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J. Phys. Chem. C 2007, 111, 262-267
Size-Dependent Catalytic Activity of Zeolite-Supported Iridium Clusters Fen Li and Bruce C. Gates* Department of Chemical Engineering and Materials Science, UniVersity of California, DaVis, California 95616 ReceiVed: July 18, 2006; In Final Form: October 4, 2006
Iridium carbonyl clusters approximated as Ir2(CO)8, Ir4(CO)12, and Ir6(CO)16 were synthesized by carbonylation under various conditions of Ir(CO)2(acac) sorbed in the cages of zeolite NaY. These supported clusters were decarbonylated by treatment in helium and used to catalyze ethene hydrogenation at atmospheric pressure and temperatures up to 80 °C. Extended X-ray absorption fine structure (EXAFS) spectroscopy was used to characterize the decarbonylated clusters, which are approximated as Ir2, Ir4, and Ir6, respectively. The data are consistent with the inference that these clusters are the catalytically active species. The catalytic activities and apparent activation energies for ethene hydrogenation depend significantly on the cluster size, with the identity of the most active of the three catalysts depending on the temperature. There appears to be no simple explanation for the variation in catalytic performance with cluster size.
Introduction
Experimental Methods
When metal particles are made so small as to become clusters of only a few atoms, they no longer have metallic character and their catalytic properties are expected to become size dependent, even for structure-insensitive reactions.1-5 Determination of the catalytic properties of supported clusters requires samples having systematically varied cluster sizes, and preparation of such samples is challenging because (a) metals on supports are susceptible to migration and aggregation and therefore difficult to prepare uniformly, (b) in the limit as the number of metal atoms in a cluster approaches one, the metal on an oxide or zeolite support typically becomes cationic (part of the mononuclear metal complex with the support as an anionic ligand) and thus not comparable to the metal in a cluster,6 and (c) the metals may incorporate ligands that vary from one cluster size to another. Attempts have been made to synthesize families of supported metal clusters of uniform size by taking advantage of zeolite cages to confine the metal and limit the cluster size.7-9 Much of the work in this direction has been focused on iridium because relatively stable supported iridium carbonyls, Ir4(CO)12 and Ir6(CO)16, can be prepared in high yields (from organometallic precursors such as Ir(CO)2(acac)) and then decarbonylated to give supported clusters approximated as Ir4 and Ir6, respectively.10-12 When zeolite-supported iridium carbonyl clusters were prepared from Ir(CO)2(acac) in the supercages of zeolite NaY, evidence from infrared (IR) and extended X-ray absorption fine structure (EXAFS) spectroscopies indicated that diiridium carbonyls, likely Ir2(CO)8,13 were formed in high yields as intermediates en route to formation of Ir4(CO)12 and then Ir6(CO)16. Thus, the opportunity arose, by decarbonylation, for preparation of a family of clusters, Ir2, Ir4, and Ir6. These clusters offer a unique opportunity for determination of the dependence of catalytic activity on cluster size for the smallest clusters. As a catalytic test reaction, we chose ethene hydrogenation because it is regarded as structure insensitive and occurs under mild enough conditions to ensure stability of the clusters during operation.
Materials. Zeolite NaY (Davison Division, W. R. Grace) was treated in flowing O2 at 300 °C and atmospheric pressure for 4 h followed by evacuation for 12 h. Synthesis of zeolite-supported iridium clusters and sample transfers were performed with exclusion of air and moisture on a double-manifold Schlenk vacuum line in a N2-filled glovebox (AMO-2032, Vacuum Atmospheres). He (Matheson, 99.999%), ethene (Matheson, 99.5%), and CO (Matheson, 99.995%) were purified by passage through traps containing particles of copper and activated zeolite to remove traces of O2 and moisture, respectively. H2 was supplied by Matheson (99.999%) or generated by electrolysis of water in a Balston generator (99.99%) and purified by traps. n-Pentane (99.0% purity, Aldrich), used as a solvent, was refluxed under N2 in the presence of Na/benzophenone ketyl to remove traces of water and then deoxygenated by sparging of dry N2. The precursor Ir(CO)2(acac) (dicarbonylacetylacetonato iridium(I), 99%, Strem) was used as received. Catalyst Preparation. Ir(CO)2(acac) in n-pentane solution was brought in contact with the treated zeolite powder to give samples that contained 1.0 wt % Ir after removal of the solvent by evacuation at 25 °C. Carbonylation to form zeolite-encaged iridium clusters of various sizes from sorbed Ir(CO)2(acac) by a ship-in-a-bottle synthesis was carried out with samples in flowing CO at 40 or 175 °C in a plug-flow reactor as before.10,11,13 In the synthesis the precursor iridium carbonyl was converted into diiridium carbonyl, which was subsequently converted into Ir4(CO)12 at a relatively low temperature (40 °C),13 or converted into Ir6(CO)16 at a higher temperature (175 °C);16 the conditions were chosen to allow isolation of each of these in high yield. The supported clusters were decarbonylated by treatment in flowing helium at 300 °C for 12 h. Ethene Hydrogenation Catalysis. Ethene hydrogenation catalysis was carried out at steady state in a temperaturecontrolled stainless-steel tubular flow reactor at atmospheric pressure. Experiments were performed at various temperatures in the range of 25-80 °C. The reactor, initially in the glovebox, was loaded with 75 mg of catalyst and mixed with 2.5 g of inert, nonporous R-Al2O3 to minimize temperature gradients in the reactor during catalysis. The catalyst bed was held in the
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Activity of Zeolite-Supported Iridium Clusters
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middle of the temperature-controlled zone of the reactor by glass wool plugs. The loaded reactor was isolated, removed from the glovebox, and installed in a flow system without contacting of the catalyst with air. An on-line gas chromatograph (HewlettPackard, HP-5890 Series II) was used to analyze the products; it was equipped with a 30-m × 0.53-mm DB-624 (J&W Scientific) capillary column (with N2 as the carrier gas, 2.5 mL (NTP) min-1) and a flame-ionization detector. Rates of reaction were determined from differential conversions of ethane with a molar ratio of H2 to ethene of 1:1 (155 Torr of H2, 155 Torr of C2H4, with the remainder being He) with total feed flow rates in the range of 27-185 mL/min. Under these conditions, the reactor was well approximated as an isothermal plug-flow reactor. X-ray Absorption Spectroscopy. EXAFS spectroscopy experiments were performed at X-ray beamline X-18B of the National Synchrotron Light Source (NSLS), Brookhaven National laboratory, Upton, NY. The storage ring energy was 2.8 GeV, and the ring current was in the range of 180-300 mA. The data were recorded in transmission mode after the cells had been cooled to nearly liquid nitrogen temperature. Each sample was scanned several times, and the typical spectrum is the average of four scans. The data were collected with a Si(111) double-crystal monochromator that was detuned by 30% to minimize effects of higher harmonics in the X-ray beam. The samples were scanned at energies near the Ir LIII absorption edge (11215 eV). Details are as reported elsewhere.10,12,14 EXAFS Data Analysis. The methods of EXAFS data analysis are essentially the same as those described previously.13,15 The data were analyzed with experimentally determined reference files obtained from EXAFS data characterizing materials of known structure16 except that the Ir-Ir reference file was calculated with the software FEFF-7. The data quality is high, and analysis was carried out without Fourier filtering, as described elsewhere.17 Results EXAFS Results Characterizing Initially Prepared Iridium Clusters Supported in Zeolite NaY. To prepare the smallest supported clusters, the supported sample prepared from Ir(CO)2(acac) was treated in flowing CO at 40 °C for 2.5 h (giving iridium carbonyl clusters approximated as Ir2(CO)8),13 and this sample was decarbonylated by treatment in flowing He at 300 °C and 1 bar for 12 h. EXAFS data characterizing the sample were measured after decarbonylation. The raw data and fits are summarized in Figure 1; the fit parameters are summarized in Table 1. The software XDAP18 was used to estimate the errors in the parameters shown in Table 1; these represent precisions, not accuracies. The accuracies are estimated to be as follows: firstshell coordination number (N), Ir-Ir (20%, and Ir-Osupport (30%; distance (R), Ir-Ir (1% and Ir-O (2%; Debye-Waller factor (∆σ2), (30%; and inner potential correction (∆E0), (10%. The Ir-Ir first-shell coordination number representing the sample (1.0) indicates iridium clusters of two atoms each on average; the Ir-Ir distance, 2.65 Å, is similar to that characterizing the larger clusters (Ir4 and Ir6), as already reported (Table 1).12,13,19 We represent the smallest clusters as Ir2, recognizing that they could consist of a mixture of Ir2 with larger clusters and remaining mononuclear iridium complexes. Treatment of the sample in CO for a longer time and at a higher temperature than required to give diiridium clusters gave larger clusters. EXAFS results (Table 1) characterizing the
Figure 1. Results of EXAFS analysis characterizing zeolite NaYsupported iridium (1.0 wt %) (treatment, CO, 40 °C for 2.5 h; He, 300 °C for 12 h): (a) experimental EXAFS (χ) function (solid line) and the sum of the calculated contributions (dotted line); (b) imaginary part and magnitude of uncorrected Fourier transform (k0 weighted, ∆k ) 2.7-15.9 Å-1) of experimental EXAFS (solid line) and sum of the calculated contributions (dotted line).
samples prepared from the sorbed precursor Ir(CO)2(acac) after treatment in CO at 40 and 175 °C, respectively, each for 12 h, followed by decarbonylation in flowing He at 300 °C for 12 h, indicate first-shell Ir-Ir coordination numbers of 2.9 ( 0.2 and 3.7 ( 0.3, respectively.10-12 The coordination number of approximately 3 indicates Ir4 tetrahedra, and the corresponding sample is represented as Ir4. The coordination number of approximately 4 indicates Ir6 octahedra, and the corresponding second-shell coordination number of approximately 1 (0.9 ( 0.5, Table 1) also indicates octahedra; thus, the corresponding sample is referred to as Ir6. These data are in good agreement with results reported earlier.12,15,19 Ethene Hydrogenation Catalysis. The only product observed was ethane, and there was no measurable conversion of ethene and H2 when the reactor contained the zeolite support without
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TABLE 1: EXAFS Results Characterizing Samples Formed by Treatment of Ir(CO)2(acac) in Zeolite NaY Followed by Decarbonylation and Ethene Hydrogenation Catalysisa EXAFS parameter s treatment conditions
shell
N
R, Å
103∆σ2, Å2
∆E0, eV
EXAFS supported cluster reference modeled as
CO, 40 °C for 2.5 h, followed by He, 300 °C for 12 h
Ir-Ir I-Os Ir-Ol
1.0 ( 0.3 2.65 ( 0.01 2.1 ( 0.1 2.15 ( 0.01 0.9 ( 0.3 2.57 ( 0.02
4.0 ( 1.2 -3.5 ( 1.4 5.1 ( 0.4 -0.3 ( 0.4 0.2 ( 3.3 7.0 ( 3.0
Ir-Ir Pt-O Pt-O
Ir2
CO, 40 °C for 2.5 h, followed by He, 300 °C for 12 h, followed by catalytic reaction (H2, 155 Torr; C2H4, 155 Torr; He, 450 Torr; 70 °C for 0.5 h)
Ir-Ir Ir-Os Ir-Ol
1.0 ( 0.3 2.63 ( 0.01 2.2 ( 0.1 2.15 ( 0.01 0.8 ( 0.2 2.59 ( 0.02
4.1 ( 1.2 4.5 ( 0.8 0.4 ( 3.1
2.3 ( 1.9 0.0 ( 0.5 2.8 ( 2.1
Ir-Ir Pt-O Pt-O
Ir2
CO, 40 °C for 12 h, followed by He, 300 °C, 12 h
Ir-Ir Ir-Os Ir-Ol
2.9 ( 0.2 2.66 ( 0.01 1.2 ( 0.1 2.15 ( 0.01 0.7 ( 0.1 2.63 ( 0.01
5.3 ( 0.4 1.0 ( 0.9 3.0 ( 3.0
-0.0 ( 1.1 -1.0 ( 0.7 -9.5 ( 0.7
Ir-Ir Pt-O Pt-O
Ir4
CO, 175 °C for 12 h followed by He, 300 °C, 12 h
Ir-Ir 3.7 ( 0.3 2.63 ( 0.01 9.4 ( 0.1 Ir-Os 1.7 ( 0.1 2.15 ( 0.01 0.4 ( 0.4 Ir-Ir (second shell) 0.9 ( 0.5 3.73 ( 0.04 10.0 ( 5.9
-0.4 ( 0.6 -2.2 ( 0.5 3.0 ( 3.1
Ir-Ir Pt-O Ir-Ir
Ir6
a Notation: N, coordination number; R, distance between absorber and backscatterer atoms; ∆σ,2 Debye-Waller factor; ∆E , inner potential 0 correction; the subscripts s and l refer to short and long, respectively.
Figure 2. Catalyst break in: initial reaction rates defined as TOF of ethene hydrogenation catalyzed by Ir4/zeolite NaY at 25 °C; the feed composition was H2:C2H4 ) 1 (molar).
iridium. When iridium was present in the catalyst, data characterizing ethene hydrogenation catalysis were collected after short transient induction periods (about 2 h in length), after which each of the supported iridium catalysts was found to have a stable activity for ethene hydrogenation (Figure 2). Conversions of ethene to ethane were
