CO and H2 adsorption on Au-Ni bimetallic surfaces: a combined experimental and DFT theoretical study

Abstract

Abstract Au-Ni bimetallic thin films were grown on refractory metal substrates. CO and H2 adsorption on Au-Ni bimetallic surfaces have been studied by a combination of in situ polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS), temperature-programmed desorption (TPD), and density functional theory (DFT) calculations. It is found CO desorption peak shifts from 413 K on pure Ni surfaces to 293 K on the isolated Ni atoms formed by alloying with Au atoms. The sharp decrease of CO desorption temperature on Au-Ni surfaces with increasing Au coverage is caused by the change of the favored CO adsorption sites from bridge/hollow sites on pure Ni surfaces to Ni top sites on Au-Ni bimetallic surfaces. In situ PM-IRRAS shows two CO adsorption bands on Au sites at 2119 cm−1 and 2103 cm−1 on Au-Ni surfaces at 80 K, which are due to CO bound on under-coordinated Au atoms and electron negatively charged Au sites modified with nearby Ni atoms, respectively. Even with the Au-Ni surface temperature at as low as 100 K, CO adsorption induced Ni surface segregation has been observed by in situ PM-IRRAS. Furthermore, DFT calculation results discover the adsorption energy of CO on Ni top sites continues to decrease with increasing Au coverage due to the geometric ensemble effect and the lowered d-band center after Ni alloying with Au. H2 desorption temperature decreases from 363 K on pure Ni thin films to 302 K with increasing Au coverage to 0.6 ML. A new H2 peak appears at around 170 K on the Au-Ni surfaces with Au coverages between 0.6 ML and 0.9 ML. This new H2 TPD peak is assigned to H2 desorption from the totally isolated Ni sites. With Au coverage above 1.5 ML, there is no any H2 desorption detected. The combined surface science studies and DFT calculations provide new insights into the surface structure-activity correlation of Ni-base bimetallic surface alloys.