Thermal Treatment of MgO-Protected Ptsn/C Catalyst: Stability and Ethanol Oxidation

Abstract

The direct ethanol fuel cell (DEFC) is an alternative solution to energy-generation problems. However, the ethanol oxidation kinetics is slow and inefficient, due to the difficult to break C–C bond at practical potentials. Furthermore, catalysts instability is a problem and it is associated with the dissolution of non-noble metal and the nanoparticles agglomeration. Therefore, the studies of stability and influence of physical and chemical properties in catalytic activity of materials are very important to the development of efficient catalysts. Even the most thoroughly studied PtSn catalyst continue to lack understanding and it is need more fundamental research to improve the performance of PtSn catalyst in fuel cell. Herein, we present a comparative study of physical and electronic properties and catalytic activities for ethanol oxidation of carbon-supported binary (PtSn) metallic nanoparticles. The PtSn-based catalysts were synthesized and coated with MgO by a polyol method. Subsequently, nanoparticles were supported on carbon powder (Vulcan XC-72) and divided into two parts. Each part was treated at 500ºC for 1 hour with different atmospheres (Hydrogen and Argon). After thermal treatment, the MgO were removed with H2SO4 solution. The materials were metal load of 20% wt, (metal = Pt, Sn). The catalysts were characterized by X-ray diffraction (XRD), transmission electronic microscopy (TEM) and EDX. Their catalytic activities for ethanol oxidation reaction were studied and the performance of stability test was evaluated by potential sweeps. It was observed XRD patterns with typical peaks of face-centered cubic structure of Pt and the catalysts showed different fases and alloy amount. The atomic composition PtSn was about 60:40. TEM analysis showed particles homogenously dispersed in carbon support and indicated average particle sizes of 3,9 and 2,9 nm for Hydrogen and Argon, respectively. The MgO coatings were effective as the particles do not accret during thermal treatment, according to DRX and TEM analyses. The voltammetry profile and value of currents were similar for all samples. Catalysts showed better performance than PtSn ETEK for CO adsorved oxidation after and before stability test. It was observed an increasing on current density in H adsorption/desorption region after stability test. It is known that CO electro-oxidation reaction on Pt is structure-sensitive and considering the sweep linear profile of oxidation CO difference, it is possible to see a change in surface structure of Sn-based catalysts after stability test. Possibly the change of surface structure is due to Sn dissolution during voltammetry cyclic sweeping and/our coalescence of nanoparticles. It was confirmed by highest on set potential of CO oxidation and decrease current density of ethanol oxidation after stability test. Two important points are evident: first, activity for CO oxidation improved with less Sn in the alloy. Second, the stability decrease can be influenced by the higher amount of Sn in the alloy. In conclusion, this study showed the catalysts with more Sn oxide had better stability and the PtSn/C catalyst treated in Hydrogen was the most active for ethanol oxidation. The sample treated in Argon was more effective for CO oxidation, evidencing that the surface processes involved in the bifunctional mechanism are relevant to the oxidation of CO.