Comparing"-t to the amount of hydroacn th:lt clesorbs follows a saturauon npo~urc: of H: in Ru(OOI ). The amount of ac:et)lene which adsorbs and dccompoaon to show (ultimately). CH , mode scos.sonna is thus auianod to an ori-CHCH, spccoes. the !osteal intermediate in the acetylene hydroplant to eth~lene. Corresponding loss features in Ru(OOl }-p(l x.2)- 0 occurs at 585 and 240 cm-1• and an additional loss feature due to the asymmetric ruthenium-oxygen stretching vibration, v.(RuO), occurs at 430 cm-1• The path mode involves the movement of oxygen aLOms mostly parallel to the surface, but not entirely thus due to the coupling of this mode with v,(RuO).
3(b)], much more type II acetylene is clearly present, as evidenced by the CH bending loss features at 740 and 960 cm-1 • EEL spectrum in Fig. It appears that virtually all of the chemisorbed acetylene is decomposed by 350 K. The 955 cm-1 loss band of Type II acetylene has completely disappeared, and the decomposition of Type I acetylene is indicated by the large decrease in intensity of the loss band near 1140 cm-1 upon annealing from 300 to 350 K. In the former case. the mechanism of Jakob et al. would produce ethylidyne primarily of the type CCH2D. while the mechanism in Hills would primarily produce ethylidyne of the CCHD2 type.
ENERGY
The heating rate in all thermal desorption spectra was approx. 8 K-s-1• The base pressure in the UHV chamber was less than 10 to Torr. Thermal desorption spectra of Hz, HD and Dz after saturation exposures of NDzCHO on Ru(OOl) (not shown) were also measured to determine the origin of the hydrogen desorbing in the four peaks in Fig. The initial desorption temperature of 315 K is consistent with the results of a previous, detailed study of ammonia adsorption on pure Ru(OOl), where it was shown that ammonia desorbed in a single peak centered at 315 K for initial ammonia coverages Jess than ca. 15% of saturation (14). The total amount of ammonia desorbing after a saturation formamide exposure is estimated to be S{).02 monolayer, based on the CO and H2 thermal desorption measurements discussed earlier.
7(a) must be of substantial character, since the highest of these four intermediate frequency modes of TI2(C.O)-ND2CO occurs at a frequency 140 cm-1 lower than it. The observed vibration frequencies and partial mode assignments for TI~C.O). The intensity of the w(!'-11;0 mode is expected to be greatest when it comes to motion of the NH2 hydrogen atoms that is largely perpendicular to the surface (25). A v(ND) loss feature is present at 2450 cm-1, and a very weak v(NH) loss is present at 3295 cm-1.
8(a) to 350 K causes the adsorbed ammonia to molecularly desorb, leaving only CO and NH in the subsequent EEL spectra (31) and allowing a more definitive characterization of the NH species. Although many of the loss features from 111(0)-NH2CHO overlap to some extent with those from 112(C,O)-NH2CO, the first-order v(CO) loss is quite strong and is clearly resolved at 1670 cm-1, the loss and v(CH) is also observed at 2880 cm-1• The frequency of v(CO) shows that the double bond of CO is preserved in molecularly adsorbed formamide. The desorption of CO (and all remaining hydrogen) is complete below 500 K, and only nitrogen adatoms remain on the surface, which are shown in the EEL spectra by v,(RuN) at 580 cm-1 • Recombinant desorption of N2 is complete at 810 K. K, leaving a clean surface of Ru(OOl) after annealing to this temperature.
This estimate uses the known coverage-intensity relationship for this CO loss function (20). The resulting estimate is that about 0.10 of the formamide monolayer decomposes via the intermediate T\~C.O)-NH2CO, while the remaining about 0.05 monolayer decomposes via Tt1(N)-NCHO. EELS below 230 K does not identify any other surface species. The only thermal desorption products detected at these low formamide exposures ~ L) are CO near 480 K, H2 near 420 K. In this case, the assignment of v(CO) is simple and makes sense because the connection to CH bending modes insignificantL [The mode identified as v(CO) is only slightly downshifted to 1160 cm-1 after deuteration of formyl.) The decay of tt2(C,O)-NH2CO is similar to Tt1-formyl on Ru(001) as carbon monoxide the only decomposition product containing oxygen or carbon.
In the decomposition of l)1(0)-NH2CHO, two stable decomposition intermediates are observed, one present from about 225 to 300 K and another from 300 to 375 K. The species that is stable from about 300 to 375 K is likely l)1(N)-NCHO, analogous to the NH species formed from . l)2(C,O)-NH2CO decomposition, but with the hydrogen atom replaced by a formyl group.
4) 7]2(N,0)- NHCHO
TEMPERATURE, K
TEMPERATURE,
2EJK
TEMPERATURE,K
A detailed study of the adsorption and decomposition of fonnamide on the pure Ru(OOl) surface will be presented elsewhere (16). The properties of the p(lx2)-0 overlay on the hexagonally close-packed Ru(OOl) surface have been discussed in detail previously (17). Annealing of the surface to temperatures between approx. 225 and 260 K results in the formation of the second form of molecular chemisorbed formamide, whose EEL spectrum is shown in Fig.
The other differences between the EEL spectra of the low and high temperature forms of molecular chemically sorbed formamide are small. The v(CO) frequencies of the two forms of molecularly chemically sorbed formamide clearly indicate that the CO double bond is preserved in each case. Thus, for both forms, surface binding occurs through single-pair donor bonds to the oxygen and/or nitrogen atoms, rather than through rehybridization of the CO bond.
The frequency of this mode is somewhat higher than the frequencies of the v.(NCO) modes in the corresponding gallium and indium complexes, although the v.(NCO) modes of a similar series of aluminum complexes are somewhat closer, ranging from 1580 to at 1621 cm-1 (12b). This mode is not resolved in the case of T\2-NDCHO due to the slight shift of the v,(NCO) mode at 1360 cm-1• The mode at 1140 cm-1 in T\2-NHCHO is lowered to 920 cm-1 in T\2-NDCHO, and is thus assigned as an NH bending mode, although without vibrational data for the T\2-NHCHO complexes it is not clear whether this is the 1t(NH) or o(NH) mode. Finally, a 11 1(0)-NH2CHO is expected to maintain a nearly planar structure (gas-phase formamide has a planar structure) and the NH2 vibrational vibration will involve movement of the NH2 hydrogen atoms out of this plane and mostly parallel to the surface .
On the other hand, 111 (N)-NH2CHO should result in a more intense NH2 rocking mode, since the NH2 group will no longer be in (or nearly in) a plane perpendicular to the surface; and this mode will involve movement of the NH2 hydrogen atoms largely perpendicular to the surface. The fact that this lower temperature desorption peak occurs at the same temperature as the conversion of. the low-temperature form of chemisorbed formamide to the high-temperature form indicates that this is a monolayer desorption state, while the proximity of the 225 K . desorption peak after the multilayer desorption peak at 210 K may indicate that it is the result of second layer desorption. We therefore (slightly) favor assigning the 225 K thermal desorption feature as a monolayer desorption state.
Structures (1), (II) and Om involve bonding through lone electron pairs of heteroatoms, while structure (IV) involves bonding through rehybridization of the CO double bond.
13L) 772( C,0)-NH 2 CHO
The electronic structure of the substrate (i.e. the metal surface) is manifested in the nature of (molecular) ligand binding and in the reactions that the ligand undergoes, just as the electronic propensities of a catalyst surface are manifested in the rates and mechanisms of its catalytic reactions. A summary of formamide reactions on the Ru(001)-p(lx2)-0 surface is outlined in Fig. When the hydrogen-presaturated surface is exposed to different flows of NH2CHO at 80 K, the following species are detected in subsequent TDM spectra: NH:zCHO, CO, H2, and N2• The exposures listed below were measured with an ionization meter and are uncorrected for the relative ionization probabilities of the different gases.
Similar to the decomposition of formamide on clean Ru(OOI) (2), the recombinative desorption of molecular nitrogen (m/e = 28 amu, with a cracking fragment at m/e = 14 amu) is observed. Note that such a small amount of T\ 1(0)-NH 2 CHO could be present in any of the EEL spectra of figure. Due to the very similar electronegativities of ruthenium (2.2) and hydrogen, the adsorption of hydrogen has only a small effect on the work function of the Ru(001) surface; a saturation hydrogen coverage at 95 K causes the work function to decrease by only 10 meV (9).
While the Fermi level of the Ru(OOI) surface is slightly raised by a saturation of hydrogen coverage (as evidenced by the slight decrease in the work function). The increase in the intensity of the loss function due to v(CO) of carbon monoxide indicates that some of the 112(C,O)-NH2CO (about 0.01 monolayer) has decomposed to CO, NH, and hydrogen atoms. The amount of formamide that decomposes (i.e., the amount invariably adsorbed after a saturation formamide exposure on hydrogen-presaturated Ru(001) is about 0.05 monolayer compared to 0.15 monolayer on pure Ru(001).
The off-spectrum EEL spectra allow the NH3 weak vibrational mode and the frustrated translation of ammonia perpendicular to the surface (i.e. metal-nitrogen stretch) of chemisorbed ammonia to be resolved near 625 and 340 cm-1. Thermal desorption measurements performed in connection with this work were obtained at a heating rate of approximately 8 K-s-1• The NH3 used in these studies was obtained from Matheson (99,990 thousand reponed purity) and purified by several cycles of freeze-pump- fusion. Since a detailed analysis of the thermal desorption of ammonia from Ru(001) has already been carried out (21), this topic was only briefly reviewed, mainly to determine the appropriate ammonia exposures and annealing temperatures to obtain the characteristics of the EEL spectrum. . of multilayer, second-layer, and (various coverages of) single-layer ammonia.
Due to the low intensity of p(NH3) and v(Ru-NH3) losses, it is impossible to say.
