NH3分子在LiH（100）表面吸附的第一性原理研究

采用第一性原理方法研究了NH₃分子在LiH（100）晶面的表面吸附情况.通过研究LiH（100）/NH₃体系的吸附位置、吸附能和电子结构,发现NH₃分子在LiH（100）晶面主要是化学吸附,初始位置为NH₃分子中N-H键在Li顶住时失去一个H原子,并在LiH（100）面形成NH₂基,其吸附能为0.511 eV,属于强化学吸附,吸附作用最强.此时NH₂基与附近H原子和Li原子之间为离子键作用,NH₂基中N—H键为共价键;NH₃分子中另一个H原子与LiH表面的一个H原子形成一个H₂分子逸出表面.H₂分子中H-H键为明显的共价键.     

NH3在TaC(0001)表面吸附和解离的第一性原理研究

采用自旋极化密度泛函理论(DFT)并结合周期平板模型的方法,研究了NH₃在TaC表面的吸附和分解反应机理.表面能计算结果显示,以Ta为终止的TaC(0001)面为最稳定的表面;NH₃分子通过其孤对电子优先吸附在顶位top位,而NH₂和H最稳定吸附位置为三重hcp位,NH和N吸附在三重fcc位.过渡态结果表明氮原子的复合反应脱附为整个反应的限速步骤.电子结构计算结果表明,NH3分子及其片段通过其N原子的2p_z轨道与底物Ta的5d_z2轨道混合吸附于表面.随着脱氢反应的进行,电荷转移现象变得逐渐明显,吸附质和底物之间的电荷转移在加速NH₃脱氢催化过程中发挥重要作用.     

氨分子在000和201带的选择光离解

本文利用脉冲紫外激光(UV)选择激发氨分子到?¹A″₂电子激发态的两个最低振动能级ν′₂=0和ν′₂=1(ν₂振动),然后检测新生态H原子的飞行谱(TOF),研究了氨分子的光碎片动力学。光谱证实了最近所测的离解能D₀⁰(H-NH₂)=4.645eV;绝大多数生成的NH₂(X²B₁)基处于非振动激发,但是具有围绕a惯性轴的高度转动激发。通过NH₃(?)的ν′₂=1光离解产生的NH₂(X)基具有较高的内部激发,并且显示了在N=K_a转动能级上的反转布居。 关键词：     

用光声谱研究大分子吸附物的振动谱

本工作在超高真空条件下用光声谱(PAS)研究了有机大分子双醋酸纤维素(C₂H₈O₃·(COOCH₃)₂)吸附在多晶Ag表面的振动模性质。测量了C-O伸缩模的谱峰位置,FWHM及线形随覆盖度、温度的变化,讨论了与此有关的吸附性质。并确定了本实验装置的表面探测灵敏度可低于一个单分子层。 关键词：     

FeOx上AsH3吸附净化的密度泛函研究

基于密度泛函理论研究了AsH₃和O₂分子在α-Fe₂O₃(001)表面和FeO(100)表面的吸附及共吸附性质.结果表明：AsH₃和O₂分子在α-Fe₂O₃(001)表面最稳定的吸附构型都是Hollow吸附位点. AsH₃分子在FeO(100)表面最稳定的吸附位点为Top O吸附位点. O₂分子在FeO(100)表面最稳定的吸附位点为Hollow吸附位点. O₂分子在α-Fe₂O₃(001)和FeO(100)表面吸附后均被活化从而促进AsH₃分子的催化氧化. AsH₃分子在α-Fe₂O₃(001)表面最小的吸附能为-0.7991 eV,在FeO(100)表面最小的吸附能为-0.9117 eV.吸附值数据表明AsH     

等离子催化重整CH4/CO2中的协同效应及积碳动力学

本文采用实验测量和数值模拟结合的方法,对NSD等离子体–催化剂协同重整CH₄/CO₂过程中的协同效应以及积碳动力学进行研究。构建了包含中性分子、自由基、振动激发态、电子激发态、带电粒子、表面吸附态等物质在内的详细动力学机理。采用ZDPlasKin-CHEMKIN耦合的方法迭代求解等离子体放电过程、气相反应动力学及表面反应动力学在内的详细动力学机理。在300～700 K的温度范围内,该动力学模型能较好地预测反应物的消耗和产物的生成,路径通量分析表明CH₃在催化剂表面上的直接吸附反应以及CH₄振动激发态分子的吸附态CH₄(vs)在催化剂表面上的解离吸附均可促进吸附态CH₃(s)的生成。积碳动力学研究表明催化剂上的积碳主要来源于吸附态CH(s)的脱氢反应CH(s)+Ni(s)→C(s)+H(s)。     

过渡金属Cu、Cr掺杂TiO2表面氧化性气体NO2光学气敏传感特性

采用密度泛函理论（DFT）体系广义梯度近似（GGA）第一性原理平面波超软赝势方法,分析锐钛矿型TiO₂（101）表面吸附NO₂分子光学气敏传感的微观机理.结果表明：Cu和Cr原子易于掺入TiO₂（101）表面,掺杂表面能稳定地吸附NO₂分子且吸附后光学性质发生显著变化.表面吸附NO₂分子后,Cu掺杂TiO₂（101）表面对分子的吸附能最大,吸附后结构更稳定,分子与表面的距离最短.通过分析差分电荷密度和电荷布居数发现,NO₂分子与基底表面间发生电荷转移,转移电子数目：Cu掺杂表面 > Cr掺杂表面 > 无掺杂表面.对比吸收光谱和反射光谱发现,在Cu掺杂表面吸附分子后,光学性质变化最明显,说明表面与吸附分子间氧化还原能力是决定光学气敏传感性能的核心因素.在过渡金属中,Cu与Cr都有4s价电子结构,其4s电子降低了材料表面氧空位的氧化性,增加了其还原性.对于氧化性气体,可以提升表面与分子的氧化还原作用,而Cu的4s电子更加活泼,从而光学气敏传感特性更加明显.因此,Cu掺杂的TiO₂对氧化性气体是一种较好的光学气敏传感材料.     

单原子催化剂Ir1/MoS2表面上的NH3吸附与直接分解的第一性原理

NH₃的催化分解一直是制备高纯度氢的有效途径之一,因此具有良好的催化活性的贵金属被广泛的应用于催化解离的研究中.然而,由于纯金属催化剂的利用效率低,增加催化成本.最近的研究发现单原子催化剂Ir₁/MoS₂以其突出的优势被认为是一种潜在的能替代现有贵金属催化剂的材料.本文采用密度泛函理论与周期性平板模型相结合的方法,研究了NH₃在单原子催化剂Ir₁/MoS₂上的吸附与活化.结果表明:NH₃的优势吸附位为Ir原子的顶位,构型为倾斜结构(atop),NH₃与体系表面的金属Ir成键,吸附能达到1.63 eV,是化学吸附;进一步分析了NH₃直接催化分解的反应路径,给出了相应的反应热、活化能,结果显示NH₃在atop位的解离比脱附有利,第一步脱氢反应活化能最小,N-H键易断裂,第二步反应能垒较高,此步为整个反应的决速步.     

CH3NO2在Cu(111)面吸附态的研究

用CNDO/2半经验量化计算方法对CH₃NO₂分子在Cu(111)面四个吸附位上25种吸附态进行了优化计算,得到以CH₃NO₂分子中的-NO₂取向吸附在Cu(111)面的桥位上,且CH₃NO₂分子中的ONO面与Cu-Cu键成60°时为最稳吸附态。计算得到的这一稳定吸附态的吸附取向和吸附体系的态密度结果与我们的实验结果是一致的;从吸附态的轨道成分分析表明, 关键词：     

基于光声光谱技术的1.5 μm波段 NH3吸收谱线测量与分析

NH₃的检测具有广泛的应用,采用光声法检测NH₃是当前研究的热点,而确定NH₃的吸收谱线则是实现光声法检测NH₃的前提。采用外腔可调谐半导体激光器构造了光声气体检测系统,检测了NH₃的近红外吸收谱线,获得了常温常压下NH₃在1 515~1 532 nm范围内的吸收光谱。实验确认了NH₃在1 515.2,1 516.0,1 518.0,1 519.9,1 522.4,1 527.0,1 531.7 nm处存在强吸收。HITRAN 2004光谱数据库在近红外1.5 μm波段NH₃的吸收谱线数据未见报道,该结果为研究光声法检测NH₃提供了更多可选择的吸收谱线。     

Vibrational spectra of ammonia chemisorbed on platinum (111): I. Identification of chemisorbed states

Ammonia chemisorption on Pt(111) has been studied with high resolution electron energy loss spectroscopy (EELS), combined with thermal desorption spectroscopy (TDS). We detect two distinct molecular states of ammonia at different coverages. Near saturation monolayer coverage, ammonia is weakly chemisorbed (ΔH_ads ～ 9 kcal mol^?1) and coordinated to the metal surface via the nitrogen atom. The vibrational frequencies are shifted from the gas phase values, but not as strongly perturbed as in stable platinum-ammine complexes. Below 40% saturation coverage, a new molecular ammonia state is detected, which has a distinct vibrational and thermal desorption spectrum. This state has a considerably reduced ν(PtN) intensity, and the other frequencies are closer to those in solid ammonia, indicating a weaker interaction with the Pt surface. The thermal desorption spectrum of this lower coverage state is broad, from 160 to 450 K, and coverage dependent. Conversion between the two molecular states appears to be only a function of coverage. We propose that the two molecular states have different adsorption sites, and convert from one form to the other as the coverage is changed. No evidence is found for significant dissociation of ammonia on the Pt surface. At very low temperatures (100 K), solid ammonia multilayers may be grown.     

The adsorption of ammonia on an Fe(110) single-crystal surface studied by high-resolution electron energy-loss spectroscopy (eels)

The EELS spectra of ammonia adsorbed on an Fe(110) surface at 110 K reveal four different adsorption states of molecular ammonia with increasing coverage : chemisorption at “on-top” sites, chemisorption at multi-coordinated sites, adsorption in a second layer, and multilayer condensation.Thermal processing of an ammonia-covered sample to 155 K causes desorption of both the condensed phase and the second layer without any fragmentation of ammonia.Further heating of the sample leads to a much weaker desorption of molecular ammonia up to a temperature of 260 K. EELS spectra recorded after heating to 290 K show only small amounts of atomic nitrogen and hydrogen present on the surface, indicating partial decomposition of ammonia.The formation of species such as NH₂ (ads) or NH (ads) during the thermal-processing experiments could not be observed.     

An electrochemical determination of oxygen adsorption on copper

The adsorption of oxygen on polycrystalline and thin film copper samples was studied with the solid state electrochemical cell: Cu,Cu₂O¦7.5wt%CaO/ZrO₂¦Cu in ultrahigh vacuum. The oxygen partial pressure in the bulk of the copper sample was controlled electrochemically by applying a voltage across the cell, while the oxygen coverage at the copper free surface was monitored by Auger Electron Spectroscopy (AES). This technique has enabled us to establish much lower oxygen partial pressures at high temperatures than normally attainable in ultrahigh vacuum. In this paper we report the results of reversible oxygen adsorption isotherms on polycrystalline copper at 928, 970 and 1093 K. The results agree reasonably well with the deductions of earlier surface energy measurements and indicate a surprising degree of stability for chemisorbed oxygen on polycrystalline copper. Isosteric heats of adsorption are calculated with and without the inclusion of the earlier surface energy measurements and are compared to previous differential heats of adsorption determined calorimetrically.     

Crystallographic orientation dependence of work function: carbon adsorption on Au surfaces

We investigate the work function (WF) variation of different Au crystallographic surface orientations with carbon atom adsorption. Ab initio calculations within density-functional theory are performed on carbon deposited (100), (110), and (111) gold surfaces. The WF behaviour with carbon coverage for the different surface orientations is explained by the resultant electron charge density distributions. The dynamics of carbon adsorption at sub-to-one- monolayer (ML) coverage depends on the landscape of the potential energy surfaces. At higher ML coverage, because of adsorption saturation, the WF will have weak surface orientation dependence. This systematic study has consequential bearing on studies of electric-field noise emanating from polycrystalline gold ion-trap electrodes that have been largely employed in microfabricated electrodes.     

A vibrational study of ammonia chemisorbed on Ni(110) and Ni(111): whither goest the metal-nitrogen stretching mode on FCC (111) surfaces?

The adsorption of ammonia on the Ni(110) and Ni(111) surfaces has been studied with high resolution (≤ 65 cm_?1) electron energy loss spectroscopy (EELS) combined with thermal desorption spectroscopy. The EELS spectra of the initial chemisorbed layer or α state on each surface are very different. Ammonia chemisorbed on the Ni(110) surface exhibits a strong Ni-N stretching mode at 570 cm^?1 which is absent on the Ni(111) surface. The Ammonia adsorption site appears to be different on the Ni(110) and Ni(111) surfaces. We suggest that the absence of the M-N stretching mode on the Ni(111) surface is a general characteristic of the ammonia adsorption site on the (111) surfaces of fcc Group VIII metals.     

Adsorption of copper on tungsten; measurements on single crystal planes

Work function changes caused by depositing and spreading of copper were measured on the (110), (100), (111) and (211) faces of tungsten crystal by means of field emission microscope. On (110) and (100) faces the work function for low coverage decreases vith increasing amount of copper deposited, passes through a minimum, then increases foi higher coverage and saturates for a thick layer. On (111) and (211) faces in the low coverage range, the work function increases when the amount of adsorbed copper increases. After reaching a maximum value the changes in work function on these planes have the same character as on (110) and (100) planes, i.e. the work function drops down when the coverage increases, passes through a minimum, increases again and saturates for a thick layer. It is proposed to connect the increase of work function on (111) and (211) faces at low coverage with the loose structure of the substrate surface. The adsorption of copper on these planes causes the smoothing of the crystal surface and this can lead to enhancement of the work function. The performed calibration of the coverage shows that changes in work function are occurring during formation of the first layer of copper adatoms.     

O在Au(111)表面吸附的密度泛函理论研究

应用密度泛函理论，本文系统地研究了O在Au(111)表面上的吸附能、吸附结构、功函数、电子密度和投影态密度，给出了覆盖度从0.11ML到1.0ML的范围内，O的吸附特性随覆盖度变化的规律.研究发现O的稳定吸附位为3重面心立方(fcc)洞位，O在fcc洞位的吸附能对覆盖度比较敏感，其值随着覆盖度的增加而减小；O诱导Au(111)表面功函数的变化量与覆盖度成近线性关系，原因是Au表面电子向O偏移，形成表面偶极子；O—Au的相互作用形成成键态和反键态，且反键态都被占据，造成O—Au键很弱，O吸附能较小. 关键词： 表面吸附 Au(111)表面 密度泛函理论 电子特性     

Adsorption and desorption of ammonia,hydrogen, and nitrogen on ruthenium (0001)

The adsorption of ammonia, hydrogen, and nitrogen on a Ru(0001) surface have been investigated by Auger electron spectroscopy, low-energy electron diffraction, and thermal flash desorption. The adsorption of ammonia on Ru(0001) can be divided into a low temperature mode (100 K) and a higher temperature mode (300–500 K). For a crystal temperature of 100 K the ammonia adsorbs into two weakly bound molecular γ states with s = 0.2. The ammonia desorbs as NH₃ molecules with desorption energies of 0.32 and 0.46 eV. At 300–500 K adsorption occurs via an activated process with a low sticking probability (s ? 2 × 10^?4).This adsorption is accompanied by dissociation and formation of an apparent (2 × 2) LEED pattern. Hydrogen adsorbs readily (s = 0.4) on Ru(0001) at 100 K and desorbs with 2nd order kinetics in the temperature range 350–450 K. Nitrogen does not appreciably adsorb on Ru(0001) even at 100 K; maximum nitrogen coverage obtained was estimated to be <2% of a monolayer. Changes in the ammonia flash desorption spectra after hydrogen preadsorption at 100 K will be discussed.     

Ammonia on Ni(1 1 1) surface studied by first principles: Bonding, multilayers structure and comparison with experimental IR and XPS data

DFT calculations have been performed on the adsorption of NH₃ on Ni(1 1 1) to obtain information on the structure of the absorbed species, the nature of the chemical interactions between the adsorbate and the surface and the structure of multilayers formed at high coverage. A cluster model, using localized basis functions as well as an approach based on plane waves and periodic boundary conditions have been considered. The two approaches lead to similar results for the relative stabilities of investigated adsorption sites (atop > fcc > hcp) with an adsorption energy of about 15-24 kcal/mol (depending on the coverage). On the atop site, α-ammonia adsorbs molecularly with an equilibrium distance between the nitrogen atom and the surface of 2.03 ± 0.02 Å and a geometry close to the one of the molecule in the gas phase. The good agreement between the two DFT approaches clearly underlines the local nature of the adsorption reaction. The vibrational frequencies computed for NH₃ adsorbed in this site are in good agreement with experimental values. They show that the interaction with the surface leads to a weakening of the strength of the N-H bond while the angular stretching is stronger. Both orbital and topological analyses were used to investigate chemical interactions between the cluster and the molecule. The results strongly suggest an electrostatic (non-covalent) interaction between the substrate and the molecule. Calculations with NH₃ coverages above 0.25 confirm that saturation occurs at a coverage of 0.25. Above the saturation coverage, ammonia molecules in excess form multiple layers (β and γ ammonia) bonded to the first layer by intermolecular hydrogen bonds. N 1s core level shifts calculations performed for the several investigated coverages are also in good agreement with experimental XPS data. It is shown that the H-bond network more than the bond to the surface allows to understand the N 1s core level variations.