Classical kinetics of catalytic reactions
Gérald Djéga-Mariadassou a,∗ and Michel Boudart b
a Laboratoire Réactivité de Surface, Université Pierre et Marie Curie, UMR CNRS 7609 Case 178, 4, Place jussieu, 75252 Paris cedex 05, France b Department of Chemical Engineering, Stanford University, Stanford, CA 94305, USAReceived 16 July 2002; revised 16 October 2002; accepted 6 November 2002
Abstract The present paper deals with a “reactivity approach” to complex catalytic reactions. It utilizes data obtained with a reactor and demonstrates the very important role of both rate constants and concentrations of adsorbed species, on catalytic cycle, activity, and selectivity. It develops a link between global kineticsand closed sequence of elementary steps. It emphasizes the aspects of “assisted” catalytic reactions, kinetic “coupling” of catalytic cycles, and selectivity. 2003 Elsevier Science (USA). All rights reserved.
Keywords: Assisted catalytic reactions; Kinetic coupling; Rate constants; Rates and selectivity
1. Introduction During the past 40 years, numerous concepts have developed in the ﬁeld ofkinetics applied to heterogeneous catalysis . In the kinetic approach no frontiers exist today between homogeneous, enzymatic, and heterogeneous catalysis. There is a consistent science which permits the deﬁnition of useful and efﬁcient rate laws describing sequences of elementary steps. Comparison between an enzyme active site and a metallic site supported on an oxide became possible owing tothe famous “turnover rate” concept. The present paper deals with the “reactivity approach” to complex catalytic reactions. It tries to show the very important role of both rate constants and concentrations of adsorbed species on the turnover rate of catalytic cycles and selectivity. It emphasizes the aspects of “assisted” catalytic reactions, kinetic “coupling” of catalytic cycles, and selectivity.The paper reviews the most important concepts and shows how they have been developed during the past 20 years in different ﬁelds of catalysis. It does not consider the “microkinetic” analysis of Dumesic et al. . Some striking applications are presented including the cleaning of automotive gas exhaust and isomerization for obtaining high-octane gasoline.
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2. Basicconcepts and deﬁnitions 2.1. The quasi-steady-state approximation The quasi-steady-state approximation (QSSA), used for computational codes such as Chemkin or Chemkin-surface, permits, in the latter case, to enter all elementary steps of a catalytic sequence and to optimize all rate constants and concentrations of all reaction intermediates. The QSSA theory is based on three fundamental rules: (i)concentrations of intermediates are very low; (ii) the variation of the concentration of one intermediate is stationary, i.e., independent of time (d[intermediate]/dt = 0); (iii) consequently, rates of all steps in a sequence have the same value, once divided by the stoichiometric number σi , the number of times we need an i-elementary step in the sequence to obtain the overall chemical equation.The QSSA theory leads to the deﬁnition of a catalytic reaction as a closed sequence, the rate of which is the turnover rate, i.e., the rate per site. The reaction rate can be calculated from any i-step according to the following equation: reaction rate = net rate of the i-step/σi . As an example, let us consider the CO oxidation on a threeway catalyst (TWC), presenting a zero-valent noble metalactive site, denoted by “∗ ,” such as Pt . The global chemical reaction is 2CO + O2 = 2CO2 ; or CO + 1 O2 = CO2 . 2 (1)
E-mail address: firstname.lastname@example.org (G. Djéga-Mariadassou).
0021-9517/03/$ – see front matter 2003 Elsevier Science (USA). All rights reserved. doi:10.1016/S0021-9517(02)00099-4
G. Djéga-Mariadassou, M. Boudart / Journal of Catalysis 216 (2003) 89–97