The chemical reduction progression behaviour of transition metals (Mo, Zr, W, Ce, and Co) doped on NiO was studied using temperature programmed reduction (TPR) analysis. A wet impregnation method was applied to synthesise the doped NiO series catalysts. The reduction progress of the catalysts was attained by using a reductant gas at the concentration of 40% v/v CO/N2. X-ray diffraction (XRD) was employed to determine the composition of the reduced phases. Undoped NiO was reduced at 384℃ to obtain a cubic phase of NiO. It was observed that Ce/NiO exhibited the lowest reduction temperature of 370℃ among all catalysts. This phenomenon might be due to a higher surface area of Ce/NiO compared to undoped NiO, which facilitated a faster reduction reaction. The rest of the doped NiO series catalysts (Co/NiO, Mo/NiO, W/NiO and Zr/NiO) demonstrated a higher reduction temperature compared to undoped NiO. New peaks in the XRD pattern were observed only for the reduced catalysts of Mo/NiO and W/NiO, which were associated with monoclinic MoO2 and WO2.72 phases, respectively. The formation of new compounds or more stable nickel alloys led to a slower reduction reaction than undoped NiO. Therefore, Ce/NiO was the most efficient catalyst in promoting the formation of Ni under the CO atmosphere.

This study aimed to investigate the influence of added chromium on the physical and chemical reduction behavior of molybdenum trioxide (MoO3) in a carbon monoxide (CO) environment. The reduction behavior of the sample was evaluated by using temperature-programmed reduction (TPR), and the phases produced by the reduced samples were analyzed using X-ray diffraction spectroscopy (XRD) and field emission scanning electron microscopy (FESEM). The TPR study was conducted using two reduction modes: non-isothermal reduction at 700°C with 20 vol. % of CO in nitrogen (N2), followed by isothermal reduction at 700°C for an additional 60 min. The TPR profile showed that the reduction of doped and undoped MoO3 was preceded by two reduction stages (MoO3 → Mo4O11 → MoO2), wherein, the reduction of doped MoO3 starting at a lower temperature (380°C500°C) than that of undoped MoO3 (550°C). Additionally, based on XRD analysis, it was shown that the conversion of MoO3 to MoO2 under CO generated an intermediate product known as Mo4O11. It is discovered that, increasing the concentration of chromium doped to MoO3 enhanced the reducibility of oxide due to the rapid production of MoO2 phases at T: 380ºC. Further heating under CO atmosphere, carbide species built up in the form of Mo2C rather than metallic Mo which might be due to excess of CO exposure to the surface layer of oxide.

The reduction behavior of cerium nickel oxide (Ce/NiO) catalyst was investigated by using temperature programmed reduction (TPR) with exposure of 40% (v/v) carbon monoxide (CO) in nitrogen atmosphere as a reductant agent. The Ce/NiO catalysts were prepared by using the conventional impregnation method. The reduction characteristics of NiO to Ni were examined up to 700 ºC and followed by isothermal reduction. The TPR profiles of doped NiO slightly shifted to a lower temperature from 375 to 366 ºC when Ce loading was increased from 3% to 10% (wt./ wt.), respectively. Whereas the undoped NiO was reduced at a higher temperature of 387 ºC. XRD diffractogram of the catalysts showed a complete reduction of NiO to Ni. The interaction between cerium and nickel ions for Ce/NiO series leads to a slight decrease in the reduction temperature. Fine sharp particles of Ce deposited on the NiO surfaces were observed through the FESEM images indicating some morphology modification occurred on NiO. It was found that the addition of 10% (w/w) of Ce on NiO also exhibited a larger BET surface area (11.31 m2g-1) and a smaller average pore diameter (17.7 nm). Based on these results, it is interesting to note that the addition of Ce to NiO has a remarkable influence in reducing the temperature of the reduction process. The 5% Ce/NiO was found sufficient to enhance the reducibility of NiO at a lower temperature.