KINETICS OF THE AMMONIA-SYNTHESIS OVER FE/TIO2, HYDRAZINE-PRETREATED FE/TIO2, AND HYDRAZINE-PRETREATED ALKALI-PROMOTED FE/TIO2 CATALYSTS

The kinetics of the synthesis of ammonia from its elements over Fe/TiO2, an hydrazine-pretreated Fe/TiO2, and hydrazine-pretreated alkali-promoted Fe/TiO2 catalysts was studied in a flow microreactor at 101 kPa pressure. A significant improvement in the accuracy and precision of kinetic parameters extracted from the data was achieved by modifying the kinetic model to account for deactivation of supported catalyst particles by Ostwald ripening. The modified form of the kinetic model also yielded information about the particle ripening kinetics. Pretreatment of Fe/TiO2, with hydrazine increased the ammonia synthesis turnover frequency by more than an order of magnitude over unpretreated Fe/TiO2. The ammonia partial pressure dependence and apparent activation energy over hydrazine-pretreated Fe/TiO2 were more representative of iron uninfluenced by the strong metal-support interaction (SMSI) which occurs in metal-titanic systems. In situ CO chemisorption measurements following the ammonia synthesis kinetics measurements showed higher CO uptake with hydrazine-pretreated Fe/TiO2 than with unpretreated Fe/TiO2. The increased turnover frequency, altered kinetic parameters, and higher CO uptake suggest that hydrazine pretreatment inhibited the onset of SMSI, which is attributed to titanium nitride formation on the support surface. Addition of the alkali promoters K and Cs to the catalysts not only increased the turnover frequency and decreased the apparent activation energy and ammonia partial pressure dependence, but acted to stabilize supported iron particles against growth by Ostwald ripening. The data suggest that physical covering of the surface by alkali inhibits Ostwald ripening of iron particles by blocking dissociation of iron atoms from supported particles thus suppressing their migration over the support surface to form larger particles. © 1991.