Physical and chemical behaviour of tungsten oxide in the presence of nickel additive under hydrogen and carbon monoxide atmospheres

The physical and chemical behaviour of bulk tungsten oxide (WO₃) and Ni doped tungsten oxide (15% Ni/WO₃) were examined by performing a temperature-programmed reduction (TPR) technique. The chemical composition, morphology, and surface composition of both samples before and after reduced were analysed by X-ray diffraction (XRD), scanning electron microscopy (FESEM), and X-ray photoelectron spectroscopy (XPS) analysis. The XRD pattern of calcined Ni doped tungsten oxide powder comprised of WO₃ and nickel tungstate (NiWO₄) phases. The reduction behaviour was investigated by a non-isothermal reduction up to 900 °C achieved under (10 and 20% v/v) hydrogen in nitrogen (H₂ in N₂) and (20 and 40% v/v) carbon monoxide in nitrogen (CO in N₂) atmospheres. The H₂-TPR were indicated the reduction of bulk WO₃ and 15% NiWO₃ proceed in three steps (WO₃ → WO₂ → WO₂ + W) and (WO₃ → WO₂ → W + Ni₄W) respectively under 20% H₂. Whereas, the reduction of 15% WO₃ under 40% CO involves of two following stages: (i) low temperature (<800 °C) transformation of WO₃ → WO_2.72 → WO₂ and, (ii) high temperature (>800 °C) transformation of WO₂ → W → WC. Furthermore, NiWO₄ alloy phase was transformed according to the sequence NiWO₄ → Ni + WO_2.72 → Ni + WO₂ → Ni + W → Ni₄W + W at temperature >700 °C and >800 °C in H₂ and CO atmospheres, respectively. It can be concluded that the reduction behaviour of WO₃ is matched with the thermodynamic data. In addition, the reduction under H₂ is more favourable and have better reducibility compared to the CO gas. It is due to the small molecule size and molecule mass of H₂ that encourages the diffusion of H₂ molecule into the internal surface of the catalyst compared to CO. Moreover, Ni additive had improved the WO₃ reducibility and enhancing the CO adsorption and promotes the formation of tungsten carbide (WC) by carburisation reaction. Besides, the formation of Ni during the reduction of 15% Ni/WO₃ under CO reductant catalysed the Boudouard reaction to occur, which disproportionated the carbon monoxide to carbon dioxide and carbon (CO → CO₂ + C).