The design of Ti-, V-, Cr-oxide single-site catalysts within zeolite frameworks and their photocatalytic reactivity for the decomposition of undesirable molecules—The role of their excited states and reaction mechanisms

Transition metal oxides (Ti, V, Mo, Cr, etc.) incorporated within the framework of zeolites were found to exhibit high and unique photocatalytic reactivity as single-site heterogeneous catalysts for various reactions such as the decomposition of NOx (NO, N₂O) into N₂ and O₂, the reduction of CO₂ with H₂O to produce CH₄ and CH₃OH, the preferential oxidation of CO in the presence of H₂ (PROX), the partial oxidation of various hydrocarbons with O₂ or NO or N₂O and the epoxidation and metathesis reaction of alkenes. In situ spectroscopic investigations of these photofunctional systems applying photoluminescence, XAFS (XANES and FT-EXAFS), ESR and FT-IR revealed that the photo-excited states of the transition metal oxides play a vital role in the photocatalytic reactions. The high photocatalytic efficiency and selectivity of these single-site catalysts for significant reactions, which could not be observed with semiconducting bulk photocatalysts, were found to depend strongly on the unique and isolated local structure of the catalysts constructed within the restricted framework structure of the zeolites.     

Photoreduction of Carbondioxide on Surface Functionalized Nanoporous Catalysts

Nanoporous photocatalysts have been designed to exhibit unique photocatalytic activities through framework substitution of titanium species or surface immobilization of rhenium complex onto mesoporous silica. This article summarizes recent work on the synthesis, characterization and photocatalytic activities of the designed porous photocatalysts performed by the present authors. Various spectroscopic investigations revealed that the photo-excited states of these catalysts play a vital role in the photocatalytic reactions and their photocatalytic reactivities are strongly dependent on structures of active sites, which are confined and immobilized in the restricted framework structure of the mesoporous silica. Highly dispersed titanium oxide species incorporated in the framework of mesoporous silica exhibited high and unique photocatalytic reactivity for the reduction of CO₂ with H₂O to produce CH₄ and CH₃OH under UV irradiation, its reactivity being much higher than bulk TiO₂. The cationic rhenium(I) complex was encapsulated into a mesoporous AlMCM-41 material by ion-exchange method, yielding a visible light photocatalyst to be active for photocatalytic reduction of CO₂.     

Effect of the local structure of Ti-oxide species on the photocatalytic reactivity and photo-induced super-hydrophilic properties of Ti/Si and Ti/B binary oxide thin films

From the results of various spectroscopic investigations of Ti-oxide-based binary oxides, it was found that tetrahedrally coordinated Ti-oxide species are formed in the thin films of Ti/Si binary oxides with low TiO₂ content, while octahedrally coordinated TiO₂ nano-particles are formed in the Ti/B binary oxide thin films, reflecting the effect of the crystalline structures of the host SiO₂ or B₂O₃ on the local structure of the guest Ti-oxide species, respectively. The photocatalytic reactivity of the TiO₂ thin films was found to be remarkably enhanced by the dispersion of the Ti-oxide moiety into both the SiO₂ and B₂O₃ matrices, whereas the photo-induced super-hydrophilic properties of the TiO₂ thin films were enhanced only by a combination or mixing of the Ti-oxide moiety with B₂O₃.     

Benzyltitanium- und -zirconium/Al2O3-Katalysatoren zur partiellen Hydrierung von Acetylen

Benzyl Titanium and Zirconium Alumina Catalysts for Partial Hydrogenation of Acetylene Benzyl titanium and zirconium compounds of the types Bzl₄M, C₅H₅MBzl₃, and (C₅H₅)₂MBzl₂ form surface compounds on dehydroxylated alumina (15% α-, and 85% δ-Al₂O₃) which on hydrogenolysis yield TiH- and ZrH-species. The supported systems were studied with respect to catalytic activity in the partial hydrogenation of acetylene. Supported (C₅H₅)₂TiBzl₂ was proved to be inactive and the systems C₅H₅TiBzl₃(Al₂O₃) and Bzl₄/Al₂O₃ catalyze the formation of polyacetylene, but catalysts containing benzyl zirconium species are suitable for the partial hydrogenation of acetylene. Acitivity and selectivity of the system C₅H₅ZrBzl₃/Al₂O₃ are comparable to industrial used Pd/Al₂O₃ catalysts.     

K promotion of Co catalysts for the two-step methane homologation reaction

The promotion of Co catalysts with K has been examined for the two-step CH₄ homologation reaction. The effect of K was strongly influenced by the catalyst support. In the case of the SiO₂ supported catalyst, addition of K increased the CH₄ decomposition activity and decreased the second-stage hydrogenation activity while the C₂₊ selectivity increased from 14 C at% to 36 C at%. With the Al₂O₃ support, addition of K increased CH₄ decomposition activity but the C₂₊ selectivity increased only marginally. These results are discussed in terms of Co dispersion, support effects and the effect of K on the reactivity of the carbon species deposited during CH₄ decomposition.     

Selective formation of CH3OH in the photocatalytic reduction of CO2 with H2O on titanium oxides highly dispersed within zeolites and mesoporous molecular sieves

Highly dispersed titanium oxide catalysts have been prepared within zeolite cavities as well as in the zeolite framework and utilized as photocatalysts for the reduction of CO₂ with H₂O to produce CH₄ and CH₃OH at 328 K. In situ photoluminescence, ESR, diffuse reflectance absorption and XAFS investigations indicate that the titanium oxide species are highly dispersed within the zeolite cavities and framework and exist in tetrahedral coordination. The charge transfer excited state of the highly dispersed titanium oxide species play a significant role in the reduction of CO₂ with H₂O with a high selectivity for the formation of CH₃OH, while the catalysts involving the aggregated octahedrally coordinated titanium oxide species show a high selectivity to produce CH₄, being similar to reactions on the powdered TiO₂ catalysts. Ti-mesoporous molecular sieves exhibit high photocatalytic reactivity for the formation of CH₃OH, its reactivity being much higher than the powdered TiO₂ catalysts. The addition of Pt onto the highly dispersed titanium oxide catalysts promotes the charge separation which leads to an increase in the formation of CH₄ in place of CH₃OH formation.     

Promotion effect of K2O and MnO additives on the selective production of light alkenes via syngas over Fe/silicalite-2 catalysts

The addition of K₂O and MnO promoters enhances catalyst activity and selectivity to light alkenes during CO hydrogenation over silicate-2 (Si-2) supported Fe catalysts. The results of CO hydrogenation and CO-TPD, CO/H₂-TPSR, C₂H₄/H₂-TPSR and C₂H₄/H₂ pulse reaction over Fe/Si-2 catalysts with and without promoters clearly show that the MnO promoter mainly prohibits the hydrogenation of C₂H₄ and C₃H₆. Therefore, it enhances the selectivity to C₂H₄ and C₃H₄ products. Meanwhile further incorporating the K₂O additive into the FeMn/ Si-2 catalyst leads to a remarkable increase in both the capacity and strength of the strong CO adspecies. These produce much more [C_ad] via their dissociation and disproportionation at higher temperatures. This results in an increase in the CO conversion and the selectivity to light olefins. Moreover, the K₂O additive modifies the hydrogenating reactivity of [C_ad] and suppresses the disproportionation of C₂H₄ that occurs as a side-reaction. Both K₂O and MnO promoters play key roles for enhancing the selective production of light alkenes from CO hydrogenation over Fe/Si-2 catalyst.     

Stepwise Conversion of Methane over Supported Metal-Boron Amorphous Alloy Catalysts

The stepwise conversion of CH₄ to higher hydrocarbons over HMCM-22 zeolite supported metal-boron amorphous alloy catalysts has been investigated, including: (1) the influence of metals (Co, Ni, Pt, Rh and Ru) of the catalysts on the reaction; (2) the promotional effect of V on the catalytic behavior of the catalysts; (3) the influence of hydrogenation pressure and CH₄ decomposition temperature; and (4) the nature of carbon species. It is found that the yield of C⁺ ₂ hydrocarbons is strongly dependent on the metals. Good yields of C⁺ ₂ hydrocarbon are reached only on the supported NiB and CoB catalysts. The probability of C--C chain growth is increased by V promotion without seriously affecting the activities of CH₄ decomposition and hydrogenation. The ease of carbon removal via hydrogenation is strongly affected by the CH₄ decomposition temperature. Increasing hydrogenation temperature has a negative effect on the yield of C⁺ ₂ hydrocarbons. XPS measurements show that a carbide(-like) carbon species is active and selective for hydrogenation to produce higher hydrocarbons. Its activity/selectivity is greatly reduced at high CH₄ decomposition temperatures, mainly due to transition of the reactive carbidic to unreactive graphitic form. Graphite/filamental carbons were found to be formed at high CH₄ decomposition temperature.     

Characterization of titanium-boron binary oxides and their photocatalytic activity for stoichiometric decomposition of water

A titanium-boron binary oxide has been prepared by sol–gel method and used as a photocatalyst for the decomposition of water. The structure of titanium oxide species in the Ti/B binary oxide was amorphous before and crystal after calcination in O₂, while the boron oxide species maintained its amorphous state. With increasing calcination temperature, the crystalline structure of titanium oxides changed from an anatase phase to a rutile phase. Pt-loaded Ti/B photocatalysts decomposed water stoichiometrically in aqueous suspension system. Their photocatalytic activity decreased markedly with increase in the calcination temperature, indicating that the photocatalytic activity of the Ti/B binary oxide was strongly dependent on the crystal phase of the titanium oxide in the Ti/B binary oxide. A remarkable yield in the reaction of water decomposition was obtained when Na₂CO₃ was added in the Pt-loaded Ti/B binary oxide suspension.     

Photocatalytic decomposition of NO on Ti-HMS mesoporous zeolite catalysts

Titanium oxide species were loaded within the framework of mesoporous materials (Ti-HMS) by hydrothermal synthesis. These Ti-HMS exhibited high and unique photocatalytic reactivity for the decomposition of NO into N₂ and O₂ at 275 K. In situ diffuse reflectance absorption and XRD investigations indicated that the titanium oxide species were dispersed well within the zeolite framework and isolated in tetrahedral coordination with low Ti content. This revised version was published online in July 2006 with corrections to the Cover Date.     

Studies on the conditions of synthesis of picolinic acid by heterogeneous catalytic oxidation of 2-picoline

Heterogeneous oxidation of 2-picoline over binary P–Ti, Sb–Ti, P–Sb, and V–Ti oxide catalysts was studied over the temperature range of 200–300°C. The vanadium–titanium catalysts based on titanium dioxide (anatase) were found to be the most selective for picolinic acid. With binary catalysts containing 20–50% of vanadium pentoxide, the selectivity for picolinic acid was 19–22% at the 36–74% conversion of 2-picoline. A distinguishing feature of these catalysts is regular surface stacking of V₂O₅ and TiO₂ crystallites.     

Activities of γ-Al2O3-Supported Metal Oxide Catalysts in Propane Oxidative Dehydrogenation

The activities of metal oxide catalysts in propane oxidative dehydrogenation to propene have been studied. The catalysts are M/-Al₂O₃ (where M is an oxide of Cr, Mn, Zr, Ni, Ba, Y, Dy, Tb, Yb, Ce, Tm, Ho or Pr). Both transition metal oxides (TMO) and rare-earth metal oxides (REO) are found to catalyze the reaction at 350-450 °C, 1 atm and a feed rate of 75 cm³/min of a mixture of C₃H₈, O₂ and He in a molar ratio of 4:1:10. Among the catalysts, Cr-Al-O is found to exhibit the best performance. The selectivity to propene is 41.1% at 350 °C while it is 54.1% at 450 °C. Dy-Al-O has the highest C₃H₆ selectivity among the REO. At 450 °C, the other catalysts show C₃H₆ selectivity ranging from 16.2 to 37.7%. In general TMO show higher C₃H₆ selectivity than REO, which, however, show higher C₂H₄ selectivity. An attempt is made to correlate propane conversion and selectivity to C₃H₆ with metal-oxygen bond strength in the catalysts. For the TMO a linear correlation is found between the standard aqueous reduction potential of the metal cation of the respective catalyst and its selectivity to propane at 11% conversion. No such correlation has been found in the case of REO. Analyses of the product distributions suggest that for TMO propane activation the redox mechanism seems to prevail while the REO activate it by adsorbed oxygen.     

Catalysis of Methane Coupling with Carbon Dioxide Over Binary Oxides

Binary oxides of Ca-Ce, Ca-Cr, and Ca-Mn exhibit good performance and similar kinetic behavior in the conversion of CH₄ to C₂ hydrocarbons with CO₂, whereas the corresponding Sr- and Ba-containing catalysts show lower activity in C₂ formation except for Sr-Mn and Ba-Mn oxides. The Sr-Mn oxide provides even higher C₂ yield than the Ca-containing catalysts. Characterization reveals that solid solution and composite oxides comprising Ca²⁺ species and the redox component (Ce, Cr, or Mn) exist at a steady state of reaction and probably account for the synergy in C₂ formation over the Ca-containing catalysts. On the other hand, Sr and Ba carbonates are formed along with Ce, Cr, or Mn oxides during the reactions over Sr- and Ba-containing oxides. The carbonates, however, can react with MnO to form SrMnO_2.5 and BaMnO_2.5, the probable active species for CH₄ activation over the Sr-Mn and Ba-Mn catalysts.     

Role of chlorine in improving selectivity in the oxidative coupling of methane to ethylene

Samarium, magnesium and manganese oxide and alkali-promoted oxide catalysts have been prepared and tested for the oxidative coupling of methane. The results show that alkali-promoted oxides inhibit total oxidation and have a higher selectivity for the formation of C₂ products than the undoped metal oxides. These catalysts have been promoted by injecting pulses of gaseous chlorinated compounds (dichloromethane and chloroform) during the reaction. It has been found that these chlorinated compounds markedly increase the selectivity for the formation of C₂ products for all the MnO₂-based catalysts and for lithium-doped MgO and Sm₂O₃ catalysts. The effect is greatest in MnO₂-based catalysts. When dichloromethane is added to a pure, unpromoted MnO₂ catalyst the selectivity for the formation of carbon dioxide decreases from 82.6% to 4.1% and the selectivity for the formation of C₂H₄ increases from virtually zero to 56.3%. The highest C₂ selectivity observed after promotion of pure MnO₂ by dichloromethane is about 93%. Promotion of these pure oxide catalysts by gaseous chlorinated compounds provides an alternative to alkali promotion as a method of inhibiting total oxidation and of increasing ethylene production.     

Supported titanium–magnesium catalysts with different titanium content: Kinetic peculiarities at ethylene homopolymerization and copolymerization and molecular weight characteristics of polyethylene

The effect of Ti content on the activity of titanium–magnesium catalysts (TMC) and molecular weight distribution (MWD) of polyethylene (PE) produced has been studied. It was found that the activity enhances sharply as Ti content decreases from 0.6 to 0.07 wt %, and shows no significant changes in the Ti content range of 0.6–5.0 wt %. The maximum activity (36 kg PE/mmol Ti × h × bar C₂H₄) was observed for TMC with the lowest Ti content. The catalyst with low titanium content (～ 0.1 wt % of Ti) produced PE with narrower MWD (M_w/M_n = 3.1–3.5) as compared to catalysts with higher titanium content (3–5 wt % of Ti; M_w/M_n = 4.8–5.0). New data on the effect of hydrogen on MWD of PE have been found. Increasing hydrogen concentration results in broadening the MWD of PE, especially in the case of TMC with high titanium content. The data presented indicate the heterogeneity of active centers of TMC in the reaction of chain transfer with hydrogen. The data on the ethylene–hexene‐1 copolymerization over TMC with different titanium content are presented. Comonomer reactivity ratios were shown to be independent of the Ti content in TMC. Presumably the difference in activity of these catalysts is mainly caused by the difference in the number of active centers. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102:5436–5442, 2006     

In situ Raman spectroscopy studies of catalysts

In situ Raman spectroscopy is rapidly becoming a very popular catalyst characterization method because Raman cells are being designed that can combine in situ molecular characterization studies with simultaneous fundamental quantitative kinetic studies. The dynamic nature of catalyst surfaces requires that both sets of information be obtained for a complete fundamental understanding of catalytic phenomena under practical reaction conditions. Several examples are chosen to highlight the capabilities of in situ Raman spectroscopy to problems in heterogeneous catalysis: the structural determination of the number of terminal M=O bonds in surface metal oxide species that are present in supported metal oxide catalysts; structural transformations of the MoO₃/SiO₂ and MoO₃/TiO₂ supported metal oxide catalysts under various environmental conditions, which contrast the markedly different oxide–oxide interactions in these two catalytic systems; the location and relative reactivity of the different surface M–OCH₃ intermediates present during CH₃OH oxidation over V₂O₅/SiO₂ catalysts; the different types of atomic oxygen species present in metallic silver catalysts and their role during CH₃OH oxidation to H₂CO and C₂H₄ epoxidation to C₂H₄O; and information about the oxidized and reduced surface metal oxide species, isolated as well as polymerized species, present in supported metal oxide catalysts during reaction conditions. In summary, in situ Raman spectroscopy is a very powerful catalyst characterization technique because it can provide fundamental molecular‐level information about catalyst surface structure and reactive surface intermediates under practical reaction conditions. This revised version was published online in August 2006 with corrections to the Cover Date.     

Low methane selectivity using Co/MnO catalysts for the Fischer-Tropsch reaction: Effect of increasing pressure and Co-feeding ethene

CO hydrogenation using cobalt/ manganese oxide catalysts is described and discussed. These catalysts are known to give low methane selectivity with high selectivity to C₃ hydrocarbons at moderate reaction conditions (GHSV < 500 h^–1, < 600 kPa). In this study the effect of reaction conditions more appropriate to industrial operation are investigated. CO hydrogenation at 1–2 MPa using catalyst formulations with Co/Mn = 0.5 and 1.0 gives selectivities to methane that are comparable to those observed at lower pressures. At the higher pressure the catalyst rapidly deactivates, a feature that is not observed at lower pressures. However, prior to deactivation rates of CO + CO₂ conversion > 8 mol/1-catalyst h can be observed. Co-feeding ethene during CO hydrogenation is investigated by the reaction of¹³C0-¹²C₂H₄-H₂ mixtures and a significant decrease in methane selectivity is observed but the hydrogenation of ethene is also a dominant reaction. The results show that the co-fed ethene can be molecularly incorporated but in addition it can generate a C, species that can react further to form methane and higher hydrocarbons.     

Effective modification of Pd surfaces with TiO2 promoters using selective chemical vapor deposition and the effect on catalytic performance improvement

In this paper, we describe a novel method for selectively attaching TiO₂ promoters on Pd surfaces in Pd/SiO₂ catalysts using selective chemical vapor deposition (CVD). Ti was selectively deposited on Pd active sites over a SiO₂ support under a hydrogen atmosphere when titanium tetraisopropoxide was used as a Ti precursor. The Pd–Ti/SiO₂ catalyst modified by CVD exhibits approximately 1.8 times higher ethylene selectivity than the un-modified Pd/SiO₂ catalyst in the selective hydrogenation of acetylene due to the effective geometric modification of the Pd surface, which is beneficial to ethylene selectivity, by the selectively deposited Ti species.     

Lewis acidity as an explanation for oxide promotion of metals: implications of its importance and limits for catalytic reactions

Sub-monolayer quantities of metal oxides are found to influence CO hydrogenation, CO₂ hydrogenation, acetone hydrogenation, ethylene hydroformylation, ethylene hydrogenation, and ethane hydrogenolysis over Rh foils. The metal oxides investigated include AlOx, TiOx, VOx, FeOx, ZrOx, NbOx, TaOx, and WOx. Only those reactions involving the hydrogenation of C-O bonds are enhanced by the oxide overlayers. The coverage at which maximum rate enhancement occurs is approximately 0.5 ML for each oxide promoter. Titanium, niobium, and tantalum oxides are the most effective promoters. XPS measurements after reaction show that of the oxides studied titanium, niobium, and tantalum oxide overlayers are stable in the highest oxidation states. The trend in promotion effectiveness is attributed to the direct relationship between oxidation state and Lewis acidity. For the oxide promoters, bonding at the metal oxide/metal interface between the O-end of adsorbed CO and the Lewis acidic oxide is postulated to facilitate C-O bond dissociation and subsequent hydrogenation.     

Methane-Propylene homologation and reactivity of surface carbon on modified Ni-Al2O3 catalysts

Modified Ni catalysts supported on alumina and reduced in CH₄ have been investigated for the synthesis of C₄ hydrocarbons from CH₄ and C₃H₆. Addition of K or P to the Ni/Al₂O₃ catalyst increased C₄ selectivity and C₃H₆ conversion. A maximum selectivity to the desired C₄ product of 9 mol % was obtained at 350°C and 101 kPa with a feed gas composition of 90 mol% CH₄/10 mol% C₃H₆, but the catalyst activity declined rapidly with time-on-stream. Large amounts of unreactive carbon were deposited on the catalyst surface following reduction in CH₄ and reaction in CH₄/C₃H₆. However, the relative amount of a much more reactive species identified from Temperature-Programmed-Surface-Reaction that was formed in the presence of CH₄ and C₃H₆, is shown to correlate with the catalyst C₄ yield. Both the C₄ yield and the relative amount of this low temperature carbonaceous species increased in the order Ni<Ni/P<Ni/K.