CO₂ hydrogenation is an attractive way to store and utilize carbon dioxide generated by industrial processes, as well as to produce valuable chemicals from renewable and abundant resources. Iron catalysts are commonly used for the hydrogenation of carbon oxides to hydrocarbons. Iron-molybdenum catalysts have found numerous applications in catalysis, but have been never evaluated in the CO₂ hydrogenation. In this work, the structural properties of iron-molybdenum catalysts without and with a promoting alkali metal (Li, Na, K, Rb, or Cs) were characterized using X-ray diffraction, hydrogen temperature-programmed reduction, CO₂ temperature-programmed desorption, in-situ ⁵⁷Fe Mossbauer spectroscopy and operando X-ray adsorption spectroscopy. Their catalytic performance was evaluated in the CO₂ hydrogenation. During the reaction conditions, the catalysts undergo the formation of an iron (II) molybdate structure, accompanied by a partial reduction of molybdenum and carbidization of iron. The rate of CO₂ conversion and product selectivity strongly depend on the promoting alkali metals, and electronegativity was identified as an important factor affecting the catalytic performance. Higher CO₂ conversion rates were observed with the promoters having higher electronegativity, while low electronegativity of alkali metals favors higher light olefin selectivity.

Bimetal-organic-framework (Bi-MOF) NH2-MIL-125(Ti/Cu)-derived nanocomposites are systematically investigated to elucidate the role of individual species TiO2, CuxO and the porous carbon matrix in photocatalytic activity. Among the studied samples, the TiO2/CuxO/C nanocomposite derived from heat processing NH2-MIL-125(Ti/Cu) under Ar/H2O vapor demonstrates the highest photocatalytic H2 evolution performance due to the formation of a phasejunction between the well-crystallized anatase/rutile TiO2 polymorph, the optimized and codoped nitrogen/carbon in the composites, the formation of p-n heterojunctions between the TiO2 and CuxO nanoparticles, as well as their uniform distribution in a hydrophilic porous carbon matrix decorated with N and carboxylic functional groups. These parameters enable the in situ-formed multi-heterostructures in these nanocomposites to not only possess relatively narrower energy band gaps and improved spatial charge separation due to the formed type-II staggered p-n heterojunctions but also offer multiple pathways for charge diffusion, resulting in lower charge-transfer resistance, suppressed bulk charge recombination, and consequently, much improved visible-light absorption. Therefore, the Bi-MOF NH2-MIL-125(Ti/Cu)-derived TiO2/CuxO/C nanocomposite provides easily accessible active sites with an excellent photocatalytic H2 evolution activity of 3147 μmol gcat-1 h-1, 99 times higher than that of bare TiO2. This work provides a simple one-step approach to producing tunable novel nanocomposites for efficient photocatalytic H2 evolution without using expensive noble metals as cocatalysts.