Heat Of Chemisorption Research Articles

Heat of molecular chemisorption from bond-order-conservation viewpoint: Why morse potentials are so efficient

Our analytic Morse-potential model of chemisorption based on bond-order conservation [Surface Sci. 150 (1985) L115; 163 (1985) L730] has been extended to calculate the heat of chemisorption Q AZ of molecules AZ coordinated parallel to a metal surface, where A and Z may be either the same or different atoms or atomic groups (“quasiatoms”). Examples include O 2, CO, H 2CCH 2, HC≡CH, H 2CO, and (CH 3) 2CO, where comparisons with the perpendicular AZ coordinations, considered earlier, are also made. The projected qualitative trends and quantitative estimates are in agreement with (scarce) experiment. It is argued that, within the bond-order-conservation framework, the efficiency of Morse potentials to describe the energetics of various chemisorption phenomena ultimately originates from the zero-energy gap between the occupied and vacant parts of the metal band.

Bond making and breaking on transition-metal surfaces: Theoretical projections based on bond-order conservation

Our analytic Morse-potential model of chemisorption based on bond-order conservation [Surface Sci. 150 (1985) L115; 163 (1985) L645, L730] has been used to calculate the heats of chemisorption of various diatomic AB and polyatomic AB x species (coordinated via A) and to estimate the activation barriers for their dissociation and transformations. Examples include adspecies such as CH x , NH x , OH x , and possible intermediates and elementary steps of reactions such as CO+O→CO 2, NO+N→N 2, N 2+H 2→NH 3, H 2+O 2→H 2O, and CO+H 2→ CH 4. Both the qualitative projections and numerical estimates are in good agreement with experiment. In particular, it is shown that (1) the most reactive adspecies should be the most weakly bound, and (2) the recombination activation barrier should primarily depend on (and may even be close to) the heat of chemisorption of the weaker bound partner.

Bond making and breaking on transition-metal surfaces: Theoretical projections based on bond-order conservation

Our analytic Morse-potential model of chemisorption based on bond-order conservation [Surface Sci. 150 (1985) L115; 163 (1985) L645, L730] has been used to calculate the heats of chemisorption of various diatomic AB and polyatomic AB x species (coordinated via A) and to estimate the activation barriers for their dissociation and transformations. Examples include adspecies such as CH x , NH x , OH x , and possible intermediates and elementary steps of reactions such as CO + O → CO 2, NO + N → N 2 + O, N 2 + H 2 → NH 3, H 2 + O 2 → H 2O, and CO + H 2 → CH 4. Both the qualitative projections and numerical estimates are in good agreement with experiment. In particular, it is shown that (1) the most reactive adspecies should be the most weakly bound, and (2) the recombination activation barrier should primarily depend on (and may even be close to) the heat of chemisorption of the weaker bound partner.

Coadsorption, promotion, and poisoning effects: Analytic modeling based on bond-order conservation

Our bond-order conservation model [ J. Am. Chem. Soc. 106 (1984) 6479; Surface Sci. 150 (1985) L115, 163 (1985) L645, L730] has been extended to treat effects of an atomic modifier D, either electronegative or electropositive, on coadsorbed atoms A and diatomic molecules AB. Taking a fcc(100) surface as an example, explicit formulas have been obtained for the relevant heats of chemisorption Q A and Q AB as a function of the D coverage θ D when D occupies the hollow site, for three typical cases: (i) a single D; (ii) a p(2 × 2) structure, θ D = 1 4 ; (iii) a c(2 × 2) structure, θ D = 1 2 . The values of Q A and ( Q AB) been estimated for various on-top, bridge, and hollow sites for a realistic range of the parameter r = Q D Q A (Q AB) . Possible corrections due to direct D-A and D-AB interactions, both attractive and repulsive, are discussed. We consider coadsorption effects on the stretch frequency ν AB and dissociation activation barrier Δ E ∗ AB and project a variety of patterns depending on the nature of D, coverages θ D and θ AB, and some other parameters. The results obtained are in encouraging agreement with experiment. The complex nature of coadsorption effects is stressed.

Anomalous temperature dependence of the electrical conductivity of zinc oxide thin films

The temperature dependence of the conductivity of Ga-doped ZnO thin films was examined in humid and dry atmospheres. The conductivity initially increases to the maximum (range 1) and then decreases to the minimum (range 2) and again increases (range 3) in the heating run under humid air atmosphere. The subsequent cooling reduces the conductivity to less than that for the heating run. The extent of the reduction becomes large as the cooling rate increases. The extrema character in the conductivity versus temperature curves does not disappear under nitrogen gas atmosphere provided it is humid. On the other hand, the conductivity in the ranges 1 and 2 decreases so significantly that the extrema character disappears in the driest air as possible. These behaviors are explained by assuming that the dissociatively chemisorbed states of water vapor act as electron donors and desorb at the higher temperature, and the desorption as well as the chemisorption require a time for attaining equilibrium. With this in view, a theoretical analysis is given of semiconducting films thinner than the depletion boundary layer thickness, and numerical curves fitting to the experimental results are obtained. The heat of chemisorption of nonionized water q° is 1.35 eV and the energy levels of water chemisorption donors and oxygen chemisorption acceptors are at 0.66 and 0.59 eV below the conduction band edge, respectively.

Chemisorption phenomena: Analytic modeling based on perturbation theory and bond-order conservation

Tremendous advances in experimental studies of chemisorption revealed that many phenomena could not be understood and projected by the current theoretical constructs. We discuss some of the experimental puzzles that prompted a development of new analytic approaches to chemisorption based on general principles such as perturbation theory (PT) and bond-order conservation (BOC). The PT results concern the periodic regularities of the heat of chemisorption, the role of the antibonding adsorbate orbitals, and universal patterns of adsorbate-induced surface polarization Some of the PT findings are further corroborated within a much broader BOC approach. The BOC model and its postulates (including the use of a Morse potential) and diverse projections are thoroughly discussed. For atomic A and diatomic AB adsorbates, it is shown how the BOC model explicitly and rigorously interrelates a variety of seemingly disparate phenomena such as preferred adsorbate sites, the activation barriers for surface migration and dissociation, relations between atomic Q A ( Q B) and molecular Q AB heats of chemisorption, coverage and coadsorption effects on Q A, overlayer phase transitions and island formation, the nature of promotion and poisoning. The model also projects possible intermediates and elementary steps of surface reactions. Although some of the findings are counter to commonly held perceptions, the whole picture of chemisorption is coherent and fits experiment well. The new conceptual understanding is stressed and some comments on the theory of chemisorption are made.

Determination of isotherms and initial heat of adsorption of CO2 and N2O in four A zeolites from infrared measurements

Measurements of the v3 band area of CO2 and N2O adsorbed on KA, NaA, CaNaA and CaA zeolites at different temperatures and pressures have allowed us to obtain ‘infrared isotherms’ at very low coverage, from which initial heats may be deduced. A measurement of the band intensity for the two sorbates leads to the true isotherms. The advantages of this method are its high sensitivity at low coverage, the verification of the absence of water and the exclusion of chemisorption heat when it occurs, as in the case of CO2. The results have allowed us to analyse the mobility of both molecules on the four zeolites studied by using different models for the partition function.

Surface migration of molecular adsorbates revisited: Morse-potential modeling based on bond-order conservation

Our bond-order-conservation model of surface migration of molecular AB adsorbates [J. Am. Chem. Soc. 106 (1984) 6479] has been generalized to provide for A-B bond-order changes under migration and to search for energy stationary points. The results are rigorous and well defined. The new conclusions corroborate most of the previous findings but also lead to important corrections (e.g., the migration patterns of donor molecules) and new projections (e.g., monotonic destabilization of AB as its coordination increases without direct relevance to the heat of chemisorption and simple interrelations between molecular and atomic heats of chemisorption), in agreement with experiment.

Surface migration of molecular adsorbates revisited: Morse-potential modeling based on bond-order conservation

Our bond-order-conservation model of surface migration of molecular AB adsorbates [J. Am. Chem. Soc. 106 (1984) 6479] has been generalized to provide for A-B bond-order changes under migration and to search for energy stationary points. The results are rigorous and well defined. The new conclusions corroborate most of the previous findings but also lead to important corrections (e.g., the mogration patterns of donor molecules) and new projections (e.g., monotonic destabilization of AB as its coordination increases without direct relevance to the heat of chemisorption and simple interrelations between molecular and atomic heats of chemisorption), in agreement with experiment.

Coverage effects under atomic chemisorption: Morse-potential modeling based on bond-order conservation

Our recent analytical Morse-potential model based on bond-order conservation has been extended to treat coverage (θ) effects on the heat of atomic chemisorption Q. For highly symmetric surfaces such as fcc(111), fcc(100), and bcc(100), explicit expressions for Q versus θ have been obtained projecting regularities of Q( θ) and of the overlayer structures, in encouraging agreement with experiment. In particular, the model predicts that Q should typically decrease with θ (though at very low θ, Q can sometimes increase) and that there may be some critical coverage θ c<1 beyond which the second-order phase transition (hollow→bridge or on-top) will occur.

Coverage effects under atomic chemisorption: Morse-potential modeling based on bond-order conservation

Our recent analytical Morse-potential model based on bond-order conservation has been extended to treat coverage (θ) effects on the heat of atomic chemisorption Q. For highly symmetric surfaces such as fcc(111), fcc(100), and bcc(100), explicit expressions for Q versus θ have been obtained projecting regularities of Q( θ) and of the overlayer structures, in encouraging agreement with experiment. In particular, the model predicts that Q should typically decrease with θ (though at very low θ, Q can sometimes increase) and that there may be some critical coverage θ c < 1 beyond which the second-order phase transition (hollow → bridge or on-top) will occur.

A computational model of the energy of dissociative chemisorption

A computational surface potential method is developed to describe the energy barrier of dissociative chemisorption on fcc(111) surfaces. The energy at any point on the reaction coordinate, involving simultaneous tipping and bond elongation of a chemisorbed diatomic fragment, is determined by conservation of total bond order. The potential energy profile is determined by a balance between repulsive closed-shell interactions between the free end of the diatomic fragment with the surface atoms bound to the opposite end of the molecule and attraction between other surface atoms with the free end of the diatomic fragment. The heat of chemisorption of the atomic constituents plays a major role in determining the activation barrier of dissociation. There is considerable surface anisotropy in the activation barrier, with bridge sites being most favorable for dissociation and hollow sites least favorable. Activation barriers are directly dependent upon molecular vibration frequency for on-top and bridge sites. Low activation barriers computed for some sites correlate with low molecular vibrational frequencies of surface species. Comparison of computed activation barriers with an analytic formalism gives good correspondence.

Hydrogen and Oxygen Chemisorptions and Titrations by Platinum on Zirconia Catalysts Reduced between 673 and 1273 K

Some 10 wt % Pt supported on zirconia catalysts were investigated by hydrogen and oxygen chemisorptions and titrations by means of automated catharometric equipment (frontal method). Microcalorimetry was further used to determine the heats of oxygen chemisorption. The temperature of reduction by hydrogen, TR, was varied between 673 and 1273 K. At 673 K, the H and O coverages of Pt were about the same as for Pt supported on SiO2 (Euro Pt catalyst). The increase in TR from 673 to 873 K was accompanied by a strong decrease of the hydrogen to oxygen chemisorption ratio. It is proposed that this is due to Pt1– xZr x alloy formation, with small values of x (≤0·05). When TR was above 873 K, x determined by X-ray diffraction analysis increased up to 0·25. The hydrogen chemisorption is confined to Pt s, while the oxygen chemisorption takes place on Pt s and Zr sO x. The surface composition may be derived and is found twice as rich in Zr as the mean composition of the alloy particles.

The heat of oxygen chemisorption on carbonaceous char

The first step in the combustion of a char that has been formed from the pyrolysis of oil shale, lignite, or coal is the chemisorption of oxygen onto the active sites formed during the fragmentation reaction. This chemisorption process, can be associated with the way that the char was formed. It is definitely a function of the rate at which the raw material was pyrolyzed and the maximum pyrolysis temperature. Studies have been made which show that the chemisorption kinetics follow the theory normally associated with the chemisorption of gases on catalytic surfaces. However, when trying to determine the energy associated with the chemisorption of oxygen on chars, experimental difficulties were encountered which necessitated equipment and experimental changes for studying this type of reaction. A discussion of the problem and some of the observations are presented in this paper.

ChemInform Abstract: NMR STUDIES ON HYDROGEN CHEMISORPTION AND WATER FORMATION ON PLATINUM BLACK CATALYSTS

AbstractDie FT‐NMR‐Methode erlaubt eine Unterscheidung und Bestimmung von adsorbierten H2O und Hbei der Reduktion von Sauerstoff‐bedecktem Platinschwarz mit H2.

Toward A Coherent Theory of Chemisorption

Studies of chemisorption phenomena, the cornerstone of heterogeneous catalysis, have become the central part of contemporary surface science. As a result of the great variety of the available experimental techniques, a backlog of information, some of which conflicts with current theoretical constructs, has accumulated. New models that combine analytical and computational facets have now begun to appear, revealing intrinsic relations among seemingly disparate chemisorption phenomena. Among the major findings are (i) the crucial role of antibonding adsorbate orbitals in bond activation and in the heat of chemisorption, (ii) adsorbate-induced surface polarization leading to a decrease of the metal work function and to an increase of the surface core binding energy, and (iii) important differences between atomic and molecular adsorbate modes of bonding and surface migration.

Dissociation activation barrier and heat of chemisorption: A Morse-type analytical approach

The activation barrier ΔE ∗ AB for dissociation AB→A+B on transition-metal surfaces is analyzed within an additive Morse-type approach based on the bond-order conservation. It is shown that ΔE ∗ AB = D AB -(Q A +Q B )+Q A Q B (Q A +Q B ) , where D AB is the gas dissociation energy and Q A( Q B) is the heat of atomic chemisorption. Estimates of †E ∗ for H 2, N 2, O 2 , and NO are shown to be in reasonable agreement with experiment. The two-dimensional potential diagram of the metal-AB interactions is defined analytically and discussed in some detail.

Dissociation activation barrier and heat of chemisorption: A morse-type analytical approach

The activation barrier Δ E ∗ ABfor dissociation AB → A + B on transition-metal surfaces is analyzed within an additive Morse-type approach based on the bond-order conservation. It is shown that Δ E ∗ AB = D AB−( Q A + Q B + Q A Q B/( Q A + Q B) where D AB is the gas-phase dissociation energy and Q A (Q B ) is the heat of atomic chemisorption. Estimates of Δ E ∗ for H 2, N 2, O 2 , and NO are shown to be in reasonable agreement with experiment. The two-dimensional potential diagram of the metal-AB interactions is defined analytically and discussed in some detail.

Adsorption and conductivity studies in oxychlorination catalysis. Part 5.—Temperature-programmed desorption

The desorption of ethylene, 1,2-dichloroethane (EDC), carbon dioxide and other adsorbates from CuCl2 has been studied using temperature-programmed desorption (t.p.d.). A chemisorptive centre corresponding to an ethylene concentration of 5 × 10–9 mol m–2 has an energy of activation for desorption (Ed) of 73 kJ mol–1. Another chemisorptive site has a value of 57 kJ mol–1 for Ed when the ethylene coverage is 10–6–10–7 mol m–2. These magnitudes are closely related to previously reported heats of chemisorption, suggesting adsorption to be only weakly activated.EDC is shown to desorb readily whereas CO2 has three distinct values of Ed(85, 94 and 97 kJ mol–1). An assumed value of 1013 s–1 for the pre-exponential factor Ad is appropriate in Redhead's method of analysis; the kinetics are consistent with unimolecular decomposition.

NMR Studies on Hydrogen Chemisorption and Water Formation on Platinum Black Catalysts

AbstractFourier Transform NMR spectroscopy has been used to study the reduction of oxygen covered Pt‐black and the chemisorption of hydrogen. The 1H Knight shift is dependent on hydrogen surface coverage and Pt particle size. T1, and T2 relaxation times were measured for adsorbed H2O and H as a function of temperature. The results are discussed with respect to the surface stoichiometry and the mechanism of the catalytic water reaction, isosteric heats of hydrogen chemisorption, estimation of metal surface areas of Pt catalysts and surface mobility of adsorbed H2O and H.