Role of SnO₂ in the Bifunctional Mechanism of CO Oxidation at Pt‐SnO₂ Electrocatalysts

Abstract

Pt-Sn bimetallic catalysts, especially Pt-Sn alloys, are considered highly CO-tolerant and are thus candidates for reformate derived hydrogen oxidation and for direct oxidation of fuel cell molecules. However, it remains unclear if this CO-tolerance originates from Sn in the Pt-Sn alloy or whether SnO 2 , present as a separate phase, also contributes. In this work, a carbonsupported Pt-SnO 2 was carefully synthesized to avoid the formation of Pt-Sn alloy phases. The resulting structure was analysed by scanning transmission electron microscopy (STEM) and detailed X-ray absorption spectroscopy (XAS). CO oxidation voltammograms of the Pt-SnO 2 /C and other SnO 2 -modified Pt surfaces unambiguously suggest that a bifunctional mechanism is indeed operative at such Pt-SnO 2 catalysts for stable CO oxidation at low overpotentials. The results from these studies suggest that the bifunctional mechanism can be attributed to the co-catalysis role of SnO 2 , in which the surface hydroxide of SnO 2 (Sn-OH) reacts with CO adsorbed on Pt surface (Pt-CO ads ) and regenerates via a Sn II /Sn IV reversible redox couple (À 0.2-0.3 V vs. reversible hydrogen electrode).

Selective Catalytic Behavior Induced by Crystal‐Phase Transformation in Well‐Defined Bimetallic Pt‐Sn Nanocrystals

“…EXAFS analysis shows that the average coordination environment of Sn–Pt is 2.2 ± 0.4 in PtSn/Al 2 O 3 and 5.1 ± 1.0 in Pt 3 Sn/Al 2 O 3 (Figure 5, Figure S9, Table S6, Supporting Information), with Sn–Pt bond lengths of 2.70 ± 0.08 Å and 2.78 ± 0.09 Å, respectively, confirming the formation of an alloyed Pt‐Sn intermetallic structure. [ 42,43 ] The slightly longer Sn‐Pt bond in Pt 3 Sn/Al 2 O 3 compared to that of PtSn/Al 2 O 3 is consistent with the structural models for fcc‐Pt 3 Sn and hcp‐PtSn (Sn–Pt bond length is 2.87 Å in Pt 3 Sn and 2.78 Å in PtSn, see Scheme S1, Supporting Information). Overall, the results indicate the distinct structural differences of the reduced (activated) PtSn/Al 2 O 3 and the Pt 3 Sn/Al 2 O 3 catalysts compared to the Pt/Al 2 O 3 with the formation of hcp‐PtSn/Al 2 O 3 and fcc‐Pt 3 Sn/Al 2 O 3 .…”

“…EXAFS analysis shows that the average coordination environment of Sn-Pt is 2.2 ± 0.4 in PtSn/Al 2 O 3 and 5.1 ± 1.0 in Pt 3 Sn/Al 2 O 3 (Figure 5, Figure S9, Table S6, Supporting Information), with Sn-Pt bond lengths of 2.70 ± 0.08 Å and 2.78 ± 0.09 Å, respectively, confirming the formation of an alloyed Pt-Sn intermetallic structure. [42,43] The slightly longer Sn-Pt bond in Pt Pt-Sn alloy formed, (detailed in the Supplementary Information, Equations S6-S7, Supporting Information), extracted from EXAFS analysis; indicates that both bimetallic Pt-Sn systems are comprised of the expected homogenous distribution of Pt and Sn atoms.…”

The Effect of SnO₂ and Rh on Pt Nanowire Catalysts for Ethanol Oxidation

“…This peak is more prominent for Pt/SnO2/C, indicating oxidation of adsorbed CO on Pt atoms adjacent to SnO2. However, the peak is less pronounced for Pt-Rh/SnO2/C and Pt-Rh/SnO2(commercial)/C, suggesting reduced contact between Pt and SnO2 due to Rh deposition on the exposed Pt surface [28,29]. During CO electro-oxidation, Tien et al [28] investigated various configurations of mixtures containing Pt/C and SnO2 across a range of molar ratios of Pt to SnO2 (3:1, 1:1, and 1:3).…”