Catalytic production of methane from CO 2 and H 2 at low temperature: Insight on the reaction mechanism A review by Shujin Jiang 3/17/2015

Introduction Increasing CO 2 emission is a significant problem. CO 2 is a stable molecule. The transformation of CO 2 is easier if CO 2 is used as a co-reactant with other substances, such as H 2 CO 2 can react with H 2 to produce CH4 in the presence of catalyst (Rh/γ-Al 2 O 3 ) at low temperature and atmospheric pressure Insight on the reaction mechanism is necessary to optimize the process and improve the performances.

Experiments Rh/-Al2O3 catalyst Catalyst test X-ray photoelectron spectrometer (XPS) In situ DRIFTS – CO2 absorption, dissociation and the reaction with H2 Micromeritics ADAP2010Chemi apparatus– CO2 and hydrogen chemisorption

Catalyst Test 1.Reduction of catalyst under H2 at 623K for 1h and cooling down to 293K 2.CO2 flushing 3.H2 injection 4.Several cycles of steps 2 and 3

Catalyst Test Results Fig. 1 Catalytic test on the Rh-γAl 2 O 3 at 298 K. The masses were followed in continuous with a MS. Y -axis represents the intensity (arbitrary units) and the X -axis the time. Mass 15- CH4 Without catalyst Cycle 1Cycle 2 Cycle 3 CH4 was effectively produced with catalyst A deactivation was observed in the 3rd cycle Water was not observed. The only product of the reaction is methane Rh/-Al2O3

Catalyst Test Results The production of CH 4 was higher when very small amounts of CO 2 were flushed (oxidation Rh by high CO2, catalyst was deactivated) The conversion increases weakly with the temperature No CH4 was produced with unreduced catalyst

XPS Results XPS result shows that Rh Binding energy (BE) decrease after reduction -Rh is reduced Rh BE peak after CO2 introduction and reaction -Rh is oxidized These indicates that CO2 oxidizes the catalyst and this oxidation deactivates the catalyst

DRIFT Results Interaction between CO and Rh 2048 Linear Rh-CO 2123 Oxidized Rh atom(Rh3+) and CO 2024 and 2092 gem-dicarbonyl Rh-(CO)2 All four bands demonstrates the dissociation of CO2 on the catalyst surface CO bands at 2048, 2024 and 2092 are still present indicating that CO2 is really dissociated into CO H2 bands at 2123,2092, and 2024 decrease or disappear, showing that H2 reacts with the dissociated CO2 species CO2 CO Reaction H2

CO2 Chemisorption Results Compared to the fresh catalyst, the reduced catalyst has significantly higher CO2 chemisorption. -Why is CH4 produced only with reduced catalyst Conclusion : - The adsorption of CO 2 can be improved by the catalyst reduction -No CH4 production from fresh catalyst might due to lower CO2 chemisorption.

Conclusion It is possible to produce methane from an exclusively inorganic way using CO 2 and H 2 on a Rh/γ-Al 2 O 3 catalyst at low temperature and atmospheric pressure. Reaction Mechanism of Methanation: 1.The chemisorption of CO 2 on the catalysts 2.The dissociation of CO 2 into CO and O adsorbed on the surface 3.The reaction of dissociated species with hydrogen The adsorption of CO 2 can be improved by the catalyst reduction CO2 oxidizes the catalyst and this oxidation deactivates the catalyst The amount of CH4 produced is higher with higher temperature. Methane is the unique hydrocarbon molecule formed

Further Direction How the hydrogen reacts with the dissociated species is not determined Hypothesis1:CO is dissociated into C and O on the surface of the catalysts and that C could react with H2 and produce CH4 Hypothesis 2: the formation of CH4 from CO2 and H2 following a Fisher-Tropsch reaction

Questions?