094
The Journal of Physical Chemistry, Vol. 82, No. 8, 1978
(13) R. M. Friedman, to be submitted for publication. (14) V. H. J. De Beer, T. H. M. Van Sint Fiet, J. F. Engelen, A. C. Van Haandel, M. W. J. Wolfs, C. H. Amgerg, and G. C. A. Schuit, J. Catal., 27, 357 (1972).
T. N. Rhodin, C. F. Brucker, and A. B. Anderson
(15) P. Canesson, 8. Delmon, G. Delvaux, P. Grange, and J. M. Zabala, 6th International Congress on Catalysis, London, 1976, Communication B 32. (16) J. M. Zabala, Ph.D. Thesis, Louvaln-la-Neuve, 1976, Belgium.
Structure and Bonding of Acetylene and Ethylene on a-Iron Surfaces at Low Temperaturest Thor N. Rhodin," Charles F. Brucker, School of Applied and Engineering Physics, Corneii University, Ithaca, New York 14853
and Alfred B. Anderson$ Chemistry Department, Yale Unlversity, New Haven, Connectlcut 06520 (Received June 24, 1977; Revised Manuscript Received January 5, 1978) Publication costs assisted by Corneii University
A combined UV photoemission and molecular orbital analysis is presented of chemisorption and reaction of acetylene and ethylene at low temperatures on a-Fe(100) iron surfaces. Both acetylene and ethylene appear to be dissociatively chemisorbed on Fe(100) at 98 K into CH, CHz,and other fragmented species. It is significant that the spectrum for fragmented ethylene is reproduced for hydrogen chemisorbed on a carbon-covered surface. It is proposed that a thin weakly chemisorbed molecular layer forms on top of the fragment-covered,more strongly chemisorbed surface. At higher temperatures, a gradual conversion of the molecular layer to the fragmented state occurs, involving the migration of loosely bound molecules to reactive surface sites and subsequent dissociative interaction. Reaction mechanisms are discussed to account for the facility of these reactions.
I. Introduction A combination of ultrahigh vacuum, low energy electron diffraction (LEED), and ultraviolet photoemission spectroscopy (UPS) has provided a unique combination of data for elucidating structures and bonding properties of chemisorbed and reacted species on iron metal surfaces. Effort is made in this paper to make a significant analysis based on quantum mechanical interpretations and bond energies and configurations. The reactive properties of iron toward acetylene, ethylene, and other small hydrocarbon molecules have A detailed been previously measured and de~cribed.l-~ molecular orbital analysis is made here to the interpretation of the chemisorption and reaction of acetylene and ethylene on iron in the 98-123-K temperature range. It is concluded that acetylene dissociatively chemisorbs primarily as CH fragments on an a-iron (100) surface and that ethylene dissociatively chemisorbes primarily as CH2 fragments. The role of other fragmented species is also discussed. 11. Experimental Section The experimental system and characterization of the clean Fe(100) surface and the corresponding UPS and LEED/MS results have been fully described previously.2t4 A four-grid LEED/AES spectrometer' modified for ups experiments by the addition of a
t Supported by the Materials Science Center of Cornell University, the American Iron and Steel Institute, and the National Science Foundation. t ~ e p a r t m e n tof Chemistry, University of California-santa Barbara. 0022-3654/78/2082-0894$01 .OO/O
cold-cathode helium discharge light source giving monochromatic radiation at 21.2 eV, was used to perform the energy analysis. A computerized data acquisition and storage system with graphic display (DEC GT-40) greatly facilitated the accumulation of UPS spectra. Precise subtraction of spectra by this method made it possible to descriminate small but significant features in the spectral data, Difficulties in cleaning iron single crystal surfaces are due not only to the high chemical reactivity of iron but also because the lattice of the allotropic a form is stable only below 910 "C. An elaborate cleaning procedure was required to reduce the levels of the chief impurities (phosphorus, sulfur, carbon, nitrogen, and oxygen) to less than -3% of a monolayer total impurity c o n c e n t r a t i ~ n . ~ ~ ~ In the final stages of cleaning, involving the removal of residual carbon and oxygen by thermally activated surface titration, surface composition was monitored using UPS. Undesirable AES beam effects were thus avoided. In comparison with AES compositional analysis, UPS analysis can be more sensitive to trace oxygen impurity on Fe(100).2
111. Theoretical Procedure The theory used in this paper is an approximate but well-tested approach which combines atomic charge density and molecular orbital theoryS6 Rigid atoms are superimposed and the electrostatic two-body repulsion energies are using the H ~ ~ force ~ formula. The atomic electron charge densities are allowed to relax, hi^ is accomplished by using molecular orbitals and diagonilizing the approximate molecular Hamiltonian which is a sum of atomic Fock potentials. It is then possible to use the new charge density to calculate the 0 1978 American Chemical Society
~
Chemisorbed and Reacted Species on a-Iron
The Journal of Physical Chemistry, Vol. 82, No. 8, 1978 895
TABLE I: Parameters b e d in the Calculationsa before Orbital Energy Shifts Discussed in the Text Principal quantum no., Slater exponent, and atomic orbital energy (eV) Atom S P d H 1 1.200 - 13.60 C 2 1.658 - 20.00 2 1.668 - 11.26 Fe 4 1.700 - 7.87 4 1.400 - 3.87 3 5.35b - 9.00 a Orbital exponents for H and C are based on E. Clementi and D.L. Raimondi, J Chem. Phys., 38, 2686 (1963). Exponents for Fe are based on J. W. Richardson, W. C. Nieuwpoort, R. R. Powell, and W. F . Edgell, J. Chem. Phys., 36, 7 9 6 This Slater func(1963). Orbital energies are based on ionization energies from W. Lotz, J. Opt. SOC.Am., 206 (1970). tion has the coefficient 0.5366 and the second one has the coefficient 0.6678 and an exponent 1.80. TABLE 11: Calculated and Experimental Bond Lengths, R e , Harmonic Force Constants, k , , and Dissociation Energies, D e , for Diatomic and Adsorbate Molecules' Moleculeb
State
c*
1x2
HZ
'Eg+
CH Fez FeH FeC Moleculee Acetylene Ethylene
zir
['E [
["a]
4
I" 1~
Re, a 1.26(1.24) 0.74(0.74) 1.16(1.12) 1.31, 1.30(-) 1.63(-) 1.78(-)
he, mdyn/A 16.3(12.17) 7.8(5.71) 5.1(4.48) 2.4, 2.5(-) 1.9(-) 4.4(-)
D e , kcal/mol 204(142) 222(110) 74(80.0) 82, 36(24 w - ) 42(- 1
*
Rcc, a
Rcn, a
LCCH, deg
1.30(1.20) 1.48(1.34)
1.14(1.06) 1.17(1.09)
180(180) 120(117.3)
5)c
a Proposed states are in square brackets. Experimental values in parentheses from B. Rosen, "Spectroscopic Data Relative to Diatomic Molecules", Pergamon, Oxford, 1970. u g z a u 46 g 4 u g 1 S u 2 u u 1 i r g z .On dissociation one electron occupies the Fe 4s orbital, as the high-lying suu orbital is empty. The second set of calculated properties corresponds to using an approximate free atom 3d valence ionization energy for Fe as discussed in ref 12. The bulk value is used in this chemisorption study. Experimental values from ref 14. e Adsorbate molecules.
attractive energy component via the Hellmann-Feynman formula. It is easier in most cases to use the one electron orbital energies. The derivation of the orbital part of this procedure provides validation for the extended Huckel procedure, which does itself not recognize the two-body repulsion energies and must be wed with model bond lengths. The molecular orbital wave functions employ atomic orbitals which are variationally determined or fitted numerically to highly accurate self-consistent-field atomic orbitals. The energy matrix employs the ionization energy of the corresponding atomic orbital for the diagonal elements. For the off-diagonal elements their average is multiplied by 2.253 exp (-0.13R), where S is the atomic overlap integral and R is the internuclear distance. Parameters used in this paper are summarized in Table I and calculated molecular properties in Table 11. The usefullness of this procedure has been demonstrated in a variety of studies dealing with bonding interactions pertinent to this paper. These include titanium, chromium, iron, and nickel diatomic and cluster molecules,6 catalytic interactions of small hydrocarbons with iron surfaces,l adsorption of acetylene and ethylene on nickel(ll1) ~ u r f a c e s chalcogen ,~ atoms on Ni surfaces,8 and bimetallic cobalt and iron carbonyl acetylene complexe~.~ Advancement is made in the application of the theoretical procedure. Previously, the Slater orbital exponents used in the iron cluster study were the same as the literature values except that the 4s exponent was raised by 0.2. On assuming an atomic d6s2atomic density and a low spin approximation, agreement with diatomic and cluster binding energies was found. The diatomic bond length was 2.4 A, which is between the sum of the atomic and covalent radii. Iron-iron bond lengths in cluster compounds are frequently in the 2.5-2.6-A range.1° However, for Fez(C0)6C2(t-Bu)2the length is 2.316 A which corresponds approximately to a double bond.ll The diatomic iron molecule, if it is high spin, has a double bond and should have about the same bond length. This is achieved by assigning the atomic charge density as d7s1,which cor-
responds more closely to the actual situation almost up to the dissociation limit. This weakens the two-body repulsion as the s orbital is diffuse lending to increasing the 4s exponent by 0.3 as previously discusseda6This results in high-spin bond length of 2.31 A, corresponding to a double bond. A recent theoretical analysis has shown the importance of atomic ionization energy shifts to reduce excess charge transfer, and spin unpairing to the determination of the correct structure for oxygen on Fe(100).13 Spin unpairing, necessary for the correct description of Fez, also improves homonuclear Fe cluster structures and charge separations and is reflected in the bulk paramagnetic moment of iron. Consequently in this paper C and H valence ionization energies are decreased by 1eV and for Fe they are raised by 1 eV, reducing charge transfer to the hydrocarbon adsorbates. Each d-band energy level in the calculations is assigned at least one electron. The resulting adsorbate charges are similar to those rep0rted.l Despite the advancement in definition of the transition metal parameterization, the conclusions in this paper are similar to those previously r e p ~ r t e d . l - ~A qualitative change is in the heat of the adsorption energy for methane which is now decreased to 10 kcal/mol. This improvement comes from increasing the 4p orbital exponent and thus reducing the hydrocarbon u framework interaction with the iron surface.
IV. Chemisorption and Reaction of Acetylene Figure 1shows the measured photoemission spectra for chemisorbed acetylene as a function of coverage, time, and temperature. Peaks A, C, and D are characteristic of weakly chemisorbed molecular a ~ e t y l e n ewhile , ~ ~ ~peak B has been associated with chemisorbed CH fragmenh2p3 It is seen that at 98 K, weakly chemisorbed acetylene molecules appear to be evaporating or converting to the fragmented state. This temperature is lower than the 190 K reported for the sublimation of pure acetylene13a t 1 Torr pressure. Under vacuum conditions, sublimation will proceed a t a lower temperature and will be kinetically
898
The Journal of Physical Chemistry, Vol. 82, No. 8, 1978
T. N. Rhodin, C. F. Brucker, and A. B. Anderson
I ' " " " ' " " ' ~ " ' ACETYLENE (C,H,)
ON Fe(100)
@@00 \-
9.\
20
\
I
up\
,
' 7
\
I
Molecular Acetylene
15 10 Ionization Energy (eV)
5
Figure 3. Calculated energy levels for chemisorbed molecular acetylene (top) and CH acetylene fragments (bottom). Shaded regions in the orbital schematics correspond to positive orbital amplitude, open regions to negative orbital amplitude.
t
I
I
16
I
I
14
I
I
I ,
12
E, ELECTRON
I
4
10
I
8
I
I
6
I
I 4
I
I
2
I
I
E,:O
BINDING ENERGY, eV
Flgure 1. UPS spectra (hv = 21.2 eV) for acetylene adsorbed on a clean Fe(100) surface as a function of time and temperature: (1) sequential exposure (1L = IO-' Torr s) of acetylene onto the clean surface at 98 K; (2) adsorption as in l(c) followed by 30 min wait at 98 K with UV-light source valved off; (3) (a) after warming the surface in (2) to 123 K or (b) direct adsorption (6 L) onto the clean surface at 123 K; and (4)after warming the surface in (3a) to (a) 223 K and (b) 773 K.
Figure 2. Five-atom iron cluster used In the calculations. The broken circle indicates an atom beneath the surface. (a-Iron is bccub with a nearest neighbor spacing of 2.48 A,)
controlled. A model has been proposed1v2whereby molecules incident on clean surface sites are dissociatively chemisorbed, thus deactivating the surface to a large degree. It is postulated that a weakly bound molecular layer forms on top of the fragmented layer. The gradual conversion process (Figure 1,curves ICand 2) is attributed either to the migration of molecules to reactive surface sites and subsequent dissociative interaction, or simply to sublimation. The high spin calculations indicate spontaneous dissociation of acetylene over a fourfold hold site into CH fragments which reside in bridging positions 1.2 8, from the surface of the five-atom cluster (Figure 2). The total adsorption energy is calculated to be 95 kcal/mol. This energy is approximate and will decrease with a corresponding decrease in the C and H ionization energies. It requires 26 kcal/mol to place CH fragments parallel to the surface in a bridging position 1.0 A from the surface. It
is postulated that when a activation energy barrier of 5 kcal/mol is reached the CH bond stretches by 0.35 A and dissociates. The overall barrier to dehydrogenation is therefore 31 kcal/mol. The overall increase in stability is about 4 kcal/mol, with the H in one coordinate and the C in bridging positions 1.6 and 1.2 A, respectively, from the surface. The reaction energies for adsorbates already on the surface are viewed as relatively accurate because any errors due to approximate handling of charge transfer occur largely during the initial adsorption operation. Calculated energy levels for chemisorbed, dissociated, and molecularly adsorbed acetylene are shown in Figure 3. All the features of the spectra but one are accounted for in terms of the B,, FeCH and H FeCH bonding levels. The unaccounted feature is the weak, but definite peak located at 9.3 eV below Ep A similar peak is also seen in the spectra of acetylene chemisorbed on Ni( 111),15 W(110),l6Fe(film),17Cu (film),17and Ir(100)18which has been interpreted as a cr-acetylene orbital, due to acetylene chemisorbed in an undissociated state. Such an interpretation in the case of iron appears untenable,2 mainly since it would imply an excessively large "relaxation" shift. Calculations of acetylene on nickel7 and copperlg suggest that barriers to dissociation into CH fragments are substantial, but less than for the free molecules. Based on the interpretation of the chemisorbed acetylene spectrum as being due to CH fragments, what is the cause of the weak, lowest lying peak? Multielectron satellites have been observed in iron oxides lying beneath the 2p energy levelse4Whether a similar situation occurs for chemisorbed CH fragments is a possibility. As the CH-covered surface is warmed to 223 K, significant changes occur in the photoemission spectrum, which can be associated with CH bond cleavage and the corresponding formation of hydrocarbon species which flash off a t high temperature (see Figure 1). It is significant that only chemisorbed carbon atoms remain on the surface at 773 K. Additional evidence for strong interaction with acetylene, leading to carbon bond scission, lies in an examination of ethylene reactions with the Fe surface. These interactions lead to the formation of methylene (CH,) fragments, the topic of the next section.
V. Chemisorption and Reaction of Ethylene Photoemission spectra, shown in Figure 4,indicate that molecularly adsorbed ethylene, stable at 98 K, has converted at 123 K to a chemisorbed surface covered with a
The Journal of Physical Chemistry, Vol. 82, No. 8, 1978 897
Chemisorbed and Reacted Species on a-Iron
I ' " " " " " " l ' i ' ~
1
Molecular Ethylene
-
J
c/-c
[-cH,
25
L.o
( 0 ) OXIDIZED
t
I
o
16
14
I
I
12
on
20
Flgure 5. Calculated energy levels for chemisorbed molecular ethylene (top) and CH, ethylene fragments (bottom). (Note that the calculations place the b,, and levels in reverse order.)
HYDROGEN ON CARBONATED Fe(100)
SURFACE
I
I
IO
I
IO
15
Ionization Energy (eV)
I
8
I
I
6
I
I
I
4
I
2
I
I
EF:O
E, ELECTRON BINDING ENERGY, eV Figure 4. UPS spectra (hv = 21.2 eV) for ethylene adsorbed on clean and oxidized Fe(100) surfaces: (1) sequential exposure of ethylene onto the clean surface at 98 K; (2) after warming the surface in (IC) to 123 K; and (3) exposure of the oxidized surface (a) to 5 L of ethylene at 98 K (b).
modified chemisorbed ethylene layer. The UPS features bear no resemblence to previously reported spectra for chemisorbed molecular ethyleneq2 The possibility of a chemisorbed acetylene species, which might be formed as a result of ethylene dehydrogenation, is also considered unlikely, The spectral features at 123 K appear to be characteristic primarily of CH2 fragments. According to our analysis chemisorbed ethylene can, with about 4 kcal/mol activation energy, dissociate into methylene (CH,) fragments, the total dissociative chemisorption energy being 92 kcal/mol. These fragments are assumed to be positioned in bridging positions, the HCH plane perpendicular to an iron-iron bond, with the carbon 1.4 A from the surface. In addition, a methylene fragment can dehydrogenate, releasing 11 kcal/mol, yielding CH H. The most favorable transition state to methylene dehydrogenation has a CH group parallel to the surface and the other CH bond pointing away. A total barrier of 25 kcal/mol results when the bond stretches 0.4
+
A.
Careful scrutiny of the molecularly adsorbed ethylene spectrum in Figure 4 shows a small peak between the bl, and 2, peaks (peaks C and B, respectively) associated with a contribution from fragmented ethylene even a t 98 K. This suggests the chemisorbed layer dissociates a t this temperature and that the molecularly adsorbed layer is very thin. Calculations for CH2formation produce (Figure 5) the 1b2,3al, and l b l levels lying within a measurable shift to the proper relative positions to account for the ethylene spectrum (Figure 4), e.g., peaks H, G, and F.
VI. Hydrogenation of Carbon-Covered a-Fe(100) A carbon-covered surface with approximately one monolayer of C produces a marked C 2p band2 (Figure 6). When this system is exposed to excess H2a t 98 K the
t 16
14
12
IO
8
6
4
2
EF=O
E, ELECTRON BINDING ENERGY, eV
Flgure 6. Comparison of UPS spectral features (hv = 21.2 eV) for (1) sequential exposure of a carbon covered Fe(100) surface to hydrogen at 98 K; (2) ethylene adsorption on clean Fe(100) at 123 K; and (3) acetylene adsorption on clean Fe(100) at 123 K. Curves 2 and 3 are reproduced segments of curves 2, Figure 4, and 3b, Figure 1, respectively.
apparent methylene spectrum associated with chemisorbed ethylene forms. Comparison of this spectrum with the chemisorbed acetylene and ethylene spectrum indicate that the small peak at 9.3 eV is not associated with methylene although the ethylene spectrum may have CH or other fragments contributing to it. It is significant that CH2 forms a t 98 K while the reaction CH2 CH .tH does not proceed at an appreciable rate even at 123 K. (Note: the calculated dehydrogenation barrier is 26 kcal/mol compared to 37 kcal/mol for the formation of CH2 from CH H.) This appears to be a coverage-dependent pheonemenon. The CH2 fragments may tilt until a CH bond is almost parallel to the surface for dehydrogenation to occur. (At high coverage, surface crowding could prevent this.) On the other hand, hydrogenation of C and CH takes place a t 98 K because of the ready excess of H atoms, which are easily produced from H2 by Fe.l It appears to be energetically advantageous for these excess H atoms to leave the over-saturated metal and combine with carbon atoms. The propensity of H to tunnel may enhance this. Calculations with large clusters of surface metal atoms and adsorbate atoms and
-
+
890
R. D. Bagnall and P. A. Arundel
The Journal of Physical Chemistry, Vol. 82, No. 8, 1978
molecules will allow more detailed exploration of this concept.
Acknowledgment. The computer calculations in this work were supported by National Science Foundation Grant No. MPS74-23245. The experimental studies were supported by AIS1 Grant No. 66-343 and by the Cornel1 Materials Science Center. A scholarship from the American Vacuum Society to C. F. Brucker is also gratefully acknowledged. The iron crystals were prepared by B. Addis. References and Notes (1) A. B. Anderson, J. Am. Chem. SOC.,90, 696 (1977). (2) C. F. Brucker and T. N. Rhodin, J . Catal., 47, 214 (1977). (3) A. B. Anderson, T. N. Rhodin, and C. F. Brucker, Bull. An. Phys. SOC.,21, (7) 940 (1976), (abstract). (4) C. F. Brucker and T. N. Rhodin, Surf. Sci., 57, 523 (1976). (5) A. B. Anderson, J . Chem. Phys., 62, 1187 (1975).
A. B. Anderson, J . Chem. Phys., 64, 4046 (1976). A. B. Anderson, J. Chem. Phys., 65, 1729 (1976); J . Am. Chem. Soc., 100, 1153 (1978). A. B. Anderson, J . Chem. Phys., 66, 2173 (1977). A. B. Anderson, Inorg. Chem., 15, 2598 (1976). R. Mason, D. M. P. Mlngos, Int. Rev. Sci.: Ser. Three(1976). This review of structures of transitlon metal cluster-ligand complexes shows the richness In varlations of ligand structures which may be reflected in chemisorbed systems. F. A. Cotton, J. D. Jamerson, and 8. R. Stuns, J. Organometal. Chem., 94, C53 (1975). A. B. Anderson, J . Chem. Phys., 66, 5108 (1977). K. 0.Legg, F. P. Jona, D. W. Jepson, and P. M. Marcus, J. Phys. C 8 L492 (1975). C. D. Hdgman, Ed., “Handbook of Chemistry and Physlcs”, Chemical Rubber Publishing Co., Cleveland, Ohio, 1962. J. E. Demuth and D. E. Eastman, Phys. Rev. Lett., 32, 1123 (1974). E. W. Plummer, B. J. Waclawski, and T. V. Vorburger, Chem. Phys. Lett.. 28. 510 (1974). KT Y.’Lu,’W. E. ‘Spice;, I. Lindau, P. Ianetta, and S. F. Lin, J . Vac. Sci. Technol., 13, 277 (1976). 0. Brod6n and T. N. Rhodin, Chem. Phys. Lett., 40, 247 (1976). A. B. Anderson, unpublished results.
The Profile Area of Pendant Drops Robert David Bagnail* Bioengineering and Medical Physics Unit, Llverpool Unlverslty, Liverpool, England
and Philip Anthony Arundei Imperial Chemical Industries Limited, Corporate Laboratory, Runcorn, Cheshire. England (Received May 12, 1977; Revised Manuscript Received December 7, 1977)
A simple technique is described for the prediction of pendant drop profile areas and a relationship is established between profile area, surface area, and drop volume. It is suggested that the measurement of profile area in situ could represent one method for automating the pendant drop technique, thus replacing the surface balance in studies of adsorption from bulk solutions. Predicted profile areas and drop volumes are shown to agree with observed values.
Introduction In a previous paper,l a simple method for estimating the surface area of pendant drops has been derived from published table^^,^ of pendant drop profiles. There is however no direct proof that the procedure is correct, but as will be shown in this paper an indirect means of testing the validity of the procedure has been developed using alternative drop parameters which are more easily determined. The basis for this new work is the calculation of pendant drop profile areas from the same published using a mathematical technique similar to that described previously for pendant drop surface areas.l Since profile areas are readily obtained from drop photographs by planimetry, it is considered that a comparison of observed and predicted pendant drop profile areas is a reasonable, albeit indirect test of the method of estimating drop surface areas. Further, it has been demonstrated that surface area and profile area are indeed related through drop shape and in order to show this relationship more completely, the previously published tables of surface areal have been extended. A particular problem during pendant drop experiments is that slight temperature changes, particularly in the (Agla) syringe body, can affect the volume of the drop and render adsorption studies inconclusive. In this paper 0022-3654f 7812082-0898$01 .OO/0
therefore profile area is also shown to be related to drop volume so that it may eventually be possible to detect changes in drop volume through automatic measurement of pendant drop profile area.
Background The profile of a pendant drop is given by2v3 l / ( p / b )+ sin G l ( x / b )= 2 + W / b ) (1) where z, z, and $I are shown in Figure 1,and p is the radius of curvature of the profile at the point (z, z). The shape-determining quantity p is given by (2 1 where 0 is the density difference between the two phases, y is the interfacial tension, and b is the radius of curvature at the apex of the drop. The unit of length is b, so that x l b and z l b are independent of drop size. Thus b determines drop size and /3 determines drop shape. Equation 1has been variously solved for selected values of /3 by either finite difference integration2” OF Taylor’s ~ e r i e s . ~The - ~ results are presented as tables of z / b and z l b for equal steps of arc length s l b or equal steps of $I (see Figure 1). In a previous paper,l we have shown that those tables which involve equal increments of arc length s lb from the
0 = -go b2/T
0 1978 American Chemical Society
