Long-term stability test of Ni-based catalyst in carbon dioxide reforming of methane

Abstract

A La-NiMgAlO catalyst, obtained after calcination of a hydrotalcite precursor, was evaluated in dry reforming of methane at 650 and 700°C and compared with a previous test performed at 750°C. At 700°C the catalyst showed no sign of deactivation during 200h, while it deactivated slowly afterwards. However, at 650°C conversion decay was detected from the beginning of the test. In both tests, CH4 and CO2 conversion were higher than thermodynamic equilibrium estimation which suggests the participation of other reactions such as methane decomposition or steam reforming of methane. The occurrence of reverse water-gas-shift reaction (RWGS) was responsible for the H2/CO ratios below unity and for the higher CO2 conversion compared with CH4.Used catalysts were characterized by several techniques (TEM, 27Al MAS NMR, XRD, TPO and Raman spectroscopy) in order to study catalyst structure and to establish whether carbon was deposited and its nature. Ni particle diameter increased when reaction temperature decreased but no differences in 27Al MAS NMR or XRD results were observed. A higher coke deposition rate was detected when the temperature was increased. At 700 and 650°C carbon species were mainly graphite ribbons, coating carbon and graphite nanoencapsulates, while at 750°C there were multi-walled carbon nanotubes (MWCNT), fibres and graphite ribbons. The deactivation of the catalysts tested at 650 and 700°C can be related to: (i) the presence of coating carbon, graphite nanoencapsulates and Ni particles embedded inside the carbon nanotubes (CNT); (ii) partial sintering and (iii) a lower hydrogen production what makes that carbon transportation away the surface was less favoured.