7191
J . Phys. Chem. 1989, 93, 7191-7194
Structure of Phosgene on Ruthenium(001) M. A. Henderson, Y. Zhou, K. G. Lloyd, and J. M. White* Department of Chemistry, University of Texas, Austin, Texas 78712 (Received: January 17, 1989; In Final Form: May 12, 1989)
The interaction of phosgene (0CCl2)with Ru(001) was studied by high-resolution electron energy loss spectroscopy (HREELS), temperature-programmeddesorption (TPD), and Auger electron spectroscopy (Am). First layer phosgene on Ru(001) desorbs at 138 K, only 16 K above its multilayer desorption temperature. A small amount of decomposition (about 0.05 ML; ML = monolayer) to adsorbed CO and C1 occurs, presumably upon adsorption at defect sites. The bonding orientation of monolayer phosgene was determined by off-specular HREELS measurements and the dipole selection rule. Of the five probable configurationsconsidered [q’(O),ql(Cl), q2(0,Cl), q2(Cl,C1), and q2(C,0)], the qI(C1) configuration, with C, or CIsymmetry, is most compatible with the highly dipole-scattered u,(CC12) HREELS loss (845 cm-I) and the absence of any red shift of the u(C0) frequency in the monolayer compared to the multilayer. An ql(Cl) configuration is also consistent with the weak phosgene-metal interaction observed by TPD.
Introduction
TABLE I: Mass Spectral Fragmentation Pattern of Phosgene
High-resolution electron energy loss spectroscopy (HREELS) is very useful in determining adsorbate structure on ordered surfaces. The growing popularity of this technique is evidenced by the large increase in such studies in recent years.I However, the determination of adsorbate bonding geometries by off-specular analysis is sometimes overlooked in HREELS work. According to the dipole selection rule, only those vibrations that are totally symmetric with respect to the adsorbate’s point group (i.e., vibrations that retain the symmetry of the adsorbed species) are “dipole allowed”.2 Electrons inelastically scattered by this mechanism are highly directed along the specular angle. Offspecular analysis can differentiate these dipole-scattered losses from other loss processes. Additionally, the total number of dipole-scattered losses decreases as the symmetry of the adsorbed species increases. By counting dipole-scattered-loss peaks, an estimation of the maximum degree of symmetry is realized. In this study, we show that simple off-specular measurements significantly aid in determining the geometry of phosgene (OCCl2) adsorbed on Ru(001). Phosgene is a good candidate molecule for HREELS study because of its high symmetry (C2&, its low number of normal modes (6), the expected strong dipoles of these modes and, as it turns out, its low reactivity toward decomposition on Ru(001). Additionally, phosgene has the potential of bonding to a substrate in several different configurations: (1) through the oxygen and/or (2) through chlorine lone pair electrons, (3) ?r bonded through the carbonyl or (4) rehybridization of the carbonyl to a di-a configuration. The chemistry of phosgene on metal surfaces is also of importance in corrosion, environmental, and health sciences. Experimental Section
The ultrahigh-vacuum chamber used in this study and the methods of data collection have been detailed e l ~ e w h e r e . ~The system is ion-pumped with a working base pressure of 1 X 1O-Io Torr. The HREEL spectrometer is a 127O cylindrical sector type with stationary monochromator and analyzer defining a total scattering angle of 120°. Off-specular analysis was performed by rotating the crystal about an axis that passes through its surface. In this manner, the total scattering angle remained at 120°, but the incident and reflected beam angles changed. All spectra were taken with a primary beam energy of 6.8 f 0.3 eV and resolution of IO-meV full width at half-maximum. Temperature-programmed ramp rates were 6.2 K/s. The mounting, cleaning, heating, and cooling of the Ru(001) are also detailed e l ~ e w h e r e . ~ (1) (a) Thiry, P. A. J . Electron Spectrosc. Relat. Phenom. 1983, 30, 261.
(b) Thiry, P. A. J . Electron Spectrosc. Relat. Phenom. 1986, 39, 213.
(2) Ibach, H.; Mills, D. L. Electron Energy Loss Speczroscopy and Surface Vibrations; Academic Press: New York, 1982. (3) Mitchell, G. E.; Radloff, P. L.;Greenlief, C. M.; Henderson, M. A.; White, J. M. Surf. Sci. 1987, 183, 403.
0022-3654/89/2093-7191.$0l.50/0
amu 12 28 35 36 37 44 47 49 63 65
70 82 98
ion C+
lit.“
*
re1 ion intens this work 0.05
co+
*
35c1+
0.37
H’5C1t
0.03
37c1+
0.12
0.00 0.1 1
c02+
0.40
*
0.62
C’7CI+ OC’5CI+ OC’7CI+
0.02
0.02
0.32 0.04
0.31
C’5CI2+ 0C’5Cl2+
0.01
0.01 0.02
c35c1+
35c12+
1.oo
0.03
0.29 0.33
0.05
1.oo
0.03
“Reference 5. Relative ion intensities that were not reported are indicated by an asterisk (*). Phosgene, as received, was contaminated with significant amounts of Ar, He, COz, and CO, presumably as carrier gases and side products in its synthesis. He and CO were easily removed by freeze-pumpthaw cycles in liquid nitrogen. The majority of the Ar and COz were removed with use of a pentane slush (140 K). Figure 1 shows the mass spectrum of 2 X Torr 0CC12 after subtraction of the background spectrum. The dominant phosgene ions are at m / e = 98, 82, 70, 63, 47, 35, 28, and 12. (Only chlorine ions pertaining to the m / e = 35 isotope are labeled in Figure 1.) Table I compares the relative ion intensities of Figure 1 with those obtained from the literature.s There is excellent agreement for each ion with the exception of the m / e = 40 and 44 ions, indicating that significant amounts of Ar and COz are still present in the OCCl2 sample. These impurities did not pose a threat since they are not adsorbed on Ru(001) at 105 K, particularly in competition with large amounts of 0CCl2. The low m / e = 70 signal and the absence of a m / e = 36 signal indicate that C12 and HC1 were not present in the phosgene sample. The H2 ( m / e = 2) signal results mainly from displacement reactions in the mass spectrometer.6 The crystal was held a t 105 K for all doses. Exposures were made through a directional doser consisting of a 0.5-cm i.d. stainless steel tube connected by a leak valve to the gas handling system. All phosgene exposures were done with the sample 1 cm in front of the doser. The phosgene flux and dosing time were sufficient to give multilayer coverages on Ru(O0 1). (4) Henderson, M. A.; Radloff, P. L.; White, J. M.; Mims, C. A. J. Phys. Chem. 1988, 92, 4111. ( 5 ) Atlas of Mass Spectral Data; Stenhagen, E., Abrahamsson, S., McLafferty, F. W., Eds.; Interscience: New York, 1969; Vol. 1. (6) Mitchell, G. E.; Henderson, M. A,; White, J. M. J. Phys. Chem. 1987, 92, 3808.
0 1989 American Chemical Society
The Journal of Physical Chemistry, Vol. 93, No. 20, 1989
7192
I
Henderson et al. MULTILAYER OCC12/RU (001)
I290
occ12 RGA 63
*
I
I
I
*I
40
-
-? 4
* IC(
v1 W I-
z z
Y
50
0
100
AMU
Figure 1. Mass spectrum of 2 X lo4 Torr of phosgene after subtraction of the background spectrum. An asterisk (*) indicates ions not due to
phosgene fragmentation. 2680 MULTILAYER OCC12/RU (0011
122
I(
0
1000 2000 3000 ELECTRON ENERGY LOSS (cm-1)
Figure 4. HREEL spectra of (a) multilayer phosgene on Ru(001) at 105 K followed by heating to (b) 120 and (c) 150 K. 0
MONOLAYER OCClp/Ru (001)
1000
2000
285 Hp (de-21
x5
OCC12 Me-63) xi
a
100
200
300
400
-
500
TEMPERATURE (KI
?
Figure 2. Phosgene ( m / e = 63) (a), CO ( m / e = 28) (b), and H2 ( m / e = 2) (c) TPD spectra from a multilayer exposure of phosgene on Ru(001) at 105 K.
4
I
> IC(
9 W I-
z
C(
0
I I
0
I
3000
ELECTRON ENERGY LOSS (cm-1)
Figure 5. Specular (a) and off-specular (b) HREEL spectra of monolayer saturation phosgene obtained by heating a multilayer exposure to xi0
I
180
c1
I 0
100
200
300
400
500
KINETIC ENERGY lev)
Figure 3. AES spectra of before (a) and after (b) the TPD spectra of Figure 2.
Results Figure 2 shows TPD spectra from a multilayer exposure of phosgene on Ru(001) at 105 K. The dominant signals are from phosgene ( m / e = 63) at 122 (multilayer) and 138 K (first layer). Small amounts of H2 (285 K) and CO (427 K) were also detected with surface coverages of about 0.01 and 0.05 monolayer (ML;
120 K.
1 ML = 1 species/surface Ru atom), respectively. No other desorption products were detected. C1 was observed by AES after the TPD (Figure 3). Figure 4 shows the HREELS annealing set from a multilayer phosgene exposure at 105 K. Isotopic shifts due to the two stable C1 isotopes ( m / e = 35 and 37) are on the order of 2 to 8 ~ m - l , ~ well below the resolving capability of the HREEL spectrometer. The prominant losses from multilayer phosgene (105 K) are at 1805, 875, and 590 cm-I, with weaker features at 2680, 1750-1700, 1440, and 300 cm-I. After desorption of the multilayer (7) Carpenter, J. H.; 1978, 74, 466.
Rimmer, D. F. J. Chem. SOC.,Faraday
Trans. 2
The Journal of Physical Chemistry, Vol. 93, No. 20, 1989 7193
Structure of Phosgene on Ru(001) TABLE 11: Vibrational Frequencies of Phosgene HREELS~ mode assimmt
IR' solid Ar. 20 K
multilayer, 105 K
monolayer, 120 K
U(C0)C U.(CCl,) ;(Co)-' VS(CC12) N O ) WCI,) ~(Ru-CI)
1803d 835 582 568 438 284
1805 815 590 nr 590 no 300
1805 845 580 nr 580 nr nr nr
-
(4)
g
"Reference 10. bThis work. cNotation: a, asymmetric; s, symmetric; Y, stretch; 6, deformation; T , in-plane bend; y, out-of-plane bend; nr, not resolved; no, not observed. dAll frequencies in cm-I.
(120 K), HREELS losses of the monolayer are detected at 1970, 1805, 1705, 845, 580,425, and 290 cm-I. Desorption of the first layer phosgene (1 50 K) leaves HREELS losses due to adsorbed C1 (290 cm-I) and atop C O (2000 and 425 cm-I). Off-specular analysis was used in determining the orientation of monolayer phosgene. Figure 5 shows the specular and offspecular (by about 20°) spectra of monolayer phosgene on Ru(001). Both spectra are normalized to the same total collection time (dwell time per channel times number of scans), enabling an accurate comparison of specular and off-specular loss intensities. Losses retaining the same intensity per unit collection time are not dipole scattered. The off-specular elastic peak was about 97% of its specular intensity. From a comparison of the X250 traces, all of the loss intensities, with the possible exception of the weak 1705-cm-' shoulder, decrease significantly in the off-specular data. The 1805- and 845-cm-' losses exhibited the most significant intensity decreases (83% and 81%, respectively), and the 580-cm-' loss decreased the least (63%).
Discussion The TPD spectrum of Figure 2 indicates that first-layer phosgene is very weakly bonded to Ru(OO1). The desorption temperature is only 16 K higher than that of the multilayer. Using the first-order Redhead approximation* with a preexponential factor of 10" s-', we estimate a desorption activation energy of 8.1 kcal/mol for the monolayer state. The small amount of decomposition is evidenced by C1 in AES (Figure 3) and CO in TPD (Figure 2). The amount of C O left after molecular phosgene desorption is about 0.05 ML based on TPD of saturation CO. This indicates that only about 0.05 M L of phosgene decomposes on the surface during adsorption. This low level of decomposition suggests that defect sites may be involved. A rough estimate of the C1 coverage left after phosgene desorption is based on the AES results of Gudde and Lambert for Clzon Ru(100)? They estimated a saturation C1 atom coverage of 0.5 M L based on a c(2 X 4) LEED structure. This coverage of CI gave a C1(180)/Ru(273) AES ratio of about 1.7. Although our crystal has a different surface lattice plane, we estimate on the basis of the C1(180)/Ru(273) AES ratio of Figure 3 (0.66) that about 0.2 ML of CI is left from phosgene decomposition. This estimate is probably high because the more open Ru( 100) face should give a lower C1(180)/Ru(273) AES ratio for 0.5 M L C1 than the close-packed Ru(001) face. Table I1 compares the HREELS frequencies of multilayer and monolayer phosgene on Ru(001) with the IR frequencies of solid phosgene in an Ar matrix.I0 The multilayer HREELS and solid IR frequencies agree fairly well with the exception of the 875-cm-' v,(CC12) HREELS loss, which is 40 cm-I higher than the IR. We have no explanation for the difference in this one band; we can only note that the Ar matrix will isolate phosgene molecules from each other. HREELS is not able to resolve the v,(CC12) and y ( C 0 ) features. In the solid multilayer phase, we expect the dipole of the v,(CClZ)mode to be much greater than the r ( C 0 ) mode?," (8) Redhead, P. A. Vacuum 1962, 12, 203. (9) Gudde, N. J.; Lambert, R. M. Sur(. Sci. 1983, 134, 703. (10) Jones, D. E. H.; Wood, J. L. J. Chem. SOC.A 1967, 1140.
n2(C90) cs Figure 6. Five probable bonding configurations and their bonding geometries for monolayer phosgene on Ru(001).
Thus, on the basis of intensity considerations and geometric factors (see text below), we assign the 590-cm-' loss of multilayer phosgene primarily to the vS(CCl2)mode, even though it is farther in frequency from the IR value than the y ( C 0 ) mode. Additionally, the r ( C 0 ) loss was not observed, presumably due to its weak dipole intensity. Several losses in Figure 4a, not shown in Table I, are due to multiple scattering events. The 2680-cm-' loss is assigned to the double-loss process of v(C0) ua(CClz) (1805 875 cm-I). Although the 1750-1700-cm-' loss is not resolved, it is probably due to the double-loss 2v,(CCIz) (875 875 cm-I). The 1440-cm-' loss is about 25 cm-' lower in frequency than expected for the double loss from the 875 590 cm-' combination. This feature is not an overtone of the v,(CC12) v,(CC12) modes, which is at 1390 cm-l for solid phosgene,12since the overtone's symmetry is bl. Therefore, it must be a double-loss event despite the 25-cm-' mismatch. Having assigned the multilayer peaks, we turn to the monolayer spectrum. Assignment of the first-layer phosgene HREELS losses (Figure 4b) is not entirely straightforward. The 1805- and 845-cm-' losses are easily assigned to the v(C0) and va(CCI2) modes, respectively, but, the remaining assignments are ambiguous. Again, because of the close proximity of the v,(CCIz) and y ( C 0 ) modes, the 580-cm-' loss of Figure 4b cannot be resolved into two loss features. The argument used above in assigning the 590-cm-' loss in the multilayer spectrum (relative dipole intensity) will depend, in the adsorbed monolayer, upon both the strength of the dipole and the adsorption geometry. For example, the y ( C 0 ) loss should be very intense if phosgene is bonded as $(C,O) (see text below) but much less intense if bonded as oz(CI,C1); the reverse applies for the v,(CC12) loss. Assignment of the 580-cm-' loss is discussed below after working out the phosgene bonding geometry. Another complication arises from the similarity in the expected frequency of the n ( C 0 ) mode and the v(RuC) loss of atop CO.I3 Because the 425-cm-' loss is present after phosgene desorption, we believe it is due to v(RuC) loss of atop C O and not the r ( C 0 ) mode of phosgene. A third problem exists in distinguishing the v(RuC1) mode of adsorbed phosgene with that of adsorbed CI from phosgene decomposition. Because the 290-cm-' loss in Figure 4b also remains after phosgene desorption, it is probably the v(RuC1) of adsorbed C1. Although these factors complicate the assignment of all the losses, a great deal can be said about the bonding geometry of monolayer phosgene based solely on the v(C0) and va(CC12) losses.
+
+
+
+
+
(11) Nikolova, B.; Galabov, B.; Lozanova, C. J. Chem. Phys. 1983, 78, 4828. (12) Catalano, E.; Pitzer, K. S.J . Am. Chem. SOC.1958, 80, 1054. (13) Thomas, G. E.; Weinberg, W. H. J . Chem. Phys. 1979, 70, 1437.
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The Journal of Physical Chemistry, Vol. 93, No. 20, 1989
Henderson et al.
TABLE III: Experted and Observed Dominant HREELS Scattering Mechanisms' exnected
U,(CC12) a(c0) VACC12) *(CO) WC12) u( Ru-CI) ~(Ru-0) a Notation:
b2 a1 b, a1
n n d n d
d d d
d
d
d
n
d n d d d d
d d d d d d
d d n d d d d d
d d d d d d d d
d n
n d n d d
d n d d n d d
d n d d n d d
d, dipole scattered; n, nondipole scattered.
Consider the 5 mast probable bonding configurations of phosgene on a metal surface (Figure 6 and Table HI),each expressed as C, C,,and/or C1symmetries. The q ' ( 0 ) and $(C,O) bonding configurations are common for adsorbed carbonyl^.'^-^^ ~ ' ( 0 ) bonding involves 0 lone pair electron donation to the surface with , ( C 4 normal to the surface) the molecule oriented in either C or C, symmetry (C=O tilted toward the surface in the plane of the molecule). Tilting the molecular plane off the surface normal also results in C, symmetry but is excluded on the basis of poor overlap between the 0 lone pair(s) and surface sites. The $(C,O) configuration involves either coordination of both the C and 0 atoms by rehybridization of the C=O bond (di-a bonding) or a direct A interaction between the C=O and Ru. Both have C, symmetry regardless of whether the C1 atoms also interact with the surface. The remaining configurations involve C1-metal interactions. The tlZ(O,C1)configuration involves lone pair electron interaction with the surface from both the 0 atom and a C1 atom. This configuration has either C,(with the molecule's plane normal to the surface) or C1symmetry (with the plane tilted). The q'(C1) configuration involves interaction of only one C1 atom with the surface and possesses either C, (with the plane normal) or CI symmetry (with the plane tilted). Finally, the $(Cl,Cl) configuration involves coordination through both C1 atoms possessing , (with the plane normal to the surface) or C,symmetry either C (plane tilted). Table I11 shows the expected scattering mechanisms for each loss in the geometries of the five configurations. Only those vibrations that are totally symmetric with respect to their point group (Le., they retain the symmetry of the molecule's bonding configuration during their vibration), can be dipole allowed. Because the v(CO), v,(CCI2), and 8(CCl2) modes have a, symmetry in the gas phase, they are dipole allowed for adsorbed phosgene regardless of its bonding geometry. In Figure 5, the v(C0) (1805 cm-I) and va(CCl2) (845 cm-l) losses are highly dipole scattered. Although the v(C0) loss is dipole allowed in all nine configuration geometries, the va(CC1,) loss is only dipole allowed in five of the geometries [$(O) C,,$(Cl) C, and C,,and $(O,C1) C, and Cl], thereby excluding the other four as possibilities. Further structural information about adsorbed phosgene is obtained from the monolayer and multilayer v(C0) frequencies. Carbonyl species (such as (CH3)2C0,14.16q17 CH3CH0,'8*'9and H2COIS),coordinated to metal surfaces as vl(0), exhibit red shifts (14) (a) Avery, N. R. Surf. Sci. 1983, 125, 771. (b) Avery, N. R. Langmuir 1985, I , 162. (c) Avery, N. R. J. Vac. Sci. Technol., A 1985, 3, 1459. (15) (a) Anton, A. B.; Parmeter, J. E.; Weinberg, W.H. J . Am. Chem. Soc. 1985, 107, 5558. (b) Anton, A. B.; Parmeter, J. E.; Weinberg, W. H. J. Am. Chem. Soc. 1986, 108, 1823. (16) Avery, N. R.; Anton, A. B.; Toby, B. H.; Weinberg, W. H. J. Electron Specirosc. Relat. Phenom. 1983, 29, 233. (1 7) Avery, N. R.; Weinberg, W. H.; Anton, A. B.; Toby, B. H. Phys. Rev. Lett. 1983, 51, 682. (18) McCabe, R. W.; DiMaggio, C. L.; Madix, R. J. J. Phys. Chem. 1985, 89, 854. (19) Henderson, M. A,; Zhou, Y.;White, J. M.; J . Am. Chem. SOC.,in press.
in their v(C0) loss frequencies of greater than 40 cm-I. Similarly, a 100-cm-l red shift in the IR v(C0) frequency has been observed for an A1C13-phosgene complex.I0 Since the v(C0) lass frequency of monolayer phosgene does not red shift, coordination through the 0 lone pair electrons seems unlikely. The 30-cm-' shift in the v,(CCl,) loss from multilayer to monolayer however supports a weak interaction of the molecule with the surface through one or both C1 atoms. Since the qz(CI,Cl) configuration has been excluded by symmetry, ql(Cl) (C,and C,)is the favored bonding configuration of phosgene on Ru(001). The C,and Cl symmetries of this configuration differ with respect to the scattering mechanism of the y(C0) loss. In Cl symmetry, the y(C0) loss is dipole allowed and in C, it is not. Although the 580-cm-l loss is significantly dipole scattered (Figure 5), we must assign its dominant contributor to the vS(CCl2)mode since the y ( C 0 ) vibration is not expected to have as strong a dipole moment normal to the surface in an s'(C1) configuration. Further arguments then cannot distinguish between these two symmetries. The 1705-cm-' shoulder in Figure 4b is not due to a doubleloss process, as in Figure 4a, because the intensity of a double-loss compared to its single-loss(es) intensity matches the intensity ratio between the single loss(es) and elastic peak. The v,(CC12) loss intensity (Figure 4b) is less than l/mth of the elastic peak intensity so a va(CCI2)double-loss feature should not be detectable. The 1705-cm-' loss is either due to a small amount of C O in 3-fold hollow sites or possibly a small amount of phosgene coordinated through the 0 lone pair electrons as ~'(0) or $(O,CI). Bonding of phosgene through C1 lone pair electrons is consistent with the very low thermal desorption temperature observed in Figure 2. The desorption energies of another chlorohydrocarbon, methyl chloride, from Ru(OO1)Mand Pt(l1 l),' are also below 150 K. The chemistry of phosgene on Ru(001) is quite different from that of acetyl chloride [CH3C(0)Cl] for which significant amounts of decomposition occur.zz Substitution of a methyl group for a C1 atom thus has a profound effect on the structure and stability of a ClC(0)-X species on Ru(001). Summary 1. Upon adsorption of phosgene on Ru(001) at 105 K, no more than 0.05 ML dissociates; the remainder desorbs molecularly in two TPD states-multilayer a t 122 K and monolayer a t 138 K. 2. Specular and off-specular HREELS analysis indicates that the most probable bonding configuration is ~ l ( C 1 )(C,or CI symmetry). This conclusion is based on the strong dipole character of the v,(CCl,) loss and the absence of a red shift of the v(C0) frequency of the monolayer compared to the multilayer value. Acknowledgment. This work was supported in part by the US. Army Research Office. Registry No. OCCl,, 75-44-5; Ru, 7440-18-8. (20) Zhou, Y.;White, J. M., to be published. (21) Henderson, M. A,; Mitchell, G. E.; White, J. M. Surf. Sci. 1987, 184, L325. (22) Henderson, M. A.; Zhou, Y.;White, J. M., to be published.
