Kazuo Shimamura

Compositional dependence of the CO2 methanation activity of Ni/ZrO2 catalysts prepared from amorphous NiZr alloy precursors

Abstract Finely grained Ni/ZrO 2 catalysts were prepared from amorphous Ni Zr alloy precursors by oxidation and subsequent reduction pretreatment, and the catalytic activity for CO 2 methanation was examined as a function of precursor alloy composition and temperature. The catalysts thus prepared produce exclusively methane, apart from water as a by-product. The conversion of CO 2 increases with temperature in the range of 373–573 K. Among the catalysts examined, the maximum methanation rate is obtained on the catalysts prepared from the amorphous alloy precursors containing 40 and 50 at% zirconium. Further, the methanation rates of all the catalysts prepared from the amorphous alloy precursors are higher than that of a 3 at% Ni/ZrO 2 catalyst prepared by wet impregnation. The number of surface nickel atoms, determined by hydrogen chemisorption, increases with zirconium content in the catalysts, while, interestingly, the turnover number decreases with increasing zirconium content. In the catalysts prepared from the amorphous alloys, two types of zirconia are present: metastable tetragonal and stable monoclinic zirconia. The former zirconia phase is present predominantly in the catalyst prepared from the Ni-30 at% Zr alloy, but the relative amount of this oxide phase, with respect to the total amounts of zirconia, gradually decreases with an increase in zirconium content of alloys. Thus, the higher turnover number of the catalysts with higher nickel content can be attributed to nickel supported on metastable tetragonal zirconia. Increasing nickel content of the precursor alloys leads to an increase in tetragonal zirconia and to a decrease in the number of surface nickel atoms on the catalysts. This is responsible for the fact that the maximum conversion appears at medium contents of zirconium in the precursor alloys.

Methanation of carbon dioxide on catalysts derived from amorphous Ni-Zr-rare earth element alloys

Abstrct The nano-grained Ni/ZrO 2 catalysts containing rare earth element oxides were prepared by oxidation-reduction pretreatment of amorphous Ni-(40-x) at% Zr-x at% rare earth element (Y, Ce and Sm; x=l – 10) alloy precursors. The conversion of carbon dioxide on the catalysts containing 1 at% rare earth elements was almost the same as that on the rare earth element-free catalyst, but the addition of 5 at% or more rare earth elements increased remarkably the conversion at 473 K. In contrast to the formation of monochnic and tetragonal ZrO 2 during pretreatment of amorphous Ni-Zr alloys containing 1 at% rare earth elements, tetragonal ZrO 2 , which is generally stable only at high temperatures, was predominantly formed during the pretreatment of the catalysts containing 5 at% or more rare earth elements. The surface area of the catalysts increased with the content of rare earth element. Thus, the increase in the surface area and stabilization of tetragonal ZrO 2 seem to be responsible for the improvement of catalytic activity of the Ni-Zr alloy-derived catalysts by the addition of rare earth elements.

Characterization of CO2 methanation catalysts prepared from amorphous Ni-Zr and NI-Zr-rare earth element alloys

Nano-grained Ni/ZrO2 and Ni/ZrO2 -Sm2O3 catalysts were prepared from amorphous Ni-Zr and Ni-Zr-Sm alloys by oxidation-reduction treatment. Their catalytic activity for methanation of carbon dioxide was examined as a function of precursor alloy composition and temperature. The addition of samarium is effective in enhancing the activity of the nickel-rich catalysts, but not effective for the zirconium-rich catalysts. The surface area and hydrogen uptake of the nickel-rich catalysts are increased by the samarium addition. In addition, tetragonal zirconia, the formation of which is beneficial to the catalytic activity, is stabilized and formed predominantly by the addition of samarium to the nickel-rich catalysts, although monoclinic zirconia is also formed in the zirconium-rich catalysts. As a consequence, the higher conversion of carbon dioxide is obtained on the Ni-Zr-Sm catalysts with relatively high nickel contents.