ADSORPTION OF ACETYLENE ON IRON(110) THE ROLE OF PRECURSOR STATES IN ADSORPTION (ADSORPTION, ACETYLENE, PRECURSOR).

摘要

Of the transition metals, iron is known as an excellent catalyst for the synthesis of ammonia and the Fischer-Tropsch synthesis. However, until recently, no detailed study for hydrocarbon adsorption on Fe(110) has been carried out, due in part to the difficulty of producing a clear iron surface.; In the present study, the adsorption of acetylene on the Fe(110) surface has been studied using Modulated Molecular Beam Relaxation Spectroscopy (MBRS). MBRS has advantages over many other techniques because it does not impose extraneous effects on the adsorption process and because it can be easily applied to the study of precursor states. It was, however, necessary to develop an analysis technique for the interpretation of MBRS data taken under transient conditions. In systems such as the one studied here, data taken under transient conditions give more information on surface reaction mechanisms than data taken under steady state conditions.; The resulting MBRS data, analyzed using this technique, led to the following conclusions: (1) Acetylene adsorption on Fe(110) takes place through a precursor state. (2) At 573(DEGREES)K, acetylene adsorbs dissociatively. The initial sticking coefficient has the relatively low value of 0.31. (3) From 344 to 473(DEGREES)K, the fraction of non-dissociative adsorption for acetylene on Fe(110) decreases from 0.984 to 0.845, with the initial sticking coefficient having a fairly constant value (0.75). (4) At 523(DEGREES)K, the MBRS data showed different characteristics from other experimental data. (5) The values of E(,a)-E(,p) and E(,a)'-E(,p) are -1130(' )(+OR-) 30 and 904 (+OR-) 4 cal/mole, respectively. Here, E(,a), E(,a)' and E(,p) are the activation energies for non-dissociative adsorption, dissociative adsorption and desorption from the precursor state. The activation energy for desorption from the dissociated state to the precursor state, E(,d)', is 44.4 (+OR-) 3 kcal/mole. The approximate activation energy for dissociation of adsorbed acetylene is 20 kcal/mole.