Rufino Manuel Navarro

In this study, Pd/TiO₂ catalysts were prepared using different methods (impregnation (IMP), chemical reduction (CR), and deposition–precipitation (DP)) to evaluate the effects of metal dispersion and metal‐support interfacial sites on the activity for the reverse water–gas shift (rWGS) reaction (CO₂ + H₂ ↔ CO + H₂O) under low‐temperature and high‐pressure conditions. Under challenging low‐temperature reaction conditions, the highest CO yield (711.2 mmol_CO·min⁻¹·mol_Pd⁻¹) was achieved with the Pd/TiO₂ catalyst prepared via deposition–precipitation method. Characterization results revealed that this method exhibited the best surface exposure of metallic Pd particles and altered the support morphology by decorating the TiO₂ and Pd surfaces with amorphous TiO₂ particles. The structure–activity correlation indicated that the key factors influencing catalyst activity and selectivity were: surface exposure and particle size of metallic Pd (from XPS, XRD, and HRTEM), the development of Pd–TiO₂ interfacial region (from XPS and DRIFTS‐CO), and surface hydroxylation (from DRIFTS‐OH). The Pd‐PD catalysts with moderate SMSI seem to develop an optimal interaction that leads to better selectivity toward CO with respect to methanation.

This study examines the factors influencing the formation and structure of ZnPd-ZnO active sites supported on TiO₂ by comparing their genesis from Pd/TiO₂ base catalysts prepared by impregnation using different Pd precursors (Pd(NH₃)₄(NO₃)₂ and Pd(acac)₂). The experimental results demonstrated that, in contrast to the production of CO over Pd/TiO₂ base catalysts, the selectivity of methanol over ZnPd-ZnO/TiO₂ catalysts was significantly affected by the dispersion of ZnPd intermetallic particles and the development of ZnPd-ZnO interfaces. These are determined by the characteristics of Pd particles supported on TiO₂ and their contact with the ZnO particles deposited on them. The Pd/TiO₂ base catalyst prepared by impregnation with neutral Pd precursor (Pd(acac)₂) produces a higher concentration and more effective dispersion of the ZnPd intermetallic phase as well as a wider ZnO-ZnPd interface region in comparison to the Pd/TiO₂ counterpart synthetized using the cationic Pd precursor (Pd(NH₃)₄(NO₃)₂). These differences in the ZnPd-ZnO active sites resulted in notable variations in the methanol yield, achieving the catalysts prepared with the neutral precursor about twice higher methanol selectivity from CO₂ hydrogenation at low temperatures.

The performance of bifunctional hybrid catalysts based on phosphotungstic acid (H₃PW₁₂O₄₀, HPW) supported on TiO₂ combined with a Cu-ZnO(Al) catalyst in the direct synthesis of dimethyl ether (DME) from syngas has been investigated. In this work, different types of TiO₂ were used as a support to study the effect of changes in the structure of the TiO₂ support on the acidity and dispersion of HPW. Various TiO₂ supports with different structural and surface characteristics have been studied and the results indicate that: (i) the crystallinity and crystallite size of the primary particles of the HPW units depend on the TiO₂ support; (ii) the pore size distribution of the TiO₂ support affects the surface segregation of the heteropolyacids; and (iii) changes in the supported HPW acid catalysts do not significantly alter the crystal structure of the CuO and ZnO phases after contact with CZA in bifunctional catalysts. The activity results indicate that the variation in the intrinsic activity of the Cu-ZnO_x centers in the bifunctional catalysts for direct DME synthesis is minimal due to the limited alteration of the crystal structure of the centers.

In the face of escalating environmental challenges, innovative solutions for purifying air and water are more critical than ever

Direct catalytic conversion of CO₂ to methanol via renewable hydrogen has emerged as a promising technology among the various CO₂ conversion techniques. However, efficient hydrogenation of CO₂ using conventional Cu‐ZnO‐based catalysts, which are currently used for industrial methanol production from synthesis gas, remains a challenge due to inefficient energy conversion, poor stability and sluggish CO₂ conversion kinetics. As the catalytic activity, stability and methanol selectivity of conventional Cu/ZnO catalysts are still insufficient for industrial applications, novel catalyst formulations using transition metals/metal oxides and supported noble metal nanostructures have emerged. Among them, intermetallic compounds are being explored for their unique electronic and crystalline structures, which can be tailored by controlled, precise, and seamless tuning of interatomic distances, specific arrangements and electronic structure to enhance their stability and activity for the selective hydrogenation of CO₂ to methanol. In this context, intermetallic catalysts containing Pd, Cu and Ni combined with metal oxide nanoparticles (ZnO, Ga₂O₃, In₂O₃, etc.) have been shown to be more effective than the classical Cu‐ZnO‐Al₂O₃. This review analyses the progress made in the study of these intermetallic catalysts by analysing different aspects of their preparation, characterization, effects of promoters, support interactions, etc. Future research perspectives are discussed in the context of potential industrial applications of intermetallics for direct methanol production via CO₂ hydrogenation.

This work is dedicated to the study of the effect of the synthesis conditions (drying and calcination) of sulfated zirconia on the final catalytic behavior of bifunctional composite catalysts prepared by the physical mixing of the sulfated zirconia (methanol dehydration catalyst) with Cu/ZnO/Al₂O₃ (CZA; methanol synthesis catalyst). The main objective was to optimize the CZA-ZrO₂/SO₄²⁻ composite catalyst for its use in the direct production of dimethyl ether (DME) from syngas. Sulfated zirconia aerogel (AZS) and xerogel (XZS) were prepared using the sol–gel method using different solvent evacuation conditions and calcination temperatures, while the Cu-ZnO(Al) catalyst was synthesized using the coprecipitation procedure. The effectivity of CZA-ZrO₂/SO₄²⁻ composite catalysts for the direct production of dimethyl ether (DME) from syngas was evaluated in a flow reactor at 250 °C and 30 bar total pressure. The characterization of the sulfated zirconia aerogels and xerogels using different techniques showed that the mesoporous aerogel (AZS_0.5300) exhibited the best textural and acidic properties due to the gel drying under supercritical conditions and calcination at 300 °C. As a result, the composite catalyst CZA-AZS_0.5300 exhibited seven times higher DME production than its xerogel-containing counterpart (364 vs. 52 μmol_DME·min⁻¹·g_cat⁻¹). This was attributed to its well-matched metal surface, mesoporous structure, optimal crystallite size and, most importantly, its higher acidity.

Mn-promoted bulk iron catalysts with a high specific surface area (82–211 m²·g⁻¹) were synthesized by coprecipitation followed by drying under supercritical conditions. The catalysts were tested in the CO₂ hydrogenation to valuable C₂-C₅ hydrocarbons. The Mn-promoted iron catalysts exhibited better textural properties than the bare Fe₂O₃ catalyst, allowing better dispersion of the active phase, easier reduction and carburization of iron oxides and, consequently, resulting in higher catalytic activity than the bare Fe₂O₃ catalyst. The best activity results were obtained by catalyst promotion with a very low amount of Mn (Mn/Fe atomic ratio of 0.05). Upon steady state conditions (T = 340 °C, total pressure of 20 bar and H₂/CO₂ = 3), this catalyst exhibited high CO₂ conversion (44.2%) and selectivity to C₂-C₄ hydrocarbons (68%, olefin to paraffin ratio of 0.54), while the selectivity to C₅₊ hydrocarbons, CH₄ and CO was about 3.2, 38.5 and 5%, respectively. A close correlation was found between catalyst textural properties and CO₂ conversion. The most active MnFe-0.05 catalyst exhibited high stability during 72 h of reaction related to a low amount of soft coke formation and catalyst activation through the formation of the χ-Fe₅C₂ phase during the on-stream reaction.

Mn-promoted bulk iron catalysts with high specific surface area (82-211 m2.g-1) were synthesized by coprecipitation followed by drying under supercritical conditions. The catalysts were tested in the CO2 hydrogenation reaction. The Mn-promoted iron catalysts showed better textural properties than the bare Fe2O3 catalyst, allowing better dispersion of the active phase, easier reduction of iron oxide, better carburization of iron oxides and higher catalytic activity than the bare Fe2O3 catalyst. The best activity results were obtained by catalyst promotion with a very low amount of Mn (Mn/Fe ratio of 0.05). Upon steady-state conditions (T=340 ºC, total pressure of 20 bar and H2/CO2=3), this catalyst showed high CO2 conversion (44.2%) and selectivity to C2-C4 hydrocarbons (68%, olefin to paraffin ratio of 0.54), while the selectivity to C5+ hydrocarbons, CH4 and CO was about 3.2, 38.5 and 5%, respectively. A close correlation was found between catalyst textural properties and CO2 conversion. The most active MnFe-0.05 catalyst shows high stability during 72 h of reaction related to low amount of soft coke formation and catalyst activation through the formation of χ-Fe5C2 phase during the on-stream reaction.