Shohei Tada

CO2-to-Methanol Hydrogenation on Zirconia-Supported Copper Nanoparticles: Reaction Intermediates and the Role of the Metal-Support Interface

Methanol synthesis by CO2 hydrogenation is a key process in a methanol-based economy. This reaction is catalyzed by supported copper nanoparticles and displays strong support or promoter effects. Zirconia is known to enhance both the methanol production rate and the selectivity. Nevertheless, the origin of this observation and the reaction mechanisms associated with the conversion of CO2 to methanol still remain unknown. A mechanistic study of the hydrogenation of CO2 on Cu/ZrO2 is presented. Using kinetics, in situ IR and NMR spectroscopies, and isotopic labeling strategies, surface intermediates evolved during CO2 hydrogenation were observed at different pressures. Combined with DFT calculations, it is shown that a formate species is the reaction intermediate and that the zirconia/copper interface is crucial for the conversion of this intermediate to methanol.

Mechanistic study and catalyst development for selective carbon monoxide methanation

Selective CO methanation has been attractive as a CO removal technique from reforming gases in polymer electrolyte fuel cell systems. The catalysts for the title reaction require the following two features: (i) high CO methanation activity at low temperatures and (ii) low CO2 methanation activity at high temperatures. In this review, we surveyed numerous studies of selective CO methanation using heterogeneous catalysts, and discussed its plausible mechanism. Furthermore, we summarized how the activity and selectivity of CO methanation can be affected by the particle size of active metals, support materials, and additives.

Surface Sites in Cu-Nanoparticles: Chemical Reactivity or Microscopy?

Copper nanoparticles are widely used in catalysis and electrocatalysis, and the fundamental understanding of their activity requires reliable methods to assess the number of potentially reactive atoms exposed on the surface. Herein, we provide a molecular understanding of the difference observed in addressing surface site titration using prototypical methods: transmission electron micrscopy (TEM), H2 chemisorption, and N2O titration by a combination of experimental and theoretical study. We show in particular that microscopy does not allow assessing the amount of reactive surface sites, while H2 and N2O chemisorptions can, albeit with slightly different stoichiometries (1 O/2CuS and 1 H2/2.2CuS), which can be rationalized by density functional theory calculations. High-resolution TEM shows that the origin of the observed difference between microscopy and titration methods is due to the strong metal support interaction experienced by small copper nanoparticles with the silica surface.

Preparation of Ru nanoparticles on TiO2 using selective deposition method and their application to selective CO methanation

Selective CO methanation was investigated over Ru/TiO2 prepared using a selective deposition method with NaOH and NH3 aqueous solution as a pH adjuster. Control of pH by NH3 solution resulted in the small particle size of Ru (average 1.2 nm) and the formation of Na-free Ru/TiO2, leading to high CO methanation activity and a wide temperature window for selective CO methanation at low temperatures.

Physical mixing of TiO2 with sponge nickel creates new active sites for selective CO methanation

Ni–α-Al2O3, Ni–SiO2, Ni–γ-Al2O3, Ni–TiO2, and Ni–ZrO2 were prepared by physical mixing of metal oxides with sponge Ni, and the effect of physical contact of the metal oxides with sponge Ni on selective CO methanation was examined. The prepared Ni–TiO2 catalyst removed CO more deeply and suppressed CO2 methanation better relative to the other catalysts. The metal oxide nature of the prepared Ni catalysts affected the CO2 methanation activity, as well as the reverse water gas shift activity. Moreover, the catalytic performance for the five catalysts was not related to contact time. These results predicted the appearance of new active sites between sponge Ni and the metal oxides.

CO 2 Hydrogenation: Supported Nanoparticles vs. Immobilized Catalysts

The conversion of CO(2) to more valuable chemicals has been the focus of intense research over the past decades, and this field has become particularly important in view of the continuous increase of CO(2) levels in our atmosphere and the need to find alternative ways to store excess energy into fuels. In this review we will discuss different strategies for CO(2) conversion with heterogeneous and homogeneous catalysts. In addition, we will introduce some promising research concerning the immobilization of homogeneous catalysts on heterogeneous supports, as a hybrid of hetero- and homogeneous catalysts.

Methanol synthesis via CO2 hydrogenation over CuO–ZrO2 prepared by two-nozzle flame spray pyrolysis

Flame made CuO–ZrO2 catalysts for CO2 hydrogenation to methanol were prepared such that only the Cu size was varied. Smaller CuO clusters in CuO–ZrO2 showed a higher activity for methanol synthesis via CO2 hydrogenation. Thus, the two-nozzle flame spray pyrolysis technique is a promising one-step preparation process for CO2 hydrogenation catalysts.

Isolated Zr Surface Sites on Silica Promote Hydrogenation of CO2 to CH3OH in Supported Cu Catalysts

Copper nanoparticles supported on zirconia (Cu/ZrO2) or related supported oxides (Cu/ZrO2/SiO2) show promising activity and selectivity for the hydrogenation of CO2 to CH3OH. However, the role of the support remains controversial because most spectroscopic techniques provide information dominated by the bulk, making interpretation and formulation of structure-activity relationships challenging. In order to understand the role of the support and in particular of the Zr surface species at a molecular level, a surface organometallic chemistry approach has been used to tailor a silica support containing isolated Zr(IV) surface sites, on which copper nanoparticles (∼3 nm) are generated. These supported Cu nanoparticles exhibit increased CH3OH activity and selectivity compared to those supported on SiO2, reaching catalytic performances comparable to those of the corresponding Cu/ZrO2. Ex situ and in situ X-ray absorption spectroscopy reveals that the Zr sites on silica remain isolated and in their +4 oxidation state, while ex situ solid-state nuclear magnetic resonance spectroscopy and catalytic performances show that similar mechanisms are involved with the single-site support and ZrO2. These observations imply that Zr(IV) surface sites at the periphery of Cu particles are responsible for promoting CH3OH formation on Cu-Zr-based catalysts and provide a guideline to develop selective CH3OH synthesis catalysts.