Mechanism and structure sensitivity of methanol synthesis from CO2 over SiO2-supported Cu nanoparticles

• Published in: Journal of catalysis Vol. 369; pp. 415 - 426
• Main Authors: Karelovic, Alejandro, Galdames, Gabriel, Medina, Juan C., Yévenes, Claudia, Barra, Yanitza, Jiménez, Romel
• Format: Journal Article
• Language: English
• Published: Elsevier Inc 01-01-2019
• Subjects: Copper; Deuterium; In situ DRIFTS; Kinetic isotope effect; Kinetic study; Methanol synthesis; Reverse water gas shift; Deuterium; Kinetic isotope effect; Methanol synthesis; In situ DRIFTS; Reverse water gas shift; Kinetic study; Copper
• ISSN: 0021-9517; 1090-2694
• DOI: 10.1016/j.jcat.2018.11.012

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•Cu/SiO2 catalysts (4–36 nm) synthesized in water-in-oil microemulsions.•CH3OH and CO formation rates increase with Cu particle size.•H/D KIE and kinetic parameters are similar for catalysts with different Cu sizes.•HCOOH* hydrogenation and COOH* dissociation rate-determining steps for CH3OH and CO formation.•Structure sensitivity due to step-edge site abundancy rather than site inhibition.
The size of copper nanoparticles affects the intrinsic rate of methanol and CO formation in the hydrogenation of CO2 at 230–270 °C and total pressure 8 bar. Cu/SiO2 catalysts with different mean Cu particle sizes were prepared by water-in-oil microemulsions and suitably pretreated to obtain samples with mean particle size between 4 and 36 nm. The intrinsic rates of methanol and CO formation are 3-fold higher for large particles (>10 nm) than for smaller ones (ca. 4 nm). The reaction path was studied by means of kinetic experiments, H/D isotopic substitution, and in situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS). Results demonstrate that the hydrogenation of formic acid is the rate-determining step (rds) for methanol synthesis, whereas for CO formation the reaction proceeds with the assistance of H atoms and the dissociation of the carboxyl intermediate is the rds. This reaction scheme implies that the ratio of CO to methanol formation rates is inversely proportional to H2 partial pressure, which is also consistent with results for similar catalysts reported in earlier studies. The similar kinetic parameters and H/D kinetic isotope effect observed for catalysts with significantly different Cu mean particle sizes suggests that active surface sites have similar topology regardless of Cu particle size. Step-edge sites whose relative abundance increases with particle size in a way similar to activity are suggested as the active sites for methanol and CO formation reactions.