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The growing concentration of CO2 in the atmosphere is a serious problem, and efforts to counter this issue are thus highly important. One of the possible approaches to solving this problem is the conversion of waste CO2 into products with added economic value. Methanol is one of these products with vast potential usage. In this study, indium oxide prepared by a simple precipitation method and modified by nanoparticles of metals from the iron triad were tested as possible catalysts to produce methanol by the method of CO2 hydrogenation. The prepared catalysts demonstrated a strong dependence of their catalytic activity on used metal. The best selectivity for the production of CH3OH was observed for the Fe/In2O3 catalyst at the value of 54.7% at 300 °C. However, due to the higher value of CO2 conversion, the highest CH3OH formation rate was observed at a value of 11.3 mmol/(h*g) at 300 °C for a composite of Ni/In2O3.

The main objective of the present study is to synthesize iron oxide catalysts with engineered crystal defects and to clarify their crucial impact on the final catalytic activity in the CO2 hydrogenation process. The method used to engineer the desired crystal defects is based on changing the precipitation reaction conditions, such as the addition rate and the order of the precipitant during the primary phase of the synthesis of iron oxide catalysts. The catalyst synthesis process is based on the formation of iron oxalates in the first step, followed by thermal decomposition into iron oxides in the second step, which were subsequently tested as catalysts in CO2 hydrogenation. The reversed double-beam photoacoustic spectroscopy used for advanced characterization of the prepared catalysts demonstrated that the observed change in catalytic activity is related to the energy and density of electron traps connected with the defects in the crystal lattice of the catalysts. These defects occur during the precipitation of oxalates, and their formation is significantly affected by changes in the precipitation conditions, i.e., the course of nucleation and growth of iron oxalate crystals. The results of the presented study thus affirmed the cardinal importance of defect engineering in heterogeneous catalysis.

Nickel oxide (NiO) is one of the most popular hydrogenation catalysts. In heterogeneous catalysis, nickel oxide is used, for example, as a suitable methanation catalyst in the Fischer–Tropsch reaction not only for CO hydrogenation but also in the modified Fischer–Tropsch reaction with CO2. However, CH4 selectivity and CO2 conversion strongly depend on NiO micro- (MPs) and nanoparticles’ (NPs) shape, size, and surface area. In this study, the synthesis of NiO micro- and nanoparticles was conducted using the simple solvothermal method. Different morphologies (microspheres, sheet clusters, hexagonal microparticles, and nanodiscs) were prepared using this method with different solvents and stabilizers. The prepared catalysts were tested in the hydrogenation of CO2 in a gas phase with excellent conversion values and high selectivity to produce CH4. The best results were obtained with the NiO with disc or sphere morphology, which produced methane with selectivity at a level near 100% and conversion close to 90%.