Desorption nitrate

After the addition of silver nitrate, potassium nitrate is added as coagulant, the suspension is boiled for about 3 minutes, cooled and then titrated immediately. Desorption of silver ions occurs and, on cooling, re-adsorption is largely prevented by the presence of potassium nitrate. 

Laboratory method using porous polymer adsorbent tubes, thermal desorption and gas chromatt raphy MDHS 32 Dioctyl phthalates in air Laboratory method using Tenax adsorbent tubes, solvent desorption and gas chromatography MDHS 33 Adsorbent tu standards Preparation by the syringe loading technique MDHS 34 Arsine in air Colorimetric field method using silver diethyl-dithiocarbamate in the presence of excess silver nitrate... 

Since the formation of NO2 can occur homogeneously, it was of interest to establish whether adsorbed NO could be oxidized. NO was adsorbed at 225 C, after which the infrared cell was purged with He and subsequently a stream of 10.1% O2 in He was allowed to flow over the catalyst. Prior to the introduction of the 02-containing stream, the only features evident were those for mono- and dinitrosyls. In the presence of O2 at 225 °C, the intensities of the bands for both mono- and dinitrosyl species attenuated and new features appeared at 1628 and 1518 cm-, corresponding to nitrate and nitrito species, respectively. A similar experiment carried out in the absence of O2, showed only a small decrease in the intensity of the nitrosyl bands due to NO desorption and the absence of bands for nitrate and nitrito species during a 30 min purge in He at 225 °C. 

Then the reduction of stored NOx with hydrogen is addressed. The bulk of data points out that the reduction of stored nitrates occurs under near isothermal conditions through a Pt-catalysed surface reaction that does not involve the thermal desorption of the stored nitrates as a preliminary step. A specific role of a Pt—Ba interaction was suggested, which plays a role in the NOx storage phase as well. 

The results of the TPD and TPSR experiments performed over the Ba/7-AI203 catalyst are displayed in Figure 6.11. In the case of the TPD experiment, no desorption peaks were observed below 350°C, i.e. below the adsorption temperature. The decomposition of nitrate species present on the catalyst surface is apparent only above 350°C, and the process is not yet completed at temperatures as high as 600°C. In line with several literature indications [25,28,33,35] decomposition of nitrates results in this case in the initial evolution of N02, followed by NO and 02. 

A different picture was obtained in the case of the reference Pt—BaAy-A Oj (1/20/100 w/w) sample (Figure 6.12). In the case of the TPD experiment performed after NO, adsorption at 350°C, the decomposition of stored NO, species was observed only above 350°C. Evolution of NO and 02 was observed in this case, along with minor quantities of N02 [25,28,33,35], Complete desorption of NO, was attained already slightly below 600°C. As in the case of the binary Ba/y-Al203 sample, the data hence indicate that nitrates formed upon N0/02 adsorption at 350°C followed by He purge at the same temperature, did not appreciably decompose below the adsorption temperature during the TPD run under inert atmosphere. 

One of the most promising processes is the active DeNO based on NO -trap materials. It has been developed for lean-burn gasoline engines. Cerium compounds are thought to intervene in different steps of the whole process (1) NO oxidation, (2) NO storage, (3) Nitrate desorption and NO reduction. Most probably, the main role of OSC materials is to accelerate HC partial oxidation during rich-spikes (giving CO and H2 as NO reducers). However, this beneficial effect of OSC compounds competes with a detrimental reaction,... 

The NH4-Beta-300 (Zeolyst International, number denote Si02/Al203 molar ratio) was transformed to corresponding proton form using a step calcination procedure at 500 °C. H-Beta-300 was partially modified with Fe by repeated ion-exchange method (Fe(III)nitrate). The surface areas as well as acidities (Bronsted and Lewis acid sites) of Fe-Beta (iron content - 0.1 wt %) were determined by nitrogen adsorption and pyridine desorption at 250, 350 and 450 °C using FTIR spectroscopy [6]. 

T. K. Dutta and S. Harayama. Time-of-Flight Mass Spectrometric Analysis of High-Molecular-Weight Alkanes in Crude Oil by Silver Nitrate Chemical Ionization after Laser Desorption. Anal. Chem., 73(2001) 864-869. 

Desorption of nitrate species leads to the formation of NO and O2 with a molar ratio... 

Experiments over Cu/ZSM-5, Centi and Perathoner[46] eoncluded that the thermal stability of adsorbed species increased with increasing oxidation state of nitrogen in nitrogen adspeeies. Valyon and Hall[41] also found that the nitrosyl speeies adsorbed on Co-ferrierite is weakly adsorbed compared to nitrate species. Therefore, the second NO desorption peak oeeurring at medium temperatures likely corresponds to the nitrosyl species, according to their thermal stability. 

Co-, Mn-, and Fe-based perovskites, NO peaks appearing at similar temperatures as those in NO-TPD experiments (apart from a minor desorption at medium temperature and an intense one at high temperature) were also found and aseribed to the desorptions of physically adsorbed NO, nitrosyl species and nitrate species in the order of their thermal stability. The NO peaks obtained at low (80-110 °C), medium (200-210 °C), and high (310-390 °C) temperatures in the present NO-TPD analyses were thus correlated to physically adsorbed NO, nitrosyl and nitrate species. 

TPD of Cu-Al-MCM-41 (after NO adsorption under 0.8% NO in He) was eondueted (Table 14). NO and NO2 are the species detected coming off the surface as the temperature of the catalyst is increased. Two features were observed in the NO desorption profile a principal peak at 149 °C and a second NO desorption feature at higher temperature (440 °C). This indicates that there are at least two types of NO adsorption sites available. The presence of two types of adsorbed NO species over Cu catalysts has been reported earlier in the literature[45]. These have been proposed to be the desorption of NO from Cu ions and nitrate (NO3 ), nitrite (NO2 ) or N02 adsorbed species, respectively. Assuming the sensitivity factors of the peaks at low and high temperature are equivalent, the areas can be used to estimate the normalized desorption of NO. As listed in Table 14, the amount of NO desorbed at low temperature is close to the total amoimt of desorbed NO. This feature indicates that copper is mainly as isolated Cu in the catalyst. During NO desorption, a small amoimt of NO2 (8.2 pmol/g) desorbed at 80 °C. 

Reaction (13.47) accounts for more than 95% of the overall H2 consumption. Notably, the H2 uptake in Figure 13.18a is seen before the evolution of the reaction products, which suggests that H2 is first adsorbed and activated on the catalyst surface and then participates in the reduction of nitrates. However, a time delay in the detection of ammonia due to its slow desorption from the catalyst surface cannot be excluded. 

A stepwise reaction mechanism which involves adsorption of nitrate at a bimetallic site, reduction to nitrite, desorption in to the aqueous phase and re-adsorption at a monometallic e.g. Pd) site has been proposed and is supported by theoretical prediction. A reaction scheme based on the use of a bimetallic catalyst is illustrated in Fig. 2. 

Bezabeh, D. Z., T. M. Allen, E. M. McCauley, P. B. Kelly, and A. D. Jones, Negative Ion Laser Desorption Ionization Time-of-Flight Mass Spectrometry of Nitrated Polycyclic Aromatic Hydrocarbons, J. Am.. Soc. Mass Spectrom., 8, 630-636 (1997). 

One additional step (added late in the desorption scheme) consisted of rinsing the column with 0.1 N HC1 in water to remove lead compounds. This step was necessary because lead nitrate at 25 ppb would precipitate, and lead was removed by the resin acting as a filter. This step may not be necessary in real field concentration-isolation of trace organic compounds. 

Oxidation of arsenic-bearing pyrite with adsorption onto iron oxides and/or other metal (oxy)(hydr)oxides Nitrate reduction by pyrite oxidation (note that Appelo and Postma, 1999 referred to pure rather than arsenian pyrite) Manganese oxide reduction and release of sorbed arsenic Fe(lll) reduction on oxide surfaces changes net charge leading to arsenic desorption Iron oxide reductive dissolution and release of sorbed arsenic catalyzed by NOM degradation... 

The nitration of aromatics in the vapour phase has two main advantages namely the continuous removal/desorption of water and the possibility to use a fixed bed reactor, which allows a continuous nitration process. 

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