Article pubs.acs.org/JPCC
Adsorption Geometry Determines Catalytic Selectivity in Highly Chemoselective Hydrogenation of Crotonaldehyde on Ag(111) Katrin Brandt,† May E. Chiu,‡ David J. Watson,*,§ Mintcho S. Tikhov, and Richard M. Lambert*,⊥ Department of Chemistry, Cambridge University, Lensfield Road, Cambridge, CB2 1EW, United Kingdom S Supporting Information *
ABSTRACT: The chemoselective hydrogenation of crotonaldehyde to crotyl alcohol was studied by temperature-programmed desorption/reaction, high-resolution XPS, and NEXAFS. The organic molecule adsorbed without decomposition, all three possible hydrogenation products were formed and desorbed, and the clean overall reaction led to no carbon deposition. Selectivities up to 95% were found under TPR conditions. The observed behavior corresponded well with selectivity trends previously reported for Ag/SiO2 catalysts, and the present findings permit a rationalization of the catalytic performance in terms of pronounced coverage-dependent changes in adsorption geometries of the reactant and the products. Thus, at low coverages, the CO bond in crotonaldehyde lies almost parallel to the metal surface, whereas the CC was appreciably tilted, favoring hydrogenation of the former and disfavoring hydrogenation of the latter. With increasing coverage of reactants, the CC bond was forced almost parallel to the surface, rendering it vulnerable to hydrogenation, thus markedly decreasing selectivity toward formation of crotyl alcohol. Butanol formation was the result of an overall two-step process: crotonaldehyde → crotyl alcohol → butanol, further hydrogenation of the desired product crotyl alcohol being promoted at high hydrogen coverage due to the CC bond in the unsaturated alcohol being driven from a tilted to a flat-lying geometry. Finally, an explanation is offered for the strikingly different behavior of Ag(111) and Cu(111) in the chemoselective hydrogenation of crotonaldehyde in terms of the different degrees of charge transfer from metal to CO π bond, as suggested by C 1s XPS binding energies.
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INTRODUCTION The chemoselective hydrogenation of α,β-unsaturated aldehydes, especially that of crotonaldehyde to crotyl alcohol, has been extensively studied by traditional methods of catalytic science due to the value of such unsaturated alcohols as versatile intermediates in the production of fine chemicals and pharmaceuticals. An extensive literature exists, and comprehensive reviews of the subject include refs 1−3. CO hydrogenation in the presence of a CC functionality is challenging and fundamentally interesting because thermodynamics favors hydrogenation of the latter to form the (undesirable) saturated aldehyde. Thus, a suitable catalyst is required in order to manipulate kinetic effects so as to favor CO hydrogenation: however, only a few direct experimental investigations of the associated phenomena carried out under well-defined conditions have been reported. Over the last 15 years, classical studies involving platinum and copper nanoparticle catalysts have dominated the field only recently, has attention turned to dispersed silver. Usually encountered as an oxidation catalyst, silver has emerged as a promising and effective practical catalyst for chemoselective hydrogenation of unsaturated aldehydes,4−7 as first demonstrated by Claus et al., who used Ag/SiO2 catalysts and found steady-state selectivities as high as 63% for the production of crotyl alcohol from crotonaldehyde.5 Thus far, no fundamental studies of this system have been reported. © 2012 American Chemical Society
Here, we give an account of the surface chemistry, adsorption geometry, and hydrogenation behavior of crotonaldehyde on the Ag(111) surface, studied by temperature-programmed desorption/reaction (TPR/TPD), high-resolution X-ray photoelectron (XPS), and near-edge X-ray absorption fine structure spectroscopy (NEXAFS). Pertinent properties of the three possible hydrogenation products, crotyl alcohol, butyraldehyde, and n-butanol, are also described. The chemoselective hydrogenation activity observed under ultra-high-vacuum conditions corresponds well with selectivity trends previously reported for Ag/SiO2 catalysts, and our findings permit a rationalization of the catalytic behavior in terms of coverage-dependent changes in adsorption geometries of the reactant and the products. The clean unmodified Ag(111) surface exhibits very high intrinsic selectivity toward crotyl alcohol formation (up to 95% under TPR conditions), in striking contrast to Cu(111).8,9 In that case, the clean surface was entirely inert, whereas addition of sulfur activated the system toward chemoselective hydrogenation, again in accord with the steady-state behavior of practical Cu nanoparticle catalysts.10 The observed high selectivity of the Ag(111) surface toward crotyl alcohol formation is in accord with studies using Received: September 13, 2011 Revised: January 23, 2012 Published: January 24, 2012 4605
dx.doi.org/10.1021/jp208831h | J. Phys. Chem. C 2012, 116, 4605−4611
The Journal of Physical Chemistry C
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supported silver nanoparticles11,12 where larger particles consisting of a greater proportion of (111) facets showed enhanced selectivity to crotyl alcohol compared to smaller particles.
Article
RESULTS AND DISCUSSION Adsorption−Desorption Chemistry. The adsorption and desorption characteristics of crotonaldehyde and its three hydrogenation products were determined by TPD and XPS in order to provide reference data necessary for analysis of the hydrogenation results. TPD profiles for each of these four species are shown in Figure 1A: submonolayer coverages were
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EXPERIMENTAL METHODS The surface chemistry and hydrogenation of crotonaldehyde on Ag(111) were studied by TPD and TPR in Cambridge, U.K., and by high-resolution fast-XPS and NEXAFS on the SuperESCA beamline at the ELETTRA synchrotron radiation facility in Trieste, Italy. TPD and TPR experiments were carried out in an ultra-high-vacuum chamber of conventional design, using a linear heating rate of 4 K s−1 with simultaneous detection of all desorbing species by a quadrupole mass spectrometer. Data are corrected for mass spectrometer sensitivity, molecular ionization cross sections, and molecular fragmentation patterns so as to be comparable with NIST reference spectra. Desorption yields and hence conversions and selectivities were calculated by means of calibration data consisting of sequences of TPD uptake experiments that established the saturation coverage of each molecule; in each case, onset of a characteristic multilayer desorption feature enabled identification of the monolayer point. Coverages are thus quoted in terms of nominal monolayers (MLs). High-resolution XPS and NEXAFS were carried out to study the electronic structure and orientation of CO and CC bonds with respect to the surface. Experimental details relevant to the synchrotron experiments are provided elsewhere,13−15 and the NEXAFS data were processed by standard methodology.16,17 XPS binding energies were corrected for monochromator error, and adsorbate coverages were calibrated by continuously monitoring the C 1s XPS signal during exposure of the sample to each adsorbate, the monolayer point being identified by shifts in the binding energies of the peak maxima. The Ag(111) sample was cleaned by cycles of Ar + bombardment at room temperature, 600 K, 800 K, and again at room temperature, followed by annealing in vacuum at 800 K. Surface condition was monitored by LEED and Auger spectroscopy in Cambridge and by XPS at ELETTRA. Crotonaldehyde (≥99.5%) and its three hydrogenation products, crotyl alcohol (≥97%), butyraldehyde (≥99%), and n-butanol (≥99.5%), were obtained from Sigma Aldrich, further purified by freeze−pump−thaw cycles and dosed by backfilling the vacuum chamber. Dissociative adsorption of dihydrogen on Ag(111) is significantly activated18,19 so that dosing with H2 is not a practical means of delivering chemisorbed hydrogen to the surface under vacuum conditions. Accordingly, atomic hydrogen was generated by means of a hot filament atomizer20,21 that delivered a flux of H atoms directly onto the Ag(111) surface. Hydrogen exposures are quoted as fractions of a monolayer, based on calibration experiments that showed that an exposure of 60 langmuirs of H2(g) with the atomizer switched on gave a coverage of ∼1 ML of H(a). In all cases, the dosing sequence of adsorbates onto the clean, liquid-nitrogen-cooled single crystal surface was (1) organic species, then (2) hydrogen. This sequence mirrors the procedure employed during high-pressure catalytic testing.22,23 Precovering the surface with hydrogen passivated it toward reaction with subsequently adsorbed crotonaldhyde, which desorbed without reaction, as also found by Zaera et al. for Pt(111) + H(a) + crotonaldehyde.24 Base temperatures were 140 K in Cambridge and 123 K at ELETTRA.
Figure 1. (A) TPD spectra of crotonaldehyde, crotyl alcohol, butyraldehyde, and butanol on Ag(111) after adsorption at 140 K. (B) C 1s XPS of the same four molecules on Ag(111) at 123 K.
used so as to simulate behavior representative of practical reaction conditions. Reported desorption profiles correspond to the most intense ion observed with our quadrupole instrument in the case of each moleculethe parent ion of crotonaldehyde and fragment ions in the case of the three products. In every case, several other m/z signals corresponding to diagnostic ions in each molecule’s fragmentation pattern were also monitored: the relative intensities and profiles of these auxiliary data confirmed that the four molecules adsorbed and desorbed intact, without measurable decomposition, as confirmed by XPS (see later). The asymmetric peak shapes indicate approximately first-order desorption kinetics, the shoulders observed at high temperatures being ascribed to desorption from defect sites. In the case of the reactant, this component corresponds to
