Adsorption-desorption heats

Figure 1. Proton adsorption-desorption heats ( 1 Standard Deviation or SD) as a function of pH for goethite at 10 g/L solid concentration in 0.05 M NaNO. All titrations were NaOH titrations starting from pH 4 followed by acid titrations back to pH 4.

XVIII-11 (the paradox of desorption heat exceeding adsorption heat is explainable in terms of a partial irreversibility of the adsorption-desorption process). 

When a molecule adsorbs to a surface, it can remain intact or it may dissociate. Dissociative chemisorption is conmion for many types of molecules, particularly if all of the electrons in the molecule are tied up so that there are no electrons available for bonding to the surface without dissociation. Often, a molecule will dissociate upon adsorption, and then recombine and desorb intact when the sample is heated. In this case, dissociative chemisorption can be detected with TPD by employing isotopically labelled molecules. If mixing occurs during the adsorption/desorption sequence, it indicates that the mitial adsorption was dissociative. 

Fig. 5.19 Adsorption of water vapour on a silica gel which had been heated at 900°C. (The water content, calculated from the loss in weight at 1000°C, was 0-2%.) First run O, adsorption ( ), desorption. Second run , adsorption desorption.

The opposite of adsorption, desorption, represents the end of the catalytic cycle. It is also the basis of temperature-programmed desorption (TPD), an important method of studying the heats of adsorption and reactions on a surface, so that the activation... 

CO oxidation is often quoted as a structure-insensitive reaction, implying that the turnover frequency on a certain metal is the same for every type of site, or for every crystallographic surface plane. Figure 10.7 shows that the rates on Rh(lll) and Rh(llO) are indeed similar on the low-temperature side of the maximum, but that they differ at higher temperatures. This is because on the low-temperature side the surface is mainly covered by CO. Hence the rate at which the reaction produces CO2 becomes determined by the probability that CO desorbs to release sites for the oxygen. As the heats of adsorption of CO on the two surfaces are very similar, the resulting rates for CO oxidation are very similar for the two surfaces. However, at temperatures where the CO adsorption-desorption equilibrium lies more towards the gas phase, the surface reaction between O and CO determines the rate, and here the two rhodium surfaces show a difference (Fig. 10.7). The apparent structure insensitivity of the CO oxidation appears to be a coincidence that is not necessarily caused by equality of sites or ensembles thereof on the different surfaces. 

Beta/montmorillonite composite was prepared under dynamic hydrothermal conditions. Firstly, montmorillonite calcined at 800 °C were added to a diluted solution of sodium hydroxide, potassium chloride and TEAOH in distilled water and the resulting mixture was vigorously stirred for 1 h secondly, silica sol was added into the above uniform mixture to allow at least 3 h stirring finally, the gel was moved into stainless steel autoclaves (1L) and heated at 413 K for 48 h. The samples were characterized by XRD, N2 adsorption-desorption, FT-IR and SEM-EDS. The catalytic assessment experiments were carried out in a flowing-type apparatus designed for continuous operation. 

Porosity characteristics were determined by N2 adsorption-desorption at 77 K (Dubinin method) with a Thermoquest Sorptomatic 1990. Powder samples were outgassed (10 4 Torr) and heated to 450°C before each test. 

As stated above, when probes with specific adsorption characteristics are used, additional chemical information can be extracted from adsorption-desorption experiments. Temperature-programmed desorption (TPD) in particular is often employed to obtain information about specific sites in catalysts [55,56], The temperature at which desorption occurs indicates the strength of adsorption, whereas either the amount of gas consumed in the uptake or the amount of desorption upon heating attests to the concentration of the surface sites. The most common molecules used in TPD are NH3 and C02, which probe acidic and basic sites, respectively, but experiments with pyridine, Oz, H2, CO, H20, and other molecules are often performed as well [57-59], As an example, the ammonia... 

Fig. 4 Nitrogen adsorption-desorption isotherms at 77 K of heat-treated NH4 -exchanged MSU-Ge-2 (solid circles, adsorption data open circles, desorption data). The hysteresis observed at P/Po > 0.8 is due to the voids between the agglomerated particles. (Inset) BJH pore size distribution calculated from the adsorption branch of the isotherm...

XRD patterns were obtained with a Rigaku D/MAX-IIA diffractometer system equipped with Ni-filtered Cu-Ka radiation. N2 adsorption/desorption isotherms were measured with a Micromeritics ASAP 2010 system. NH3-TPD was conducted under He flow of 30 ml/min and a heating rate of 20 °C/min. 

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