H/D exchange experiments on a Cu/ZnO/Al2O3 catalyst have shown that methanol synthesis and RWGS
display a strong thermodynamic isotope effect, which is attributed to differences in the zero-point energy
of hydrogenated vs. deuterated species. The effect is larger for methanol synthesis and substantially
increases the equilibrium yield in deuterated syngas. In the kinetic regime of CO2 hydrogenation, an
inverse kinetic isotope effect of H/D substitution was observed, which is stronger for methanol synthesis
than for CO formation suggesting that the two reactions do not share a common intermediate. Similar
observations were also made on other catalysts such as Cu/MgO, Cu/SiO2, and Pd/SiO2. In contrast to
CO2 hydrogenation, the CO hydrogenation on Cu/ZnO/Al2O3 did not show such a strong kinetic isotope
effect indicating that methanol formation from CO2 does not proceed via consecutive reverse water
gas shift and CO hydrogenation steps. The inverse KIE is consistent with formate hydrogenation being
the rate-determining step of methanol synthesis from CO2. Differences in the extent of product inhibition
by water, observed for methanol synthesis and reverse water gas shift indicate that the two reactions
proceed on different surface sites in a parallel manner. The consequences for catalyst design for effective
methanol synthesis from CO2 are discussed.
H/D exchange experiments on a Cu/ZnO/Al2O3 catalyst have shown that methanol synthesis and RWGSdisplay a strong thermodynamic isotope effect, which is attributed to differences in the zero-point energyof hydrogenated vs. deuterated species. The effect is larger for methanol synthesis and substantiallyincreases the equilibrium yield in deuterated syngas. In the kinetic regime of CO2 hydrogenation, aninverse kinetic isotope effect of H/D substitution was observed, which is stronger for methanol synthesisthan for CO formation suggesting that the two reactions do not share a common intermediate. Similarobservations were also made on other catalysts such as Cu/MgO, Cu/SiO2, and Pd/SiO2. In contrast toCO2 hydrogenation, the CO hydrogenation on Cu/ZnO/Al2O3 did not show such a strong kinetic isotopeeffect indicating that methanol formation from CO2 does not proceed via consecutive reverse watergas shift and CO hydrogenation steps. The inverse KIE is consistent with formate hydrogenation beingthe rate-determining step of methanol synthesis from CO2. Differences in the extent of product inhibitionby water, observed for methanol synthesis and reverse water gas shift indicate that the two reactionsproceed on different surface sites in a parallel manner. The consequences for catalyst design for effectivemethanol synthesis from CO2 are discussed.
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