Boudart reaction

Applied Statistical Mechanics Thomas M. Reed and Keith E. Gubbins Elementary Chemical Reactor Analysis Rutherford Aris Kinetics of Chemical Processes Michel Boudart Reaction Kinetics for Chemical Engineers Stanley M. Walas... 

M. Boudart and G. Djega-Mariadassou, Kinetics of Heterogeneous Catalytic Reactions, Princeton University Press, Wnceton, NJ, 1984. 

Boudart M and Djega-Mariadassou G 1984 Kinetics of Heterogeneous Catalytic Reactions (Prinoeton, NJ Prinoeton University Press)... 

Complete or very high conversion requires the study of catalyst at very low concentrations. At such conditions, close to equilibrium (Boudart 1968), all reactions behave according to first order kinetics. Study at very low concentrations is also helped by the very small heat generation, so these studies can be executed in small tubular reactors, placed in simple muffle furnaces. Such studies were made by Kline et al (1996) at Lafayette College and were evaluated by Berty (1997). 

The systematic use of classical catalytic kinetics is always a useful approach in modeling (Boudart 1986). Even if these models do not reflect the true mechanism in the case of structure-sensitive catalysts, they are a formally correct representation of the observed facts. As Boudart sees it in the case of structure-insensitive reactions, it can also be the real thing. 

For a sequenee of reaetion steps two more eoneepts will be used in kinetics, besides the previous rules for single reaetions. One is the steady-state approximation and the seeond is the rate limiting step eoneept. These two are in strict sense incompatible, yet assumption of both causes little error. Both were explained on Figure 6.1.1 Boudart (1968) credits Kenzi Tamaru with the graphical representation of reaction sequences. Here this will be used quantitatively on a logarithmic scale. 

Boudart, M., and Diega-Mariadassou, G. (1984). Kinetics of Heterogenous Catalytic Reactions. Princeton, NJ Princeton University Press. 

The decomposition of NO is a very slow catalytic reaction. Amirazmi, Benson, and Boudart recently studied the kinetics over platinum and over oxides of copper, cobalt, nickel, iron and zirconium from 450 to 900°C. They found that the kinetics is first order in NO with concentrations from 1.5 to 15%, and that oxygen has a strong inhibiting effect. Even at these temperatures, the kinetics is about a factor of 1000 too low for automotive usage (97). 

The positive values obtained in practically all cases indicate that all these models may be plausible representations of the data and indeed, the correlation coefBcients, R, are greater than 0.9. Thus, statistical compliance is not a sufficient basis for model discrimination. Specifically, the thermodynamic consistency of the estimates, as proposed by Boudart et al. [3], is appropriate further scrutinizing criterion during kinetic modelling and has been gainfully employed in other reactions [4-6]. 

Steam reforming proceeds according to the following reactions [J. Rostrup-Niel-sen in J.R. Anderson, M. Boudart (Eds.), Catalysis - Science and Technology, Vol. 5, Springer, Berlin (1984)] ... 

The kinetics of ethylene hydrogenation on small Pt crystallites has been studied by a number of researchers. The reaction rate is invariant with the size of the metal nanoparticle, and a structure-sensitive reaction according to the classification proposed by Boudart [39]. Hydrogenation of ethylene is directly proportional to the exposed surface area and is utilized as an additional characterization of Cl and NE catalysts. Ethylene hydrogenation reaction rates and kinetic parameters for the Cl catalyst series are summarized in Table 3. The turnover rate is 0.7 s for all particle sizes these rates are lower in some cases than those measured on other types of supported Pt catalysts [40]. The lower activity per surface... 

Attempts to determine how the activity of the catalyst (or the selectivity which is, in a rough approximation, the ratio of reaction rates) depends upon the metal particle size have been undertaken for many decades. In 1962, one of the most important figures in catalysis research, M. Boudart, proposed a definition for structure sensitivity [4,5]. A heterogeneously catalyzed reaction is considered to be structure sensitive if its rate, referred to the number of active sites and, thus, expressed as turnover-frequency (TOF), depends on the particle size of the active component or a specific crystallographic orientation of the exposed catalyst surface. Boudart later expanded this model proposing that structure sensitivity is related to the number of (metal surface) atoms to which a crucial reaction intermediate is bound [6]. 

Boudart [4] TOFs can either be independent of particle size (structure-insensitive reactions), increase (antipathetic structure sensitivity) or decrease (sympathetic structure sensitivity) with growing particle size, or cross a maximum (Figure 2). 

Hanson FV, Boudart M. 1978. The reaction between H2 and O2 over supported platinum catalysts. J Catal 53 56-67. 

Djega-Mariadassou, G. and Boudart, M. (2003) Classical kinetics of catalytic reactions, J. Catal., 216, 89. 

Table 3 Madon-Boudart test for the absence of mass transfer effects, reaction conditions 45 psig hydrogen / 298 K / 700 RPM.

Several generalizations can be made about the variables that influence reaction rates. Those that follow are in large measure adapted from Boudart s text (1). 

The temperature dependence of the selectivity for isomerization versus hydrogenolysis depends on the type of catalyst. Thus, over thick platinum film catalysts this selectivity was temperature independent for the reaction of the butanes and neopentane (24). However, in Boudart and Ptak s (122) reaction of neopentane over platinum/carbon the selectivity to isomerization decreased slightly with increasing temperature while Kikuchi et al. (128) found an increased trend for isomerization in the reaction of n-pentane over platinum/silica and platinum/carbon catalysts. 

In quite a number of cases the particle size was not found to have an effect on the specific reaction rate, whereas in some others this effect was clearly observed. Boudart et al. (7) coined the term facile for reactions in which the specific activity does not depend on the particle size, using the term demanding in referring to those cases where such a dependence exists. 

Various catalytic reactions are known to be structure sensitive as proposed by Boudart and studied by many authors. Examples are the selective hydrogenation of polyunsaturated hydrocarbons, hydrogenolysis of paraffins, and ammonia or Fischer-Tropsch synthesis. Controlled surface reactions such as oxidation-reduction reactions ° or surface organometallic chemistry (SOMC) " are two suitable methods for the synthesis of mono- or bimetallic particles. However, for these techniques. 

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