Adsorption microcalorimetry surface properties

Measurement of heat of adsorption by means of microcalorimetry has been used extensively in heterogeneous catalysis to gain more insight into the strength of gas-surface interactions and the catalytic properties of solid surfaces [61-65]. Microcalorimetry coupled with volumetry is undoubtedly the most reliable method, for two main reasons (i) the expected physical quantities (the heat evolved and the amount of adsorbed substance) are directly measured (ii) no hypotheses on the actual equilibrium of the system are needed. Moreover, besides the provided heat effects, adsorption microcalorimetry can contribute in the study of all phenomena, which can be involved in one catalyzed process (activation/deactivation of the catalyst, coke production, pore blocking, sintering, and adsorption of poisons in the feed gases) [66]. 

The pretreatment temperature is an important factor that influences the acidic/ basic properties of solids. For Brpnsted sites, the differential heat is the difference between the enthalpy of dissociation of the acidic hydroxyl and the enthalpy of protonation of the probe molecule. For Lewis sites, the differential heat of adsorption represents the energy associated with the transfer of electron density toward an electron-deficient, coordinatively unsaturated site, and probably an energy term related to the relaxation of the strained surface [147,182]. Increasing the pretreatment temperature modifies the surface acidity of the solids. The influence of the pretreatment temperature, between 300 and 800°C, on the surface acidity of a transition alumina has been studied by ammonia adsorption microcalorimetry [62]. The number and strength of the strong sites, which should be mainly Lewis sites, have been found to increase when the temperature increases. This behavior can be explained by the fact that the Lewis sites are not completely free and that their electron pair attracting capacity can be partially modified by different OH group environments. The different pretreatment temperatures used affected the whole spectrum of adsorption heats... 

The AlGaPON samples were used as catalysts of the Knoevenagel condensation reaction and the authors [211] found that the -NH2 groups present at the surface of the samples were the basic sites responsible for the condensation properties of the catalysts. The catalytic performances of the studied samples increased with their basic character observed by SO2 adsorption microcalorimetry. 

Some very important surface properties of solids can be properly characterized only by certain wet chemical techniques, some of which are currently under rapid improvement. Studies of adsorption from solution allow determination of the surface density of adsorbing sites, and the characterization of the surface forces involved (the energy of dispersion forces, the strength of acidic or basic sites and the surface density of coul-ombic charge). Adsorption studies can now be extended with some newer spectroscopic tools (Fourier-transform infra-red spectroscopy, laser Raman spectroscopy, and solid NMR spectroscopy), as well as convenient modern versions of older techniques (Doppler electrophoresis, flow microcalorimetry, and automated ellipsometry). 

In the present work the behaviour of zirconia samples doped with oxides of alkali metals and alkaline-earth metals was investigated, in order to better understand the role of both the nature and the amount of the doping cation. Li-, K-, Ca-, and Ba-doped zirconia samples were prepared. Their surface acid-base properties were assessed by means of adsorption microcalorimetry, using ammonia and carbon dioxide as probe molecules. Their catalytic activity for the 4-methylpentan-2-ol dehydration was tested in a flow microreactor. 

Thus adsorption microcalorimetry continues to provide valuable information on the surface properties of solids and retains its potential for further applications in research work and routine industrial work on catalysts, pigments, fillers, cements, clays, other minerals, and particulates in general. 

The solids in the form of fine powders or dispersions of fine powders in minerals, which lower friction and wear when added to lubricating oils, possess certain surface properties in common which are responsible for their anti-wear action. Flow adsorption microcalorimetry was instrumental in discovering some important surface properties of the solid powders that make them either lubricating or abrasive... 

This area has recently been extensively investigated by flow adsorption microcalorimetry, mostly in respect of the strength of adsorption of surfactants and resins on the surfaces of the above materials and the effect on the stability of pigment dispersion and adhesion properties as a function of the surface chemistry of solids. A major contribution in this field has been made by the late Prof. F. Fowkes whose work was recently summarised by D. P. Ashton and D. Briggs [30],... 

The most promising approach to this problem is the use of suitable probe molecules for the quantitative characterization of site density and strength by means of adsorption microcalorimetry. The best-known appUcations of chemisorption involve the use of bases such as NH3 or pyridine to probe the acidity of zeoUtes. Moreover, it is well known that adsorption influences all phenomena depending on surface properties, since it constitutes the primary step for every catalytic reaction involving sohd catalysts. Adsorption is generally exothermic (AH < 0) and the heat evolved is called heat of adsorption. This heat is related to the ability of the sites to interact with the probe molecule, i.e. to their basic or acidic character. 

Experimentally, the physicochemical properties of the solid per se, obtained by solid-state NMR spectroscopy [27-31], or IR [32, 33] spectroscopy, give insight into the surface properties related to the acidity. However, more detailed information is accessible if probe molecules are brought into contact with the surface sites and the mode of interaction is studied. Among techniques funded on adsorption of specific probe molecules, several methods such as temperature-programmed desorption (TPD) of amines [34-37] UV-Vis [38-40], IR [41-43] or XPS spectroscopies [44, 45] of adsorbed probe molecules and adsorption microcalorimetry [46-52] are applicable for the characterization of the solid materials acidities. It is important to point out that none of mentioned methodologies can reveal all previously listed acidity concepts. In any case ithas to be specified whether the concentration, strength or nature of acid sites is estimated. 

Adsorption microcalorimetry allows an accurate determination of amount, strength and strength distribution of surface sites, based on the heats of adsorption of appropriate probe molecules and differential heats as a function of surface coverage. Following text discuss the applicability of adsorption microcalorimetry for the determination of zeolites acidity, and the estimation of different factors that can influence this important property. 

Abstract To date, microcalorimetry of CO adsorption onto supported metal catalysts was mainly used to study the effects induced by the nature and the particle size of supported metallic clusters, the conditions of pretreatment and the support materials on the surface properties of the supported metallic particles. The present chapter focuses on the employ of adsorption microcalorimetry for studying the interaction of carbon monoxide with platinum-based catalyst aimed to be used in proton exchange membrane fuel cells (PEMFCs) applications. 

Adsorption isotherms. Isothermal microcalorimetry, in conjunction with an RH perfusion device, is a powerful method for mapping surface properties of solids and especially drugs [32]. The principle of the study is to adsorb and desorb water vapour onto and off the surface of a solid in small steps and measure the associated enthalpy change. At low RH values, monolayer water sorption conforms to a BET (Brunauer, Emmett and Teller) model and can therefore be used to determine surface properties. The analysis of the data can be achieved by plotting the water sorption isotherm as a function of RH and fitting to a modified BET type equation [33]. This can provide information about the surface affinity for water and the hydrophilic surface area, parameters... 

Chemical composition was determined by elemental analysis, by means of a Varian Liberty 200 ICP spectrometer. X-ray powder diffraction (XRD) patterns were collected on a Philips PW 1820 powder diffractometer, using the Ni-filtered C Ka radiation (A, = 1.5406 A). BET surface area and pore size distribution were determined from N2 adsorption isotherms at 77 K (Thermofinnigan Sorptomatic 1990 apparatus, sample out gassing at 573 K for 24 h). Surface acidity was analysed by microcalorimetry at 353 K, using NH3 as probe molecule. Calorimetric runs were performed in a Tian-Calvet heat flow calorimeter (Setaram). Main physico-chemical properties and the total acidity of the catalysts are reported in Table 1. 

The measurement of heats of adsorption by means of microcalorimetry has been used extensively in heterogeneous catalysis in the past few decades to gain more insight into the nature of gas-surface interactions and the catalytic properties of solid surfaces. Specific attention will be focused on group IIIA containing samples in this section. 

The major advantage of protein adsorption studies on high surface area materials is that changes of some extensive properties which accompany the process of adsorption are large enough to be directly measured heat of adsorption through microcalorimetry 141), uptake or release of small ions by a combination of electrokinetic methods and titration 142), thickness of adsorbed layer or an increase of the volume fraction of solid phase by a hydrodynamic method like viscometry 143). Chromatographiclike analysis can also be applied to protein adsorption 144). 

The effect of Ca loading on the acid-base and redox properties of chromia catalysts supported on alumina has been investigated by microcalorimetry of NH3 adsorption and TPR. This alkaline promoter strongly decreases the acidity of the chromia catalyst, particularly suppressing the medium and strong acid sites. No clear correlations were found between the surface acidic properties and the catalytic behavior of the investigated samples in the oxidative dehydrogenation of isobutene, while clear trends were observed between reducibihty and catalytic activity [52]. 

Hydrogenations of acetonitrile and pentylnitrile (valeronitrile) in gas and liquid phases respectively were carried out on catalysts obtained from Ni/Mg/Al layered double hydroxides (LDHs) precursors of various Mg/Ni molar ratios. Their catalytic properties were compared with those of a commercial Ni/Al203 catalyst. Selectlvities to primary amines, higher than 90% were obtained on catalysts with Mg/Ni molar ratios in the range 0.3-1. This behaviour was correlated with the acido-basic properties of the solids characterized by TPD of NH3 and microcalorimetry of monoethylamine adsorption. Both studies show that upon Mg addition, the surface acidity, which is responsible for secondary amine formation decreases. 

The adsorption of CO2 has been studied in order to compare the surface acid-base properties of yttria-stabilized tetragonal Zr02, either plain or sulfated to various extents [104]. The site populations and their energy distributions were studied by microcalorimetry, which evidenced the modification of the basicity of zirconia induced by the sulfation process. Upon sulfation die initial heats of CO2 adsorption decreased fi om about 120 kJ mol to less than 85 kJ mol" , depending on the amount of loaded sulfate. 

This paper reviews the work of the author and that of other users of flow microcalorimetry (FMC). which led to discoveries of new phenomena at solid-liquid and solid-gas interfaces [7, 8] including the behaviour of adsorbates at different surface coverages [9, 10], and many, previously unknown, interactions of surfaces of metals, metal oxides, graphites [11], active carbons, zeolites, clays, and other minerals with organic and inorganic adsorptives. The work led to development of new selective adsorption processes [12], prediction of the performance of liquid lubricants and its components [13], surface reactions [14, 15], dispersion technology [16] and determinations of specific areas of surface sites with different chemical properties [17, 18]. 

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