ESCA study of "model" allyl-based molybdenum/silica catalysts

Abstract

Stoichiometric redox measurements and X-ray photoelectron spectroscopy (XPS or ESCA) studies were performed on an allyl-based Mo/SiOz catalyst (1.7 wt % Mo) obtained by the sublimation of Mo(v3-C3H5)4 onto Si02 at 40 OC. The average oxidation state estimated from stoichiometric measurements correlated well with those reported in the literature. ESCA results indicated that reduction of the Mo/SiOz catalyst at 550 "C primarily led to the formation of Mo2+. Also, the results were consistent the reported reversibility of the redox cycle. However, the reported formation of discrete Mo4+ by oxidation of the reduced catalyst at room temperature could not be substantiated.

?Vibrational frequencies of CO adsorbed on silica-supported Mo atoms from density functional calculations?: a re-interpretation of results reported by N. L�pez, F. Illas, G. Pacchioni

“…On the other hand, in our previous work [2,3] the notation Mo n+ designated the +n O-S of a real species (as probed by XPS). It was assumed that such species are of the type described by López et al as neutral, because they should be more stable than the charged ones.…”

“…Although in most related work, e.g. [3][4][5][6][7][8][9], the Mo n+ notation is generally employed to designate the O-S of the postulated surface species, to facilitate the comparison we will adhere in this letter to the use of the Mo(R) notation to indicate the formal O-S of a given species.…”

“…Model supported catalysts synthesized by grafting of molecular allylic molybdenum compounds, such as Mo( 3 -C 3 H 5 ) 4 , and Mo 2 ( 3 -C 3 H 5 ) 4 , on the surface of silica or alumina, have been shown to lead to active species with narrow distributions of Mo O-S; thus, a single, well defined species in a chosen O-S would be prepared with the correct reductive pretreatment [3][4][5]. On the other hand, conventional catalysts prepared by impregnation techniques generally show broader distributions of Mo O-S, with several species simultaneously present at any reduced state of the catalyst [6,7].…”

“…An example of a set of catalysts derived Scheme 1. from the reaction of Mo( 3 -C 3 H 5 ) 4 with the OH groups of silica and submitted to red-ox pretreatments, is depicted in Scheme 1. In this particular case, stoichiometry red-ox cycles (volumetric measurements of gas consumption performed after heating under H 2 or O 2 ) suggested [3,4] the exclusive presence of Mo(IV) in state C, and Mo(II) in B; Mo(VI) is assumed to be the only possible species in D, the fully oxidic catalyst. By means of XPS it was confirmed [3] that a single Mo(VI) species was present in D, as expected.…”

“…In this particular case, stoichiometry red-ox cycles (volumetric measurements of gas consumption performed after heating under H 2 or O 2 ) suggested [3,4] the exclusive presence of Mo(IV) in state C, and Mo(II) in B; Mo(VI) is assumed to be the only possible species in D, the fully oxidic catalyst. By means of XPS it was confirmed [3] that a single Mo(VI) species was present in D, as expected. In B there was more than 90% of Mo(II) and a much lower amount of higher O-S Mo species, most likely Mo(IV).…”