Elaboration and characterization of nickel and/or ruthenium based catalysts for CO2 valorization through the methanation or the dry reforming of methane reactions

Abstract

The aim of this work is to investigate the valorization of carbon dioxide through the CO2 methanation and the CO2 reforming of methane reactions. Alumina, ceria, and mesoporous silica were chosen as the catalytic support materials. The wet impregnation technique was used in order to add an active phase of 15 wt% nickel (Ni), 1 wt% ruthenium (Ru) or a combination of the two metals (Ni-Ru) on each of the stabilized supports. The obtained catalysts were calcined at 550 oC, and then characterized using X-Ray Diffraction (XRD), Nitrogen adsorption/desorption, H2-Temperature Programmed Reduction (H2-TPR), and CO2-Temperature Programmed Desorption (CO2-TPD). Characterization results of Ni and/or Ru catalysts supported on CeO2, Al2O3, and KIT-6 showed that all supports and catalysts present a type IVa isotherms typical to mesoporous materials and that the formed RuO2 species were well-dispersed when CeO2 is used as a support and in the bi-metallic catalysts. The reduction of NiO was enhanced in the presence of CeO2 support given its redox properties. The presence of RuO2 also led to a facilitated NiO reduction in the bi-metallic catalysts. In the CO2 methanation reaction, a first part compared the catalytic activity of the different active phases on the same support. In each case, the bi-metallic Ni-Ru catalyst exhibited the highest conversions at 350 oC which was linked to a possible good RuO2 dispersion and NiO reducibility observed in these catalysts. Moreover, regardless of the active phase used, the catalytic activity of the catalysts depended on the type of the support. The order of reactivity obtained was: CeO2 supported catalysts > KIT-6 supported catalysts > Al2O3 supported catalysts. In the second part, KIT-6 was promoted with different CeO2 percentages and impregnated with the same studied active phases in an attempt to create more economical and stable catalysts. A good dispersion of RuO2 species, an ameliorated active phase reducibility at lower temperatures, and an enhancement in the basic properties were observed following the promotion with Ce and as the percentage of Ce in the catalyst increased. This ultimately led to higher catalytic performances especially for the bi-metallic 15Ni1Ru/CexKIT-6 catalysts. The stability study showed that the 15Ni1Ru/CeO2 catalyst exhibited good catalytic activity in terms of CO2 conversion (70 %) and CH4 selectivity (99 %) and showed no deactivation for 24 h on stream. In the CO2 reforming of methane, CeO2 supported catalysts showed lower catalytic activity when compared to Al2O3 supported catalysts. However, they showed more resistance to carbon formation as proven by the thermal analyses and the XRD performed on the spent catalysts. The effect of adding Ce to Al2O3 on the physico-chemical properties and the catalytic performances was then investigated. The addition of Ce was found to cause a partial destruction of the porous structure, strengthen the weak basic sites of the catalysts and enhance the catalytic activity. The 15Ni/Ce-Al2O3 catalyst showed higher activity and stability during a 12 hour stability test compared to the non-promoted counter-part. The three different mesoporous silicas (15Ni/KIT-6, 15Ni/SBA-15 and 15Ni/SBA-16) were then synthesized using the wet impregnation technique, characterized and compared for their activity and stability in the CO2 reforming reaction. In the dynamic tests, 15Ni/SBA-16 was the best performing catalyst due to its strong metal-support interactions and high contribution from strong basic sites. However, the 15Ni/KIT-6 catalyst was able to maintain a good activity and stability even at higher gas hourly space velocities GHSVs. Finally, among the 15Ni/Ce-KIT-6, 1Ru/Ce-KIT-6, and 15Ni1Ru/Ce-KIT-6 catalysts, the latter was the most active in the DRM reaction and maintained a stable high CO2 conversion (97 % for 12 hours). Despite the deposition of graphitic carbon during the stability test, the catalyst was not deactivated.