| dc.description.abstract | The effect of lanthanum (La) on the catalytic behavior of nickel and cobalt catalysts de- rived from hydrotalcites in total combustion of methane was investigated. 12 wt. % Co (12Co), 12 wt. % Ni (12Ni) and 3.6 wt. % 8.4 wt. % Co (3.6Ni-8.4Co) were prepared by co-precipitation method followed by calcination and promoted further by 0.5 2 wt. % lanthanum using incipient wetness impregnation method. The final hydrotalcite-derived catalysts were characterized by x-ray diffraction (XRD), x-ray fluorescence (XRF), induc- tively coupled plasma-mass spectrometer (ICP-MS), temperature-programmed reduction (TPR), N2 adsorption and hydrogen chemisorption techniques. The XRD profiles con-firmed the hydrotalcite structure of dried samples. The nitrogen adsorption demonstrated high surface area, 136 172 m2/g and small pore diameter of 10.4 13.2 nm. Hydrogen chemisorption presented slightly varied dispersion and metallic surface area.
The addition of La promoter to the catalysts showed slightly decreased dispersion, as well as BET surface area, except from 3.6Ni-8.4Co catalyst which its BET surface area was observed to increase with 0.6 5 %. On the other hand, the La promoter increased the interaction between the cobalt oxides and the hydrotalcite (HT) support, while for the nickel oxides, the opposite was observed.
The methane combustion over the nonpromoted and promoted catalysts was performed in a fixed-bed quartz reactor followed by gas chromatography (GC), mass spectrometery (MS) and ultraviolet-visible spectrometry (UV-Vis). The UV-Vis measurements were per- formed to study the oxidation states of the hydrotalcite-derived catalysts. This technique enabled to observe that oxidation state increased with temperature, where the oxidation state fraction was observed to decrease by La promotion in 12Ni catalyst.
The catalysts were mainly analyzed for methane conversion. Methane conversion showed to increase with the cobalt-contained catalysts and the temperature, where 3.6Ni- 8.4Co catalyst was found to exhibit the highest activity on methane combustion among the catalysts. However, all the catalysts displayed decreasing methane conversion over time due to deactivation assigned to carbon formation on catalysts, as the TGA results showed. No significant changes were observed in the methane conversion with the addition of lan- thanum, except from 12Ni-2La which showed lower methane conversion. Indeed, La promoter had two main effects on the catalysts. It blocked/covered parts of the nickel/cobalt active sites, at the same time it increased the remaining active sites. It also decreased the oxidation state fraction of the catalysts.
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