Optimisation and characterisation of nickel based nano-catalysts for the dry reforming of methane with carbon dioxide

摘要

Two of the main components of biogases, methane and carbon dioxide, are regularly flared or released into the environment due to the compositions of these gases being unsuitable for further processing using currently available technologies. One process that can utilise the two aforementioned gases is catalytic dry reforming, which involves the conversion of these two gases into syngas (carbon monoxide and hydrogen). This process however is not commercially viable due to the lack of discovery of a suitable catalyst. The aim of this research project was to develop a nickel supported material which showed significantly increased catalytic ability compared with conventionally prepared materials within the dry reforming reaction, thus increasing the economic viability of the reaction in the hope it will one day be used in industry. The first two sections of the study concentrate on improving monometallic Ni catalysts by considering the effect of improved supports over two distinct classes of carrier. Firstly the preparation, characterisation, activity and stability of Ni-incorporated mesoporous alumina (MAl) materials with different Ni loading (7, 10 and 15 wt %) are discussed. The most active catalyst tested (Ni(10wt%)/MAl) showed a very high stability over 200 h compared to a Ni(10wt%)/γ-Al 2 O 3 prepared using a wet-impregnation method which had a significant loss in activity after only ~4 hours of testing. The high stability of the Ni/MAl materials prepared was due to the Ni nanoparticles in these catalysts being highly stable towards migration / sintering under the reaction conditions used (700 o C, 52000 mL.h -1 g -1  GHSV). The low susceptibility of the Ni nanoparticles in these catalysts to migration / sintering was most likely due to a strong Ni-support interaction and / or active metal particles being confined to the mesoporous channels of the support. Secondly, the preparation, characterisation, activity and stability of Ni-incorporated silica sol-gel (SiO 2 SG) materials with various Ni loading (10, 20 and 30 wt %) are discussed. It was found that all Ni/SiO 2 SG materials showed a comparable initial activity and increased stability over 15 h reaction period (700 o C, 52000 mL.h -1 g -1 GHSV) compared to a Ni(10 wt %)/SiO 2 prepared using a wet-impregnation method, which lost ability to catalyse the reaction after a very short testing time. The high activity and stability of the Ni/SiO 2 SG materials were attributed to the decreased Ni particle size with increased distribution, and the increased resistance to sintering of the Ni nanoparticles. The final section focuses on further optimisation of the materials reported in the previous sections by considering the positive doping effect of the lanthanide ytterbium (Yb) on the reported catalysts. The preparation, characterisation, activity and stability of Ni(10 wt %)/MAl and Ni(10 wt%)/SiO 2 SG materials with different Yb doping concentrations (1 – 4 wt %) are discussed. The materials showed a further increased conversion of CO 2 and CH 4 over 15 h reaction time over undoped materials. The further increased activity and stability of the Yb doped Ni/MAl materials were thought to be due to the decreased Ni particle size and increased Ni-support interaction observed in the characterisation of these materials.