The selectivity of hydrocarbon oxidation on metal surfaces can be significantly affected by changes in pressure and temperature. These changes can alter the reaction pathways, intermediates, and products formed during the oxidation process. The underlying mechanisms that govern these changes are related to the adsorption, desorption, and reaction kinetics of the hydrocarbon molecules on the metal surface.1. Temperature effects:Increasing the temperature can have several effects on the selectivity of hydrocarbon oxidation on metal surfaces:a. Enhanced desorption: As temperature increases, the desorption rate of hydrocarbon molecules from the metal surface also increases. This can lead to a decrease in the residence time of the hydrocarbon molecules on the surface, which may result in lower selectivity towards certain oxidation products.b. Altered reaction kinetics: Higher temperatures can increase the rate of reaction between the hydrocarbon molecules and the metal surface, leading to a change in the selectivity of the oxidation products. This is because the activation energy for different reaction pathways may vary, and increasing the temperature can favor pathways with higher activation energies.c. Formation of new intermediates: At higher temperatures, new intermediates may be formed during the oxidation process, which can lead to the formation of different products. This can also affect the selectivity of the reaction.2. Pressure effects:Changes in pressure can also affect the selectivity of hydrocarbon oxidation on metal surfaces:a. Adsorption equilibrium: Increasing the pressure can shift the adsorption equilibrium of hydrocarbon molecules on the metal surface, leading to a higher surface coverage. This can affect the selectivity of the reaction, as the availability of active sites on the surface may be altered.b. Reaction kinetics: Changes in pressure can affect the reaction kinetics of the hydrocarbon oxidation process. For example, increasing the pressure can lead to a higher collision frequency between the hydrocarbon molecules and the metal surface, which may result in a higher reaction rate and altered selectivity.c. Diffusion limitations: At high pressures, the diffusion of reactants and products to and from the metal surface can become limited, which can affect the selectivity of the reaction. This is because the rate of reaction may be controlled by the diffusion of species rather than the intrinsic reaction kinetics.In summary, the selectivity of hydrocarbon oxidation on metal surfaces can be influenced by changes in pressure and temperature through various mechanisms, including altered adsorption/desorption behavior, reaction kinetics, and the formation of new intermediates. Understanding these mechanisms can help in designing more efficient and selective catalysts for hydrocarbon oxidation processes.