Adsorption Dynamics Of CO Research Articles

Assessing the dynamics of CO adsorption on Cu(110) using the vdW-DF2 functional and artificial neural networks

In this work, we revisit the dynamics of carbon monoxide molecular chemisorption on Cu(110) by using quasi-classical trajectory calculations. The molecule–surface interaction is described through an atomistic neural network approach based on Density Functional Theory calculations using a nonlocal exchange–correlation (XC) functional that includes the effect of long-range dispersion forces: vdW-DF2 [Lee et al. Phys. Rev. B, 82, 081101 (2010)]. With this approach, we significantly improve the agreement with experiments with respect to a similar previous study based on a semi-local XC functional. In particular, we obtain excellent agreement with molecular beam experimental data concerning the dependence of the initial sticking probability on surface temperature and impact energy at normal incidence. For off-normal incidence, our results also reproduce two trends observed experimentally: (i) the preferential sticking for molecules impinging parallel to the [1̄10] direction compared to [001] and (ii) the change from positive to negative scaling as the impact energy increases. Nevertheless, understanding the origin of some remaining quantitative discrepancies with experiments requires further investigations.

Influence of {111} nanofaceting on the dynamics of CO adsorption and oxidation over Au supported on CeO2 nanocubes: An operando DRIFT insight

Abstract CO adsorption and further reaction with O2 on two Au/CeO2 model systems have been investigated by operando diffuse reflectance infrared Fourier transform spectroscopy (DRIFT) in order to unveil the active sites for CO oxidation. One of the ceria supports was constituted by crystallites depicting a nanocube shape bounded mostly by {100} planes, whilst the second one was consisted of nanocubes whose {110} surfaces were reconstructed into {111}-nanofacets with a zig-zag shape. The deconvolution of the CO adsorption bands and a detailed analysis of the time-evolution of these bands were also carried out. These studies evidenced a fast adsorption of CO over Au° and then Auδ--CO bands appeared during exposure to CO, in parallel with the appearance of Ce3+ features. Such changes suggest that CO adsorption process starts on Au sites on the low coordinated sites of the surface of the Au nanoparticles and their transfer to the interface sites. By comparing the nanostructure of the two catalysts, the Auδ--CO species could be fruitfully related to CO adsorption on sites located at the Au||CeO2 perimeter. The sample containing gold nanoparticles dispersed over the {111}-nanofaceted CeO2 nanocubes, which retained a higher amount of gold nanoparticles, depicts higher contribution of Auδ−−CO bands than the other Au catalyst in the DRIFT spectra. The dynamic DRIFT study clearly revealed that the CO species adsorbed on Auδ- sites were the most reactive ones in both gold supported systems for CO oxidation. This explains much higher TOF values observed on the catalyst prepared on the surface reconstructed support. A detailed analysis on the reactivity was performed by modulation excitation spectroscopy (MES). The presence of peroxide surface species (O22-) in nanofaceted {111} could lead to higher catalytic activity than that of gold on the non-reconstructed catalyst.

Simultaneous time-resolved ATR-SEIRAS and CO-charge displacement experiments: The dynamics of CO adsorption on polycrystalline Pt

The high sensitivity of ATR-SEIRAS allowed simultaneous recording of time-resolved infrared spectra of the Pt-electrolyte interface and of the current flowing during CO-charge displacement experiments. These experiments revealed the dynamics of CO adsorption on Pt, as well as of the rearrangement of the interfacial water layer. Our experiments show that COL sites are populated first, COB appearing only at slightly higher coverages. At potentials sufficiently separated from the potential of zero total charge, the charge displaced upon CO adsorption on Pt and the integrated intensity of the COL band run approximately parallel, as also do the current transient and the time derivative of the integrated intensity of the COL band. This suggests that, for low to medium CO coverage, the current flowing at a given instant during the CO-charge displacement experiment corresponds to the instantaneous rate of CO adsorption, while the charge displaced when a CO submonolayer is formed is a good estimate of the corresponding relative coverage. Although the structure of the interfacial water layer is affected at low coverages, the band corresponding to interfacial water on top of a CO-covered Pt surface only appears at a relatively high CO coverage, suggesting that this kind of interfacial water is linked to the presence of compact islands of adsorbed CO.

Adsorption dynamics of CO on copper and gold clusters supported on silica – How special is nanogold?

A twist about an atomistic explanation for the unusual particle size-dependent reactivity enhancement of supported gold clusters still exists in the surface science and catalysis communities. A molecular beam scattering technique allowed the active sites of a catalyst to be mapped. In doing so, copper and gold clusters, both supported on silica, were studied. Indeed, Au clusters showed quite unusual behavior, however, that would be consistent with the influence of defects rather than quantum size effects.

Modeling and parameter estimation for a fixed-bed adsorption process for CO 2 capture using zeolite 13X

Modeling of a fixed-bed adsorption process for CO 2 capture using zeolite 13X was performed through parameter estimation. An empirical mass transfer rate model is proposed as a relevant description of the adsorption dynamics of CO 2/N 2 on zeolite 13X. An efficient objective function for the estimation of the mass transfer rate parameters was devised to avoid the local minimum and to improve the convergence to the desired solution. Langmuir isotherm equation coefficients, bed porosity, average particle size, and heat transfer coefficients were determined in separately designed experiments. The porosity estimate was updated during the estimation of mass transfer rate parameters to account for the effect of pore diffusion on bulk flow. To ensure numerically stable computation, the gradient-directed adaptive predictive collocation method was adopted with a cubic spline interpolation function and far-side boundary conditions. The model was experimentally evaluated in an adsorption bed with a height of 70 cm and an inner diameter of 2.54 cm and shown to predict the dynamic behaviors of the process variables with high accuracy.

Adsorption Dynamics of CO on Silica-Supported Cu Clusters: A Molecular Beam Scattering Study

The adsorption kinetics and dynamics of CO on silica-supported Cu clusters were studied with thermal desorption spectroscopy and molecular beam scattering. In addition, the electronic properties of the Cu clusters and sample cleanliness were characterized with X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES). Physical vapor deposition was used to deposit the clusters; according to XPS, the Cu clusters remain metallic. The CO impact energy, surface temperature, and Cu coverage dependence of the adsorption dynamics can be discussed in the framework of the capture zone model. The XPS/AES and thermal desorption spectroscopy (TDS) data are consistent with a standard growth mode of Cu on silica: a nucleation phase is followed by a cluster growth phase until thick and rather smooth Cu films are formed.

Adsorption Dynamics of CO on Silica Supported Gold Clusters: Cluster Size Effects in Molecular Beam Scattering Experiments

We report on particle size effects in the adsorption dynamics (gas-surface energy transfer) of CO, studied by molecular beam scattering. The initial adsorption probability shows a maximum as a function of Au exposure, χAu, i.e., cluster size. This reactivity maximum appears to coincide with the enhancement of catalytic activity seen for the CO oxidation reaction for small clusters.

Gas–surface interactions with nanocatalysts: Particle size effects in the adsorption dynamics of CO on supported gold clusters

We present evidence that adsorption probabilities (gas–surface adsorption dynamics) of CO depend distinctly on the Au cluster size, with a reactivity maximum at ∼3 nm. This cluster size is consistent with the legendary reactivity enhancement seen for the CO-oxidation reaction. Electron beam lithography has been used to fabricate some of the samples (down to 12 nm) as well as physical vapor deposition.

Adsorption Kinetics and Dynamics of CO, NO, and CO2 on Reduced CaO(100)

The adsorption kinetics and dynamics of typical combustion gases have been studied on CaO(100) by means of temperature programmed desorption and molecular beam scattering. In addition, the sample has been characterized by Auger electron spectroscopy. Whereas CO interacts rather weakly with CaO and adsorbs molecularly on pristine and oxygen vacancy sites, CO2 and NO interact strongly with CaO, leading to a variety of structures in TPD. CO2 adsorption leads to the formation of carbonates. NO adsorption results in the desorption of N2 and N2O species presumably by the formation of NO dimers stabilized at defect sites, which is consistent with prior reports. In assisting the assignment of the TPD structures that have been seen, density functional cluster calculations have been performed and are compared with results from prior studies. Trends seen for the initial adsorption probabilities, S0, are consistent with a simple mass-match model, S0−CO > S0−CO2. The coverage dependence of the adsorption dynamics is dominated by precursor effects for CO2 but shows direct Langmuirian adsorption dynamics of CO.

Molecular beam scattering of CO on iron oxide clusters supported on graphite (HOPG)

The adsorption dynamics of CO has been studied by measuring adsorption probabilities by means of the molecular beam scattering technique. Iron has been vapor deposited on ordered pyrolytic graphite (HOPG) with the vacuum chamber at a base pressure below 3 × 10 −10 mbar. However, the Fe clusters partially oxidize, according to Auger electron spectroscopy (AES). Indications for a transition from two-dimensional to three-dimensional cluster growth modes with increasing Fe exposure have been obtained by AES and thermal desorption spectroscopy (TDS). The initial adsorption probabilities, S 0, decrease with increasing impact energy and are systematically larger for larger clusters, consistent with the capture-zone model (CZM) and molecular, non-activated adsorption. S 0 initially increases with adsorption temperature, T s, and decreases above T s = 100 K. The latter is not fully consistent with the simplest version of the CZM. The coverage dependence of the adsorption probabilities obeys the Kisliuk precursor model, as expected in the framework of the CZM.

Adsorption dynamics of CO 2 on Cu(1 1 0): A molecular beam study

Molecular beam scattering measurements have been conducted to examine the adsorption dynamics of CO 2 on Cu(1 1 0). The initial adsorption probability, S 0, decreases exponentially from 0.43 ± 0.03 to a value close to the detection limit (∼0.03) within the impact energy range of E i = (0.12–1.30) eV. S 0 is independent of the adsorption temperature, T s, and the impact angle, α i, i.e., the adsorption is non-activated and total energy scaling is obeyed. The coverage, Θ, dependent adsorption probability, S( Θ), agrees with precursor-assisted adsorption dynamics (Kisliuk type) above T s ∼ 91 K. However, below that temperature adsorbate-assisted adsorption ( S increases with Θ) has been observed. That effect is most distinct at large E i and low T s. The S( Θ) data have been modeled by Monte Carlo simulations. No indications of CO 2 dissociation were obtained from Auger Electron Spectroscopy or the molecular beam scattering data.

Particle size effects in the adsorption dynamics of CO: The structure–activity relationship in the case of Cu/Zn–ZnO

We provide evidence for particle size effects and shed light on the structure–activity relationship for the Cu-on-ZnO(0 0 0 1) system. The energy transfer processes governing the adsorption of CO depend distinctly on the morphology of the metal-on-metal oxide system as evident by impact energy dependent variations in the shape of the coverage dependent adsorption probabilities.

Adsorption dynamics of CO on Pd(1 1 0): energy dependence, structure insensitivity and the role of the surface electronic structure

The translational energy ( E T) dependence of CO chemisorption has been investigated on Pd(1 1 0). A steep increase from 0.61 to 0.93 is observed in the initial sticking probability S 0 with increasing E T from 27 to 93 meV. Comparison to density functional theory calculations suggests a steering-mediated adsorption channel, driving slow molecules towards less favorable surface sites. This long-range interaction between CO and the Pd d-orbitals screens the potential influence of the surface structural corrugations on the CO adsorption and thus provides an explanation to the well-known structure insensitivity of CO adsorption on Pd surfaces. For E T>93 meV the adsorption reverts back to the direct chemisorption mechanism found on other Group VIII metals.

MONTE CARLO SIMULATION OF THE ADSORBATE- ASSISTED ADSORPTION IN THE CASE OF CO/ZnO

Recently, a detailed study of the adsorption dynamics of CO on metal oxides, i.e. for the polar ZnO surfaces, has been published [Becker et al., JCP113, 6334 (2000)]. For both polar surfaces, an increase in the adsorption probabilities with increasing surface coverage (adsorbate-assisted adsorption) has been observed. A Monte Carlo (MC) version of the so-called modified Kisliuk model is presented, which accounts for the main features observed experimentally. The model, which includes two fit parameters, is based on the differences in the mass differences in the mass mismatch of the gas phase species and the surfaces. Therefore, in particular, the polar surfaces of ZnO, which differ by the mass of the atoms in the first surface layer, are perfectly suited for discussing the proposed MC algorithm.

Adsorption dynamics of CO on Cu(1 1 0): a molecular beam study

Molecular beam measurements of initial adsorption probabilities, S 0, as well as the coverage dependence of the adsorption probability, S( Θ), of CO on Cu(1 1 0) are presented. The influence of kinetic energy, polar impact angle, α i, azimuthal orientation ([1 0 0] and [ 1 1 ̄ 0 ]), and adsorption temperature, T s, on the adsorption dynamics have been studied. With regard to microkinetical models used to describe processes in heterogeneous catalysis, the dependence of the adsorption probability on the density of defects has been investigated. The surface has been characterized by measuring He atom angular distributions, He atom reflectivity curves, and LEED. The shape of the S( Θ) curves are consistent with a precursor mediated adsorption mechanism. For low temperatures T s⩽90 K, the adsorption dynamics are dominated by adsorbate assisted adsorption. Despite the pronounced difference in the surface corrugation along the different high-symmetry directions no significant differences in S 0 and S( Θ) were observed for the different azimuths studied.

Adsorption dynamics of CO on the polar surfaces of ZnO

Measurements of initial adsorption probabilities, S0, as well as the coverage dependence of the adsorption probability, S(ΘCO), of CO on Zn–ZnO [ZnO(0001)] and O–ZnO [ZnO(0001̄)] are presented. The samples have been characterized by He atom scattering, He atom reflectivity measurements, LEED, and XPS. Samples with different densities of defects were examined, either by investigating different samples with identical surface termination (for O–ZnO) or by inducing defects by ion sputtering at low temperatures (for Zn–ZnO). The influence of kinetic energy and impact angle (for Zn–ZnO) as well as adsorption temperature on the adsorption dynamics have been studied. For both polar surfaces the shape of the coverage dependent adsorption probability curves are consistent with a precursor mediated adsorption mechanism. Adsorbate assisted adsorption dominates the adsorption dynamics for high impact energies and low adsorption temperatures, especially for Zn–ZnO. The He atom reflectivity measurements point to the influence of an intrinsic precursor state. In contrast to the Zn–ZnO surface, for O–ZnO a weak thermal activation of the CO adsorption was observed. Total energy scaling is obeyed for Zn–ZnO. The heat of adsorption for CO on both polar faces varies between 7 kcal/mol (low coverage) and 5 kcal/mol (high coverage). A comparison of He atom reflectivity with S(ΘCO) curves demonstrates that CO initially populates defect sites on both surfaces. For O–ZnO an increase in S0 with decreasing density of defects was observed, whereas for the Zn-terminated surface S0 was independent of the defect density within the range of parameters studied.

A molecular beam study of the adsorption dynamics of CO on Ru(0001), Cu(111) and a pseudomorphic Cu monolayer on Ru(0001)

We have investigated the sticking coefficient of CO on Ru(0001), a pseudomorphic Cu monolayer on Ru(0001), and a fully relaxed Cu(111) multilayer as function of kinetic energy, surface coverage, and surface temperature. At a low kinetic energy of 0.09 eV, the initial sticking coefficients, S 0, on these surfaces are determined to be 0.92, 0.96 and 0.87, respectively. In all cases, a decrease of S 0 with increasing beam energy was observed, yielding values of 0.58, 0.14 and 0.07, respectively, at a kinetic energy of ∼2.0 eV. For all three surfaces the coverage dependent sticking coefficients, S( Θ), display very characteristic behavior at low kinetic energies: S( Θ) remains more or less constant up to coverages close to saturation, indicative of precursor adsorption kinetics. However, characteristic minima at intermediate coverages are observed, which are correlated to the formation of well ordered adsorbate phases. For high kinetic energies we observe a transition towards a linear decrease of S( Θ) for Ru(0001). In contrast, for the pseudomorphic Cu monolayer and for Cu(111) we find an increase in the sticking coefficients at low coverages, followed by a decrease close to saturation. This behavior is attributed to adsorbate assisted sticking, that is, to a higher sticking coefficient on adsorbate covered regions than on the bare surface. The comparison between the pseudomorphic monolayer and Cu(111) reveals that the CO bond strength to the former is larger by ∼40%. The initial sticking coefficients for both surfaces are very similar at low kinetic energies; at high kinetic energies, S 0 for the pseudomorphic Cu monolayer is, however, larger by a factor of two.

Site resolved adsorption dynamics of CO on Pt(111)

Abstract Site resolved sticking coefficients were measured by reflection absorption infrared spectroscopy for room temperature CO adsorbing on Pt(111) at 20 K where thermal diffusion was completely suppressed. CO adsorbed directly onto the top sites but not the bridge sites of the bare Pt(111) surface. Only through collisions between impinging CO and preadsorbed CO could bridge sites be occupied in the absence of thermal diffusion. Isotopes studies revealed two mechanisms for collisional occupation of the bridge sites: one in which impinging CO collisionally converts preadsorbed CO from top sites into bridge sites, and another in which occupation of bridge sites by impinging CO is facilitated by a scattering collision with preadsorbed CO. Cross sections for these two processes were large, σ T → B = 11 ± 2 A 2 and σ S = 20 ± 4 A 2 .