Chemisorption ammonia

Acid properties. It was reported that the acidity of aluminum-deficient mordenite, measured by ammonia chemisorption, decreases linearly with aluminum content (78,82,83). 

In this communication, the results of a systematic study of ammonia chemisorption on V2O -Ti02 (anatase) catalysts of different vanadia loading is reported." Low temperature oxygen chemisorption is also utilized to determine the monolayer loading of 20 on Ti02 (anatase). Partial oxidation of methanol to formaldehyde is studied as a model reaction on these catalysts and the activities of the catalysts are correlated with NH and O2 uptakes. 

Ammonia Chemisorption. Ammonia gas has been widely employed as a basic adsorbate to count the number and strength of acid sites on various solid surfaces (28,29). The nature and strength of these sites may relate the activity and selectivity character of the catalysts. Further, all acid sites on catalyst surface are easily accessible to the small molecules of NH (kinetic dla 0.26 nm) and... 

Ammonia chemisorption experiments were carried out at different temperatures ranging from 25 to 400°C to establish conditions for the adsorption studies on the catalysts. The NH chemisorption experiments were performed on an all glass high vacuum system according to the procedure described by Kanta Rao et al. (31). In a typical experiment, about 0.3 g of the catalyst... 

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,... 

Thus ammonia chemisorption technique can be used for the estimation of active sites as well as monolayer capacity of Ti02 (anatase) supported vanadla catalysts. 

Acidity by Ammonia Chemisorption. Relative catalyst acidity was measured by the quantity of ammonia chemisorbed at the arbitrary conditions of 250°C, 11.2 torr. An Isorpta analyzer (Engelhard model 3A-2) was used the procedure was that described in Ref. 8. 

The purpose of the present work is to incorporate aluminum into the framework of SBA-15 during the synthesis in order to create acid sites on the surface of the material directly and to enhance its activity in acid-catalyzed reactions and to study the stability of SBA and AlSBA molecular sieves under various treatments. The influence of these treatments on the pore size, wall thickness and the environment of Al in these materials are investigated in detail. X-ray diffraction (XRD), Electron Microscopy (TEM) and N2 adsorption were used to characterize the structure, the porosity and the stability of these materials. 27Al MAS NMR was used to ascertain the nature and environment of Al, cumene cracking to test the catalytic activity of parent materials and ammonia chemisorption to probe their surface acidity. 

Ammonia chemisorption only occurs at reaction temperatures above 673 K,1,6,22 and proceeds according to two reaction mechanisms. The main reaction (A) is a substitution reaction ... 

Catalyst Acidity bv Ammonia Chemisorption. Plant performances below expectations were reported by two units, in which the original alumina based catalysts were replaced. Surface area specifications were 150-200 m2/g for both catalysts. Acidity was measured by ammonia chemisorption using a recording vacuum thermoanalyzer with a TGA sample holder, and was correlated with BET surface area, as shown in Table V. 

Titanium sulfate supported on zirconia catalysts were prepared by drying of powdered Zr(OH)4 with titanium sulfate aqueous solution followed by calcining in air at high temperature. The characterization of prepared catalysts was performed using Fourier transform infrared (FTIR), X-ray diffraction (XRD), differential scanning calorimetry (DSC), and by the measurement of surface area. The addition of titanium sulfate to zirconia shifted the phase transition of ZrOa from amorphous to tetragonal to higher temperature because of the interaction between titanium sulfate and zirconia. The catalytic activities for both reactions, 2-propanol dehydration and cumene dealkylation were correlated with the acidity of catalysts measured by ammonia chemisorption method. 

The acid properties of the oxide precursors and the oxynitride samples have been measured by ammonia chemisorption at 35, 100 and 350°C using an Asap... 

Fig. 1. Total, weak, medium and stron acid sites distribution measured by ammonia chemisorption as a function of the catalysts nitrogen content on AlPON (A) and AlGaPON (B).

Alford, J.M., Laaksonen, R.T., Smalley, R.E., Ammonia chemisorption studies on silicon cluster ions. J. Chem. Phys. 1991, 94, 2618. 

Fig. 16 Differential molar heats of ammonia chemisorption at 423 K on MFl zeolites as a function of the adsorbed amount Al-SU, o Fe-Sil, 0 In-Sil, A silicalite. Pretreatment temperature 673 K (from [245])...

The acid properties of A1-, Ga-, and Fe-substituted MCM-41-type mesoporous silicates have been probed using ammonia adsorption at 423 K [285]. Substitution led to the formation of Bronsted and Lewis acid sites of different type and strength. Initial heats of ammonia chemisorption decreased in the same order (185 (Al) > 162 (Ga) > 144 (Fe) kJ mol ) as the degree of isomorphous substitution of framework silicon by trivalent atoms. The ratio of strong to medium-strong Brdnsted sites fell in the same order. 

Centi, G., Marchi, F. and Perathoner, S. (1997). Effect of Ammonia Chemisorption on the Surface Reactivity of V-Sb-Oxide Catalysts for Propane Ammoxidation, Appl. Catal. A Gen., 149, pp. 225-244. 

Zajac, J., Dutrartre, R., Jones D.J., and Rozi res, J. Determination of surface acidity of powdered porous materials based on ammonia chemisorption comparison of flow microcalorimetry with bath volumetric method and temperature programmed desorption. Thermochim. Acta 2001, 379, 123-130. 

Test Method for Determination of Catalyst Acidity by Ammonia Chemisorption... 

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