Characterization of Ti-Beta zeolites and their reactivity for the photocatalytic reduction of CO_2 with H_2O

A characterization of Ti-Beta zeolites synthesized under various conditions as well as an investigation of their photocatalytic properties for the reduction of CO_2 with H_2O at 323 K to produce CH_4 and CH_3OH were carried out. In situ XAFS spectra measurements indicated that a highly dispersed tetrahedral titanium oxide species was present in the zeolite framework and an increase in the coordination number of the titanium oxide species by the addition of H_2O and CO_2 molecules could be detected. The Ti-Beta zeolite having a hydrophilic property (Ti-Beta(OH)) exhibited a more dramatic increase in the coordination number than the Ti-Beta(F) zeolite which had a hydrophobic property. These results suggest that CO_2 and H_2O molecules can be adsorbed efficiently onto the highly dispersed tetrahedrally coordinated titanium oxide species. UV irradiation of these Ti-Beta zeolite catalysts in the presence of H_2O and CO_2 led to the formation of CH_4 and CH_3OH. Ti-Beta(OH) exhibited a higher reactivity than Ti-Beta(F), while the selectivity for the formation of CH_3OH on Ti-Beta(F) was higher than that for Ti-Beta(OH). These results indicated that the reactivity and selectivity of the zeolite catalyst can be determined by the hydrophilic and hydrophobic properties of the zeolites

Characterization of Ti-Beta zeolites and their reactivity for the photocatalytic reduction of CO_2 with H_2O

A characterization of Ti-Beta zeolites synthesized under various conditions as well as an investigation of their photocatalytic properties for the reduction of CO_2 with H_2O at 323 K to produce CH_4 and CH_3OH were carried out. In situ XAFS spectra measurements indicated that a highly dispersed tetrahedral titanium oxide species was present in the zeolite framework and an increase in the coordination number of the titanium oxide species by the addition of H_2O and CO_2 molecules could be detected. The Ti-Beta zeolite having a hydrophilic property (Ti-Beta(OH)) exhibited a more dramatic increase in the coordination number than the Ti-Beta(F) zeolite which had a hydrophobic property. These results suggest that CO_2 and H_2O molecules can be adsorbed efficiently onto the highly dispersed tetrahedrally coordinated titanium oxide species. UV irradiation of these Ti-Beta zeolite catalysts in the presence of H_2O and CO_2 led to the formation of CH_4 and CH_3OH. Ti-Beta(OH) exhibited a higher reactivity than Ti-Beta(F), while the selectivity for the formation of CH_3OH on Ti-Beta(F) was higher than that for Ti-Beta(OH). These results indicated that the reactivity and selectivity of the zeolite catalyst can be determined by the hydrophilic and hydrophobic properties of the zeolites

Elucidation of the local structure of active titanium(iv) sites on silica-based phase-boundary catalysts for alkene epoxidation with aqueous hydrogen peroxide

Structural and functional aspects of active titanium sites for phase boundary catalytic (PBC) epoxidation of 1-octene with hydrogen peroxide (H2O2) were investigated in detail using X-ray absorption fine structure (XAFS) analysis and ultraviolet and visible-light diffuse reflectance (UV-Vis-DR) spectroscopy. By analysis of the Ti K-edge X-ray absorption near edge fine structure (XANES) spectra of several titanium-loaded silica catalysts, the ratio of the amount of 4-coordinated titanium oxide (Ttet) to that of a 6-coordinated one (Toct) was determined. Monotonic increments of catalytic activity for epoxide production and efficiency of H2O2 utilization with the ratio Ttet/(Ttet + Toct) revealed that the highly active catalysts mainly include Ttet but not Toct. UV-Vis-DR spectra of samples with external surfaces partially covered with alkylsilyl groups indicated that there are at least two kinds of Ttet sites with different ligands. One site anchors an acidic hydroxyl (Ttet(OH)), giving absorption centered at the wavelength of ca. 230 nm and the other, exhibiting an absorption peak at the wavelength of ca. 210 nm, is directly attached to an alkylsilyl group (Ttet(OSiR)) formed via reaction of an acidic hydroxy of Ttet(OH) with an alkylsilane reagent. Since the catalysts have both an alkylsilyl-grafted hydrophobic surface and a hydroxy-terminated hydrophilic surface, it is postulated that the former is located on both hydrophilic (Ttet(OH,w)) and hydrophobic (Ttet(OH,o)) surfaces and the latter, Ttet(OSiR), exists only on the hydrophobic surface. From analyses of catalytic activities of several catalysts with different distributions of these Ttet sites, both Ttet(OH,o) and Ttet(OSiR) sites on the hydrophobic surface were proved to be active, while Ttet(OH,w) on the hydrophilic surface did not work for the present PBC system. Among the sites on the hydrophobic surface, moreover, it was found that a Ttet(OSiR) site acted as a more effective site for selective epoxidation when compared with Ttet(OH,o), which induced ring-opening of epoxide to give a by-product, 1,2-octanediol

Asymmetrically modified titanium(IV) oxide particles having both hydrophobic and hydrophilic parts of their surfaces for liquid–liquid dual-phase photocatalytic reactions

Titanium(IV) oxide (TiO2)-based photocatalyst particles assembled at the phase boundary of a liquid–liquid dual-phase mixture were prepared by partial modification of the external surface of each particle with alkylsilyl groups. The average surface coverage of alkylsilyl groups was estimated by elemental analyses of carbon and ash components of the samples and floatability on aqueous ethanol solutions. Results revealed that the hydrophobicity–hydrophilicity of asymmetrically modified samples was comparable to that of samples fully covered with alkylsilyl groups. TiO2 particles asymmetrically or fully modified with alkylsilyl groups showed photocatalytic activity for benzene oxidation to produce phenol from an aerated benzene–water dual-phase mixture even without agitation, while bare TiO2 required mechanical agitation to induce the photocatalytic reaction. However, prolonged irradiation precipitated some of the surface-modified particles in the aqueous layer due to photocatalytic decomposition of surface alkylsilyl groups. The photostability was improved by employment of TiO2 particles coated with porous silica (SiO2) as a starting material. Compared with the SiO2-coated TiO2 particles fully modified with alkylsilyl groups (o-Si/Ti), the asymmetrically modified SiO2–TiO2 particles (w/o-Si/Ti) showed slightly higher photocatalytic activity for benzene oxidation. On the other hand, a notable difference between the two types of particles was observed in photocatalytic hydrogen evolution in the presence of sacrificial donors from a benzene–water mixture and from an aqueous solution under deaerated conditions; w/o-Si/Ti showed the activity more than two-fold greater than that of o-Ti/Si, presumably because of efficient contact of w/o-Si/Ti with both aqueous and organic phases compared with o-Si/Ti, which was rather difficult to contact with the aqueous phase

Catalytic NO-H2-O2 reaction over Pt/Mg-Al-O prepared from PtCl62-- and Pt(NO2)42--exchanged hydrotalcites

The catalytic NO–H2–O2 reaction at low temperatures (50–130 °C) has been investigated over Pt-supported Mg–Al binary oxides, which were prepared from PtCl62− and Pt(NO2)42−–exchanged hydrotalcite(HT)-like compounds by heating at 600 °C in H2. Although the NOx conversion of both catalysts was similarly high (>80%) at around 70 °C, the Pt(NO2)42−–HT catalyst exhibited the higher selectivity to N2 (53% N2 and 47% N2O), compared to 12% N2 and 88% N2O for the PtCl62−–HT catalyst. The XANES/EXAFS spectroscopy showed that Pt prepared from PtCl62−–HT is partly oxidized due to the coordination of residual chlorides, whereas Pt(NO2)42−–HT yielded highly dispersed metallic Pt. Because of the presence of chloride strongly bound to Pt, the oxidative NO adsorption as NO2/NO3 was inhibited for PtCl62−–HT. The improved N2 selectivity with an increase of oxidative NO adsorption is in accord with the catalytic property of Na–Pt/ZSM-5 in our previous work [M. Machida, T. Watanabe, Appl. Catal. B: Environ. 52 (2004) 281.], suggesting that the N2 would be formed via NO2/NO3 intermediates

Oxidation of hydrophobic alcohols using aqueous hydrogen peroxide over amphiphilic silica particles loaded with titanium(IV) oxide as a liquid-liquid phase-boundary catalyst

Phase-boundary catalysis (PBC), a new concept of a heterogeneous catalytic system for oxidation of various hydrophobic alcohols with aqueous hydrogen peroxide (H2O2), has been investigated. A part the external surface of silica (SiO2) particles loaded with titanium(IV) oxides was modified with hydrophobic alkyl groups to obtain amphiphilic particles, having both hydrophobic and hydrophilic surfaces on each particle. The amphiphilic particles were spontaneously assembled at interfaces between dual phase mixtures of aqueous solutions and water-immiscible organic compounds. Upon addition to a dual phase mixture of aqueous H2O2 and toluene-containing hydrophobic alcohols, these particles acted as an efficient catalyst for the reaction, to produce corresponding aldehydes and ketones selectively. Notable features of the PBC system are that the oxidation proceeds even without agitation and that only a few percent of titanium species was detected as dissolved species. Productions of aldehydes and ketones were also observed when titanium loaded SiO2 without modification with alkyl groups was employed for the reaction. However, a large amount of titanium loaded on the material was leached during the reaction. These results indicate that surface-covered alkyl groups not only bring about effective contact with hydrophobic alcohols in the organic phase but also give stability against leaching, leading to heterogeneous catalytic functions