Catalytic Pyrolysis of Methane to Hydrogen over Carbon (from cellulose biochar) Encapsulated Iron Nanoparticles

Abstract

The catalytic performance of carbon encapsulated iron nanoparticles (CEINPs) for methane decomposition was investigated over a range of temperatures between 700 and 800 °C with varying iron concentrations (20, 30, and 40 wt %) in a semicontinuous flow fixed bed reactor. CEINPs were prepared by impregnating a cellulose biochar support (carbon source) with iron nitrate solution. This mixture was dried at 110 °C for 72 h in air followed by pyrolysis at 1000 °C in a N2 atmosphere. Unlike conventional iron-based catalysts, in CEINPs, the iron nanoparticles were encapsulated in a graphitic shell that prevented atmospheric oxidation and sintering at high temperature, improving the thermal stability of the catalysts. The fresh CEINPs did not contain any metal oxides and had only the following phases: elemental iron, iron carbides, activated carbon, and graphitic carbon, which allowed for the catalyst reduction step (in H2) to be eliminated. The decomposition of methane being an endothermic reaction showed better catalytic activity with an increase in temperature with the highest conversion of 95.7% observed at 800 °C. There are four different active sites in the catalyst, namely, graphite, graphite encapsulated iron nanoparticles, uncovered iron nanoparticles, and activated carbon. In the case of 20 wt % CEINP, the presence of higher amounts of activated carbon and graphite with a lower amount of Fe negatively impacted methane decomposition, confirming the importance of Fe as an active site. At 800 °C, the maximum catalytic performance was exhibited by 30 wt % CEINP where the initial methane conversion was 95.7% and dropped to 36.8% after 180 min of reaction. The higher conversion was due to the combined optimum amounts of Fe, graphite, and activated carbon active sites. In the case of 40 wt % CEINP, the higher amount of Fe resulted in the agglomeration of Fe nanoparticles and reduction in surface area, leading to lower methane conversions. The surface area of the spent catalysts reduced appreciably in all cases due to carbon deposition from CH4 in the form of coke and graphite (on non-encapsulated Fe nanoparticles), resulting in catalyst deactivation.