Chemisorption temperature

A catalyst sample, between 1 to 10 grams, was loaded into the apparatus, evacuated, reduced in 1 atm of H2 at 250 or 450°C overnight and then evacuated at the reduction temperature before cooling and admitting H2 at the chemisorption temperature. Initial equilibration was very slow for all catalysts indicating the presence of states which are, kinetlcally, relatively inaccessible W. In some cases, 10-20 hrs were required for pressure equilibration. 

The effect of chemisorption temperature on the ammonia uptake capacity of 6.5 wt% V20c/Ti02 is shown in Fig. 1. Ammonia chemisorption capacities increase with temperature upto 150°C and then decrease with further Increase up to 400°C. It is worth noting that there is considerable NH uptake even at 400°C. These results are in accordance with the reported literature. A number of studies have been reported on the acidic character of supported transition-metal oxides (22,34-38). Ammonia on V20g can be either adsorbed in the form of NH species on Bronsted acid sites or coordlnatively bonded to vanadium ions on Lewis acid sites (39,40). The latter species were observed up to 250°C,... 

In order to understand better these interesting systems without complications that might arise due to different preparation procedures, we compared oxygen-treated WC and Mo2C prepared by similar reduction/ carburization procedures from their respective oxides. The effects of pretreatment conditions were also studied. An attempt was made to correlate the kinetic behavior of these catalysts in n-hexane-H2 reactions with their physical properties obtained from X-ray diffraction (XRD), CO chemisorption, temperature-programed reaction (TPR) with flowing H2 or He, temperature programed desorption (TPD) of adsorbed NH3, and X-ray photoelectron spectroscopy (XPS). 

In order to establish the optimum chemisorption temperature, a series of isothermal chemisorption experiments were performed at different temperatures between 200 and 300 °C. The sample was first outgassed in Ar (15 °C min, 1000 C, 5 hours). The temperature was then lowered to the chemisorption temperature, and a flow rate of 75 mL min of oxygen was introduced to the TGA. In this way, an optimised chemisorption temperature of 250 °C was found, so that equilibrium could be achieved in a reasonable period of time, and simultaneous carbon gasification could be avoided. 

Once the chemisorption temperature was optimised, the ASA evaluation by the gravimetric method could be performed, and the mass increase was assigned to the chemisorbed oxygen. 

Alkali metal promoters are known to control acidity in supported metal catalysts. Our studies on alkali promoted Pt/Al203 catalysts through H2-O2 chemisorption. Temperature Programmed Reduction and ammonia TPD techniques have shown that besides the attenuation of acidity, added alkali affects the binding of Pt species on the support, thereby influencing its reducibility and dispersion. Based on the studies above, several aspects of promoter effects in supported platinum catalysts are discussed. 

Static volumetric hydrogen chemisorption experiments were carried out with Coulter OMNISORP lOOCX equipment. The chemisorption temperatures were 303, 333 and 363 K. The catalysts were pretreated in a similar manner as in the TPD experiments, except that the heating and cooling were done under vacuum. After the measurement of the total hydrogen adsorption the sample was degassed to remove weakly adsorbed hydrogen, and the isotherm for the reversible adsorption was measured. 

Saturation densities of Zr and Ti on Si02 preheated at 450°C. Chemisorption temperature of... 

Earneth, W.E., Staley, R.H., and Sleight, A.W. Stoichiometry and structuraleffeets in alcohol chemisorption/temperature-programmed desorption on M0O3.,/ Am. Chem. Soc. 1986,108, 2327-2332. 

Vibrational energy states are too well separated to contribute much to the entropy or the energy of small molecules at ordinary temperatures, but for higher temperatures this may not be so, and both internal entropy and energy changes may occur due to changes in vibrational levels on adsoiption. From a somewhat different point of view, it is clear that even in physical adsorption, adsorbate molecules should be polarized on the surface (see Section VI-8), and in chemisorption more drastic perturbations should occur. Thus internal bond energies of adsorbed molecules may be affected. 

As also noted in the preceding chapter, it is customary to divide adsorption into two broad classes, namely, physical adsorption and chemisorption. Physical adsorption equilibrium is very rapid in attainment (except when limited by mass transport rates in the gas phase or within a porous adsorbent) and is reversible, the adsorbate being removable without change by lowering the pressure (there may be hysteresis in the case of a porous solid). It is supposed that this type of adsorption occurs as a result of the same type of relatively nonspecific intermolecular forces that are responsible for the condensation of a vapor to a liquid, and in physical adsorption the heat of adsorption should be in the range of heats of condensation. Physical adsorption is usually important only for gases below their critical temperature, that is, for vapors. 

Chemisorption may be rapid or slow and may occur above or below the critical temperature of the adsorbate. It is distinguishable, qualitatively, from physical adsorption in that chemical specihcity is higher and that the energy of adsorption is large enough to suggest that full chemical bonding has occurred. Gas that is chemisorbed may be difficult to remove, and desorption may be... 

Restructuring of a surface may occur as a phase change with a transition temperature as with the Si(OOl) surface [23]. It may occur on chemisorption, as in the case of oxygen atoms on a stepped Cu surface [24]. The reverse effect may occur The surface layer for a Pt(lOO) face is not that of a terminal (100) plane but is reconstructed to hexagonal symmetry. On CO adsorption, the reconstruction is lifted, as shown in Fig. XVI-8. 

It was noted in Section XVII-1 that chemisorption may become slow at low temperatures so that even though it is favored thermodynamically, the only process actually observed may be that of physical adsorption. Such slowness implies an activation energy for chemisorption, and the nature of this effect has been much discussed. 

Sequences such as the above allow the formulation of rate laws but do not reveal molecular details such as the nature of the transition states involved. Molecular orbital analyses can help, as in Ref. 270 it is expected, for example, that increased strength of the metal—CO bond means decreased C=0 bond strength, which should facilitate process XVIII-55. The complexity of the situation is indicated in Fig. XVIII-24, however, which shows catalytic activity to go through a maximum with increasing heat of chemisorption of CO. Temperature-programmed reaction studies show the presence of more than one kind of site [99,1(K),283], and ESDIAD data show both the location and the orientation of adsorbed CO (on Pt) to vary with coverage [284]. 

The microcalorimetric measurements of Della Gatta and his co-workers in their investigation of the interaction of water vapour with highly dehydroxylated y-alumina confirm that in this system also, the nondissocia-tive chemisorption of water is nonactivated, whilst the dissociative chemisorption is always activated. Thus the pseudo-equilibrium between the two chemisorbed states is displaced towards dissociative chemisorption as the temperature is increased above 150 C. 

Adsorption and Desorption Adsorbents may be used to recover solutes from supercritical fluid extracts for example, activated carbon and polymeric sorbents may be used to recover caffeine from CO9. This approach may be used to improve the selectivity of a supercritical fluid extraction process. SCF extraction may be used to regenerate adsorbents such as activated carbon and to remove contaminants from soil. In many cases the chemisorption is sufficiently strong that regeneration with CO9 is limited, even if the pure solute is quite soluble in CO9. In some cases a cosolvent can be added to the SCF to displace the sorbate from the sorbent. Another approach is to use water at elevated or even supercritical temperatures to facilitate desorption. Many of the principles for desorption are also relevant to extraction of substances from other substrates such as natural products and polymers. 

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