Controlling the selectivity of CO2 hydrogenation catalysts is a fundamental challenge. In this study, the selectivity of supported Ni catalysts prepared by the traditional impregnation method was found to change after a first CO2 hydrogenation reaction cycle from 100 to 800 °C. The usually high CH4 formation was suppressed leading to full selectivity toward CO. This behavior was also observed after the catalyst was treated under methane or propane atmospheres at elevated temperatures. In situ spectroscopic studies revealed that the accumulation of carbon species on the catalyst surface at high temperatures leads to a nickel carbide-like phase. The catalyst regains its high selectivity to CH4 production after carbon depletion from the surface of the Ni particles by oxidation. However, the selectivity readily shifts back toward CO formation after exposing the catalysts to a new temperature-programmed CO2 hydrogenation cycle. The fraction of weakly adsorbed CO species increases on the carbide-like surface when compared to a clean nickel surface, explaining the higher selectivity to CO. This easy protocol of changing the surface of a common Ni catalyst to gain selectivity represents an important step for the commercial use of CO2 hydrogenation to CO processes toward high-added-value products.

Controlling the selectivity of CO2 hydrogenation is a challenge for the application of earth-abundant-based metal catalysts. Herein, we show that a high-temperature pyrolysis protocol can be applied to prepare a nickel catalyst embedded in N-doped carbon (Ni@NC), which provides an enhanced selectivity to CO in the nickel-catalyzed CO2 hydrogenation reaction under atmospheric pressure. The nitrogen-containing carbon overlayer was obtained through controlled pyrolysis of the nickel–1,10-phenanthroline complex impregnated on SiO2. The Ni@NC/SiO2 catalyst is more selective for CO, following the reverse water-gas shift reaction pathway, while an analogous “naked” Ni catalyst was more selective toward CH4, following the CO2/CO complete hydrogenation pathway. Although the Ni@NC/SiO2 catalyst has larger particle sizes than the naked Ni/SiO2 catalyst, the binding strength of CO to the Ni@NC surface is significantly weaker. Consequently, the CO intermediate easily desorbs instead of being converted into methane. Hence, selectivity toward CO, which is an important intermediate to methanol and C2+ products, or CH4 can be modulated by a simple modification of the Ni surface through controlled carbon deposition.

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Using niobium compounds as heterogeneous catalysts in biodiesel production is a promising methodology from economic and environmental viewpoints. However, the application of niobium catalysts still is a challenge due to the high temperatures and pressures for moderate biofuel yields. Therefore, easily handled and applied materials have been developed to optimize biofuel production, which is the goal of this study. Nb 2 O 5 and ammonium niobium oxalate (AmNO) were activated in reflux and ultrasound-assisted system. Nb 2 O 5 showed better activity under reflux, using methanol. The characterizations conclude that the Lewis-acid sites are determinant for higher conversion rather than surface area. AmNO has better activity also in the reflux system at 70 °C, against 170 °C for Nb 2 O 5 , reaching above 70% conversion. In addition, reactions in ultrasound-assisted systems are also appealing due to the lower time and temperature, with conversion rates above 40%. Both catalysts showed interesting results under milder conditions than those in the literature.