We explored how single-reactant gases such as CO and O2 and a mixed reactant environment of a catalytic reaction such as CO oxidation in terms of catalytic conditions influence compositional and structural restructurings of intermetallic catalyst surfaces by using advanced surface-sensitive techniques, including ambient pressure X-ray photoelectron spectroscopy and high-pressure scanning tunneling microscopy. Stepped intermetallic surface, Pt3Ni(557) was chosen as a model system of intermetallic catalysts for exploring how the pressure of a single reactant and a mixture of all reactants of a catalytic reaction and the temperature of a catalyst could modulate the surface of an intermetallic catalyst under a reaction or catalytic condition. It is found that the surface of Pt3Ni(557) in UHV is Pt-rich. Ready dissociation of CO on singly dispersed Ni1 atoms on Pt3Ni(557) suggests that Pt3Ni(557) is a potential catalyst of Fischer–Tropsch synthesis. Atomic fraction of Ni in the surface layer of Pt3Ni(557) in CO or O2 increases along with the increase of CO or O2 pressure at 25 °C, resulting from a preferential segregation of Ni atoms due to the stronger binding of CO or atomic O on Ni atoms than that on Pt. Compared to the CO or O2 pressure-driven change of surface composition at 25 °C, the catalyst temperature triggers a much stronger segregation of Ni atoms to the surface region at a temperature higher than 25 °C in pure CO or O2. The greater extent of segregation arises from the surface reaction with CO to form NixCy patches or with O2 to form NiaOb patches at a temperature >100 °C. In pure O2, NiOx and PtOy patches form on Pt3Ni(557) at 250 °C. Similar to pure CO or O2, the atomic fraction of Ni of Pt3Ni(557) in a mixture of CO and O2 under a catalytic condition increases with the pressure of the gaseous mixture even at 25 °C but decreases along with the increase of catalysis temperature. The restructured Pt3Ni(557) surfaces are active for CO oxidation in the whole temperature range of 25–250 °C. Different from Pt3Ni(557) at 150–250 °C in pure CO or O2, the active phase in the mixture of CO and O2 consists of Pt clusters anchored singly dispersed Ni1 atoms. At 250 °C, the active phase transforms to a reverse catalyst, PtOy and NiOx patches supported on Pt-rich clusters, which are highly active for CO oxidation. These findings demonstrate that compositional and geometric restructurings can occur at the topmost atomic layer on an intermetallic catalyst even at temperatures as low as 25 °C, driven by reactant gas, despite the ordered atomic arrangement of constituent metals in the bulk lattice. This dynamic behavior reveals that the topmost atomic layers of intermetallic catalysts are in fact quite structurally flexible under reaction and catalytic conditions than commonly assumed, highlighting the significance of compositional and structural explorations under specific catalytic conditions.
Tao et al. (Thu,) studied this question.