Developing Surface Reaction Mechanisms

In recent years the surface science approach has led to a dramatic increase in our knowledge of the surface chemistry and kinetics. Processes that have been studied using the surface science approach include the deposition and etching of semiconductors (e.g., Si, Ge, 

metals (e.g., W, Cu, Al), and insulators (e.g., SiC 2). The information gained from these studies may be both mechanistic and kinetic in nature. 

In the traditional surface science approach the surface chemistry and physics are examined in a UHV chamber at reactant pressures (and sometimes surface temperatures) that are normally far from the actual conditions of the process being investigated (catalysis, CVD, etching, etc.). This so-called pressure gap has been the subject of much discussion and debate for surface science studies of heterogeneous catalysis, and most of the critical issues are also relevant to the study of microelectronic systems. By going to lower pressures and temperatures, it is sometimes possible to isolate reaction intermediates and perform a stepwise study of a surface chemical mechanism. Reaction kinetics (particularly unimolecular kinetics) measured at low pressures often extrapolate very well to real-world conditions. 

In addition to experiments, a range of theoretical techniques are available to calculate thermochemical information and reaction rates for homogeneous gas-phase reactions. These techniques include ab initio electronic structure calculations and semi-empirical approximations, transition state theory, RRKM theory, quantum mechanical reactive scattering, and the classical trajectory approach. Although still computationally intensive, such techniques have proved themselves useful in calculating gas-phase reaction energies, pathways, and rates. Some of the same approaches have been applied to surface kinetics and thermochemistry but with necessarily much less rigor. 

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. 

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The cases discussed above represent only a small fraction of the surface reaction mechanisms which might be considered. Yang and Hougen (12) have considered several additional surface reaction mechanisms and have developed tables from which rate expressions for these mechanisms may be determined. They approached this problem by writing the rate expression in the following form. 

By combining rate parameters from these 0-H studies with rate parameters from the hterature for the various steps in CO oxidation over Pt and Rh catalysts, we have developed a model based on the surface reaction mechanism outlined in... 

For each reaction in a surface chemistry mechanism, one must provide a temperature dependent reaction probability or a rate constant for the reaction in both the forward and reverse directions. (The user may specify that a reaction is irreversible or has no temperature dependence, which are special cases of the general statement above.) To simulate the heat consumption or release at a surface due to heterogeneous reactions, the (temperature-dependent) endothermicity or exothermicity of each reaction must also be provided. In developing a surface reaction mechanism, one may choose to specify independently the forward and reverse rate constants for each reaction. An alternative would be to specify the change in free energy (as a function of temperature) for each reaction, and compute the reverse rate constant via the reaction equilibrium constant. 

Theoretical. In deriving a theoretical expression for k, we have developed a reaction mechanism model for calcite dissolution which expands on the adsorption layer heterogeneous reaction model of Mullin ( ). We assume that a thin (possibly only a few molecules thick) "adsorption layer" (or "surface layer") exists adjacent to the crystal surface, between the crystal surface and the hydrodynamic boundary layer. Species in the adsorption layer are loosely bound to the crystal surface and have relatively low mobility, particularly in comparison with species mobility in the boundary layer. The crystal surface is believed to be sparsely covered by reaction sites at discontinuities in the surface ( 3). To distinguish between species activities in the bulk fluid, at the base of the boundary layer (near the crystal surface), and in the adsorption layer, we use the subscripts (B), (o), and (s), respectively. 

Recently, methods of preparing catalyst and characterizing the surface properties have been developed. Deep insight into the surface reaction mechanisms and the functions required to promote reactions will enable to design the solid base catalysts active for desired reactions. 

The good agreement between the model and both steady state and ignition experiments indicates that the surface reaction mechanism describes the essential steps of the catalytic reaction very accurately. The next step in developing this model further will now be to fill in the remaining gaps in the surface mechanism for methane oxidation and to extend the mechanism towards C2 chemistrv on the catalvst surface. 

The previous sections described techniques employed for parameter estimation. These thermodynamic and kinetic parameters are input to a microkinetic model that is solved numerically to describe material balances in a chemical reactor (e.g., a PFR). This section describes tools for the subsequent model analysis, which can be used in multiple ways. Initially during mechanism development, they can be used to assess which reactions and reactive intermediates are important in the model, which helps the modeler to focus on important features of the surface reaction mechanism. During this process, simulated macroscopic observables, for example, global reaction orders and apparent activation energies can be compared directly to experimental data. Then, once the model describes experimental data reasonably well, analytical tools can be used to develop further insights into the reaction mechanism, with apphcations that include catalyst design [50]. 

Based on a model proposed by Harris and Goodwin [36], an elementary surface reaction mechanism for the diamond (lOO)-surface reconstructed to the (100)-(2xl) H form is developed [11]. The mechanism assumes that diamond growth takes place at surface step sites as shown in Fig. 5. The principle features... 

One of the major issues in developing detailed surface reaction mechanisms is thermodynamic consistency. Even though the recently published reaction mechanisms ensure enthalpic consistency, many of them are not consistent with respect to entropy, which is due to the lack of knowledge about the transition states of the individual reaction steps. Thus, there is not sufficient information for a theory-hased determination of preexponential factors in the rate equations. However, an independent choice of the rate coefficients causes an inconsistent entropy change in the overall reaction, which leads to an incorrect prediction of equilibrium states. There are two approaches to avoid this inconsistency by adjusting the rate expressions, which are described in the Hterature (Deutschmann, 2008 Maier et al., 2011 Mhadeshwar et al., 2003). 

Maier L, Schadel B, Delgado KH, Tischer S, Deutschmann O Steam reforming of methane over nickel development of a multi-step surface reaction mechanism, Top Catal 54 845-858, 2011. 

Mhadeshwar AB, Aghalayam P, Papavassihou V, Vlachos DG Surface reaction mechanism development for platinum-catalyzed oxidation of methane, Proc Combust Inst 29 997-1004, 2002. 

Mhadeshwar AB, Wang H, Vlachos DG Thermodynamic consistency in microkinetic development of surface reaction mechanisms, J Phys Chem B 107 12721—12733, 2003. 

As mentioned in Section 2.1, the development of surface reaction mechanisms proceeds with optimization of theoretical reaction parameters through comparisons with measurements obtained in a variety of reactors. Catalytic ignition of hydrogen over noble metals has been extensively investigated in stagnation point flow geometries. 

Mbadeshwar AB, Vlachos DG Hierarchical, multiscale surface reaction mechanism development CO and H2 oxidation, water-gas shift, and preferential oxidation of CO on Rh, J Catal 234 48-63, 2005. 

The lack of detailed surface reaction mechanisms for propane on platinum necessitated the use of a single-step catalytic reaction coupled to a detailed gas-phase reaction mechanism. The global reaction step of Garetto et al. [1] has been developed for the total oxidation of propane to H2O and CO2 over Pt at... 

Recent developments in the mechanisms of corrosion inhibition have been discussed in reviews dealing with acid solutions " and neutral solu-tions - . Novel and improved experimental techniques, e.g. surface enhanced Raman spectroscopy , infrared spectroscopy. Auger electron spectroscopyX-ray photoelectron spectroscopyand a.c. impedance analysis have been used to study the adsorption, interaction and reaction of inhibitors at metal surfaces. 

The potential that develops in an electrochemical system such as a fuel cell can also act to significantly influence the energies, kinetics, pathways, and reaction mechanisms. The double-reference potential DFT method [Cao et al., 2005] described earlier was used to follow the influence of an external surface potential on the reaction... 

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