Molybdenum bronzes, formation

The reduction of molybdate salts in acidic solutions leads to the formation of the molybdenum blues (9). Reductants include dithionite, staimous ion, hydrazine, and ascorbate. The molybdenum blues are mixed-valence compounds where the blue color presumably arises from the intervalence Mo(V) — Mo(VI) electronic transition. These can be viewed as intermediate members of the class of mixed oxy hydroxides the end members of which are Mo(VI)02 and Mo(V)0(OH)2 [27845-91-6]. MoO and Mo(VI) solutions have been used as effective detectors of reductants because formation of the blue color can be monitored spectrophotometrically. The nonprotonic oxides of average oxidation state between V and VI are the molybdenum bronzes, known for their metallic luster and used in the formulation of bronze paints (see Paint). 

Special mention must be given to the numerous studies concerning hydrogen bronze formation induced by spillover. In particular, tungsten and molybdenum oxide hydrogen bronzes have been thoroughly studied since Khoobiar s experiments (4). 

The kinetics and mechanism of the formation of molybdenum bronzes was studied in detail and published in a series of three papers by Erre et al. (86-88), who studied the (100) face of Mo03. Pt was added and the sample was exposed to H2 under very low partial pressures (10 8-10-5 Torr), forming monoclinic Hj 6Mo03. The kinetics unexpectedly exhibited three steps (i) activation of H2 by Pt with an increasing rate with time on stream (ii) a kinetic plateau where the rate remained constant and finally (iii) a decrease in the rate of bronze formation. The salient results were that once step (i) starts, Pt is not required for the insertion of hydrogen into the lattice,... 

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