Gas Adsorption Calorimetry Research Articles

Energetics of CO2 and H2O adsorption on alkaline earth metal doped TiO2.

The process of CO2 and H2O adsorption on the surface of nano-oxide semiconductors is important in the overall performance of artificial photosynthesis and other applications. In this study, we explored the thermodynamics of CO2 and H2O adsorption on TiO2 as a function of surface chemistry. We applied gas adsorption calorimetry to investigate the energetics of adsorption of those molecules on the surface of anatase nanoparticles. In an attempt to increase TiO2 surface affinity to CO2 and H2O, TiO2 was doped with alkaline earth metals (MgO, CaO, SrO, and BaO) by manipulating the chemical synthesis. Adsorption studies using diffuse reflectance infrared spectroscopy at different temperatures indicate that due to the segregation of alkaline earth metals on the surface of TiO2 nanoparticles, both CO2 and subsequent H2O adsorption amounts could be increased. CO2 adsorbs in two different manners, forming carbonates which can be removed at temperatures lower than 700 °C, and a more stable linear adsorption that remains even at 700 °C. Additionally to the surface energetic effects, doping also increased specific surface area, resulting in further improvement in net gas adsorption.

CO2 gas-adsorption calorimetry applied to the study of chemically activated carbons

In this work, the microporous structure of a series of H3PO4 chemically activated carbons from peach stones with increased activation degree were investigated. CO2 Adsorption equilibrium isotherms and differential enthalpy curves were simultaneously measured at 300K using a Tian-Calvet microcalorimeter coupled to an adsorption manometric setup. Temperature programmed decomposition experiments were used to assess density of oxygen functional groups and determine the impact of surface chemistry on CO2 adsorption capacity. Computer based theoretical calculations were also performed to attempt to predict the adsorption enthalpy profiles. The most activated sample (Xp=0.90) has an average adsorption enthalpy which is approximately 8kJ/mol lower than that of the non-activated samples carbonized under the same conditions. The combination of techniques enabled a better understanding of the pore filling regimes with increasing coverage, since the use of CO2 as a probe gas allows accessing small pores, which otherwise would not be identified from N2 isotherms at 77K. The oxygen content on the carbon surface decreased almost 80% with the increasing degree of activation and did not influence in the CO2 adsorption. Besides providing information about carbon chemistry, CO2 adsorption calorimetry can also be successfully applied to the screening of carbons intended for CO2 capture.

Probing the Energetics of Molecule–Material Interactions at Interfaces and in Nanopores

During the past decades, advances in interfacial chemistries at the molecular level are shaping our world by playing crucial roles in balancing global scale energy crisis and critical environmental concerns. However, systematic investigations into the binding energies, site distribution, and their correlation with the molecular-level surface assemblages and structures at interfaces and in nanopores are rarely documented. In this review, we summarize a set of systematic calorimetric studies on surface energetics we performed during the past decade. These studies demonstrate how thermochemistry can reveal crucial energetic insights into a series of molecule–material interactions relevant to a number of applications, including carbon capture and sequestration, energy production, sustainable chemical processing, catalysis, and nanogeoscience. Calorimetric methodologies developed and applied include direct gas adsorption calorimetry, near-room temperature solvent immersion/solution calorimetry, and high temper...

Thermodynamics of Methane Adsorption on Copper HKUST-1 at Low Pressure.

Metal-organic frameworks (MOFs) can be engineered as natural gas storage materials by tuning the pore structures and surface properties. Here we report the direct measurement of CH4 adsorption enthalpy on a paddlewheel MOF (Cu HKUST-1) using gas adsorption calorimetry at 25 °C at low pressures (below 1 bar). In this pressure region, the CH4-CH4 intermolecular interactions are minimized and the energetics solely reflects the CH4-MOF interactions. Our results suggest moderately exothermic physisorption with an enthalpy of -21.1 ± 1.1 kJ/mol CH4 independent of coverage. This calorimetric investigation complements previous computational and crystallographic studies by providing zero coverage enthalpies of CH4 adsorption. The analysis of the new and literature data suggests that in initial stages of adsorption the CH4-HKUST-1 interaction tends to be more sensitive to the pore dimension than to the guest polarizability, suggesting a less specific chemical binding role for the open Cu site.

Energetics of CO2 Adsorption on Mg–Al Layered Double Hydroxides and Related Mixed Metal Oxides

Energetics of CO2 uptake by layered double hydroxides (LDH) of Mg and Al and their corresponding mixed metal oxides (MMO) of two different compositions (Mg/Al = 2:1 and 3:1) were investigated by gas adsorption calorimetry. The initial adsorption enthalpies for all the LDH and MMO are similar and in the range −90 to −120 kJ/mol, indicating strong chemisorption of CO2. However, the differential adsorption enthalpy varies substantially with increasing coverage for different samples. LDH prepared by coprecipitation (both Mg/Al = 2 and 3) and their corresponding MMO exhibit similar CO2 uptake, which are in the working sorption capacity range (0.6–1 mmol/g), but the one prepared by urea hydrolysis shows poor sorption capacity (0.02–0.35 mmol/g). An integral enthalpy of adsorption of −54 kJ/mol was observed for [Mg–Al–CO3] LDH prepared by urea hydrolysis and −59 kJ/mol and −57 kJ/mol for the ones prepared by coprecipitation with composition 3:1 and 2:1, respectively. Attenuated total reflectance spectroscopy measurements of samples after CO2 uptake aid in identifying the mode of binding of adsorbed carbonate, which gives information about strength of basic sites. The presence of monodentate carbonate in all samples suggests that strong basic sites are available in LDH and MMO, in agreement with the large exothermic initial adsorption enthalpies observed, indicating strong chemisorption.

Energetics of CO2 and H2O adsorption on zinc oxide.

Adsorption of H2O and CO2 on zinc oxide surfaces was studied by gas adsorption calorimetry on nanocrystalline samples prepared by laser evaporation in oxygen to minimize surface impurities and degassed at 450 °C. Differential enthalpies of H2O and CO2 chemisorption are in the range -150 ±10 kJ/mol and -110 ±10 kJ/mol up to a coverage of 2 molecules per nm(2). Integral enthalpy of chemisorption for H2O is -96.8 ±2.5 kJ/mol at 5.6 H2O/nm(2) when enthalpy of water condensation is reached, and for CO2 is -96.6 ±2.5 kJ/mol at 2.6 CO2/nm(2) when adsorption ceases. These values are consistent with those reported for ZnO prepared by other methods after similar degas conditions. The similar energetics suggests possible competition of CO2 and H2O for binding to ZnO surfaces. Exposure of bulk and nanocrystalline ZnO with preadsorbed CO2 to water vapor results in partial displacement of CO2 by H2O. In contrast, temperature-programmed desorption (TPD) indicates that a small fraction of CO2 is retained on ZnO surfaces up to 800 °C, under conditions where all H2O is desorbed, with adsorption energies near -200 kJ/mol. Although molecular mechanisms of adsorption were not studied, the thermodynamic data are consistent with dissociative adsorption of H2O at low coverage and with several different modes of CO2 binding.

A predictive model for the enthalpies of hydration of zeolites

A compilation of the average hydration enthalpies per mole of water of 145 diversely originating zeolites measured using different technical methods [76 data from transposed-temperature drop calorimetry (TTDC), 57 data from immersion calorimetry (IC), 6 data from phase equilibria (PE), 5 data from gas-adsorption calorimetry (GAC), and 3 data from hydrofluoric acid solution calorimetry (HF)] was generated. Statistical regressions between three parameters involving the average hydration enthalpy per mole of water {Δ H hyd-W , Δ H hyd-W /(Al/Si), ln[−Δ H hyd-W /(Al/Si)]}and six parameters namely: (1) the charge defined by the Al/Si ratio; (2) the ratio of the framework charge to the number of H 2 O molecules (Al/H 2 O); (3) the framework density (FD) calculated from the molecular volume of the anhydrous zeolite, FD anh , and hydrated zeolite, FD hyd ; (4) the average cation electronegativity in the exchange site characterized by parameter Δ H O (site A) aq ; and (5) the intracrystalline water porosity (WP) determined from the volume of liquid water that can be recovered upon thorough outgassing of the hydrated zeolite. The regressions were performed by taking into account either the nature of the measurement technique, or the nature of the zeolite family. Within the zeolites from the TTDC and IC populations (133 data), the best results were obtained with ln[−Δ H hyd-W /(Al/Si)] and Al/(Al Si). Whatever the measurement technique, considering the nature of the zeolite family having a constant framework density of the anhydrous form (129 data), the Al/(Al Si) ratio remains the best parameter and the enthalpy of hydration can be expressed as follows: \[{\Delta}\mathit{H}_{hyd-w} = {-}(Al/Si){\ast}e^{{\{}5.491 {-} 4.674{\ast}[Al/(Al + Si)]{\}}}\] This general relationship can be improved by considering the following parameters: FD anh , Δ H O (site A) aq , WP and a new parameter that is the product of three parameters Al/Si, Δ H O (site A) aq , and WP weighing the variation of the water porosity related to the nature of the cation and to the total charge of the exchange site. Therefore, an understanding of the chemical formulae and unit-cell volumes of anhydrous and hydrated zeolites is required to evaluate the enthalpy of hydration with an accuracy of ±3.25 kJ/mol H 2 O.

Microcalorimetry and ellipsometry in surfactant adsorption studies

The applications of calorimetric and ellipsometric methods to the understanding of adsorption phenomena occurring at the solid/liquid interface are presented. Gas adsorption calorimetry and immersion calorimetry can be used to characterise powdered adsorbents in terms of surface area, surface energy and pore size distribution, whereas adsorption from solution calorimetry is well suited to follow in situ the building steps of the surfactant adsorbed layer. On the other hand, ellipsometry allows the adsorption isotherms as well as the thickness of the adsorbed layer to be determined on well defined flat surfaces. Some applications for surfactant studies are reviewed and a recent result, obtained in the case of the adsorption of a cationic surfactant below its Kraft temperature onto silica, is presented.