Initial Heat Of Adsorption Research Articles

Adsorption and desorption of different gases on Ru film in the dark and under UV irradiation

The adsorption and desorption of some vapors and gases (water, hydrogen, dinitrogen oxide, carbon monoxide, oxygen) on Ru film has been investigated at 305 K using a Tian-Calvet microcalorimeter. The initial heat of adsorption, the irreversible coverage and the temperature of maximum desorption rate indicate the following binding energy order: H2>N2O>H2O>CO>O2. The amount of photoadsorption (for H2O, N2O, H2) and photodesorption (for CO and O2) has also been determined by recording the pressure changes induced by UV irradiation of the Ru film.

Calorimetric heats for CO and oxygen adsorption and for the catalytic CO oxidation reaction on Pt{111}

Single crystal adsorption calorimetry was applied to investigate the heats of adsorption of CO and oxygen and the reaction heats for the CO oxidation process on Pt{111} at room temperature. Both sticking probabilities and heats of adsorption for CO and oxygen are presented as a function of coverage. These results are used to interpret the subsequent measurements taken for the CO oxidation process on the same surface. The initial heats of adsorption of CO and oxygen on Pt{111} are 180±8 and 339±32 kJ/mol, respectively. In addition the pairwise lateral repulsive interaction between CO molecules in a (√3×√3)R30° ordered layer at θ=1/3 is found to be 4 kJ/mol. A detailed Monte Carlo modeling of the dissociative adsorption and sticking probability of oxygen on Pt{111} is performed. The initial rapid fall in heat is attributed to adsorption on defect sites, and subsequent adsorption on the planar {111} surface proceeds with a third neighbor interaction energy between the oxygen adatoms ω3∼22 kJ/mol. When gaseous CO reacts with preadsorbed oxygen adatoms, the CO2 produced has an excess energy of 16±8 kJ/mol.

Microcalorimetric study of ethylene adsorption on the Pt{111} surface

The reaction of C 2H 4 on Pt{111} at 300 K has been investigated by single crystal adsorption calorimetry. The initial heats of adsorption and the initial sticking probability are 174 kJ/mol and 0.67 respectively. The adsorption heat versus coverage data shows a remarkable minimum which serves as a basis for a mechanism involving ethylidene (CHCH 3) as an intermediate for the formation of ethylidyne (C 2H 3) on the Pt{111} surface. The coverage-dependent sticking probability strongly indicates that adsorption is precursor mediated and is well-fitted by the King and Wells expression. From the structure of the decomposition species known to form on the Pt{111} surface, the average CPt bond strength is found to be about 245 kJ/mol, although the actual value is found to be species-dependent.

Adatom bond energies and lateral interaction energies from calorimetry: NO, O2, and N2 adsorption on Ni{100}

Heats of adsorption and sticking probabilities have been measured for NO and O2 on clean and oxygen precovered Ni{100} at room temperature by single crystal adsorption calorimetry (SCAC). Adsorption is initially dissociative for both species and the initial heat of adsorption is 536 kJ/mol for O2 and 426 kJ/mol for NO. From these data the initial heat of dissociative adsorption of N2 on Ni{100} is estimated to be 136 kJ/mol. With increasing coverage the adsorption heat for both NO and O2 decreases due to repulsive interactions between adsorbed adatoms; the magnitude of the pairwise repulsive interactions between oxygen adatoms in nnn sites is estimated to be about 40 kJ/mol by fitting the experimental data for O2 dissociative adsorption with a Monte Carlo simulation. The pairwise repulsive interaction between nitrogen and oxygen and between nitrogen adatoms in nnn sites is obtained as about 100 kJ/mol. Due to the high magnitude of lateral repulsive interactions between adatoms, a critical coverage of NO is reached for which molecular adsorption is energetically favored with respect to dissociative adsorption. This coverage is ∼0.16 ML for a 1:1 O:N adatom coverage decreasing to ∼0.07 ML for 3:1 O:N adatom ratio. For O2 adsorption, the lower magnitude of repulsive interactions allows dissociative adsorption to take place up to a coverage of about 0.3–0.4 ML; above this coverage the bulk oxide is more energetically favorable and an oxide layer is formed. NO dissociative adsorption is precursor mediated, whereas for O2 the precursor plays only a limited role.

Energetics and kinetics of CO and NO adsorption on Pt{100}: Restructuring and lateral interactions

Calorimetric heats of adsorption and sticking probabilities are reported for NO and CO on both the reconstructed hex and the unreconstructed (1×1) surfaces of Pt{100} by single crystal adsorption calorimetry (SCAC), at room temperature. The hex surface reverts to the (1×1) structure during adsorption of both gases, as previously reported. The initial heat of adsorption on the (1×1) surface is 215 kJ/mol for CO and 200 kJ/mol for NO. Adsorbate–adsorbate interactions determine not only the dependence of the heat of adsorption on coverage but also the formation of different ordered structures. A model is suggested to explain the observed dependence of the differential heat on coverage and the LEED patterns, and a Monte Carlo simulation is performed to derive the corresponding differential heat, thus allowing estimates to be made of the magnitude of adsorbate–adsorbate interactions. For CO adsorption, the critical contribution is the pairwise interaction energy εd between molecules in nnn sites while for NO triplet formation is suggested with significant repulsive interaction between molecules in the same triplet (εt) and an even stronger repulsion between triplet pairs (εtt). NO–NO repulsive interactions (εt=20 kJ/mol, εtt=80 kJ/mol) are considerably stronger than CO–CO interactions (εd=5 kJ/mol); thus, at half monolayer coverage CO gives rise to a c(2×2) pattern while NO gives a c(2×4) pattern. Moreover, with CO the coverage can be increased to 0.75 ML, with the formation of compressed structures, while for NO the saturation coverage is just 0.5 ML. The differential heat on the hex surface is also discussed showing the possible role of adsorption at defect sites in the energetics of the system. The surface energy difference between the clean (1×1) and hex surfaces is obtained as 20 kJ(mol Pts)−1 by comparing the integral heats of adsorption of CO on both surfaces at θ=0.5, when the final states of the two systems are identical.

Microcalorimetric Heats of Adsorption for CO, NO, and Oxygen on Pt{110}

The reactions of CO, NO, and oxygen on platinum surfaces have received a great deal of research attention, because of both their industrial importance in the three-way automobile catalytic converter and the academic interest surrounding the kinetic oscillations observed under some conditions. Crucial parameters in many of the kinetic models of these systems are the coverage-dependent heats of adsorption of the reacting species, which, with the exception of isosteric data for CO, have not previously been determined experimentally. Here the results of microcalorimetric measurements of the heats of adsorption of CO, NO, and O2 as a function of coverage on Pt{110} are presented. The initial adsorption heats are 183, 160, and 335 kJ mol-1, respectively.

Influence of the Framework Density and the Effective Electronegativity of Silica and Aluminophosphate Molecular Sieves on the Heat of Adsorption of Nonpolar Molecules

The influence of the framework density, the chemical composition, and derived from those the influence of the effective electronegativities of silica and aluminophosphate molecular sieves on the adsorption properties have been investigated experimentally and theoretically. By using the electronegativity equalization method (EEM), the average effective electronegativities have been determined. A correlation of the initial heats of adsorption (as measured by calorimetry) of nonpolar molecules such as ethane, n-hexane, and benzene is found with the framework density as well as with the effective electronegativity of the uncharged frameworks. An increasing density of the microporous solids and a corresponding decrease of the effective electronegativity results in an increase of the initial heats of adsorption. Because of a compensation effect of the entropy, the free enthalpy of adsorption increases less with rising framework densities than the enthalpy of adsorption does.

Kinetic modeling of hydrogen adsorption/absorption in thin films on hydrogen-sensitive field-effect devices: Observation of large hydrogen-induced dipoles at the Pd-SiO2 interface

A kinetic modeling of the hydrogen interaction with a Pd-SiO2-Si (Pd-MOS) device is reported. The model is fitted to a number of experimental results, mainly from mass spectrometric desorption measurements. The new results verify an older description of the energetics of hydrogen adsorption states at the Pd/SiO2 interface in the sense that the adsorption may be described by a so-called Temkin isotherm; however, several new findings have to be incorporated into the model in order to obtain a consistent picture. The initial heat of adsorption at the interface is around 0.8 eV/hydrogen atom. The number of adsorption sites at the interface is considerably smaller than at the surface, 6×1017 m−2 versus 1.5×1019 m−2. Furthermore, the interface hydrogen atoms are strongly polarized. An average value of 2 Debye is obtained. It is the large hydrogen polarization at the Pd/SiO2 interface and not a large concentration of adsorbed hydrogen atoms per se which accounts for the very high sensitivity of a Pd-MOS device as hydrogen sensor.

On the kinetic interpretation of metal-support interaction

Metal-support interaction (MSI) in catalysis is discussed in terms of the surface electron gas model (SEGM), which accounts for the case of inhomogenous surface. In the case of MSI the initial heat of adsorption as well as the effective charge of adsorbed particles can differ from that in the absence of metal-support interaction. Conditions at which either negligible small adsorption coefficients (reaction rate) or their enhancement compared to non-MSI conditions are presented.

Model of atomic hydrogen adsorption on thin gold film surface

A model for hydrogen adsorption on surface of thin gold film was proposed on the basis of measured parameters: (i) initial heat of adsorption; (ii) activation energy for desorption at low coverages (θ ≊ 0.01); (iii) heat of hydrogen solution in the bulk of thin gold film; and (iv) calculated activation energy for dissociative H 2 adsorption.

Effect of cations on methane adsorption by NaY, MgY, CaY, SrY, and BaY zeolites

Methane isotherms of NaY, MgY, CaY, SrY, and BaY zeolites are measured between 25 and 70 o C and upto 6760 kPa. All isotherms are of type I. The initial heat of adsorption for divalent cationic forms decreased with decreasing charge density (CaY>MgY>SrY>BaY) except for MgY. Similar anamoly was observed for methane adsorption on MgX (Zhang, S. Y.; Talu, O.; Hayhurst, D. T. J. Phys. Chem. 1991, 95, 1722) and it is attributed to incomplete dehydration (activation) at normal activation temperatures.. The cation type significantly affected adsorption properties even at loadings as high as 7.5 molecules/cavity

Adsorption of CO2over univalent cation-exchanged ZSM-5 zeolites

Isotherms and IR spectra were measured for CO2-MZSM-5 (M = Li+, Na+, K+, Rb+, Cs+) adsorption systems. The observed adsorption characteristics were well approximated by a physical adsorption where the van der Waals force and an electrostatic interaction force are operating: the degree of irreversible adsorption (chemisorption) was less than 10% of the total adsorption. Although the adsorbed CO2 molecule interacts with both the cation and the pore wall, only the CO2-cation interaction can convert the IR inactive ν1 vibration into an IR active state. The adsorption model proposed enables us to calculate the initial heat of adsorption as well as the molecular orientation angle of CO2 against the cation site.

Calorimetric heats of adsorption for CO on nickel single crystal surfaces

An adsorption calorimeter for studies on well-defined single crystal surfaces under ultrahigh vacuum conditions is now available, based on supersonic molecular beam dosing onto ultrathin metal single crystals. Here we discuss the relationship between the calorimetric heat of adsorption as measured in this system and the related parameters: the differential heat of adsorption, the isosteric heat, and the Arrhenius desorption energy. Coverage-dependent calorimetric heats of adsorption and sticking probabilities for CO on Ni{111}, {110}, and {100} are presented, and comparisons made with literature values for isosteric heats and Arrhenius desorption energies. At intermediate coverages some significant discrepancies occur which are attributed to a temperature-dependent adlayer structure. By combining sticking probability with heat measurements at high coverage, at 300 K, where significant desorption occurs, the desorption preexponential has been accurately determined; differential entropies of adsorption are also obtained. Differences in initial heats of adsorption and in the coverage dependencies for the three crystal planes are discussed, particularly in relation to surface stoichiometry, and to CO–CO interactions.

Complementary study by calorimetry and infrared spectroscopy of alkali metal doped Pd/SiO 2 solids: adsorption of hydrogen and carbon monoxide

Alkali metal (Li, Na, K) promoted Pd/SiO 2 catalysts were prepared by adding nitrates to the Pd/SiO 2 precursor and subsequent hydrogen reduction. The initial heats of adsorption of carbon monoxide and of hydrogen at 298 K and at 473 K and the variations of the differential heats with coverage were measured by calorimetry. The changes caused by the presence of the alkali: decrease in the initial heat of hydrogen adsorption at 298 K in the presence of lithium and increase in the initial heat of carbon monoxide adsorption at 298 K in the presence of potassium and sodium are very small (ca. 10%). These results, and the conclusions deduced from the infrared spectra of adsorbed carbon monoxide on the same samples, are compared. They are discussed assuming that alkali cations interact with the oxygen atom of adsorbed carbon monoxide. Calorimetry clearly shows that adsorption heats of hydrogen and carbon monoxide do not change appreciably when alkali cations are present on palladium although infrared spectfoscopy of adsorbed carbon monoxide and catalytic properties are modified. Thus initial adsorption heats determined by calorimetry and catalytic activities are not directly related.

Correlation between initial heats of adsorption and structural parameters of molecular sieves with different chemical composition - a calorimetric study

Abstract

Heat of adsorption of hydrogen on evaporated iron layers

The differential heats of adsorption of hydrogen on iron films prepared under UHV conditions have been measured calorimetrically at 298, 323 and 363 K. The initial heat of adsorption was 97±6 kJ/mol at 298 K, 100±4 kJ/mol at 323 K and 97±7 kJ/mol at 363 K. The maximal degree of coverage of the deposited iron surfaces with hydrogen was 0.85±05 at 298 K, 0.64±0.05 at 323 K and 0.61±0.06 at 363 K. The minimal degree of coverage to which hydrogen could be isothermally desorbed and readsorbed was 0.55±0.04 at 298 K, 0.33±0.05 and 0.30±0.06 at 363 K. The coverage range can be divided into three distinct regions. The extent of the first region, where the heat of adsorption is constant and equal to the initial heat of adsorption, is increasing with temperature and at 363 K is equal to about 70% of the whole iron surface allowed for hydrogen adsorption, about 40% of that amount can be isothermally desorbed and readsorbed. The extent of the second region, where the heat of adsorption was changing with coverage from 85 to 65 kJ/mol, is decreasing with temperature and at 363 K is vanishingly small. At the third region the heat of adsorption is continuously dropping to zero, the rate of heat evolution measured calorimetrically is much higher than the corresponding rate calculated from the sticking probability data, which can be assumed as caused by an adsorption-induced iron film reconstruction.

Potential energies and IR spectra for homopolar diatomic adsorbates over ion-exchanged ZSM-5 zeolites

Potential energies for N2, D2, and O2 entrapped in pores of MeZSM-5 (Me = Li, Na, K, Rb, and Cs) zeolites have been investigated theoretically as a basis for understanding the characteristic IR spectra of the adsorption systems. The adsorption potential proposed gives a reasonable value for the initial heat of adsorption q st/0 and enables us to predict the mode of orientation of an admolecule at the cation site. Specific predictions include an end-on orientation for N2, a lying-down orientation for D2, and a random orientation for O2. Analysis of the IR spectral intensity of the adsorbed molecule proves also that the predicted molecular orientation is realistic. Furthermore, the IR peak shifts predicted theoretically by using the adsorption potential are in good agreement with the experimental results. The influence of the orientation of the admolecule upon the IR peak shift is thus clarified.

Lead selenide as a polar adsorbent for gas chromatography

The properties of lead selenide (PbSe), a crystalline adsorbent with a simple cubic lattice, were investigated by means of gas chromatography. The retention volumes V g (per gram of adsorbent), the relative retention volumes V rel (with respect to n-alkane), Kováts retention indices and differential initial heats of adsorption with a close-to-zero filling of the surface for different classes of compounds were determined on PbSe. The presence of π-bonds in aromatic compounds, a free electron pair on the oxygen atom in the esters and hydroxyl groups in the alcohols is shown to make considerable contributions to adsorption on PbSe. The characteristics of adsorption on PbSe were compared with those on adsorbents and liquid stationary phases of different polarity. It is shown that PbSe can be classified as a medium-polarity adsorbent with a homogeneous surface. Columns with PbSe were used for the separation of oxygen- and nitrogen-containing compounds.

Calculation of heat adsorption of certain halomethanes on graphitized thermal black

Initial heats of adsorption of a number of halomethanes on graphite have been calculated by a semiempirical molecular-statistical method and from the proportionality of the heat of adsorption to critical parameters of the adsorbate. The results from these calculations are in satisfactory agreement with each other and with the available experimental data.

Hydrogen adsorption and hydrogen-deuterium equilibration on sulfided ruthenium and bulk ruthenium sulfide catalysts

The effect of presulfidization on hydrogen adsorption over ruthenium catalysts has been studied using volumetric adsorption and H 2D 2 equilibration. On reduced ruthenium catalysts (101 kPa of H 2 at 673 K for 8 h) 10.2 μmol of H 2/m 2 of ruthenium (measured at 373 K) was irreversibly adsorbed, the activation energy for adsorption was negligible and the initial heat of adsorption was estimated to be between −109 and −126 kJ/mole H 2. On “mildly sulfided” ruthenium catalysts (101 of kPa 10% H 2S H 2 at 673 K for 2 h, S a Ru (s) = 0.25 ) both reversible and irreversible hydrogen adsorption were suppressed at temperatures below 473 K. Results of H 2D 2 equilibration kinetics on these catalysts (508 ≤ T <- 623 K) revealed an activation energy of 71 ± 2 kJ/mole. On “extensively sulfided” ruthenium catalysts (101 kPa of 100% H 2S at 673 K for 2 h, i.e., crystalline RuS 2 at the surface) partial restoration of the H 2 adsorption capacity was observed (6.5 μmol/m 2 at 373 K). H 2D 2 exchange rates at 573 K were ca. two orders of magnitude faster than those over mildly sulfided samples. The activation energy for hydrogen adsorption was 32 ± 2 kJ/mole (on RuS 2-like surfaces) and the heat of adsorption was calculated to be −269 kJ/mole H 2. It is suspected that this large heat of adsorption may result in higher hydrogen coverages at HDS reaction conditions. The above noted differences in hydrogen accommodation provide a unique means for understanding the selectivity toward hydrogenated products which extensively sulfided ruthenium Catalysts possess.