Modification of Co3O4 by Al2O3: Influence on the reducibility

In this study, we used a coprecipitation method followed by calcination at 500 °C to synthesize undoped and Al³⁺-doped Co₃O₄ nanoparticles with different aluminum fractions (x = Al/(Co + Al) = 1/60, 1/30, 1/15, 1/75, 1/6 and 1/5). An addition of Al³⁺ ions led to a decrease in the average crystallite size from 29 to 11 nm, and growth of the specific surface area from 28 to 91 m²/g. TEM images indicated round and platelet shapes of the nanoparticles. According to HAADF-STEM combined with EDS elemental mapping, the platelet shape particles are Al³⁺-enriched, while the round shape particles are Al³⁺-depleted. The origin of Al³⁺ distribution over the oxide volume is conditioned by the state of the hydroxide precursor. It was shown by XRD that the coprecipitation yielded homogeneous hydroxides only for Al fractions x = 0 and x = 1/5. For the intermediate compositions, the precursors represent a mixture of Co₆(CO₃)₂(OH)₈*H₂O and Co_0.8Al_0.2(OH)₂(CO₃)_0.1*nH₂O. On the TPR-H₂ profiles, reduction peaks for three (Co,Al)₃O₄ oxides differing in the Al³⁺ concentration (y) can be found. Two of these oxides with y = 0 and y = 0.2 are formed from different hydroxides, and third one with y ∼0.05 is the result of their mutual interaction. In situ XRD allowed us to interpret the TPR peaks correctly and showed that the reduction of all the oxides occurs in two steps. In the first step, Co³⁺ → Co²⁺, and (Co_1-yAl_y)₃O₄ oxides transform to (Co,Al)O. In the second step, Co²⁺ → Co⁰, and (Co,Al)O is reduced into metallic cobalt. In undoped Co₃O₄, Co³⁺ → Co²⁺ and Co²⁺ → Co⁰ reduction steps occur at T₁ = 280 and T₂ = 325 °C, respectively. For Al-depleted (Co_1-yAl_y)₃O₄ (y ∼ 0.05 in the interior of particles), both reduction steps shift toward higher temperatures T₁ = 305 and T₂ = 405 °C, respectively. The reduction of Al-enriched (Co_0.8Al_0.2)₃O₄ is more difficult; first and second reduction steps occur at T₁ = 345 and T₂ = 610−690 °C. Therefore, Al³⁺ ions have a little effect on the first step and very significantly influence the second one. Additionally, it was shown by TEM that after the reduction at 700 °C metallic cobalt particles were surrounded by the Al-enriched oxide shell. Apparently, that is why the addition of even a small amount of Al³⁺ ions prevents a quick sintering of metallic cobalt observed for pure Co₃O₄.