Please cite this article in press as: Wang et al., Strong Evidence of the Role of H2O in Affecting Methanol Selectivity from CO2 Hydrogenation
over Cu-ZnO-ZrO2, Chem (2019), https://doi.org/10.1016/j.chempr.2019.10.023
distributions also exhibit high methanol selectivity owing to the larger metal-oxide
At present, the mechanistic understanding of methanol synthesis over Cu-ZnO-ZrO2
is insufficient. In general, formate (*HCOO) produced by the reaction between CO2
and adsorbed atomic H and CO generated from the reverse water-gas shift (RWGS)
are considered to be two major intermediates.9,28 Regardless of the formate and
RWGS + CO hydrogenation pathways, methanol is finally generated from the me-
thoxy (*CH3O) intermediate, but there is still lack of basic knowledge on how me-
thoxy is converted to methanol. In addition, the role of water, which is a byproduct
in both the hydrogenation process (CO2 + 3H2 / CH3OH + H2O) and in the main
side reaction (RWGS: CO2 + H2 / CO + H2O), in affecting the formation of methanol
and intermediates is not well understood.
It has been reported that the presence of water in the hydrogenation of CO could
help in maintaining the catalyst in an active state (a partially oxidized state) under
a steady-state operation, enhancing the methanol yield.29 When using CO-
CO2-H2 as the feed, either positive or negative effect of water in the feed on meth-
anol synthesis is observed, which mainly depends on the ratio of CO to CO2 and the
partial pressure of water.30–32 For the CO2+H2 system, the methanol formation is
suppressed with the presence of water in the feed, which is presumably because
of the competitive adsorption on the active sites.31,33 It is also found that water pro-
duced during methanol synthesis from a CO2-rich feed could accelerate the crystal-
lizations of Cu and ZnO in the catalyst, leading to the deactivation of the catalyst.34
On the other hand, a thermodynamic analysis reveals that the in situ water adsorp-
tion on the CO2 hydrogenation process could lead to 15% higher methanol
The role of water in the reaction pathway of CO2 hydrogenation to methanol is also un-
der debate. Surface hydroxyl group originating from water might directly react with CO
to produce formate, and alternatively active surface oxygen could be produced by the
H2O-H2 redox mechanism that promotes methanol synthesis from CO2 via carbonate.32
In addition, it is proposed that some water-derived adsorbates might assist in the hydro-
genation of adsorbed formate to methoxy.36 A density functional theory (DFT) calcula-
tion suggests that CO2 hydrogenation to trans-COOH is kinetically more favorable
1State Key Laboratory of Complex Nonferrous
Metal Resources Clean Utilization Engineering,
than formate in the presence of water via a unique hydrogen transfer mechanism.37
Notably, isotope-tracing experiments show that water and water-derived species might
serve as reactants to form critical active intermediates or might serve as a ‘‘catalyst’’ for
one or more hydrogenation steps in CO2 hydrogenation.38 In particular, the conversion
of methoxy species to methanol by hydrolysis is believed to be more rapid than by
reductive elimination and is considered as a primary pathway to methanol generation.15
Although substantial evidence verify the crucial role of water in CO2 hydrogenation,
direct evidence for water-assisted step(s) is still missing, and little is known about the ef-
fect of water on the methional selectivity.
Kunming University of Science and Technology,
Kunming 650093, China
2Faculty of Metallurgical and Energy Engineering,
Kunming University of Science and Technology,
Kunming 650093, China
3Department of Earth and Environmental
Engineering, Columbia University, New York, NY
10027, USA
4Chemistry Division, Brookhaven National
Laboratory, Upton, New York, NY 11973, USA
5School of Pharmacy and Chemistry, Dali
University, Dali 671003, China
Here, we provide strong evidence for deeper understanding of the role of water
during CO2 hydrogenation over CZZ catalysts by using in situ Fourier transform
infrared and transient isotope-tracing experiments. Water is an actual participant
in the transformation of intermediates (i.e., methoxy), and a suitable amount of
additional water in the system can improve the methanol generation. The enhance-
ment on the water vapor diffusion in catalysts by forming a three-dimensional (3D)-
ordered macroporous (3DOM) structure strongly promotes the methanol selectivity
without lowering the CO2 conversion.
6Department of Chemical Engineering, Columbia
University, New York, NY 10027, USA
7Lead Contact
*Correspondence:
2
Chem 6, 1–12, February 13, 2020