Base, ammonia adsorption

Another thermal analysis method available for catalyst characterization is microcalorimetiy, which is based on the measurement of the heat generated or consumed when a gas adsorbs and reacts on the surface of a solid [66-68], This information can be used, for instance, to determine the relative stability among different phases of a solid [69], Microcalorimetiy is also applicable in the measurement of the strengths and distribution of acidic or basic sites as well as for the characterization of metal-based catalysts [66-68], For instance, Figure 1.10 presents microcalorimetry data for ammonia adsorption on H-ZSM-5 and H-mordenite zeolites [70], clearly illustrating the differences in both acid strength (indicated by the different initial adsorption heats) and total number of acidic sites (measured by the total ammonia uptake) between the two catalysts. 

Cu(NH3)2BTC2/3 and finally copper hydroxide in the presence of water. The formation of the BTC salts was supported by the collapse of the structure after interaction of ammonia with unsaturated copper centers. The release of BTC and copper oxide centers provides sites for reactive adsorption of ammonia during the course of the breakthrough experiments. Interestingly, even though the structure collapses, some evidence of the structural breathing of the resulting materials caused by reactions with ammonia was found, based on the ammonia adsorption at equilibrium and the analysis of the heat of interactions [51]. 

Katada, N., Igi, H., Kim, J.H., and Niwa, M. (1997) Determination of the acidic properties of zeolite by theorecti-cal analysis of temperature programmed desorption of ammonia based on adsorption equilibrium. J. Phys. Chem. B, 101, 5969-5977. 

Typical catalysts for SCR include supported vanadia, and iron or copper supported on zeolite. Here the application of a model to the design and understanding of vanadia catalyst systems is presented. Over the vanadia-based catalyst system, a Rideal-Eley approach has been adopted by most workers in the field, in which the first step is ammonia adsorption on the catalyst. This stored ammonia can then either react with NOx or be desorbed. Some important contributions to the SCR modelling literature are Andersson et al. (1994), Lietti and Forzatti (1994), Dumesic et al. (1996), Lietti et al. 

The reaction of ammonia with oxygen over V-based catalysts produces mainly nitrogen, according to the stoichiometry of R5 in Table V. Analogously to the case of the ammonia adsorption-desorption, specific runs were carried out in order to extract the intrinsic kinetics of ammonia oxidation and at the same time to validate the previously fitted kinetics of the ammonia adsorption-desorption process. 

The depletion width can play a role in analyte-induced modulation of the semiconductor PL [4]. As molecules adsorb onto the surface of the semiconductor, the dead-layer thickness can change, resulting in what can be described as a luminescent litmus test When Lewis bases adsorb onto the semiconductor surface, they donate electron density to the solid, which decreases the electric field and thus decreases the dead-layer thickness. The reduction in D causes an enhancement in the PL intensity from the semiconductor. Figures 2a and 2b present typical PL enhancements observed from an etched n-CdSe substrate Relative to a nitrogen reference ambient, adsorption of the Lewis bases ammonia and trimethylamine cause a reversible increase in PL intensity. In contrast, when Lewis acids adsorb onto the surface, they can withdraw additional electron density, causing the electric field to increase and the PL intensity to decrease. Such effects have been observed with gases like sulfur dioxide [5]. 

The acid properties of NaH zeolite (NaHY, NaHMOR, NaHMFI) series with different Na contents were investigated with adsorption microcalorimetry23. Whatever the zeolite, the initial heat of base adsorption (NH3, pyridine, etc.) as well as the total acidity increases with the exchange of Na+ by FT Thus for a series of NaHMOR zeolites24, the initial heat of ammonia adsorption passes from 145 to 184 kJ mol"1 when the exchange rate passes from 33 to 98 % and the number of acid sites with adsorption heat greater than 110 kJ mol 1 passes from 0.4 to 9.3 1020 g"1 i.e. from 7% of the theoretical number of protonic sites to 56 %. 

The poisoning of the strongest acid sites was also shown indirectly by ammonia adsorption and TPD or butene adsorption and TPD. The coked sample adsorbed a smaller amount of each base than did the fresh catalyst. 

The acid-base properties of the decationated HY zeolites have been extensively studied with adsorption microcalorimetry. Tables II and III present a summary of calorimetric studies of the adsorption of ammonia and other probe molecules on HY zeolites with different Si/AI ratios, preparation methods, pretreatments, adsorption temperatures, and sodium contents. The large variety of conditions used in these studies complicates the comparison of the materials. For example, the initial differential heat of ammonia adsorption at... 

The acid-base properties of decationated ZSM-5 zeolite have been studied in some detail using adsorption microcalorimetry, as shown in Table VIII (169-173). As the calcination temperature for HZSM-5 zeolites was increased from room temperature to 1073 K, a maximum in acidity was observed while the initial differential heat of ammonia adsorption increased continuously. Vedrine et al. (92) also found a maximum in the intensity of the IR hydroxyl bands (169) of HZSM-5 at 673 K. The IR absorption band of pyridine adsorbed on Brpnsted sites followed the same trend as that found for the hydroxyl stretching bands, confirming that above 673 K the Bronsted acidity decreased as the dehydration temperature increased. 

Not only the CN region gives information about the acidity but also the shift of the OH stretching modes of the zeolites upon adsorption of a base (Table 2). The high-frequency shift of the v(OH) from 3610-3640 cm after isomorphous substitution of the lattice corresponds with the decreasing heat of ammonia adsorption (Al>Fe>In). But more important the decreasing heats combine with a lower shift of the v(OH), see column 2 and... 

The acid-base properties of the samples were investigated using adsorption of appropriate probe molecules, namely ammonia and sulfur dioxide, monitored by microcalorimetry. The microcalorimetric studies were performed at 353 K for sulfur dioxide adsorption and at 423 K for ammonia adsorption in a heat flow calorimeter of Tian-Calvet type (Setaram C80), linked to a conventional volumetric apparatus. Before each experiment the samples were outgassed overnight at 673 K. 

Carbon dioxide removal in ammonia plants is usually accomplished by organic or inorganic solvents with suitable activators and corrosion inhibitors. In a few circumstances, C02 is removed by pressure swing adsorption (PSA) (see Chapter 3). The removed C02 is sometimes vented to the atmosphere, but in many instances it is recovered for the production of urea and dry ice. Urea is the primary use of carbon dioxide and, in case of a natural gas feed, all of the C02 is consumed by the urea plant. This practice is especially significant since C02 is a proven greenhouse gas. Typically, 1.3 tons of C02/ton of NH3 is produced in a natural gas-based ammonia plant. The C02 vented to the atmosphere usually contains water vapor, dissolved gases from the absorber (e.g., H2, N2, CH4, CO, Ar), traces of hydrocarbons, and traces of solvent. Water wash trays in the top of the stripper and double condensation of the overhead help to minimize the amount of entrained solvent. The solvent reclaimer contents are neutralized with caustic before disposal. Waste may be burned in an incinerator with an afterburner and a scrubber to control NOx emissions. 

Sulfided samples were characterized with XRD, BET surface area, NO sorption capacity, ESR and FTIR spectroscopy. The details concerning the characterization procedures as well as certain properties of USY based samples can be found elsewhere (ref. 9, 10). The ammonia adsorption capacity of sulfided and non-sulfided catalysts and supports was measured from the desorption peak obtained during 3the temperature programmed desorption (heating rate 30 K min ). Each sample (0.1 g) after activation or sulfidation was saturated with ammonia (a series of 1 cm NH3 injections) at 375 K until full saturation was achieved. This was monitored as a sharp GC peak detected by thermal conductivity detector. Next, sample was purged 1 hour in purified helium at 375 K to remove the excess of weakly held ammonia and TPD started. 

A largely accepted view to date has been that the acid strength of sulfonated resins is low compared to many inorganic acid catalysts. Our data on dry catalysts, based on enthalpies of ammonia adsorption (Figure 1), still supports this view, showing that the strongest sites... 

The acid-base properties of zeolites or oxides are often studied by measuring the selectivities to the different products in the decomposition of alcohols and particularly isopropanol. The rate of propene formation can very often be correlated to the number of acidic sites determined by ammonia adsorption. A relationship has been found between the strength of the acid sites of bulk oxides, as determined by ammonia adsorption microcalorimetry [95], and the activation energy of dehydration, while the activation energy of dehydrogenation was independent of the strength of the sites [149]. 

What are the factors that determine the acid-base properties of solid surfaces such as metal oxides On the basis of the discussion thus far it seems appropriate to relate the appearance of Lewis acidity and disappearance of Bronsted acidity to the increase in the degree of dehydroxylation. Indeed, the interconversion of Lewis and Bronsted acid sites has been demonstrated for some oxides, such as ZnO or supported Mo03 Cr203, or WO3, by IR studies of pyridine or ammonia adsorption [59]. But which factors determine the strength of acid sites ... 

More complex kinetic models, which relax the hypothesis of equilibrated ammonia adsorption and the participation of a single reactive site. Dumesic and co-workers proposed (104), on the basis of the reaction mechanism reported in the section 7.1, proposed a kinetic model, which is based on the following three steps ... 

Evaluation of surface acidity can also be carried out in the gas phase by contacting adsorbents saturated with a carrier gas, such as He, with gaseous ammonia or another base. The adsorption and the determination of the heats of adsorption can be carried out at high pressures which may produce results altogether different to the measurements of adsorption at very low surface coverages and the associated low partial pressures of gaseous bases at which they may totally saturate the adsorbent surface. 

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