The influence of Si-modified TiO2 on the activity of Ag/TiO2 in CO oxidation

Nanocrystalline TiO₂ and Si-modified TiO₂ with Si/Ti ratios 0.01, 0.05, 0.1, and 0.3 were prepared by the solvothermal method and employed as the supports for Ag/TiO₂ catalysts for CO oxidation reaction. The incorporation of Si into the TiO₂ lattice in the form of Ti–O–Si as revealed by FT-IR results could inhibit the agglomeration of TiO₂ crystallites, resulting in an increase of both surface area and metal dispersion. However, there existed an optimum content of Si/Ti at ca. 0.05–0.1 which resulted in an improved catalytic activity of Ag/TiO₂ in CO oxidation. Based on the O₂-temperature program desorption (O₂-TPD) results, the catalysts with appropriate amounts of Si/Ti exhibited higher amount of O₂ adsorption and much lower desorption temperature. It is suggested that the presence of Ti–O–Si promoted the formation of active oxygen species and increased the mobility of lattice oxygen so that the catalytic activity was enhanced. There was no improvement in CO oxidation activity of the Ag/TiO₂ catalyst when the Si/Ti was further increased to 0.3 due probably to the formation of amorphous SiO₂ instead of the Ti–O–Si bond.     

In situ FTIR Study of Cobalt Oxides for the Oxidation of Carbon Monoxide

A high-valance cobalt oxide, CoO _x , was prepared from cobalt nitrate aqueous solution through precipitation with sodium hydroxide and oxidation by hydrogen peroxide. Further, other pure cobalt oxide species were refined from the CoO _x by temperature-programmed reduction (TPR) to 170, 230 and 300 °C. They were characterized by TPR and X-ray diffraction (XRD). Adsorption of CO and the co-adsorption of CO/O₂ over the cobalt oxides were further tested by in situ FTIR. It was shown that Co₃O₄ is quite active for the oxidation of CO at room temperature in the presence of oxygen, leading to the formation of CO₂. The variation in the oxidation of CO was interpreted with a mechanism involving two kinds of oxygen species, i.e., *-O₂ on the CoO _x surface and *-O_L on the surface of Co₃O₄ spinel structure.     

Characterization and Catalytic Oxidation of Carbon Monoxide Over Supported Cobalt Catalysts

Three types of supported cobalt catalysts (CoO_x/SiO₂, CoO_x/TiO₂ and CoO_x/Al₂O₃) were prepared by incipient wetness impregnation with aqueous Co(NO₃)₂·6H₂O solution. The phase composition and the interactions of cobalt with supports under different calcined temperatures were investigated using thermogravimetry (TG), N₂-adsorption at −196 °C, X-ray diffraction (XRD), temperature-programmed reduction (TPR) and diffuse reflectance spectroscopy (DRS). Their catalytic activities towards the CO oxidation were further studied in a continuous flow micro-reactor. The results showed that the interaction of cobalt oxide with supports was much stronger in the kinds of Al₂O₃ and TiO₂, while no conclusive evidence of any interaction was found for SiO₂. Besides the crystalline Co₃O₄ which was formed in three supported catalysts, both high-temperature phases CoAl₂O₄ and CoTiO₃ spinel were also detected under XRD, DRS and TPR analysis. The degree of interaction between cobalt oxide and the support not only affected the surface area and reduction behavior of the catalysts, the catalytic activity toward the CO oxidation also affected simultaneously. As the CoAl₂O₄ and CoTiO₃ spinel formed, both the surface area and catalytic activity decreased significantly.     

Carbon Monoxide Hydrogenation on Cobalt/Zeolite Catalysts

The synthesis of hydrocarbons from catalytic hydrogenation of CO/H₂ was investigated over Co/zeolite catalysts at 1 atm, 493–553 K, H₂/CO = 2, and GHSV = 1200. Various zeolites, such as NaA, NaX, NaY, KL and NaMordenite, were used as the supports. The catalysts were prepared by impregnation and were characterized by H₂/CO chemisorption and temperature-programmed reduction (TPR). Based on TPD measurements, the CO/H₂ adsorption ratio can be used as an index for the extent of metal-zeolite interaction. The stronger the metal-zeolite interaction is, the higher the Co/H₂ adsorption ratio on metal is. The activity and selectivity of cobalt supported in zeolites were affected by complex factors such as framework structure, Si/Al ratio, and the complementary cations. The activity of the catalyst is in the order: Co/KL > Co/NaX > Co/NaY > Co/NaMordenite > Co/NaA. All of the Co/zeolite catalysts had a very high selectivity to C₂–C₄ olefins, which would decrease with increasing reaction temperature. Cobalt oxide supported in zeolite was difficult to reduce. Increasing the reduction temperature could increase the reducibility of cobalt and resulted in the increase of activity.     

Selective Oxidation of Diphenylmethane Over Cobalt Doped Mesoporous Titania–Silica Catalyst with High Ti Content

Cobalt doped mesoporous titania–silica with Ti/Si molar ratio of 0.5 (Co–TiO₂–SiO₂) was synthesized for the oxidation of diphenylmethane in acetic acid using aqueous hydrogen peroxide as oxidant for the first time. Fast hot catalyst filtration experiment proved that the catalyst acted as a heterogeneous one. Recycling of the catalyst indicates that the catalyst can be used a number of times without losing its activity to a greater extent. The effects of reaction time and reaction temperature on the performance of the catalyst were investigated. Moreover, cobalt doped mesoporous titania with a crystalline structure and cobalt doped mesoporous titania–silica with different molar ratio were also studied. It was found that Co–TiO₂–SiO₂ with Ti/Si molar ratio of 0.5 showed the highest activity.     

TPD study of the reaction of CH3CH2SH and (CH3CH2)2S2 on ZnO(0001) and ZnO

Superior activation of on 1 wt% Pt/TiO₂ catalysts for the oxidation of CO was attained by loading a large amount of Fe-oxide (100 wt%) and TiO₂. In situ IR spectra of CO proved that the structural transformation is brought about on the Pt-sites by loading of Fe-oxide, where predominant Pt-sites giving linear CO change to highly reactive bridge CO Pt-sites. In contrast, no transformation of the linear CO sites to the bridge CO sites takes place by loading of TiO₂ but the environment of Pt-sites for linear CO is changed.     

Synthesis,Characterization and Application of Co–MgO Mixed Oxides in Oxidation of Carbon Monoxide

The present work deals with the synthesis of nanostructured Co–MgO mixed oxides with different weight ratios of cobalt by a facile co-precipitation method as a catalyst for low-temperature CO oxidation. The prepared samples were characterized by X-ray diffraction (XRD), N₂ adsorption/desorption (BET), Fourier transform infrared spectroscopy (FTIR), and transmission and scanning electron microscopies (TEM and SEM) techniques. The results revealed that inexpensive cobalt–magnesium mixed metal oxide nanoparticles have a high potential as catalyst in low-temperature CO oxidation. The Co–MgO mixed oxide with 30 wt.% cobalt had the highest activity. The results showed that the catalysts pretreated under O₂-containing atmosphere possessed higher activity compared to the catalyst pretreated under H₂ atmosphere. Co–MgO catalyst showed a good repeatability in reaction condition. The stability test exhibited that the Co–MgO mixed oxides were highly stable for CO oxidation over a 30 h time on stream in the feed gas containing a high amount of moisture and CO₂.     

Electrocatalytic activity of Pt nanoparticles deposited on porous TiO2 supports toward methanol oxidation

Porous TiO₂ thin films were prepared on the Si substrate by hydrothermal method, and used as the Pt electrocatalyst support for methanol oxidation study. Well-dispersed Pt nanoparticles with a particle size of 5–7 nm were pulse-electrodeposited on the porous TiO₂ support, which was mainly composed of the anatase phase after an annealing at 600 °C in vacuum. Cyclic voltammetry (CV) and CO stripping measurements showed that the Pt/TiO₂ electrode had a high electrocatalytic activity toward methanol oxidation and an excellent CO tolerance. The excellent electrocatalytic performance of the electrode is ascribed to the synergistic effect of Pt nanoparticles and the porous TiO₂ support on CO oxidation. The strong electronic interaction between Pt and the TiO₂ support may modify CO chemisorption properties on Pt nanoparticles, thereby facilitating CO oxidation on Pt nanoparticles via the bifunctional mechanism and thus improving the electrocatalytic activity of the Pt catalyst toward methanol oxidation.     

Self oscillatory behaviour in N2O decomposition over Co-ZSM-5 catalysts

Isothermal oscillations developed during N₂O decomposition over Co-ZSM-5 catalysts with different Si/Al ratios have been investigated. Spontaneous oscillations were observed between 350 and 450 °C. The maximum amplitude has been obtained for the catalysts having Si/Al of 40 and 50. The activation energies of the obtained oscillations were calculated in respect to cobalt concentration. The results showed that the E_a values increase linearly with an increasing Si/Al ratio of the zeolite. For Co-ZSM-5 catalyst (Si/Al = 25), increasing cobalt content in the catalyst led to a decrease in the frequency as well as the amplitude of the oscillations. Meanwhile, the increase in the E_a values was observed. The calculated reaction rate was found to be first order with respect to nitrous oxide concentration. Moreover, the developed oscillations were found to be sensitive to inlet N₂O concentration, catalyst weight and milling time duration. Decreasing the N₂O inlet concentration as well as the catalyst weight and increasing the milling time would lead to a quenching of the developed oscillations.     

Enhancement of catalytic activity of Au/TiO2 by thermal and plasma treatment

A significant enhancement in the catalytic activity of Au/TiO₂ in CO oxidation and preferential oxidation reaction by creating the active sites on the catalyst surface by thermal treatment as well as by producing small gold particles by plasma treatment has been studied. Au/TiO₂ catalyst (Au (1 wt%) supported on TiO₂) was prepared by conventional deposition-precipitation method with NaOH (DP NaOH) followed by washing, drying and calcination in air at 400 °C for 4 h. Thermal treatment of Au/TiO₂ was carried out at 550 °C under 0.05 mTorr. A small amount of Au/TiO₂ catalyst was taken from the untreated and thermally treated Au/TiO₂ and both kinds of catalysts were treated with plasma sputtering at room temperature. The activity of the catalysts has been examined in the reaction of CO oxidation and preferential oxidation (PROX) at 25–250 °C. Thermally treated Au/TiO₂ showed better catalytic activity as compared to the untreated catalyst. There is also an additional enhancement in the catalytic activity due to plasma sputtering on the both kinds of catalysts. Thermally treated Au/TiO₂ followed by plasma sputtering Au/TiO₂ showed higher conversion rates for CO oxidation reaction compared with untreated, thermally treated and plasma sputtered Au/TiO₂ catalysts. It may be concluded that the enhancement of catalytic activity of thermally treated Au/TiO₂ followed by plasma sputtering is owing to the generation of active sites such as oxygen vacancies/defects in TiO₂ support using thermal treatment as well as by producing small gold particles using plasma treatment.     

Promotional effect of H2 on CO oxidation over Au/TiO2 studied by operando infrared spectroscopy

The oxidation of carbon monoxide in the presence of various concentrations of molecular hydrogen has been studied over a Au/TiO₂ reference catalyst by combining diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and mass spectrometry. It is shown for the first time that H₂ enhances the CO oxidation rate on Au/TiO₂ without leading to any major loss of selectivity. Increasing the H₂ pressure induces higher CO and H₂ oxidation rates. Under H₂-free conditions, the surface species detected are Au^δ+–CO, Ti⁴⁺–CO, carbon dioxide and carbonates. Upon the addition of H₂, Au⁰–CO, water and hydroxyl groups become the main surface species. The occurrence of a preferential CO oxidation mechanism involving H_xO_y species under the present experimental conditions is proposed.     

Catalytic oxidation of hydrogen over Au/TiO2 catalysts

Selective catalytic oxidation of hydrogen in the presence of hydrocarbons was studied in a fixed bed quartz reactor, over 3 wt%Au/TiO₂ and 5 wt%Au/TiO₂ catalysts. This reaction can be utilised in the production of light alkenes via catalytic dehydrogenation, providing in situ heat to the endothermic dehydrogenation reaction and simultaneously removing a fraction of the produced hydrogen. It is important to avoid the non-selective combustion of the hydrocarbons in the mixture. Both 3 wt%Au/TiO₂ and 5 wt%Au/TiO₂ are active for the combustion of hydrogen, but in a gas mixture with propane and oxygen the selectivity is dependent upon the feed ratio of hydrogen and oxygen. At 550 °C, with propane present, no carbon oxides are formed when the H₂:O₂ ratio is four, but at lower ratios some CO₂ and some CO is formed.     

A spectroscopic study of amorphous and crystalline Ti-containing silicas and their surface acidity

Different titanium silicalite (TS-1) samples and TiO₂/SiO₂ aerogels samples have been prepared and characterized structurally by XRD, skeletal FT-IR and FT-Raman spectroscopies and diffuse reflectance UV spectroscopy. The surface acidity of all materials has been investigated by FT-IR spectroscopy of the surface hydroxy groups and of adsorbed acetonitrile. The TS-1 samples are found to be totally free from extra-framework Ti oxide species, while the characteristic features of framework tetrahedrally coordinated Ti cations are well evident. Framework Ti cations substituting for Si into the silicalite MFI structure act as quite strong Lewis acid sites towards acetonitrile. The presence of Ti neither changes significantly the Brønsted acidity of the silicalite silanols, nor results in the formation of new hydroxy groups. TS-1 samples present lower defectivity (smaller amount of clustered hydrolyzed Si–OH OH–Si bonds) than pure silicalite prepared in the same way. Extra-framework TiO₂, instead, is well detectable into aerogel samples, although framework Ti cations are likely also present. The surface of Ti-containing aerogels is covered by large amounts of silanol groups, although these sites do not present enhanced Brønsted acidity. Additionally, Lewis sites are detected whose strength is similar to that observed on TS samples, and is superior to that of the predominant sites on TiO₂. It is suggested that silica tends to cover the TiO₂ core in the case of aerogels. The surface is consequently dominated by silica with Ti sites in substitutional position.     

Effect of rapid thermal annealing on the structural and electrical properties of TiO2 thin films prepared by plasma enhanced CVD

Titanium oxide thin films were deposited on p-type Si(100), SiO₂/Si, and Pt/Si substrates by plasma enhanced chemical vapor deposition using high purity Ti(O-i-C₃H₇)₄ and oxygen. As-deposited amorphous TiO₂ thin films were treated by rapid thermal annealing (RTA) in oxygen ambient, and the effects of RTA conditions on the structural and electrical properties of TiO₂ films were studied in terms of crystallinity, microstructure, current leakage, and dielectric constant. The dominant crystalline structures after 600 and 800 ‡C annealing were an anatase phase for the TiO₂ film on SiO₂/Si and a rutile phase for the film on a Pt/Si substrate. The dielectric constant of the as-grown and annealed TiO₂ thin films increased depending on the substrate in the order of Si, SiO₂/Si, and Pt/ Si. The SiO₂ thin layer was effective in preventing the formation of titanium silicide at the interface and current leakage of the film. TEM photographs showed an additional growth of SiO_x from oxygen supplied from both SiO₂ and TiO₂ films when the films were annealed at 1000 ‡C in an oxygen ambient. Intensity analysis of Raman peaks also indicated that optimizing the oxygen concentration and the annealing time is critical for growing a TiO₂ film having high dielectric and low current leakage characteristics.     

Characterization of Ti/Si binary oxides prepared by the sol-gel method and their photocatalytic properties: The hydrogenation and hydrogenolysis of CH3CCH with H2O

Titanium-silicon (Ti/Si) binary oxides having different Ti content were prepared by the sol-gel method and utilized as photocatalysts for the hydrogenation and hydrogenolysis of CH₂CCH with H₂O. The photocatalytic reactivity and selectivity of these catalysts were investigated as a function of the Ti content and it was found that the hydrogenolysis reaction (C₂H₆ formation) was predominant in regions of low Ti content, while the hydrogenation reaction (C₃H₆ formation) proceeded in regions of high Ti content. The in situ photoluminescence, diffuse reflectance absorption, FT-IR, XAFS (XANES and EXAFS), and XPS spectroscopic investigations of these Ti/Si binary oxides indicated that the titanium oxide species are highly dispersed in the SiO₂ matrices and exist in a tetrahedral coordination exhibiting a characteristic photoluminescence spectrum. The charge transfer excited state of the tetrahedrally coordinated titanium oxide species plays a significant role in the efficient photoreaction with a high selectivity for the hydrogenolysis of CH₃CCH to produces mainly C₂H₆ and CH₄, while the catalysts involving the aggregated octahedrally coordinated titanium oxide species show a high selectivity for the hydrogenation of CH₃CCH to produce C₃H₆, being similar to reactions of the powdered TiO₂ catalysts. The good parallel relationship between the yield of the photoluminescence and the specific photocatalytic reactivity of the Ti/Si binary oxides as a function of the Ti content clearly indicates that the high photocatalytic reactivity of the Ti/Si binary oxides having low Ti content is associated with the high reactivity of the charge transfer excited state of the isolated titanium oxide species in tetrahedral coordination, [Ti³⁺-O⁻]*.     

Synergism Between Pt/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> and Au/TiO<Subscript>2</Subscript> in the Low Temperature Oxidation of Propene

CO impedes the low temperature (<170 °C) oxidation of C₃H₆ on supported Pt. Supported Au catalysts are very effective in the removal of CO by oxidation, although it has little propene oxidation activity under these conditions. Addition of Au/TiO₂ to Pt/Al₂O₃ either as a physical mixture or as a pre-catalyst removes the CO and lowers the light-off temperature (T ₅₀) for C₃H₆ oxidation compared with Pt catalyst alone by ~54 °C in a feed of 1% CO, 400 ppm C₃H₆, 14% O₂, 2% H₂O.     

Studies on the interaction between ruthenium and cobalt in supported catalysts in favor of hydroformylation

The interaction between ruthenium and cobalt atoms in SiO₂‐supported catalysts prepared from various precursors by H₂ treatment at 350 °C has been studied by ethylene hydroformylation, temperature‐programmed reduction (TPR) technique and IR spectroscopy. Incorporation of cobalt with ruthenium gives a catalyst with remarkably enhanced hydroformylation activity with respect to those of monometallic catalysts, irrespective of the ruthenium and cobalt precursors used. The synergistic effect of ruthenium and cobalt on the catalysis is consistent with TPR and IR results. TPR analysis shows regularly a promoted reduction of cobalt due to the “hydrogen spillover” effect, which indicates that ruthenium and cobalt atoms are in intimate contact in the catalysts. CO adsorption IR study demonstrates a strong decrease of CO chemisorption on Ru in the presence of cobalt, proposing that ruthenium and cobalt atoms interact on the SiO₂ surface to form Ru–Co bimetallic particles. The results suggest that the catalysts thus obtained contain Ru–Co bimetallic particles, at least atoms of the two metals in intimate contact. However, in situ surface IR spectra of ethylene hydroformylation exhibit little modification by the presence of cobalt on Ru/SiO₂. This revised version was published online in August 2006 with corrections to the Cover Date.     

Properties of polyimide hybrids with mixed metal oxide

Polyimide/silica–titania (PI/SiO₂–TiO₂) hybrid films were prepared via an in situ sol–gel process. The PI precursor, poly(amic acid) (PAA), which contains 2,2'‐bis[4‐(4‐aminophenoxy)‐phenyl]propane (p‐BAPPP), 3,3',4,4'‐ benzophenetetracarboxylic anhydride (BTDA) and 3‐aminopropyltrimethoxysilane (APrTMOS), was first synthesized; this was followed by the addition of phenyltrimethoxysilane (PTMS) and/or tetraethyl orthotitanate (Ti(OEt)₄) to fabricate PI/SiO₂–TiO₂ films. The relative content of SiO₂ to TiO₂ has remarkable effects on the crosslink structure and resultant properties of the hybrids. XPS results confirm that the amount of Si on the surface of the hybrids is higher than that in the bulk. The distribution of Ti in the hybrid films is contrary to the above trend because of the formation of three‐dimensional Si? O? Si, Si? O? Ti, and Ti? O? Ti networks. The SiO₂ content of the hybrids containing only silica significantly affects their refractive index, contact angle, and dielectric constant. The films with added PTMS show higher contact angles than pure PI because nonpolar segments, ? C₂H₆ or benzene groups, tend to distribute on the surface. Upon the addition of (Ti(OEt)₄), some hydrophilic segments on the surface of the hybrids are induced because of the formation of a crosslinked structure. The denser crosslinked molecular structure, and consequently lower CTE and higher T_g are obtained from hybrids containing more TiO₂. By comparing the above properties and flexibility, the best composition of metal oxides (SiO₂/TiO₂) in hybrids is 20/80. That is, an optimum ratio of metal oxides in PI hybrids induces superior properties for advanced practical applications. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Characterization of α-Al2O3–(Al–Si)3Ti composites

The reactions that take place during the formation of ceramic matrix composites that are based on α-Al₂O₃–(Al–Si)₃Ti interpenetrating networks were analysed. A reactive preform was pressure infiltrated with an Al–Si alloy. After pressure infiltration, the composite did not react in a full manner and further thermal annealing was required. The reduction of TiO₂ by the liquid Al–Si alloy results in the formation of (Al-Si)₃Ti (Al₆₀Si₁₂Ti₂₈). The formation of (Al–Si)₃Ti is governed by the consumption of TiO that is formed as an intermediate phase during the reduction of TiO₂.     

Improvement in corrosion resistance of micro-arc oxidation coating on PVD Ti-coated aluminum alloy 7075

Surface treatments are always needed to enhance corrosion-resistant performance of aluminum (Al) alloys when they are used in seawater environments. The paper aimed to prepare the composite oxide ceramic coating on Al alloy 7075 by combining micro-arc oxidation (MAO) and magnetron sputtering technology. The Al substrate was precoated with titanium (Ti) layer by using the magnetron sputtering technology and then treated by MAO in the alkaline aluminate electrolyte, resulting in a composite MAO coating, which is composed of Al₂O₃ and TiO₂ along with the complex oxide (Al₂TiO₅). The potentiodynamic polarization and electrochemical impedance spectroscopy were carried out to evaluate the corrosion performance of the MAO coatings in 3.5 wt% NaCl solution. Better corrosion resistance was observed for composite oxide coating than the reference MAO coating on the bare Al, as evidenced by the higher corrosion potential of −0.664 V versus Ag/AgCl and the lower corrosion current density of 4.41 × 10^-6 A/cm².