J. Phys. Chem. 1993,97, 484-488
484
Formation of Metallic Particles during Temperature-Programmed Reduction of Silica-Supported Platinum and Nickel Chlorides Andreas Jentys,? Gary L. Holler,#md Johmoes A. Lercber'vt Institut fur Physikalische Chemie und Christian Doppler Laboratorium fur Heterogene Katalyse, Technische UniversitBt Wien, Getreidemarkt 9, A - 1060 Vienna, Austria, and Department of Chemical Engineering, Yale University, P.O. Box 2159 YS,9 Hillhouse Avenue, New Haven, Connecticut 06520 Received: August 6, 1992; In Final Form October 13, I992
The formation of (bi-) metallic particles during temperature-programmed reduction of silica-supported mixtures of Pt and Ni chlorides was followed by X-ray absorption spectroscopy. The concentration and composition of the metal particles in the final catalysts were derived from the results of the EXAFS analysis. The reduction kinetics followed by XANES were related to the formation of ordered metallic phases. During temperatureprogrammed reduction, most of the bimetallic phase was formed simultaneously with the reduction of the catalyst precursor. The bimetallic catalysts consisted of at least one bimetallic phase and of the pure phase of the metal in excess. For catalysts containing more than 50 atom 9% Ni, we concluded that there was either an enrichment of the bimetallic phase in Ni or the formation of a second bimetallic phase (Ni3Pt). For catalysts containing 50 atom % Ni or less we concluded that the stoichiometric NiPt phase was the only bimetallic phase.
Introduction Ordered and nonordered metal alloys are frequently used as catalysts for reforming and isomerization reactions.'+ Because most of these reactions are structure sensitive, the detailed knowledge of the kind and concentration of the metallic phases present is indispensable to understand the properties of these materials. Moreover, the knowledge of the processes involved during the reduction of the precursor of the metal catalyst should provide the information necessary to modify these procedures to optimize the dispersion while maintaining the necessary degree of interaction between the catalyst components. For the analysis of small metal particles, X-ray absorption spectroscopyis an important technique, because it does not suffer the usual limitations with respect to the particle size, e.g., of X-ray diffraction. Moreover, the X-ray absorption experiment may be performed under both vacuum and high-pressure condition^.^ The analysis of EXAFS allows the determination of the average number of atoms around the X-ray absorber atom and their distance. In some circumstances, deduction about geometric arrangements can be made. This might be used to determine the average particle size of highly dispersed metal catalysts? Besides average coordination numbers and distances, the geometric arrangement of metal atoms, the composition, and the concentration of phases forming the metallic particles may beestimated from EXAFS' by using assumptionsfor the chemical composition and/or the number of phases present as additional constraining variables. In a previous paper we described the reduction kinetics of platinum and nickel chlorides supported on silica by means of semiquantitative XANES8 We showed that the addition of platinum to nickel chlorides increased the rate of nickel chloride reduction. Those Ni2+ ions that were in close proximity to Pt, i.e., atoms in catalysts samples with compositionsup to a 1:1 ratio between Pt and Ni, were reduced at significantly lower temperatures than theother Ni2+ions. While we wereable to specifically attribute the overall hydrogen consumption to the reduction of a specific kind of metal cations, the XANES measurements did not allow us to decide whether or not the bimetallic phase was formed at the reduction temperature. In this paper we describe the EXAFS analysis of X-ray absorption measurements during the reduction of chloride t Technische Univenitat Wien. t
Yale University.
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precursors of silica-supported Pt/Ni catalysts, The goal was to asseas the structural parameters of the phases formed during the reduction and the composition of the resulting metal particles.
Experimental Section Catalysts. A series of silica-supported bimetallic Pt/Ni catalysts with a metal loading of 4 X 10-4 mol@ was prepared by coimpregnation of silica support with metal chlorides by means of theincipientwetnessmethod.9Thecompositionsofthereduced samples and the dispersion of the catalysts, determined by H2 chemisorption, are compiled in Table I. A detailed characterization of the catalysts is compiled in refs 8 and 9. X-my AbsorptionS ~ ~ a e c o p The y . X-ray absorptionspectra (XAS) were measured at the beamline X18B at the NSLS, Brookhaven National Laboratory, NY. The sampleswere pressed into self-supportingwafers and placed in a cell that permitted the collection of XAS in H2 and inert gas atmosphere (He) betwan 77 and 723 K. The cell had two sample positians, allowing two samplestobe treated simultaneously. The thicknessof all samples was selected to achieve an absorption of x = 2.5 to optimize the signal to noise ratio.1° The precursors were dried in He at 323 K and subsequentlyreduced in Hz flow. X-ray absorption spectra were collected at 77 K after (i) drying at ambient temperature, (ii) temperatureprogrammed reduction (tpr) up to 573 K,and (iii) tpr up to 723 K. Pt and Ni foils were used as references for the metal-metal contributions to the EXAFS and PtC4 and NiCL (stable phases: NiC12-7H20 G33.3 OC; NiC12~6H20
