(93d) Ceria Supported Bimetallic Overlayer Catalysts to Aid in the Dry Reforming of Methane

Teneil A. Schumacher^a, Jing Zhou^b, and Joseph H. Holles^a

^aDepartment of Chemical Engineering, University of Wyoming, Laramie, WY 82071

^bDepartment of Chemistry, University of Wyoming, Laramie, WY 82071

The dry reforming of methane (DRM) utilizes two abundantly available greenhouse gases, methane (CH₄) and carbon dioxide (CO₂), to produce industrially important syngas, hydrogen (H₂) and carbon monoxide (CO) in unity. Syngas can be further treated to produce synthetic petroleum as fuels or chemicals. Most importantly, DRM serves as an important prototype reaction for sustainable chemical recycling and conversion by utilizing the major atmospheric pollutant, CO₂. It is commonly accepted that in the DRM reaction mechanism, CH₄ and CO₂ activate on the metal and support, respectively. Thus, the interface between the metal and the metal oxide support is also important as it provides sites to complete the reaction.

Supported noble metals, such as palladium, are highly active toward the DRM reaction at high temperatures and are more resistant to carbon formation than other transition metals. However, these elements are costly for practical applications. While nickel proves to be economically favorable, deposited carbon species can accumulate on the metal which leads to loss of active sites. Furthermore, Ni is subject to sintering which also causes rapid deactivation during DRM. Previous research has shown that alloying Ni with Pd will increase catalytic activity and enhance stability against carbon formation. However, the strong Pd binding, which is required to activate CH₄, also results in Pd binding too strongly to the CO and H₂ products. This results in Pd surface sites being blocked and a decrease in catalytic activity. Therefore, the current global challenge for DRM is to develop thermally stable and active catalysts which can show good resistance to deactivation while minimizing use of expensive metals.

Computational and experimental literature has shown that by synthesizing an overlayer of Pd on Ni (Ni@Pd), the coordination of metal atomic orbitals on the surface of a catalyst can increase. By increasing orbital coordination, the d-band center shifts down and the energy band widens. This results in a decrease of binding strength. By synthesizing a Ni@Pd overlayer, the binding strength of the overlayer metal, Pd, can be controllably decreased. By decreasing the binding strength to reactants and products, the catalyst design can then increase catalytic activity and enhance stability against carbon formation.

Moreover, ceria supports provide a solution to improve the stability and catalytic performance of metal catalysts. Due to its unique redox properties, ceria may act as the active phase and remove carbon deposits on the metal by the oxidation of surface carbon to CO and prevent metal deactivation. A strong metal-support interaction between the metal and ceria can modify the structure and electronic properties of active metals which could improve the performance of metal-ceria systems. Due to ceria’s unique redox properties and oxygen storage capacities, it can better anchor the base metal as small clusters and inhibit coke formation. Furthermore, the introduction of the metal dopant, Ti, into ceria could promote its redox properties as well as enhance its thermal stability at high temperatures.

The Ce_0.7Ti_0.3O₂ mixed metal oxide support was synthesized using the sol gel method. Bimetallic overlayer catalysts, Ni@Pd, supported on CeO₂ and Ce_0.7Ti_0.3O₂ have been prepared by the directed deposition synthesis technique. Monometallic Ni and Pd catalysts supported on CeO₂ and Ce_0.7Ti_0.3O₂ were also synthesized using the co-impregnation method. Hydrogen chemisorption, ethylene hydrogenation descriptor reaction, and XRD studies were employed to characterize the catalysts. The CeO₂ and Ce_0.7Ti_0.3O₂ supported catalysts have also been studied for DRM. Additionally, preliminary data regarding the reforming of ethane activity over the Ni/Ce_0.7Ti_0.3O₂ catalyst was investigated for future studies introducing impurities into the DRM feed.

H₂ chemisorption studies showed that CeO₂ and Ce_0.7Ti_0.3O₂ supported Ni@Pd overlayer catalysts had reduced adsorption strength compared to the monometallic Pd catalyst, which agrees with the computational prediction. Similarly, the ethylene hydrogenation descriptor reaction further proved the CeO₂ and Ce_0.7Ti_0.3O₂ supported Ni@Pd overlayer catalysts experienced a reduced adsorption strength when compared to the Pd only catalyst. The Ni@Pd overlayer catalyst supported on CeO₂ displayed an enhanced DRM activity when compared to both monometallic Pd and Ni catalysts. Similarly, at higher temperatures (>425⁰C) the Ni@Pd overlayer catalyst supported on Ce_0.7Ti_0.3O₂ showed an increased DRM activity when compared to the monometallic Pd and Ni catalysts. The DRM stability studies showed that the Ce_0.7Ti_0.3O₂ support enhanced activity, stability, and H₂:CO production selectivity for all catalysts when compared to the CeO₂ support. The enhanced reactivity and stability of the Ce_0.7Ti_0.3O₂ catalysts can be attributed to the unique interaction between the metal and support.