Molybdenum actions/effects

As indicated in Fig. 25-18, free adenine released from catabolism of nucleic acids can be deaminated hydrolytically to hypoxanthine, and guanine can be deaminated to xanthine.328 The molybdenum-containing xanthine oxidase (Chapter 16) oxidizes hypoxanthine to xanthine and the latter on to uric acid. Some Clostridia convert purine or hypoxanthine to xanthine by the action of a selenium-containing purine hydroxylase.3283 Another reaction of xanthine occurring in some plants is conversion to the trimethylated derivative caffeine. 328b One of the physiological effects of caffeine in animals is inhibition of pyrimidine synthesis.329 However, the effect most sought by coffee drinkers may be an increase in blood pressure caused by occupancy of adenosine receptors by caffeine.330... 

Haltner and Oliver found that several metallic sulphides brought about an improvement in the load-carrying capacity when mixed with molybdenum disulphide. The sulphides included stannic and stannous sulphides, lead sulphide, ferrous suiphide and cuprous and cupric sulphides, and in a standard test procedure there was up to a ten-fold increase in load-carrying capacity. They speculated that the action of the added sulphides was similar to that of extreme-pressure additives in liquid lubricants. This would imply the formation of some protective film on the substrate surface. Pardee later suggested that the effective mechanism was more likely to be oxidation inhibition. An alternative would seem to be the possibility that certain sulphides can act as an additional source of sulphur to form sulphide on the substrate surface, and thus improve adhesion of the molybdenum disulphide, as discussed in the previous chapter. 

Figure 19-5. Effect of load and sliding speed on the lubricating action of molybdenum disulfide. From data by Haltner and Oliver [36].

Variations in the selectivity of propene oxidation as a function of the catalyst composition are shown in Fig. 19a and b. If the suggested electronic mechanism of the action of mixed catalysts is true, the electron work function () of mixtures should be higher than that of pure molybdenum and bismuth oxides. The dependence of A on the composition of a molybdenum-bismuth catalyst is shown in Fig. 19b. The maximum change in the electron work function corresponds to highest selectivity. Such a proportional change in catalytic and electronic properties seems to provide evidence for the electronic mechanism of the effect of these mixed catalysts. 

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