Catalysts and adsorbents for CO2 capture and conversion with dual function materials: Limitations of Ni-containing DFMs for flue gas applications

Abstract

Dual Function Materials (DFM) capture CO2 from flue gas followed by catalytic conversion to methane all at 320 °C using renewable H2. DFM is composed of a catalytic metal intimately in contact with alkaline metal oxides supported on high surface area carriers. The catalyst is required to methanate the adsorbed CO2 after the capture step is carried out in an O2-and steam-containing flue gas. Ruthenium, Rhodium and Nickel are known CO2 methanation catalysts provided they are in the reduced state. Ni is a preferred methanation catalyst based on price and activity, however, its inability to be reduced to its active state during the DFM process (capture and hydrogenation at 320 °C) was compared with Ru and Rh as methanation candidates. The performance of a variety of alkaline adsorbents was also studied and the strengths and weaknesses of candidate catalysts and adsorbents were evaluated. All samples were tested in a fixed bed reactor to quantify the extent and rate of methane generation.Complementing fixed bed testing, thermogravimetric analysis (TGA) was used to evaluate the extent of CO2 adsorption and rate of catalytic methanation. Pre-reduced (at 650 °C) Ni-containing DFM is highly active for CO2 methanation. However, the hydrogenation with 15% H2/N2 is completely inactive after exposure to O2 and steam, in a flue gas simulation, during the CO2 capture step at 320 °C. Rh and Ru DFMs were effective methanation catalysts with Ru being superior based on capture capacity, hydrogenation rate and price. In contrast to Ni – containing DFM, Ru remained active towards methanation even after exposure to flue gas simulation. Alkaline adsorbents (“Na2O”, CaO, “K2O” and MgO) in combination with reduced Ru were tested for adsorption and methanation. Ru – “Na2O”/Al2O3 DFMs showed the highest rates for methanation although CaO is also a reasonable candidate. To date, we have demonstrated that γ-Al2O3 is the most suitable carrier for DFM application relative to other materials studied.