Apparent Desorption Energy Research Articles

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.

H2 splitting on Pt, Ru and Rh nanoparticles supported on sputtered HOPG

The equilibrium hydrogen exchange rate between adsorbed and gas phase hydrogen at 1bar is measured for Pt, Ru and Rh nanoparticles supported on a sputtered HOPG substrate. The particles are prepared by Electron Beam Physical Vapor Deposition and the diameter of the particles varies between 2 and 5nm. The rate of hydrogen exchange is measured in the temperature range 40–200°C at 1bar, by utilization of the H–D exchange reaction. We find that the rate of hydrogen exchange increases with the particle diameter for all the metals, and that the rate for Ru and Rh is higher than for Pt. In the case of Pt, the equilibrium dissociative sticking probability, S, is found to be nearly independent of particle diameter. For Ru and Rh, S is found to depend strongly on particle diameter, with the larger particles being more active. The apparent energy of desorption at equilibrium, Eapp, shows a dramatic increase with decreasing particle diameter for diameters below 5nm for Ru and Rh, whereas Eapp is only weakly dependent on particle diameter for Pt. We suggest that the strong variation in the apparent desorption energy with particle diameter for Ru and Rh is due to the formation of compressed hydrogen adlayers on the terraces of the larger particles. Experiments are also carried out in the presence of 10ppm CO. Pt is found to be very sensitive to CO poisoning and the H–D exchange rate drops below the detection limit when CO is added to the gas mixture. In the case of Ru and Rh nanoparticles, CO decreases the splitting rate significantly, also at 200°C. The variation of the sensitivity to CO poisoning with particle diameter for Ru and Rh is found to be weak.

Hydrogen adsorption on palladium and palladium hydride at 1 bar

The dissociative sticking probability for H 2 on Pd films supported on sputtered Highly Ordered Pyrolytic Graphite (HOPG) has been derived from measurements of the rate of the H–D exchange reaction at 1 bar. The sticking probability for H 2, S, is higher on Pd hydride than on Pd (a factor of 1.4 at 140 °C), but the apparent desorption energy derived from S is the same on Pd and Pd hydride within the uncertainty of the experiment. Density Functional Theory (DFT) calculations for the (1 1 1) surfaces of Pd and Pd hydride show that, at a surface H coverage of a full mono layer, H binds less strongly to Pd hydride than to Pd. The activation barrier for desorption at a H coverage of one mono layer is slightly lower on Pd hydride, whereas the activation energy for adsorption is similar on Pd and Pd hydride. It is concluded that the higher sticking probability on Pd hydride is most likely caused by a slightly lower equilibrium coverage of H, which is a consequence of the lower heat of adsorption for H on Pd hydride.

The sticking probability for H2 on some transition metals at a hydrogen pressure of 1bar

The sticking probability for hydrogen on films of Co, Ni, Cu, Ru, Rh, Pd, Ir, and Pt supported on graphite has been measured at a hydrogen pressure of 1 bar in the temperature range 40-200 degrees C. The sticking probability is found to increase in the order Ni, Co, Ir, Pd, Pt, Rh, and Ru at temperatures below 150 degrees C, whereas at higher temperatures, the sticking probability for Pd is higher than for Pt. The sticking probability for Cu is below the detection limit of the measurement. The measured sticking probabilities are slightly lower than those obtained at high hydrogen coverage under ultrahigh vacuum conditions. This could be a consequence of the higher hydrogen pressure used here. The apparent desorption energies extracted from the steady-state desorption rate are found to agree reasonably well with published values for the heat of adsorption at high coverage. However, the sticking probability is not related in a simple way to published values for the heat of adsorption at low coverage, with Ru and Rh giving exceptionally high values for the sticking probability. It is suggested that this is due to the presence of adsorption sites with very low desorption energy on Ru and Rh.

Effect of oxygen on the hydrogenation properties of magnesium films

The effect of magnesium oxide on the magnesium and hydrogen desorption properties of magnesium films have been investigated. We find that by capping metallic magnesium films with oxide overlayers the apparent desorption energy of magnesium is increased from 146 kJ/mol to 314 kJ/mol. The results are discussed in light of previous investigations of ball-milled magnesium powders.

OH desorption energies for a palladium catalyst characterised by kinetic modelling and laser-induced fluorescence

Abstract A kinetic model for the H 2 /O 2 reaction on a polycrystalline palladium catalyst has been constructed using CHEMKIN in order to understand the coverage-dependent OH desorption energy. Each adsorbed oxygen atom was modelled to cover four Pd surface sites. The yield of OH and the water production were measured with laser-induced fluorescence (LIF) and microcalorimetry respectively as a function of the relative hydrogen concentration, α H 2 . The temperature of the catalyst was 1300 K, the total pressure was 13 Pa and the flow was set to 100 SCCM. In fitting the model to the experimental data, the OH desorption energy E d OH was found to have a first-order coverage dependence according to: E d OH ( θ )= E d OH (0)− Bθ , where B is a constant set to 92 kJ/mol. The desorption energy at zero coverage E d OH (0) was determined to be 226 kJ/mol. The model could also qualitatively and quantitatively reproduce the apparent desorption energy as a function of α H 2 ; therefore it is believed that the coverage could be predicted by the model. The values for E d OH ( θ ) were calculated as a function of α H 2 . From the results of a sensitivity analysis and rate of production calculations, there are strong reasons to believe that the main water-forming reaction on Pd at 1300 K is the hydrogen addition reaction, H + OH ⇌ H 2 O. Enthalpy diagrams for the water-forming reactions are also presented.

C58on HOPG: Soft-landing adsorption and thermal desorption

Electron-impact induced dissociation/ionization of C60 molecules was used to produce an intense beam of C58+ ions. This was directed towards a HOPG (highly oriented pyrolytic graphite) substrate under nominally perpendicular impact conditions in order to generate deposits by soft-landing (kinetic energy < 0.1 eV atom−1). Deposited C58 molecules could subsequently be thermally desorbed intactly. Thermal desorption mass spectra of the deposits exhibit only C58. Surface deposited C58 can react with background gases to generate hydride derivatives C58Hn which are also desorbable. The apparent desorption energy of C58 and C58Hn molecules from the HOPG surface varies with increasing adsorbate coverage from 2 ± 0.1 to 2.2 ± 0.1 eV as determined by a Redhead analysis. These values are 0.7 ± 0.1 eV larger than found for C60 desorbed from the same substrate.

A Study of the Apparent Desorption Energy for OH in the Water Formation Reaction on a Palladium Catalyst

In the H2/O2 reaction, the desorption of OH from a polycrystalline palladium foil has been studied with Laser-Induced Fluorescence (LIF) as a function of the hydrogen mixing ratio, αH2, and the temperature. The water production as a function of αH2 was also monitored with microcalorimetry. The apparent desorption energy, EOH a, for OH has been measured from αH2 = 0% to αH2 = 74%. At αH2 = 0%, EOH a was measured to be 143 ± 7 kJ/mol; the apparent desorption energy then increases linearly with αH2 to a maximum value of 223 ± 6 kJ/mol at αH2 = 40%. At higher αH2, the apparent desorption energy was measured to be constant at about 200 kJ/mol. The maximum in apparent desorption energy at αH2=40% occurs at the same αH2 as the maximum in water production. This phenomenon has also been reported for the platinum metal, although at an αH2 value of 20%. The relative coverage of intermediates and products on the palladium catalysts was obtained using Chemkin.

A comparative study of high-temperature water formation and OH desorption on polycrystalline palladium and platinum catalysts

The water formation reaction has been studied on a hot polycrystalline palladium catalyst and a theoretical model has been derived with Chemkin in order to fit the experimental data. The results were compared with similar studies for platinum. The H 2/O 2 reaction was measured with LIF and microcalorimetry on palladium at a temperature of 1300 K, pressures of 13 and 26 Pa and flows of 100 and 200 SCCM. The experiments were performed in a stagnation-point flow field, which was also the geometry modelled with Chemkin. The maximum in water production as a function of α H 2 ( α H 2,H 2O max ), occurs at about 20% on platinum, while it occurs at about 40–45% on palladium; α H 2 is the hydrogen mixing ratio. However, the maximum in OH desorption ( α H 2,OH max ), occurs at about the same α H 2 for both palladium and platinum. In order that the theoretical model would match the experimental data for palladium it was necessary to assume a coverage dependent OH desorption energy, E a d,OH, according to E a d,OH( θ)= E a d,OH(0)− Bθ, where B is a constant equal to 100 kJ/mol. The apparent desorption energy for OH on palladium was also measured at α H 2 =17% to 200 kJ/mol and α H 2 =45% to 220 kJ/mol. The main water-forming reaction on platinum is thought to be the hydrogen addition reaction H * + OH * ⇔ H 2O *. For palladium, there are difficulties to determine the main water formation route because the OH desorption energy is believed to be coverage dependent. The reverse of reaction H * + OH * ⇔ H 2O * seems to have greater importance for the OH formation on palladium than on platinum. The initial sticking coefficient of hydrogen and oxygen is found to be greater on palladium than on platinum by a factor of about 10.

Kinetics of OH formation on Pt in H 2O/O 2 mixtures at high temperatures

The formation of OH from adsorbed oxygen and water on a polycrystalline platinum foil at high temperatures (≈ 1200 K) and at a total pressure of 50 mTorr, has been investigated as a function of H 2O/O 2 ratio and surface temperature. The maximum in the OH desorption rate occurs at the H 2O/O 2 ratio 2.3, and is about 7 times weaker than from the H 2+ 1 2 O 2 reaction. The apparent desorption energy determined from Arrhenius plots of the data varies with the H 2O/O 2 ratio and deviates from the “true” activation energy OH desorption, determined in the water formation reaction. The experimental data can be qualitatively understood by kinetic considerations and are quantitatively reproduced by a kinetic model previously developed for the water formation reaction. The latter suggests that H 2O→OH+H dominates over H 2O→2OH in the water decomposition.

ChemInform Abstract: Laser Diagnostics of Nascent Gaseous Product Species Formed in Chemical Reactions on Surfaces

Examples from this laboratory illustrating laser probing of gaseous products formed in chemical reactions on surfaces are taken from the areas of catalytic reactions, silicon etching, and surface photochemistry. The emphasis will be on the unique advantages of laser diagnostics in these dynamical and mechanistic studies in which the chemical reaction product species undergo no collisions after leaving the surface before detection by the laser. UHV experiments on the catalytic oxidation of H2, D2 and NH3 by N02 and H2 by NO2 used the laser-induced fluorescence (LIF) technique to measure the apparent desorption energies of OH radicals and the energy accommodation of the desorbing OH, OD and NO reaction products. The observed different degrees of rotational cooling in the different products will be discussed. In multiphoton ionization mass spectrometry experiments (MPI/MS) spectroscopies of three different MPI processes for the SiF2 radical were studied and two of these were applied to the study of silicon etching by XeF2, F2, and NF3. The only observed gaseous products were SiF4 and the SiF2 radical and their apparent production energies correlated with the case of surface fluorine atom production. In the area of surface photochemistry, an excimer laser was used to photolyze iodobenzene, trimethylgallium, and copper acetylacetonate adsorbed on a cold surface. The time-of-flight (TOF) spectra of the fragments or reaction products were measured by using a mass spectrometer equipped with multiphoton (MPI) or electron (EI) ionization.

Rotational energy accommodation and apparent desorption energy of OH formed in the catalytic reaction of NO2 and H2 over Pt(111): Effect of surface oxygen coverage

Laser induced fluorescence has been used to study the desorption dynamics of OH radicals produced by the reaction of NO2 and H2 on Pt(111), under ultrahigh vacuum conditions. We measure an apparent Arrhenius activation energy for desorption that increases from 25 to 38 kcal/mol as the NO2 to H2 ratio in the dosing mixture decreases from 2 to 0.05. Boltzmann-like rotational energy distributions with relatively high rotational to surface temperature ratios (Tr/Ts =0.69−0.87) are reported for crystal temperatures between 1078 and 1433 K. We have also observed a small but definite dependence of the Tr/Ts ratio on both Ts and the dosing mixture ratio. These results are compared with earlier data from the O2+H2 reaction over Pt(111) and interpreted in terms of O coverage.

Laser probing of nascent gaseous product species formed in chemical reactions on surfaces

A review of research conducted at the Naval Research Laboratory illustrates the unique advantages of using lasers to probe the newly formed gaseous products from chemical reactions on surfaces. Examples to be discussed are taken from the areas of catalytic reactions, silicon etching, and surface photochemistry. UHV experiments on the catalytic oxidation of H2, D2, and NH3 by 02 and of H2 by NO2 used the laser-induced fluorescence technique to measure the apparent desorption energies of OH radicals and the rotational energy accommodation of the desorbing OH, OD, and NO reaction products. The observed different degrees of "rotational cooling" in the different products are discussed. In multiphoton ionization mass spectrometry (MPI/MS) experiments, spectroscopies of three different MPI processes for the SiF2 radical were studied and two of these were applied to the study of silicon etching by XeF2, F2, and NF3. The only observed gaseous products were SiF4 and the SiF2 radical and their apparent production energies correlated with the ease of surface fluorine atom production. In the area of surface photochemistry, an excimer laser was used to photolyze iodobenzene, trimethylgallium, and trimethylindium adsorbed on a cold surface. The time-of-flight spectra of the fragments or reaction products were measured by using a mass spectrometer equipped with MPI or electron ionization (El).

A comparative study of OH radical desorption in the H 2 + O 2 reaction on Pt, Pd, Rh, Ir, and Ni

OH radicals emitted from Pt, Pd, Rh, Ir, and Ni during the reaction between H 2 and O 2 have been studied by laser-induced fluorescence. The OH emission yields from the different catalysts were determined as a function of total pressure (5 × 10 −3−0.2 Torr), temperature of the catalyst (950–1300 K), and H 2 O 2 partial pressure ratio. When varying the total pressure at a constant mixing ratio of the reactants, a maximum intensity of OH yield is obtained at 0.7 ± 0.1 Torr for all catalysts. The OH production is very sensitive to the mixing ratio of H 2 and O 2, and shows a maximum at a specific ratio, which varies for the different catalysts. The maximum in the OH production does not coincide with the maximum in the H 2O production. The absolute intensity of desorbed OH radicals varies strongly with the different catalysts in the order Pt > Pd > Rh > Ir > Ni. When the H 2 gas supply is closed, OH radicals are still detected, indicating that the reaction H 2O ads + O ads → 2OH ads contributes to the observed OH intensity. The apparent desorption energy of OH shows a weak dependence on total pressure but a strong dependence on the mixing ratio.

Dynamics of OH desorption from single crystal Pt(111) and polycrystalline Pt foil surfaces

The laser-induced fluorescence technique in conjunction with an UHV apparatus was used to probe the desorption dynamics of OH radicals formed in the association reaction of chemisorbed O and H atoms on single crystal Pt(111) and polycrystalline Pt foil surfaces. The apparent activation energies of desorption and the rotational energy distributions of OH radicals were measured in these experiments. The apparent OH desorption energies for both surfaces were observed to increase by about 10 kcal/mol as the O/H reactant gas mixture ratio is decreased by a factor of 100, with the values for the Pt(111) single crystal at 5–10 kcal/mol below those for the Pt foil. Computer kinetic modeling of the temperature dependence of OH desorption rate as a function of O/H ratio showed similar trends as those observed in the experiments. Boltzmann-like OH rotational energy distributions with relatively high rotational to surface temperature ratios, Tr/Ts=0.85, were measured over the temperature range 1227–1479 K for the Pt(111) single crystal and over 1283–1475 K for the polycrystalline Pt foil. For both surfaces the OH spin-orbit temperatures were essentially the same as the rotational temperatures and no preference for any lambda-doublet state was observed. The Tr/Ts ratio does not seem to depend on the O/H reactant gas mixture ratio for each surface, suggesting that the rotational energy accommodation is insensitive to the apparent Pt-OH bond strength.

Diffusion and desorption steps in the transport of Br and Br2 from a graphite effusion source at high temperatures, observed by molecular-beam mass spectrometry

The transport of bromine out of a graphite source at 300–1350 K was studied with molecular-beam mass spectrometry. The mass-spectrometer signals from both Br and Br2 varied approximately linearly with the total pressure in the source. A theoretical comparison with the gas-phase density of Br in the source gave good fits. A pressure-dependent grain-boundary diffusion part and a pressure-independent, probably bulk diffusion related part of the signals can be separated in the analysis of the results. Apparent desorption energies for Br and Br2 from the graphite surface were found to be 0.7±0.1 and 0.3±0.1 eV, respectively, from the temperature dependence of the bulk diffusion signal.

Laser diagnostics of radicals in catalytic reactions

OH radicals emitted from a Pt foil surface during the reaction between H 2 and O 2 have been studied by laser-induced fluorescence. The experiments were performed in a flow system with a O 2/H 2 mixing ratio of 6/1 and Ar as the carrier gas. The total pressure and Pt-sample temperature ranges were 0.01 - 2 torr and 850 – 1150 K, respectively. Some preliminary results are reported. The OH yield increases linearly with pressure up to about 0.3 torr, where collisional quenching starts to be significant. Assuming Arrhenius behaviour an apparent desorption energy of OH of (50 ± 5) kcal/mole was obtained. The OH rotational population gives a temperature of only (525 ± 50) K at a sample temperature of (975 ± 50) K. The most likely source of OH is the thermally activated desorption of OH radicals formed by recombination of H and 0 atoms on the surface (forward reaction), but contributions from H 2O decomposition (backward reaction H 2O + O → 2OH) on the surface cannot be excluded.

Initial oxidation of Zn(0001) surfaces: H2 adsorption and XPS studies

H2 adsorption capacity measurements have been combined with XPS intensity measurements to study the kinetics of O2 uptake on Zn(0001) surfaces. On clean Zn(0001) the temperature programmed desorption spectrum of H2 shows half-order desorption kinetics and an apparent desorption energy of 11 kcal/mole. As the surface is oxidized by progressive dosing with O2, two features are noted: The H2 desorption spectrum characteristic of clean Zn(0001) is attenuated in a nonlinear fashion and a new H2 desorption state with an activation energy of 18 kcal/mole is observed for surfaces with less than saturation oxygen coverage. XPS results indicate that H2 adsorption is not completely blocked until an oxygen coverage corresponding to approximately 3 monolayers is achieved. Both H2 uptake and XPS intensity measurements also indicate that the oxygen sticking coefficient increases with oxygen coverage up to about one half of the O2 saturation coverage. These results indicate that the initial oxidation of Zn surfaces involves the formation and growth of multilayer oxide islands, where oxygen incorporation at the perimeter of the islands is the rate limiting step.

Adsorption-desorption properties and surface structural chemistry of chlorine on Cu(111) and Ag(111)

The adsorption, desorption, and structural properties of chlorine adlayers on Cu(111) and Ag(111) have been studied by LEED, Auger, Δϑ, and thermal desorption measurements. Ancillary experiments were also carried out on cuprous chloride for purposes of comparison with the Cu(111)-Cl data. Chlorine adsorption is rapid on both metals and follows precursor kinetics, the absolute initial sticking probabilities being ~1.0 (Cu) and ~0.5 (Ag). Δϑ results suggest that significant depolarisation of the chemisorption bond occurs at high coverages, the maximum values being + 1.2 eV (Cu) and + 1.8 eV (Ag). On Cu(111), adsorption leads to the formation of a sequence of well-ordered phases; in order of increasing coverage, these are as follows: (√3 × √3)R30°, (12√3 × 12√3)R30°, (4√7 × 4√7)R19.2°, and (6√3 × 6√3)R30°. On Ag(111) (√3 × √3)R30°, and (10 × 10) structures are observed. All six structures are susceptible to a straightforward interpretation in terms of coincidence lattices resulting from the progressive uniform compression of a hexagonal layer of Cl atoms. This interpretation is consistent with all the experimental results, and gives values for the nearest-neighbour ClCl spacing on both Cu(111) and Ag(111) which are in good agreement with other work on other surfaces. Chlorine desorbs exclusively as atoms from both metals with first-order desorption kinetics, and apparent desorption energies of 236 (Cu) and 209 (Ag) kJ mol −1. These values, which depend on an assumed pre-exponential factor of 10 13 s −1, are shown to be inconsistent with the thermochemical constraints on the system necessitated by the complete absence of Cl 2 desorption. Lower limits for the pre-exponential factors are then deduced, and the values are found to be consistent with the differences between the CuCl and AgCl systems.

Adsorption studies based on thermionic emission measurements: II. Oxygen on single-crystal tungsten

A simplified version of the cylindrical thermionic emission microscope has been used to investigate the adsorption and desorption of oxygen on a single-crystal tungsten filament. Data are presented for the dependence of the effective work functions of the (100), (110), (111) and (112) crystallographic directions on oxygen pressure and filament temperature in the range 1×10 −9–5×10 −6 Torr and 1900–2200 °K. The data for each crystallographic direction may be correlated quite well by means of the parameter T T R ∗ , where T is the filament temperature and t r ∗ is the saturation temperature corresponding to the oxygen pressure. In an attempt to determine the oxygen desorption energies for the various crystallographic directions, we have measured the rate of change of thermionic emission following a sudden change in filament temperature and/or oxygen pressure. We conclude that this technique does not yield the true desorption energy for individual crystallographic directions because of surface migration of oxygen between patches. The apparent desorption energies for the different directions are approximately equal, the average value being ~ 5.9 eV(~135 kcal mole ) . The work functions of the various directions were measured in vacuum (<1 ×10 −9 Torr) both before and after the filament was exposed at 2200°K to oxygen at 1×10 −6 Torr for 63 hours. Since this exposure did not produce significant changes in the work functions, we conclude that (i) the concentration of carbon impurities in the filament was not sufficient to affect the work function and (ii) the oxygen exposure did not produce significant irreversible changes in the filament surface.