Ammonia adsorption-desorption

FIGURE 8.26 Ammonium adsorption isotherm. Relationship between ammonium adsorbed on cation exchange complex and pore water ammonium concentration. 

At concentrations found in many wetlands, the relationship between S and C is typically linear, and can be described as follows  

ENH4C0 (equilibrium ammonium concentration) is defined as the ammonium concentration in solntion at which adsorption equals to desorption (S = 0). The ENH4C0 can be estimated as follows  

Low ENH4C0 values are observed in soils with high CEC snch as clay or organic soils. Soils with low CEC or soils where exchange sites are occupied with other cations have high ENH4C0. 

Biogeochemistry of Wetlands Science and Applications Ammonium adsorption—aerobic 

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Langmuir kinetics have usually been considered for ammonia adsorption-desorption kinetics [48, 51, 52]. On the other hand, Andersson et al. ruled out that... 

Lietti and co-workers studied the kinetics of ammonia adsorption-desorption over V-Ti-O and V-W-Ti-O model catalysts in powder form by transient response methods [37, 52, 53[. Perturbations both in the ammonia concentration at constant temperature in the range 220-400 °C and in the catalyst temperature were imposed. A typical result obtained at 280 °C with a rectangular step feed of ammonia in flowing He over a V2O5-WO3/TiO2 model catalyst followed by its shut off is presented in Figure 13.5. Eventually the catalyst temperature was increased according to a linear schedule in order to complete the desorption of ammonia. 

To describe the NH3 + NO/NO2 reaction system over a wide range of temperatures and NO2 NOxfeed ratios in addition to ammonia adsorption-desorption, ammonia oxidation and standard SCR reaction with the associated kinetics already discussed in Section 2.3.2, the following reactions and kinetics have been considered by Chatterjee and co-workers [79] ... 

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 kinetic parameters of ammonia oxidation were fitted by multiresponse nonlinear regression, while the parameter estimates for the ammonia adsorption-desorption kinetics were kept unchanged with respect to those obtained from the fit in the previous section. Notably, in this case both the NH3 and the N2 outlet concentrations were regarded as regression responses. 

Catalytic properties Phosphorus is known to have deactivation effects for some automotive catalysts and the formation of CeP04 has been identified in phosphorus contaminated catalysts (Uy et al., 2003). Nanocrystalline LaP04 would act as Lewis acid in a catalytic process, which could be determined by a temperature-programmed ammonia adsorption/desorption process (Onoda et al., 2002 Rajesh et al., 2004, 2007). In addition, the rare earth phosphate NCs could act as supports for example, Pd, Pt, or Rh supported on RPO4 show excellent catalytic reduction of NO into N2 and O2 (Tamai et al., 2000), and gold supported on RPO4 shows catalytic activity and stability for CO oxidation. 

Ammonia adsorption-desorption + Temperature Programmed Desorption (TPD) runs [11] were performed in order to study the adsorption-desorption of NH3 onto the catalyst. Experiments were typically performed at a GHSV of 92,000 h by feeding 1,000 ppm of ammonia in the presence of 2 % O2 and 1 % H2O at constant adsorption temperature (between 200 and 400 °C) when the catalyst adsorption capacity was saturated, NH3 and O2 were shut off and a temperature ramp from 50 to 550 °C at 15 °C/min was started. Temperature Programmed Reaction (TPR) runs [11] were performed to study the gas-phase reactivity on increasing temperature the reactants were fed at constant temperature, then a temperature ramp at 2, 10, or 20 °C/min was run. TPR experiments were typically carried out in the presence of oxygen (2 %) and water vapor (1 %) with GHSV between 90,000 and 230,000 h the reactant feed concentration varied between 250 and 1,000 ppm. 

The capabihty to adsorb ammonia is an important characteristic of SCR catalysts. Indeed all authors who had previously studied the SCR process for stationary applications agreed that in the SCR reactions ammonia reacts from a strongly adsorbed state [16-21]. Moreover, the aflSnity of such a reactant for the catalyst surface decreases the so-called ammonia slip that is the undesired release of unreacted ammonia from the reactor. Accordingly, a good understanding of the ammonia adsorption-desorption process is strictly required for a correct description of the SCR process. 

Specific runs were carried out in order to extract the intrinsic kinetics of ammonia oxidation and at the same time to validate the kinetics of the ammonia adsorption-desorption process, previously fitted. 

It is worthy of note that the kinetic model was capable to capture both the light-off temperature of ammonia oxidation and the slope with which it proceeds upon increasing the temperature. Moreover, the ammonia adsorption-desorption... 

The same estimates of kinetic parameters for ammonia adsorption/desorption and oxidation as previously obtained were retained with no further adjustment. 

To describe the full NH3-NO/NO2-O2 reacting system in the whole range of temperatures and NO2/NOX feed ratios, other reactions apart from those that describe the redox cycles, resulting in the Standard and Fast SCR reactions, had to be incorporated in the kinetic model. Such additional reaction steps included ammonia adsorption/desorption and oxidation, nitrates adsorption and desorption, NO2 SCR, and N2O formation (see Table 10.1). 

The first step is ammonia adsorption/desorption and this step was tuned to an ammonia TPD experiment described in an earlier section (Eq. 12.21, see Fig. 12.4). The second step is ammonia oxidation (Eq. 12.22) and the third step NO oxidation (Eq. 12.23). The parameters for these three steps were determined by separate experiments. In Eq. (12.24), the reaction for standard SCR is described and in Eq. (12.25), the rapid SCR with equimolar amounts of NO and NO2 is covered. NH3 also reacts with NO2 alone, which is described in Eq. (12.26). The final step (see Eq. 12.27) is N2O production from a reaction between adsorbed ammonia and NO2. The results from exposing the Cu-ZSM-5 catalyst to 500 ppm NO, 500 ppm NH3, 8 % O2 and 5 % H2O, while increasing the temperature stepwise are shown in Fig. 12.14 [10]. Initially, there is a total uptake of ammonia for an extended time period, but due to minor NO storage, NO breaks through immediately. The temperature is then increased to about 150 °C, while loosely bound ammonia is released. At this temperature, the standard SCR reaction is active and equimolar consumption of NO and NH3 is seen. At 200 °C, the conversion of NOx is close to 100 %. At high temperature, the NOx conversion again decreases due to the ammonia oxidation. Also, some NO2 is produced, since the back part of the catalyst is not exposed to ammonia, NO oxidation may occur. The global model described... 

A simple approach to capture the ammonia adsorption/desorption kinetics is the single-site approach, where NH3 is assumed to adsorb on a global single-surface site. A nonactivated ammonia adsorption process is considered while a Temkin-type coverage dependence of the activation energy is assumed for the desorption process [24]. The reaction rate expression of adsorption is given in Eq. (3.21) ... 

A dual-site model for the storage and the release of NH3 over a Fe-zeolite catalyst has been proposed by Colombo et al. [23]. The acid sites where ammonia is either weakly adsorbed or physisorbed are denoted as Site-1 while the strong adsorption sites are denoted as Site-2. The following rate expressions describe the rates of ammonia adsorption/desorption for each site ... 

Colombo M, Koltsakis G, Nova I, Tronconi E (2011) Modelling the ammonia adsorption-desorption process over an Fe-zeolite catalyst for SCR automotive applications. Catalysis... 

With an adequate amount of ammonia being adsorbed on the SCR substrate, high NOx reduction rate can be realized. It is thus believed that consistent SCR NOx reduction can be ensured by ammonia storage control. However, besides the NOx reduction, tailpipe ammonia slip constraint is another objective needs to be taken into account. From the SCR model in Eq. (14.29) and the ammonia adsorption/desorption reactions in Eq. (14.4), it can be seen that high ammonia... 

Ammonia adsorption/desorption— The interaction of NH3 with the catalyst surface is obviously important for SCR applications, as it is well known that the DeNOx performances and the dynamics of SCR converters are governed by the reactivity of adsorbed NH3. The adsorption-desorption behavior of NH3 (R.1-R.2 in Table 18.1) was first studied performing isothermal NH3 feed... 

Ammonia adsorption/desorption— First of aU, the ammonia adsorption and desorption properties of the PGM catalyst have been investigated by transient runs, involving a concentration step-change mn at 150 °C, stepwise feeding 500 or 1,000 ppm of NH3 in the presence of water (8 % v/v) and balance helium, followed by NH3 shut off and by a temperature ramp in order to complete ammonia desorption (TPD). 

Data, not reported, indicated that ammonia does not appreciably adsorb onto the PGM catalyst. However, literature reviews on the ammonia oxidation mechanism over PGM catalysts unanimously consider the adsorption of both reactant molecules (NH3 and O2) [16], with ammonia adsorbing in on-top position on Pt [17]. Indeed, the lack of a detectable storage capacity for ammonia during our runs is not sufficient to mle out the adsorption of the same species on the catalyst surface accordingly, notwithstanding the experimental evidence, ammonia adsorption/desorption steps were included in the developed kinetic model (R.16 and R.17 in Table 18.2). 

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