The as-prepared (Co₃O₄) and hydrazine-treated (Co₃O₄(H)) cobalt catalysts were prepared using the precipitation method and evaluated at a temperature range of 40–220 °C for preferential oxidation (PROX) of CO in excess hydrogen. An improved surface reducibility with smaller crystallite size was noted on hydrazine-treated cobalt species (i.e., Co₃O₄(H) catalyst), which indicates some surface transformation. This finding correlates with the surface roughness formation (as depicted by scanning electron microscope (SEM) and transmission electron microscope (TEM) data), which was further confirmed by an increase in the Brunauer–Emmett–Teller (BET) surface area. The mesoporous structure of the Co₃O₄(H) catalyst remained intact, as compared to that of the Co₃O₄ catalyst. Interestingly, the in situ treatment of the standalone Co₃O₄(H) catalyst decreased the maximum CO conversion temperature (T_100%) from 160 °C (over Co₃O₄) to 100 °C, with good selectivity. The Co₃O₄(H) catalyst showed good stability, with approximately 85% CO conversion at 100 °C for 21 h, as compared to a faster deactivation of the Co₃O₄ catalyst. However, the Co₃O₄(H) catalyst was unstable in both CO₂ and the moisture environment. Based on the evaluation of spent hydrazine-treated (CoO(H)) cobalt catalyst, the high PROX activity is associated with the formation of Co³⁺ species as confirmed by X-ray diffraction (XRD), X-ray photoelectron spectra (XPS), and temperature-programmed reduction (TPR) data.

The Co ₃ O ₄ -based catalyst was prepared via precipitation method, followed by lanthanum incorporation through incipient wetness impregnation. All catalysts were evaluated in the preferential oxidation (PROX) of carbon monoxide in hydrogen-rich streams. Catalysts were characterised using XRD, BET surface area, TGA, SEM, TEM, FTIR, TPR, and XPS. The XRD and FTIR results confirmed the formation of the spinel CO ₃ O ₄ phase. The TPR measurements suggest the formation of LaCo linkage, which evidences strong electronic and structural interaction between La species and the Co ₃ O ₄ lattice. An increase in the Co ³⁺ /Co ²⁺ ratio was observed over the catalyst loaded with 1 wt% La species. As a result, an improvement in PROX performance was observed for the 1 wt% La/Co ₃ O ₄ catalyst, which exhibited optimum activity at relatively lower temperatures (~60 to 120 °C). A slight decrease in CO conversion with time on stream was observed for 1wt.% La/Co ₃ O ₄ in the presence of moisture, attributable to La doping, which suppresses OH-vacancy-assisted dissociative adsorption of H ₂ O. Furthermore, the introduction of CO ₂ into the feed resulted in an approximately 30% decrease in catalytic activity with time on stream. Overall, La doped Co ₃ O ₄ appears as a promising catalyst for low temperature (~80–120 °C) CO reduction in hydrogen rich streams, aligning with requirements for efficient proton exchange membrane fuel cells.

Copper doped on cobalt oxide-based catalysts prepared by hydrothermal and reflux methods were investigated for preferential oxidation of carbon monoxide (CO(PrOx) in hydrogen rich stream. The FTIR spectra of as-prepared catalysts showed the presence of Co³⁺–O²⁻ and Co²⁺–O²⁻ stretching vibrations arising from Co₃O₄ spinel crystal structure, as confirmed by the XRD, TEM, and H₂-TPR data. The reflux prepared 5 wt% CuOx/Co₃O₄ catalyst demonstrated a notable CO conversion of 91% at 80 °C, compared to its hydrothermal counterpart. The activity is correlated with strong Cu–Co interactions within the spinel matrix and the unique catalyst morphology. These favoured the formation of high surface oxygen vacancies and small crystallite size (~ 5.5 nm), contributing to large pore structure and higher BET surface area of 64.9 m²/g. The surface oxidation states of Cu and Co were revealed to consists of Cu⁺, Cu²⁺, Co²⁺ and Co³⁺ by the XPS spectra, which drove Cu⁺+ Co³⁺↔Cu²⁺+ Co²⁺ redox synergy. A decrease in content of active species (Cu⁺ and Co³⁺) with time on stream was observed during CO(PrOx). The as prepared 5 wt% CuO_x/Co₃O₄(Ref) catalyst showed good stability under dry and CO₂ environments, and its activity deactivate in moisture environment. This work offers a novel strategy for preparation of active CO(PrOx) catalysts with the aim to promote adoption of proton exchange membrane fuel cell (PEMFC) technology.