High-temperature Desorption Research Articles

Isotopic studies of the reaction of NO on silver surfaces

Abstract The isotopomers 14 N 16 O and 15 N 18 O were used to obtain information on the mechanisms of NO reaction on silver layers on Ru(001), by observation of the isotopic distributions of the products. NO adsorption on Ag in the 75–100 K range produces adsorbed N 2 O and oxygen atoms. Very different isotopic distributions of product N 2 O are observed, depending on whether the NO isotopomers are dosed simultaneously or sequentially. The observed N 2 O distributions favor a non-dissociative reaction pathway with a NO dimer as the reaction intermediate. The isotopic distribution of a high temperature NO desorption state can be explained by dissociation of a NO 2 surface species that is formed at a low temperature. The N 2 O which is produced by reaction of NO on the surface is found to have a higher desorption temperature than N 2 O adsorbed on either the clean Ag surface or on an oxygenated Ag surface. This is attributed to a strong binding interaction with surface NO 2 .

Low-temperature (490 °C)GaAs epitaxial growth on (100)Si by molecular beam epitaxy

Low-temperature (490 °C)GaAs epitaxial growth on (100) silicon by molecular beam epitaxy is reported in this letter. Silicon wafers were cleaned by spin-etch technique to passivate silicon surface with hydrogen, by which the conventional high-temperature (≳850 °C) oxide desorption step for pre-epitaxial substrate preparation is eliminated. The possibility of epitaxial growth on such a passivated surface to obtain good quality epitaxy is investigated. Epitaxial films are characterized by cross-sectional transmission electron microscopy (XTEM), plan-view transmission electron microscopy, and double crystal diffraction (DCD).

Photon- and electron-stimulated desorption of excited alkali-metal atoms from alkali halide surfaces.

We report and review progress in photon- and electron-stimulated desorption studies of excited-state alkali-metal atoms from alkali halide crystal surfaces. Recent experimental results indicate the existence of two temperature regimes each having a different mechanism for the excited-atom production. In the low-temperature regime, excited-atom desorption is shown to depend on the presence of neutral metal atoms on the surface. In the high-temperature regime excited-state production involves gas-phase excitation of the ground-state alkali-metal atoms. Based on experimental results, we propose two separate and distinct mechanisms for the excited-alkali-metal-atom production. The first proposed mechanism appropriate to the low-temperature regime for both electron- and photon-stimulated desorption, assumes that surface exoergic reactions between alkali dimers and halogen atoms produce desorbed excited alkali-metal atoms. The surface reactants are formed through an irradiation-initiated and defect-mediated process. The second proposed mechanism assumes that the gas-phase excitations of ground-state alkali-metal atoms by primary electrons, and perhaps to a small extent by secondary electrons, produce the excited alkali-metal atoms. The gas-phase excitation model applies to high-temperature electron-stimulated desorption, where the desorption yield for ground-state alkali-metal atoms is high.

Cluster formation in the adsorbate-induced reconstruction of the O/Mo(001) surface

An X-ray crystallographic structure determination has been carried out for the square root 5* square root 5 Mo(001)/O surface. This is the penultimate structure in the sequence of ordered states obtained by high-temperature desorption from the oxidized surface. The oxygen induces a reconstruction with one Mo atom missing from the surface unit cell and O atoms occupying the exposed threefold sites. The O sits asymmetrically in these sites and induces a rotational distortion of the remaining top layer Mo atoms, which cluster together. The asymmetry can be attributed to partial direct bond formation between surface Mo-O clusters, since apparently this is not mediated by any significant second-layer displacements.

The adsorption of H 2O on K precovered Ni(111) studied by ARUPS and TPD

The adsorption of H 2O on a Ni(111) surface precovered with various amounts of potassium has been studied by TPD, work-function measurements, and angle-resolved UPS using linearly polarized synchrotron radiation. Coadsorption of potassium leads to major changes in the desorption characterics of water which depend strongly on the amount of preadsorbed potassium. At potassium precoverages below θ K ≈ 0.14 ML water is molecularly adsorbed; for higher potassium precoverages dissociation of H 2O into an OH species as well as molecular adsorption is observed at 110 K. The molecularly bound species is significantly stabilized on the surface, as concluded from a high temperature desorption state (170–280 K) in TPD. At low potassium precoverages approximately three H 2O molecules are affected by one potassium atom. Using symmetry selection rules the symmetry of the adsorbed H 2O molecules is determined to C s(σ v, yz ), which is probably due to a tilting in the HOH plane. At high potassium precoverages H 2O decomposes upon adsorption forming an OH species; this OH species is stable up to temperatures above 400 K. The ratio of K to OH formed by dissociation is found to be about 1. From the angle and polarization dependences of the molecular orbitals a tilted orientation of the OH species is derived. The results are compared to those obtained for H 2O adsorption on clean and oxygen-precovered Ni(111) surfaces, and other alkali/metal systems.

The adsorption of H 2 on K precovered Ni(111) studied by ARUPS and TPD

The adsorption of H2O on a Ni(111) surface precovered with various amounts of potassium has been studied by TPD, work-function measurements, and angle-resolved UPS using linearly polarized synchrotron radiation. Coadsorption of potassium leads to major changes in the desorption characterics of water which depend strongly on the amount of preadsorbed potassium. At potassium precoverages below θK ≈ 0.14 ML water is molecularly adsorbed; for higher potassium precoverages dissociation of H2O into an OH species as well as molecular adsorption is observed at 110 K. The molecularly bound species is significantly stabilized on the surface, as concluded from a high temperature desorption state (170–280 K) in TPD. At low potassium precoverages approximately three H2O molecules are affected by one potassium atom. Using symmetry selection rules the symmetry of the adsorbed H2O molecules is determined to Cs(σv,yz), which is probably due to a tilting in the HOH plane. At high potassium precoverages H2O decomposes upon adsorption forming an OH species; this OH species is stable up to temperatures above 400 K. The ratio of K to OH formed by dissociation is found to be about 1. From the angle and polarization dependences of the molecular orbitals a tilted orientation of the OH species is derived. The results are compared to those obtained for H2O adsorption on clean and oxygen-precovered Ni(111) surfaces, and other alkali/metal systems.

Growth of gallium arsenide on hydrogen passivated Si with low-temperature treatment (∼600 °C)

Epitaxial growth of GaAs on Si commonly employs a high-temperature (≳850 °C) oxide desorption step. In this letter, we report the first epitaxial growth of GaAs on Si without the need for this high-temperature treatment. This method utilizes a final HF treatment whereby the Si surface dangling bonds are terminated by hydrogen with a resultant (1×1) bulk-like surface structure. Upon medium temperature heat treatment (×500 °C), hydrogen leaves the surface leading to the common orthogonal 2×1 surface reconstruction. High quality GaAs epitaxial layers were successfully grown on these 2×1 reconstructed Si surfaces with the pregrowth substrate preparation temperatures of as low as 600 °C.

Powder metallurgical processing of Tb0.27Dy0.73Fe2−x (0<x≤0.5) by hydrogen decrepitation

This paper reports on the characterization of giant magnetostrictive alloys, of the general formula Tb0.27Dy0.73Fe2−x (0<x≤0.5), at various stages during their powder processing by hydrogen decrepitation, in order to try to optimize the process. Mass spectrometer degassing studies on hydrogenated material have shown that disproportionated material exhibits two desorption peaks and is not degassed completely with respect to hydrogen until above 600 °C. The RFe2 (R=Tb0.27Dy0.73) hydride powder appears to degas at a lower temperature; however, it is thought that a high-temperature desorption peak is not present due to oxidation. Metallographic studies have shown that there is much less rare-earth-rich grain-boundary phases in sintered compacts of initial composition RFe1.5 than in cast RFe1.5 and also reveal the microstructure of hydrogenated and disproportionated RFe1.5. Wavelength dispersive x-ray analysis seems to indicate preferential loss of Dy during sintering.

Defect-induced dissociation of CO on palladium

The adsorption and reaction of carbon monoxide with a polycrystalline Pd foil, containing variable amounts of defect sites, has been studied by means of temperature-programmed desorption and reaction (TPD, TPR), temperature-programmed static secondary ion mass spectrometry (TPSSIMS) and X-ray photoelectron spectroscopy (XPS). On a well-annealed Pd surface a molecular adsorption state is found, with no evidence for dissociation. In contrast, the same but Ar + ion sputtered Pd sample is seen to dissociate part of the adsorbed CO. It is concluded that a high amount of defect sites is responsible for this phenomenon. The intensity ratios, Pd 2 O + Pd 2 + and Pd 2 C + Pd 2 + , measured by TPSSIMS, have been used to monitor the build-up of surface car and oxygen under in situ reaction conditions. The onset temperature for CO ad dissociation is found to be ∼ 410 K at which temperature nearly half of the molecular adsorption state has already undergone thermal desorption. C1s XPS provides corroborating evidence for carbon deposition on the surface. Recombination with oxygen followed by thermal desorption of CO is associated with the occurrence of high temperature desorption features at ∼ 640 and ∼ 725 K in the TPD spectra and with the disappearance of the carbidic and oxidic relative intensities in TPSSIMS. Evidence has been obtained for reversible subsurface diffusion of oxygen and carbon.

Interaction of atomic hydrogen with cluster models of Pd, Rh and bimetallic PdSn and RhSn catalysts

Results of LCGTO-MCP-LSD (Linear Combination of Gaussian Type Orbitals-Model Core Potentials-Local Spin Density) calculations are reported for chemisorption and for bulk and surface diffusion of atomic hydrogen on clusters simulating the (111) and (100) surfaces of pure transition metals (Pd and Rh) and bimetallic catalysts (PdSn, RhSn). The replacement of a Pd or Rh atom near the hydrogen atom by Sn always decreases the binding energy. Hydrogen surface diffusion has been calculated with the aim of explaining the nature of the second, high-temperature desorption peak recently reported for RhSn/SiO 2 catalysts. Our results, for Rh 8Sn 3-H, show that hydrogen diffusion into the tin region is clearly unfavored. A consistent pattern has emerged from the calculations; the proximity of tin to hydrogen leads to destabilisation. This should affect the recombination process to form H 2 and the implications of this for catalysis are discussed.

Interaction of manganese with the ruthenium(001) surface and chemisorption of carbon monoxide on the manganese/ruthenium(001) interface

The interaction of vapor-deposited Mn with the Ru(001) surface and its effect on CO chemisorption were studied under ultrahigh vacuum by Auger electron spectroscopy (AES), thermal desorption spectroscopy (TDS), isotopic exchange, and low-energy electron diffraction (LEED). At room temperature, Mn grows layer by layer. The first three layers are pseudomorphic with respect to the Ru substrate. A complete analysis of Mn thermal desorption data shows that there is a decrease of the heat of adsorption with increasing coverage within the first monolayer (59 to 55 kcal/mol) and zero-order desorption for higher coverages with desorption energy 54 kcal/mol. A new, higher temperature CO desorption state at 650-800 K appears at submonolayer coverages. The C-O bond of CO chemisorbed on Mn/Ru is severely weakened and probably dissociated as evidenced by a high CO desorption temperature and random isotope mixing of coadsorbed isotopes of carbon monoxide.

Electrical properties of K/sub 2/O-doped Ba/sub 0.5/Sr/sub 0.5/TiO/sub 3/ ceramic humidity sensor

A ceramic humidity sensor using Ba/sub 0.5/Sr/sub 0.5/TiO/sub 3/ doped with a few molar percent of potassium oxide was studied. It was characterized by long life, was reversible without repeated high-temperature thermal desorption processes, and conductance against relative humidity (RH) sensitivity, as high as four orders of magnitude, was found at low frequencies. This sensor showed a good exponential relationship between the conductance, RH, and temperature at low frequencies. The plots of the relative dielectric dispersion against RH showed that it had an apparent dielectric constant increase at low frequency as a result of the electrode and water molecular polarization effects. By complex impedance plots with a non-Debye capacitor concept, an equivalent circuit model was established which could well simulate all the electrical properties of the sensor in the range of 65-95% RH for all measured temperatures (25 degrees C-85 degrees C) and frequencies (5 Hz-13 MHz). In the measurement of this sensor, an inductance loop over the low RH (15% and 30%) and low frequencies (5 to 40 Hz) range was observed, which might be due to the desorption of water vapor at the electrode surface for reduction reactions. >

Growth of GaAs on Si using ionized cluster beam technique

A single-crystalline epitaxial film of GaAs has been grown on Si using an ionized cluster beam technique. The native oxide layer on the silicon substrate was removed at 550 °C by use of an accelerated arsenic ion beam, instead of a high-temperature desorption. During the growth the substrate temperature was maintained at 550 °C. Transmission electron microscopy and electron diffraction data suggest that the GaAs layer is an epitaxially grown single-crystalline layer.

Oxygen adsorption on Ag powder

Oxygen adsorption on a Ag powder sample is different from that on a supported Ag catalyst. The powder shows the presence of a very high temperature desorption state (at 900 K) which diminishes adsorption into the state which is common to all types of silver samples (desorption peak at 560 K). This behaviour, together with the increased sticking probability which is observed on the powder, indicates that the powder has a very much rougher surface structure than a catalyst, which consists of mostly (111) planes. The powder probably consists of a high proportion of (111) planes with a high density of steps and defects since the high-temperature desorption state has been seen on a stepped single-crystal plane [Ag(331)].

Activated adsorption of deuterium on Ni(110): evidence for a high temperature desorption state

Cooling a clean Ni(110) sample in D 2 from T ≥ 350 K results in the appearance of a new high temperature peak ( β 3) centred at ∼ 435 K in the thermal desorption spectrum. The Ni surface is also reconstructed to the streak (St) phase. For exposures up to ∼ 10 −1 Pa s during slow cooldown, the β 3 state saturates at ∼ 0.06 monolayers (ML). It is not formed upon exposure at T ≤ 320 K even though the St phase may form under these conditions. The desorption of the ß 3 state occors without change in work function. Nuclear reaction analysis (NRA) has shown that the total coverage obtained on cooling from T ≥ 350 K to 170 K is 1.7±0.05 ML, significantly higher than obtained upon saturation at 170 K (1.5 ML). The ß 3 state probably arises from deuterium at sites just below the surface. The desorption is limited by activated migration to the surface at temperatures at which the equilibrium coverage would be very small. The ß 3 state might be associated with the final stages of removal of the St phase.

The oxidation of ethylene over silver-based alloy catalysts: 3. Silver-gold alloys

Supported silver-gold alloy catalysts were prepared by impregnation of low surface area α-Al 2O 3 with mixed silver cyanide and gold cyanide solutions. Alloying was achieved by calcination at 250 °C. X-ray diffraction analysis indicates that complete alloying was achieved. Surface composition of alloy particles was determined as a function of bulk composition by high resolution ESCA analysis. Surfaces were found to be significantly enriched in silver. Turnover numbers of ethylene epoxidation and combustion were found to exhibit maxima on surfaces containing 10–15% Au and drop to zero on surfaces containing 30% Au or more. Temperature-programmed desorption of oxygen from Ag-Au alloy surfaces revealed that oxygen desorbs in two modes from surfaces containing less than 30% Au. The high-temperature desorption peak was assigned to multicoordinated adsorbed atomic oxygen and the low-temperature peak to monocoordinated atomic oxygen. Based on these results a mechanism of ethylene epoxidation and combustion is formulated.

Thermodynamic study of sulphur-silver-gold solid solutions

Supported silver-gold alloy catalysts were prepared by impregnation of low surface area α-Al2O3 with mixed silver cyanide and gold cyanide solutions. Alloying was achieved by calcination at 250 °C. X-ray diffraction analysis indicates that complete alloying was achieved. Surface composition of alloy particles was determined as a function of bulk composition by high resolution ESCA analysis. Surfaces were found to be significantly enriched in silver. Turnover numbers of ethylene epoxidation and combustion were found to exhibit maxima on surfaces containing 10–15% Au and drop to zero on surfaces containing 30% Au or more. Temperature-programmed desorption of oxygen from Ag-Au alloy surfaces revealed that oxygen desorbs in two modes from surfaces containing less than 30% Au. The high-temperature desorption peak was assigned to multicoordinated adsorbed atomic oxygen and the low-temperature peak to monocoordinated atomic oxygen. Based on these results a mechanism of ethylene epoxidation and combustion is formulated.

The acidity of trivalent cation-exchanged montmorillonite. Temperature-Programmed desorption and infrared studies of pyridine and n-butylamine

AbstractTemperature-programmed desorption (TPD) and IR spectroscopy were used to characterize the number and strength of acid sites in Al3+-, Cr3+- and Fe3+-exchanged montmorillonite. The bases pyridine and n-butylamine occupied three different sites in the interlamellar space: (i) physisorbed base, (ii) base bound to Lewis acid sites, and (iii) protonated base. TPD profiles for pyridine were characterized by maxima at 40°, 150° and 340°C, whilst those for n-butylamine occurred at 30°, 200° and 410°C. The Al3+- and Cr3+-exchanged forms were stable up to pretreatment temperatures of 300°C, but the Fe3+-form required > 3 day exposure to base vapour to re-establish the high-temperature desorption peak. Variable-temperature IR studies showed that the number of Brönsted-bound pyridine molecules increased with increased outgassing temperature.

A hreels, tpd study of nitric oxide adsorption, desorption and reaction on clean and sulfur covered palladium (100)

The effect of sulfur poisoning on the adsorption, desorption and reactions of NO on the Pd(100) surface was studied using TPD, EELS, and LEED at sulfur precoverages ranging from zero to saturation (0.5 monolayer) and for NO coverages between zero and saturation (0.65 ML). NO adsorption is primarily associative. Simple steric blocking accounts for the decrease in NO saturation coverage with adsorbed sulfur. Correspondingly, a 25% decrease in the saturation coverage of NO was observed at a sulfur coverage of 0.1 ML, as expected for simple atop site blockage by sulfur. Thereafter the NO coverage declined more slowly, with detectable NO adsorption at θ S(sat). Three different vibrational modes were detected by EELS at 1510, 1720, and 1750 cm −1, which were assigned respectively to a bridge site, an atop site and a weakly bound NO. The 1720 cm −1 loss frequency is coverage dependent. NO adsorption on the clean surface gives rise to atop site occupation in a c(2 × 2) LEED structure at 300 K, and a p(4 × 2) structure at 410 K, corresponding to coverages of 0.5 and 0.25 ML, respectively. The p(4 × 2) structure was generally observed to form a single domain. In the presence of 0.03 ML of sulfur the p(4 × 2) LEED pattern is not observed at any temperature, but the c(2 × 2) LEED pattern persists up to at least 0.2 ML sulfur. Correspondingly, the atop to bridge site transfer was not observed, and the high temperature desorption state was eliminated by adsorbed sulfur, implying that the most strongly bound state is bridge bonded. Only small amounts of N 2 and N 2O are observed to desorb during heating of adsorbed NO, and oxygen is not observed as a product. At sulfur coverages above 0.10 ML these side reactions were suppressed.

Adsorption and reaction on strained-metal overlayers Cu/Ru(0001)

These studies have addressed the adsorption of CO on very thin (submonolayer to multilayer) deposits of Cu on an Ru(0001) single crystal as well as the measurement of the high-pressure kinetics of the methanation, ethane hydrogenolysis and cyclohexane dehydrogenation reactions on this model bimetallic catalyst. Temperature-programmed desorption (t.p.d.) spectra of CO at submonolayer Cu coverages show a monotonic diminution of features due to CO adsorbed on Ru with a concomitant apperance of new t.p.d. peaks, which can be assigned to desorption from the top and edges of [1 × 1]‘strained-layer’ Cu islands. Importantly, these new CO features have significantly higher desorption temperatures than those observed on bulk Cu(111), indicating a stabilization relative to bulk Cu of CO on monolayer Cu films supported on Ru. This behaviour can be correlated with LEED, ARUPS and work-function measurements, which indicate the unique character of the first layer of Cu on Ru(0001).For methanation and ethane hydrogenolysis the overall surface activity is found to decrease monotonically with decreasing Ru surface area upon Cu addition. For cyclohexane dehydrogenation, however, the Ru specific activity is observed to increase by ca. 40 at a copper coverage of 0.75 of a monolayer. The rate enhancement observer for submonolayer Cu deposits may relate to the modified activity of the strained Cu overlayer owing to its altered geometric and electronic properties.