Time-resolved sum-frequency generation spectroscopy of methoxy and deuterated methoxy on Ni(111) using near-infrared laser pulses

Methoxy (CH3O-) and deuterated (d-) methoxy (CD3O-) species on Ni(111) are investigated by sum-frequency generation (SFG) spectroscopy. Methoxy adsorbed on the Ni(111) surface is confirmed by SFG spectroscopy to be oriented normal to the surface. Two resonant peaks produced by methoxy, at 2921 and 2821 cm(-1), are assigned to Fermi resonance between the CH symmetric stretching and overtone modes. Deuterated methoxy exhibits a single strong peak at 2051 cm(-1) assigned to the CD symmetric stretching mode. Investigation of the sub-nanosecond transient behavior of methoxy and d-methoxy species on Ni(111) under short-pulse laser pumping at 1064 nm reveals a clear weakening and recovery of the SFG peaks upon heating. The observed temporal profile is reproduced by simulation assuming that the original methoxy in the ground state is in chemical equilibrium with a new state produced by instantaneous heating. The dependence of the SFG spectra on the initial substrate temperature is also reproduced by the simulation. The simulation suggests a temperature jump of 250 K upon laser pumping, inducing a change in the molecular orientation or adsorption site of methoxy on the Ni(111) surface without decomposition of methoxy to adsorbed CO and hydrogen, which occurs under normal heating at 200 K.     

CH3O decomposition on PdZn(111), Pd(111), and Cu(111). A theoretical study

Methanol steam re-forming, catalyzed by Pd/ZnO, is a potential hydrogen source for fuel cells, in particular in pollution-free vehicles. To contribute to the understanding of pertinent reaction mechanisms, density functional slab model studies on two competing decomposition pathways of adsorbed methoxide (CH(3)O) have been carried out, namely, dehydrogenation to formaldehyde and C-O bond breaking to methyl. For the (111) surfaces of Pd, Cu, and 1:1 Pd-Zn alloy, adsorption complexes of various reactants, intermediates, transition states, and products relevant for the decomposition processes were computationally characterized. On the surface of Pd-Zn alloy, H and all studied C-bound species were found to prefer sites with a majority of Pd atoms, whereas O-bound congeners tend to be located on sites with a majority of Zn atoms. Compared to Pd(111), the adsorption energy of O-bound species was calculated to be larger on PdZn(111), whereas C-bound moieties were less strongly adsorbed. C-H scission of CH(3)O on various substrates under study was demonstrated to proceed easier than C-O bond breaking. The energy barrier for the dehydrogenation of CH(3)O on PdZn(111) (113 kJ mol(-)(1)) and Cu(111) (112 kJ mol(-)(1)) is about 4 times as high as that on Pd(111), due to the fact that CH(3)O interacts more weakly with Pd than with PdZn and Cu surfaces. Calculated results showed that the decomposition of methoxide to formaldehyde is thermodynamically favored on Pd(111), but it is an endothermic process on PdZn(111) and Cu(111) surfaces.     

High-pressure ammonia adsorption and dissociation on clean fe(111) and oxygen-precovered fe(111) studied by sum frequency generation vibrational spectroscopy

The adsorption of gases N2, H2, O2, and NH3 that play a role in ammonia synthesis have been studied on the Fe(111) crystal surface by Sum Frequency Generation (SFG) vibrational spectroscopy using an integrated ultrahigh vacuum/high-pressure system. SFG spectra are presented for the dissociation intermediates, NH2 ( approximately 3325 cm-1) and NH ( approximately 3235 cm-1) under high pressure of ammonia (200 Torr) on the clean Fe(111) surface. Addition of 0.5 Torr of oxygen to 200 Torr of ammonia does not significantly change the bonding of dissociation intermediates to the surface. However, it leads to a phase change of nearly 180 degrees between the resonant and nonresonant second-order nonlinear susceptibility of the surface, demonstrated as a reversal of the SFG spectral features. Heating the surface in the presence of 200 Torr of ammonia and 0.5 Torr of oxygen reduces the oxygen coverage, which can be seen from the SFG spectra as another relative phase change of 180 degrees . The reduction of the oxide is also supported by Auger electron spectroscopy. The result suggests that the phase change of the spectral features could serve as a sensitive indicator of the chemical environment of the adsorbates. Clean Fe(111) is found to have a large SFG nonresonant signal. The magnitude of the nonresonant signal was dependent on the adsorption species; O2 and N2 decrease, while H2 and NH3 increase the SFG nonresonant signal. The change in nonresonant signal is correlated to the change in work function for Fe(111) upon adsorption. Adsorption-induced changes in the SFG nonresonant signal was used as an indicator of surface conditions and to monitor surface reactions.     

Alkyl chain conformation and the electronic structure of octyl heavy chalcogenolate monolayers adsorbed on Au(111)

Adsorption states of dioctyl dichalcogenides (dioctyl disulfide, dioctyl diselenide, and dioctyl ditelluride) arranged on Au(111) have been studied by X-ray photoelectron spectroscopy (XPS), infrared-visible sum-frequency generation (SFG), and ultraviolet photoelectron spectroscopy (UPS). XPS measurements suggest that dioctyl dichalcogenides dissociatively adsorbed on Au(111) surfaces to form the corresponding monolayers having chalcogen-gold covalent bonds. The elemental compositions of octanechalcogenolates on Au(111) indicate that the saturation coverages of the octyl heavy chalcogenolate (OcSe, OcTe) monolayers are lower than that of the octanethiolate (OcS) self-assembled monolayers (SAMs). The SFG observations of the CH(2) vibrational bands for the heavy chalcogenolate monolayers strongly suggest that a discernible amount of gauche conformation exists in the monolayers, while OcS SAMs adopt highly ordered all-trans conformation. The intensity ratio of the symmetric and asymmetric CH(3) stretching vibration modes measured by SFG shows that the average tilt angle of the methyl group of the OcSe monolayers is greater than that of the OcS SAMs. The larger tilt angle of the methyl group and the existence of a discernible amount of gauche conformation in the OcSe monolayers are due to the lower surface coverage of the OcSe monolayers compared with the OcS SAMs. The smaller polarization dependence in the angle-resolved UPS (ARUPS) spectra of the OcSe monolayers than that of the OcS SAMs is caused by the more disordered structures of the alkyl chain in the former. XPS, SFG, and ARUPS measurements indicate a similar tendency for the OcTe monolayers. The density of states (DOS) observed by UPS at around 1.3 eV for OcS adsorbed on Au(111) is considered to be the antibonding state of the Au-sulfur bond. Similar DOS is also observed by UPS at around 1.0 eV for the OcSe monolayers and at approximately 1.6 eV for the OcTe monolayers on Au(111).     

Methane dissociative adsorption on the Pt(111) surface over the 300-500 K temperature and 1-10 Torr pressure ranges

The dissociative adsorption of methane on the Pt(111) surface has been investigated and characterized over the 1-10 Torr pressure and 300-500 K temperature ranges using sum frequency generation (SFG) vibrational spectroscopy and Auger electron spectroscopy (AES). At a reaction temperature of 300 K and a pressure of 1 Torr, C-H bond dissociation occurs in methane on the Pt(111) surface to produce adsorbed methyl (CH(3)) groups, carbon, and hydrogen. SFG results suggest that C-C coupling occurs at higher reaction temperatures and pressures. At 400 K, methyl groups react with adsorbed C to form ethylidyne (C(2)H(3)), which dehydrogenates at 500 K to form ethynyl (C(2)H) and methylidyne (CH) species, as shown by SFG. By 600 K, all of the ethylidyne has reacted to form the dissociation products ethynyl and methylidyne. Calculated C-H bond dissociation probabilities for methane, determined by carbon deposition measured by AES, are in the 10(-8) range and increase with increasing reaction temperature. A mechanism has been developed and is compared with conclusions from other experimental and theoretical studies using single crystals.     

Kinetics and dynamics in physisorption of CH3Cl on HOPG: surface temperature and molecular orientation dependence

We report a study of kinetics and dynamics in physisorption of CH(3)Cl on a highly-oriented pyrolytic graphite (HOPG). Thermal energy atom scattering (TEAS) was used to probe the kinetics of thermal CH(3)Cl adsorption on HOPG during the coverage evolution. The desorption energy of CH(3)Cl on HOPG changes from 0.25 to 0.30 eV with increasing surface coverage, suggesting the attractive interaction between CH(3)Cl molecules on the surface. On the other hand, the oriented molecular beam scattering was used to monitor the dynamical interaction of CH(3)Cl with HOPG at zero coverage, demonstrating that the CH(3)Cl scattering intensity depends on the molecular orientation of the incident CH(3)Cl. The observed steric preference is not sensitive to the surface temperature. These results suggest that the moderate anisotropy in the interaction potential induces the molecular-orientation dependence of energy dissipation during the transient trapping into the physisorption well.     

In situ ATR-IR study of prochiral 2-methyl-2-pentenoic acid adsorption on Al2O3 and Pd/Al2O3

Adsorption and desorption of trans-2-methyl-2-pentenoic acid (MPeA) in dichloromethane (CH(2)Cl(2)) were investigated by using in situ attenuated total reflection infrared (ATR-IR) spectroscopy. A liquid flow-through spectroscopic cell allowed for high quality spectra to be obtained from deposited thin films of Al(2)O(3) and 1 wt% Pd/γ-Al(2)O(3) on a ZnSe internal reflection element. The MPeA molecules adsorb on both Al(2)O(3) and Pd surfaces molecularly and dissociatively under the concentration range examined (2-16 mM). In the case of molecular adsorption, both monomer (ν(C=O) ~ 1720 cm(-1)) and dimer (ν(C=O) ~ 1685 cm(-1)) species are observed to adsorb, with the relative amount of monomer to dimer dependent on the surface and the liquid phase acid concentration. In the case of dissociative adsorption, the acid adsorbs predominantly in a bridged bidentate configuration, as adjudged by the ca. 150-220 cm(-1) separation between asymmetric and symmetric vibrational bands. All of these species are found to be strongly adsorbed on both Al(2)O(3) and 1 wt% Pd/γ-Al(2)O(3) surfaces, even under pure solvent flow after adsorption.     

Density functional theory study of the adsorption of alkanethiols on Cu(111), Ag(111), and Au(111) in the low and high coverage regimes

The structure, the surface bonding, and the energetics of alkanethiols adsorbed on Cu(111), Ag(111), and Au(111) surfaces were studied under low and high coverages. The potential energy surfaces (PES) for the thiol/metal interaction were investigated in the absence and presence of externally applied electric fields in order to simulate the effect of the electrode potential on the surface bonding. The electric field affects the corrugation of the PES which decreases for negative fields and increases for positive fields. In the structural investigation, we considered the relaxation of the adsorbate and the surface. The highest relaxation in a direction perpendicular to the surface was observed for gold atoms, whereas silver atoms presented the highest relaxation in a plane parallel to the surface. The surface relaxation is more important in the low coverage limit. The surface bonding was investigated by means of the total and projected density of states analysis. The highest ionic character was observed on the copper surface whereas the highest covalent character occurs on gold. This leads to a strong dependence of the PES with the tilt angle of the adsorbate on Au(111) whereas this dependence is less pronounced on the other metals. Thus, the adsorbate-relaxation and the metal-relaxation contributions to the binding energy are more important on gold. The adsorption of thiols on gold was investigated on the 111 surface as well as on a surface with gold adatoms in order to elucidate the effect of thiols on the surface diffusion of gold. The CH(3)CH(2)S radical adsorbs ontop of the gold adatom. The diffusional barrier of the CH(3)CH(2)SAu species is lower than that for a bare gold adatom and is also lower than that for the bare thiol radical. The adsorption of the molecular species CH(3)SH and CH(3)CH(2)SH was also investigated on Au(111). They adsorb via the sulfur atom ontop of a gold atom. On the other hand, the adsorption of the alkanethiol radicals on the perfect 111 surfaces occurs on the face centered cubic (fcc)-bridge site in the low coverage limit for all metals and shifts toward the fcc site at high coverage on copper and silver.     

Zn modification of the reactivity of Pd(111) toward methanol and formaldehyde

The adsorption and reaction of methanol and formaldehyde on two-dimensional PdZn alloys on a Pd(111) surface were studied as a function of the Zn content in the alloy in order to understand the role of Zn in Pd/ZnO catalysts for the steam reforming of methanol (SRM). Temperature programmed desorption (TPD) and high resolution electron energy loss spectroscopy (HREELS) data show that Zn atoms incorporated into the Pd(111) surface dramatically decrease the dehydrogenation activity and alter the preferred bonding sites for adsorbed CO, CH3O, and CH2O intermediates. The experimental results obtained in this study are consistent with previous theoretical studies of this system and provide new insight into how Zn alters the reactivity of Pd.     

Molecular packing of lysozyme,fibrinogen, and bovine serum albumin on hydrophilic and hydrophobic surfaces studied by infrared-visible sum frequency generation and fluorescence microscopy

Infrared-visible sum frequency generation (SFG) vibrational spectroscopy, in combination with fluorescence microscopy, was employed to investigate the surface structure of lysozyme, fibrinogen, and bovine serum albumin (BSA) adsorbed on hydrophilic silica and hydrophobic polystyrene as a function of protein concentration. Fluorescence microscopy shows that the relative amounts of protein adsorbed on hydrophilic and hydrophobic surfaces increase in proportion with the concentration of protein solutions. For a given bulk protein concentration, a larger amount of protein is adsorbed on hydrophobic polystyrene surfaces compared to hydrophilic silica surfaces. While lysozyme molecules adsorbed on silica surfaces yield relatively similar SFG spectra, regardless of the surface concentration, SFG spectra of fibrinogen and BSA adsorbed on silica surfaces exhibit concentration-dependent signal intensities and peak shapes. Quantitative SFG data analysis reveals that methyl groups in lysozyme adsorbed on hydrophilic surfaces show a concentration-independent orientation. However, methyl groups in BSA and fibrinogen become less tilted with respect to the surface normal with increasing protein concentration at the surface. On hydrophobic polystyrene surfaces, all proteins yield similar SFG spectra, which are different from those on hydrophilic surfaces. Although more protein molecules are present on hydrophobic surfaces, lower SFG signal intensity is observed, indicating that methyl groups in adsorbed proteins are more randomly oriented as compared to those on hydrophilic surfaces. SFG data also shows that the orientation and ordering of phenyl rings in the polystyrene surface is affected by protein adsorption, depending on the amount and type of proteins.     

Photochemistry of vinyl chloride physisorbed on Ag(111) through molecular anion formation induced by substrate electron attachment

Pulsed 266 and 355 nm ultraviolet laser irradiation of monolayer vinyl chloride physisorbed on Ag(111) results in molecular dissociation leading to C2H3 and Cl, much of which is adsorbed to the surface. On the basis of observations made on dissociation dependences on chlorine isotope and photon energy, it is deduced that upon excitation vinyl chloride forms a transient negative ion through a substrate mediated, vertical electron attachment mechanism. The anion either dissociates or relaxes through energy transfer to the neutral state causing the neutral molecule to desorb. The threshold for vertical attachment of substrate electron is estimated to be 0.8 eV below the vacuum level, in agreement with the experimentally observed wavelength dependence in photoinduced dissociation. Chemisorbed Cl on the Ag(111) surface inhibits the photodissociation process by increasing the substrate work function and consequently the energy threshold for electron vertical attachment. Upon heating the Ag(111) surface, adsorbed vinyl combines to produce 1,3-butadiene in a first order, diffusion limited, process with an activation energy of 10.4 kcal/mol.     

In situ adsorption studies of a 14-amino acid leucine-lysine peptide onto hydrophobic polystyrene and hydrophilic silica surfaces using quartz crystal microbalance, atomic force microscopy, and sum frequency generation vibrational spectroscopy

The adsorption of a 14-amino acid amphiphilic peptide, LK14, which is composed of leucine (L, nonpolar) and lysine (K, charged), on hydrophobic polystyrene (PS) and hydrophilic silica (SiO2) was investigated in situ by quartz crystal microbalance (QCM), atomic force microscopy (AFM), and sum frequency generation (SFG) vibrational spectroscopy. The LK14 peptide, adsorbed from a pH 7.4 phosphate-buffered saline (PBS) solution, displayed very different coverage, surface roughness and friction, topography, and surface-induced orientation when adsorbed onto PS versus SiO2 surfaces. Real-time QCM adsorption data revealed that the peptide adsorbed onto hydrophobic PS through a fast (t < 2 min) process, while a much slower (t > 30 min) multistep adsorption and rearrangement occurred on the hydrophilic SiO2. AFM measurements showed different surface morphologies and friction coefficients for LK14 adsorbed on the two surfaces. Surface-specific SFG spectra indicate very different ordering of the adsorbed peptide on hydrophobic PS as compared to hydrophilic SiO2. At the LK14 solution/PS interface, CH resonances corresponding to the hydrophobic leucine side chains are evident. Conversely, only NH modes are observed at the peptide solution/SiO2 interface, indicating a different average molecular orientation on this hydrophilic surface. The surface-dependent difference in the molecular-scale peptide interaction at the solution/hydrophobic solid versus solution/hydrophilic solid interfaces (measured by SFG) is manifested as significantly different macromolecular-level adsorption properties on the two surfaces (determined via AFM and QCM experiments).     

Analyzing velocity map images to distinguish the primary methyl photofragments from those produced upon C-Cl bond photofission in chloroacetone at 193 nm

We use a combination of crossed laser-molecular beam scattering experiments and velocity map imaging experiments to investigate the three primary photodissociation channels of chloroacetone at 193 nm: C-Cl bond photofission yielding CH(3)C(O)CH(2) radicals, C-C bond photofission yielding CH(3)CO and CH(2)Cl products, and C-CH(3) bond photofission resulting in CH(3) and C(O)CH(2)Cl products. Improved analysis of data previously reported by our group quantitatively identifies the contribution of this latter photodissociation channel. We introduce a forward convolution procedure to identify the portion of the signal, derived from the methyl image, which results from a two-step process in which C-Cl bond photofission is followed by the dissociation of the vibrationally excited CH(3)C(O)CH(2) radicals to CH(3) + COCH(2). Subtracting this from the total methyl signal identifies the methyl photofragments that result from the CH(3) + C(O)CH(2)Cl photofission channel. We find that about 89% of the chloroacetone molecules undergo C-Cl bond photofission to yield CH(3)C(O)CH(2) and Cl products; approximately 8% result in C-C bond photofission to yield CH(3)CO and CH(2)Cl products, and the remaining 2.6% undergo C-CH(3) bond photofission to yield CH(3) and C(O)CH(2)Cl products.     

Sensitivity of methyl thiolate desulfurization selectivity to reaction temperature and hydroxyl coverage

The adsorption and reaction of methanethiol (CH3SH) and dimethyl disulfide (CH3SSCH3) on Mo(110)-(1 x 6)-O have been studied using temperature-programmed reaction spectroscopy and reflection-absorption infrared spectroscopy over the temperature range of 110-550 K. The S-H bond is broken upon adsorption to form adsorbed OH, water, and methyl thiolate (CH3S-) at low temperature. Water is evolved at 210 and 310 K via molecular desorption and disproportionation of OH, respectively. Some hydroxyl remains on the surface up to 350 K. Methyl thiolate is also formed from CH3SSCH3 on Mo(110)-(1 x 6)-O. Methyl thiolate undergoes C-S cleavage above 300 K, yielding methane and methyl radicals. There is also a minor amount of nonselective decomposition leading to the formation of carbon and hydrogen. Methane production is promoted by adsorbed hydroxyl. As the hydroxyl coverage increases, the yield of methyl radicals relative to methane diminishes. Accordingly, there is more methane produced from methanethiol reaction than from dimethyl disulfide, since S-H dissociation in CH3SH produces OH. The maximum coverage of the thiolate is approximately 0.5 monolayers, based on the amount of sulfur remaining after reaction measured by Auger electron spectroscopy. In contrast to cyclopropylmethanethiol (c-C3H5CH2SH), for which alkyl transfer from sulfur to oxygen is observed, there is no evidence for transfer of the methyl group of methyl thiolate to oxygen on the surface. Specifically, there is no evidence for methoxy (CH3O-) in infrared spectroscopy or temperature-programmed reaction experiments.     

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.     

贵金属原子在CeO2(111)表面吸附的密度泛函理论研究

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 4f、O 2p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O 2p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。     

Adsorption of carbon on Pd clusters of nanometer size: a first-principles theoretical study

Adsorbed atomic C species can be formed in the course of surface reactions and commonly decorate metal catalysts. We studied computationally C adsorption on Pd nanoclusters using an all-electron scalar relativistic density functional method. The metal particles under investigation, Pd(55), Pd(79), Pd(85), Pd(116), Pd(140), and Pd(146), were chosen as fragments of bulk Pd in the form of three-dimensional octahedral or cuboctahedral crystallites, exposing (111) and (100) facets as well as edge sites. These cluster models are shown to yield size-converged adsorption energies. We examined which surface sites of these clusters are preferentially occupied by adsorbed C. According to calculations, surface C atoms form strongly adsorbed carbide species (with adsorption energies of more than 600 kJ mol(-1)) bearing a significant negative charge. Surface sites allowing high, fourfold coordination of carbon are overall favored. To avoid effects of adsorbate-adsorbate interaction in the cluster models for carbon species in the vicinity of cluster edges, we reduced the local symmetry of selected adsorption complexes on the nanoclusters by lowering the global symmetry of the nanocluster models from point group O(h) to D(4h). On (111) facets, threefold hollow sites in the center are energetically preferred; adsorbed C is calculated to be slightly less stable when displaced to the facet borders.     

A molecular beam study of the NO + CO reaction on Pd(111) surfaces

Nitric oxide (NO) reduction with carbon monoxide (CO) on the Pd(111) surface was studied under isothermal conditions by molecular beam techniques as a function of temperature, NO:CO beam composition, and beam flux. Systematic experiments were performed under transient and steady state conditions. Displacement of adsorbed CO by NO in the transient state of the reaction was observed at temperatures between 375 and 475 K for all the NO:CO compositions studied. NO accumulation occurs on Pd(111) surface under steady state conditions, below 475 K, due to stronger chemisorption of NO. The steady state reaction rates attain a maximum at about 475 K, nearly independent of beam composition. N2 was found to be the major product of the reduction, along with a minor production of N2O. The production of N2 and N2O indicates molecular and dissociative adsorption of NO on Pd(111) at temperatures up to 525 K. Postreaction TPD measurements were performed in order to determine the nitrogen coverage under steady-state conditions. Finally, the results are discussed with respect to the rate-controlling character of the different elementary steps of the reaction system.     

Cl/Ag(111)体系研究——Ⅲ.Cl吸附对O_2吸附的影响

考察了Ag(111)表面和吸氯的Ag(111)表面上氧的吸附行为.结果表明在Ag(111)及低暴露量氯吸附的Ag(111)上氧吸附时,表面上均存在弱的分子氧和原子氧物种.但在高暴露量氯吸附的Ag(111)表面上氧吸附时则选择性地只产生表面分子氧物种,这种选择性只与氯的吸附程度有关,而与氯的存在与否无关.结合以前的实验结果,对氯吸附至(c)阶段时的Ag(111)表面上氧的选择性吸附行为的本质作了详细讨论.     

CO2在金属表面活化的能学方法研究

应用UBI-QEP方法, 估算了CO2-在金属表面的吸附热, 并计算了CO2在Cu(111)、Pd(111)、Fe(111)、Ni(111)表面的各种反应途径的活化能垒. 结果表明, CO2-在4种过渡金属表面相对的稳定性和CO2解离吸附的活性顺序一致,均为Fe>Ni>Cu>Pd. 说明CO2-可能是CO2解离吸附的关键中间体. 在Cu、Pd、Ni表面上, CO2解离吸附的最终产物是CO,而在Fe表面其最终会解离成C和O. 在Cu、Fe、Ni表面, CO2加氢活化是一种有效模式, 而在Pd上则不容易进行. 在Cu和Pd表面,碳酸盐物种也可能是CO2活化的重要中间体.