Surface hydroxylation induced by alkaline-earth metal doping in NiO nanocrystals and its application in achieving a wide temperature operation window for preferential CO oxidation

Abstract

A series of Pt/Ni_1−xMg_xO catalysts with a low Pt loading of 0.5 wt% are prepared by multistep co-precipitation and annealing processes followed by wet impregnation. Mg²⁺ ions are doped in the NiO lattice, forming a nanoscale solid solution as evidenced by systematic variations of lattice parameters in terms of Vegard's law. The stack of nanoparticles led to a mesoporous feature as represented by a broad pore size distribution from 2 to 80 nm. X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, and H₂-temperature-programmed reduction analysis demonstrate that the presence of Mg²⁺ ions in mesoporous nanoscale solid solutions created more surface hydroxyls. Strikingly, these surface hydroxyls stabilized the ionic platinum (Pt²⁺) under preferential carbon monoxide oxidation (CO-PROX) conditions at temperature as high as 100–240 °C. Pt/Ni_0.5Mg_0.5O exhibits a wide temperature window and an excellent stability for CO conversion. The analysis of in situ diffuse reflectance infrared Fourier-transform spectra further reveals that CO adsorbed preferentially on Pt²⁺, and the formation of CO₂ proceeds through bicarbonate intermediate species with the help of hydroxyl groups during the CO-PROX reaction of Pt/Ni_0.5Mg_0.5O. The enhanced adhesion of Pt particles, which is due to the formation of strong Pt–O interfacial bonds, is responsible for the observed higher stability of CO-PROX toward thermal sintering on hydroxylated Ni_0.5Mg_0.5O surfaces. The results reported here could inspire one to find more new hydroxyl-enriched supports that improve the noble metal utilization efficiency for enhancing the catalyst performance in CO-PROX and other relevant catalytic fields.