An infrared study on the reduction processes of dodecamolybdophosphates

The reduction processes of dodecamolybdophosphoric acid and its potassium salt were investigated by infrared (ir) spectroscopy. The changes of ir bands were analyzed in detail as a function of the degree of reduction x (electron/anion) for the vapor phase reduction of the potassium salt with H2 (A) and of the acid with cyclohexane and methacrolein (B) as well as for the liquid phase reduction of the acid with SnCl2 solution (C) and with an electrochemical method (D). It was shown that reduction was initiated with the formation of 4 electron reduced polyanions in A and D while 2e reduction preceded the 4e state in B and C. It was commonly observed in all cases that, of the three bonding types of oxygen of polyanions, the bridging oxygen (Ob) had exclusive reactivity until x = 4 and the terminal oxygen (Ot) was consumed at x > 4. However, the actual reduction scheme was somewhat different between the vapor phase reduction (A and B) and the liquid phase reduction. Polyanions were deprived of oxygen in the former case while protons were introduced to them in the latter case. The introduced protons were bonded rather strongly and eliminated as water molecules at 200–420 °C on heating. In connection with the difference of reduction processes in A and B, X-ray photoelectron spectroscopy measurements of the surface Mo5+ fraction revealed that in A the reduction took place almost homogeneously within each grain of the sample while in B the surface region was reduced in preference to the bulk. It is suggested that H2 enters the pore of the crystal lattice reducing the whole grain homogeneously, but cyclohexane and methacrolein with larger molecular sizes reduce the grains by a surface reaction.