The Mechanism of HDS Catalysis

The mechanism of heterogeneous catalytic reactions is much more difficult to elucidate than that of homogeneous systems. Despite the facilities provided by physical methods for investigating the surface of solids, obtaining detailed information on the structure of the active component in real heterogeneous catalysts presents difficulties due to the nonuniform chemical composition of the surface species. Some of these surface species are totally inactive in catalysis, and others can catalyze the given chemical reaction by different pathways and according to different mechanisms. This results in a change of selectivity to the desired product and the appearance of intermediates and reaction by-products. Furthermore, the effect of the reaction medium on the catalyst gains importance during a catalytic process when, at high temperature and pressure, one type of surface species is transformed into another, thus changing the mechanism and direction of the catalyzed reaction.     

水处理多相催化臭氧氧化技术研究现状

在对多相催化臭氧氧化水处理技术的除污染效能评述的基础上,针对多相催化臭氧氧化机理研究的难点问题进行了分析,其中对有机物吸附过程在多相催化臭氧氧化反应中的作用、可能存在的活性氧化物种等问题进行了重点讨论。此外还对该技术的应用前景和研究中所存在的问题进行了分析。     

Modeling of transport and chemistry in channel flows of automotive catalytic converters

A novel comprehensive numerical study is presented for a better understanding of mass transfer in channel flows with catalytically active walls at moderate temperatures and surface reaction rates. Altogether, 18 different numerical models are compared, which represent mass transfer in single channels of a honeycomb-type automotive catalytic converter operated under direct oxidation conditions. Three different channel geometries have been investigated: circular cross-section, square cross-section, and square cross-section with rounded corners (fillets). 1D plug-flow, 2D boundary-layer and Navier–Stokes, and 3D Navier–Stokes equations are applied to model the reactor geometry. The diffusion limitation within the porous washcoat has been modeled by a simplified zero-dimensional effectiveness factor model as well as multidimensional reaction–diffusion models. Furthermore, simulations are also carried out for cases with instantaneous diffusion within the washcoat. All numerical models account for the coupled interactions of mass-transfer and heterogeneous chemistry within the channels. The chemical conversion of the pollutants on the platinum catalyst is described by an elementary-step-like heterogeneous reaction mechanism consisting of 74 reactions among 11 gas-phase species and 22 adsorbed surface species. The results of numerical simulations are compared with experimental data.     

非均相催化臭氧化水中药物与个人护理品研究进展

药物和个人护理用品（PPCPs）是一类新兴的有机污染物，与常见的污染物相比，在水环境中浓度很低但化学结构复杂，种类多，性质差异大，具有毒性，常规处理技术很难完全去除。非均相催化臭氧化技术的固相催化剂可回收重复利用，二次污染少，目前此处理技术在PPCPs领域的研究已经非常广泛。文章详细描述了非均相催化臭氧化技术降解PPCPs时常遵循的表面反应机理、自由基反应机理、协同反应机理，进一步阐释了自由基反应机理中的4种途径，简要介绍了催化剂等因素对PPCPs降解的影响，归纳总结了非均相催化臭氧化在PPCPs治理领域的应用进展，提出了现今非均相催化臭氧化技术存在的问题，最后展望了非均相催化臭氧化技术的未来研究方向及应用前景。     

Characterisation of active phases of a copper catalyst for methanol oxidation under reaction conditions: an in situ X-ray absorption spectroscopy study in the soft energy range

The oxidation of methanol over copper is investigated by X-ray absorption spectroscopy in the soft X-ray range under reaction conditions. This in situ method allows the surface electronic structure of the catalyst to be correlated with its performance. The correlation reveals information about the catalytic function of various oxygen species on the surface. Oxide and metastable suboxide species affect in distinctly different ways the multiple action of copper as selective or unselective heterogeneous catalyst.     

Reactivity of surface alkoxy species on acidic zeolite catalysts

[Reaction: see text]. A solid understanding of the mechanisms involved in heterogeneously catalyzed reactions is of fundamental interest for modern chemistry. This information can help to refine modern theories of catalysis and, in a very practical way, can help researchers to optimize existing industrial processes and develop new ones. To understand the mechanisms of heterogeneous catalysis, we need to observe and identify reaction intermediates on a working catalyst. Motivated by this goal, we have monitored the catalytic events in heterogeneous systems using in situ magic-angle-spinning (MAS) NMR under flow conditions. In this Account, we describe the reactivity and possible intermediate role of surface alkoxy species in a variety of zeolite-catalyzed reactions. First, we isolate the surface alkoxy species on a working zeolite catalyst and then investigate the chemical reactivity with different probe molecules under reaction conditions. Finally, we investigate reaction mechanisms facilitated by these intermediate surface alkoxy species. We examined the reactivity of surface methoxy species (SMS) in terms of C-O bond and C-H bond activation. SMS on acidic zeolite catalysts act as an effective methylating agent when reacted with different probe molecules (including methanol, water, ammonia, alkyl halides, hydrochlorides, aromatics, carbon monoxide, and acetonitrile) through C-O bond activation. At higher reaction temperatures (ca. 523 K and above), the C-H bond activation of SMS may occur. Under these conditions, intermediates such as surface-stabilized carbenes or ylides are probably formed. This C-H bond activation is directly related to the initiation mechanism of the methanol-to-olefin (MTO) process and invites further investigation. Based on our experimental results, we also discuss the reactivity and the carbenium-ion-like nature of surface alkoxy species and recent theoretical investigations in this area.     

Kinetics of catalytic reactions with two types of sites: nonuniform surfaces

Several aspects of heterogeneous catalytic kinetics over induced nonuniform surfaces are considered. The reaction mechanism is thought to occur through a surface collision of species, adsorbed on two distinct surface sites, which display nonuniform behavior. The expressions for rates of elementary reactions have been deduced within the framework of the surface electronic gas model, which accounts for the case of inhomogeneous surface. Equations for catalyst activity in the range of medium coverage have been derived and compared with the power-law model.     

非热等离子体催化转化C1分子及其催化剂研究进展

非热等离子体催化具有反应条件温和、启动快和反应器结构紧凑等特点,在C₁分子催化转化领域（如CO₂加氢、甲烷活化、水煤气变换反应和甲醇重整制氢）有着广阔的应用前景。具体来说,等离子体特有的高能电子可在气相中快速活化稳定性极强的C₁分子并生成活性物质,接着与催化剂结合发生表面化学反应,从而实现常温常压下C₁分子的高效转化。然而,等离子体与催化剂之间的协同作用机制以及催化机理极为复杂,仍有待进一步研究。本综述简单介绍了非热等离子体催化转化C₁分子的近期研究进展,重点探讨了适用于非热等离子 体的催化剂研究以及催化机理的高级原位表征。最后,提出了非热等离子体催化转化C₁分子的未来发展方向：①设计并构筑适用于非热等离子体催化的高效催化剂,并研究其构效关系;②发展高级原位表征技术,揭示活性物质的作用机理以及催化机理;③设计并构建高效的等离子体催化反应器,并建立反应器的理论模型和数值模拟方法,科学指导等离子体反应器的设计、优化和放大。     

Adaptation and application of the In Situ Adaptive Tabulation (ISAT) procedure to reacting flow calculations with complex surface chemistry

In this article, the In Situ Adaptive Tabulation (ISAT) procedure, originally developed for the efficient computation of homogeneous reactions in chemically reacting flows, is adapted and demonstrated for reacting flow computations with complex heterogeneous (or surface) reactions. The treatment of heterogeneous reactions within a reacting flow calculation requires solution of a set of nonlinear differential algebraic equations at boundary faces/nodes, as opposed to the solution of an initial value problem for which the original ISAT procedure was developed. The modified ISAT algorithm, referred to as ISAT-S, is coupled to a three-dimensional unstructured reacting flow solver, and strategies for maximizing efficiency without hampering accuracy and convergence are developed. These include use of multiple binary tables, use of dynamic tolerance values to control errors, and periodic deletion and/or re-creation of the binary tables. The new procedure is demonstrated for steady-state catalytic combustion of a methane-air mixture on platinum using a 24-step reaction mechanism with 19 species, and for steady-state three-way catalytic conversion using a 61-step mechanism with 34 species. Both reaction mechanisms are first tested in simple 3D channel geometry with reacting walls, and the impact of various ISAT parameters is investigated. It is found that the temperature of the reacting wall dictates the retrieval rate from the ISAT table. As a final step, the catalytic combustion mechanism is demonstrated in an laboratory-scale monolithic catalytic converter geometry with 57 channels discretized using 354,300 control volumes (4.6 million unknowns) after employing quarter symmetry. For this particular case, the use of ISAT-S resulted in reduction of the overall CPU time from 19.3 to 13.6 h. For all of the cases considered, the reduction in the time taken to perform surface chemistry calculations alone was found to be a factor of 5-11.     

Catalytic steam reforming of methane using Rh supported on Sr-substituted hexaaluminate

This paper reports the synthesis, characterization, and performance of steam-reforming catalysts based upon dispersed Rh particles on Sr-substituted hexaaluminate supports. As confirmed by electron microscopy and X-ray diffraction, the Sr-substituted hexaaluminate provides a plate-like support structure that resists sintering and occlusion of the Rh. The hexaaluminate is synthesized using an alumoxane process, with the cation substitution accomplished by exchange with metal acetylacetonates. The Rh is dispersed using impregnation with metal-nitrate salts. A stagnation-flow reactor is used to measure catalytic activity. In these experiments, the catalyst is applied to a flat surface that is held at a fixed temperature. Reactive gases (methane, steam, and diluent) impinge on the catalytic stagnation surface. Microprobe mass spectrometry is used to measure gas-phase species profiles in the boundary layer normal to the catalyst surface. The experiments are interpreted using a chemically reacting flow model, including an elementary heterogeneous chemical reaction mechanism. Results confirm that the Rh on Sr-substituted hexaaluminate is a highly stable and active reforming catalyst.     

动态方法测定吸附和表面反应速率

本文较详细地综述了在近些年来刚刚发展起来的能用于分离测定非均相催化反应中的吸附和表面反应速率的三种动态分析方法:催化反应色谱技术,动态一稳态法和浆化反应器中的动态分析。文中不仅介绍了三种方法的基本理论和一些有代表性的实验结果,也指出了它们在研究催化反应动力学和机理以及催化剂制备变量对其影响方面的重要意义和前景。     

High-throughput preparation and screening of rhenium oxide-alumina catalysts in olefin metathesis

High-throughput (HT) experimentation in heterogeneous catalysis exemplifies a multidisciplinary approach to address in an efficient manner different aspects of catalyst development – from synthesis to testing and characterization. The present work reports on preparation systems for rapid parallel synthesis of well defined heterogeneous catalysts and catalyst screening in olefin metathesis. Metathesis of ethene and but-2-ene to propene was chosen as a test reaction. The influences of various parameters such as rhenium oxide loading, catalyst calcination conditions, catalyst pre-treatment, as well as the reaction temperature and contact time on the catalytic performance are discussed. Sample characterization by UV–vis and FT-IR gave an evidence for the formation of active surface perrhenate species, very sensitive to humidity and pre-treatment conditions applied.     

Asymmetric Hydrogenation of Activated Ketones on Platinum: Relevant and Spectator Species

The heterogeneous enantioselective hydrogenation of activated ketones over chirally modified platinum is reviewed with emphasis on identifying the role of the various species observed in this catalytic system. The past years have witnessed a continuous broadening of the scope of this catalytic system including new reactants and modifiers affording over 97% ee. New reaction pathways have been uncovered and the kinetic and mechanistic studies have been faced with a number of complicating factors caused by spectator species and interactions in solution and on the Pt surface. The previously proposed mechanistic models are critically assessed in the light of these new findings.     

轻质烷烃异构化催化剂及机理研究进展

简介了近年来国内外轻质烷烃异构化新工艺和新催化剂,综述了近年来的经典及新提出的异构化反应机理。根据反应机理针对不同催化剂体系总结了提高异构催化剂反应活性和选择性的途径,并展望了反应机理在制备新型催化剂领域以及提高催化剂的活性和选择性方面的重要指导意义。     

Solid-state NMR for metal-containing zeolites: from active sites to reaction mechanism

Metal-containing zeolite catalysts have found a wide range of applications in heterogeneous catalysis. To understand the nature of metal active sites and the reaction mechanism over such catalysts is of great importance for the establishment of structure-activity relationship. The advanced solid-state NMR (SSNMR) spectroscopy is robust in the study of zeolites and zeolite-catalyzed reactions. In this review, we summarize recent developments and applications of SSNMR for exploring the structure and property of active sites in metal-containing zeolites. Moreover, detailed information on host-guest interactions in the relevant zeolite catalysis obtained by SSNMR is also discussed. Finally, we highlight the mechanistic understanding of catalytic reactions on metal-containing zeolites based on the observation of key surface species and active intermediates.     

芳烃催化加氢反应机理的研究进展

对萘、菲、蒽及芘根据不同计算方法所得的反应活性位进行总结,对苯在金属表面的催化加氢机理-芳烃交换机理进行概括,对菲在MoS₂/Al₂O₃催化剂表面生成二氢菲的基元反应步骤和在Ni-MoS₂/Al₂O₃催化剂表面加氢生成四氢菲的过程进行总结,还给出了蒽的催化加氢反应网络图,分析了在不同催化条件下蒽是否有中环断裂情况的发生,给出了芘在RaneyNickel（W-7）催化剂表面生成二氢芘、四氢芘和六氢芘的催化加氢机理反应图,以及根据能量最低理论,判断芘发生加氢反应的优先位置情况,分别计算了芘加氢生成二氢芘、四氢芘、六氢芘和十氢芘可能存在的加氢路径。并总结了蒽、菲等多环芳烃发生催化加氢反应的规律。加深对多环芳烃催化加氢机理的研究将更好地指导重质油轻质化的进行,因此对多环芳烃催化加氢机理的研究具有重要意义。     

The nature of surface species on modified Pt-based catalysts for the SCR of NO by C<Subscript>3</Subscript>H<Subscript>6</Subscript> under lean-burn condition

The catalytic activity behavior for the selective catalytic reduction of NO by C₃H₆ under excess oxygen and the nature of surface species on the active sites of Pt/Al₂O₃ catalyst after adding a second metal (Fe, Sn, Co, Cr or W) were investigated. It has been found that an important role of second metals is on TONs of C₃H₆ and NO conversions and the nature of surface species produced on the catalyst surface at low temperature instead of the catalytic activity behavior towards the temperature programmed reaction. Although the introduction of each second metal distinctly disturbs the characteristic of surface species, the reaction mechanism is presumably similar. The observation of few surface species and the investigation about their reactivity indicate that few mechanisms are simultaneously proceeding at the same reaction condition.     

Mesopore-encaged V-Mn oxides: Progressive insertion approach triggering reconstructed active sites to enhance catalytic oxidative desulfuration

The oxidative desulphurization (ODS) has become mainly popular by rapid catalytic oxidation of dibenzothiophene (DBT) relied on efficient heterogeneous catalyst. V-based catalytic active species were regarded as the potential option in the activity-preferred ODS systems. Herein, we reported the re-dispersion of vanadium oxide (VO_x) on the mesoporous silica modified with manganese oxide (Mn₃O₄) through one progressive insertion approach of metal oxides in the silica. Impressively, mesopore-encaged vanadium-manganese oxides in the silica (VMn-MS) as the admirable output of excellent ODS catalyst was demonstrated compared to other monometal-modified counterparts and one-pot implanted one. The characterization results revealed the post-implanted VO_x species not only deposited around the pre-covered Mn₃O₄ on the mesoporous surface but also inserted the surface layer of Mn₃O₄ inducing the amorphous evolution of aggregated Mn₃O₄ and the reconstruction of final active sites. This integrated approach made the reconstructed active species afford more exposed catalytic sites and the tailored surface redox cycles owing to the electronic communication of V-Mn. The catalytic results demonstrated the excellent catalytic desulphurization efficiency (~100%) during 60 min at 80 ℃, which made the sulphur content reduce to 6 mg·L^-1, remarkably superior to other comparative samples. The outstanding catalytic performance of VMn-MS catalyst can be ascribed to the synergistic effect of V-Mn dual metals rendering two different reaction pathways, which includes free-radical reaction and ring-forming reaction, where Mn site acted as active center triggering reactive free radicals which could be further optimized by surrounded V sites around Mn sites to promote the ODS process.     

Catalytic oxidation of sulfide ions over nickel hydroxides

The catalytic sulfide ion oxidation by oxygen to elemental sulfur over β-Ni(OH)₂ and LiNiO₂ has been studied. As a result of experimental investigation performed, a reaction mechanism is suggested which involves heterogeneous and homogeneous processes. Dioxygen activation in the heterogeneous process proceeds via a redox Ni²⁺ ↔ Ni³⁺ transition and participation of OH⁻ groups. The active HO⁻₂ species thus formed carries on the reaction in homogeneous phase. Nickel hydroxides are promising catalysts for practical application.