J. Phys. Chem. C 2010, 114, 16885–16891
16885
Oxidation of Supported PtRh Particles: Size and Morphology Effects† M. Dalmiglio,‡,§ M. Amati,‡ L. Gregoratti,*,‡ T. O. Mentes¸,‡ M. A. Nin˜o,‡ L. Felisari,| and M. Kiskinova‡ Sincrotrone Trieste, Area Science Park, I-34012 BasoVizza-Trieste, Italy, Graduate School of Nanotechnology, UniVersita` degli Studi di Trieste, Piazzale Europa, 1, 34127 Trieste, Italy, and Laboratorio TASC, INFM-CNR, S.S. 14, Km 163.5, 34012 Trieste, Italy ReceiVed: October 21, 2009; ReVised Manuscript ReceiVed: January 27, 2010
The chemical and structural evolution of supported PtRh alloy nano- and microparticles during oxidation has been studied by means of scanning photoelectron microscopy (SPEM) and low energy electron microscopy (LEEM). The SPEM Rh 3d and Pt 4f images and microspot spectra revealed a higher reactivity of the Rh atoms and coexistence of several stoichiometric and nonstoichiometric Rh and Pt oxide phases with well developed lateral variations in the oxidation states with a single microparticle. The observed variations in the local reactivity were correlated to the complex initial surface morphology of the particles and the following structural changes upon oxidation monitored by LEEM. 1. Introduction Alloys have often catalytic properties superior of those of single metals and have found broad applications in many industrial synthesis and exhaust gas converters.1 Exploring the behavior of the alloys, it has been shown that the constituent metal atoms generally maintain the desired catalytic properties, modified by ligand effects in order to improve the catalyst efficiency and stability.2–4 As discussed in ref 5 for identifying the proper alloys that will show superior catalytic behavior for specific reactions, it is necessary to understand the electronic structure evolving upon mixing two or more metals. A textbook example in this respect is the PtRh alloy, one of the most efficient automobile gas converter catalysts, and recently also considered as a promising electrocatalyst for use in fuel cells.6 In this alloy, the Pt has a higher catalytic activity for CO oxidation, whereas Rh adds the high activity needed for NO reduction to N2. PtRh alloy exhibits excellent catalytic performance at high temperatures, e.g., in the steady state, the automotive catalytic converters operate above 700 °C, but its behavior at lower temperatures, for instance, the cold-start emissions in automobiles or in fuel cells, has to be deeply delved.7 A lot of basic knowledge for the catalytic properties of the individual Pt and Rh metals in oxidation/reduction reactions has been obtained by bottom-up investigations, aiming at bridging the “material gap” by starting with simple model systems and gradually increasing the complexity to approach the real catalyst systems. The studies using single crystal, vicinal, and highly defective Pt or Rh surfaces have provided a detailed picture of the oxygen adsorption, transient surface, and bulk oxide structures, including the strong effects of the surface morphology and in particular the higher activity of the more open planes in the presence of under-coordinated atoms.8,9 Under oxidation conditions, the structure of the initial metal plane can undergo drastic changes; e.g., oxide formation on Pt(110) can result in a disordered phase.10 Moving to bimetallic alloy systems, the †
Part of the “D. Wayne Goodman Festschrift”. Sincrotrone Trieste. § Universita` degli Studi di Trieste. | INFM-CNR. ‡
complexity increases with the addition of other variables, such as the local organization of the two metals and the actual surface composition, which is strongly dependent on the gas environment and temperature. As reported recently, due to the presence of different species, the balance between the energetic terms, which determine the catalytically active surface composition of PtRh catalysts under oxidation-reduction reaction conditions, is rather complex.11 The effect of surface structure and composition becomes less predictable with decreasing dimensionality, as evidenced by the studies of model supported metal particles of different size and shape.12–15 It turned out that even in such model systems the particles are not identical and due to geometric or electronic reasons they may behave as individual microreactors exhibiting different catalytic activity and/or selectivity. Recent in situ XPS studies comparing the CO oxidation on Rh nanoparticles of different sizes showed increased oxide formation over smaller particles and correlated to that higher catalytic activity.16 These findings are concomitant with the size distribution in real catalyst particles, containing an appreciable number of co-ordinatively unsaturated surface atoms, which can promote surface stress and elevated atomic mobility.17 For the majority of catalyst particles, being in the sub-10 nm range, quantum size and support interfacial effects can play an important role as well. For the particular case of the PtRh bimetallic particles, a large number of electron microscopy and X-ray diffraction studies has been focused on looking for correlations between their morphology, structure, and catalytic activity in specific reactions, based exclusively on the particle structure determined by the preparation procedures.6,18,19 Thermodynamic calculations and a limited number of X-ray photoelectron spectroscopy (XPS) studies revealed temperature and gas adsorption effects on the surface composition, similar to those observed with single crystal surfaces.19,20 There is a consensus that before being exposed to reactive gases the outer shell of Pt0
