Thermodynamic Control of Product Formation During the Reaction Between CH4 and Pt Promoted Ceria-zirconia Solid Solutions

Abstract

The reaction between CH 4 and framework oxygen of Pt promoted Ce x Zr 1-x O 2 (x = 0.25, 0.5, 0.75, 1) solid solutions was studied at 873 K for syngas (CO ? H 2 ) production. The homogeneity and cubic/tetragonal structure of the solid solutions were confirmed by powder X-ray diffraction. The thermodynamic properties of the solid solutions, connected with Ce 4? /Ce 3? redox reactions, were measured by titrating the oxygen content of the oxides after equilibration in flowing mixtures of H 2 and CO 2 with 10 -31 \ pO 2 \ 10 -22 atm. The transient reactions between the oxides and discrete pulses of CH 4 , as well as of isotopically labeled 13 CH 4 ? 12 CO mixtures (limited to the material with x = 0.25) were investigated and reactivity, conversion and selectivity were measured. A strong correlation between the thermodynamic properties of the oxides and the syngas selectivity was found. The 13 C scrambling in CO and CO 2 during co-feed experiments confirmed equilibration of these molecules with the oxides surfaces.

Crystal Structure and Oxide-Ion Diffusion of Nanocrystalline, Compositionally Homogeneous Ceria–Zirconia Ce0.5Zr0.5O2 up to 1176 K

“…It is considered that the nonequilibrium tetragonal forms are important also in the ceria–zirconia catalysts for reforming ethanol and CH 4 at high temperatures to produce hydrogen for fuel cells. The existing phase and crystal structure of compositionally homogeneous, nonequilibrium, or metastable Ce x Zr 1– x O 2 solid solutions have been investigated by many researchers. − There exist three forms, t, t′, and t″, in the tetragonal phase. ,,,, All three forms have the distorted fluorite-type structure with the tetragonal P 4 2 / nmc space group (Figure ). The t-form is stable in the equilibrium state at high temperatures, and the solubility limit of CeO 2 in t-ZrO 2 is x = 0.18 in Ce x Zr 1– x O 2 .…”