Surface systems, thermodynamics

A large surface area of the dispersed particles is associated with a high surface free energy that renders the system thermodynamically unstable. The surface free energy, AG, can be calculated from the total surface area, A A, as follows ... 

For the non-oxidative activation of light alkanes, the direct alkylation of toluene with ethane was chosen as an industrially relevant model reaction. The catalytic performance of ZSM-5 zeolites, which are good catalysts for this model reaction, was compared to the one of zeolite MCM-22, which is used in industry for the alkylation of aromatics with alkenes in the liquid phase. The catalytic experiments were carried out in a fixed-bed reactor and in a batch reactor. The results show that the shape-selective properties of zeolite ZSM-5 are more appropriate to favor the dehydroalkylation reaction, whereas on zeolite MCM-22 with its large cavities in the pore system and half-cavities on the external surface the thermodynamically favored side reaction with its large transition state, the disproportionation of toluene, prevails. 

Thermodynamics of Surface Systems (Eriksson) Towards an Analytic Theory of Chemical Reactions 6 145... 

Not all of these features will appear for every system. For example, a kinetically nonreactive system can be thought of as having a very large value of either A or D. Of more relevance for molecule-surface reaction dynamics is the fact that all of these features may not appear in every geometry of a particular molecule-surface system. For example, if the H2 bond was perpendicular to the surface, feature M would either disappear or be much smaller the barrier D would disappear since point B would lie considerably above ( 2.3-2.5eV) the reactant Hj molecule. In this configuration, the PES would appear similar to that of a thermodynamically nonreactive system. 

A high free energy excess, particularly in systems with a fine degree of dispersion, is the cause of thermodynamic instability, which is the most important feature of a majority of disperse systems. Thermodynamic instability in turn entails various processes aimed at decreasing the surface energy, which results in the saturation of surface forces. Such processes may occur in a number of ways. For example, in a free disperse system partial saturation of the surface forces may take place in the contact zone between the... 

Figure 1.5 Various states of system on the surface of thermodynamic potential a - metastable equilibrium b - imstable equihbrium c - unstable state d - stable equilibrium.

Quantitative SNIFTIRS was introduced in 2002 and so far has been applied to the investigation of the orientation of adsorbed molecules in three systems pyridine at Au(llO) [40], citrate at Au(lll) [53], and 2-mercaptobenzimidazole at Au(lll) [54, 55]. Pyridine adsorption at gold single-crystal surfaces has long been used as a model system to study the coordination of organic molecules to metal electrode surfaces. The thermodynamics of pyridine adsorption has been thoroughly investigated with the help of the chronocoulometric technique [56-64]. The availability of the thermodynamic data made this an ideal system to test the performance of the quantitative SNIFTIRS. 

GC has been used widely to study properties of catalysts, such as diffusivity, dispersion, and selective surface area of supported catalytic systems, surface acidity, thermodynamics of adsorption, and various aspects of catalytic reactions. 

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