CO2 Desorption Kinetics Research Articles

Effects of preadsorbed oxygen on the bonding and desorption kinetics of CO on Ni(110)

The effects of oxygen preadsorption on CO adsorption and CO desorption kinetics have been studied by temperature programmed desorption (TPD) and high resolution electron energy loss spectroscopy (HREELS). It is found that preadsorbed oxygen decreased total CO uptake at 120 K by no more than 20% even though the adsorption and desorption energies are strongly affected. The presence of oxygen causes several new desorption states to appear in the TPD whose populations depend sensitively upon oxygen predose. Using published results for the structure of oxygen covered Ni(110) and the present HREELS measurements, the desorption states have been assigned to structurally distinct bonding sites. Using the variation in heating rates method, the first order desorption energies and preexponential factors are obtained as a function of CO coverage for various oxygen predoses. The relationship between the desorption energies and the bonding sites, especially with regard to the distance of CO from neighboring oxygen atoms, is discussed.

Adsorption ? desorption kinetics on epitaxially oriented palladium clusters

Palladium clusters with a low size dispersion and a single crystallographic shape have been obtained by epitaxial growth on (100) MgO. The isothermal adsorption — desorption kinetics of CO is obtained by a molecular beam method allowing the direct determination of the global adsorption probability and of the mean life time of CO molecules on the clusters. Three main results are deduced from these measurements: — an increase of the adsorption rate of CO on the clusters by the capture of CO molecules physisorbed on the substrate, — a faster desorption rate from (100) than from (111) facets at high coverage, — an increase of the adsorption energy for clusters smaller than 3 nm.

The kinetics of CO desorption from Ni(110)

The kinetics of CO desorption from clean Ni(110) have been measured by thermal desorption mass spectrometry (TDMS) using the method of variation of heating rates. Analysis of the desorption spectra gives the coverage dependence of the preexponential factor νd, and the activation energy Ed, of the desorption rate constant. For 0<θ<0.5, Ed is constant at 30.6 kcal/mol, allowing analysis of the coverage dependence of the preexponential factor under conditions where the compensation effect is absent. We observe only small changes in the preexponential factor under these conditions, where log[νd(s−1)] rises slowly from 14.6 to 15.3 from 0<θ<0.65. Near θ=0.8, a synchronous drop in νd by 0.6 orders of magnitude and in Ed by 5 kcal/mol is observed at the onset of formation of the (2×1)p2mg structure. These results are discussed in light of the structures of CO in Ni(110) and the effects of ordering transitions on the desorption kinetics. The experimental results for νd(θ) are compared with those predicted for desorption by three precursor mechanisms and for desorption of a lattice gas with lateral interactions.

CO desorption kinetics from clean and sulfur-covered Ru(001) surfaces

The desorption of CO from clean and sulfur-covered Ru(001) surfaces was studied using laser-induced thermal desorption (LITD) and temperature programmed desorption (TPD) techniques. CO was observed to desorb from clean Ru(001) with coverage-dependent kinetics. The isothermal desorption of CO was monitored with LITD measurements. The rates for CO desorption were determined using a simple Padé approximant method to evaluate coverage-dependent kinetic parameters. On the clean Ru(001) surface, the desorption activation energy and preexponential dropped sharply from Ed=34 kcal/mol and νd=5×1015s−1 for ΘCO<0.33 ML to Ed=23 kcal/mol and νd=2×1012s−1, for ΘCO>0.33 ML. The clean Ru(001) surface results agreed very well with earlier studies of CO desorption. The presence of surface sulfur shifted the TPD peaks for CO on Ru(001) to lower temperatures. Likewise, isothermal LITD measurements revealed that the CO desorption parameters at ΘCO=0.06 ML decreased from Ed=36 kcal/mol and νd=1×1016s−1 to Ed=22 kcal/mol and νd=5×1010s−1 as the sulfur coverage increased from ΘS=0.00 to 0.24 ML. The kinetic parameters derived from the isothermal LITD experiments were able to predict accurately the CO TPD spectra as a function of CO coverage, sulfur coverage and heating rate. The dramatic effect of sulfur on the CO desorption kinetics at low sulfur coverages argued for a long-ranged electronic effect for sulfur interacting with CO on Ru(001).

Desorption kinetics on an alkali metal-precovered surface: CO and K on Pt(111)

The desorption kinetics of CO initially coadsorbed with K on Pt(111) at 100 K has been measured as a function of both CO and K coverage (θ) using temperature programmed desorption (TPD). CO and K desorb separately when the initial θCO is low, supporting the theory that CO and K interact via a strong Coulombic attraction and do not form an oxocarbon (salt-like) compound. Although precoverage with ≥0.15 ML K does induce a few percent of the desorbing CO to undergo atomic exchange, the effect is very small in comparison to that observed on other surfaces. The desorption is found to be first order in θCO on both clean and K-precovered surfaces. With low initial θCO the activation energy (E) and preexponential factor (ν) for desorption from the K-free surface are found to be 28.8±0.7 kcal mol−1 and 1013.3±0.2 s−1, respectively, in excellent agreement with previously reported results. As observed with most alkali metal-precovered surfaces, the low coverage CO desorption peak temperature increases monotonically with K coverage. We find that the higher desorption temperatures result from the combined effects of increasing E and simultaneously decreasing ν, with the change in ν causing most of the shift; when low θCO are initially coadsorbed with 0.15 ML K E=31.3±0.6 kcal mol−1 and ν=1011.4±0.2 s−1. For all surfaces studied both E and ν decreased approximately linearly with increasing θCO>0.1 ML. The θCO and θK dependence of the kinetics is generally consistent with the proposed structural model for this surface based on the formation of CO+K islands. Possible mechanisms for the K-induced change in the kinetics are discussed and further investigations are proposed.

Summary Abstract: Direct, real-time measurement of the adsorption and desorption kinetics of CO and Cu(100)

Summary Abstract: Direct, real-time measurement of the adsorption and desorption kinetics of CO and Cu(100)

A thermal desorption study of the adsorption of CO on Fe(110); enhancement of dissociation by surface defects

The adsorption and desorption kinetics of CO on Fe(110) was studied by Thermal Desorption Spectroscopy. Our results indicate that the adsorption takes place via a mobile weakly bound precursor state that also influences the desorption kinetics as concluded by analysing the desorption spectra. We have also investigated the role of surface defects produced by argon sputtering on the rate of dissociation of carbon monoxide and we have found that, on a slightly sputtered surface 36% of the adsorbed CO molecules will undergo dissociation, whereas on an annealed surface only 20% of the adsorbed CO molecules will dissociate. This seems to suggest that surface defects act as active sites for carbon monoxide dissociation on Fe(110) probably through a decrease in the activation barrier for dissociative chemisorption.

Phase-sensitive detection of electron-beam-induced desorption from ZnO

We report a method of phase-sensitive detection of desorption from surfaces bombarded by a properly modulated electron beam. Mass-spectrometer signals detected by this method assure a high sensitivity and selectivity of the results. This method is used here for studying surfaces of ZnO powders, and it found that CO2 is the only desorbed species during electron bombardment of such surfaces. Following the CO2 desorption kinetics simultaneously with the associated surface conductivity change, we find one-to-one correspondence between both quantitites. Furthermore, the rates of the electron-induced CO2 desportion and the surface conductivity are in a very good agreement with a theoretical model, previously developed for photodesorption. This model predicts a slowing of the desorption rate due to the buildup of an electron accumulation layer and a corresponding band bending which hinder irradiation-generated holes from reaching the surface and neutralizing CO−2 species chemisorbed on it. Our observations seem to support this model together with pointing out the similarities between electron- and photon-induced desorption in this case.

Binding states of CO and H 2 on clean and oxidized (111)Pt

The binding states and sticking coefficients of CO and H 2 on clean and oxide covered (111)Pt are examined using flash desorption mass spectrometry and Auger electron spectroscopy (AES). On the clean surface at 78 K there is one major binding state of CO with a desorption activation energy which decreases with coverage plus a second smaller state, while H 2 exhibits three binding states with peak temperatures of 140, 230 and 310 K and saturation density ratios of 0.5 : 1 : 1. Desorption kinetics of CO are consistent with a first order state with a normal pre-exponential factor of 10 13 ± 1 sec −1, while all three peaks of H 2 are broader than expected. Interpretations in terms of anomalous pre-exponential factors, coverage dependent desorption activation energies, and desorption orders are considered. On the oxidized surface saturation densities of both gases are nearly identical to those on the clean surface, but desorption temperatures are increased significantly and the initial sticking coefficient on the oxide decreases slightly for CO and increases slightly for H 2.

Thermal desorption studies of CO and H 2 from the CuNi alloy

The desorption order and activation energies of CO and H 2 from a (110) oriented CuNi single crystal have been studied by thermal desorption spectroscopy. The surface composition of the alloy can be varied by annealing (after sputtering treatment which reduces the surface Cu concentration) in a temperature range where the surface composition is controlled by bulk diffusion. The surface composition is determined by the relative line intensities of the Auger M 1M 4,5M 4,5 Cu and Ni transitions at ∼-100 eV electron energy, and therefore represents an average concentration measurement within the electron escape length (approximately the first couple of atomic layers). The CO desorption spectra are mainly interpreted in terms of the ensemble effect, which relates the heat of chemisorption to the number and the species of the surface atoms in contact with the CO molecule. The CO desorption kinetics is first order. Pour desorption states are resolved, two of which are readily identified as desorption from a pure Cu or pure Ni ensemble (i.e., pure Cu or pure Ni binding site). The pure Ni binding state activation energy decreases linearly with increasing surface Cu concentration, indicating that a ligand (long range) effect is also present in the bonding. The H 2 desorption kinetics is second order. Two binding states are resolved, and both of them are shown to have a pure Ni origin. The activation energy of the high temperature bonding state remains constant with changing surface composition.