J. Phys. Chem. C 2008, 112, 18039–18049
18039
Time-Resolved Structural Characterization of Formation and Break-up of Rhodium Clusters Supported in Highly Dealuminated Y Zeolite Ann J. Liang and Bruce C. Gates* Department of Chemical Engineering and Materials Science, UniVersity of California at DaVis, DaVis, California 95616 ReceiVed: July 4, 2008; ReVised Manuscript ReceiVed: September 2, 2008
Mononuclear rhodium complexes incorporating two ethylene ligands and anchored to dealuminated zeolite Y by two Rh-O bonds were characterized by transient extended X-ray absorption fine structure (EXAFS) spectroscopy and infrared (IR) spectroscopy as they were converted in the presence of H2. EXAFS spectra indicate reduction of the rhodium in the complex at 298 K to form rhodium clusters less than 3 Å in average diameter. Contacting of the resultant clusters with C2H4 led to their oxidative fragmentation, and the process was reversible. When the H2 treatment was carried out at a higher temperature (373 K), larger clusters formed. The reduction and oxidation of the rhodium were confirmed by X-ray absorption near edge spectra. During the ethylene treatments, ethyl groups formed on the rhodium, as indicated by IR spectra; treatment in H2 led to hydrogenation of these groups to form ethane, and the ethyl groups are inferred to be intermediates in the catalytic hydrogenation of ethylene. Ethylene in the gas phase helps to stabilize rhodium in the form of mononuclear complexes on the zeolite during catalysis, hindering the cluster formation. Introduction Rhodium complexes are among the most important soluble catalysts, for reactions including hydrogenation and hydroformylation of alkenes and carbonylation of methanol.1-6 Analogues of these complexes anchored to solids are among the most widely investigated supported catalysts, as they offer the advantages of minimized corrosion and ease of separation from products.7-9 But most supported metal complexes are challenging to characterize structurally, because they are intrinsically nonuniform, being anchored to nonuniform surfaces that are ligands. Furthermore, metals such as rhodium have a strong tendency to form clusters,10,11 complicating the characterization.12,13 There are only few data characterizing the formation of clusters from metal complexes on supports. Infrared (IR) and X-ray absorption fine structure (EXAFS) spectroscopies were used to follow the conversion of iridium complexes formed from Ir(CO)2(acac) (acac ) CH3COCHCOCH3) in zeolite NaY into Ir4(CO)12 as the sample was treated in CO;14a and gold complexes formed from Au(CH3)2(acac) on TiO2 were converted into gold nanoclusters as the sample was treated in helium and the temperature increased.14b The formation of platinum particles on TiO2 from supported Pt4+ ions formed from PtCl62+ was monitored with X-ray diffraction.14c These results provide clear representations of the cluster formation processes, but they leave the chemistry only partially characterized. Our goal was to elucidate such chemistry by characterizing both the cluster formation and its reverse and determining changes in the ligation of the metal during the changes. The methods were IR, EXAFS, and X-ray absorption near edge structure (XANES) spectroscopies, as these provide evidence of the changes in the metal-metal and metal-ligand bonds, and characterization of the gas-phase products by mass spectrometry. The starting material was a well-defined mono* To whom correspondence should be addressed. E-mail: bcgates@ ucdavis.edu.
nuclear rhodium complex on a well-defined (crystalline) support, a zeolite; it was prepared by reaction of the precursor Rh(C2H4)2(C5H7O2) [acetylacetonatobis(ethylene)rhodium(I)] with dealuminated Y zeolite having a low density of bonding sites for cationic metal complexes; EXAFS spectra and calculations at the density functional level indicated bonding of rhodium diethylene complexes at aluminum sites in the zeolite.15,16 The ligands on the Rh atoms in the mononuclear rhodium complexes formed initially on the support have been shown15 to be two ethylene ligands and two support oxygen atoms. The ethylene ligands exhibit the uniform fluxionality of molecular species, as shown by 13C NMR spectra.15 The results summarized here show that these supported complexes catalyze ethylene hydrogenation, and the only rhodium species detected by EXAFS spectroscopy during catalysis are mononuclear complexes; these complexes in the presence of H2 alone are reduced to form clusters, but when ethylene is present, the cluster formation is inhibited. Ethylene alone fragments the clusters. Thus, the composition of an ethylene-H2 mixture determines the structure of the supported catalyst for ethylene hydrogenation. Results Stability of Zeolite-Supported Site-Isolated Rhodium Complex in Ethylene. The supported sample formed by adsorption of Rh(C2H4)2(acac) on dealuminated Y zeolite (calcined at 773 K) contained 1 wt % Rh, corresponding to one Rh atom per five Al atoms in the zeolite. The sample in He was characterized by EXAFS spectroscopy, and the data provide no evidence of Rh-Rh contributions (Table 1). The results thus indicate site-isolated mononuclear rhodium complexes (in agreement with EXAFS spectra and XANES, presented elsewhere).15-17 The 16-electron RhI complex is expected to have a square planar structure,16,18–20 as does the precursor.21 IR spectra (Supporting Information) measured as the supported metal complex was treated in flowing ethylene at 298 K
10.1021/jp805917g CCC: $40.75 2008 American Chemical Society Published on Web 10/28/2008
18040 J. Phys. Chem. C, Vol. 112, No. 46, 2008
Liang and Gates
TABLE 1: EXAFS Results at the Rh K Edge Characterizing the Zeolite DAY-Supported Rhodium Complex during C2H4 Treatment and during C2H4 Hydrogenation Catalysis at 298 K and Atmospheric Pressurea,b gas in contact with sample He
C2H4
C2H4 + H2 (during catalytic hydrogenation of ethylene)
shell
N
Rh-Rh Rh-O Rh-C Rh-Rh Rh-O Rh-C Rh-Ald Rh-Rh
-c 2.1 3.8 -c 2.1 2.8 0.6 -c
Rh-O Rh-C Rh-Ald
2.0 1.9 0.5
R (Å)
103 × ∆σ2 (Å2)
∆E0 (eV)
2.07 2.03
0.5 0.9
4.2 4.2
2.06 2.21 3.07
4.9 7.0 9.2
2.2 2.2 0.8
2.11 2.27 2.95
2.1 0.6 0.6
0.5 0.5 14
a Notation: N, coordination number; R, distance between absorber and backscatterer atoms; ∆σ2, Debye-Waller factor; ∆E0, inner potential correction; the subscript s refers to short. b The catalysis experiments were carried out at 298 K with a molar ratio of H2 to ethylene in the feed to a flow reactor of 1:1. c Contribution not detectable. d The interpretation of the Rh-Al contribution characterizing the sample after ethylene treatment and ethylene hydrogenation catalysis is tempered by the small coordination number; the uncertainty in the parameters characterizing the Rh-Al contribution is large as a consequence of the small coordination number and therefore not well established.
and atmospheric pressure include peaks at 3086, 3062, 3015, 2979, 2963, 2932, and 2875 cm-1.22 Those at 3086, 3062, 3015, and 2979 cm-1 are assigned to π-bonded ethylene,16 and the others are assigned to ethyl, as the frequencies approximately match those of ethyl chloride,23 ethyl iodide adsorbed on Pt(111),24 and ethyl groups on the metal particles in Pt/SiO225,26 and Ni/SiO2.26 Blank experiments with ethylene in contact with the zeolite without rhodium gave no evidence of bonding of ethylene, and it is concluded that the spectra referred to in this paragraph indicate species bonded to rhodium in the zeolite. Mass spectra of the effluent gas were recorded periodically after the start of flow of ethylene over the sample; no changes were observed during the flow. EXAFS data recorded during the first hour of treatment of the initial supported sample in flowing ethylene gave no evidence within error of Rh-Rh contributions (Table 1, Supporting Information). These data also show that each supported rhodium complex remained bonded to approximately two oxygen atoms of the support, on average, as indicated by the Rh-O coordination number of 2.1 (Table 1). They also indicate changes in the metal-hydrocarbon ligands during the treatment, as the Rh-C coordination number decreased from ∼4 (3.8) to ∼3 (2.8) after 1 h of contact with C2H4. These data are consistent with the IR evidence that both ethylene and ethyl ligands were bonded to the rhodium. In summary, the IR, mass spectrometry, and EXAFS data, taken together, indicate retention of ethylene ligands by the Rh centers and the formation of ethyl ligands, presumably by reaction of ethylene ligands with OH groups of the support.27 Conversion of Zeolite-Supported Site-Isolated Rhodium Complexes to Clusters in H2. When the supported rhodium complex was treated in H2 at 298 K and atmospheric pressure, the rhodium was reduced and aggregated to form small clusters. The evidence is as follows. Figure 1 shows normalized XANES spectra at the Rh K-edge characterizing the zeolite-supported rhodium recorded during
Figure 1. XANES spectra at the Rh K-edge of the sample formed from Rh(C2H4)2(acac) and zeolite DAY during H2 treatment (for 80 min) at 298 K and atmospheric pressure. The XANES spectrum of rhodium foil is shown as a reference. The inset shows changes in the white line peak intensity during the treatment.
treatment for 80 min at 298 K in flowing H2 (at atmospheric pressure, with a H2 partial pressure of 55 mbar and the balance being He).28 The intensity of the white line (which originates from the transition of electrons from s states to unoccupied p states) (Figure 1, inset), centered at 15 eV higher than the absorption edge, decreased gradually throughout the 80-min treatment. Two isosbestic points, at 25 and 54 eV above the absorption edge (Figure 1), indicate the transformation of one species to another, suggesting a near uniformity of the resultant species, inferred below to be clusters. For comparison, XANES spectra characterizing rhodium foil (Figure 1) show a low-intensity white line centered 10 eV higher than the absorption edge and an intense peak located 35 eV higher than this edge. The spectrum of the sample formed by the H2 treatment is clearly different from that of rhodium metal. Nor do the XANES features characterizing the H2-treated sample match those of relatively large rhodium clusters formed by treatment of the supported mononuclear complex under harsh conditions, as discussed below. Consequently, we infer that the rhodium species in the H2-treated sample were different from the complexes initially present, but were not metallic; the XANES features show more resemblance to those of Rh2O329 than to those of rhodium foil. The EXAFS data recorded during the H2 treatment (Table 2, Figures 2 and 3) demonstrate the formation of rhodium clusters, indicated by the growing in of a Rh-Rh contribution illustrated by the increasing intensity of the peak at 2.4 Å (Figure 2B; this peak is not phase corrected, nor are the other peaks shown in Figure 2). The Rh-Rh contribution was detected after the supported sample had been in contact with H2 for only 8 min, indicating the rapid formation of rhodium clusters; then the Rh-Rh coordination number was found to be 0.2, with a Rh-Rh distance of 2.69 Å. Thereafter, the Rh-Rh coordination number increased with time on stream, reaching a value of 1.1 after 36 min. Thus, each rhodium species, on average, then comprised ∼2 Rh atoms. If we assume that the sample was structurally uniform and incorporated rhodium exclusively in dimers, the average rate of dimer formation over the 36-min treatment was ∼3 × 1017 dimers s-1 (g of zeolite)-1. After 80 min of continued treatment in H2, the Rh-Rh coordination number increased to 1.6, at a distance of 2.68 Å (Table 2). These results indicate that the rhodium reacted further
Rhodium Clusters in Highly Dealuminated Y Zeolite
J. Phys. Chem. C, Vol. 112, No. 46, 2008 18041
TABLE 2: EXAFS Results at the Rh K Edge Characterizing the Effects of H2 and C2H4 Treatments at 298 K on Zeolite DAY-Supported Rhodium Samplesa Rh-Rh contribution time on stream, min
feed gas present during scan
0 80 160 240 296
He H2 C2H4 H2 C2H4
a
Rh-O contribution
N
R (Å)
103 × ∆σ2 (Å2)
∆E0 (eV)
0.0 1.6 0.4 1.6 0.6
0.00 2.68 2.69 2.68 2.68
5.8 4.4 5.3 4.1
-8.3 3.9 3.7 4.3
Rh-C contribution
N
R (Å)
103 × ∆σ2 (Å2)
∆E0 (eV)
2.2 1.7 1.9 1.1 2.2
2.08 2.07 2.06 2.15 2.09
0.1 4.3 2.3 1.4 6.0
5.6 -5.9 1.7 -4.7 1.1
N
R (Å)
103 × ∆σ2 (Å2)
∆E0 (eV)
3.6 0.9 1.7 0.6 1.8
2.01 1.98 2.22 2.05 2.23
0.5 1.5 0.7 0.0 3.5
5.6 -5.9 1.7 -4.7 1.1
Notation as in Table 1. Experiments were performed in the sequence listed.
Figure 2. Fourier transform functions (k3-weighting) determined from time-resolved EXAFS spectra recorded at 298 K and atmospheric pressure characterizing the sample prepared from Rh(C2H4)2(acac) and zeolite DAY as it was treated in flowing (A) He, (B) H2 after A, (C) C2H4 after B, (D) H2 after C, and (E) C2H4 after D. Following the treatments mentioned above, the sample was then treated in flowing (F) H2 when temperature was ramping from 298 to 373 K, (G) H2 at 373 K for 1 h, (H) He as temperature cooled from 373 to 298 K, and (I) C2H4 at 298 K. The partial pressures of H2 and C2H4 were 55 and 55 mbar, with the remainder being He. The total feed flow rate was 35 mL/min (except during the period when temperature was ramped to 373 K; the total feed flow rate was then 22 mL/min and partial pressure of H2 was 500 mbar), and the catalyst mass was 0.45 g. The radial positions were not corrected for phase shifts.
with H2 and formed larger clusters. The distance of the Rh-Rh contribution (2.68 Å) matches that of the initially H2-treated sample and that of bulk rhodium metal,30 within the experimental uncertainty. The average number of Rh atoms per cluster was in the range of 2-3 (presuming that all the mononuclear rhodium species had been converted). The EXAFS data also show how the metal-support interface changed as the clusters formed, as indicated by a decrease in intensity of the peak at 1.6 Å representing metal-low-Z scatterer contributions (Figure 2B). Both Rh-O and Rh-C bonds broke as the Rh-Rh bonds formed (Figures 2B and 3). Within the first 4 min of H2 flow, the value of the Rh-O distance (a bonding distance) apparently increased from 2.08 to 2.11 Å (Figure 2B) (but this change may be insignificant within error), while the Rh-O coordination number increased from 2.2 to 2.8 (Figure 3). Thus, there was a significant structural change during just the first few minutes of reduction. After 8 min, the value of the Rh-O coordination number had passed through a maximum, decreasing to 2.3 (as the distance remained unchanged within error) (Figure 3), indicating restructuring of the metal-support interface. As the Rh-Rh coordination number
Figure 3. Cluster formation. Results of EXAFS analysis characterizing sample formed from Rh(C2H4)(acac) and zeolite DAY during H2 treatment at 298 K and atmospheric pressure; changes in coordination number of Rh-Rh ([), Rh-O (9), and Rh-C (2) contributions estimated from time-resolved EXAFS spectra.
continued to increase, reaching a value of 1.6 at the end of the H2 treatment, the Rh-O coordination number continued to decrease, to 1.5. These results indicate clearly that Rh-O bonds broke as Rh-Rh bonds formedsthe Rh atoms became increasingly bonded to each other rather than the support as clusters formed. Simultaneous changes were observed for the Rh-C contribution (Figure 3); within 4 min after the start of the H2 treatment, the value of the Rh-C coordination number had increased from 3.6 to 4.2, while the Rh-C distance remained essentially unchanged. But the Rh-C coordination number passed through a maximum, after 8 min having declined to 2.8, and at the end of the H2 treatment having declined to about 1. These results indicate that hydrocarbon ligands were removed from the rhodium. IR spectra representing the sample during treatment in H2 also demonstrate the removal of ethylene ligands, and the mass spectra of the effluent products demonstrate the formation of ethane, indicating that the ligands were hydrogenated. Details follow. (The times stated in the next paragraph do not correspond exactly to those in the preceding paragraphs because of the different lag times in the two flow systems.) IR spectra were recorded as the initially prepared sample was treated in H2 under the same conditions mentioned above.31 The spectra in the C-H stretching region before H2 treatment (Figure 4) include bands at 3085, 3060, 3014, and 2979 cm-1, characteristic of ethylene ligands bonded to Rh in the assynthesized sample (Figure 4a). During H2 flow, these bands
18042 J. Phys. Chem. C, Vol. 112, No. 46, 2008
Figure 4. IR spectra of sample formed from Rh(C2H4)2(acac) and zeolite DAY treated at 298 K and atmospheric pressure in the following sequence: (a) He, (b) H2, and (c) C2H4 (after evacuation of gas phase C2H4).
Figure 5. Changes in intensity of the mass spectral (MS) signals (× m/e ) 2; b, m/e ) 27; and 2, m/e ) 30) of the effluent gases from the flow reactor when the sample formed from Rh(C2H4)2(acac) and partially dehydroxylated zeolite DAY was treated in H2 at 298 K and atmospheric pressure.
decreased in intensity (Figure 4b), disappearing within the first 2 min. Simultaneously, mass spectra of the effluent gas gave evidence of ethane (Figure 5). The evidence that ethylene ligands were hydrogenated to form ethane is consistent with earlier results characterizing the equivalent sample treated under similar conditions.16 The removal of hydrocarbon shown by the IR and mass spectrometry data and matching the EXAFS results is important in confirming that Rh-C contribution in the EXAFS spectra were identified correctly. In summary, the ethylene ligands of the zeolite-supported rhodium complex reacted with H2 to form ethane. During this process, the mononuclear rhodium complex was reduced, and as the Rh-O and Rh-C bonds broke, Rh-Rh bonds formed. With an Rh-Rh coordination number of 1.6, the resultant clusters contained two or three Rh atoms each, on average, and the Rh atoms in them were bonded on average to two oxygen atoms of the zeolite, as demonstrated by the EXAFS data (Table 2). The Rh-C coordination number at the end of the H2 treatment was still substantially greater than zero (Table 2), and so we infer that some hydrocarbon remained bonded to the Rh centers in the clusters.
Liang and Gates
Figure 6. XANES spectra at the Rh K-edge of the sample, formed from Rh(C2H4)2(acac) and zeolite DAY and treated in H2 for 80 min, during the C2H4 treatment at 298 K and atmospheric pressure. The duration of the C2H4 treatment was 80 min. The inset shows the white line peak intensity during the treatment.
Fragmentation of Supported Rhodium Clusters in C2H4. Figure 6 shows the XANES spectra of the sample containing the small rhodium clusters that had formed during the treatment in H2 as they were subsequently treated in flowing C2H4 (at a partial pressure of 55 mbar in He) at 298 K. The data indicate a partial reversal of the cluster formation that occurred in H2, as the white line intensity increased during the treatment. Two isosbestic points, at 25 and 54 eV above the absorption edge, were again observed, consistent with the conversion from essentially one rhodium species (clusters) to another and indicating changes in the oxidation state and/or ligand environment of rhodium. At the end of the treatment, the white-line intensity of the sample (Supporting Information) was still lower than that of the initial mononuclear sample (Supporting Information), indicating that the reduction/aggregation process of the H2 treatment had not been completely reversed by the treatment in C2H4. Furthermore, the shapes of the white lines characterizing the sample under the two sets of conditions mentioned above are slightly different, suggesting different ligand environments around the rhodium in them. EXAFS results (Figure 7) recorded as the supported rhodium clusters were treated in flowing C2H4 confirm the fragmentation, indicated by the decreased Rh-Rh coordination number and illustrated by the diminishing intensity of the Rh-Rh contribution indicated by the peak at 2.4 Å (Figure 2C). Figure 2C shows a sharp decrease in intensity of this peak within 4 min of the start of C2H4 flow. Further changes in the intensity of the peak after 8 min of treatment were insignificant, indicating that the fragmentation process had then essentially stopped. The final Rh-Rh coordination number (after 80 min) was 0.4, at a distance of 2.69 Å. Thus, the fragmentation in C2H4 was only partial, but the value of the coordination number shows that some mononuclear species had formed, consistent with the observation of the same isosbestic points in the XANES spectra during both the H2 and C2H4 treatments. Structural changes in the metal-support interface during the C2H4 treatment were insignificant, as indicated by the average Rh-O coordination number of 2 and the average Rh-O distance of 2.07 Å; these two values remained unchanged (within error), indicating Rh atoms in the supported sample still remained bonded to two oxygen atoms of the support, on average.
Rhodium Clusters in Highly Dealuminated Y Zeolite
Figure 7. Results of EXAFS analysis characterizing sample formed from Rh(C2H4)2(acac) and zeolite DAY treated in H2 for 80 min, followed by C2H4 treatment at 298 K and atmospheric pressure: changes in coordination number of Rh-Rh ([), Rh-O (9), and Rh-C (2) contributions estimated from time-resolved EXAFS spectra recorded during C2H4 treatment.
In contrast, structural changes were observed in the metalhydrocarbon ligand environment during the cluster fragmentation, illustrated by the increasing intensity of the peak at 1.6 Å (Figure 2C). The data (Figure 7) show that the Rh-C coordination number increased as the Rh-Rh coordination number decreased. At the end of the treatment, the Rh-C coordination number was ∼2, at a distance of 2.22 Å (Table 1), suggesting formation of ethyl ligands, inferred to occur by bonding of ethylene followed by reaction with support OH groups. The IR spectra provide complementary information about the hydrocarbon ligands. The intensities of the bands in the C-H stretching region during the C2H4 treatment (Supporting Information), at 2962, 2931, and 2877 cm-1, had increased substantially after 80 min of treatment, and they remained unchanged after subsequent evacuation (Figure 4c). These bands are assigned to ethyl groups stably bonded to rhodium,23-26,32 consistent with the EXAFS results. Ethane was detected in the effluent gas by mass spectrometry right after the beginning of C2H4 flow (there was a short time lag in the flow system31). This result suggests that rhodium hydride species that had formed during the preceding treatment in H2 reacted with ethylene to form ethane. In summary, the rhodium clusters that had formed from the supported mononuclear complexes were fragmented during C2H4 treatment and converted into a mixture of rhodium complexes and clusters. The remaining clusters and the mononuclear complexes present with them remained bonded to oxygen atoms of the support, and ethyl ligands formed on the Rh centers. These results show that ethylene stabilizes rhodium in a highly dispersed form, whereas H2 leads to reduction and aggregation of the rhodium. Reaggregation of the Fragmented Rhodium Clusters in H2. When the sample containing the partially fragmented rhodium clusters was re-exposed to H2 (55 mbar in He) at 298 K and atmospheric pressure, EXAFS spectra were observed that almost match those observed when the sample was treated in flowing H2 for the first time (Figure 2B and D). The results of the EXAFS analysis (Supporting Information) show that the structural changes nearly matched those observed in the initial H2 treatment. Specifically, the Rh-Rh coordination number gradually increased, reaching a value of 1.6 after the sample
J. Phys. Chem. C, Vol. 112, No. 46, 2008 18043 had been exposed to H2 for 80 min, essentially the same as that of the sample after the first H2 treatment (Table 2). Again, the changes in the Rh-O and Rh-C coordination numbers occurred concomitant with the changes in the Rh-Rh coordination number during the treatment, as Rh-O and Rh-C bonds broke during the cluster formation (the Rh-O and Rh-C coordination numbers decreased to 1.1 and 0.6, respectively; these values are smaller than those of the sample treated in H2 for the first time, indicating different ligation of the metal in the sample that was re-exposed to H2). Furthermore, ethane was again found in the effluent gas, consistent with the inference that ethyl ligands that had formed from the C2H4 treatment were hydrogenated by the H2. Refragmentation of Rhodium Clusters in C2H4. When the reformed rhodium clusters were again brought in contact with C2H4 (55 mbar in He), EXAFS spectra were observed matching those of the aforementioned samples treated in C2H4 (Figure 2C and E). Thus, the results (also see Supporting Information) show that, after the first process of cluster formation, the aggregation and (partial) fragmentation of the rhodium was reversible at 298 K. Reaction of the Fragmented Rhodium Clusters in H2 at Elevated Temperature. The supported rhodium species, after twice being cycled through the processes of cluster formation and fragmentation, were subjected to a more severe treatment in H2. XANES spectra were recorded as the sample underwent the further H2 treatment (at a partial pressure of 500 mbar in He and a total flow rate of 22 mL (NTP)/min) with the temperature ramped at a rate of 3 K/min from 298 to 373 K, followed by a period of 60 min at 373 K (Supporting Information). The final spectra show features similar to those of rhodium foil (Supporting Information), demonstrating that the supported rhodium had now been further reduced and formed clusters larger than those mentioned above, having electronic properties closely approaching those of metallic rhodium. Consistent with these XANES results, the EXAFS data (Table 3, Figure 8) show that the clusters remained relatively small when the temperature was raised to 319 K, as indicated by the Rh-Rh coordination number of 0.5. As the temperature increased further, however, the clusters grew (Figure 8): following the heating in H2 to 373 K, the Rh-Rh coordination number after 1 h was 4.4, with a distance of 2.67 Å; thus, the data indicate clusters with an average diameter of about 5 or 6 Å in this sample; this average size corresponds to a metal dispersion of nearly 100% and indicates that each cluster incorporates on average approximately nine atoms if spherical clusters are assumed.33,34 Again, structural changes in the metal-ligand environment were indicated by the EXAFS spectra recorded during the aggregation. The data (Figure 2F) show that the intensity of the peak at 1.6 decreased soon after the sample was brought in contact with H2. After the temperature reached 329 K (Table 3), no detectable Rh-C contribution remained, and thus we infer that the hydrocarbon species either reacted completely with H2 (or decomposed) at these higher temperatures. Effect of C2H4 on Larger Rhodium Clusters. Further treatment of the sample incorporating the larger rhodium clusters, after cooling, in C2H4 at 298 K for 60 min (C2H4 partial pressure 55 mbar in He) led to partial fragmentation of the clusters. The EXAFS data (Table 3) show that the Rh-Rh coordination number decreased from 4.8 to 3.3, with the average Rh-Rh distance being 2.67 Å after 60 min, and thereafter remaining unchanged within error. Again, there was a partial
18044 J. Phys. Chem. C, Vol. 112, No. 46, 2008
Liang and Gates
TABLE 3: EXAFS Results at the Rh K Edge Characterizing Zeolite DAY-Supported Rhodium Species in H2, He, and C2H4 at Elevated Temperaturesa Rh-Rh contributions temperature, K 319 329 339 349 359 369 373 373 354 332 318 309 304 298
time on feed gas present stream, min during scan 300 304 308 312 316 320 324 376 380 384 388 392 396 452
H2 H2 H2 H2 H2 H2 H2 H2 He He He He He C2H4
N 0.5 0.9 1.7 2.0 2.4 3.0 3.6 4.4 4.3 4.8 4.8 4.7 4.6 3.3
R 103 × ∆σ2 (Å) (Å2) 2.68 2.67 2.67 2.67 2.67 2.66 2.67 2.67 2.67 2.67 2.67 2.68 2.68 2.67
4.0 5.0 6.3 6.5 6.5 7.3 8.0 9.2 8.8 9.1 9.1 8.8 8.7 8.4
Rh-O contribution ∆E0 (eV)
N
2.2 3.4 5.3 5.8 6.5 6.4 6.4 4.0 5.3 4.4 4.5 4.4 4.3 5.7
2.2 2.0 1.6 1.3 1.2 0.9 0.7 0.6 0.6 0.6 0.6 0.6 0.6 -
R 103 × ∆σ2 (Å) (Å2) 2.10 2.12 2.14 2.16 2.17 2.19 2.20 2.18 2.19 2.19 2.20 2.19 2.19 -
3.9 8.3 8.6 7.3 7.6 5.4 3.3 1.6 1.2 1.2 1.6 1.3 1.5 -
Rh-C contribution ∆E0 (eV)
N
R (Å)
103 × ∆σ2 (Å2)
∆E0 (eV)
-1.3 -4.4 -6.9 -8.5 -10.6 -12.5 -12.6 -7.2 -7.0 -8.4 -9.5 -9.5 -8.4 -
0.9 2.5
2.26 2.24
0 5.9
-1.3 -4.8
a Notation as in Table 1. Experiments were performed in the sequence listed, and these are the continuation of the experiments listed in Table 1.
Figure 8. Results of EXAFS analysis characterizing sample formed from Rh(C2H4)2(acac) and zeolite DAY treated in flowing H2 and C2H4 for two cycles, with subsequent treatment in flowing H2 at elevated temperature: the three lower curves indicate changes in coordination number of the Rh-Rh ([), Rh-O (9), and Rh-C (2) contributions estimated from time-resolved EXAFS spectra recorded during C2H4 treatment; the upper curve indicates the temperature profile.
breakup of the clusters in C2H4, but when the clusters were larger, the degree of fragmentation was less. Catalytic Hydrogenation of C2H4 and Stabilization of Mononuclear Rhodium Complexes as the Catalytically Active Species. The results presented above raise the question of the state of rhodium in the sample during catalysis of ethylene hydrogenation. The catalyst performance was measured with the initially prepared sample in a packed-bed once-through plug-flow reactor operated essentially isothermally at 294 K and atmospheric pressure, with a 1:1 molar ratio of H2 to C2H4 in the feed. The methods are as described elsewhere.16 The only reaction product was ethane, and there was no detectable conversion in the absence of the supported rhodium catalyst. When the catalyst mass was 50 mg and the feed flow rate 100 mL(NTP)/min, the operation of the reactor and the activity of the catalyst stabilized after about 20 min, with the conversion being 0.5%. A similar experiment was carried out with the EXAFS cell35 serving as a flow reactor and spectra recorded as the catalyst functioned. The conditions matched those of the plug-flow
reactor experiments, except that the catalyst was present as a pressed wafer rather than a powder, and the mass of catalyst in the spectroscopy experiment was about 10 times greater than that used in the plug-flow reactor experiment (the additional catalyst was needed to provide a satisfactory EXAFS signalto-noise ratio); consequently, the conversion in the flow-through cell is inferred to have been roughly 10 times greater than that in the plug-flow reactor (this conversion is assumed to have still been differential).36 The EXAFS data characterizing the working catalyst are shown in Table 1 and in the Supporting Information. They give no evidence of Rh-Rh bonds, within error, and thus point to the continued presence of mononuclear rhodium complexes as the catalytic species, even though H2 was present in the feed stream. Nor do the EXAFS data give evidence of significant changes in the metal-support interface during catalysis, as indicated by the Rh-O coordination number of 2.0 (Table 1). However, changes were observed in the metal-hydrocarbon ligand environment, as the Rh-C coordination number decreased from about 4 to 1.9 after the supported rhodium complex had been exposed to C2H4 + H2 for 1 h, suggesting that (two) ethylene ligands were no longer bonded to the rhodium, having been replaced by (two) ethyl ligands, which are inferred to be reaction intermediates in ethylene hydrogenation catalysis. We emphasize, however, that the errors in the EXAFS parameters characterizing the Rh-C coordination numbers (Table 1) are so large that we cannot be confident of the number and nature of the hydrocarbon ligands on the rhodium or entirely rule out the presence of a small number of rhodium clusters. IR spectra were also recorded to characterize the functioning catalyst in a flow reactor (Supporting Information).32 They show the disappearance of IR bands associated with ethylene ligands bonded to the rhodium, consistent with the conversion of C2H4 with H2. New IR bands, at 2962, 2931, and 2876 cm-1, formed, similar to those observed when the sample was treated in flowing ethylene; these are assigned to ethyl groups.37 These IR data are consistent with the EXAFS results characterizing the working catalyst, and with the hypothesis that ethyl groups are intermediates in the catalytic reaction. The evidence that the rhodium was dispersed as mononuclear species during catalysis demonstrates that ethylene stabilizes the rhodium in this form; evidently the tendency of H2 to cause
Rhodium Clusters in Highly Dealuminated Y Zeolite reduction and aggregation of the rhodium was not strong enough to overcome the tendency of the ethylene to prevent these changes under the conditions of our catalytic reaction experiments (but we expect that aggregation would occur under other catalytic reaction conditions (such as at high H2/C2H4 ratios)). Discussion Changes Occurring During Formation of Supported Rhodium Clusters. Noble metal complexes are readily converted into metal clusters, with the aggregation depending on the environment and reaction conditions, such as temperature and H2 partial pressure. Formation of clusters from such complexes, either in solution or on the surfaces of supports, has often gone unnoticed because of the smallness of the clusters and the challenge of detecting them. The results presented here provide new evidence of the chemistry occurring during the cluster formation process that accounts for changes in the oxidation state of the metal and the structure and ligation of the metal species (Figure 1). The XANES of the initially prepared sample clearly indicates cationic rhodium, with a white line centered at 15 eV above the absorption edge.29 These data are not sufficient to determine the exact rhodium oxidation state, although the changes in the XANES spectra clearly demonstrate that the H2 treatment caused reduction of the rhodium, indicated by the decrease in intensity of the white line (Figure 1). During the reduction, the Rh-Rh coordination number increased, demonstrating the formation of rhodium clusters (Figure 3, Table 2). The clusters, comprising approximately two or three Rh atoms each, on average, have XANES features that are clearly different from those of rhodium foil (Figure 1), indicating that the clusters are too small to be considered metallic and thus raising the question of the oxidation state of the rhodium. The oxidation state of rhodium in small zeolite-supported clusters was addressed by Vayssilov et al.,38 who used density functional theory (DFT) to determine the most stable structure and to estimate the Mulliken charges on rhodium. Rh6 was chosen as a model cluster because it contains metal atoms in a stable octahedral framework (as in Rh6(CO)16) and could be synthesized in high yield on the support, as indicated by EXAFS results.39 The supported Rh6 clusters were shown to be markedly stabilized by hydride ligands (arising from OH groups of the support), with the Rh atoms at the metal-support interface being positively charged (Mulliken charge: 0.76 e). Support OH groups were inferred to oxidize the Rh atoms bonded to the support, but not the other Rh atoms in the clusters, which were found to have Mulliken charges of nearly zero.38 The reaction of bare Rh6 clusters with support OH groups (by reverse spillover) was found to be markedly downhill energetically.40 Vayssilov’s results38 are in qualitative agreement with our XANES results suggesting that (some) Rh atoms in our clusters were positively charged. On the basis of his results, we suggest that the Rh atoms bearing the charges are those at the metal-support interface; with clusters as small as ours (on average, Rh2 or Rh3), essentially all of the Rh atoms are suggested to be bonded to the support. (The change in the XANES spectra as these clusters formed are inferred to indicate changes in the rhodium ligation as well as the change in rhodium oxidation state.) The Rh-O coordination numbers close to 2 indicate that the Rh atoms in these clusters were bonded to two support oxygen atoms. The Rh-O distance of 2.07 Å bears out this picture, being indicative of bonds between cationic rhodium and oxygen atoms of the zeolite.19,41
J. Phys. Chem. C, Vol. 112, No. 46, 2008 18045 Similar inferences have been drawn for oxide-supported clusters of noble metals other than rhodium. For example, Hammer et al.42,43 used DFT and scanning tunneling microscopy to investigate gold clusters on TiO2, showing that clusters with a range of sizes and an average diameter of 2.4 Å were bonded more strongly to an oxygen-rich TiO2 support than a reduced TiO2 support (as was evident from the sintering of gold clusters supported on the latter at high temperatures, but not for the clusters on the former); these results were confirmed by DFT calculations for gold clusters of various nuclearities (1-10 atoms). The gold-oxygen distance calculated for Au7 on TiO2 was 2.10 Å,42 nearly matching the Rh-O distances characterizing our samples. Our postulate that fragmentation of the rhodium clusters in our experiments involved OH groups of the support is consistent with the aforementioned theoretical evidence of reverse hydrogen spillover in the oxidation of zeolite-supported Rh6 clusters.38 Cluster Fragmentation Induced by Ethylene. The EXAFS data show that ethylene caused fragmentation of the small zeolite-supported rhodium clusters. Ours is the first evidence that C2H4 induces disintegration of metal clusters on a support, and the data indicate why the rhodium remains dispersed as mononuclear complexes during ethylene hydrogenation catalysissat least under the conditions of our experiments. Fragmentation of clusters in solution by ethylene has been demonstrated. For example, according to Adams et al.,44 thermal reaction of ethylene with Ru5(µ5-C2PPh2)(µ-PPh2)(CO)13 (Ph is phenyl) under pressure caused fragmentation of the pentaruthenium clusters, yielding tetraruthenium clusters, Ru4{µ4η2,O,P-C5H4(O)(PPh2)}(µ-PPh2)(CO)11, and two isomers of the pentaruthenium cluster Ru5(µ4-PPh){µ4-η3-C2(CHCH2)CHMe} (µ-PPh2)(CO)12 (Me is methyl). Kampe et al.45,46 showed that Ru3{µ-H,µ-X}(CO)10 (X ) Cl, Br, I) reacted with ethylene to form trinulcear ruthenium clusters, which underwent partial fragmentation in a further reaction with C2H4 + CO to form Ru3(CO)12 and metal-metal bonded dimers; the reaction pathway and details of how ethylene induces the metal-metal bond breaking remain to be determined. Some preliminary insight into the chemistry of cluster fragmentation induced by ethylene is provided by a comparison of Rh-Rh and Rh-ligand bond energies. Bonds between π-bonded alkenes and transition metals are usually weak relative to metal-metal bonds and to the bonds between metals and σ-bonded alkyl ligands.47,48 Specifically, values of the Rh-Rh, Rh-CO, and Rh-C2H4 bond dissociation energies have been found to be 56.4,49,50 39.4,51 and 24.8 kcal/mol,52 respectively. On the basis of these data alone, we might not expect C2H4 to break the Rh-Rh bond. Consequently, we suggest that the reaction pathway for cluster fragmentation may involve other species, such as the support. There is precedent for such a suggestion, as follows. The conversion of rhodium clusters on metal oxide surfaces to give RhI species occurs in the presence of CO, and a number of researchers have suggested the involvement of support hydroxyl groups in the transformations.53-62 By extension, we suggest the involvement of support hydroxyl groups in our observed rhodium cluster fragmentation by C2H4. The CO-induced fragmentation of rhodium clusters by CO on various oxide surfaces has been characterized by IR spectroscopy,63-65 EXAFS spectroscopy,54 energy-dispersive X-ray absorption fine structure (DXAFS) spectroscopy,66 and scanning tunneling microscopy.67,68 This class of reaction has been shown to occur rapidly at 298 K, as illustrated by the DXAFS results of Suzuki et al.66 showing that the breaking of
18046 J. Phys. Chem. C, Vol. 112, No. 46, 2008 Rh-Rh bonds in rhodium clusters on γ-Al2O3 took place in as little as 3 s after the introduction of CO at 298 K. Their results indicate that rhodium clusters were completely fragmented to form anchored rhodium gem-dicarbonyl (RhI(CO)2) species in less than 7 s.69 Another example of fragmentation of supported metal clusters was reported by Bal et al.,72 who used transient EXAFS spectroscopy to follow the process. Results characterizing the zeolite-supported rhenium species, prepared by chemical vapor deposition of methyl trioxorhenium on the zeolite HZSM-5 followed by treatment with He at 673 K, showed that treatment with NH3 led to the formation of rhenium clusters. These clusters were found to be catalytically active for benzene oxidation, but they were deactivated and fragmented to give mononuclear rhenium species in the presence of O2. In this case, surface hydroxyl groups were evidently not involved in the oxidation reaction. Experimental Methods Materials and Sample Preparation. Details of the sample preparation have been reported.16 Sample syntheses and handling were performed with the exclusion of moisture and air. The supported rhodium complex was prepared by bringing the precursor Rh(C2H4)2(C5H7O2) (Strem, 99%) in contact with highly dealuminated HY zeolite (DAY zeolite) (Zeolyst International) in dried and deoxygenated n-pentane (Fisher, 99%). The zeolite, with a Si/Al atomic ratio of ∼30, was calcined in flowing O2 at 773 K for 4 h and then evacuated for 16 h at 773 K prior to contacting with the precursor. The resultant pale yellow powder contains 1.0 wt % Rh (approximately one Rh atom per seven R-cages in zeolite and one Rh atom per five Al atoms). H2 was supplied by Airgas (99.995%) or generated by electrolysis of water in a Balston generator (99.99%) and purified by passage through traps containing reduced Cu/Al2O3 and activated zeolite 4A to remove traces of O2 and moisture, respectively. He (Airgas, 99.997%) and C2H4 (Airgas, 99.99%) were purified by passage through similar traps. IR Spectroscopy. A Bruker IFS 66v/S spectrometer with a spectral resolution of 4 cm-1 was used to collect transmission IR spectra of powder samples. Approximately 1 mg of powder sample, handled with exclusion of moisture and air, was pressed between two KBr windows for optical optimization that allowed detection of minor peaks. IR spectra were recorded at room temperature under vacuum, with an average of 64 scans per spectrum. Experiments with samples in the presence of inert or reactive gases were carried out at atmospheric pressure. In an N2- or Ar-filled glovebox, each sample (typically, 20 mg) 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 10-4 mbar, which allowed recording of spectra while the gases (He, H2, and/or C2H4) flowed through the cell as the temperature was ramped from 298 to a designated final temperature at a rate of 3 K/min. Each spectrum is the average of a number of scans, ranging from 8 to 128. Mass Spectrometry. Mass spectra of the effluent gases introduced into the flow system or produced by reaction with the sample were measured with an online Balzers OmniStar mass spectrometer running in multi-ion monitoring mode; these data were collected as IR spectra were being recorded. Specifically, changes in the signal intensities of the major fragments of C2H4 (m/e ) 26, 27, and 28) and C2H6 (m/e ) 26, 27, 28, and 30) were recorded.
Liang and Gates X-ray Absorption Spectroscopy. The X-ray absorption spectra were recorded at X-ray beam line MR-CAT at the Advanced Photon Source (APS) at the Argonne National Laboratory. The storage ring electron energy and ring currents were 7.0 GeV and 105 mA, respectively. The cryogenic doublecrystal Si(111) monochromator was detuned by 15-20% at the Rh K edge to minimize the effects of higher harmonics in the X-ray beam. In an argon-filled glovebox at the APS, powder samples were pressed into self-supporting wafers. Each wafer was loaded into a flow-through cell,33 and the cell was sealed in the inert atmosphere. The mass of each wafer (∼0.5 g) was chosen to yield optimal absorption measurements at the Rh K edge (23 220 eV) (giving an X-ray absorbance of approximately 2.0 calculated at an energy 20 eV greater than the absorption edge). X-ray absorption spectra were recorded with a rhodium foil and each sample at atmospheric pressure and at temperatures between 298 and 373 K. The samples were scanned in the presence of flowing gases, as follows: He followed by H2 for 80 min; then C2H4 for 80 min. The sample was then treated in H2 for another 80 min followed by C2H4 for 60 min. The respective partial pressures of H2 and C2H4 in the gas stream were 55 mbar each, with the remainder being He. These treatments took place at 298 K and atmospheric pressure; the total gas flow rate was 35 mL/min unless specified otherwise. Spectra were also recorded to characterize the sample in flowing H2 or He at 373 K. All spectra were collected in transmission mode during the treatments, and a spectrum was recorded every 4 min. EXAFS Data Analysis. Method of Data Fitting. The X-ray absorption edge energy was calibrated with the measured signal of a rhodium foil (scanned simultaneously with the sample) at the Rh K edge, which was taken to be the inflection point at 23 220 eV. The data were normalized by dividing the absorption intensity by the height of the absorption edge. Analysis of the EXAFS data was carried out with the software ATHENA of the IFEFFIT73,74 package and the software XDAP. ATHENA was used for edge calibration, deglitching, data normalization, and conversion of the data into an EXAFS (chi) file. A background function was subtracted from the normalized data by using spline points between the values of the wave vector (k) of 1.2 and 15.9 Å-1, with a strong spline clamp made to the end point. An Rbkg value (the value at which ATHENA removes the low- frequency Fourier components) in the range of 1.0 to 1.1 (typical for removal of background noise) was used for all spectra. XDAP allowed the efficient application of a difference-file technique75,76 for determination of optimized fit parameters. Structural models postulated for the supported rhodium species were compared with the EXAFS data; the models included the plausible contributions Rh-Rh, Rh-O, and Rh-C. Reference files, with backscattering amplitudes and phase shifts for Rh-Rh, Rh-O, and Rh-C contributions, were calculated with the software FEFF7.077 from crystallographic coordinates of the unit cells of the reference compounds rhodium metal30 and Rh(C2H4)2(acac).21 Data analysis was carried out with unfiltered data; iterative fitting was performed until optimum agreement was attained between the calculated k0-, k1-, k2-, and k3-weighted EXAFS data and each postulated model. The data were fitted in r space with the Fourier-transformed chi data (r is the distance from the absorbing atom; chi is the EXAFS function). The EXAFS data were analyzed with a maximum of 12 free parameters over the ranges 2.53 < k < 15.67 Å-1 and 1.2 < r < 4.0 Å. The
Rhodium Clusters in Highly Dealuminated Y Zeolite TABLE 4: Qualitative Summary of EXAFS Fitting Results for Three Candidate Models Representing the Supported Rhodium Sample in H2 at 373 K for 60 min and Atmospheric Pressure
model no.
absorberbackscatterer contributions
I
Rh-O
II
Rh-Rh Rh-C
III
Rh-Rh Rh-O
comments regarding the quality of fit of EXAFS data good overall fit; physically realistic values of all parameters good overall fit; physically realistic values of all parameters; individual Rh-C shell not well fitted when phase- and amplitude-corrections applied good overall fit indicated by goodness of fit parameters, but comparison of fit with data indicates less than adequate fit; unrealistic Debye-Waller factors characterizing Rh-C and Rh-O contributions (Debye-Waller factor approaches zero for each contribution)
Rh-C Rh-Rh
number of parameters used in the fitting was always less than the statistically justified number, computed with the Nyquist theorem:78 n ) (2∆k∆r/π) +1, where ∆k and ∆r, respectively, are the k and r ranges used in the fitting. The most thorough EXAFS data analyses were done for selected samples after selected treatments, and less thorough analyses were done for intermediate samples (those with intermediate structures formed during the transformations) by interpolation on the basis of the most thorough analyses. This procedure is justified because the samples underwent gradual structural changes during the treatments. Detailed Examples of Data Fitting. Described below, as an example, is the detailed analysis carried out for the data characterizing the sample at the end of the H2 treatment at 373 K (this is a sample characterized by a relatively large Rh-Rh contribution). The initial data fitting with the plausible absorberbackscatterer contributions (Rh-O, Rh-C, and Rh-Rh) led to a narrowed list of candidate fits (models) on the basis of the goodness of fit. The fits carried out in the most detail are summarized in Table 4. No fit including only a single shell (contribution) was adequate. Each of the three models found to be most successful in fitting the data included a Rh-Rh contribution. The three models (Table 4) differ from each other with respect to the candidate Rh-low-Z-scatterer contributions, such as Rh-O and Rh-C. Besides the Rh-Rh contribution, the two-shell model I (Table 4) includes a Rh-O contribution; model II includes a Rh-C contribution along with a Rh-Rh contribution. Similarly, in addition to the Rh-Rh contribution, the three-shell model III includes a Rh-O contribution and a Rh-C contribution. All contributions were fitted with reference files that best represent the contributions in the measured sample. Details of the fits with all these models are given in the Supporting Information. Each of the models among those considered finally (except model III) provided a satisfactory overall representation of the data, with the quality of the fit varying slightly from one model to another. Model I fits the data best and model III the worst; details are given in the Supporting Information (Table S2), where the values of the fit diagnostic parameters, εv2 (goodness
J. Phys. Chem. C, Vol. 112, No. 46, 2008 18047 of fit) and the variances between the data and postulated models for the EXAFS function, χ, and the Fourier transform of χ are summarized. Further scrutiny of the models showed the following: Model I provided the best overall fit and model II provided a good overall fit as well; however, the shell characterizing the Rh-C contribution in model II was found not to fit well after the contribution was phase- and amplitude-corrected. Model III includes both Rh-O and Rh-C contributions in the fit; addition of the Rh-C contribution to the two-shell model did not improve the fit substantially. Furthermore, the Debye-Waller factors for both of the Rh-low-Z-scatterer contributions were close to zero, and these values are unrealistic for the sample scanned at 373 K, because structural disorder is expected to be significantly higher at this relatively high temperature than that representing samples scanned at room temperature. Thus, the contributions added to generate the three-shell model did not enhance the overall fits significantly. Of the two two-shell models, model I (with Rh-O and Rh-Rh contributions) is the one that fits the data better with physically realistic parameters. As a second example, we describe the results of the detailed EXAFS analysis carried out for the sample during ethylene hydrogenation catalysis at 298 K (this illustrates results for a typical sample for which no Rh-Rh contribution was indicated). The initial fitting was carried out with the plausible absorberbackscatterer contributions (Rh-O, Rh-C, Rh-Al, and Rh-Rh). A summary of the fits is presented in the Supporting Information. No fit with only two contributions was adequate. Each of the three models found to be most successful in representing the data included a Rh-O and a Rh-C contribution. Model I′ includes Rh-O, Rh-C, and Rh-Al contributions; model II′ includes Rh-O, Rh-C, and Rh-Rh contributions; the fourshell model (model III′) includes Rh-O, Rh-C, Rh-Al, and Rh-Rh contributions. Model I′ provides the best overall fit, and models II′ and III′ provide adequate fits; however, the Rh-Rh contribution in model II′ was found not to fit well after the contribution was phase- and amplitude-corrected; the Rh-Rh coordination number was very small (0.2). The additional Rh-Rh contribution in model III′ was discounted because the distance (3.07 Å) is too long for a bonding distance in clusters (the Rh-Rh coordination number was also low, 0.3). The contribution added to generate the four-shell model did not enhance the overall fit significantly. Of the two three-shell models, model I′ (with Rh-O, Rh-C, and Rh-Al contributions) is the one that fits the data best with physically realistic parameters. It is not surprising that the Rh-Al contribution is indicated for the supported mononuclear species, as the charge-compensating cationic rhodium is expected to be associated with the aluminum sites, as discussed elsewhere.17 The lack of a clear indication of Rh-Al contributions for the samples containing the supported rhodium clusters is not surprising either, as more than one Rh atom on average was evidently present in association with Al sites, and there was not one unique Rh-Al distance, but rather more than one Rh-Al distance. Conclusions A site-isolated rhodium diethylene complex with a high degree of uniformity that was supported on a dealuminated faujasite zeolite was characterized with IR, XANES, and EXAFS spectroscopies, as were the products formed from it during treatments in various gases. The zeolite-supported mononuclear rhodium diethylene complex formed initially remained mononuclear during treatments in flowing C2H4 and
18048 J. Phys. Chem. C, Vol. 112, No. 46, 2008 during ethylene hydrogenation catalysis in a flow reactor. Treatment of this sample in H2 led to formation of rhodium clusters (approximately dimers, on average), and when the sample containing these clusters was brought in contact with ethylene, Rh-Rh bonds broke and the clusters were partially fragmented. Treatment of the fragmented clusters in H2 at 298 K led to reconstruction of the clusters. Treatment of the sample in H2 at a higher temperature (373 K) caused the formation of larger rhodium clusters (∼5.4 Å in average diameter), which had bulk-like character. These larger clusters fragmented partially upon contact with ethylene, providing confirming evidence that ethylene was able to break Rh-Rh bonds and stabilize the mononuclear complexes during ethylene hydrogenation catalysis. Thus, these results indicate that the structure of the rhodium species on the support depends sensitively on the composition of the gas phase during ethylene hydrogenation catalysis. Acknowledgment. The research was supported by the U.S. Department of Energy, Office of Energy Research, Office of Basic Energy Sciences, Grant No. DE-FG02-04ER15600. We acknowledge beam time and the support of the DOE Division of Materials Sciences for its role in the operation and development of beam line MR-CAT at the Advanced Photon Source at the Argonne National Laboratory. Supporting Information Available: Additional figures and tables of data. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) James, B. R. Coord. Chem. ReV. 1966, 1, 505. (2) Trzeciak, A. M.; Ziolkowski, J. J. Coord. Chem. ReV. 1999, 192, 883. (3) Evans, D.; Osborn, J. A.; Wilkinson, G. J. Chem. Soc. A 1968, 3133. (4) Wilkinson, G. Bull. Soc. Chim. Fr. 1968, 5055. (5) Cui, X.; Burgess, K. Chem. ReV. 2005, 105, 3272. (6) Robinson, K. K.; Hershman, A.; Roth, J. F.; Craddock, J. H. J. Catal. 1972, 27, 389. (7) Lee, T. J.; Gates, B. C. Catal. Lett. 1991, 8, 15. (8) Lee, T. J.; Gates, B. C. J. Mol. Catal. 1992, 71, 335. (9) Solymosi, F.; Tombacz, I.; Kocsis, M. J. Catal. 1982, 75, 78. (10) Muetterties, E. L.; Rhodin, T. N.; Band, E.; Brucker, C. F.; Pretzer, W. R. Chem. ReV. 1979, 79, 91. (11) Foley, H. C.; Decanio, S. J.; Tau, K. D.; Chao, K. J.; Onuferko, J. H.; Dybowski, C.; Gates, B. C. J. Am. Chem. Soc. 1983, 105, 3074. (12) Jarrell, M. S.; Gates, B. C.; Nicholson, E. D. J. Am. Chem. Soc. 1978, 100, 5727. (13) Oh, S. H.; Eickel, C. C. J. Catal. 1991, 128, 526. (14) (a) Li, F.; Gates, B. C. J. Phys. Chem. B 2004, 108, 11259. (b) Fierro-Gonzalez, J. C.; Gates, B. C. J. Phys. Chem. B 2005, 109, 7275. (c) Chupas, P. J.; Chapman, K. W.; Jennings, G.; Lee, P. L.; Grey, C. P. J. Am. Chem. Soc. 2007, 129, 13822. (15) Ehresmann, J. O.; Kletnieks, P. W.; Liang, A.; Bhirud, V. A.; Bagatchenko, O. P.; Lee, E. J.; Klaric, M.; Gates, B. C.; Haw, J. F. Angew. Chem., Int. Ed. 2006, 45, 574. (16) Liang, A. J.; Bhirud, V. A.; Ehresmann, J. O.; Kletnieks, P. W.; Haw, J. F.; Gates, B. C. J. Phys. Chem. B 2005, 109, 24236. (17) Kletnieks, P. W.; Liang, A. J.; Craciun, R.; Ehresmann, J. O.; Marcus, D. M.; Bhirud, V. A.; Klaric, M. M.; Hayman, M. J.; Guenther, D. R.; Bagatchenko, O. P.; Dixon, D. A.; Gates, B. C.; Haw, J. F Chem. Eur. J. 2007, 13, 7294. (18) This assignment of the formal oxidation state is based on the inference that the rhodium complex was bonded to two oxygen atoms next to the Al3+ site in the zeolite framework, in a structure similar to that of Rh(CO)2 on dealuminated zeolite Y, for which the Mulliken charge (calculated by density functional theory) on the Rh atom was found to be 0.53 e.19,20 The electron counting is based on the 18-electron rule and the assumption that each ligand provided two electrons to the metal. The support zeolite was treated as a bidentate ligand providing a total of four electrons and a negative charge, comparable to the acac ligand in the precursor RhI(C2H4)2(acac).
Liang and Gates (19) Goellner, J. F.; Gates, B. C.; Vayssilov, G. N.; Ro¨sch, N. J. Am. Chem. Soc. 2000, 122, 8056. (20) Fierro-Gonzalez, J. C.; Kuba, S.; Hao, Y. L.; Gates, B. C. J. Phys. Chem. B 2006, 110, 13326. ¨ hrstro¨m, L. (21) Bu¨hl, M.; Håkansson, M.; Mahmoudkhani, A. H.; O Organometallics 2000, 19, 5589. (22) No spectral changes were observed in the region where the features of the acac ligand were displaced, indicating that the acac ligand remained bonded to the Al site in the zeolite. (23) Shimanouchi, T. Table of Molecular Vibrational Frequencies. Natl. Stand. Ref. Data Ser. (US Natl. Bur. Stand.) 1967, 1, 39. (24) Hoffmann, H.; Griffiths, P. R.; Zaera, F. Surf. Sci. 1992, 262, 141. (25) McGee, K. C.; Driessen, M. D.; Grassian, V. H. J. Catal. 1995, 157, 730. (26) De La Cruz, C.; Sheppard, N. J. Mol. Struct. 1991, 247, 25. (27) (a) Kondo, J.; Domen, K.; Maruyak, K.; Onishi, T. J. Chem. Soc., Faraday Trans. 1990, 86, 3021. (28) After a switch from one feed gas to another, there was a lag of ∼4 min before a new component was evident in the effluent stream analyzed by mass spectrometry. (29) Fernandez-Garcia, M.; Martinez-Arias, A.; Rodriguez-Ramos, I.; Ferreira-Aparicio, P.; Guerrero-Ruiz, A. Langmuir 1999, 15, 5295. (30) Singh, H. P Acta Crystallogr., Sect. A 1968, A 24, 469. (31) A lag of ∼2 min in the flow system was observed after a switch from one feed gas to another in this experiment. (32) Argo, A. M.; Goellner, J. F.; Phillips, B. L.; Panjabi, G. A.; Gates, B. C. J. Am. Chem. Soc. 2001, 123, 2275. (33) Jentys, A. Phys. Chem. Chem. Phys. 1999, 1, 4059. (34) Kip, B. J.; Duivenvoorden, F. B. M.; Koningsberger, D. C.; Prins, R. J. Catal. 1987, 105, 26. (35) Odzak, J. F.; Argo, A. M.; Lai, F. S.; Gates, B. C.; Pandya, K.; Feraria, L. ReV. Sci. Instrum. 2001, 72, 3943. (36) Xu, Z.; Gates, B. C. J. Catal. 1995, 154, 335. (37) The IR bands observed here are similar to those of ethyl chloride23 and ethyl groups adsorbed on Pt(111)24 and on the metal particles in Pt/SiO225,26 and Ni/SiO2.26. (38) Vayssilov, G. N.; Gates, B. C.; Ro¨sch, N. Angew. Chem., Int. Ed. 2003, 42, 1391. (39) Weber, W. A.; Gates, B. C. J. Phys. Chem. B 1997, 101, 10423. (40) The reverse spillover of hydroxyl groups from the zeolite was also observed for our sample. Ethane was detected by mass spectrometry in the effluent gas after the start of C2H4 flow through the sample. (41) Koningsberger, D. C.; Gates, B. C. Catal. Lett. 1992, 14, 271. (42) Wang, J. G.; Hammer, B. Top. Catal. 2007, 44, 49. (43) Matthey, D.; Wang, J. G.; Wendt, S.; Matthiesen, J.; Schaub, R.; Laegsgaard, E.; Hammer, B.; Besenbacher, F. Science 2007, 315, 1692. (44) Adams, C. J.; Bruce, M. I.; Liddell, M. J.; Tiekink, E. R. T.; Skelton, B. W.; White, A. H. J. Organomet. Chem. 1993, 445, 187. (45) Kampe, C. E.; Boag, N. M.; Kaesz, H. D. J. Mol. Catal. 1983, 21, 297. (46) Kampe, C. E.; Boag, N. M.; Kaesz, H. D. J. Am. Chem. Soc. 1983, 105, 2896. (47) Chatt, J.; Duncanson, L. A. J. Chem. Soc. 1953, 2939. (48) Cramer, R.; Kline, J. B.; Roberts, J. D. J. Am. Chem. Soc. 1969, 91, 2519. (49) Langenberg, J. D.; Morse, M. D. J. Chem. Phys. 1998, 108, 2331. (50) Luo, Y. ComprehensiVe Handbook of Chemical Bond Energies; CRC Press: Boca Raton, FL, 2007; p 882. (51) Wei, D. H; Skelton, D. C.; Kevan, S. D. Surf. Sci. 1997, 381, 49. (52) Wang, K.; Goldman, A. S.; Li, C.; Nolan, S. P. Organometallics 1995, 14, 4010. (53) Metal Clusters in Catalysis; Gates, B. C., Guczi, L, Kno¨zinger, H, Eds.; Elsevier: Amsterdam, 1986. (54) van’t Blik, H. D. J.; van Zon, J. B. A. D; Huizunga; T.; Koningsberger, J. C.; Prins, R. J. Am. Chem. Soc. 1985, 107, 3139. (55) Hucul, D. A.; Brenner, A. J. Phys. Chem. 1981, 85, 496. (56) Hucul, D. A.; Brenner, A. Inorg. Chem. 1979, 18, 2836. (57) Brenner, A.; Hucul, D. A. J. Catal. 1980, 61, 216. (58) Solymosi, F.; Pasztor, M. J. Phys. Chem. 1985, 89, 4789. (59) Solymosi, F.; Pasztor, M. J. Phys. Chem. 1986, 90, 5312. (60) Thornton, E. W.; Kno¨zinger, H.; Tesche, B.; Rafalko, J. J.; Gates, B. C. J. Catal. 1980, 62, 117. (61) Brenner, A.; Burwell, R. L. J. Am. Chem. Soc. 1975, 97, 2565. (62) Brenner, A.; Burwell, R. L. J. Catal. 1978, 52, 353. (63) Yang, A. C.; Garland, C. W. J. Phys. Chem. 1957, 61, 1504. (64) Cavanagh, R R; Yates, J. T., Jr J. Chem. Phys. 1981, 74, 4150. (65) Basu, P.; Panayotov, D.; Yates, J. T., Jr. J. Am. Chem. Soc. 1988, 110, 2074. (66) Suzuki, A.; Inada, Y.; Yamaguchi, A.; Chihara, T.; Yuasa, M.; Nomura, M.; Iwasawa, Y. Angew. Chem., Int. Ed. 2003, 42, 4795. (67) Berko, A.; Menesi, G.; Solymosi, F. J. Phys. Chem. 1996, 100, 17732. (68) Berko, A.; Solymosi, F. J. Catal. 1999, 183, 91.
Rhodium Clusters in Highly Dealuminated Y Zeolite (69) Others62,70 have also found evidence that fragmentation of the metal frameworks of clusters on supports involves oxidation of the metal by surface hydroxyl groups, which are also involved in reactions of supported metal complexes. Brenner et al.55,56,60,61 used temperature-programmed decomposition to investigate the stoichiometry of the decomposition of Mo(CO)6 on γ-Al2O3, showing that the metal carbonyl complexes could be oxidized as follows. M(CO)j + n(σ-OH) f (σ-O-)nMn+ + (n/2)H2 + jCO (1) On the basis of quantitative measurements of the evolution of H2 during high- temperature decomposition of the metal complexes on supports, these authors55,56 determined the oxidation states of the metal atoms. Basu et al.71 reported similar observations for silica- and alumina-supported rhodium particles on the basis of IR spectroscopy, finding direct evidence of the involvement of OH groups in the oxidation of Rh0 to RhI in the presence of CO. Their results show that the intensities of the IR bands associated with OH groups of the support decreased with increasing CO partial pressure during the formation of RhI(CO)2. (70) Burwell, R. L.; Brenner, A. J. Mol. Catal. 1976, 1, 77.
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