CH4-H2 mixtures production through thermochemical reaction between CO2 and MgH2 doped with Ni and CNTs

AMICA, GUILLERMINA; F. C GENNARI

Buenos Aires

Congreso; 11th Word Congress of Chemical Engineering (WCCE11); 2023

Asociación Argentina de Ingenieros Químicos

The CO2 transformation through catalytic processes to products with added value is attractive from an environmental and economic point of view. Hydrides-based systems that store green H2 safely can react with CO2 under certain conditions to generate significant amounts of methane or CH4-H2 mixtures. In this work the performance of nickel-catalyzed MgH2 with the incorporation of carbon nanotubes was analyzed, seeking to achieve high CO2 conversion rates under moderate conditions, with reduced operating temperature and reaction time. Ball milling in a planetary mill was employed to prepare four samples: (i) MgH2 and (ii) MgH2-10wt.% Ni, (iii) MgH2-5wt.%Ni-5wt.%CNT and (iv) MgH2-5wt.%CNT. The reaction between the samples and CO2 was studied under static conditions performed in a stainless steel reactor coupled to a Sieverts volumetric equipment that allows the selection of the pressure and temperature. A specific mass of each sample was heated up to different temperatures (300, 350, 375 and 400 oC) under CO2 pressure during different reaction times (6, 12, 24 and 48 h), considering a molar ratio MgH2:CO2 of 4:1. The composition of the gases after thermal treatment under CO2 was studied through gas-phase FTIR analyses and GC, whereas the solid phase was characterized by solid-state FTIR and XRPD. The methane yield was calculated as the ratio between the produced CH4 moles and the total CO2 moles at the beginning of the reaction. When metallic Ni is available, the methanation process takes place through the Sabatier reaction with Ni-containing species as active catalysts (Ni0, Mg2NiH4 and Mg2NiH0.3), capable of dissociating the H2 and the CO2 adhered to the surface of the hydride, without formation of higher hydrocarbons. The contribution of direct reduction of CO2 to produce carbonaceous deposits in the solid phase and the generation of CH4 via C as an intermediary can be considered as a simultaneous and complementary process. The synergistic effect of the nickel and carbon nanotubes was demonstrated. By reducing the metallic charge from 10wt.%Ni, replacing 5wt.% with CNTs, the amount of produced methane through thermal treatment at ~350-375 oC under CO2 increased more than 30%. A COx-free mixture of methane (70.9%) and hydrogen (29.1%) with a CH4 yield of 79% was obtained by reaction at 350 oC during 48 h. Ni-containing species act as selective catalyst promoting the occurrence of the Sabatier reaction, which competes with the direct reduction of CO2 to generate CH4 via C as an intermediary. The role of CNTs is not catalytic but rather it could be related to the protection of the hydride. It is proposed that when MgH2 is neighbored by graphitic planes, its oxidation due to direct contact with CO2 and/or the generated H2O could be impeded or retarded. Then, this investigation provides a thermochemical method for CO2 recycling to synthesize COx-free mixture of CH4 and H2 employing a doped- metal hydride.