First-principles studies of vanadia-titania catalysts: beyond the monolayer

Periodic density functional calculations have been used to investigate the structure and stability of epitaxial vanadium oxide films grown on the TiO(2)(001) anatase surface. The formation energy of films of V(2)O(5) stoichiometry, initially low, is found to rapidly increase with the film thickness, at variance to what is obtained for reduced pseudomorphic VO(2) films. This is in tune with results of oxygen-assisted molecular beam epitaxy. The oxidation of thick, viz. >2 monolayers (ML), VO(2) films yields a c(2 x 2) reconstructed surface, in agreement with low energy electron diffraction. These films are composed by partially reduced inner V atoms in a distorted-octahedral environment, and by isolated surface dioxovanadium centers exhibiting a distorted trigonal-bipyramidal coordination. Single scattering simulations of X-ray photoelectron diffraction patterns have also been performed, taking both 2- and 3-ML surface surface-oxidized films as models. Results are in fair agreement with experiments referring to films grown in oxidizing conditions, which suggests that coherent vanadia ultrathin films could be formed in vanadia-titania catalysts. The electronic structure of the films has been finally studied, finding that the terminal oxygens carried by the surface dioxovanadium species have a strong nucleophilic character, which makes them potential active centers for selective oxidation catalysis.     

Formation of vanadia-titania oxide catalysts

The formation of vanadia-titania catalysts was studied with a complex of physicochemical methods. The use of highly dispersed anatase with a defect structure results in the formation of coherent boundaries of coalescence of the V₂O₅ and TiO₂ crystallites with the ratio V : Ti =1 : 1 in a wide range of vanadium and titanium concentrations. The catalysts containing coherent boundaries are active and selective in Β-picoline oxidation to nicotinic acid.     

Overview on the selective lean NO x reduction by hydrocarbons over Pt-based catalysts

Selective catalytic reduction of NO _x with hydrocarbons (HC-SCR) has received much attention as one of potential technologies for reducing NO _x emissions under lean-burn conditions. Pt/ZSM-5 prepared by sublimation method and Pt/V/MCM-41 catalysts have been introduced for the wider activity temperature window than those Pt catalysts reported previously. The influence of pre-treatment, oxygen concentration, water and SO₂ on the activities of Pt-based catalysts has been discussed. Combinatorial catalysis, which has been developed recently for discovering the practical HC-SCR catalysts quickly, has been introduced too. Finally, the reaction mechanism of HC-SCR over Pt-based catalysts has been briefly discussed.     

Co-doped MnCeOx/ZrO2 catalysts for low temperature selective catalytic reduction of NO

Co-doped MnCeO_x/ZrO₂ catalysts were synthesized by impregnation method and their low temperature deNOx performance were evaluated. The physicochemical properties of the catalysts were studied. The results showed that the doped Co could promote the deNOx performance of MnCeO_x/ZrO₂ significantly, and the doped catalyst with the Co/Mn molar ratio of 1:2 possessed the best catalytic performance. Compared with pure MnCeO_x/ZrO₂ catalyst, the deNOx efficiency of the optimal 1Co₂MnCeO_x/ZrO₂ was higher to 93% at 100 °C, improved nearly by 17%. The complete removal of NO was achieved at the temperature range of 120–250 °C. The promoted catalytic performance of Co-doped MnCeO_x/ZrO₂ catalyst was mainly attributed to the improvement of the catalyst support structure and surface acidity by Co. The catalytic reaction of NO with NH₃ over 1Co2MnCeO_x/ZrO₂ catalyst follows both Eley–Rideal mechanism and Langmiur–Hinshelwood mechanism.

     

Car exhaust catalysis from first principles: selective NO reduction under excess O2 conditions on Ir

Combining energetic data from density functional theory with thermodynamic calculations, we have studied in detail selective NO reduction under excess O2 conditions on Ir. We show that excess O2 can readily poison the Ir catalyst for NO reduction and the poisoning starts from a low O coverage on the surface. The adsorbed O switches the reaction selectivity from reduction (N2 production) to oxidation (NO2 production). As the O coverage is built up, Ir metal can eventually be oxidized to IrO2, which is predicted to be thermodynamically possible under reaction conditions. To prevent O poisoning the surface, the presence of reductants is thus essential. We demonstrate that NO reduction is sensitive to the choice of reductant, and that alkenes are the most effective, mainly because they are able to produce surface C atoms that can selectively remove O atoms from Ir steps. On the basis of our analyses of the electronic structures, the mechanism of O-poisoning is elucidated and the reactant sensitivity in NO reduction is also discussed in terms of the bonding competition effect. We found that for different adsorbates, such as NO, O, and N, their bondings with surface d-states are remarkably similar. This gives rise to an indirect repulsion between adsorbates whenever they may bond with the same metal atoms. This energy cost can be qualitatively correlated with the valency of the adsorbate, and this is the key to understand the O-poisoning effect and the structure sensitivity in NO reduction.     

Toward an understanding of the formation of vanadia-titania catalysts

Structures of hydrated vanadia species on the TiO2-anatase surfaces were investigated using the semiempirical molecular orbital method MSINDO. The (101), (001), and (100) surfaces of anatase were considered. They were modeled by appropriate two-dimensional cyclic clusters of TiO2. Monomeric and dimeric hydrated vanadia species on the anatase surfaces were simulated by adsorbing VO4H3 and V2O7H4 molecules, respectively. Different adsorption structures were considered, and their stabilities at 300 and 600 K were tested by constant-temperature Born-Oppenheimer molecular dynamics simulations in the framework of MSINDO. Structural features of the vanadia-titania catalysts found in extended X-ray absorption fine structure, secondary ion mass spectrometry, IR, Raman, and NMR spectroscopy and conductivity experiments can be explained by the present calculations.     

Metal oxide promoted TiO2 catalysts for photo-assisted selective catalytic reduction of NO with NH3

The photo-assisted selective catalytic reduction (SCR) of NO with NH₃ (Photo-SCR) was performed over TiO₂ modified by supporting 1 wt% of various transition metal (V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb, Mo, Ta or W) oxides aiming at the improvement of the photo-SCR activity. The addition of Nb, Mo or W oxide to TiO₂ was found to enhance the photo-SCR activity. We have reported that the amount of acid sites on TiO₂ is one of the key factors to the photo-SCR activity. The increase in the activity depends on the enhancement of acidity of catalyst by the addition of Nb, Mo or W oxide. In contrast, the addition of V, Cr, Mn, Fe, Co, Ni or Cu oxide to TiO₂ lowered the photo-SCR activity, although addition of metal cations also changed the acidity of TiO₂. We guess that the reduction of the activity was caused by two reasons; the first is that the sites newly formed on these transition metal oxides is not photoactive and the second is that TiO₂ supporting V, Cr, Mn, Fe, Co, Ni or Cu oxides had low stabilities under the reaction conditions, i.e., the chemical state of the cations changed during the reaction. Therefore, we concluded that the increase in the acid sites that are active sites for photo-SCR and the stability of the catalysts are important for the photo-SCR.     

Production from ilmenite of TiO2-supported catalysts for selective catalytic reduction of NO with NH3

Economic production of titanium dioxide (yield >98 %) from ilmenite has been achieved by use of a modified sulfate reduction process. A series of samples were prepared by varying the concentration of titanium dioxide nuclei (0.2, 0.3, and 0.6 %) and further impregnation with antimony and vanadia. The structural and acidic properties of the samples were comprehensively studied by X-ray diffraction (XRD), transmission electron microscopy, BJH pore size distribution, and temperature-programmed desorption of NH₃. The XRD results revealed the presence of intense peaks from anatase titanium dioxide. Enhancement of surface area was observed for second-time filtered samples, possibly because of loss of iron from the bulk. As a result, formation of additional micropores was apparent from N₂ adsorption and desorption isotherms. Among all the antimony and vanadia-doped samples, the first-time filtered sample with the low concentration of nuclei (0.2 %) had the highest catalytic activity at low temperatures, owing to its larger pore size and abundant acidic species.     

Low temperature selective catalytic reduction (SCR) of NO with NH3 over Fe-Mn based catalysts

Fe-Mn based transition metal oxides (Fe-Mn, Fe-Mn-Zr and Fe-Mn-Ti) show nearly 100% NO conversion at 100-180 degrees C for selective catalytic reduction of NO with NH3 under the applied conditions with a space velocity of 15,000 h-1.     

Fe-Mn/Al2O3催化剂低温 NH3选择性催化还原 NO的性能

本文制备了一系列 Fe-Mn/Al2O3催化剂,并在固定床上考察了其 NH3低温选择性催化还原 NO的性能.首先考察了不同 Fe负载量制备的催化剂的脱硝性能,优选出最佳的 Fe负载量;在此基础上,研究了 Mn负载量对催化剂脱硝效率的影响;最后,对优选催化剂的抗 H2O和抗 SO2性能进行了实验研究;同时,对催化剂由于 SO2所造成的失活机制进行了考察.采用 N2吸附-脱附、X射线衍射、透射电镜、能量弥散 X射线谱、程序升温还原、程序升温脱附、X射线光电子能谱、热重和傅里叶变换红外光谱等方法对催化剂进行了表征.结果表明,最佳的 Fe和 Mn负载量均为8%,所制的8Fe-8Mn/Al2O3催化剂在150°C的脱硝效率可达近99%;同时,在整个低温测试区间(90–210°C)的脱硝效率均超过了92.6%. Fe在催化剂表面主要以 Fe3+形态存在,而 Mn主要包括 Mn4+和 Mn3+; Mn的添加提高了 Fe在催化剂表面的积累,促进了催化剂比表面积增大和活性物种分散,改善了催化剂氧化还原性能和对 NH3的吸附能力.催化剂的高活性主要是由于其具有较大的比表面积、高度分散的活性物种、增加的还原特性和表面酸性、较低的结合能、较高的 Mn4+/Mn3+和增强的表面吸附氧.此外,8Fe-8Mn/Al2O3的催化性能受 H2O和 SO2影响较小,抗 H2O和 SO2能力较强.同时,反应温度对催化剂的抗硫性有重要影响,在较低的反应温度下,催化剂抗硫性更好; SO2造成催化剂活性降低主要是由于催化剂表面硫酸盐物种的生成.一方面,表面硫酸铵盐的生成造成催化剂孔道堵塞和比表面积降低,减少了反应中的气固接触从而导致活性降低;另一方面,催化剂表面的活性物种被硫酸化,造成反应中的有效活性位减少,从而降低了催化剂活性.     

Active sites for NO reduction over Fe-ZSM-5 catalysts

A study of Fe-ZSM-5 catalysts with variable amounts of isolated, oligomeric and heavily aggregated Fe3+ oxo sites (as evidenced by UV-Vis and EPR spectroscopic data) and their catalytic properties in the selective catalytic reduction of NO by isobutane or by NH3 is presented, which allows development of a unified concept of the active Fe sites in these reactions, according to which isolated Fe sites catalyse both SCR reactions while oligomeric sites, though also involved in the selective reduction path, limit the catalyst performance by causing the total oxidation of the reductant.     

Modified montmorillonites as potential NO decomposition catalysts: Characterization by temperature-programmed reduction

Na-montmorillonite has been modified by ion exchange of simple Fe³⁺ or Keggintype cations obtainedvia the partial hydrolysis of AlCl₃ or the co-hydrolysis of AlCl₃+FeCl₂ mixture, in order to prepare catalysts for NO decomposition. Temperature-programmed reduction, the most important characterizing method in this work, revealed that isomorphous substitution of iron for aluminium was not feasible, instead, co-hydrolysis and co-pillaring occurred, resulting in Fe,Al mixed pillared clays.     

环境友好的选择性催化还原氮氧化物催化剂

富氧条件下的氮氧化物（NO。）选择性催化还原（SCR）是目前环境催化领域的研究热点,其核心问题是研发环境友好、高效稳定的SCR催化剂．目前,以NH3为还原剂的NH3-SCR已经大规模应用于固定源烟气脱硝和柴油车尾气净化,以碳氢化合物（Hc）为还原剂的HC-SCR也有望实际应用．针对NH3-SCR,本文以作者研究的铁钛复合氧化物催化剂、铈基氧化物催化剂以及国际上新兴的小孔分子筛催化剂为例,从催化剂结构、SCR反应机理、催化剂低温活性改进以及抗中毒性能等诸多方面对该领域的研究进展做了较为全面的论述．针对HC-SCR催化剂,本文在综述长链HC及柴油选择性还原NQ研究现状的基础上,结合作者在HC-SCR反应机理方面的研究成果,展望了实现柴油-SCR的发展方向．     

Effect of oxygen on NO reduction over nickel chromite catalysts

Reduction rates of NO_x in HO_x+CO(H₂)+O₂ mixtures over a pure nickel chromite catalyst and samples supported on -Al₂O₃ and faience are high. At a space velocity of 10,000 h^–1, the complete reduction of nitrogen oxides by hydrogen and by carbon monoxide is achieved at 400–450°C and 450–500°C, respectively. Hence these catalysts can be recommended as a basis to develop commercial catalysts for NO_x removal from oxygen-containing exhaust gases.

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-Al₂O₃ NO_x NO_x–CO (H₂)–O₂ . 10 ·^–1 400–450°C, — 450–500°C. .

     

Fe/Al-PILC催化C_3H_6选择性还原NO的实验研究

采用浸渍法制备了负载于铝柱撑黏土的铁基催化剂(Fe/Al-PILC),在固定床反应器上测试其催化C3H6选择性还原NO的性能。通过N2吸附-脱附、X射线衍射(XRD)、H2的程序升温还原(H2-TPR)、紫外可见光谱(Uv-vis)、吡啶吸附红外光谱(Py-FTIR)等手段对催化剂的物理化学性质进行表征。结果表明,9Fe/Al-PILC在400-550℃能够还原98%以上的NO,而且SO2和水蒸气对其催化性能的影响很小。XRD、N2吸附-脱附表征结果表明,Fe/Al-PILC催化剂中铁氧化物高度分散在载体表面,催化剂有较大的比表面积和孔容。H2-TPR结果表明,催化剂的活性主要由Fe_2O_3物相的还原性能决定。Uv-vis结果表明,催化剂的活性与铁氧低聚物种FexOy呈正相关性。Py-FTIR结果表明,催化剂表面同时存在Lewis酸和Brnsted酸,L酸性位是NO和C3H6反应的主要催化活性中心。     

First principles studies on boron sites in zeolites

A systematic computational investigation on protonated and nonprotonated boron-containing zeolites (boralites), performed by using different periodic density functional theory approximations, is presented. Both minimum energy structures and finite temperature behavior of model boron sodalites were analyzed. All of the adopted computational schemes agree in predicting an acid site composed of a silanol Si-OH group loosely linked to a planar BO(3) structure in the protonated system and a BO(4) tetrahedral site in the sodium-containing zeolite. Calculated structural and vibrational properties are in line with experimental data. Comparisons of the protonated boralite site with Al and Ga zeolitic acid sites are discussed as well. Results indicate that this class of mild acid catalysts is characterized by significant framework flexibility and pronounced thermal effects due to the loosely bound acid site.     

Catalytic reduction of NO with decomposed methanol on alumina-supported Mn-Ce catalysts

A series of manganese-ceria supported on alumina catalysts with various Mn/Ce ratios are investigated in both methanol decomposition to CO and hydrogen and SCR of NO(x) with CO. The study is aimed at the potential application of both reactions in integrated devices, where NO(x) is reduced with the products of the decomposed methanol. The samples are characterized by nitrogen physisorption, XRD, TEM, XPS, UV-Vis, and TPR. It was established that manganese-ceria supported on alumina catalysts are perspective in both methanol decomposition and NO reduction at temperatures above 723 K, which are typical for exhausted gases from the vehicles and some stationary stations. The best catalytic activity and selectivity to the desired products under these conditions was found for the samples with Mn/Mn+Ce ratio of 0.5 and 0.7. This superior catalytic performance is related to the formation of mixed valence Mn(3+)/Ce(4+) and Mn(4+)/Ce(3+) active sites.     

First principles based mean field model for oxygen reduction reaction

A first principles-based mean field model was developed for the oxygen reduction reaction (ORR) taking account of the coverage- and material-dependent reversible potentials of the elementary steps. This model was applied to the simulation of single crystal surfaces of Pt, Pt alloy and Pt core-shell catalysts under Ar and O(2) atmospheres. The results are consistent with those shown by past experimental and theoretical studies on surface coverages under Ar atmosphere, the shape of the current-voltage curve for the ORR on Pt(111) and the material-dependence of the ORR activity. This model suggests that the oxygen associative pathway including HO(2)(ads) formation is the main pathway on Pt(111), and that the rate determining step (RDS) is the removal step of O(ads) on Pt(111). This RDS is accelerated on several highly active Pt alloys and core-shell surfaces, and this acceleration decreases the reaction intermediate O(ads). The increase in the partial pressure of O(2)(g) increases the surface coverage with O(ads) and OH(ads), and this coverage increase reduces the apparent reaction order with respect to the partial pressure to less than unity. This model shows details on how the reaction pathway, RDS, surface coverages, Tafel slope, reaction order and material-dependent activity are interrelated.     

Influence of chromium modification on the properties of MnOx-FeOx catalysts for the low-temperature selective catalytic reduction of NO by NH3

Catalytic properties of MnO_x-FeO_x complex oxide (hereafter denoted as Mn-Fe) catalysts modified with different loadings of chromium oxide were investigated by using the combination of physico-chemical techniques, such as N₂ physisorption, X-ray diffraction (XRD), high-resolution transmission electron microscope (HRTEM), in situ Fourier transform infrared spectroscopy (in situ FT-IR) and temperature-programmed reduction (TPR) and their catalytic activities were evaluated with the selective catalytic reduction (SCR) of NO_x by NH₃. It was found that with the addition of Cr, more NO could be removed in the low-temperature window (below 120 °C). Among the tested catalysts, Mn-Fe-Cr (2 : 2 : 1) catalyst exhibited the best catalytic performance at 80 °C with the NO conversion higher than 90%. The combination of the reaction and characterization results indicated that (1) the strong interaction among tertiary metal oxides existed in the catalysts when Cr was appropriately added, which made the active components better dispersed with less agglomeration and sintering and the largest BET specific surface area could be obtained; (2) Cr improved the low-temperature reducibility of the catalyst and promoted the formation of the active intermediate (–NH⁺₃), which favored the low-temperature SCR reaction.