The impact of dopants on the activity and selectivity of a Ni-based methanation catalyst

The influence of different dopants in varying contents on the activity and selectivity of Ni-based methanation catalysts under proton exchange membrane fuel cell (PEMFC) relevant conditions (hydrogen-rich gas reformate with low concentrations of CO and excess CO2) was investigated. The modification of Ni100Ox with 2.2 mol% of Re resulted in a catalyst highly inactive for both the methanation of CO and CO2 while the addition of Zr led to an enhanced CO and CO2 methanation activity. Further modification of Zr10Ni90Ox with small amounts of Re caused a drastically decreased reactivity towards the undesired hydrogenation of CO2 while that for CO was practically unchanged. Solo methanation experiments unambiguously reveal that this increase in selectivity is based on a loss of the intrinsic CO2 reactivity and not necessarily associated with any competition for active sites between the different kinds of carbon oxides. A detailed study of the impact of the compositional variations on structural and chemical properties was performed using nitrogen physisorption, hydrogen chemisorption, temperature-programmed reduction (TPR), temperature-programmed desorption experiments of CO2 (CO2 TPD), X-ray diffraction (XRD), X-ray photoelectron spectra (XPS) as well as TEM. As a part of the catalytically active Ni particles, Re changes the surface of the catalyst resulting in drastically altered catalytic properties while Zr or the Ni–ZrO2 interfacial region seems to play a decisive role in the activation of the CO molecule.

Well-proportioned Re–Zr-doped Ni-based mixed metal oxides (MMOs) are the ideal precursor for highly active and selective methanation catalysts for the final removal of CO from hydrogen-rich gas reformates with an excess of CO2. Comparisons of the catalytic properties of these ternary oxides with those of the respective binary oxides as well as the base oxide reveal a strong synergism between the different components. A detailed characterization of the various materials was undertaken. The role of the different dopants is discussed.Figure optionsDownload as PowerPoint slide