The aim of this work was to study the influence of copper content and particle morphology on the performance of Cu/FeO_x catalysts in the gas-phase conversion of CO₂ with hydrogen. All four investigated catalysts with a copper content between 0 and 5 wt% were found highly efficient, with CO₂ conversion reaching 36.8%, and their selectivity towards C₁ versus C₂-C₄, C₂-C₄=, and C₅₊ products was dependent on catalyst composition, morphology, and temperature. The observed range of products is different from those observed for catalysts with similar composition but synthesized using other precursors and chemistries, which yield different morphologies. The findings presented in this paper indicate potential new ways of tuning the morphology and composition of iron-oxide-based particles, ultimately yielding catalyst compositions and morphologies with variable catalytic performances.

Nickel and nickel oxide are widely used as heterogeneous catalysts in various processes involving the hydrogenation or reduction of organic compounds, and also as excellent methanation catalysts in the hydrogenation of CO₂. As heterogeneous catalysis is a surface-dependent process, nickel compounds in the form of microparticles (MPs), and particularly nanoparticles (NPs), improve the catalytic activity of Ni-based catalysts due to their high specific surface area. Solvothermal synthesis, which has so far been neglected for the synthesis of Ni-based methanation catalysts, was used in this study to synthesize nickel and nickel oxide MPs and NPs with a narrow size distribution. Solvothermal synthesis allows for the control of both the chemical composition of the resulting Ni catalysts and their physical structure by simply changing the reaction conditions (solvent, temperature, or concentration of reactants). Only non-toxic substances were used for synthesis in this study, meaning that the whole synthesis process can be described as environmentally friendly. Solvothermally prepared Ni compounds were subsequently transformed into nickel oxide by means of high-temperature decomposition, and all of the prepared Ni-based compounds were tested as catalysts for CO₂ methanation. The best catalysts prepared in this study exhibited a CO₂ conversion rate of nearly 95% and a selectivity for methane close to 100%, which represent thermodynamic limits for this reaction at the used temperature. These results are commonly achieved with much more complex catalytic composites containing precious metals, while here we worked with pure nickel and its oxides, in the form of micro- or nanoparticles, only.

The influence of hydrothermal treatment (HTT) and subsequent mechanochemical treatment (MChT, milling) on the porous and crystalline structure of precipitated zirconium dioxide was studied. It has been established that HTT at 300 °C promotes the transformation of amorphous ZrO₂ into a pure monoclinic phase, as well as the formation of a uniform mesoporous structure which has higher thermal stability. Soft dry milling (300 rpm, 0.5–1 h) of hydrothermally modified monoclinic ZrO₂ causes the introduction of defects into its structure without a noticeable change in the phase composition. The presence of defects is confirmed directly using UV-vis spectra and indirectly by the manifestation of photocatalytic activity of milled samples under visible irradiation. Importantly, it is found that after calcination of milled samples at 500 °C a high fraction of defects remains preserved which opens up the potential of using zirconium dioxide modified in this way as a catalyst or catalytic support with added specific properties offered by defects.

Alteration of the support structure via milling is a feasible yet rarely applied strategy for boosting the performance of the catalyst in dry methane reforming for syngas production. In this study, we introduce stable oxygen vacancies in the zirconia structure, which enhance the activation of the feedstock, specifically CO₂, while preserving the specific surface area and porosity of the material under reaction conditions. The activity of the tested Ni/ZrO₂ assemblies shows a clear dependence on the milling intensity of ZrO₂, with mild milling at 400 rpm yielding the most active catalyst. At 600 °C, this material achieved the highest feedstock conversion among the tested samples, with 29% for CH₄ and 39% for CO₂. Spectroscopic characterization indicates that the activity of the tested catalysts is controlled by a partial change in the phase composition of the support from monoclinic to tetragonal under reaction conditions, as well as the nature and population of O₂ ^– species, oxygen vacancies, Zr³⁺ defects, and Ni–ZrO₂ interfacial interactions.

This study reports on the performance of alumina-supported copper-based catalysts in the oxidative dehydrogenation of propane, with copper dispersed on two distinct commercial aluminium oxide supports made of micro- and nanosized alumina, respectively. The activity and selectivity of the two catalysts was investigated at temperatures between 250 and 550 °C. At a propane-to-O₂ ratio of 1:1, Cu/nanoAl₂O₃ achieves propylene selectivity of 35–48% at low temperatures (250–300 °C), while Cu/Al₂O₃ only exhibits activity starting at 350 °C with about 40% propylene selectivity. Altering the propylene-to-oxygen ratio to 3:1 enhances selectivity towards propylene in both catalysts, up to about 64% on Cu/Al₂O₃ at temperatures of 250–350 °C. The switch to the mild oxidant CO₂ boosts propylene selectivity to 100%. In case of Cu/nanoAl₂O₃, the rate of propylene formation doubles that of the obtained with O₂ used as oxidant. While with CO₂ the Cu/nanoAl₂O₃ catalyst retains 100% propylene selectivity up to 500 °C, on the less active Cu/Al₂O₃ cracking sets off already at 400 °C. The different size of copper particles in the two catalysts is seen as a primary factor determining the observed differences in the performance of the studied catalysts.