Recently, climate change associated with the increasing level of atmospheric CO2 provoked growing interest in the recycling of captured CO2 to produce methanol and DME using renewable sources of hydrogen. For CO2 hydrogenation, numerous catalysts have been investigated. Among them, bimetallic catalysts are promising due to their unique properties. In this work, the performance of Ni-Ga based catalysts for CO2 hydrogenation to methanol and DME was investigated at elevated pressures. A passivation technique using N2O was developed for the air-sensitive Ni-Ga catalysts to facilitate transfer to the reactor. The effect of Pd promotion on the reducibility and performance of a Ni-Ga catalyst was also studied. For the single-stage DME synthesis, Ni-Ga and Pd-Ga based catalysts were investigated as the methanol synthesis component with the zeolites ferrierite (FER) and ZSM-5 as the acid component.
Ni-Ga(5:3 and 1:1)/SiO2 catalysts were found to have higher selectivity to methanol but lower activity compared to a commercial Cu/ZnO/Al2O3 catalyst at elevated pressures. Loss in activity and selectivity is attributed in part to the decomposition of the Ni-Ga intermetallic phase. The Ni-Ga catalyst was reversibly poisoned significantly in the presence of reaction products (especially CO). Addition of Pd to the Ni-Ga(5:3)/SiO2 showed an enhanced methanol synthesis activity, possibly due to hydrogen spillover. Ferrierite and ZSM-5 worked well as the methanol dehydration component in single-stage DME synthesis with Ni-Ga or Pd-Ga catalysts under CO2/H2. FER based mixtures demonstrated good stability and selectivity due to the unidirectional channel structure that suppresses hydrocarbon formation, while the performance of ZSM-5 was greatly dependent on the water concentration. The effect of proximity between the two catalyst components in physical mixtures was found to be more pronounced in the Ni-Ga based mixtures due to metal transfer, which appeared to be facilitated by the relatively high re-activation temperature.