UPD Compared with OPD First-Order Phase Transitions

Adsorption isotherms were calculated for the different systems in the case of defect-free surfaces. For a fixed temperature and chemical potential, a simulation has to be performed to evaluate the average coverage degree corresponding to the equilibrium state of the system. By repeating this procedure for different values of the chemical potential, one obtains the isotherms, such as those 

In all examples shown in Fig. 8, each isotherm exhibits an abrupt jump in the coverage degree, as expected for the case of a first-order phase transition. 

The isotherms were obtained at T = 300 K and each graph includes two of them, which can genetically be symbolized as A/S(100) and A/A(100), where A and S denote adsorbate and substrate, respectively. Thus, the comparison of the behavior of the heteroatomic A/S(100) system with that of the pure metal A/A(100) system allows us to determine the existence of either UPD or OPD. 

The difference between the chemical potentials A/i = /aa/s -IXX, at which the transition occurs for the systems A/S(100) and A/A(100), is then a measure of the underpotential shift, namely. 

Chemical potential (eV) at coexistence ficoex, which is identified by the abrupt step observed in the adsorption isotherms. Results corresponding to different systems and obtained at T = 300 K. (Reprinted from Ref. [16], with permission from the the American Chemical Society.) 

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