High surface-area nitrides

McGee. RC Bej. SK Thompson. LT. High surface area nitrides new base cataly.sts. Proceedings of the 13 th ICC. Paris (France) 2004, Reference PI-453. 

Gas-SoUd Reactions Metal oxides are popular precursors for the preparation of high surface area nitrides and carbides. It has been found that the specific surface area of the product strongly depends on the ammonia or methane flow, and the method is often referred to as a temperature programmed reduction process (TPR) ... 

Catalysis. The development of high surface area porous M02N and W2N has spurred interest in nitrides for catalytic appHcations (52—58). High... 

Re-use of the catalyst The high activity of the solid catalysts also allows one to re-use high surface area titanium nitride several times. By centrifugation, the solid can be separated and used in a new reaction. The catalyst was used up to four times. Figure 19.10 shows the conversion of DPE with time. 

Mechanism The understanding of mechanisms in catalytic reactions is sometimes crucial for the creative development of new applications. In a first approach, the main interest was to develop high surface area titanium nitride as a material for catalytic applications and, therefore, evaluation of catalysts prepared under different conditions was performed. 

When controlled nitridation of surface layers is required, as for example in the modification of the chemical properties of the surface of a support, the atomic layer deposition (ALD) technique can be applied." This technique is based upon repeated separate saturating reactions of at least two different reactants with the surface, which leads to the controlled build-up of thin films via reaction of the second component with the chemisorbed residues of the first reactant. Aluminium nitride surfaces have been prepared on both alumina and silica supports by this method wherein reaction cycles of trimethylaluminium and ammonia have been performed with the respective supports, retaining their high surface areas." This method has been applied to the modification of the support composition for chromium catalysts supported on alumina." ... 

Table 13.12 Nitridation of high surface area MoS2 at different temperatures...

High surface area MoS2 powders show an interesting reactivity with ammonia (Table 13.12). Significant nitridation occurs as low as 923 K with formation of L-MoN which can be easily prepared as single phase below 973 K. As seen before, formation of H-MoN is observed at higher temperatures, and from 1123-1173 K y-Mo2N appears. 

The TPR method has now been applied to TlN O [52], VN [23], and NbC [53]. Although the TPR method produces high surface area materials, the pore structure of these is usually not controllable. Often, the pores are in the micropore regime (< 3 nm). However, a number of the solid state transformations that lead to carbides and nitrides are topotactic and exhibit pseudomorphism (retention of external particle size and shape) This provides a possible means of engineering the pore structure by preparing oxide precursors with the desired external morphology. 

Thompson, L.T. High surface area carbides and nitrides. Abstracts of Papers, 227th ACS National Meeting. Anaheim, CA, March 28-April 1, 2004. 

However, there have been few reports on the successful fabrication of ordered mesoporous carbides and nitrides with high surface areas and large adjustable pores. The difficulties are mainly the extremely high synthesis temperatures required and the lack of proper precursors. A few of examples are shown here for the successful synthesis of mesoporous carbides and nitrides. 

In the present work, porous boron nitride samples have been prepared from different molecular precursors. The nature and the pretreatment of the precursors have been investigated in order to determine accurately the optimal conditions leading to high surface area porous boron nitride materials suitable to be used as catalysts supports. The best results led to BN powders presenting surface areas of about 250 to 300 m. g. The porosity resulted from the presence of two kind of pores mesopores down to 2 nm, and a microporosity corresponding to pores from 0.5 to 1 nm. Attempts were made to incorporate directly a noble metal precursor into the BN precursor. Up to 1100°C, Pd or Pt were kept in the porous structure, but the very high temperature required to stabilise BN powders lowered drastically the metal concentration on the catalyst As a consequence, only the platinum sample could be obtained and characterized this catalyst is currently being tested. 

Elaboration of high surface area boron nitride. 

The preparation of high surface area boron nitride powders and foams from various molecular precursors has been studied. The highest specific areas were obtained using the poly-borazylenic precursor. However the difference with the measured surface area of the samples prepared from the polyhaloborazinic precursor was specially related to an enhanced micro-porosity of the samples prepared from the borazinic precursor. Curiously, the samples obtained from aminoborazinic precursors always exhibited a smaller specific surface area. This has been related to the ability of this precursor to melt and stay under a waxy form at... 

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