Utilização de Ni ancorado em argila bentonítica na reação de decomposição do metano para produção de carbono ordenado e “hidrogênio azul”

Abstract

The need to find ways to mitigate the degrading effects on the environment caused by the use of fossil fuels has motivated searches for energy generation alternatives that are clean and efficient. The catalytic decomposition of methane (CDM) using clay as catalytic support becomes shows promising for the production of hydrogen with low environmental impact, free of CO2, generating solid carbon as a by-product of high added value. Clays, due to their characteristics, can be used as effective catalytic support in CDM. The present work describes the synthesis and characterization of Ni-based catalysts anchored in bentonite clay, in natura and thermally modified, and also, for comparison purposes, anchored in alumina, for the production of ordered carbon and hydrogen via CDM. The catalysts were synthesized via wet impregnation, with 5, 10 and 20% Ni (m/m), and a single catalyst with 20% Ni and in natura bentonite clay, BSI, was prepared via the electrospinning technique. All catalysts were characterized by energy dispersion X-ray fluorescence spectrometry (EDX), X-ray diffraction (XRD), thermogravimetric analysis (TGA), N2 adsorption and desorption at 77 K, electron microscopy and programmed temperature reduction. (TPR) and tested in the DCM reaction at 500 °C varying the reaction time (30, 180 and 240 min) and methane concentration (CH4/N2 = 1/6 and 1/1 (v/v)). The catalytic performance was studied from the methane conversion, catalyst stability, yield in carbonaceous materials and hydrogen production. The carbon formed was characterized by TGA, Raman spectroscopy, transmission electron microscopy (TEM) and scanning (SEM). The results indicated that the anchoring of 10 and 20 % in BSI followed by calcination promoted structural changes, total delamination, of the clay layers. While the NiO anchoring in the thermally modified clay, BSI500, kept its delaminated structure. The final location of NiO particles on the surface of the bentonite clay was driven by the type of delamination, partial or total, that the clay layers underwent, influencing the strength of metalsupport interaction (TPR). The synthesis of the sample via electrospinning technique, 20NiBSI-EC, facilitated a strong interaction between NiO-BSI, preventing the agglomeration of NiO into larger particles. The textural changes and the mesoporosity of the materials were confirmed by adsorption and desorption of N2 at 77 K. All synthesized catalysts showed catalytic activity forming ordered carbon and hydrogen. For the catalysts tested in DCM for 180 min, CH4 /N2 = 1/6 (v/v), the sample with 10% Ni anchored in BSI500, 10Ni-BSI500, showed the highest methane conversion, with 62%, followed by of the alumina-anchored catalyst 20Ni-Al2O3 (54%). In terms of yields of carbonaceous materials and hydrogen, under these reaction conditions, 10Ni-BSI (32%) and 20Ni-Al2O3 (30%) showed similar yields, but when tested for 30 min, 20Ni-Al2O3 showed 95% yield, and 20Ni-BSI and 10Ni-BSI500 showed 74 and 70%, respectively. With a feed of CH4 /N2 = 1/1 (v/v) during 240 min of reaction, 20Ni-BSI showed 99% yield in carbonaceous materials and also in hydrogen. In quantitative terms of the mass of H2 generated in the reactions with feed CH4 /N2 = 1/6 (v/v) the following decreasing sequence of production was observed: 20Ni-Al2O3, 9 mg; 20Ni-BSI, 7mg; 10Ni-BSI500, 6,5 mg in 30 min reactions, and 10Ni-BSI, 20Ni-BSI, ~18 mg; 10Ni-BSI500, 20Ni-Al2O3, ~ 17,5mg in the 180 min reactions. In the reaction in 240 min with flow CH4 /N2 = 1/1 (v/v) the 20Ni-BSI produced 767 mg of free H2. It was highlighted that the metal-support interaction allowed the formation of carbon by growth mechanisms at the base and at the tip, whose carbons presented ordering in the form of carbon nanotubes with defective multiple walls.