Kinetic study of CO oxidation on copper modified Pt(111)

The kinetics of CO oxidation on Pt(111) modified with submonolayer copper adlayers were studied using mass spectrometry (MS) and photoemission electron microscopy (PEEM). The submonolayer 2D copper adlayers were formed on approximately half of the Pt(111) crystal, by evaporating copper through a mask. More precisely, the article deals with the modifications of the hysteresis in the reaction rate compared to the hysteresis occurring under certain reaction conditions on pure Pt(111) when the CO pressure is cyclically varied. When copper was evaporated onto the Pt crystal, the kinetic phase transition between the high and low rate regimes shifted to higher CO pressures and the reaction rate increased. In addition, the CO pressure range in which bistability was observed became wider. Upon increasing the copper coverage, the shift showed a maximum at a coverage of 0.3 ML until at 0.7 ML no clear first-order transition in the reaction rate could be observed any more. The results suggest that the active site responsible for the increased activity lies at the copper-platinum bimetallic border sites. (C) 1998 Elsevier Science B.V. All rights reserved

CO oxidation on a copper-modified Pt(111) surface

The catalytic oxidation of carbon monoxide is studied in situ by means of reaction-rate measurements on a macroscopic scale and photoelectron emission microscopy (PEEM) on a mesoscopic scale on a Pt(111) single-crystal surface partially modified by evaporated Cu. By using a special preparation technique it is possible to cover selectively certain parts of the Pt(111) surface with copper, thus enabling a simultaneous PEEM investigation of the behavior of each separate catalyst component as well as their mutual influence. The objective of the work is to investigate the possible influence of deposited copper on CO oxidation on Pt(111) and to compare the results with pure Pt(111). Cu-modified regions preferentially form oxygen adsorption layers under the reaction conditions. The well-documented behavior of CO oxidation on Pt(111) is modified by the presence of the Cu domains. This modification concerns the transition between the GO-covered and oxygen-covered state of the Pt surface. The macroscopic effect of the modification by Cu is to shift the whole hysteresis in the reaction rate to higher CO pressures. On the mesoscopic scale the nucleation of oxygen islands at the beginning of the phase transition takes place on Cu-modified areas. On a Pt(111) sample on which submonolayer quantities of copper were uniformly deposited, the propagation velocity of the reaction/diffusion front of a growing oxygen island increases with the amount of deposited Cu. These experiments demonstrate the possibility of creating special geometric adsorbate patterns during CO oxidation on the Pt(111) surface, and of modifying the velocity of propagation of the reaction/diffusion front. (C) 1997 Elsevier Science B.V

Interaction between Cu and Pt(111) in the reaction CO+O-2 modification by Cu sub-monolayers and cooperation between pure and Cu-modified Pt(111)

Sub-monolayers of Cu modify the characteristics of the CO + CO2 reaction on Pt(111), in particular (i) the bistability region where either dense CO or oxygen adsorption layers can form according to the direction of variation of the external parameters, and (ii) the progression velocity of the reaction front as observed by PEEM. In addition, a cooperation takes place between Pt modified by Cu sub-monolayers and pure Pt(lll). This highlights two different roles of copper in modifying the kinetics of CO oxidation, namely (i) triggering the reaction and (ii) modifying the rate of reaction front propagation. In addition, a completely new effect occurring at the border between Cu-covered and pure Pt is reported. (C) 1997 Elsevier Science B.V