Coverage-dependent Desorption Energies Research Articles

Understanding the Binding of Aromatic Hydrocarbons on Rutile TiO2(110)

The adsorption of cyclohexane, benzene, and alkyl-substituted benzene derivatives is studied on rutile TiO2(110) by a combination of molecular beam dosing, temperature-programmed desorption, and density functional theory (DFT). An inversion analysis is used to extract the coverage-dependent desorption energies from TiO2(110). The values of the suitable prefactors are derived from simple statistical mechanical models assuming different limits in the adsorbate mobility on the surface. The prefactor values determined using the vibrational frequencies from DFT calculations corroborate this analysis and show that the adsorbates are mobile in one or two dimensions on a corrugated TiO2(110) surface. The adsorption of benzene derivatives is found to be dominated by the dative Lewis acid–base interactions of the π system with the surface Ti ions. While the desorption energy generally increases with increasing the length and the number of substituents, the difference between the desorption energies decreases as the...

Temperature- and pressure-dependent adsorption configuration of NO molecules on Rh(111) surface: A theoretical study

Understanding the behavior of NO molecules on Rh metal surfaces is necessary for gaining fundamental knowledge about catalytic chemistry and surface science. The present theoretical study details the configuration of NO molecules adsorbed on Rh(111) surface on the basis of density functional theory (DFT) calculations. The impact of coverage dependences on the adsorption/desorption energies is examined systematically: multiple configurations of NO adsorption structures are investigated with NO surface coverage from 0.06 to 0.75. A phase diagram for the various NO configurations with temperature and pressure is obtained by considering the adsorption equilibrium. Adsorption energies of NO molecules generally decrease with increasing surface coverage, reflecting the repulsive interactions between the adsorbed NO molecules. Microkinetic simulations for temperature-programmed desorption (TPD) spectra are performed. Simulated TPD spectra on the basis of the surface coverage-dependent desorption energies are in reasonable agreement with the experimental results.

Thermal desorption of astrophysically relevant molecules from forsterite(010)

We present temperature programmed desorption (TPD) measurements of CO, CH4, O2 and CO2 from the forsterite(010) surface in the sub-monolayer and multilayer coverage regimes. In the case of CO, CH4 and O2, multilayer growth begins prior to saturation of the monolayer peak, resulting in two clearly distinguishable desorption peaks. On the other hand, a single peak for CO2 is observed which shifts from high temperature at low coverage to low temperature at high coverages, sharpening upon multilayer formation. The leading edges are aligned for all the molecules in the multilayer coverage regime indicating zero order desorption. We have extracted multilayer desorption energies for these molecules using an Arrhenius analysis. For sub-monolayer coverages, we observe an extended desorption tail to higher temperature. Inversion analysis has been used to extract the coverage dependent desorption energies in the sub-monolayer coverage regime, from which we obtain the desorption energy distribution. We found that owing to the presence of multiple adsorption energy sites on the crystalline surface the typical desorption energies of these small molecules are significantly larger than obtained in previous measurements for several other substrates. Therefore molecules bound to crystalline silicate surfaces may remain locked in the solid state for a longer period of time before desorption into the gas phase.

Desorption Kinetics of Ar, Kr, Xe, N2, O2, CO, Methane, Ethane, and Propane from Graphene and Amorphous Solid Water Surfaces.

The desorption kinetics for Ar, Kr, Xe, N2, O2, CO, methane, ethane, and propane from graphene-covered Pt(111) and amorphous solid water (ASW) surfaces are investigated using temperature-programmed desorption (TPD). The TPD spectra for all of the adsorbates from graphene have well-resolved first, second, third, and multilayer desorption peaks. The alignment of the leading edges is consistent the zero-order desorption for all of the adsorbates. An Arrhenius analysis is used to obtain desorption energies and prefactors for desorption from graphene for all of the adsorbates. In contrast, the leading desorption edges for the adsorbates from ASW do not align (for coverages < 2 ML). The nonalignment of TPD leading edges suggests that there are multiple desorption binding sites on the ASW surface. Inversion analysis is used to obtain the coverage dependent desorption energies and prefactors for desorption from ASW for all of the adsorbates.

Thermal desorption of ammonia from crystalline forsterite surfaces

The thermal desorption of ammonia (NH$_3$) from single crystal forsterite (010) has been investigated using temperature-programmed desorption. The effect of defects on the desorption process has been probed by the use of a rough cut forsterite surface prepared from the cleaved forsterite sample. Several approaches have been used to extract the desorption energy and pre-exponential factor describing the desorption kinetics. In the sub-monolayer coverage regime, the NH$_3$ desorption shows a broad distribution of desorption energies, indicating the presence of different adsorption sites, which results in an apparent coverage-dependent desorption energy. This distribution is sensitive to the surface roughness with the cut forsterite surface displaying a significantly broader distribution of desorption energies compared to the cleaved forsterite surface. The cut forsterite surface exhibits sites with desorption energies up to 62.5 kJ mol$^{-1} $ in comparison to a desorption energy of up to 58.0 kJ mol$^{-1} $ for the cleaved surface. Multilayer desorption is independent of the nature of the forsterite surface used, with a desorption energy of ($25.8\pm0.9$) kJ mol$^{-1} $. On astrophysically relevant heating time-scales, the presence of a coverage dependent desorption energy distribution results in a lengthening of the NH$_3$ desorption time-scale by $5.9\times 10^4$ yr compared to that expected for a single desorption energy. In addition, the presence of a larger number of high-energy adsorption sites on the rougher cut forsterite surface leads to a further lengthening of ca. 7000 yr.

Microkinetic Simulation of Temperature-Programmed Desorption

The temperature-programmed desorption (TPD) spectra were simulated by combining density functional theory (DFT) calculations and microkinetic modeling. In the microkinetic analyses, all kinetic and thermodynamic parameters were obtained from DFT calculations to minimize artificial assumptions. For the case study, the desorptions of NH3 and H2O from the RuO2(110) surface were simulated. The coverage-dependent desorption energies were introduced into the microkinetic model because different adsorbates on the surface will exhibit different desorption behaviors. In addition, temperature-dependent pre-exponential factors were applied to the desorption rate equations. The calculated pre-exponential factors are ranged from 1014 to 1017 s–1, which are greatly larger than 1013 s–1, a generally accepted empirical value for desorption processes. The desorption temperatures obtained from microkinetic simulations consist with experimental results, and the simulated TPD patterns are also similar to the experimental observations.

Surface Chemistry of Methanol on Clean and Atomic Oxygen Pre-Covered Au(111)

Desorption kinetics of methanol on Au(111) and the oxidation of methanol on atomic oxygen pre-covered Au(111) have been investigated employing temperature programmed desorption (TPD) and molecular beam reactive scattering (MBRS). On clean Au(111), methanol is weakly adsorbed and desorbs molecularly. Three adsorption states, which are assigned to methanol molecules desorbing from the monolayer (β phase), amorphous multilayers (α2 phase), and crystallized multilayers (α1 phase), are observed. For the β peak, detailed numerical analysis, based on mathematical inversion of the Polanyi-Wigner equation, leads to an optimized pre-exponential factor (log ν = 11.3 ± 1.1 s-1) and coverage-dependent desorption energies, which are further used to accurately simulate TPD spectra for submonolayer initial methanol coverages. For methanol coverages of 0.2 and 1.0 ML, desorption energies of 40.6 ± 1.6 kJ/mol and 34.8 ± 1.4 kJ/mol are extracted. In the presence of atomic oxygen, TPD of adsorbed methanol exhibits desorption of only CH3OH, H2O, CO, and CO2. No other partial oxidation products or derivatives such as hydrogen, formaldehyde, formic acid, or methyl formate are detected during TPD or MBRS measurements. On the basis of our experimental results and previous similar studies on other coinage metals, we suggest that abstraction of hydrogen is the initial step in the surface decomposition of methanol and that the adsorbed/surface bound methoxy group [CH3O(a)] is the primary intermediate in the oxidation of methanol.

N -alkanes on MgO(100). II. Chain length dependence of kinetic desorption parameters for small n-alkanes

Coverage-dependent desorption-kinetics parameters are obtained from high-quality temperature-programmed desorption data for seven small n-alkane molecules on MgO(100). The molecules, CNH2N+2 (N=1-4,6,8,10), were each studied for a set of 29 initial coverages at a heating ramp rate of 0.6 K/s as well as at a set of nine ramp rates in the range of 0.3-10.0 K/s. The inversion analysis method with its least-squares preexponential factor (prefactor) optimization discussed in the accompanying article is applied to these data. This method allows for accurate determination of prefactors and coverage-dependent desorption energies. The prefactor for desorption increases dramatically with chain length from 10(13.1) to 10(19.1) s(-1) over the range of N=1-10. We show that this increase can be physically justified by considering the increase in rotational entropy available to the molecules in the gaslike transition state for desorption. The desorption energy increases with chain length as Ed(N)=6.5+7.1N, which implies an incremental increase of 7.1+/-0.2 kJ/mol per CH2.

N -alkanes on MgO(100). I. Coverage-dependent desorption kinetics of n-butane

High-quality temperature-programmed desorption (TPD) measurements of n-butane from MgO(100) have been made for a large number of initial butane coverages (0-3.70 ML, ML-monolayers) and a wide range of heating ramp rates (0.3-10 K/s). We present a TPD analysis technique which allows the coverage-dependent desorption energy to be accurately determined by mathematical inversion of a TPD spectrum, assuming only that the preexponential factor (prefactor) is coverage independent. A variational method is used to determine the prefactor that minimizes the difference between a set of simulated TPD spectra and corresponding experimental data. The best fit for butane desorption from MgO is obtained with a prefactor of 10(15.7+/-1.6) s(-1). The desorption energy is 34.9+/-3.4 kJ/mol at 0.5-ML coverage, and varies with coverage approximately as Ed(theta)=34.5+0.566theta+8.37 exp(-theta/0.101). Simulations based on these results can accurately reproduce TPD experiments for submonolayer initial coverages over a wide range of heating ramp rates (0.3-10 K/s). Advantages and limitations of this method are discussed.

A thermal desorption study of iodine on Pt( [formula omitted

Thermal desorption spectra of iodine atoms adsorbed on Pt(1 1 1) are reported for coverages θ up to 0.43 monolayers (ML), the saturation coverage at 300 K. Analysis of the spectra as a function of heating rate and initial coverage shows that for θ<0.28 ML, desorption is first order with a coverage-dependent desorption energy. These data are fitted quantitatively to a Bragg–Williams statistical–mechanical transition-state model that includes the effect of vibrations of the adsorbed iodine layer according to the Debye theory. Measurements of the temperature variation of low-energy electron diffraction beam intensities gave surface Debye temperatures of 105±15 K for the (√3×√3) R30° phase and 155±15 K for the (√7×√7) R19.1° phase. Lateral repulsions between iodine atoms are 10 kJ mol −1 for θ<0.28 ML. A feature at θ=0.28 ML is interpreted in the hard-hexagon model as a phase transition between 2-D gas and 2-D solid phases. Another such phase transition occurs at θ>0.38 ML, and explains the zero-order desorption peak observed at these coverages. Thermodynamic quantities were estimated from the data, including Δ des H o=219±20 kJ mol −1 for (√3×√3) R30°-I→I(g) and Δ des H o=201±20 kJ mol −1 for (√7×√7) R19.1°-I→I(g).

Thermal Desorption of Au from W(001) Surface

Adsorption of Au on W(001) at 450 K up to multilayer structures was investigated. Temperature programmed desorption technique was used in determination of coverage dependent desorption energy (region up to one monolayer).Results were discussed in terms of competitive interactions of Au-Au and Au-W atoms. Simple procedure for prediction of faceting behavior on the interface, basing on the desorption data, was postulated. It was deduced that the Au/W(001) interface should not show faceting tendency after thermal treatment.

Adsorption and desorption kinetics of tetrakis(dimethylamino)titanium and dimethylamine on TiN surfaces

Titanium nitride (TiN) can be deposited using the organometallic precursor tetrakis(dimethylamino)titanium (Ti[N(CH 3) 2] 4) (TDMAT). Deviations from conformal TiN film growth have been observed in trench structures using TDMAT. This nonconformal deposition may be associated with readsorption and site blocking by the dimethylamine (HN(CH 3) 2) (DMA) reaction product. To understand the deviations from conformal TiN deposition in trench structures, the adsorption and desorption kinetics for TDMAT and DMA were measured on a sputter-deposited TiN surface using laser induced thermal desorption (LITD) techniques. The LITD measurements revealed that DMA has a higher sticking coefficient than TDMAT. The sticking coefficients for both TDMAT and DMA were also dependent on surface coverage. The initial sticking coefficient for TDMAT is S 0=0.23 with a coverage-dependence approximated by S( θ)=0.25 exp(−4.7 θ) where θ is the normalized surface coverage. The initial sticking coefficient for DMA is S 0=0.70 and the coverage-dependence is approximated by S( θ)=0.86 exp(−3.7 θ). The observed desorption kinetics of DMA following TDMAT and DMA exposures on TiN were also coverage-dependent. The isothermal desorption measurements could be fit using a simple first-order desorption rate expression, k d= ν dexp[− E d( θ)/ RT] where E( θ) is the coverage-dependent desorption energy, E d( θ)= E 0− E 1 θ. Assuming a desorption preexponential of ν d=1×10 13 s −1, the observed isothermal desorption measurements for DMA desorption following both TDMAT and DMA exposures could be fit using E 0=29.1 kcal mol −1 and E 1=8.2 kcal mol −1. These measured adsorption and desorption kinetics are consistent with DMA readsorption as a major contributor to nonconformal TiN growth in trench structures using TDMAT.

Model of lateral forces and compensation effect in TDS spectra analysis

A model of a coverage-dependent desorption energy, E d( Θ), and a preexponential factor, v( Θ), is proposed in which lateral forces and a compensation effect are taken into account. Two parameters are applied: a coverage-dependent energy E lat( Θ) of lateral forces and a constant T c which describes the relation between v and E d. Together with the Polanyi-Wigner equation, formulas for E lat( Θ) and v( E d) as taken from literature allow to obtain a modified Arrhenius plot. With values of E lat and T c properly chosen, the plot makes again a straight line with parameters which correspond to zero-coverage values of desorption energy and preexponential factor. As the analysis is based on the application of two mutually independent parameters ( E lat, T c ) the ambiguity of results can occur. It can be reduced if a set of TDS spectra with different initial values of coverage are analyzed. The model has been applied to a set of CO desorption spectra from small Pd supported particles as well as from a Pd single crystal surface.

Desorption energies averaged over the desorption process by model calculations

Model calculations are used to investigate the relation between the coverage-dependent desorption energies for the atoms of A 2B 6 compounds and the desorption energies averaged over the desorption process. The desorption energy of B atoms is assumed to decrease linearly with increasing surface coverage of B atoms. The desorption energy of A atoms is assumed to be independent of the coverage. Variation of the surface coverage and the time necessary to obtain the equilibrium coverage after deposition of one element are calculated. The desorption energies averaged over the desorption process are determined. The calculations show that there is almost no difference between the averaged desorption energies of different elements. The desorption energy averaged over the desorption process is independent of the initial surface coverage.

Effects of K, O, and H adatoms on the adsorption kinetics of CO on Pt(111)

The sticking coefficient, S, of CO on clean and K-, O- and H-modified Pt(111) surfaces has been measured directly using a simple kinetic uptake method, as a function of CO and modifier coverage and surface temperature. A simple method is also introduced to determine the coverage-dependent desorption energy from these measurements without the need to know the pre-exponential factor or desorption order for those systems where S is only a function of coverage or only weakly dependent on temperature. The initial sticking coefficient of CO on clean Pt(111) is 0.9 and is independent of substrate temperature. The major influence of adsorbed K on the initial sticking coefficient of CO on Pt(111) at 320 K is site blocking, and the magnitude of this effect correlates well with the size of the adsorbed K ion or atom. The shape of S as a function of CO coverage for low K coverages in the ionic regime is similar to that of CO on clean Pt(111). A dramatic change occurs for K coverages near or greater than 0.24 ML, where S passes through a maximum at about case1 3 of the saturation CO coverage and coincident CO and K desorption is observed. This behavior of S, which has not been seen before, can be fit quite adequately by a simple model that incorporates the reduced site blocking effects of K due to the CO-induced transition of K from a metallic neutral to an ionic state and the accompanying size change of K atoms. These results are best described by a coverage-dependent alkali-CO interaction. In additional studies, we find that adsorption of 0.25 ML O on Pt(111) has no effect on S for low CO coverages at 100 K, and only the saturation CO coverage is reduced from 0.59 to 0.31 ML. In contrast, adsorbed H reduces not only the CO saturation coverage but also S even at low CO coverages.

Adsorption of chlorine on the Pt(100) face: structural aspects and desorption kinetics

The interaction of chlorine with the Pt(100) single crystal face has been studied using LEED and thermal desorption mass spectrometry. Cl 2 molecules adsorb readily on the initially clean reconstructed Pt(100)-hex surface to form a chemisorbed adlayer. Adsorption of chlorine effects the removal of surface reconstruction which is completed at about 4 × 10 14 Cl atoms/cm 2. At higher concentrations a c(2 × 2) structure is formed. Beyond saturation of the c(2 × 2) structure the rate of further chlorine adsorption is largely diminuished. Eventually a (2 × 3) structure develops after heating to 500 K., corresponding to the limiting observed chlorine to platinum surface atom ratio of 2 3 . In this stage the adlayer is interpreted to consist of a distorted hexagonal arrangement of adsorbed chlorine atoms, which are in part out of registry with the substrate. Temperature programmed thermal desorption is observable from 580 to 1100 K and yields exclusively Cl atoms and Cl 2 molecules. The desorption kinetics is influenced by the coverage dependent desorption energy and by the substrate reconstruction taking place during desorption. A kinetic model encompassing surface diffusion of Cl atoms, substrate reconstruction and atomic and molecular desorption was proven by calculating desorption traces on the basis of a lattice gas model with lateral repulsive interactions using Monte Carlo simulations as well as an analytical treatment.

Adsorption and growth modes of Bi on Pt(111)

The vapor deposition of Bi on Pt(111) at 110 and ∼600 K have been characterized by Auger electron spectroscopy (AES), thermal desorption mass spectroscopy (TDMS), low-energy electron diffraction (LEED), and changes in the work function (Δφ). At 110 K Bi growth follows a layer-by-layer mechanism. At ∼600 K Bi fills the first monolayer (θBi≂0.56) relatively uniformly, followed by 3D island growth. Bi desorption is characterized by a large, coverage-dependent desorption energy, Edes =(81−34.2 θBi ) kcal mol−1, in the first monolayer, and zero-order kinetics with constant activation energy (Edes =53–56 kcal mol−1) for the multilayer. Many LEED patterns are observed within the first monolayer for both cold and hot substrates. Structural models for these are proposed which are consistent with coverages obtained by AES. Annealed structures show continuous compression of hexagonal Bi overlayers with increasing coverage, subject to mild substrate constraints. At 110 K and θBi &amp;gt;0.33, uniaxial compression is instead seen, due to an unsurmounted energy barrier. Weakly repulsive lateral Bi–Bi interactions (due to dipole repulsions) dominate submonolayer growth. These results for the semimetal Bi are intermediate in behavior between alkali and transition metal overlayers on Pt(111). This is consistent with the relative strengths of the surface dipole of these adsorbed metals.

Flash desorption activation energies: DCOOH decomposition and CO desorption from Ni (110)

Accurate values of activation energies were measured by flash desorption methods without assumptions about preexponential factors, reaction orders or specific reaction mechanisms. The activation energies were determined by two methods; one method employed a relationship for the shift in peak temperature with change in heating rate, and the other utilized the change in peak amplitude with shift in peak temperature for different heating rates. Agreement between the two methods was excellent. A series of flash curves at different heating rates were obtained for the CO 2 and CO products from DCOOH flash decomposition following adsorption on Ni (110) at 37°C. Adsorbed DCOOH decomposed autocatalytically with an activation energy of 26.6 kcal/mol to form CO 2 and D 2. Carbon monoxide formation from DCOOH decomposition, which corresponded identically to CO desorption from this surface, showed a first order activation energy of 32.7 kcal/mol; this activation energy was used to fit a series of CO flash desorption curves obtained for CO adsorption at −55°C. The preexponential factor was found to be 8.5 × 10 15 s −1. The desorption was first order with a slightly coverage dependent desorption energy. In addition the CO flash curves showed additional binding states at coverages at which changes in isosteric heats of adsorption have been observed. The results illustrate the sensitivity of flash desorption for the determination of binding energies over a wide range of coverages.

Abstract: Flash desorption activation energies: DCOOH decomposition and CO desorption from Ni(110)

Accurate values of activation energies were measured by flash desorption methods without assumptions about preexponential factors, reaction orders, or specific reaction mechanisms. The activation energies were determined by two methods; one method employed a new relationship for the shift in peak temperature with change in heating rate, and the other utilized the change in peak amplitude with shift in peak temperature for different heating rates. Agreement between the two methods was excellent. A series of flash curves at different heating rates were obtained for the CO2 and CO products from DCOOH flash decomposition following adsorption on Ni(110) at 37°C. Adsorbed DCOOH decomposed autocatalytically with an activation energy of 26.6 kcal/mol to form CO2 and D2. Carbon monoxide formation from DCOOH decomposition, which corresponded identically to CO desorption from this surface, showed a first order activation energy of 32.7 kcal/mol; this activation energy was used to fit a series of CO flash desorption curves obtained for CO adsorption at −55°C. The preexponential factor was found to be 8.5×1015 sec−1. The desorption was first order with a coverage dependent desorption energy. The results illustrate the sensitivity of flash desorption for the determination of binding energies over a wide range of coverages.