Chemisorbed Species And Fixation Of Dinitrogen To Ammonia On Iron Catalysts

The mechanism of heterogeneous catalysis in ammonia synthesis on iron catalysts has been studied for over seventy years, but a well unified theory has not yet been reached so far. For better appraisal of all different mechanistic interpretations, more detailed knowledge about the nature of N-containing chemisorbed species and their relative concentrations on the surface of Iron catalysts under ammonia synthesis reaction conditions is very much needed. In the present work, some experimental or theoretical methods, including molecular vibrational spectroscopy, molecular orbital approximate calculation, and reaction kinetics are used to study the Important problems in molecular level.The complementary in-situ dynamical Fourier transform infrared and laser Raman spectra indicated that underconditions of ammonia synthesis reaction of N₂/3N₂ with high space velocity at high temperature ( 400 — 450℃ ) and atmospheric pressure, there were a large number of chemi-sorbed hydrogen or even interstitial hydrogen ( V(FeH) = 1950— 1958 cm^-1 (IR, R);2052 — 2056 cm^-1 (IR, R);1892 — 1902 cm^-1 (IR, R) etc. ) on the surface of commercial doubly promoted fused iron catalyst A_110-3 and the most abundant chemisorbed N-containing species was molecular N₂(ad.) with nultinuclear coordination ( V(NN) = 2036 — 2040 cn^-1 (IR), 2032 — 2042 cm^-1 (R);1935 cm^-1 (IR)? 1934 —1944 cm^-1 (R);V (FeN) = 434 cm^-1 (IR), 423 — 432 cm^-1 (R);450 cm^-1 (IR), 443 — 450 cm^-1 (R) ), rather than atomic N(ad.) ( V(FeN) = 1087 cm^-1 (IR), 1088 cm^-1 (R) ) or NH(ad.) ( V (FeK) = 886 cm^-1 (IR), 890 cm^-1 (R) ).The results of extented Huckel molecular orbital approximate calculations indicated that the symmetrical flat-lying mode with bridging double-end-on plus multiple-side-on coordination activation of dinitrogen on the active center with Fe₇ cluster of d-Fe(111) surface appeared to be more favorable to the coordination and activation of chemisorbed N₂ than other two possible nultinuclear coordination activation modes, the perpendicular-insertion mode and the inclined-mounting mode. The results also indicated that the dinitrogen sufficiently activated with this mode could react with coordination activation 2H(ad.) or H₂(ad.) in a way of symmetrical hydrogenation.Therefore, it may be suggested that in the reorienta- _? ? tion process of chemisorbed N"2 from perpendicular-insertion to inclined-mounting mode, and further to flat-lying mode before dissociation, most chemisorbed Np may be sufficiently actived, and react with certain nearby adspecies of hydrogen, and undergo the first hydrogenation in a way of associative mechanism, and immediately undergo hydrogenolysis, followed by rapid further hydrogenation to ammonia, and only a little chemisorbed 17,, may be finally dissociated to N(ad,) , followed by rapid further hydrogenation to ammonia in a dissociative mechanism. Such as two parallel competitive major and minor reaction pathways may be likely a more satisfactory explanation about the mechanism of ammonia synthesis reaction on iron catalysts.In this paper, a slightly modified form of the extended Temkin kinetic equation of ammonia synthesis reaction was â–  also derived from associative mechanism, and some known important experimental facts, including the inverse deuterium isotope effect, the relationship of the reaction rate with partial pressure of hydrogen, ajid the structure sensitivity of ammonia synthesis reaction, were more adquately correlated and elucidated.Finally, some similarities of iron catalysts to nitroge-nase were suggested in the way that they activate the sturdy NsN triple bond and bring about the conversion to ammonia. The preliminary organic relationship of fixation of dinitrogento ammonia via enzymatic and non-enzymatic catalysis was set up. The modification and desing of ammonia synthesis catalysts v/ere discussed in order to change the stringent conditions of traditional iron catalysts operated at high temperature and pressure, and to find a high-powerful ammonia synthesis catalyst, which can be operated at a temperate condition.