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Abstract
Approaches to enhance the performance of nickel catalysts for the carbon dioxide, or dry, reforming of methane were systematically investigated.
The implementation of a reduction-oxidation-reduction (ROR) activation strategy on Ni-SiO2 was examined for the first time. ROR pre-treatment decreased the average nickel deposit size and altered the interaction between the nickel and silica, which ultimately increased conversion. The interaction between carbon monoxide and the Ni sites was altered by varying the deposits through ROR pre-treatment which significantly enhanced selectivity.
The ROR studies elucidated the influence of nickel deposit properties on catalyst selectivity and conversion. However, the study did not examine the impact of support properties. Flame spray pyrolysis (FSP) was subsequently investigated as a single-step method of producing novel supports whilst directly influencing nickel properties.
Silica particles were prepared by FSP at varying precursor feed rates which successfully tuned the surface area, altered the surface silanol groups and affected residual carbon-based surface species. Notably, at the highest feed rate residual carbon species enhanced nickel dispersion and consequently activity and selectivity.
To supress carbon formation on the silica supports, ceria-zirconia was included. FSP was used to design novel silica/ceria-zirconia structures. The synergy invoked by the silica/ceria-zirconia was evident when comparing to the separate oxides. The improved properties, including oxygen storage capacity and thermal stability, allowed the unique structures to outperform the individual supports in terms of both activity and selectivity.
Despite the enhancements in performance facilitated by the silica/ceria-zirconia supports, the structures produced were not optimum in terms of accessing the advantages of the separate oxides. A novel, asymmetrically variable, double-FSP (DFSP) system was developed to produce silica/ceria-zirconia with targeted structures. The oxygen storage capacity and surface chemistry of the structures were tuned by varying the intersection distance between the flames. The DFSP materials exhibited enhanced selectivity compared to the physically-mixed and single-FSP prepared supports.
Overall, this study demonstrated the impact of varying Ni size and structure on the activity and selectivity for dry reforming. Novel methods of pre-treatment and support synthesis proved the viability of economical support combinations and synthesis methods whilst providing new insights into single and DFSP systems.