SYNTHESIS AND CHARACTERIZATION OF HETEROGENEOUS CATALYSTS FOR CO2 VALORIZATION

Abstract

Conversion of environmental pollutant CO2 emitted by anthropogenic activities into valuable products offers a feasible way to reduce the CO2 accumulation in the atmosphere and balances the carbon cycle. CO2 conversion with renewable H2 to storable chemical fuel CH4, so called power to gas technology (PtG), has received increasing attention due to its many advantages. Dry reforming of methane (DRM) with CO2 offers valuable environmental benefits such as biogas utilization, removal of GHG and production of syn-gas with ratio of 1 (H2/CO2=1) which is suitable for the synthesis of oxygenated chemicals such as methanol, dimethyl ether (DME) and hydrocarbons by Fischer-Tropsch synthesis. CO2 methanation over Ni catalyst is a well-known process since 20th century, but many aspects concerning the catalysts are still under debate, such as role of particle size, role of support, role of metal concentration and preparation method etc. A part of this thesis is devoted to the development of suitable Ni based catalyst with high performance and stability for CO2 methanation and to understand the role of Ni0 metal concentration and of Ni0 particle size on the catalytic activity. Ni/YSZ catalysts were prepared by electroless plating method with different metal concentration and the impact of Ni0 metal concentration on CO2 methanation was studied. The CO2 conversion was directly dependent on the metal loading and the highest CO2 conversion and CH4 selectivity were achieved for the catalyst with largest Ni (12 wt%) loading. To investigate the role of Ni0 particle size, Ni(10 wt%)/YSZ catalysts were prepared by different methods, such as wet impregnation, wet impregnation with [NiEDTA]2- , electroless plating and mechanical mixing, in order to obtain different Ni0 particle size. The as prepared and used catalysts were characterized by complementary synchrotron (in-situ XAS) and laboratory based techniques (XRD, TPR, XPS, BET, HR-TEM). Among them, the catalysts prepared by wet impregnation with [NiEDTA]2- complex showed high activity (60 % CO2 conversion and 98 % CH4 selectivity at 350 °C) and stable performance. The Ni0 nanoparticle (19 nm) remained stable even after the 30 h time on stream (TOS) experiment without sintering and without forming any carbon filaments. The kinetic measurements confirmed that both reactants CO2 and H2 were activated on the Ni0 sites. To develop highly stable Ni based catalyst for DRM reaction, Co was added to Ni-CeO2 catalyst to suppress the carbon deposition and the role of Co was investigated. Five Ni-Co/CeO2 catalysts with Co molar concentration varying from 0 to 100 % were prepared by auto combustion method. The Ni-Co alloy formation and the reducibility of the catalyst were confirmed by H2-TPR, in-situ XRD and in-situ XAS. All catalysts showed remarkable stability during 30 h TOS experiment. HR-TEM and TGA analysis of spent catalysts showed very low carbon formation (<1 %) for Co rich catalyst and high carbon formation (17 %) for Ni rich catalyst. The kinetic measurements showed very low conversion at high GHSV (ml g-1 s -1 ) for Co rich catalyst. This clearly indicates that the Co was oxidized when CO2 stream was high. The partial pressure analysis further proved that O adatoms are adsorbed on Co and that the Co2+/0 redox cycle removes the intermediate carbonaceous species in subsequent step. The role of CeO2 nanostructures support in Ni-CeO2 and Ru-CeO2 catalysts on CO2 methanation and DRM reactions is presently under investigation and the preliminary results are highlighted in the future perspectives section.