Flow adsorption microcalorimetry

Heat-flow adsorption microcalorimetry. The most important type of isothermal calorimeter in current use is that based on the principle of the heat flowmeter, which was first applied by Tian (1923) and improved by Calvet (Calvet and Prat, 1958,... 

Static or flow adsorption microcalorimetry, less extensively used, is a very powerful method for describing surface reactions and merits a more detailed overview. 

An alternative method is flow adsorption microcalorimetry, which involves the use of a carrier gas passing continuously through the adsorption cell. The catalyst is placed on a glass frit in a gas circulation cell in the calorimeter. In order to determine the amounts of gas adsorbed, flow calorimetry must be used in combination with another technique, most frequently TG, MS or GC [8, 18]. 

Flow adsorption microcalorimetry has been used to measure the heats of adsorption of ammonia in a nitrogen carrier on the H and Na forms of a Y zeolite [21]. The calorimeter was linked to a thermal conductivity detector in which the rates of adsorption and desorption and the associated rates of heat evolution or absorption were measured simultaneously at atmospheric pressure. The authors found that, as surface coverage increased, the sites covered first were not necessarily those with the highest molar heats of adsorption. 

Ammonia accessibility to the porosity of several activated carbons measured by flow adsorption microcalorimetry... 

Among them, flow adsorption microcalorimetry has been successfully used for this purpose proving to be very useful to obtain adsorption and thermodynamic data under experimental conditions close to those operating in industrial practice. 

Groszek [56, 57] studied the adsorption of simple gases (COj, CH4, SOj, O2, He, and Nj) on microporous carbons using flow adsorption microcalorimetry. Shen and Biilow [58] demonstrated that the isosteric adsorption technique (Eqns (3.11) or (3.31)) is a useful and effective tool to obtain highly accurate thermodynamic data for microporous adsorption systems like the heat of adsorption given by Eqn (3.47). They studied the adsorption of CO2 and N2—O2 mixtures on a super-activated, almost entirely microporous, carbon (M-30, from Osaka Gas) and three faujasite-type zeolites. They also estimated the energetic heterogeneity of the solids due to specific interactions between the adsorbate and the solid. 

Groszek, A.J. (1998). Flow adsorption microcalorimetry. Thermochim. Acta, 313,133—43. 70. Moreno-Castilla, C. (2004). Adsorption of organic molecules from aqueous solutions on carbon materials. Carbon, 42, 83—94. 

An alternative method is flow adsorption microcalorimetry, which involves the use of a carrier gas passing continuously through the adsorption cell [25-29]. 

In many cases the increases in surface areas of carbons by oxidation or by comminution were accompanied by the creation of microporosity which influenced the correlation between the heats of adsorption and surface area estimates as shown below in section 3.4. For non-porous purely hydrophobic solids there is an excellent agreement between the determinations of surface areas by flow adsorption microcalorimetry and the BETfN i) method as can be seen for Graphon in Table 2. 

An extensive study of the acidity and basicity of minerals by flow adsorption calorimetry over the past decade has been carried out by F. Fowkes and his group [25, 29]. His conclusion was that flow adsorption microcalorimetry was the only method that can correctly evaluate the distribution and strength of acid and base sites in solids, and predict the effect of these parameters on their catalytic and adhesive properties. This was fully supported by later work done by D. P. Ashton and D. Briggs [30]. 

In relatively simple cases of non-admission into the pores that are too small for the adsorptive, flow adsorption microcalorimetry can produce striking results as exemplified by the heats of adsorptions of normal and tertiary butyl alcohols shown in Figure 18 indicating a major reduction in adsorptive accessibility due to a... 

One of the first applications of flow adsorption microcalorimetry has been the study of interactions of n-heptane solutions of long chain alcohols, carboxylic acids, and amines with metal surfaces and metal oxides. Strong correlation was found between the heats of adsorption of their long chain compounds and anti-wear action. Later work was extended to the study of adsorption on freshly formed metal surfaces, which behaved quite differently from the metal oxides [46]. Freshly formed... 

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... 

Tests for the quality of the solid lubricants can be conveniently carried out by flow adsorption microcalorimetry whereby the ratio of the basal planes to the polar sites can be determined, and the affinity of the basal planes for n—paraffins, as well as the affinity for transition metal ions can both be confirmed. The samples with excessive amounts of edge sites are not likely to be acceptable for lubrication [51, 52]. 

Further work by the author [42] compared the adsorption of PtCle and AuCU ions on a basic ion exchanger and a graphitised carbon adsorbent and has confirmed that flow adsorption microcalorimetry can rapidly determine the capacity and the kinetics of sorption of the ions on such adsorbents under dynamic conditions. The rate of sorption of the Pt and the Au ions was limited by their diffusion into the resin matrix and the high affinity of the resin for water. High surface area graphitic carbons suffer much less from these effects and appeared to be much more effective adsorbents. 

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],... 

Flow adsorption microcalorimetry has now been developed to a point at which it provides accurate and reliable adsorption and desorption data for events occurring at solid-liquid and solid-gas interfaces within a wide range of temperatures, pressures, and/or solution concentrations. 

---