Low temperature and high pressure are thermodynamically more favorable conditions to achieve high conversion and high methanol selectivity in CO₂ hydrogenation. However, low-temperature activity is generally very poor due to the sluggish kinetics, and thus, designing highly selective catalysts active below 200 °C is a great challenge in CO₂-to-methanol conversion. Recently, Re/TiO₂ has been reported as a promising catalyst. We show that Re/TiO₂ is indeed more active in continuous and high-pressure (56 and 331 bar) operations at 125-200 °C compared to an industrial Cu/ZnO/Al₂O₃ catalyst, which suffers from the formation of methyl formate and its decomposition to carbon monoxide. At lower temperatures, precise understanding and control over the active surface intermediates are crucial to boosting conversion kinetics. This work aims at elucidating the nature of active sites and active species by means of in situ/operando X-ray absorption spectroscopy, Raman spectroscopy, ambient-pressure X-ray photoelectron spectroscopy (AP-XPS), and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). Transient operando DRIFTS studies uncover the activation of CO₂ to form active formate intermediates leading to methanol formation and also active rhenium carbonyl intermediates leading to methane over cationic Re single atoms characterized by rhenium tricarbonyl complexes. The transient techniques enable us to differentiate the active species from the spectator one on TiO₂ support, such as less reactive formate originating from spillover and methoxy from methanol adsorption. The AP-XPS supports the fact that metallic Re species act as H₂ activators, leading to H-spillover and importantly to hydrogenation of the active formate intermediate present over cationic Re species. The origin of the unique reactivity of Re/TiO₂ was suggested as the coexistence of cationic highly dispersed Re including single atoms, driving the formation of monodentate formate, and metallic Re clusters in the vicinity, activating the hydrogenation of the formate to methanol.

@en

The direct synthesis of methanol via the hydrogenation of CO₂, if performed efficiently and selectively, is potentially a powerful technology for CO₂ mitigation. Here, we develop an active and selective Cu-Zn/SiO₂ catalyst for the hydrogenation of CO₂ by introducing copper and zinc onto dehydroxylated silica via surface organometallic chemistry and atomic layer deposition, respectively. At 230 °C and 25 bar, the optimized catalyst shows an intrinsic methanol formation rate of 4.3 g h^-1 g_Cu^-1 and selectivity to methanol of 83%, with a space-time yield of 0.073 g h^-1 g_cat^-1 at a contact time of 0.06 s g mL^-1. X-ray absorption spectroscopy at the Cu and Zn K-edges and X-ray photoelectron spectroscopy studies reveal that the CuZn alloy displays reactive metal support interactions; that is, it is stable under H₂ atmosphere and unstable under conditions of CO₂ hydrogenation, indicating that the dealloyed structure contains the sites promoting methanol synthesis. While solid-state nuclear magnetic resonance studies identify methoxy species as the main stable surface adsorbate, transient operando diffuse reflectance infrared Fourier transform spectroscopy indicates that μ-HCOO*(ZnO_x) species that form on the Cu-Zn/SiO₂ catalyst are hydrogenated to methanol faster than the μ-HCOO*(Cu) species that are found in the Zn-free Cu/SiO₂ catalyst, supporting the role of Zn in providing a higher activity in the Cu-Zn system.

@en

Since the industrial revolution in the 1760s, the CO2 concentration in the atmosphere has been rising incessantly driving global warming closer to the point of no return. The world requires urgent actions to not only reduce CO2 emissions but also capture the CO2 for utilization to mitigate the future environmental crisis. CO2 hydrogenation to CH3OH offers an alternative to produce a feasible and economic substitute for oil. This technology also resembles the nearly 100 years old CH3OH synthesis processes from syngas containing H2, CO, and CO2. The conventional Cu/ZnO/Al2O3 catalyst has also been applied for more than 50 years, and its high performance stems from synergies between Cu and ZnO. However, the true nature of the interfacial sites is still extensively debated. Moreover, lower temperature and higher pressure are thermodynamically favorable for maximum CO2 conversion and CH3OH selectivity according to Le Châtelier’s principle and beneficial in terms of energy consumption and catalyst stability against sintering. The limitation in the catalytic performance of Cu/ZnO/Al2O3 in such conditions demands the exploration of novel catalysts. Part I of this dissertation is dedicated to gaining a deeper understanding of Cu-ZnO synergistic structure as well as other Cu-based catalysts. In Chapter 2, we proposed a greener synthesis route for Cu/ZnO catalysts via urea hydrolysis of acetate precursors that can achieve comparable activity to commercial Cu/ZnO/Al2O3 catalysts without producing wastewater. Co-precipitated Cu-Zn hydroxycarbonate mineral-like precursors are crucial for a high inter-dispersion between CuO and ZnO after calcination and providing Cu-ZnO interfacial sites for the reaction. In Chapter 3, the effects of key process conditions, namely temperature and pressure, on CO2 hydrogenation over a commercial Cu/ZnO/Al2O3 catalyst were investigated using a space-resolved study. The gradients of reactants/products concentration and catalyst bed temperature within the catalytic reactor can reveal the significant effect of temperature on the dominant reaction pathways. CH3OH is formed through direct CO2 hydrogenation at low temperatures, while CH3OH formation is mediated via CO which is formed by a reverse water–gas shift reaction at a high temperature. Although pressure did not influence the reaction pathway, higher pressure helped suppress CH3OH decomposition to CO. In Chapter 4, the decisive roles of peripheral promoters to Cu nanoparticles in promoting CH3OH selectivity were elucidated. The model Cu-based catalysts (Cu-M/SiO2, M = Zn, Ga, and In) were prepared via surface organometallic chemistry (SOMC). The M+ sites played important roles in stabilizing formate species spillovered from Cu and determining the reactivity of formate hydrogenation. Improving the spillover and tuning the reactivity of formate help suppress formate decomposition to CO over Cu and ultimately boost CH3OH selectivity. Part II is dedicated to exploring the novel catalysts for low-temperature CO¬2 hydrogenation, as well as, gaining a deeper understanding of the state-of-the-art Re/TiO2 catalyst. In Chapter 5, the bifunctionality of Re supported on TiO2 was deciphered, where metallic Re functions as the H2 activator and cationic Re as the CO2 activator. Re/TiO2 suffers from additional CH4 formation, and the active intermediates and reaction pathways for CH3OH and CH4 were identified. Understanding the nature of active sites and reaction mechanisms over Re/TiO2 led to approaches for CH4 selectivity mitigation in Chapter 6. Exploring various transition metals under low-temperature conditions provided insights into the formate stabilization of the coinage metals (Cu, Ag, and Au). Since the balance between metallic and cationic Re limited the CH3OH selectivity of Re/TiO2, the addition of Ag complemented the role of cationic Re. A synergistic interplay between Ag and Re did not only improve CH3OH selectivity significantly by suppressing intermediates in the reaction pathways toward CH4 but also exhibited superior stability. Finally, the dissertation conveys a message that obtaining the definitive synthesis of well-defined active sites, expansive structure-activity relationships, and comprehensive reaction mechanisms are the major prerequisites for the rational design of novel catalysts. @en

On page 452, column 1 (lines 12?23) reads: H2 and CO chemisorption show an uptake of 0.91 molH2 molPd?1 and 0.61 molCO molPd ?1, respectively (Table 1, Supporting Information S6). Considering a 1:1 CO/Pd stoichiometry,32 the dispersion from CO chemisorption (D?CO) equals 61%, in a reasonable agreement with the dispersion from TEM (D?TEM 70%; Supporting Information S9).32 While H2 chemisorption is not effective for a determination of the metal dispersion of Pd nanoparticles due to the formation of a stable bulk hydride with larger particles (2.6 nm),32 a comparison of the H2 uptake and D?CO would correspond to approximately three hydrogen atoms per surface Pd.

@en

The direct conversion of CO₂to CH₃OH represents an appealing strategy for the mitigation of anthropogenic CO₂emissions. Here, we report that small, narrowly distributed alloyed PdGa nanoparticles, prepared via surface organometallic chemistry from silica-supported Ga^IIIisolated sites, selectively catalyze the hydrogenation of CO₂to CH₃OH. At 230 °C and 25 bar, high activity (22.3 mol_MeOHmol_Pd^-1h^-1) and selectivity for CH₃OH/DME (81%) are observed, while the corresponding silica-supported Pd nanoparticles show low activity and selectivity. X-ray absorption spectroscopy (XAS), IR, NMR, and scanning transmission electron microscopy-energy-dispersive X-ray provide evidence for alloying in the as-synthesized material. In situ XAS reveals that there is a dynamic dealloying/realloying process, through Ga redox, while operando diffuse reflectance infrared Fourier transform spectroscopy demonstrates that, while both methoxy and formate species are observed in reaction conditions, the relative concentrations are inversely proportional, as the chemical potential of the gas phase is modulated. High CH₃OH selectivities, across a broad range of conversions, are observed, showing that CO formation is suppressed for this catalyst, in contrast to reported Pd catalysts.

@en

Cu/ZnO-based catalysts for methanol synthesis by COx hydrogenation are widely prepared via co-precipitation of sodium carbonates and nitrate salts, which eventually produces a large amount of wastewater from the washing step to remove sodium (Na+) and/or nitrate (NO3-) residues. The step is inevitable since the remaining Na+ acts as a catalyst poison whereas leftover NO3- induces metal agglomeration during the calcination. In this study, sodium- and nitrate-free hydroxy-carbonate precursors were prepared via urea hydrolysis co-precipitation of acetate salt and compared with the case using nitrate salts. The Cu/ZnO catalysts derived from calcination of the washed and unwashed precursors show catalytic performance comparable to the commercial Cu/ZnO/Al2O3 catalyst in CO2 hydrogenation at 240-280 °C and 331 bar. By the combination of urea hydrolysis and the nitrate-free precipitants, the catalyst preparation is simpler with fewer steps, even without the need for a washing step and pH control, rendering the synthesis more sustainable. This journal is

@en

The reaction pathway of high-pressure CO₂ hydrogenation over a Cu/ZnO/Al₂O₃ catalyst is investigated through the gradients of reactants/products concentration and catalyst temperature within the catalytic reactor. This study reveals that methanol is formed through direct CO₂ hydrogenation at low temperature, while above 260 °C methanol formation is mediated via CO which is formed by reverse water-gas shift reaction.

@en