Temperature-programmed Desorption Mass Spectrometry Research Articles

Surface chemistry of alkylsilanes on Si(100)2 × 1

The adsorption and thermal decomposition of ethylsilane, diethylsilane, triethylsilane, tetraethylsilane, dimethylsilane and di-isobutylsilane were studied on the Si(100)2 × 1 reconstructed surface using temperature-programmed desorption mass spectrometry and high-resolution electron energy-loss spectroscopy. Ethylsilane, diethylsilane and di-isobutylsilane each adsorbs dissociatively onto the surface up to a saturation coverage of approximately 0.25 ML. Thermal decomposition proceeds via β-hydride elimination. Ethylene and hydrogen desorption occurs at 700 and 800 K, respectively, for ethylsilane and diethylsilane. Isobutylene desorption is observed at 635 K, followed by hydrogen desorption at 800 K for di-isobutylsilane. Dimethylsilane adsorbs dissociatively up to a saturation coverage of 0.25 ML. Thermal decomposition proceeds via dehydrogenation of the methyl groups. Hydrogen desorption is observed at 800 K for low coverages with a 5–10 K peak shift to higher temperatures for higher coverages. Triethylsilane adsorbs dissociatively, but saturation coverage could not be established in these experiments. Thermal decomposition proceeds via β-hydride elimination, resulting in the desorption of ethylene at 700 K and hydrogen at 800 K. Adsorption of tetraethylsilane is not observed. For every precursor, except tetraethylsilane, hydrogen adsorbs exclusively as the (2 × 1) monohydride. No adsorption of any of these precursors on the native SiO 2 surface is observed at 100 K.

Synthesis and characterization of ZrO2 as an acid–base catalyst Dehydration–dehydrogenation of propan-2-ol

Two ZrO 2 -based catalysts have been synthesized by thermal treatment of a gel prepared from ZrOCl 2 ·8H 2 O. The effect of different preparation parameters on the final properties of the solids was studied. Textural characterization was carried out from nitrogen adsorption–desorption isotherms. XRD and FT-Raman spectroscopy have been used to elucidate the physical state of the catalysts. Their acid–base properties were studied by using temperature-programmed desorption–mass spectrometry (TPD–MS) and diffuse reflectance FTIR techniques (DRIFT), employing pyridine, 2,6-dimethylpyridine and carbon dioxide as probe molecules. Both solids were tested for the dehydration–dehydrogenation of propan-2-ol. The reactivity of the catalysts was studied through isotherm and temperature–programmed surface reactions (TPSR), in a flow reactor. In addition, temperature–programmed oxidation (TPO) and temperature–programmed desorption (TPD) experiments revealed that catalyst deactivation was caused by carbon deposition and irreversible adsorption of propene formed on the catalytic surface.

NO-Intercalation Properties of a Double-Layered Cuprate, La2-xBaxSrCu2O6. Dissociative Desorption of Intercalated NO

The desorption process of nitric monoxide intercalated into the double layered cuprate, La1.4Ba0.6SrCu2O6, was studied by temperature programmed desorption-mass spectrometry (TPD-MS). The desorption profile consisted of NO (<600 °C), O2 (>600 °C) and N2 (>800 °C) peaks, the latter two implying dissociative desorption of the intercalated NO. Qualitative relations between NO uptake and each desorption suggests the presence of two different NO sites in the layer structure. One is the site bound to Ba, where NO is converted to NO2- ions, and produces a reversible NO desorption on heating (<620 °C). Another site is ascribable to an anion vacancy in the CuO5 pyramid layer, being occupied by NO strongly bonding to Cu. Instead of this NO ligand, lattice oxygens are eliminated as O2 above 600 °C. A further increase of temperature results in cleavage of the N−O bond and subsequent elimination of N2 while leaving the NO oxygen in the lattice. The dissociative NO desorption proceeds via the oxygen-exchange between the intercalated NO and the cuprate lattice.

Temperature-Programmed Desorption Mass Spectrometry (TPDMS) of Dispersed Oxides

Temperature-programmed desorption mass spectrometry (TPDMS) is discussed with reference to studies of modified pyrogenic oxides. Simple theoretical models are presented in order to allow an approximate analytical treatment of TPDMS data. The ‘unimolecular’ decomposition of alkoxide groups was studied. The rate constants of the reactions were derived from TPDMS measurements. The kinetics of thermal transformations of phosphorous and phosphoric acids on a pyrogenic silica surface were investigated by TPDMS. Oxidation–reduction reactions in samples containing phosphorous acid resulted in the removal of molecular phosphorus (P4). Two stages of phosphorus evolution were observed which were attributed to the disproportion of phosphorus in the surface monolayer (high-temperature stage) and in the bulk layers (low-temperature stage). The biologically active samples obtained by adsorption of cyanocobalamine, glucose and raffinose on ultrafine oxide surfaces were studied via a combination of temperature-programmed desorption and solid-state thermal analysis mass spectrometry (TPD/TA MS). The role of the charged cobalt atom and of the propionamide and benzimide groups in the adsorption mechanism is discussed. It was shown that the adsorption of cyanocobalamine on the silica surface transformed the propionamide group to a weakly bound form. TPD/TA MS data were obtained confirming the stable adsorption of glucose and raffinose on an ultrafine pyrogenic silica surface modified by amino groups.

Synthesis and characterization of various MgO and related systems

Various magnesium oxide catalysts have been prepared by thermal treatment of two different precursors: Mg(OH)2 and Mg5(OH)2(CO3)4. An additional system based on MgO and doped with B2O3 was also prepared. The textural and acid–base properties of the catalysts were investigated. The synthesized solids were characterized from adsorption isotherms, X-ray diffraction (XRD), 1H MAS NMR, diffuse reflectance IR Fourier transform spectroscopy (DRIFT) and temperature-programmed desorption–mass spectrometry (TPD–MS) of adsorbed probe molecules (pyridine, 2,6-dimethylpyridine and carbon dioxide). The surface properties of the solids were found to be strongly influenced by the magnesium oxide preparation conditions (viz. the precursor and calcination method used). Use of Mg(OH)2 as the precursor and in vacua calcination provided highly basic solids (those with the highest proportions of strong basic sites). Various types of Lewis-acid sites were observed in the catalysts prepared in vacuo, probably as the result of the presence of Mg2+ cations of low coordination after the calcination.

Temperature-programmed desorption mass spectrometry of butyloxysilyl groups on silica surfaces

The decomposition reactions of the alkoxide groups SiO(CH 2) 3CH 3 on dispersed silica (Butosil) surfaces have been studied by temperature-programmed desorption mass spectrometry (TPD) and quantum chemical (AM1) methods. The rate constants were calculated on the basis of experimental data and by the RRKM theory. The potential energy surface profiles were found by adiabatic and dynamic reaction coordinate methods. The main desorption channel corresponds to the elimination of 1-butene above 600 K. The TPD data suggest that this reaction is of first order and that it can be viewed as “unimolecular”.

NO dissociation on rhodium

The desorption and dissociation of nitric oxide on a polycrystalline Rh foil were studied by means of combined temperature-programmed desorption (TPD) and static secondary ion mass spectrometry (SSIMS). The latter method was employed either under isothermal conditions or in a temperature-programmed mode (TPSSIMS). After carefully calibrating the secondary ion intensities by means of TPD, the NO dissociation kinetics were quantitatively evaluated for low NO coverages. Fitting the SSIMS data with a Polanyi-Wigner ansatz and assuming first-order kinetics, we found an activation energy, E = 8 4- 2 kJ/mol, and a preexponential, v = 10 -o.2 s -1. The data were confirmed by independent isothermal SSIMS measurements at several temperatures between 80 and 330 K. The fairly unusual rate parameters are discussed in relation to similar results obtained recently from measurements on Rh single crystal surfaces.

Adsorption and reactions of ethylsilane on Si(100)

The adsorption and thermal decomposition of ethylsilane was studied on the Si(100) 2 × 1 reconstructed surface using high resolution electron energy loss spectroscopy and temperature programmed desorption mass spectrometry. Ethylsilane dissociatively adsorbed on the surface at a saturation coverage of approximately 0.25 monolayers. Thermal decomposition of ethylsilane proceeded by β-hydride elimination, resulting in the desorption of ethylene at 700 K. Hydrogen adsorbed on the substrate as the 2 × 1 monohydride and desorbed at approximately 800 K, leaving behind a carbon-free surface.

Adsorption and reactions of diethylsilane on Si(100)

The adsorption and thermal decomposition of diethylsilane on the Si(100)2 × 1 surface was studied using temperature programmed desorption mass spectrometry and high resolution electron energy loss spectroscopy. Diethylsilane dissociatively adsorbed to a saturation coverage of ≈ 0.25 ML. Adsorbate thermal decomposition produced ethylene and hydrogen which desorbed at 700 and 800 K, respectively. Hydrogen desorption occurred exclusively from the 2 × 1 monohydride state. Thermal decomposition proceeded by β-hydride elimination which facilitated the clean removal of the ligand groups, resulting in a carbon-free layer of silicon. An additional adsorption process, which eliminated H 2 during the dissociative adsorption, occurred at surface defect sites.

Adsorption and Reaction of TiCl4 on W(100)

AbstractThe adsorption and reaction of titanium tetrachloride (TiC14) on W(100) was investigated using temperature programmed desorption mass spectrometry (TPRS), x-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), and low energy electron diffraction (LEED). TiC14 adsorbs molecularly on W(100) at 100 K. Desorption from the molecularly bound state occurs near 220 K. Competing with desorption is dissociation to adsorbed TiCl3, which reacts to form gaseous TiCl4 near 450 K. TiC13 also decomposes into atomically adsorbed Ti and Cl on the surface upon heating to 700 K.

Copper metalorganic chemical vapor deposition reactions of hexafluoroacetylacetonate Cu(I) vinyltrimethylsilane and bis (hexafluoroacetylacetonate) Cu(II) adsorbed on titanium nitride

Cu is receiving increasing attention as a replacement for Al in future Si ultra-large-scale integrated circuits due to its lower resistivity and potentially better reliability in terms of electromigration resistance and stress-induced voiding. Metalorganic chemical vapor deposition (MOCVD) of Cu offers the advantages of conformal coverage and selective growth. Whatever the means of deposition, a diffusion barrier and adhesion layer such as TiN is required. To understand the nucleation mechanisms of Cu film growth on TiN during MOCVD, we have studied the thermal decomposition of two metalorganic precursors, hexafluoroacetylacetonate Cu(I) vinyltrimethylsilane [CuI(hfac)(vtms)], and bis (hexafluoroacetylacetonate) Cu(II) [CuII(hfac)2] by x-ray photoelectron spectroscopy and temperature programmed desorption mass spectrometry in an ultrahigh vacuum (UHV) system. Experiments were carried out on polycrystalline air-exposed (i.e., oxidized) TiN and N2 ion beam sputter-cleaned TiN. These surfaces are representative of those found in the traditional stand-alone and newer integrated metallization processing sequences, respectively. Room temperature dosing of TiN with CuI(hfac)(vtms) is accompanied by rapid desorption of the vtms ligand, leaving chemisorbed CuI(hfac). This CuI(hfac) decomposes upon heating to form CO, CO2 and CF4 desorption products, leaving about 90% of the Cu on the surface, along with some F and C. CII(hfac)2 also dissociatively chemisorbs at room temperature to form CuI(hfac) and an adsorbate with the stoichiometry of hfac. Unlike CuI(hfac)(vtms), a Cu(hfac)-containing species desorbs upon heating above 75 °C. Near 200 °C this process ceases and decomposition (and possibly some intact desorption) of hfac takes place, leading to desorption of CO, CO2, and CF4. After desorption of all C-containing species, about 30% of the initially deposited Cu remains on the surface, along with a submonolayer coverage of F. Regardless of the precursor, Cu begins to diffuse into the TiN film, above 280 °C. The results of our studies offer insights into the different mechanisms that may play a role in the nucleation of MOCVD film growth and incorporation of impurities at the interface. Significantly, the disproportionation reaction that dominates the deposition of pure Cu from CuI(hfac)(vtms) at the higher pressures typical of MOCVD does not occur under UHV conditions.

Characterization of carbon deposited from carbon dioxide on oxygen-deficient magnetites

Oxygen-deficient magnetite (ODM) powder has been reacted with CO2 at 300 °C; black particles of carbon separated from the ODM were studied by Raman, energy-dispersive X-ray (EDX) and wave-dispersive X-ray (WDX) spectroscopies. The carbon was a mixture of graphite and amorphous carbon in at least two levels of crystallization. Temperature-programmed desorption–mass spectrometry (TPD-MS) showed that a large portion of the carbon deposited on the ODM was released as CO and CO2 gas in a flow of Ar at ca. 520 °C, indicating that the carbon particles formed on the ODM are reactive. X-Ray photoelectron spectra of the carbon-deposited ODM showed no indication of carbide (Fe3C) or α-Fe phase formation.

Temperature programmed desorption mass spectrometry of organic matrix compounds

The desorption of organic matrix compounds from a stainless steel sample probe in vacuum has been studied using temperature programmed desorption (TPD) to learn about their useful lifetimes and stabilities at temperatures and vacuum environments typically used in secondary ion mass spectrometry (SIMS) or fast atom bombardment mass spectrometry (FABMS) of organic samples. Glycerol, thioglycerol, triethanolamine, and 3-nitrobenzyl alcohol were chosen for study since they are commonly used as matrix compounds in FABMS. For all matrix compounds studied, molecular species desorb within a relatively narrow temperature range which is unique for each matrix, but consistent with the known liquid–gas phase transition temperature of each compound. For one compound (glycerol), additional species desorb which are attributed to decomposition of the compound during TPD. These results suggest that for each matrix compound used in SIMS or FABMS an optimum useful temperature range can be defined. The ultimate performance of each matrix, however, may also be affected by the thermal stability, which varies for the compounds studied in this work.

Quantum chemistry study on the nature of the CO and H 2 activation over CuO-ZnO catalysts for methanol synthesis

Self-consistent charge-discrete variation-X α (SCC-DV-X α) quantum chemical calculations were used to study the nature of the activation of CO and H 2 on CuO-ZnO catalysts for methanol synthesis. Based on X-ray photoelectron spectroscopy, electron spin resonance and temperature programmed desorption mass spectrometry investigations (L.F. Chen and K.H. Huang, J. Catal. (in Chinese), 10 (1989) 14; W.J. Lai and K.H. Huang, J. Chem. Phys. (in Chinese), 3 (2) (1990) 86.) into CO and H 2 activation adsorption, a model of CO adsorption on the CuO-ZnO cluster and a model of H 2 adsorption on the (101̄0) surface cluster of ZnO were proposed. The results show that CO is terminally bonded to the Cu atom of the CuO-ZnO cluster and its activation is caused through transfer of 0.30 electrons from the σ molecular orbitais of CO to the Cu and back-donation of 0.23 electrons from Cu to the lowest unfilled 2π ∗ orbital of the adsorbed CO molecule. Moreover, DV-X α calculations for the Zn 8O 8 cluster model on the (101̄0) surface of ZnO for the proposed adsorption of the hydrogen species show that the density of states of Zn 8O 8 is in good agreement with known ultraviolet photoelectron spectroscopy results. The lowest unoccupied molecular orbital (LUMO) of cluster model is mainly composed of surface states Zn(4s + 4p), and consists of a linear combination of the sp 3 hybridized orbitals centred on the surface Zn ions and directed from the surface along the tetrahedral direction. The highest occupied molecular orbital (HOMO) is mainly made up of a 2p x + 2p y + 2p z surface orbital on O 2p. Both frontier orbitals play a role in H − or H 0 adsorption on Zn 2+ and H 0 or H + adsorption on O 2−. The calculations also show that H 2 adsorbs heterolytically on ZnO, yielding Zn 2+-H − and O 2−-H +, which are consistent with the IR spectra.

Low-temperature adsorption and decomposition of borazine on the Si(100)-(2 × 1) surface

Borazine adsorption at a surface temperature ( T s) of − 120°C and the thermal decomposition of borazine have been investigated on the Si(100)-(2 × 1) surface using temperature-programmed desorption (TPD) and static secondary ion mass spectrometry (SSIMS). The initial reactive sticking coefficient ( S 0) at this temperature was measured as 0.08. Borazine dissociates by breaking BH and/or NH bonds to form silicon monohydride species, SiH, even at −120°C because of the highly reactive silicon dangling bonds. The monohydride then decomposes at 550°C to desorb H 2. After the initial decomposition stage, subsequent borazine molecules (α 1-borazine) can coadsorb intact with the partially decomposed borazine (β-borazine). α 1-borazine eventually desorbs molecularly with a peak temperature of 110°C. Taking into account the coexistence of both α 1- and β-borazine, the saturation coverage of borazine (θ BZ) was estimated as 1.2 molecules per surface Si atom. After borazine adsorption and subsequent high-temperature annealing (900°C), the surface contains boron and nitrogen originating from decomposed borazine. This surface is less reactive (“poisoned”) towards borazine adsorption. Above 900°C, the nitrogen- and boron-containing Si surface further decomposes to evolve SiN or Si 2N.

Preparation and spectroscopic characterization of a thin polymer film in ultra high vacuum: 2,5-distyrylpyrazine

The UHV-preparation and -characterization of thin, highly ordered polymer films of 2,5-distyrylpyrazine is reported. Monomer films were evaporated from a Knudsen cell. Subsequent photo-oligomerization occurs by illumination with UV light. The desorption behavior of the monomers and the lower oligomers was studied by temperature-programmed desorption mass spectrometry (TDS). The photoreaction was monitored in situ by high resolution electron energy loss spectroscopy (HREELS) in the range of electronic transitions. The information obtained from the loss spectra are compared to those from absorption and fluorescence spectroscopy. The results are discussed in the framework of the calculated electronic structure of related compounds.

Surface photochemistry. II. Wavelength dependences of photoinduced dissociation, desorption, and rearrangement of O2 on Pt(111)

A Pt(111) surface saturated with molecular oxygen was irradiated by UV light from a mercury arc lamp with various cutoff filters. High-resolution electron energy-loss spectroscopy (HREELS) and temperature programmed desorption mass spectroscopy (TPD) were used to characterize the products retained on the surface. Upon UV irradiation at 95 K, chemisorbed O2 (peroxo) undergoes dissociation, desorption and rearrangement. Different wavelength dependences were observed for the three processes: dissociation was not observed at wavelengths longer than 295 nm, in agreement with gas phase photodissociation of the O–O bond in hydrogen peroxide; desorption and rearrangement became negligible at wavelengths longer than 420 nm, in agreement with the photolysis of an organometallic peroxoplatinum complex. For wavelengths between 230 and 315 nm, the average cross sections of dissociation and desorption were estimated to be 5.7×10−20 and 1.2×10−19 cm2 , respectively. The possible origins of the three processes are discussed.

Combined SIMS/TPD investigations of UHV-prepared tolyltriazole overlayers on Cu, Ni and Au

The corrosion inhibitor tolyltriazole (TTA) has been investigated by combined secondary ion mass spectrometry (SIMS) and temperature programmed desorption mass spectrometry (TPD). TTA overlayers in the submonolayer to multilayer range are produced by in situ molecular beam exposure of Cu, Ni and Au substrates under UHV conditions. TPD yields information on different binding states and layer thickness. Molecular cluster ions as (2M−2H+Me) −, indicating the formation of TTA molecules in quasipolymeric chains, are emitted from all three substrates. Only for Cu we found a TTA desorption peak at high target temperatures (580 K), correlated to the emission of characteristic secondary ions, mainly (MH) − and (2M−2H+Cu) −. It is obviously the corresponding stable TTA monolayer on Cu that is responsible for the strong corrosion protection effect.

Flue gas desulphurisation: Catalytic removal of sulphur dioxide by carbon monoxide on sulphided La 1− xSr xCoO 3: I. Adsorption of sulphur dioxide, carbon monoxide and their mixtures

Temperature-programmed desorption mass spectrometry (TPD-MS) and thermogravimetric analysis of freeze-dried La 1− x Sr x CoO 3 ( x=0.3, 0.5, 0.6, 0.7) on exposure to sulphur dioxide and carbon monoxide was performed for the gases alone and their mixtures. The theory of TPD-MS is presented and the method used to determine activation energies of desorption. The ease of oxygen loss from the catalysts heated in vacuo to 650°C increased with x. Subsequent adsorption of carbon monoxide leads to a further mass loss for x=0.3 but a gain in mass occurs for x=0.7 suggesting strong adsorption at oxygen vacancies. Mass increases were recorded for the adsorption and reaction of sulphur dioxide at 500°C that increased with lanthanum content. Exposure to mixtures of sulphur dioxide and carbon monoxide were accompanied by reaction yielding elemental sulphur, carbonyl sulphide and great increases of mass of the catalyst. The X-ray diffraction patterns of the catalysts after adsorption of carbon monoxide at 500°C showed little change from the perovskite structure. However a mixture of sulphates and sulphides in addition to the underlying perovskite were found after exposure to sulphur dioxide, and for exposure to both gases the original structure was lost and replaced by a mixture of sulphides and oxosulphides of the metals. No perovskite sulphides were detected.

Lanthanide oxides: preparation and ageing

La2O3, CeO2, Sm2O3, Dy2O3, and Yb2O3 were obtained by calcining in air phases prepared by precipitation with ammonia from solutions of the corresponding nitrates. During the calcination to oxides, under the experimental conditions used, the nitrate ions are removed to an extent which could not be detected by temperature programmed desorption–mass spectrometry or i.r. spectroscopy, contradictory to the established literature. For comparative purposes, Yb2O3 has also been prepared by thermal decomposition of oxalate. The Brunauer–Emmett–Teller surface area was always lower for the oxide obtained from oxalate. This was so even after thermal treatment at 1 173 K. Independent of the preparation method, the heaviest 4f oxides undergo partial conversion to bulk carbonate hydroxides when exposed to air. According to our results and those reported in the literature, lanthanide oxides, when used as catalysts, are often activated at temperatures lower than those necessary for the complete elimination of CO2 and H2O.