Science - USA (2022-05-06)

surface. Spectroscopically, we have demon-
strated that Zn alloyed into the surface of Cu(211)
to form a structure in which the Zn-Zn inter-
action is minimized. This observation is en-
tirely consistent with the work of Behrenset al.
( 4 ), which suggests that Zn alloying into the
Cu steps is energetically favorable.
With increasing temperature under reac-
tion conditions, the surface gradually depleted
formate and methoxy species, which is ex-
pected for a transient population of reaction
intermediates. We inferred that the catalyst
was very active in this state and that the ob-
served low coverages were consistent with the
findings of Kuldet al.( 3 ). We conclude that
the oxidation state of Zn was dictated by the
redox chemical potential of the gas phase,
rationalizing the autocatalytic behavior ob-
served by Thraneet al.( 42 ). Surface-alloyed
Zn and ZnOxacted as a dynamically revers-
ible acceptor and donor, respectively, of O for
the O atoms produced by the reductive activ-
ation of CO 2.
In a similar manner, Zn appeared to inhibit
Cu oxidation, an effect related to the higher
oxophilicity of Zn relative to Cu. Under CO-
rich conditions (Fig. 5A), ZnO transformed
into Zn at temperatures as low as 60°C by
the formation of CO 2 and H 2 O. If, alterna-
tively, CO 2 is increasingly admixed to the re-
action together with CO (Fig. 5B), it can access
the active interfacial and/or metallic Zn sites
and contribute to their oxidative depletion.
The admixture of CO 2 to the reaction affected
the local ZnO⇄Zn redox equilibrium by push-
ing it toward ZnO with increasing admixture
of CO 2. In the absence of CO (Fig. 5C), mostly
ZnO was stabilized and even enriched at 180°C.
Formation of ZnO occurred at the expense of
an active alloyed surface, which was effectively
needed for both product-forming reactions—
CO 2 reduction and direct CO hydrogenation—
to proceed.
We can conclude that the most-active state
is stabilized in the simultaneous presence of
balanced amounts of CO, CO 2 , and H 2. An
enhanced CO:CO 2 ratio kept the surface more

metallic, and allowed methanol formation

through CO 2 to proceed more efficiently, as

seen from isotope labeling experiments ( 16 ).

The results thereby indicate that the most-

active state involved an optimized surface-

near abundance of redox-active surface Zn-Cu

alloy sites for the reduction of CO 2 , fully con-

sistent with the proposal from Behrenset al.

( 4 ) and other studies ( 3 , 11 , 43 ), whereas com-

putation studies indicate that Zn poisons CO

hydrogenation ( 15 ).

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ACKNOWLEDGMENTS

We thank A. Gloskovskii for experimental support at the beamline

and G. L. Silva Rodrigues and L. G. M. Pettersson for helpful

discussions.Funding:This work was supported by the Swedish

Research Council under grant no. 2013-8823, the Knut & Alice

Wallenberg (KAW) foundation under grant no. 2016.0042, the

Global Climate and Energy Project (GCEP) at Stanford University,

and the Swedish Foundation for strategic research (Stiftelsen

för Strategisk Forskning, SSF) under proj. nr. ITM 17-0034. We

acknowledge DESY (Hamburg, Germany), a member of the

Helmholtz Association HGF, for the provision of experimental

facilities. Parts of this research were carried out at PETRA III using

beamline P22. Beamtime was allocated for proposals I-2018014

EC and II-20190003 EC.Author contributions:P.A. and B.K.

designed the study for beamtime I and II; P.A. and A.N. designed

the study for beamtime III; P.A., B.K., and C.R. designed the

experimental parameters for beamtime I, P.A. defined the

parameters for beamtime II and III; P.A., B.K., C.R., N.K., and

T.G. designed and implemented the evaporator; P.A., B.K., N.K.,

T.G., P.L., K.P., D.B., C.R., H.-Y.W., M.So., M.B., D.D., M.Sh.,

C.M.G., J.H.S., and J.G. conducted the experiments; P.A. analyzed

the XPS data; D.D. analyzed the mass spectrometry data; P.A.

created the figures; P.A., B.K., and A.N. interpreted the data;

and P.A., A.N., B.K., and D.D. wrote the manuscript.Competing

interests:The authors declare no competing interests.

Data materials availability:All of the data necessary for

evaluating the conclusions of the study are included in the

supplementary materials.

SUPPLEMENTARY MATERIALS

science.org/doi/10.1126/science.abj7747

Materials and Methods

Supplementary Text

Figs. S1 to S16

Table S1

References ( 44 – 55 )

Data S1

Submitted 1 June 2021; resubmitted 9 December 2021

Accepted 29 March 2022

10.1126/science.abj7747

608 6 MAY 2022•VOL 376 ISSUE 6593 science.orgSCIENCE

Fig. 5. Schematic illustration of the reaction mechanism.(AtoC) Behavior of the Zn/ZnO/Cu(211) surface as dependent on reaction mixture and conditions,
which range from CO-rich, reducing conditions in (A) to CO 2 -rich, more-oxidizing conditions in (C). These illustrations show how temperature and gas mixture critically
affect the ZnO⇄Zn equilibrium.

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