Investigation of the effect of thermal annealing of Ni-cobaltite nanoparticles on their structure, electronic properties and performance as catalysts for the total oxidation of dimethyl ether†

Herein, we report the influence of thermal annealing on the structural and redox properties, optical band gap (E^Opt_g) and electrical conductivity (σ) of Ni-doped Co₃O₄ nanoparticles (NPs). The influence of annealing on the catalytic performance is also studied. Coprecipitation technique was employed to prepare single oxides (Ni₂O₃ and Co₃O₄) and Ni-cobaltite oxides (NiCo₂O₄), which were annealed at different temperatures. The samples were characterized in terms of structure (XRD), surface specific area (S_BET), morphology (SEM), chemical composition (EDS/XPS), redox properties (H₂-TPR), optical band gap (E^Opt_g) and conductivity σ. In addition, the conversion of dimethyl ether (DME) was achieved over all samples at low temperature. Overall, NiCo₂O₄ exhibits outstanding catalytic behaviour, surpassing single oxides. However, a notable difference in performance among NiCo₂O₄ samples was observed. The catalytic behaviour is discussed with respect to the quality of reducibility, the ratio of active species (O_Lat/O_Ads), the lowest E^Opt_g and the good σ. H₂-TPR, E^Opt_g and σ analysis were used to gain a deeper insight into the reaction process occurring at the surface of each of the catalysts. NiCo₂O₄ samples annealed at 450 and 500 °C cannot be reduced in the reaction temperature range (150 to 225 °C), suggesting that the process occurs via a surface mechanism. However, NiCo₂O₄ annealed at 350 °C is reduced under the reaction conditions within the temperature range from 200 to 350 °C, attesting that DME oxidation proceeds through an intrafacial redox process in this case. Using density functional theory (DFT) calculations, the adsorption and dissociation of DME on the catalytically relevant NiCo₂O₄ (001) surface was investigated. DME adsorbs preferentially on top the Co³⁺ site in a degenerate state: intact and single dehydrogenated molecules have very similar binding energies (−0.76 eV and −0.77 eV, respectively). The presence of surface vacancies in the vicinity of the adsorption site leads to the most interesting catalytic pathway. The activation energy for the dehydrogenation of one DME carbonyl group, considered as the first step to the complete oxidation, shows a lower value on the half-metallic surface with vacancies (0.9 eV) compared to 1.5 eV on the vacancy-free metallic surface.