Evidence of a molecular chemisorption-mediated mechanism for high translational energy oxygen adsorption on Pt(100)-hex-R0.7°

Surface Alloy Composition Controlled O2 Activation on Pd–Au Bimetallic Model Catalysts

“…To relate the O 2 dissociation behavior to the reactivity of the resulting oxygen adatoms (which depends on Pd coverage), we estimated the dissociation energy barriers for O 2 and compared these values to the reaction energy barriers for CO oxidation, which indirectly indicates the reactivity of the adsorbed atomic oxygen. To determine an activation barrier to dissociation, we adopted a simple model used to describe molecular chemisorption-mediated oxygen dissociation, which has been used previously for O 2 dissociation on transition metals such as Pd, ,, Pt, − Ir, and Ru . It is based on a two potential energy well model for O 2 : one for the molecularly chemisorbed O 2 and the other for the dissociated (atomic) oxygen (ignoring a well for physical adsorption).…”

“…Without explicit consideration of the kinetic effects of physically adsorbed molecules, S 0 in the limit of zero coverage is modeled as where α indicates the probability of molecular chemisorption and k a and k d are the rate constants for the dissociation and desorption of chemisorbed molecular species, respectively. Assuming that k a and k d can be accurately described by the Arrhenius equation, S 0 can be rewritten as where k B is Boltzmann’s constant, T s represents the sample temperature, E d and E a indicate activation energies for desorption and dissociation, and v d /v a is the ratio of pre-exponential factors. − ,, Arrhenius plots of ln (α/ S 0 – 1) vs 1/ T for 2, 2.5, 3.0, and 4 ML Pd-deposited surfaces are shown in Figure b assuming α as the adsorption probability at 77 K, which allows us to calculate the value of E d – E a from the slope. By comparing the 4 ML Pd-deposited surface with the 2 ML Pd-deposited surface, we notice that the 4 ML case shows a higher E d – E a value (0.086 eV, 8.3 kJ/mol) and lower v d / v a ratio (1.002) in comparison to the corresponding values for the 2 ML case, 0.058 eV and 5.6 kJ/mol for E d – E a and 2.411 for v d / v a .…”

“…where k B is Boltzmann's constant, T s represents the sample temperature, E d and E a indicate activation energies for desorption and dissociation, and v d /v a is the ratio of preexponential factors. [55][56][57][58][59][60]64,65 Arrhenius plots of ln (α/S 0 − 1) vs 1/T for 2, 2.5, 3.0, and 4 ML Pd-deposited surfaces are shown in Figure 4b dissociation (confirmed from no measurable CO 2 generation during the CO-RMBS measurements), its E d − E a value could be negative, suggesting that E a is higher than the vacuum level energy (=0). The 1.5 ML Pd-deposited surface does show CO 2 generation by CO-RMBS measurements, but it is still difficult to directly observe the dissociative adsorption of O 2 via King and Wells measurements; thus, we believe that the E d − E a value for the 1.5 ML case is close to zero but is probably a positive value.…”