Insight into the Effect of Sn on CO and Formic Acid Oxidation at PtSn Catalysts

Abstract

The role of Sn on the catalytic activity for CO and formic acid oxidation is studied by comparing the activities of differently treated PtSn/C and Pt/C catalysts. The catalysts are prepared by a microwave-assisted polyol synthesis method. As revealed by scanning tunneling and transmission electron microscopic (STM and TEM) characterization, the outcomes of the synthesis procedure for both Pt and PtSn are small particles, ∼1.5 nm in diameter. Upon deposition on the carbon support, the particle size increases to ∼2.5 nm due to sintering. X-ray diffraction (XRD) analysis shows that PtSn/C has a low alloying degree and is mainly composed of Pt and Pt 3 Sn phases. The remaining Sn is present in the form of very small tin oxide particles. Different surfaces are obtained by double-layer, oxide, and CO annealing of the Pt/C and PtSn/C catalysts and by modifying the CO-annealed surfaces with irreversibly adsorbed tin, Sn irr . The presence of Sn in any form (oxide, alloyed, or Sn irr ) on the surface shifts the onset potential for the CO oxidation negatively by more than 0.4 V in comparison to equivalently treated Pt/C catalysts. For the CO-annealed PtSn/C catalyst, a so-called skeleton structure, Sn is present only in the subsurface layers. The subsurface Sn has a mild effect on the CO activity, and hence the onset potential is only marginally shifted to cathodic potentials by ∼50 mV compared to that on Pt/C. The formic acid oxidation is enhanced at any of the PtSn/C surfaces with Sn in the surface layer. The activity enhancement is explained by a reduced CO poisoning of the surface Pt sites. As a consequence, the current is not entering plateau as on the Pt/C catalysts. Furthermore, the skeleton PtSn/C is ∼2 times more active than similarly treated Pt/C. The results have been substantiated and explained by comprehensive density functional theory (DFT) simulations. The DFT results indicate that the increased oxidation rates are not only due to surface Sn but also due to a weakened CO binding in the vicinity of the surface SnOH x moieties and SnO 2 particles.

Glycerol Electrooxidation on Platinum-Tin Electrodeposited Films: Inducing Changes in Surface Composition by Cyclic Voltammetry

“…Hence, the surface reconstructs and becomes very rough and full of defects. In this new surface, Sn is present only in the subsurface layers [20]. On the other hand, in the same work, the authors also showed that the degradation of these structures results in a significant loss of ECSA [20], such as we observe in the present case.…”

“…In this new surface, Sn is present only in the subsurface layers [20]. On the other hand, in the same work, the authors also showed that the degradation of these structures results in a significant loss of ECSA [20], such as we observe in the present case. Here, the loss of ECSA is particularly evident for those electrodes richer in Sn, as shown in Table 2.…”

“…Here, it is important to point out that we are not assuming that Sn adsorbs CO; hence, the surface area estimated by CO stripping refers only to Pt. Regardless, our results can be understood on the basis of a mechanism similar to the formation of the Bskeleton structure,^as proposed by Stevanovic et al [20]. According to these authors, a Bskeleton^structure is formed after the dissolution of Sn from PtSn/C (during CO annealing).…”

“…In both figures, we note that CO is oxidized over a broad potential range with two poorly resolved peaks. Stevanovic et al attributed the peak at low potentials to the Pt-Sn phase and the peak at higher potentials to the Pt phase [20]. As the electrodes are cycled 100 and 500 times (Figs.…”

“…4. Assuming that this BCO preoxidation^is due to the presence of Sn atoms [20], its disappearance indicates that surface Sn decreases with cycling. However, we cannot affirm that this phenomenon is solely provoked by Sn dissolution because the currents associated to the double layer region are still significantly large for PtSn, even after they are cycled (Figs.…”