Precursor-mediated Adsorption Research Articles

Spatial and Temporal Self-organization During CO Oxidation Over Ni

Oscillatory behavior during CO oxidation over a Ni foil in a continuous-flow catalytic reactor has been discovered in the reaction mixture containing CO excess at atmospheric pressure in the temperature range of 500–630 °C. The oscillations are accompanied by propagation of the oxidation and reduction fronts which were recorded by a photo–video camera. Dark fronts of the oxidized state appear periodically in the upstream part of the catalyst and propagate to the downstream region in the direction of the gas flow. Reduction fronts move in the opposite direction. A mathematical model has been developed to simulate the oscillatory dynamics. The developed model simulates experimental data almost quantitatively. The origin of oscillations is connected with periodic oxidation and reduction of the nickel surface. It is shown that a precursor-mediated adsorption of CO on Ni surface is required to simulate the oscillations under reducing conditions.

Mathematical modeling of oscillations during CO oxidation on Ni under reducing conditions

This paper presents the first mathematical model simulating the recently discovered oscillatory behavior during CO oxidation on a Ni foil in a continuous-flow catalytic reactor. These oscillations occur in the presence of excess CO; thus, the well-known Sales–Turner–Maple model, simulating oscillations in oxygen excess, cannot be applied. We suggest a modification of the Sales–Turner–Maple model by introducing the precursor-mediated adsorption of CO. This made it possible to simulate the oscillations under reducing conditions. The suggested microkinetic model coupled with a continuous-flow reactor model predicts that the oscillations proceed on the partially oxidized catalyst because of the periodic formation and reduction of the surface oxide. The introduction of oxygen diffusion into the subsurface layers allows the description of the experimentally observed variation in Ni color during the oscillatory cycle. The developed model successfully reproduces experimental data as the temperature range of the oscillations together with their shape and amplitudes.

Formation of Si/organic interfaces using alkyne-functionalized cyclooctynes—precursor-mediated adsorption of linear alkynes versus direct adsorption of cyclooctyne on Si(0 0 1)

Adsorption of ethynyl-cyclopropyl-cyclooctyne (ECCO), an alkyne-functionalized cyclooctyne, on Si(0 0 1) was studied by means of x-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM). Together, XPS and STM results clearly indicate chemoselective adsorption of ECCO on Si(0 0 1) via a [2+2] cycloaddition of the strained triple bond of cyclooctyne without reaction of the ethynyl group. The results are compared to the adsorption of acetylene on Si(0 0 1): C2H2 adsorbs on Si(0 0 1) via a precursor-mediated reaction channel as it was shown by means of temperature dependent measurements of the sticking probability as well as by means of STM experiments at variable temperature. On the other hand, cyclooctyne adsorbs on Si(0 0 1) via a direct reaction channel. This qualitative difference in the reaction pathways of the two functionalities leads to the observed chemoselective adsorption of ECCO via the strained triple bond of cyclooctyne. As the ethynyl group stays intact, monolayers of ECCO on Si(0 0 1) form a well defined interface between the silicon substrate and further organic molecular layers which can be attached to the ethynyl functionality.

First-Principles Study on O2 Adsorption and Dissociation Processes over Rh(100) and Rh(111) Surfaces.

Through DFT calculations, a systematic description of O2 adsorption and dissociation processes over Rh(100) and Rh(111) surfaces has been provided. The dominance of parallel orientation in molecularly adsorbed states and during the impinging processes has been identified, along with the explicit adsorption configurations and preferred impinging trajectories on both surfaces. The dissociation of O2 is found to occur either by precursor-mediated adsorption or by direct dissociation. O2 in its initial precursor state dissociates facilely on Rh(100), but this is a little harder on Rh(111) by going through a two-step process. The latter can be described as a preliminary rotation and subsequent dissociation, with the final locations of two O atoms disturbed easily by coadsorbed O atoms surrounding the dissociating O2 molecule due to the existence of a relatively flat potential energy surface stage along the way. The present work may provide the basis for kinetic modeling to investigate the catalytic properties on a realistic scale.

Steady-State CO Oxidation on Pd(111): First-Principles Kinetic Monte Carlo Simulations and Microkinetic Analysis

Using a kinetic Monte Carlo (KMC) approach with parameters derived from first-principles calculations, we modeled the steady-state of CO oxidation on Pd(111), a prototypical catalytic system with various practical applications, including the treatment of automotive gas exhausts. Focusing on the metallic phase of the catalyst, we studied how the rate of CO oxidation depends on temperature and pressure, at fixed gas phase composition. Comparing the results of our simulations with experimental data, we found that all the qualitative features of this catalytic system are correctly reproduced by our model. We show that, when raising the temperature, the system transitions from a CO-poisoned regime with high apparent activation energy to a regime where the rate is almost independent of the temperature. The almost zero apparent activation energy at high temperature stems from approximately equal and opposite values of the O2 adsorption energy and dissociation barrier, as revealed by a simple microkinetic analysis. In the CO-poisoned regime, the precursor-mediated dissociative adsorption of oxygen plays a crucial role: we find that small changes (within DFT error) in the parameters controlling this elementary step have large effects on the kinetics of CO oxidation at low temperature.

The Role of Precursor States in Adsorption, Surface Reactions and Catalysis

Among the many concepts which have been developed in surface science and catalysis to explain the nature of adsorption, the role of weakly-held states, so-called precursor states, is one of the most important. The kinetics of precursor-mediated adsorption is described, together with examples showing how significant such effects can be, not just in adsorption itself, but also in surface reactions on crystals and on model supported catalysts. These examples include CO adsorption on Pd single crystals and catalysts and implications for CO oxidation, and extreme examples of the precursor effect, where adsorbate molecules can appear to ‘seek out’ active sites which are in very low concentration on the surface. The decomposition and oxidation of formic acid is just such an example. Such reactivity patterns then also lead to concepts such as the collection zone’ or ‘diffusion circle’ describing how much of an area of surface is ‘swept’ by incoming weakly-held molecules during their brief sojourn there.

Carbon dioxide adsorption on MgO(001)–CO2 kinetics and dynamics

Thermal desorption spectroscopy (TDS) and molecular beam scattering were used to characterize the adsorption kinetics and dynamics, respectively, of CO2 on MgO(001). X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy as well as low energy electron diffraction were used for sample characterization. TDS revealed two CO2 desorption peaks at 200K and 260K, consistent with carbonate decomposition. XPS of adsorbed CO2 did reveal one broad C1s structure at 289.4eV which also indicates carbonate formation. Carbonates decompose into CO2, CO, and O2 as evident from multi-mass TDS. CO2 beam scattering indicates non-activated and precursor-mediated adsorption with a low impact energy adsorption probability of 0.74.

Oxidation of TiAl surface with hyperthermal oxygen molecular beams

We report results of our detailed studies on the initial oxidation process of TiAl with a 2eV hyperthermal oxygen molecular beam (HOMB) and thermal O2 in the backfilling. The oxidation processes are monitored by X-ray photoemission spectroscopy (XPS) measurements in conjunction with synchrotron radiation (SR). Oxidation of both Al and Ti occurs during the oxidation. The incident-energy and surface-temperature dependences of oxidation reveal that the precursor-mediated dissociative adsorption is the dominant initial step of oxidation in the thermal O2 backfilling. Thus, the efficiency of oxidation is higher for the thermal O2 backfilling than for the HOMB dose. The result is quite different from that on TiNi where the HOMB dose has the advantages in oxide layer growth at high O coverage. We succeeded in fabricating blue colored TiO2 and Al2O3 containing layers, combining HOMB and surface annealing.

Ab initio Simulation of 1D Pattern Formation of Adsorbates on the Ge(100)-2 × 1 Surface

ABSTRACTIt is known that methanol and ethylene form distinct one-dimensional patterns along the dimer row on the Ge(100)-2 × 1 surface. A unified explanation for the pattern formation is attempted in this study through theoretical methods. Kinetic parameters of the precursor-mediated adsorption of the two molecules are calculated using density functional theory methods. The potential energy surface along the reaction channel was found to vary according to nearest-neighbor occupation. Monte Carlo simulations were performed with calculated kinetic coefficients and assumptions of a one-dimensional lattice with nearest neighbor interactions. The simulation results effectively reproduce the coverage-dependent evolution of longer-range adsorption patterns.

Thermodynamics and kinetics of CO and benzene adsorption on Pt(111) studied with pulsed molecular beams and microcalorimetry

The adsorption and desorption of the system CO/Pt(111) and C 6H 6/Pt(111) at 300 K has been investigated with a pulsed molecular beam method in combination with a microcalorimeter. For benzene the sticking probability has been measured in dependence of the coverage θ. For coverages θ > 0.8 transient adsorption is observed. From an analysis of the time-dependence of the molecular beam pulses the rate constant for desorption is determined to be 5.6 s − 1 . With a precursor-mediated kinetic adsorption model this allows to obtain also the hopping rate constant of 95.5 s − 1 . The measured adsorption enthalpies could be best described by (199 − 77 θ − 51 θ 2) kJ/mol, in good agreement with the literature values. For CO on Pt(111) also transient adsorption has been observed for θ > 0.95 at 300 K. The kinetic analysis yields rate constants for desorption and hopping of 20 s −1 and 51 s −1, respectively. The heats of adsorption show a linear dependence on coverage (131 − 38 θ) kJ/mol between 0 ≤ θ ≤ 0.3, which is consistent with the desorption data from the literature. For higher coverage (up to θ = 0.9ML) a slope of −63 kJ/mol describes the decrease of the differential heat of adsorption best. This result is only compatible with desorption experiments, if the pre-exponential factor decreases strongly at higher coverage. We found good agreement with recent quantum chemical calculations made for ( θ = 0.5ML).

Active oxidation of Cu 3Au(1 1 0) using hyperthermal O 2 molecular beam

Oxidation of Cu 3Au(1 1 0) using a hyperthermal O 2 molecular beam (HOMB) was investigated by X-ray photoemission spectroscopy in conjunction with a synchrotron light source. From the incident energy dependence of the O-uptake curve, the precursor-mediated dissociative adsorption occurs, where the trapped O 2 molecule can migrate and dissociate at the lower activation-barrier sites, dominantly at thermal O 2 exposures. Dissociative adsorption of O 2 on Cu 3Au(1 1 0) is as effective at the thermal O 2 exposure as on Cu(1 1 0). On the other hand, at the incident energies of HOMB where the direct dissociative adsorption is dominant, it was determined that the dissociative adsorption of O 2 implies a higher activation barrier and therefore less reactivity due to the Au alloying in comparison with the HOMB oxidation of Cu(1 1 0). The dissociative adsorption progresses with the Cu segregation on Cu 3Au(1 1 0) similarly as on Cu 3Au(1 0 0). The growth of Cu 2O for 2 eV HOMB suggests that the diffusion of Cu atoms also contribute to the oxidation process through the open face, which makes the difference from Cu 3Au(1 0 0).

Si(100)-2×1 표면과 개질된 Si(100) 표면 상에서 실릴 (Silyl) 흡착충의 형성과 안정성

깨끗한 Si(100)-<TEX>$2{\times}1$</TEX>, D를 먼저 흡착시킨 Si(100)-<TEX>$2{\times}1$</TEX>, 그리고 이온 빔에 의해 원자 수준으로 거칠어진 Si(100) 등의 세 가지 표면에 각각 <TEX>$Si_2H_6$</TEX>의 흡착시켜 포화 실릴(<TEX>$-SiH_3(a)$</TEX>) 흡착층을 형성시키고 실험적으로 비교 고찰하였다 전구체 흡착 거동(기작)과 함께 <TEX>$Si_2H_6$</TEX>의 표면 분해(화학)흡착 반응성은 개질을 시켜주지 않은 깨끗한 Si(100)-<TEX>$2{\times}l$</TEX> 표면에서 가장 크게 나타났다. 이 결과는 화학흡착 반응 즉, <TEX>$H_3Si-SiH_3$</TEX> 결합 분해와 두 개의 Si-<TEX>$SiH_6(a)$</TEX> 표면결합 형성이 표면의 Dangling Bond Pair 상에서 동시적으로(Concertedly) 일어나는 <TEX>$Si_2H_6$</TEX>의 분해흡착 기작으로 설명될 수 있었다. 또한 Si(100)-<TEX>$2{\times}l$</TEX> 표면에 흡착된 <TEX>$-SiH_3(a)$</TEX>의 매우 논은 열적 안정성은 <TEX>${\sim}0.9\;ML$</TEX>나 되는 표면 덮힘도와 함께 실릴기로 조밀하게 흡착된 표면에 Dangling Bond가 존재하지 않는 것에 의한 것으로 밝혀졌다. Saturation-coverage silyl, <TEX>$-SiH_3(a)$</TEX>, overlayers were prepared from <TEX>$Si_2H_6$</TEX> adsorption on three comparative surfaces: clean unmodified; D-precovered; and atomically roughened Si(100). Together with its precursor-mediated adsorption behavior, the surface reactivity of <TEX>$Si_2H_6$</TEX> was found to be the highest on the unmodified Si(100)-<TEX>$2{\times}1$</TEX> surface. This was correlated with its dissociative adsorption mechanism, in which both the <TEX>$H_3Si-SiH_3$</TEX> bond scission and the dual surface <TEX>$Si-SiH_3(a)$</TEX> bond formation require a surface dangling bond 'pair'. The unusually high thermal stability of <TEX>$-SiH_3(a)$</TEX> on the unmodified surface was ascribed to a nearly close-packed <TEX>$-SiH_3(a)$</TEX> coverage of <TEX>${\sim}0.9$</TEX> monolayer and the consequent lack of dangling bonds on the silyl-packed surface.

Reaction-Path Selection with Molecular Orientation of CH3Cl on Si{100}

We report direct evidence of steric effects in the dissociative adsorption of CH3Cl on Si{100} during the evolution of the coverage. The sticking probability (S) as a function of the amount of accumulated CH3Cl on the surface reveals the typical mobile precursor-mediated adsorption. For the Cl end collision, the decrease of S starts with a lower CH3Cl accumulation than for the CH3 end collision. Moreover, this steric effect depends on the surface temperature.

Molecular Layer Deposition of Nylon 66 Films Examined Using in Situ FTIR Spectroscopy

Molecular layer deposition (MLD) techniques can utilize sequential, self-limiting surface reactions with bifunctional monomers to grow polymer films. In this study, transmission in situ Fourier transform infrared (FTIR) investigations were utilized to monitor the growth of nylon 66 MLD films using adipoyl chloride (AC) and 1,6-hexanediamine (HD) as the bifunctional monomers. In situ FTIR measurements on high surface area SiO2 powders monitored the linear growth of the integrated absorbance of the N−H, C−H, C−N, and CO stretching vibrations and the CO−N−H bending vibrations versus the number AC and HD sequential exposures. The growth of the integrated absorbance for the nylon 66 features was also self-limiting as a function of the individual AC and HD exposures. In addition to the vibrational modes expected for nylon 66, vibrational features were also observed that were consistent with some ammonium chloride salt incorporated in the film during nylon 66 MLD. The temperature dependence of the nylon 66 growth rate for constant exposures revealed that the highest growth rates were observed at the lowest sample temperature of 62 °C. This temperature dependence was consistent with a precursor-mediated adsorption model. Nylon 66 MLD was studied with and without surface functionalization of the SiO2 substrate using (3-aminopropyl)triethoxysilane (APS). The APS functionalization facilitated the reaction of AC with the amine-terminated surface. The growth rate of the nylon 66 MLD films was estimated using in situ transmission FTIR measurements on flat KBr substrates with an amine-terminated Al2O3 ALD overlayer. The integrated absorbance of the C−H vibrational stretches was utilized to estimate the nylon 66 MLD growth rate per AC/HD reaction cycle. The observed growth rates were both above and below the expected growth rate for one nylon 66 bilayer. This range of growth rates illustrates the difficulties involved in using bifunctional monomers for MLD.

Adsorption Dynamics of Alkanes on Single-Wall Carbon Nanotubes: A Molecular Beam Scattering Study

The adsorption dynamics of n-/isobutane on “closed”-end (as-prepared, c-CNTs) and open-end (vacuum-annealed) carbon nanotubes (o-CNTs) have been studied by molecular beam scattering adsorption probability measurements. Thermal desorption spectroscopy (TDS) and scanning electron microscopy have been used to characterize the CNTs. Evident from TDS data, o-CNTs allow internal adsorption sites to be populated, which is correlated with an increase in the initial adsorption probabilities as compared with c-CNTs, consistent with the enhancement in the surface area by opening the tube ends. Furthermore, precursor-mediated adsorption dynamics were observed.

A supersonic molecular beam study of the reaction of tetrakis(dimethylamido)titanium with self-assembled alkyltrichlorosilane monolayers

The reaction of a transition metal coordination complex, Ti[N(CH(3))(2)](4), with self-assembled monolayers (SAMs) possessing-OH, -NH(2), and -CH(3) terminations has been examined using supersonic molecular beam techniques. The emphasis here is on how the reaction probability varies with incident kinetic energy (E(i)=0.4-2.07 eV) and angle of incidence (theta(i)=0 degrees -60 degrees ). The most reactive surface is the substrate underlying the SAMs-SiO(2) with a high density of -OH(a) (>5 x 10(14) cm(-2)), "chemical oxide." On chemical oxide, the dynamics of adsorption are well described by trapping, precursor-mediated adsorption, and the initial probability of adsorption depends only weakly on E(i) and theta(i). The dependence of the reaction probability on substrate temperature is well described by a model involving an intrinsic precursor state, where the barrier for dissociation is approximately 0.2-0.5 eV below the vacuum level. Reaction with the SAMs is more complicated. On the SAM with the unreactive, -CH(3), termination, reactivity decreases continuously with increasing E(i) while increasing with increasing theta(i). The data are best interpreted by a model where the Ti[N(CH(3))(2)](4) must first be trapped on the surface, followed by diffusion through the SAM and reaction at the SAMSiO(2) interface with residual -OH(a). This process is not activated by E(i) and most likely occurs in defective areas of the SAM. On the SAMs with reactive end groups, the situation is quite different. On both the-OH and -NH(2) SAMs, the reaction with the Ti[N(CH(3))(2)](4) as a function of E(i) passes through a minimum near E(i) approximately 1.0 eV. Two explanations for this intriguing finding are made-one involves the participation of a direct dissociation channel at sufficiently high E(i). A second explanation involves a new mechanism for trapping, which could be termed penetration facilitated trapping, where the Ti[N(CH(3))(2)](4) penetrates the near surface layers, a process that is activated as the molecules in the SAM must be displaced from their equilibrium positions.

Sensitivity and selectivity of Pt electrodes for hydrocarbon sensing in an ultra high vacuum environment

X-ray photoelectron spectroscopy (XPS) and temperature-programmed reaction (TPR) were used to investigate the suitability of polycrystalline platinum as a sensing electrode for sensitive and selective detection of hydrocarbons in a vacuum environment by electrochemical means. At 870 K, all the species investigated undergo precursor-mediated dissociative adsorption to yield graphite, though at widely different rates. It is found that polycrystalline platinum discriminates effectively between “sticky” hydrocarbons such as alkenes, aromatics and alkanes >C 5 on the one hand, and “benign” low molecular weight alkanes on the other. The proposed methodology offers a promising way forward with respect to the intended application.

Adsorption dynamics of CO2 on Zn-ZnO(0001): A molecular beam study

Presented are initial S(0) and coverage Theta dependent, S(Theta), adsorption probability measurements, respectively, of CO(2) adsorption on the polar Zn-terminated surface of ZnO, parametric in the impact energy E(i), the surface temperature T(s), the impact angle alpha(i), varied along the [001] azimuth, the CO(2) flux, and the density of defects, chi(Ar(+)), as varied by rare gas ion sputtering. S(0) decreases linearly from 0.72 to 0.25 within E(i)=0.12-1.33 eV and is independent of T(s). Above E(i)=0.56 eV, S(0) decreases by approximately 0.2 with increasing alpha(i). The shape of S(Theta) curves is consistent with precursor-mediated adsorption (Kisliuk shape, i.e., S approximately const) for low E(i); above E(i)=0.56 eV, however, a turnover to adsorbate-assisted adsorption (S increases with Theta) has been observed. The initial slope of S(Theta) curves decreases thereby with increasing alpha(i), chi(Ar(+)), and T(s), i.e., the adsorbate-assisted adsorption is most distinct for normal impact on the pristine surface at low T(s) and is independent of the CO(2) flux. The S(Theta) curves have been parametrized by analytic precursor models and Monte Carlo simulations have been conducted as well. The temperature dependence of the saturation coverage shows two structures which could be assigned to adsorption on pristine and intrinsic defect sites, respectively, in agreement with a prior thermal desorption spectroscopy study. The heat of adsorption E(d) for the pristine sites amounts to 34.0-5.4Theta, whereas for adsorption on the intrinsic defect sites E(d) of approximately 43.6 kJ/mol could be estimated. Thus, a kinetic structure-activity relationship was present.

ADSORPTION DYNAMICS OFCO2ON HYDROGEN PRECOVEREDZn-ZnO(0001): A MOLECULAR BEAM STUDY

Presented are initial, S0, and coverage, Θ, dependent, S(Θ), adsorption probability measurements, respectively, of CO2adsorption on a hydrogen precovered, polar, Zn -terminated surface of ZnO , parametric in the impact energy, Ei, and atomic hydrogen precoverage, ΘH. Furthermore, CO2Thermal Desorption Spectroscopy has been used to estimate ΘHas well as the binding energy of CO2on H / Zn - ZnO . The S(Θ) curves are below Ei=0.56 eV , consistent with precursor-mediated adsorption (S~ const ), and above that impact energy with adsorbate-assisted adsorption (S increases with Θ). Although a decrease in the CO2binding energy from 32.5 to 28.8 kJ/mol with ΘHis present, S(Θ, ΘH) curves are consistent with a physical site blocking, as demonstrated by Monte Carlo Simulations.

Fast x-ray spectroscopy study of ethene on clean and SO4 precovered Pt{111}

The adsorption and decomposition of ethylene over a Pt{111} single crystal surface has been investigated by fast x-ray spectroscopy. At 100 K ethene displays precursor-mediated adsorption kinetics, adopting a single environment with a saturation C2H4 coverage of 0.25 ML and binding energy of 283.2 eV. Thermal decomposition proceeds above 240 K via dehydrogenation to ethylidyne with an activation barrier of 57±3 kJ mol−1 and preexponential factor ν=1×1010±0.5 s−1. Site-blocking by preadsorbed SO4 reduces the saturation ethene coverage but induces a new, less reactive π-bonded ethene species centered around 283.9 eV, which in turn decomposes to ethylidyne at 350 K.