample 2 (Dk = 2.8–13.0 Å 1, DR = 0.8–3.0 Å). Sample 3 (Dk = 2.7–11.6 Å 1, DR = 0.8–3.2 Å). Sample 4 (Dk = 2.5–11.7 Å 1, DR = 0.8–3.2 Å). Sample 5 (Dk = 3.3–13.7 Å 1, DR = 0.8–3.2 Å). Sample 6 (Dk = 3.2–11.4 Å 1, DR = 0.8–3.3 Å). Sample 7 (Dk = 3.4–11.6 Å 1, DR = 0.8–3.3 Å). Sample 8 (Dk = 3.1–13.9 Å 1, DR = 0.8–3.5 Å). Sample 9 (Dk = 3.1–14.8 Å 1, DR = 0.8–3.3 Å). b Notation: T, temperature; N, coordination number; R, distance between absorber and backscatterer atoms; Ds2, disorder term; DE0, inner potential correction. Error bounds (accuracies) characterizing the structural parameters obtained by EXAFS spectroscopy are estimated to be as follows: N, 20%; R, 0.02 Å; Ds2, 20%; and DE0, 20%, but these values do not pertain to the Rh–Mg and Rh–Al contributions, for which the errors are greater. c The goodness of fit calculation method is described in the ESI.

approximately 3, indicating, on average, tetrarhodium species— clusters slightly larger than those formed at 298 K (Table 1). The absence of isosbestic points in the family of XANES spectra recorded during the H2 treatment (Fig. 4A) indicates a mixture of species with various rhodium nuclearities. The Rh(C2H4)2 species on MgO also underwent cluster formation at 353 K, but in this case the surface reaction evidently took place in a stoichiometrically simple way, as shown by the isosbestic points in the XANES spectra (Fig. 4B). Steady state, as indicated by a cessation of changes in these spectra, was


attained 30 min after the temperature reached 353 K with the sample in flowing H2, and the final species was characterized by a Rh–Rh coordination number of nearly 1, as shown by the EXAFS data (Table 1). Consistent with the greater average nuclearity of the clusters formed on the zeolite at 353 K relative to those formed on MgO under these conditions, the Rh–O and Rh–C coordination numbers characterizing the former are also lower than those characterizing the latter (Table 1), because, we infer, the formation of Rh–Rh bonds was accompanied by the breaking


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Fig. 1 Evolution of the Fourier transform function determined from time-resolved EXAFS data measured as a function of time on stream for zeolite- (A) and MgO- (B) supported Rh(C2H4)2 species in flowing H2.The experiment with the zeolite-supported sample was performed at a constant temperature of 298 K and a constant pressure of 1 bar. The experiment with the MgO-supported sample was performed at increasing temperatures (in the range 298–353 K), as indicated in the plot, and a constant pressure of 1 bar. In the plot, the Rh–backscatterer distances are not corrected for phase shifts. The colour bar represents the magnitude of each contribution, corresponding to the abundance of backscatterer atoms at a particular Rh–backscatterer distance. The peak at approximately 2.5 Å includes the Rh–Rh contribution, together with Rh–Al and Rh–Clong contributions, comparatively much less intense, and that at approximately 1.6 Å includes the Rh–low-Z-backscatterer contributions, Rh–O and Rh–C. Fits of the data characterizing the initial (time = 0) and final samples (time = 80–100 min) are provided in Table 1 and Table S1 and Fig. S7, S8, S12 and S14 (ESI†).

Fig. 2 IR spectra characterizing the C–H stretching region of HY zeolite-supported (A) and MgO-supported (B) Rh(C2H4)2 complexes after the following treatments: flow of helium at 298 K and 1 bar (a); flow of H2 at 298 K and 1 bar (b); flow of H2 at 353 K and 1 bar (c).

Fig. 3 IR spectra characterizing the CO stretching region of the following samples: MgO-supported Rh(CO)2 complexes in flowing helium at 298 K and 1 bar (a); MgO-supported Rh(CO)2 complexes after treatment in H2 at 298 K and 1 bar (b); HY zeolite-supported Rh(CO)2 complexes in flowing helium at 298 K and 1 bar (c); and HY zeolite-supported Rh(CO)2 complexes after treatment in H2 at 298 K and 1 bar (d).


of Rh–O bonds to allow migration of the rhodium and breaking of Rh–C bonds as ethene ligands were hydrogenated. As observed for the zeolite-supported Rh(C2H4)2 species, but at temperatures approaching 353 K rather than 298 K, EXAFS spectra of the MgO-supported Rh(C2H4)2 species recorded during the H2 treatment, characterizing the changes of the Rh–C and Rh–Rh contributions as a function of time on stream (Fig. 1B), indicate that the reaction of the ethene ligands with H2 precedes the formation of Rh–Rh bonds. The transformation of ethene into ethyl ligands was confirmed by IR spectra of the sample in the presence of H2 at increasing temperatures. The data show disappearance of the initial bands at 3059 and 2999 cm 1 at 353 K, with the concomitant growth of bands at 2963 and 2924 cm 1, ascribed to CH3 groups. A residual band at 2999 cm 1 was still observed after the treatment attributed to the presence of Hacac and Mg–acac species on the surface of the support (formed upon adsorption of the Rh(C2H4)2 precursor), which also produce that band.23


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Fig. 4 Normalized XANES spectra at the Rh K edge characterizing the samples during cluster formation from Rh(C2H4)2 complexes supported on (A) zeolite HY and (B) MgO16 as the temperature was ramped from 298 to 353 K with the sample in flowing H2 at 1 bar. The insets show changes in the whiteline peak intensities.

In contrast, treatment of the supported Rh(CO)2 complexes with H2 at temperatures in the range of 423–453 K resulted in the formation of rhodium clusters when the support was MgO, but not when it was the zeolite. On MgO, small clusters, characterized by an average Rh–Rh coordination number of approximately 1.5, formed from the Rh(CO)2 complexes at 423 K (Table 1). When the support was the zeolite, however, no Rh–Rh contribution was detected in the EXAFS spectra after such a treatment. Indeed, the zeolite-supported rhodium carbonyl complexes were stable even at higher temperatures (453 K), as demonstrated by the EXAFS and IR data (Table 1 and Fig. 5). These EXAFS results are consistent with IR data characterizing the carbonylated rhodium complexes on each support in flowing H2 at increasing temperatures. On the zeolite, the Rh(CO)2 species were highly stable in H2 as the IR spectra remained unchanged over the whole range of temperatures investigated (the two initial bands at 2053 and 2118 cm 1 were red-shifted by approximately 2 cm 1) as a result of the temperature increase from 298 to 453 K, and they shifted back to the initial positions after the sample was cooled to 298 K (Fig. 5A). The presence of rhodium gem-dicarbonyl species at the end of the H2 treatment (Fig. 5A) is consistent with the EXAFS data characterizing this sample (Table 1). The observations that the rhodium dicarbonyl bands during the entire H2 treatment were significantly blue-shifted (by approximately 40 cm 1) with respect to those of unsupported Rh(CO)2(acac) species,27 and the sharpness of these bands, measured as full width at half maximum (fwhm) o 5, indicate highly uniform rhodium carbonyl species and suggest that the rhodium remained tightly bonded at the Al sites of the zeolite, in agreement with the EXAFS results showing a constant Rh–Al coordination number of approximately 1 during the H2 treatment. In contrast, evidence of changes in the ligation of Rh(CO)2 species on MgO appeared at a relatively low temperature (353 K), as the IR spectra show that the initial bands ascribed to rhodium dicarbonyls, at 2077 and 2001 cm 1, decreased in intensity with the sample in flowing H2. The decrease in intensity of the carbonyl bands was accompanied by the appearance of a broad band at lower frequencies.


Fig. 5 IR spectra characterizing the CO stretching region of the following samples: (A) HY zeolite-supported Rh(CO)2 species in flowing helium at 298 K and 1 bar (a); the preceding sample in flowing H2 at 453 K and 1 bar for 30 min (b); HY zeolite-supported Rh(CO)2 species in flowing helium at 298 K and 1 bar after being treated in H2 at 453 K and 1 bar for 30 min (c). (B) MgO-supported Rh(CO)2 species in flowing H2 as the temperature was ramped from 298 to 353 K in 1 h ((a), time-resolved spectra); the sample resulting from the preceding treatment after subsequent exposure to a pulse of CO at 298 K and 1 bar (b); MgO-supported Rh(C2H4)2 species treated in flowing H2 at 353 K and 1 bar for 1 h after subsequent exposure to a pulse of CO at 298 K and 1 bar (c).

3.5

Reaction of H2-treated samples with CO

Each of the samples that had been treated with H2 was brought in contact with a stream consisting of 5% CO by volume in helium to determine the resistance of the rhodium frame to


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oxidative fragmentation by CO (a process that leads to the formation of mononuclear rhodium carbonyls on supports28). When the rhodium clusters that had formed upon treatment of the zeolite-supported Rh(C2H4)2 complexes with H2 at 298 K for 1.5 h were exposed to CO flowing at 298 K and 1 bar, the IR spectra show that the latter were oxidatively fragmented, as expected,29,30 to give a mixture of mononuclear species and clusters (predominantly the former). Within 5 min after the introduction of CO, the Rh–Rh coordination number had decreased from approximately 3 to approximately 0.3 (Table 1), giving supported species that were characterized by strong IR bands at 2053 and 2118 cm 1, which are characteristic of mononuclear zeolite-supported rhodium gem-dicarbonyls (Fig. S3B, ESI†). The lack of stability of the rhodium clusters on the zeolite is contrasted with the stability of the rhodium clusters on MgO. When the MgO-supported dimers formed from Rh(C2H4)2 complexes treated in H2 at 353 K for 1 h were treated with CO at 298 K and 1 bar, the resultant nCO spectrum consisted of bands at 2074, 2025, 2001, 1958, 1893, and 1853 cm 1 (Fig. 5), indicating the formation of partially carbonylated dinuclear rhodium clusters.16,31 Formation of these species did not alter the Rh–Rh coordination number indicated by EXAFS spectroscopy (Table 1), a result that demonstrates that the dirhodium clusters remained intact.16 The nCO spectra are characterized by sharp, intense bands, with those at 1893 and 1853 cm 1 indicating bridging CO ligands and thus confirming the existence of Rh–Rh bonds.26,32 These results reinforce the importance of the support in the stabilization of the metal species. In contrast, the MgO-supported clusters that formed from the supported rhodium gem-dicarbonyls in H2 at 393 K were characterized by relatively broad nCO bands (Fig. 5B). Exposure of the sample to CO to further carbonylate the rhodium resulted in the appearance of new nCO bands indicating bridging CO ligands and therefore neighbouring rhodium centres and rhodium clusters.26 The nCO bands of these

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bridging carbonyls are, however, much less intense than those produced by dimeric species formed from Rh(C2H4)2 in H2 at 353 K, and the rest of the bands are also narrower and more resolved in the spectrum of the latter. On the basis of these observations and the results shown in Table 1, demonstrating an average Rh–Rh coordination number of approximately 1.5, we infer the likely presence of a mixture of clusters of various sizes when the ligands on rhodium prior to the H2 treatment were CO, potentially together with some unconverted mononuclear complexes.

4. Discussion The data presented here provide evidence leading to a picture of the first steps of rhodium cluster formation on the zeolite and MgO supports (Fig. 6). The temperatures required for the onset of cluster formation (o423 K) are markedly lower than those typically applied in investigations of the mechanisms of metal nanoparticle sintering in supported metal catalysts;10,11 thus, our data characterize the first steps of cluster formation rather than sintering. The data clearly demonstrate that the ligands on the rhodium (including the support) affect the first steps of cluster formation. The effects of the support and the other ligands are not simply resolvable. The less electron-donating support, zeolite HY, facilitates the formation of Rh–Rh bonds, provided that ethene is bonded to the rhodium in the supported complexes. The observed temperatures of onset of cluster formation show that the rhodium ethene complexes are more stable on MgO than on the zeolite. In contrast, when the ligands on the rhodium complexes are CO, the rhodium complexes are more resistant to aggregation on the zeolite than on MgO. Because the surface area of the zeolite is substantially greater than that of the MgO, we used a lower metal loading

Fig. 6 Schematic representation summarizing the set of transformations observed for MgO-supported Rh(C2H4)2 complexes; MgO-supported Rh(CO)2 complexes; HY zeolite-supported Rh(C2H4)2 complexes; and HY zeolite-supported Rh(CO)2 complexes upon treatment in H2 at various temperatures (low temperature = 298 K; high temperature = 353–423 K) and subsequent treatment in CO at 298 K and 1 bar. Coordination numbers (CN) and catalyst stability were determined by EXAFS, XANES and IR data.



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on the latter (0.4 vs. 1 wt%). We rule out surface density of the supported metal species as a significant influence on the observed process of cluster formation—we stress that notwithstanding the difference in rhodium loading per unit surface area, (a) both the initially prepared MgO- and HY zeolite-supported samples incorporated site-isolated rhodium complexes with no evidence of Rh–Rh bonds and (b) the formation of rhodium clusters from Rh(C2H4)2 species occurred more facilely on the zeolite than on MgO (Table 1). Furthermore, the observation that Rh(CO)2 species on the zeolite remained virtually unchanged for long times in the presence of H2, even when the temperature was higher than that needed to form clusters on MgO, points to the inference that the resistance to aggregation is a consequence principally of the stability of the CO ligands in the reductive atmosphere and the strength of the interaction between rhodium and the acidic sites of the zeolite. The role of the metal loading on the surface is expected to be more important, in contrast, in processes of sintering that follow the initial cluster formation characterized here. Beyond the differences in stability of the rhodium complexes on the different supports, the XANES data indicate a significant contrast in the chemistry leading to cluster formation. Most significant, nearly unique species—rhodium dimers—form when the support is MgO and the ligands on rhodium are ethene. Formation of larger clusters evidently did not occur on this support under the mild conditions of dimer formation (353 K in H2). The structural simplicity of this supported dimeric rhodium species has provided the opportunity for investigations of the role of metal– metal bonds in catalysis; for example, a 58-fold increase in the catalytic activity for ethene hydrogenation resulted from the transformation of Rh(C2H4)2 species into the rhodium dimers, which are stable in reacting ethene–H2 mixtures.16 The data characterizing rhodium cluster formation on the zeolite, in contrast, demonstrate a mixture of species, suggesting the occurrence of a sequence of transformations whereby a fraction of the small clusters formed initially (presumably including rhodium dimers) keep migrating and undergoing further aggregation. On the other hand, the data also demonstrate differences in the stability of the rhodium clusters in contact with CO, depending on the nature of the support. This observation is important for catalysis of reactions involving CO (such as CO oxidation and alkene hydroformylation, which are large-scale processes), especially insofar as some such reactions involve neighbouring metal sites for activation of multiple reactants.33 The oxidative fragmentation of metal clusters on acidic supports caused by CO,29,30 which we observed for the zeolitesupported sample, is avoided when the support is the more electron-donating MgO, enabling the preparation of rhodium carbonyl dimers on the latter support. This sample shows excellent catalytic performance in the selective partial hydrogenation of 1,3-butadiene to give n-butenes upon partial decarbonylation of the rhodium species sites—while retaining the dimeric structure, a result that emphasizes the importance of investigations like the one reported here;7,8 we infer that results such as ours may be useful in guiding the design of stable, highly dispersed


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supported metal catalysts by choice of the support and other ligands on the metal.

5. Conclusions The data reported here characterize the first steps of aggregation of supported metal species in the most highly dispersed state. Our approach was to synthesize samples that incorporate supported rhodium complexes bonded to ligands with different reactivities (including the support, as a ligand), and then investigate the formation of extremely small rhodium clusters in the presence of H2 by using IR, EXAFS, and XANES spectroscopies to track changes in the structures of the surface species. The stability of the rhodium complexes and the stoichiometry of the surface-mediated transformations are regulated by the support and the other ligands bonded to the rhodium, being prompted at lower temperatures with zeolite HY than the better electron-donor MgO when the rhodium complexes incorporate ethene ligands, but occurring more facilely on the MgO than on the zeolite when the ligands are CO. The preparation of highly uniform rhodium dimers—active, selective and stable catalysts for the hydrogenation of various alkenes—is possible provided that MgO is used as the support and the initial ligands bonded to rhodium are ethene. Results such as the ones presented here may be useful in guiding the design of stable, highly dispersed supported metal catalysts.

Acknowledgements The research was supported by the European Union Seventh Framework Programme (FP7/2007-2013) under grant agreement PIOF-GA-2009-253129 (P.S.) and by DOE Basic Energy Sciences (Contract No. FG02-04ER15513) (DY) and DE-SC0005822 (JDK). We thank the DOE Division of Materials Sciences for its role in the operation and development of beamline 4-1 at the Stanford Synchrotron Radiation Lightsource. We thank the beamline staff for valuable support.

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Enantioselective Rhodium-Catalyzed Alkenylation of Aliphatic Imines.

Interaction of rhodium(III) with DNA.