ARTICLE pubs.acs.org/JPCC
Adsorption of Acid, Ester, and Ether Functional Groups on Pt: Fast Prediction of Thermochemical Properties of Adsorbed Oxygenates via DFT-Based Group Additivity Methods M. Salciccioli, S. M. Edie, and D. G. Vlachos* Department of Chemical Engineering, Catalysis Center for Energy Innovation and Center for Catalytic Science and Technology, University of Delaware, Newark, Delaware 19716-3110, United States
bS Supporting Information ABSTRACT: Density functional theory (DFT) calculations are utilized to systematically study the geometric and energetic trends of adsorbed carboxylic acid, ether, and ester intermediates on Pt(111). Specifically, the properties of formic acid, acetic acid, propionic acid, isobutyric acid, dimethyl ether, ethyl methyl ether, isopropyl methyl ether, methyl formate, methyl acetate, and their dehydrogenation intermediates are calculated. Subsequent statistical mechanical calculations are performed to reveal systematic trends in thermochemical properties of oxygenate adsorbates on Pt(111). These trends are utilized to develop and parametrize a new semiempirical group additivity scheme that can be used to predict thermochemical properties of acids, ethers, esters, alcohols, aldehydes, and hydrocarbons and their decomposition intermediates on Pt(111). This scheme is both rapid and accurate and can easily be implemented as a tool to parametrize and screen large oxygenate reaction mechanisms for identification of important reaction steps that can subsequently be refined with more accurate computational methods.
1. INTRODUCTION The production of fuels and chemicals from biomass, as a means of alleviating dependence on fossil resources, has recently attracted significant attention in heterogeneous catalysis research.1,2 An important aspect of technology development in this area centers around gaining a fundamental understanding of the surface mechanisms involved in chemical transformations.3 Understanding and controlling the bond-breaking sequences of biomass-derived oxygenates on metal surfaces could lead to the development of more active and selective processes for biomass transformations to fuels and chemicals.4 Specifically, understanding the interactions of specific oxygenate functional groups and their reactive intermediates with catalytic surfaces may enable processing of more complex biomass derivative mixtures.5 Gayubo et al. have recently shown that alcohol, carboxylic acid, ester, and ether functional groups (as well as the combination of multiple functional groups) are representative of lignocellulose derived bio-oil.6 Density functional theory (DFT) can be an effective tool to probe the energetic and geometric trends of these functional groups and their reactive intermediates. Combining experimental studies with quantum mechanical calculations and kinetic models offers insights into surface intermediates and reaction fluxes that cannot currently be observed experimentally and offer systematic methods to discover new and improve existing catalysts.79 Platinum (Pt) is a promising single metal catalyst for biomassderived oxygenate reforming.2 While decomposition of small acids and alcohols on Pt has been studied previously via DFT for r 2012 American Chemical Society
applications in fuel cells,1014 limited work has been conducted on the intermediates containing these functional groups, along with ethers and esters on Pt. To address this lack of understanding, in the first part of this work we extend our previous work on alcohols and their dehydrogenation intermediates4,15,16 to present a comprehensive set of DFT geometric optimizations and thermochemical property calculations of formic acid, acetic acid, propionic acid, isobutyric acid, dimethyl ether, ethyl methyl ether, isopropyl methyl ether, methyl formate, methyl acetate, and their dehydrogenation intermediates on Pt(111). This set of data allows for full analysis of energetic trends in the conversion of important oxygenate functional groups in biomass processing over Pt. Further, our previous research has shown that comprehensive sets of DFT-calculated thermochemical properties of surface intermediates can lead to the development of semiempirical methods that drastically reduce the cost of studying large reaction networks.9,15,17 Specifically, the Benson type group additivity schemes,1821 when applied to surface reaction mechanisms,15,22,23 can effectively screen out energetically unfavorable and improbable reaction pathways and allow for computational resources to be used on realistic and prevalent reaction pathways.17 With this as a motivation, the second part of this work uses the comprehensive set of DFT calculations to define a broader set of group Received: September 21, 2011 Revised: December 2, 2011 Published: January 05, 2012 1873
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contributions which can be used to calculate thermochemical properties of biomass-derived oxygenate reactive intermediates containing alcohol, acid, ether, and ester functional groups.
2. DFT-BASED THERMOCHEMICAL PROPERTY CALCULATIONS All density functional theory (DFT) calculations employed the SIESTA code24,25 which utilized TroullierMartins normconserving scalar relativistic pseudopotentials26 and a double-ζ plus polarization (DZP) basis set. The DZP basis set is chosen as a balance of accuracy and computational expense (given the large number of calculations of reasonably large molecules). Previous work has shown that the “standard” DZP basis offers wellconverged results, comparable to those used in practice in most plane-wave calculations.24 A DFT supercell approach used the PerdewBurkeErnzerhof (PBE) form of the generalized gradient approximation (GGA) functional,27 and a mesh cutoff of 200 Ry was used. Further calculation methods are identical to our previous published work.15 Due to the use of only Pt metal atoms, the nonspin version of the code was utilized, as previous research has shown that this does not significantly affect the results.28 Methods for vibrational frequency calculations were described previously.15 The structure from complete geometry optimization with all forces on atoms being less than 0.05 eV/ Å was used for this analysis. All metal atoms were frozen to the original geometry as atoms of the adsorbate were systematically displaced by 0.03 Bohr in all three Cartesian directions. Temperature-dependent thermochemical properties of surface intermediates were calculated from the vibrational frequencies using statistical mechanics as described previously.15 All adsorbates are assumed to contain two frustrated translational degrees of freedom, corresponding to lateral movement in orthogonal directions parallel to the surface, which were converted to vibrational frequencies as previously described.15,29 Zero-point energy (ZPE) corrections were applied to all calculations unless otherwise noted, and the PV contribution to Gibbs free energy was neglected.30 Heats of formation of adsorbates were defined from the heats of formation of the most highly hydrogenated species in each set and the reaction enthalpy at 298 K for dehydrogenation to that intermediate. For example, energies for all HxCOOHy surface species were defined with respect to the heat of formation of formic acid in the gas phase and hydrogen atoms adsorbed on infinitely separated slabs. Heats of formation of gasphase molecules were taken from the NIST database31 (gas-phase properties of isobutyric acid were calculated with Benson's method18). Calculated thermochemical properties of all adsorbates on Pt(111) are listed in the Supporting Information. 3. ADSORPTION OF BIOMASS-DERIVED OXYGENATE FUNCTIONAL GROUPS ON PT This section presents the geometric and energetic results of DFT calculations on acid, ether, and ester adsorption on Pt(111). The Supporting Information includes additional geometric information (optimized structure bond lengths) not included in the tables in this section. Snap binding energies23 are included for species with gas-phase structures that cannot be fully relaxed without significant rearrangement. 3.1. Carboxylic Acid Dehydrogenation Intermediates on Pt(111). Formic acid, the simplest carboxylic acid, adsorbs to
Pt(111) through three stable configurations. Figure 1 shows these configurations, along with the optimized gas-phase species.
Figure 1. Stable gas-phase (A) and stable adsorption configurations of formic acid (HCOOH) on Pt(111). Adsorption configurations include (B) weakly bound η2μ2(O,O), (C) weakly bound η2μ2(C,O), and (D) the most stable η1μ1(O).
The most stable configuration has a binding energy of ∼13 kcal/mol (see Table 1). This configuration (η1μ1) involves binding through the lone pair orbital, where the adsorbate undergoes little structural change from the optimized gas-phase formic acid. Additional stabilization is provided from the hydrogenPt interaction (Figure 1D). The other stable configurations include a similar mode where the hydroxyl group is rotated 180 degrees so that there is interaction between both of the oxygen atoms and the metal (η2μ2) (Figure 1B) and a di-σ type η2μ2(C,O) configuration which brings the CO molecule backbone much closer to the metal surface (see Figure 1C). The 180 degree rotation of the hydroxyl group of formic acid in vacuum (from the structure shown in Figure 1A) results in a destabilization of around 3 kcal/mol, a topic that has been covered in depth in a previous study.32 These adsorption configurations are consistent with previous studies of formic acid on Pt,1214 where differences in relative stability can arise based on surface facet and hydrogen bonding networks from explicit water molecules. In fact, experimental results show that the condensation of 35 monolayers of HCOOH on Pt(111) results in significant changes in the vibrational spectra when compared to the monomer frequencies.33 Specifically, Avery found that the ν(OH) mode appears at ∼2900 cm1, whereas the monomer value is at 3570 cm1.33 The DFT calculations in this work also show a reduced frequency of ∼3000 cm1 (see Table 1). Avery explains this reduction in frequency from the expected ν(OH) acid mode as the effect of the strong intermolecular hydrogen bonding that occurs in the molecules. While this would not apply to the lowcoverage DFT calculations presented here, the PtH interaction in the adsorption geometry shown in Figure 1D is a potential contributor to the decrease in frequency. Dehydrogenation of formic acid can occur through two paths. CH bond activation results in formation of the hydroxyl carbonyl species (COOH) (shown in Figure 2A), which has been shown to be an important intermediate in watergas shift chemistry on Pt.34 This intermediate bonds in a η1μ1(C) configuration with the hydrogen atom pointing down at a neighboring Pt atom. The alternative dehydrogenation product of formic acid is through OH bond scission and results in the formate (HCOO) intermediate (shown in Figure 2B). This intermediate binds to the surface through two PtO bonds and is slightly less stable thermodynamically than 1874
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Table 1. DFT Calculated Structural and Energetic Properties of Adsorbed Acid Intermediates on Pt(111)a binding adsorbate
mode
Q
E Eo ν [cm1]
[kcal/mol] [kcal/mol] HCOOH (formic acid)
HCOOH
η1μ1(O)
12.9
12.7
31, 31, 132, 194, 233, 251, 636, 737, 947, 1125, 1276, 1298, 1615, 2745, 3002
COOH
η1μ1(C)
62.0
24.7
69, 69, 189, 264, 312, 523, 597, 651, 1066, 1239, 1681, 3125
HCOO
η2μ2(O,O)
56.4
14.2
66, 66, 162, 290, 323, 355, 720, 909, 1270, 1277, 1532, 2963
H2COOH
η2μ2(O,O)
40.4
12.7
54, 54, 126, 238, 257, 394, 432, 555, 747, 999, 1054, 1173, 1192, 1300, 1355, 2859, 2940, 3572
H2COO
η2μ2(O,O)
64.9
27.1
70, 70, 155, 190, 383, 425, 610, 923, 956, 1016, 1087, 1242, 1306, 2816, 2848
CH3COOH
η1μ1(O)
14.0
13.8
28, 28, 86, 144, 157, 166, 188, 450, 545, 602, 711, 876, 961, 983, 1168, 1279, 1341, 1362, 1386, 1616, 2866, 2984, 3079, 3122
CH2COOH
η2μ2(C,O)
46.8
10.4
52, 52, 114, 198, 220, 302, 439, 594, 612, 645, 707, 877, 1039, 1059, 1156, 1276, 1346, 1616,
CHCOOH
η2μ3(C,O)
96.5
12.1
76, 76, 214, 251, 305, 377, 573, 630, 644, 679, 799, 918, 1095, 1167, 1350, 1535, 3020, 3558
CCOOH
η1μ3(C)
132.7
18.0
87, 87, 155, 253, 304, 441, 587, 596, 629, 706, 941, 1118, 1315, 1666, 3539
CH3COO
η2μ2(O,O)
52.1
13.4
55, 55, 95, 189, 200, 236, 258, 516, 573, 650, 923, 966, 977, 1276, 1343, 1358, 1380, 1481,
CH2COO CHCOOb
η2μ2(C,O) η2μ3(C,O)
42.4 105.3
0.9 2.8
CCOO
η2μ4(C,O)
146.0
0.5
CH3CH2COOH
η1μ1(O)
13.8
13.9
25, 25, 45, 93, 117, 144, 165, 226, 287, 482, 532, 591, 721, 746, 816, 952, 1033, 1068, 1157,
CH3CHCOOH
η2μ2(C,O)
37.9
9.0
42, 42, 80, 114, 211, 226, 242, 291, 401, 491, 578, 610, 695, 814, 854, 977, 1005, 1077, 1144,
CH3CCOOH
η2μ3(C,O)
83.9
10.9
63, 63, 154, 190, 229, 244, 278, 337, 398, 539, 594, 627, 679, 837, 936, 1020, 1084, 1141, 1287,
CH3CH2COO
η2μ2(O,O)
52.6
14.5
1317, 1330, 1349, 1533, 2830, 2979, 3039, 3564 50, 50, 90, 142, 147, 184, 240, 248, 375, 545, 562, 651, 745, 874, 976, 1030, 1076, 1199, 1250,
CH3CHCOO
η3μ3(C,O,O)
38.3
4.4
43, 43, 161, 194, 208, 277, 316, 318, 400, 502, 591, 665, 824, 866, 975, 988, 1082, 1174, 1272,
CH3CCOO
η2μ3(C,O)
60.5
6.3
55, 55, 152, 175, 189, 284, 325, 363, 389, 522, 664, 673, 809, 904, 1002, 1073, 1158, 1275,
CH3COOH (acetic acid)
2939, 3028, 3562
2982, 3077, 3113 50, 50, 129, 228, 293, 317, 456, 601, 628, 709, 845, 1036, 1049, 1200, 1292, 1623, 2960, 3040 90, 90, 184, 304, 314, 395, 622, 650, 658, 796, 865, 1101, 1149, 1613, 2982 95, 95, 140, 287, 333, 474, 597, 641, 657, 887, 1012, 1632 CH3CH2COOH (propionic acid) 1204, 1258, 1322, 1335, 1361, 1388, 1399, 1603, 2905, 2975, 2982, 3032, 3066, 3082 1223, 1299, 1324, 1354, 1363, 1611, 2913, 2966, 3007, 3011, 3563
1320, 1338, 1371, 1385, 1401, 1493, 2976, 2981, 3023, 3079, 3080 1280, 1326, 1340, 1357, 2903, 2994, 3046, 3058 1290, 1328, 1618, 2778, 2982, 3042 CH3CH(CH3)COOH (isobutyric acid) CH3CH(CH3)COOH η1μ1(O)
17.4
17.4
26, 26, 45, 73, 102, 133, 151, 205, 230, 304, 335, 358, 492, 545, 617, 691, 777, 874, 908, 925, 1034, 1111, 1127, 1150, 1232, 1247, 1313, 1326, 1341, 1381, 1384, 1397, 1411, 1594, 2844,
η1μ1(C)
37.7
13.4
39, 39, 94, 156, 203, 210, 226, 245, 263, 265, 332, 397, 475, 544, 592, 694, 744, 887, 899,
2936, 2971, 2973, 3062, 3065, 3083, 3086 CH3C(CH3)COOH
931, 1011, 1042, 1151, 1182, 1280, 1286, 1294, 1320, 1335, 1343, 1359, 1699, 2871, 2898, 2965, 2992, 2998, 3015, 3533 CH3CH(CH3)COO
η2μ2(O,O)
51.8
17.9
45, 45, 64, 129, 146, 182, 213, 218, 235, 335, 371, 380, 544, 603, 694, 819, 870, 917,
10.2
1488, 2948, 2966, 2972, 3062, 3066, 3081, 3081 37, 37, 152, 177, 203, 224, 249, 278, 296, 326, 360, 415, 488, 548, 665, 770, 878, 910, 921, 996,
920, 1029, 1122, 1128, 1230, 1247, 1310, 1319, 1347, 1375, 1381, 1393, 1406, CH3C(CH3)COO
η3μ3(C,O,O)
34.3
1118, 1185, 1255, 1277, 1295, 1299, 1324, 1340, 1350, 1366, 2878, 2913, 2972, 3003, 3019, 3044
ηxμy notation refers to x adsorbate atoms binding to y metal atoms. Q is the molecular binding energy calculated from XM* X M*, where XM* is the adsorbate/slab complex, X is the adsorbate isolated in vacuum, and M* is the clean slab. Binding energies are not corrected for ZPE. b Binding energy reported is a snap binding energy due to instability of gas-phase species. E Eo is a ZPE-corrected dehydrogenation/hydrogenation reaction energy from the reference molecule at 0 K. For example, E Eo for COOH corresponds to the DFT reaction energy of HCOOH(g) + 2* f COOH* + H*. a
the COOH intermediate. Avery experimentally identified this η2μ2(O,O) geometry (shown in Figure 2B) as the most likely bonding configuration for HCOO.33 Electron energy loss
spectroscopy (EELS) results of the formate are in good agreement with the DFT-calculated vibrational frequencies in Table 1.33 Further activation of HCOO to carbon dioxide (CO2) carries 1875
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Figure 2. Schematics of remaining adsorbed acid intermediates on Pt(111) calculated via DFT. Pt, C, O, and H atoms are represented by gray, black, red, and white circles, respectively. Insets in the upper left show the top view of the adsorbate. For simplicity, only one layer of Pt is shown. Species shown include (A) COOH, (B) HCOO, (C) H2COOH, (D) H2COO, (E) CH3COOH, (F) CH2COOH, (G) CHCOOH, (H) CCOOH, (I) CH3COO, (J) CH2COO, (K) CHCOO, (L) CCOO, (M) CH3CH2COOH, (N) CH3CHCOOH, (O) CH3CCOOH, (P) CH3CH2COO, (Q) CH3CHCOO, (R) CH3CCOO, (S) CH3CH(CH3)COOH, (T) CH3C(CH3)COOH, (U) CH3CH(CH3)COO, and (V) CH3C(CH3)COO.
high activation barriers, and thus, it is suggested that formic acid more readily decomposes through the COOH pathway.12 Hydrogenation of formic acid leads to the dioxymethylene (H2COO) and the hydroxymethoxy (H2COOH) intermediates (shown in Figure 2C and D, respectively). While these intermediates are thermodynamically unstable on Pt compared to formic acid in the gas phase (see Table 1), they are important in methanol synthesis on Cu.35 The next largest carboxylic acid of interest is acetic acid (CH3COOH). The adsorption modes of CH3COOH are similar to that of formic acid. The most stable configuration (shown in Figure 2E) is the result of interaction between the oxygen lone pair and the metal. In fact, our DFT results show that regardless of further methyl substitution propionic (CH3CH2COOH) and isobutyric (CH3CH(CH3)COOH) acid adsorb through a similar oxygen lone pairPt interaction (see Figure 2M and S, respectively). Energetic and geometric trends emerge when comparing dehydrogenation intermediates of these carboxylic acids with varying levels of methyl substitution. As seen with formic acid, initial OH scission results in a formate-type intermediate that adsorbs through two PtO bonds on neighboring Pt atoms. Energetically, dehydrogenation reaction energies from the gasphase acid species to the formate species (HCOO, CH3COO, CH3CH2COO, and CH3CH(CH3)COO) all consist of around
15 kcal/mol (Table 1), which is consistent with previous DFT results for both formic and acetic acid dehydrogenation on Pt.7,14 CH3COO has been observed experimentally and binds in the η2μ2(O,O) configuration as predicted by the present DFT calculations.36,37 Similar observations have been made for CH3CH2COO.37 The calculated binding energy of 52 kcal/mol for CH3COO agrees well with the 48 kcal/mol value in a previous DFT study of acetic acid hydrogenation by Olcay et al.38 Alpha-hydrogen elimination of the larger acid species can lead to two stable adsorption configurations. The first is the η2μ2(C,O) configuration (shown for CH2COO in Figure 2J), and the second is the η3μ3(C,O,O) configuration (shown for CH3CHCOO and CH3C(CH3)COO in Figure 2Q and V, respectively). While only the most stable configuration is shown in Table 1 and Figure 2 for each species (information on other configurations can be found in the Supporting Information), for all three of these species these two competing configurations are within 4 kcal/mol. The η3μ3(C,O,O) configuration systematically gets more stable relative to the η2μ2(C,O) configuration as methyl substitution occurs. As the β-carbon becomes more highly methyl substituted, the η3μ3(C,O,O) facilitates stabilization through geometrically convenient interactions between methyl groups and neighboring Pt atoms. This could be related to the experimental observation that thermal decomposition 1876
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Figure 3. Schematics of adsorbed ether intermediates on Pt(111) calculated via DFT. Pt, C, O, and H atoms are represented by gray, black, red, and white circles, respectively. Insets in upper left show the top view of the adsorbate. For simplicity only one layer of Pt is shown. Species shown include (A) CH3OCH3, (B) CH3OCH2, (C) CH3OCH, (D) CH3OC, (E) CH2OCH2, (F) CHOCH2, (G) COCH2, (H) CHOCH, (I) COCH, (J) CH3CH2OCH3, (K) CH3CHOCH3, (L) CH3COCH3, (M) CH3CH(CH3)OCH3, and (N) CH3C(CH3)OCH3.
reactivity of carboxylates decreases from formate to propionate (HCOO > CH3COO > CH3CH2COO).37 Surface intermediates resulting from further dehydrogenation through β-hydrogen elimination are consistent with previously reported DFT trends of the decomposition of alcohols and hydrocarbons on Pt.11,15,22,23,28,40 Species follow gas-phase bond order rules where the β-carbon bonds accordingly to the surface (atop, bridge, or 3-fold) to ensure that the carbon center is tetravalent. The next section of this work describes the geometric and energetic trends of ether adsorption on Pt. 3.2. Ether Dehydrogenation Intermediates on Pt(111). Dimethyl ether (DME) adsorbs to Pt through interaction between the lone pair of electrons on the oxygen atom and the Pt surface site (see Figure 3A). Figure 3 includes schematics of calculated surface intermediates resulting from dehydrogenation of DME, ethyl methyl ether, and isopropyl methyl ether, while Table 2 contains geometric and energetic information for each adsorbate. Our findings are consistent with alcohol adsorption on Pt(111) that has been studied extensively via DFT.4,7,10,15,39,40,4143 The predicted relatively weak binding energy (∼5 kcal/mol) is consistent with a previous DFT result for DME on Pt(111).44 Ethyl methyl ether and isopropyl methyl ether also bind in the η1μ1(O) mode with increasing binding energies due to stabilization through methyl group interactions with neighboring Pt sites. CH bond cleavage of DME results in the CH3OCH2 species. The binding energy of this species is nearly 50 kcal/mol (see Table 2), which agrees well with the previous DFT work of Ishikawa et al. who found the binding energy to be slightly weaker at ∼45 kcal/mol. Additionally, the binding mode and energy are very similar to previously calculated values for hydroxymethyl (CH2OH) on Pt(111).11,45 Further parallels can be drawn between alcohol dehydrogenation intermediates and ether dehydrogenation intermediates when observing the next level of dehydrogenation. The structure and energetics of CH3OCH are similar to that of hydroxymethylene (CHOH). In both cases, the molecules adsorb in the η1μ2(C) bridge bound configuration and have binding energies of around 75 kcal/mol. 11
CH3COCH3 and 1-hydroxyethylidene (CH3 COH) have binding energies of 90 kcal/mol and around 100 kcal/mol,15 respectively. The 10 kcal/mol difference likely arises from steric hindrance from substitution of the hydroxyl hydrogen in CH3COH with a methyl group in CH3COCH3 (Figure 3L). Interestingly, both of these species depart in binding mode from their C1 analog in that they adsorb in a η1μ1(C) configuration.15,39 This can be traced to the favorable interaction of the substituted methyl group with neighboring Pt atoms, as discussed in our previous publication.15 The energies of the η1μ2(C) binding configuration of these molecules are within 3 kcal/mol of the η1μ1(C) configuration energies. Along with the CH3COCH3 intermediate, strain is attributed to higher energies for CHyOCHx ethers that form cyclic patterns through the surface. This will be discussed later in the paper, but as the level of dehydrogenation increases, the destabilization due to bond angle strain increases. This is an observation seen in gasphase chemistries.18 Specifically, looking at the COCH intermediate (Figure 3I), it is clear from the energy that this species breaks the trend of gas-phase bond order connectivity. This is due to the constraint of having two highly dehydrogenated carbon centers around an ether linkage. In general, the gas-phase bond order connectivity rules can be broken for oxygenates and hydrocarbons at very high levels of dehydrogenation.15,23,28,39 3.3. Ester Dehydrogenation Intermediates on Pt(111). A final set of DFT calculations were conducted for oxygenates with ester functional groups (Table 3). Ester formation is prevalent through reaction of alcohol and acid groups.7 In general, these groups resemble, geometrically and energetically, a combination of acids and ethers. The smallest ester, methyl formate (HCOOCH3), adsorbs most stably in the η1μ1(O) configuration similar to that of the acid adsorbates discussed previously (Figure 4A). Similar to the acids, there are also two less stable adsorption configurations (η2μ2(O,O) and η2 μ2(C,O) configurations). The same trend is observed for the larger methyl acetate (CH3COOCH3) species (shown in Figure 4H). Methyl acetate adsorbs with a slightly higher binding energy than methyl formate due to the stability offered by the alpha methyl group. 1877
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Table 2. DFT Calculated Structural and Energetic Properties of Adsorbed Ether Intermediates on Pt(111)a binding adsorbate
mode
Q
E Eo ν [cm1]
[kcal/mol] [kcal/mol] CH3OCH3 (dimethyl ether)
CH3OCH3
η1μ1(O)
6.2
5.6
CH3OCH2
η1μ1(C)
49.8
15.7
55, 55, 111, 171, 213, 290, 416, 520, 722, 919, 1015, 1071, 1091, 1178, 1204, 1320,
CH3OCH
η1μ2(C)
77.6
18.7
75, 75, 178, 192, 239, 370, 399, 572, 787, 909, 1057, 1088, 1145, 1269, 1325, 1341, 1356, 2749,
CH3OC CH2OCH2
η1μ3(C) η2μ2(C,C)
109.4 98.0
37.9 24.9
93, 93, 164, 246, 285, 386, 486, 627, 851, 1077, 1093, 1132, 1347, 1352, 1361, 2908, 3023, 3092 87, 87, 162, 308, 419, 523, 550, 705, 748, 935, 945, 1105, 1108, 1183, 1203, 1332, 1354,
CHOCH2
η2μ2(C,C)
138.7
21.8
105, 105, 173, 318, 451, 587, 663, 697, 756, 777, 1094, 1150, 1255, 1286, 1389, 2959, 3005, 3062
COCH2
η2μ4(C,C)
133.5
24.8
101, 101, 243, 407, 470, 524, 635, 673, 704, 1010, 1088, 1187, 1320, 2976, 3082
CHOCHb
η2μ3(C,C)
166.2
20.0
COCH
η2μ3(C,C)
127.8
1.8
104, 104, 342, 390, 466, 642, 692, 705, 887, 1068, 1220, 3076
CH3CH2OCH3
η1μ1(O)
7.6
7.4
17, 17, 34, 77, 118, 150, 180, 203, 278, 361, 460, 735, 772, 934, 1021, 1058, 1080, 1096, 1140, 1227, 1271, 1309, 1335, 1345, 1350, 1357, 1367, 1385, 2882, 2887, 2950, 2953, 2993,
CH3CHOCH3
η1μ1(C)
47.0
17.4
51, 51, 58, 106, 151, 183, 227, 257, 321, 466, 504, 836, 864, 1007, 1024, 1063, 1073, 1112,
CH3COCH3
η1μ1(C)
90.0
18.1
73, 73, 85, 110, 149, 231, 248, 304, 417, 441, 553, 868, 908, 1014, 1075, 1091, 1138, 1222,
21, 21, 65, 102, 154, 171, 184, 285, 404, 892, 1061, 1085, 1103, 1135, 1190, 1339, 1345, 1350, 1357, 1362, 1387, 2880, 2893, 2985, 2998, 3005, 3008 1331, 1342, 1365, 2662, 2934, 2969, 3045, 3067 2915, 2962, 3045
2899, 2907, 3057, 3058
117, 117, 287, 468, 481, 601, 704, 713, 754, 785, 1072, 1219, 1276, 3031, 3050
CH3CH2OCH3 (ethyl methyl ether)
3025, 3053, 3063 1176, 1248, 1303, 1318, 1327, 1334, 1342, 1362, 2722, 2858, 2935, 2964, 2970, 3022, 3067 1269, 1302, 1319, 1329, 1339, 1383, 2842, 2848, 2976, 2985, 3044, 3070 CH3CH(CH3)OCH3 (isopropyl methyl ether) CH3CH(CH3)OCH3 η1μ1(O)
8.5
8.4
19, 19, 35, 52, 82, 114, 173, 195, 242, 256, 300, 369, 402, 533, 719, 860, 873, 886, 999, 1075, 1084, 1108, 1109, 1145, 1257, 1296, 1309, 1313, 1335, 1341, 1364, 1371, 1373, 1382, 1390, 2868, 2878, 2942, 2945, 2980, 3030, 3044, 3047, 3049, 3053
CH3C(CH3)OCH3
η1μ1(C)
43.9
13.8
42, 42, 73, 91, 137, 207, 218, 226, 253, 277, 325, 374, 474, 496, 769, 877, 891, 918, 1011, 1055, 1066, 1096, 1173, 1191, 1281, 1285, 1317, 1332, 1336, 1338, 1343, 1359, 1368, 2742, 2903, 2910, 2953, 2994, 3010, 3026, 3035, 3038
Binding energies are not corrected for ZPE. b Binding energy reported is a snap binding energy due to instability of gas-phase species. E Eo is a ZPEcorrected dehydrogenation/hydrogenation reaction energy from the reference molecule at 0 K. For example E Eo for CH3OCH2 corresponds to the DFT reaction energy of CH3OCH3(g) + 2* f CH3OCH2* + H*. a
While structurally similar to the acids, these esters adsorb with slightly lower binding energies. The ZPE-corrected binding energies for methyl formate and methyl acetate are 3.4 and 6.2 kcal/mol, respectively. The slight destabilization is due to the lack of direct HPt interaction observed in the case of acids. Upon dehydrogenation, adsorbed methyl formate HCOOCH3 can form either the more stable COOCH3 (Figure 4E) or the less stable HCOOCH2 (Figure 4B). Both of these intermediates bind to the surface through the carbon atom from which the hydrogen was extracted. Surface COOCH3 is approximately 5 kcal/mol more stable than HCOOCH2. Methyl-substituted CH3COOCH2 is very similar structurally and energetically to HCOOCH2. In comparison, both are just over 11 kcal/mol more stable than the fully hydrogenated gas-phase reference and have binding energies around 48 kcal/mol. The binding configuration is η2μ2(C,O), where the dehydrogenated ester carbon binds to the surface and the carbonyl oxygen forms a weak interaction with a neighboring Pt atom (Figure 4B and Figure 4I).
Further dehydrogenation of the ester carbon in both methyl formate and methyl acetate follows structural trends expected based on gas-phase bond order rules. Further, the energetic trends of these species are similar to those seen in the ether data set. The ester carbon of both HCOOCH and CH3COOCH adsorbs to a bridge site, while the carbonyl group adsorbs in a di-σ configuration to adjacent Pt atoms (see Figure 4C and Figure 4J). Both species have binding energies between 95 and 100 kcal/mol and are 15 and 12 kcal/mol more stable than the gas-phase reference, respectively. The next level of dehydrogenation results in the 3-fold hollow adsorbed intermediates of HCOOC and CH3COOC (see Figure 4D and Figure 4K). Both of these intermediates have binding energies of ∼116 kcal/mol and are 24 kcal/mol more stable than their gas-phase reference. Similar to the ether data set, dehydrogenation results in the formation of species that include cyclic sequences of atoms through the surface. For example, COOCH2 and COOCH bind to the surface through both carbon atoms. In the case of COOCH, the ester carbon binds in a bridge site (see Figure 4F and Figure 4G). 1878
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Table 3. DFT Calculated Structural and Energetic Properties of Adsorbed Ester Intermediates on Pt(111)a binding
Q
E Eo
adsorbate
mode
[kcal/mol]
[kcal/mol]
HCOOCH3
η1μ1(O)
HCOOCH2
ν [cm1] HCOOCH3 (methyl formate)
4.5
3.9
η2μ2(C,O)
48.5
11.8
53, 53, 101, 132, 164, 278, 325, 567, 663, 774, 840, 901, 1040, 1164, 1199, 1293, 1337, 1622,
HCOOCH
η3μ4(C,C,O)
99.2
15.2
77, 77, 248, 328, 369, 391, 497, 541, 637, 661, 786, 938, 1020, 1142, 1218, 1262, 2966, 2974
HCOOC
η1μ3(C)
116.1
24.0
84, 84, 112, 153, 239, 321, 513, 565, 657, 889, 906, 1106, 1254, 1715, 2932
COOCH3
η1μ1(C)
52.8
17.1
56, 56, 135, 164, 207, 247, 252, 405, 546, 619, 976, 1081, 1088, 1125, 1321, 1338, 1348, 1588, 2772, 2881, 3078
15, 15, 58, 90, 109, 122, 165, 337, 366, 750, 861, 938, 1093, 1112, 1166, 1274, 1363, 1370, 1375, 1659, 2968, 3005, 3070, 3120 2980, 3000, 3076
COOCH2
η2μ2(C,C)
98.4
25.1
77, 77, 153, 186, 312, 421, 526, 588, 598, 683, 911, 970, 1157, 1213, 1355, 1622, 2972, 3060
COOCH
η2μ3(C,C)
155.7
32.9
97, 97, 152, 207, 274, 433, 517, 573, 619, 753, 823, 1072, 1236, 1665, 2989
CH3COOCH3
η1μ1(O)
6.8
6.2
18, 18, 51, 64, 68, 105, 150, 167, 254, 303, 437, 562, 633, 836, 942, 989, 999, 1103, 1139, 1232,
CH3COOCH2
η2μ2(C,O)
47.4
11.1
47, 47, 82, 86, 130, 161, 169, 228, 427, 540, 567, 605, 708, 804, 922, 963, 984, 1121, 1184, 1220,
CH3COOCH
η3μ4(C,C,O)
95.8
12.4
1301, 1344, 1354, 1363, 1636, 2960, 2983, 3056, 3074, 3123 68, 68, 185, 208, 248, 310, 349, 388, 413, 490, 558, 589, 632, 821, 858, 944, 993, 1113, 1175, 1233,
CH3COOCb
η1μ3(C)
116.2
24.0
75, 75, 60, 134, 162, 217, 286, 445, 499, 513, 523, 664, 713, 945, 974, 1002, 1139, 1291, 1344, 1359,
CH3COOCH3 (methyl acetate) 1310, 1349, 1351, 1373, 1380, 1385, 1661, 2960, 2972, 3067, 3075, 3077, 3105
1285, 1344, 1354, 2956, 2969, 3060, 3082 1708, 2957, 3065, 3121
Binding energies are not corrected for ZPE. b Binding energy reported is a snap binding energy due to instability of gas-phase species. E Eo is a ZPEcorrected dehydrogenation/hydrogenation reaction energy from the reference molecule at 0 K. For example E Eo for HCOOCH2 corresponds to the DFT reaction energy of HCOOCH3(g) + 2* f HCOOCH2* + H*. a
Figure 4. Schematics of adsorbed ester intermediates on Pt(111) calculated via DFT. Pt, C, O, and H atoms are represented by gray, black, red, and white circles, respectively. Insets in upper left show the top view of the adsorbate. For simplicity only one layer of Pt is shown. Species shown include (A) HCOOCH3, (B) HCOOCH2, (C) HCOOCH, (D) CHOOC, (E) COOCH3, (F) COOCH2, (G) COOCH, (H) CH3COOCH3, (I) CH3COOCH2, (J) CH3COOCH, and (K) CH3COOC.
Overall, the trends established in this section suggest that systematic parametrization can allow for DFT-based semiempirical prediction of the properties of acids, ethers, and
esters. The next section of this work discusses the formulation of Benson type group additive contributions, derived from these calculations, which allow for fast prediction of 1879
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thermochemical properties of surface species with these functional groups.
4. GROUP ADDITIVITY FOR THERMOCHEMICAL PROPERTY PREDICTION OF ADSORBED OXYGENATES ON PT The results in the previous sections illustrate the trends associated with various oxygenate molecules of various sizes (number of carbon and oxygen atoms). These trends can be quantified in group additivity schemes originally developed by Benson for fast prediction of properties of species of gasphase mechanisms1821 and later extended to surface hydrocarbons22,23 and alcohols and polyols.15 Recent work showed the utility of this method in reducing the computational cost associated with understanding the reaction pathway of glycerol decomposition on Pt.17 The remainder of this article discusses the development and adaptation of these methods. 4.1. Parameterization of Group Contributions. While our previous group additivity scheme has proven useful as a method for reducing the computational cost involved in accurately describing the energetics of mechanisms of alcohol and polyol chemistry,15 the extension of the diatomic centered group contributions to the functional groups described in this work is not possible. In the case of alcohols and polyols, all oxygen atoms are associated with one carbon atom, and the alcohol function is terminal. This is not the case for acid, ether, and ester functions described in this work. As seen in Figure 5, basing groups on CO centers results in a group identification problem. In the first example, the carbon in HCOO is shared by two identical CO centered groups. The second example shows that for the dehydrogenated ether, CH2OCH2, the oxygen is shared by two identical CO centered groups. The motivation to classify oxygenate surface groups on the basis of CO centers in prior work15 was to increase the accuracy in accounting for the weak OPt interactions prevalent in saturated alcohol functions. Our motivation here is to adjust the surface oxygenate group additivity scheme to accurately encompass all functional groups that are indicative of biomass feedstock and processing. In this work, we show that defining single atom centered Benson type groups can allow for accurate predictions of a wide range of surface hydrocarbons and oxygenates on Pt. The major adjustment made to the original Benson scheme is the addition of oxygenmetal weak binding groups (CdO groups are still treated as single centers consistent with the Benson framework18). These can consist of either saturated alcohol bonding through lone pair electrons or η1 type bonding of aldehyde, acid, or ester carbonyl groups. In essence, the groups are two (or three in the case of carbonyl functions) atom centered groups, where the metal atom is taken into account implicitly through the oxygen. For example, adsorbed ethanol contains three groups: the group with a direct surfaceoxygen interaction, [Owk(C)(H)], the group which has this surfaceoxygen interaction as a ligand, [C(C)(H)2(Owk)], and the original Benson gas-phase group [C(C)(H)3]. The nomenclature [X(Y)(Z)] refers to a group centered around atom X with ligands Y and Z. For instance, in the adsorbed ethanol example, the [Owk(C)(H)] group is centered around the weakly surface-bound oxygen which is directly bound to a carbon and a hydrogen atom. More information on this nomenclature can be found elsewhere.18 Group contributions for adsorbed oxygenates and hydrocarbons were regressed
Figure 5. Schematics showing the diatomic centered groups of (A) HCOO and (B) CH2OCH2 based on our previous group additivity method.15 The overlap in the (red) ellipsoids demonstrates the difficulty in extending this scheme to these types of adsorbates.
from DFT calculated thermochemical properties listed in the Supporting Information and in previous work.4,15,16 This framework is fully compatible with the Benson gas-phase framework with a minor adjustment to take into account the lack of free translation entropy of surface intermediates. As described in our previous publication,15 the entropy of gasphase derived groups cannot be directly applied to surfaceintermediate predictions. This is due to the contribution of translational entropy which is a function of total mass of the molecule. To apply gas-phase groups for surface entropy estimations, the translational component of entropy was calculated and subtracted from the raw thermochemical data of molecules used in the original gas-phase group regression.1821 The groups were then reregressed using only the vibrational and rotational contributions, as it is often assumed that the local entropy of these degrees of freedom is approximately equal to that of the corresponding surface species.40,46 We report these nonsurface bound groups, along with surface atom containing groups from Pt(111), in Table 4. Gas-phase groups were not adjusted from Benson’s work with the exception of the extraction of translational entropy described above. An advantage of the group additive scheme presented in this work is the ability to automate group additive predictions. We utilized this concept in this work to identify and parametrize the group contributions in Table 4 from the thermochemical data of surface intermediates listed in the Supporting Information. This flexibility can be advantageous to researchers who are interested in using this framework to extend sets of DFT calculations they have performed to larger molecules on diverse surfaces of interest. 4.2. Accounting for Cyclic Strain through the Surface. Aside from the ability to more generally predict the properties of oxygenate adsorbates, the other major improvement in this work is in a new model developed herein for accounting for cyclic ring strain through the surface. Previous work, including the original Benson framework, uses the properties of cyclic compounds to empirically correct for the impact intramolecular rings have on thermochemical properties.15,18,22 The major drawback of this approach is the unique nature of these corrections being applied to specific atom sequences forming rings. While Benson describes some flexibility with substituting similar atom sequences of the same ring size for empirical corrections,18 there is no systematic way to make this substitution. Further, as we move to using this semiempirical method to describe surface oxygenate intermediates, the number and combination of cyclic patterns of atoms through the surface increase as the level of decomposition increases. The sheer number of these rings through the surface makes empirical derivation of the strain effect on thermochemical properties impractical because of the impact these fitted corrections have on the independence of the overall regression given the limited data set. To combat this problem, we have devised a simple “ring-strain model” which adds only one degree of 1880
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Table 4. List of Group Contributions for Predicting Thermochemical Properties of Adsorbed Oxygenates on Pt(111) Cp [cal/mol/K] groupa
Hf298 [kcal/mol]
S298 [cal/mol/K]
300
400
500
600
800
1000
Surface Metal Containing Groups C(C)(CO)(H)(M)
5.6
2.0
6.7
8.6
9.9
10.8
12.0
12.8
C(C)(CO)(M)2
0.1
1.4
5.5
6.8
7.6
8.1
8.6
8.9
10.3
0.3
4.0
5.6
6.7
7.5
8.7
9.4
4.7
6.2
5.2
6.9
8.4
9.7
11.7
13.1
C(C)(COwk)(H)(M) C(C)(COwk)(H)2 C(C)(COwk)(M)2
18.2
5.2
2.8
3.9
4.6
5.0
5.4
5.6
C(C)(H)(M)(O)
18.3
3.3
6.3
8.6
10.1
11.1
12.4
13.1
C(C)(H)(M)2 C(C)(H)2(M) C(C)(H)2(Owk)
3.6
1.6
5.3
6.9
8.0
8.8
9.8
10.6
8.4 9.3
3.2 3.6
6.6 4.3
8.6 6.1
10.2 7.7
11.5 8.9
13.3 10.7
14.6 12.0
C(C)(M)(O)2
33.7
1.4
4.9
7.3
8.6
9.4
10.0
10.2
C(C)(M)2(O)
13.8
6.7
5.7
7.5
8.5
9.1
9.6
9.9
6.2
2.3
5.1
6.5
7.3
7.9
8.5
8.8
C(C)2(CO)(M)
5.4
4.3
6.3
8.0
9.0
9.7
10.5
10.9
C(C)2(COwk)(H)
4.1
2.9
4.8
6.4
7.8
8.8
10.4
11.3
C(C)2(COwk)(M)
12.1
2.8
4.2
5.6
6.5
7.1
7.8
8.0
C(C)2(H)(M) C(C)2(H)(Owk)
6.7 10.2
3.1 3.9
6.7 5.6
8.5 7.5
9.8 8.9
10.8 9.9
12.1 11.3
13.0 12.2
C(C)2(M)(O)
C(C)(M)3
12.7
2.0
7.1
9.1
10.4
11.1
11.7
12.0
C(C)2(M)2
3.7
1.1
6.9
8.2
9.1
9.7
10.4
10.7
C(C)3(M)
0.2
0.2
5.5
6.8
7.8
8.3
9.0
9.3
17.9
4.0
6.5
8.7
10.2
11.2
12.3
13.1
C(CO)(H)(M)2
1.4
0.0
5.7
7.4
8.5
9.4
10.4
11.1
C(CO)(H)2(M)
8.8
4.2
7.0
9.0
10.5
11.7
13.3
14.6
C(CO)(M)2(O) C(CO)(M)3
16.7 2.3
8.6 3.5
5.9 5.6
7.9 6.8
9.0 7.6
9.6 8.0
10.2 8.4
10.5 8.6
C(CO)(H)(M)(O)
C(COwk)(H)(M)(O) C(COwk)(H)(M)2
3.7
6.0
3.9
5.8
6.9
7.8
8.7
9.4
11.0
3.5
2.6
4.0
5.0
5.7
6.6
7.2 11.2
C(COwk)(H)2(M)
4.6
1.6
4.5
6.2
7.5
8.6
10.0
C(COwk)(M)2(O)
3.4
12.0
2.8
4.3
5.1
5.6
6.0
6.2
C(H)(M)(O)2
30.3
2.9
4.9
7.2
8.8
9.9
10.9
11.6
C(H)(M)2(O)
3.4
2.1
5.3
6.7
7.7
8.5
9.4
10.0
C(H)(M)3
3.8
5.1
6.3
7.5
8.2
8.7
9.4
9.9
11.4
1.7
6.4
8.5
10.1
11.4
13.0
14.3
C(H)2(M)(O)
1.2
6.8
7.6
9.3
10.6
11.5
12.8
13.9
C(H)2(O)(Owk)
19.8
0.5
4.3
6.5
8.0
9.2
10.8
12.0
C(H)3(M)
12.3
11.0
9.1
11.1
12.7
13.9
16.0
17.5
C(M)3(O)
8.0
5.3
5.2
6.4
7.1
7.5
7.8
7.9
CO(C)(M) CO(C)(Owk)
46.5 33.3
7.6 7.2
8.0 6.1
9.2 6.9
10.1 7.7
10.8 8.3
11.6 9.3
12.1 9.9
CO(CO)(M)
44.2
7.0
8.6
9.8
10.6
11.2
12.0
12.5
CO(H)(M)
43.3
12.2
9.6
11.0
12.1
12.9
14.2
15.0
CO(M)(O)
36.8
0.5
8.6
9.3
10.0
10.5
11.3
11.7
COwk(C)(H)
43.1
16.9
10.7
12.2
13.5
14.5
16.1
17.1
COwk(C)(O)
48.1
15.5
10.2
11.7
12.7
13.4
14.5
15.0
C(H)2(M)2
[C.](M)3
18.9
4.4
4.6
5.1
5.4
5.5
5.7
5.8
O(C)(COwk)
27.5
5.3
1.6
1.9
2.2
2.5
2.9
3.1
O(C)(M)
24.7
6.7
4.3
4.6
4.9
5.3
5.7
6.0
O(CO)(M)
38.9
4.5
3.9
4.7
5.3
5.5
5.8
6.0
O(COwk)(H)
59.0
4.1
3.6
4.4
5.1
5.7
6.6
7.3
O(COwk)(M)
49.2
1.7
1.8
2.3
2.6
2.9
3.2
3.3
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Table 4. Continued Cp [cal/mol/K] groupa Owk(C)(CO) Owk(C)(H)
Hf298 [kcal/mol] 42.7 42.9
S298 [cal/mol/K] 17.6 12.5
300
400
500
600
800
1000
5.4 7.1
6.5 7.7
7.2 8.4
7.6 8.9
8.1 9.7
8.4 10.3 10.1
Owk(C)2
27.7
14.5
7.2
7.9
8.5
9.0
9.7
Owk(CO)(H)
59.3
14.5
5.8
7.2
7.9
8.5
9.2
9.7
25.3
10.6
4.9
6.4
7.5
8.3
9.0
9.3
surface-ring strain
Nonsurface Metal Atom Containing Groupsb C(C)(CO)(H)2
5.2
8.6
6.2
7.7
8.7
9.5
11.1
12.2
C(C)(Cd)(H)2
4.8
9.6
5.1
6.9
8.3
9.5
11.2
12.5
C(C)(H)2(O) C(C)(H)3
8.1 10.2
5.8 9.0
5.0 6.2
6.9 7.8
8.3 9.4
9.4 10.8
11.1 13.0
12.3 14.8
C(C)2(CO)(H)
1.7
7.4
-
-
-
-
-
-
C(C)2(Cd)(H)
1.5
5.8
4.2
5.9
7.3
8.2
9.5
10.2
C(C)2(H)(O)
7.2
4.8
4.8
6.6
8.1
8.7
9.8
10.4
C(C)2(H)2
4.9
9.0
5.5
7.0
8.3
9.4
11.1
12.3
C(C)3(H)
1.9
6.5
4.5
6.0
7.2
8.1
9.3
10.1
C(C)3(O)
6.6
1.9
4.3
6.2
7.3
7.7
8.2
8.2
C(C)4 CO(C)(H)
0.5 29.1
0.5 17.3
4.4 7.0
6.1 7.8
7.4 8.8
8.1 9.7
8.8 11.2
8.8 12.2
CO(C)(O)
35.1
11.3
6.0
6.7
7.3
8.0
8.9
9.4
CO(C)2
31.4
15.0
5.6
6.3
7.1
7.8
8.9
9.6
8.6
9.1
4.2
5.0
5.8
6.5
7.7
8.5
10.3
6.4
4.1
4.6
5.0
5.3
5.8
6.1
Cd(C)(H) Cd(C)2 Cd(H)2
6.3
8.3
5.1
6.4
7.5
8.5
10.1
11.3
O(C)(CO)
43.1
17.9
2.6
3.5
4.0
4.4
4.9
5.2
O(C)(H) O(C)2
37.9 23.2
14.2 8.4
4.3 3.4
4.4 3.7
4.8 3.7
5.2 3.8
6.0 4.4
6.6 4.6
O(CO)(H)
58.1
6.9
3.8
5.0
5.8
6.3
7.2
7.8
a
Several groups have been remapped to other groups in accordance with.18 Please see the Supporting Information for more details on remapping of groups. b Nonentropic group values for nonmetal atom containing groups are taken from refs 18 and 20. Entropic group contributions for nonsurface metal groups are regressed from local entropy (S Strans) of gas-phase molecules in data used by Benson and co-workers.1821
freedom to the group regression and encompasses all potential added strain through the surface for these adsorbed molecules. This model utilizes the qualitative observations that ring strain increases as the number of atoms in the ring decreases18 and that ring strain is closely tied to deformation of natural bond angles. Further, if a molecule binds to the surface in two locations and has a sequence of atoms in between, which are geometrically situated so that the natural bond angles are approximately intact, the strain effect on the thermochemical properties is very small. We can then parametrize the total strain effect of each adsorbate as a function of the deformation of the natural bond angles in the molecule. Part of the utility of the group additivity method is the ability to accurately predict thermochemical properties without the requirement of complex spatial information, including specific geometric coordinates of molecules. This means that it is necessary to predict the deformation of natural bond angles without exact knowledge of the bond angles. This can be done with reasonable accuracy by looking at the sum of natural supplementary angles of the adsorbate. Assuming that non-negligible strain comes only from adsorbate atoms, the supplementary angles of a cyclic molecule with no strain should sum to 180°. Building on this approximation, cyclic molecules with summed supplementary angles of over 180° are assumed
Figure 6. Schematic of cyclic surface intermediates (A) COOCH and (B) CH2OC.
to have negligible strain contributions. Equation 1 describes the implementation of this model used to account for these contributions. N
χs ¼ 1
∑i θi 180
ð1Þ
χs is the strain coefficient to be multiplied by the contributions shown for “surface-ring strain” in Table 4 and added to the other group contributions for property prediction. This coefficient is equal to unity minus the sum of natural supplementary angles in the ring (θi) divided by 180°. χs values of less than zero are neglected. This first-order model is used, as implementation of higher-order models showed negligible improvement in the R2 values of property prediction. 1882
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Examples of two cyclic surface intermediates which potentially require a strain correction for property estimation are shown in Figure 6. The dehydrogenated ester, COOCH (shown in Table 5. Common Atomic Vertices of Bonds and Assumed Natural Supplementary Angles for the Simple Ring-Strain Modela atomic vertices CX2
70.50
O
70.50
(CdO)
60.00
M(CdO)
60.00
MO
70.50
MCX2 M2CX
70.50 54.75
M3C a
natural supplementary angle (deg)
0.00
Atomic vertices show atom combinations making up vertices in potential cyclic sequences. For example, CX2 represents a tetrahedral carbon which has an assumed angle of 109.5° (supplementary angle is 70.5°). M, M2, and M3 notate top, bridge, and three-fold binding atoms.
Figure 6A and in more detail in Figure 4G), contains three nonsurface vertices: M2CX, O, and M(CdO). For illustration of the notation, M2CX refers to the connectivity from the surface bridge site (two metal atoms), through the carbon vertex (or in the case of COOCH the CH segment), to the oxygen atom in Figure 6A. Summing the natural supplementary angles on these bond combinations from Table 5 gives a total of 185.25°. This leads to a strain coefficient (χs) of 0.03, which is subsequently neglected in group property prediction. The second example, CH2OC (shown in Figure 6B and in more detail in Figure 3G), also contains three nonsurface vertices: M3C, O, and MCX2. This time, summing the natural supplementary angles on these bond combinations from Table 5 gives a total of 141°. The corresponding strain coefficient (χs) of 0.22 will need to be multiplied by the properties listed for “surfacering strain” in Table 4 and added to the other group contributions for property prediction. The major advantage of this ring-strain model in the group additivity scheme is the reasonable accuracy of capturing the trends and the ability to parametrize the model based only on information available in the group additivity scheme in the first place. No additional information is necessary to include the strain
Figure 7. Parity plots comparing group additivity predictions to DFT calculated heat of formation at 298 K (A), entropy at 298 K (B), heat capacity at 300 K (C), and heat capacity at 800 K (D) for adsorbed alcohol (red circles), ether (green triangles), acids (blue triangles), esters (magenta diamonds), and hydrocarbon (black squares) intermediates on Pt(111). 1883
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Figure 8. Normalized histograms of deviations between group additivity predicted and DFT calculated (A) heat of formation and (B) entropy at 298 K.
corrections based on eq 1. It needs to be noted, however, that there are limitations to this simple model. The major one is the assumption that the adsorbate atoms are solely responsible for the ring strain. While a necessary approximation given the information available in the group additivity scheme, this means that the strain correction of cyclic adsorbates does not change as a function of surface facet or surface atom type. This is not entirely correct as shown in our previous publication.15 For increased accuracy of these corrections, one would need to include information on surface metal atomatom distance and develop a model to adjust strain contributions as a function of this surfaceatom distance. Such models can quickly become too complex given the large number of cyclic surface intermediates present in oxygenate reaction mechanisms. 4.3. Prediction of Oxygenates on Pt via Group Additivity. In this work, the more general surface oxygenate group additivity scheme is applied to Pt(111). The comparison of group additivity predicted and DFT calculated thermochemical properties of alcohol, ether, acid, ester, and hydrocarbon intermediates is shown in Figure 7. R2 values are shown in the parity plots in Figure 7, and normalized histograms showing the deviations between group additivity estimated and DFT calculated heat of formation and entropy are shown in Figure 8. Figure 8 shows that for most adsorbates calculated in this study group additivity predicts heats of formation at 298 K within 5 kcal/mol, and it is very rare for a deviation to be greater than 10 kcal/mol. These outliers generally consist of highly dehydrogenated hydrocarbons
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which break gas-phase bond order rules due to ring stain from multiple surface binding atoms. The addition of the simple ring strain model reduces the maximum error of these outliers from nearly 20 kcal/mol to just over 12 kcal/mol. Additionally, Figure 8 shows that group additive predictions of surface species’ entropy is within 5 cal/mol/K for the majority of species calculated. The R2 value for entropy predictions (0.88, shown in Figure 7B) is slightly worse than the same metric for heat of formation and heat capacity. This can be traced to the inaccuracies in the assumptions that went into the calculations of surface species entropy. The combination of neglecting internal rotational degrees of freedom, assuming frustrated translation of all intermediates, and using the gas-phase entropic contributions (despite subtracting translational degrees of freedom) for nonmetal containing groups can result in considerable errors. However, as the histogram in Figure 8 shows, this method is still adequate for use as a screening method for identifying important species and reactions of large kinetic mechanisms whereupon more expensive computational methods can refine kinetic parameters.9 Not only do the comparisons shown in Figure 7 suggest that this new scheme for surface additivity predictions is at least as accurate as the one previously proposed for surface polyols15 but also this new scheme has the benefits of being easier to implement and can be more widely applied to relevant surface oxygenates. As has been shown in previous research,17 the major utility of this type of semiempirical method is as a screening tool to give an inexpensive, yet reasonably accurate, estimation of the thermochemical parameters of a large chemical reaction network from which important parameters of interest can be studied with more expensive methods.9 Additionally, group additivity predictions in combination with automated mechanism and reaction pathway generation47 can provide a powerful tool for fast, inexpensive, and accurate mappings of complex reaction networks. Finally, this semiempirical method can be used as a thermochemical property input for intermediates on single metals in combination with linear scaling techniques, for utilizing atomic binding energy descriptor-based catalyst design schemes.16 To illustrate the method, as a brief example, the Hf,298 of the dehydrogenated ether CH2OCH2 is calculated from the group properties listed in Table 4. Schematics of this adsorbate are shown in Figure 3E and Figure 5B. First, adding the three groups in the adsorbate, 2[C(H)2(M)(O)] + [O(C)2], gives a value of 2 11.4 + 23.2 = 46 kcal/mol. This gives the heat of formation of the groups alone, but it is important to notice the cyclic nature of the adsorbate through the surface. This means that the strain correction described in Section 4.2 must be implemented. The vertices connecting the two neighboring metal atoms through the adsorbate are 2[CX2] and [O], which carries natural supplementary angle values (Table 5) of 2 70.5 + 70.5 = 211.5°. When inserting this value into eq 1, we find that the strain coefficient will be negative, and strain can be neglected. Therefore, the group additive predicted heat of formation of CH2OCH2 is 46 kcal/mol. This compares well with the calculated value of 48.8 kcal/mol. Another simple example in computing the heat capacity at 500 K of CH3CH(CH3)COO (shown in Figure 2U) is discussed next. This is an important example as it shows the treatment of the formate type function. This adsorbate contains five groups: 2 [C(C)(H)3] + [C(C)2(COwk)(H)] + [COwk(C)(O)] + [O(COwk)(M)]. As is seen by this selection of groups, the formate function is treated as the combination of an OM bond and a weak carbonyl interaction with the surface. While this is not 1884
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The Journal of Physical Chemistry C entirely correct from a physical-chemistry standpoint,33 it is the most appropriate classification given the working framework and the lack of properties and accuracy of unsaturated carbon centered groups18,21 and yields accurate results. Summing these groups gives a heat capacity at 500 K of 2 9.4 + 7.8 + 12.7 + 2.6 = 41.9 cal/mol/K. This compares well with the DFT calculated value of 41.8 cal/mol/K. These examples underscore the ease at which these reasonably accurate thermochemical property predictions can be implemented. While the computation of individual group properties requires the systematic calculation of properties of smaller oxygenates, once the groups are parametrized, this method offers a direct extension of the trends and properties of smaller oxygenates to large biomass-derived oxygenates that are more indicative of the surface intermediates involved in biomass processing chemistry.
5. CONCLUSIONS In the first part of this work, DFT calculations were performed to study the energetic and geometric trends of acid, ether, and ester functional groups on Pt(111). Dehydrogenation products of formic acid, acetic acid, propionic acid, isobutyric acid, dimethyl ether, ethyl methyl ether, isopropyl methyl ether, methyl formate, and methyl acetate were studied to determine thermodynamic trends of decomposition reactions of these species on the Pt(111) surface. Subsequent statistical mechanical calculations were performed to reveal systematic trends in thermochemical properties of oxygenate adsorbates on Pt(111). It was found that acids adsorb most favorably in the η1μ1 configuration where binding occurs through the metal and the carbonyl oxygen. Further, experimental thermal decomposition reactivity of carboxylates decreases from formate to propionate (HCOO > CH3COO > CH3CH2COO),37 which follows the DFT calculated trends of stabilization through methyl substitution of the β-carbon. Calculations of saturated ether intermediates on Pt(111) verify adsorption in the η1μ1 configuration similar to that of alcohols on Pt(111). Upon dehydrogenation, surface intermediates of ethers follow gas-phase bond order rules for the most part, and decomposition trends closely match those observed for alcohol intermediates. Additionally, very highly dehydrogenated ether intermediates, which are multidentate, show destabilization through cyclic strain. Finally, energetic and geometric trends of surface ester intermediates generally resemble combinations of acids and ethers on Pt(111). This observation leads to the conclusion that semiempirical parametrization of thermochemical properties based on addition of molecular subparts is possible. The second part of this work focused on carrying out this parametrization based on a new Benson type group additivity scheme. This scheme shows to be at least as, if not more, accurate as the previous scheme for prediction of alcohol and polyol properties,15 has additional superiority in the breadth of surface oxygenate intermediates that can be studied (acids, ethers, and esters in addition to alcohols, aldehydes, and hydrocarbons), and is more easily implementable because of the simplistic nature of single atom centered groups and the possibility of automated group matching and calculation. A new model for computing strain in cyclic sequences contained in surface molecules was developed that allows for more accurate predictions of highly dehydrogenated, multidentate surface oxygenates without the necessity of previous calculation of molecules containing the identical cyclic sequence. This group additivity scheme can be used in future studies of larger biomass oxygenate mechanisms
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that would otherwise be computationally intractable to parametrize via DFT. This scheme has adequate accuracy to be used as a screening method to determine important reaction pathways in large oxygenate mechanisms. Finally, this method offers a blueprint for implementing this scheme to other catalytic surfaces of interest and nonoxygenate reaction mechanisms of interest that have previously been too computationally demanding to study in-depth.
’ ASSOCIATED CONTENT
bS
Supporting Information. Additional geometric and energetic information of surface acid, ether, and ester intermediates, along with thermochemical properties of adsorbates used to parametrize group contributions. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail address:
[email protected]. Telephone number: 302-8312830.
’ ACKNOWLEDGMENT This material is based upon work financially supported as part of the Catalysis Center for Energy Innovation, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0001004. DFT calculations were performed using the TeraGrid resources provided by University of Illinois’ National Center for Supercomputing Applications (NCSA). We also thank Professor Jingguang Chen for many fruitful discussions. ’ REFERENCES (1) Huber, G. W.; Iborra, S.; Corma, A. Chem. Rev. 2006, 106, 4044. (2) Davda, R. R.; Shabaker, J. W.; Huber, G. W.; Cortright, R. D.; Dumesic, J. A. Appl. Catal. B-Environ. 2005, 56, 171. (3) Vlachos, D. G.; Caratzoulas, S. Chem. Eng. Sci. 2010, 65, 18. (4) Salciccioli, M.; Yu, W.; Barteau, M. A.; Chen, J. G.; Vlachos, D. G. J. Am. Chem. Soc. 2011, 133, 7996. (5) Medlin, J. W. ACS Catal. 2011, 1284. (6) Gayubo, A. G.; Valle, B.; Aguayo, A. T.; Olazar, M.; Bilbao, J. J. Chem. Technol. Biotechnol. 2010, 85, 132. (7) Gursahani, K. I.; Alcala, R.; Cortright, R. D.; Dumesic, J. A. Appl. Catal., A 2001, 222, 369. (8) Nørskov, J. K.; Bligaard, T.; Rossmeisl, J.; Christensen, C. H. Nature Chem. 2009, 1, 37. (9) Salciccioli, M.; Stamatakis, M.; Caratzoulas, S.; Vlachos, D. G. Chem. Eng. Sci. 2011, 66, 4319. (10) Greeley, J.; Mavrikakis, M. J. Am. Chem. Soc. 2002, 124, 7193. (11) Greeley, J.; Mavrikakis, M. J. Am. Chem. Soc. 2004, 126, 3910. (12) Neurock, M.; Janik, M.; Wieckowski, A. Faraday Discuss. 2008, 140, 363. (13) Wang, H. F.; Liu, Z. P. J. Phys. Chem. C 2009, 113, 17502. (14) Yue, C.; Lim, K. H. Catal. Lett. 2009, 128, 221. (15) Salciccioli, M.; Chen, Y.; Vlachos, D. G. J. Phys. Chem. C 2010, 114, 20155. (16) Salciccioli, M.; Vlachos, D. G. ACS Catal. 2011, 1, 1246. (17) Chen, Y.; Salciccioli, M.; Vlachos, D. G. J. Phys. Chem. C 2011, 115, 18707. (18) Benson, S. W. Thermochemical Kinetics, 2 ed.; John Wiley & Sons, Inc.: New York, 1976. 1885
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(19) Benson, S. W.; Buss, J. H. J. Chem. Phys. 1958, 29, 546. (20) Benson, S. W.; Cruicksh, Fr; Golden, D. M.; Haugen, G. R.; Oneal, H. E.; Rodgers, A. S.; Shaw, R.; Walsh, R. Chem. Rev. 1969, 69, 279. (21) Cohen, N.; Benson, S. W. Chem. Rev. 1993, 93, 2419. (22) Kua, J.; Faglioni, F.; Goddard, W. A. J. Am. Chem. Soc. 2000, 122, 2309. (23) Kua, J.; Goddard, W. A. J. Phys. Chem. B 1998, 102, 9492. (24) Soler, J. M.; Artacho, E.; Gale, J. D.; Garcia, A.; Junquera, J.; Ordejon, P.; Sanchez-Portal, D. J. Phys.: Condens. Matter 2002, 14, 2745. (25) Ordejon, P.; Artacho, E.; Soler, J. M. Phys. Rev. B 1996, 53, 10441. (26) Troullier, N.; Martins, J. L. Phys. Rev. B 1991, 43, 8861. (27) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. Rev. Lett. 1996, 77, 3865. (28) Chen, Y.; Vlachos, D. G. J. Phys. Chem. C 2010, 114, 4973. (29) Gokhale, A. A.; Kandoi, S.; Greeley, J. P.; Mavrikakis, M.; Dumesic, J. A. Chem. Eng. Sci. 2004, 59, 4679. (30) Reuter, K.; Scheffler, M. Phys. Rev. B 2002, 65. (31) Burgess, D. R. In NIST Standard Reference Database Number 69; Linstrom, P. J., Mallard, W. G., Eds.; National Institute of Standards and Technology: Gaithersburg, MD, 2009. (32) Hermida-Ramon, J. M.; Mosquera, R. A. Chem. Phys. 2006, 323, 211. (33) Avery, N. R. Appl. Surf. Sci. 1982, 1112, 774. (34) Grabow, L. C.; Gokhale, A. A.; Evans, S. T.; Dumesic, J. A.; Mavrikakis, M. J. Phys. Chem. C 2008, 112, 4608. (35) Grabow, L. C. G. L. C.; Mavrikakis, M. ACS Catal. 2011, 1, 365. (36) Gao, Q.; Hemminger, J. C. J. Electron Spectrosc. Relat. Phenom. 1990, 5455, 667. (37) Avery, N. R. J. Vac. Sci. Technol. 1982, 20, 592. (38) Olcay, H.; Xu, L.; Xu, Y.; Huber, G. W. ChemCatChem 2010, 2, 1420. (39) Alcala, R.; Mavrikakis, M.; Dumesic, J. A. J. Catal. 2003, 218, 178. (40) Salciccioli, M.; Chen, Y.; Vlachos, D. G. Ind. Eng. Chem. Res. 2011, 50, 28. (41) Skoplyak, O.; Barteau, M. A.; Chen, J. G. Surf. Sci. 2008, 602, 3578. (42) Skoplyak, O.; Menning, C. A.; Barteau, M. A.; Chen, J. G. J. Chem. Phys. 2007, 127, 114707. (43) Stottlemyer, A. L.; Ren, H.; Chen, J. G. Surf. Sci. 2009, 603, 2630. (44) Ishikawa, A.; Neurock, M.; Iglesia, E. J. Am. Chem. Soc. 2007, 129, 13201. (45) Mhadeshwar, A. B.; Vlachos, D. G. Ind. Eng. Chem. Res. 2007, 46, 5310. (46) Santiago, M. A. N.; Sanchez-Castillo, M. A.; Cortright, R. D.; Dumesic, J. A. J. Catal. 2000, 193, 16. (47) Rangarajan, S.; Bhan, A.; Daoutidis, P. Ind. Eng. Chem. Res. 2010, 49, 10459.
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