Initial Differential Heat Research Articles

Experimental Study and Thermodynamic Analysis of Carbon Dioxide Adsorption onto Activated Carbons Prepared from Biowaste Raw Materials

Nutshells are regarded as cost-effective and abundant raw materials for producing activated carbons (ACs) for CO2 capture, storage, and utilization. The effects of carbonization temperature and thermochemical KOH activation conditions on the porous structure as a BET surface, micropore volume, micropore width, and pore size distribution of ACs prepared from walnut (WNS) and hazelnut (HNS) shells were investigated. As a result, one-step carbonization at 900/800 °C and thermochemical KOH activation with a char/KOH mass ratio of 1:2/1:3 were found to be optimal for preparing ACs from WNS/HNS: WNS-AC-3 and HNS-AC-2, respectively. The textural properties of the WNS/HNS chars and ACs were characterized by low-temperature nitrogen vapor adsorption, XRD, and SEM methods. Dubinin’s theory of volume filling of micropores was used to evaluate the microporosity parameters and to calculate the CO2 adsorption equilibrium over the sub- and supercritical temperatures from 216.4 to 393 K at a pressure up to 10 MPa. The CO2 capture capacities of WNS- and HNS-derived adsorbents reached 5.9/4.1 and 5.4/3.9 mmol/g at 273/293 K under 0.1 MPa pressure, respectively. A discrepancy between the total and delivery volumetric adsorption capacities of the adsorbents was attributed to the strong binding of CO2 molecules with the adsorption sites, which were mainly narrow micropores with a high adsorption potential. The high initial differential heats of CO2 adsorption onto ACs of ~32 kJ/mol confirmed this proposal. The behaviors of thermodynamic functions (enthalpy and entropy) of the adsorption systems were attributed to changes in the state of adsorbed CO2 molecules determined by a balance between attractive and repulsive CO2–CO2 and CO2–AC interactions during the adsorption process. Thus, the chosen route for preparing ACs from the nutshells made it possible to prepare efficient carbon adsorbents with a relatively high CO2 adsorption performance due to a substantial volume of micropores with a size in the range of 0.6–0.7 nm.

The influence of the potassium promoter on the kinetics and thermodynamics of CO adsorption on a bulk iron catalyst applied in Fischer–Tropsch synthesis: a quantitative adsorption calorimetry, temperature-programmed desorption, and surface hydrogenation study

The adsorption of carbon monoxide on an either unpromoted or potassium-promoted bulk iron catalyst was investigated at 303 K and 613 K by means of pulse chemisorption, adsorption calorimetry, temperature-programmed desorption and temperature-programmed surface reaction in hydrogen. CO was found to adsorb mainly molecularly in the absence of H(2) at 303 K, whereas the presence of H(2) induced CO dissociation at higher temperatures leading to the formation of CH(4) and H(2)O. The hydrogenation of atomic oxygen chemisorbed on metallic iron was found to occur faster than the hydrogenation of atomically adsorbed carbon. At 613 K CO adsorption occurred only dissociatively followed by recombinative CO(2) formation according to C(ads) + 2O(ads)→ CO(2(g)). The presence of the potassium promoter on the catalyst surface led to an increasing strength of the Fe-C bond both at 303 K and 613 K: the initial differential heat of molecular CO adsorption on the pure iron catalyst at 303 K amounted to 102 kJ mol(-1), whereas it increased to 110 kJ mol(-1) on the potassium-promoted sample, and the initial differential heat of dissociative CO adsorption on the unpromoted iron catalyst at 613 K amounted to 165 kJ mol(-1), which increased to 225 kJ mol(-1) in the presence of potassium. The calorimetric CO adsorption experiments also reveal a change of the energetic distribution of the CO adsorption sites present on the catalyst surface induced by the potassium promoter, which was found to block a fraction of the CO adsorption sites.

ChemInform Abstract: Acid Strength of Silica-Supported Oxide Catalysts Studied by Microcalorimetric Measurements of Pyridine Adsorption.

Microcalorimetric measurements of the differential heat of pyridine adsorption were used to probe the distribution of acid strength on a series of silica-supported oxide catalysts. Depositing oxides of the following cations onto silica increased the acid strength of the catalyst: Ga{sup 3{plus}}, Zn{sup 2{plus}}, Al{sup 3{plus}}, Fe{sup 3{plus}}, Fe{sup 2{plus}}, Mg{sup 2{plus}}, and Sc{sup 3{plus}}. The acid strength distribution curves for the supported oxide samples showed either two or three regions of constant heat of adsorption while silica had an energetically homogeneous surface. The Ga, Al, and Sc samples were found to have both Bronsted and Lewis acidity while the remaining samples showed only Lewis acidity. Incremental adsorption of pyridine indicated that the initial region of highest heat corresponds to strong Lewis acidity while intermediate heats seemed to be due to weaker Lewis acid sites or a combination of Lewis and Bronsted acid sites. The final region of lowest heat was due to H-bonded pyridine on silica. Estimates of the entropies of adsorption were determined, providing information about the mobility of the adsorbed pyridine molecules. The initial differential heat of adsorption increases proportionally to the Sanderson electronegativity of the added oxide.

Thermodynamics of Carbon Monoxide Adsorption on Polycrystalline Titania Studied by Static Adsorption Microcalorimetry

The adsorption of CO on polycrystalline TiO2 was investigated by static adsorption microcalorimetry. The initial differential heat of adsorption (qdiff,0) of CO on polycrystalline titania is 40 kJ/mol, and the standard adsorption entropy (Deltas0) is -104 J mol(-1) K(-1). These results are consistent with those derived from temperature-programmed desorption and FTIR results in the literature. The good reproducibility of the isotherms and the stable qdiff indicate that the lattice oxygen and hydroxyl groups on titania surface are basically not reactive to adsorbed CO.

Comparative study on the gas-phase adsorption of hexane over zeolites by calorimetry and inverse gas chromatography

The scope of this work is to carry out a systematic comparison of inverse gas chromatography (IGC) and microcalorimetry as tools for the study of the gas-phase adsorption of organic vapours (using hexane as model compound) on zeolitic materials (using different Mn, Co and Fe-exchanged NaX and CaA zeolites). Adsorption isotherms were recorded using both techniques in the temperature range of 150–250 °C, being observed that the shape of the isotherms obtained with the dynamic (IGC) and static (microcalorimetry) techniques was surprisingly similar in the pressure range at which both techniques are applicable (low surface coverages). Concerning to the measurement of the strength of the adsorption, calorimetric data provide two parameters related to the adsorption enthalpy: the initial differential heat and the isosteric adsorption enthalpy. A great coincidence was found between the last one and the adsorption enthalpy determined by IGC (4–20% of difference, depending on the studied material). The behaviour of the initial differential heat depends strongly on the studied material, being in some cases closely related to the other two parameters and temperature-independent (in the case on Mn-exchanged zeolites), whereas for the Co-CaA and Fe-CaA zeolites, it is temperature-dependent, being not correlated with the other parameters in this case. The main conclusion of this work is that IGC is an attractive alternative to the static microcalorimetric data for obtaining information on the adsorption of organic compounds on microporous materials.

Studies of the roles of Sn or Fe on γ-Al 2 O 3 -supported Pt catalysts by CO adsorption microcalorimetry and dehydrogenation reaction of C 4 alkanes

CO adsorption microcalorimetry was employed in the study of γ-Al2O3-supported Pt, Pt-Sn and Pt-Fe catalysts. The results indicated that the initial differential heat of CO adsorption of the Pt/γ-Al2O3 catalyst was 125 kJ/mol. As CO coverage increased, the differential heat of adsorption decreased. At higher coverages, the differential heat of adsorption decreased significantly. 60% of the differential heat of CO adsorption on the Pt/γ-N2O3 catalyst was higher than 100 kJ/mol. No significant effect on the initial differential heat was found after adding Sn and Fe to the Pt/γ-Al2O3 catalyst. The amount of strong CO adsorption sites decreased, while the portion of CO adsorption sites with differential heat of 60–110 kJ/mol increased after increasing the Sn or Fe content. This indicates that the surface adsorption energy was changed by adding Sn or Fe to Pt/γ-N2O3. The distribution of differential heat of CO adsorption on the Pt-Sn(C)/γ-Al2O3 catalyst was broad and homogeneous. Comparison of the dehydrogenation performance of C4 alkanes with the number of CO adsorption sites with differential heat of 60–110 kJ/mol showed a good correlation. These results indicate that the surface Pt centers with differential heats of 60–110 kJ/mol for CO adsorption possess superior activity for the dehydrogenation of alkanes.

Adsorption and Microcalorimetric Measurements on the Interaction of CO and H2with Polycrystalline Ru and Ru/TiO2Catalyst

A microcalorimeter equipped with gas circulation cells and coupled atoutlet to a gas chromatograph was used for the simultaneous measurements of the uptake and the differential heat (qd) evolvedduring the adsorption of CO and H2pulses over polycrystallineruthenium metal and a Ru/TiO2catalyst in the temperature range300–475 K and as a function of surface coverage. The initialdifferential heat for the adsorption of CO and H2overpolycrystalline ruthenium at 300 K was 120 and 65 kJ mol−1,respectively, the corresponding values in the case of Ru/TiO2beingaround 130 and 57 kJ mol−1. With the rise in sampletemperature, theqdfor CO adsorption over Ru metal remained almost constant, while inthe case of Ru/TiO2it decreased substantially. The fraction of COor H2adsorbed, conversion of COadto CO2, and the correspondingvalues of heat evolved showed different trends, when these sampleswere exposed to the successive CO or H2pulses at differenttemperatures. The H2adsorption is found to be suppressed onRu/TiO2, particularly at the low sample temperatures. Also, the COadsorption over Ru/TiO2at temperatures above 400 K resulted in thepartial reduction of the support, and this is facilitated by the heatevolved at the metal/support interfaces during CO chemisorption. Onthe other hand, the CO dissociation followed by CO(ad)+O(ad)reaction was a predominant step giving rise to CO2formation in thecase of Ru metal. This study also confirms that, for both thesamples, while the CO adsorption remains uninhibited by thepreadsorbed H2, the catalyst surface covered with the CO wascompletely inaccessible to subsequent H2adsorption.

Heats of Adsorption of Different Gases on Polycrystalline Transition Metals

The adsorption of several vapours and gases (ammonia, oxygen, hydrogen, nitrogen, dinitrogen oxide, carbon dioxide, carbon monoxide, water) on titanium, palladium and silver powder has been investigated at 305 K over the equilibrium pressure range 1 Pa to 20 kPa, using a Tian-Calvet microcalorimeter. Some magnetic measurements have also been performed on the titanium and palladium samples. The consistency of the measurements is supported by the fact that the initial differential heat of adsorption of all the gases and the irreversibly adsorbed fraction may be correlated. However, the chemisorption energy does not follow any general order of bond energy for the interaction of the series of gases with the three transition metals. Magnetic measurements suggest that the change in suceptibility may be an indicator of the direction of electron transfer between the gas and the paramagnetic powder.

Estimation of cellulose polarity by gas chromatography

The interaction between bead cellulose and methanol, ethanol, acetonitrile, nitromethane, acetone, 2-butanone, chlorobenzene and tetrahydrofuran during gas chromatography has been studied. From the specific retention volumes, initial differential heats of adsorption and the contributions of the specific interaction energies of molecules adsorbed to the cellulose surface to the total adsorption energy have been estimated. The calculated Kováts indices and Rohrschneider's equation constants have been employed for comparison of the polarity and selectivity of the cellulose surface with various polymer sorbents.

Acid strength of silica-supported oxide catalysts studied by microcalorimetric measurements of pyridine adsorption

Microcalorimetric measurements of the differential heat of pyridine adsorption were used to probe the distribution of acid strength on a series of silica-supported oxide catalysts. Depositing oxides of the following cations onto silica increased the acid strength of the catalyst: Ga 3+, Zn 2+, Al 3+, Fe 3+, Fe 2+, Mg 2+, and Sc 3+. The acid strength distribution curves for the supported oxide samples showed either two or three regions of constant heat of adsorption while silica had an energetically homogeneous surface. The Ga, Al, and Sc samples were found to have both Brønsted and Lewis acidity while the remaining samples showed only Lewis acidity. Incremental adsorption of pyridine indicated that the initial region of highest heat corresponds to strong Lewis acidity while intermediate heats seemed to be due to weaker Lewis acid sites or a combination of Lewis and Brønsted acid sites. The final region of lowest heat was due to H-bonded pyridine on silica. Estimates of the entropies of adsorption were determined, providing information about the mobility of the adsorbed pyridine molecules. The initial differential heat of adsorption increases proportionally to the Sanderson electronegativity of the added oxide.

Retention and heats of adsorption of n-alkanes, aromatic hydrocarbons, aliphatic alcohols and ketones on macroporous silicas

Adsorption of molecules of n-alkanes, aromatic hydrocarbons, alcohols and ketones on the hydroxylated surface of S-80, S-120, and SX-3 silochromes was studied by gas chromatography in the temperature range 80–200°C. The retention volumes and initial differential heats of adsorption were calculated. The contribution of the specific interaction of aromatic compounds with the silica surface in the course of adsorption was evaluated. The properties of the surfaces of the silicas under study were compared on the basis of the chromatographic data.

Gas chromatographic retention on carbon adsorbents coated with monolayers of polynuclear aromatic hydrocarbons

Monolayers of polynuclear aromatic hydrocarbons (PAHs) were used to modify carbon adsorbents (non-graphitized and graphitized thermal carbon blacks). The thermodynamic adsorption characteristics, retention volumes and initial differential heats of adsorption, of molecules belonging to different classes of organic compounds were measured by gas chromatography on these sorbents. The nature of the retention of molecules with different chemical properties on PAH-modified surfaces is determined predominantly by dispersion interaction by competing electrostatic interaction. The effect of PAH modification is the greatest on the retention of oxygen- and nitrogen-containing compounds. Exposure of thermal carbon blacks, modified with monomolecular PAH layers, to an high-frequency low-temperature plasma helps to improve the symmetry of chromatographic peaks.

Retention and heats of adsorption of hydrocarbons and aliphatic alcohols on cellulose

The interaction of methanol, n-propanol, n-butanol, n-hexane, n-octaine, n-nonane, n-decane, benzene, toluene and ethylbenzene molecules with the surface of microspherical cellulose beads has been studied by gas chromatography. The retention volumes and their dependences on the temperature and molecular parameters of the adsorbate have been determined. The initial differential heats of adsorption have been calculated. The character of interaction of these organic substances has been shown to be determined by the electronic and geometric structures of the molecules.

ChemInform Abstract: DIRECT MEASUREMENT OF THE INTERACTION ENERGY BETWEEN SOLIDS AND GASES. VI. CALORIMETRIC STUDIES ON ACIDIC PROPERTIES OF SOLID SUPER ACIDS PREPARED FROM SILICA‐ALUMINA

AbstractFeste Supersäuren mit unterschiedlichen Sb‐Gehalten werden durch Umsetzung von gasförmigem SbF5 mit SiO2‐Al,O3‐Substraten dargestellt.

Direct Measurement of the Interaction Energy between Solids and Gases. VI. Calorimetric Studies on Acidic Properties of Solid Super Acids Prepared from Silica–Alumina

Abstract Solid super acids were prepared by the reaction of gaseous antimony pentafluoride with silica–alumina and heats of adsorption of ammonia on their surfaces were measured. Solid super acids gave about 170 kJ/mol of an initial differential heat of adsorption and their adsorption energies were mainly distributed in the vicinity of 137 kJ/mol, while the initial differential heat of adsorption on original silica–alumina was about 150 kJ/mol and its adsorption energy was chiefly distributed in the vicinity of 117 kJ/mol. Therefore, it was shown that solid super acid had stronger, more homogeneous and much more acid sites than original silica–alumina.

Nitrogen and oxygen adsorption on NaA zeolite

1. The rate of establishing adsorption equilibrium at temperatures below 170°K decreases sharply, so that the equilibrium adsorption of a mixture of gases with nitrogen on NaA zeolite can be investigated only above 170°K. 2. The presence of a Henry region on the nitrogen and oxygen adsorption isotherms at various temperatures gives yet another independent method for the determination of the initial differential heats and can simplify to a considerable degree the calculation of the thermodynamic functions which characterize the adsorption equilibrium of a binary mixture.