This review paper is devoted to an extended analysis of ammonia gas sensors based on carbon nanomaterials. It provides a detailed comparison of various types of active materials used for the detection of ammonia, e.g., carbon nanotubes, carbon nanofibers, graphene, graphene oxide, and related materials. Different parameters that can affect the performance of chemiresistive gas sensors are discussed. The paper also gives a comparison of the sensing characteristics (response, response time, recovery time, operating temperature) of gas sensors based on carbon nanomaterials. The results of our tests on ammonia gas sensors using various techniques are analyzed. The problems related to the recovery of sensors using various approaches are also considered. Finally, the impact of relative humidity on the sensing behavior of carbon nanomaterials of various different natures was estimated.

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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 − ( 3 ) 2 − ( 3 ) 2 − ( 3 ) 3 − 2 system have been derived. On the basis of those equations the adiabatic combustion temperature ( ) and the amount of gaseous products ( ) have been calculated depending on the ignition temperature ( 1 ), water content ( ), excess fuel coefficient ( ), and the composition of the obtained solid product. The calculations have shown that , depending on and , changes from hundreds to thousands of degrees Kelvin. Increase of 2 3 content in the catalyst up to 0.6 increases by hundreds of degrees, and that increase of the Ni:NiO ratio up to 0.5 lowers 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 = 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 < 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.

Highly porous expanded graphite was synthesized by the programmable heating technique using heating with a constant rate (20 °C/min) from room temperature to 400–700 °C. The samples obtained were analyzed by scanning electron microscopy, energy-dispersive spectroscopy, low-temperature nitrogen adsorption, X-ray photoelectron spectroscopy, Raman spectroscopy, thermogravimetry, and differential scanning calorimetry. A comparison between programmable heating and thermal shock as methods of producing expanded graphite showed efficiency of the first one at a temperature 400 °C, and the surface area reached 699 and 184 m2/g, respectively. The proposed technique made it possible to obtain a relatively higher yield of expanded graphite (78–90%) from intercalated graphite. The experiments showed the advantages of programmable heating in terms of its flexibility and the possibility to manage the textural properties, yield, disorder degree, and bulk density of expanded graphite.

Summary The catalyst for decomposition of methane to carbon nanofibers and hydrogen was prepared using the solution combustion technique. Dynamics of change of temperature of solution (gel) during the passing of redox reaction of synthesis of catalyst was found experimentally. Testing of catalyst was carried out in a flow through installation. Catalyst with high content of active component 90Ni‐10Al2O3 (wt%) was tested in a reaction of methane decomposition in a temperature range 535°С to 675°С and pressures 1 to 5 atm. The samples of carbon nanofibers were investigated by scanning electron microscopy, energy‐dispersive X‐ray spectroscopy, and X‐ray diffraction. The optimal parameters of catalytic reaction (550°С, 3 atm) providing the high specific yield of hydrogen (287.7 mol/g) were established. The increase of pressure above 1 atm led to prolonged operation of the catalyst. The negative role of temperature rise on the yield of CNFs was found.