The adsorption of ammonium heptamolybdate (AHM) on phosphated alumina and of P2Mo50231 23456~a nd PMo^O^3-on 7-Al203 has been studied by using FTIR, Raman, and 31P solid-state NMR spectroscopies, and TPR. Adsorption of phosphate prior to contacting of 7-Al203 with AHM leads to a deactivation of the alumina surface still remaining. Phosphate reacts with 7-Al203 to form an A!P04-type surface phase, which itself is capable of adsorbing molybdate, with the formation of an irreducible surface molybdophosphate, its adsorption capacity being lower, however, than that of the original alumina. The primary phosphomolybdate adsorption reaction on alumina is shown to be the same as that observed in the case of AHM, viz., a reaction with the basic surface OH groups, leading to the decomposition of the adsorbing species. Thiophene hydrodesulfurization (HDS) activity data of variously prepared Mo(P)/A1203 catalysts are in agreement with the idea that HDS activity increases with increasing Mo reducibility.

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