NH2 Et C Me NH2 C N H Me C N Me MeCN(g) NH2 Et 1 2 3 4 5 6 C Me NH • C Me NH 7 8 324 J. CHEM. RESEARCH (S), 1997 J. Chem. Research (S), 1997, 324–325† The Enthalpy Changes in the Course of Ethylamine Decomposition on a Ni(111) Surface† Fereydoon Gobal* and Saied Azizian Department of Chemistry, Sharif University of Technology, P.O. Box 11365-9516, Tehran, Iran The BOC-MP method is capable of calculating the enthalpies of various adsorbates in the catalytic decomposition of ethylamine on Ni(111) with results in good agreement with the experiments and superior to the extended H�uckel approximation predictions.Surface-sensitive spectroscopic methods are capable of providing extensive information regarding the structure of observed intermediates and reaction mechanisms. However, little information about the energies involved in these processes is provided. The present study applies the method of bond order conservation–Morse potential analysis1 (BOCMP) to investigate the heat of adsorption of ethylamine and the enthalpies of the possible intermediates in the decomposition of this species to acetonitrile and hydrogen on a Ni(111) surface.This process has been studied in some detail by Somorjai et al.2 where the methods of temperature programmed desorption (TPD) and high resolution electron energy loss spectroscopy (HREELS) were employed. It has been concluded that the dehydrogenation starts by an a-H·C cleavage, and the subsequent dehydrogenation of the ensuing MeCNH2 (2) and MeCNH (4) species gives rise to MeCN.On the basis of the TPD and HREELS results, the enthalpies of all the species were calculated (Table 1). In a further endeavour, Somorjai et al.3 attempted to calculate the enthalpies by the extended H�uckel method with the inclusion of a repulsive correction4 for various pairs of adsorbate atoms but ignoring the repulsion between the metal and adsorbate (Table 1). Although the treatment reproduces the trend, it largely over-estimates the enthalpies.We have employed the BOC-MP method to calculate the heats of adsorption of the surface species proposed by Somorjai.2,3 Table 2 shows the values of the gas phase bond energies (D) and the heats of atomic and molecular chemisorption (Q) calculated using the equations in the appendix of ref. 5 neglecting the destabilizing/stabilizing effects of the changes of the surface coverages of the adsorbates.6 From these results and using eqn.(1) the enthalpies were calculated and the results are presented in Table 1. In eqn. (1), P and R refer to the products and the reactant [MeCH2NH2(g)] respectively. DH=µ[S(Q+D)PµS(Q+D)R] (1) The agreement with the experimental results is very good and the deviations never exceed 0.3 eV except for the case of the acimidoyl group 4 where the BOC-MP analysis gives a highly under-estimated value. It must be emphasized that the reported enthalpy of 4 is not an experimental finding and is calculated on the basis of bond energies.On the basis of the BOC-MP analysis it seems that 4 is unlikely to be the intermediate. This intermediate has been proposed2 on the basis of the HREELS peaks at 1350 and 3300 cmµ1 assigned to the C�N·H structure adsorbed on the surface via C and N and by comparison with the corresponding organometallic compound. 2 Although this structure with the C�N ‘softened’ by adsorption may be present on the surface, it is probably not the one taking part in the reactions and desorbing as MeCN in the course of a TPD run.It seems that the interaction with the surface which relaxes the double bond should strengthen the N·H bond and shift it to higher wavenumbers, yet the reverse has been observed. Assuming structure 7, the enthalpy of adsorption was calculated (Table 1). Considering the structure 8 where the C·N bond has been somewhat strengthened, the enthalpy is around µ1.41 eV which is far better than the H�uckel value.In this structure the C·N bond order is certainly higher than that in 7 and could possess the stretching frequency around 1350 cmµ1, midway between that of C·N (ca. 1100 cmµ1) and that of C�N (ca. 1650 cmµ1). In fact, owing to the exothermicity of the surface reactions, the enthalpy of this species must lie in *To receive any correspondence (e-mail: Gobal@alborz.sharif.ac.ir). †This is a Short Paper as defined in the Instructions for Authors, Section 5.0 [see J.Chem. Research (S), 1997, Issue 1]; there is therefore no corresponding material in J. Chem. Research (M). Table 1 Enthalpies of the gas phase (g) and adsorbed (a) species Species DH (BOC-MP)/ eV DH (exp·)/ eVa DH (ext· H�uckel)/ eVa 123 +2H(a) 4+3H(a) 5+4H(a) 6+4H(a) 6+2H(a)+H2(g) 6+2H2(g) 0.00 µ0.86 µ1.12 0.66 (µ1·86) µ1.85 µ1.03 µ0.08 0.87 0.00 µ0.78 µ1.04 µ1.65b µ1.72 µ0.82 0.17 1.17 0.00 µ1.71 µ2.00 µ4.08 µ4.79 µ2.37 µ0.53 1.31 aRef. 3. bCalculated on the basis of experimental bond energies.2 Table 2 Heat of adsorption (Q) and total bond energies in the gas phase (D) and chemisorbed state (Q+D) on Ni(111) Species D/eV Q/eV Q+D/eV C NH2345678 H2(a) H2(g) ——— 35.43 27.36 25.93 25.54 25.54 25.93 23.93 4.51 4.51 7.42 5.85 2.73 0.86 3.73 0.65 0.82 — 3.17 2.73 0.30 — 7.42 5.85 2.73 36.29 31.09 26.58 26.36 25.54 29.10 28.66 4.81 4.51 Fig. 1J. CHEM. RESEARCH (S), 1997 325 the range µ1.12 to µ1.85 eV (Fig. 2). The calculated value for structure 8 is well within this range.Table 3 compares the heat of adsorptions of ethylamine and acetonitrile in both n1- and n2-coordinations where again the values obtained by the BOC-MP method are very close to the experimental values and the method is by far superior to the extended H�uckel approximation. We believe that the good agreement between the BOC-MP-based calculation and the experimental findings stems from the use of fairly accurate atomic heats of adsorption and dissociation energies used in the calculations. Received, 17th January 1997; Accepted, 12th May 1997 Paper E/7/00408G References 1 E. Shustorovich, Adv. Catal., 1990, 37, 101. 2 D. E. Gardin and G. A. Somorjai, J. Phys. Chem., 1992, 96, 9424. 3 P. D. Ditlevsen, D. E. Gardin, M. A. Van Hove and G. A. Somorjai, Langmuir, 1993, 9, 1500. 4 A. B. Anderson, R. W. Grimes and S. Y. Hong, J. Phys. Chem., 1987, 91, 4245. 5 E. Shustorovich, Surface Science, 1992, 279, 355. 6 S. Azizian and F. Gobal, Langmuir, submitted for publication. Fig. 2 Enthalpies of the surface reactions Table 3 Heat of adsorption (Q) Species Q (BOC-MP)/eV Q (exp.)/eV Q (H�uckel)/eV MeCH2NH2 2 MeCN-n2 MeCH-n1 0.86 0.82 0.95 ca. 0.8 ca. 0.9 &midd