NH3 pretreatment

Notably, NH3 is indispensable for the catalytic phenol synthesis. In the absence of NH3, neither benzene combustion nor phenol formation occurred on the Re-CVD/HZSM-5 catalyst (Table 10.6). Other amine compounds such as pyridine and isopropylamine did not promote the catalytic reaction at aU, which indicates that the role of NH3 in the catalysis is not due to its basic function. Fe/ZSM-5 has been reported to be active and selective for phenol synthesis from benzene using N2O as an oxidant [90, 91], but N2O did not act as an active oxidant on the Re-CVD/ HZSM-5 catalyst Furthermore, no positive effects were observed by the addition of both N2O and H2O. Notably, the NH3-pretreated Re-CVD/HZSM-5 catalyst selectively converted benzene into phenol with O2 in the absence of NH3, as discussed below. 

Table X. Influence of NH3 Pretreatment and of Mercerization on Enzymatic Degradation of Cellulose Powders...

Acid hydrolysis of these samples led to the results to be expected, i.e., a lowering of LODP and residue value by NH3 pretreatment and mercerization as well as by mechanical disintegration prior to the hydrolytic treatment (Table XII). With regard to residue value, an NH3 pretreatment again proved to be more efficient in enhancing accessibility than a mercerization, while the rate constant of chain-length degradation was increased somewhat more by mercerization. 

Chlorine is desirable as a bulk pretreatment biocide for inlet water, but its subsequent removal upstream of the membrane is absolutely necessary ana difficult. NaHSO,3 is a common additive to dechlorinate before membranes. It is customarily added at 3-5 mg/1, an excess over the stoichiometric requirement. NH3 is sometimes added to convert the chlorine to chloramine, a much less damaging biocide. Heavy metals present in seawater seem to amplify the damaging effects of chlorine and other oxidants. 

When the thermokinetic parameter was plotted versus the amount of NH3 adsorbed for samples of H-ZSM5 (Si/Al = 10.3) pretreated at 400 and 800°C it was found that the maximum time constant is higher for the sample pretreated at 800°C than for that pretreated at 400°C [103]. In fact, the increase of the pretreatment temperature caused dealumination extra-framework aluminum species were created that restricted the access to the channels and created diffusional limitations. 

As can be seen in Table 13.2, the heats of NH3, pyridine, CO2 or SO2 adsorption clearly show that these molecules are chemisorbed on all aluminas (heats of adsorption higher than 100 kJ/mol) in spite of the different origins of AI2O3 and different pretreatment and adsorption temperatures used. 

Preparation of Catalysts. A large amount of pure omega was prepared as described above. Platinum and palladium catalysts were made from this material by slurrying variously pretreated forms with aqueous solutions of Pt and Pd(NH3)4Cl2 of appropriate concentrations. Zeolites thus obtained were dried and calcined to a final temperature of 500° C. 

X-ray diffraction patterns of powdered catalysts were recorded with a Rigaku RINT 1200 diffractometer using a radiation of Ni-filtered Cu-Ka. BET surface area and pore size distribution were calculated from the adsorption isotherm of N2 at 77 K. The BJH method was used for the latter. Aluminum content was determined by ICP spectrometer. FTIR spectra of adsorbed NH3 were recorded with a JASCO FT/IR-300 spectrometer. The self-supporting wafer was evacuated at prescribed temperatures, and 25 Torr of NH3 was loaded at 473 K. After NH3 was allowed to equilibrate with the wafer for 30 min, non-adsorbed NH3 was evacuated and a spectrum was collected at 473 K. The differential heat of adsorption of NH3 was measured with a Tokyo-riko HTC-450. The catalyst was pretreated in the presence of 100 Torr oxygen and evacuated at 873 K. The measurements were run at 473 K. 

Fig. 30. Effect of sequential pretreatment on magnetically split spectral area versus temperature of 3% Fe/MgO MOssbauer spectra in H2 N2. Pretreatment sequence , H2 reduction O, NH3 A, H2 0. NH3. Figure according to Dumesic et al. (165).

Extrelut column pretreated with tartaric acid, cleaning up by hexane isopropilic ether (1 1), elution with CH2C12 after basification with gaseous NH3. 

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). 

With the samples at first mechanically disintegrated, all the statements regarding the influence of an NH3 and an NaOH pretreatment were confirmed (Table XII). In any case, the previous mechanical disintegration decreased the rate of enzymatic attack as compared with the appropriate nondisintegrated samples. Probably some homification and/or change in pore structure occurs during mechanical disintegration in the dry state. 

In these terms, the ammonia adsorption is not entirely reversible. Figure 12.1 shows the total and irreversible ammonia adsorption (at room temperature) on Kieselgel 60, thermally pretreated at 473, 673 and 973 K. The adsorption capacity is exceptionally expressed as /nm2. The figure clearly demonstrates that the total ammonia adsorption on silica is determined by the silanol number, in a 1 1 relationship. This means that ammonia adsorption on silica involves an attachment (by a hydrogen bond) of 1 NH3 molecule to 1 silanol group. Such species produce an infrared band at 3419 cm1, assigned by Peri1 to the v3 valence vibration of ammonia. 

When the reaction temperature is raised to temperatures above 873 K, the 3450 cm 1 Si-NH2 band shows a tailing effect. Figure 12.3 illustrates this phenomenon by a FTIR spectrum of Kieselgel 60, thermally pretreated at 973 K and reacted with NH3 at 923 K for 5 hours. A band centred around 3405 cm 1 can be observed, attributable to silazane (Si-NH-Si) species. 

Figure 12.3 FTIR spectrum of Kieselgel 60, thermally pretreated at 973 K and reacted with NH3 at 923 Kfor 5 h.

Dalla Betta and Boudart21 studied the pretreatment of Pt(NH3)421 exchanged on zeolite Y. They report that direct reduction in H2 leads to the formation of neutral R(NH3)2H2 hydride in the temperature range of 80-100°C, ultimately resulting in agglomeration and thus large particles. They conclude that decomposition of the complex in 02 should be carried out prior to the reduction. 

Van den Broek et a/.18 studied the pretreatment of ion-exchanged R(NH3)42+ on zeolite HZSM-5 in He and 02 with UV-Vis spectroscopy and mass spectrometry. For the pretreatment in He, autoreduction was found to occur via to the formation of H2 and N2 from the NH3 ligands. Calcination in 02 led to the production of NOx in several different steps. The presence of H20 was found to play a crucial role in the pretreatment, replacing NH3 as a ligand on the Pt complex. 

Keegan et al. studied the calcination and reduction of the same Pt(NH3)4 on HZSM-5 system with energy dispersive Extended X-ray Absorption Fine Structure (EXAFS). They showed that during calcination the Pt-Pt coordination rises, indicating agglomeration. The final metal particle size obtained after direct reduction (no calcination prior to the reduction) was smaller than the particle size obtained after calcination prior to the reduction. The authors did not clarify the chemistry of the pretreatment process. 

All of these literature reports mainly deal with exchanged Pt(NH3)4 on zeolite. The main difference between exchanged and impregnated [Pt (NH3)4](N03 )2 is the presence of nitric groups (N03 ) on the support in the impregnated case. As will be shown, these groups play a vital role in the pretreatment of the impregnated catalyst precursor. 

All in all, little has been reported on the reactions taking place for impregnated [Pt2+(NH3)4](N03 )2 on macroporous supports. Moreover, the relation of the pretreatment to the final particle size distribution is rarely investigated. In our view, knowledge of the reactions occurring during pretreatment is a crucial step towards the development of a process leading to uniform small particle sizes. 

In this study, a powerful combination of high resolution transmission electron microscopy (HRTEM), mass spectrometry (MS) and Quick EXAFS (QEXAFS) is used to study the reactions of [Pt2+(NH3)4](N03 )2 impregnated on Si02 during different pretreatment processes. MS is used to monitor which gases are produced during the pretreatment. QEXAFS is used to study the local structure of the Pt complex during the pretreatment. The timescale of... 

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