Add time:07/22/2019         Source:sciencedirect.com

It was shown that hexamethylenetetramine (HMT) is a new effective fuel for single-step solutions combustion synthesis (SCS) of supported Ni catalysts for methane decomposition into hydrogen and nanofibrous carbon. Several generalized chemical equations reflecting different ideas about combustion of the HMT−Ni(NO3)2−Cu(NO3)2−Al(NO3)3−H2O system have been derived. On the basis of those equations the adiabatic combustion temperature (Tad) and the amount of gaseous products (ng) have been calculated depending on the ignition temperature (T1), water content (m), excess fuel coefficient (φ), and the composition of the obtained solid product. The calculations have shown that Tad, depending on m and φ, changes from hundreds to thousands of degrees Kelvin. Increase of Al2O3 content in the catalyst up to 0.6 increases Tad by hundreds of degrees, and that increase of the Ni:NiO ratio up to 0.5 lowers Tad by tens of degrees. Three samples of the supported unreduced 0.97NiO/0.03Al2O3 catalyst were successfully prepared with the help of the SCS method using HMT as the fuel at φ=0.7. Those samples, obtained at reaction mixture preliminary heating rates V = 1, 10, 15 K/min were characterised using XRD, TEM, and SEM, and further tested in a pure methane decomposition reaction (100 LCH4/h/gcat, 823К, 1 bar). Nanoparticles of metal Ni were found in the SCS products, in contrast to cases when other types of fuel were used with φ<1. The experimental results showed that the higher is V, the higher is the maximum SCS temperature, the larger is the average size of Ni nanoparticles in unreduced catalyst, the higher is the stability of unreduced catalyst (up to 14 h) and the higher is the specific yield of hydrogen (up to 818 mol/molcat) during the deactivation period. The activity value of our unreduced catalyst (0.7 mol/h/gcat) in the methane decomposition reaction is close to maximum activity values of pre-reduced Ni catalysts of different nature reported in published articles.

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