J. Phys. Chem. C 2010, 114, 21457–21464
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Destabilizing Effects of Thiols on Bonding to a Noble Metal: The Effects of Methanethiolate on the Bonding of Aldehydes and Alcohols on Cu(111) A. Mulligan and M. Kadodwala* Department of Chemistry, Joseph Black Building, UniVersity of Glasgow, Glasgow G12 8QQ, U.K. ReceiVed: July 14, 2010; ReVised Manuscript ReceiVed: NoVember 3, 2010
We demonstrate that the presence of coadsorbed methanethiolate (CH3S-) dramatically destabilizes the bonding toward Cu(111) of two higher alcohols (butanol and crotyl alcohol) but in contrast has little effect on the strength of interaction of the analogous aldehydes (butanal and crotonaldehyde). This behavior contrasts with the stabilizing effects that coadsorbed sulfur has on the same species. The destabilizing effects of CH3S- are rationalized in terms of its ability to influence bonding through the electric fields associated with its surface dipole moment. This study provides evidence for an alternative mechanism by which sulfur containing organic species can improve the selectivity of Cu catalysts toward unsaturated alcohols in the hydrogenation of R, β-unsaturated aldehydes. Introduction Controlling the selectivity in the hydrogenation of multifunctional molecules is a key problem in the industrial production of fine chemicals. This has consequently motivated basic research, model catalytic and surface science studies, into issues surrounding selectivity. Hutchings and co-workers, demonstrated in a model catalytic study that in the selective hydrogenation of the R, β-unsaturated aldehyde crotonaldehyde, sulfur modification of Cu catalysts enhances the selectivity toward the unsaturated alcohol, crotyl alcohol.1-3 In the context of sulfur being perceived as being detrimental to catalyst performance, this was a somewhat surprising result. The authors concluded that the presence of sulfur in any form resulted in an increased selectivity to crotyl alcohol, although the type of sulfur moiety did have a marked result upon the magnitude of the effect with selectivity changing in the order: CS2 ∼ SO2 < dimethyl disulfide (DMS) ∼ tetrahydrothiophene∼ dimethyl sulfoxide (DMSO) < thiophene. The effect was also concluded to be a general one for Cu catalysts and was found to be independent of catalyst support, and that the enhanced selectivity of sulfur derived from electronic effects. Hutchings and co-workers found similar behavior on supported gold catalysts.4-6 Recently, in model single crystal studies, Lambert and co-workers7,8 have shown that preadsorbed atomic sulfur enhances the bonding of the CdO group of crotonaldehyde on Cu(111), which they concluded was the origin of the selective hydrogenation observed in catalytic studies. In related work we have previously shown that preadsorbed sulfur enhances the bonding of cyclohexene and benzene on a Cu surface.9 We rationalized this enhanced bonding in terms of the ability of adsorbed sulfur to influence the balance between charge donation from the adsorbate to metal, and back-donation from the metal to adsorbate. The motivation for this study was the observation by Hutchings and co-workers that superior selectivities are achieved with sulfur-containing organic species than with CS2 and SO2. In this study we have contrasted the influences of preadsorbed methanethiolate (generated by the decomposition of dimethyl disulfide (DMDS)) and atomic sulfur on the bonding of crotonaldyde, butanal, crotyl alcohol, and butanol toward * Corresponding author. E-mail:
[email protected].
Cu(111), with the intention of determining whether the nature of the sulfur containing species strongly influences the bonding of the aldehydes and alcohols. We have observed that preadsorbed methanethiolate and atomic sulfur have dramatically different effects on the bonding of alcohols. The bonding of both alcohols is significantly destabilized by methanethiolate (in the case of crotyl alcohol, its interaction with the copper surface is completely inhibited), while atomic sulfur enhances the bonding of both alcohols to bare Cu(111). Preadsorbed methanethiolate does not significantly affect the strength of the interaction of the aldehydes. In contrast, atomic sulfur increases the strength of aldehyde bonding interaction. Consequently, we propose that the presence of chemisorbed alkyl thiolates on Cu catalyst particles could enhance selectivity toward the formation of alcohols in the selective hydrogenation of R,β-unsaturated aldehydes because they destabilize the product, whereas the selectivity induced by chemisorbed sulfur derives from its ability to preferentially activate the CdO group. Experimental Section Experiments were performed in two separate UHV systems, where temperature programmed desorption (TPD) data were collected in a system that has been described in detail previously,10 desorption profiles were collected using a heating rate of 0.5 K s-1. UV photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS) measurements were performed in a second system, which will be briefly described. The system was equipped with the usual sample preparation facilities and low energy electron diffraction (LEED) optics for confirming crystal quality. In addition, it has a discharge lamp (VG Ltd.) that provides He(I) radiation and a concentric hemispherical analyzer (CHA) (CLAM 2 VG Ltd.). For the collection of UPS and XPS spectra the analyzer operated at pass energies of 20 and 40 eV, respectively. All UPS spectra were collected in normal emission and with radiation incident at 47° to the surface normal. Work function change (∆φ) measurements were made by monitoring the shifts in the secondary electron cutoff from UP spectra collected from a sample biased at -9.0 V.
10.1021/jp106522f 2010 American Chemical Society Published on Web 11/22/2010
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Figure 1. (a) TPD profile showing the desorption of DMDS (parent ion 94 amu) from a multilayer coverage on Cu(111). Desorption is observed from DMDS multilayers at 154 K. (b) TPD profiles showing desorption of methane (16 amu) from Cu(111) exposed to 0.1 (blue), 0.25 (green), 0.5 (red), and 2 (black) L of DMDS. Desorption is observed between 325 and 475 K. (c) TPD profiles showing desorption of ethane (30 amu) from Cu(111) exposed to 0.1 (blue), 0.25 (green), 0.5 (red), and 2 (black) L DMDS. Desorption is observed between 375 and 450 K. (d) TPD profiles showing desorption of hydrogen (2 amu) from Cu(111) exposed to 0.1 (blue), 0.25 (green), 0.5 (red), and 2 (black) L DMDS. Desorption is observed at 430 K in 0.5 and 2 L profiles. All profiles were collected with a heating rate of 0.5 K s-1.
In both UHV systems the Cu(111) surface was cleaned by cycles of Ar+ bombardment (1 keV, 40 min, ca. 16 µA) followed by annealing to 900 K. Surface cleanliness was monitored by electron beam AES (collected using an retarding field analyzer (RFA)) in the TPD system, while XPS was used in the second system. In both cases LEED was used to monitor surface quality and adsorption was carried out at crystal temperatures of ca. 110 K. Results DMDS/Cu(111). The adsorption of DMDS (CH3S)2 on Cu(111) has been investigated in numerous photoemission studies.11-15 This work has shown that at room temperature DMDS adsorbs dissociatively, generating a chemisorbed thiolate moiety (CH3S-). At cryogenic temperatures adsorption is predominantly molecular, with a small amount of dissociation occurring at defect sites, upon heating the molecularly adsorbed
DMDS completely dissociates producing thiolate. We have used TPD, UPS, and XPS to assess the thermal stability of the thiolate moiety in the temperature range required for our study. In Figure 1 is a TPD profile monitoring evolution of intact DMDS (parent ion 94 amu) from a surface which had multilayers deposited at ca. 110 K. Only a single desorption peak is observed with a Tmax ) 154 K, Figure 1a, attributed to multilayer desorption. The complete dissociation of the DMDS overlayer into chemisorbed thiolate, is demonstrated by the lack of molecular desorption of DMDS from a monolayer state. Dissociation of the thiolate moiety at temperatures above 325 K and the subsequent evolution of CH4 and C2H6 are evidenced by profiles, Figure 1b,c, obtained by monitoring the 16 and 30 amu fragments. Desorption of methane occurs over a broad temperature range, 325-475 K, while ethane evolution occurs over a somewhat narrower range, 375-450 K. In Figure 2 are UPS and XPS spectra collected that show the effects of sequential
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Figure 2. (a) S2s XP spectra of clean Cu(111) surface (black), methanethiolate on Cu(111) heated to 298 K (red) and 823 K (green). (b) C1s XP spectra of clean Cu(111) surface (black), methanethiolate on Cu(111) heated to 298 K (red) and 823 K (green). (c) Normal emission UPS spectra of mulyilayers of DMS (and methanethiolate) adsorbed on Cu(111) at 108 K (red), heated to sequentially higher temperatures. The gas phase DMS UPS spectrum (black) is also shown for comparison.11
annealing on a condensed multilayer of DMDS deposited at ca. 110 K. As expected, the spectrum collected from the multilayer correlates well with one obtained from the gas phase.11 Upon heating to 163 K there is a significant change, including an increase in emission from the Cu d-band 2.0-4.2 eV, attributable to multilayer desorption, and consistent with the desorption observed in Figure 1a. Further annealing of the overlayer up to 298 K leads to little significant change, the structure in the Cu d-band does become a little better resolved; however, by XPS there is no measurable change in C 1s signal intensity, indicative of an absence of significant desorption from the overlayer. The UPS spectrum collected at 423 K is consistent with the onset of dissociation of the thiolate overlayer, in particular the appearance of a feature between 4.5 and 6.5 eV, which can be attributed to emission from S3p orbitals of chemisorbed atomic sulfur.10 Finally, by 823 K the complete dissociation of the thiolate overlayer has occurred, with the UPS spectrum being identical to that of chemisorbed atomic sulfur; this is confirmed by XPS spectra, Figure 2, which show the absence of adsorbed C and a S 2p peak characteristic of chemisorbed atomic sulfur.9 In totality, these data clearly show that in the temperature range of interest in the current study 110-290 K the thiolate moiety remains intact. Finally, we have measured workfunction changes (∆Φ) induced by sequentially larger coverages of thiolate at room temperature, the absolute coverage of the overlayers has been calibrated against an ordered sulfur overlayer of known coverage ((7×7)R19.1°16). There are no reported workfunction (Φ) data on this system in the literature. In contrast to the effects of chemisorbed atomic sulfur, thiolate adsorption leads to the progressive linear decrease in Φ, Figure 3, with a saturated thiolate layer inducing a ∆Φ of -1.8 ( 0.1 eV. Initially, the results of these measurements would appear to be counterintuitive, since one may have presumed that methanethiolate acts, like atomic sulfur, as an electronegative adsorbate where an increase in work function upon adsorption is observed, Figure 3b. Indeed, a decrease in work function is more commonly associated with the adsorption of electropositive elements such as alkali metals. For example,
Na adsorption on Cu(111)17 causes a decrease in work function of up to 2.5 eV. However, we can rationalize our observations by consideration of the theoretical work of Ferral et al.18 In this study the authors concluded the bonding of the methanethiolate on Cu(111) is primarily through the sulfur p-orbitals, but they also suggested a lesser interaction with the orbitals of the methyl group. This results in a charge transfer from the metal to the adsorbate causing a depletion of electron density around the primary site of chemisorption, accompanied by accumulation of charge around the S atom. In addition, a small degree of charge depletion is also observed at the methyl group. Hence this study suggested a strong molecular dipole running along the molecule from the methyl group toward the more electronegative sulfur atom. Previous work has shown methanethiolate adsorbs on Cu(111) with the S-C bond in an orientation close to perpendicular to the surface.11-13 Hence, the dipole associated with the molecule will be antiparallel to that associated with the copper surface. Thus, unlike the situation for atomic sulfur where the direction of charge transfer increases the surface dipole, the antiparallel dipole associated with methanethiolate will have the opposite effect, reducing the magnitude of the surface dipole and therefore causing a decrease in work function. Crotonaldehyde, Butanal, Crotyl Alcohol, and 1-Butanol/ Cu(111). We have used TPD, XPS, and UPS to characterize the adsorption behavior of crotonaldehyde, butanal, crotyl alcohol, and 1-butanol on clean Cu(111). The adsorption of crotonaldehyde on Cu(111) has been previously studied with TPD and XPS by Chiu and co-workers;8 our data are qualitatively similar to that observed by them. There have been no previously reported studies of the three other molecules on Cu(111). The UPS, XPS, and TPD data obtained for the four molecules is consistent with them undergoing predominantly reversible molecular adsorption. The weak nature of interaction between the four molecules and the Cu(111) substrate is revealed by the UPS data; representative data are shown in Figure 4 (see Supporting Information for further data). The spectral emission in the range 2.0-4.2 eV originates from the d-band of the underlying substrate; multilayer spectra display less emission
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Figure 3. (a) ∆φ as a function of thiolate coverage. The line is intended to guide the eye. (b) For comparison the ∆φ as a function of Sads coverage. The structural phases observed by LEED are also labeled; the lines are intended to guide the eye.
Figure 4. Difference spectra, obtained by subtracting the spectra obtained prior to adsorption from the monolayer spectra. For butanol and butanal monolayers on clean Sads and CH3S-ads preadsorbed layers are shown. The position of the lone pair oxygen (HOMO) is shown.
in this region due to greater levels of attenuation. The bands observed in all the spectra at binding energies >4 eV are associated with molecular orbitals of the adsorbates. On the basis of previous photoemission studies, we can attribute the adsorbate bands observed at lowest binding energies in the butanal and butanol spectra to emission from oxygen lone pair electrons.19,20 In the case of crotyl alcohol and crotonaldehyde the equivalent low binding energy band not only has a contribution from the
oxygen lone pairs but also has a contribution due to emission from the π orbitals;21 this is most apparent in the multilayer spectrum of crotyl alcohol. For all four molecules a small shift to higher binding energy relative to the unperturbed multilayers is observed in the position of the oxygen lone pair (HOMO) orbital, Table 1; all other orbitals retain the same relative positions. The largest level of shift is observed for crotonaldehyde, 0.5 eV, which is smaller than the values, typically between
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TABLE 1: Shifts in the Positions of the Oxygen Lone Pair (HOMO) Orbitals, Relative to the Unperturbed Molecules, Listed for the Four Molecules Adsorbed on Clean and Sads and CH3S-ads Co-adsorbed Layers
crotonaldehyde crotyl alcohol butanol butanal
Clean [eV] ((0.05)
S [eV] ((0.05)
CH3S- [eV] ((0.05)
0.50 0.30 0.20 0.30
0.80 0.50 0.50 0.60
0.00 0.00 0.10
1-2 eV, observed in previous photoemission studies of more strongly chemisorbed systems.37 The increase in binding energy of the lone pair electron HOMO orbital upon adsorption indicates that it is perturbed through a bonding interaction with the substrate. A small amount of dissociation is observed for both alcohols, as evidenced by the evolution of small amounts of aldehydes. This is comparable to the previously reported behavior of higher alcohols on Cu surfaces,22-24 where dissociation at defect sites to alkoxy moieties, which subsequently decompose upon heating to aldehydes/ketones, is observed; in the present case small amounts of crotonaldehyde and butanal are evolved. Chiu and co-workers have demonstrated that crotonaldehyde bonds to bare Cu(111) via the CdO bond, with the molecular plane roughly parallel to the surface.8 The same group also illustrated that crotyl alcohol adopts a similar geometry, bonding to the surface through its oxygen atom. Given that the CdC bonds in crotyl alcohol and crotonaldehyde do not significantly interact with the substrate,7,8 we expect that butanol and butanal bond to the surface in a similar manner, via the OH and CO groups, and adopt similar adsorption geometries. Co-adsorption with Methanethiolate. In Figure 5, in blue, are TPD profiles showing the desorption of the four intact molecules from surfaces that had ca. 0.15 ML precoverage of adsorbed thiolate, which corresponds to
