Elementary Surface Reactions Between Adsorbates

TABLE 6.2 Free energy changes for aqueous-phase adsorption at a temperature (r=298.15K) 

Species Experimental A, solv G of isolated species (eV) Calculated A, G A, H of solv solv adsorbed species (eV) A,..G ,(eV) 

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ELEMENTARY SURFACE REACTIONS BETWEEN ADSORBATES 6.5.1 Reaction Thermodynamics... 

The kinetic model comprises the following elementary steps irreversible adsorption of oxygen and reversible adsorption of ethene on the noble metal surface, followed by a surface reaction between adsorbed ethene and oxygen. The values of the kinetic parameters, i.e. preexponential factors and activation energies, were estimated by non-linear regression of the ethene conversion and found to be physically meaningful. 

The surface reaction between adsorbed molecular hydrogen and germanium dichloride is believed to be rate-limiting. The reaction follows an elementary rate law with the rate being proportional to the fraction of the surface covered by GeCU times the square of the fraction of the surface covered by molecular hydrogen. 

In the case of coupled heterogeneous catalytic reactions the form of the concentration curves of analytically determined gaseous or liquid components in the course of the reaction strongly depends on the relation between the rates of adsorption-desorption steps and the rates of surface chemical reactions. This is associated with the fact that even in the case of the simplest consecutive or parallel catalytic reaction the elementary steps (adsorption, surface reaction, and desorption) always constitute a system of both consecutive and parallel processes. If the slowest, i.e. ratedetermining steps, are surface reactions of adsorbed compounds, the concentration curves of the compounds in bulk phase will be qualitatively of the same form as the curves typical for noncatalytic consecutive (cf. Fig. 3b) or parallel reactions. However, anomalies in the course of bulk concentration curves may occur if the rate of one or more steps of adsorption-desorption character becomes comparable or even significantly lower then the rates of surface reactions, i.e. when surface and bulk concentration are not in equilibrium. 

The extent to which such reactions take place in parallel with the dominant reaction (4.1) is, in general, difficult to quantify as the overall reaction (4.3a) may consist of the elementary step (4.1) followed by reaction between adsorbed CO and adsorbed oxygen on the metal surface ... 

Liquid phase hydrogenation catalyzed by Pd/C is a heterogeneous reaction occurring at the interface between the solid catalyst and the liquid. In our one-pot process, the hydrogenation was initiated after aldehyde A and the Schiff s base reached equilibrium conditions (A B). There are three catalytic reactions A => D, B => C, and C => E, that occur simultaneously on the catalyst surface. Selectivity and catalytic activity are influenced by the ability to transfer reactants to the active sites and the optimum hydrogen-to-reactant surface coverage. The Langmuir-Hinshelwood kinetic approach is coupled with the quasi-equilibrium and the two-step cycle concepts to model the reaction scheme (1,2,3). Both A and B are adsorbed initially on the surface of the catalyst. Expressions for the elementary surface reactions may be written as follows ... 

An alternate approach is to specify an elementary chemical reaction mechanism at the surface. In this case one can have reactions between gas-phase species and surface species, as well as reactions between adsorbed species. At this level of specification, surface reaction mechanisms often become very complex, including dozens of elementary reactions. Such complex surface chemistry reaction mechanisms have been used in models for many CVD systems, for example. 

We now propose a mechanism for the hydrodemethylation of toluene. We assume that toluene is adsorbed on the surface and then reacts with hydrogen in the gas phase to produce benzene adsorbed on the surface and methane in the gas phase. Benzene is then desorbed from the surface. Since approximately 75% of all heterogeneous reaction mechanisms are surface-reaction-limited rather than adsorption- or desorption-limited, we begin by assuming the reaction between adsorbed toluene and gaseous hydrogen to be reaction-rate-limited. Symbolically, this mechanism and associated rate laws for each elementary step are ... 

The kinetic studies of ethylene hydrogenation up to the year 1995 are summarized in (ref. fl) The new approaches are in the mere recent papers (refs. 9, 10) It is evident that the ethylene hydrogenation can be reasonably described in terms of several elementary steps when more than one is not at equilibrium. From the practical point of view we prefer the simplified model which provides quite a good description of our experimental kinetic data in the investigated interval. This model corresponds to the surface reaction between CDiipetitively adsorbed reactants and the kinetic expression is of the form... 

Microkinetic modeling assembles molecular-level information obtained from quantum chemical calculations, atomistic simulations and experiments to quantify the kinetic behavior at given reaction conditions on a particular catalyst surface. In a postulated reaction mechanism the rate parameters are specified for each elementary reaction. For instance adsorption preexponential terms, which are in units of cm3 mol"1 s"1, have been typically assigned the values of the standard collision number (1013 cm3 mol"1 s 1). The pre-exponential term (cm 2 mol s 1) of the bimolecular surface reaction in case of immobile or moble transition state is 1021. The same number holds for the bimolecular surface reaction between one mobile and one immobile adsorbate producing an immobile transition state. However, often parameters must still be fitted to experimental data, and this limits the predictive capability that microkinetic modeling inherently offers. A detailed account of microkinetic modelling is provided by P. Stoltze, Progress in Surface Science, 65 (2000) 65-150. 

In the absence of chemical reaction between adsorbed species, it is instructive to analyze adsorption/desorption equilibria via steps 3 and 6. The overall objective here is to develop expressions between the partial pressnie / a of gas A above a solid surface and the fraction of active sites a on the catalyst that are occnpied by this gas when it adsorbs. The phenomenon of chemisorption and the relation between pa and a apply to a unimolecular layer of adsorbed molecnles on the catalytic surface. This is typically referred to as a monolayer, where the intermolecular forces of attraction between adsorbed molecules and active snrface sites are characteristic of chemical bonds. When complete monolayer coverage of the surface exists, subsequent adsorption on this saturated surface corresponds to physisorption, which is analogous to condensation of a gas on a cold snbstrate. The enthalpy change for chemisorption is exothermic with valnes between 10 and 100 kcal/mol. The Langmuir adsorption isotherm, first proposed in 1918 (see Langmuir, 1918), is based on the following reversible elementary step that simulates single site adsorption on a catalytic surface when there is only one adsorbate (i.e., gas A) present ... 

Investigations in the first step define the surface structure and composition on the atomic scale and the chemical bonding of adsorbates. Studies in the second step, which are carried out at low pressures, reveal many of the elementary surface reaction steps and the dynamics of surface reactions. Studies in the third and fourth steps establish the similarities and differences between the model system and the dispersed catalyst under practical reaction conditions. 

Numerical simulations of the reaction system for preferential oxidation of carbon monoxide was performed by Ouyang et cd. who applied CHEMKIN software and a network of 8 species in the gas phase, 8 surface species and 28 elementary reaction steps, not provided here in detail [117]. The simulation described the experimental performance of the reactor of Ouyang et al. very well. It revealed that the oxidation of carbon monoxide occurred by reaction between adsorbed CO and O H species and not by the reaction between adsorbed CO and O species, because the latter reaction rate was ten orders of magnitude lower. Thus a simplified mechanism of the reaction network could be formulated according to Ouyang et al. as follows ... 

A L-H model with a bimolecular surface reaction between two adsorbed NO molecules as the RDS was proposed as follows, where S is an active site and the stoichiometric number for an elementary step lies outside the brackets around that step, i.e.. 

In this book, we demonstrate the use of transition-state theory to describe catalytic reactions on surfaces. In order to do this we start by treating the kinetics of catalytic reactions (Chapter 2) and provide some background information on important catalytic processes (Chapter 3). In Chapter 4 we introduce the statistical mechanical basis of transition-state theory and apply it to elementary surface reactions. Chapter 5 deals with the physical justification of the transition-state theory. We also discuss the consequences of media effects and of lateral interactions between adsorbates on surfaces for the kinetics. In the final chapter we present the principles of catalytic kinetics, based on the application of material given in earlier chapters. 

If particles enter the surface activated complexes directly from the volume, e.g., when the reaction occurs as a result of impact of molecules from the gas phase upon the adsorbed molecules, the expression for the reaction rate will contain, together with surface concentrations, the values of volume concentrations. These impact mechanisms were long ago proposed by Langmuir (22) for the reactions of CO and H2 with 02 on the surface of platinum the reaction occurs at the impact of a CO or H2 molecule against an adsorbed O atom. Such reactions seem to be numerous (23). Along with this, the above-mentioned adsorption mechanisms that involve the reaction between two adsorbed particles are possible. Elementary acts of surface reactions in which more than two particles participate are hardly probable. 

Perhaps one of the important conclusions of these studies that points to the unique chemistry of surface irregularities, steps, and kinks, which appear to be active sites, is the controlling influence of the local atomic structure, local surface composition, and local bonding between adsorbates and surface sites. The microstructure of the metal surface controls bond scission and thus the rate and path of chemical reactions. Calculations taking into account this local bonding picture should help to unravel the elementary bond-breaking steps in catalytic surface reactions. 

Changes in the state of the adsorbent-adsorbate system which, at the atomic-molecular level, is described by the lattice-gas model are caused by variations in the occupancy of its individual sites as a result of the elementary processes. The following elementary processes occur on the adsorbent surface adsorption and desorption of the gaseous phase molecules, reaction between the adspecies, migration of the adspecies over the surface and their dissolution inside the solid. The solid s atoms are capable of participating in the chemical reactions with the gaseous phase molecules, as well as migrating inside the solid or on its surface. 

In the Langmuir-Hinshelwood (L-H) mechanism for surface-catalyzed reactions, the reaction takes place between two surface-adsorbed species [4,5], As a substitute for concentration, we use surface coverage, and the rate is expressed in this term. We consider that the elementary reaction in the L-H mechanism is the bimolecular surface reaction expressed by the following equations ... 

Figure 62. A selection of possible elementary steps for the incorporation of oxygen in an oxide. The surface reaction, in particular, is made up of complex individual steps. In reality the ionization degree of adsorbed atoms lies between zero and the bulk value (here -2). T transport, R chemical reaction, E electrochemical reaction.

Consider the entire sequence of elementary steps comprising a surface-catalyzed reaction adsorption of reactant(s), surface reaction(s), and finally desorption of product(s). If the surface is considered uniform (i.e., all surface sites are identical kinetically and thermodynamically), and there are negligible interactions between adsorbed species, then derivation of overall reaction rate equations is rather straightforward. 

These difficulties have stimulated the development of defined model catalysts better suited for fundamental studies (Fig. 15.2). Single crystals are the most well-defined model systems, and studies of their structure and interaction with gas molecules have explained the elementary steps of catalytic reactions, including surface relaxation/reconstruction, adsorbate bonding, structure sensitivity, defect reactivity, surface dynamics, etc. [2, 5-7]. Single crystals were also modified by overlayers of oxides ( inverse catalysts ) [8], metals, alkali, and carbon (Fig. 15.2). However, macroscopic (cm size) single crystals cannot mimic catalyst properties that are related to nanosized metal particles. The structural difference between a single-crystal surface and supported metal nanoparticles ( 1-10 nm in diameter) is typically referred to as a materials gap. Provided that nanoparticles exhibit only low Miller index facets (such as the cuboctahedral particles in Fig. 15.1 and 15.2), and assuming that the support material is inert, one could assume that the catalytic properties of a... 

The mechanistic sequence for adsorption of Mn + on the surface and then oxidation proposed by Davies and Morgan (1989) is illustrated in Fig. 9.8. After Mn + adsorption there is adsorption of O2 molecules on the surface (Fig. 9.8b) and then an electron transfer between adsorbed O2 and Mn + (Fig. 9.8c). The mechanism is completed by reaction of the adsorbed Mn(III)-superoxide complex (Fig. 9.8d). If the rate-limiting step is the reaction of the adsorbed Mn(III)-superoxide complex to products, then the rate for the elementary reaction is... 

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