Surface reactions Mechanisms

For the description of surface reactions two well-established approaches are used, namely (i) the Eley-Rideal (ER) mechanism and (ii) the Langmuir-Hinshelwood (LH) mechanism. 

The ER type of mechanism is associated with a direct process, and atypical example would be the interaction 

More generally, let us assume that we were asked to find the reaction rate for the bimolecular reaction A(s) + B(g)— products, where A(s) means that species A is adsorbed on a surface to describe this, the ER mechanism and Langmuir adsorption isotherm for the coverage degree are given as the model conditions. The solution to this problem would look as follows. The reaction rate r for a direct ER mechanism will be proportional to the concen- 

In order to determine Op, one needs to find its dependence on Pa this dependence can be calculated using the model described in Box 26.2. 

In the study of chemical reactions at surfaces, an important step is to investigate how an atom or molecule sticks to surfaces, i.e. how they adsorb on a given surface before reaction takes place. In these types of smdy two important quantities are used, namely the extent of surface coverage and the rate of adsorption. 

A knowledge of the change in work function which occurs during physical adsorption or chemisorption processes on a metal surface may help to elucidate the mechanism of a surface reaction. 

N2O on a metal surface may take place in accordance with the reactions, 

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The physical chemist is very interested in kinetics—in the mechanisms of chemical reactions, the rates of adsorption, dissolution or evaporation, and generally, in time as a variable. As may be imagined, there is a wide spectrum of rate phenomena and in the sophistication achieved in dealing wifli them. In some cases changes in area or in amounts of phases are involved, as in rates of evaporation, condensation, dissolution, precipitation, flocculation, and adsorption and desorption. In other cases surface composition is changing as with reaction in monolayers. The field of catalysis is focused largely on the study of surface reaction mechanisms. Thus, throughout this book, the kinetic aspects of interfacial phenomena are discussed in concert with the associated thermodynamic properties. 

Figure 1.55. The relationships between the concentration product, (Ba " )i(S04 )i, at the initiation of barite precipitation, and morphologies of barite crystals (Shikazono, 1994). The dashed line represents the boundary between dendritic barite crystals and well-formed rhombohedral, rectangular, and polyhedral barite crystals. The 150°C data are from Shikazono (1994) the others from other investigations. D dendritic (spindle-like, rodlike, star-like, cross-like) barite Dp feather-like dendritic barite W well-formed rectangular, rhombohedral, and polyhedral barite. The boundary between the diffusion-controlled mechanism (Di) and the surface reaction mechanism (S) for barite precipitation at 25°C estimated by Nielsen (1958) The solubility product for barite in 1 molal NaCl solution at 150°C based on data by Helgeson (1969) and Blount (1977). A-B The solubility product for barite in 1 molal NaCl solution from 25 to 150°C based on data by Helgeson (1969).

Massive barite crystals (type C) are also composed of very fine grain-sized (several xm) microcrystals and have rough surfaces. Very fine barite particles are found on outer rims of the Hanaoka Kuroko chimney, while polyhedral well-formed barite is in the inner side of the chimney (type D). Type D barite is rarely observed in black ore. These scanning electron microscopic observations suggest that barite precipitation was controlled by a surface reaction mechanism (probably surface nucleation, but not spiral growth mechanism) rather than by a bulk diffusion mechanism. 

The various morphological features of barites from the Kuroko and Mariana deposits, when combined with the experimental studies on barite precipitation, suggest that the surface reaction mechanism was dominant for the formation of these barites. This implies that the concentration product, m 2+) msol-), at the initiation of barite precipitation was probably less than ca. 100 times that for equilibrium. 

A quick survey of the literature reveals a confusing picture of the mechanism or mechanisms of surface reactions and the role or roles of the catalyst surface. A contributing factor is that different investigators are approaching surface reaction mechanisms from different points of view. In a very general way, there are three groups of investigators. 

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. 

Duprez, D. 2006. Study of surface reaction mechanisms by lsO/lsO and H/D isotopic exchange. Catal. Today 112 17-22. 

In reality, it is believed that the oxidation of carbonaceous surfaces occurs through adsorption of oxygen, either immediately releasing a carbon monoxide or carbon dioxide molecule or forming a stable surface oxygen complex that may later desorb as CO or C02. Various multi-step reaction schemes have been formulated to describe this process, but the experimental and theoretical information available to-date has been insufficient to specify any surface oxidation mechanism and associated set of rate parameters with any degree of confidence. As an example, Mitchell [50] has proposed the following surface reaction mechanism ... 

With appropriate choices of kinetic constants, this approach can reproduce the NSC experimental data quite well. Park and Appleton [63] oxidized carbon black particles in a series of shock tube experiments and found a similar dependence of oxidation rate on oxygen concentration and temperature as NSC. Of course, the proper kinetic approach for soot oxidation by 02 undoubtedly should involve a complex surface reaction mechanism with distinct adsorption and desorption steps, in addition to site rearrangements, as suggested previously for char surface combustion. 

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... 

Subscript (ads) denotes adsorption via a thiolate linkage, while (ps) stands for a physisorbed and/or adsorbed state via different interactions. However, large dimensions of the studied molecules and their amphiphilic nature make the surface reaction mechanism more complex than in case of cystine/cysteine. Interfacial microstructure plays an important role in the determination of the surface behavior of the adsorbed molecules. From the study on the charge-transfer kinetics, the transfer coefficient a was calculated as slightly less than 0.50, while the rate constant (based on Laviron s derivations [105]) was of the order of 10 s k The same authors [106] have shown earlier that the adsorption rate constant of porcine pancreatic phospholipase A2 at mercury via one of its disulfide groups is of the order of 10 s h... 

As in the case of adsorption, a variety of surface reaction mechanisms exists (Fogler,... 

Although the division of surface reaction mechanisms into LH or ER dates to the early days of catalysis, ER/HA surface reactions have only been demonstrated recently and only for strongly reactive atomic gas phase species, e.g., H, O. There are many differences between the ER/HA mechanism and the LH mechanism that can be used to separate them experimentally. For example, ER/HA reactions of reactive incident atoms are very exothermic relative to the equivalent LH reaction, typically by several eV. Much of this released energy should end up in the gas-phase product molecule. ER/HA are direct non-activated reactions whose final state properties depend on the initial conditions of the incoming atom and not Ts. This is of course the exact opposite of LH properties. 

When a simple, fast and robust model with global kinetics is the aim, the reaction kinetics able to predict correctly the rate of CO, H2 and hydrocarbons oxidation under most conditions met in the DOC consist of semi-empirical, pseudo-steady state kinetic expressions based on Langmuir-Hinshelwood surface reaction mechanism (cf., e.g., Froment and Bischoff, 1990). Such rate laws were proposed for CO and C3H6 oxidation in Pt/y-Al203 catalytic mufflers in the presence of NO already by Voltz et al. (1973) and since then this type of kinetics has been successfully employed in many models of oxidation and three-way catalytic monolith converters... 

For example, the Langmuir adsorption isotherm specifically describes adsorption of a single gas-phase component on an otherwise bare surface. When more than one species is present or when chemical reactions occur, the functional form of the Langmuir adsorption isotherm is no longer applicable. Thus, although such simple functional expressions are very useful, they are not generally extensible to describe arbitrarily complex surface reaction mechanisms. 

It is important to recognize that Kp is unitless, and is related to thermodynamic quantities by Eq. 9.93, for example. However, Eq. 11.17 has exactly the same form as the classic Langmuir adsorption isotherem, Eq. 11.11, if we take K = Kp/p°. Thus the two approaches are entirely equivalent. In addition the discussion above shows how the more restrictive form that is usually written for the Langmuir adsorption isotherm can be converted to the extensible mass-action kinetics form to be used, for example, within a more extensive surface reaction mechanism. 

Assume a surface reaction mechanism involving I (reversible or irreversible) surface reactions with K chemical species that can be represented in the general form... 

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. 

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. 

Heterogeneous reaction mechanisms range from the very simple to the complex. Many features of the formalism presented in this chapter are illustrated by the catalytic combustion reaction mechanism given in Table 11.1. The surface-reaction mechanism is due to Sidwell et al. [361], which in turn had its origins with the work of Schmidt [173,174] and Deutschmann [96,97,101],... 

Table 11.1 Catalytic Combustion Surface Reaction Mechanism [361]...

The chemical vapor deposition (CVD) of CdTe thin films is used in the manufacture of highly efficient solar cells. To model this deposition process, a surface reaction mechanism is needed. 

An experiment was preformed by Piccolo et al. [315] to study catalytic oxidation of CO on supported Pd clusters. The following three-step surface reaction mechanism was proposed ... 

In the experiment, a bare Pd surface was exposed to oxygen, until the surface attained a saturation coverage of O(s) of 0glt=O.4. The oxygen source was then turned off, and the surface was exposed to a constant flux of CO of Fco beginning at time t = 0 s. A quadrupole mass spectrometer was used to monitor the flux of the oxidation product C02, as well as CO, from the surface. The coverages of O(s) and CO(s) were deduced as a function of time through analysis of the data and the surface reaction mechanism above. 

Table 11.2 Surface Reaction Mechanism for Growth of Diamond and Point Defects...

Assume that the rate constants for the surface-reaction mechanism are given as... 

The baseline reactor conditions in the following reactor analysis are susceptor temperature Ts = 1273 K, inlet temperature Tm = 333 K, reactor pressure p = 400 mTorr, gas velocity through the inlet manifold Vin = 100 cm/s, and the gap between inlet and susceptor L = 1 cm. Incoming gas-mixture mole fractions (e.g., from a gas-cylinder) are TEOS 0.25 and N2 (carrier gas) 0.75. You may use the files teos. gas and teos. surf for the gas-phase and surface reaction mechanisms. (Hint You may need the following initial guesses at the surface species site fractions SiG3(OH) 0.98, SiGsE 0.02, SiG(OH)2E 0.001. More details on the surface reaction mechanism and nomenclature are found in Ref. [69].)... 

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