J. Phys. Chem. 1990, 94, 8451-8456
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Mononuclear, Trinuclear, and Metallic Rhenium Catalysts Supported on MgO: Effects of Structure on Catalyst Performance P. S. Kidin,+ H. Knozinger,*and B. C. Gates*'+ Center for Catalytic Science and Technology, Department of Chemical Engineering, University of Delaware, Newark, Delaware 19716, and Institut f u r physikalische Chemie, Universitat Munchen, Sophienstrasse 11, 8000 Miinchen 2, West Germany (Received: January 23, 1990)
MgO-supported Re catalysts were prepared from [HRe(CO)5] and [H3Re3(C0)1z]to give isolated Re subcarbonyls and ensembles of three Re subcarbonyls, respectively. The latter sample was reduced in Hz to give Re metal crystallites on the support. The reactivities of the various surface species were characterized by temperature-programmed reduction, temperature-programmed desorption, and wet chemical experiments. The samples were probed as catalysts for the structureinsensitive propene hydrogenation and the structure-sensitive cyclopropane isomerization/hydrogenolysis. The supported Re complexes formed from [HRe(CO)5]and MgO catalyze the alkene hydrogenation but not the C-C bond rupture. The ensembles of these complexes formed from [H3Re3(C0)lz]catalyze both, in this respect resembling supported crystallites of Re metal partially poisoned with CO. It is inferred that neighboring metal centers are required for the C-C bond rupture, whereas isolated metal centers are active for the hydrogenation. The catalyst containing Re metal is also active for both of these conversions and is the only one of these samples that is active for the isomerization of cyclopropane to give propene.
Introduction One of the central challenges of research in catalysis is to determine relations between surface structure and catalytic properties. The first step in meeting the challenge is synthesis of catalyst surfaces having relatively well-defined structures; determination of the structures by spectroscopic methods then opens the opportunity for relating structure and catalytic properties. In this work, simple structures on the MgO surface have been synthesized from organorhenium precursors and characterized with extended X-ray absorption fine structure and other spectroscopic methods, as described in an accompanying publication.' Re was chosen as the catalytic metal because (1) it is important in industrial catalysts, including those used for alkene metathesis2 and petroleum naphtha reforming3 and (2) it is oxophilic, forming cationic complexes that are stably bonded to the surfaces of metal oxides such as MgOS4 MgO was chosen as the support because ( I ) in a partially dehydroxylated form, it is basic and capable of chemisorbing even weakly acidic precursors such as [HRe(CO)5] and [H3Re3(C0)lz]sand (2) more than 90% of the surface, even of the powder, evidently consists of highly stable, highly ordered MgO( 1OO).6 The synthetic strategy was to adsorb each of these two organometallic precursors on MgO and then to oxidize it, forming Re subcarbonyls. Results of characterization experiments with EXAFS spectroscopy and other methods are consistent with the hypothesis that [HRe(C0)5] gives isolated Re subcarbonyls on the surface whereas [H3Re3(C0)1z]gives ensembles of three of the subcarbonyls.' Reduction of the latter in H2 gives samples incorporating metallic Re crystallites.' Here we report experiments to determine the reactivities and catalytic activities of the surface species. The reactivities have been investigated in temperature-programmed reduction (TPR) and temperature-programmed desorption (TPD) experiments, in addition to wet-chemical experiments. The catalytic properties have been probed with the following test reactions: the hydrogenation of propene and the isomerization and hydrogenolysis of cycl~propane.'-~ Experimental Methods Synthesis and Structural Characterization of the Catalysts. Samples were prepared from a solution of either [HRe(CO)5]or [H3Re3(CO),,] brought into contact with MgO powder. The structures of the resulting adsorbed Re species were investigated 'University of Delaware. Universitit Mlinchen.
*
spectroscopically. Details are given in an accompanying publication. I Temperature-Programmed Reduction and Decomposition. The stabilities of the surface complexes derived from [HRe(CO)5] and from [H3Re3(C0)12]were evaluated by heating the samples under flowing helium (Matheson UHP grade, 99.995%) or hydrogen (Matheson U H P grade, 99.999%) and monitoring the infrared absorptions of the solids. Complementary TPR and TPD experiments provided evidence of the stoichiometry of the reaction of the surface-bound trirhenium cluster to give Re subcarbonyls. The infrared experiments were done as described elsewhere.' The TPR and TPD experiments were performed with apparatus described by Knozinger.'O A mixture of 9.25% H2 in He was used for the reduction. The temperature program in both TPR and TPD experiments usually involved heating the sample from 25 to 225 'C at 6 OC/min, followed by 4 h at 225 'C. The gas flow rates were 0.435 and 0.455 mL/s for TPR and TPD experiments, respectively. The mass of catalyst sample was in the range 0.4-0.6 g. The H2 evolved (or consumed) was continuously monitored as H2 was transferred to (or from) a stream of N2 (sometimes containing H2) across a Pd membrane in a diffusion cell; a thermal conductivity detector was used. The CO, C02, and CH4 produced were eluted on a Carbosieve 6 column (1.5 m in length); the elution times limited the minimum sampling interval to 4 min (or 2 min if only C O was detected). Catalytic Reaction Experiments. A . Kinetics. The catalytic reaction experiments were carried out with the supported Re catalysts in a microflow reactor interfaced to a gas chromatograph (GC, Antek 300). The flow system allowed monitoring of the reactants, namely, Hz (Matheson UHP grade, 99.999%), propene (Matheson C P grade, 99.0%), and cyclopropane (Airco Medical grade, 99.0%) with He (Matheson UHP grade, 99.995%) in a ( I ) Kirlin, P. S.; van Zon, F. B. M.;Koningsberger, D. C.; Gates, B. C. J . Phys. Chem., previous article in this issue. (2) Banks, R. L. In Catalysis (Specialist Periodic Report, Vd 4 ) ; The Chemical Society: London, 1980; p 101. (3) Sinfelt, J. H. Bimetallic Catalysts: Discoveries, Concepts, and Applications; Wiley: New York, 1983. (4) Kirlin, P. S.; DeThomas, F. A,; Bailey, J. W.; Moller, K.; Gold, H.S.; Dybowski, C.; Gates, B. C. Surf Sci. 1986, 175, L707. (5) Scott, J. P.; Budge, J. R.; Rheingold, A. L.; Gates, B. C. J . Am. Chem. SOC.1987, 109, 7736. (6) Henrich, V. C. Rep. P r o p . Phys. 1985, 48, 1481. (7) Boudart, M. Adu. Catal. 1969, 20, 153. (8) Somorjai, G. A,; Carrazza, J. Ind. Eng. Chem. Fundam. 1986, 25,63. (9) Sinfelt, J. H. Acc. Chem. Res. 1977, 10, 15. (IO) Knozinger, H. In Metal Clusters in Catalysis; Gates, B. C., Guczi, L., Knozinger, H., Eds.; Elsevier: Amsterdam, 1986; p 259.
0022-3654/90/2094-845 1 %02.50/0 0 1990 American Chemical Society
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Kirlin et al.
The Journal of Physical Chemistry, Vol. 94, No. 22, 1990
TABLE I: Results of Temperature-ProgrammedDecomposition (in He) and Reduction (in H2) of [H2Re3(C0),J{MgO) To Form IRe(CO)dOMnllHOMel,1" (mol of Co)b/ (mol of C02)*/ (mol of H2)c*d/ (mol of C)b.r/ eas Dhase (mol of Re) (mol of Re) (mol of Re) (mol of Re) 0.8 0.94 0.59 0.35 He 12 0.98 He+Hf 0.64 0.34 ~~
'Samples were heated from 25 to 225 OC at 6 OC/min and then held at 225 OC for 4 h. balance He.
* f 10%. 'H, evolution. d*20%. C M ~ lofe CO ~
+ CO,.
f9.2'% H;,
stream fed to a quartz tubular reactor enclosed in a Lindberg furnace equipped with a thermostat. The impurities in the propene (0.38% propane) and the cyclopropane (0.18% propene) were analyzed by GC. The feed H2 and H e flowed through traps to remove traces of O2(activated Cu) or water (activated zeolite 5A). The hydrocarbons were dried by flowing through a bed of activated Si02. To begin a catalysis experiment, the reactor was loaded under N 2 with 100-200 mg of catalyst powder supported on a quartz frit. The reactor (approximately 30 mL in volume) was placed in the flow system without contamination of the sample by air and purged with He (1 mL/s) for 600 s at 15 OC. Two methods of catalyst pretreatment were employed. In one set of experiments, the He purge was replaced with a 1:l (molar) mixture of H 2 and propene (or H2 and cyclopropane, 0.33 mL/s), and the sample containing the supported complexes, initially in the form of adsorbed [HRe(CO),] or adsorbed Re clusters, was pretreated for 4 h at 80 "C or for 4 h at 225 OC, respectively. A separate set of experiments was carried out to give Re metal crystallites on the MgO support; the He purge was placed with H2 (0.33 mL/s), and the catalysts, prepared from [H3Re,(C0)12]as described above, were then treated for 5 h a t 550 O C in flowing H2.Ii The reactor effluent stream was sampled periodically for analysis by GC. The products were separated in a 3.18 mm X I O m Supelco SP1700 column or in a 3.18 mm X 1.5 m Porapak Q column with pressure drops of 7.8 and 2 atm, respectively. The GC was equipped with dual flame ionization detectors; the response was calibrated for Ci-C6 hydrocarbons. Products were identified by their retention times. Rates of the catalytic reactions were measured with various reactant partial pressures, which were set by the H2 and hydrocarbon flow rates in the atmospheric-pressure reactor; He was used as a diluent to maintain constant contact times in the reactor. Blank experiments demonstrated the lack of reaction in the absence of the catalysts. The catalyst that had been treated in H2 at 550 OC exhibited extremely high initial activity followed by rapid deactivation and then a period of slowly falling reaction rates. Since steady state was not achieved with this catalyst, the kinetics data were taken during the period of slow deactivation; the data collection procedure of Sinfelt et al.I2 was adopted, with each datum being bracketed by a pair of data taken under standard conditions, and the reaction rate being corrected for any loss of activity. The other kinetics data were collected at steady state, unless otherwise noted. To obtain reaction rates directly, differential conversions of the reactants (
