INFLUENCE OF ALLOYING PLATINUM FOR THE HYDROGENATION OF PARA-CHLORONITROBENZENE OVER PTM/AL2O3 CATALYSTS WITH M = SN, PB, GE, AL, ZN

Hydrogenation of p-chloronitrobenzene (CNB) has been studied, in methanol suspension, at 303 K and atmospheric pressure, over a series of Pt/Al2O3 and PtM/Al2O3 catalysts (M = Sn, Pb, Ge, Al, Zn). The bimetallic catalysts were prepared by the so-called controlled surface reaction method by reacting organometallic compounds on the well-dispersed Pt/Al2O3 parent sample. The kinetics obeys a modified Langmuir-Hinshelwood model with competitive adsorption between CNB and hydrogen. The competition of adsorption no longer holds at high CNB concentration: on the Pt surface saturated by CNB, there are still Pt sites available for the chemisorption of the smaller hydrogen molecule. A rate law is then proposed which allows both adsorption and rate constants to be determined. Upon addition of the second metal M, the specific activity per Pt surface atom goes through a maximum value around a chemical composition M/Pt congruent-to 0.1-0.2. It is suggested that an electron-deficient species of the second metal promotes the reaction rate by activating the nitrogen-oxygen bond; ionic tin species are better promoters for this purpose. At high CNB conversion (> 98%) the yield of the desired product, p-chloroaniline (CAN), increases from 82% on pure Pt/Al2O3 to 97.5% on PtSn/Al2O3 (Sn/Pt = 0.36). The improvement of selectivity to CAN is not due to a decrease in CAN dehalogenation rate, which is not affected by alloying. Actually, the promoting effect of the second metal decreases the relative strength of adsorption between CAN and CNB up to a factor of 10: CAN is then easily removed from the surface by CNB, thus preventing hydrodechlorination.