Photocatalytic reduction of CO2 with H2O on Ti-MCM-41 and Ti-MCM-48 mesoporous zeolite catalysts

Titanium oxide species included within the framework of mesoporous zeolites (Ti-MCM-41 and Ti-MCM-48) prepared by a hydrothermal synthesis exhibited high and unique photocatalytic reactivity for the reduction of CO₂ with H₂O at 328 K to produce CH₄ and CH₃OH in the gas phase. In situ photoluminescence, diffuse reflectance absorption, ESR and XAFS investigations indicated that the titanium oxide species are highly dispersed within the zeolite framework and exist in tetrahedral coordination. The charge transfer excited state of the highly dispersed titanium oxide species played a significant role in the reduction of CO₂ with H₂O exhibiting a high selectivity for the formation of CH₃OH.     

Characterization of self-standing Ti-containing porous silica thin films and their reactivity for the photocatalytic reduction of CO2 with H2O

Self-standing porous silica thin films with different pore structures were synthesized by a solvent evaporation method and used as photocatalysts for the photocatalytic reduction of CO₂ with H₂O at 323 K. UV irradiation of these Ti-containing porous silica thin films in the presence of CO₂ and H₂O led to the formation of CH₄ and CH₃OH as well as CO and O₂ as minor products. Such thin films having hexagonal pore structure exhibited higher photocatalytic reactivity than the Ti-MCM-41 powder catalyst even with the same pore structure. From FTIR investigations, it was found that these Ti-containing porous silica thin films had different concentrations of surface OH groups and showed different adsorption properties for the H₂O molecules toward the catalyst surface. Furthermore, the concentration of the surface OH groups was found to play a role in the selectivity for the formation of CH₃OH.     

Catalytic reduction of NOx with H2/CO/CH4 over PdMOR catalysts

Conversion of NO_x with reducing agents H₂, CO and CH₄, with and without O₂, H₂O, and CO₂ were studied with catalysts based on MOR zeolite loaded with palladium and cerium. The catalysts reached high NO_x to N₂ conversion with H₂ and CO (>90% conversion and N₂ selectivity) range under lean conditions. The formation of N₂O is absent in the presence of both H₂ and CO together with oxygen in the feed, which will be the case in lean engine exhaust. PdMOR shows synergic co-operation between H₂ and CO at 450–500 K. The positive effect of cerium is significant in the case of H₂ and CH₄ reducing agent but is less obvious with H₂/CO mixture and under lean conditions. Cerium lowers the reducibility of Pd species in the zeolite micropores. The catalysts showed excellent stability at temperatures up to 673 K in a feed with 2500 ppm CH₄, 500 ppm NO, 5% O₂, 10% H₂O (0–1% H₂), N₂ balance but deactivation is noticed at higher temperatures. Combining results of the present study with those of previous studies it shows that the PdMOR-based catalysts are good catalysts for NO_x reduction with H₂, CO, hydrocarbons, alcohols and aldehydes under lean conditions at temperatures up to 673 K.     

TiO2基催化剂还原CO2研究进展

能源危机和环境污染是当今世界发展面临的两大挑战,如何有效缓解煤、石油等不可再生化石资源过度消耗所引发的能源危机,以及由此造成CO₂过量排放引起的温室效应问题,是当前人类发展亟待解决的重大科学问题之一。基于此,本文综述了近年来以TiO₂为光催化剂,以绿色、清洁的太阳光能催化还原CO₂成低价态含碳燃料(如CH₄、CH₃OH、HCHO、HCOOH、C₂H₅OH等)研究进展。在TiO₂光还原CO₂机理基础上,对元素掺杂、半导体复合与染料敏化、高活性晶面调控、低维纳米结构设计、助催化剂、Z型结构设计和单原子催化等方法来提高光还原CO₂反应效率和选择性进行分析,并指出目前研究存在的关键问题和未来CO₂光还原的发展方向。     

锆基双金属氧化物催化剂硫中毒的研究

在工业二氧化碳加氢制甲醇过程中,硫化氢气体的引入将对该过程中使用的催化剂活性及稳定性带来负面的影响。基于此,采用微反应合成法成功制备了InZrO_x和ZnZrO_x锆基催化剂,并研究了在二氧化碳加氢反应中,硫化氢气体对锆基催化剂的结构性质及其催化性能的影响规律。结果表明,在T=573 K、p=3.0 MPa和GHSV=18 000 mL/(g_cat·h)条件下,仅通入二氧化碳/氢气反应气时,InZrO_x和ZnZrO_x催化剂的二氧化碳转化率和甲醇选择性分别为7.2%、9.3%和93%、92%。在二氧化碳/氢气原料气中通入体积分数为5×10^-3硫化氢气体时,InZrO_x和ZnZrO_x催化剂的二氧化碳转化率和甲醇选择性都降为0,这主要是因为硫化氢气体占据了氧空位,导致锆基双金属氧化物催化剂硫中毒失活。当停止通硫化氢气体时,InZrO_x和ZnZrO_x催化剂的二氧化碳转化率和甲醇选择...     

Production of hydrogen via partial oxidation of methanol over Au/TiO2 catalysts

Selective production of hydrogen by partial oxidation of methanol (CH₃OH + (1/2)O₂ → 2H₂ + CO₂) over Au/TiO₂ catalysts, prepared by a deposition–precipitation method, was studied. The catalysts were characterized by XRD, TEM, and XPS analyses. TEM observations show that the Au/TiO₂ catalysts exhibit hemispherical gold particles, which are strongly attached to the metal oxide support at their flat planes. The size of the gold particles decreases from 3.5 to 1.9 nm during preparation of the catalysts with the rise in pH from 6 to 9 and increases from 2.9 to 4.3 nm with the rise in calcination temperature up to 673 K. XPS analyses demonstrate that in uncalcined catalysts gold existed in three different states: i.e., metallic gold (Au⁰), non-metallic gold (Au^δ+) and Au₂O₃, and in catalysts calcined at 573 K only in metallic state. The catalytic activity is strongly dependent on the gold particle size. The catalyst precipitated at pH 8 and uncalcined catalysts show the highest activity for hydrogen generation. The partial pressure of oxygen plays an important role in determining the product distribution. There is no carbon monoxide detected when the O₂/CH₃OH molar ratio in the feed is 0.3. Both hydrogen selectivity and methanol conversion increase with increasing the reaction temperature. The reaction pathway is suggested to consist of consecutive methanol combustion, partial oxidation and steam reforming.     

CH4/CO2 reforming over La2NiO4 and 10%NiO/CeO2–La2O3 catalysts under the condition of supersonic jet expansion via cavity ring-down spectroscopic analysis

CH₄/CO₂ reforming over La₂NiO₄ and 10%NiO/CeO₂–La₂O₃ catalysts under the condition of supersonic jet expansion was studied via direct monitoring of the reactants and products using the sensitive technique of cavity ring-down spectroscopy. Vibration–rotational absorption lines of CH₄, H₂O, CO₂ and CO molecules were recorded in the near infrared spectral region. Our results indicated that La₂NiO₄ is superior to 10%NiO/CeO₂–La₂O₃ in performance. In addition, we observed enhanced reverse-water-gas-shift reaction at augmented reaction temperature. The formation of reaction intermediates was also investigated by means of time-of-flight mass spectrometry and there was the detection of CH_x⁺, OH⁺ and H⁺ species.     

Decomposition of nitric oxide over perovskite oxide catalysts: effect of CO2, H2O and CH4

Direct nitric oxide decomposition over perovskites is fairly slow and complex, its mechanism changing dramatically with temperature. Previous kinetic study for three representative compositions (La_0.87Sr_0.13Mn_0.2Ni_0.8O_3−δ, La_0.66Sr_0.34Ni_0.3Co_0.7O_3−δ and La_0.8Sr_0.2Cu_0.15Fe_0.85O_3−δ) has shown that depending on the temperature range, the inhibition effect of oxygen either increases or decreases with temperature. This paper deals with the effect of CO₂, H₂O and CH₄ on the nitric oxide decomposition over the same perovskites studied at a steady-state in a plug-flow reactor with 1 g catalyst and total flowrates of 50 or 100 ml/min of 2 or 5% NO. The effect of carbon dioxide (0.5–10%) was evaluated between 873 and 923 K, whereas that of H₂O vapor (1.6 or 2.5%) from 723 to 923 K. Both CO₂ and H₂O inhibit the NO decomposition, but inhibition by CO₂ is considerably stronger. For all three catalysts, these effects increase with temperature. Kinetic parameters for the inhibiting effects of CO₂ and H₂O over the three perovskites were determined. Addition of methane to the feed (NO/CH₄=4) increases conversion of NO to N₂ about two to four times, depending on the initial NO concentration and on temperature. This, however, is still much too low for practical applications. Furthermore, the rates of methane oxidation by nitric oxide over perovskites are substantially slower than those of methane oxidation by oxygen. Thus, perovskites do not seem to be suitable for catalytic selective NO reduction with methane.     

Partial oxidation of ethanol on Ru/Y2O3 and Pd/Y2O3 catalysts for hydrogen production

The partial oxidation of ethanol was investigated over Ru and Pd catalysts supported onto yttria over a wide range of temperatures (473–1073 K). The product distributions obtained over these catalytic systems were correlated with diffuse reflectance infrared spectroscopy analyses (DRIFTS). Results showed that reaction route depended strongly on the type of metal. The decomposition of ethoxy species to CH₄ and CO or oxidation to CO₂ was promoted by Pd, and the acetaldehyde desorption was predominant over Ru in the low temperature region. Furthermore, the acetate and carbonate formation prevailed over Pd, which explained the lower acetaldehyde selectivity. The presence of CH₄ and CO₂ at high temperature is assigned to the decomposition of acetate species via carbonates over Pd-based catalysts. Ru was more suitable system for H₂ production than Pd by achieving a selectivity of about 59%.     

Fischer–Tropsch synthesis over Re-promoted Co supported on Al2O3, SiO2 and TiO2: Effect of water

The activity and selectivity of rhenium promoted cobalt Fischer–Tropsch catalysts supported on Al₂O₃, TiO₂ and SiO₂ have been studied in a fixed-bed reactor at 483 K and 20 bar. Exposure of the catalysts to water added to the feed deactivates the Al₂O₃ supported catalyst, while the activity of the TiO₂ and SiO₂ supported catalysts increased. However, at high concentrations of water both the SiO₂ and TiO₂ supported catalyst deactivated. Common for all catalysts was an increase in C₅₊ selectivity and a decrease in the CH₄ selectivity by increasing the water partial pressure. The catalysts have been characterized by scanning transmission electron microscope (STEM), BET, H₂ chemisorption and X-ray diffraction (XRD).     

Liquid phase methanol reforming with water over silica supported Pt–Ru catalysts

The mechanism of the liquid phase methanol reforming reaction over silica supported Pt–Ru catalyst was investigated by kinetic studies, employing a pyrex glass reactor with reflux condensers connected to a closed gas circulation system under ambient pressure. The rate of H₂ formation over Pt–Ru/SiO₂ catalysts was more than 20 times faster than that over Pt/SiO₂ catalysts with high selectivity for CO₂ (72.3%), indicating a marked addition effect of Ru. In the case of HCHO–H₂O reaction over Pt–Ru/SiO₂, the H₂ formation rate was five times larger than that in the CH₃OH–H₂O reaction but selectivity to CO₂ was only 4%. On the contrary, in the HCOOCH₃–H₂O and HCOOH–H₂O reactions, both high activity and selectivity were observed over Pt–Ru/SiO₂. These results clearly indicate that the CO₂ formation does not proceed via HCHO decomposition and following water gas shift reaction. We propose the following pathway for liquid phase methanol reforming reaction over Pt–Ru/SiO₂; a partly dehydrogenated methanol (CH₂OH^*) is the initial reaction intermediate, from which H₂ and CO₂ are formed through HCOOCH₃ and HCOOH as the successive reaction intermediates.     

A comparative study of the water-gas shift activity of Pt catalysts supported on single (MOx) and composite (MOx/Al2O3, MOx/TiO2) metal oxide carriers

The water-gas shift (WGS) activity of platinum catalysts dispersed on a variety of single metal oxides as well as on composite MO_x/Al₂O₃ and MO_x/TiO₂ supports (M = Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Y, Zr, La, Ce, Nd, Sm, Eu, Gd, Ho, Er, Tm) has been investigated in the temperature range of 150–500 °C, using a feed composition consisting of 3% CO an 10% H₂O. For Pt catalysts supported on single metal oxides, it has been found that both the apparent activation energy of the reaction and the intrinsic rate depend strongly on the nature of the support. In particular, specific activity of Pt at 250 °C is 1–2 orders of magnitude higher when supported on “reducible” compared to “irreducible” metal oxides. For composite Pt/MO_x/Al₂O₃ and Pt/MO_x/TiO₂ catalysts, it is shown that the presence of MO_x results in a shift of the CO conversion curve toward lower reaction temperatures, compared to that obtained for Pt/Al₂O₃ or Pt/TiO₂, respectively. The specific reaction rate is in most cases higher for composite catalysts and varies in a manner which depends on the nature, loading, and primary crystallite size of dispersed MO_x. Results are explained by considering that reducibility of small oxide particles increases with decreasing crystallite size, thereby resulting in enhanced WGS activity. Therefore, evidence is provided that the metal oxide support is directly involved in the WGS reaction mechanism and determines to a significant extent the catalytic performance of supported noble metal catalysts. Results of catalytic performance tests obtained under realistic feed composition, consisting of 3% CO, 10% H₂O, 20% H₂ and 6% CO₂, showed that certain composite Pt/MO_x/Al₂O₃ and Pt/MO_x/TiO₂ catalysts are promising candidates for the development of active WGS catalysts suitable for fuel cell applications.     

Efficient adsorption and photocatalytic degradation of organic pollutants diluted in water using the fluoride-modified hydrophobic titanium oxide photocatalysts: Ti-containing Beta zeolite and TiO2 loaded on HMS mesoporous silica

Using the F⁻ media, the hydrophobic zeolite and mesoporous silica can be synthesized. These hydrophobic porous materials exhibit the high ability for the adsorption of organic compounds diluted in water and become the useful supports of photocatalyst. The hydrophobic Ti-Beta(F) zeolite prepared in the F⁻ media exhibited high efficiency than the hydrophilic Ti-Beta(OH) zeolite prepared in OH⁻ media for the liquid-phase photocatalytic degradation of 2-propanol diluted in water to produce CO₂ and H₂O. The TiO₂ loaded on the hydrophobic mesoporous silica HMS(F) (TiO₂/HMS(F)), which was synthesized using tetraethyl orthosilicate, tetraethylammonium fluoride as the source of the fluoride and dodecylamine as templates, also exhibited the efficient photocatalytic performance for the degradation. The amount of adsorption of 2-propanol and the photocatalytic reactivity for the degradation increased with increasing the content of fluoride ions on these photocatalysts. The efficient photocatalytic degradation of 2-propanol diluted in water on Ti-Beta(F) zeolite and TiO₂/HMS(F) mesoporous silica can be attributed to the larger affinity for the adsorption of propanol molecules on the titanium oxide species depending on the hydrophobic surface properties of these photocatalysts.     

Photocatalytic conversion of CH4 and CO2 to oxygenated compounds over Cu/CdS–TiO2/SiO2 catalyst

Coupled semiconductor (CS) Cu/CdS–TiO₂/SiO₂ photocatalyst was prepared using a mutli-step impregnation method. Its optical property was characterized by UV–vis spectra. BET, XRD, Raman and IR were used to study the structure of the photocatalyst. Fine CdS was found dispersed over the surface of anatase TiO₂/SiO₂ substrate. Chemisorption and IR analysis showed methane absorbed in the molecular state interacted weakly with the surface of catalyst, and the interaction of CO₂ with CS produced various forms of absorbed CO₂ species that were primarily present in the form of formate, bidentate and linear absorption species. Photocatalytic direct conversion of CH₄ and CO₂ was performed under the operation conditions: 373 K, 1:1 of CO₂/CH₄, 1 atm, space velocity of 200 h⁻¹ and UV intensity of 20.0 mW/cm². The conversion was 1.47% for CH₄ and 0.74% for CO₂ with a selectivity of acetone up to 92.3%. The reaction mechanisms were proposed based on the experimental observations.     

Direct synthesis of organic carbonates from the reaction of CO2 with methanol and ethanol over CeO2 catalysts

The catalytic properties of CeO₂ catalysts in direct synthesis of dimethyl carbonate (DMC) from CH₃OH and CO₂ were investigated. The formation rate of DMC over the catalysts calcined at 873 K and above was almost proportional to the surface area of catalysts. However, CeO₂ calcined at 673 K showed lower activity than expected from the surface area. From the results of catalyst characterization, CeO₂ calcined at 673 K contained considerable amount of amorphous phase. In contrast, the ratio of amorphous phase decreased on the catalysts calcined at 873 K and above. This suggests that stable crystallite surface is active for the reaction.

In the CH₃OH + C₂H₅OH + CO₂ reaction at low temperature, ethyl methyl carbonate (EMC) was formed, and selectivity of EMC formation was comparable to that of DMC. The formation route is discussed by the comparison with transesterification reaction.     

异丙醇钛控制水解的小角X射线散射研究

由金属醇盐水解制备溶胶的方法已广泛应用于溶胶-凝胶法制备纳米孔无机膜,但对金属醇盐水解机理的认识十分有限。通过控制异丙醇钛[Ti（i-OC₃H₇）₄]在异丙醇（i-C₃H₇OH）中水解制备TiO₂溶胶,利用小角X射线散射（SAXS）方法研究了由不同H₂O/Ti（i-OC₃H₇）₄的反应混合物[Ti（i-OC₃H₇）₄：H₂O：i-C₃H₇OH=1：m：30（摩尔比）]形成TiO₂溶胶的过程,探讨了控制Ti（i-OC₃H₇）₄水解的过程中胶粒形成与长大的规律。研究结果表明,所合成的TiO₂溶胶的胶粒粒径小于10 nm,胶粒的形成和长大与H₂O/Ti（i-OC₃H₇）₄摩尔比密切相关。H₂O/Ti（i-OC₃H₇）₄（摩尔比） ≥ 2.0时,随着H₂O/Ti（i-OC₃H₇）₄增加,溶胶的稳定性下降。     

Potential of zeolite supported rhodium catalysts for the CO2 reforming of CH4

Zeolite Y supported rhodium catalysts were prepared by ion-exchange starting from an aqueous solution of [Rh[(NH₃)₅Cl]Cl₂·6H₂0]. Previous work in this laboratory had shown that this procedure results in a Rh dispersion of near 100%. The catalysts were tested for their activity in the CO₂ reforming of CH₄. They were found to combine extraordinary stability with high activity and selectivity. At 923 K, 90 mol-% of the CH₄ was converted giving a H₂/CO ratio near unity. A weight loading of 0.5 to 0.93% Rh gives the highest turnover frequencies. Thermodynamic equilibrium is reached near 873 K. With a given Rh loading, the zeolite supports are superior to amorphous supports and NaY is superior to the HY. No deactivation was observed in tests of 30 h time on stream at atmospheric pressure or after repeated thermal cycles. No coke deposition was detected by temperature programmed oxidation of used catalysts. Temperature programmed reduction indicates the presence of three discernible Rh species.     

Designing the activity/selectivity of surface acidic, basic and redox active sites in the supported K2O–V2O5/Al2O3 catalytic system

The supported K₂O–V₂O₅/Al₂O₃ catalytic system was designed to create surfaces that were 100% acidic, 100% basic, 100% redox, mixed redox-acidic and mixed redox-basic. The resulting nature of the surface sites was controlled by the impregnation of the specific additives (K-basic or V-redox/acidic), their order of impregnation and their surface coverage. The exact locations of the surface methoxy intermediates (AlOCH₃, KOCH₃ or VOCH₃) on the mixed oxide catalyst surfaces during methanol oxidation were determined with in situ Raman spectroscopy. The surface chemistry of the various surface sites and their surface reaction intermediates were chemically probed by CH₃OH oxidation steady-state and temperature programmed surface reaction (TPSR) spectroscopy studies. The specific reactivity order and the product selectivity of the various surface sites were found to be: VOCH₃ (HCHO) AlOCH₃ (CH₃OCH₃) KOCH₃ (primarily CO₂ and minor amounts of HCHO). Formation of dimethoxy methane, (CH₃O)₂CH₂, required the presence of dual surface redox-acidic sites surface redox sites to yield H₂CO and surface acidic sites to insert the surface methoxy into H₂CO to form dimethoxy methane, (CH₃O)₂CH₂. The addition of basic surface potassium oxide to Al₂O₃ possessing surface acid sites completely suppressed reactions from the surface acidic sites and formed a surface with only basic characteristics. The addition of redox surface vanadia to the supported K₂O/Al₂O₃ catalyst was able to completely suppress reactions from surface basic sites and formed a surface with only redox characteristics. These studies demonstrate that it is possible to determine the specific surface site requirements for each reaction pathway for methanol oxidation to products, and that this informative approach should also be applicable to other reactant molecules.     

Tri-reforming of methane: a novel concept for catalytic production of industrially useful synthesis gas with desired H2/CO ratios

A novel process concept called tri-reforming of methane has been proposed in our laboratory using CO₂ in the flue gases from fossil fuel-based power plants without CO₂ separation [C. Song, Chemical Innovation 31 (2001) 21–26]. The proposed tri-reforming process is a synergetic combination of CO₂ reforming, steam reforming, and partial oxidation of methane in a single reactor for effective production of industrially useful synthesis gas (syngas). Both experimental testing and computational analysis show that tri-reforming can not only produce synthesis gas (CO + H₂) with desired H₂/CO ratios (1.5–2.0), but also could eliminate carbon formation which is usually a serious problem in the CO₂ reforming of methane. These two advantages have been demonstrated by tri-reforming of CH₄ in a fixed-bed flow reactor at 850 °C with supported nickel catalysts. Over 95% CH₄ conversion and about 80% CO₂ conversion can be achieved in tri-reforming over Ni catalysts supported on an oxide substrate. The type and nature of catalysts have a significant impact on CO₂ conversion in the presence of H₂O and O₂ in tri-reforming in the temperature range of 700–850 °C. Among all the catalysts tested for tri-reforming, their ability to enhance the conversion of CO₂ follows the order of Ni/MgO > Ni/MgO/CeZrO > Ni/CeO₂ ≈ Ni/ZrO₂ ≈ Ni/Al₂O₃ > Ni/CeZrO. The higher CO₂ conversion over Ni/MgO and Ni/MgO/CeZrO in tri-reforming may be related to the interaction of CO₂ with MgO and more interface between Ni and MgO resulting from the formation of NiO/MgO solid solution. Results of catalytic performance tests over Ni/MgO/CeZrO catalysts at 850 °C and 1 atm with different feed compositions confirm the predicted equilibrium conversions based on the thermodynamic analysis for tri-reforming of methane. Kinetics of tri-reforming were also examined. The reaction orders with respect to partial pressures of CO₂ and H₂O are different over Ni/MgO, Ni/MgO/CeZrO, and Ni/Al₂O₃ catalysts for tri-reforming.     

A comparative study of TiO2 supported noble metal catalysts for the oxidation of formaldehyde at room temperature

The TiO₂ supported noble metal (Au, Rh, Pd and Pt) catalysts were prepared by impregnation method and characterized by means of X-ray diffraction (XRD) and BET. These catalysts were tested for the catalytic oxidation of formaldehyde (HCHO). It was found that the order of activity was Pt/TiO₂  Rh/TiO₂ > Pd/TiO₂ > Au/TiO₂  TiO₂. HCHO could be completely oxidized into CO₂ and H₂O over Pt/TiO₂ in a gas hourly space velocity (GHSV) of 50,000 h⁻¹ even at room temperature. In contrast, the other catalysts were much less effective for HCHO oxidation at the same reaction conditions. HCHO conversion to CO₂ was only 20% over the Rh/TiO₂ at 20 °C. The Pd/TiO₂ and Au/TiO₂ showed no activities for HCHO oxidation at 20 °C. The different activities of the noble metals for HCHO oxidation were studied with respect to the behavior of adsorbed species on the catalysts surface at room temperature using in situ DRIFTS. The results show that the activities of the TiO₂ supported Pt, Rh, Pd and Au catalysts for HCHO oxidation are closely related to their capacities for the formation of formate species and the formate decomposition into CO species. Based on in situ DRIFTS studies, a simplified reaction scheme of HCHO oxidation was also proposed.