Surface abundant reaction intermediate

The Most Abundant Reaction Intermediate (MARI) approximation is a further development of the quasi-equilibrium approximation. Often one of the intermediates adsorbs so strongly in comparison to the other participants that it completely dominates the surface. This intermediate is called the MARI. In this case Eq. (156) reduces to... 

Propose a mechanism where the rate-limiting step is the recombination of adsorbed carbon C and adsorbed oxygen O and write up an equation for the rate. In the following we assume that only one adsorbate dominates the surface. The so-called MARI for the most abundant reaction intermediate. Here we assume that it is oxygen O. Is that reasonable ... 

In a case such as this one where only one species is present in appreciable concentration on the surface, that species is often referred to as the most abundant reaction intermediate, or mari. The overall rate of reaction can be expressed as the rate of dissociative adsorption of N2 ... 

Derive a rate expression for the hydrogenation of ethylene on Pt assuming steps 1, 2, and 3 are quasi-equilibrated, step 4 is virtually irreversible, and C2H5 is the most abundant reaction intermediate covering almost the entire surface ([ ]o [ C2H5]). Discuss why the rate expression cannot properly account for the experimentally observed half order dependence in H2 and zero-order dependence in ethylene. Could the observed reaction orders be explained if adsorbed ethylene ( C2H4 ) were the most abundant reaction intermediate Explain your answer. 

MASI) or most abundant reaction intermediate (MARI) are terms to denote the (reactive) intermediate, which is present in the highest concentration on the surface. If the concentration of the MASI/MARI sufficiently exceeds that of the other surface species, simplified rate expressions can be obtained.)(13,14,23). As a result of this microscopic vuiderstanding of the macroscopic phenomena, that is, the observed catalytic activity and selectivity, a more rational design of new catalysts becomes possible, provided that relations can be established between the catalyst synthesis procedure and the surface phenomena on the catalyst (see Fig. 8). 

Sabatier-type volcano plots have been constructed for a number of different commercially relevant systemsl l. A simple kinetic expression that simulates the Sabatier result is found when one realizes that the decomposition of molecules requires a vacant site for molecular fragments to adsorb on. For instance, in the N2O decomposition reaction, the dominant surface species (most abundant reaction intermediate) is atomic oxygen (O), which is in equilibrium with the gas phase. When the slow step in the reaction is dissociative adsorption of N2O, the mean-field kinetic rate expression for N2O decomposition, normalized per unit surface area of catalyst, becomes ... 

At low temperatures, the only products that form are N2O and N2. In situ spectroscopic studies of working Cu and Ag catalysts show that apart from adsorbed oxygen, there is a high surface coverage of nitrite and nitrate speciesl . Hence, on these metals at low temperature, N2 and N2O production is likely the result of consecutive reactions of NOj, the most abundant reaction intermediate (MARI), with NH3. N2 is formed by the reaction of nitrite with NH3, whereas N2O can also form via reaction of nitrate with ammonia (see also Section 6.4.1). 

The overall reaction is A = B with the species B being the most abundant reaction intermediate, and A requires n adjacent surface atoms in the rate-determining adsorption step. The rate is then ... 

If, under reaction conditions, one of the adsorbed species dominates on the surface and the fractional coverage of this intermediate on the catalytic sites is much greater than any other species, then it is said to be the most abundant reaction intermediate (MARI). Technically, it may not be the most abundant surface intermediate (MASI) because some adsorbed species may not be participating in the reaction sequence [2], although these two terms tend to be used interchangeably [1]. 

In modeling reactions, in general, and catalytic reactions, in particular, the kineticist must draw on as many tools at his disposal as possible. Some of the most important concepts that are routinely used to derive, simplify and evaluate complicated rate expressions are 1) Transition-state theory 2) The steady-state approximation 3) Bond-order conservation calculations for surface species 4) A rate determining step 5) A most abundant reaction intermediate and 6) Criteria to evaluate parameters in derived rate expressions. Let us examine these topics prior to their utilization in deriving and evaluating reaction models and rate equations. 

N2 has been observed using infra-red spectroscopy [512] and secondary-ion mass-spectroscopy [379] on Fe surfaces exposed to N2 + H2. Arguments in opposition to N2 being the most abundant reaction intermediate the weakness of adsorption for this species [650] and the rapid decomposition of N2H4 to NH3 over Fe [651]... 

Evidence for the existence of significant amounts of has been deduced from interpretations of the reaction orders [512, 614, 658, 659], laser fluorescence [656, 660], and from the study of the dissociation of NH3 /Fe(110) by electron spectroscopy [661]. It is a complication in the deduction of the surface coverages by intermediates that the coverages depend on the operating conditions of the catalyst [396]. These variations may be sufficient to change the nature of the most abundant intermediate [396]. Experimental evidence has been found for changes in the nature of the most abundant reaction intermediates with temperature or promoter concentration [614]. 

However, the model of ammonia synthesis of Stoltze nd Norskov shows that even if N is the most abundant reaction intermediate at typical reaction condition, N, NH, NH2, H, and NH3 are all more abundant than. This explains why a Langmuir-Hinschelwood expression with only one surface intermediate is not particularly successful [396, 645]. 

In the case of semiconductor assisted photocatalysis organic compounds are eventually mineralized to carbon dioxide, water, and in the case of chlorinated compounds, chloride ions. It is not unusual to encounter reports with detection of different intermediates in different laboratories have been observed. For example, in the degradation of 4-CP the most abundant intermediate detected in some reports was hydroquinone (HQ) [114,115,123], while in other studies 4-chloro-catechol, 4-CC (3,4-dihydroxychlorobenzene) was most abundant [14,116-118, 121,163]. The controversy in the reaction intermediate identification stems mainly from the surface and hydroxyl radical mediated oxidation processes. Moreover, experimental parameters such as concentration of the photocatalyst, light intensity, and concentration of oxygen also contribute in guiding the course of reaction pathway. The photocatalytic degradation of 4-CP in Ti02 slurries and thin films... 

In summary, these XPS experiments have shown that the active catalyst surface is metallic copper that contains a subsurface oxygen species. The abundance of subsurface oxygen correlates with the amount of formaldehyde produced in the catalytic reaction. The active surface can be observed only in the experiments involving reactive mixtures in contact with the catalyst. Under conditions such as T = 673 K and Ptotai = 0.6 mbar, no reaction intermediates could be observed on the catalyst surface. 

FT synthesis kinetics are similar on Co and Ru catalysts and reflect similar CO activation and chain growth pathways on these two metals. These kinetic expressions are consistent with the stepwise hydrogenation of surface carbon formed in fast CO dissociation steps (26). Chemisorbed CO and CH t species are the most abundant reactive intermediates, as expected from the high binding energy of CO and carbonaceous deposits on Co and Ru surfaces (7, 76-80). As a result, reaction orders in CO remain negative at the usual inlet pressures but can become positive as CO reactants are depleted by transport limitations within pellets or by high levels of conversion within the catalyst bed. 

The oxidation of N02 eventually leads to the formation of nitric acid and aerosol nitrate, which are deposited at the earth surface. The relevant oxidation pathways are indicated in Fig. 9-6. The following discussion deals first with observations of reaction intermediates then with tropospheric abundances of N02, PAN, and HN03/ aerosol nitrate, and finally with the budget of nitrogen oxides and their oxidation products in the troposphere. 

---