Roger A. Bennett

Catalysis at the metal-support interface: exemplified by the photocatalytic reforming of methanol on Pd/TiO2

Some types of catalytic reaction may take place at the boundary between a metal and a support. In this paper we describe the mathematical relationships between observed reaction rates and the loading of the supported phase. The reaction rate shows a sharp maximum with increasing loading, and at high loadings the rate is reduced to zero, due to the lack of active sites (the support is completely covered). We report a study of a particular reaction where the kinetic data indicate rather clearly that the active site is at the boundary between such phases. The particular reaction is the photocatalytic degradation of methanol on a Pd/TiO2 catalyst under anaerobic conditions. The reaction produces CO2 and hydrogen and only proceeds at steady state when light is introduced to the system. We propose that this is due to blockage of the Pd by adsorbed CO, and the role of light is to produce a reactive form of oxygen on the support, which cleans off CO from the Pd at the boundary, thus allowing a catalytic cycle to be completed. The larger the extent of this boundary, the higher the reaction rate. A general mathematical model of such reactions is given and applied to these data.

Reactive re-oxidation of reduced TiO2(110) surfaces demonstrated by high temperature STM movies

The re-oxidation of slightly reduced TiO2(110) surfaces by exposure to an oxygen pressure of ~2 × 10-7 mbar in the temperature range 473-1000 K occurs by re-growth of TiO2 overlayers by diffusion of Tin+ interstitials from the bulk. Starting with a (1 × 2) reconstructed surface, scanning tunnelling microscope images of the surface reacting under these conditions show that the (1 × 1) islands nucleate within the (1 × 2) layer and grow laterally. As the islands reach a critical size, which is temperature dependent, a new (1 × 2) layer begins to nucleate and grow. At both extremes of the temperature range nucleation of the second layer occurs before coalescence of the (1 × 1) islands, however, at temperatures between 673 and 773 K large areas of (1 × 1) surface form before growth of the second layer. The reaction is cyclic and several layers of TiO2 can be grown in this way.

Pd nanoparticle enhanced re-oxidation of non-stoichiometric TiO2: STM imaging of spillover and a new form of SMSI

We have used STM imaging in situ to demonstrate two fundamental steps in catalytic processes on model catalysts at elevated temperature. We show that Pd nanoparticles on sub-stoichiometric TiO2(110) dissociatively adsorb O2 at 673 K which “spills over” onto the support where further reaction takes place. The spillover oxygen re-oxidises the surface by removing Tin+ interstitial ions trapped in the crystal lattice, preferentially re-growing TiO2 around and over the particles. The identification of the metal enhanced re-oxidation mechanism may have important and general consequences for the understanding of catalysis and gas sensing.

Formic acid adsorption and decomposition on TiO2(1 1 0) and on Pd/TiO2(1 1 0) model catalysts

The reaction of formic acid with TiO2(1 1 0)-(1×1) and with model catalysts with Pd nanoparticles on that oxide has been investigated using a molecular beam reactor. On the oxide surface only the dehydration reaction is seen, and it proceeds at steady state with a very high reaction probability above 500 K. There is no evidence for a dehydrogenation pathway. However, when the surface is loaded with Pd nanoparticles then the dehydrogenation reaction is also seen, beginning at about 350 K. The rate of formic acid decomposition is slower than on a Pd single crystal because the nanoparticles are much less reactive than the bulk metal, probably due to alloying with Ti originating from interstitial Ti3+ cations in the oxide. This results in reduced electron density at the Fermi level of the nanoparticles. However, the adsorption probability of the formic acid with the Pd is higher than expected due to diffusion of formic acid, in a weakly held state, from the support to the metal particles.

Methanol oxidation to formate on Cu(110) studied by STM

Abstract We present an STM study of methanol oxidation to formate by co-dosing of oxygen and methanol onto Cu(110) at room temperature. Formate results from the reaction of adsorbed methoxy and isolated oxygen adatoms generated during dosing. Methanol-rich gas-phase mixtures have the highest efficiency for formate production generating characteristic alternating formate c(2 × 2) and O-(2 × 1) islands in the 〈001〉 direction. An oxygen-rich gas-phase composition does not produce formate due to preferential growth of O-(2 × 1) islands. Formate production at 300 K during co-dosing experiments contrasts with the case of sequential dosing as oxygen (2 × 1)-reconstructed Cu(110) surfaces are seen to have negligible activity toward formate production when subsequently dosed with methanol at 300 K.

A combined STM/molecular beam study of formic acid oxidation on Cu(110)

Abstract In this paper, we describe a powerful combination of methods used to elucidate the mechanisms and kinetics of surface reactions, namely, molecular beam reactor work and scanning tunnelling microscopy (STM). These two combine microkinetic measurements with a technique which has atomic spatial resolution for structural determination during the course of a reaction. This gives the ability to link the worlds of macroscale and nanoscale surface chemistry. A particular application is highlighted here, namely, the adsorption and oxidation of formic acid on Cu(110), in principle, a simple reaction, which turns out to be very complicated. The findings can be summarised as follows: (i) predosed oxygen enhances formic acid sticking by at least an order of magnitude; (ii) oxygen predosed to a coverage below 0.25 monolayers is completely removed as water by the reaction with formic acid to produce formate; (iii) above this oxygen coverage, some oxygen remains on the surface after reaction and is compressed up into the locally higher coverage c (6×2) structure, otherwise only obtained by extremely high exposures of oxygen; (iv) the reaction stoichiometry changes from 2 HCOOH: 1 O (a) to 1:1 at high temperature where the formate is unstable; (v) gross rearrangement and redistribution of Cu atoms occurs during the course of the reaction; and (vi) the formate adsorbate structure varies considerably with substrate temperature, oxygen precoverage and formic acid exposure, five distinct structures being observed with STM.

Formic acid adsorption and decomposition on non-stoichiometric TiO2(110)

We have investigated the adsorption and decomposition of formic acid on the non-stoichiometric TiO 2 (110) surface using sequential scanning tunneling microscopy (STM) imaging at variable temperature. Formic acid adsorption on the surface displaying the cross-linked (1 × 2) reconstruction shows no ordered structures upon adsorption at room temperature. The added rows of the reconstructed surface appear disrupted or obscured. Upon ramping the temperature to 673 K the ( 1 × 2) reconstruction becomes visible and small islands of ( 1 × 1) termination grow within the ( 1 × 2) terraces. This behaviour is reminiscent of the ( 1 × 2) surface reacting with O 2 at elevated temperature. We attribute the formation of the ( 1 × 1) islands to the decomposition of the formate and insertion of an oxygen atom into the lattice as the surface re-oxidises. The growth of the surface implies interstitial Ti ions, diffusing out from the bulk, react with either an intact formate molecule or its decomposition product to form the new TiO 2 surface.

HIGH-RESOLUTION X-RAY PHOTOELECTRON SPECTROSCOPY STUDY OF THE ETHANOL OXIDATION REACTION ON PD(110)

The adsorption, decomposition, and oxidation of ethanol on Pd(110) has been studied using high-resolution x-ray photoelectron spectroscopy (XPS) and temperature-programmed XPS. The decomposition pathways of ethanol on the clean surface (to methane, hydrogen and carbon monoxide; and to methane, hydrogen, and carbon and oxygen adatoms) previously studied using molecular beam and thermal desorption spectroscopy were confirmed by this study. The presence of an overlayer of oxygen did not significantly alter the major or minor decomposition pathways observed on the clean surface, except for the production of water and, at temperatures above 380 K, carbon dioxide as oxidation products. It also resulted in the formation of acetate, which was first seen during temperature-programmed desorption as coincident carbon dioxide and hydrogen desorption, and was confirmed by XPS. Two C 1s peaks, one assigned to the methyl carbon in acetate and the other to the carboxylate carbon, developed simultaneously during TPXPS. Th...

Contrasting reaction pathways in methanol oxidation on Cu(110) studied by STM

Scanning tunnelling microscopy (STM) has been employed in studying the adsorption and reaction of methanol and oxygen on Cu(110) during co-dosing. We have observed the production of formate c and structures at 300 K during co-dosing. This strongly contrasts with the established case of sequential dosing, since oxygen precovered Cu(110) surfaces are seen to have negligible activity toward formate production when subsequently dosed with methanol. During co-dosing formate and methoxy groups are seen as the main reaction products while oxygen reconstructed areas poison the formate production channel. Competition for isolated oxygen adatoms between the O islands and methoxy groups is seen as determining the rate of formate production. Methanol rich gas mixtures have the highest efficiency for formate production whereas an oxygen rich gas phase composition does not produce formate due to preferential growth of O islands.

Surface and interstitial Ti diffusion at the rutile TiO2(110) surface

Diffusion of Ti through the TiO(2)(110) rutile surface plays a key role in the growth and reactivity of TiO(2). To understand the fundamental aspects of this important process, we present an analysis of the diffusion of Ti ad-species at the stoichiometric TiO(2)(110) surface using complementary computational methodologies of density functional theory corrected for on-site Coulomb interactions (DFT + U) and a charge equilibration (QEq) atomistic potential to identify minimum energy pathways. We find that diffusion of Ti from the surface to subsurface (and vice versa) follows an interstitialcy exchange mechanism, involving exchange of surface Ti with the 6-fold coordinated Ti below the bridging oxygen rows. Diffusion in the subsurface between layers also follows an interstitialcy mechanism. The diffusion of Ti is discussed in light of continued attempts to understand the re-oxidation of non-stoichiometric TiO(2)(110) surfaces.