Catalytic methane decomposition is acquiring a great interest for the hydrogen production as an alternative process to steam reforming. Methane decomposition can be considered as a CO2-free method, leading to the production of high purity hydrogen that could be directly applied in fuel cells [1]. Among the wide range of catalysts investigated in this reaction, carbon catalysts present the advantages of combining significant activity with a lower cost compared to metal catalysts. It has been found that the catalytic activity and sustainability are favored in those carbons exhibiting low crystallinity and high external surface area, respectively [1]. Recently, we have demonstrated the remarkable catalytic behavior of ordered mesoporous carbons, belonging to the CMK-family, for hydrogen production via methane decomposition, when compared with the most active commercial carbons tested until now [2].

In this communication, the results obtained using two different ordered mesoporous carbons, CMK-3 (single mesoporosity) and CMK-5 (bimodal mesoporosity), are presented and compared with those of commercial carbon blacks. CMK-type carbons were synthesized via replication of a hard template, according to literature [3]. Methane decomposition reactions were carried out using a thermobalance, operating with a mixture of 10% CH4 in Ar as reactive gas. Hydrogen production was determined from the weight increase of the catalyst during the reaction due to the carbon deposition. A variety of characterization techniques were utilized for a better comprehension of the relationship between the catalytic results and the carbon properties.

In both temperature programmed and isothermal experiments, the CMK-5 carbon exhibited the highest activity. This result is attributed to two contributions: a) higher proportion of surface defects which act as active sites of the reaction, and b) better textural properties consisting in a BET surface area close to 2000 m2 g-1, together with a double system of pores with sizes of 3 and 5.8 nm, respectively, which favors a rapid access of the methane molecules to the active sites. In order to evaluate the stability of the carbon catalysts, long term reactions were performed under isothermal conditions. Thus, while a commercial carbon black sample was almost completely deactivated after 24 hours of reaction, a continuous and significant hydrogen production was still observed after 72 hours for CMK-5 catalyst. It is important to note that after this time more than 25 g of carbon (co-product of methane decomposition) per gram of raw catalyst were deposited. Although the reaction mechanism is not yet completely understood, the existence of an important fraction of surface active sites that remain accessible to the methane molecules is assumed to justify this result

References

[1] Muradov et al., Catal. Today, 102-103 (2005) 225.

[2] Serrano et al., Chem. Commun., (2008) 6585.

[3] Ryoo et al., Stud. Surf. Sci. Catal., 135 (2001), 50.