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Role of surface hydroxyl species in copper catalyzed hydrogenation of ketones Jenoff De Vrieze, Joris W. Thybaut, and Mark Saeys ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b01652 • Publication Date (Web): 03 Jul 2018 Downloaded from http://pubs.acs.org on July 5, 2018
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ACS Catalysis
Role of surface hydroxyl species in copper catalyzed hydrogenation of ketones Jenoff E. De Vrieze, Joris W. Thybaut, Mark Saeys* Laboratory for Chemical Technology, Technologiepark 914, B-9052 Gent, Belgium
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1.0
θ (-)
0.8
OH
0.6 0.4 0.2 0.0
O TOF = 6 10-5 s-1 δGrepulsion
1.0 0.8
θ (-)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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free
0.6 0.4 0.2
OH
0.0
TOF = 2 10-3 s-1
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Abstract A comprehensive, coverage-dependent mean-field microkinetic model is developed for the hydrogenation of carbonyl compounds on Cu(111). In the model, hydrogenation by surface hydrogen, surface hydroxyl species and adsorbed water molecules is considered, including a reaction pathway via keto-enol tautomerization. The model parameters were calculated by VdWDF2 density functional theory and account for inter- and intra-species repulsion. Accounting for these coverage effects changes the surface from completely covered with 25% oxygen atoms and 75% hydroxyl groups to a surface with 65% free sites. Including coverage effects also surprisingly increases the calculated turnover frequency from 6 10-5 s-1 to 2 10-3 s-1. In the dominant reaction path, the carbonyl group is hydrogenated to an alkoxy intermediate by surface hydrogen, followed by a proton transfer from either a surface hydroxyl species or an adsorbed water molecule to form the alcohol product. The addition of small amounts of water suffices to open this pathway. The pathway in which acetone is converted to 2-hydroxypropylene via ketoenol tautomerisation is kinetically irrelevant at the considered conditions. Regeneration of the hydroxyl groups is the rate-controlling step in the mechanism, suggesting an alternative role for the reducible oxide promotors which are often encountered for Cu-based carbonyl hydrogenation catalysts. Keywords: Hydrogenation, copper, catalysis, acetone, microkinetic modeling, coverage
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1. Introduction Copper-based catalysts are widely employed as catalysts for the hydrogenation of aldehyde and ketone compounds.1 Because of their superior selectivity, promoted copper catalysts are often preferred over conventional palladium and platinum catalysts in the hydrogenation of unsaturated and aromatic ketones or aldehydes.2-3 Moreover, copper-based catalysts are significantly cheaper and easy to synthesize.1 As a result, the use of copper-based catalysts for this reaction has steadily increased over the past decade.3-6 The reaction mechanism and the nature and type of active sites for the copper-catalyzed hydrogenation of carbonyl compounds remain debated.7-8 While Sharma et al. propose a singlesite mechanism on metallic copper,9 others have advocated a multiple-site mechanism including multiple copper species or active sites on the support.5, 10 A synergetic effect between Cu0 and Cu+ is often proposed, where Cu+ sites serve as Lewis acid sites.7,
11
Even though it is often
hypothesized that Cu+ contributes to the activity and selectivity,12-13 the presence of Cu+ under reducing hydrogenation conditions has not been demonstrated. Two main pathways are typically considered for the hydrogenation of carbonyl compounds, depending on the atom that is hydrogenated in the first step (Figure 1).14
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H O Alkoxy patway
Carbene pathway OH
H OH
O
enol 2-carbene pathway
OH
H OH
enol 1-carbene pathway
Figure 1. Possible pathways in the hydrogenation of ketones.
In the alkoxy pathway, the carbon atom is hydrogenated first, resulting in an alkoxy intermediate. In the carbene pathway, oxygen is hydrogenated first to form a hydroxycarbene intermediate. When keto-enol tautomerism is considered,15-16 two additional reaction pathways open up. In the first enol pathway, the carbon atom attached to the oxygen atom is hydrogenated first. This pathway results in formation of the 2-carbene species from the carbene pathway. Alternatively, the other sp2 carbon atom of the enol form can be hydrogenated. This results in the formation of an alternative 1-carbene species. Using hydrogen labeling experiments, Vila et al.16 showed that the enol pathways slightly dominate for the hydrogenation of hydroxyacetone. Sitthisa et al.17 reported DFT calculations for furfural hydrogenation on Cu(111), considering only hydrogen as the hydrogenating species and concluded that the carbene pathway is preferred because the aromatic furan ring stabilizes the carbene intermediate. Wang et al.18 found that the carbene pathway is also dominant on Pd(111) for this reaction. Loffreda et al.19 performed first principles calculations to investigate the hydrogenation of unsaturated aldehydes on Pt(111). Using an ab initio kinetic model, these authors showed that the carbene pathway dominates on
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Pt(111). Cao et al.20 focused on the hydrogenation of crotonaldehyde using first principles microkinetics. Similar to Loffreda et al.19, they found that the carbene pathway is the dominant hydrogenation pathway on Pt(111). Cyclohexanol dehydrogenation on the other hand
was
reported to follow the alkoxy pathway on Cu(111), while the carbene pathway is preferred on Cu2O(111).21 In addition, the calculations suggest a higher dehydrogenation reaction rate on the cupric oxide surface. Even though the hydrogenation of carbonyl compounds on Cu(111) and Pt(111) has been thoroughly explored, the role of surface hydroxyl species and of water has received much less attention and keto-enol tautomerization is often ignored. The active participation of surface hydroxyl and oxygen species in (de)hydrogenation reactions is an emerging concept.22-23 Our group recently showed how surface hydroxyl species play a crucial role in the activation of CO and RCO in Fischer-Tropsch synthesis on cobalt catalysts.22 The hydrogenation of the oxygen atom in adsorbed CO is facilitated via proton transfer from surface hydroxyl group, forming a COH surface species. The latter species is then easily hydrogenated to a CHOH surface species, which rapidly dissociates into adsorbed CH and OH, avoiding the formation of formaldehyde. In the current work we present an ab initio microkinetic model to assess the role of surface hydroxyl groups and the relevance of the enol pathway in the hydrogenation of ketones on copper catalysts, using acetone as a model compound. Analysis of the kinetic model furthermore provides suggestions for the role of promoters in copper catalysis. While acetone hydrogenation is selected as a model reaction for the hydrogenation of ketones, the acetone-isopropanolhydrogen system is increasingly applied in chemical heat pumps24 due to its high selectivity.
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2. Computational details Stabilities, activation barriers and Gibbs free energies were calculated using the VdW-DF225-26 functional as implemented in the Vienna ab initio simulation package (VASP),27-28 and a planewave basis set with a cutoff kinetic energy of 400 eV. The selection of the VdW-DF2 functional was motivated by several benchmark calculations for CO, CH3O, and CH3OH adsorption on Cu(111) and Pt(111), Supporting Information Table S1 and S2. In all cases, the VdW-DF2 adsorption energy was within 8 kJ/mol of experimental Single Crystal Adsorption Calorimetry data.29 Since hydrogenation reactions are generally structure insensitive,30-32 the reactions were modeled on the Cu(111) surface. This surface was modeled as a five-layer Cu(111) slab, using a large p(4x4) unit cell. The top two layers and the adsorbates were relaxed, while the bottom three layers were fixed at bulk positions. The optimized lattice parameter, 3.74 Å slightly exceeds the experimental lattice, 3.62 Å.33 A 15 Å vacuum layer separates repeated slabs. The Brillouin zone was sampled with a 3 x 3 x 1 Monkhorst-Pack grid. Transition states were identified with the climbing image nudged elastic band (ci-NEB)34-35 method and refined with the dimer36 method. Enthalpy corrections and entropies for gas phase species, surface intermediates and transition states were obtained from frequency calculations for the full structure. Thermodynamic consistency was imposed for all elementary reactions, including coverage effects, i.e. the reverse rate coefficients, k-, were calculated from the forward rate coefficients, k+, and the equilibrium coefficients, Keq. A microkinetic model was constructed for the hydrogenation of acetone to isopropanol, using the rate coefficients from density functional theory. The conversion was simulated for an ideal plug flow reactor:
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+ = ,
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(1)
with Ci the concentration of species i in mol/m³, Ct the total site concentration in mol/kgcat and Rw,i, the net formation rate of species i in s-1. The latter was integrated in a transient manner to ensure a smooth mathematical convergence to the solution. For the surface intermediates, no axial dispersion is present in the mass balance.
= ,
(2)
The equations were solved till the steady state was reached using the DASPK37 solver implemented in an in-house FORTRAN code. Central differencing was used to discretize the axial reactor coordinate. A typical active site concentration of 1 mmol kgcat-1 was used.38 To provide an accurate description of the stability of surface intermediates, repulsion between the adsorbates should be taken into account.39-40 Therefore, coverage-dependent stabilities were computed using the approach described by Jorgensen and Grönbeck.41 In this approach, the stability of each species is calculated for a range of coverages of the relevant (high coverage) species. For example, the stability of the alkoxy species is calculated for O* and OH* coverages of 1/9, 2/9, 3/9 and 4/9 ML. By comparing these values to the stability of the intermediate on a clean surface, the destabilization by interspecies repulsion is determined to construct so-called ‘destabilization correlations’. These correlations are used to correct the low coverage reaction free energies and activation free energies and establish a coverage-dependent microkinetic model. To identify the surface species subject to kinetically relevant destabilization effects, a sensitivity analysis was performed for the low-coverage microkinetic model (Figure S1). The microkinetic model appeared only to be sensitive to the coverage-dependent stabilities of a limited number of species, i.e., alkoxy, oxygen and hydroxyl. 7 ACS Paragon Plus Environment
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3. Results and Discussion Reaction pathways for acetone hydrogenation on a clean surface As illustrated in Figure 1 in the Introduction, hydrogenation of the carbonyl group can follow several pathways. The transition states for the relevant elementary steps are shown in Figure 2 and the corresponding energy profiles in Figure 3. The carbon atom of the carbonyl group in the keto-form (TS1 and TS4) or the sp2 carbon atoms in the double bond of the enol-form (TS9, TS10, and TS11) are preferentially hydrogenated by surface hydrogen, while the oxygen atom can be hydrogenated by surface hydrogen (TS2 and TS3), by hydroxyl species (TS 5 and TS6) and by water molecules (TS 7 and TS8) with reasonable barriers.
Figure 2. Transition states for the various possible acetone hydrogenation pathways (keto- and enol-form) on Cu(111) in Figure 1. Top row: Hydrogenation of the carbonyl C and O atom by atomic hydrogen; second row: hydrogenation of the carbonyl O atom by a surface hydroxyl and by water. bottom row: hydrogenation of the C atoms by atomic hydrogen in the enol pathway. 8 ACS Paragon Plus Environment
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A: Hydrogen as a hydrogenating agent
Energy [kJmol-1]
150
103 TS 10
100
112 TS 3
50 0
79 TS 11
74 TS 9
65 TS 1
+ 2H*
33 TS4 + H*
132 TS 2
-50 + H*
-100 -150
B: Hydroxyl as a hydrogenating agent 150
Energy [kJmol-1]
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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100 108 TS 5
50 0 -50
33 TS4 +H* +O*
65 TS 1
+H* +OH*
72 TS 6
+H* +OH*
+O*
-100 +OH*
-150
C: Water as a hydrogenating agent
Figure 3. Energy profiles for the hydrogenation of acetone to isopropanol on Cu(111) with surface hydrogen (A), with surface hydroxyl (B) and with surface water (C). The green line corresponds to the alkoxy pathway, the red line to the carbene pathway, the blue and orange lines to the enol pathways. The corresponding transition state structures are shown in Figure 2. 9 ACS Paragon Plus Environment
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With hydrogen as hydrogenating agent (Figure 3A), the stable alkoxy intermediate is formed with a relatively low barrier. Subsequent hydrogenation of the alkoxy species is, however, highly activated. Hydrogenation of the oxygen atom via the carbene pathway, irrespective of the hydrogenating agent, is highly activated and results in an unstable carbene intermediate. Clearly, hydrogenation of the oxygen atom is most highly activated in both pathways. Tautomerization to the enol form could, to a certain extent, circumvent this difficult O hydrogenation step. The barrier to hydrogenate the primary carbon atom in the enol (TS9) has a relatively low barrier, typical for hydrogenation of a carbon atom (TS1 and TS11). The limited stability of the enol form, however, renders the enol pathway for acetone hydrogenation less important and indicates the alkoxy pathway as the preferred one, at least from an energetic perspective. In addition, the surface is expected to be highly covered with the alkoxy species due to its high stability compared to acetone. Grabow and Mavrikakis investigated the hydrogenation of formaldehyde as part of a mechanistic study of CO2 hydrogenation on Cu(111).42 Our energy profile is similar to theirs. Hydrogenation of formaldehyde to the methoxy species has a barrier of 23 kJ mol-1, while hydrogenation to the carbene species (hydroxymethylene) has a barrier of 79 kJmol-1. Hydrogenation of the methoxy species is difficult, with a high barrier of 113 kJmol-1, while hydrogenation of the carbene species is relatively easy, 54 kJmol-1. Comparable profiles were also obtained for the hydrogenation of cyclohexanone to cyclohexanol on Cu(111).21 When hydroxyl groups are available, a new reaction path opens (Figure 3B), and hydrogenation of the oxygen atom in the alkoxy species is significantly facilitated. Proton transfer to the O atom in the carbonyl group also becomes possible, but the limited stability of the hydroxycarbene intermediate still limits the importance of the carbene pathway. A similar reduction in barriers is observed in oxidative dehydrogenation of ethanol on gold and platinum surfaces due to surface 10 ACS Paragon Plus Environment
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oxygen.23 The barrier for ethanol dehydrogenation to an ethoxy species decreases from 204 kJmol-1 to 22 kJmol-1 on Au(111) and from 116 kJmol-1 to 18 kJmol-1 on Pt(111) when oxygen is available as the proton acceptor. The relevance of the hydroxyl pathway depends on the surface hydroxyl concentration and on the interconversion between oxygen and hydroxyl species. These oxygen species can originate from the presence of water, the spillover of oxygen from the support or incomplete reduction of the metal surface. To evaluate the relevance of this pathway, a microkinetic model accounting for the surface coverages is therefore required. In addition to hydroxyl groups, also adsorbed water can act as hydrogenating agent. The barriers for proton transfer from adsorbed water to the O atom in the alkoxy and the carbonyl species are significantly lower than the barriers for hydrogen, Figure 3C. In the first step, hydrogenation of the carbon atom is preferred over proton transfer from water to the O atom, resulting in the formation of the alkoxy intermediate. Subsequent proton transfer from adsorbed water to the alkoxy species has a lower barrier than hydrogenation with either hydrogen or hydroxyl groups. Based on these energy profiles, water as hydrogenating agent might be expected to dominate. Very similar energy profiles were obtained for the hydrogenation of formaldehyde on Cu(111), see Supporting information Figure S2 and S3, demonstrating the general validity of the energy profiles for the hydrogenation of carbonyl groups on Cu(111). To close the catalytic cycle for the hydroxyl and water pathways, the resulting O and OH species need to be hydrogenated. The relevant reactions are summarized in Table 1.
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Table 1. Transition states and activation energies for the hydrogenation of O and OH species on Cu(111). Reaction
∗ + ∗ ⇋ ∗ + ∗
∗ ⇋ ∗ + ∗
∗ + ∗ ⇋ ∗
Transition state
Ea,+ [kJmol-1]
130
62
118
∆Hr [kJmol-1]
5
62
-57
As evident from Table 1, hydrogenation of hydroxyl groups is highly activated. This is consistent with previous calculations in the context of methanol synthesis and the water-gas-shift reaction.43-44 Hydrogenation of the oxygen atoms also has a high activation barrier. However, regeneration of the hydroxyl groups via proton transfer from water to surface oxygen has an activation barrier close to zero. To evaluate the balance between the different reaction paths, a DFT-based microkinetic model is constructed next. Microkinetic simulations with first principle rate coefficients Based on the elementary steps contained in the energy profiles in Figure 3, as well as those in Table 1, a microkinetic model was constructed. The rate and equilibrium coefficients for all these steps are provided in Table 2 and Table 3. Interestingly, a wide range of pre-exponential factors is obtained, from as low as 1.5 107 s-1 for proton transfer from water to carbonyl, to the more typical 1013 s-1 for hydrogenations involving surface hydrogen. The pre-exponential factors are determined by the entropy difference between the reactant and the transition state. Hydrogenation 12 ACS Paragon Plus Environment
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by surface hydroxyl or adsorbed water is associated with a significant entropy loss from the reactant to the transition state of -12 to -21 Jmol-1K-1 for surface hydroxyl and -45 Jmol-1K-1 for water, leading to low pre-exponential factors. This is associated with the fairly high mobility of hydroxyl groups and the high mobility of water on Cu(111) at low coverage, as also illustrated by the rather small entropy loss for water adsorption as compared to acetone and isopropanol (Table 2). The kinetic coefficients presented in Table 3 provide an indication of the important reaction steps. Along the enol-pathways, hydrogenation of the carbon atom (TS 1, TS 4) is orders of magnitude faster than hydrogenation of the oxygen atom (TS 2, TS 3). However, proton transfer from either hydroxyl or water to the O-atom in the carbonyl group, via the carbene pathway, has a lower rate coefficient than hydrogenation by surface hydrogen, despite the higher barrier for the latter pathway. Hydrogenation of the O-atom of the alkoxy intermediate (TS 2) has a rate coefficient of only 6.5 10-2 s-1, while the rate coefficient for proton transfer from a hydroxyl species (TS 6) or from adsorbed water (TS 7) is 106 or 109 times faster.
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Table 2. Adsorption and isomerization rate and equilibrium coefficients at 473 K for the gas phase molecules in the model. The rate coefficients for acetone, isopropanol and water adsorption are calculated using collision theory and the equilibrium coefficients by DFT. Since dissociative adsorption of H2 on Cu(111) is activated, the corresponding rate coefficient was calculated using transition state theory. ∆Hads [kJmol-1]
Reaction
∆Sads [Jmol-1K-1]
Keq [Pa-1]
k+ [s-1 Pa-1]
k- [s-1]
+ ∗ ⇋ ∗
-32
-135
2.8 ∙ 10'(
8.6 ∙ 10*
3.5 ∙ 10++
+ ∗ ⇋ ∗
-38
-133
1.7 ∙ 10'-
8.4 ∙ 10*
4.9 ∙ 10+0
+ ∗ ⇋ ∗
-6
-134
4.2 ∙ 10'1
4.2 ∙ 10'-
1.0 ∙ 10(
+ ∗ ⇋ ∗
-21
-104
9.0 ∙ 10'(
1.7 ∙ 10-
6.1 ∙ 10+0
∗ ⇋ ∗
54
-15
1.8 ∙ 10'1
1.7 ∙ 102
9.9 ∙ 10+*
Table 3. Pre-exponential factors, activation energies and rate coefficients at 473 K for the surface reactions in the microkinetic model. The corresponding transition state structures (Figure 2) are indicated. A+ [s-1]
Reaction ∗ + ∗ ⇋ ∗ + ∗
Ea+ [kJmol-1]
A[s-1]
Ea[kJmol-1]
k+ [s-1]
k[s-1]
6.5 ∙ 10+0
62
9.6 ∙ 10+*
131
9.2 ∙ 10-
3.5 ∙ 10'*
2.8 ∙ 10+-
133
4.1 ∙ 10+*
124
6.5 ∙ 10'*
7.9 ∙ 10'-
6.2 ∙ 10+*
112
1.8 ∙ 10+3
48
2.8
1.0 ∙ 10+0
3.1 ∙ 10+(
46
2.2 ∙ 10+*
171
2.4 ∙ 104
3.1 ∙ 10'1
∗ + ∗ ⇋ ∗ + ∗ (TS5)
6.5 ∙ 105
99
1.9 ∙ 10+(
0
7.4 ∙ 10'-
1.9 ∙ 10+(
∗ + ∗ ⇋ ∗ + ∗
(TS6)
1.5 ∙ 101
73
4.3 ∙ 10+*
14
1.2 ∙ 10'+
1.3 ∙ 10++
∗ + ∗ ⇋ ∗ + ∗
(TS7)
1.9 ∙ 10+*
67
1.0 ∙ 10++
2
6.9 ∙ 10(
6.0 ∙ 10+0
7.4 ∙ 10+0
19
2.5 ∙ 10+*
16
1.2 ∙ 105
4.6 ∙ 104
2.0 ∙ 10+*
71
9.5 ∙ 10+-
61
2.8 ∙ 10(
1.8 ∙ 101
∗ + ∗ ⇋ ∗ + ∗ (TS10)
2.5 ∙ 10++
100
1.1 ∙ 10+-
103
2.0
4.8 ∙ 10+
∗ + ∗ ⇋ ∗ + ∗
1.4 ∙ 10+-
79
1.1 ∙ 10++
191
2.7 ∙ 10(
9.3 ∙ 10'++
∗ + ∗ ⇋ ∗ + ∗
4.1 ∙ 10+0
125
4.2 ∙ 10+-
130
6.6 ∙ 10'(
2.0 ∙ 10'+
∗ + ∗ ⇋ ∗
1.8 ∙ 104
0
2.9 ∙ 10+*
60
1.8 ∙ 104
7.6 ∙ 103
∗ + ∗ ⇋ ∗ + ∗
1.9 ∙ 10+-
118
5.1 ∙ 10+*
175
1.6 ∙ 100
2.4 ∙ 10'1
(TS1)
∗ + ∗ ⇋ ∗ + ∗ ∗ + ∗ ⇋ ∗ + ∗
(TS2)
(TS3)
∗ + ∗ ⇋ ∗ + ∗
(TS4)
∗ + ∗ ⇋ ∗ + ∗ ∗ + ∗ ⇋ ∗ + ∗
(TS8) (TS9)
(TS11)
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To evaluate the relative importance of the different pathways, microkinetic simulations were performed for a typical acetone space time of 500 kgcat s molacetone-1, a temperature of 473 K and a total pressure of 100 bar, for which intrinsic kinetic data were obtained experimentally38 An inlet molar H2:acetone ratio of 2 and a molar acetone:H2O ratio of 2 were used. At these operating conditions, the microkinetic model simulates a turnover frequency of only 6 10-5 s-1 and a corresponding acetone conversion of 0.005%. This value is orders of magnitude lower than typical experimental values for acetone hydrogenation over Cu catalysts, which are in the range of 10-3 to 10-1 s-1.45 The low TOF could be caused by the very high O* and OH* surface coverages calculated by the microkinetic model. As shown in Figure 4, the surface is completely occupied by oxygen and hydroxyl groups, inhibiting the adsorption and hydrogenation of acetone. When no H2O is fed, the surface is completely covered with alkoxy species and the TOF drops to 8 10-6 s-1 because of the low rate coefficient for the hydrogenation of the alkoxy species (TS 2).
These high coverages are not in line with the assumptions made in the ab initio
calculations for the rate coefficients, i.e., particularly that of a clean surface. To overcome the inconsistency, coverage dependent rate coefficients are computed in the next section.
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ACS Catalysis
Figure 4. Coverages of oxygen (green) and hydroxyl (blue) species along the reactor. Conditions: 500 kgcat s molacetone-1, 473 K, 100 bar, inlet molar ratio H2:acetone:H2O = 2:1:0.5.
Figure 5. Reaction path analysis for the hydrogenation of acetone to isopropanol. The numbers indicate the fraction of each component consumed in the particular reaction step. Dotted arrows indicate reaction steps that are insignificant, i.e., a fraction below 0.5%, while green arrows indicate reaction steps with a fraction above 90%. Conditions: 500 kgcat s molacetone-1, 473 K, 100 bar, inlet molar ratio: H2:acetone:H2O = 2:1:0.5. The reaction path analysis in Figure 5 shows the four possible pathways for the hydrogenation of acetone: the alkoxy path and the carbene path and the enol pathways. As expected from the energy profiles (Figure 2), the alkoxy pathway dominates in the consumption of acetone. Carbene
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formation is insignificant both via hydrogenation with surface hydrogen and via proton transfer from hydroxyl species and water. The enol pathways have no significant contribution to the acetone consumption because of the relatively low stability of the adsorbed enol species. The alkoxy species are converted quantitatively to the isopropanol product; dehydrogenation of alkoxy species back to the reactant does not occur to a significant extent. This originates from the high stability of the alkoxy intermediate. Conversion of the alkoxy species essentially occurs via hydroxyl species. Despite the lower barrier for proton transfer from water (19 kJ/mol (TS8) vs. 67 kJ/mol (TS7), Table 3), the low surface concentration and the low pre-exponential factor limit its role. The relative importance of water as hydrogenating agent slightly depends on the operating conditions. At higher temperatures, the importance of the water path slightly increases. The reaction path analysis furthermore shows that the forward rate for the hydrogenation of the alkoxy intermediate is 6 orders of magnitude faster than the forward rate for the hydrogenation of acetone. The hydrogenation of the carbonyl group to the alkoxy intermediate is irreversible along the hydrogenation path and the second step is quasi-equilibrated. Hydrogenation via the hydroxyl pathway converts one hydroxyl group to an oxygen atom. The fate of the oxygen and hydroxyl groups is, hence, crucial for the microkinetic simulations. Surface O and OH groups are interconverted by the quasi-equilibrated reaction with water (8). The reaction path analysis furthermore shows that 50% of the surface hydrogen is consumed for the hydrogenation of surface oxygen, and less than 0.1% is consumed for the hydrogenation of hydroxyl groups, due to the higher rate coefficient for O* hydrogenation (Table 3). Based on these insights, the reaction mechanism can be simplified.
6- * = 78 + ∗ ⇋ 6- * = 7∗
quasi-equilibrated
(3)
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ACS Catalysis
6* 78 + ∗ ⇋ 6* 7∗
quasi-equilibrated
(4)
6* 8 + 2 ∗ ⇋ 26 ∗
quasi-equilibrated
(5)
6- * = 7∗ + 6 ∗ → 6- * 67∗ + ∗
irreversible
(6)
6- * 67∗ + 76 ∗ ⇋ 6- * 676 ∗ + 7∗
quasi-equilibrated
(7)
7∗ + 6* 7∗ ⇋ 276 ∗
quasi-equilibrated
(8)
7∗ + 6 ∗ → 76 ∗ + ∗
irreversible
(9)
The findings of the reaction path analysis are reflected in the reaction orders determined by fitting a polynomial rate equation to the calculated TOFs at operating conditions:
:;< = ?;< @A0.3 @'0.3 @0 B AB C DAE B DC
(10)
The half-order in H2 results from the consumption of hydrogen in the irreversible step (9). The inhibiting effect of water results from its effect on the ratio of the O* and OH* coverages. The higher the water pressure, the lower the equilibrium ratio of the O* and OH* coverages, thus lowering the O* hydrogenation rate. In the absence of water, acetone is also hydrogenated via the alkoxy pathway, and the TOF decreases by a factor 5. The rate-limiting step in this case becomes the hydrogenation of the alkoxy species, and the first hydrogenation step now becomes reversible. Even in the absence of water, the enol pathway remains insignificant. Only when the enol tautomer would be 20 kJ/mol more stable, the reaction path tends towards this pathway (Figure S4). Interestingly, the enol pathway would lead to the formation of the 2-carbene intermediate, which converts back to acetone rather than form isopropanol even though the rate coefficient for carbene hydrogenation is high (2.4 109 s-1, Table 3).
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Accounting for coverage dependence effects Stability of the important reaction intermediates as a function of the surface coverage The microkinetic model parameters as reported in Table 2 and 3 were based on low-coverage DFT calculations, i.e., a clean surface. Such calculations tend to overestimate the stability of adsorbed species under real operating conditions and, hence, often result in surface coverages that violate the assumptions made in the first principles calculations, see Figure 4. In reality, repulsion between adsorbates reduces the stability of the intermediates, which reduces the coverages. To account for these coverage effects, the stability of the different species was adjusted by introducing coverage-dependent intra- and inter-species correction factors, δGcorr: L
FGH I J∀ = FGH,LM> NH O + P FGH,,L M
(11)
QH
As indicated in Figure S1, sensitivity analyses identified the stability of oxygen, hydroxyl and alkoxy species as the most important parameters (Figure S1). Activation free energies and rate coefficients were adjusted using a Bronsted-Evans-Polanyi relationship with a transfer coefficient of 0.5. Changing the transfer coefficient by 20% had a limited effect on the TOF (Table S3), and no effect on the reaction path analysis. Intra-species repulsion corrections were estimated by calculating the change in adsorption free energy for a range of coverages, using the adsorption free energy at 1/16 ML as the reference (Figure 6). The resulting data points were fitted to a 2-parameter power law, see Table 4. Interspecies repulsions between X and Y were determined by calculating the adsorption free energy of X for different coverages of Y, and subtracting that value from the adsorption free energy of X on a clean surface (Figure 6). These data points were also fitted to two-parameter power laws, Table 19 ACS Paragon Plus Environment
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4. The power law functions provide a good fit for the repulsion correction with R² values all exceeding 0.94 (Table S4). The effective destabilization was calculated as the sum of the interspecies and intra-species repulsion terms. The structures that were used to calculate the repulsion corrections are shown in the Supplementary Info, Figure S6 – S13. For each coverage several structures were investigated. The most stable ones were selected to construct the coverage corrections and were presented in Figure S6 – S13. With this approach, the correction factor for the oxygen adsorption free energy, δGO, for (θO=0.2, θOH=0.2, θalkoxy=0.2), becomes 5 + 10 + 15 = 30 kJ/mol.
50
0
0.2 θO (-)
δGalkoxy (kJ/mol)
100
0
100
100 50 0
0.4
0.2 θOH (-)
0.4
50
100
0
0.2 θOH (-)
50
0
0.4
0.2
0
0.2 θO (-)
0.4
0.2 θOH (-)
0.4
50 0
0.4
100 50
150 δGalkoxy,OH (kJ/mol)
δGOH,alkoxy (kJ/mol)
150 100 50
0.4
100 50 0
0
0 0.2 θalkoxy(-)
0.4
θO (-)
150
0
0.2 θalkoxy (-)
100
0
0
0 150 δGalkoxy,O (kJ/mol)
δGOH,O (kJ/mol)
150
100
50
0 0
150 δGO,OH (kJ/mol)
150
150 δGOH (kJ/mol)
δGO (kJ/mol)
150
δGO,alkoxy (kJ/mol)
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0
0.2 θalkoxy (-)
0.4
0
Figure 6. Coverage-dependent intra- (top row) and inter-species (middle and bottom row) correction factors for oxygen (left column), hydroxyl (middle column) and alkoxy species (right column). The dashed lines show the fitted repulsion model, the 20 ACS Paragon Plus Environment
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circles represent the calculated datapoints. The optimized structures are shown in Supplementary Info, Fig S5-S12. Table 4. Intra-species and inter-species repulsion correction relationships included in the microkinetic model. Intra-species repulsion [kJmol-1] θO on O*
θOH on OH*
θalkoxy on alkoxy
FGC θC = 974 SC −
1 -.3 U 16
FGCA θCA = 32 SCA −
1 +.*3 U 16
FG>
