- Email: [email protected]
Understanding adsorbate bonding through quantitative surface structure determination
Applied Surface Science 237 (2004) 13–20 www.elsevier.com/locate/apsusc
Understanding adsorbate bonding through quantitative surface structure determination D.P. Woodruff* Physics Department, University of Warwick, Coventry CV47AL, UK Available online 30 July 2004
Abstract Quantitative structural studies of adsorption structures, especially using scanned-energy mode photoelectron diffraction (PhD) and low energy electron diffraction (LEED), are now capable of providing interatomic bondlengths with sufficient precision to provide chemically significant information regarding the nature of adsorbate–substrate bonding. Selected examples of such studies of molecular adsorbates on mainly metal surfaces are presented which provide insight into the relationship between adsorption energies, coordination site and both adsorbate–substrate and intramolecular bond lengths. Specific examples include simple hydrocarbons on transition and noble metal surfaces, and pure and coadsorbed CO on Ni and NiO surfaces. # 2004 Elsevier B.V. All rights reserved. PACS: 68.43.Fg; 68.43.Bc; 82.65.+r; 61.14.Pq; 61.14.Hg Keywords: Surface structure; Chemisorption; Molecular bonding; Bondlengths; Adsorption energy
1. Introduction A knowledge of the structure of a surface is generally seen as the starting point from which one can try to understand the electronic and chemical properties, and there have been huge advances in this area in the last 20–30 years. Indeed, the latest edition of the Surface Structure Data Base [1] contains rather complete structural information for over 1200 surface phases, although the great majority of these are concerned with clean surfaces and atomic adsorbate * Tel.: +44 24 76523378; fax: +44 24 76692016. E-mail address: [email protected].
systems. In addition there are many more fragmentary structural studies, many aimed at simply determining the adsorption site or some aspect of (molecular) adsorbate orientations, but lacking substantial quantitative information. Of course, the adsorption site is commonly seen as the key feature of structure, and most obviously relates to the notion of ‘active sites’ commonly referred to in traditional studies of heterogeneous catalysis. The adsorbate–substrate coordination associated with specific surface sites may also give some insight into the character of the bonding to the surface. Significantly greater insight, however, may be achieved with quantitative information on interatomic bondlengths, although achieving this
0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2004.06.071
14
D.P. Woodruff / Applied Surface Science 237 (2004) 13–20
information with sufficient precision to be chemically significant is far from trivial [2]. In the case of atomic adsorbates, commonly involving surfaces with good long-range order, a relatively small surface unit mesh, and only a single well-defined high-symmetry adsorption site, this kind of quantitative information is now widely available, especially through the use of quantitative low energy electron diffraction (LEED) and some systematics of the associated adsorbate–substrate bondlengths have been identified [3,4]. For molecular adsorbates the problem is more challenging; the locations of several different atoms within the adsorbate need to be determined, and molecular adsorbates often do not involve high-symmetry adsorption sites and good long-range order. In those cases which lack long-range order conventional LEED cannot be used, and even in cases in which this is not true the large number of structural parameters which must be optimised simultaneously renders the method less than optimal. Nevertheless, the required structural precision is now being achieved by a number of methods (including LEED), and the results are providing some interesting surprises. In this short review a few illustrations of both the problems and successes in this area are reviewed, with a particular emphasis on molecular adsorption systems, and on some our own work using the technique of scannedenergy mode photoelectron diffraction (PhD) [5].
2. Hydrocarbon adsorbates on metals The enormous industrial importance of the chemical modification of hydrocarbons in the processing of oil and natural gas and the extensive use of heterogeneous catalysis in these processes means that there is significant interest in model studies of simple hydrocarbons on transition and noble metal surfaces. The simplest such hydrocarbons which interact significantly with such surface are the unsaturated C2 species ethylene (ethene—C2H4) and acetylene (ethyne—C2H2). Probably the first demonstration of the ability to follow a simple catalytic reaction on a single-crystal surface by a modern surface spectroscopy was the use of ultraviolet photoemission to identify the dehydrogenation of ethylene to acetylene when an adsorbed layer on Ni(1 1 1) at 100 K was heated to 230 K [6]. Much later a PhD investigation
Fig. 1. Plan view of the local adsorption sites of acetylene and ethylene adsorbed on Ni(1 1 1) as determined by scanned-energy mode photoelectron diffraction (PhD) [7], including the value of the C–C bondlengths. The location of the H atoms are not determined in this analysis and are shown schematically only.
[7] determined the local structure of the initial reactant and final produce molecules on this surface (Fig. 1), providing some insight into the possible geometrical reaction path. The PhD technique exploits the fact that when a photoelectron is emitted from an adsorbate atom (in this case C 1s emission) the detected photoemission signal comprises a coherent sum of the directly emitted photoelectron wavefield and components of the same wavefield elastically (back-) scattered by the surrounding (substrate) atoms. If one measures the angle-resolved photoemission signal as a function of photoelectron energy, the associated changes in electron wavelength cause scattering paths to switch in and out of phase, leading to intensity modulations which are characteristic of the emitter site relative to the surrounding substrate atoms. These modulations can be simulated by multiple scattering calculations for a series of structural models in order to determine the adsorption geometry. Evidently the physical process giving rise to the structural information is quite similar to that involved in LEED, but the fact that the electron source is localised on an adsorbate atom, and is specific to both the elemental species and local ‘chemical’ character of this atom through its characteristic photoelectron binding energy, provides information significantly more sensitive to the location of this atom and also significantly more local in character.
D.P. Woodruff / Applied Surface Science 237 (2004) 13–20
While Fig. 1 shows the two distinct adsorption sites occupied by these two adsorbates, one particularly interesting quantitative feature of this structure determination is the value of the C–C intramolecular bondlengths. Because of the scattering geometries which are usually exploited, both PhD and LEED are generally significantly more sensitive to changes in interatomic spacings perpendicular to a surface than parallel to it, so the ‘lying-down’ configuration of these species means that the C–C bondlengths are not very precise. Nevertheless, for both species the C– C bondlength was found to be significantly longer than ˚ for in the equivalent gas-phase species (0.26 0.18 A ˚ for acetylene), consistent ethylene and 0.23 0.15 A with more indirect evidence of a weakening of the C– C bond strengths through a softening of the C–C stretching vibrations [8]. Perhaps more surprising, however, is a remarkably similar finding for acetylene adsorbed on Cu(1 1 1) in which the C atoms occupy the same two inequivalent (‘hcp’ and ‘fcc’) hollow sites (above second and third layer substrate atoms respectively) found on Ni(1 1 1), leading to a C–C ˚ [9]. This result bondlength extension of 0.27 0.10 A seems to be quite inconsistent with the fact that the adsorption energy of acetylene on Cu(1 1 1) is rather low, as manifested by its desorption temperature of 323 K [10]; if the adsorption energy is so low, how can the interaction with the substrate cause such a pronounced change in the intramolecular bonding (producing a bondlength change consistent with a C–C bond order of only slight more than one, compared with a value of three in the gas phase)? The solution to this apparent dilemma seems to be provided by some ab initio cluster calculations [11] which show that in fact the interaction of the acetylene with Cu(1 1 1) is strong, but the large energy cost of the intramolecular bonding modification in stretching the C–C bond means that the total adsorption energy is low. This concept is one we will return to later in the light of new results on quite a different problem. A somewhat more complex surface structural problem is presented by the adsorption of benzene on transition metal surfaces. A wide range of spectroscopic studies have established that in general benzene adsorbs with its molecular plane essentially parallel to such surfaces, and there have been quite a number of quantitative structure determinations, mainly of longrange ordered phases by LEED, but also a few studies
15
by PhD [1]. Apart from establishing the local adsorption site, the azimuthal orientation, and the moleculesubstrate spacing, these studies have also investigated the intramolecular C–C distances. There are two obvious questions to address: does adsorption lead to an extension of the C–C bonds, and does the reduced symmetry of the adsorption site lead to variations in the C–C distance around the benzene ring? From a chemical view these are reasonably distinct questions, but from the point of view of structure determination they are somewhat related. This is because if one allows the full range of symmetrically plausible distortions to be included in the modelling, there is a significant increase in the number of structural variables, and the effect of these extra parameters on the quality of the theory/experiment fit may be partially coupled, i.e. a degradation of the fit by changing one parameter may be rectified by adjusting a second parameter. The net effect of this is that the estimated precision of each of several parameters is typically worse than the single parameter of a more symmetric adsorbate. For example, in a study of benzene adsorption on Ni(1 1 0) (without long-range order), analysis of PhD data on the assumption that the benzene ring is fully symmetric led to an optimum ˚, value of the C–C bondlengths of 1.45 0.03 A significantly larger than the value in the gas phase ˚ ) [12]. However, in the same study molecule (1.39 A two different types of ring distortion were considered which involved two different C–C bondlengths. In this case zero C–C bondlengths differences were still found to be optimal, but the precision of the individual ˚ , meaning that even values was degraded to 0.05 A the ring expansion became marginally significant. Of course, the most interesting type of ring distortion is the three-fold symmetric Kekule´ distortion with alternating C–C bondlengths, a structure most readily reconciled with three-fold symmetric substrates (e.g. Fig. 2). In fact most structural studies of adsorbed benzene have been on such surfaces, and generally the best-fit structures have shown this type of distortion, yet the precision estimates are such that even the ring expansions found are not formally significant. For example, LEED studies of benzene adsorption found ˚ on ring radii in the distorted ring of 1.43 and 1.46 A ˚ on Ni(1 1 1) Ru(0 0 0 1) [13] and 1.48 and 1.50 A ˚ [14], although error estimates of 0.10 and 0.15 A
16
D.P. Woodruff / Applied Surface Science 237 (2004) 13–20
[17], somewhat smaller than the changes indicated in the various surface studies, but perhaps highlighting the need for extreme precision to solve this problem convincingly.
3. CO adsorption and coadsorption on Ni and NiO
Fig. 2. Plan view of the local adsorption site of benzene on Ni(1 1 1) in the (H7 H7) ordered phase. C–C1 and C–C2 denote the two inequivalent C–C bonds in this geometry centred on the hcp hollow site which may differ in length (and which are repeated as alternating bondlengths around the ring).
respectively render both the distortions and the expansions formally not significant. Similarly, a PhD study of benzene adsorbed on Ni(1 1 1) [15] found alternat˚ at low covering C–C bondlengths of 1.40 and 1.44 A ˚ at high coverage, but again age and 1.40 and 1.46 A ˚ or larger. The reality with estimated errors of 0.10 A of such distortions is thus as yet unproved. On the other hand, there does seem to be evidence of ring expansion beyond the one formally significant finding on Ni(1 1 0) mentioned above. Notice, in particular, that while all of these earlier measured expansions (in analyses which allow ring distortion) lie within the error estimates, the consistent trend strongly suggests that a real adsorption-induced ring expansion does occur, and indeed perhaps implies that the error estimates may be unnecessarily pessimistic. Expansion of the benzene ring associated with such adsorption is fully compatible with simple molecular orbital considerations. On the assumption that the bonding scheme is similar to that for benzene ligands in organometallic complexes, the synergic Dewar– Chatt–Duncanson model [16] predicts that there is s-donation from the filled p-bonding orbital of benzene to the metal and p-backbonding from filled metal d-states into the antibonding p*-orbital of benzene. This lowers the C–C bond order and increases the C–C bondlength. For the ‘classic’ benzene complex ˚ Cr(C6H6)2 this increase in C–C distance is 0.03 A
Possibly the most studied of molecular adsorption systems in surface science is that of CO on Ni surfaces. Despite this, this model system continues to reveal new depths of understanding in the general area of molecular chemisorption. Moreover, while the Ni(1 0 0)c(2 2)-CO surface phase provided the first example of a quantitative surface structure determination in the early days of the development of quantitative LEED [18–22] (albeit not without some early problems), showing the CO to adsorb with its axis perpendicular to the surface bonding through the C atom which occupies at atop site, it was only some 15 years later that the correct (hollow) adsorption site was firmly identified for CO on Ni(1 1 1) [23–25]. This delay was brought about by a widespread belief that the local adsorption site of CO could be established in a reliable fashion by the indirect method of vibrational spectroscopy, the value of the internal C–O stretching frequency being regarded as characteristic of the CO bonding coordination on the basis of extensive calibration in metal coordination compounds. Furthermore, the 0.5 ML ordered phase of CO on Ni(1 1 1) is c(4 2), implying two inequivalent CO molecules per primitive surface unit mesh, and a rather elegant model based on bridge site adsorption satisfies this requirement and was consistent with the assignment of vibrational spectroscopy. In fact the two inequivalent sites are found to be the fcc and hcp hollow sites; with hindsight it is certainly possible to rationalise the vibrational spectroscopic data in these terms, yet the result clearly sounds a note of caution for indirect spectroscopic methods of surface structure assignment. More recently, however, the Ni/CO adsorption system has proved a valuable model system with which to explore the relationship between chemisorption bondlengths and the associated bond strengths. The first, and simplest, example of this is the influence of bond order. In the previous section the well-known
D.P. Woodruff / Applied Surface Science 237 (2004) 13–20
˚ [28]) given in the table. The value (1.93 0.03 A experimental bondlength differences in going from ˚ and from two- to three-fold one- to two-fold is 0.16 A ˚ is 0.04 A if we take the PhD value for the bridge site (for the Ni(1 0 0) (3H2 H2)R458-CO phase); taking some average bridging bondlength from the table gives values for the bondlength changes with bond ˚ , respectively. coordination change of 0.14 and 0.06 A Either way, it is clear that the trend is entirely consistent with the predictions of the simple Pauling formula, although the actual values are somewhat smaller. In evaluating similar data for atomic adsorbate on surfaces, Mitchell et al. have argued in terms of a prefactor of 0.80 [3] or 0.85 [4] rather than the value of 0.60 in the equation above; in our case the optimum value of the prefactor for CO chemisorption appears to be 0.5, although the data set is clearly too small to try to make broad quantitative predictions. Notice, incidentally, that as shown in Table 1, the adsorption energy for CO in all of these phases is about the same, so the increasing bondlength with decreasing bond order reflects the sharing of this energy. A rather different question which we might ask of a CO–Ni surface bond is ‘how does the bond length depend on bond strength for a fixed coordination number?’ In general terms it seems unlikely that there is a simple (general) answer to this question, since marked changes in adsorption energy for fixed coordination presumably imply some change of bonding character, and the exact nature of this change presumably influences the bondlength change. On the other hand, we would intuitively expect a reduction in bond energy to lead to an increase in bondlength, so it is certainly interesting to try to quantify this effect. Table 1 contains two entries at the bottom of the table which give two specific answers to this question. The first case quantified in the table is that of the c(2 2) coadsorption phase of CO and hydrogen on
influence of bond order on C–C bondlengths in saturated and unsaturated hydrocarbons was remarked upon; in this case the bond order changes from one in the saturated hydrocarbon (e.g. ethane, C2H6) to two (e.g. in ethylene, C2H4) to three (e.g. in acetylene, C2H2) in the unsaturated species. In the case of CO on Ni, the changes of interest involve fractional bond orders; from CO in a single-coordinated atop state (as on Ni(1 0 0) in the c(2 2) phase) with a bond order of one, to CO in two-fold coordinated bridge sites (as in Ni(1 1 0)(2 1)pg-CO and in the high coverage (3H2 H2)R458-CO phase on Ni(1 0 0)) with bond order 0.5, to CO in three-fold coordinated hollow sites (as on Ni(1 1 1)) with a bond order of 0.33. In discussing the relationship between C–C bondlengths and bond order in free molecules Pauling [26] concluded that a simple relationship for estimating fractional order bondlengths was DðnÞ ¼ Dð1Þ 0:60 log10 n where D(n) is the bondlength for bond order n. What happens if we take values of n of 1.00, 0.50 and 0.33 corresponding to one-, two- and three-fold coordinated sites of CO on Ni? This equation gives a difference in bondlength from one- to two-fold coordination of ˚ , and from two- to three-fold of 0.10 A ˚ . Table 1 0.18 A shows the experimental values for CO on Ni surfaces. In this table we have concentrated on values obtained from PhD experiments simply to ensure a common methodology and thus to reduce the possible problems of systematic errors which should be common to all such values. The exceptions are the values for CO on Ni(1 1 0) (in bridging sites) which derive from LEED and theoretical studies, but clearly yield bondlengths in good agreement with the PhD value for the bridging site on Ni(1 0 0). A LEED study of the Ni(1 1 1)c(4 2)-CO surface also gave a value (1.94 ˚ [27]) in excellent agreement with the PhD 0.03 A
Table 1 Summary of Ni–C bondlengths and adsorption energies for CO adsorption on various Ni and NiO surfaces ˚) Substrate Phase Site Bond order Bondlength (A Ni(1 0 0) Ni(1 1 0) Ni(1 1 1) Ni(1 0 0)/H NiO(1 0 0)
c(2 2)-CO (3H2 H2)R458-CO (2 1)pg-CO c(4 2)-CO c(2 2)-H/CO Disordered CO
Atop Bridge Bridge Hollow Atop Atop
17
1.00 0.50 0.50 0.33 1.00 1.00
1.73 0.03 [33] 1.89 0.02 [33] 1.85–1.89 [42,43] 1.93 0.03 [28] 1.79 0.02 [33] 2.07 0.02 [38]
adsorption energy (eV) 1.2 [41] 1.1 [41] 1.1 [41] 1.2 [41] 0.4–0.6 0.30 [44]
18
D.P. Woodruff / Applied Surface Science 237 (2004) 13–20
Ni(1 0 0). This phase [29,30] is formed by initially exposing the Ni(1 0 0) surface to hydrogen, followed by CO exposure; the phase is only stable at temperatures below about 140 K, although above this temperature the order is lost but no desorption occurs until the temperature reaches about 210 K. Simple application of the Redhead formula [31] assuming first-order desorption and a standard pre-exponential desorption factor leads to an estimate for the desorption energy of 0.6 eV at a temperature of 210 K, although one might argue that the true adsorption energy of the ordered phase is even lower. Nevertheless, the adsorption energy is almost certainly a factor of 2 or more smaller than for CO on Ni in the absence of coadsorbed hydrogen. Moreover, the c(2 2) phase has generally been believed to involve atop site adsorption on the basis of spectroscopic information and this has, indeed, been proved to be the case in a recent PhD structure determination [32,33]. As may be seen from Table 1, however, the reduction in bond strength by a factor of 2 or more leads to an increase in the Ni–C bondlength for atop adsorption of ˚ (from 1.73 to 1.79 A ˚ ), only about half only 0.06 A the value of the bondlength extension associated with reducing the bond order by a factor of 2. Some insight into this surprising result is provided by the results of a density functional theory (DFT) calculation which fully reproduces the structural results, with excellent agreement for the Ni–C bondlengths in the c(2 2) phases with and without coadsorbed hydrogen [33]. This calculation also reproduces at least semi-quantitatively the different adsorption energies of the CO in these atop sites with values of 1.43 eV for the pure CO layer and 0.73 eV for the coadsorption layer. However, these calculations also showed a pronounced change in the interlayer spacings of both the H and the outermost Ni layer when the CO was added to form the coadsorption phase. In the pure Ni(1 0 0)(1 1)-H surface phase the H atoms were found to occupy the hollow sites with an optimum H– ˚ , in good agreement with the Ni layer spacing of 0.43 A results of both experiment [34] and recent calculations [35,36]. The Ni(1 0 0)c(2 2)-H/CO coadsorption structure showed an unusually large rumpling of the outermost Ni layer, with the Ni atoms bonded to the CO ˚ higher above the surface than the surbeing 0.40 A rounding outermost Ni atoms which are not bonded to CO molecules; this same effect was found in the PhD experiment, although the amplitude of the distortion
˚ ) seems slightly smaller, but is also (0.24 0.08 A unusually large. Moreover, the CO adsorption caused ˚ the H atoms to sink down into the surface, just 0.08 A above the uncovered outermost layer Ni atoms and ˚ below the CO-covered Ni surface atoms. 0.32 A Clearly, these structural changes have an energy cost which is incorporated into the total adsorption energy, so the implication is that the local Ni–CO bond strength may be quite large, but the total adsorption energy is low because of the energy cost of the distortions which the CO adsorption produces on the Ni(1 0 0)–H surface. As such, the Ni–C bondlength presumably reflects the local strength of the Ni–CO bond, and not the total adsorption energy. In some ways this situation therefore is somewhat similar to the case of acetylene adsorption on Cu(1 1 1) discussed in the previous section, although in that case it is the distortion of the adsorbed molecule itself, and not the modification of the surface, which provides the energy cost leading to an overall low adsorption energy. The final entry in Table 1 shows results for quite a different situation, that of CO adsorption on NiO rather than Ni metal. Nevertheless, the CO does bond atop surface Ni atoms, yet the Ni–C bondlength is a ˚ longer than for atop adsorption on Ni huge 0.34 A ˚ longer than the Ni–C metal, and is even 0.14 A bondlength for CO adsorbed in a three-fold coordinated hollow site. However, the adsorption energy is extremely low, at only 0.30 eV. Bearing in mind that the adsorption energy of CO on Ni metal is 1.2 eV, then in the three-fold coordinated site one might crudely assign 0.4 eV to each Ni–C bond, so the further increase in the bondlength in lowering the bond energy to 0.3 eV is consistent with a simple qualitative correlation of individual bondlength and bond strength. Of course, this analysis is highly simplistic, yet achieving a proper theoretical description of CO bonding to NiO has proved a major challenge [37,38]. So far calculations for the NiO(1 0 0)–CO adsorption system have found unrealistically long Ni– ˚ [39] and 2.86 A ˚ [40]. C bondlengths of 2.49 A
4. Conclusions A key challenge in surface structure determination has been to determine interatomic distances with a precision which is chemically significant; the problem
D.P. Woodruff / Applied Surface Science 237 (2004) 13–20
is especially demanding, but also especially interesting, in the case of molecular adsorbates in which intramolecular bondlength changes may also be a strong ‘fingerprint’ of bonding behaviour. While this problem is far from being solved in a routine fashion, it is clear that, at least in a limited range of problems, information is emerging which is leading to new levels of understanding. For CO on Ni surfaces, one particularly simple result is the Pauling-like correlation of Ni–C bondlengths and bond order. Introducing a hydrogen coadsorbate or placing the Ni atom in a compound clearly has a significant impact on both bond strength and bondlength, with some of the systematics broadly consistent with expectations. On the other hand, for the systems Ni(1 0 0)–CO/H and Cu(1 1 1)–C2H2 in particular, the combination of the experimental structural results and theoretical total energy calculations has proved of great value. In both of these systems the theoretical calculations indicate that the adsorption energy is not, in fact, a reliable indicator of the local bonding strength because the bonding leads to energy costs for surface or adsorbate distortions which must accompany the adsorption. This highlights the fact that the desorption temperature may not generally be a reliable indicator of ‘strong’ and ‘weak’ chemisorption.
Acknowledgements The author acknowledges the financial support of the Engineering and Physical Sciences Research Council (UK) and the invaluable role of many collaborators, notably at the University of Warwick and the Fritz Haber Institute in Berlin, whose names appear in the citations at the end of this paper.
References [1] P.R. Watson, M.A. Van Hove, K. Hermann, NIST Surface Structure Database Ver. 4.0, NIST Standard Reference Data Program, Gaithersburg, MD, USA, 2002. [2] D.P. Woodruff, Surf. Sci. 500 (2002) 147. [3] K.A.R. Mitchell, Surf. Sci. 149 (1985) 93. [4] K.A.R. Mitchell, S.A. Schlatter, R.N.S. Sodhi, Can. J. Chem. 64 (1986) 1435. [5] D.P. Woodruff, A.M. Bradshaw, Rep. Prog. Phys. 57 (1994) 1029.
19
[6] J.E. Demuth, D.E. Eastman, Phys. Rev. Lett. 32 (1974) 1123. [7] S. Bao, Ph. Hofmann, K.-M. Schindler, V. Fritzsche, A.M. Bradshaw, D.P. Woodruff, C. Casado, M.C. Asensio, J. Phys. Condens. Matter 6 (1994) 93. [8] N. Sheppard, Rev. Phys. Chem. 39 (1988) 589. [9] S. Bao, K.-M. Schindler, Ph. Hofmann, V. Fritzsche, A.M. Bradshaw, D.P. Woodruff, Surf. Sci. 291 (1993) 295. [10] N.D.S. Canning, M.D. Baker, M.A. Chesters, Surf. Sci. 111 (1981) 441. [11] K. Hermann, M. Witko, Surf. Sci. 337 (1995) 205. [12] J.-H. Kang, R.L. Toomes, J. Robinson, D.P. Woodruff, O. Schaff, R. Terborg, R. Lindsay, P. Baumga¨ rtel, A.M. Bradshaw, Surf. Sci. 448 (2000) 23. [13] C. Stellwag, G. Held, D. Menzel, Surf. Sci. 325 (1995) 379. [14] G. Held, M.P. Bessent, S. Titmuss, D.A. King, J. Chem. Phys. 105 (1996) 11305. [15] O. Schaff, V. Fernandez, Ph. Hofmann, K.-M. Schindler, A. Theobald, V. Fritzsche, A.M. Bradshaw, R. Davis, D.P. Woodruff, Surf. Sci. 348 (1996) 89. [16] I.S. Butler, J.F. Harrod, Inorganic Chemistry: Principles and Applications, Benjamin/Cummings, Redwood City, CA, 1989 (pp. 679–689). [17] E. Uhlig, Organometallics 12 (1993) 4751. [18] S. Andersson, J.B. Pendry, Surf. Sci. 71 (1978) 75. [19] K. Heinz, E. Lang, K. Mu¨ ller, Surf. Sci. 87 (1979) 595. [20] M. Passler, A. Ignatiev, F. Jona, D.W. Jepsen, P.M. Marcus, Phys. Rev. Lett. 43 (1979) 360. [21] S. Andersson, J.B. Pendry, J. Phys. C: Solid State Phys. 13 (1980) 3547. [22] S.Y. Tong, A. Maldonado, C.H. Li, M.A. Van Hove, Surf. Sci. 94 (1980) 73. [23] L. Becker, S. Aminpirooz, B. Hillert, M. Pedio, J. Haase, D.L. Adams, Phys. Rev. B 47 (1993) 9710. [24] K.-M. Schindler, Ph. Hofmann, K.-U. Weiss, R. Dippel, V. Fritzsche, A.M. Bradshaw, D.P. Woodruff, M.E. Davila, M.C. Asensio, J.C. Conesa, A.R. Gonzalez-Elipe, J. Electr. Spectros. Rel. Phenom 64–65 (1993) 75. [25] L.D. Mapledoram, M.P. Bessent, A.D. Wander, D.A. King, Chem. Phys. Lett. 228 (1994) 527. [26] L. Pauling, The Nature of the Chemical Bond, Cornell University Press, Ithaca, NY, 1939, 1940. 1960. [27] L.D. Mapledoram, M.P. Bessent, A. Wander, D.A. King, Chem. Phys. Lett. 228 (1994) 527. [28] M.E. Davila, M.C. Asensio, D.P. Woodruff, K.-M. Schindler, Ph. Hofmann, K.-U. Weiss, R. Dippel, P. Gardner, V. Fritzsche, A.M. Bradshaw, J.C. Conesa, A.R. Gonza´ lez-Elipe, Surf. Sci. 311 (1994) 337. [29] H.C. Peebles, D.E. Peebles, J.M. White, Surf. Sci. 125 (1983) 87. [30] B.E. Koel, D.E. Peebles, J.M. White, Surf. Sci. 125 (1983) 709. [31] P.A. Redhead, Vacuum 12 (1962) 203. [32] D.I. Sayago, J.T. Hoeft, M. Polcik, M. Kittel, R.L. Toomes, J. Robinson, D.P. Woodruff, M. Pascal, C.L.A. Lamont, G. Nisbet, Phys. Rev. Lett. 90 (2003) 116104. [33] J.T. Hoeft, M. Polcik, D. Sayago, M. Kittel, R. Terborg, R.L. Toomes, J. Robinson, D.P. Woodruff, M. Pascal, G. Nisbet, C.L.A. Lamont, Surf. Sci. 540 (2003) 441.
20
D.P. Woodruff / Applied Surface Science 237 (2004) 13–20
[34] I. Stensgaard, F. Jakobsen, Phys. Rev. Lett. 54 (1985) 711. [35] T.R. Mattsson, G. Wahnstro¨ m, L. Bengtsson, B. Hammer, Phys. Rev. B 56 (1997) 2258. [36] G. Kresse, J. Hafner, Surf. Sci. 459 (2000) 287. [37] J.-T. Hoeft, M. Kittel, M. Polcik, S. Bao, R.L. Toomes, J.-H. Kang, D.P. Woodruff, M. Pascal, C.L.A. Lamont, Phys. Rev. Lett. 87 (2001) 086101. [38] M. Kittel, J.-T. Hoeft, M. Polcik, S. Bao, R.L. Toomes, J.-H. Kang, D.P. Woodruff, M. Pascal, C.L.A. Lamont, Surf. Sci. 499 (2002) 1.
[39] G. Pacchioni, G. Cogliandro, P.S. Bagus, Surf. Sci. 255 (1991) 344. [40] M. Po¨ hlchen, V. Staemmler, J. Chem. Phys. 97 (1992) 2583. [41] J.T. Stuckless, N. Al-Sarraf, C. Wartnaby, D.A. King, J. Chem. Phys. 99 (1993) 2202. [42] C. Zhao, M.A. Passler, Surf. Sci. 320 (1994) 1. [43] F. Favot, A.D. Corso, A. Baldereschi, Phys. Rev. B 63 (2001) 115416. [44] R. Wichtendahl, M. Rodriguez-Rodrigo, U. Ha¨ rtel, H. Kuhlenbeck, H.-J. Freund, Surf. Sci. 423 (1999) 90.
Understanding adsorbate bonding through quantitative surface structure determination D.P. Woodruff* Physics Department, University of Warwick, Coventry CV47AL, UK Available online 30 July 2004
Abstract Quantitative structural studies of adsorption structures, especially using scanned-energy mode photoelectron diffraction (PhD) and low energy electron diffraction (LEED), are now capable of providing interatomic bondlengths with sufficient precision to provide chemically significant information regarding the nature of adsorbate–substrate bonding. Selected examples of such studies of molecular adsorbates on mainly metal surfaces are presented which provide insight into the relationship between adsorption energies, coordination site and both adsorbate–substrate and intramolecular bond lengths. Specific examples include simple hydrocarbons on transition and noble metal surfaces, and pure and coadsorbed CO on Ni and NiO surfaces. # 2004 Elsevier B.V. All rights reserved. PACS: 68.43.Fg; 68.43.Bc; 82.65.+r; 61.14.Pq; 61.14.Hg Keywords: Surface structure; Chemisorption; Molecular bonding; Bondlengths; Adsorption energy
1. Introduction A knowledge of the structure of a surface is generally seen as the starting point from which one can try to understand the electronic and chemical properties, and there have been huge advances in this area in the last 20–30 years. Indeed, the latest edition of the Surface Structure Data Base [1] contains rather complete structural information for over 1200 surface phases, although the great majority of these are concerned with clean surfaces and atomic adsorbate * Tel.: +44 24 76523378; fax: +44 24 76692016. E-mail address: [email protected].
systems. In addition there are many more fragmentary structural studies, many aimed at simply determining the adsorption site or some aspect of (molecular) adsorbate orientations, but lacking substantial quantitative information. Of course, the adsorption site is commonly seen as the key feature of structure, and most obviously relates to the notion of ‘active sites’ commonly referred to in traditional studies of heterogeneous catalysis. The adsorbate–substrate coordination associated with specific surface sites may also give some insight into the character of the bonding to the surface. Significantly greater insight, however, may be achieved with quantitative information on interatomic bondlengths, although achieving this
0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2004.06.071
14
D.P. Woodruff / Applied Surface Science 237 (2004) 13–20
information with sufficient precision to be chemically significant is far from trivial [2]. In the case of atomic adsorbates, commonly involving surfaces with good long-range order, a relatively small surface unit mesh, and only a single well-defined high-symmetry adsorption site, this kind of quantitative information is now widely available, especially through the use of quantitative low energy electron diffraction (LEED) and some systematics of the associated adsorbate–substrate bondlengths have been identified [3,4]. For molecular adsorbates the problem is more challenging; the locations of several different atoms within the adsorbate need to be determined, and molecular adsorbates often do not involve high-symmetry adsorption sites and good long-range order. In those cases which lack long-range order conventional LEED cannot be used, and even in cases in which this is not true the large number of structural parameters which must be optimised simultaneously renders the method less than optimal. Nevertheless, the required structural precision is now being achieved by a number of methods (including LEED), and the results are providing some interesting surprises. In this short review a few illustrations of both the problems and successes in this area are reviewed, with a particular emphasis on molecular adsorption systems, and on some our own work using the technique of scannedenergy mode photoelectron diffraction (PhD) [5].
2. Hydrocarbon adsorbates on metals The enormous industrial importance of the chemical modification of hydrocarbons in the processing of oil and natural gas and the extensive use of heterogeneous catalysis in these processes means that there is significant interest in model studies of simple hydrocarbons on transition and noble metal surfaces. The simplest such hydrocarbons which interact significantly with such surface are the unsaturated C2 species ethylene (ethene—C2H4) and acetylene (ethyne—C2H2). Probably the first demonstration of the ability to follow a simple catalytic reaction on a single-crystal surface by a modern surface spectroscopy was the use of ultraviolet photoemission to identify the dehydrogenation of ethylene to acetylene when an adsorbed layer on Ni(1 1 1) at 100 K was heated to 230 K [6]. Much later a PhD investigation
Fig. 1. Plan view of the local adsorption sites of acetylene and ethylene adsorbed on Ni(1 1 1) as determined by scanned-energy mode photoelectron diffraction (PhD) [7], including the value of the C–C bondlengths. The location of the H atoms are not determined in this analysis and are shown schematically only.
[7] determined the local structure of the initial reactant and final produce molecules on this surface (Fig. 1), providing some insight into the possible geometrical reaction path. The PhD technique exploits the fact that when a photoelectron is emitted from an adsorbate atom (in this case C 1s emission) the detected photoemission signal comprises a coherent sum of the directly emitted photoelectron wavefield and components of the same wavefield elastically (back-) scattered by the surrounding (substrate) atoms. If one measures the angle-resolved photoemission signal as a function of photoelectron energy, the associated changes in electron wavelength cause scattering paths to switch in and out of phase, leading to intensity modulations which are characteristic of the emitter site relative to the surrounding substrate atoms. These modulations can be simulated by multiple scattering calculations for a series of structural models in order to determine the adsorption geometry. Evidently the physical process giving rise to the structural information is quite similar to that involved in LEED, but the fact that the electron source is localised on an adsorbate atom, and is specific to both the elemental species and local ‘chemical’ character of this atom through its characteristic photoelectron binding energy, provides information significantly more sensitive to the location of this atom and also significantly more local in character.
D.P. Woodruff / Applied Surface Science 237 (2004) 13–20
While Fig. 1 shows the two distinct adsorption sites occupied by these two adsorbates, one particularly interesting quantitative feature of this structure determination is the value of the C–C intramolecular bondlengths. Because of the scattering geometries which are usually exploited, both PhD and LEED are generally significantly more sensitive to changes in interatomic spacings perpendicular to a surface than parallel to it, so the ‘lying-down’ configuration of these species means that the C–C bondlengths are not very precise. Nevertheless, for both species the C– C bondlength was found to be significantly longer than ˚ for in the equivalent gas-phase species (0.26 0.18 A ˚ for acetylene), consistent ethylene and 0.23 0.15 A with more indirect evidence of a weakening of the C– C bond strengths through a softening of the C–C stretching vibrations [8]. Perhaps more surprising, however, is a remarkably similar finding for acetylene adsorbed on Cu(1 1 1) in which the C atoms occupy the same two inequivalent (‘hcp’ and ‘fcc’) hollow sites (above second and third layer substrate atoms respectively) found on Ni(1 1 1), leading to a C–C ˚ [9]. This result bondlength extension of 0.27 0.10 A seems to be quite inconsistent with the fact that the adsorption energy of acetylene on Cu(1 1 1) is rather low, as manifested by its desorption temperature of 323 K [10]; if the adsorption energy is so low, how can the interaction with the substrate cause such a pronounced change in the intramolecular bonding (producing a bondlength change consistent with a C–C bond order of only slight more than one, compared with a value of three in the gas phase)? The solution to this apparent dilemma seems to be provided by some ab initio cluster calculations [11] which show that in fact the interaction of the acetylene with Cu(1 1 1) is strong, but the large energy cost of the intramolecular bonding modification in stretching the C–C bond means that the total adsorption energy is low. This concept is one we will return to later in the light of new results on quite a different problem. A somewhat more complex surface structural problem is presented by the adsorption of benzene on transition metal surfaces. A wide range of spectroscopic studies have established that in general benzene adsorbs with its molecular plane essentially parallel to such surfaces, and there have been quite a number of quantitative structure determinations, mainly of longrange ordered phases by LEED, but also a few studies
15
by PhD [1]. Apart from establishing the local adsorption site, the azimuthal orientation, and the moleculesubstrate spacing, these studies have also investigated the intramolecular C–C distances. There are two obvious questions to address: does adsorption lead to an extension of the C–C bonds, and does the reduced symmetry of the adsorption site lead to variations in the C–C distance around the benzene ring? From a chemical view these are reasonably distinct questions, but from the point of view of structure determination they are somewhat related. This is because if one allows the full range of symmetrically plausible distortions to be included in the modelling, there is a significant increase in the number of structural variables, and the effect of these extra parameters on the quality of the theory/experiment fit may be partially coupled, i.e. a degradation of the fit by changing one parameter may be rectified by adjusting a second parameter. The net effect of this is that the estimated precision of each of several parameters is typically worse than the single parameter of a more symmetric adsorbate. For example, in a study of benzene adsorption on Ni(1 1 0) (without long-range order), analysis of PhD data on the assumption that the benzene ring is fully symmetric led to an optimum ˚, value of the C–C bondlengths of 1.45 0.03 A significantly larger than the value in the gas phase ˚ ) [12]. However, in the same study molecule (1.39 A two different types of ring distortion were considered which involved two different C–C bondlengths. In this case zero C–C bondlengths differences were still found to be optimal, but the precision of the individual ˚ , meaning that even values was degraded to 0.05 A the ring expansion became marginally significant. Of course, the most interesting type of ring distortion is the three-fold symmetric Kekule´ distortion with alternating C–C bondlengths, a structure most readily reconciled with three-fold symmetric substrates (e.g. Fig. 2). In fact most structural studies of adsorbed benzene have been on such surfaces, and generally the best-fit structures have shown this type of distortion, yet the precision estimates are such that even the ring expansions found are not formally significant. For example, LEED studies of benzene adsorption found ˚ on ring radii in the distorted ring of 1.43 and 1.46 A ˚ on Ni(1 1 1) Ru(0 0 0 1) [13] and 1.48 and 1.50 A ˚ [14], although error estimates of 0.10 and 0.15 A
16
D.P. Woodruff / Applied Surface Science 237 (2004) 13–20
[17], somewhat smaller than the changes indicated in the various surface studies, but perhaps highlighting the need for extreme precision to solve this problem convincingly.
3. CO adsorption and coadsorption on Ni and NiO
Fig. 2. Plan view of the local adsorption site of benzene on Ni(1 1 1) in the (H7 H7) ordered phase. C–C1 and C–C2 denote the two inequivalent C–C bonds in this geometry centred on the hcp hollow site which may differ in length (and which are repeated as alternating bondlengths around the ring).
respectively render both the distortions and the expansions formally not significant. Similarly, a PhD study of benzene adsorbed on Ni(1 1 1) [15] found alternat˚ at low covering C–C bondlengths of 1.40 and 1.44 A ˚ at high coverage, but again age and 1.40 and 1.46 A ˚ or larger. The reality with estimated errors of 0.10 A of such distortions is thus as yet unproved. On the other hand, there does seem to be evidence of ring expansion beyond the one formally significant finding on Ni(1 1 0) mentioned above. Notice, in particular, that while all of these earlier measured expansions (in analyses which allow ring distortion) lie within the error estimates, the consistent trend strongly suggests that a real adsorption-induced ring expansion does occur, and indeed perhaps implies that the error estimates may be unnecessarily pessimistic. Expansion of the benzene ring associated with such adsorption is fully compatible with simple molecular orbital considerations. On the assumption that the bonding scheme is similar to that for benzene ligands in organometallic complexes, the synergic Dewar– Chatt–Duncanson model [16] predicts that there is s-donation from the filled p-bonding orbital of benzene to the metal and p-backbonding from filled metal d-states into the antibonding p*-orbital of benzene. This lowers the C–C bond order and increases the C–C bondlength. For the ‘classic’ benzene complex ˚ Cr(C6H6)2 this increase in C–C distance is 0.03 A
Possibly the most studied of molecular adsorption systems in surface science is that of CO on Ni surfaces. Despite this, this model system continues to reveal new depths of understanding in the general area of molecular chemisorption. Moreover, while the Ni(1 0 0)c(2 2)-CO surface phase provided the first example of a quantitative surface structure determination in the early days of the development of quantitative LEED [18–22] (albeit not without some early problems), showing the CO to adsorb with its axis perpendicular to the surface bonding through the C atom which occupies at atop site, it was only some 15 years later that the correct (hollow) adsorption site was firmly identified for CO on Ni(1 1 1) [23–25]. This delay was brought about by a widespread belief that the local adsorption site of CO could be established in a reliable fashion by the indirect method of vibrational spectroscopy, the value of the internal C–O stretching frequency being regarded as characteristic of the CO bonding coordination on the basis of extensive calibration in metal coordination compounds. Furthermore, the 0.5 ML ordered phase of CO on Ni(1 1 1) is c(4 2), implying two inequivalent CO molecules per primitive surface unit mesh, and a rather elegant model based on bridge site adsorption satisfies this requirement and was consistent with the assignment of vibrational spectroscopy. In fact the two inequivalent sites are found to be the fcc and hcp hollow sites; with hindsight it is certainly possible to rationalise the vibrational spectroscopic data in these terms, yet the result clearly sounds a note of caution for indirect spectroscopic methods of surface structure assignment. More recently, however, the Ni/CO adsorption system has proved a valuable model system with which to explore the relationship between chemisorption bondlengths and the associated bond strengths. The first, and simplest, example of this is the influence of bond order. In the previous section the well-known
D.P. Woodruff / Applied Surface Science 237 (2004) 13–20
˚ [28]) given in the table. The value (1.93 0.03 A experimental bondlength differences in going from ˚ and from two- to three-fold one- to two-fold is 0.16 A ˚ is 0.04 A if we take the PhD value for the bridge site (for the Ni(1 0 0) (3H2 H2)R458-CO phase); taking some average bridging bondlength from the table gives values for the bondlength changes with bond ˚ , respectively. coordination change of 0.14 and 0.06 A Either way, it is clear that the trend is entirely consistent with the predictions of the simple Pauling formula, although the actual values are somewhat smaller. In evaluating similar data for atomic adsorbate on surfaces, Mitchell et al. have argued in terms of a prefactor of 0.80 [3] or 0.85 [4] rather than the value of 0.60 in the equation above; in our case the optimum value of the prefactor for CO chemisorption appears to be 0.5, although the data set is clearly too small to try to make broad quantitative predictions. Notice, incidentally, that as shown in Table 1, the adsorption energy for CO in all of these phases is about the same, so the increasing bondlength with decreasing bond order reflects the sharing of this energy. A rather different question which we might ask of a CO–Ni surface bond is ‘how does the bond length depend on bond strength for a fixed coordination number?’ In general terms it seems unlikely that there is a simple (general) answer to this question, since marked changes in adsorption energy for fixed coordination presumably imply some change of bonding character, and the exact nature of this change presumably influences the bondlength change. On the other hand, we would intuitively expect a reduction in bond energy to lead to an increase in bondlength, so it is certainly interesting to try to quantify this effect. Table 1 contains two entries at the bottom of the table which give two specific answers to this question. The first case quantified in the table is that of the c(2 2) coadsorption phase of CO and hydrogen on
influence of bond order on C–C bondlengths in saturated and unsaturated hydrocarbons was remarked upon; in this case the bond order changes from one in the saturated hydrocarbon (e.g. ethane, C2H6) to two (e.g. in ethylene, C2H4) to three (e.g. in acetylene, C2H2) in the unsaturated species. In the case of CO on Ni, the changes of interest involve fractional bond orders; from CO in a single-coordinated atop state (as on Ni(1 0 0) in the c(2 2) phase) with a bond order of one, to CO in two-fold coordinated bridge sites (as in Ni(1 1 0)(2 1)pg-CO and in the high coverage (3H2 H2)R458-CO phase on Ni(1 0 0)) with bond order 0.5, to CO in three-fold coordinated hollow sites (as on Ni(1 1 1)) with a bond order of 0.33. In discussing the relationship between C–C bondlengths and bond order in free molecules Pauling [26] concluded that a simple relationship for estimating fractional order bondlengths was DðnÞ ¼ Dð1Þ 0:60 log10 n where D(n) is the bondlength for bond order n. What happens if we take values of n of 1.00, 0.50 and 0.33 corresponding to one-, two- and three-fold coordinated sites of CO on Ni? This equation gives a difference in bondlength from one- to two-fold coordination of ˚ , and from two- to three-fold of 0.10 A ˚ . Table 1 0.18 A shows the experimental values for CO on Ni surfaces. In this table we have concentrated on values obtained from PhD experiments simply to ensure a common methodology and thus to reduce the possible problems of systematic errors which should be common to all such values. The exceptions are the values for CO on Ni(1 1 0) (in bridging sites) which derive from LEED and theoretical studies, but clearly yield bondlengths in good agreement with the PhD value for the bridging site on Ni(1 0 0). A LEED study of the Ni(1 1 1)c(4 2)-CO surface also gave a value (1.94 ˚ [27]) in excellent agreement with the PhD 0.03 A
Table 1 Summary of Ni–C bondlengths and adsorption energies for CO adsorption on various Ni and NiO surfaces ˚) Substrate Phase Site Bond order Bondlength (A Ni(1 0 0) Ni(1 1 0) Ni(1 1 1) Ni(1 0 0)/H NiO(1 0 0)
c(2 2)-CO (3H2 H2)R458-CO (2 1)pg-CO c(4 2)-CO c(2 2)-H/CO Disordered CO
Atop Bridge Bridge Hollow Atop Atop
17
1.00 0.50 0.50 0.33 1.00 1.00
1.73 0.03 [33] 1.89 0.02 [33] 1.85–1.89 [42,43] 1.93 0.03 [28] 1.79 0.02 [33] 2.07 0.02 [38]
adsorption energy (eV) 1.2 [41] 1.1 [41] 1.1 [41] 1.2 [41] 0.4–0.6 0.30 [44]
18
D.P. Woodruff / Applied Surface Science 237 (2004) 13–20
Ni(1 0 0). This phase [29,30] is formed by initially exposing the Ni(1 0 0) surface to hydrogen, followed by CO exposure; the phase is only stable at temperatures below about 140 K, although above this temperature the order is lost but no desorption occurs until the temperature reaches about 210 K. Simple application of the Redhead formula [31] assuming first-order desorption and a standard pre-exponential desorption factor leads to an estimate for the desorption energy of 0.6 eV at a temperature of 210 K, although one might argue that the true adsorption energy of the ordered phase is even lower. Nevertheless, the adsorption energy is almost certainly a factor of 2 or more smaller than for CO on Ni in the absence of coadsorbed hydrogen. Moreover, the c(2 2) phase has generally been believed to involve atop site adsorption on the basis of spectroscopic information and this has, indeed, been proved to be the case in a recent PhD structure determination [32,33]. As may be seen from Table 1, however, the reduction in bond strength by a factor of 2 or more leads to an increase in the Ni–C bondlength for atop adsorption of ˚ (from 1.73 to 1.79 A ˚ ), only about half only 0.06 A the value of the bondlength extension associated with reducing the bond order by a factor of 2. Some insight into this surprising result is provided by the results of a density functional theory (DFT) calculation which fully reproduces the structural results, with excellent agreement for the Ni–C bondlengths in the c(2 2) phases with and without coadsorbed hydrogen [33]. This calculation also reproduces at least semi-quantitatively the different adsorption energies of the CO in these atop sites with values of 1.43 eV for the pure CO layer and 0.73 eV for the coadsorption layer. However, these calculations also showed a pronounced change in the interlayer spacings of both the H and the outermost Ni layer when the CO was added to form the coadsorption phase. In the pure Ni(1 0 0)(1 1)-H surface phase the H atoms were found to occupy the hollow sites with an optimum H– ˚ , in good agreement with the Ni layer spacing of 0.43 A results of both experiment [34] and recent calculations [35,36]. The Ni(1 0 0)c(2 2)-H/CO coadsorption structure showed an unusually large rumpling of the outermost Ni layer, with the Ni atoms bonded to the CO ˚ higher above the surface than the surbeing 0.40 A rounding outermost Ni atoms which are not bonded to CO molecules; this same effect was found in the PhD experiment, although the amplitude of the distortion
˚ ) seems slightly smaller, but is also (0.24 0.08 A unusually large. Moreover, the CO adsorption caused ˚ the H atoms to sink down into the surface, just 0.08 A above the uncovered outermost layer Ni atoms and ˚ below the CO-covered Ni surface atoms. 0.32 A Clearly, these structural changes have an energy cost which is incorporated into the total adsorption energy, so the implication is that the local Ni–CO bond strength may be quite large, but the total adsorption energy is low because of the energy cost of the distortions which the CO adsorption produces on the Ni(1 0 0)–H surface. As such, the Ni–C bondlength presumably reflects the local strength of the Ni–CO bond, and not the total adsorption energy. In some ways this situation therefore is somewhat similar to the case of acetylene adsorption on Cu(1 1 1) discussed in the previous section, although in that case it is the distortion of the adsorbed molecule itself, and not the modification of the surface, which provides the energy cost leading to an overall low adsorption energy. The final entry in Table 1 shows results for quite a different situation, that of CO adsorption on NiO rather than Ni metal. Nevertheless, the CO does bond atop surface Ni atoms, yet the Ni–C bondlength is a ˚ longer than for atop adsorption on Ni huge 0.34 A ˚ longer than the Ni–C metal, and is even 0.14 A bondlength for CO adsorbed in a three-fold coordinated hollow site. However, the adsorption energy is extremely low, at only 0.30 eV. Bearing in mind that the adsorption energy of CO on Ni metal is 1.2 eV, then in the three-fold coordinated site one might crudely assign 0.4 eV to each Ni–C bond, so the further increase in the bondlength in lowering the bond energy to 0.3 eV is consistent with a simple qualitative correlation of individual bondlength and bond strength. Of course, this analysis is highly simplistic, yet achieving a proper theoretical description of CO bonding to NiO has proved a major challenge [37,38]. So far calculations for the NiO(1 0 0)–CO adsorption system have found unrealistically long Ni– ˚ [39] and 2.86 A ˚ [40]. C bondlengths of 2.49 A
4. Conclusions A key challenge in surface structure determination has been to determine interatomic distances with a precision which is chemically significant; the problem
D.P. Woodruff / Applied Surface Science 237 (2004) 13–20
is especially demanding, but also especially interesting, in the case of molecular adsorbates in which intramolecular bondlength changes may also be a strong ‘fingerprint’ of bonding behaviour. While this problem is far from being solved in a routine fashion, it is clear that, at least in a limited range of problems, information is emerging which is leading to new levels of understanding. For CO on Ni surfaces, one particularly simple result is the Pauling-like correlation of Ni–C bondlengths and bond order. Introducing a hydrogen coadsorbate or placing the Ni atom in a compound clearly has a significant impact on both bond strength and bondlength, with some of the systematics broadly consistent with expectations. On the other hand, for the systems Ni(1 0 0)–CO/H and Cu(1 1 1)–C2H2 in particular, the combination of the experimental structural results and theoretical total energy calculations has proved of great value. In both of these systems the theoretical calculations indicate that the adsorption energy is not, in fact, a reliable indicator of the local bonding strength because the bonding leads to energy costs for surface or adsorbate distortions which must accompany the adsorption. This highlights the fact that the desorption temperature may not generally be a reliable indicator of ‘strong’ and ‘weak’ chemisorption.
Acknowledgements The author acknowledges the financial support of the Engineering and Physical Sciences Research Council (UK) and the invaluable role of many collaborators, notably at the University of Warwick and the Fritz Haber Institute in Berlin, whose names appear in the citations at the end of this paper.
References [1] P.R. Watson, M.A. Van Hove, K. Hermann, NIST Surface Structure Database Ver. 4.0, NIST Standard Reference Data Program, Gaithersburg, MD, USA, 2002. [2] D.P. Woodruff, Surf. Sci. 500 (2002) 147. [3] K.A.R. Mitchell, Surf. Sci. 149 (1985) 93. [4] K.A.R. Mitchell, S.A. Schlatter, R.N.S. Sodhi, Can. J. Chem. 64 (1986) 1435. [5] D.P. Woodruff, A.M. Bradshaw, Rep. Prog. Phys. 57 (1994) 1029.
19
[6] J.E. Demuth, D.E. Eastman, Phys. Rev. Lett. 32 (1974) 1123. [7] S. Bao, Ph. Hofmann, K.-M. Schindler, V. Fritzsche, A.M. Bradshaw, D.P. Woodruff, C. Casado, M.C. Asensio, J. Phys. Condens. Matter 6 (1994) 93. [8] N. Sheppard, Rev. Phys. Chem. 39 (1988) 589. [9] S. Bao, K.-M. Schindler, Ph. Hofmann, V. Fritzsche, A.M. Bradshaw, D.P. Woodruff, Surf. Sci. 291 (1993) 295. [10] N.D.S. Canning, M.D. Baker, M.A. Chesters, Surf. Sci. 111 (1981) 441. [11] K. Hermann, M. Witko, Surf. Sci. 337 (1995) 205. [12] J.-H. Kang, R.L. Toomes, J. Robinson, D.P. Woodruff, O. Schaff, R. Terborg, R. Lindsay, P. Baumga¨ rtel, A.M. Bradshaw, Surf. Sci. 448 (2000) 23. [13] C. Stellwag, G. Held, D. Menzel, Surf. Sci. 325 (1995) 379. [14] G. Held, M.P. Bessent, S. Titmuss, D.A. King, J. Chem. Phys. 105 (1996) 11305. [15] O. Schaff, V. Fernandez, Ph. Hofmann, K.-M. Schindler, A. Theobald, V. Fritzsche, A.M. Bradshaw, R. Davis, D.P. Woodruff, Surf. Sci. 348 (1996) 89. [16] I.S. Butler, J.F. Harrod, Inorganic Chemistry: Principles and Applications, Benjamin/Cummings, Redwood City, CA, 1989 (pp. 679–689). [17] E. Uhlig, Organometallics 12 (1993) 4751. [18] S. Andersson, J.B. Pendry, Surf. Sci. 71 (1978) 75. [19] K. Heinz, E. Lang, K. Mu¨ ller, Surf. Sci. 87 (1979) 595. [20] M. Passler, A. Ignatiev, F. Jona, D.W. Jepsen, P.M. Marcus, Phys. Rev. Lett. 43 (1979) 360. [21] S. Andersson, J.B. Pendry, J. Phys. C: Solid State Phys. 13 (1980) 3547. [22] S.Y. Tong, A. Maldonado, C.H. Li, M.A. Van Hove, Surf. Sci. 94 (1980) 73. [23] L. Becker, S. Aminpirooz, B. Hillert, M. Pedio, J. Haase, D.L. Adams, Phys. Rev. B 47 (1993) 9710. [24] K.-M. Schindler, Ph. Hofmann, K.-U. Weiss, R. Dippel, V. Fritzsche, A.M. Bradshaw, D.P. Woodruff, M.E. Davila, M.C. Asensio, J.C. Conesa, A.R. Gonzalez-Elipe, J. Electr. Spectros. Rel. Phenom 64–65 (1993) 75. [25] L.D. Mapledoram, M.P. Bessent, A.D. Wander, D.A. King, Chem. Phys. Lett. 228 (1994) 527. [26] L. Pauling, The Nature of the Chemical Bond, Cornell University Press, Ithaca, NY, 1939, 1940. 1960. [27] L.D. Mapledoram, M.P. Bessent, A. Wander, D.A. King, Chem. Phys. Lett. 228 (1994) 527. [28] M.E. Davila, M.C. Asensio, D.P. Woodruff, K.-M. Schindler, Ph. Hofmann, K.-U. Weiss, R. Dippel, P. Gardner, V. Fritzsche, A.M. Bradshaw, J.C. Conesa, A.R. Gonza´ lez-Elipe, Surf. Sci. 311 (1994) 337. [29] H.C. Peebles, D.E. Peebles, J.M. White, Surf. Sci. 125 (1983) 87. [30] B.E. Koel, D.E. Peebles, J.M. White, Surf. Sci. 125 (1983) 709. [31] P.A. Redhead, Vacuum 12 (1962) 203. [32] D.I. Sayago, J.T. Hoeft, M. Polcik, M. Kittel, R.L. Toomes, J. Robinson, D.P. Woodruff, M. Pascal, C.L.A. Lamont, G. Nisbet, Phys. Rev. Lett. 90 (2003) 116104. [33] J.T. Hoeft, M. Polcik, D. Sayago, M. Kittel, R. Terborg, R.L. Toomes, J. Robinson, D.P. Woodruff, M. Pascal, G. Nisbet, C.L.A. Lamont, Surf. Sci. 540 (2003) 441.
20
D.P. Woodruff / Applied Surface Science 237 (2004) 13–20
[34] I. Stensgaard, F. Jakobsen, Phys. Rev. Lett. 54 (1985) 711. [35] T.R. Mattsson, G. Wahnstro¨ m, L. Bengtsson, B. Hammer, Phys. Rev. B 56 (1997) 2258. [36] G. Kresse, J. Hafner, Surf. Sci. 459 (2000) 287. [37] J.-T. Hoeft, M. Kittel, M. Polcik, S. Bao, R.L. Toomes, J.-H. Kang, D.P. Woodruff, M. Pascal, C.L.A. Lamont, Phys. Rev. Lett. 87 (2001) 086101. [38] M. Kittel, J.-T. Hoeft, M. Polcik, S. Bao, R.L. Toomes, J.-H. Kang, D.P. Woodruff, M. Pascal, C.L.A. Lamont, Surf. Sci. 499 (2002) 1.
[39] G. Pacchioni, G. Cogliandro, P.S. Bagus, Surf. Sci. 255 (1991) 344. [40] M. Po¨ hlchen, V. Staemmler, J. Chem. Phys. 97 (1992) 2583. [41] J.T. Stuckless, N. Al-Sarraf, C. Wartnaby, D.A. King, J. Chem. Phys. 99 (1993) 2202. [42] C. Zhao, M.A. Passler, Surf. Sci. 320 (1994) 1. [43] F. Favot, A.D. Corso, A. Baldereschi, Phys. Rev. B 63 (2001) 115416. [44] R. Wichtendahl, M. Rodriguez-Rodrigo, U. Ha¨ rtel, H. Kuhlenbeck, H.-J. Freund, Surf. Sci. 423 (1999) 90.