Sulfur, normally a poison, strongly promotes chemoselective catalytic hydrogenation: Stereochemistry and reactivity of crotonaldehyde on clean and S-modified Cu(111)

Article abstract of DOI:10.1039/b517154g

Sulfur adatoms strongly activate the otherwise inert Cu(111) surface towards chemoselective hydrogenation of crotonaldehyde by electronically perturbing and strongly tilting the reactant. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b517154g

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Sulfur, normally a poison, strongly promotes chemoselective
catalytic hydrogenation: stereochemistry and reactivity of
crotonaldehyde on clean and S-modified Cu(111){
May E. Chiu, Georgios Kyriakou, Federico J. Williams, David J. Watson, Mintcho S. Tikhov and
Richard M. Lambert*
Received (in Cambridge, UK) 2nd December 2005, Accepted 24th January 2006
First published as an Advance Article on the web 7th February 2006
DOI: 10.1039/b517154g
Sulfur adatoms strongly activate the otherwise inert Cu(111)
surface towards chemoselective hydrogenation of crotonalde-
hyde by electronically perturbing and strongly tilting the
reactant.
measurements were carried out on the SuperESCA beamline at the
ELETTRA synchrotron radiation facility in Trieste, Italy using
4
well-established procedures detailed elsewhere. The data were
5
processed in accordance with standard methodology.
+
The Cu(111) surface was cleaned by Ar bombardment at 800 K,
Unsaturated alcohols are valuable intermediates in the fine
chemicals and pharmaceuticals industries. Their production by
chemoselective heterogeneous hydrogenation of the corresponding
followed by annealing in vacuum at 800 K. Surface integrity was
checked by LEED and Auger electron spectroscopy in Cambridge,
and by XPS in Trieste. Reagent grade crotonaldehyde (>99.9%,
Sigma Aldrich) was purified before use by means of freeze-pump-
thaw cycles; surface coverages were calibrated by temperature
programmed desorption (Cambridge) and by fast XPS (Trieste) as
shown in the supplementary information.{ The monolayer point
was easily identifiable by both techniques, enabling crotonaldehyde
doses to be delivered with good accuracy (¡5%). Dissociative
chemisorption of dihydrogen on Cu(111) is activated and therefore
extremely slow under ultra-high vacuum conditions; accordingly a
1
unsaturated aldehydes is difficult because thermodynamics
favours CLC hydrogenation over CLO hydrogenation.
Therefore, the desired selectivity can only be achieved by exercising
kinetic control over the competing reaction pathways. Previously,
in the case of Pt(111), we showed by means of NEXAFS that
single crystal studies can provide insight into the observed
dependence on reactant partial pressures of CLC/CLO hydrogena-
2
tion chemoselectivity. However, in that case we were unable to
6
investigate the reactive properties of the system under vacuum
conditions due to decomposition of crotonaldehyde on platinum at
T > 300 K. Here, with Cu(111) we are able to examine both the
reactive behaviour and the corresponding adsorption geometries of
crotonaldehyde in the presence of (i) H(a) alone and (ii) H(a) +
S(a). It is found that sulfur, normally a potent catalyst poison,
strongly activates the otherwise inactive copper surface towards
crotyl alcohol formation, a result that is understandable in terms of
its effect on the electronic structure and orientation of the CLC
and CLO bonds. Our findings are in excellent accord with, and
provide an explanation for, the observations of Hutchings et al.,
hot filament source was used to generate a flux of H atoms, thus
enabling the required hydrogen coverages to be achieved on a
convenient timescale. Sulfur was deposited using an electrochemi-
7
cal source that produced a beam of (largely) S2 molecules,
enabling precise control of S coverage.
In every case, the dosing sequence was (a) sulfur at room
temperature (if used), (b) hydrogen at 200 K and (c) crotonalde-
hyde at 150 K. The effects of sulfur were studied by working with
the (!7 6 !7)R19u-S surface, its formation being monitored by
LEED and Auger spectroscopy. Adsorbate fractional coverages
X
(k ; X = crotonaldehyde, S, H) are specified relative to the number
2 4
who showed that addition of sulfur to Cu/Al O catalysts operated
density of metal atoms in the Cu(111) surface.
at atmospheric pressure greatly increased selectivity towards crotyl
3
alcohol production.
Fig. 1A shows TPR spectra acquired after co-adsorption of
crotonaldehyde and hydrogen (k = 0.23 and 0.67 respectively) on
clean Cu(111). It is apparent that no hydrogenation reactions
occur in this case: all the initially adsorbed crotonaldehyde
desorbed intact at y200 K, accompanied by dihydrogen
desorption at y325 K. Fig. 1B shows the corresponding TPR
data obtained from the sulfur pre-dosed (!7 6 !7)R19u-S surface
Crotonaldehyde hydrogenation was studied in an ultra-high
vacuum chamber (Cambridge) by means of temperature pro-
2
1
grammed reaction (TPR; 3 K s ) with mass spectrometric
detection of the desorbing species. The data are corrected for mass
spectrometer sensitivity, molecular ionisation cross-sections, and
fragmentation patterns. High resolution X-ray photoelectron
spectroscopy (XPS) and near-edge X-ray absorption fine structure
spectroscopy (NEXAFS) were used to examine surface composi-
tion, adsorbate electronic structure and orientation of the CLC and
CLO bonds in crotonaldehyde in the presence of H(a). These
(k = 0.43). A mixture of partial hydrogenation products desorbed;
crucially, crotyl alcohol, the desired and thermodynamically
disfavoured product was formed, along with the saturated
aldehyde. Quantification of these data via appropriate calibration
of the mass spectrometer showed that the selectivities towards
crotyl alcohol and butyraldehyde formation were y56% and
y34% respectively. This is in remarkable agreement with the
Department of Chemistry, University of Cambridge, Cambridge, UK
CB2 1EW. E-mail: rml1@cam.ac.uk; Fax: +44 1223 336362;
Tel: +44 1223 336467
3
findings of Hutchings et al. who investigated the same reaction
2 3
with S-promoted Cu/Al O catalysts operated under practical
{
Electronic supplementary information (ESI) available: TPD and XPS
crotonaldehyde uptake. See DOI: 10.1039/b517154g
conditions and found that S caused a substantial increase in
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Fig. 1 TPR spectra for (A) crotonaldehyde + H(a) (k = 0.23 and 0.67) and (B), the same as A, in the presence of S (k = 0.43). m/z 70 is scaled by 1/30.
selectivity towards crotyl alcohol formation. Specifically, at the
lowest temperature they used (y330 K) which is closest to the
temperature regime of our experiments, they observed selectivities
towards crotyl alcohol and butyraldehyde formation of 44% and
and CLO p* orbitals, weakening these bonds and thus activating
them towards hydrogenation.
The presence of both CLC and CLO resonances in the
NEXAFS (Fig. 3) provides strong confirmation that crotonalde-
hyde adsorbs intact on both the clean surface and the S-modified
surface; their intensities were invariant with time indicating the
absence of beam damage effects.
25% respectively. It is also apparent from Fig. 1 that S induced a
more strongly bound state of adsorbed crotonaldehyde, which
desorbed at y225 K. We suggest that this is the catalytically active
species—stronger binding of the molecule to the surface is
accompanied by a weakening of the intramolecular bonding, thus
Crotonaldehyde C K-edge NEXAFS data were acquired at five
photon incidence angles (h) for both the clean and S-modified
8
surfaces in the presence of hydrogen adatoms (kCr y 0.25; k
S
=
activating the molecule to hydrogenation; as we shall see, this
0.43; k = 0.67); they should therefore be relevant to an
H
view accords well with the XPS results.
understanding of the catalytic system. The dominant spectral
features are two sharp peaks marked A and B (Fig. 3) assigned to
The synchrotron XPS data are also consistent with the presence
of two distinct states of crotonaldehyde on the S-modified surface.
Fig. 2 shows C 1s XP spectra acquired at 77 K for crotonaldehyde
on the clean Cu(111) surface and in the presence of sulfur (k =
2
the CLC and CLO p* transitions, respectively. From the
characteristic changes in their intensities between h = 10u and
5
h = 90u it is immediately apparent that in the absence of sulfur
0
.43). In both cases, two peaks appear with 3 : 1 intensity ratio
2
consistent with the presence of intact crotonaldehyde molecules; it
both the CLC bond and the CLO bonds are essentially parallel to
the metal surface. Strikingly, in the presence of S, it is clear that the
molecular geometry is strongly altered (Fig. 3, bottom spectra) as
discussed in detail below.
is also apparent that S causes a very significant decrease in the
apparent binding energy of both components, pointing to the
presence of an S-perturbed state of crotonaldehyde. The emission
due to the S-induced state may be extracted from the raw data as a
difference spectrum, indicated by the red curve in Fig. 2 which
shows that S causes an equal reduction in the binding energy of
both components by y0.38 eV, consistent with S A molecule
charge transfer. This could imply partial population of the CLC
5
The results of a full analysis of the NEXAFS intensity data are
shown in Fig. 4 which permits an estimation of the CLC and CLO
tilt angles (a) with respect to the metal surface. In the absence of S,
both carbonyl and alkene functions are parallel to the surface—the
molecule lies flat. In the presence of sulfur, detailed interpretation
of the NEXAFS data is more complicated. Here, both the CLO
and CLC intensities initially decrease with h but then increase
Fig. 2 C 1s XP spectra at 77 K for crotonaldehyde submonolayer on
clean Cu(111) (top) and S-treated surface (bottom). Energy scale
referenced to the Cu 3p3/2 line at 75.0 eV. Difference spectrum shown in
red (scaled top spectrum (blue) subtracted from bottom spectrum).
Fig. 3 C K-edge NEXAFS spectra at 77 K for crotonaldehyde and H(a)
(k
H
= 0.67, crotonaldehyde and S coverages as in Fig. 2).
1
284 | Chem. Commun., 2006, 1283–1285
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Fig. 5 Suggested adsorption geometries for crotonaldehyde on Cu(111)
showing sulfur-induced tilting. The molecule is viewed down a C–C bond,
with CLO shown in blue and CLC shown in red.
and tilting of the CLC and CLO groups with respect to the surface,
thus facilitating interaction with adsorbed hydrogen. This re-
orientation is most pronounced for the CLC bond, favouring the
unsaturated alcohol, thus providing a plausible explanation for the
effects of promotion by S on the performance of dispersed Cu
Fig. 4 p* resonance intensities normalised to intensity at 10u for the CLC
and CLO bonds on clean Cu(111) and S/Cu(111) (k = 0.43).
S
again at normal incidence (h = 90u). This indicates the presence of
more than one type of chemisorbed crotonaldehyde—as expected
on the basis of the TPD and XPS results. Some molecules lie
essentially flat and some, perturbed by S, have both CLO and CLC
tilted with respect to the surface. Inspection of Fig. 4 shows that
the manifestation of tilting at h > 60u are much more pronounced
for CLC than for CLO. The implications are that (i) in the tilted
molecules CLC is much more inclined than CLO or (ii) there are
some molecules in which only CLC is tilted and CLO is unaffected
or (iii) a combination of both effects. We infer that co-adsorbed
sulfur electronically perturbs crotonaldehyde (XPS) activating it
for hydrogenation by strengthening its interaction with the surface
3
catalysts operated at atmospheric pressure. Our findings are also
2
in harmony with earlier observations on un-promoted Pt(111) in
which case it was shown that increased reactant coverage induced
tilting of the CLO bond thus favouring crotyl alcohol formation.
MEC acknowledges financial support from the Cambridge
University Oppenheimer Trust. The authors thank Mark Turner
for his assistance with figures, and Dr S. Lizzit and Dr L. Petaccia
for their assistance during the synchrotron experiments.
Notes and references
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2 A. J. Urquhart, F. J. Williams, O. P. H. Vaughan, R. L. Cropley and
R. M. Lambert, Chem. Commun., 2005, 1977–1979.
(TPD) and weakening the intramolecular bonding. The NEXAFS
results show that this process involves substantial tilting of the
CLC bond accompanied by some CLO tilting. The result is an
overall increase in reactivity. Possible adsorption geometries are
shown in Fig. 5 which depicts the more tilted CLC bond and the
less tilted CLO bond, a configuration that should favour carbonyl
hydrogenation relative to alkene hydrogenation as observed under
3
G. J. Hutchings, F. King, I. P. Okoye, M. B. Padley and C. H. Rochester,
J. Catal., 1994, 148, 453–463; G. J. Hutchings, F. King, I. P. Okoye,
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6069–6076.
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and D. A. Outka, Phys. Rev. B: Condens. Matter , 1987, 36, 7891–7905.
3
practical reaction conditions.
In summary, S adatoms on Cu(111) promote the activation of
crotonaldehyde towards hydrogenation, notably with respect to
formation of the unsaturated alcohol, which is the desired product.
This is the result of S-induced rehybridisation of the adsorbed
reactant which results in weakening of the intermolecular bonding
6 M. F. Luo, D. A. MacLaren, I. G. Shuttleworth and W. Allison,
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