Mesoporous zirconia-modified clays supported nickel catalysts for CO and CO2 methanation

Abstract

The Ni catalysts supported on a new structure with zirconia nanoparticles highly dispersed on the partly damaged clay layers has been prepared by the incipient wetness impregnation method and the new structure of the support has been prepared in one pot by the hydrothermal treatment of the mixture of the clay suspension and the ZrO(NO3)2 solution. The catalytic performances for the CO and CO2 methanation on the catalysts have been investigated at a temperature range from 300 °C to 500 °C at atmospheric pressure. The catalysts and supports have been characterized by X-ray diffraction (XRD), transmittance electron microscopy (TEM), H2 temperature-programmed reduction (H2-TPR), nitrogen adsorption–desorption, and thermogravimetry and differential thermal analysis (TG-DTA). It is found that the zirconia-modified clays have the typical bimodal pore size distribution. Most of the pores with the sizes smaller than 10 nm are resulted from the zirconia pillared clays and the mesopores with the sizes larger than 10 nm and the macropores with the sizes larger than 50 nm are resulted from the partly damaged clay layers. The bimodal pore structure is beneficial to the dispersion of Ni on the layers of the zirconia-modified clays and the increase in Ni loading. The zirconia nanoparticles are highly dispersed on the partly damaged clay layers. Nickel oxide in cubic phase is the only Ni species that can be detected by XRD. The nickel oxide nanoparticles with the sizes of 12 nanometers or more are well dispersed on the zirconia-modified clay layers, which are observed to be buried in the stack layers of zirconia. The presence of nickel oxide in six different forms could be perceived on the new structure. Five of them except the Ni species that forms the spinel phase with Al in clays can be reduced to the active Ni species for the CO and CO2 methanation. But the activity of the Ni species is different, which is associated with the chemical environment at which the Ni species is located. The catalyst with the higher zirconia content, which also has the larger specific surface area and pore volume, exhibits the better catalytic performance for the CO or CO2 methanation. Zirconia in the catalyst is responsible for the dispersion of the Ni species, and it prevents the metallic Ni nanoparticles from sintering during the process of the reaction. In addition, it is also responsible for the reduction of the inactive carbon deposition. The catalyst with 15 wt.% zirconia content has the highest CO conversion of about 100% and the highest methane selectivity of about 93% at 450 °C for CO methanation, and the catalyst with 20% zirconia content has the CO2 conversion of about 80% and the highest methane selectivity of about 99% for CO2 methanation at 350 °C. The catalyst with 15 wt.% zirconia possesses promising stability and no distinct deactivation could be perceived after reaction for 40 h. This new catalyst has great potential to be used in the conversion of the blast furnace gas (BFG) and the coke oven gas (COG) to methane.