Study of CO Dissociative Adsorption over Pt and Rh Catalysts by Inverse Gas Chromatography D. Gavril & , V. Loukopoulos, G. Karaiskakis Department of Chemistry, University of Patras, 26504 Patras, Greece; E-Mail: D.Gavril@upatras.gr Received: 7 November 2003 / Revised manuscript Received: 20 January 2004 / Accepted: 30 January 2004 Online publication: 20 May 2004 Abstract Carbon monoxide dissociative adsorption was studied over silica-supported platinum, rhodium and Pt-Rh alloy catalysts, by Reversed Flow-Gas Chromatography. Using appropriate mathe- matical analysis physicochemical quantities such as fractional catalytic conversions of CO to CO 2 , as well as rate constants for the adsorption, desorption and surface reaction, describing the dissociative adsorption of CO, were determined. From the variation of the above param- eters against the nature of the studied catalysts (Rh content) useful conclusions concerning the mechanism of CO dissociative adsorption were extracted. Keywords Inverse gas chromatography CO adsorption and dissociation Platinum Rhodium catalysts Rate constants Introduction The dissociation of molecules is one of the most important reactions in catalysis. It is the first step and often the rate-deter- mining step in many catalytic processes. Therefore, dissociation reactions have been extensively studied both experimen- tally and theoretically [1–3]. Two factors [3] are found to affect the dissociation reactions. The first one is the electronic factor: It has been found experimentally that the reactivity of transition metals for dissociation decreases from left to right in the periodic table [2]. Hammer and Norskov [3] have succesfully correlated the reactivity of metal for dissociation reactions with the metal d band center. The second factor is the geometrical one: Both experimental and theoretical work shows that dissociation reaction occur much more efficiently on corrugated surfaces than on flat surfaces [3, 4]. CO dissociation is an important first step in various catalytic processes such as the methanation reaction, Fisher-Trop- sch synthesis [5, 6]. These reactions take place over a number of transition metals. We have chosen to focus here on silica supported Platinum, Rhodium and Pt 0.50 –Rh 0.50 alloy catalysts, because there are detailed experimental data for CO dissociation [4, 7–8]. On the other hand Rh and Pt are by far from the most expensive precious metals [9]. Hence, it is of great importance to find new catalysts with high activity and selectivity to decrease in Pt and Rh usage. The util- isation of bimetallic catalysts can be advantageous, resulting in a drastic decrease in Pt or Rh usage. For example under oxidising conditions the surface composition of a Pt-Au alloy is Pt rich because of the strong Pt-O bond [10]. Therefore, high Pt surface concentrations can be achieved even for catalyst formu- lations with a low bulk concentration of platinum. In particular understanding critical elementary reaction steps such as adsorption, desorption and bond break- ing over platinum, rhodium and plati- num-rhodium catalysts can provide substantial insights into why Rh and Pt are so essential in catalysis and how they can best be replaced. In the present work, we present kinetic quantities for CO adsorption, desorption and dissociative surface reaction over Pt, Rh and Pt-Rh alloy catalysts, by means of an inverse gas chromatographic tech- nique called reversed flow gas chroma- tography (RF-GC). In this technique, instead of basing physicochemical mea- surements on retention volumes of elu- tion peaks, their broadening and their shape distortion, due to physicochemical processes are under study. Such mea- surements can easily and accurately be done if the chromatographic column, being at a steady-state situation, is perturbed so that it deviates from equilibrium for a short time interval and DOI: 10.1365/s10337-004-0278-9 2004, 59, 721–728 0009-5893/04/06 Ó 2004 Friedr. Vieweg & Sohn Verlagsgesellschaft mbH Original Chromatographia 2004, 59, June (No. 11/12) 721