Mechanism of the Reaction of Nitric Oxide , Ammonia , and Oxygen over Vanadia Catalysts . 1 . The

calculation for each term and their sum which contribute to AG are reported in Table 11. In any case, the entropy of the reactions is unfavorable and particularly for photoreduction in CO. The enthalpy, AH, is calculated in Table 111 from the energy balance of the bonds created and destroyed in reactions 13-16. The Ni-O and Ni-CO bond energies are taken as 24 and 30 kcal mol-' from theoretical calculation where Ni is in a zerovalent states4' These values are not known for Ni2+ and Ni+ ions, but one can expect that the difference of the bond energies does not change drastically with the oxidation state of nickel. The Ni+-H2 bond energy is evaluated to be 8.5 kcal mol',32 and the 0-H and 0-C bond energies are taken from the l i t e r a t ~ r e . ~ ~ In conclusion, whatever the pressure (reactions 13-15) and the nature (reactions 15 and 16) of the gas, the photoreduction is thermodynamically highly favorable since AG ranges from -1 16.5 to -146 kcal mol-'.