甲酸在Au(997)台阶表面的氧化反应（英文）

金催化是纳米催化的代表性体系之一,但对金催化作用的理解还存在争议,特别是金颗粒尺寸对其催化作用的影响.金颗粒尺寸减小导致的表面结构主要变化之一是表面配位不饱和金原子密度的增加,因此研究金原子配位结构对其催化作用的影响对于理解金催化作用尺寸依赖性具有重要意义.具有不同配位结构的金颗粒表面可以利用金台阶单晶表面来模拟.我们研究组以同时具有Au(111)平台和Au(111)台阶的Au(997)台阶表面为模型表面,发现Au(111)台阶原子在CO氧化、NO氧化和NO分解反应中表现出与Au(111)平台原子不同的催化性能.负载型Au颗粒催化甲酸氧化反应是重要的Au催化反应之一.本文利用程序升温脱附/反应谱(TDS/TPRS)和X射线光电子能谱(XPS)研究了甲酸在清洁的和原子氧覆盖的Au(997)表面的吸附和氧化反应,观察到Au(111)台阶原子和Au(111)平台原子不同的催化甲酸根氧化反应行为.与甲酸根强相互作用的Au(111)台阶原子表现出比与甲酸根弱相互作用的Au(111)平台原子更高的催化甲酸根与原子氧发生氧化反应的反应活化能.在清洁Au(997)表面,甲酸分子发生可逆的分子吸附和脱附.甲酸分子在Au(111)台阶原子的吸附强于在Au(111)平台原子的吸附.TDS结果表明,吸附在Au(111)台阶原子的甲酸分子的脱附温度在190 K,吸附在Au(111)平台原子的甲酸分子的脱附温度在170 K.XPS结果表明,分子吸附甲酸的C 1s和O 1s结合能分别位于289.1和532.8 e V.利用多层NO_2的分解反应在Au(997)表面控制制备具有不同原子氧吸附位和覆盖度的原子氧覆盖Au(997)表面,包括氧原子吸附在(111)台阶位的0.02 ML-O(a)/Au(997)、氧原子同时吸附在(111)台阶位和(111)平台位的0.12 ML-O(a)/Au(997)、氧原子和氧岛吸附在(111)平台位和氧原子吸附在(111)台阶位的0.26 ML-O(a)/Au(997).TPRS和XPS结果表明,甲酸分子在105 K与Au(997)表面原子氧物种反应生成甲酸根和羟基物种,但甲酸根物种的进一步氧化反应依赖于Au原子配位结构和各种表面物种的相对覆盖度.在0.02 ML-O(a)/Au(997)表面暴露0.5 L甲酸时,Au(111)台阶位氧原子完全反应,甲酸过量.表面物种是Au(111)台阶位吸附的甲酸根、羟基和甲酸分子.在加热过程中,甲酸分子与羟基在181 K反应生成甲酸根和气相水分子(HCOOH(a)+OH(a)=H_2O+HCOO(a)),甲酸根在340 K发生歧化反应生成气相HCOOH和CO_2分子(2HCOO(a)=CO_2+HCOOH).在0.12 ML-O(a)/Au(997)和0.26 ML-O(a)/Au(997)表面暴露0.5 L甲酸时,甲酸分子完全反应,原子氧过量.表面物种是Au(111)平台位和Au(111)台阶位吸附的甲酸根、羟基和原子氧.在加热过程中,Au(111)平台位和Au(111)台阶位的甲酸根分别在309和340 K同时发生氧化反应(HCOO(a)+O(a)=H_2O+CO_2)和歧化反应(2HCOO(a)=CO_2+HCOOH)生成气相CO_2,H_2O和HCOOH分子.在0.26 ML-O(a)/Au(997)表面暴露10 L甲酸时,甲酸分子和原子氧均未完全消耗.表面物种是Au(111)平台位和Au(111)台阶位吸附的甲酸根、羟基、甲酸分子和原子氧.在加热过程中,除了上述甲酸根的氧化反应和歧化反应,还发生171 K的甲酸分子与羟基的反应(HCOOH(a)+OH(a)=H_2O+HCOO(a))和216 K的羟基并和反应(OH(a)+OH(a)=H_2O+O(a)).     

NO分子在金表面吸附的理论研究

气体分子在过渡族金属表面吸附是异相催化过程中的一个重要步骤.研究其在金属表面的吸附特性是了解其催化性能的基础,多年来一直是表面科学领域的研究热点.理论研究在解释吸附机理、实验现象以及证实实验结论的可靠性方面发挥着越来越重要的作用.本文使用密度泛函理论(DFT)研究了NO分子在中性及带正、负电荷的Au(111),Au(100),Au(310)和Au/Au(111)表面的吸附行为.研究结果表明,NO倾斜地吸附在金表面.在这种吸附构型中,Au原子的dz2轨道和NO分子的2π*轨道对称性匹配,并达到最大重叠.中性及带正、负电荷的Au(111),Au(100),Au(310)和Au/Au(111)表面不同吸附位对NO的反应活性不同,NO易吸附于各个金表面的顶位.计算结果显示,NO分子在Au(111)面几乎不吸附,而在Au/Au(111)的吸附能高达0.89eV.对表面金原子d态电子分波态密度分析表明,金表面对NO分子的吸附活性随着金原子配位数的减少而增强,这是由于低配位数的金原子的d态电子更靠近费米能级.当金表面增加或减少一个电子时,金表面对NO的吸附能有明显变化.正电荷的金表面对NO吸附的活性比中性的表面活性高,而带...     

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.     

DFT法研究HCOOH在Pd/WC(0001)上的分解机理（英文）

作为便携式电子设备的动力源,直接甲酸燃料电池(DFAFC)具有燃料跨界范围小、电动势大、甲酸无毒、低温下功率密度大等优点,因而引起了人们的极大兴趣.DFAFC商业化的主要挑战之一是阳极电催化剂材料的高成本和低CO耐受性.阳极通常需要高负载的贵金属电催化剂(Pt或Pd)氧化甲酸(HCOOH)以获得所需的电能.完全电氧化甲酸在Pt和Pd表面上会产生强吸附的CO,从而降低了Pt或Pd催化剂的活性.Pt和Pd储量少且价格昂贵,减少Pt和Pd含量且保持催化性能的燃料电池催化剂一直是研究者的奋斗目标.本文用周期性密度泛函理论(DFT)系统地研究了WC负载的单分子层Pd(Pd/WC(0001))催化剂对甲酸的分解机理,这可为所需的反应路径设计、筛选催化剂提供指导.Trans-HCOOH通过C-H,O-H,C-O键的活化发生分解.关于吸附,确定了可能反应中间体的最稳定吸附构型.trans-HCOOH,HCOO,mHCOO,cis-COOH,trans-COOH,CO,H2O,OH和H的吸附过程是化学吸附,而cis-HCOOH和CO2与Pd/WC(0001)表面的相互作用较弱,是物理吸附.此外,提出了trans-HCOOH分解的不同途径来探索分解机理.trans-HCOOH中O-H,C-H和C-O键的活化能垒分别为0.61,0.77和1.05 eV,O-H键断裂的能垒最小,则trans-HCOOH优先通过O-H键断裂生成HCOO.双齿HCOO是HCOOH分解的主要中间体,它可以转变为单齿HCOO,这条路线生成CO2的能垒比双齿HCOO的低0.04 eV.CO2是HCOO主要解离产物,这一步是总反应的决速步骤.对于cis-COOH和trans-COOH,CO是其主要解离产物.此外,trans-HCOOH也能直接生成CO,但克服的能垒较大.在Pd/WC(0001)表面上分解trans-HCOOH的最有利途径是HCOOH→HCOO→CO2,其中HCOO脱氢形成CO2的步骤是速率决定步骤.本文提供了HCOOH在Pd/WC(0001)表面上分解的活性中间体、能垒和机理的推测,CO形成主要是通过cis-COOH、trans-COOH及HCO的分解,CO2的形成主要是通过HCOO的分解,CO2占主导.该结论与Pd(111)面上甲酸分解结果一致,说明WC作为Pd载体没有改变Pd对甲酸的催化性能,但降低了Pd的使用量.综上,本文阐明了WC负载单分子层Pd催化剂上甲酸催化分解机理,得出甲酸分解的最佳反应路径,为直接甲酸燃料电池设计低贵金属含量、高活性的负载型Pd催化剂提供了理论指导;可用于预测不同载体负载Pd催化剂的性能,大大减少实验成本,以验证提出的实验假设.     

纳米金催化一氧化碳氧化反应的理论研究

近年来,纳米金催化剂独特的催化性质,特别是其优异的低温催化氧化活性,引起了人们极大的研究热情.除低温选择氧化外,在精细化学品合成、大气污染物消除、氢能的转换和利用等领域也开发出了一系列有广泛应用前景的金催化反应.此外,体相金的化学惰性和纳米金的超高活性之间差异的“鸿沟”也引起了理论工作者浓厚兴趣,试图从原理上理解体相金和纳米金活性差异的根源. CO催化氧化是最具有代表性的研究金催化活性的化学反应,本文主要综述了近十多年来金催化 CO氧化反应理论计算方面的研究工作.一般认为, CO在纳米金表面的吸附是 CO氧化反应的初始步骤.密度泛函理论研究表明, CO在金表面的吸附强度主要与被吸附金原子的配位数有关：金配位数越低, CO的吸附能越强,部分研究结果表明两者之间存在近似的线性关系.我们研究发现, CO吸附强度也与被吸附金周围配位金原子的相对位置有关,其中位于正下方的配位金原子加强 CO吸附,而位于侧位的配位金原子则弱化 CO吸附,这显然削弱了 CO吸附与金配位数线性关系的可靠性.理论研究表明,在纯金表
　　面上 O2吸附强度一般很弱,只有在一些特殊结构的金团簇上才有较强的吸附,但在 Au/TiO2界面及 CeO2表面上 O2吸附较强.金表面原子氧的吸附和金的表面结构有关.我们发现,原子氧倾向于在金的表面形成一种线性的 O–Au–O结构以增加其稳定性.当金表面的氧覆盖度增大时,会形成一种金氧化物薄膜结构,其结构依赖于氧的化学势和金的表面结构.纳米金催化 CO氧化反应机理可能因体系、载体等的差异而不同.大部分理论计算结果表明,在纯金表面上 O2很难直接解离形成原子氧,因此反应机理可能是吸附的 CO先与 O2反应形成了一种 CO–O2中间体,然后解离形成 CO2.在 Au/TiO2和 Au/CeO2催化剂上 CO催化氧化机理争议很大,均有计算结果支持 LH机理和 M–vK机理.另外,根据实验上观察到了负载型纳米金能直接活化分子氧的结果,理论上也提出了分子氧先解离为原子氧再与 CO反应的氧解离机理.针对如何解离分子氧问题,人们分别提出了低配位金模型、正方形金结构模型、Ti5c模型及 Au/Ti5c模型等.我们也提出了一种独特的双直线 O–Au–O模型来理解 Au/TiO2或 Au/CeO2界面解离活化分子氧.理论计算结果表明,低配位的金,金和载体之间的电荷转移,以及金所表现出的强相对论效应对于纳米金的活性影响很大.需要特别指出的是,金的强相对论效应有助于理解金表面的 CO吸附与金配位的关系、金表面原子氧的吸附特性、金氧化物薄膜的结构和分子氧的活化等过程.我们认为,金的强相对论作用导致了体相金的化学惰性以及纳米金的活性,因此相对论效应的深入研究将有助于理解金催化 CO氧化反应机理,从而有助于深层次理解纳米金催化活性来源.     

纳米金催化一氧化碳氧化反应的理论研究（英文）

近年来,纳米金催化剂独特的催化性质,特别是其优异的低温催化氧化活性,引起了人们极大的研究热情.除低温选择氧化外,在精细化学品合成、大气污染物消除、氢能的转换和利用等领域也开发出了一系列有广泛应用前景的金催化反应.此外,体相金的化学惰性和纳米金的超高活性之间差异的"鸿沟"也引起了理论工作者浓厚兴趣,试图从原理上理解体相金和纳米金活性差异的根源.CO催化氧化是最具有代表性的研究金催化活性的化学反应,本文主要综述了近十多年来金催化CO氧化反应理论计算方面的研究工作.一般认为,CO在纳米金表面的吸附是CO氧化反应的初始步骤.密度泛函理论研究表明,CO在金表面的吸附强度主要与被吸附金原子的配位数有关:金配位数越低,CO的吸附能越强,部分研究结果表明两者之间存在近似的线性关系.我们研究发现,CO吸附强度也与被吸附金周围配位金原子的相对位置有关,其中位于正下方的配位金原子加强CO吸附,而位于侧位的配位金原子则弱化CO吸附,这显然削弱了CO吸附与金配位数线性关系的可靠性.理论研究表明,在纯金表面上O_2吸附强度一般很弱,只有在一些特殊结构的金团簇上才有较强的吸附,但在Au/Ti O_2界面及CeO_2表面上O_2吸附较强.金表面原子氧的吸附和金的表面结构有关.我们发现,原子氧倾向于在金的表面形成一种线性的O–Au–O结构以增加其稳定性.当金表面的氧覆盖度增大时,会形成一种金氧化物薄膜结构,其结构依赖于氧的化学势和金的表面结构.纳米金催化CO氧化反应机理可能因体系、载体等的差异而不同.大部分理论计算结果表明,在纯金表面上O_2很难直接解离形成原子氧,因此反应机理可能是吸附的CO先与O_2反应形成了一种CO–O_2中间体,然后解离形成CO_2.在Au/TiO_2和Au/Ce O_2催化剂上CO催化氧化机理争议很大,均有计算结果支持LH机理和M–v K机理.另外,根据实验上观察到了负载型纳米金能直接活化分子氧的结果,理论上也提出了分子氧先解离为原子氧再与CO反应的氧解离机理.针对如何解离分子氧问题,人们分别提出了低配位金模型、正方形金结构模型、Ti5c模型及Au/Ti5c模型等.我们也提出了一种独特的双直线O–Au–O模型来理解Au/TiO_2或Au/CeO_2界面解离活化分子氧.理论计算结果表明,低配位的金,金和载体之间的电荷转移,以及金所表现出的强相对论效应对于纳米金的活性影响很大.需要特别指出的是,金的强相对论效应有助于理解金表面的CO吸附与金配位的关系、金表面原子氧的吸附特性、金氧化物薄膜的结构和分子氧的活化等过程.我们认为,金的强相对论作用导致了体相金的化学惰性以及纳米金的活性,因此相对论效应的深入研究将有助于理解金催化CO氧化反应机理,从而有助于深层次理解纳米金催化活性来源.     

甲酸在轻微氧化的Nb(110)表面吸附的HREELS研究

用高分辨电子能量损失谱(HREELS)研究了甲酸在轻微氧化的Nb(110)表面(O/Nb原子比=0.2)上的吸附与分解,提出了相应的表面反应模式.140K时,低暴露量的甲酸在该表面解离生成甲酸根(HCOO),生成的甲酸根以单齿形式键合在Nb上,同时也有少量甲酸分解生成吸附态的CO;高暴露量时则生成多层物理吸附的固体甲酸.升温至～190K,物理吸附的甲酸脱附,此时的表面为单齿键合的HCOO和CO所覆盖.温度升至250～300K时,HCOO的吸附态由单齿式转变成桥式,同时表面吸附的CO分子消失.升温过程的HREELS表明HCOO的分解导致了Nb的氧化.暴露量较高时表面的甲酸根比较稳定以致于在540K的高温时仍不完全分解.     

HCOO在Cu(110)、Ag(110)和Au(110)表面的吸附

采用密度泛函理论(DFT)以及广义梯度近似方法(GGA)计算了甲酸根(HCOO)在Cu(110)、Ag(110)和Au(110)表面的吸附. 计算结果表明, 短桥位是最稳定的吸附位置, 计算的几何参数与以前的实验和计算结果吻合. 吸附热顺序为Cu(110)(-116 kJ·mol-1)>Ag(110)(-57 kJ·mol-1)>Au(110)(-27 kJ·mol-1), 与实验上甲酸根的分解温度相一致. 电子态密度分析表明, 吸附热顺序可以用吸附分子与金属d-带之间的Pauli 排斥来关联, 即排斥作用越大, 吸附越弱. 另外还从计算的吸附热数据以及实验上HCOO的分解温度估算了反应CO2+1/2H2→HCOO的活化能, 其大小顺序为Au(110)>Ag(110)>Cu(110).     

A density functional theory study of the noble metal adsorption on the CeO2(111) surface

利用密度泛函理论系统研究了贵金属原子(Au、Pd、Pt和Rh)在CeO2( 111)表面的吸附行为.结果表明,Au吸附在氧顶位最稳定,Pd、Pt倾向吸附于氧桥位,而Rh在洞位最稳定.当金属原子吸附在氧顶位时,吸附强度依次为Pt ＞Rh＞ Pd＞Au.Pd、Pt与Rh吸附后在Ce 4 f、O2p电子峰间出现掺杂峰;Au未出现掺杂电子峰,其d电子峰与表面O2p峰在-4 -1 eV重叠.态密度分析表明,Au吸附在氧顶位、Pd与Pt吸附在桥位、Rh吸附在洞位时,金属与CeO2(111)表面氧原子作用较强,这与Bader电荷分析结果相一致.     

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.     

Reaction heat-driven CO2 desorption during CO oxidation on Au(997) at low temperatures

Adsorption and reaction of CO and CO₂ were studied on oxygen-covered Au(997) surfaces by means of temperatureprogrammed desorption/reaction spectroscopy. Oxygen atoms (O(a)) on Au(997) enhances the CO₂ adsorption and stabilizes the adsorbed CO₂(a), and the stabilization effect also depends on the CO₂(a) coverage and involved Au sites. CO₂(a) desorption is the rate-limiting step for the CO+O(a) reaction to produce CO₂ on Au(997) at 105 K and exhibits complex behaviors, including the desorption of CO₂(a) upon CO exposures at 105 K and the desorption of O(a)-stabilized CO₂(a) at elevated temperatures. The desorption of CO₂(a) from the surface upon CO exposures at 105 K to produce gaseous CO₂ depends on the surface reaction extent and involves the reaction heat-driven CO₂(a) desorption channel. CO+O(a) reaction proceeds more easily with weakly-bound oxygen adatoms at the (111) terraces than strongly-bound oxygen adatoms at the (111) steps. These results reveal complex rate-limiting CO₂(a) desorption behaviors during CO+O(a) reaction on Au surfaces at low temperatures which provide novel information on the fundamental understanding of Au catalysis.     

Self-assembly of mixed Pt and Au nanoparticles on PDDA-functionalized graphene as effective electrocatalysts for formic acid oxidation of fuel cells

Pt and Au nanoparticles with controlled Pt?:?Au molar ratios and PtAu nanoparticle loadings were successfully self-assembled onto poly(diallyldimethylammonium chloride) (PDDA)-functionalized graphene (PDDA-G) as highly effective electrocatalysts for formic acid oxidation in direct formic acid fuel cells (DFAFCs). The simultaneously assembled Pt and Au nanoparticles on PDDA-G showed superb electrocatalytic activity for HCOOH oxidation, and the current density associated with the preferred dehydrogenation pathway for the direct formation of CO(2) through HCOOH oxidation on a Pt(1)Au(8)/PDDA-G (i.e., a Pt?:?Au ratio of 1?:?8) is 32 times higher than on monometallic Pt/PDDA-G. The main function of the Au in the mixed Pt and Au nanoparticles on PDDA-G is to facilitate the first electron transfer from HCOOH to HCOO(ads) and the effective spillover of HCOO(ads) from Au to Pt nanoparticles, where HCOO(ads) is further oxidized to CO(2). The Pt?:?Au molar ratio and PtAu nanoparticle loading on PDDA-G supports are the two critical factors to achieve excellent electrocatalytic activity of PtAu/PDDA-G catalysts for the HCOOH oxidation reactions.     

Water-enhanced low-temperature CO oxidation and isotope effects on atomic oxygen-covered Au(111)

Water-oxygen interactions and CO oxidation by water on the oxygen-precovered Au(111) surface were studied by using molecular beam scattering techniques, temperature-programmed desorption (TPD), and density functional theory (DFT) calculations. Water thermally desorbs from the clean Au(111) surface with a peak temperature of approximately 155 K; however, on a surface with preadsorbed atomic oxygen, a second water desorption peak appears at approximately 175 K. DFT calculations suggest that hydroxyl formation and recombination are responsible for this higher temperature desorption feature. TPD spectra support this interpretation by showing oxygen scrambling between water and adsorbed oxygen adatoms upon heating the surface. In further support of these experimental findings, DFT calculations indicate rapid diffusion of surface hydroxyl groups at temperatures as low as 75 K. Regarding the oxidation of carbon monoxide, if a C (16)O beam impinges on a Au(111) surface covered with both atomic oxygen ( (16)O) and isotopically labeled water (H 2 (18)O), both C (16)O (16)O and C (16)O (18)O are produced, even at surface temperatures as low as 77 K. Similar experiments performed by impinging a C (16)O beam on a Au(111) surface covered with isotopic oxygen ( (18)O) and deuterated water (D 2 (16)O) also produce both C (16)O (16)O and C (16)O (18)O but less than that produced by using (16)O and H 2 (18)O. These results unambiguously show the direct involvement and promoting role of water in CO oxidation on oxygen-covered Au(111) at low temperatures. On the basis of our experimental results and DFT calculations, we propose that water dissociates to form hydroxyls (OH and OD), and these hydroxyls react with CO to produce CO 2. Differences in water-oxygen interactions and oxygen scrambling were observed between (18)O/H 2 (16)O and (18)O/D 2 (16)O, the latter producing less scrambling. Similar differences were also observed in water reactivity toward CO oxidation, in which less CO 2 was produced with (16)O/D 2 (16)O than with (16)O/H 2 (16)O. These differences are likely due to primary kinetic isotope effects due to the differences in O-H and O-D bond energies.     

Pathways for the Non‐CO‐Involved Oxidation of Methanol on Pt(111)

The non‐CO‐involved oxidation of methanol (NCOIOM) on a Pt(111) surface is investigated by using density functional theory. Relative energy diagrams for the NCOIOM are established in which the reaction mechanisms for a catalytic cycle—including the associated barriers, the reactive energies, the intermediates, and the transient states—are shown. The results indicate that the reaction proceeds via the kinetically favored pathways: A) HCOH→HC(OH)₂→HCOOH→HCOO‐ [‐COOH]→CO₂ and B) CHO→HCOOH→HCOO‐ [‐COOH]→CO₂, with OH playing a key role in the entire process. The vibrational frequencies of the intermediate states derived from the calculations are in agreement with the experimental measurements.     

Enhancement of O2 dissociation on Au111 by adsorbed oxygen: implications for oxidation catalysis

We show that the dissociation probability of O2 on the reconstructed, Au111-herringbone surface is dramatically increased by the presence of some atomic oxygen on the surface. Specifically, at 400 K the dissociation probability of O2 on oxygen precovered Au111 is on the order of 10(-3), whereas there is no measurable dissociation on clean Au111, establishing an upper bound for the dissociation probability of 10(-6). Atomic oxygen was deposited on the clean reconstructed Au111-herringbone surface using electron bombardment of condensed NO2 at 100 K. The dissociation probability for dioxygen was measured by exposing the surface to 18O2. Temperature programmed desorption (TPD) was used to quantify the amount of oxygen dissociation and to study the stability of the oxygen in all cases. Oxygen desorbs as O2 in a peak centered at 550 K with pseudo-first-order kinetics; i.e., the desorption peak does not shift with coverage. Our interpretation is that the coverage dependence of the activation energy for dissociation (deltaE(dis)) and/or preexponential factor (nu(d)) may be responsible for the unusual desorption kinetics, implying a possible energy barrier for O2 dissociation on Au111. These results are discussed in the context of Au oxidation chemistry and the relationship to supported Au nanoparticles.     

Silver residues as a possible key to a remarkable oxidative catalytic activity of nanoporous gold

Recently, several forms of unsupported gold were shown to display a remarkable activity to catalyze oxidation reactions. Experimental evidence points to the crucial role of residual silver present in very small concentrations in these novel catalysts. We focus on the catalytic properties of nanoporous gold (np-Au) foams probed via CO and oxygen adsorption/co-adsorption. Experimental results are analyzed using theoretical models represented by the flat Au(111) and the kinked Au(321) slabs with Ag impurities. We show that Ag atoms incorporated into gold surfaces can facilitate the adsorption and dissociation of molecular oxygen on them. CO adsorbed on top of 6-fold coordinated Au atoms can in turn be stabilized by co-adsorbed atomic oxygen by up to 0.2 eV with respect to the clean unsubstituted gold surface. Our experiments suggest a linking of that most strongly bound CO adsorption state to the catalytic activity of np-Au. Thus, our results shed light on the role of silver admixtures in the striking catalytic activity of unsupported gold nanostructures.     

碱性介质中CO对甲醇在PtAu(111)和Pt(111)表面氧化生成甲酸的影响的第一性原理研究

采用密度泛函理论计算研究了碱性介质中甲醇在清洁的PtAu(111)和Pt(111)表面、及有CO存在的PtAu(111)和Pt(111)表面的氧化。计算结果表明,在碱性介质中,预吸附的CO促进了甲醇在PtAu(111)和Pt(111)表面氧化的每一步反应,这与其在Au(111)表面的作用相似。究其原因,是由于CO的吸附增强了OH的稳定性和碱性,从而增强了OH夺取氢原子的能力。     

Water activated by atomic oxygen on Au(111) to oxidize CO at low temperatures

The Au(111) surface was populated with atomic oxygen [16O] followed by oxygen-labeled water [H218O] at surface temperatures as low as 77 K. When a CO beam was impinged on this surface, both [C16O16O] and [C16O18O] were produced. The results strongly suggest the direct involvement and promoting role of water in CO oxidation on oxygen covered Au(111) at low temperatures.     

Application of Si‐doped graphene as a metal‐free catalyst for decomposition of formic acid: A theoretical study

Using density functional theory calculations, the adsorption and catalytic decomposition of formic acid (HCOOH) over Si‐doped graphene are investigated. For the stable adsorption geometries of HCOOH over Si‐doped graphene, the electronic structure properties are analyzed by adsorption energy, density of states, and charge density difference. A comparison of the reaction pathways reveals that both dehydration and dehydrogenation of HCOOH can occur over Si‐doped graphene. The estimated reaction energies and the activation barriers suggest that for the dehydration of HCOOH on the Si‐doped graphene, the rate‐controlling step is H + OH → H₂O reaction. For the dehydrogenation of HCOOH, the rate‐determining step is the breaking of the C? H bond of the HCOO group to form the CO₂ molecule and the atomic H. Our results reveal that the low cost Si‐doped graphene can be used as an efficient nonmetal catalyst for O? H bond cleavage of HCOOH. © 2015 Wiley Periodicals, Inc.