CO2在La2O3(011)表面吸附和活化的DFT计算

运用广义梯度密度泛函理论方法(density functional theory,DFT),结合周期性平板模型,研究了CO2在La2O3(011)表面的吸附和活化,计算了CO2吸附的吸附能和Mulliken电荷数.结果表明,CO2在La2O3(011)表面吸附形成6种平衡构型,分别为CO2-η1、CO2-η2、CO2-...     

甲醛在CeO_2(110)表面的吸附

利用密度泛函理论计算了甲醛在洁净CeO_2(110)表面的吸附行为,发现甲醛在表面存在化学与物理吸附。甲醛化学吸附时,甲醛的碳、氧原子分别与表面氧、铈原子发生作用,生成CH_2O_2,平面结构变为四面体构型;吸附能随覆盖度的减小而增大;电子结构分析表明,其最高占据轨道nO的电子进入表面铈原子的空轨道,形成Ce-Os化学键;而表面氧原子的电子填充到甲醛的C-Of反键轨道,形成新的C-Os键,甲醛的C-Of键长伸长。而物理吸附的甲醛,平面结构不变,吸附能较小,电子结构无明显变化。     

密度泛函理论研究NO在CuO(111)的表面吸附

为了深入研究纳米CuO与含能材料分解产生的NO间的相互作用,采用密度泛函理论研究了NO在CuO(1 1 1)表面的吸附行为,同时在NO吸附的最稳定位点研究NO_2在Cu表面的吸附及其对NO的影响;通过DMol~3模块中广义梯度密度泛函理论(GGA)的Perdew-Burke-Ernzerh (PBE)方法对吸附构型进行了计算。结果表明,NO以分子形式吸附在CuO(1 1 1)表面,较为稳定的吸附构型为NO的N原子与CuO表面的Cu、O原子相互作用,均为化学吸附;最稳定的吸附构型为NO吸附在Cu1位点上,吸附能为-0.89eV;HOMO-LUMO轨道能隙值和态密度图分析均表明NO和CuO表面有强烈相互作用;Mulliken电荷布局分析显示,电荷从NO转移到Cu表面,NO带部分正电荷;在含能材料分解过程中,NO气体稳定吸附在CuO表面,但当存在NO_2时,NO的吸附位点可能会被吸附能力更强的NO_2占据。     

内外担载的Pd-CNT对CO吸附的理论研究

本文以Pd掺杂的碳纳米管为切入点,基于密度泛函理论,模拟了CO在单壁碳纳米管上的吸附作用,结合吸附能、DOS和轨道理论对吸附体系进行分析。结果表明,内担载的Pd-4聚碳纳米管对CO的吸附能力比外担载时强。Pd-4CNT传感器,其电导率会在内担载的Pd-4CNT环境中增加,在外担载的Pd-4CNT环境中降低。当Pd-4CNT与CO分子进行接触时,Pd-4CNT更容易接受电子而不是给予电子,从而形成p-型半导体。     

甲醇在金红石型TiO_2(110)表面的吸附解离

金红石型二氧化钛在光催化中有许多应用。运用周期性密度泛函理论计算方法研究了甲醇在金红石型二氧化钛(110)表面的吸附和解离方式。分别考察了饱和固定和饱和放开模型上甲醇的吸附、解离及解离过程中过渡态能垒的情况。通过对计算能量和能垒、解离前后各键长的分析比较,得出结论:在金红石型二氧化钛(110)表面上,甲醇的分子吸附形式和解离吸附形式能够同时存在,但是解离吸附的存在形式略为稳定。     

N_2O在Rh(100)表面吸附机理的量子化学研究

在对三效催化剂(TWC)表面N2O的催化反应实验及理论研究的基础上,选用了在过渡金属催化体系量子化学理论研究中常用的密度泛函方法,采用CASTEP计算程序,交换相关函数采用PW91函数,计算了N2O在催化剂Rh(100)面上的化学吸附。计算结果显示:通过对N端吸附位和O端吸附位的N-N、N-O、Rh-N、Rh-O等键长和吸附能的比较,说明NO最有可能发生的是N端顶位吸附。     

CO分子和C、O和H原子在Ni_n(n=4～13)团簇表面吸附的密度泛函理论研究

采用密度泛函理论(DFT)对Nin(n=4~13)团簇吸附CO分子和C、O和H原子进行了系统研究。结果表明:Nin(n=4~13)团簇对它们的吸附强度的大小顺序始终是COHCO,择优吸附位点是谷位;吸附后,CO的键长与自由的C-O键长(1.140?)相比有所增加(0.016~0.071?),这表明C-O键被削弱,CO被活化;比结合能(稳定性)由大到小的顺序是NinCONinCNinONinH和Nin,吸附后,团簇的稳定性普遍增加。     

CO分子和C、O和H原子在Ni_n(n=4～13)团簇表面吸附的密度泛函理论研究

采用密度泛函理论(DFT)对Nin(n=4~13)团簇吸附CO分子和C、O和H原子进行了系统研究。结果表明:Nin(n=4~13)团簇对它们的吸附强度的大小顺序始终是C>O>H>CO,择优吸附位点是谷位;吸附后,CO的键长与自由的C-O键长(1.140?)相比有所增加(0.016~0.071?),这表明C-O键被削弱,CO被活化;比结合能(稳定性)由大到小的顺序是NinCO>NinC>NinO>NinH和Nin,吸附后,团簇的稳定性普遍增加。     

NiM双金属和γ-Al2O3的相互作用及对CO2吸附的影响

用密度泛函理论(DFT)研究了单金属Ni2及NiMn,NiFe,NiCo和NiCu四种双金属与γ-Al2O3之间的相互作用及其对CO2吸附的影响.通过计算NiM在MgO上结合能、电子结构以及CO2在NiM/γ-Al2O3上的吸附能发现:NiM和γ-Al2O3之间的作用是电子的,NiM和γ-Al2O3之间电子的转移数以及NiM的d-带中心的变化能表现了NiM和γ-Al2O3之间相互作用的强弱;NiM和γ-Al2O3之间相互作用的强弱直接影响催化剂对CO2的吸附能力,相互作用越强,CO2的吸附越强;除了NiCu/γ-Al2O3,其他三种负载型双金属对CO2的吸附能力均强于负载的单金属Ni催化剂,其中,NiFe/γ-Al2O3对CO2的吸附能力最强.     

Adsorption of atomic and molecular oxygen on Cu(1 0 0)

We have studied the initial stages of the oxidation of the Cu(1 0 0) surface using ab initio calculations. Both atomic and molecular oxygen are addressed. We show that subsurface oxygen is not energetically favourable, but gets stabilized by on-surface O. We discuss the adsorption of molecular oxygen using elbow plots, which can be used in order to qualitatively understand the measured sticking coefficient at different oxygen coverages.     

Determination of Site Specific Adsorption Energies of CO on Copper

The binding energies of (isolated) CO molecules adsorbed at several atomic sites (terrace, step, kink) on a number of differently oriented copper surfaces have been measured by thermal desorption spectroscopy (TDS). In addition to the three low-indexed Cu surfaces several regular stepped and kinked single crystal surfaces have been employed. Using LEED measurements together with available data in the literature allowed identification of the various different CO adlayers and to assign the different TDS binding energies to the different adsorbate sites. For the close-packed surfaces binding energies between 47 kJ/mol (Cu(111)) and 51 kJ/mol (Cu(100)) were observed, which increased to 58 kJ/mol for CO molecules bound to step edges. Unexpectedly, for kink sites the same binding energy (to within 1 kJ/mol) as for step edges was observed. Moreover, a very similar binding energy of 58 kJ/mol was also measured for random defect sites on sputtered and on poly-crystalline substrates.     

甲醇在金红石型Ti02（11O）表面的吸附解离

金红石型二氧化钛在光催化中有许多应用。运用周期性密度泛函理论计算方法研究了甲醇在金红石型二氧化钛（110）表面的吸附和解离方式。分别考察了饱和固定和饱和放开模型上甲醇的吸附、解离及解离过程中过渡态能垒的情况。通过对计算能量和能垒、解离前后各键长的分析比较,得出结论：在金红石型二氧化钛（110）表面上,甲醇的分子吸附形式和解离吸附形式能够同时存在,但是解离吸附的存在形式略为稳定。     

Kinetic Monte Carlo modeling of CO desorption and adsorption on Pd(110) surface

Desorption and adsorption of carbon monoxide on Pd(110) is modeled and simulated, aiming at gaining atomic level understanding of experimentally observed rates. The model parameters are fitted to reproduce the temperature programmed desorption spectra and molecular beam surface scattering data. Desorption turns out to be best described as thermally activated, the activation energy depending on the detailed nearest neighbor site occupation configuration. For a good fit, the adsorption induced surface reconstruction needs to be included in the model. Also, desorption needs to be modeled with a precursor state included. However, surface diffusion was not found to be essential. With these ingredients the coverage dependent sticking coefficient can be successfully simulated in the temperature range from 300 to 500 K. Furthermore, the experimentally observed saturation coverage—temperature dependence is correctly predicted from the balance between simultaneous adsorption and desorption.     

Electro-oxidation kinetics of adsorbed CO on platinum electrocatalysts

We describe the voltammetric measurement of the full oxidation of adsorbed CO on unsupported platinum electrocatalysts, with concomitant cyclic voltammetry of the hydrogen adsorption and desorption. The hydrogen region of platinum is used to parse the platinum surface into sites associated with weakly bound (WB) hydrogen and strongly bound (SB) hydrogen. By monitoring changes in the hydrogen region while following the two observed CO oxidation peaks, we are able to identify the WB sites as being the most active sites for CO_ads electro-oxidation. The full oxidation peak is fitted to a model based on a modified Butler-Volmer equation that includes the two families of sites. Excellent agreement with experimental results is obtained, and the resulting fits yield the kinetic parameters for the two families of sites. When combined with coulometry, these kinetic analyses also show the importance of linear- and bridged-CO_ads species in the electro-oxidation process. Limitations of the model and the role of CO_ads dynamics amongst the various surface sites are discussed.     

CO oxidation on a Cu(100) catalyst

The oxidation of carbon monoxide by molecular oxygen on a single crystal Cu(100) catalyst was studied at 458 K using reactant gas mixtures with CO/O₂ ratios of 2/1, 10/1 and 25/1 at a total pressure of 10 Torr. The catalytic activities were found to be strongly dependent upon the CO/O₂ ratio. Under stoichiometric reaction conditions (CO/O₂ = 2), the initial CO oxidation activity decreased sharply; with a highly reducing reaction mixture (CO/O₂ = 25), the initial activity gradually increased. These changes in catalytic activities with reactant gas mixture composition correlate with changes in surface composition, namely an increase in the surface oxygen coverage. Post-reaction TPD revealed the presence of a carbonate-like species which decomposed at ca. 630 K.     

Adsorption prediction for three binary supercritical gas mixtures on activated carbon based on a NDFT/PSD approach

A non-local density functional theory (NDFT) in combination with the pore size distribution (PSD) analysis has been applied to make a comprehensively theoretical study for correlating and predicting the adsorption equilibria of pure supercritical gases and the corresponding binary supercritical gas mixtures on activated carbon. In this approach, the required PSDs were determined from an input of experimental adsorption data of pure components. By comparing with the experimental data of three different binary systems, CH₄/N₂, CH₄/CO₂, and CO₂/N₂, at high pressure up to 13.6 MPa and temperature 318.2 K with various concentration ranges, the predictive performance of the theoretical approach was evaluated. The adsorption of the mixtures has also been predicted by applying the ideal adsorbed solution (IAS) theory. It was shown that the NDFT/PSD method could be used to predict the mixture adsorption behaviors under high pressure. The developed method has greater superiority over the IAS theory in the prediction of the mixture adsorption.     

Vibrational Stark tuning rates from periodic DFT calculations: CO/Pt(1 1 1)

DFT periodic calculations have been used to study the influence of an external electric field on the adsorption of CO on Pt(1 1 1). Particular attention has been focused on the determination of the CO and metal-CO vibrational Stark tuning rates. Stark tuning rates have been calculated at various CO coverages; a linear dependence between the CO Stark tuning rate and the CO surface coverage has been found. We have calculated a value of 68.94 cm⁻¹/(V/Å) for the zero-coverage limit CO Stark tuning rate, in good agreement with the experimental value of 75 ± 9 cm⁻¹/(V/Å). Like the CO Stark tuning rate, the metal-CO vibrational Stark tuning rate also increases as CO surface coverage decreases. In addition, we have found (at 0.25 ML) that the CO Stark tuning rate is similar at different adsorption sites, being only slightly larger at high-coordinated sites. CO vibrational Stark tuning rates of 45.58, 47.96, 47.61 and 48.49 cm⁻¹/(V/Å) have been calculated for ontop, bridge, hcp and fcc hollow sites, respectively. Calculations at high coverage using a (2 × 2)-3CO model yield a CO Stark tuning rate of 21.08 and 25.93 cm⁻¹/(V/Å) for ontop and three-fold hollow CO, respectively. These results show that the CO Stark tuning rate for CO adsorbed at high coordinated sites is only slightly larger than that at ontop sites. This result is in contradiction with experiments, which reported larger CO Stark tuning rates at high-coordinates sites than at ontop sites. Furthermore, the calculated metal-CO stretch is larger for ontop sites than for high-coordinated sites; this result is in disagreement with previous DFT cluster model calculations. Unfortunately, there is not experimental information available to support either result. Finally, we have also studied the CO adsorption site preference dependence on electric fields. We have found that CO adsorbs preferentially at high coordinated sites at more negative fields, and at ontop sites at more positive fields, in agreement with previous experiments and DFT cluster model calculations.