Data in Fig. 3A show that the process of
mixing chloroplatinic acid (H 2 PtCl 6 ) and tin(II)
chloride (SnCl 2 ) in 0.1 M hydrochloric acid
solution formed a heterometallic Pt-Sn coordi-
nation complex. The UV-Vis absorption spec-
trum of H 2 PtCl 6 is characterized by a prominent
absorption peak at ~260 nm; SnCl 2 exhibits no
absorption in the visible range. The combined
precursor solution shows several additional UV-
Vis absorption peaks at 279, 363, 420, and 488 nm
that are characteristic of a heterometallic Pt-Sn
complex ( 55 ), indicating the mixing of Sn and
Pt atoms before reduction and growth of NPs
on the support.
A bright-field TEM image of Pt-Sn NPs formed
on SiO 2 upon the reduction of the Pt-Sn complex
shows high metal dispersion on the support, with
an average NP size of ~1.3 ± 0.6 nm (Fig. 3B). The
NP size distribution (Fig. 3C) shows that most
NPswere<2nm.TheXRDspectrainFig.3D
show that the formed NPs were essentially alloys
of Pt and Sn. This finding is supported by the
presence of the XRD peaks corresponding to
Pt 1 Sn 1 (41.8° and 44.1°) and Pt 3 Sn 1 (39.0° and
45.3°) stoichiometries.
To establish that the mixing of Sn and Pt
atoms in PtSn NPs extended to their surface,
we used DRIFTS to measure the infrared ab-
sorption spectra of carbon monoxide (CO), as
its adsorption on Pt surfaces has been well
characterized. The CO-DRIFTS spectra for Pt/
SiO 2 and Pt 1 Sn 1 /SiO 2 (Fig. 4A) revealed linear
(on top) and bridge-bonded CO on Pt/SiO 2 with
infrared adsorption peaks at 2073 and 1821 cm−^1 ,respectively ( 56 , 57 ). On Pt 1 Sn 1 /SiO 2 , the bridge-
bonded CO was not present, and the linearly
bonded peak had much lower intensity. Because
CO does not adsorb at the bridge sites between
Sn and Pt, the data suggest that Sn broke the
ensembles of Pt atoms on the NP surfaces and
created a checkerboard Pt-Sn surface structure
( 58 ). The decreased intensity of the linearly
bonded CO adsorption peak was consistent
with reduced surface coverage of CO on the
PtSn compared with Pt ( 59 ), further demon-
strating the mixing of Pt and Sn in Pt 1 Sn 1 /SiO 2.
Introduction of Sn to the catalyst also slightly
shifted the vibrational frequency of the linearly
bonded CO band to 2078 cm−^1 , indicating the
perturbation of the electronic structure of sur-
face Pt atoms by neighboring Sn atoms ( 56 ).220 9JULY2021•VOL 373 ISSUE 6551 sciencemag.org SCIENCE
Fig. 3. Pt 1 Sn 1 /SiO 2 catalysts characterization.(A) UV-Vis absorption
spectrum of precursor solutions. (Inset) Photographs of chloroplatinic acid
solution (top) and a mixture of chloroplatinic acid and tin(II) chloride (bottom).
a.u., arbitrary units. (B) Bright-field TEM image of Pt-Sn NPs formed on SiO 2.
(Inset) Enlarged area of the image identifying small Pt-Sn NPs. (C) Particle
size distribution of PtSn NPs shown in (B). (D) X-ray diffraction pattern for
Pt 1 Sn 1 /SiO 2. The peak positions for Pt, PtSn, and Pt 3 Sn were obtained from the
International Centre for Diffraction Data. 2q, diffraction angle.RESEARCH | REPORTS