Combination of temperature-programmed thermal desorption and laser-induced thermal desorption

The combination of temperature-programmed thermal desorption with heating rates of the order of 1 K s^–1 and laser-induced thermal desorption with of the order of 10¹⁰ K s^–1 has proved to be a suitable method to determine both the frequency factor v and the activation energy E _d of desorption. The systems Ni(110)/N₂ and Ni(110)/CO serve as examples.     

Intermittent temperature-programmed desorption study of perovskites used for catalytic purposes

A differential desorption technique, called intermittent temperature-programmed desorption (ITPD), was used to give new insights into the properties of La_1−xSr_xCo_0.8Fe_0.2O₃ perovskites as a contribution to improve their performances with respect to various important application fields such as catalysis, electrocatalysis and solid oxide fuel cells (SOFC). Both ITPD and interrupted TPD (carried out at different heating rates) evidenced two distinct oxygen adsorbed states, desorbing at temperatures lower than 400 °C, corresponding to less than 5% of a compact monolayer of oxide ions. The first one, for low desorption temperatures (lower than 290 °C) exhibits a heat of adsorption (ΔH) distribution from 101 to 121 kJ mol⁻¹. The second one, for higher desorption temperatures (between 290 and 400 °C) corresponds to ΔH = 146 ± 4 kJ mol⁻¹. Additionally, for temperatures higher than 400 °C, we observed a continuous desorption of oxygen species, probably originating from the sub-surface or semi-bulk, with an associated activation energy of desorption ≥175 kJ mol⁻¹.     

Qualitative and quantitative analysis of complex temperature-programmed desorption data by multivariate curve resolution

The substantial amount of information carried in temperature-programmed desorption (TPD) experiments is often difficult to mine due to the occurrence of competing reaction pathways that produce compounds with similar mass spectrometric features. Multivariate curve resolution (MCR) is introduced as a tool capable of overcoming this problem by mathematically detecting spectral variations and correlations between several m/z traces, which is later translated into the extraction of the cracking pattern and the desorption profile for each desorbate. Different from the elegant (though complex) methods currently available to analyze TPD data, MCR analysis is applicable even when no information regarding the specific surface reaction/desorption process or the nature of the desorbing species is available. However, when available, any information can be used as constraints that guide the outcome, increasing the accuracy of the resolution. This approach is especially valuable when the compounds desorbing are different from what would be expected based on a chemical intuition, when the cracking pattern of the model test compound is difficult or impossible to obtain (because it could be unstable or very rare), and when knowing major components desorbing from the surface could in more traditional methods actually bias the quantification of minor components. The enhanced level of understanding of thermal processes achieved through MCR analysis is demonstrated by analyzing three phenomena: i) the cryogenic desorption of vinyltrimethylsilane from silicon, an introductory system where the known multilayer and monolayer components are resolved; ii) acrolein hydrogenation on a bimetallic Pt–Ni–Pt catalyst, where a rapid identification of hydrogenated products as well as other desorbing species is achieved, and iii) the thermal reaction of Ti[N(CH₃)₂]₄ on Si(100), where the products of surface decomposition are identified and an estimation of the surface composition after the thermal reaction is afforded. Since this work constitutes, to the best of our knowledge, the first effort to introduce multivariate analysis to TPD data, the procedures, algorithms and strategies employed are described in full detail.     

Evaluation of flash desorption spectra

It is shown that desorption energies may be extracted from thermal desorption spectra without any assumption on the preexponential, ν, regardless of coverage. This allows a determination of ν, at least at low coverages. Experimental TD spectra of CO adsorbed at $\text{Co(10}\text{1}\text{0)}$ are evaluated to illustrate the applicability of the procedure.     

Analysis of complex thermal desorption spectra: PTCDA on copper

Organic molecules often show very complex thermal desorption spectra. If there is an ordered structure the activation energy for desorption E_des will decrease within the first few layers because of the decreasing van der Waals interaction between molecules and substrate. This is specially true for the system 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) on Cu(1 1 1). The thermal desorption spectra for this particular system consist of three different signals which can be attributed to the second layer, the multilayer and a phase consisting of nanocrystals. We did not see any desorption from the first layer of PTCDA on Cu(1 1 1). In the first part of this paper, we will outline a numerical algorithm to evaluate the spectra with respect to the desorption energy of the second layer () and the multilayer (). In agreement with the transition state theory, we found a pre-exponential factor of . Furthermore, we will show that nanocrystals have a contribution to the thermal desorption spectra different from the one of the multilayer. By evaluation of the third, high temperature signal it is possible to get parameters which describe the distribution of the nanocrystals and therefore gain further understanding of their growth.     

Adsorption and desorption of dibenzothiophene on Ag-titania studied by the complementary temperature-programmed XPS and ESR

Adsorption, desorption and structure of the surface chemical compounds formed upon interaction of dibenzothiophene (DBT) in solution of n-octane with the sulfur-selective Ag/Titania sorbent for the ultradeep desulfurization of liquid fuels was characterized by the temperature-programmed X-ray photoemission spectroscopy (XPS) and Electron Spin Resonance. Adsorption of DBT proceeds via chemisorption via the oxygen-containing surface groups. Desorption of DBT and thermal regeneration of the “spent” Ag/Titania were studied by the complementary temperature-programmed XPS and ESR from 25 °C to 525 °C, in the high vacuum vs. air. The XPS spectrum of the pure DBT is reported for the first time.     

Surface characterization of oxygen-functionalized multi-walled carbon nanotubes by high-resolution X-ray photoelectron spectroscopy and temperature-programmed desorption

Multi-walled carbon nanotubes were exposed either to nitric acid or to an oxygen plasma to synthesize oxygen-containing functional groups which were characterized by high-resolution X-ray photoelectron spectroscopy (XPS). The C 1s spectra revealed that the treatment with nitric acid mainly resulted in the formation of carboxylic (COOR) and phenolic (COR) groups, whereas the plasma treatment led to a higher amount of carbonyl (CO) groups. Furthermore, the nitric acid treatment yielded a 60% higher surface oxygen concentration compared to the plasma treatment, and created a minor amount of nitrogen-containing functional groups. Thus, the nitric acid treatment was found to be more effective in creating acidic functional groups. The presence and the thermal stability of these groups was also investigated by temperature-programmed desorption (TPD). The release of carbon dioxide was detected at about 350 and 450 °C, indicating the decomposition of COOR groups. The CO groups were more stable decomposing even above 600 °C. In addition, ammonia was adsorbed as probe molecule followed by TPD to derive the amount and the acidity of the carboxylic and phenolic groups.     

Energy and angle dependence of desorption spectra

We present a simple microscopic model that qualitatively explains recent experimental desorption spectra. The key features of the model are an activation barrier and frictional forces.     

P-branch emission spectra of the A~1π-X~1Σ~+ system of IrN

The P-branch emission spectra of (4,1) and (3,1) bands of the A～1π-X～1Σ～+ system of IrN molecule are studied using an analytical formula which is derived from elementary expression of molecular total energy by taking multiple spectral differences.It not only reproduces the known experimental transition lines,but also predicts the unknown spectral lines up to J=80 for each band by using a group of fifteen known experimental transition lines.     

Nonequilibrium thermal desorption spectra of dissociatively adsorbed molecules

Thermal desorption spectra of dissociatively adsorbed molecules with a low surface mobility and with lateral interaction between adspecies are studied. A method for the solution of a system of master equations for a nonequilibrium system is developed. It is shown that the model reproduces qualitative features of the thermal desorption spectra of CO/W(100).     

Structure of the Si(011)-(16 × 2) surface and hydrogen desorption kinetics investigated using temperature-programmed desorption

D₂ temperature-programmed desorption (TPD) was used to probe the structure of the Si(011)-(16 × 2) surface. Deuterium was adsorbed at 200°C to coverages θ_D ranging up to complete saturation (approximately 1.1 ML) and the sample heated at 5°C s⁻¹. TPD spectra exhibited three second-order desorption peaks labelled β₂, β*₁ and β₁ centered at 430, 520 and 550°C. Of the proposed models for the Si(011)-(16 × 2) reconstruction, the present TPD results as a function of θ_D provide support for the adatom/dimer model with the β₂ peak assigned to D₂ desorption from the dihydride phase, while the β*₁ and β₁ peaks arise from adatom and surface-atom monohydride phases.     

纳米NaY1-x-yF4：Yby3+, Tmx3+材料的上转换光学特性

用水热法合成了NaY_1-x-yF₄:Yb_y³⁺,Tm_x³⁺上转换纳米粒子材料。实验发现,样品的粒径多数在100nm以内;当加热温度180℃、反应时间120min、稀土离子Tm³⁺的摩尔分数为0.04%时,所制备的纳米材料在980nm半导体激光激发下,其主要上转换发光的辐射峰值位于476.54nm处。     

Manifestation of oxygen desorption in photoluminescence spectra of ZnO

The influence of oxygen desorption on the photoluminescence of zinc oxide has been investigated. Photodesorption and thermal desorption of oxygen have been controlled using mass spectrometry. It has been found that the removal of oxygen from the ZnO surface leads to a manifold increase in the exciton photoluminescence intensity. The process has a reversible character. The mechanism of increase in the exciton luminescence intensity of ZnO during the oxygen desorption has been discussed.     

Time of flight spectra in thermal desorption of helium

Time of flight spectra are calculated within the cascade model of phonon-mediated desorption for helium desorbing from graphite, sapphire and Nichrome. One and three-dimensional theories are compared. Deviations from Knudsen's cosine law are traced to dynamical effects. The spectra are — as a function of time — narrower than a Maxwellian distribution and their maxima are generally shifted to shorter times indicating that the desorbed molecules are hotter than the solid temperature. Relevant experiments are discussed. Experimental geometries are analysed.     

The effect of three-body interactions on thermal desorption spectra

Thermal desorption spectra are calculated for a one-dimensional chain and for a two-dimensional square lattice using the transfer-matrix technique and Monte Carlo simulations. Lateral interactions of adsorbed particles cause a splitting of spectra. The repulsive three-body interactions are shown to lead to an inequality of the integral intensities of the thermal desorption peaks.     

A simple parametrization of thermal desorption spectra: Y-factor

The parameter Y, the ratio between the full width at half maximum (FWHM) and the temperature at peak maximum (T_p), may be easily obtained from thermal desorption (TD) spectra. In this paper we exploit the functional dependences of Y on the coverage θ, the adsorption energy E(θ), and the pre-exponential factor v(θ) using computer simulations, and demonstrate their usefullness for a quick evaluation of TD spectra. (1) Since Y is independent of E as well as of the applied heating rate it may serve for comparison of spectra from different sources. (2) For a fixed value of v, Y(θ) is a smooth function if E(θ) = E₀(1 + bθ) (Bragg-Williams approximation), which makes it possible to extract b values from the Y/b relationship. (3) If both E and v are constant the simple Y/v correlation yields v values. This relationship may be used even in the case when both kinetic parameters are coverage dependent, but only in the very low coverage regime. (4) Any deviation from the trends of the Y(θ) dependences simulated for various values of b indicates that either the Bragg-Williams approximation is not justified or that both E and v are coverage dependent.     

Simulation of kinetics of water temperature-programmed desorption from n-GaAs(100) and n-GaP(100) semiconductor surfaces

The energy spectra of adsorption centers on the n-GaAs(100) and n-GaP(100) surfaces are studied using temperature-programmed desorption of water. The desorption spectra are analyzed in terms of the model of discrete adsorption centers, which assumes the presence of a loosely bound precursor state. The values of frequency factors and activation energies of desorption are in good agreement with the frequency of electronic transitions and energies of surface electronic states in gallium arsenide and gallium phosphide. It is concluded that the water desorption kinetics is limited by slow electronic processes on the surface of the semiconductors.     

Analysis of broadband dielectric spectra in a Na+ mordenite

The surface properties of a sodic exchanged mordenite has been studied by means of dielectric measurements. The imaginary part of the permittivity is correctly explained by using a model based on the existence of a distribution function of the interaction energies between sodium ions and the zeolitic lattice. Taking into account the sites involved in the mordenite, four elementary distribution functions have been used. The obtained global distribution function has been compared with that resulting from the application of an “ill-posed and inverse method”. A satisfactory coherence is observed between both approaches. Paper presented at the 4th Euroconference on Solid State Ionics, Renvyle, Galway, Ireland, Sept. 13–19, 1997