Correction to Rutile-Deposited Pt–Pd Clusters: A Hypothesis Regarding the Stability at 50/50 Ratio

Research Article pubs.acs. org/acscatalysis Rutile-Deposited Pt−Pd clusters: A Hypothesis Regarding the Stability at 50/50 Ratio Mai-Anh Ha, †,§ Jonny Dadras, †,§ and Anastassia Alexandrova* ,†,‡ Department of Chemistry and Biochemistry, University of California, 607 Charles E Young Drive East, Box 951569, Los Angeles, California 90095, United States California NanoSystems Institute, 570 Westwood Plaza, Los Angeles, California 90095, United States S Supporting Information ABSTRACT: Mixed Pt−Pd clusters deposited on oxides have been of great interest to catalysis. Clusters containing Pt and Pd in roughly equal proportions were found to be unusually stable against sintering, one of the major mechanisms of catalyst deactivation. After aging of such catalysts, the 50/50 Pt−Pd and Pd−O clusters appeared to be the two most prevalent phases. The reason for the enhanced stability of these equally proportioned clusters has remained unclear. In the following, sintering of mixed Pt−Pd clusters on TiO 2 (110) for various initial atomic concentrations of Pt and Pd and at a range of catalytically relevant temperatures was simulated. It is conﬁrmed that equally mixed clusters have the relatively highest survival rate. Surprisingly, subnanoclusters containing Pt and Pd in all proportions have very similar geometries and chemical bonding, revealing no apparent explanation for favoring the 1:1 Pt/ Pd ratio. However, it was discovered that at high temperatures, the 50/50 clusters have considerably more thermally accessible isomers than clusters containing Pt and Pd in other proportions. Hence, one of the reasons for stability is entropic stabilization. Electrostatics also plays a key role as a subtle charge redistribution, and a shift of electron density to the slightly more electronegative Pt results in the partially charged atoms being further stabilized by intracluster Coulomb attraction; this eﬀect is greatest for 1:1 mixtures. KEYWORDS: PtPd clusters on titania, density functional theory, sintering, Monte Carlo, theory, chemical bonding 1. INTRODUCTION Supported Pt nanoclusters have shown great catalytic activity and selectivity for alkane dehydrogenation and cracking. 1−4 Recently, it has been shown that Pt and Pd on zeolite (ZSM-5) greatly increased these catalytic properties beyond that of ZSM- 5 alone. 5,6 Additional dehydrogenation to alkynes can occur, as well as methane formation from cracking, and both of these can result in coke deposition. For dehydrogenation catalyzed by deposited clusters, two mechanisms for catalysis deactivation are the prime suspects: coke deposition and cluster sintering. Coke fouling reduces catalytic activity of the clusters, and therefore, a primary approach to reduce coking involves alloying Pt with main group metals, as well as some transition metals. 7−12 Gutierrez et al. have demonstrated that deactivation of Pt−Pd clusters resulting from deposited coke can be reduced by controlling the acidity of the support. 13 Sintering of metal nanoparticles is a result of the particles minimizing their surface energy; hence, populations of small sized particles decrease with a corresponding increase of large-sized particle popula- tions. Graham et al. have shown that mixed Pt−Pd clusters, supported on alumina, have improved stability over those of just pure Pt. 14 However, Johns et al. showed conﬂicting results of the sintering rate of Pt−Pd nanoparticles. 15 It is noted that the latter study observed no core−shell structure of nano- © 2014 American Chemical Society particles under conditions relevant for industrial applications, in contrast with results of, for example, Anderson et al. on smaller- sized (approximately 2 nm) bimetallic nanoparticles. 16 A proper understanding of the catalytic properties of these Pt− Pd bimetallic clusters as well as their structure, stability, and mobility requires deeper knowledge of this system at the nano and subnano scales. For subnanoclusters, the role of the surface goes beyond that of a stage on which the action of catalysis is played. The rutile TiO 2 (110) is a popular model support of the transition metal− oxide interface, providing controllable surface conditions (e.g., adsorption sites) to study various catalytic reaction path- ways. 17−19 Previously, one of the authors (Alexandrova) has shown that pure Pd clusters deposited on TiO 2 will readily sinter, by the Ostwald ripening mechanism, 20 greatly decreasing the catalytic activity of the clusters by reducing the number of edge or step sites. Ostwald ripening, as opposed to particle coalescence, is indeed the mechanism expected to be operational for Pt, Pd, and mixed Pt−Pd clusters on titania. 21 Received: June 21, 2014 Revised: September 2, 2014 Published: September 4, 2014 dx.doi. org/10.1021/cs5011426 | ACS Catal. 2014, 4, 3570−3580