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 degrees C achieved under (10 and 20% v/v) hydrogen in nitrogen (H-2 in N-2) and (20 and 40% v/v) carbon monoxide in nitrogen (CO in N-2) atmospheres. The H-2-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% H-2. Whereas, the reduction of 15% WO3 under 40% CO involves of two following stages: (i) low temperature (<800 degrees C) transformation of WO3 -> WO2.72 -> WO2 and, (ii) high temperature (>800 degrees C) transformation of WO2 -> W -> WC. Furthermore, NiWO4 alloy phase was transformed according to the sequence NiWO4 -> Ni + WO2.72 -> Ni + WO2 -> Ni + W-> Ni4W + W at temperature >700 degrees C and >800 degrees C in H-2 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 H-2 is more favourable and have better reducibility compared to the CO gas. It is due to the small molecule size and molecule mass of H-2 that encourages the diffusion of H-2 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). (C) 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Automated summary

The physical and chemical behavior of bulk tungsten oxide and Ni doped tungsten oxide were analyzed using temperature-programmed reduction technique, with XRD, FESEM, and XPS analysis. Reduction studies showed different pathways for WO3 and Ni-doped WO3 under H2 and CO atmospheres, with Ni additive enhancing WO3 reducibility and promoting tungsten carbide formation.