氧在Co3O4催化剂上的化学吸附

测定了氧在氨氧化催化剂Co_3O_4上于100—400℃下的吸附动力学和室温-600℃的热脱附谱,并与离子探针质谱测定的结果进行了比较。动力学曲线指出氧具有快速和慢速不同类型的吸附,热脱附谱显示有相应的脱附曲线。快速吸附对应于两个脱附峰,峰的极大值温度各为165℃,380—420℃;相应的吸附氧粒子可指定为O_2~-,O~-;脱附活化能分别为13.3千卡/克分子,26.7千卡/克分子。慢速吸附服从Elovich吸附规律,吸附过程可能是快速吸附的氧粒子在催化剂表面上迁移生成O~(2-)并入氧化物表面晶格,脱附温度高于500℃时可由表面晶格中逸出。吸附中心可能是表面Co~(2+)离子。讨论了氧吸附与氨催化氧化反应之间的关系。     

CO在Pd/Al2O3壳型催化剂上的吸脱附和表面氧化反应

在高真空系统中采用化学吸附,T2D和T2SR技术研究了CO在Pd/Al_2O_3壳型催化剂上的吸脱附行为和表面氧化反应。结果表明在室温下CO不仅吸附在Pd原子上,还可以吸附在Al_2O_3上,其中一部分是易于脱附的弱吸附,在室温～450℃之间出现相当于三种吸附型式的程脱峰。低温和中温程脱峰中以CO为主,CO_2占少量。高温程脱峰中CO_2显著增多。当先吸附氧,后吸附CO时,则发生剧烈的表面氧化反应。还发现CO在PdO/Al_2O_3上的化学吸附键合能力比在Pt/Al_2O_3上弱。Pd在Al_2O_3上的壳层厚度太薄,晶粒增大至一定程度时,CO化学吸附中心数目明显减少。 根据实验结果讨论了CO在Pd/Al_2O_3壳型催化剂上的表面氧化反应机理,Pd价态和晶粒大小与CO吸脱附行为之间的联系。     

乙酸在Rh(100)表面吸附和分散的HREELS和TDS研究

 用高分辨电子能量损失谱（HREELS）和热脱附谱（TDS）研究了乙酸在Rh（100）表面上的吸附和分解,提出了乙酸吸附和分解反应的模式．130K时,高暴露量的乙酸在Rh（100）表面上形成多层吸附的凝聚态乙酸．升温至290K时,部分乙酸以分子形式直接脱附,另一部分乙酸分子通过O—H键断裂形成乙酸根和氢吸附在表面上;在升温至400K的过程中,乙酸根在表面发生两个相互竞争的反应,即乙酸根分解成CO,CHx,O和H,以及乙酸根分解成CHx,H和CO2;升温至500K,只剩下CHx,O和CO吸附在表面上;600K后,表面吸附的CHx完全解离,同时表面吸附的碳原子和氧原子结合成CO脱附．     

CO在Pt/A12O3催化剂上歧化的研究

利用程序升温脱附(TPD)和差分红外光谱方法研究了CO在Pt/Al_2O_3上的吸附和脱附。发现CO在Pt/A1_2O_3上的TPD谱存在一系列的极大值,由TPD气相产物的组成分析证明除了CO外,尚有CO_2。从差分红外光谱研究发现CO在Pt/Al_2O_3上被吸附时,随温度升高发生歧化反应,生成的CO_2同Al_2O_3表面形成一系列的羧基、羧化物、碳酸盐吸附态,它们的热稳定性不同,热脱附时产生的CO_2则是由这些吸附态脱附而形成的。TPD图谱中的一系列的极大值同这些吸附态的脱附有关。     

红外光谱法测定CO在Ni/AL2O3催化剂上的吸附热和脱附活化能

本文介绍了用红外光谱(IR)和程序升温脱附原位红外(TPD-IR)技术测定吸附热(Q_a)和脱附活化能(E_d)的实验方法,并测定了CO在Ni/Al_2O_3催化剂上弱吸附线式态的吸附热和强吸附线式态的脱附活化能。结果表明弱吸附线式态的吸附热约为21kJ/mol,强吸附线式态的脱附活化能在10%Ni/Al_2O_3上约为84kJ/mol,在20%Ni/Al_2O_3上约为100kJ/mol。对本方法及实验结果进行了讨论。     

CuCl等在γ-Al_2O_3表面的分散及其对乙烯吸附的研究

本文研究了CuCl在γAl_20_3表面上的分散和乙烯在CuCl/γ-Al_2O_3上的吸附和程序升温脱附情况.X射线相定量法和紫外漫反射法都表明用烘烤法或浸渍法制备的CuCl/γ-Al_2O_3中CuCl在γ-Al_2O_3表面可达单层分散.X射线相定量法测得CuCl在γ-Al_2O_3表面上的最大分散量为0.095克CuCl/100米~2γ-Al_2O_3表面.吸附和程序升温脱附研究表明,分散在γ-Al_2O_3表面上的CuCl在室温常压下便可吸附乙烯,比纯CuCl吸附乙烯的能力提高千倍以上,分散后的CuCl吸附乙烯的分子比可高达0.19C_2H_4分子/Cu~ 离子.分散在γ-Al_2O_3表面上的CuCl可与C_2H_4,C_3H_6等形成表面π络合物,但要求有合适的几何环境.这些结果可供利用CuCl/γ-Al_2O_3分离含不饱和键分子时参考。     

担载的NaRuCo_3(CO)_(12)络合物及其分散型Ru-Co双金属催化剂

担载于Al_2O_3上的NaRuCO_3(CO)_(12)络合物在IR光谱上出现类似于Ru(CO)_2O_2络合物特征带的羰基带,在300～400℃左右变强,在CO中尤为明显。在以ZrO_2为载体时出现与原来络合物相近的弱羰基带。当CO吸附于以Al_2O_3为载体的脱羰基催化剂上时,发现在氧化态催化剂上也出现了与担载络合物相似的吸收带,在还原态催化剂上只出现线式带。吸附于以ZrO_2为载体的脱羰基催化剂上的CO不出现CO的IR吸收带,在He中亦无TPD脱附物,在H_2中却可发生加氢反应生成CH_4。从Na~+和ZrO_2协同效应角度,探讨了CO在上述两种分散型催化剂上吸脱附行为不同的原因。     

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.     

锌电极上2-巯基吡啶自组装单层的原位表面增强拉曼光谱电化学观察

利用原位表面增强拉曼散射(SERS)技术, 观察了2-巯基吡啶在锌电极上单层吸附和脱附行为. SERS实验结果表明, 2-巯基吡啶分子主要通过巯基上的硫原子垂直吸附于锌表面. 原位SERS光谱电化学发现, 当外加电位达到－1.4 V vs. SCE时, 分子开始从表面脱附, 并伴随吸附构型变化, 在－1.6 V时发生完全脱附.     

表面活性剂修饰对MCM-41负载离子液体吸附CO2影响的研究

以表面活性剂修饰的MCM-41为载体,采用浸渍法制备了负载离子液体[NH2p-mim][PF6]的二氧化碳吸附剂,考察了表面活性剂对离子液体在MCM-41上分散的影响以及所导致的CO2吸附性能的变化.利用红外光谱(FT-IR),X-射线衍射(XRD),高分辨透射电子显微镜(HRTEM),热重分析(TG)技术对所合成的负载型离子液体吸附剂进行了表征研究,并与其吸附CO2的性能变化、离子液体与表面活性剂相互作用方式等因素进行了关联.结果表明:MCM-41负载离子液体后对CO2的吸附性能略有提高,而经表面活性剂修饰的MCM-41负载离子液体后,对CO2的吸附容量较载体本身提高了2.5倍.这一方面是因为表面活性剂胶束改善了MCM-41上离子液体的分散性,另一方面是表面活性剂胶束对离子液体分子上电荷分布的影响,导致离子液体内部阴阳离子之间的相互作用减弱,从而引起离子液体中-NH2上N原子电子云密度增大,使其与CO2作用更容易.CO2在经表面活性剂修饰后的MCM-41负载离子液体[NH2p-mim][PF6]吸附剂上的吸附受扩散控制,其吸附-脱附CO2所需能量较小,经过5次吸附-脱附循环后,其吸附性能仍保持稳定.热重分析结果表明,经表面活性剂修饰后的MCM-41负载离子液体吸附剂在100℃下氮气气氛再生时不会发生性质改变.     

The nature of the oxidation states of gold on ZnO

The interaction between gold in the 0, i, ii and iii oxidation states and the zinc-terminated ZnO(0001) surface is studied via the QM/MM electronic embedding method using density functional theory. The surface sites considered are the vacant zinc interstitial surface site (VZISS) and the bulk-terminated island site (BTIS). We find that on the VZISS, only Au(0) and Au(i) are stable oxidation states. However, all clusters of i to iii oxidation states are stable as substitutionals for Zn2+ in the bulk terminated island site. Au(OH)(x) complexes (x= 1-3) can adsorb exothermically onto the VZISS, indicating that higher oxidation states of gold can be stabilised at this site in the presence of hydroxyl groups. CO is used as a probe molecule to study the reactivity of Au in different oxidation states in VZISS and BTIS. In all cases, we find that the strongest binding of CO is to surface Au(i). Furthermore, CO binding onto Au(0) is stronger when the gold atom is adsorbed onto the VZISS compared to CO binding onto a gas phase neutral gold atom. These results indicate that the nature of the oxidation states of Au on ZnO(0001) will depend on the type of adsorption site. The role of ZnO in Au/ZnO catalysts is not, therefore, merely to disperse gold atoms/particles, but to also modify their electronic properties.     

Surface dependence of CO2 adsorption on Zn2GeO4

An understanding of the interaction between Zn(2)GeO(4) and the CO(2) molecule is vital for developing its role in the photocatalytic reduction of CO(2). In this study, we present the structure and energetics of CO(2) adsorbed onto the stoichiometric perfectly and the oxygen vacancy defect of Zn(2)GeO(4) (010) and (001) surfaces using density functional theory slab calculations. The major finding is that the surface structure of the Zn(2)GeO(4) is important for CO(2) adsorption and activation, i.e., the interaction of CO(2) with Zn(2)GeO(4) surfaces is structure-dependent. The ability of CO(2) adsorption on (001) is higher than that of CO(2) adsorption on (010). For the (010) surface, the active sites O(2c)···Ge(3c) and Ge(3c)-O(3c) interact with the CO(2) molecule leading to a bidentate carbonate species. The presence of Ge(3c)-O(2c)···Ge(3c) bonds on the (001) surface strengthens the interaction of CO(2) with the (001) surface, and results in a bridged carbonate-like species. Furthermore, a comparison of the calculated adsorption energies of CO(2) adsorption on perfect and defective Zn(2)GeO(4) (010) and (001) surfaces shows that CO(2) has the strongest adsorption near a surface oxygen vacancy site, with an adsorption energy -1.05 to -2.17 eV, stronger than adsorption of CO(2) on perfect Zn(2)GeO(4) surfaces (E(ads) = -0.91 to -1.12 eV) or adsorption of CO(2) on a surface oxygen defect site (E(ads) = -0.24 to -0.95 eV). Additionally, for the defective Zn(2)GeO(4) surfaces, the oxygen vacancies are the active sites. CO(2) that adsorbs directly at the Vo site can be dissociated into CO and O and the Vo defect can be healed by the oxygen atom released during the dissociation process. On further analysis of the dissociative adsorption mechanism of CO(2) on the surface oxygen defect site, we concluded that dissociative adsorption of CO(2) favors the stepwise dissociation mechanism and the dissociation process can be described as CO(2) + Vo → CO(2)(δ-)/Vo → CO(adsorbed) + O(surface). This result has an important implication for understanding the photoreduction of CO(2) by using Zn(2)GeO(4) nanoribbons.     

簇模型ZnO(100)面上吸附CO_2的密度泛函理论研究

采用电荷自洽方法, 以嵌入原子簇Zn4O4为模型, 使用量子化学的密度泛函理论, 研究了二氧化碳在六方ZnO非极化的(1010)面的可能吸附态。计算表明, CO2垂直底物表面吸附, 氧原子只能与Zn原子配位, 并且吸附能为很弱的1.8 kJ/mol;吸附质分子平行于底物表面时, 得到了5种平衡吸附构型, 其中采用CZn配位和η2O, O二齿配位时, 吸附很弱, 经BSSE校正后的吸附能在8.8~6.6 kJ/mol。 采用η2C, O方式分别与O和Zn配位时, 吸附能为31.1 kJ/mol; C原子与表面O配位时计算得到了唯一的一个化学吸附态, 吸附能为139.6 kJ/mol, 与实验结果一致。     

H2O and H2 interaction with ZnO surfaces: A MNDO,AM1, and PM3 theoretical study with large cluster models

We investigated the adsorption and heterolytic dissociation of H₂O and H₂ molecules on a (ZnO)₂₂ cluster corresponding to ZnO (0001), (000(OVERBAR)1), and (10(OVERBAR)10) surfaces using MNDO , AM 1 and PM 3 semiempirical procedures. The geometry of the adsorbed molecule has been optimized in order to analyze binding energies, charge transfer, and preferential sites of interaction. The adsorbed species interact most strongly when it is bonded to the twofold coordinated zinc atom of the cluster surface. The interaction of the H₂O molecule with the surface of ZnO has a charge transfer from H₂O to the surface ranging between 0.17 and 0.27 au. The neighboring atoms of the surface are the main receptors during the process of charge transfer. Our results indicate that there is a weak bonding of the hydrogen atom from OH with the oxygen surface atom that could produce the O(SINGLE BOND)H·O band. The interaction of the H₂ molecule with the surface is generally weak and only the PM 3 method yields a strong binding energy for this interaction. There is a charge transfer from the H₂ molecule to the surface. The chemisorption of H on oxygen atom of the surface transfer charge from the surface to the H. We also calculated the vibrational analyses for these interactions on ZnO surface and compared our results with available experimental data. © 1996 John Wiley & Sons, Inc.     

Adsorption of single Cu atoms at differently stabilized polar ZnO surfaces: An ab initio study

In this study, the adsorption of a single Cu atom on the polar ZnO(0001) and ZnO(0001) surfaces was investigated by means of highly accurate wave function based ab initio methods. Different modifications of the polar surfaces were investigated: unstabilized surfaces, surfaces with hydrogen and OH overlayers, and defectious surfaces. Cu is strongly bound at the unstabilized surfaces by a charge transfer mechanism which induces a stabilization of the surface. At regular adsorption sites of stabilized surfaces the binding energies are much smaller. At oxygen defects, the Cu electron is transferred to the defect and a Cu(+) ion sits on top of the defect. At the Zn terminated surface the Cu moves into the Zn defects.     

Ab initio cluster calculations on the electronic structure of oxygen vacancies at the polar ZnO(0001) surface and on the adsorption of H2, CO, and CO2 at these sites

Oxygen vacancies at the polar O terminated (0001) surface of ZnO are of particular interest, because they are discussed as active sites in the methanol synthesis. In general, the polar ZnO surfaces are stabilized by OH groups, therefore O vacancies can be generated by removing either O atoms or OH or H2O groups from the surface. These defects differ in the number of electrons in the vacancy and the number of OH groups in the neighborhood. In the present study, the electronic structure and the adsorption properties of four different types of oxygen vacancies have been investigated by means of embedded cluster calculations. We performed ab initio calculations on F+ like surface excitations for the different defect types and found that the transition energies are above the optical band-gap, while F+ centers in bulk ZnO show a characteristic optical excitation at 3.19 eV. Furthermore, we studied the adsorption of CO2 and CO at the different defect sites by DFT calculations. We found that CO2 dissociates at electron rich vacancies into CO and an O atom which remains in the vacancy. At the OH vacancy which contains an unpaired electron CO2 adsorbed in the form of CO2-, while it adsorbed as a linear neutral molecule at the H2O defect. CO adsorbed preferentially at the H2O defect and the OH defect, both with a binding energy of 0.3 eV.     

Pd(Cu)在MgO(100)表面吸附CO和O2的理论研究

采用固体镶嵌势能模型和DFT/B3LYP方法研究了在Pd/MgO和Cu/MgO表面吸附CO和O₂分子的电子性质. 计算结果表明, 在完美MgO(100)表面Pd原子对CO和O₂的吸附能分别为206.5和84.8 kJ/mol, 因此可知Pd原子更容易吸附CO分子; 而当Pd原子附着于有氧缺陷的MgO表面时, 它对两种分子的吸附都非常弱. 相反, 附着于MgO表面的Cu原子对O₂分子的吸附更为有利, 其吸附能在140～155 kJ/mol之间. 研究结果还表明, 对于双分子吸附体系, 即CO＋CO, CO＋O₂, O₂＋O₂体系, 双分子之间的结合力可减小完美MgO表面上Pd原子与被吸附分子的相互作用, 使吸附能减少了46～96 kJ/mol. 而对于在MgO表面上的Cu原子, 只有O₂＋O₂ 体系使吸附能减少了大约50～71 kJ/mol.     

Density function theory study of CO adsorption on Fe3O4(111) surface

Density functional theory calculations have been carried out for CO adsorption on the Fe(oct2)- and Fe(tet1)-terminated Fe(3)O(4)(111) surfaces, which are considered as active catalysts in water-gas shift reaction. It is found that the on-top configurations are most stable on these two surfaces. Some bridge configurations are also stable in which the new C-O bond formed between the surface O atom and the C atom of CO. The adsorption on the Fe(oct2)-terminated surface is more stable than on the Fe(tet1)-terminated surface. The density of state reveals the binding mechanism of CO adsorption on the two surfaces. Our calculations have also shown that the absorbed CO can migrate from the on-top site to the bridge site or 3-fold site. The oxidation of CO via surface oxygen atoms is feasible, which is in good agreement with experimental results.     

Zn adsorption on Pd(111): ZnO and PdZn alloy formation

The adsorption and thermal desorption of Zn and ZnO on Pd(111) was studied in the temperature range between 300 and 1300 K with TDS, LEED, and CO adsorption measurements. At temperatures below 400 K, multilayer growth of Zn metal on the Pd(111) surface takes place. At a coverage of 0.75 ML of Zn, a p(2 x 2)-3Zn LEED structure is observed. Increasing the coverage to 3 ML results in a (1 x 1) LEED pattern arising from an ordered Zn multilayer on Pd(111). Thermal desorption of the Zn multilayer state leads to two distinct Zn desorption peaks: a low-temperature desorption peak (400-650 K) arising from upper Zn layers and a second peak (800-1300 K) originating from the residual 1 ML Zn overlayer, which is more strongly bound to the Pd(111) surface and blocks CO adsorption completely. Above 650 K, this Zn adlayer diffuses into the subsurface region and the surface is depleted in Zn, as can be deduced from an increased amount of CO adsorption sites. Deposition of >3 ML of Zn at 750 K leads to the formation of a well-ordered Pd-Zn alloy exhibiting a (6 x 4 square root 3/3)rect. LEED structure. CO adsorption measurements on this surface alloy indicate a high Pd surface concentration and a strong reduction of the CO adsorption energy. Deposition of Zn at T > 373 K in 10(-6) mbar of O2 leads to the formation of an epitaxial (6 x 6) ZnO overlayer on Pd(111). Dissociative desorption of ZnO from this overlayer occurs quantitatively both with respect to Zn and O2 above 750 K, providing a reliable calibration for both ZnO, Zn, and oxygen coverage.     

CO2选择性氧化甲烷制C2烃催化剂La2O3/ZnO的氧化还原和CO2吸附性能

Ｌａ２Ｏ３／ＺｎＯ催化剂体系在以二氧化碳作为氧化剂的甲烷氧化偶联反应中具有很高的Ｃ２烃选择性和稳定性．采用ＣＯ２－ＴＰＤ－ＭＳ和ＴＰＲ技术考察了Ｌａ２Ｏ３／ＺｎＯ对ＣＯ２的吸附性质及其氧化还原行为．结果表明：（１）Ｌａ２Ｏ３／ＺｎＯ催化剂体系存在着强、弱两种碱中心，其中弱碱中心数量随样品中Ｌａ２Ｏ３含量增加而减少，强碱中心强度随样品中Ｌａ２Ｏ３含量增加而增强．（２）由于组分相互作用，高温下，Ｌａ２Ｏ３／ＺｎＯ易产生晶格氧空位，使之对ＣＯ２的吸附增强，吸附后的ＣＯ２与晶格氧作用形成立方晶型Ｌａ２Ｏ２ＣＯ３．（３）Ｌａ２Ｏ３／ＺｎＯ表面的Ｌａ３＋和Ｚｎ２＋可以部分被还原，由于组分间的相互作用，使得二者的还原都较单一组分存在时更难．（４）Ｈ２－ＣＯ２－Ｈ２氧化还原循环实验表明，Ｌａ２Ｏ３／ＺｎＯ表面被部分还原后，ＣＯ２可以将部分被还原的表面再氧化．在此基础上对Ｌａ２Ｏ３／ＺｎＯ催化剂上甲烷与ＣＯ２转化为Ｃ２烃的机制也进行了讨论．