PtOx-SnOx-TiO2 catalyst system for methanol photocatalytic reforming: Influence of cocatalysts on the hydrogen production

Effects of modification of PtOx-TiO2 photocatalysts by tin were elucidated by exploring relationships between the structural properties of variously prepared tin-loaded catalysts and their catalytic activity in methanol photocatalytic reforming. Tin free and amorphous tin-oxide decorated TiO2 samples were prepared by sol-gel method from titanium-isopropoxide. In other approach, Sn was loaded onto the sol-gel prepared TiO2 by impregnation followed by calcination. Pt was introduced by impregnation followed by either reduction in H2 at 400 °C or calcination at 300 °C. TEM, XRD and Raman spectroscopic measurements proved that TiO2 existed in the form of aggregates of polycrystalline anatase with primary particle size of 15–20 nm in all samples. Photocatalytic hydrogen production was influenced by the combined effect of many parameters. Both the presence of Sn and the way of Pt co-catalyst formation played important role in the activity of these photocatalysts. The Sn introduction by both sol-gel method and impregnation clearly enhanced the photocatalytic activity. 1H MAS NMR measurements revealed that the Sn introduction reduced the amount of the terminal Ti-OH groups of relatively basic character considered to be unfavorable for the photocatalytic reaction. Presence of SnOx decreased the signal of the undesirable vacancies observed by ESR. Furthermore surface SnOx enhanced the dispersion of Pt. Formation of the Pt co-catalyst by calcination was more favorable than by H2 treatment. In case of the calcined samples in situ reduction of the Pt nanoparticles at the beginning of the photocatalytic reaction was found to be favorable for the hydrogen production. The relatively modest photocatalytical activity obtained after high temperature H2 treatment could be related to at least two processes in this system: (i) creation of unfavorable oxygen vacancies and (ii) segregation of SnOx to the surface of the Pt cocatalyst as the result of the air exposure of the alloy type Pt-Sn nanoparticles formed during the H2 treatment, resulting in a decreased number of active sites for reduction of H+

Composites of Titanium–Molybdenum Mixed Oxides and Non-Traditional Carbon Materials: Innovative Supports for Platinum Electrocatalysts for Polymer Electrolyte Membrane Fuel Cells

TiO2-based mixed oxide–carbon composite support for Pt electrocatalysts provides higher stability and CO tolerance under the working conditions of polymer electrolyte membrane fuel cells compared to traditional carbon supports. Non-traditional carbon materials like graphene nanoplatelets and graphite oxide used as the carbonaceous component of the composite can contribute to its affordability and/or functionality. Ti(1−x)MoxO2-C composites involving these carbon materials were prepared through a sol–gel route; the effect of the extension of the procedure through a solvothermal treatment step was assessed. Both supports and supported Pt catalysts were characterized by physicochemical methods. Electrochemical behavior of the catalysts in terms of stability, activity, and CO tolerance was studied. Solvothermal treatment decreased the fracture of graphite oxide plates and enhanced the formation of a reduced graphene oxide-like structure, resulting in an electrically more conductive and more stable catalyst. In parallel, solvothermal treatment enhanced the growth of mixed oxide crystallites, decreasing the chance of formation of Pt–oxide–carbon triple junctions, resulting in somewhat less CO tolerance. The electrocatalyst containing graphene nanoplatelets, along with good stability, has the highest activity in oxygen reduction reaction compared to the other composite-supported catalysts

Strategies to Improve CO Tolerance and Corrosion Resistance of Pt Electrocatalysts for Polymer Electrolyte Membrane Fuel Cells: Sn-doping of the Mixed Oxide–Carbon Composite Support

Design of composite support materials based on Sn-doped TiO2 and carbon is one of the strategies to develop corrosion-resistant and CO-tolerant Pt electrocatalysts for polymer electrolyte membrane (PEM) fuel cells. As the synthesis methodology may have crucial influence on the structural and functional properties of the composites, different preparation routes for the novel support materials are explored and compared. Ti(1-x)SnxO2–C (x: 0.1–0.3) composites with different mixed oxide/carbon ratios were prepared by two sol-gel-based synthesis routes, namely (i) the introduction of a Sn precursor after the formation of the TiO2-rutile nuclei on the carbon backbone (route A), and (ii) simultaneous introduction of Ti and Sn precursors, resulting in good mixing of the Sn- and Ti-sol before the addition of the carbon (route B). The bulk and surface microstructure of the composites and the electrocatalysts obtained by their Pt-loading were investigated in detail. The incorporation of tin into the TiO2-rutile unit cell was confirmed by X-ray powder diffraction and Raman spectroscopy; the results indicated doping levels in good accordance with the amount of tin precursor. The advantages of composites and Pt electrocatalysts obtained via synthesis route B were that they do not contain segregated Sn0 or SnO2 phases, have a more homogeneous/uniform mixed oxide distribution over the carbon backbone, and the electrochemically active surface area values (~60–80 m2/gPt) are twice as high as those of catalysts with similar compositions synthesized by method A. A common feature of the composites prepared by routes A and B was the presence of a tin oxide-rich overlayer identified by X-ray photoelectron spectroscopy. As a consequence, the electrocatalytic behavior of the catalysts was not influenced by the Ti/Sn ratio and was mainly dependent on the synthesis method used in the preparation of composite support materials. Elemental maps confirmed the formation of areas where Pt and the Sn doping element were in atomic proximity to each other, which means a favorable interaction either for the bifunctional mechanism or the electronic ligand effect. An increase in carbon content in composite materials led to an increase in both catalytic activity and long-term stability. The results of electrochemical studies showed that Sn-containing Pt catalysts with a high carbon content (75 wt%) are the most promising for potential use both as an anode and a cathode for PEM fuel cells