Metal promotion is broadly applied to enhance the performance of heterogeneous catalysts to fulfill industrial requirements. Still, generating and quantifying the effect of the promoter speciation that exclusively introduces desired properties and ensures proximity to or accommodation within the active site and durability upon reaction is very challenging. Recently, In 2 O 3 was discovered as a highly selective and stable catalyst for green methanol production from CO 2 . Activity boosting by promotion with palladium, an efficient H 2 -splitter, was partially successful since palladium nanoparticles mediate the parasitic reverse water–gas shift reaction, reducing selectivity, and sinter or alloy with indium, limiting metal utilization and robustness. Here, we show that the precise palladium atoms architecture reached by controlled co-precipitation eliminates these limitations. Palladium atoms replacing indium atoms in the active In 3 O 5 ensemble attract additional palladium atoms deposited onto the surface forming low-nuclearity clusters, which foster H 2 activation and remain unaltered, enabling record productivities for 500 h.

Indium oxide has emerged as a highly effective catalyst for methanol synthesis by direct CO 2 hydrogenation. Aiming at gathering a deeper fundamental understanding, mechanistic and (micro)kinetic aspects of this catalytic system were investigated. Steady-state evaluation at 5 MPa and variable temperature indicated a lower apparent activation energy for CO 2 hydrogenation than for the reverse watergas shift reaction (103 versus 117 kJ mol À1), which is in line with the high methanol selectivity observed. Upon changing the partial pressure of reactants and products, apparent reaction orders of À0.1, 0.5, À0.2, and À0.9 were determined for CO 2 , H 2 , methanol, and water, respectively, which highlight a strong inhibition by the latter. Co-feeding of H 2 O led to catalyst deactivation by sintering for partial pressures exceeding 0.125 MPa, while addition of the byproduct CO to the gas stream could be favorable at a total pressure below 4 MPa but was detrimental at higher pressures. Density Functional Theory simulations conducted on In 2 O 3 (1 1 1), which was experimentally and theoretically shown to be the most exposed surface termination, indicated that oxygen vacancies surrounded by three indium atoms enable the activation of CO 2 and split hydrogen heterolytically, stabilizing the polarized species formed. The most energetically favored path to methanol comprises three consecutive additions of hydrides and protons and features CH 2 OOH and CH 2 (OH) 2 as intermediates. Microkinetic modeling based on the DFT results provided values for temperature and concentration-dependent parameters, which are in good agreement with the empirically obtained data. These results are expected to drive further optimization of In 2 O 3-based materials and serve as a solid basis for reactor and process design, thus fostering advances towards a potential large-scale methanol synthesis from CO 2 .

Monoclinic zirconia has been uncovered as a carrier able to substantially boost the activity of indium oxide for CO2 hydrogenation to methanol. Here, electronic, geometric, and interfacial phenomena associated with this unique effect are investigated. Generating mixed In-Zr oxides by coprecipitation does not improve performance, excluding a primary role of electronic parameters.Since even only 1 mol% of indium stabilizes the metastable tetragonal phase of zirconia, the relevance of its crystalline structure is explored in impregnated solids. Both tetragonal and monoclinic ZrO2 permit epitaxial growth of In2O3, but a delicate lattice mismatching leads to a slightly lower dispersion of the oxide on the second, which is observed in the form of subnanometric islands on the carrier. More importantly, compressive and tensile forces are exerted on In2O3, respectively, which inhibit and foster oxygen vacancy formation, in line with the low and greatly enhanced indium-specific activity of the catalysts prepared with the two polymorphs.Hence, a deposition synthesis method is essential to unlock the role of monoclinic zirconia.According to analyses with reference In2O3-based catalysts supported on alumina and ceria, which display diverse ability to activate CO2 on their surface, the direct participation of monoclinic zirconia in a parallel pathway to methanol is put forward as a second origin of activity boosting.The latter is likely enabled by the abundancy of indium active sites vicinal to the interface and/or by a more favorable CO2 adsorption geometry onto this carrier or onto an alternative bimetallic site possibly produced.