The state of adsorption of diatomic molecules has been analyzed thermodynamically. It has been shown that molecular and dissociative adsorption are competing processes and that the state of adsorption depends on pressure and temperature. Molecular adsorption is preferred at low temperature and high pressure, and dissociative adsorption is preferred at high temperature and low pressure. The analysis has been applied to the adsorption of CO, NO and N//2 on transition metal surfaces. The heat of dissociative adsorption has been estimated from the heats of formation of transition metal carbides, nitrides and oxides. This analysis was able to predict previous correlations relating dissociative adsorption with position in the periodic table. It was clearly shown that dissociative adsorption was a thermodynamic phenomena. The effect of surface structure on dissociation was discussed, and it was shown that defects can cause dissociation by forming stronger metal-carbon, metal-nitrogen, or metal-oxygen bonds at the defect.
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