Isothermal desorption of hydrogen from polycrystalline diamond films

The kinetics of isothermal H₂ desorption from polycrystalline diamond are studied in real time. The surface H coverage (θ_H) is measured by mass analyzing the recoiled H₊ ion signal during the desorption. We find that the H₂ desorption is 1st order in θ_H with an activation energy of 69 ± 6 kcal/mol and a prefactor of 10^{10.5 ± 0.9} s⁻¹. We suggest that formation of a C---C π-bond on the clean surface plays a key role in H₂ desorption from diamond, a view consistent with previous theoretical calculations of H₂ desorption from diamond.     

The sticking and dissociation of NH3 on W(110): a three-state model

The kinetics of the adsorption of NH₃ on W(110) and its subsequent dissociation have been investigated using molecular beam techniques and temperature programmed desorption (TPD) for surface temperatures ranging from 140 to 700 K. NH₃ shows a wide desorption peak around 270 K and a smaller peak at 170 K while H₂ and N₂, produced by dissociation, desorbed at 550 and 1350 K, respectively, with kinetic parameters similar to those reported for H and N generated by adsorption of H₂ and N₂. At normal incidence and for a surface temperature of 140 K, the NH₃ sticking coefficient was found to decrease from unity at a beam energy of 0.8 kcal/mol to 0.5 for a beam energy of 5.4 kcal/mol. The sticking coefficient generally decreases with surface temperature to a value of 0.05 at 700 K, but, for a 5.4 kcal/mol beam, it exhibits a relative minimum near 300 K. The reflection coefficient of NH₃, for an angle of incidence of 49°, increases with temperature and incident beam energy in agreement with the sticking measurements. The TPD peak positions, sticking and reflection data are all well reproduced by a three-state model based on simple kinetics. The model assumes that NH₃ initially traps in a molecular state and that dissociation occurs by thermal activation into an intermediate state. At no temperature is the sticking probability enhanced by increasing the kinetic energy of the incident molecules and there is no evidence for a direct dissociation channel which has a translational energy barrier less than 5.4 kcal/mol.     

Glycine on Pt(111): a TDS and XPS study

The adsorption and desorption of in situ deposited glycine on Pt(111) were investigated with thermal desorption spectroscopy (TDS) and X-ray photoelectron spectroscopy (XPS). Glycine adsorbs intact on Pt(111) at all coverages at temperatures below 250 K. The collected results suggest that the glycine molecules adsorb predominantly in the zwitterionic state both in the first monolayer and in multilayers. Upon heating, intact molecules start to desorb from multilayers around 325 K. The second (and possibly third) layer(s) are somewhat more strongly bound than the subsequent layers. The multilayer desorption follows zero order kinetics with an activation energy of 0.87 eV molecule⁻¹. From the first saturated monolayer approximately half of the molecules desorbs intact with a desorption peak at 360 K, while the other half dissociates before desorption. Below 0.25 monolayer all molecules dissociate upon heating. The dissociation reactions lead to H₂, CO₂, and H₂O desorption around 375 K and CO desorption around 450 K. This is well below the reported gas phase decomposition temperature of glycine, but well above the thermal desorption temperatures of the individual H₂, CO₂, and H₂O species on Pt(111), i.e. the dissociation is catalyzed by the surface and H₂, CO₂, and H₂O immediately desorb upon dissociation. For temperatures above 500 K the remaining residues of the dissociated molecules undergo a series of reactions leading to desorption of, for example, H₂CN, N₂ and C₂N₂, leaving only carbon left on the surface at 900 K. Comparison with previously reported studies of this system show substantial agreement but also distinct differences.     

Adsorption of hydrogen and deuterium on potassium-promoted Pd(100) surfaces

The adsorption of H₂ and D₂ has been studied on clean and K-promoted Pd(100) surfaces using thermal desorption, work function changes, ultraviolet photoelectron and Auger spectroscopy. The potassium adlayer significantly lowers the sticking coefficient (from 0.6 to 0.06 at θ_k = 0.2), and the uptake of hydrogen, but increases the desorption energy for H₂ desorption. Calculation showed that each potassium adatom blocks approximately 4–5 adsorption sites for H₂ adsorption. Atomization of hydrogen led to an increase of hydrogen uptake. The adsorption of potassium on the H-covered surface caused a significant decrease in the amount of hydrogen adsorbed on the surface (as indicated by less desorbing hydrogen below 500 K) and promoted the dissolution of H atoms into the bulk of Pd. The dissolved hydrogen was released only above 600–650 K. In the interpetation of the results the extended charge transfer from K-dosed Pd to the adsorbed H atoms and the direct interaction between adsorbed H and K adatoms are taken into account.     

Desorption of H2 from W(110) and Fe covered W(110) surfaces

The kinetics of H₂ desorption from H/W(110) and H/Fe₁/W(110) were studied by measuring work function changes Δø vs time at a number of temperatures. Combination with previously determined Δø vs coverage data and differentiation at various fixed coverages gave rate vs T data from which activation energies of desorption could be obtained. E vs coverage results agree well with previously determine ΔH_des results. In the case of H/Fe₁/W(110) this includes a rise from 20 to 30 kcal mol⁻¹ of H₂ at H/Fe = H/W > 0.3. Plots of rate −dθ/dt vs θ (θ being coverage in units of H/W) vary much more steeply than θ² at most coverages for both systems. The θ dependence can be explained almost quantitatively in terms of the variations of ΔH_des and surface entropy S_s with coverage, by assuming that rates of desorption are equal to the equilibrium rates of adsorption. The latter can be formulated thermodynamically, except for a sticking coefficient, s. Values for s(θ, T) can also be obtained and show relatively little temperature dependence.     

The reaction of H2S with surface oxygen on Ni(110)

The reactions of H₂S with predosed surface oxygen on Ni(110) surfaces were studied for a variety of coverage conditions. The primary reaction product is H₂O, but the details of the water formation and desorption depends on the coverage of both O and H₂S.

For high coverages of oxygen (p(2 × 1)−O; 0.5 ML), the reaction to form water is quantitative. The loss of oxygen from the surface (as measured by AES) is equal to the increase in sulfur coverage. XPS and HREELS measurements indicate the presence of chemisorbed H₂O immediately following large exposures of H₂S on the oxygen predosed surface at 110 K. Deuterium incorporation results suggest that the primary mechanism for these coverage conditions involves direct transfer of hydrogen from SH or H₂S moieties to the oxygen.

A second mechanism involving reaction of surface hydroxyl groups with surface hydrogen was also identified. This mechanism is particularly important for high coverages of oxygen (0.5 ML) and low coverages of H₂S (0.15 ML), where water desorption was observed at 235 K, but was not observed spectroscopically at 110 K. The sequential addition of two surface hydrogen atoms to surface oxygen is not an important mechanism in this system.

These reactions were modeled using a bond-order conservation method, and the model successfully reproduced the important mechanistic conclusions.     

Evidence for bicarbonate formation on vacuum annealed TiO2(110) resulting from a precursor-mediated interaction between CO2 and H2O

The reaction of CO₂ and H₂O to form bicarbonate (HCO⁻₃) was examined on the nearly perfect and vacuum annealed surfaces of TiO₂(110) with temperature programmed desorption (TPD), static secondary ion mass spectrometry (SSIMS) and high resolution electron energy loss spectrometry (HREELS). The vacuum annealed TiO₂(110) surface possesses oxygen vacancy sites that are manifested in electronic EELS by a loss feature at 0.75 V. These oxygen vacancy sites bind CO₂ only slightly more strongly (TPD peak at 166 K) than do the five-coordinated Ti⁴⁺ sites (TPD peak at 137 K) typical of the nearly perfect TiO₂(110) surface. Vibrational HREELS indicates that CO₂ is linearly bound at the latter sites with a ν_a(OCO) frequency similar to the gas phase value. In contrast, oxygen vacancies dissociate H₂O to bridging OH groups which recombine to liberate H₂O in TPD at 490 K. No evidence for a reaction between CO₂ and H₂O is detected on the nearly perfect surface. In sequentially dosed experiments on the vacuum annealed surface at 110 K, CO₂ adsorption is blocked by the presence of preadsorbed H₂O, adsorbed CO₂ is displaced by postdosed H₂O, and there is little or no evidence for bicarbonate formation in either case. However, when CO₂ and H₂O are simultaneously dosed, a new CO₂ TPD state is observed at 213 K, and the 166 K state associated with CO₂ at the vacancies is absent. SSIMS was used to tentatively assign the 213 K CO₂ TPD state to a bicarbonate species. The 213 K CO₂ TPD state is not formed if the vacancy sites are filled with OH groups prior to simultaneous CO₂+H₂O exposure. Sticking coefficient measurements suggest that CO₂ adsorption at 110 K is precursor-mediated, as is known to be the case for H₂O adsorption on TiO₂(110). A model explaining the circumstances under which the proposed bicarbonate species is formed involves the surface catalyzed conversion of a precursor-bound H₂O–CO₂ van der Waals complex to carbonic acid, which then reacts at unoccupied oxygen vacancies to generate bicarbonate, but falls apart to CO₂ and H₂O in the absence of these sites. This model is consistent with the conditions under which bicarbonate is formed on powdered TiO₂, and is similar to the mechanism by which water catalyzes carbonic acid formation in aqueous solution.     

Effects of surface treatments and annealing on carbon-based molecular sieve membranes for gas separation

A new method in preparing carbon-based molecular sieve (CMS) membranes for gas separation has been proposed. Carbon-based films are deposited on porous Al₂O₃ disks using hexamethyldisiloxane (HMDSO) by remote inductively coupled plasma (ICP) chemical vapor deposition (CVD). After treating the film with ion bombardment and subsequent pyrolysis at a high temperature, carbon-based molecule sieve membranes can be obtained, exhibiting a very high H₂/N₂ selectivity around 100 and an extremely high permeance of H₂ around 1.5 × 10⁻⁶ mol m⁻² s⁻¹ Pa⁻¹ at 298 K. The O₂/N₂ selectivity could reach 5.4 with the O₂ permeance of 2 × 10⁻⁷ mol m⁻² s⁻¹ Pa⁻¹ at 423 K.

During surface treatments, HMDSO ions were found to be more effective than CH₄, Ar, O₂ and N₂ ions to improve the selectivity and permeance. Short and optimized surface treatment periods were required for high efficiency. Without pyrolysis, surface treatments alone greatly reduced the H₂ and N₂ permeances and had no effect on the selectivity. Besides, without any surface treatment, pyrolysis alone greatly increased the H₂ and N₂ permeances, but had no improvement on the selectivity, owing to the creation of large pores by desorption of carbon. A combination of surface treatment and pyrolysis is necessary for simultaneously enhancing the permeance and the selectivity of CMS membranes, very different from the conventional pore-plugging mechanism in typical CVD.     

Effects of preadsorbed oxygen on the formation and decomposition of NCO on Rh(111) surfaces

The interaction of HNCO with oxygen dosed Rh(111) surface has been investigated by Auger electron, electron energy loss and thermal desorption spectroscopy. The presence of adsorbed oxygen exerted no apparent influence on the weakly adsorbed HNCO (T_p = 130 K). It promoted, however, the dissociative adsorption of HNCO by forming a strong O—H bond which prevented the associative desorption of HNCO. As a result no H₂ and NH₃ formation occurred, in contrast with the clean surface, and the surface concentration of irreversibly bonded NCO was also increased. New products of the surface reaction were H₂O and CO₂, in addition to CO and N₂ observed on a clean surface. From the behavior of the losses characteristic for the adsorbed NCO it appeared that the preadsorbed oxygen exerted a significant stabilizing effect on the NCO bonded to the Rh.     

The adsorption and decomposition of methanol on the Rh(100) surface

The adsorption and decomposition of methanol on the Rh(100) surface have been studied using high-resolution electron energy loss spectroscopy and thermal desorption mass spectrometry. Below 200 K, methanol is molecularly adsorbed and bonds to the surface via the oxygen atom. At 200–220 K, a saturated methanol layer undergoes two competing reactions: desorption and OH bond cleavage to form an O-bonded methoxy species. The methoxy species is stable to approximately 250 K. Between 250 and 320 K, a fraction of the methoxy species decomposes to form coadsorbed CO and hydrogen adatoms while the remainder recombines with hydrogen adatoms to desorb as molecular methanol. The hydrogen adatoms remaining on the surface desorb as H₂ between 270 and 400 K, and the CO desorbs between 450 and 550 K. Following a saturation exposure, approximately 0.2 monolayers of methanol decompose to eventually yield CO and H₂ as desorption products. These results are compared to the chemistry of methanol on other metal surfaces.     

Scattering of N2 from Ni(111)

A cold (T_rot<10 K) beam of N₂ with an initial translational energy of 0.40 eV strikes an Ni(111) surface at surface temperatures from 300 to 873 K at several incident angles from 15 to 60°. The rotational energy and angular distributions of the scattered molecules are probed using (2+1) resonance-enhanced multiphoton ionization. Molecules scattered in the specular direction have mean rotational energies that are independent of surface temperature, whereas those scattered at angles far from the specular show a dependence on surface temperature, caused likely by multiple collisions with the surface before escape. A rotational rainbow, seen in systems such as CO–Ni(111) and N₂–Ag(111), is not seen in this system. For molecules that scatter close to the specular direction, approximately 10% of the initial translational energy is converted into rotational energy of the scattered N₂. For surface temperatures above room temperature, the angular distributions indicate that molecules that scatter into low-J states also tend to exit in a broad peak (10–20° FWHM) near the specular, and this peak is broadened with increasing incident angle. The molecules that scatter into high-J states have a much broader distribution, indicating that they are trapped rotationally during the initial collision and suffer multiple collisions before leaving the surface.     

Photochemistry in ordered physisorbed monolayers of dinitrogen tetroxide

The effects of physisorption and two-dimensional ordering on the photochemistry of N₂O₄ were investigated. Ordered monolayers were prepared by adsorption of NO₂ at 100 K on a water-ice surface. Irradiation with a continuous light source in the wavelength region 300–400 nm or with pulsed laser radiation at 355 nm resulted in exclusive desorption of NO₂. This desorption was induced by electronic absorption directly in the adsorbate via a transition corresponding to the ( )¹B_2u←( )¹A_g transition in N₂O₄, as in the gas phase. However, the subsequent dynamics in the excited state were markedly different from the gas-phase counterpart. Time-of-flight mass spectrometry of NO₂ photodesorbed at 355 nm revealed a most probable fragment translational energy of ca. 17 meV; and the angular distribution of the nascent NO₂ was peaked sharply in a direction around 10° from the normal. It is apparent that, despite the weak interaction with the substrate, significant energy transfer occurs in the ordered physisorbed monolayer to yield nascent NO₂ with very low translational energy and a constrained angle of escape which is consistent with a high degree of adsorbate order and alignment.     

Adsorption of oxygen on Au(111) by exposure to ozone

Atomic oxygen coverages of up to 1.2 ML may be cleanly adsorbed on the Au(111) surface by exposure to O₃ at 300 K. We have studied the adsorbed oxygen layer by AES, XPS, HREELS, LEED, work function measurements and TPD. A plot of the O(519 eV)/Au(239 eV) AES ratio versus coverage is nearly linear, but a small change in slope occurs at Θ_O=0.9 ML. LEED observations show no ordered superlattice for the oxygen overlayer for any coverage studied. One-dimensional ordering of the adlayer occurs at low coverages, and disordering of the substrate occurs at higher coverages. Adsorption of 1.0 ML of oxygen on Au(111) increases the work function by +0.80 eV, indicating electron transfer from the Au substrate into an oxygen adlayer. The O(1s) peak in XPS has a binding energy of 530.1 eV, showing only a small (0.3 eV) shift to a higher binding energy with increasing oxygen coverage. No shift was detected for the Au 4f_7/2 peak due to adsorption. All oxygen is removed by thermal desorption of O₂ to leave a clean Au(111) surface after heating to 600 K. TPD spectra initially show an O₂ desorption peak at 520 K at low Θ_O, and the peak shifts to higher temperatures for increasing oxygen coverages up to Θ_O=0.22 ML. Above this coverage, the peak shifts very slightly to higher temperatures, resulting in a peak at 550 K at Θ_O=1.2 ML. Analysis of the TPD data indicates that the desorption of O₂ from Au(111) can be described by first-order kinetics with an activation energy for O₂ desorption of 30 kcal mol⁻¹ near saturation coverage. We estimate a value for the Au–O bond dissociation energy D(Au–O) to be 56 kcal mol⁻¹.     

Formaldehyde decomposition and oxidation on Pt(110)

The decomposition reactions of formaldehyde on clean and oxygen dosed Pt(110) have been studied by LEED, XPS and TPRS. Formaldehyde is adsorbed in two states, a monolayer phase and a multilayer phase which were distinguishable by both TPRS and XPS. The saturated monolayer (corresponding to 8.06 × 10¹⁴ molecules cm⁻²) desorbed at 134 K and the multilayer phase (which could not be saturated) desorbed at 112 K. The only other reaction products observed at higher temperatures were CO and H₂ produced in desorption limited processes and these reached a maximum upon saturation of the formaldehyde monolayer. The desorption spectrum of hydrogen was found to be perturbed by the presence of CO as reported by Weinberg and coworkers. It is proposed that local lifting of the clean surface (1 × 2) reconstruction is responsible for this behaviour. Analysis of the TPRS and XPS peak areas demonstrated that on the clean surface approximately 50% of the adsorbed monolayer dissociated with the remainder desorbing intact. Reaction of formaldehyde with preadsorbed oxygen resulted in the formation of H₂O (hydroxyl recombination) and CO₂ (decomposition of formate) desorbing at 200 and 262 K, respectively. The CO and H₂ desorption peaks were both smaller relative to formaldehyde decomposition on the clean surface and in particular, H₂ desorbed in a reaction limited process associated with decomposition of the formate species. No evidence was found for methane or hydrocarbon evolution in the present study under any circumstances. The results of this investigation are discussed in the light of our earlier work on the decomposition of methanol on the same platinum surface.     

H3正三角形和H4正四面体结构的变分计算

采用改进排列通道量子力学(Modified Arrangement Channel Quantum Mechanics,简称MACQM)方法和变分法,计算了H₃体系正三角形和H₄体系的正四面体结构的能量曲线.当H₃体系原子核的间距R=1.74a₀,波函数变分参数α=1.03时,体系能量有最低值-1.58161 a.u.;当H₄体系原子核的间距R=1.60a₀,波函数变分参数α=1.07时,体系能量有最低值-2.28097 a.u.,这表明H₃体系的正三角形构型和H₄的正四面体结构是可以稳定存在的.     

Photodesorption and photofragmentation of disilane adsorbed on a hydrogen terminated Si(100) surface

The photochemical mechanisms leading to the desorption and fragmentation of Si₂H₆ adsorbed on a hydrogen terminated Si(100) surface have been explored by recording the time-of-flight distributions of products escaping from the surface and by using electron energy loss spectroscopy to probe possible electronic excitations. Photodesorption of intact Si₂H₆ involves hot electrons that lose energy and move to the conduction band edge before initiating desorption. When the wavelength of the incident light is 193 nm, Si₂H₆ fragments give mostly Si, SiH₂, H₂ and SiH₄, but this pathway is quenched at longer wavelengths. This is consistent with direct excitation, but we also show that a negative ion resonance is accessible to substrate electrons that have been excited by 193 nm light.     

Study of ion desorption induced by a resonant core-level excitation of condensed H2O using Auger electron photo-ion coincidence (AEPICO) spectroscopy combined with synchrotron radiation

Ion desorption induced by a resonant excitation of O 1s of condensed amorphous H₂O has been studied by total ion and total electron yield spectroscopy, nonderivative Auger electron spectroscopy (AES) and Auger electron photo-ion coincidence (AEPICO) spectroscopy. The spectrum of total ion yield divided by total electron yield exhibits a characteristic threshold peak at hν = 533.4 eV, which is assigned to the 4a₁ ← O 1s resonant transition. The AES at the 4a₁ ← O 1s resonance is interpreted as being composed of the spectator-AES of the surface H₂O, and the normal-AES of the bulk H₂O, where the 4a₁ electron is delocalized before Auger transitions. H⁺ is found to be the only ion species in AEPICO spectra measured at the 4a₁ ← O 1s resonance and at the O 1s ionization (hν = 560 eV). The electron kinetic energy dependence of the AEPICO yield (AEPICO yield spectrum) at the 4a₁ ← O 1s resonance is found to be greatly different from that at the O 1s ionization. The peak positions of the AEPICO yield spectrum at the 4a₁ ← O 1s resonance are found to correspond to those of the spectator-AES of the surface H₂O, which is extracted from the AES at the 4a₁ ← O 1s resonance. Furthermore, the AEPICO yield is greatly enhanced at the 4a₁ ← O 1s resonance as compared with that at the O 1s ionization. On the basis of these results, a spectator-Auger-stimulated ion desorption mechanism and/or ultra-fast ion desorption mechanism are concluded to be responsible for the H⁺ desorption at the 4a₁ ← O 1s resonance. The enhancement of the H⁺ yield is ascribed to the O---H anti-bonding character of the 4a₁ orbital.     

吸附氢分子的振动态及熵的计算

用第一性原理方法研究了H_2在(MgO)_9及(AlN)_(12)团簇上的吸附态、振动模式及熵.分析表明,吸附体系的振动中有六个简正模式可归为氢分子的振动;由于氢分子质量很小,零点能修正对吸附能有重要影响.利用振动配分函数计算了吸附氢分子的熵,表明吸附态H_2的熵主要决定于较低的同相振动的频率,并不完全与吸附强度相关;在标准大气压下70—350 K的温度范围内,吸附H_2的熵与气态H_2的熵之间存在很好的线性关系,吸附后H_2的熵减小约10.2R.     

A THEORETICAL MODEL FOR H2 DISSOCIATIVE ADSORPTION ON Cu SURFACES

A theoretical model for describing H₂ dissociative chemisorption on Cu surfaces is proposed. The sticking probability S is calculated as a function of vibrational state, average kinetic energy and incident angle of hydrogen molecular beam. Within the theoretical frame of this model, the different contributions to S from H₂(v = 0) and H₂(v = 1) can be clearly distinguished. The calculated results indicate that vibrational energy significantly promotes the chemisorption of H₂ on Cu surfaces in the region of low translational energy. The equations derived can be used to analyze the experimental data for both pure and seeded molecular beams.     

Hydrogen adsorption and desorption on the Pt and Pd subnano clusters – a review

In this review, we present our recent first principles studies on the sequential H₂ dissociative chemisorption and H desorption on the Pt_n and Pd_n clusters (n=2-9, 13). Upon full saturation by H atoms, the calculated H₂ dissociative chemisorption energy and H desorption energy on Pt₁₃ and Pd₁₃ clusters are similar to the corresponding values on smaller close-packed clusters. Indeed, the catalytic performances of these subnano clusters do not vary significantly with the particle sizes or shapes. Instead, they are dependent on the surface metal atoms which can be accessed by H atoms. In addition to the coverage dependency of the H₂dissociative chemisorption and H sequential desorption energies, the phase transition of both Pt₁₃ and Pd₁₃from the icosahedral to fcc-like structures at certain H coverage was also investigated.