Science - USA (2021-07-09)

To further confirm the presence of PtSn
surface alloys, XPS of the freshly reduced
Pt 1 Sn 1 /SiO 2 catalyst showed that the atomic
ratio of Pt and Sn was ~1, consistent with the
nominal loading (fig. S8). Figure 4B shows the
XPS spectra associated with the Pt 4f7/2elec-
tronic state, demonstrating that the Pt atoms
were in the metallic Pt(0) state. Additionally,
the Pt 4f7/2peak shifted toward lower binding
energy (70.5 eV) relative to a monometallic Pt
reference (71.0 eV), in agreement with prior
reports that the formation of Pt–Sn caused a
slight electron transfer from Sn to Pt that in-
creased the local electron density on Pt and
local Coulombic repulsion, thereby lowering
the electron binding energy ( 60 – 62 ). Further-
more, the Sn 3d5/2XPS spectra in Fig. 4C show
that most Sn atoms were metallic Sn, and a
small fraction of Sn existed as SnO or SnO 2 ,
further confirming the formation of PtSn in-
termetallic NPs ( 63 , 64 ).
The real-space Pt LIIIEXAFS spectra for the
Pt 1 Sn 1 /SiO 2 catalyst (Fig. 4D) are substantially
different from the spectra of monometallic Pt
and indicate the mixing of Pt and Sn atoms.
Thebest-fitspectrumforthePt 1 Sn 1 /SiO 2 catalyst
is characterized by the presence of the Pt-Pt and
Pt-Sn scattering paths, consistent with the for-
mation of PtSn intermetallic phases. The param-
eters used to obtain the best-fit spectra are
provided in the Fig. 4D caption.
When assigning EXAFS spectra to a geomet-
ric structure, it is instructive to compare the
measured bond distances to those correspond-
ing to thermodynamically stable phases of a
material. For Pt 3 Sn, both Pt-Pt and Pt-Sn bond
distances are 2.83 Å. By contrast, the Pt-Pt bond
distance in PtSn is 2.72 Å, whereas the Pt-Sn
bond distance is 2.73 Å ( 63 , 64 ). The Pt-Pt and
Pt-Sn bond distances experimentally measured
for the Pt 1 Sn 1 /SiO 2 catalyst (2.75 and 2.71 Å,
respectively) suggest that the catalyst was likely
a mix of Pt 3 Sn and PtSn, with a higher concen-
tration of the PtSn phase, as no peaks were
observed at 2.83 Å.
Data in Fig. 4, E and F, show the XANES
spectra associated with the Pt LIII-edge and
Sn K-edge, respectively, of the Pt 1 Sn 1 /SiO 2 cat-
alyst and relevant controls. The Pt LIII-edge
XANES spectra in Fig. 4E are consistent with
the analysis of the EXAFS data, indicating that
the mixing of Pt and Sn took place in the re-
duced Pt 1 Sn 1 /SiO 2 catalyst. Indeed, the forma-
tion of a stable Pt-Sn phase was complete by
390°C—above this temperature, no further
changes were observed (Fig. 4E, inset). A slight
increase in the Pt LIII-edge energy and a de-
crease in white-line intensity and its broad-
ening observed for Pt 1 Sn 1 /SiO 2 compared with
pure Pt could be attributed to the forma-
tion of the Pt-Sn phase, consistent with
XPS results that showed slight electron
transfer from Sn to Pt. Likewise, the slight
increase in Sn K-edge energy and the in-

crease in the white-line intensity relative

to Sn foil in (Fig. 4F) indicated that Sn existed

in both metallic and oxide forms on the cat-

alyst surface.

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ACKNOWLEDGMENTS

We thank R. J. Meyer, S. F. Liu, and J. R. Johnson at ExxonMobil for

their help in obtaining EXAFS data and fruitful discussion on

reaction data and catalyst characterization. We thank T. Ma and

H. Sun at University of Michigan for their help in collecting TEM

data.Funding:This material is based on work supported by RAPID

under award DE-EE0007888. The materials characterization part

of the work was supported by the US DOE Office of Basic Energy

Sciences, Division of Chemical Sciences (DE-SC0021008). The

authors acknowledge the financial support of the University of

Michigan College of Engineering and technical support from the

Michigan Center for Materials Characterization. R.A. acknowledges

support from the National Science Foundation Graduate Research

Fellowship under grant DGE 1256260.Author contributions:S.L.

and A.H.M. conceived the project and designed the experiments.

A.H.M., R.A., J.W., and V.O.I. designed and constructed the reactor

used to obtain reaction data. A.H.M. and R.A. performed the

propane dehydrogenation experiments. R.A., A.H.M., J.W., and

V.O.I. performed catalyst characterization. All authors analyzed the

reaction data and catalyst characterization data. S.L., A.H.M.,

and R.A. wrote the paper. All authors edited the manuscript.

Competing interests:The authors declare no competing interests.

Data and materials availability:All data needed to evaluate the

conclusions in the paper are present in the paper and/or the

supplementary materials. Additional data related to this paper may

be requested from the authors.

SUPPLEMENTARY MATERIALS

science.sciencemag.org/content/373/6551/217/suppl/DC1

Materials and Methods

Supplementary Text

Figs. S1 to S10

Tables S1 to S3

References

27 January 2021; accepted 28 May 2021

10.1126/science.abg7894

222 9JULY2021•VOL 373 ISSUE 6551 sciencemag.org SCIENCE

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