Mechanism of Selective Hydrogenation of α,β-Unsaturated Aldehydes

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J. Phys. Chem. C 2009, 113, 13231–13240

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Mechanism of Selective Hydrogenation of r,β-Unsaturated Aldehydes on Silver Catalysts: A Density Functional Study Kok Hwa Lim,†,‡ Amjad B. Mohammad,† Ilya V. Yudanov,†,§ Konstantin M. Neyman,*,| Michael Bron,⊥ Peter Claus,⊥ and Notker Ro¨sch*,† Department Chemie and Catalysis Research Center, Technische UniVersita¨t Mu¨nchen, 85747 Garching, Germany, School of Chemical and Biomedical Engineering, Nanyang Technological UniVersity, Singapore 637459, BoreskoV Institute of Catalysis, Russian Academy of Sciences, 630090 NoVosibirsk, Russia, Institucio´ Catalana de Recerca i Estudis AVanc¸ats (ICREA), 08010 Barcelona, Spain, Departament de Quı´mica Fı´sica i Institut de Quı´mica Teo`rica i Computacional, UniVersitat de Barcelona, 08028 Barcelona, Spain, and Ernst-Berl-Institut, Technische Chemie II, Technische UniVersita¨t Darmstadt, 64287 Darmstadt, Germany ReceiVed: March 7, 2009; ReVised Manuscript ReceiVed: May 26, 2009

Supported silver catalysts exhibit a remarkably high selectivity in the industrially important hydrogenation of R,β-unsaturated aldehydes to unsaturated alcohols. We carried out density functional calculations to clarify factors that affect the catalytic function of silver in hydrogenating unsaturated aldehydes. We examined the activity and the selectivity of model silver catalysts for acrolein, the simplest, yet most difficult unsaturated aldehyde to be selectively hydrogenated. We focused on describing bulky catalyst particles, represented by sites on extended silver surfaces, on the regular clean Ag(110) surface and the surface Osub/Ag(111) with subsurface oxygen centers. On Ag(110) our results imply propanal, the undesired saturated aldehyde, to be the main product. In contrast, the calculations suggest a very high selectivity of Osub/Ag(111) for the corresponding unsaturated alcohol, allyl alcohol, although the activity of this system is lower than that of clean silver. At variance with Pt(111), where the selectivity to allyl alcohol is strongly reduced by the hindered desorption of the latter, allyl alcohol and propanal products are predicted to desorb easily from both Ag(110) and Osub/Ag(111) at common reaction temperatures. We also analyzed inherent limitations for an accurate description of the chemical regioselectivity by contemporary computational methods. 1. Introduction The partial hydrogenation of R,β-unsaturated aldehydes (UALs) is an industrially important route to unsaturated alcohols (UOLs),1,2 yielding valuable fine chemicals for producing perfumes, pharmaceuticals, and flavoring materials. However, conventional catalysts of group VIII metals exhibit a very low selectivity to the desired UOLs (e.g., ∼2% on Pt), compared to saturated aldehydes (SALs) and alcohols (SOLs).1,2 The targeted selectivity is somewhat larger for higher UALs, R1R2Cd CHsCHdO, as their bulky substituents R1, R2 sterically disfavor the activation of the CdC bond.1,2 Therefore, the transformation of the simplest UAL acrolein (Figure 1, A, R1 ) R2 ) H) to the corresponding UOL allyl alcohol (AA, propenol) appears to be particularly difficult compared to the corresponding SAL propanal (PA) because space-filling substituents at the CdC bond are lacking. In general, the intramolecular selectivity, i.e., the preferred hydrogenation of the CdO vs the CdC bonds, can be controlled by the nature of the dominant metal (Pd, Pt vs Ag, Au), the presence of a second metal (Sn, In, and Cd), the size of the metal particles, electrondonating or -withdrawing effects of ligands induced by the catalyst support material, steric constraints in the metal environ* Corresponding authors. E-mail: [email protected]; roesch@ ch.tum.de. † Technische Universita¨t Mu¨nchen. ‡ Nanyang Technological University. § Boreskov Institute of Catalysis. | ICREA & Universitat de Barcelona. ⊥ Technische Universita¨t Darmstadt.

Figure 1. Elementary steps of the partial hydrogenation of adsorbed acrolein by atomic H studied on silver catalysts.

ment, strong metal-support interactions, etc.3 Various factors affecting the selectivity for UOL on Pt(111) surface have been discussed, in particular the orientation of CdC and CdO bonds of crotonaldehyde.4,5 Silver is a widely used oxidation catalyst.6,7 Surprisingly, supported silver catalysts8 exhibit a rather high selectivity (up to 42%) in the hydrogenation of A to AA.9 Further studies substantiated the crucial role of silver as active component in the selective hydrogenation.9-11 Experimentally ascertained key features of this catalytic process are as follows:11 (i) the formation of UOL is not observed below a threshold pressure of ∼100 mbar; (ii) an abundance of defect sites significantly

10.1021/jp902078c CCC: $40.75  2009 American Chemical Society Published on Web 07/07/2009

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improves the catalytic performance and the selectivity to UOL, the latter being below 10% over bulk silver materials; (iii) the activity depends on the number of sites that are able to activate H2 and are concluded to be electron deficient; (iv) oxygen incorporated in silver catalysts during the reaction promotes hydrogen activation and notably affects (and recovers) both the activity and the selectivity of UAL hydrogenation. Reaction conditions were found to influence strongly the state of the catalysts, which further complicates the challenge of uncovering performance/properties relationships.11 In particular, a beneficial effect of oxygen pretreatment on the activity and the selectivity of Ag/SiO2 catalysts for the partial hydrogenation of acrolein has recently been demonstrated.12 Nevertheless, the mechanism of UAL hydrogenation on silver catalysts remained unclear at the molecular level. Even the activity of a clean silver surface appears to be surprising in light of the weak adsorption of acrolein,13 coupled with the low probability of H2 activation on silver surfaces according to a variety of experimental surface science14,16-18 and theoretical19-24 studies of different Ag systems. This lack of knowledge of the hydrogenation mechanism is in contrast to Pt-based catalysts where recent density functional (DF) calculations on entire chains of acrolein transformations in the course of the (partial) hydrogenation on Pt(111)25,26 demonstrated that theoretical methods coupled with kinetic models are able to provide useful insights into “realworld” catalytic reactions. To clarify activity and selectivity issues of UAL hydrogenation (exemplified by A) to UOL (AA) and SAL (PA) over silver catalysts, we carried out DF calculations of the corresponding reaction pathways. In view of the complexity of the catalytic system we focused on describing bulky catalysts, represented by sites on extended silver surfaces. To reveal the effect of catalyst pretreatment in an oxygen atmosphere on the activity and the selectivity, we also addressed oxygen-modified silver besides the clean metal. Having probed a series of clean and modified silver models, we studied in detail the regular clean Ag(110) surface and the Ag(111) surface with incorporated subsurface oxygen atoms. We determined reaction and activation energies for essential elementary steps of A hydrogenation (Figure 1) on these two model Ag surfaces. 2. Models and Computational Details The DF calculations were performed with the plane-wave based Vienna ab initio simulation package (VASP)27-29 using a generalized gradient approximation, the exchange-correlation functional PW91.30 The effect of the core electrons was taken into account with the projector augmented wave (PAW) method.31,32 We employed an energy cutoff of 400 eV for the plane-wave basis set. For the integrations over the Brillouin zone, we combined (5 × 5 × 1) Monkhorst-Pack grids33 with the first-order Methfessel-Paxton smearing technique (broadening 0.15 eV).34 The final energies were extrapolated to zero smearing. Previously, the adsorption of acrolein on clean planar silver surfaces was found to be very weak and hence not accompanied by any essential bond activation.13 Therefore, anticipating more open Ag surfaces to be more reactive, we selected the Ag(110) surface to represent a clean silver catalyst. A five-layer slab with a (4 × 2) surface unit cell (eight Ag atoms per layer) was employed to describe the Ag(110). The Ag(111) surface with oxygen atoms, Osub, located in interstitial octahedral subsurface (oss) positions was also modeled by a five-layer slab with a (3 × 3) unit cell (1/3 of nine oss sites are occupied by Osub). For each type of models, we allowed the structure of the “top” two

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Figure 2. Sketches of the substrate models studied in the present work: (a) five-layer slab model of Ag(110) with a (4 × 2) surface unit cell, (b) five-layer slab model of Osub/Ag(111) with a (3 × 3) unit cell and oxygen atoms, Osub, located in interstitial octahedral subsurface positions, (c) the added-row structure p(4 × 1)O/Ag(110) obtained by depositing two Ag and two O atoms per unit cell on model a, (d) model O/Ag(110) of isolated oxygen adatoms on a unit cell (3 × 2).

Ag layers to relax in the absence of adsorbates; the remaining three layers at the “bottom” of the metal slab were kept fixed at the optimized geometry of the bulk material (Ag-Ag ) 293 pm). The added-row structure p(4 × 1)O/Ag(110) was obtained by formally depositing two Ag and two O atoms per surface unit cell of the same Ag(110) slab. To model isolated oxygen adatoms, a smaller unit cell, (3 × 2), was used in the model O/Ag(110). All four substrate models employed in this study are sketched in Figure 2. Adsorbed moieties were placed on one side of each slab, which was separated from neighboring slabs by a vacuum spacing of about 1 nm. Previously, we used this type of models to study acrolein and hydrogen adsorption on silver.13,23,24 Here, we studied single reagent or product molecules, monohydrated intermediates (mhx, x ) 0-3, Figure 1), and transition state (TS) structures, one complex per unit cell. The Cartesian coordinates of all structures considered are available as the Supporting Information. TSs of acrolein hydrogenation were located with the nudged elastic band method.35 We calculated activation energies with respect to noninteracting adsorbed reactant species. Coadsorption states are less favorable by ∼10 kJ mol-1 than adsorption at formally infinite separation [cf. 3-18 kJ mol-1 for the same reaction on Pt(111)26]; we neglected this difference. Each of the TS structures presented was probed by a normal-mode analysis to ensure that it indeed exhibited only one vibrational mode with an imaginary frequency. 3. Adsorption of Reagents and Products on Silver Catalyst We start with examining adsorption parameters of reactants and products of the partial hydrogenation of acrolein on silver catalyst.

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TABLE 1: Calculated Energiesa of Formation from Gas Phase H2 and Acrolein A (kJ mol-1) for the Intermediates and Products of A Hydrogenation in the Gas Phase and at Ag(110) and Osub/Ag(111) Surfaces A 1/2H2c mh0 mh1 mh2 mh3 AA PA

gas phase

Ag(110)

Osub/Ag(111)

Pt(111)b

0 0 35 147 80 14 -80 -141

-16 24 -33 -38 -33 -108 -98 -160

-12 -10 -7 -1 -15 -54 -100 -152

-102 -47 -178 -96 -112 -137 -199 -174

a The energies of A (trans isomer) and H2 in the gas phase are taken as reference. b Data for Pt(111) from ref 26 are given for comparison. In that study, adsorption energies of AA (104 kJ mol-1) and PA (22 kJ mol-1) are reported that correspond to gas phase energies of AA and PA, which differ by 10-15 kJ mol-1 from the results given here. c For adsorbed species this is the energy for the dissociative adsorption, calculated according to 1/2H2 + * f H*.

3.1. Hydrogen. Recently we addressed in detail the problem of H2 activation on Ag surfaces.23,24 We calculated the dissociation of H2 to be thermodynamically and kinetically unfavorable on clean silver surfaces (endothermic by ∼40 kJ mol-1, activation barriers of ∼125 kJ mol-1).23 This inactivity of Ag for hydrogen activation is strikingly different from the Pt-group metals, where hydrogen readily dissociates almost without a barrier.36 Thus, to rationalize why Ag is active at all as hydrogenation catalyst, one has to consider possible surface modifications, taking place in the course of the reaction or as result of a pretreatment. With the goal to identify potential sources of atomic hydrogen for the hydrogenation of organic molecules on silver catalysts, we studied computationally the activation (dissociation) of H2 on different model oxygen species adsorbed on various Ag surfaces as well as trapped in the subsurface region: O/Ag(110), OH/Ag(110), p(2 × 1)O/ Ag(110), p(2 × 1)OH/Ag(110), Osub/Ag(111).24 We found that all these oxygen species, present on or underneath a silver surface, are able to promote the dissociation of H2. Other computational studies also suggested that H2 dissociation should proceed easier on partially oxidized Ag surface.37 In the framework of the present study we considered hydrogenation of acrolein by H preferentially adsorbed on the Ag(110) surface at bridge sites above rows of silver atoms with a binding energy -196 kJ mol-1 [the coverage of H is 1/8 ML (monolayer)], and H at Osub/Ag(111) surface adsorbed at 3-fold hollow site with Ead ) -230 kJ mol-1 (H coverage 1/9 ML). We calculated the dissociation energy of H2 in the gas phase at 439 kJ mol-1. 3.2. Acrolein. In a previous study we found acrolein to interact only weakly with low-index Ag surfaces.13 On the clean Ag(110) surface, at a coverage of 1/6 ML, we calculated the adsorption energy at only -13 kJ mol-l; cf. Ead ) -16 kJ mol-l for a coverage of 1/8 ML in the present work (Table 1). Essentially vanishing adsorption interactions of acrolein molecules were also calculated on the close-packed clean surface Ag(111) and even on the stepped surface Ag(221).13 Similarly to clean Ag, on oxygen-exposed model surfaces Osub/Ag(111), p(4 × 1)O/Ag(110), and O/Ag(110) acrolein was calculated to be bound very weakly, with adsorption energy of only ∼-10 kJ mol-1. (More detailed results on the latter two structures can be found in our previous study,24 where we discussed them as models of oxygen-modified silver surfaces.) Concomitant with the weak adsorption of A on silver we found no indication of preferential activation of the CdO vs the CdC bond, which is

commonly assumed as prerequisite for the selective hydrogenation of the carbonyl.1,2 The geometry of adsorbed acrolein remains almost unchanged (Table 2) with the trans isomer being more stable as in the gas phase.13 On all model silver substrates under scrutiny, the molecular plane of adsorbed A was found to be oriented almost parallel to the surface plane as one expects at low coverage. According to ab initio thermodynamics modeling of A on Pt(111), starting at low pressure (or high temperature) and increasing the pressure (or decreasing the temperature), the adsorption of A evolves from a flat η4 or η3 form at low coverage to higher coverage phases exhibiting η3 modes with a gradual decoordination of the CdO group from the surface.38 That theoretical view agrees with experimental HREELS data.39 In line with the weak adsorption, A forms rather long contacts with the substrates: on Ag(110) Ag-O ) 252 pm and Ag-C2 ) 249 pm, the distances from C1 and C3 atoms to Ag centers are above 300 pm. On Osub/Ag(111) all distances from acrolein to Ag atoms are even longer, more than 370 pm. We calculated a somewhat stronger binding of A when hydroxyl groups are present at the surface: Ead ∼-30 kJ mol-1 for OH/Ag(110) and ∼-60 kJ mol-1 for p(4 × 1)OH/Ag(110). According to our recent computational study, hydroxyl groups can result from H2 activation on oxygen centers of regular Ag(110) or added-row reconstructed p(4 × 1)O/Ag(110).24 This strengthening of the adsorption bonding of acrolein goes along with the formation of a hydrogen bond (of 200 pm) between the H atom of the hydroxyl group and the oxygen atom of acrolein. However, even in these most strongly bound adsorption complexes, the CdC and CdO bonds of acrolein are only weakly activated, as measured by the corresponding adsorptioninduced bond elongations of ∼1 pm. In summary, as in the case of hydrogen, A exhibits on Ag surfaces adsorption behavior very different from that on Ptbased catalysts, where a much stronger interaction of A with the surface was calculated, up to ∼100 kJ mol-1 for Pt(111).40,41 We emphasize this difference because the strength of interaction and the adsorption mode may affect the selectivity of the hydrogenation. For instance, we assume that a strong interaction of the CdC bond with the surface may hinder the hydrogenation of this bond, as will be discussed in Section 5. In contrast to earlier expectations,12 we did not find any enhancement of the interaction between a carbonyl group and silver surfaces that may be classified as “electropositive” due to electron withdrawing oxygen centers. 3.3. Propenol and Propanal. Similarly to acrolein, both products of its partial hydrogenation, propenol (allyl alcohol, AA) and propanal (PA), bind weakly to both clean Ag(110) and oxygen-modified Osub/Ag(111) surfaces. The calculated adsorption energies do not exceed -20 kJ mol-1; see Table 1 for the formation energies of AA and PA in the gas phase and on silver substrates. In general, these interactions should be qualified as physisorption, as indicated also by the nearest adsorbate-surface distances, which are longer (in some cases significantly) than 250 pm. Also, the adsorption-induced structural distortion compared to the gas phase is minor, at most 1 pm for the C-C and CdO bonds (Table 2) for both products. Such weak adsorption of the products of a partial hydrogenation of acrolein represents a crucial finding for understanding the selectivity of silver catalysts in this reaction (see below). In contrast, on Pt(111), propenol adsorbs (with a binding energy of about -100 kJ mol-1) much stronger than the alternative product propanal, for which an adsorption energy of about -20 kJ mol-1 was

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TABLE 2: Calculated Bond Lengths (pm) of Acrolein A, Its Monohydrogenated Derivatives mhx (x ) 0-3), and Products of Partial Hydrogenation, Allyl Alcohol (AA) and Propanal (PA), in the Gas Phase and in Adsorption Complexes on Silver Substrates gas phase

Ag(110)

p(4 × 1)O/Ag(110)

Osub/Ag(111)

O/Ag(110)

O-C1 C1-C2 C2-C3 O-C1 C1-C2 C2-C3 O-C1 C1-C2 C2-C3 O-C1 C1-C2 C2-C3 O-C1 C1-C2 C2-C3 A mh0 mh1 mh2 mh3 AA PA

123 137 135 122 125 144 122

147 138 150 152 142 149 151

134 138 134 148 148 133 153

124 138 141 123 129 143 123

145 138 150 149 140 149 150

137 145 136 155 152 134 153

123 138 143 122 127 144 122

147 144 150 152 143 150 150

134 137 134 153 151 134 154

124 138 142 123 128 142 122

145 137 150 147 141 150 150

136 143 136 154 152 135 154

125 138 142 124 128 143 122

145 137 150 151 141 150 150

138 144 136 154 152 135 154

TABLE 3: Calculated Adsorption Energies (Ead, kJ mol-1) and Selected Interatomic Distances X-Ag (pm)a for Intermediates of the Partial Hydrogenation of Acrolein on Various Surfaces Modeling Silver Catalysts Ag(110) mh0 mh1

mh2 mh3

a

p(4 × 1)O/Ag(110)

Osub/Ag(111)

O/Ag(110)

X

Ead

X-Ag

Ead

X-Ag

Ead

X-Ag

Ead

X-Ag

C1 C2 C3 O O O C2 C3 O C3 O O C2

-68

260 257 228 235 254

-42

241 270 259 228 229 231

-59

279 316 233 233 252

-26

302 331 247 236 257

-185

-113 -122

269 252 242 221 237 263 243

-148

-95 -68

-195

-112 225 238 298 237

-122

265 254 221 248 239 278 238

-189

-83 -113

316 280 248 238 237 247

X is the atomic center of the adsorbed species that is closest to a surface atom Ag.

calculated.25,26 This large difference evidently is due to the strong coordination of the CdC π-bond of propenol to the Pt surface, while on Ag the contribution from this interaction channel is very small. 4. Intermediates of Acrolein Hydrogenation In the first reaction step, when a hydrogen atom attacks either the CdO or the CdC bond of A, four monohydrated intermediates can be formed, depending on the site of the attack (Figure 1): hydroxyallyl (mh0), allyloxy (mh1), 2-formylethyl (mh2), and 1-formylethyl (mh3). Because these mhx (x ) 0-3) intermediates are radical species in the gas phase, they bind strongly on the metal surface. Attack of mhx by a second hydrogen atom leads to the formation of AA (pathways a and b) or PA (c, d) (Figure 1). In the gas phase the species mh0 and mh3 with hydrogen attached to terminal centers, O and C3, respectively, are considerably more stable than mh1 and mh2 (Table 1), obviously due to delocalization effects in a conjugated π system. For species in the gas phase (Table 2) one can monitor this effect in particular by a shortening of the C1-C2 bonds from 147 pm (A) to 142 pm (mh3) and 138 pm (mh0). Moreover, hydroxyallyl (mh0) exhibits equally long C1-C2 and C2-C3 bonds, 138 pm, clearly indicating delocalization of π electrons. On Ag(110) and Osub/Ag(111) surfaces, the adsorption complexes of the intermediates mh0, mh1, and mh2 have very similar energies (Table 1), while intermediate mh3 is considerably more stable on both substrates. In this sense, the silver substrates considered behave differently from Pt(111), where mh0 and mh3 form significantly more stable complexes than mh1 and mh2 (Table 1).25,26 4.1. Hydroxyallyl (mh0). This intermediate is of crucial importance for the selectivity because it is involved in the most

favorable pathway that leads to AA on Pt(111)25,26 as well as on the Ag substrates considered in the present work (see Section 5). The stabilization of hydroxyallyl by resonance effects reduces its reactivity and, thus, its ability to adsorb on silver surfaces: both on Ag(110) and Osub/Ag(111) substrate models mh0 exhibits the weakest binding with respect to the gas phase, -68 and -42 kJ mol-1, respectively (Table 3). Despite of this relatively weak interaction, the electronic structure of hydroxyallyl is notably modified by the coupling with the surface. Whereas the two C-C bonds of a hydroxyallyl radical are equally long in the gas phase, 138 pm (Table 2), adsorbed mh0 exhibits significantly reduced π character of one of the C-C bonds, as monitored by an elongation to 144-145 pm (C2-C3 on Ag(110), C1-C2 on Osub/Ag(111); Table 2). Concomitantly, the position of the C-C bond with remaining strong π character (double bond) varies with the substrate: on Ag(110) it is the C1-C2 bond, 138 pm, while on Osub/Ag(111) it is the C2-C3 bond, 137 pm. Thus, on the Ag(110) surface the position of the double bond of mh0, C1-C2, is different from that in acrolein, C2-C3. Intermediate mh0 interacts with the support via two channels: the σ-like interaction via the carbon center not involved in the CdC double bond, i.e. C3 on Ag(110) and C1 on Osub/Ag(111) with bond lengths C-Ag of 228 and 241 pm, respectively (Table 3, Figure 3a). The second channel is the π interaction of the CdC bond with a single Ag center; pertinent bond lengths Ag-C are in the range 260-270 pm (Table 3). To inspect how the structure of the surface affects the adsorption mode of hydroxyallyl, we calculated this species also adsorbed on Ag(111). Apparently, in the absence of oxygen the structure of mh0 on Ag(111) is very similar to that on Ag(110). Center C3 forms the strongest bond to the surface,

Hydrogenation of R,β-Unsaturated Aldehydes

Figure 3. Calculated adsorption complexes of monohydrated intermediates of acrolein hydrogenation on the surfaces Ag(110) (left-hand column) and Osub/Ag(111) (right-hand column): (a) hydroxyallyl (mh0), (b) allyloxy (mh1), (c) 2-formylethyl (mh2), and (d) 1-formylethyl (mh3). Only the top three layers of Ag(110) slab and two layers of Osub/Ag(111) are shown.

235 pm; the C1-C2 bond, 137 pm, exhibits a notable π character with C1 and C2 atoms coordinated to the same Ag center on the surface, with bond lengths of 284-287 pm. The C2-C3 bond, 144 pm, exhibits less π contribution than the C1-C2 bond. Thus, subsurface oxygen likely affects the adsorption mode of mh0 on Osub/Ag(111). 4.2. Allyloxy (mh1). This intermediate, least stable in the gas phase (Table 1), binds most strongly on all surfaces considered (Table 3). It is of very similar stability on silver substrates as its mh0 congener, which also leads to AA. Obviously, the unpaired electron at the O center of an allyloxy radical in the gas phase cannot be noticeably stabilized by delocalization to a conjugated π-system, at variance with 1-formethyl (mh3) and hydroxyallyl (mh0). Therefore, the allyloxy radical interacts most strongly with the surface: the adsorption energy on Ag(110) was calculated at -185 kJ mol-1 (Table 3). mh1 binds to the examined silver surfaces via the activated O atom, similarly, e.g., to the adsorption complexes of methoxy species on metal surfaces.42-45 The O center of mh1 is located above the trough of Ag(110), at a so-called long bridge, forming bonds of 235 and 254 pm to the two Ag centers (Table 3, Figure 3b). The double bond C2-C3 (136 pm, with a planar arrangement of the H atoms at the centers C2 and C3) interacts with a single Ag center forming coordinative bonds

J. Phys. Chem. C, Vol. 113, No. 30, 2009 13235 of 269 on (C2) and 252 pm (C3, Table 3). For mh1 on Osub/ Ag(111), we calculated a weaker interaction, -148 kJ mol-1 with respect to the gas phase. Here, the O atom of mh1 occupies an fcc 3-fold hollow site, without an Osub species located below it (Table 3, Figure 3b). The weaker adsorption on Osub/Ag(111) can be partly assigned to lacking interactions of the C3-C2-C1 part of mh1 with the substrate. 4.3. 2-Formylethyl (mh2). Similarly to the allyloxy radical, the radical 2-formylethyl can hardly be stabilized by delocalizing the unpaired electron of center C3. Therefore, this species binds to the surfaces via the C3 center, which features a high spin density in the gas phase; see Figure 3c for the adsorption complex of mh2 on the Ag(110) surface. In line with the reactivity inherent to radicals, 2-formylethyl binds rather strongly to the surfaces under consideration. The calculated adsorption energies are -113 kJ mol-1 on Ag(110) and -95 kJ mol-1 on Osub/Ag(111); the Ag-C3 distances on these substrates vary by 4 pm only, from 221 to 225 pm (Table 3). On Ag(110) the O center of mh2 also coordinates to the surface, with a Ag-O distance of 242 pm (Table 3). These data again indicate a slight destabilizing effect (of at most ∼30 kJ mol-1) of subsurface oxygen on the adsorption of this intermediate. This mechanism of reducing the adsorption strength in the presence of an electronegative heteroatom in a subsurface position is reminiscent of the effect that subsurface atomic C has on the nearby adsorption of CO on a Pd(111) surface46 or (111) facets of a Pd nanoparticle.47 The difference between the adsorption energies of allyloxy mh0 and 2-formylethyl mh2 intermediates is partly due to the different affinities of C and O centers to surface Ag atoms, just as in cases of methyl and methoxy adsorption on the Cu(111) surface.42 Indeed, according to our calculations, a methyl species CH3 adsorbs at Ag(111) with an energy of -103 kJ mol-1; this value is very similar to those of 2-formylethyl adsorption on silver substrates. 4.4. 1-Formylethyl (mh3). This intermediate is most stable in the gas phase (Table 1), partly due to a delocalization of the unpaired electron of the C2 center involving the nearby CdO double bond. mh3 remains the most stable intermediate on clean Ag(110) and Osub/Ag(111) substrates (Table 1), forming surface complexes with adsorption energies of -122 and -68 kJ mol-1, respectively (Table 3). This intermediate interacts with the Ag(110) surface via the atoms C2 and O (Figure 3d), with bond lengths of 243 and 237 pm, respectively (Table 3). On the Osub/ Ag(111) surface the interaction is significantly weaker than on Ag(110). Yet, the adsorption mode remains similar: on the Osub/Ag(111) surface mh3 forms a C2-Ag bond of 237 pm and an O-Ag bond of 238 pm (Table 3); the Ag centers involved belong to different cages occupied by subsurface oxygen atoms (Figure 3d). As mh3 is a key intermediate on the pathway to PA, as is mh0 along the pathway to AA, we also investigated the adsorption complex of mh3 on Ag(111) in the absence of subsurface oxygen. The structural characteristics of mh3/ Ag(111) are very similar to those on Ag(110) and Osub/Ag(111): binding to the surface via C2 and O centers, 249 and 235 pm; C-O ) 129 pm, C1-C2 ) 140 pm, and C2-C3 ) 151 pm (cf. Table 2, 3). On Ag(111) mh3 is 57 kJ mol-1 more stable than mh0, while on Ag(110) and Osub/Ag(111) the energy difference between these two species is 75 and 47 kJ mol-1, respectively. Note that in contrast to the silver substrates considered here, on the Pt(111) surface mh3 intermediate is ∼40 kJ mol-1 less stable than mh0 (Table 1).25,26 This change in the order of stability on different substrates of key intermediates along

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competing pathways, to PA or AA, without a doubt is an important factor that affects the selectivity of the process, as will be discussed in the following section. As there is experimental evidence for important effect of oxygen pretreatment on activity and selectivity of silver catalyst for acrolein hydrogenation,12 we also considered two substrates with oxygen deposited on the surface, O/Ag(110) and p(4 × 1)O/Ag(110). The system O/Ag(110) represents isolated oxygen centers adsorbed at so-called long-bridge positions between two rows of Ag atoms in [001] direction, whereas p(4 × 1)O/ Ag(110) exhibits a reconstructed added-row structure; see our previous study of H2 activation for a detailed description of these substrates.24 Structure and energetic of adsorbed mhx intermediates on these surfaces are very similar to those on oxygen-free Ag(110) because the adsorption complexes are formed at the oxygen-free parts of the substrate and thus oxygen centers are not involved in the binding of mhx adsorbates. Moreover, most adsorption complexes in the vicinity of surface oxygen centers are considerably destabilized; only mh1 exhibits slightly higher adsorption energies on the substrates p(4 × 1)O/Ag(110) and O/Ag(110) (Table 3). On the other hand, hydrogen atoms bind rather strongly to surface-adsorbed oxygen centers with binding energy up to -330 kJ mol-1;24 such hydrogen centers can hardly participate in hydrogenation processes. In a previous study24 we had pointed out that an oxygen center on a silver surface is able to promote the dissociation of an H2 molecule, producing an OH group and releasing an H atom that can undergo facile diffusion on the metallic part of the surface. Then, the subsequent hydrogenation process can be considered to take place on the part of the oxygen-free Ag(110). In contrast, the influence of a subsurface oxygen is indirect: it also promotes H2 dissociation, but the resulting H centers are not so strongly bound as in surface OH groups and thus both are able to participate in hydrogenation processes. For these reasons we confined the following consideration of reaction mechanisms to two substrates: Ag(110) and Osub/Ag(111).

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Figure 4. Calculated transition state structures for the first steps a1-d1 (top to bottom; see Figure 1) of the partial acrolein hydrogenation on the surfaces Ag(110) (left-hand column) and Osub/Ag(111) (right-hand column). Also given are distances (in pm) from the attacking atom H to the nearest atom of the reactant (H-X) and to the substrate (H-Ag).

5. Activation Barriers for Acrolein Hydrogenation A recent computational study of acrolein hydrogenation by molecular hydrogen in the gas phase48 shows very high activation barriers of 260-280 kJ mol-1 at the PW91 level; B3LYP and CBS-APNO approaches yield even higher values, by ∼40 kJ mol-1. Such high barriers are not surprising, as most of the energy is spent to activate a H2 molecule, which in that study is added directly to the CdC or the CdO function of A. Note, however, that no clear preference was found for the kinetics of hydrogenating either the CdC or the CdO function.48 One should keep in mind that the concerted addition of H2 considered for the gas phase is completely different from the stepwise addition of H atoms on metal surfaces. 5.1. Ag(110). We start by analyzing the activation energies and the transition state (TS) structures of the partial hydrogenation of acrolein (reaction steps a1-d2, Figure 1) on model Ag(110), representative for clean defect-free silver surfaces. The geometries of the TSs on the Ag(110) surface are displayed in the left-hand columns of Figures 4 and 5. Table 4 collects the bond lengths of the C-C-C-O backbone of molecule A for the TSs of the various hydrogenation steps. As discussed above, acrolein interacts only weakly with both silver substrates considered. Therefore, one will not expect any preference for the attack by H of either CdO or CdC bond just from the orientation of A at the surface. Indeed, the TSs of all first hydrogenation step on Ag(110) (Figure 4) exhibit an almost planar structure of A. The molecule is somewhat lifted

from the surface where the adsorbed hydrogen atom, Ha, approaches the attacked center of A from below. During the formation of mh0 (reaction step a1), acrolein moves toward the adsorbed hydrogen, Ha, to reach a distance Ha-O of 149 pm in the TS (Figure 4a). With the formation of this O-Ha bond, the C1-O distance of the attacked A molecule is elongated by 6 to 129 pm (Table 4), indicating a weakening of the carbonyl bond. Concomitantly center C3 starts to form a bond with a surface Ag center, 250 pm, while the C1-C2 distance is shortened by 4 pm, indicating a bond strengthening due to electron delocalization. On the way to mh1 (TS of step b1), A moves toward Ha and the C1-Ha contact shortens to 157 pm in the TS (Figure 4b). While forming the C1-Ha bond, the C1-O bond is weakened, as indicated by its elongation by 5 pm, and the O atom becomes available for a stronger interaction with the Ag surface. In this TS the C2-C3 double bond of A remains almost intact, while C1-C2 slightly elongates, to 149 pm (Table 4). Intermediate mh2 is formed in a similar way (step c1). A moves toward the adsorbed Ha with the C2-Ha distance shortened to 142 pm in the TS (Figure 4c). Likewise, during the formation of 1-formylethyl mh3 (reaction d1), the C3-Ha contact shortens to 170 pm (Figure 4d) and the bond C2-C3 elongates to 140 pm. In summary, the attack of the terminal centers of A, O or C3, in the TSs of the first hydrogenation step induces significant changes in the whole

Hydrogenation of R,β-Unsaturated Aldehydes

Figure 5. Calculated transition state structures for the second steps a2-d2 (top to bottom; see Figure 1) of the partial acrolein hydrogenation on the surfaces Ag(110) (left-hand column) and Osub/Ag(111) (righthand column). Also given are distances (in pm) from the attacking atom H to the nearest atom of the reactant (H-X) and to the substrate (H-Ag).

structure of reactant A indicating the formation of a delocalized π system. On the other hand, the attack of C1 and C2 centers has more local consequences, influencing mainly the structure of the functional group affected, C1dO or C2dC3, respectively. The structures of the TSs of the second hydrogenation step, i.e., the TSs for the hydrogenation of mh0-mh3 species, are shown in Figure 5. Unlike steps a1-d1, where reactant A under attack interacts only weakly with the surface, during steps a2-d2 the adsorbed H attacks species that are strongly bound to the metal substrate (Table 3). The bond lengths in the backbone of these transition structures are very close to those in the corresponding mhx species, especially for structures b2 and c2 where the changes of C-C and C-O bond lengths do not exceed 1 pm compared to mh1 and mh2, respectively (Tables 2 and 4). The most pronounced structural change occurs in TS a2 where the C2-C3 bond shortens by 5 pm, obviously due to the formation of a π-system on this bond. In general, the TSs for the second hydrogenation step can be classified as early transition states because their structures are closer to the initial mhx adsorption complexes than to the products. However, also note that the structures of the backbone for mh1 and mh2 are already rather close to those of the corresponding products, AA and PA (Table 2).

J. Phys. Chem. C, Vol. 113, No. 30, 2009 13237 Figure 6 shows the reaction profile calculated on the clean Ag(110) surface, where all eight elementary steps of A hydrogenation are exothermic. The desorption of both products, AA and PA, from the Ag(110) substrate was calculated to require essentially the same low energy, ∼20 kJ mol-1, which is small enough for rapid release of the products to the gas phase at the reaction temperature 523 K.8-11 Therefore, the relative surface coverage of the products defines the partial pressure of each product in the gas phase and, hence, the selectivity of the process. Thus, when discussing the selectivity, one has to focus on the activation barriers of these surface reactions. On the model Ag(110), the activation energies E‡ of all hydrogenation routes leading to AA, a and b, were calculated below 80 kJ mol-1 (Table 4, Figure 6), indicative of rather rapid transformations. Both routes a and b exhibit notably higher barriers for attaching the first hydrogen atom, H1, than for the second one, H2, rendering the initial steps a1 and b1 ratelimiting. The barrier E‡(b1) is 13 kJ mol-1 lower than E‡(a1). Thus, the kinetics of the A f AA transformations studied is dominated by route b. In contrast to the formation of AA, along routes c and d to the undesired product PA, barriers E‡(c2) ) 79 kJ mol-1 and E‡(d2) ) 57 kJ mol-1 for attaching the second hydrogen atom H2 are somewhat higher than those of the initial steps c1 (74 kJ mol-1) and d1 (36 kJ mol-1) of attaching H1. For the latter routes, the kinetics can be approximated by the more favorable chain d (Figure 6). Note that the heights of the activation barriers of the first hydrogenation steps a1-d1 correlate with the reaction energies of these steps. Indeed, the lowest activation barrier d1, 36 kJ mol-1, leads to the most stable intermediate mh3 while step d2 exhibits a higher activation energy. The relative rates of forming AA and PA under stationary conditions are determined by the rate constants of the first steps (see the Supporting Information). The rates evaluated within transition-state theory based on these DF energies (without empirical parameters) show the undesired PA to be the dominating product by far. The selectivity of the partial hydrogenation of A to AA is limited to 9%, even under nonsteady conditions which are more favorable for the formation of AA (see the Supporting Information). Indeed, experimental observations also point to a low selectivity (∼10%) of bulklike silver catalysts.11 Inherent approximations of the DF method and the surface models employed are expected to yield even relative reaction and activation energies with a precision of at most ∼5-10 kJ mol-1. As the selectivity is very sensitive to the E‡ values, altering the latter by mere 5-10 kJ mol-1 can significantly change the resulting selectivity. The interaction of the CdC double bond of A, mh0, mh1, and AA with the Ag surface appears to be underestimated in our calculations, reflecting the well-known problem of current gradient-corrected exchangecorrelation functionals when describing weak interactions.49 For instance, from the temperature 148 K of ethylene thermodesorption,50 one estimates the adsorption energy of ethylene on Ag(110) at ∼40 kJ mol-1, whereas our calculations yielded just ∼10 kJ mol-1. Thus, one may assume that dispersive forces will increase the adsorption energies of A and AA, calculated to only ∼20 kJ mol-1. In consequence, the stability of intermediates mh0 and mh1, with the CdO group monohydrated and the CdC bond preserved, should be underestimated. As activation barriers appear to correlate with reaction energies, we can expect lower activation barriers for steps a1 and b1 than obtained in our calculations (Figure 1). Evidently, limitations of contemporary first-principles approaches49 prevent a quantitative description of the chemo-selectivity of reactions with

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TABLE 4: Pertinent Inter-Atomic Distances (pm) Characterizing the Adsorbed Reactant/Intermediate in the Transition States of Various Stepsa of the Partial Hydrogenation of Acrolein A, on the Models Ag(110) and Osub/Ag(111)b Ag(110)

Osub/Ag(111)

reaction step

O-C1

C1-C2

C2-C3

H-X

H-Ag

E‡

Er

O-C1

C1-C2

C2-C3

H-X

H-Ag

E‡

Er

a1A + H f mh0 b1A + H f mh1 c1A + H fmh2 d1A + H f mh3 a2 mh0 + H fAA b2 mh1+ HfAA c2 mh2 + H f PA d2 mh3 + H f PA

129 128 122 126 138 141 123 130

143 149 150 142 141 150 150 138

139 134 142 140 140 136 154 150

149 157 142 170 189 182 202 174

185 189 188 188 192 188 202 211

78 65 74 36 33 39 79 57

-41 -46 -41 -115 -89 -84 -151 -77

126 127 129 128 135 140 122 126

146 147 149 143 139 150 152 146

136 134 149 150 139 134 153 153

169 164 176 142 197 152 159 231

199 178 171 214 214 176 168 170

90 198 200 128 103 149 181 90

14 20 6 -33 -82 -88 -128 -89

a For the designations of the atomic centers and the reaction steps, see. b H-X and H-Ag are distances of the attacking H atom to the nearest atom X of the acrolein derivative and of the substrate, respectively. Activation and reaction energies E‡ and Er (kJ mol-1) are also shown.

Figure 6. Reaction profile of the partial hydrogenation of acrolein (A) to propenol (AA) and propanal (PA) on Ag(110). Reaction and activation energies in kJ mol-1.

comparable rates. In fact, lowering the activation barrier b1 by only 10 kJ mol-1 results in a selectivity of 47% to AA, if one assumes a nonsteady kinetic regime. Nevertheless, qualitatively our computational results yield a low selectivity for AA on the regular Ag(110) surface, hence agree with experimental data for bulk-like silver catalysts.11 We found barrier heights also to be rather sensitive to the structure of the surface. For instance, on Ag(111) the barriers for steps a1 and d1 increase to 99 and 61 kJ mol-1, compared to 78 and 36 kJ mol-1 on Ag(110), respectively. Recently, the hydrogenation of A on the surface Pt(111) was studied by DF calculations and kinetic modeling.25,26 There, the observed low selectivity to AA in the gas phase was attributed to its very slow desorption. AA was calculated to bind ∼80 kJ mol-1 stronger at Pt(111) than the alternative product PA. At variance with the often-assumed kinetic preference for the formation of adsorbed PA, the rate for producing adsorbed AA was calculated notably larger. This result was rationalized by the rather strong interaction A-Pt(111), ∼100 kJ mol-1, which preferentially activates the CdO bond of A rather than its CdC bond. Evidently, the strong binding of A and AA is mainly due to the interaction of the CdC bond with the Pt surface. Overall, the rate-determining activation barriers in the A f AA process on Pt(111) were calculated25,26 ∼15 kJ mol-1 lower than on Ag(110) in this work, implying an enhanced reactivity of a Pt substrate for this route. In contrast, the calculated crucial barriers in the A f PA transformation were calculated noticeably higher, by ∼35 kJ mol-1, on Pt(111) than on Ag(110). Interestingly, the features that define the hydrogenation kinetics of A on silver, modeled as Ag(110), are rather different from those of the analogous reactions on Pt(111). On silver,

the double bonds of A exhibit a very limited (if any) adsorptioninduced activation, in line with the weak adsorption of that molecule, and an essentially unhampered desorption of both product molecules AA and PA. This variation of characteristics clearly illustrates the rich opportunities for tuning the performance of metal catalysts for selective hydrogenation. Comparing Pt and Ag we note the dual role of the CdC interaction with the metal surface. When too strong, as on Pt, this interaction hinders the desorption of an UOL leading to its further hydrogenation to a SOL. In contrast, when too weak, as on our model Ag surface, the CdC bond predominantly undergoes hydrogenation to SAL. To produce UOLs with high selectivity the optimal balance is necessary. To rationalize the higher selectivity for AA (up to ∼50%) on dispersed Ag catalysts compared to bulky Ag (while relying on the accuracy of the calculated barriers at hand), one has to consider either low-coordinated sites or impurities. Lowcoordinated sites can stabilize the CdC bond that leads to the hydrogenation of the CdO group as discussed above. However, in the following we will focus on the possible role of impurities, exemplified by subsurface oxygen, in controlling acrolein hydrogenation. 5.2. Osub/Ag(111). Experiments demonstrated oxidative pretreatment of supported Ag catalysts to increase notably both the activity and the selectivity of the partial hydrogenation of acrolein: the selectivity to AA exceeded 50% while the activity increased by a factor of two to three.11,12 On the basis of computational results23,24 this increase of activity can be assigned to a noticeable enhancement of the dissociation rates of H2 to produce adsorbed atomic H on oxygenated Ag surfaces. In this section, we explore the effect of oxygen centers on the selectivity

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Figure 7. Reaction profile for the partial hydrogenation of acrolein (A) to propenol (AA) and propanal (PA) on the model surface Osub/Ag(111). Pathways b and c are not shown as they exhibit significantly higher barriers than pathways a and d (Table 4). Reaction and activation energies in kJ mol-1.

of silver catalysts for the hydrogenation of A. To represent such catalyts, we chose the Osub/Ag(111) system (with subsurface oxygen centers), but one should keep in mind that more complicated structures may occur under catalytic conditions.51,52 Although isolated subsurface oxygen species occupying octahedral interstitial sites beneath clean Ag surface are unstable,24 a recent experimental and computational study suggests that at surface steps subsurface oxygen can be stabilized by the presence of a neighboring oxygen center.53 Yet, under hydrogenation conditions, oxygen centers at the surface are transformed to water, which subsequently may easily desorb.24 Determining the exact nature of oxygen species that enhance the performance of Ag catalyst for acrolein hydrogenation12 is beyond the present study. Rather, we addressed how oxygen in silver catalysts, exemplified by subsurface atomic species Osub on Ag(111), affects the activity and the selectivity of A hydrogenation. Figure 7 shows the calculated reaction profile for pathways a (A f AA) and d (A f PA) of A hydrogenation on the model Osub/Ag(111). Routes b and c exhibit significantly higher activation barriers for the first hydrogenation steps, ∼200 kJ mol-1 (Table 4), and, therefore, are not considered in the following. Hence, routes a and d determine the selectivity A f AA and A f PA on Osub/Ag(111). Already for pathways a and d one also notices considerably higher activation barriers on Osub/Ag(111) (Figure 7), implying a decreased activity, compared to the clean Ag(110) surface. This correlates with the stronger binding of hydrogen in the vicinity of subsurface oxygen. The structures of TSs for both steps of A hydrogenation on Osub/Ag(111) substrate (Table 4, Figures 4 an 5, right-hand column) strongly resemble the corresponding structures on clean metal Ag(110). Therefore we will not repeat the detailed structural analysis of TSs as given in Section 5.1. However, there are few noteworthy differences that are likely to affect the heights of the activation barriers. In steps c1 and d1, when a first hydrogen attacks C2dC3, this bond elongates in the transition states to 149-150 pm. This is a significantly larger effect than calculated on Ag(110), 140-142 pm (Table 4). Thus, in contrast to Ag(110), the C2-C3 bond in TSs c1 and d1 on Osub/Ag(111) approaches a structure that is closer to the intermediates mh2 and mh3. On Osub/Ag(111) the C2-C3 bond in these species is slightly shorter, by 1-2 pm, than on the clean metal (Table 2). Therefore on Osub/Ag(111), the TSs c1 and d1 can be classified as “late”. This structural difference of the TSs for steps c1 and d1 is associated with significantly higher

activation barriers, 200 and 128 kJ mol-1, respectively. Note that on Osub/Ag(111) all activation barriers were calculated higher than on the clean surface Ag(110). Even more importantly, the order of the barriers heights for the competing pathways to AA and PA is reversed. This change induces an alteration of the selectivity as the analysis below will show. On Osub/Ag(111), the channel a1-a2 exhibits the lowest barrier a1 for the first hydrogenation step, 90 kJ mol-1, while a slightly higher barrier of 103 kJ mol-1 was calculated for attaching the second hydrogen to mh0 in a2 step. As step a1 is endothermic by 14 kJ mol-1, one should account for the reverse dehydrogenation of mh0 to A when setting up a kinetic model (see the Supporting Information). The calculated barriers of the channel d1-d2, for the route A f PA, are 128 and 90 kJ mol-1; that is, formation of mh3 is much slower than formation of mh0. Putting the calculated energetics and activation barriers (Table 4) into a system of kinetic equations and assuming stationary concentrations of mh0 and mh3 (see the Supporting Information), we estimated the route A f AA as kinetically dominant over route A f PA, with the concomitant selectivity to AA being ∼90%. As on Ag(110), the desorption of the products AA and PA from Osub/Ag(111) is only slightly activated, by 20 and 13 kJ mol-1, respectively. This implies similar relative concentrations of the products under typical conditions on the surface and in the gas phase, i.e., close to 90% of AA. The present calculated results for Osub/Ag(111) provide an indication how one may rationalize the experimentally observed selectivity enhancement of acrolein hydrogenation to propenol on Ag catalysts in the presence of oxygen.11,12 This model mechanism assumes that more active parts of a silver catalysts (as judged by lower hydrogenation barriers), free of oxygen [e.g., model Ag(110); section 5.1], in effect do not dominate the hydrogenation process. As one reason for this scenario, one may mention the likely insufficient supply of atomic hydrogen. As the DF results show, H2 preferentially dissociates on oxygenmodified silver and dissociated H atoms have been calculated to remain strongly bound in the neighborhood of subsurface oxygen centers on Osub/Ag(111).24 6. Conclusions We addressed the intriguing problem of rationalizing the selectivity of silver catalysts in the hydrogenation of R,βunsaturated aldehydes to unsaturated alcohols by DF model calculations. The experimentally explored hydrogenation cata-

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lysts based on supported Ag particles are quite complex; their properties are noticeably altered under reaction conditions and reliable information on the active sites is still lacking.11 Therefore, to improve our understanding of the catalytic action of the silver component, we modeled in detail two limiting situations, extended clean and O-modified silver surfaces, represented by Ag(110) and Osub/Ag(111). On Ag(110) our results imply propanal to be the main product, in line with the observed selectivity, ∼10%, of acrolein hydrogenation to allyl alcohol bulk-like silver catalysts.11 We found the Osub/Ag(111) model system to be less active with respect to hydrogenation of acrolein by adsorbed H atoms than the clean Ag surface. However, we calculated the selectivity of Osub/Ag(111) to allyl alcohol to be extremely high and we related this finding to the experimentally observed beneficial effect of an oxidative pretreatment on the selectivity of the catalyst.12 In contrast to the model catalyst Pt(111),25,26 where the calculated strongly reduced selectivity to allyl alcohol was assigned to the hindered desorption of the latter, the hydrogenation produces allyl alcohol and propanal are predicted to desorb easily from both Ag(110) and Osub/Ag(111) at common reaction temperatures. These conclusions are conceptually important for designing more efficient catalysts for the selective hydrogenation of R,βunsaturated aldehydes and other multiply unsaturated organic compounds in industrial applications. Clearly, more work is needed to build a sufficiently complete microscopic picture of the unusual hydrogenation properties of silver catalysts. In particular, it is desirable to detect and characterize sites that are responsible for the formation of atomic hydrogen as reactant and to determine the reactivity of various defects on such catalysts. Acknowledgment. This work was supported by Deutsche Forschungsgemeinschaft and Fonds der Chemischen Industrie (Germany). K.M.N. is grateful for financial support of the Spanish Ministry of Science and Innovation (Grant FIS200802238). Supporting Information Available: Cartesian coordinates of pertinent structures, results of vibrational analyses of the transition states, and outline of the kinetic models applied. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Ponec, V. Appl. Catal., A 1997, 149, 27–48. (2) Gallezot, P.; Richard, D. Catal. ReV. Sci. Eng. 1998, 40, 81–126. ¨ nal, Y.; Claus P. Regioselective Hydrogenations. In Handbook (3) O of Heterogeneous Catalysis; Ertl, G., Kno¨zinger, H., Schu¨th, F., Weitkamp, J., Eds.; Wiley-VCH: Weinheim, Germany, 2007; pp 3308-3329. (4) Englisch, M.; Jentys, A.; Lercher, J. A. J. Catal. 1997, 166, 25– 35. (5) Urquhart, A. J.; Williams, F. J.; Vaughan, O. P. H.; Cropley, R. L.; Lambert, R. M. Chem. Commun. 2005, 1977–1979. (6) Grant, R. B.; Lambert, R. M. J. Catal. 1985, 92, 364–375. (7) Lambert, R. M.; Williams, F. J.; Cropley, R. L.; Palermo, A. J. Mol. Catal. A 2005, 228, 27–33. (8) Claus, P. Top. Catal. 1998, 5, 51–62. (9) Bron, M.; Teschner, D.; Knop-Gericke, A.; Steinhauer, B.; Scheybal, A.; Ha¨vecker, M.; Wang, D.; Fo¨disch, R.; Ho¨nicke, D.; Wootsch, A.; Schlo¨gl, R.; Claus, P. J. Catal. 2005, 234, 37–47.

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