Methanol Decomposition on Pt/ZnO(0001)-Zn Model Catalysts Ann W. Grant, Jane H. Larsen, † C. A. Perez, ‡ S. Lehto, Martin Schmal, ‡ and Charles T. Campbell* Department of Chemistry, UniVersity of Washington, Seattle, Washington 98195 ReceiVed: May 16, 2001; In Final Form: July 5, 2001 The relationship between metal particle structure and the reactivity and selectivity of oxide-supported metal catalysts has been explored here with well-defined Pt particles on the Zn-terminated ZnO(0001)-Zn surface, using X-ray photoelectron spectroscopy (XPS), ion scattering spectroscopy (ISS), and temperature-programmed desorption (TPD). These are model catalysts for the steam reforming and selective dehydrogenation of methanol. Vapor-deposited Pt on ZnO(0001) grows in two-dimensional (2D) islands up to ∼0.7ML. The effect of the 2D island size and 3D island height are studied. Even 0.01 ML of Pt suppresses the dissociative adsorption of methanol to methoxy on Zn sites by preferential decoration of defects. Tiny amounts of Pt also affect the selectivity of methanol conversion at the Zn sites: the dehydration pathway of its Zn-bound formate intermediate is suppressed and its dehydrogenation pathway to formaldehyde is eliminated. Even Pt sites on 2D Pt islands catalyze the partial oxidation of methanol to CO and H 2 , characteristic of low-index Pt facets. When the Pt islands are 2-3 layers thick, methanol decomposition is similar to that on low-index Pt faces. I. Introduction Platinum on ZnO or Pt-Zn combinations on other oxides are active catalysts in a variety of reactions: methanol steam reforming to CO 2 and H 2 , 1,2 selective methanol dehydrogenation to methyl formate, 2,3 propane conversion to aromatic hydrocar- bons, 4 highly selective dehydrogenation of alkanes, 5,6 selective dehydrogenation of R,-unsaturated aldehydes, 7 and the vapor- phase hydrogenation of crotonaldehyde to crotyl alcohol. 8 Copper-based catalysts are active for methanol steam reform- ing and methanol dehydrogenation to methyl formate. 9,10 Recently, it has been shown that Pt and Pd supported on ZnO are also active and selective for these reactions. 1-3 This is surprising since most group VIII noble metals catalysts primarily decompose methanol to CO and H 2 . 1,3 The excellent selectivity (70%) for methanol dehydrogenation to methylformate was shown to be due to the presence of PtZn and PdZn alloys. 3,2,11 It was speculated that PdZn alloys stabilize a monodentate H 2 - CO intermediate as typically found on Cu catalysts, rather than the usual bidentate intermediate found on group VIII metals. 2 Methanol decomposition has been studied previously on a number of low index Pt single crystal surfaces and on Zn- terminated ZnO(0001). A small fraction of the adsorbed methanol dehydrogenates completely to CO(a) and H(a) on pure Pt surfaces, and the rest desorbs during TPD. 12-17 The temper- atures at which the CO(a) and H(a) desorb depends on the surface plane and on the presence or absence of steps. It has been found that the reactivity of methanol depends on the Pt single-crystal surface. 18 On ZnO(0001)-Zn, methanol dehy- drogenates to methoxy, which undergoes two competing reac- tions, partial dehydrogenation to formaldehyde and oxidation by lattice oxygen to formate. This formate also decomposes through two competing reactions, dehydrogenation to CO 2 (g) and H 2 (g), and dehydration to CO(g) and H 2 O(g). Previous studies of the growth of Pt on ZnO(0001)-Zn with Auger electron spectroscopy (AES), 19 high-resolution electron energy loss spectroscopy (HREELS) and low energy electron diffraction (LEED), 20 and Li + -ion scattering spectroscopy (ISS) 21 all found approximately layer by layer growth, (except that Li + -ISS estimated the critical coverage to be ∼0.90). The 2D Pt films have a similar lattice constant to Pt(111) and are incommensurate with the underlying substrate. 20,21 After an- nealing above 650 K, the 2D Pt films demonstrate improved azimuthal alignment. 21 In the current work, we have characterized the growth of Pt on ZnO(0001)-Zn with He + ISS, which is a more surface sensitive probe than used previously. Pt grows with nearly layer- by-layer growth up to ∼0.7 ML, covering the surface completely after vapor-deposition of ∼2 ML. Methanol dehydrogenation was studied with TPD as a detailed function of the Pt coverage on the ZnO(0001)-Zn surface. The Pt exists mainly in the form of 2D islands below ∼0.7 ML, where the Pt coverage controls the island’s lateral dimensions. (This assumes that their number density becomes nearly constant below 0.1 ML, which is typical of such systems. 22,23 ) Even tiny 2D particles dehydrogenate methanol to CO and H 2 much like low index Pt surfaces, although the products show a higher desorption temperature, similar to stepped Pt surfaces. The addition of even tiny amounts of Pt changes the reaction pathways for methanol decomposition on the ZnO(0001)-Zn surface sites, eliminating the production of formaldehyde, attenuating the production and dehydration of formate species, and enhancing the formate dehydrogenation. II. Experimental Section The experiments were performed in an ultrahigh vacuum (UHV) apparatus described previously 24,25 with a base pressure of less than 2 × 10 -10 Torr. It has capabilities for XPS, LEED, * To whom correspondence should be addressed. E-mail: campbell@ chem.washington.edu. Fax: (206) 616-6250. † Current address: Interdisciplinary Research Center for Catalysis (ICAT), Department of Physics, Building 307 Technical University of Denmark DK-2800 Lyngby, Denmark. ‡ Current address: Univ Federal do Rio de Janeiro, Nucleo de Catalise, Programa de Engenharia Quimica/COPPE, Bloco G, Sala 115, Caixa Postal 68502, CEP 21945-945, Rio De Janeiro, Brazil. 9273 J. Phys. Chem. B 2001, 105, 9273-9279 10.1021/jp0118928 CCC: $20.00 © 2001 American Chemical Society Published on Web 08/25/2001