Titania-silica mixed oxides are intriguing catalysts and catalyst supports since their surface reactivities depend strongly on composition and homogeneity of mixing. In this review, the synthesis, characterization, and catalytic activity of titania-silica are described, and important structure/property relationships for these materials are emphasized.

A series of high surface area, microporous, T i S i mixed oxides having a wide range of elemental compositions was prepared by a sol-gel method. Since the materials were noncrystalline, X-ray absorption spectroscopy a t the Ti K edge was used to probe the first-shell coordination environment around Ti. Incorporation of Si into the mixed oxide shortened the average Ti-0 bond distance derived from EXAFS and disrupted the normal octahedral coordination of pure titania. The shortest Ti-O bond distance was 1.82 A and was attributed to Ti primarily residing in tetrahedral sites, which was also indicated by a significant pre-edge peak present in Ti K edge XANES. Shifts in the U V absorption edges for the mixed oxides were attributed to titania domain sizes decreasing with increasing Si content in accord with the well-known quantum size effect. The smallest domain size was estimated as less than 1 nm for a sample with a Ti:Si atomic ratio of 1:8. Lattice vibrations in the materials were probed with FT-Raman and FT-IR absorption spectroscopies, and the results wereconsistent with a shortening of the Ti-0 bond and the formation of T i -O S i linkages in the mixed oxides. In situ experiments showed that adsorbed water profoundly affects the local environment around surface Ti atoms.

PtMo alloy and MoO(x)@Pt core-shell nanoparticles (NPs) were successfully synthesized by a chemical coreduction and sequential chemical reduction method, respectively. Both the carbon-supported alloy and core-shell NPs show substantially higher CO tolerance, compared to the commercialized E-TEK PtRu alloy and Pt catalyst. These novel nanocatalysts can be potentially used as highly CO-tolerant anode electrocatalysts in proton exchange membrane fuel cells.

The poisoning of Pt electrocatalysts by carbon monoxide (CO), a major impurity in H 2 fuels derived from reformed hydrocarbons, limits the commercialization of Nafion-based proton-exchange membrane (PEM) fuel cells. [1] To mitigate the CO-poisoning effect in PEM fuel cells, one straightforward strategy is to replace Pt with Pt-based bimetallic electrocatalysts that can tolerate small amounts of CO (typically < 100 ppm). Three promising classes of Pt-based bimetallic NPs have been investigated for this purpose: 1) Pt-M alloys (e.g. PtRu) with metal atoms randomly distributed in face-centered-cubic (fcc) lattices, [2][3][4] 2) ordered intermetallics (e.g. PtBi) that have well-defined compositions and crystal structures, [5] and 3) core-shell bimetallics, e.g. Ru-core/Ptshell (Ru@Pt) in which Pt is concentrated on the Ru nanoparticle surface. [6] Each of these bimetallic architectures has potential advantages and disadvantages in terms of synthetic accessibility, performance, and stability in electrocatalytic applications. However, a direct comparison of these three architectures in a specific Pt-M series has not been reported.We describe here the synthesis, characterization, electrocatalytic performance, and stabilities of PtSn alloy, core-shell, and intermetallic nanoparticles (NPs) of the same composition and size. These studies show that the PtSn intermetallic is significantly more stable and has superior performance relative to the PtSn alloy in acidic electrolyte solutions. In addition, the intermetallic can be converted to a PtSn@Pt core-shell particle through a successive potential cycling process in CO-saturated H 2 SO 4 solutions, while no such coreshell structure forms from PtSn random alloys. The PtSn@Pt and PtSn intermetallic NP electrocatalysts show significantly better CO -tolerance than commercial E-TEK PtRu and Pt catalysts but presumably involve different CO oxidation mechanisms.PtSn intermetallic NPs were prepared by the co-reduction of [Pt(acac) 2 ] (acac = acetylacetonate) and SnCl 4 in octadecene by using NaBEt 3 H, a strong reducing agent. Oleylamine and oleic acid were employed as capping agents to control the particle size and shape. The transmission electron microscopy (TEM) image of as-prepared NPs (Figure 1 a) shows the particles have an average size of 3.5 nm with a narrow size distribution. To make electrocatalysts, the as-prepared intermetallic particles were loaded onto carbon supports and subsequently heated at 450 8C in Ar/H 2 (5 % H 2 ) to remove the capping agents. Despite the high annealing temperatures, no significant particle growth or aggregation was observed. Most of the post-treated NPs were still in the size range of 3-5 nm, with only a few particles larger than 5 nm (Figure 1 b). The NP lattice fringes (insert of Figure 1 b) show an average lattice separation of 0.21 nm, which corresponds to the (102) plane of the hexagonal (P6 3 /mmc) PtSn intermetallic. The Xray diffraction (XRD) pattern of the intermetallic catalyst (Figure 1 c) displays the distinct hexagonal pattern a...

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