Mechanism and Nature of Active Sites for Methanol Synthesis from CO/CO₂ on Cu/CeO₂

Abstract

CO2 hydrogenation to methanol can play an important role in meeting the sustainability goals of the chemical industry. In this study, we investigated in detail the role of the Cu–CeO2 interactions for methanol synthesis, emphasizing the role of the copper surface and interface sites between copper and ceria for the hydrogenation of CO2 and CO. A combined CO2–N2O titration approach was developed to quantify the exposed metallic copper sites and ceria oxygen vacancies in reduced Cu/CeO2 catalysts. Extensive characterization shows that copper dispersion is strongly enhanced by strong Cu–CeO2 interactions in comparison to Cu/SiO2. CO2 hydrogenation activity data show that the Cu/CeO2 catalysts displayed higher methanol selectivity compared to a reference Cu/SiO2 catalyst. The improved methanol selectivity stems from inhibition of the reverse water-gas-shift activity. The role of CO in CO2-to-methanol conversion was studied by steady-state and transient cofeeding activity measurements together with (quasi) in situ characterization (TPH, XPS, SSITKA, and IR spectroscopy). The Cu–CeO2 interface provides active sites for the direct hydrogenation of CO to methanol via a formyl intermediate. Cofeeding of small amounts of CO2 to a CO/H2 mixture poisons these interfacial sites due to the formation of carbonate-like species. Methanol synthesis proceeds mainly via CO2 hydrogenation in which the metallic Cu surface provides the active sites.

Flame Synthesis of Cu/ZnO–CeO2 Catalysts: Synergistic Metal–Support Interactions Promote CH3OH Selectivity in CO2 Hydrogenation

“…The blocking of oxygen vacancies by in situ formed carbonates may explain their limited role in CO 2 hydrogenation. 21 , 45 , 75 …”

“…We propose that the ceria-induced inhibition of the rWGS is associated with the higher formate coverage, which blocks metallic Cu sites active in the rWGS as proposed before. 45 , 78 − 81 Besides the blocking of Cu surface by formate, in situ CO-to-CH 3 OH conversion during CO 2 hydrogenation can also contribute to the ceria-induced inhibition of the rWGS. It has been reported that Cu–CeO 2 interface is highly active for CO hydrogenation to CH 3 OH.…”

Mechanistic and multiscale aspects of thermo-catalytic CO2conversion to C1products

“…2. In the formate (HCOO*) mechanism, 259,302,303 Regarding the Cu/ZnO system, many hypotheses on the nature of the active sites were proposed. 259,304,305 Fujita et al 306 investigated the system with diffuse reflectance FT-IR spectroscopy and temperature-programmed desorption, showing that two types of formate species and zinc methoxide -Zn(CH 3 O) 2form during the reaction.…”

“…. In the formate (HCOO*) mechanism,259,302,303 CO* 2 is hydrogenated to HCOO*, H 2 COO* (or HCOOH*), and to H 2 COOH*. Then, an OH* is removed, yielding H 2 CO*, which is hydrogenated to H 3 CO* and CH 3 OH.…”

A computational investigation of the adsorption of small copper clusters on the CeO2(110) surface

“…The calculated adsorption energies of Cu n on CeO 2 (110), absolute values of 2.971-3.492 eV, are generally higher than the abovementioned values for other surfaces, suggesting strong interactions between copper and ceria. In addition, experimental studies reported that nanostructured Cu/CeO 2 catalysts had a copper particle size-activity dependence 51 , which thus strongly stimulates further study of larger Cu cluster adsorption on CeO 2 (110).…”