Study of the interaction between hydrogen and the MOO3-ZRO2 catalyst

Abstract

The interaction of molecular hydrogen with the surface of MoO3–ZrO2 was observed using infrared IR and electron spin resonance (ESR) spectroscopy, and the hydrogen adsorption was quantitatively evaluated in the temperature range of 323–573 K. The hydrogen adsorbed IR results confirmed the formation of a new broad band in the range of 3700–3400 cm−1, which corresponds to hydrogen-bonded OH groups. A decrease in the ESR signals indicated the formation of electrons that have been trapped by the electron-deficient metal cations and/or oxygen radicals. The hydrogen adsorbed IR and ESR results suggested that the protons and electrons were formed on the surface of MoO3–ZrO2 from molecular hydrogen enhancing the isomerization of n-heptane. A quantitative study of the hydrogen adsorption showed that the rate of hydrogen uptake was high for the first few minutes at 473 K and above, and the rate reached an equilibrium value within 10 h. At 423 K, different features of the hydrogen adsorption were observed on MoO3–ZrO2, where the hydrogen uptake increased slowly with time and did not reach equilibrium after 10 h. The rate of hydrogen adsorption increased slightly at 373 K and below. Hydrogen adsorption on MoO3–ZrO2 involves two successive steps. The first step involves hydrogen dissociation on a specific site on the MoO3–ZrO2 catalyst to form hydrogen atoms, and the second step involves the surface diffusion of the hydrogen atoms on the MoO3–ZrO2 surface. Then the hydrogen atom becomes a proton by donating an electron to an adjacent Lewis acid site. The rate-controlling step involves the surface diffusion of hydrogen atoms and has an activation energy of 62.8 kJ/mol. A comparison of the hydrogen adsorption on SO42−–ZrO2, WO3–ZrO2 and MoO3–ZrO2 catalysts is discussed.