Structural aspects of the oxidative coupling of methane over europium oxide catalysts

Gregg Johnston Howsmon, Purdue University

Abstract

Structure-function relationships for the oxidative coupling of methane (OCM) reaction have been investigated for the cubic and monoclinic crystal forms of europium sesquioxide, $\rm Eu\sb2O\sb3.$ It has been established that the two crystal forms differ markedly in their ability to selectively activate CH$\sb4$ to produce C$\sb2$ hydrocarbons, with the cubic structure more selective under all conditions tested. X-ray diffraction clearly shows differences in the bulk crystal structure, however, Mossbauer spectroscopy shows almost the exact same bulk Eu-O bond strength for the two materials, although some differences in lattice stiffness are evident. Temperature programmed desorption (TPD) of O$\sb2$ from the catalysts reveals that the surface Eu-O bond strength of monoclinic $\rm Eu\sb2O\sb3$ is less than that for cubic $\rm Eu\sb2O\sb3$ and TPD of CO$\sb2$ implies that the surface of the cubic crystal structure is more basic than that of the monoclinic structure. In situ XPS analysis of the surface of the $\rm Eu\sb2O\sb3$ materials heated to temperatures up to 773 K reveals that in the monoclinic surface layer Eu$\sp{3+}$ is readily reduced to Eu$\sp{2+},$ while the reduction is more difficult for the cubic material. The surface reducibility of the cubic crystal structure is, however, sensitive to the catalyst pretreatment conditions. Analysis of the surface oxygen type by XPS implies the surface oxygen of monoclinic $\rm Eu\sb2O\sb3$ may be more weakly bound than the surface oxygen of cubic $\rm Eu\sb2O\sb3.$ The overall evidence provides a consistent rationale for the superiority of the cubic crystal structure for the OCM reaction and suggests that for $\rm Eu\sb2O\sb3$ both surface and bulk structure are important.

Degree

Ph.D.

Advisors

Delgass, Purdue University.

Subject Area

Chemical engineering

COinS