Non-dissociative Adsorption Research Articles

Rate expressions for adsorption and desorption kinetics with precursor states and lateral interactions

Experimental and theoretical evidence for the existence of mobile, transitory precursor states and for the importance of lateral interactions between chemisorbed species is briefly surveyed. Generalised rate expressions for adsorption and desorption are derived which specifically take into account the existence of intrinsic and extrinsic precursor states, and which can be readily adapted to account for dissociative or non-dissociative adsorption, with and without interactions, and which also account for the possibility of direct transfer to or from the chemisorbed state, without trapping in the intrinsic precursor state. These expressions, derived using kinetic schemes, are shown to be identical with rate expressions obtained from a successive site statistical model. Thermodynamic conditions are used to restrict the number of parameters in these expressions.

Chemisorption of diatomic molecules (H 2, N 2, CO) on transition d-metals

The interaction energies of diatomic molecules, H 2, N 2, and CO with transition d-metal surfaces are empirically calculated using a bond energy bond order (BEBO) approach developed in earlier papers. The two problems, i.e., (i) does a molecule chemisorb on a given surface and (ii) whether the adsorbed species is in a molecular form or dissociative form, are examined from the potential energy curves obtained. The metals are then classified approximately into four groups, A, B, C, and D. The A group metals, which consist mainly of IV B, V B, and VI B group metals in the periodic table of the elements, strongly chemisorb all the above gases including O 2 and NO molecules in a dissociative form at room temperature. The group D metals consisting of noble d-metals, on the other hand, chemisorb NO and CO in a molecular form and do not decompose CO and N 2 molecules. The group B (Fe, Re) and C (Co, Ni, Tc) metals which are known as main elements of catalysts for the reactions of Fischer-Tropsch and ammonia syntheses have intermediate adsorption properties between the A and D group metals. These qualitative and quantitative results of heats of dissociative and nondissociative adsorption are summarized in tables and compared with the experiments.

Adsorption and decomposition of ammonia on Fe(110)

The interaction of NH 3 with a Fe(110) surface was studied by means of LEED, AES, UPS, work function and thermal desorption measurements. Below room temperature non-dissociative adsorption takes place into two states with bond strengths of about 17 and 10 kcal/mole leading to the formation of a partially disordered overlayer with a maximum coverage of θ = 1 6 (≙ 2.9 × 10 14 molecules/cm 2) and a maximum work function decrease by 2.4 eV. The dipole moment of the adsorbate is independent of coverage (2.2 Debye) and even larger than that of the free NH 3 molecule, which indicates that chemisorption is predominantly taking place through coupling of the N lone electron pair to the metal. NH 3,ad is characterized by valence ionization potentials of 6.7 and 11.2 eV below E F. The adsorption kinetics follows a simple first-order rate law with an initial sticking coefficient S 0 = 0.16. At 350 K a stable surface intermediate characterized by ionization potentials of -5.2 and -8.4 eV and a 2 × 2-LEED pattern is formed which is identified with NH ad. Above 400 K complete dissociation and desorption of H 2 takes place. The remaining adsorbed N-atoms recombine and desorb as N 2 only above 850 K. In contrast to the previously investigated Fe(111) plane adsorbed ammonia exhibits no isotopic exchange with adsorbed deuterium which is attributed to somewhat different rate constants of the consecutive steps of dissociation and recombination. This effect is also made responsible for the fact that with Fe(111) and (100) presumably NH 2,ad is observed as a transient intermediate, whereas with Fe(110) NH ad may be isolated on the surface. Preadsorption of N ad influences the adsorption energy and sticking coefficient of NH 3, but has no strong influence on the totally adsorbed amount.

Decomposition of ethylene on W(100) studied by Auger spectroscopy

The interaction of ethylene with a W(100) surface was studied in the temperature range 80–500 K by monitoring changes in the carbon Auger peak shape. On adsorption at 80 K decomposition to acetylene occurred at low coverage followed by non-dissociative adsorption. Heating the adsorbate to 300 K resulted in further decomposition to acetylene. Final decomposition to chemisorbed carbon atoms was detectable above 300 K and rapid above 400 K.

Entropies of adsorption in heterogeneous catalytic reactions

Langmuir-Hinshelwood (or Hougen-Watson) rate expressions are frequently used to correlate kinetic data. The equilibrium adsorption constants contained in these equations can be utilized to abstract values for standard enthalpies and entropies of adsorption. By using limiting, model cases to describe dissociative, dual-site adsorption, we have shown that the values obtained for Δ S a 0 must conform to certain rules in order to have any physical meaning and thereby support the proposed reaction model. Rules proposed earlier by Boudart, Mears, and Vannice for nondissociative adsorption have also been found to be applicable to dissociative adsorption.

Molecular orbital treatment of gas adsorption on 3d-metal clusters III. Adsorption of nitrogen

This third part of the series presents results for nitrogen atomic and molecular adsorption on 3d-metal clusters, received with the Hückel method in the frontier molecular orbitals approximation. Non-dissociative adsorption of nitrogen above the clusters of the metals from Sc to Cr proves energetically preferred. Above the clusters of Fe, Co and Ni the process is expected to be activated and endothermic. The activation barrier for dissociation in adsorbed state is however lower than that for the rupture of the N-N bond in an isolated nitrogen molecule. Dissociative adsorption above the clusters of Mn and Cu is activated. Reference is made to the experimental results, concerning 3d-metal adsorption and catalytic properties in the reaction of ammonia synthesis, and a satisfactory agreement with the numerical data is found.

Temperature range for growth of autoepitaxial GaAs films by MBE

The temperature range over which autoepitaxial films of GaAs can be prepared by molecular beam epitaxy (MBE) from beams of As 4 and Ga has been investigated by monitoring the growth process with in-situ RHEED. In this way it is possible to assess continuously during growth the crystallographic state of the surface. The results obtained have been compared with a chemical model of the growth process obtained from modulated beam mass spectrometric measurements. This model suggests that the minimum temperature should be determined by the onset of non-dissociative adsorption of As 4, while the upper limit will be set by thermal dissociation of GaAs. The results of the present investigation are consistent with this model, and in addition provide information on the temperature dependence of surface reconstruction.

The interaction of H 2S with Si(111) 7 × 7 surfaces by energy loss and Auger electron spectroscopies

Low energy electron loss spectroscopy (ELS) and Auger electron spectroscopy (AES) have been applied for the studies of the interaction of H 2S molecules with Si(111)7 × 7 surfaces. The observations are consistent with the interpretation that the room temperature non-dissociative adsorption state of H 2S molecules changes substantially after annealing at 550°C, resulting in the desorption of hydrogen and the covalent bond formation between silicon and sulfur atoms. The silicon disulfide films formed on Si(111) surfaces have been identified by the characteristic loss peaks in comparison with those of silicon dioxide.

Adsorption, coadsorption, and reactivity of sodium and nitric oxide on Ag(111)

The adsorption, desorption, and surface structural properties of Na and NO on Ag(111), together with their coadsorption and surface reactivity, have been studied by LEED, Auger spectroscopy, and thermal desorption. On the clean surface, non-dissociative adsorption of NO into the a-state occurs at 300 K with an initial sticking probability of ~0.1, saturation occurring at a coverage of ~ 1 20 . Desorption occurs reversibly without decomposition and is characterised by a desorption energy of E d ~ 103 kJ mol −1. In the coverage regime 0 < θ Na < 1, sodium adsorbs in registry with the Ag surface mesh and the desorption spectra show a single peak corresponding to E d ~ 228 kJ mol −1. For multilayer coverages (1 < θ Na < 5) a new low temperature peak appears in the desorption spectra with E d ~ 187 kJ mol −1. This is identified with Na desorption from an essentially Na surface, and the desorption energy indicates that Na atoms beyond the first chemisorbed layer are significantly influenced by the presence of the Ag substrate. The LEED results show that Na multilayers grow as a (√7 × √7) R19.2° overlayer, and are interpreted in a way which is consistent with the above conclusion. Coadsorption of Na and NO leads to the appearance of a more strongly bound and reactive chemisorbed state of NO (β-NO) with E d ~ 121 kJ mol −1. β-NO appears to undego surface dissociation to yield adsorbed O and N atoms whose subsequent reactions lead to the formation of N 2, N 2O, and O 2 as gaseous products. The reactive behaviour of the system is complicated by the effects of Na and O diffusion into the bulk of the specimen, but certain invariant features permit us to postulate an overall reaction mechanism, and the results obtained here are compared with other relevant work.

Interaction of NO and O 2 with Pd(111) surfaces. I.

The adsorption of NO on Pd(111) was studied by means of LEED, UPS and thermal desorption measurements. Non-dissociative adsorption is characterized by additional maxima in the photomission spectra at 2.6, 9.2 and 14.6 eV below the Fermi level originating from chemisorption levels which are derived from the highest occupied molecular orbitais of NO. Thermal desorption takes place from three distinct states (α, β and γ) corresponding to binding energies of about 15, 17 and 31 kcal/mole, respectively, with about equal populations. The α-state is associated with a 2 × 2 LEED pattern and the β-state with a c4 × 2 structure, whereas the γ-state corresponds to disordered adsorption at low coverages. Plausible structure models are proposed for the ordered structures with θ = 0.75 for the α-state and θ = 0.5 for the β-state. The strong decrease of the adsorption energy is explained in terms of pronounced short-range repulsive interactions between neighbouring adsorbate molecules.

Chemisorption and surface reactivity of nitric oxide on clean and sodium-dosed Ag(110)

The chemisorption of NO on clean and Na-dosed Ag(110) has been studied by LEED, Auger spectroscopy, and thermal desorption. On the clean surface, non-dissociative adsorption into the α-state occurs at 300 K with an initial sticking probability of ∼0.1, and the surface is saturated at a coverage of about 1 25 . Desorption occurs without decomposition, and is characterised by an enthalpy of E d ∼104 kJ mol −1 — comparable with that for NO desorption from transition metals. Surface defects do not seem to play a significant role in the chemistry of NO on clean Ag, and the presence of surface Na inhibits the adsorption of αNO. However, in the presence of both surface and subsurface Na, both the strength and the extent of NO adsorption are markedly increased and a new state (β 1NO) with E d ∼121 kJ mol −1 appears. Adsorption into this state occurs with destruction of the Ag(110)-(1 × 2)Na ordered phase. Desorption of β 1NO occurs with significant decomposition, N 2 and N 2O are observed as geseous products, and the system's behaviour towards NO resembles that of a transition metal. Incorporation of subsurface oxygen in addition to subsurface Na increases the desorption enthalpy (β 2NO), maximum coverage, and surface reactivity of NO still further: only about half the adsorbed layer desorbs without decomposition. The bonding of NO to Ag is discussed, and comparisons are made with the properties of α and βNO on Pt(110).

Oxygen (1s) binding energies in carbon monoxide adsorption on metals

The oxygen (1s) binding energy in the non-dissociative adsorption of CO on five metals (Ni, Pt, Cu, W and Mo) is shown to decrease as the heat of adsorption increases. When the heat of adsorption is the same on different metals the O(1s) binding energy is also the same. It is suggested that π bonding (back donation) is the major contribution to the M-CO bond. Correlation of the binding energy with the heat of adsorption suggests that, at 295 K, dissociative adsorption of CO occurs when ΔH ads ≳ 300 kJ mol −1.

Fem and volumetric studies of silver-nitrous oxide and silver-oxygen interactions

The non-dissociative and the dissociative adsorption of nitrous oxide and the adsorption of oxygen on silver have been studied by field-emission microscopy using whiskers and epitaxial layers on tungsten tips and volumetrically, with the aid of ultraclean thin films. At 77 K non-dissociative adsorption of nitrous oxide takes place, leading to a decrease in work function. At 273–473 K slow face-specific dissociative adsorption of nitrous oxide occurs, which causes an increase in work function and proceeds with an activation energy at low coverages of 29 ± 5 kJ mol −1. The adsorption of oxygen in this temperature range is more than 10 4 times faster and for low coverages work function-oxygen exposure plots yield an activation energy of 16 ± 3 kJ mol −1. The coverages reached above 1 Pa are constant and occur in the ratio 1:2:3.5 at 296, 373 and 473 K, the corresponding increases in work function being approximately 0.4, 0.6 and 0.8 eV. The oxygen adsorbed at low temperatures (≈ 273 K) is bound more loosely than that adsorbed at higher temperatures, which is shown by the partial desorption upon evacuation to low pressures (10 −8 Pa) at 273 K and application of high electric fields (5 V/nm). The adsorbate formed in the presence of oxygen at 273 K can further be distinguished from the adsorbates formed in the presence of nitrous oxide at 273 K and oxygen at 473 K (both probably O = ads) by the higher reactivity towards hydrogen reduction and the easier thermal desorption, indicating that at 273 K molecular adsorption (O − 2, ads) occurs.

Kinetics and Morphological Development of the Oxide Scale on Iron at 800°C in Oxygen at 2.5 × 10 − 3 to 3.0 × 10 − 1 Torr Pressure

A thermogravimetric technique was employed to investigate the oxidation of iron in oxygen over the pressure range . The oxidation curves exhibited distinct intervals of linear kinetics followed by transitions to intervals of parabolic kinetics during exposures extending to 125 min. Linear kinetics governed the growth of uniformly thick wustite scales; the linear rate constants showed a proportional dependence on oxygen pressure due to reaction control by a phase boundary reaction involving nondissociative adsorption of oxygen. The sticking coefficient of oxygen on wustite was equal to 0.067. Parabolic kinetics governed growth of wustite‐magnetite scales containing magnetite as outermost layers. The values of the parabolic rate constants were independent of oxygen pressure since scale growth was directly dependent on the iron vacancy gradient in wustite established by the oxygen activities at the and interfaces.

An infrared study of nitrous oxide adsorption on α-chromia

Room temperature adsorption of nitrous oxide on α-chromia has been studied by means of IR spectroscopy. Two different processes occur: (i) A rapid decomposition leading to a selective coverage of surface chromium sites with oxygen adsorbed through a quasi-double bond. (ii) A nondissociative adsorption on surface sites that are not covered with oxygen. Two types of N 2O ads species are observed and discussed.

The kinetics of adsorption on a nonuniform surface

The Roginsky-Zeldovich-Elovich equations describing the rates of chemisorption of gases onto solid surfaces are adequately accounted for in the case of nondissociative adsorption by assuming a surface consisting of only three to five sets of sites, each set comprising about the same number of sites. The rate constants for the adsorption onto these sets must fall within certain limits but the variation permissible is sufficient to allow appreciable variation between the sets in the activation energy for adsorption. The model can be extended to include a desorption process.

Chemisorption of Nitrogen on Tungsten

Chemisorption studies are made in a closed system by a mass spectrometer operated statically and it is found that tungsten surface has three chemisorption states α, e , and ζ for nitrogen. These chemisorption states have properties as follows; α is about 5% of the total saturation coverage and an unstable dissociative adsorption state of which the heat of chemisorption is 0.7 8 eV, e is 60%∼65% and a non-dissociative adsorption of 3.6 5 eV, and ζ is 35%∼30% and a dissociative adsorption of 3.0 eV. “Activation energies” of chemisorption for ζ and e -states are -0.07 5 and about -0.08 eV, respectively. The sticking probabilities are 0.5 for ζ and 0.02∼0.04 for e and about 0.2 for the apparent value of e at room temperature. At room temperature, adsorption to the three states occurs simultaneously at low coverages and the average sticking probability is 0.2. At the vicinity of the completion of the monolayer, however, the e -adsorption only takes place. As it is verified that the strongest chemisorption stat...