APCATA 413-2012-176

Study of the interaction between hydrogen and the MoO3–ZrO2 catalyst

The interaction of molecular hydrogen with the surface ofMoO3–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 3700–3400cm−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 electrondeficient
metal cations and/or oxygen radicals. The hydrogen adsorbed IR and ESR results suggested that
the protons and electrons were formed on the surface ofMoO3–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 473K 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 373K 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 SO4
2−–ZrO2, WO3–ZrO2 and MoO3–ZrO2 catalysts is discussed.