半极性ZnO(101)表面负载Au催化CO氢助氧化的计算研究

本文利用半极性ZnO(101)表面负载Au团簇模型,通过密度泛函理论计算,对比研究Au/ZnO界面催化CO氢助氧化机理,探究H和Au的协同催化作用.结果表明:Au/ZnO(101)催化CO氧化活性较低,能垒为1.45eV;在氢助条件下,活性大大提高,能垒仅为0.48eV,这主要归因于H有助于促进O2解离.该研究为理解Au/ZnO界面H/OH的协同催化作用提供了新的认识.     

三种Au(111)催化水煤气变换反应机理的比较

采用密度泛函理论对三种水煤气变换反应(WGSR)机理(氧化还原机理、羧基机理、甲酸基的生成机理)在Au(111)面上的反应历程进行详细讨论.通过对表面吸附物种(H2O、CO、OH、O、H、CO2、COOH、HCOO)的吸附行为进行研究,得到最佳活性吸附中心.对三种机理中的14个基元反应的活化能进行分析,得出WGSR在Au(111)上按照羧基机理和氧化还原机理进行的可能性较大,按照甲酸基的生成机理进行的可能性较小.相比较羧基机理和氧化还原机理,反应更有可能按照羧基机理进行,最佳反应途径为H2O-H→OH+CO→COOH+OH→CO2.     

Au_n团簇催化水煤气变换反应机理的密度泛函理论研究

利用密度泛函理论(DFT)研究了Au10、Au13和Au20三类团簇的稳定性和对水煤气变换(WGSR)反应的催化活性,考察了各物质在Aun团簇上的吸附行为和微观反应机理。结果表明,三类Aun团簇的稳定性顺序为Au10Au13Au20,而Aun团簇中电子离域性及吸附能力大小趋势为Au13Au10Au20。在三类Aun团簇上,水煤气变换反应的控速步骤均为H2O的解离,但其反应机理路径有所不同。Au10团簇上为羧基机理,COOH*中间体直接解离;Au13团簇上为氧化还原机理,两个OH*发生歧化反应;Au20团簇上为羧基机理,COOH*和OH*发生歧化反应。通过对三类团簇上的最佳反应路径进行比较发现,Au13团簇在低温下具有较好的催化活性。     

气相金属催化反应综述

金属催化剂在工业、环境、能源以及生物等过程具有重要的应用.设计具有特定活性、环境友好型以及室温下具有反应活性的催化剂,需要在分子水平对金属催化剂的基元步骤,活性位点的结构以及催化反应微观机理有充分的认识.然而,由于宏观催化剂表面结构异常复杂,催化反应常受到溶剂、压力、金属颗粒团聚、催化剂表面缺陷等因素的干扰,利用现有实验仪器,从微观角度探索金属催化反应机理仍具有较大挑战,因此,对金属催化剂活性位的结构以及反应微观机理的认识还不十分清楚.质谱方法结合现代量子化学理论计算,提供了在气相条件下实验探索化学反应微观机理的有力工具,团簇反应可在隔离外界条件、可控以及可重复条件下进行,可以排除一些难以控制因素的干扰,可在化学键和分子结构水平认识金属活性位的结构以及催化反应的微观机理.气相金属团簇离子可用多种实验方法制备,与反应物分子反应后可利用多种质谱仪器探测,根据实验上所得的具有反应活性的团簇,结合现代量子化学理论模拟,得到金属催化反应的基元步骤以及微观反应机理信息,所得反应机理信息为宏观催化剂的设计提供重要理论研究基础.本综述总结了团簇实验上已经探测到的金属单原子离子、金属团簇、金属氧化物团簇和金属化合物催化的气相反应.反应物分子囊括了大量的无机和有机分子,包括CO,H2,CH4,C2H2,C2H4,C6H6,CH3OH,HCOOH,CH3COOH等.本综述主要介绍了以下三类催化反应:(1)CO催化氧化;(2)CH4催化转化;(3)催化脱羧反应,并重点关注贵金属单原子掺杂团簇独特的催化反应性.单原子催化剂可最大限度地利用有限的贵金属.在化学反应方面,单原子催化剂具有特异的反应活性,选择性以及稳定性.本综述对气相团簇反应中报道的两个重要的贵金属单原子掺杂团簇的催化反应进行了详细介绍:(1)金原子掺杂的AuAl3O3-5+团簇为首次报道的可以利用分子氧催化氧化CO的团簇单原子催化剂,我们对Au原子起催化作用的本质原因进行了介绍:(2)铂原子掺杂的PtAl3O5-7-团簇能利用分子氧催化氧化CO,该研究提出了"电负性阶梯"效应来解释Pt原子催化的微观机理,且此效应有望对大部分贵金属适用.此外,本综述对CO催化氧化反应和CH4催化转化反应的研究现状以及尚未解决的问题进行了剖析.相比CO的催化氧化反应,科学家对CH4催化转化反应机理的认识还不够深入,还需要进一步实验研究,而团簇单原子催化剂有望在此领域有所突破.     

氢氧化铁负载亚纳米铂族金属催化剂室温湿氧化消除CO（英文）

室温条件下高效消除CO具有重要的意义,但目前仍具有极大的挑战.考虑到实际应用环境中存在的水汽,实现具有应用价值的CO消除过程的关键是设计耐湿性好,且能够在室温甚至更低温度下具有较高CO氧化活性的催化剂.以Hopcalite(Cu-Mn-Ox)和Co3O4为代表的氧化物和负载型Au基催化剂具有优异的低温CO氧化活性,但存在耐湿性差、催化性能重复性不好等缺点,因而限制了其实际应用.铂族金属催化剂凭借优异的稳定性和耐湿性成为目前最广泛应用的尾气净化催化剂.但是由于铂金属位点强吸附CO的毒化作用,CO氧化工作温度多在200℃以上,从而限制了其在室内空气净化、燃料电池工业氢源净化以及汽车发动机冷启动阶段尾气净化等过程中的实际应用.研究人员尝试调节金属粒子尺寸、金属-载体界面、双金属结构及助剂效应等以促进O2的活化或者削弱CO的吸附,尽管取得了一定的进展,但仍缺少一种具有普适性和实际CO消除应用前景的铂族金属基催化体系.本文利用新型Fe(OH)x负载亚纳米Rh催化剂作为室温条件下CO湿氧化的典型例子,研究H2O对CO氧化反应的影响并探索其反应机制,旨在为发展实际可用的CO氧化消除催化体系提供参考.活性测试结果表明,H2O的存在可以大幅提高Fe(OH)x负载亚纳米Rh催化剂的CO氧化速率,并在室温条件下实现CO的长效稳定消除;而相似共沉淀法制备的Rh/Al2O3催化剂上H2O并没有促进CO氧化.原位红外表征发现,Fe(OH)x在湿氧化CO过程中的重要作用在于为O2和H2O的吸附提供位点,促进二者反应生成羟基物种,并与亚纳米Rh团簇上吸附的CO反应生成CO2.此外,H2O的存在使得CO氧化的表观活化能由22降至9 kJ mol^-1,说明反应路径或决速步骤由CO+O转变为反应能垒更低的CO+OH,从而大幅提高了CO氧化反应速率和反应活性.随后,时间分辨CO滴定红外实验证明,Rh/Fe(OH)x催化剂表面OH可以与CO反应生成CO2,而Al2O3负载的纳米Rh催化剂则不能,从而进一步揭示了Fe(OH)x载体在高效湿氧化消除CO过程中的重要作用.最后,通过拓展实验证明该反应机理可以适用于Fe(OH)x负载的其它铂族金属催化剂,提供了一种具有普适性和实际CO消除应用前景的铂族金属基催化体系.     

金基二元合金团簇Au_(12)M(M=Cu,Pt,Ni)催化水煤气变换反应的理论研究

利用密度泛函理论(DFT)对Au12M(M=Cu,Pt,Ni)3种合金团簇的结构稳定性、热力学稳定性和反应活性进行研究,并对金基二元合金团簇催化水煤气变换反应(WGSR)的反应机理进行探讨.研究发现,Au12Ni合金团簇的稳定性及电子活性最优.考察了WGSR在金基二元合金团簇上的氧化还原机理和羧基机理,表明Au12Cu合金团簇上WGSR按照氧化还原机理A进行,水解离后产生的OH*会继续解离为O*和H*(*代表吸附物质);Au12Pt及Au12Ni合金团簇上按照氧化还原机理B进行,2个OH*发生歧化反应.比较3种团簇上的最佳反应路径发现,Au12Cu团簇对WGSR表现出较好的催化活性.     

单晶Au表面和Au/TiO2（110）模型催化剂表面CO氧化反应机理和活性位

在分子尺度上介绍了Au/TiO2(110)模型催化剂表面和单晶Au表面CO氧化反应机理和活性位、以及H2O的作用.在低温(<320 K), H2O起着促进CO氧化的作用, CO氧化的活性位位于金纳米颗粒与TiO2载体界面(Auδ+–Oδ––Ti)的周边. O2和H2O在金纳米颗粒与TiO2载体界面边缘处反应形成OOH,而形成的OOH使O–O键活化,随后OOH与CO反应生成CO2.300 K时CO2的形成速率受限于O2压力与该反应机理相印证.相反,在高温(>320 K)下,因暴露于CO中而导致催化剂表面重组,在表面形成低配位金原子.低配位的金原子吸附O2,随后O2解离,并在金属金表面氧化CO.     

Au4团簇上甲酸分解反应机理的理论研究

采用密度泛函理论方法对Au4团簇上甲酸分解反应的反应机理进行了研究,并考察了Au4团簇的两个催化活性位点。在路径Ⅰ和路径Ⅱ中,HCOOH分解的产物是CO2和H2。在路径Ⅲ和路径Ⅳ中,HCOOH分解的最终产物为CO和H2O。此外,本文研究了CO2、H2和CO、H2O两种产物的相互转化,即路径Ⅴ和路径Ⅵ。研究结果表明,路径Ⅰ和路径Ⅱ的活化自由能垒较低,即在Au4团簇上HCOOH更易分解得到CO2和H2,此外两种产物之间不容易转化。进一步研究发现团簇的大小及CeO2载体对HCOOH分解脱氢路径的活化自由能垒有一定的影响。     

气相金属催化反应综述（英文）

金属催化剂在工业、环境、能源以及生物等过程具有重要的应用.设计具有特定活性、环境友好型以及室温下具有反应活性的催化剂,需要在分子水平对金属催化剂的基元步骤,活性位点的结构以及催化反应微观机理有充分的认识.然而,由于宏观催化剂表面结构异常复杂,催化反应常受到溶剂、压力、金属颗粒团聚、催化剂表面缺陷等因素的干扰,利用现有实验仪器,从微观角度探索金属催化反应机理仍具有较大挑战,因此,对金属催化剂活性位的结构以及反应微观机理的认识还不十分清楚.质谱方法结合现代量子化学理论计算,提供了在气相条件下实验探索化学反应微观机理的有力工具,团簇反应可在隔离外界条件、可控以及可重复条件下进行,可以排除一些难以控制因素的干扰,可在化学键和分子结构水平认识金属活性位的结构以及催化反应的微观机理.气相金属团簇离子可用多种实验方法制备,与反应物分子反应后可利用多种质谱仪器探测,根据实验上所得的具有反应活性的团簇,结合现代量子化学理论模拟,得到金属催化反应的基元步骤以及微观反应机理信息,所得反应机理信息为宏观催化剂的设计提供重要理论研究基础.本综述总结了团簇实验上已经探测到的金属单原子离子、金属团簇、金属氧化物团簇和金属化合物催化的气相反应.反应物分子囊括了大量的无机和有机分子,包括CO,H_2,CH_4,C_2H_2,C_2H_4,C_6H_6,CH_3OH,HCOOH,CH_3COOH等.本综述主要介绍了以下三类催化反应:(1)CO催化氧化;(2)CH4催化转化;(3)催化脱羧反应,并重点关注贵金属单原子掺杂团簇独特的催化反应性.单原子催化剂可最大限度地利用有限的贵金属.在化学反应方面,单原子催化剂具有特异的反应活性,选择性以及稳定性.本综述对气相团簇反应中报道的两个重要的贵金属单原子掺杂团簇的催化反应进行了详细介绍:(1)金原子掺杂的Au Al_3O_(3-5)~+团簇为首次报道的可以利用分子氧催化氧化CO的团簇单原子催化剂,我们对Au原子起催化作用的本质原因进行了介绍:(2)铂原子掺杂的Pt Al_3O_(5-7)~-团簇能利用分子氧催化氧化CO,该研究提出了"电负性阶梯"效应来解释Pt原子催化的微观机理,且此效应有望对大部分贵金属适用.此外,本综述对CO催化氧化反应和CH_4催化转化反应的研究现状以及尚未解决的问题进行了剖析.相比CO的催化氧化反应,科学家对CH4催化转化反应机理的认识还不够深入,还需要进一步实验研究,而团簇单原子催化剂有望在此领域有所突破.     

二元铜族团簇负离子催化CO氧化反应机理

用密度泛函理论B3LYP方法研究了二元铜族团簇负离子AuAg^-, AuCu^-和AgCu^-催化CO氧化反应的详细机理. 计算结果表明: CO在混合团簇中的吸附位顺序为Cu>Au>Ag; O₂也优先吸附到Cu上, 其次为Ag, 最难的为Au; 另外, O₂分子较CO分子易于吸附到混合团簇上. CO氧化反应有三条反应通道, 在热力学和动力学上均容易进行. AuAg-团簇催化CO氧化反应的最优反应通道为CO插入AuAgO₂^-中的Ag―O键形成中间体[Au―AgC(O―O)O]^-, 然后直接分解形成CO₂和AuAgO^-, 或另一个CO分子进攻中间体[Au―AgC(O―O)O]^-形成两分子的CO₂和AuAg^-. 而AuCu^-和AgCu^-催化CO氧化反应的最优反应通道为CO和O₂共吸附到团簇上,然后形成四元环中间体,最后四元环中间体分解形成产物或另一个CO分子进攻四元环中间体从而形成产物. 第二个CO分子的协同效应不明显. AuAg^-和AuCu^-对CO氧化反应催化活性强于Au₂^-团簇, 因此, Ag和Cu掺杂可以提高金团簇的催化活性, 与之前实验研究结果一致.     

Cu_2O(111)催化水煤气变换反应机理的理论研究

采用密度泛函理论结合周期平板模型方法系统地研究了水煤气变换反应在Cu_2O(111)表面上的反应机理,包括氧化还原机理、羧基机理和甲酸根机理.结果表明,在Cu_2O(111)表面,羧基机理和甲酸根机理均可行,且甲酸根机理更为有利,其最佳反应途径为H_2O~*→H~*+OH~*;CO(g)+H~*+OH~*→trans-HCOOH~*(1)→cis-HCOOH~*→CO_2~*+H_2(g).其中trans-HCOOH~*(1)→cis-HCOOH~*为其决速步,该基元反应的能垒仅为59 kJ·mol~(-1).羧基机理的最优反应路径同样是以H_2O的解离反应开始,随后CO(g)+OH~*→cis-COOH~*→trans-COOH~*→CO_2(g)+H~*,最后产生的两个吸附的H原子先迁移再结合生成H_2,整个反应的控速步骤为H原子的迁移,迁移能垒为96 kJ·mol~(-1).氧化还原机理则由于OH解离需要越过一个很高的能垒(254 vs.187 kJ·mol~(-1))而不可行.     

Origin and activity of gold nanoparticles as aerobic oxidation catalysts in aqueous solution

Whether gold is catalytically active on its own has been hotly debated since the discovery of gold-based catalysis in the 1980s. One of the central controversies is on the O(2) activation mechanism. This work, by investigating aerobic phenylethanol oxidation on gold nanoparticles in aqueous solution, demonstrates that gold nanoparticles are capable to activate O(2) at the solid-liquid interface. Extensive density functional theory (DFT) calculations combined with the periodic continuum solvation model have been utilized to provide a complete reaction network of aerobic alcohol oxidation. We show that the adsorption of O(2) is very sensitive to the environment: the presence of water can double the O(2) adsorption energy to ~0.4 eV at commonly available edge sites of nanoparticles (~4 nm) because of its strongly polarized nature in adsorption. In alcohol oxidation, the hydroxyl bond of alcohol can break only with the help of an external base at ambient conditions, while the consequent α-C-H bond breaking occurs on pure Au, both on edges and terraces, with a reaction barrier of 0.7 eV, which is the rate-determining step. The surface H from the α-C-H bond cleavage can be easily removed by O(2) and OOH via a H(2)O(2) pathway without involving atomic O. We find that Au particles become negatively charged at the steady state because of a facile proton-shift equilibrium on surface, OOH + OH ? O(2) + H(2)O. The theoretical results are utilized to rationalize experimental findings and provide a firm basis for utilizing nanoparticle gold as aerobic oxidation catalysts in aqueous surroundings.     

Theoretical investigation of water gas shift reaction catalyzed by iron group carbonyl complexes M(CO)5 (M = Fe, Ru, Os)

We have investigated the mechanism of M(CO)(5) (M = Fe, Ru, Os) catalyzed water gas shift reaction (WGSR) by using density functional theory and ab initio calculations. Our calculation results indicate that the whole reaction cycle consists of six steps: 1 → 2 → 3 → 4 → 5 → 6 → 2. In this stepwise mechanism the metals Fe, Ru, and Os behave generally in a similar way. However, crucial differences appear in steps 3 → 4 → 5 which involve dihydride M(H)(2)(CO)(3)COOH(-) (4') and/or dihydrogen complex MH(2)(CO)(3)COOH(-) (4). The stability of the dihydrogen complexes becomes weaker down the iron group. The dihydrogen complex 4_Fe is only 11.1 kJ/mol less stable than its dihydride 4'_Fe at the B3LYP/II(f)++//B3LYP/II(f) level. Due to very low energy barrier it is very easy to realize the transform from 4_Fe to 4'_Fe and vice versa, and thus for Fe there is no substantial difference to differentiate 4 and 4' for the reaction cycle. The most possible key intermediate 4'_Ru is 38.2 kJ/mol more stable than 4_Ru. However, the barrier for the conversion 3_Ru → 4'_Ru is 23.8 kJ/mol higher than that for 3_Ru → 4_Ru. Additionally, 4'_Ru has to go through 4_Ru to complete dehydrogenation 4'_Ru → 5_Ru. The concerted mechanism 4'_Ru → 6_Ru, in which the CO group attacks ruthenium while H(2) dissociates, can be excluded. In contrast to Fe and Ru, the dihydrogen complex of Os is too unstable to exist at the level of theory. Moreover, we predict Fe and Ru species are more favorable than Os species for the WGSR, because the energy barriers for the 4 → 5 processes of Fe and Ru are only 38.9 and 16.2 kJ/mol, respectively, whereas 140.5 kJ/mol is calculated for the conversion 4' → 5 of Os, which is significantly higher. In general, the calculations are in good agreement with available experimental data. We hope that our work will be beneficial to the development and design of the WGSR catalyst with high performance.     

CO oxidation catalyzed by a single gold atom: benchmark calculations and the performance of DFT methods

Quantum chemical calculations were carried out on CO oxidation catalyzed by a single gold atom. To investigate the performance of density functional theory (DFT) methods, 42 DFT functionals have been evaluated and compared with high-level wavefunction based methods. It was found that in order to obtain accurate results the functionals used must treat long range interaction well. The double-hybrid mPW2PLYP and B2PLYP functionals are the two functionals with best overall performance. CAM-B3LYP, a long range corrected hybrid GGA functional, also performs well. On the other hand, the popular B3LYP, PW91, and PBE functionals do not show good performance and the performance of the latter two are even at the bottom of the 42 functionals. Our accurate results calculated at the CCSD(T)/aug-cc-pVTZ//mPW2PLYP/aug-cc-pVTZ level of theory indicate that Au atom is a good catalysis for CO oxidation. The reaction follows the following mechanism where CO and O(2) adsorb on Au atom forming an Au(OCOO) intermediate and subsequently O(2) transfer one oxygen atom to CO to form CO(2) and AuO. Then AuO reacts with CO to form another CO(2) to complete the catalytic cycle. The overall energy barrier at 0 K for the first CO oxidation step (Au + CO + O(2)→ AuO + CO(2)) is just 4.8 kcal mol(-1), and that for the second CO oxidation step (AuO + CO → Au + CO(2)) is just 1.6 kcal mol(-1).     

Partial oxidation of propylene to propylene oxide over a neutral gold trimer in the gas phase: a density functional theory study

We report a B3LYP study of a novel mechanism for propylene epoxidation using H(2) and O(2) on a neutral Au(3) cluster, including full thermodynamics and pre-exponential factors. A side-on O(2) adsorption on Au(3) is followed by dissociative addition of H(2) across one of the Au-O bonds (DeltaE(act) = 2.2 kcal/mol), forming a hydroperoxy intermediate (OOH) and a lone H atom situated on the Au(3) cluster. The more electrophilic O atom (proximal to the Au) of the Au-OOH group attacks the C=C of an approaching propylene to form propylene oxide (PO) with an activation barrier of 19.6 kcal/mol. We predict the PO desorption energy from the Au(3) cluster with residual OH and H to be 11.5 kcal/mol. The catalytic cycle can be closed in two different ways. In the first subpathway, OH and H, hosted by the same terminal Au atom, combine to form water (DeltaE(act) = 26.5 kcal/mol). We attribute rather a high activation barrier of this step to the breaking of the partial bond between the H atom and the central Au atom in the transition state. Upon water desorption (DeltaE(des) = 9.9 kcal/mol), the Au(3) is regenerated (closure). In the second subpathway, H(2) is added across the Au-OH bond to form water and another Au-H bond (DeltaE(act) = 22.6 kcal/mol). Water spontaneously desorbs to form an obtuse angle Au(3) dihydride, with one H atom on the terminal Au atom and the other bridging the same terminal Au atom and the central Au atom. A slightly activated rearrangement to a symmetric triangular Au(3) intermediate with two equivalent Au-H bonds, addition of O(2) into the Au-H bond, and rotation reforms the hydroperoxy intermediate in the main cycle. On the basis of the DeltaG(act), which contains contribution from both pre-exponetial factor and activation energy, we identify the propylene epoxidation step as the actual rate-determining step (RDS) in both the pathways. The activation barrier of the RDS (epoxidation step: DeltaE(act) = 19.6 kcal/mol) is in the same range as that in the published computationally investigated olefin epoxidation mechanisms involving Ti sites (without Au involved) indicating that isolated Au clusters and possibly Au clusters on non-Ti supports can be active for gas-phase partial oxidation, even though cooperative mechanisms involving Au clusters/Ti-based-supports may be favored.     

Methanol oxidation on the PtPd(111) alloy surface: A density functional theory study

The mechanisms of methanol (CH₃OH) oxidation on the PtPd(111) alloy surface were systematically investigated by using density functional theory calculations. The energies of all the involved species were analyzed. The results indicated that with the removal of H atoms from adsorbates on PtPd(111) surface, the adsorption energies of (i) CH₃OH, CH₂OH, CHOH, and COH increased linearly, while those of (ii) CH₃OH, CH₃O, CH₂O, CHO, and CO exhibited odd‐even oscillation. On PtPd(111) surface, CH₃OH underwent the preferred initial C H bond scission followed by successive dehydrogenation and then CHO oxidation, that is, CH₃OH → CH₂OH → CHOH → CHO → CHOOH → COOH → CO₂. Importantly, the rate‐determining step of CH₃OH oxidation was found to switch from CO → CO₂ on Pt(111) to COOH → CO₂ + H on PtPd(111) with a lower energy barrier of 0.96 eV. Moreover, water also decomposed into OH more easily on PtPd(111) than on Pt(111). The calculated results indicate that alloying Pt with Pd could efficiently improve its catalytic performance for CH₃OH oxidation through altering the primary pathways from the CO path on pure Pt to the non‐CO path on PtPd(111).     

Comparison of the catalytic activity of Au3, Au4+, Au5, and Au5- in the gas-phase reaction of H2 and O2 to form hydrogen peroxide: a density functional theory investigation

We report a detailed density functional theory (B3LYP) analysis of the gas-phase H2O2 formation from H2 and O2 on Au3, Au4+, Au5, and Au5-. We find that H2, which interacts only weakly with the Au clusters, is dissociatively added across the Au-O bond, upon interaction with AunO2. One H atom is captured by the adsorbed O2 to form the hydroperoxy intermediate (OOH), while the other H atom is captured by the Au atom. Once formed, the hydroperoxy intermediate acts as a precursor for the closed-loop catalytic cycle. An important common feature of all the pathways is that the rate-determining step of the catalytic cycle is the second H2 addition to form H2O2. The H2O2 desorption is followed by O2 addition to AunH2 to form the hydroperoxy intermediate, thus leading to the closure of the cycle. On the basis of the Gibbs free energy of activation, out of these four clusters, Au4+ is most active for the formation of the H2O2. The 0 K electronic energy of activation and the DeltaGact at the standard conditions are 12.6 and 16.6 kcal/mol respectively. The natural bond orbital charge analysis suggests that the Au clusters remain positively charged (oxidic) in almost all the stages of the cycle. This is interesting in the context of the recent experimental evidence for the activity of cationic Au in CO oxidation and water-gas shift catalysts. We have also found preliminary evidence for a correlation between the activation barrier for the first H2 addition and the O2 binding energy on the Au cluster. It suggests that the minimum activation barrier for the first H2 addition is expected for the Au clusters with 7.0-9.0 kcal/mol O2 binding energy, i.e., in the midrange of the expected interaction energy. This represents a balance between more favorable H2 dissociation when the Aun-O2 interaction is weaker and high O2 coverage when the interaction is stronger. On the basis of this work, we predict that the hydroperoxy intermediate formation can be both thermodynamically and kinetically viable only in a narrow range of the O2 binding energy (10.0-12.0 kcal/mol)-a useful estimate for computationally screening Au-cluster-based catalysts. We also show that a competitive channel for the OOH desorption exists. Thus, in propylene epoxidation both OOH radicals and H2O2 can attack the active Ti in/on the Au/TS-1 and generate the Ti-OOH sites, which can convert propylene to propylene oxide.     

Spectroscopic and Kinetic Studies of the Reaction of CO+H(2)O and CO+O(2) and Decomposition of HCOOH on Au/H-Mordenite Catalysts

The surface species formed from the reaction of CO+H(2)O and CO+O(2) and decomposition of HCOOH on Au incorporated into H-mordenite zeolite have been studied by means of in situ FTIR spectroscopy. On H-mordenite, a bidentate formate species (2912, 1536, and 1390 cm(-1)) is produced upon exposure to the CO+H(2)O gas mixture at 323 K, as well as different carbonate-like species (1956, 1852, 1705, and 1360 cm(-1)). The latter species was extensively formed in a short time and was responsible for hindering the CO(2) adsorbed species. However, Au/H-mordenite presented different vibration modes of formate species with a high emphasis on the monodentate ones (2950, 2916, 2896, 1690, and 1340 cm(-1)). The HCOOH adsorption on Au/H-mordenite showed two bands at 1622 and 1590 cm(-1) of the nu(as)(OCO) species, suggesting the formation of two types of formate species. The decomposition rate of the formate species formed on Au moieties was faster than that formed on H-mordenite. This was consistent with the calculated activation energies of CO(2) formation that showed a lower value (40.1 kJ/mol) on the former sample than on the latter one (63.3 kJ/mol). A dehydrogenation mechanism is proposed (HCOOH-->H(2)+CO(2)) for the decomposition of HCOOH on the Au/H-mordenite catalyst. On the other hand, the Au/H-mordenite catalyst activated the CO oxidation reaction. This reaction proceeded mainly through the formation of carboxylate species at first, which tended to obviate with time, preferring the formate species. The latter species resulted from the interaction of CO with OH stretching of the zeolite assisted by the presence of gas phase O(2). The formate species is further decomposed with time to carbonate species. Copyright 2000 Academic Press.