Kinetic measurement surface science experiments

CO oxidation catalysis is understood in depth because potential surface contaminants such as carbon or sulfur are burned off under reaction conditions and because the rate of CO oxidation is almost independent of pressure over a wide range. Thus ultrahigh vacuum surface science experiments could be done in conjunction with measurements of reaction kinetics (71). The results show that at very low surface coverages, both reactants are adsorbed randomly on the surface CO is adsorbed intact and O2 is dissociated and adsorbed atomically. When the coverage by CO is more than 1/3 of a monolayer, chemisorption of oxygen is blocked. When CO is adsorbed at somewhat less than a monolayer, oxygen is adsorbed, and the two are present in separate domains. The reaction that forms CO2 on the surface then takes place at the domain boundaries. 

The link between the microscopic description of the reaction dynamics and the macroscopic kinetics that can be measured in a catalytic reactor is a micro-kinetic model. Such a model will start from binding energies and reaction rate constants deduced from surface science experiments on well defined single crystal surfaces and relate this to the macroscopic kinetics of the reaction. 

With the surface science approach reactions occur at a vacuum-sohd interface on an atomically ordered surface maintained at ultrahigh-vacuum (UHV) conditions. The objective is to accurately define the structure and composition of the surface, measure its chemical or kinetic properties, and establish the relationship between structure and properties. Kinetic parameters determined under UHV conditions using molecular beam scattering (MBS) and thermal desorption experiments on an atomically ordered surface can be directly related to elementary processes and can be used to construct detailed models that describe how different atomic or molecular species interact with the surface. In recent years, studies have moved beyond single crystals, and a number of groups have fabricated model catalysts with greater complexity to more closely represent the important features of technical materials (1, 2]. 

With the TAP approach reactions also occur at a vacuum-solid interface. However, unlike the surface science approach, the solid surface in a TAP experiment is generally a disordered technical material such as industrial catalyst particles that are generally studied in flow reactors. The objective of a TAP experiment is to measure nonsteady-state kinetic properties and track how changes in reaction conditions and sample composition alter these properties. Incrementally increasing or decreasing the amount of a single component while simultaneously making kinetic measurements charts the correspondence between a component... 

At noble metals, the growth of submonolayer and monolayer oxides can be studied in detail by application of electrochemical techniques such as cyclic-voltammetry, CV 11-20) and such measurements allow precise determination of the oxide reduction charge densities. Complementary X-Ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), infra-red (IR) or elUpsommetry experiments lead to elucidation of the oxidation state of the metal cation within the oxide and estimation of the thickness of one oxide monolayer 12,21-23), Coupling of electrochemical and surface-science techniques results in meaningful characterization of the electrified solid/liquid interface and in assessment of the relation between the mechanism and kinetics of the anodic process under scrutiny and the chemical and electronic structure of the electrode s surface 21-23). 

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