Contribution to the mechanism of NO reduction by CO over Pt/NaX zeolite

The reaction of NO + CO was studied over Pt/NaX prepared by the decomposition of [Pt(NH₃)₄]²⁺. The decomposition was carried out via calcination followed by reduction, by vacuum decomposition, and by decomposition in hydrogen, by ways which are known to lead to the formation of Pt clusters of different sizes and location. The NO reduction by CO was studied under static conditions for longer (20–30 min) and shorter (100 s) time intervals, and the reaction was followed by temperature programmed decomposition (TPD) of species adsorbed during the preceding isothermal reactions. The effect of various NO/CO ratios and of added oxygen was examined. The reactions of N₂O + CO were compared with those of NO + CO. The increasing size of Pt clusters enhances the reduction of NO by CO, but it is complicated at lower reaction temperatures (below 230°C) by the poisoning of active Pt centres, especially by adsorbed CO. Smaller Pt clusters exhibit higher preference towards NO adsorption from NO + CO mixtures than the larger Pt clusters. The incomplete reduction of NO to N₂O proceeds under our experimental conditions below 230°C, and is accompanied by the formation of adsorbed species. N₂O formation is enhanced by the increased NO/CO ratio and by the addition of oxygen. The reduction of nitrous oxide occurs much slower than that of nitric oxide, and therefore N₂O could play a role only as a surface intermediate in the CO + NO reaction.     

Activity during reduction of NO by CO over bimetallic palladium-rhodium/silica catalysts

A study of the activity of bimetallic Pd-Rh catalysts supported on silica in the reduction of NO by CO is presented. The catalysts were prepared by three different methods: (1) Pd and Rh were coimpregnated on the support, (2) Rh was impregnated first and, after calcining, the sample was impregnated with Pd, (3) the monometallic Pd and Rh catalysts were physically mixed. The results showed that the activity of the catalysts prepared by coimpregnation was much lower than that of the other two catalysts.     

Catalytic property of Pt/AlSBA-15 in selective catalytic reduction of NO

Pt-supported on mesoporous SBA-15 catalysts (Pt/SBA-15) have been investigated for selective catalytic reduction of NO by propene in excess oxygen. Catalytic activity of Pt/SBA-15 can be improved by the post-synthesis modification of Al into SBA-15. The resulting material (AlSBA-15) retains a hexagonal order and physical properties of the parent SBA-15 and shows new acid sites. Increased acid sites are confirmed by NH₃-TPD. Medium acid site appeared at 200 °C and broad strong acid site appeared above 250 °C. A certain degree of support acidity of Pt/AlSBA-15 increased the NO conversion, but when there are too many acid sites, carbon deposition becomes extensive and lead to decrease the NO conversion. The influence of different Pt loadings on the catalytic properties was also investigated.     

Reduction of NO by hydrogen versus reduction by CO over Pt, Pd and Rh clusters in NaX zeolite

The activation of hydrogen and CO was examined using D₂+H₂ equilibration at room temperature and by ¹³CO+C¹⁸O scrambling at 170°C, respectively, the adsorption of NO at room temperature and its TPD were used to characterise the activation of NO. These reactions were compared with the NO reduction by carbon monoxide and by hydrogen. It appeared that the M/NaX clusters (M=Pt, Rh or Pd) exhibit opposite behaviour in the NO reduction by these two reductants: with CO the sequence was RhPdPt, while platinum (Pt/NaX) was the most active catalyst (PtPd>Rh) when hydrogen was employed.

The CO scrambling was found to be most rapid over Pt, while the adsorption and dissociation of NO was most extensive over Rh; in the NO reduction by CO the weak CO activation over Rh was overwhelmed by the strong NO dissociation. On the other hand, the extensive NO adsorption and dissociation over Rh hindered the dissociation of hydrogen, which resulted in the lowest activity in the NO reduction by H₂ accompanied by an intermediate formation of N₂O. This was not the case with Pt, over which easily dissociated hydrogen reacted with probably molecularly adsorbed NO.

The reduction of N₂O by hydrogen proceeded readily over all metallic clusters at room temperature, being thus, either of the same activity as that of NO+H₂ reaction, or even of higher activity over Pd and especially over Rh. The easy reduction of N₂O by hydrogen does not agree with the reduction by CO, which was found to proceed worse than that of NO.

In some cases, also bimetallic species (PtRh/NaX, PtPd/NaX, PdRh/NaX) were employed, as well as oxidized M clusters.     

Catalytic reduction of NO by CO over NiO/CeO2 catalyst in stoichiometric NO/CO and NO/CO/O2 reaction

A new catalyst composed of nickel oxide and cerium oxide was studied with respect to its activity for NO reduction by CO under stoichiometric conditions in the absence as well as the presence of oxygen. Activity measurements of the NO/CO reaction were also conducted over NiO/γ-Al₂O₃, NiO/TiO₂, and NiO/CeO₂ catalysts for comparison purposes. The results showed that the conversion of NO and CO are dependent on the nature of supports, and the catalysts decreased in activity in the order of NiO/CeO₂ > NiO/γ-Al₂O₃ > NiO/TiO₂. Three kinds of CeO₂ were prepared and used as support for NiO. They are the CeO₂ prepared by (i) homogeneous precipitation (HP), (ii) precipitation (PC), and (iii) direct decomposition (DP) method. We found that the NiO/CeO₂(HP) catalyst was the most active, and complete conversion of NO and CO occurred at 210 °C at a space velocity of 120,000 h⁻¹. Based on the results of surface analysis, a reaction model for NO/CO interaction over NiO/CeO₂ has been proposed: (i) CO reduces surface oxygen to create vacant sites; (ii) on the vacant sites, NO dissociates to produce N₂; and (iii) the oxygen originated from NO dissociation is removed by CO.     

Nanocrystalline CeO2 in SBA-15: Performance of Pt/CeO2/SBA-15 Catalyst for Water-gas-shift Reaction

CeO₂ particles confined within the pores of an SBA-15 mesoporous silica host were prepared by incipient wetness impregnation (IMP) and deposition precipitation (DP) methods. The materials were characterized by XRD, N₂-adsorption and temperature programmed reduction (TPR) to evaluate the structure, texture, and redox properties. The preparation procedure had significant impact on the assembling mode of CeO₂ inside the SBA-15 mesopores. A high dispersion of CeO₂ particles was achieved via DP, whereas the dispersion of CeO₂ prepared by IMP was found to be inhomogeneous and CeO₂ partially blocked the pores. The CO conversion in the water-gas-shift reaction was enhanced over 1 wt% Pt supported on CeO₂-modified SBA-15 obtained by DP.     

金属氧化物催化CO还原NO的研究进展

一氧化碳（CO）广泛存在于烧结/球团/焦化烟气或汽车尾气中,应用CO-选择性催化还原（SCR）技术同时脱除烟气中CO和NO是烟气治理的理想方案之一。目前,在NO-CO反应研究中较多的是贵金属催化剂,但由于其价格昂贵、高温失活、易中毒等问题难以在工业中实现应用。本文将近几年来金属氧化物催化CO还原NO的研究成果进行了系统的梳理与总结,重点介绍Fe基、Ce基、Co基、Cu基这4种金属氧化物催化剂的研究进展,分析催化剂的制备方法、掺杂助剂种类和比例、NO-CO反应条件等因素与催化活性之间的关系,总结催化剂抗水抗硫性能及可能的CO-SCR反应机理,并探讨O₂存在的条件下对催化剂活性的影响,为提高金属氧化物催化剂抗氧性研究提供理论参考。     

Characterization of Pt/SBA-15 prepared by the deposition–precipitation method

Mesoporous silica SBA-15 was prepared and loaded with Pt using the deposition–precipitation method (DP). The Pt loaded material was characterized by N₂ sorption, and X-ray diffraction (XRD) at low scattering angles as well as XRD at wide angles, in order to monitor the impact of the metal deposition pathway on the mesoporous texture. After DP the material contains ordered mesoporous silica as well as a fraction appearing as non-ordered amorphous silica. This is most likely caused by the hydrothermal treatment involved in the DP. The material was also characterized using NIR and ²⁹Si MAS NMR spectroscopy. The NIR results of the calcined materials indicate that the silanol groups of SBA-15 may act as anchoring groups for the metallic Pt particles. The NMR spectroscopy data shows that the Pt/SBA-15 sample prepared by the DP method posseses a better short-range regularity of SBA-15 walls as compared to the parent SBA-15. This is suggested to be caused by dissolution and possible re-precipitation of siliceous species.     

Effect of Pt dispersion on the reduction of NO by propene over alumina-supported Pt catalysts under lean-burn conditions

The hydrogen exchange in the propane‐d ₈ loaded zeolite H‐ZSM‐5 was monitored by in situ ¹H MAS NMR spectroscopy within the temperature range 457–543 K. Measurements of the H/D exchange between the acidic hydroxyl groups of the zeolite and the adsorbed deuterated propane molecules show that both methyl and methylene groups of alkane are involved in the exchange. The comparison of the experimentally obtained apparent activation energies for the exchange in methyl groups (108 ± 7 kJ mol^-1) and methylene groups (117 ± 7 kJ mol^-1) with theoretical values for methane and ethane supports the assumption that the H/D exchange for methyl and methylene groups takes place via a pentavalent transition state. This revised version was published online in July 2006 with corrections to the Cover Date.     

水泥分解炉高CaO/CO2环境CO还原NO机制

通过流化床反应器模拟水泥分解炉高CaO/CO_2环境,并结合分子动力学广义梯度密度泛函理论,研究了CaO对CO还原NO的催化特性和CO_2促使CaO催化失效规律。在900℃、体积分数15%CO_2下,CaO催化作用可使NO脱除效率提高18%左右。在900℃时,当CO_2体积分数由5%升至30%,CO对NO还原率由97.0%降低到23.4%,且随反应时间增长CaO对CO还原NO的催化作用不断减弱。基于广义梯度密度泛函理论计算,CO_2,CO,NO在CaO表面活性位点的吸附能依次为CO_2(-1.869 eV)>NO(-0.781 eV)>CO(-0.669 eV),随着CO_2体积分数升高或反应时间增长,吸附在CaO表面的CO和NO减少,高体积分数CO_2促使CaO催化作用失效。CO还原NO的反应势垒(10.84 eV)大于CaO表面CO还原NO反应势垒(2.06 eV),CaO易催化CO还原NO。     

Mechanism of selective catalytic reduction of NO in the presence of excess O2 over Pt/Si-MCM-41 catalyst

The reaction mechanism for the selective catalytic reduction (SCR) of nitric oxide (NO) over Pt/Si-MCM-41 catalyst has been studied by XPS and in-situ Fourier transform infrared (FTIR) spectroscopies as well as TPR. The XPS results indicate that Pt species are reduced during the reaction of NO-SCR in the presence of excess oxygen. The reduced Pt species are found not to be completely oxidised after they have been in contact with NO or O₂ at the reaction temperature. FTIR results indicate that CO is one of the reaction intermediates taking part in the selective reduction of NO. The possible reaction mechanism for the selective catalytic reduction of NO over Pt/Si-MCM-41 catalyst is proposed.     

Adsorbate-assisted NO decomposition in NO reduction by C3H6 over Pt/Al2O3 catalysts under lean-burn conditions

The rates and product selectivities of the C₃H₆-NO-O₂ and NO-H₂ reactions over a Pt/Al₂O₃ catalyst, and of the straight, NO decomposition reaction over the reduced catalyst have been compared at 240C. The rate of NO decomposition over the reduced catalyst is seven times greater than the rate of NO decomposition in the C₃H₆-NO-O₂ reaction. This is consistent with a mechanism in which NO decomposition occurs on Pt sites reduced by the hydrocarbon, provided only that at steady state in the lean NO _x reaction about 14% of the Pt sites are in the reduced form. However, the (extrapolated) rate of the NO-H₂ reaction at 240C is about 10⁴ times faster than the rate of the NO decomposition reaction thus raising the possibility that NO decomposition in the former reaction is assisted by H_ads. It is suggested that adsorbate-assisted NO decomposition in the C₃H₆-NO-O₂ reaction could be very important. This would mean that the proportion of reduced Pt sites required in the steady state would be extremely small. The NO decomposition and the NO-H₂ reactions produce no N₂O, unlike the C₃H₆-NO-O₂ reaction, suggesting that adsorbed NO is completely dissociated in the first two cases, but only partially dissociated in the latter case. It is possible that some of the associatively adsorbed NO present during the C₃H₆-NO-O₂ reaction may be adsorbed on oxidised Pt sites.     

The selective catalytic reduction of NO by propylene over Pt supported on dealuminated Y zeolite

The reactivity of Pt supported on a stable dealuminated Y zeolite (Pt/DeY) for the Selective Catalytic Reduction of NO by hydrocarbons (HC-SCR) has been investigated with a monolith sample. The results show that the Pt/DeY catalyst has substantial activity for this reaction at temperatures between 200 and 300°C. Furthermore, the presence of water and sulfur dioxide, at levels similar to the ones expected in vehicle exhaust gas, does not significantly affect the performance of the catalyst, which makes it a promising candidate for further commercial development. In the same temperature range, oxygen promotes the rate of the NO reduction by assisting in the activation of the hydrocarbon. NO₂ is also formed under the conditions studied as a result of the oxidation of NO. In the presence of the hydrocarbon however, it is preferentially reacting with the hydrocarbon, and reduces primarily back to NO. High selectivities were observed toward the formation of N₂O, which is a primary product of the hydrocarbon-SCR reaction.     

Catalytic aerobic oxidation of ethylbenzene over Co/SBA-15

Co(II)O was highly dispersed in the mesopores of SBA-15 by alcoholic impregnation method and characterized by XRD, TEM, UV–VIS DRS, TPR, and XRF techniques. It was found that tetrahedral coordinated Co(II)O was stabilized by SBA-15 at low Co-loading. Co/SBA-15 showed much higher activity than Co(OAc)₂ or Co₃O₄ in the liquid-phase aerobic oxidation of ethylbenzene under solvent-free condition.     

HPWA/SBA-15催化合成对叔丁基苯酚

以SBA-15负载磷钨酸（HPWA）为催化剂、甲基叔丁基醚（MTBE）和苯酚为原料在钢密封间歇反应釜中进行催化合成对叔丁基苯酚的实验研究。考察了HPWA负载量、催化剂用量、反应温度、反应时间及原料配比对烷基化反应的影响。实验结果表明,反应温度为160 ℃、负载HPWA质量分数30%、反应时间3 h、催化剂用量为原料质量的5%和原料配比n(MTBE)∶n(苯酚)＝2∶1时,苯酚的转化率为85.1%, 对叔丁基苯酚的选择性66.87%。实验中HPWA/SBA-15分子筛表现出较高的活性及选择性。     

MoOx/SBA-15丙烷选择氧化催化剂的研究

采用浸渍法制备了一系列不同Mo含量的MoOx/SBA-15催化剂,并考察了丙烷选择氧化生成醛类含氧化合物（甲醛、乙醛和丙烯醛）的催化剂性能,利用FT-IR和XRD等技术对催化剂及载体的物化性能进行了表征。结果表明,催化剂的活性主要取决于Mo的负载量,当x(Mo)=1.75%［n(Mo)∶n(Si)=1.75∶100］时,醛类选择性约30%,总醛收率为10.4%,高分散催化剂尤其有利于醛类含氧化合物的定向生成。     

NO/CH4 reaction over nanodispersed Pt particles

The catalytic behavior of the cubic (≈70%) Pt nanoparticles supported on alumina, with an average diameter of 132 nm, was investigated for NO/CH₄ reaction. It was observed that the formation of reaction products (N₂O, CO and NH₃) is related to the size as well to the shape (facet) of the Pt nanoparticles.

     

Mesoporous TiO2/SBA-15, and Cu/TiO2/SBA-15 Composite Photocatalysts for Photoreduction of CO2 to Methanol

A series of mesoporous TiO₂/SBA-15, Cu/TiO₂ and Cu/TiO₂/SBA-15 composite photocatalysts were prepared by sol–gel synthesis for photoreduction of CO₂ with H₂O to methanol. It was found that optimum amount of titanium loading of TiO₂/SBA-15 was 45 wt% which exhibited higher photoreduction activity than pure TiO₂. An addition of copper on TiO₂ or TiO₂/SBA-15 catalyst as cocatalyst was found to enhance the catalytic activity because copper serves as an electron trapper and prohibits the recombination of hole and electron.