Oxygen Availability in Zn_xCe_1–xO₂ Nanocrystallites as a Function of Zinc Concentration

Abstract

Oxygen availability, an important property of cerium oxide catalysts, has been investigated over a wide range of concentrations for the substitutional solid solution Zn x Ce1–x O2 in nanocrystalline (∼2.5 nm) form. X-ray diffraction data of the as-synthesized materials indicates Zn x Ce1–x O2 to be a single-phase system up to approximately x = 0.16, with the lattice parameter decreasing linearly with zinc concentration, signifying a homogeneous distribution of the Zn2+ cation within the bulk of the nanoparticle. X-ray photoelectron spectroscopy (XPS) of the Zn x Ce1–x O2 samples shows stoichiometric to slightly enriched zinc surface concentrations accompanied by oxygen surface reduction and an increased ratio of Ce3+/Ce4+ upon zinc incorporation, which correlate with increased oxygen surface reactivity. The ability of Zn x Ce1–x O2 to oxidize chemical reactants, taken here as a measure of oxygen availability, is measured by two different reactions, H2 temperature-programmed reduction (H2-TPR) to measure total oxygen storage capacity and the oxidation of methanol by temperature-programmed desorption (MeOH-TPD), both of which show enhancement in total oxidation and lower peak reaction temperatures with increased zinc concentration. H2-TPR gives evidence of involvement of both the outermost surface and sub-surface lattice oxygens, whereas XPS of methanol adsorption indicates the presence of both molecular and dissociative methoxy adsorbates as well as formate and carbonate intermediates. Methoxy decomposes into the intermediate species, leading to both CO partial oxidation and total oxidation to CO2 at higher zinc concentrations. Both H2-TPR and MeOH-TPD are sensitive to zinc concentration, indicating increasing oxygen availability as zinc is added up to the 16% miscibility limit.

Surface Cleaning and Protection Engineering via Functionalization to Awaken the Intrinsic Catalytic Activity of Co3O4

“…The opposite variation patterns of O w and O chem suggest that the reduction of hydroxyl concentration contributes to the formation of oxygen vacancies, which is agree with the previous studies. [8,9] It is worth noting that the hydroxyl peaks of organic functional groups overlap with the surface OH groups, [39,40] so it is difficult to determine whether the surface hydroxyl groups have been completely removed by XPS. Interestingly, the XPS for Co 2p orbital (Figure S11, Supporting Information) has little changed after functionalization, suggesting that the surface has not been oxidized or reduced by reflux treatment, which is consistent with the previous analysis.…”