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Commercially available Pt metal catalysts supported on carbon black (Pt/CB) for polymer electrolyte fuel cell (PEFC) cathodes were covered with silica layers to improve their durability under the severe cathode operating conditions. The Pt metal particles in the Pt/CB catalyst grew in size during the accelerated durability tests (potential cycling between 0.6 and 1.0 V vs. RHE in an aqueous HClO 4 electrolyte). Thus, the Pt/CB catalyst was seriously deactivated for the oxygen reduction reaction over the course of the durability tests. In contrast, the silica layers, which wrapped around the Pt metal particles in the silica-coated Pt/CB catalyst, prevented the migration of the Pt metal particles on the carbon supports and the diffusion of Pt cations out of the silica layers. Thus, the silica-coated Pt/CB catalysts maintained a high activity for the oxygen reduction reaction over the course of the durability tests. In addition, the silica-coated Pt/CB prepared from methyltriethoxysilane showed a higher activity than that prepared from tetraethoxysilane. The porous structures and hydrophobicity of silica prepared from methyltriethoxysilane promoted the diffusion of oxygen and water molecules in the silica layers of the silica-coated Pt catalysts. [ABSTRACT FROM AUTHOR]

Abstract: Carbon black-supported Pt (Pt/CB) catalysts that have been used at the cathode in state of the art polymer electrolyte fuel cell (PEFC) were covered with silica layers to improve the durability of the Pt catalysts under PEFC cathode conditions. The durability of silica-coated Pt/CB to potential cycling between 0.6 and 0.9V (vs. reversible hydrogen electrode (RHE)) was strongly dependent on the thickness of the silica layers, i.e., the durability of Pt/CB improved after coverage with thick silica layers. However, the coverage of the whole surface of the Pt/CB catalysts with silica layers produced electrochemically inactive Pt catalysts. Silica-coated Pt/CB catalysts with an optimal silica layer thickness showed similar activity for the oxygen reduction reaction compared to Pt/CB catalysts without a silica coating, and they had excellent durability at the cathode in a PEFC single cell. Coverage with silica layers improved the durability of the Pt/CB cathode catalysts without a decrease in the catalytic activity for the oxygen reduction reaction. [Copyright &y& Elsevier]

Abstract: Pd metal supported on carbon nanotubes (Pd/CNT) for use as a cathode catalyst in polymer electrolyte fuel cells was covered with silica layers a few nanometers thick to improve the durability of the catalyst under severe cathode conditions. The silica-coated Pd/CNT cathode catalyst showed similar activity for the oxygen reduction to Pd/CNT despite coverage of the Pd metal with a silica insulator. In addition, the silica-coated Pd/CNT was highly durable during potential cycling between 0.05 and 1.20V (vs. reversible hydrogen electrode) in aqueous HClO4 while Pd/CNT suffered serious deactivation under the same conditions. The silica layers in the silica-coated Pd/CNT prevent the diffusion of dissolved Pd species from the Pd metal out of the layers. Therefore, silica-coated Pd/CNT was highly durable under the cathode conditions. Furthermore, coverage of Pd/CNT with silica layers inhibited the two-electron reduction of oxygen to H2O2. [Copyright &y& Elsevier]

Abstract: Pt-based alloy catalysts such as carbon black-supported Pt–Co catalysts (Pt–Co/CB) are used in the oxygen reduction reaction in polymer-electrolyte fuel cells (PEFCs) and are generally treated at high temperatures to allow for alloy formation. However, treatment of these catalysts at high temperatures results in serious aggregation of alloy particles. In this study, a Pt–Co/CB catalyst was covered with silica layers to inhibit the aggregation of alloy particles during treatment of the catalyst at high temperature for alloy formation. Coverage of Pt–Co/CB with silica promoted Pt–Co alloy formation with a high alloying degree and a small particle size, whereas the alloying degree of Pt–Co alloys in Pt–Co/CB without silica-coating was low, and these alloy particles were seriously aggregated during treatment at high temperature. These silica-coated Pt–Co/CB catalysts showed high activity and excellent durability for the oxygen reduction reaction. [Copyright &y& Elsevier]

Carbon black (CB)-supported Pt metal nanoparticles were covered with microporous silica layers using successive hydrolysis of 3-aminopropyl-triethoxysilane (APTES) and methyltriethoxysilane (MTES). The Pt/CB nanoparticles covered with microporous silica layers showed the high sintering resistance of Pt metal particles to thermal treatment at 973 K in a H2 atmosphere as compared with Pt/CB. As a result, this catalyst showed higher catalytic activity for cyclohexane decomposition than Pt/CB after thermal treatment. [Figure not available: see fulltext.] [ABSTRACT FROM AUTHOR]

Abstract: Multi-walled carbon nanotube-supported Pt metal particle (Pt/CNT) catalysts were covered with silica layers with different thickness and density. In the present study, the Pt/CNT covered with silica layers (SiO2/Pt/CNT) was prepared by the successive hydrolysis of 3-aminopropyl-triethoxysilane (APTS) and tetraethoxysilane (TEOS). The thickness of silica layers in SiO2/Pt/CNT could be controlled by varying the concentration of silica sources during the preparation of SiO2/Pt/CNT. The density of silica layers in SiO2/Pt/CNT was changed by varying the treatment temperature for the catalysts as well as by adjusting the pH for the hydrolysis of silica sources during the preparation of the catalysts. The catalytic performance of these SiO2/Pt/CNT as cathode catalysts in polymer electrolyte fuel cells was examined. Pt/CNT covered with silica layers of adequate thickness (ca. 6nm) and high density had a high electrochemically active surface area as well as excellent durability. Dense silica layers in SiO2/Pt/CNT prevented the diffusion of cationic Pt species dissolved from Pt metal particles out of the catalysts. [Copyright &y& Elsevier]

Carbon nanotube (CNT)-supported Pt metal nanoparticles were covered with silica layers by hydrolysis of 3-aminopropyl-triethoxysilane (APTES) and/or tetraethoxysilane (TEOS). The hydrolysis of only APTES resulted in a uniform coverage of silica layers on Pt/CNT, but the thickness of the silica layers was very thin (<1 nm). Pt/CNT could also be coated with silica layers of a few nanometers in thickness by hydrolysis of TEOS, but exposed surfaces of CNTs in the sample were frequently observed. In contrast, the successive hydrolysis of APTES and TEOS brought about a uniform coverage of silica layers of a few nanometers in thickness on Pt/CNT. The silica-coated Pt/CNT showed high catalytic activity for electrochemical reactions in aqueous H2SO4 electrolyte, in spite of a uniform coverage of Pt metal with silica layers. In addition, the coverage of Pt/CNT with silica layers improved its durability in electrochemical reactions. [ABSTRACT FROM AUTHOR]

Formation of carbon nanofibers (CNFs) and carbon nanotubes (CNTs) through the decomposition of ethylene at 973 K was achieved using various metal catalysts covered with silica layers. CNFs of various diameters were formed by ethylene decomposition over a Co metal catalyst supported on the outer surface of the silica. In contrast, silica-coated Co catalysts formed CNTs with uniform diameters by ethylene decomposition. Silica-coated Ni/SiO2 and Pt/carbon black also formed CNTs with uniform diameters, while CNFs and CNTs with various diameters were formed over Ni/SiO2 and Pt/carbon black without a silica coating. These results indicate that silica layers that envelop metal particles prevent sintering of the metal particles during ethylene decomposition. This results in the preferential formation of CNTs with a uniform diameter. [ABSTRACT FROM AUTHOR]

Abstract: Ethylene decomposition was performed over supported Pt catalysts to fabricate composites of Pt metal nanoparticles and carbon nanotubes (CNTs). All supported Pt catalysts (Pt/carbon black, Pt/CNT, Pt/MgO, Pt/Al2O3 and Pt/SiO2) showed catalytic activity for ethylene decomposition at 973K to form CNTs. Pt metal particles were found at tips of CNTs. These results indicate that Pt metal particles have catalytic activity for growth of CNTs through hydrocarbon decomposition. A broad range (5–50nm) of CNT diameters were formed from the use of supported Pt metal catalysts although Pt metal particles in the catalysts before ethylene decomposition were relatively uniform in size (2–5nm). These results imply that Pt metal particles in the catalysts aggregated during ethylene decomposition at 973K. Aggregation of Pt metal particles in catalysts during ethylene decomposition could be suppressed by covering catalysts with silica layers that were a few nanometers thick. Silica-coated Pt catalysts showed high activity for ethylene decomposition to form CNTs with uniform diameters (8–10nm) despite the uniform coverage of Pt metal particles with silica layers. [Copyright &y& Elsevier]

Abstract: The catalytic performance of Ni metal catalysts uniformly covered with layers of silica for propane steam reforming was compared with the performances of Ni metal catalysts supported on substrates of magnesia (MgO), alumina (Al2O3) and silica (SiO2). The silica-coated Ni catalysts showed high activity for propane steam reforming at 873K. This catalytic activity is higher than the activity of effective light alkane steam reforming Ni/Al2O3 and Ni/MgO catalysts and we found that the catalytic activity of the silica-supported Ni metal was poor. The coverage of Ni metal particles with silica thus improves their catalytic activity for this reaction. Carbon deposition on silica-coated Ni catalysts during the steam reforming of propane at 873K was lower than that on Ni/MgO and Ni/Al2O3. Ni K-edge XANES and EXAFS spectra of silica-coated Ni catalyst showed a strong interaction between Ni metal and silica. This interaction prevents the sintering of Ni metal particles during the propane steam reforming, which results in the reduction of carbon deposition on silica-coated Ni catalysts. These properties of silica-coated Ni catalysts result in high catalytic activity and improved catalyst stability. [Copyright &y& Elsevier]

Carbon nanotube (CNT)-supported Pt metal (Pt/CNT) was covered with silica layers using hydrolysis of 3-aminopropyl-triethoxysilane (APTES) and/or tetraethoxysilane (TEOS). The hydrolysis of APTES resulted in a uniform coverage of Pt/CNT with silica layers, but the silica layers were very thin. Pt/CNT could be coated with silica layers of a few nanometers thickness by hydrolysis of TEOS, but the thickness was not uniform. In contrast, the successive hydrolysis of APTES and TEOS resulted in a uniform coverage of Pt/CNT with silica layers of a few nanometers thickness. Pt metal particles in silica-coated Pt/CNT adsorbed CO chemically despite uniform coverage of Pt metal particles with silica layers. Therefore, silica-coated Pt/CNT demonstrated high catalytic activity in methane combustion. [Copyright &y& Elsevier]

Abstract: Pt–SiO2 catalysts were prepared by using water-in-oil type microemulsion. The catalysts were prepared by hydrolysis of tetraethylorthosilicate in the microemulsion containing Pt species, followed by calcination in air and reduction with H2. This preparation method for Pt–SiO2 provided Pt metal nanoparticles with a diameter of ca. 6nm covered uniformly with silica layers, while a conventional impregnation method formed Pt metal particles supported on the silica surface. The Pt catalysts coated with silica (coat-Pt) showed different catalytic performance for the competitive oxidation of mixed hydrocarbons from Pt catalysts supported on silica (imp-Pt), i.e. in the competitive oxidation of methane and iso-butane, coat-Pt catalysts preferentially oxidized methane, while iso-butane was oxidized selectively over imp-Pt catalysts. The results of the adsorption of Ar on coat-Pt showed that silica in coat-Pt had porous structures of pore diameters <1nm. Because the porous structure of silica, which wrapped Pt metal particles, controlled the diffusion rate of reactant molecules, coat-Pt showed a specific catalytic performance for competitive oxidation. [Copyright &y& Elsevier]

Pd cathode catalysts for polymer electrolyte fuel cells have been covered with silica layers a few nanometres thick. The silica-coated Pd catalysts showed high activity and excellent durability for the oxygen reduction under the severe cathode conditions of PEFCs, while Pd catalysts without silica-coating were seriously deactivated under the same conditions. The coverage of Pd metal with silica prevents the diffusion of Pd species out of the silica layers. [ABSTRACT FROM AUTHOR]

Pt metal catalyst covered with silica layers oxidized methane preferentially in the competitive oxidation of methane and isobutane with gaseous oxygen. Selectivity in the competitive oxidation of mixed hydrocarbons over the catalyst was controlled by the diffusion rates of the reactants in the silica layers which wrapped Pt metal particles. [ABSTRACT FROM AUTHOR]

Carbon-supported Pt-Co catalyst for the oxygen reduction reaction was covered with silica layers to inhibit sintering of the metal particles. Coverage of the Pt-Co catalyst with silica layers promoted the formation of alloys with small diameters during treatment at 973 K. The silica-coated Pt-Co electrocatalyst showed a large electrochemically active surface area (ECSA) and excellent durability. [ABSTRACT FROM AUTHOR]

Hollow polymeric nanospheres with shells consisting of inner hydrophilic and outer hydrophobic polymer chains are prepared by a surface-initiated living radical polymerization of 2-trimethylsilyloxyethyl acrylate and methyl methacrylate from initiator-modified silica nanoparticles; these nanospheres are subsequently prepared by intraparticle cross-linking and by dissolution of silica matrices with HF. The translation behavior of hydrophilic substrates from an aqueous phase to an oil phase through the nanocapsules is investigated. [ABSTRACT FROM AUTHOR]

Carbon nanofibers with uniform diameter of 10--15 nm were produced by the decomposition of ethylene over silica-supported Ni catalysts covered with silica layers. [ABSTRACT FROM AUTHOR]

Carbon-supported Pd catalysts (Pd/CBs) for oxygen reduction reaction (ORR) in polymer electrolyte fuel cells (PEFCs) were modified with transition metals to enhance their catalytic activity. Pd-M/CB(N) (M = Co, Cu, Y, and Ag) was prepared by the reduction of metal precursors with NaBH 4 in the presence of ethylene glycol (EG) without heat treatment. Pd-Ag/CB(N) showed the highest catalytic activity for the ORR among all the Pd-based catalysts tested in the present study, and it had twice higher activity than Pd/CB. The Pd-Ag/CB(N) also had the higher ORR activity than the Pd-Ag/CB prepared by a conventional impregnation method (Pd-Ag/CB(I)) due to the formation of smaller Pd-Ag alloy particles with 1–2 nm diameters. Pd-Ag/CB(N) was covered with silica layers (SiO 2 /Pd-Ag/CB(N)) in order to improve their durability under severe cathode conditions. The SiO 2 /Pd-Ag/CB(N) had high catalytic activity for the ORR during the durability test because silica layers prevented the diffusion of metal species from the catalysts. [ABSTRACT FROM AUTHOR]

Pd metal particles supported on a high surface area carbon black (Pd/CB) were covered with silica layers to improve the durability under severe cathode condition of proton exchange membrane fuel cells (PEMFCs). The performance and the durability of the silica-coated Pd/CB (SiO 2 /Pd/CB) were investigated by rotating disk electrode (RDE) in aqueous HClO 4 and single cell test of the membrane-electrode assemblies (MEAs). SiO 2 /Pd/CB showed excellent durability exceeding Pt/CB during potential cycle in single cell test as well as in RDE measurement while Pd/CB significantly degraded. Furthermore, the MEA using SiO 2 /Pd/CB as the cathode catalyst showed higher performance than that using Pd/CB even in the initial state. The catalytic activity of SiO 2 /Pd/CB was higher than that of Pd/CB, and the drop of the cell performances due to the inhibition of electron conduction, proton conduction, and oxygen diffusion by the silica layer was not significant. It has been shown that the silica-coating is a very practical technique that can stabilize metal species originally unstable in the cathode condition of PEMFCs without a decrease in the cell performance. [ABSTRACT FROM AUTHOR]

Abstract: Carbon black-supported Pt metal particles (Pt/CB) were covered with microporous silica layers using different organosilanes like methyltriethoxysilane (MTES) (SiO2(MTES)/Pt/CB) and phenyltriethoxysilane (PhTES) (SiO2(PhTES)/Pt/CB). Pt metal particles in Pt/CB covered with microporous silica layers were stable at high temperature up to 973K in a hydrogen atmosphere, because the microporous silica layers that wrapped around the Pt metal particles essentially prevent particle sintering. Methyl or phenyl groups were introduced into the silica layers that covered the Pt metal particles using MTES or PhTES hydrolysis. Micropores were formed in the silica layers effectively after thermal treatment at 973K. The microporous silica-coated Pt catalysts were used as model catalysts for the dehydrogenation of cyclohexane. The microporous silica-coated Pt catalysts with thermal treatment at 973K exhibited a higher conversion of cyclohexane compared with Pt/CB. Especially, the SiO2(PhTES)/Pt/CB catalysts showed relatively higher conversion of cyclohexane compared with the other silica-coated Pt catalysts even when the SiO2 loading was high. Microporous silica layers with a larger micropore volume promoted the diffusion of cyclohexane during the catalytic reaction. [Copyright &y& Elsevier]

The effect of precursor on the dispersion and photocatalytic performance of titanium oxide supported on silica has been investigated. The catalysts were prepared by a simple impregnation method with three kinds of titanium complexes of different ligands (dipyvaroylmethanato (DPM), acetylacetonato (acac), isopropoxide (O-iPr)) abbreviated to D-TS, A-TS, and I-TS. The UV/Vis diffuse reflectance, Raman, XAFS spectroscopic study showed that surface titanate of D-TS is isolated TiO4 species, and that of I-TS is a mixture of rutile and anatase crystallites. The surface state of A-TS is the intermediate between D-TS and I-TS. TiO2/SiO2 samples were more active than bulk TiO2 and partial oxidation selectivity of TiO2/SiO2 samples are higher than that of TiO2 in photo-oxidation of propane. Products distribution over TiO2/SiO2 was drastically varied with the surface titanates. The activity for photo-assisted selective catalytic reduction (SCR) of NO with ammonia was found for D-TS. [Copyright &y& Elsevier]