Review pubs.acs.org/CR
Surface Chemistry of Late Transition Metal Oxides Jason F. Weaver* Department of Chemical Engineering, University of Florida, Gainesville, Florida 32611, United States 3.2.2. Adsorption of CO2 on PdO(101) 3.3. Oxidation and Dehydrogenation Reactions on PdO(101) 3.3.1. CO Oxidation on PdO(101) 3.3.2. Adsorption and Oxidation of H2 on PdO(101) 3.3.3. Methanol Oxidation on PdO(101) 3.4. Alkane Adsorption and Activation on PdO(101) 3.4.1. Facile C−H Bond Activation of Propane on PdO(101) 3.4.2. Formation and C−H Bond Activation of Alkane σ-Complexes on PdO(101) 3.4.3. DFT Calculations of a Series of n-Alkane σ-Complexes on PdO(101) 3.4.4. Reactivity of Higher n-Alkanes and Surface Poisoning Effects 3.4.5. High Selectivity for Primary C−H Bond Cleavage of Propane on PdO(101) 4. Surface Chemistry of IrO2(110) 4.1. Preparation of IrO2(110) Surfaces 4.2. Adsorption and Oxidation of NH3 on IrO2(110) 4.3. Methane Adsorption and Activation on IrO2(110) 5. Bulk Oxide Films of Platinum and Rhodium 5.1. Formation of Pt Oxide Layers: α-PtO2(0001) 5.2. CO Oxidation on PtO2 Layers 5.3. Structure and Reactivity of Rh2O3 Layers 6. Summary and Outlook Author Information Corresponding Author Notes Biography Acknowledgments References
CONTENTS 1. Introduction 2. Surface Chemistry of RuO2(110) 2.1. Interactions with Oxygen: Formation of RuO2 2.1.1. Preparation of RuO2(110) on Ru(0001) 2.1.2. Structure of the RuO2(110) Surface 2.1.3. Structure of RuO2(100)/Ru(101̅0) 2.1.4. O-Rich RuO2(110) Surface 2.1.5. O2 Dissociation and Reoxidation of RuO2(110) 2.2. Reversible Adsorption on RuO2(110): H2O and CO2 2.3. Oxidation and Dehydrogenation Reactions on RuO2(110) 2.3.1. CO Oxidation on RuO2(110): A Prototype Reaction on a Reducible Oxide 2.3.2. CO Oxidation on RuO2(100) 2.3.3. NO Adsorption and Reaction on RuO2(110) 2.3.4. H 2 Adsorption and Oxidation on RuO2(110) 2.3.5. Ammonia Oxidation: Selectivity toward N2 vs NO Production 2.3.6. Oxidation of HCl and Surface Chlorination of RuO2(110) 2.3.7. Adsorption and Oxidation of Ethylene on RuO2(110) 2.3.8. Oxidation of Methanol on RuO2 Surfaces 2.3.9. Alkane Adsorption on RuO2(110) 3. Surface Chemistry of PdO(101) 3.1. Interactions with Oxygen: Formation of PdO 3.1.1. Stability of PdO(101) 3.1.2. Surface Structure of PdO(101) 3.1.3. Preparation of PdO(101) Thin Films on Pd(111) 3.1.4. Chemisorption of O2 on PdO(101) 3.2. Reversible Adsorption 3.2.1. Water Adsorption on PdO(101): Formation of a HO-H2O Complex © 2013 American Chemical Society
4164 4166 4166 4166 4166 4167 4167 4169 4170 4171 4171 4175 4176 4178 4180
4191 4192 4192 4194 4197 4198 4198 4199 4201 4202 4203 4205 4205 4206 4206 4206 4207 4208 4209 4210 4211 4211 4211 4211 4211 4211
4182
1. INTRODUCTION The surface chemistry of late transition metal oxides is scientifically interesting and can play a central role in commercial applications of heterogeneous catalysis. The late transition metals, including Ru, Rh, Ir, Pd, and Pt, are used as catalysts to promote oxidation reactions in numerous and widespread applications, including exhaust gas remediation in automobiles and power plants,1,2 the catalytic combustion of natural gas,3−16 fuel cell catalysis,17 and the selective oxidation
4183 4185 4185 4186 4186 4186 4187 4187 4188 4190
Special Issue: 2013 Surface Chemistry of Oxides Received: August 7, 2012 Published: February 15, 2013
4190 4164
dx.doi.org/10.1021/cr300323w | Chem. Rev. 2013, 113, 4164−4215
Chemical Reviews
Review
of organic compounds,18 to name a few. In practical applications of oxidation catalysis, oxygen can react with a metallic catalyst to create a metal oxide layer at the catalyst surface. Compared with the original metal, the oxide layer represents an entirely new chemical compound. Because oxides and their parent metals usually have very different chemical properties, the transformation of a metal surface to a metal oxide can cause the performance of the catalyst to change dramatically. Indeed, understanding the growth and chemical properties of oxide layers on transition metal surfaces is essential for bridging the pressure gap in many applications of heterogeneous catalysis since metal oxide formation occurs while the catalyst is operating in a realistic gaseous environment. Several oxygen phases develop on a transition metal surface during the course of oxidation. Oxygen atoms initially chemisorb on the metal surface and can arrange in multiple ordered phases prior to the onset of oxide formation. In general, above a critical oxygen coverage, chemisorbed oxygen induces surface restructuring and the formation of so-called surface oxides, which correspond to ordered two-dimensional (2D) or monolayer oxide structures.19−45 Surface oxides on late transition metals have been investigated extensively over the past decade or so30 and typically exhibit chemical properties that differ from bulk oxides of the same metal. Continued oxygen adsorption causes the surface oxide to transform to a multilayer bulk oxide.38,40,42,43,46−48 At moderate temperature, bulk oxides can form as ultrathin films on metal single crystal surfaces, reaching thicknesses of only 1−2 nm due to kinetic constraints.40,42,49−52 Thicker, poorly ordered clusters tend to form when oxidation is conducted at higher temperature.25,48 Since each surface oxygen phase can have different chemical properties, the phase evolution during oxidation can cause the catalytic performance to depend sensitively on the reaction conditions. This idea has provided substantial motivation for pursuing a detailed understanding of the oxidation of late transition metal surfaces and the chemical properties of the resulting oxygen phases. The importance of oxide formation in applications of catalysis has long been recognized. However, significant progress in developing an atomic-level understanding of the growth and surface chemical properties of late transition metal oxides has been made only over the past decade or so. The reasons are that well-defined surfaces of late transition metal oxides are difficult to prepare for characterization in ultrahigh vacuum (UHV), and in situ techniques for studying the surfaces of working catalysts have been advanced relatively recently. Late transition metals are generally resistant to oxidation, and the bulk oxides have low thermodynamic stability compared with oxides of the earlier transition metals and the main group elements. In the context of a Brønsted−Evans−Polanyi (BEP) relation53 an inverse relationship between the activation energy and enthalpy change of reaction the low exothermicity of bulk oxide formation translates into high kinetic barriers for oxidizing a late transition metal. For example, at typical dosing pressures employed in UHV studies (
