Bulk carbon nitrides

Hitherto known 2-D polymers include graphene [1], boron nitride [51] as well as metal oxides, hydroxides, and chalcogenides [2,23c]. These inorganic 2-D polymers are usually obtained by exfoliation from their parent laminar crystals this can be achieved using physical methods such as the scotch tape approach [1] or intercalation [52]. Many reports have been made on the preparation of laminar crystals which can, in principle, be regarded as the parent materials for 2-D polymers. For example, Antonietti and coworkers reported the details of graphitic carbon nitrides based on heptazine motifs, prepared by the thermal condensation ofcyanamide (>560 °C) [53] however, individual layers have not yet been separated from the bulk products. 

The characterisation of both bulk and thin-film diamond phases by vibrational (chiefly Raman) spectroscopy has been the subject of a large number of publications. Carbon nitride thin films have also been extensively reported on,... 

Motivated by the reports of the S5mthesis of die IV-V nitrides, P principles calculations were performed on a hypothetical carbon nitride phase, C3N4, patterned on the P-SijN stmcture t) e [28]. A semi-empirical formula for bulk modulus developed out of this work, predicted the latter nitride would be of hardness comparable to diamond [28]. Inspection of... 

In finely divided form, hafnium is pyrophoric, igniting in air spontaneously. However, bulk metal reacts slowly in oxygen or air above 400°C. The rate of oxidation increases with temperature. The product is hafnium dioxide, Hf02. It combines with nitrogen, carbon, boron, sulfur and silicon at very high temperatures to form hafnium nitride HfN, hafnium boride HfB, hafnium sulfide HfSi2, respectively. Nitride formation occurs at 900°C. 

Most borides are chemically inert in bulk form, which has led to industrial applications as engineering materials, principally at high temperature. The transition metal borides display a considerable resistance to oxidation in air. A few examples of applications are given here. Titanium and zirconium diborides, alone or in admixture with chromium diboride, can endure temperatures of 1500 to 1700 K without extensive attack. In this case, a surface layer of the parent oxides is formed at a relatively low temperature, which prevents further oxidation up to temperatures where the volatility of boron oxide becomes appreciable. In other cases the oxidation is retarded by the formation of some other type of protective layer, for instance, a chromium borate. This behavior is favorable and in contrast to that of the refractory carbides and nitrides, which form gaseous products (carbon oxides and nitrogen) in air at high temperatures. Boron carbide is less resistant to oxidation than the metallic borides. 

For reactive metals, sintering is performed under vacuum or under inert atmospheres containing sufficiently low partial pressures of gases containing oxygen, carbon and nitrogen in order to minimize formation of brittle bulk oxides, carbides and nitrides. In some cases, however, residual thin adherent oxides of the reactive metals may be beneficial and act as intermediary layers to aid wetting with the structural ceramic oxide in the cermet. 

The dimensions of the added nanoelements also contribute to the characteristic properties of PNCs. Thus, when the dimensions of the particles approach the fundamental length scale of a physical property, they exhibit unique mechanical, optical and electrical properties, not observed for the macroscopic counterpart. Bulk materials comprising dispersions of these nanoelements thus display properties related to solid-state physics of the nanoscale. A list of potential nanoparticulate components includes metal, layered graphite, layered chalcogenides, metal oxide, nitride, carbide, carbon nanotubes and nanofibers. The performance of PNCs thus depends on three major attributes nanoscopically confined matrix polymer, nanosize inorganic constituents, and nanoscale arrangement of these constituents. The current research is focused on developing tools that would enable optimum combination of these unique characteristics for best performance of PNCs. 

The utility of carbide and nitride catalysts has prompted numerous studies of their reactivity that use carbide and nitride overlayers as the catalyst rather than bulk carbides or nitrides. This approach permits careful manipulation of the surface metal/nonmetal stoichiometry, which is crucial to probing reactivity. These studies consistently reveal the catalytic activity of carbide and nitride overlayers and, in several cases, the similarities between their behavior and that of noble metal catalysts. For example, the same benzene yield and reaction pathway for the dehydrogenation of cyclohexane was observed for both p(4x4)-C/Mo(110) and Pt(l 11) surfaces. Furthermore, carbon-modified tungsten may be a more desirable catalyst for direct methanol fuel cells than Pt or Ru surfaces because the transition metal carbide exhibits higher activity toward methanol and water dissociation and is more CO-tolerant. ... 

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