No abstract

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 .

Electrochemical reduction of carbon dioxide (CO 2 RR) is an attractive route to close the carbon cycle and potentially turn CO 2 into valuable chemicals and fuels. However, the highly selective generation of multicarbon products remains a challenge, suffering from poor mechanistic understanding. Herein, we used operando Raman spectroscopy to track the potential-dependent reduction of Cu 2 O nanocubes and the surface coverage of reaction intermediates. In particular, we discovered that the potential-dependent intensity ratio of the Cu–CO stretching band to the CO rotation band follows a volcano trend similar to the CO 2 RR Faradaic efficiency for multicarbon products. By combining operando spectroscopic insights with Density Functional Theory, we proved that this ratio is determined by the CO coverage and that a direct correlation exists between the potential-dependent CO coverage, the preferred C–C coupling configuration, and the selectivity to C 2+ products. Thus, operando Raman spectroscopy can serve as an effective method to quantify the coverage of surface intermediates during an electrocatalytic reaction.

The development of catalysts for electrochemical reduction of carbon dioxide (eCO 2 RR) with high activity and selectivity remains a grand challenge to render the technology useable. As promising candidates, metal−nitrogen−carbon (MNC) catalysts with metal atoms present as atomically dispersed metal−N x moieties (MN x , M = Mn, Fe, Co, Ni, and Cu) were investigated as model catalysts. The distinct activity for CO formation observed along the series of catalysts is attributed to the nature of the transition metal in MN x moieties because of otherwise similar composition, structure, and morphology of the carbon matrix. We identify a volcano trend between their activity toward CO formation and the nature of the transition metal in MN x sites, with Fe and/or Co at the top of the volcano, depending on the electrochemical potential. Regarding selectivity, FeNC, NiNC, and MnNC had Faradaic efficiency for CO >80%. To correctly model the active sites in operando conditions, experimental operando X-ray absorption near edge structure spectroscopy was performed to follow changes in the metal oxidation state with electrochemical potential. Co and Mn did not change the oxidation state with potential, whereas Fe and Ni were partially reduced and Cu largely reduced to Cu(0). Computational models then led to the identification of M 2+ N 4 −H 2 O as the most active centers in FeNC and CoNC, whereas Ni 1+ N 4 was predicted as the most active one in NiNC at the considered potentials of −0.5 and −0.6 V versus the reversible hydrogen electrode. The experimental activity and selectivity could be rationalized from our density functional theory results, identifying in particular the difference between the binding energies for CO 2 * − and H* as a descriptor of selectivity toward CO. This indepth understanding of the activity and selectivity based on the speciation of the metals for eCO 2 RR over atomically dispersed MN x sites provides guidelines for the rational design of MNC catalysts toward eCO 2 RR for their application in highperformance devices.