Nanolayer carbides

Microwave-Assisted Synthesis of Nanolayer Carbides and Nitrides... 

An XRD pattern for an MAFBS-prepared sample that was irradiated for fifteen 10 s pulses did not contain any carbide peaks. The TEM micrograph of this sample, in Figure 7.13, revealed that a nanolayer had formed. The d-spacing of this nanolayer was 2.76 A, which matches a standard value for CrjCj. The depth and coverage of this nanolayer were not uniform. In fact, TEM analysis of several samples from this experiment revealed that some samples did not experience carburization, whereas amorphous carbon was deposited on the surface of others. 

Another sample that was processed continuously for 1 h also did not exhibit a carbide peak in its XRD pattern. TEM analysis of a sample from this experiment (Figure 7.16), however, revealed a nanolayer roughly 2 nm deep with a d-spacing that matches a literature value for M02C (3.05 A). 

TEM analysis of molybdenum that reacted with ethylene (Figure 7.20) indicated that the d-spacing of the nanolayer on the metal powder was 2.378 A. A standard value for MOjC matches this result. The thickness of the nanolayer was roughly 20 nm. These results demonstrate that using the fluidizing gas as a carbon source is more effective than fluidizing carbon black and metal powders in an inert gas. They also illustrate that MAFBS is a technically feasible technology to produce MOjC carbide nanolayers on molybdenum powders. 

This work clearly demonstrates that it is possible to produce carbide and nitride nanolayers on early transition metals with MAFBS. The extent of reaction, however, varied from metal to metal. CrjN was prepared more readily than MoN, but CrjN was less catalytically active. Using ethylene rather than carbon black as the carbon source was a more effective technique to prepare carbides. MojC and WC nanolayers were prepared with this method. Although none of the MAFBS-prepared catalysts were competitive with the commercial Cu-Zn-Al catalyst for the WGS reaction, M02C had the most promising catalytic properties of the compounds that were prepared. 

Incorporating two new pieces of equipment will move this research program towards achieving controlled synthesis of catalytically active carbide and nitride nanolayers on early transition metals. Whereas the current microwave is multimode, like all domestic microwave ovens, a single axial-mode cylindrical cavity microwave oven will be acquired. This apparatus provides uniform continuously variable microwave heating that will eliminate the hot and cold spots that likely exist in the current oven. 

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