CO oxidation on unsupported Au55, Ag55, and Au25Ag30 nanoclusters

Using density functional calculations, we demonstrate a catalytic reaction path with activation barriers of less than 0.5 eV for CO oxidation on the neutral and unsupported icosahedral nanoclusters of Au(55), Ag(55), and Au(25)Ag(30). Both CO and O(2) adsorb more strongly on these clusters than on the corresponding bulk surfaces. The reaction path consists of an intermediate involving OOCO complex through which the coadsorption energy of CO and O(2) on these clusters is expected to play an important role in the reaction. Based on the studies for the Au and Ag nanoclusters, a model alloy nanocluster of Au(25)Ag(30) was designed to provide a larger coadsorption energy for CO and O(2) and was anticipated to be a better catalyst for CO oxidation from energetic analysis.     

Catalytic role of gold in gold-based catalysts: a density functional theory study on the CO oxidation on gold

Gold-based catalysts have been of intense interests in recent years, being regarded as a new generation of catalysts due to their unusually high catalytic performance. For example, CO oxidation on Au/TiO(2) has been found to occur at a temperature as low as 200 K. Despite extensive studies in the field, the microscopic mechanism of CO oxidation on Au-based catalysts remains controversial. Aiming to provide insight into the catalytic roles of Au, we have performed extensive density functional theory calculations for the elementary steps in CO oxidation on Au surfaces. O atom adsorption, CO adsorption, O(2) dissociation, and CO oxidation on a series of Au surfaces, including flat surfaces, defects and small clusters, have been investigated in detail. Many transition states involved are located, and the lowest energy pathways are determined. We find the following: (i) the most stable site for O atom on Au is the bridge site of step edge, not a kink site; (ii) O(2) dissociation on Au (O(2)-->2O(ad)) is hindered by high barriers with the lowest barrier being 0.93 eV on a step edge; (iii) CO can react with atomic O with a substantially lower barrier, 0.25 eV, on Au steps where CO can adsorb; (iv) CO can react with molecular O(2) on Au steps with a low barrier of 0.46 eV, which features an unsymmetrical four-center intermediate state (O-O-CO); and (v) O(2) can adsorb on the interface of Au/TiO(2) with a reasonable chemisorption energy. On the basis of our calculations, we suggest that (i) O(2) dissociation on Au surfaces including particles cannot occur at low temperatures; (ii) CO oxidation on Au/inactive-materials occurs on Au steps via a two-step mechanism: CO+O(2)-->CO(2)+O, and CO+O-->CO(2); and (iii) CO oxidation on Au/active-materials also follows the two-step mechanism with reactions occurring at the interface.     

Mechanism of CO Oxidation on Cu2O (111) Surface: A DFT and Microkinetic Study

Catalytic oxidation has been recognized as one of the most efficient and promising techniques for the abatement of CO and volatile organic compounds. In the present work, the CO oxidation mechanism on perfect Cu₂O (111) surface was investigated by using density functional theory (DFT) calculations with the periodic surface model. The unsaturated singly coordinated Cu⁺ site of Cu₂O (111) surface could effectively adsorb gaseous CO molecule with a strong adsorption energy of −1.558 eV. The adsorbed O on Cu₂O (111) surface is very active toward CO oxidation with only 0.269 eV energy barrier. The reaction between CO and lattice O is the rate‐determining step of Mars‐van‐Krevelen (MvK) type CO oxidation with the energy barrier of 1.629 eV. The CO oxidation cycle initiated by the reaction between coadsorbed CO and O₂ at the Cu_I site has a relatively lower energy barrier of 1.082 eV and is, therefore, more likely to proceed compared with the MvK cycle. Microkinetic rate constants of elementary reaction steps based on the transition state theory were deduced, which could be helpful in the kinetic modeling of CO oxidation on Cu₂O surface.     

Reaction mechanism of CO oxidation on Cu(2)O(111): A density functional study

The possible reaction mechanisms for CO oxidation on the perfect Cu(2)O(111) surface have been investigated by performing periodic density functional theoretical calculations. We find that Cu(2)O(111) is able to facilitate the CO oxidation with different mechanisms. Four possible mechanisms are explored (denoted as M(ER1), M(ER2), M(LH1), and M(LH2), respectively): M(ER1) is CO((gas))+O(2(ads))→CO(2(gas)); M(ER2) is CO((gas))+O(2(ads))→CO(3(ads))→O((ads))+CO(2(gas)); M(LH1) refers to CO((ads))+O(2(ads))→O((ads))+CO(2(ads)); and M(LH2) refers to CO((ads))+O(2(ads))→OOCO((ads))→O((ads))+CO(2(ads)). Our transition state calculations clearly reveal that M(ER1) and M(LH2) are both viable; but M(ER1) mechanism preferentially operates, in which only a moderate energy barrier (60.22 kJ/mol) needs to be overcome. When CO oxidation takes place along M(ER2) path, it is facile for CO(3) formation, but is difficult for its decomposition, thereby CO(3) species can stably exist on Cu(2)O(111). Of course, the reaction of CO with lattice O of Cu(2)O(111) is also considered. However, the calculated barrier is 600.00 kJ/mol, which is too large to make the path feasible. So, we believe that on Cu(2)O(111), CO reacts with adsorbed O, rather than lattice O, to form CO(2). This is different from the usual Mars-van Krevene mechanism. The present results enrich our understanding of the catalytic oxidation of CO by copper-based and metal-oxide catalysts.     

CO在CeO2(111)表面的吸附与氧化

采用密度泛函理论计算了CO在CeO2(111)表面的吸附与氧化反应行为. 结果表明, O2在洁净的CeO2(111)表面为弱物理吸附, 而在氧空位表面是强化学吸附, 且O2分子活化程度较大, O—O键长为0.143 nm. CO在CeO2(111)表面吸附行为的研究表明, CO在洁净表面及氧空位表面上为物理吸附, 吸附能均小于0.42 eV; 当表面氧空位吸附O2后, CO可吸附生成二齿碳酸盐中间体或直接生成CO2, 与原位红外光谱结果相一致. 表面碳酸盐物种脱附生成CO2的能垒仅为0.28 eV. 计算结果表明, 当CeO2表面存在氧空位时, Hubbard参数U对CO吸附能有一定的影响. CeO2载体在氧化反应中可能的催化作用为, 在氧气氛下, CeO2表面氧空位吸附O2分子, 形成活性氧物种, 参与CO催化氧化反应.     

Nanoclusters Au_(19)Pd and Au_(19)Pt Catalyzing CO Oxidation: a Density Functional Study

The gold atoms on the Au20 cluster had been substituted by the palladium and platinum atoms to obtain the doped clusters with more stable geometries as a function of the bind energy and interaction energy in the previous study. Therefore, we investigated the catalytic activities of the Au_(19)Pd and Au_(19)Pt clusters for CO oxidation along the Langmuir-Hinshelwood mechanism. It is found that the coadsorption of CO and O2 on the doped clusters is obviously stronger than on the Au20 cluster, especially on the doped atom, which makes potential energy of the transition state lower than the total energy of the reactants so that it can promote CO oxidation. The reaction on these doped clusters with the heteroatom on the vertex is more difficult. However, the Au_(19)Pd(S) is more prone to catalyzing the CO oxidation, in which the rate-limiting step has the lower energy barrier of 38.84 kJ/mol for this study. Therefore, the single atom can be modified to change the catalytic activity of the cluster for the CO oxidation. Meanwhile, the different sites on the clusters have different strengths of activity for the reaction.     

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).     

Structure Sensitivity of CO Oxidation on Co3O4: A DFT Study

The reaction mechanism of CO oxidation on the Co₃O₄ (110) and Co₃O₄ (111) surfaces is investigated by means of spin‐polarized density functional theory (DFT) within the GGA+U framework. Adsorption situation and complete reaction cycles for CO oxidation are clarified. The results indicate that 1) the U value can affect the calculated energetic result significantly, not only the absolute adsorption energy but also the trend in adsorption energy; 2) CO can directly react with surface lattice oxygen atoms (O_2f/O_3f) to form CO₂ via the Mars–van Krevelen reaction mechanism on both (110)‐B and (111)‐B; 3) pre‐adsorbed molecular O₂ can enhance CO oxidation through the channel in which it directly reacts with molecular CO to form CO₂ [O₂(a)+CO(g)→CO₂(g)+O(a)] on (110)‐A/(111)‐A; 4) CO oxidation is a structure‐sensitive reaction, and the activation energy of CO oxidation follows the order of Co₃O₄ (111)‐A(0.78 eV)>Co₃O₄ (111)‐B (0.68 eV)>Co₃O₄ (110)‐A (0.51 eV)>Co₃O₄ (110)‐B (0.41 eV), that is, the (110) surface shows higher reactivity for CO oxidation than the (111) surface; 5) in addition to the O_2f, it was also found that Co³⁺ is more active than Co²⁺, so both O_2f and Co³⁺ control the catalytic activity of CO oxidation on Co₃O₄, as opposed to a previous DFT study which concluded that either Co³⁺ or O_2f is the active site.     

Carbonate formation and decomposition on atomic oxygen precovered Au(111)

Experimental results supported by density functional theory calculations show carbonate formation and reaction on atomic oxygen precovered Au(111). Oxygen mixing is observed in temperature-programmed desorption measurements when a Au(111) precovered with 16O is exposed to isotopically labeled CO2 (C18O2). The presence of 16O18O is attributed to surface carbonate formation and decomposition at surface temperatures ranging from 77-400 K and initial oxygen coverages ranging from 0.18-2.1 ML. A reaction probability on the order of 10(-4) and an activation energy of -0.15+/- 0.08 eV are estimated for this reaction.     

CeO2(110)负载Au纳米颗粒催化CO+NOx反应的DFT+U研究

通过在位库伦校正的密度泛函理论（DFT+U）方法计算,我们研究了CO和NO_x分子在Au负载CeO₂（110）表面的吸附. 结果表明,CO在Au纳米颗粒的顶位有很强的吸附能,大约为1.2 eV,而NO在Au纳米颗粒上或者Au与CeO₂载体界面处都是弱吸附. 然而,当NO_x在界面处形成N₂O₂二聚体之后,通过断裂末端的N-O键能够有效地被降解. 纵观整个反应过程,第一步CO+N₂O₂的反应遵循了Langmuir-Hinshelwood机理,活化能只有0.4 eV,通过形成ONNOCO的中间物种最终产生N₂O和CO₂. 不同的是,第二步消除N₂O反应遵循了Eley-Rideal碰撞机理,需要相当高的能垒,约为1.8 eV. 通过进一步分析表明,稀土Ce元素独特的电子特性能够使电子从Au上转移并且局域到载体表面的Ce阳离子上,并且有助于形成带负电的N₂O₂分子. 而且Au纳米颗粒有很强的结构流动性,能够促进吸附的CO分子靠近界面处的N₂O₂并与之反应.     

First principles investigations of Pd-on-Au nanostructures for trichloroethene catalytic removal from groundwater

Catalytic groundwater remediation from chlorinated organic solvents like trichloroethene (TCE) has been found to be more effective and sustainable than traditional non-destructive methods. Among the experimentally studied catalyst materials, Pd-decorated Au nanoparticles show the highest activity and selectivity combined with the best resistance towards poisoning by chemicals present in groundwater. In this study the thermochemistry and adsorption geometries of TCE and its hydrodechlorination products are investigated via density functional theory calculations. Various model systems for Pd-supported Au nanoparticles are addressed. The adsorption of TCE is endothermic on bare Au(111), almost thermoneutral or slightly exothermic on Pd-Au surface alloys and clearly exothermic on Pd overlayer structures on Au(111). The strongest chemisorption is on the di-σ configuration between Pd atoms over the smallest 2D Pd clusters containing only a few Pd atoms. These are not, however, the best catalysts as they are too small to co-adsorb hydrogen needed for hydrodechlorination reaction. We demonstrate good correlation between adsorption energies and the d-band center of the system. The variation of adsorption energy from the one Pd-Au composition to the other can be tentatively assigned to be due to the ligand and coordination effects. Also, the ensemble effects are important; without the right ensemble the adsorption is weak or endothermic.     

Au/Pd(111)双金属表面催化噻吩加氢脱硫的反应机理

采用密度泛函理论(DFT)计算了Pd(111)表面含有N(N=1-4)个Au原子数目时的表面形成能,选取最优构型进一步研究了噻吩在Au/Pd(111)双金属表面的吸附模式及加氢脱硫反应过程.结果表明:当Pd(111)表面含有1个Au原子时,其形成能最低.在Au/Pd(111)双金属表面噻吩初始吸附于Pd-Hcp-30°位时,其构型最稳定.在各加氢脱硫过程中,反应总体均放出热量.对于直接脱硫机理,其所需活化能较低,但脱硫产物较难控制;对于间接脱硫机理,反应最有可能按照顺式加氢方式进行,C―S键断裂开环时所需活化能最高,是反应的限速步骤.此外,与单一Au(111)面及Pd(111)面相比,Au/Pd(111)双金属表面限速步骤的反应能垒最低,表明AuPd双金属催化剂比Au、Pd单金属催化剂更有利于噻吩加氢脱硫反应的进行.     

Au/Pd(111)双金属表面催化噻吩加氢脱硫的反应机理

采用密度泛函理论（DFT）计算了Pd（111）表面含有N（N=1-4）个Au原子数目时的表面形成能,选取最优构型进一步研究了噻吩在Au/Pd（111）双金属表面的吸附模式及加氢脱硫反应过程. 结果表明：当Pd（111）表面含有1个Au原子时,其形成能最低. 在Au/Pd（111）双金属表面噻吩初始吸附于Pd-Hcp-30°位时,其构型最稳定. 在各加氢脱硫过程中,反应总体均放出热量. 对于直接脱硫机理,其所需活化能较低,但脱硫产物较难控制;对于间接脱硫机理,反应最有可能按照顺式加氢方式进行,C―S键断裂开环时所需活化能最高,是反应的限速步骤. 此外,与单一Au（111）面及Pd（111）面相比,Au/Pd（111）双金属表面限速步骤的反应能垒最低,表明AuPd双金属催化剂比Au、Pd单金属催化剂更有利于噻吩加氢脱硫反应的进行.     

In situ studies of methanol decomposition and oxidation on Pd(111) by PM-IRAS and XPS spectroscopy

Methanol decomposition and oxidation on Pd(111) at millibar pressure were studied by in situ polarization-modulation infrared reflection absorption spectroscopy (PM-IRAS), on-line gas chromatography and pre- and postreaction X-ray photoelectron spectroscopy (XPS). Various dehydrogenation products such as methoxy CH3O, formaldehyde CH2O, formyl CHO, and CO could be spectroscopically identified. Methanol oxidation proceeds via dehydrogenation to formaldehyde CH2O, which either desorbs or is further dehydrogenated to CO, which is subsequently oxidized to CO2. Carbonaceous overlayers that are present during the reaction may favorably affect the selectivity toward CH2O. The reaction takes place on metallic Pd, and no indications of an involvement of Pd surface oxide were observed.     

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.     

Cu-Pt-Au三元合金催化水煤气变换反应的密度泛函研究

利用密度泛函理论（DFT）研究了不同掺杂量的Cu-Pt-Au催化剂性质及水煤气变换反应（WGSR）在催化剂表面上的反应机理。首先对Cu-Au和Pt-Au二元催化剂的稳定性和电子活性进行研究,发现Pt-Au催化剂的协同效应较优,稳定性更优,结合能为77.15 eV,d带中心为-3.18 eV。当将Cu继续掺杂到Pt-Au合金中构成Cu-Pt-Au三元催化剂时,Cu₃-Pt₃-Au（111）结合能为77.99 eV,且d带中心为-3.05 eV,表明其具有较优的稳定性和电子活性。探讨了WGSR在Cu₃-Pt₃-Au（111）上的反应历程,氧化还原机理因CO氧化的能垒达到4.84 eV而不易进行。CHO和COOH两个中间体为竞争关系,且形成CHO中间物时的能垒较小,因此,反应相对容易按照甲酸机理进行。     

Promoter effect of BaO on CO oxidation on PdO surfaces

The effect of bulk BaO promoter on CO oxidation activity of palladium oxide phase was studied by density functional calculations. A series of BaO(100) supported Pd(x)O(y) thin layer models were constructed, and energy profiles for CO oxidation on the films were calculated and compared with corresponding profiles for the most stable PdO bulk surfaces PdO(100) and PdO(101). The most stable of the thin films typically exhibit the same PdO(100) and PdO(101) surface planes; the PdO(100) dominates already with double layer thickness. The supporting promoter improves the CO oxidation activity of the Pd(x)O(y) phase via a direct electronic effect and introduced structural strain and corrugation. Changes in CO adsorption strength are reflected in oxidation energy barriers, and the promoting effect of even 0.3 eV can be seen locally. Easier oxygen vacancy formation may partially facilitate the reaction.     

The mechanism of N2O formation via the (NO)2 dimer: a density functional theory study

Catalytic formation of N(2)O via a (NO)(2) intermediate was studied employing density functional theory with generalized gradient approximations. Dimer formation was not favored on Pt(111), in agreement with previous reports. On Pt(211) a variety of dimer structures were studied, including trans-(NO)(2) and cis-(NO)(2) configurations. A possible pathway involving (NO)(2) formation at the terrace near to a Pt step is identified as the possible mechanism for low-temperature N(2)O formation. The dimer is stabilized by bond formation between one O atom of the dimer and two Pt step atoms. The overall mechanism has a low barrier of approximately 0.32 eV. The mechanism is also put into the context of the overall NO + H(2) reaction. A consideration of the step-wise hydrogenation of O(ads) from the step is also presented. Removal of O(ads) from the step is significantly different from O(ads) hydrogenation on Pt(111). The energetically favored structure at the transition state for OH(ads) formation has an activation energy of 0.63 eV. Further hydrogenation of OH(ads) has an activation energy of 0.80 eV.     

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).