Kinetic modelling of multiple steady-states for the oxidation of aqueous ethanol with oxygen on a carbon supported platinum catalyst

Abstract

-Multiple steady-states data were used for the construction of a kinetic model for the oxidation of aqueous ethanol with oxygen on a carbon supported platinum catalyst. A model, incorporating reversible creation of oxygen adatoms on the catalyst surface from surface hydroxyl as well as reversible formation of subsurface oxygen from oxygen adatoms, couid describe quantitatively all the observations. It was essential that the reaction rate coefficients for the formation of atomic and subsurface oxygen strongly depend on the corresponding degrees of coverage introducing positive and negative feedback features. INTRODUCTIONThe selective catalytic oxidation of alcohols and carbohydrates with dioxygen has received considerable attention in the recent past ( Mallat and Balker, 1994). Supported noble metals are the favourite catalysts. The reaction can be performed selectively at relatively mild conditions i.e. in aqueous media at a pH around 7 and at temperatures around 300 K.The major bottleneck for the commercial application consists in so-called catalyst deactivation. Several causes have been put forward for the latter ( Schuurman et al., 1992b;Mallat and Baiker, 1994;Vleeming et al., 1994). There is evidence for well known deactivation mechanisms such as metal dissolution and/or sintering (Schuurman et al., 1992b) and coverage of active sites by side or intermediate products of the desired reaction ( Mallat and Baiker, 1994). A phenomenon more specific to the considered type of reactions has also been suggested. It is referred to as overoxidation of the metal surface. The term stems from the observation that the loss in activity occurs in parallel with an increase of the catalyst potential (Vleeming et al., 1994).Recently steady-state multiplicity of the selective oxidation rate of aqueous ethanol over a platinum on carbon catalyst has been reported (Jelemensky et al., 1995). Up to three stable steady-state rates could be established over a feed ethanol concentration range from 300 to 400 mol m 3 in a continuous flow stirred tank reactor. The upper steady-state was reached by a reductive start-up procedure while the two lower steady-states required an oxidative start-up. The existence of steady-state multiplicity is believed to be rather common for selective oxidations and hydrogenations by noble metals in the liquid phase.Bifurcation phenomena, such as multiple steady-states (R~zon and Schmitz,1987;Zhdanov and Kasemo, 1994), autonomous oscillations ( Gray and Scott, 1990) or chaos ( Scott, 1991) are well documented for heterogeneous catalytic reactions. In the absence of heat and/or mass transport limitations such phenomena have to be caused by purely chemical feedback features such as autocatalysis. The observations of Jelemensky et al. (1995) were free of transport limitations.The present paper concerns the development of an intrinsic kinetic model for the platinum catalyzed selective oxidation of ethanol which allows a quantitative description of the complete set of data reported by Jelemensky et al. (...

A simple discrimination of the promoter effect in alcohol oxidation and dehydrogenation over platinum and palladium

“…Despite the intensive effort by several research groups, the real nature of the promoter effect is still debated. Several strategies have been applied to understand the behavior of the bi-and trimetallic catalysts, including the detailed kinetic analysis of the reaction [4,32,33], in situ study of the oxidation state of the metals by electrochemical methods [34][35][36][37] and X-ray absorption spectroscopy (XAS) [38][39][40][41], and the synthesis and catalytic test of intermetallic compounds [42,43].…”

Relaxation Processes during the Selective Oxidation of Aqueous Ethanol with Oxygen on a Platinum Catalyst

“…Reaction Kinetics and Mass Balances for the Surface Species. The modeling of steady-state multiplicity of the rate of selective oxidation of aqueous ethanol over a platinum on a carbon catalyst has been reported recently by Jelemensky et al (1996). A model which sufficiently describes the experimental data is based on the assumption that three different forms of oxygen exist on the platinum: hydroxyl, atomic oxygen, and subsurface oxygen, the former being the reaction intermediate for the ethanol oxidation.…”

“…A model which sufficiently describes the experimental data is based on the assumption that three different forms of oxygen exist on the platinum: hydroxyl, atomic oxygen, and subsurface oxygen, the former being the reaction intermediate for the ethanol oxidation. For details on the kinetics of the selective oxidation of ethanol which are described by the classical Hougen-Watson-Langmuir-Hinshelwood approach the reader is referred to Jelemensky et al (1996). Only the features important for the relaxation will be briefly presented here.…”

“…starting with oxygen in the absence of ethanol. A model considering the reversible formation of less reactive oxygen adatoms and even less reactive subsurface oxygen from the reaction intermediates, OH species, which could describe the multiplicity of steady state quantitatively was developed (Jelemensky et al, 1996).…”

On-line characterization of platinum/graphite catalysts during liquid phase oxidations using cyclic voltammetry

“…A low steady-state activity was established with an oxidative start-up and a high steady-state activity with a reductive start-up. This behaviour could only be described when the formation of subsurface oxygen was taken into account [13]. Under oxygen transfer limited conditions, it is less likely that this sub-surface oxygen will be formed.…”