Segregation dynamics of a Pd-Ag surface during CO oxidation investigated by NAP-XPS

Regeneration of Active Surface Alloys during Cyclic Oxidation and Reduction: Oxidation of H2 on Pd/Ag(111)

“…Specifically, the adsorption of oxygen on Pd provides a strong driving force for Pd resegregation. 64 Similar resegregation phenomena as a function of the adsorbate and temperature have been observed for CO 39,47,66,89 and acetylene 65 on Pd−Ag systems as well as other bimetallic systems that rearrange under oxidizing environments. 17,44,49,[54][55][56][57][58]60,61,64 By the third redox cycle, the intensities of the Pd and Pd x Ag 1−x alloy transitions remain constant, indicating that the near surface reaches a metastable surface phase and hence maintains Pd to within the top few surface layers.…”

“…35 Likewise, a study of the oxidation and reduction of the Pd 75 Ag 25 (100) bulk alloy previously showed a reversed hysteresis for the formation of CO 2 during heating and cooling as a result of dynamic changes in the composition of the surface. 66,77 An improvement in the activity for acrolein hydrogenation catalyzed by supported PdAg nanoparticles was also attributed to Pd/Ag intermixing, specifically, increased dispersion of Pd on the surface after high-temperature pretreatment in H 2 . 7 Taken together, these studies raise the question of whether the chemical functionality of Ag/Pd alloys will be retained over repeated oxidation/ reduction cycles.…”

“…The specific pretreatment can influence the degree of alloying . For example, oxygen treatments often induce the segregation of metal oxides arising from differences in metal–oxygen bonding. , In addition, adsorbates such as O 2 , CO, and other reactants result in resegregation of a metal that would otherwise dissolve into the bulk. ,,,,,− ,− ,− Hence, excursions between oxidizing and reducing conditions (cyclical redox conditions) can lead to substantial changes in catalyst morphology and composition and therefore catalyst performance. ,,, …”

PdAg/Ag(111) Surface Alloys: A Highly Efficient Catalyst of Oxygen Reduction Reaction

“…For example, the composition of the near-surface region of a Pd 75% Ag 25% (100) single crystal during CO oxidation under oxygen-rich conditions using near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) confirms that the amount of Pd in the surface region decreases with increasing temperature. CO causes Pd segregation into the topmost surface layer [ 32 ]. The strong effects of reactive gases such as CO or O 2 on the alloy structure and composition were investigated by scanning tunneling microscope (STM) and Fourier transform infrared spectroscopy (FTIR) to illustrate the correlation of chemical properties with structural aspects of bimetallic surfaces of a given composition at ambient pressure [ 33 , 34 , 35 ].…”