A temperature-programmed reduction (TPR) study of three series of biphasic hydrotreating (HDT) catalysts-PtS/γ-Al 2 O 3/(PtS/γ-Al 2 O 3+ MoS 2); Rh 2 S 3/γ-AlOOH/(Rh 2 S 3/γ-AlOOH+ WS 2) and PdS/γ-Al 2 O 3/(PdS/γ-Al 2 O 3+ WS 2)-is presented. In all cases, the quantity of H 2 S produced is larger than that which would correspond to the arithmetic addition of the quantities produced by each of the sulfide phases alone. This fact suggests that the noble metal sulfide dissociates the hydrogen molecule to spill-over hydrogen (H
