Temperature-programmed Desorption Spectra Research Articles

Adsorption and desorption of hydrogen on graphene with dimer conversion

We apply non-equilibrium thermodynamics to describe the adsorption and desorption of molecular hydrogen on graphene. Lateral interactions, precursor states in both adsorption and desorption, and limited dimer conversion are important to explain semi-quantitatively the main features of temperature-programmed desorption spectra. All energy and vibrational parameters are taken from density functional calculations. Deficiencies in previous attempts are discussed. We also point out the need for a multi-dimensional dynamic theory.

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.

Adsorption and Thermal Reaction of Short-Chain Alcohols on Ge(100)

The adsorption and thermal decomposition of alcohols (CH3OH, C2H5OH, and C4H9OH) on Ge(100) were investigated with temperature-programmed desorption and X-ray photoelectron spectra. At 105 K, CH3OH adsorbs both molecularly and dissociatively on Ge(100). Chemisorbed CH3OH molecules dissociate to form surface CH3O and hydrogen in a temperature range 150–300 K. Surface CH3O can dehydrogenate to yield CH2O as two desorption features, which depend on coverage. At small coverage, surface CH3O undergoes mainly α-hydrogen elimination to desorb CH2O at 490 K. At large coverage, another desorption of CH2O occurs predominantly at 525 K, which is initiated by a recombinative desorption of CH3OH. A calculation with density functional theory at the B3LYP/6-311+G** level shows that the dissociation of the O–H bond has a much smaller barrier (<40 kJ/mol) than those for C–O bond cleavage (>150 kJ/mol). Desorption of CH2O results from the moderate barriers (∼110 kJ/mol) for cleavage of the C–H bond of surface CH3O and weak adsorption energy of CH2O (−56 kJ/mol). The recombination of surface CH3O with H occurs at large coverage with an energy barrier 127–140 kJ/mol. Similarly to CH3OH, C2H5OH and C4H9OH undergo the mechanism of thermal reactions through formation of alkoxyl intermediates. The longer-chain alkoxyl decomposes to desorb aldehyde at lower temperature because the interaction of its alkoxyl chain with the surface is stronger. On annealing to ∼570 K, all alkoxyl groups are completely removed from the surface via dehydrogenation and recombination to desorb aldehyde and alcohol, respectively. At a large coverage, the longer-chain alkoxyl undergoes dehydrogenation to a larger extent than recombinative desorption.

Deuterium trapping and release in Be(0001), Be(11–20) and polycrystalline beryllium

Abstract The atomistic understanding of retention and release processes of deuterium in beryllium is reached by comparing well-defined experiments on Be(0 0 0 1) and Be(11–20) single crystals, as well as polycrystalline Be to simulations. Temperature programmed desorption spectra are recorded after implantation of D fluences of ∼3 × 10 19 m −2 . The experimental spectra are modelled as a coupled reaction diffusion system (CRDS). The single atomistic steps are described by a set of rate equations. Activation energies for the single processes are calculated from density functional theory. The changes in retained amounts and desorption temperatures for different crystal orientations, implantation depths and temperature ramps were successfully reproduced with the CRDS code. Anisotropic self-interstitial diffusion and fast diffusion along grain boundaries are among the important mechanisms that have to be taken into account in the simulations.

Water desorption from nanostructured graphite surfaces

Water interaction with nanostructured graphite surfaces is strongly dependent on the surface morphology. In this work, temperature programmed desorption (TPD) in combination with quadrupole mass spectrometry (QMS) has been used to study water ice desorption from a nanostructured graphite surface. This model surface was fabricated by hole-mask colloidal lithography (HCL) along with oxygen plasma etching and consists of a rough carbon surface covered by well defined structures of highly oriented pyrolytic graphite (HOPG). The results are compared with those from pristine HOPG and a rough (oxygen plasma etched) carbon surface without graphite nanostructures. The samples were characterized using scanning electron microscopy (SEM) and atomic force microscopy (AFM). The TPD experiments were conducted for H2O coverages obtained after exposures between 0.2 and 55 langmuir (L) and reveal a complex desorption behaviour. The spectra from the nanostructured surface show additional, coverage dependent desorption peaks. They are assigned to water bound in two-dimensional (2D) and three-dimensional (3D) hydrogen-bonded networks, defect-bound water, and to water intercalated into the graphite structures. The intercalation is more pronounced for the nanostructured graphite surface in comparison to HOPG surfaces because of a higher concentration of intersheet openings. From the TPD spectra, the desorption energies for water bound in 2D and 3D (multilayer) networks were determined to be 0.32 ± 0.06 and 0.41 ± 0.03 eV per molecule, respectively. An upper limit for the desorption energy for defect-bound water was estimated to be 1 eV per molecule.

Theoretical and experimental studies of hydrogen adsorption and desorption on Ir surfaces

We report adsorption and desorption of hydrogen on planar Ir(210) and faceted Ir(210), consisting of nanoscale {311} and (110) facets, by means of temperature programmed desorption (TPD) and density functional theory (DFT) in combination with the ab initio atomistic thermodynamics approach. TPD spectra show that only one H2 peak is seen from planar Ir(210) at all coverages whereas a single H2 peak is observed at around 440 K (F1) at fractional monolayer (ML) coverage and an additional H2 peak appears at around 360 K (F2) at 1 ML coverage on faceted Ir(210), implying structure sensitivity in recombination and desorption of hydrogen on faceted Ir(210) versus planar Ir(210), but no evidence is found for size effects in recombination and desorption of hydrogen on faceted Ir(210) for average facet sizes of 5-14 nm. Calculations indicate that H prefers to bind at the two-fold short-bridge sites of the Ir surfaces. In addition, we studied the stability of the Ir surfaces in the presence of hydrogen at different H coverages through surface free energy plots as a function of the chemical potential, which is also converted to a temperature scale. Moreover, the calculations revealed the origin of the two TPD peaks of H2 from faceted Ir(210): F1 from desorption of H2 on {311} facets while F2 from desorption of H2 on (110) facets.

Hydrogen adsorption and desorption at the Pt(110)-(1×2) surface: experimental and theoretical study

The interaction of hydrogen with the Pt(110)-(1×2) surface is studied using temperature programmed desorption (TPD) measurements and density functional theory (DFT) calculations. The ridges in this surface resemble edges between micro-facets of Pt nano-particle catalysts used for hydrogen evolution (HER) and hydrogen oxidation reactions (HOR). The binding energy and activation energy for desorption are found to depend strongly on hydrogen coverage. At low coverage, the strongest binding sites are found to be the low coordination bridge sites at the edge and this is shown to agree well with the He-atom interaction and work function change which have been reported previously. At higher hydrogen coverage, the higher coordination sites on the micro-facet and in the trough get populated. The simulated TPD spectra based on the DFT results are in close agreement with our experimental spectra and provide microscopic interpretation of the three measured peaks. The lowest temperature peak obtained from the surface with highest hydrogen coverage does not correspond to desorption directly from the weakest binding sites, the trough sites, but is due to desorption from the ridge sites, followed by subsequent, thermally activated rearrangement of the H-adatoms. The reason is low catalytic activity of the Pt-atoms at the trough sites and large reduction in the binding energy at the ridge sites at high coverage. The intermediate temperature peak corresponds to desorption from the micro-facet. The highest temperature peak again corresponds to desorption from the ridge sites, giving rise to a re-entrant mechanism for the thermal desorption.

Influence of Step Defects on Methanol Decomposition: Periodic Density Functional Studies on Pd(211) and Kinetic Monte Carlo Simulations

Methanol decomposition on noble metal surfaces is an important industrial process and prototype for understanding heterogeneous catalysis. Despite many advances, the role played by surface defects and structural sensitivity is still not fully understood. In this work, methanol decomposition on a stepped palladium surface, Pd(211), is investigated using periodic density functional theory (DFT). The activation barriers and thermochemistry for relevant elementary steps leading to the final decomposition products CO and H2 are obtained. Similar to the previous theoretical results on flat Pd surfaces, the initial C–H bond scission is preferred on Pd(211) because it has a lower barrier than those for the initial O–H and C–O scissions. It was also found that the barriers for the C–H or O–H bond scissions are lowered at the step sites. Finally, kinetic Monte Carlo simulations on a realistic Pd surface reproduce the temperature-programmed desorption spectrum for methanol decomposition but only when modified DFT da...

Submonolayer sensitive adsorption study of trichloroethene on single crystal surfaces by means of MIES, UPS and TPD

By means of ultraviolet photoelectron spectroscopy (UPS) and metastable impact electron spectroscopy (MIES), combined with temperature programmed desorption (TPD) experiments, the coverage dependent adsorption properties of trichloroethene (TCE) on Mo(100), Mo(112) and Pt(111) support were probed and are compared to findings on MgO(100). Peaks in the PE spectra corresponding to different MOs of the TCE molecule were fitted and their respective energy positions could be determined. Comparison with the reported IP energies of the gaseous TCE shows a uniform shift with coverage (save for small deviations on Pt for the submonolayer regime) of all orbital energies for the adsorbed molecule on each studied surface; evidence of a weak interaction of the molecule with the substrate. Additionally, the relative peak areas of the TCE MOs were investigated and showed signal saturation depending on the probe depth of the respective spectroscopic technique. The acquired TPD spectra are in accordance with weakly interacting TCE, which physisorbs on each substrate and condenses into multilayers without geometric reorganization of the molecule. The presented electron emission (EE) data demonstrates the detection limit of our UPS and MIES setup: At best UPS is sensitive to 0.23ML of TCE (detection of peak energy position and intensity of all five TCE MOs), while the MIES data still monitors all spectral features from TCE at 0.13ML (corresponding 1.1×1014TCEcm−2 for the less sensitive cases). Concerning peak energies only, the detection limit can in the best case be further extended down to 0.02 molecules per adsorption site (equal to θ=13%ML). The achieved (TCE) sensitivity is superior to NEXAFS and UPS data available in the literature (detection limit θ=1ML).

A simple method for CO chemisorption studies under continuous flow: Adsorption and desorption behavior of Pt/Al2O3 catalysts

A simple and rapid method for the determination of metal dispersion of technical catalysts is presented, which is based on temperature-programmed desorption (TPD) of pre-adsorbed CO in a continuous-flow reactor at atmospheric conditions. A commercial Pt/Al2O3 diesel oxidation catalyst is studied as an example. In the TPD spectra, desorption of CO as well as CO2 is considered. Furthermore, a pulse adsorption technique is applied to better understand the adsorption–desorption behavior and the oxidation of CO. Metal dispersion based on the TPD methods presented agrees well with data derived from H2 and CO chemisorption measurements using commercial set-ups.

Reduction of Different GeO2 Polymorphs

A combination of volumetric adsorption, thermal desorption and structure-determining methods was used to study and compare the hydrogen reduction behavior of three different GeO2 polymorphs: tetragonal, water-free hexagonal, and water-containing (hydroxylated) commercial hexagonal GeO2. Marked differences in the onset and extent of reduction between the two water-free polymorphs have been observed. Tetragonal GeO2 adsorbs more hydrogen at temperatures T ≤ 673 K and tends to be more easily reducible at low temperatures, but extended Ge metal formation at elevated temperatures is rather suppressed compared to both hexagonal GeO2 phases. Temperature-programmed hydrogen desorption spectra indicate the reduction-induced formation of weakly bonded hydrogen adsorption sites both on hydroxylated and pure hexagonal GeO2. The existence of the most weakly bonded hydrogen is linked to the transformation of initially present hydroxylated species into a tetragonal structure fraction upon annealing at ∼500 K. Thus, analogous forms of hydrogen were not observed on any of the pure (dehydroxylated) structures.

Reentrant Mechanism for Associative Desorption:H2/Pt(110)−(1×2)

Calculations of the desorption of hydrogen from Pt(110)-(1×2), a surface used to model nanoparticle edge sites, show the activation energy varying strongly with hydrogen coverage, from 0.8 to 0.3eV. The predicted temperature programed desorption spectra agree well with experiments, but the formation of the hydrogen molecules occurs only at two types of sites on the surface even though three peaks are observed. The lowest and highest temperature peaks result from desorption from the same strong binding sites at the ridge, while desorption from the weakest binding trough sites is insignificant.

Thermal Desorption from Heterogeneous Surfaces

When a given gas can adsorb onto a surface with multiple binding energies, temperature-programmed desorption (TPD) spectra may become relatively more complicated to analyze, making more difficult the extraction of energy and kinetic parameters from the experimental results. Here we present results from kinetic Monte Carlo simulations used to investigate gas desorption from external surfaces of carbon nanotube bundles. By varying the initially adsorbed coverage and keeping track of the particles as they desorb and diffuse across the surface, we identify specific features on the TPD spectra originated by the effects of the surface heterogeneity on the desorption kinetics.

Toward Low-Temperature Dehydrogenation Catalysis: Isophorone Adsorbed on Pd(111).

Adsorbate geometry and reaction dynamics play essential roles in catalytic processes at surfaces. Here we present a theoretical and experimental study for a model functional organic/metal interface: isophorone (C9H14O) adsorbed on the Pd(111) surface. Density functional theory calculations with the Perdew-Burke-Ernzerhoff (PBE) functional including van der Waals (vdW) interactions, in combination with infrared spectroscopy and temperature-programmed desorption (TPD) experiments, reveal the reaction pathway between the weakly chemisorbed reactant (C9H14O) and the strongly chemisorbed product (C9H10O), which occurs by the cleavage of four C-H bonds below 250 K. Analysis of the TPD spectrum is consistent with the relatively small magnitude of the activation barrier derived from PBE+vdW calculations, demonstrating the feasibility of low-temperature dehydrogenation.

Oxidation of Platinum in the Epitaxial Model System Pt(111)/YSZ(111): Quantitative Analysis of an Electrochemically Driven PtOxFormation

The oxidation of platinum (Pt) plays a key role in electrochemistry, both in the solid and liquid state, and in surface science. This work provides a comprehensive comparison of current knowledge of Pt oxidation in these three fields. By presenting new data of the solid state epitaxial model-type interface Pt(111)/yttria-stabilized zironia (YSZ(111)), fundamental insights are obtained: (i) analogous features in cyclic voltammograms of the interfaces Pt(111)/acid aqueous electrolyte and Pt(111)/YSZ(111) are correlated and their differences explained. (ii) By comparing the cathodic peak shapes of cyclic voltammograms of Pt/YSZ to temperature-programmed desorption spectra, the peaks can be attributed to an adsorption and an oxide-like state of oxygen. (iii) The linearity between the accumulation of oxygen at the Pt electrode and the logarithm of polarization time for sufficiently high anodic potentials, which is well-known in aqueous electrochemistry, is also found at the interface Pt/YSZ for the first time,...

The effect of oxygen vacancies on the binding interactions of NH3 with rutile TiO2(110)-1 × 1

A series of NH(3) temperature-programmed desorption (TPD) spectra were taken after dosing NH(3) at 70 K on rutile TiO(2)(110)-1 × 1 surfaces with oxygen vacancy (V(O)) concentrations of ~0% (p-TiO(2)) and 5% (r-TiO(2)), respectively, to study the effect of V(O)s on the desorption energy of NH(3) as a function of coverage, θ. Our results show that in the zero coverage limit, the desorption energy of NH(3) on r-TiO(2) is 115 kJ mol(-1), which is 10 kJ mol(-1) less than that on p-TiO(2). The desorption energy from the Ti(4+) sites decreases with increasing θ due to repulsive NH(3)-NH(3) interactions and approaches ~55 kJ mol(-1) upon the saturation of Ti(4+) sites (θ = 1 monolayer, ML) on both p- and r-TiO(2). The absolute monolayer saturation coverage is determined to be about 10% smaller on r-TiO(2) than that on p-TiO(2). Additionally, the trailing edges of the NH(3) TPD spectra on the hydroxylated TiO(2)(110) (h-TiO(2)) appear to be the same as that on r-TiO(2) while those on oxidized TiO(2)(110) (o-TiO(2)) shift to higher temperatures. We present a detailed analysis of the results and reconcile the observed differences based on the repulsive adsorbate-adsorbate dipole interactions between neighboring NH(3) molecules and the surface charge associated with the presence of V(O)s.

Physical Properties of Aliphatic Monolayer on Indium–Tin Oxide and SnO2(110) Relevant to Thermal Stability of Soft-Landed Cr(benzene)2

The physical properties of surface modified indium–tin oxide (ITO) and SnO2(110) with an adsorbed aliphatic chain monolayer were studied by soft-landing experiments coupled with infrared (IR) spectroscopy, temperature programmed desorption (TPD) spectroscopy, and scanning probe microscopy. The IR spectra showed that the aliphatic chains of palmitic acid on both the metal oxide surfaces form a crystalline-phase monolayer with an all-trans conformation at room temperature. Gas-phase synthesized Cr(benzene)2 cations deposited with a hyperthermal energy (20 eV) were used to probe the surface morphology. The IR spectrum after deposition showed that the cations soft-landed after being neutralized and oriented to cant the molecular axes far from the surface normal direction through penetration into the monolayer matrix. The TPD spectrum showed that the desorption activation energy of Cr(benzene)2 from an aliphatic monolayer on ITO is much lower than that from the aliphatic monolayer on SnO2 or a self-assembled m...

Kinetics of Nitric Oxide Adsorption on Pd(111) Surfaces through Molecular Beam Experiments: A Quantitative Study

A detailed kinetic picture derived by molecular beam studies of the adsorption–desorption of the NO/Pd(111) system is presented. Numerical simulations and detailed kinetic analysis show that the precursor state model of adsorption provides a valid picture of the sticking coefficient variation with surface coverage, especially at low temperatures. At higher temperatures, the precursor model gives way to the Langmuir molecular model of adsorption. All the parameters of the precursor state model have been quantified. Temperature programmed desorption (TPD) studies further show that there is a slight repulsive interaction between adsorbed NO molecules and there is only a negligible fraction of dissociated molecules on the surface for temperatures less than 500 K, as the Pd(111) surface is defect free. A Bragg–Williams (BW) lattice gas model with repulsive interactions, within the framework of mean field approach (MFA), is shown to describe the TPD spectra reasonably well.

CO2FORMATION IN QUIESCENT CLOUDS: AN EXPERIMENTAL STUDY OF THE CO + OH PATHWAY

The formation of CO2 in quiescent regions of molecular clouds is not yet fully understood, despite CO2 having an abundance of around 10-34 % H2O. We present a study of the formation of CO2 via the non-energetic route CO + OH on non-porous H2O and amorphous silicate surfaces. Our results are in the form of temperature-programmed desorption spectra of CO2 produced via two experimental routes: O2 + CO + H and O3 + CO + H. The maximum yield of CO2 is around 8 % with respect to the starting quantity of CO, suggesting a barrier to CO + OH. The rate of reaction, based on modelling results, is 24 times slower than O2 + H. Our model suggests that competition between CO2 formation via CO + OH and other surface reactions of OH is a key factor in the low yields of CO2 obtained experimentally, with relative reaction rates k(CO+H) \ll k(CO+OH) < k(H2O2+H) < k(OH+H), k(O2+H). Astrophysically, the presence of CO2 in low AV regions of molecular clouds could be explained by the reaction CO + OH occurring concurrently with the formation of H2O via the route OH + H.

Surface reactions of TiCl4 and Al(CH3)3 on GaAs(100) during the first half-cycle of atomic layer deposition

GaAs(100) was exposed to pulses of trimethylaluminum (TMA, Al(CH3)3) and titanium tetrachloride (TiCl4) to mimic the first half-cycle of atomic layer deposition (ALD). Both precursors removed the 9.0±1.6Å-thick mixed oxide consisting primarily of As2O3 with a small Ga2O component that was left on the surface after aqueous HF treatment and vacuum annealing. In its place, TMA deposited an Al2O3 layer, but TiCl4 exposure left Cl atoms adsorbed to an elemental As layer. This suggests that oxygen was removed by the formation of a volatile oxychloride species. A small TiO2 coverage of approximately 0.04 monolayer remained on the surface for deposition temperatures of 89°C to 135°C, but no TiO2 was present from 170°C to 230°C. The adsorbed Cl layer chemically passivated the surface at these temperatures and blocked TiO2 deposition even after 50 full ALD cycles of TiCl4 and water vapor. The Cl and As layers desorbed simultaneously at higher temperature producing peaks in the temperature programmed desorption spectrum in the range 237–297°C. This allowed TiO2 deposition at 300°C in single TiCl4 pulse experiments. On the native oxide-covered surface where there was a higher proportional Ga oxide composition, TiCl4 exposure deposited TiO2.