Nitridation temperature

Physicochemical attributes of catalysts were mostly controlled by nitridation temperature although there was a little influence on catalyst reducibility and acidity, better nitride species were formed at 973 K and TGA results revealed that complete nitridation occurs between 750-973 K and the feed gas stream containing H2 NH3=1 1 is preferably better mixture for the nitridation of Co-Mo bimetallic catalysts. 

Composition, nitridation temperature and time, surface area and nitrogen content of the "AlPONs"... 

Composition Nitridation temperature (K) Nitridation Surface area time (h) (m2.g-l) Total N (wt.%) Surface N (wt.%)... 

A differential flow microreactor was used for the preparation of the nitrided catalyst and the TPD, TPR, and NH3-TPD measurements. Nitriding of the molybdena-alumina and alumina was carried out by temperature-programed reaction with NH3 (NH3-TPR).1719 The MoCV A1203 precursor was oxidized at 723 K for 24 h, cooled to 573 K, reacted with NH3 at 49.6 (xmols-1 from 573 to 773, 973 or 1173 K at a rate of 0.0167 Ks-1, held at the nitriding temperature for 3 h, and then cooled to room temperature (RT) in flowing NH3. The catalysts were characterized by TPD, TPR, and NH3-TPD under in situ conditions, while BET and diffuse reflectance FTIR measurements were carried out after passivation. For the diffuse reflectance FTIR study, the catalysts after NH3 treatment... 

Figure 17.10 The amount of desorbed NH, per unit specific surface area in NHrTPD at 573 K as a function of nitriding temperature.

Figure 17.12 Surface acidity of 12.5 M0O3/AI2O3 catalyst as a function of nitriding temperature. O, total , Lewis acidity , Bronsted acidity.

Figure 4.32. A. Nitridation of elemental Si monitored by Si spectroscopy. As the SiaN4 peak (—48 ppm) grows during the reaction, the elemental Si peak at — 80 ppm progressively shifts to less negative values. At the highest temperatures some oxidation to Si02 (—112 ppm) is also apparent. B. Shift of the Si peak for elemental Si as a function of nitriding temperature. From MacKenzie et al. (1999) by permission of copyright owner.

The LTN catalyst was 1.5 and 1.6 times more active than MTN and HTN catalysts, respectively. The HDS activity of nitrided M0/AI2O3 catalysts decreased with increasing nitriding temperature. 

Nitrided zeolites were prepared by thermal treatment of ultrastable zeolite Y (Si/Al ratio equal to 13) under ammonia flow for a prolonged time. The effects of the nitridation parameters on the extent of the nitrogen incorporation and on the structural and catalytic properties of the resulting zeolites were investigated. The amount of nitrogen incorporated in the zeolite increases with temperature and duration. Structural order and porous volume are preserved if the nitridation temperature and the nitridation time do not exceed 800°C and 72 h, respectively. Nitrided zeolites are base catalysts as revealed by the reaction of Knoevenagel condensation between benzaldehyde and malononitrile. 

The commercially available ultrastable zeolite Y CBV720 (Si/Al = 13) was provided by Zeolyst, and used as received. The zeolite was placed in a quartz fixed bed micro-reactor, and heated under flowing ammonia (Indugas 99.9% 60 ml/min). Different temperatures and times were used. At the end of the treatment, the samples were cooled down to room temperature under a pure and dry nitrogen flow. All the samples will be referred to as 13USY600N48 where 13 stands for the Si/Al ratio, 600 for the nitridation temperature in °C and 48 for the nitridation time in hours. 

Textural properties of the starting and nitrided zeolites are presented in Table 1. The increase of the nitridation temperature causes a decrease in Vmjcroj Vt and Sbet, while Vmeso and Sext increase slightly. Thus the increase of the nitridation temperature leads to the collapse of part of the microporous structure, to the formation of mesopores and consequently to the extension of the "external" surface. The Vj decreases at the same time as Vmicro> indicating that the fall of Vmicrois not Compensated by the rise of Vmeso- No correlation was found between these results and XRD, except in the case of the sample nitrided at 900°C for which a decrease of crystallinity occurs simultaneously with an important loss of Vmicro-... 

Fig4. shows the microstructures of the fracture surfaces. Some holes can be observed in both composites as a result of the fibers-pull-out, however, the lengths of the pulled-out fibers are very short. Compared to the nitridation temperature of 1800°C, the carbonization temperature of carbon fibers in the preparation process is relatively low, therefore, in the nitridation process, some damages may be done to the fibers and lead to the degradation in the strengths of fiber reinforcements and result in the low strength of composites. Some phases with layered structures can also be observed in the miciostructures of composite with boron, which corresponds to h-BN in situ formed in the nitridation process. When the cracks propagate to h-BN, they will deflect. 

Contradictory data have been published on the nitriding temperature for Ca. The reaction has been reported to proceed at a measurable rate only above 800°C. The discrepancies, it seems, can be traced to use of metal which is not quite pure, or of Ng still containing traces of Og. 

Titanium Nitrides. Titanium nitride (TiN) with a surface area of 28 m /g was synthesized from TPR of Ti02 with ammonia at the final reaction temperature of 1223 K (48). TiN showing a higher surface area was fabricated by nitridation with anatase Ti02 nanoparticles (10-15 nm) (49). The reaction was carried out with a NHs flow rate of 1 L/min at 1073 K. The surface area was 55 m /g originated from the TiN nanoparticles of 20 nm, but it decreased (from 33 to 15 m /g) as the nitridation temperature increased from 1173 to 1373 K. 

Type of nitriding Temperature, °C Time, min e-nitride, % y -nitride, %... 

According to these, both structural analysis and hardness measurement indicate that under the fluidized bed nitriding conditions, there exists an incubation time for the initiation of nitriding reactions. Nitriding must be carried out for a duration longer than the incubation time in order to produce an effective nitrided layer. From the experimental results, it is also evident that the incubation time is temperature dependent increasing nitriding temperature reduces the incubation time. 

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