An improved microkinetic model for the water gas shift reaction on copper Caitlin Callaghan a, * , Ilie Fishtik a , Ravindra Datta a , Michael Carpenter b , Matthew Chmielewski c , Anibal Lugo c a Department of Chemical Engineering, Worcester Polytechnic Institute, 100 Institute Road, Worcester, MA 01609-2280, USA b General Motors Fuel Cell Activities, Warren, MI 48090-9055, USA c Aerotek Automotive Warren, MI 48093, USA Received 3 February 2003; accepted for publication 10 July 2003 Abstract A previously developed 13 elementary step microkinetic mechanism for the WGSR on Cu(1 1 1) [Surf. Sci. 512 (2002) 229] is extended by including four new elementary steps. Employing a systematic reaction route analysis, the micro- kinetic model is further analyzed, simplified and reduced. It is shown that from the total of 70 possible overall reaction routes (RRs), the kinetics of the WGS reaction on Cu is still dominated by only three RRs. Two of them, namely, the formate and associative RRs, are dominant at lower temperatures. The third one, referred to as a modified redox RR, is dominant at higher temperatures. Utilizing the quasi-equilibrium and steady-state assumptions, the microkinetic model is further reduced to a two-step mechanism similar to that of Temkin. The corresponding explicit rate expression with all parameters predicted a priori provides excellent agreement with the complete microkinetic model as well as with new experimental data provided here. Ó 2003 Elsevier B.V. All rights reserved. Keywords: Models of surface chemical reactions; Model of surface kinetics; Copper; Water; Carbon monoxide; Carbon dioxide; Hydrogen molecule 1. Introduction Development of both low temperature and high temperature water-gas-shift (WGS) catalysts is of current interest with a view toward increasing the yield of hydrogen and lowering the content of carbon monoxide in the fuel reforming process to be used in conjunction with hydrogen fuel cells. Until recently, the approach for designing cata- lysts has been based on guess work, heuristics, or on random or combinatorial screening of single, binary, or tertiary catalysts of varying composi- tions yielding an infinite number of possibilities to be screened. In recent years, furthermore, much emphasis has been placed on developing detailed and microkinetic models for the WGS reaction [1– 11]. A deeper understanding of the sequence of molecular events on the surface of the catalyst might eventually provide a more systematic and efficient theory-guided catalyst design. Recently [11], we have developed a 13 elementary step * Corresponding author. Tel.: +1-5088315445; fax: +1- 5088315853. E-mail address: caitlin@wpi.edu (C. Callaghan). 0039-6028/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0039-6028(03)00953-1 Surface Science 541 (2003) 21–30 www.elsevier.com/locate/susc