气相存在下过渡金属表面脱附动力学机理的研究

作者利用同位素跳跃技术来探讨气相压强促进过渡金属表面吸附分子脱附这一新现象的机理。获得了３５３Ｋ下饱和吸附Ｃ１６Ｏ的Ｒｅ（０００１）表面的超高真空等温脱附和不同气相压强的同位素Ｃ１８Ｏ交换的谱图。从相对覆盖度及其对数随时间的变化曲线可以看出,真空等温脱附过程为一级动力学过程。而在气相同位素存在下交换脱附过程可用一级加二级来近似,拟合的结果与实验符合很好。作者还发现了交换速率远大于真空等温脱附速率,而且随压强的增加而增加,这说明气相压强直接促进了表面吸附分子的脱附。并提出了协同吸附－脱附机理来解释这一新现象     

预吸附氧对NO在Pt(110)表面吸附和分解的影响

 利用程序升温反应谱、X射线光电子能谱和高分辨电子能量损失谱研究了NO在清洁和预吸附氧的Pt(110)表面的吸附和分解. 在清洁的Pt(110)表面,室温下低覆盖度时NO以桥式吸附为主,高覆盖度时NO以线式吸附为主. 加热过程中部分NO(主要是桥式吸附物种)分解,生成N2和N2O. 室温下O2在Pt(110)表面发生解离吸附. Pt(110)表面预吸附氧会抑制桥式吸附NO的生成,并导致其脱附温度降低40 K. 降低脱附温度有利于桥式吸附NO的分子脱附,从而抑制分解反应. 这些结果从表面化学的角度合理地解释了铂催化剂在富氧条件下对NO分解能力的降低.     

气相存在下CO在Pd上非线性脱附动力学的研究

利用同位素技术测定了预吸附在Pd表面上的CO在单一及混合同位素气氛下的脱附动力学．实验发现CO的脱附动力学是非线性的，在饱和表面上为一级加二级．在易位化学吸附模型和协同交换机理的基础上，对表面吸附物种的动态结构进行了深入分析讨论，并推导出了非线性脱附动力学的解析表达式．     

CO气体分子在金属氧化物SrO(001)表面吸附和脱附机理

研究了高纯度金属氧化物SrO粉末表面吸附C¹⁸O后在程序升温热脱附过程中的脱附反应规律,讨论了CO气体分子在SrO(100)表面吸附和脱附之间的关系及机理.SrO吸附C¹⁸O后,在升温过程中可通过氧的同位素交换生成C¹⁶O而脱附,同时伴随少量C¹⁸O的直接脱附,并与C¹⁶O具有相近的脱附活化能.C¹⁶O的脱附量随C¹⁸O气体暴露量增加而增加,但当气体覆盖度超过一定值后,脱附量趋于定值.脱附峰值温度随气体暴露量的增加而下降.     

乙酸丁酯废气在活性炭上的吸附和脱附行为研究

对4种商品化活性炭进行了SEM、N_2低温物理吸附等分析表征,结合吸附穿透曲线、饱和吸附量等筛选出适用于乙酸丁酯废气吸附的活性炭(编号为AC4)。分别研究了空塔气速、废气湿度等对吸附过程的影响,同时测定了25℃、40℃和60℃下不同进气浓度的饱和吸附量,并采用Langmuir和Freundlish等温吸附方程对实验数据进行拟合,结果表明,langmuir等温吸附方程能更好地描述乙酸丁酯废气在活性炭上的吸附。采用热空气脱附法对活性炭进行再生并回收乙酸丁酯,设计出在空气流速为0.2m/s时,先在140℃下脱附1.5h,然后在170℃下脱附1.5h的脱附再生工艺。在此条件下,乙酸丁酯脱附率达97.99%,回收率达89.51%。     

分子氧在锐钛矿型TiO2表面的光助吸附和光助脱附

本文采用真空-质谱技术研究了紫外线照射下分子氧在锐钛矿型TiO₂表面的吸附和脱附机理。氧光助吸附后,锐钛矿表面成为活化表面。活化表面的O₂^-(分子氧在锐钛矿表面的吸附态)在1.33×10^-3Pa的真空中,在能量大于锐钛矿禁带宽度2.9eV的紫外线照射下成为分子氧脱附,氧脱附后的表面在无紫外线照射的氧气氛中对分子氧有吸附作用,该O₂^-饱和吸附量大于相同氧压下紫外线照射下O₂的饱和吸附量。在氧压和光强度相同的条件下,O₂^-吸附量与表面羟基化程度呈线性关系。     

CO在轻微氧化的Mo(100)表面上的吸附和反应

应用HREELS和TDS研究了120K时CO在轻微氧化的Mo(100)上的吸附和脱附状况.120K时,CO在轻微氧化的Mo(100)上存在顶位垂直吸附(ν_CO=2016～2050cm^-1)、四重空位倾斜吸附(ν_CO=1088cm^-1)和通过π键与表面发生作用的倾斜吸附(ν_CO=1600cm^-1).当表面温度升高时,顶位吸附的CO在低覆盖度下发生解离,但在较高覆盖度下,可以同时发生脱附(T_p=319K)和解离;而后两种吸附态在温度升高时只发生解离.CO解离产生的C原子和O原子在930K和1320K时可重新结合成CO脱附.     

研究NO在SrTiO3表面的吸附特性

采用TPD实验方法测定了N¹⁸O在SrTiO₃表面吸附后的脱附谱, 揭示了气体脱附量的变化规律. 结果表明N¹⁸O在SrTiO₃表面吸附后脱附出大量的N¹⁶O气体, 还脱附出少量的N₂, N¹⁸O和N₂¹⁶O气体. 这说明了在吸附和脱附过程中N¹⁸O与SrTiO₃表面发生氧的同位素交换. N¹⁶O的脱附量随N¹⁸O气体暴露量增加而增加, 但当气体覆盖度超过一定值后, 脱附量趋于定值.     

丙烯在铁锑氧化物上的吸附和氨氧化

用吸附和程序升温脱附的方法,研究了不同组成铁锑氧化物催化剂上丙烯的吸附和脱附以及催化剂的再氧化。催化剂中Sb原子含量由零增加至90％时,单位表面丙烯的吸附量总的变化趋势是增加的,脱附后催化剂的再氧化吸氧量也呈现同样的规律。不同组成铁锑氧化物上丙烯氨氧化合成丙烯腈的催化活性和选择性与丙烯的吸附量无直接对应关系。丙烯程序升温脱附和对脱附产物的分析表明,不同组成的催化剂的表面氧化能力和脱附性质有明显的区别。由Mossbauer谱的分析结果可以认为锑酸铁是催化剂表面的主要提供者。根据丙烯吸附和催化反应的结果,推测选择性氧化的吸附中心可能与锑离子相关。     

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。     

Reactions of ammonia on stoichiometric and reduced TiO(2)(001) single crystal surfaces

The reaction of NH(3) on the surface of the 011-faceted structure of the TiO(2)(001) single crystal is studied and compared to that on the O-defected surface. Temperature-programmed desorption (TPD) conducted after NH(3) adsorption at 300 K shows only molecular desorption at 340 K. Modeling of TPD signals as a function of surface coverage indicated that the activation energy, E(d), and pre-exponential factor, v(eff), decrease with increasing coverage. Near zero surface coverage, E(d) was found to be equal to 92 kJ/mol and v(eff) to be close to 10(13) /s. Both parameters decreased to approximately 52 kJ/mol and approximately 10(7) /s at saturation coverage. The decrease is due to a repulsive interaction of adsorbed NH(3) molecules on the surface. Computing of the TPD results show that saturation is obtained at 1/2 monolayer coverage (referred to Ti atoms). Both the amount and shape of NH(3) peak change on the reduced (Ar(+)-sputtered) surfaces. The desorption peak at 340 K is considerably attenuated on mildly reduced surfaces (TiO( approximately )(1.9)) and has totally disappeared on the heavily reduced surfaces (TiO(1.6)(-)(1.7)), where the main desorption peak is found at 440 K. This 440-K desorption is most likely due to NH(x) + H recombination resulting from ammonia dissociation upon adsorption on Ti atoms in low oxidation states.     

Characterizing the adsorption of proteins on glass capillary surfaces using electrospray-differential mobility analysis

We quantify the adsorption and desorption of a monoclonal immunoglobulin-G antibody, rituxamab (RmAb), on silica capillary surfaces using electrospray-differential mobility analysis (ES-DMA). We first develop a theory to calculate coverages and desorption rate constants from the ES-DMA data for proteins adsorbing on glass capillaries used to electrospray protein solutions. This model is then used to study the adsorption of RmAb on a bare silica capillary surface. A concentration-independent coverage of ≈4.0 mg/m(2) is found for RmAb concentrations ranging from 0.01 to 0.1 mg/mL. A study of RmAb adsorption to bare silica as a function of pH shows maximum adsorption at its isoelectric point (pI of pH 8.5) consistent with literature. The desorption rate constants are determined to be ≈10(-5) s(-1), consistent with previously reported values, thus suggesting that shear forces in the capillary may not have a considerable effect on desorption. We anticipate that this study will allow ES-DMA to be used as a "label-free" tool to study adsorption of oligomeric and multicomponent protein systems onto fused silica as well as other surface modifications.     

Dewetting growth of crystalline water ice on a hydrogen saturated Rh(111) surface at 135 K

We investigated the water (D(2)O) adsorption at 135?K on a hydrogen pre-adsorbed Rh(111) surface using temperature programmed desorption and infrared reflection absorption spectroscopy (IRAS) in ultrahigh vacuum. With increasing the hydrogen coverage, the desorption temperature of water decreases. At the saturation coverage of hydrogen, dewetting growth of water ice was observed: large three-dimensional ice grains are formed. The activation energy of water desorption from the hydrogen-saturated Rh(111) surface is estimated to be 51 kJ/mol. The initial sticking probability of water decreases from 0.46 on the clean surface to 0.35 on the hydrogen-saturated surface. In IRAS measurements, D-down species were not observed on the hydrogen saturated surface. The present experimental results clearly show that a hydrophilic Rh(111) clean surface changes into a hydrophobic surface as a result of hydrogen adsorption.     

Adsorption and desorption of macromolecules on solid surfaces studied by on-line size exclusion chromatography 2. Preferential adsorption and exchange processes

Preferential and exchange adsorption of polymers differing in molar mass and/or chemical nature under dynamic conditions were investigated using on-line size-exclusion chromatography (SEC). The sample investigated dissolved in an appropriate solvent was injected into a small adsorption–desorption column packed with nonporous silica. A nonadsorbed or desorbed fraction of the polymer was directed into an SEC column for determination of both the amount and the molecular characteristics. This approach is in many aspects superior to other techniques for studies of polymer adsorption onto solid surfaces due to its low sample and time consumption. At a low degree of surface coverage, adsorption and desorption of macromolecules were rapid and were affected by the rate of supply of macromolecules to the adsorbent surface. The exchange between macromolecules at the stage of surface saturation was found to depend on the mean molar masses of preadsorbed and displacing polymer species and possibly also on the chain flexibility of the macromolecules. It was shown that the preferential adsorption driven by the chain-length difference upon saturation of the adsorbent surface was more noticeable if the preadsorbed macromolecules were smaller. Received: 7 April 1999 Accepted in revised form: 21 July 1999     

法拉第吸脱附偶联过程循环伏安行为的有限元分析

法拉第吸脱附偶联过程的电化学行为较为复杂,难以定量获得其表界面反应动力学信息. 本文通过COMSOL有限元软件对法拉第吸脱附偶联过程的循环伏安行为进行数值分析,研究了反应物或产物不同吸附条件下的循环伏安行为. 结果表明：当反应物或产物弱吸附时,可通过阴、阳极峰电流之差实现饱和吸附量的定量表征. 随着吸附平衡常数的增大,反应由弱吸附向强吸附过渡,峰电流由扩散峰与吸脱附峰相互重叠过渡到相互分离的吸脱附“前波”或“后波”特征. 该吸脱附特征峰的形状和位置与电势依赖的吸附平衡常数有关. 吸附平衡常数及其电势依赖程度越大,吸脱附峰偏离扩散峰越远,吸脱附峰越尖锐. 该模型为法拉第吸脱附偶联过程的循环伏安研究提供了一种定量研究方法,能够帮助研究者从复杂的吸脱附伏安行为中定量获得饱和吸附量和吸附平衡常数等信息,并对涉及吸脱附的电催化研究具有一定指导意义.     

A site-selective in situ study of CO adsorption and desorption on Pt(355)

Using time-dependent high-resolution x-ray photoelectron spectroscopy at BESSY II, the adsorption and desorption processes of CO on stepped Pt(355) = Pt[5(111) x (111)] were investigated. From a quantitative analysis of C 1s data, the distribution of CO on the various adsorption sites can be determined continuously during adsorption and desorption. These unique data show that the terrace sites are only occupied when the step sites are almost saturated, even at temperatures as low as 130 K. The coverage-dependent occupation of on-top and bridge adsorption sites on the (111) terraces of Pt(355) is found to differ from that on Pt(111), which is attributed to the finite width of the terraces and changes in adsorbate-adsorbate interactions. In particular, no long-range order of the adsorbate layer could be observed by low-energy electron diffraction. Further details are derived from sticking coefficient measurements using the method devised by King and Wells [Proc. R. Soc. London, Ser. A 339, 245 (1974)] and temperature-programmed desorption. The CO saturation coverage is found to be slightly smaller on the stepped surface as compared to that on Pt(111). The initial sticking coefficient has the same high value of 0.91 for both surfaces.     

Desorption related to adsorption of hydrogen via detailed balance on the Si(1 0 0) surfaces

Recent progress on desorption and adsorption dynamics of hydrogen (deuterium) on monohydride and dihydride Si(1 0 0) surfaces is reviewed and discussed. The dynamics experiments reveal that the desorption dynamics of hydrogen is well related to the adsorption dynamics via detailed balance. Dependence of time-of-flight (TOF) distributions of desorbed molecules on H(D) coverage is noticed to be important in understanding the kinetics mechanism of the adsorption/desorption reactions of hydrogen on the Si(1 0 0) surface. The desorption dynamics varies from the situation of strongly translational heating to the other situation of less translational heating with D coverage. This trend seems to be consistent with the 2H/3H/4H interdimer mechanism. However, despites by far the richest 4H configuration at high H coverage, the 2H desorption prevails over the 4H desorption already at 0.8 ML. To reconcile this unexpected desorption kinetics, a diffusion-promoted desorption mechanism is proposed. Height of the adsorption barriers for the 2H and 3H pathways could be reduced by the H-atom diffusion along the Si dimer rows, but that for the 4H pathway could not be the case because of no capability of diffusion on the H saturated surface. The desorption dynamics of hydrogen from the (3 × 1) dihydride surface is also reviewed and compared with the case on the monohydride surface. The sticking coefficients of hydrogen molecules onto the monohydride surfaces are evaluated from the TOF curves and found to be strongly activated by the kinetic energy. Not only the degrees of freedom of the molecules but also the vibrational degrees of freedom of substrate Si atoms determine the barrier height for adsorption. The desorption dynamics of hydrogen from the monohydride and dihydride surfaces appears to be quite similar, but the dynamics of substrate Si atoms is expected to be quite dissimilar between the two desorption pathways.     

Reversible adsorption of spherical particles from binary mixtures: long-time behaviour

Formation of monolayers of spherical particles in processes with reversible adsorption from mixtures of large and small particles was simulated in computer experiments. Computer program was based on an algorithm that took into account random sequential adsorption, desorption and lateral diffusion of adsorbed particles (RSA–DLD model). Computer experiments were performed for systems with rate constants of particle adsorption at least 10³ times higher than rate constants of desorption. In processes with very fast adsorption and slow desorption, formation of monolayer can be divided into two stages. During the first stage, the total surface coverage (the coverage with particles of both types) increases very fast and becomes very close to that at equilibrium. During the second stage, the total coverage changes very slowly and the system approaches equilibrium mainly by the replacement of large particles with the small ones. A simple kinetic model for evolution of the monolayer composition during the second stage has been proposed. Kinetic equations related to this model allow the determination of large particles’ desorption rate constants on the basis of changes in the surface concentrations of adsorbed large and small microspheres. The validity of the model has been tested comparing large particles’ desorption rate constants values that had been used for simulations with values of the corresponding rate constants determined using analytical equations, with a view to analysing the simulation results. To cite this article: S. Slomkowski et al., C. R. Chimie 6 (2003).     

Acetone and water on TiO2(110): competition for sites

The competitive interaction between acetone and water for surface sites on TiO2(110) was examined using temperature programmed desorption (TPD). Two surface pretreatment methods were employed, one involving vacuum reduction of the surface by annealing at 850 K in ultrahigh vacuum (UHV) and another involving surface oxidation with molecular oxygen. In the former case, the surface possessed about 7% oxygen vacancy sites, and in the latter, reactive oxygen species (adatoms and molecules) were deposited on the surface as a result of oxidative filling of vacancy sites. On the 7% oxygen vacancy surface, excess water displaced all but about 20% of a saturated d6-acetone first layer to physisorbed desorption states, whereas about 40% of the first layer d6-acetone was stabilized on the oxidized surface against displacement by water through a reaction between oxygen and d6-acetone. The displacement of acetone on both surfaces is explained in terms of the relative desorption energies of each molecule on the clean surface and the role of intermolecular repulsions in shifting the respective desorption features to lower temperatures with increasing coverage. Although first layer water desorbs from TiO2(110) at slightly lower temperature (275 K) than submonolayer coverages of d6-acetone (340 K), intermolecular repulsions between d6-acetone molecules shift its leading edge for desorption to 170 K as the first layer is saturated. In contrast, the desorption leading edge for first layer water (with or without coadsorbed d6-acetone) shifted to no lower than 210 K as a function of increasing coverage. This small difference in the onsets for d6-acetone and water desorption resulted in the majority of d6-acetone being compressed into islands by water and displaced from the first layer at a lower temperature than that observed in the absence of coadsorbed water. On the oxidized surface, the species resulting from reaction of d6-acetone and oxygen was not influence by increasing water coverages. This species was stable up to 375 K (well past the first layer water TPD feature) where it decomposed mostly back to d6-acetone and atomic oxygen. These results are discussed in terms of the influence of water in inhibiting acetone photo-oxidation on TiO2 surfaces.     

Solid phase adsorption of crystal violet lactone on silica nanoparticles to probe mechanochemical surface modification

The solid phase adsorption of crystal violet lactone (CVL) on five types of Stober silica nanopowders with BET specific surface areas in the range of 50-800 m2/g under dry milling conditions was described for the first time. The hydrogen bonding between surface silanol and the carboxylate of the ring-opened triphenylmethane dye (CVL+) led to the formation of monolayers of CVL+ in a flat-laid configuration. The lambda max of CVL+ in diffusive reflection visible spectra was influenced by the particle size of silica powders, suggesting that the microenvironmental polarity of adsorbed CVL+ is considerably reduced along with the decrease of the particle size. The solid phase adsorption of CVL obeyed Langmuir adsorption isotherms to give a saturated amount of CVL+ for every silica nanoparticle. The surface concentration of CVL+ on nanoparticles at the saturation was estimated to be 0.31 mg/m2 on average, disclosing that about 52% of the surface can be covered by CVL+ under the assumption that the BET-specific surface areas are equivalent to the real surfaces active for the CVL adsorption. The generation of the blue color of CVL provided a convenient means to estimate qualitative and quantitative analysis of the surface coverage with surface-active reagents, which conceal surface silanols. Subsequently, silica nanoparticles were milled with a surface modifier, followed by milling with CVL to observe the intensity of the blue color in order to disclose that the surface coverage with oligo- and polyethylene glycols as well as with nonionic surfactants by dry milling was specifically determined by the number of repeating oxyethylene units. Although the surface-active reagents were easily desorbed in water, the desorption was notably suppressed by milling with CVL, suggesting that the surface-modified particles with the surface-active reagents are covered with ultrathin films of CVL.