Synthesis and Characterization of Bimetallic Pt-Au Cluster-Derived Catalysts

Gold 2003
Lorna B Ortiz-Soto, Oleg S. Alexeev,
Abstract Synthesis and Characterization of Bimetallic Pt-Au Cluster-derived Catalysts

L. B. Soto, O. S. Alexeev, M. D. Amiridis,
University of South Carolina, Columbia, SC 29208.


Introduction

Supported bimetallic catalysts are widely used in large-scale applications such as naphtha reforming,1 automobile exhaust control,2 and direct methanol fuel cells3 because they typically exhibit higher activities and/or longer lifetimes than those containing just the individual metals. Supported bimetallic catalysts incorporating gold have drawn special attention, since gold clusters have been found to be surprisingly active for low temperature oxidation reactions.4 While the limited understanding of the nature of the bimetallic interactions contributes to the difficulty of preparation of stable, highly dispersed, and well-defined bimetallic structures by conventional preparation techniques, organometallic chemistry provides opportunities to prepare supported bimetallic catalysts with maximized bimetallic interactions. The goal of this work was to prepare and characterize SiO2- and TiO2- supported Ptƒ{Au catalysts starting from a [Pt2Au4(C„kCBut)8] cluster precursor that exclusively contains easily removable acetylide ligands. The resulting cluster-derived catalysts were compared to catalysts of identical compositions obtained by conventional impregnation methods from individual Pt and Au salt precursors.

Through in situ FTIR studies we have monitored the ligand removal process for the preparation of highly dispersed Pt-Au clusters. The results of these studies suggest that most of the acelylide ligands in the SiO2- and TiO2-supported systems were removed by H2 treatment at temperatures below 300„aC. TEM results indicate that Pt and Au remained in a highly dispersed form following the ligand removal process with an average metal particle size of approximately 2nm. In contrast, the average metal particle size in catalysts prepared by co-impregnation from individual salts was approximately 5nm (following a 300„aC H2 reduction step)

Results of FTIR studies of adsorbed CO, indicate no CO uptake by Au in either the monometallic Au or bimetallic Pt-Au samples prepared by co-impregnation from individual salts. These results are confirming the TEM data and indicate that Au crystallites in these samples are fairly large, and hence, not able to chemisorb CO. In contrast, FTIR spectra of adsorbed CO on cluster-derived catalysts contain a characteristic band at approximately 2110 cm-1, which can be assigned to CO linearly bonded on highly dispersed Au. Furthermore, a shift is observed in the same spectra in the position of the band assigned to CO linearly bonded on Pt as compared to the same species on either monometallic Pt or bimetallic Pt-Au samples prepared by co-impregnation from individual salts. Analysis of this shift by the use of 12CO/13CO mixtures, suggests that it can be attributed to an electronic rather than a coverage effect. These results suggest that in the cluster-derived catalysts Pt and Au are intimately mixed and probably are both present in bimetallic particles.
Catalytic activity measurements were performed for the low temperature oxidation of CO in a fixed-bed one pass flow reactor. Monometallic Pt/SiO2 and Pt/TiO2 catalysts yielded similar lightoff curves (Figures 1 and 2), indicating that the Pt-support interaction and the nature of the support used are not important factors for this reaction. Similar lightoff curves were also obtained with the bimetallic Pt-Au catalysts prepared by co-impregnation from individual salts. These results further suggest that in these samples, Pt and Au remain largely segregated and the Au is present in the form of larger crystallites with low CO oxidation activity. In contrast, substantial differences were observed in the catalytic activity of the cluster-derived catalysts. In this case, the lightoff curve of the SiO2-supported catalyst was shifted by 50C towards higher temperatures (i.e., the catalyst was less active), while the lightoff curve of the TiO2-supported catalyst was shifted by 50C towards lower temperatures (i.e., the catalyst was more active).
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Figure 1: Lightoff curves characterizing Figure 2. Comparison of lightoff curves for SiO2 and
SiO2-supported samples. TiO2-supported Pt-Au of the cluster-derived origin.

Taking under account the characterization results discussed in previous paragraphs, these kinetic results can be rationalized as follows: While in both catalysts Au sites that are capable of chemisorbing CO are present, only those on TiO2 can catalyze the oxidation of CO, presumably due to the availability of oxygen form the support (consistent with similar assumption made earlier by other workers in this area). The oxidation of CO over the SiO2-supported system continues to be catalyzed only by Pt, but in this case, the rate of the reaction is decreased presumably due to the smaller Pt ensemble, which results from the bimetallic nature of the metal particles. Other literature reports have already shown that oxygen activation on Pt is structure sensitive and is favored over larger Pt ensembles.
References
1. G. A. Antos, A. M. Aitani, and J. M. Parera, (Eds.), Catalytic Naphtha Reforming, Dekker, New York, 1995.
2. M. Shelef and G. W. Graham, Catal. Rev.--Sci. Eng., 36 (1994) 433.
3. L.-W. Leung and M. J. Weaver, Langmuir 6 (1990) 323.
4. M. Haruta, Catal. Today 36(1997) 153.


Keywords: Low temperature CO oxidation, cluster-derived Pt-Au catalyst
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