Calorimetric Heats Of Adsorption Research Articles

Room-Temperature Oxygen Dissociative Chemisorption on Carbon Surface-Experimental Evidence.

Oxygen interaction with the carbon surface is one of the most important topics of study in the field of material chemistry. In this work, experimental evidence for molecular oxygen dissociative chemisorption on a carbon surface at room temperature is shown for the first time. It was determined that the process occurs only on the bare carbon surface, and the quantitative description of the phenomena is possible using the Temkin model, which explains an almost linear decrease in the calorimetric heat of adsorption. The results provided by in situ infrared studies show that surface carbonyl oxides appear as intermediates of final functionality, i.e., carbonyl structures. Examining the thermal stability of surface structures shows that all surface species decompose at temperatures below 500 °C, leaving a pristine carbon surface.

Energetics of Adsorbed CH2 and CH on Pt(111) by Calorimetry: The Dissociative Adsorption of Diiodomethane

The enthalpies of molecular and dissociative adsorption of CH2I2 on Pt(111) at 100–210 K were studied using single-crystal adsorption calorimetry and density functional theory (DFT). Gaseous CH2I2 was found to adsorb on the Pt(111) surface at 100 K to form CH2,ad + 2Iad, with a calorimetric heat of adsorption that decreases with coverage as 222–480θ kJ/mol for θ < 1/8, where θ is the coverage in monolayers (ML), defined as the number of dissociatively adsorbed CH2I2 molecules per Pt(111) surface atom. These coadsorbed iodine atoms greatly destabilize the methylene species even at the lowest coverage, which we attribute to their inability to diffuse away from the near-neighbor sites where they are initially produced on the short time scale of the heat measurement. A mixture of dissociative adsorption fragments of methylene and methylidyne were detected at elevated temperatures between 125 and 190 K. At 210 K, CH2I2 adsorption produced CHad, Had, and 2Iad, with the Iad now able to diffuse away to minimize r...

Adsorption equilibrium of binary methane/ethane mixtures in BPL activated carbon: isotherms and calorimetric heats of adsorption.

The adsorption of pure methane and ethane in BPL activated carbon has been measured at temperatures between 264 and 373 K and at pressures up to 3.3 MPa with a bench-scale high-pressure open-flow apparatus. The same apparatus was used to measure the adsorption of binary methane/ethane mixtures in BPL at 301.4 K and at pressures up to 2.6 MPa. Thermodynamic consistency tests demonstrate that the data are thermodynamically consistent. In contrast to two sets of data previously published, we found that the adsorption of binary methane/ethane in BPL behaves ideally (in the sense of obeying ideal adsorbed solution theory, IAST) throughout the pressure and gas-phase composition range studied. A Tian-Calvet type microcalorimeter was used to measure low-pressure isotherms, the isosteric heats of adsorption of pure methane and ethane in BPL activated carbon, and the individual heats of adsorption in binary mixtures, at 297 K and at pressures up to 100 kPa. The mixture heats of adsorption were consistent with IAST.

Calorimetric Heats of Adsorption and Adsorption Isotherms. 3. Mixtures of CH4 and C2H6 in Silicalite and Mixtures of CO2 and C2H6 in NaX

Individual heats of adsorption and selectivity of mixtures have been measured simultaneously in a calorimeter. After the weakly adsorbed component was preloaded the dosing of both components to the sample cell was adjusted to hold the loading of the weakly adsorbed component constant. Agreement with ideal adsorbed solution theory is quantitative for mixtures of CH4 and C2H6 at low coverage in an adsorbent with a weak electric field (silicalite). Mixtures of quadrupolar (CO2) and nonpolar (C2H6) gases in an adsorbent with a strong electric field (NaX) exhibit strong negative deviations from ideality. The experimental heats of adsorption are larger than those for an ideal solution, in agreement with the results of molecular simulation.

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.

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.

Heats of adsorption of xenon and krypton on NaX zeolite at high pressures

1. The absolute differential calorimetric heats of adsorption were measured for Xe and Kr on NaX zeolite at 304.5 K and pressures up to ∼10 MPa. 2. Good agreement was found between the measured heats of adsorption and the calculated isosteric heats within experimental error.

Calorimetric and Isosteric Heats of Adsorption of Substances with Molecules of Different Electronic Structure

Calorimetric heats of adsorption have been measured for iso- and cyclo-pentane on NaX(86) zeolite at 140°C and for acetonitrile on carbon black and silica gel at 150°C. These heats have been compared with the corresponding isosteric values, calculated from isotherms measured over temperature ranges whose average values correspond to the above temperatures. The isosteric heats of adsorption coincide well with the calorimetric values in those cases where no change occurs in the mechanism for adsorbate–adsorbent interaction within the temperature range over which the isotherms have been measured. If this condition is not observed, the difference between the two sets of values can be quite considerable.

Calorimetric heats of adsorption at the silica/isooctane interface

The heats of immersion of a well-characterized mesoporous silica gel (Davisil 62) in solutions of six organic compounds (2-methylnaphthalene, benzothiophene, 2-methylbenzothiophene, pyridine, isoquinoline, and quinaldine) in isooctane (2,2,4-trimethylpentane) have been measured as a function of surface coverage by batch solution microcalorimetry. Adsorption isotherms at the silica/isooctane interface were obtained by conventional surface excess methods. The heat of adsorption of gaseous pyridine on silica was derived by correcting the heat of adsorption from the liquid for the bulk heats of mixing and phase change and the measured heats of wetting of silica covered with varioius preadsorbed amounts of pyridine in pyridine/isooctane solution. Connection was then made to the gas-phase adsorption data available for pyridine by the use of a rigorous thermodynamioc cycle. All of the compounds studied appeared to have site-limited adsorption (∼ 1 adsorbate per 2 surface silanol groups). 2-Methylnaphthalene and its sulfur aromatic analogs had very similar isotherms and low heats of adsorption from isooctane, in agreement with chromatographic experience. The one- and two-ring N-bases had high adsorption heats from isooctane which varied strongly with coverage (pyridine) or with adsorbate size (pyridine vs two-ring). It is argued, based upon the heats and entropies of adsorption, that the optimum hydrogen bond is the plane of the silica surface and that the observed large adsorbate steric effect reflected the geometry sensitivity of the hydrogen bond.

Temperature programmed desorption of sorbates from zeolites. Part I. Constant coverage, variable heating rate method

A new method of analysing temperature programmed desorption (t.p.d.) curves measured at different heating rates has been developed. The activation energy for desorption ( E d) and pre-exponential factor ( A) have been evaluated at constant coverages by a minimization procedure for ethanol, n-hexane, n-octane, and p-xylene ZSM-5/silicalite-1 pairs. The t.p.d. curves were recorded on a conventional thermogravimetric balance interfaced to a microcomputer, which was used for both data collection and analysis. The E d values compare favourably with published isoteric and calorimetric heats of adsorption. Advantages of this method are the rapidity of data collection and ease of analysis. The quantitative desorption information obtained could be of valuable assistance to catalyst screening for the characterization of sorbent-sorbate interaction, and in particular to the study of zeolitic acid sites by ammonia desorption.

Relaxation processes on graphitic surfaces. Part 2.—Adsorption of 4He on Spheron and Grafoil

An analysis of the after-periods of the calorimetric heat of adsorption determined for 4He on Spheron at temperatures close to 12 K and for 4He on Grafoil between 8 and 12 K provides information on the processes by which adsorption takes place with increasing coverage. Within the lower coverage range the exothermic processes which follow the steep rise of temperature occurring when the adatoms come into contact with the adsorbent surface are related to the migration and ‘trapping’ of the adatoms at the ‘high-energy’ sites of these adsorbents. Above a certain coverage, depending on structure (especially of the outermost surface layers of the substrate), the steep rise of temperature is followed by endothermic processes. These processes are related to the rearrangement of the adatoms in different configurations within the films superimposed on the surface of the adsorbent. At higher coverages, during the completion of the first monolayer and the formation of the second, both the relaxation times and the energies involved in the endothermic processes provide information on the processes by which the formation of the superimposed second monolayer takes place.

Entropy of adsorption and related thermodynamic functions in an open isothermal system

The reversible gas-solid interaction in an open isothermal system in studied by means of a classic thermodynamic treatment based on calorimetric heats of adsorption. An expression of the integral molar entropy of adsorption closely related to experimental heats and adsorbed amounts are thus obtained. Similar relationships deduced for closed systems are compared. Other thermodynamic functions connected with entropy, such as free energies, arc also given.

The thermal desorption of surface species from sulfur dioxide adsorption on natural chrysotile asbestos and calorimetric sorption heats at 298, 323, 373, and 423 K

Calorimetric heats of adsorption have been determined for sulfur dioxide adsorbed at 298, 323, 373, and 423 K on both Californian and Quebec chrysotiles. Adsorption isotherms have been measured and some differential molar entropies calculated. Thermal desorption of surface species resulting from the adsorption of sulfur dioxide on Californian chrysotile at 298 K have been studied from 25 to 450 °C. Sulfurous and carbonic acids, in addition to sulfites and other trace species, have been shown to result from the adsorbent–adsorbate interactions. A mechanism for sulfur dioxide adsorption on chrysotile has been prepared which accounts for the observed high heats of adsorption measured at low surface coverages.

Heterogeneous Interactions of Methylamines on Porous Adsorbents. Part III. The Adsorption Characteristics of Methylamines on γ-alumina and Types '3A' and '13X' 'Molecular Sieves'

The adsorption characteristics of mono-, di-, and trimethylamine on γ-alumina have been investigated and adsorption isotherms in the regions of their boiling points determined, along with isothermal calorimetric heats of adsorption up to monolayer coverage (θ = 1). From the initial heats of adsorption of 21.6 to 32.4 kcal/mol θ = 0.05, and the strong retention of the amines adsorbed at low pressures, chemisorption is considered to occur at θ &lt; 0.20. The maxima and minima displayed in the heat curves are discussed in terms of adsorbate–adsorbate interactions at sub-monolayer coverage levels. Adsorption isotherms for di-and tri-methylamine on Linde 'molecular sieves' '13X' and '3A' have been measured also, along with the trimethylamine heat curves for these materials.

Adiabatic Calorimetric Studies of the Adsorption of Sulfur Dioxide at 423 °K on Heat-treated Magnesias Characterized by Electron Microscopy

Calorimetric heats of adsorption of sulfur dioxide at surface coverages from 0.2 to 7.7 μmol m−2 have been measured on a series of magnesias heat-treated at 600, 700, and 800 °C. Structural features of the adsorbents have been characterized by analysis of nitrogen adsorption–desorption isotherms and by transmission electron microscopy.Heats of adsorption at 423 °K for adsorbed amounts of 0.3 μmol m−2 were respectively, 86.0, 87.0, and 77.0 kcal mol−1 for the 600, 700, and 800 °C samples. The values for the 600 and 700 °C samples fell rapidly above a coverage of 2.0 μmol m−2 to around 44.0 kcal mol−1 at 5.0 to 7.0 μmol m−2, while the values for the 800 °C sample decreased continuously to 39.0 kcal mol−1 at the same coverage.Crystallite shapes tended increasingly towards the cubic form as the temperature of heat-treatment was increased and average particle sizes were 80, 240, and 320 Å for the 600, 700, and 800 °C samples, respectively.It is proposed that the very high heats obtained on all the samples are due to interactions with high-energy defect centers in the oxide and with the formation of magnesium sulfite initially, and magnesium sulfate towards monolayer completion.

Heterogeneous Interactions of Methylamines on porous Adsorbents Part II. Interactions on Silica–Alumina and Silica Gel Surfaces of Di- and Tri-methylamine in the Region of their Boiling Points

The adsorption of di- and tri-methylamine has been studied at 280 and 276°K, respectively, on a range of silica gels and on silica–alumina. Adsorption–desorption isotherms and isothermal calorimetric heats of adsorption were measured. Heats of adsorption for di-methylamine on silica gel varied from 29.0 kcal/mol at θ = 0.05 to 12.0 kcal/mol at monolayer completion, while for silica–alumina the heats fell from 31.4 to 9.0 kcal/mol between these same values of surface coverage. For tri-methylamine on silica gel, heats fell from 21.0, θ = 0.05, to 10.3 kcal/mol, θ = 1.00, while heats of 21.4 and 9.9 kcal/mol, respectively, were found at these same surface coverages on silica–alumina. The values of the heats of adsorption are discussed in terms of interactions in the adsorbed layer which are believed to be influenced by the pore sizes in the adsorbent and by the basicity of the amine molecules.

The Effect of Heat Treatment of Silica Gels on the Calorimetric Heats of Adsorption of Sulfur Dioxide up to One-twentieth Monolayer Coverage at 423 °K

Calorimetric heats of adsorption of sulfur dioxide have been determined at 423°K for a series of silica gels heat-treated at 240, 550, 700, 800, and 900 °C. At the lowest surface coverage of 0.01 μmol m−2, heats of 25 to 30 kcal mol−1 were observed. These values dropped rapidly with increasing coverage and approached "limiting" values of 6 to 7 and 12 kcal mol−1 for dehydroxylated and hydroxylated surfaces, respectively. To explain the results at lowest coverages an adsorbed complex involving multiple hydrogen bonds is proposed while at higher coverages it is suggested that single and dual site adsorbed species predominate for the dehydroxylated and hydroxylated surfaces, respectively. Sulfur dioxide adsorption isotherms on all gels at 423 °K obeyed the Langmuir equation.Adsorbents were characterized by nitrogen adsorption–desorption isotherms at 77 °K and pore size distribution data were calculated from the desorption branch.

Thermodynamic properties of argon adsorbed on porous glass plus preadsorbed water

Precise isotherms and calorimetric heats of adsorption have been determined for argon adsorbed on a sample of porous Vycor glass containing one monolayer of preadsorbed frozen water and on the sample containing sufficient water to form a triple layer. Pore size distributions were calculated from the isotherms for these two systems as well as for the bare porous glass adsorbent. Hysteresis effects were observed in the calorimetric heats of adsorption and desorption; these are compared with hysteresis in the isotherms, and in the Clausius—Clapeyron heats. The data indicate that most of the first increment of preadsorbed water is located in micropores and pore necks, but that pore filling occurs when three statistical layers of water are adsorbed.

Heats of adsorption of organic bases on aerosilogels containing small quantities of aluminium

Calorimetric heats of adsorption of the organic bases triethylamine and benzene on aerosilogels have been measured. The aerosilogels were prepared from industrial aerosils and dehydroxylated in vacuo at temperatures up to 1150°C. One sample of aerosilogel, which contained 0.4 % by weight of aluminium impurity, produced very active surface sites, which were characterized by a high initial heat of adsorption—about 50 kcal mol–1(209 kJ mol–1) for triethylamine and 20 kcal mol–1(84 kJ mol–1) for benzene. Other samples containing less than 0.02 % aluminium did not show this activity. Aluminium was artificially introduced into a pure sample of aerosilogel using aluminium nitrate solution. The adsorption properties of this sample were similar to those of the aerosilogel prepared from the impure aerosil containing 0.4 % aluminium.

The sorption of furans on silica gels

The adsorption of furan and tetrahydrofuran on a range of thermally modified silica gels has been studied at temperatures close to the boiling points of the two adsorbates. Adsorption–desorption isotherms have been measured along with isosteric and isothermal calorimetric heats of adsorption up to and including monolayer coverage. For furan adsorbed at 304 °K on gel heat treated at 240° the variation in calorimetric heat is from above 30 kcal. mole–1 at θ= 0·01 to near 7 kcal. mole–1 at θ= 1·00. On the same material at 338 °K, the variation for tetrahydrofuran is from 26·5 kcal. mole–1(θ < 0·1) to 12·0 kcal. mole–1(θ= 1·00). Data are included also for the adsorption of furan on γ-alumina, aluminium-impregnated and hydrothermally treated silica gels. The nature of the adsorbed complexes is believed to depend on the degree of surface coverage with adsorbed molecules and also on surface hydroxy-group concentrations.