Reduction on Transition Metal (Fe, Co, Ni, and Cu .../67531/metadc...homogeneous catalysis of CO 2 reduction display the same energetic trend as a function of metal. INTRODUCTION Adsorption,

CO2 Reduction on Transition Metal (Fe, Co, Ni, and Cu) Surfaces: InComparison with Homogeneous CatalysisCong Liu, Thomas R. Cundari,* and Angela K. Wilson*

Department of Chemistry and Center for Advanced Scientific Computing and Modeling (CASCaM), University of North Texas,Denton, Texas 76203-5070, United States

*S Supporting Information

ABSTRACT: Reduction of CO2 to CO on Fe, Co, Ni, and Cu surfaces has been studied using density functional theory (DFT)methods. Three reaction steps were studied: (a) adsorption of CO2 (M + CO2 = CO2/M) (M = transition metal surface), (b)decomposition of CO2 (CO2/M = (CO + O)/M), and (c) desorption of CO ((CO + O)/M = O/M + CO). Binding energiesand reaction energies were calculated using the generalized gradient approximation (GGA) via the Perdew−Burke−Ernzerhof(PBE) functional. Calculations show an interesting trend for reaction energies and total reaction barriers, as a function of metal:from Fe to Cu, reactions tend to be less exergonic; the metals earlier in the 3d series have lower total barriers for CO2 reduction.However, “overbinding” of CO2 on Fe causes a thermodynamic sink on the reaction coordinate, and Co and Ni are morefavorable in terms of a smaller fluctuation in reaction energies/barriers for these elementary catalytic steps. A Brønsted−Evans−Polanyi (BEP) relationship was analyzed for C−O bond scission of CO2 on the metal surfaces. Heterogeneous catalysis is alsocompared with the homogeneous models using transition metal β-diketiminato complexes, showing that both heterogeneous andhomogeneous catalysis of CO2 reduction display the same energetic trend as a function of metal.

■ INTRODUCTIONAdsorption, activation, and conversion of CO2 using transitionmetal (TM) catalysts have been of interest for many years.Studies have focused on both heterogeneous (e.g., on metal/metal oxide surfaces1−5) and homogeneous reactions (e.g.,using metal−organic frameworks6−9 and TM complexes10−15).Such studies have shown that the properties of TM catalystsdepend on not only the internal chemical properties of TMelements but also on other properties, such as the structures ofclusters (e.g., surfaces and frameworks) and the properties ofligands (e.g., for complexes).Heterogeneous catalysts are widely used in industry. Carbon

dioxide adsorption and activation on clean surfaces has becomea subject of considerable investigation to probe fundamentalissues in the heterogeneous catalysis of CO2. A broad range ofTMs have been studied, including middle−late 3d metals.16−21Iron,16 cobalt,17 nickel,18 and copper19 were found to adsorb/activate CO2. Computational studies have helped to betterunderstand the structures and energetics of metal surface/CO2chemistry. For example, Glezakou et al.20 reported the reaction

mechanism of CO2 absorption and corrosion on an Febcc(100) surface using generalized gradient approximation(GGA) with Perdew−Burke−Ernerhof (PBE)22 functional,showing that CO2 is spontaneously activated when in proximityto a clean Fe(100) surface. In addition, Ding et al.23 studied theadsorption of CO2 on Ni(110) surfaces, using bothexperimental (temperature-programmed desorption (TPD))and computational (local spin density approximation (LSDAand GGA with PBE22 functional) methods. The research ofDing et al.23 suggested that CO2 is weakly adsorbed onNi(110), and different chemisorbed structures with similaradsorption energies are possible. Recent studies have also beenfocused on comparing catalytic properties on different faces of ametal surface. For instance, de la Peña O’Shea et al.17 studiedthe adsorption of CO2 on Co(100), Co(110), and Co(111) fccsurfaces using GGA methods with the exchange and correlation

Received: November 1, 2011Revised: February 3, 2012Published: February 22, 2012

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functional of Perdew and Wang (PW91).24 Their resultsshowed that the interaction with CO2 is surface sensitive; the(110) surface showed the strongest interaction and CO2activation involves charge transfer from the metal surfaces tothe substrate.In addition to heterogeneous catalysts, a number of

molecular TM complexes have also shown favorable catalyticproperties for CO2 activation, especially middle to late 3dmetals.13−15,25−29 For example, Isaacs et al.13 studied thereduction of CO2 using aza-macrocyclic complexes of Ni(II),Co(II), and Cu(II). The electronic spectra showed that CO2reduction is due to the presence of a formal 1+ metal, althoughthe reduced ligand is also important. Also, Sadique et al.25

reported reduction of CO2 to CO and carbonate mediated by alow-coordinate Fe(I)−dinitrogen complex, LtBuFeNNFeLtBu(where LtBu = 2,2,6,6-tetramethyl-3,5-bis[(2,6-diisopropylphen-yl) imino]hept-4-yl), via a dicarbonyl, LtBuFe(CO)2, and acarbonate, LtBuFe(μ-OCO2)FeL

tBu. Recently, Ariafard et al.27

have reported a DFT study (B3LYP) of the reductive cleavageof CO2 by L

MeFeNNFeLMe. Our group reported DFT studieson the reverse water-gas shift (RWGS) reaction using first-rowTM β-diketiminato complexes.28,29 A systematic investigationwas carried out for the thermodynamics and kinetics of RWGScatalysis as a function of metal. The calculations showed thatthe β-diketiminate metal catalysts are thermodynamicallyfeasible for the coordination of CO2; moreover, CO2 activationand reduction becomes less thermodynamically favorable fromthe left to the right in the 3d series. Thus, if the transition metalis the key factor for determining the kinetics and thermody-namics of CO2 homogeneous catalysis, then a number ofinteresting questions need to be addressed: would theseenergetic trends be similar or different for heterogeneous 3dmetal catalysts? Do the TMs play similar/different structuraland electronic roles in heterogeneous and homogeneouscatalysts? Although several researchers16−21 have reportedCO2 interaction with TM surfaces or TM complexes, there isa paucity of research on the comparison (a) of heterogeneoussystems among different metals or (b) between heterogeneousand homogeneous catalysts.In this paper, we report a study of CO2 adsorption and

decomposition on Fe, Co, Ni, and Cu(100) surfaces. In orderto more closely investigate the intrinsic catalytic properties ofthe 3d elements, the energetics were compared among the fourmetals, keeping all the other variables the same; i.e., the (100)surface of a face centered cubic (fcc) structure is used for allsolid-state simulations. The ground states of Fe (bcc) and ofCo (hcp) were also investigated for the sake of completeness aswell as calibration of the present plane-wave DFT results versusliterature data.17,19−21 A strong correlation was discoveredbetween the intrinsic activity of heterogeneous and homoge-neous CO2 catalysis as a function of 3d metal.

■ COMPUTATIONAL DETAILSThe structures and total free energies were determined usingplane-wave density functional theory (PW-DFT) calculationswithin the Vienna Ab initio Simulation Package (VASP).30,31

Construction of surfaces was done with Materials Studio,32 andvisualization of structures and electron densities was done withMedeA33 and Mercury.34 Spin polarization and dipolecorrections were considered for all calculations. The total freeenergy was calculated using the GGA, with the PBEfunctional.22 The electron density was expanded in a planewave basis set, whereas the effect of the inner core was taken

into account through the projected augmented wave (PAW)method.35 After convergence tests for energy cutoff and k-points, a 550 eV cutoff energy was chosen, and a 5 × 5 × 1 k-points grid within the Monkhorst−Pack scheme36 was used tosample the Brillouin zone of the surface unit cell. Allcalculations were done at 0 K. Transition state (TS) searcheswere carried out by the climbing image nudged elastic band(CI-NEB)37−39 method. Along each reaction pathway, 6−8images were taken into account (only 4 images are shown inthe figures in this paper). Transition states were further verifiedthrough frequency calculations. A Bader analysis was carriedout for all the systems, using the code developed by Henkelmanand co-workers40−42 in order to quantify the charge transferbetween the surfaces and CO2 moiety. All the surfaces arerelaxed before and after the addition of the adsorbates.

■ SURFACE MODELSThe chemisorption of carbon dioxide to fcc(100) surfaces wasmodeled for Fe, Co, Ni, and Cu. Identical surfaces weremodeled for the convenience of comparison of intrinsic metalproperties and their impact on CO bond activation.However, surfaces derived from the ground state of Fe (bcc)and of Co (hcp) were also studied. In all cases, the slab modelswere initially constructed from the optimized lattices (Table 1).

The PW-DFT calculated lattice parameters were in goodagreement with the experimental values. A five-layer periodicslab was constructed along the (100) Miller plane, with an ∼10Å vacuum along the c-axis in order to avoid interactionsbetween the surface slab and its periodic image, and betweenthe small molecules (CO2, CO, or O) and their images. Thetop three layers of the slab were allowed to relax, while thebottom two layers were fixed. A 3 × 3 supercell was used, whichcontained nine metal atoms per layer, and a total of 45 metalatoms. One CO2 molecule was adsorbed at a time on eachmetal surface, which is enough to avoid interactions betweenCO2 and its periodic image. Similar protocols have been used inmodeling studies of transition metal surfaces and carbondioxide.17−21

For CO2 adsorption, eight different starting configurationswere considered, including different adsorption sites (top,bridge, 3-fold, and 4-fold) and various molecular orientations,to help identify the location of the global minimum for theCO2/M system. The binding energy of CO2 was calculated asin eq 1:

= − −E E E E(CO /M ) (M ) (CO )ads 2 slab slab 2 (1)

where the first term is the calculated energy of CO2 adsorbedmetal system, and the second and third terms are the energy ofthe metal surface and the energy of CO2 molecule, respectively.For (CO + O)/M systems, four conformations were initially

Table 1. Calculated Lattice Parameters (Å) of Unit Cells vsExperimental Data

metal calcd (this work) exptl43

Fe fcc 3.649 3.652bcc 2.861 2.860

Co fcc 3.520 3.544hcp 2.251 (a, b), 4.070 (c) 2.251 (a, b), 4.070 (c)

Ni fcc 3.523 3.516Cu fcc 3.639 3.636

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considered, although optimizations eventually yielded a singleconformation.

■ RESULTS AND DISCUSSION1. Adsorption of CO2: M + CO2 = CO2/M. The first

modeled process is CO2 chemisorption on a metal (100)surface. Therefore, a reduction in the CO2 angle from the linearOCO may be expected during the process, particularly ifCO2 is reduced. Although several conformations of CO2/Mwere found for each metal, only the most stable structures foreach metal are shown in Figure 1 (the remaining conformationsand calculated binding energies are given in the SupportingInformation).

a. Fe fcc(100) Surface. Four stable configurations were foundfor the CO2/Fe system: 4-fold (A), 4-fold (B), short-bridge(C), and top (D) (picture of A in Figure 1; pictures of B, C,and D in the Supporting Information). In configurations A, B,and C, the adsorbed CO2 molecule displays a bent structure(O−C−O angle ∼120°), which is close to that expected forCO2

−. However, in configuration D, the CO2 moiety maintainsa near linear arrangement and shows a weak interaction withthe metal surface. In most configurations (A, B, and C), theCO2 moiety is stabilized on the surface by several bonds tosurface iron atoms, involving all three centers of CO2. All theconfigurations show negative adsorption energies, of which Ashows the lowest adsorption energy, −33.4 kcal/mol (Table 2).

It is notable that O2 (Figure 1) also has a weak interaction withFe1 (O2−Fe1 = 1.99 Å) (Table 2). One may surmise thatadditional energy may be necessary to “pull” O2 from theFe(100) surface, and this might increase the kinetic barrier toCO2 decomposition. We return to this point in section 2.b.b. Co fcc(100) Surface. For the CO2/Co(100) system, four

configurations were obtained: 4-fold (A), 3-fold (B), short-bridge (C), and top (D) (picture of A in Figure 1; pictures ofB, C, and D in Supporting Information). As with Fe, the globalminimum of CO2/Co(100) also has a 4-fold configuration(Figure 1), but with a much less negative adsorption energy of−9.6 kcal/mol (versus −33.4 kcal/mol for CO2/Fe(100)). Thisvalue is in excellent agreement with the study by de la PeñaO’Shea et al.,17 who also observed a 4-fold configuration forCO2/Co(100), with an adsorption energy of −9.7 kcal/mol. Itis also worth noting that O2 has a very weak interaction withCo1 (O2−Co = 2.13 Å) on CO2/Co(100).c. Ni fcc(100) Surface. Three stable configurations were found

for the CO2/Ni(100) system: 4-fold (A), 3-fold (B), and top(C) (picture of A in Figure 1; pictures of B and C inSupporting Information). Only the 4-fold configuration (A) hasa negative adsorption energy, −1.09 kcal/mol. Unlike the Coand Fe surfaces, O2 does not have any direct interaction withthe Ni(100) surface, and the CO2 plane is orthogonal to themetal surface. Also, the C−O1 bond (1.38 Å) on the Ni surfaceis slightly longer than the C−O1 bonds on the other surfaces(Table 2, Fe; 1.34 Å, Co: 1.35 Å, Cu: 1.33 Å).d. Cu fcc(100) Surface. Only one configuration was obtained

for the CO2/Cu(100) system: 4-fold. The 4-fold configurationhas a high positive binding energy 17.3 kcal/mol. It suggeststhat the attraction interaction between CO2 and Cu surface isso weak that spontaneous adsorption of CO2 will not occur onCu(100) surface. However, in this paper, the reactions betweenCO2 and Cu(100) are still compared with the reactions of theother metals. In summary, the adsorption energies of the four3d metal systems follow a periodic trend, i.e., lowerchemisorption energy as one traverses from left to right inthe 3d series: Fe (−33.4 kcal/mol) < Co (−9.6 kcal/mol) < Ni(−1.1 kcal/mol) < Cu (+17.3 kcal/mol), which indicates thatthe early metals tend to have stronger interaction with CO2than the late metals.e. Charge Transfer Analysis. As alluded to above, CO2 bending

upon surface chemisorption suggests electron transfer from themetal surface to the CO2 moiety. Hence, the valence chargedensity was calculated for isolated CO2

− as well as for the CO2/Fe(100) system (configuration A). The valence charge densityof isolated CO2

− has great similarity with that of the boundCO2 moiety (Supporting Information). Figure 2a shows thevalence charge density plot of configuration A for CO2/Fe(100), where the x-axis shows the distance measured fromthe bottom of the slab and the y-axis shows the valence chargedensity ρ. For CO2/Fe(100) and Fe′(100) (i.e., the isolatedsurface of the CO2/Fe system, shown by purple solid and reddot lines, respectively), five high density peaks were observed,each of which represents the valence charge density of a metallayer. In the CO2/Fe(100) system, a relatively small densitypeak (at ∼10 Å along the x-axis) was observed, representing thevalence charge density of the CO2 moiety on the surface, and itmatches the density plot of the isolated CO2′ moiety (theisolated CO2 structure from CO2/Fe, green dashed line, Figure2). It is clear that the interaction between Fe(100) and CO2 isprimarily limited to the Fe interface region (z = 8−9 Å). Figure2b shows the valence charge density difference (note the

Figure 1. The most favorable calculated binding conformations(conformation A) of CO2 on fcc(100) surfaces.

Table 2. Geometrical Parameters and Calculated AdsorptionEnergies of CO2 Species on fcc(100) Surfaces

parameter Fe Co Ni Cu

dC−O1 (Å) 1.34 1.35 1.38 1.33dC−O2 (Å) 1.29 1.27 1.23 1.22dC−M1 (Å) 2.19 2.08 2.03 2.15dC−M2 (Å) 1.95 1.95 2.03 2.15dO1−M3 (Å) 2.08 2.07 1.97 2.09dO1−M4 (Å) 2.04 2.02 1.97 2.09dO2−M1 (Å) 1.99 2.13O1−C−O2 (deg) 120.5 122.9 124.2 127.7Eads (kcal/mol) −33.4 −9.6 −1.1 17.3



change in scale) between absorbed CO2/Fe (conformer A) andthe sum of the Fe′ surface and the CO2′ moiety. The densitydifference Δρ changes from slightly positive to negative uponcrossing of the Fe interface (z = 8−9 Å) and reaches aminimum at z ∼ 8.5 Å; it then becomes positive around theCO2 region (z = 9−10 Å). This evolution illustrates that thevalence charge density flows from the Fe surface toward theCO2 moiety, which is transformed effectively to a CO2

−.To further quantify the electron transfer between the surface

and CO2, a Bader analysis40−42 was carried out for all the

systems (see Table 3 and Supporting Information). Shown inTable 3, in the adsorbed systems, the CO2 moiety was reducedto about −1.2, −0.99, −0.80, and −0.63|e| on Fe, Co, Ni, and

Cu surface, respectively. Most of the charge was transferredtoward the carbon atom. Earlier metal surfaces tend to transfermore charge to the CO2 moiety than the later metal surface,which correlates well with the adsorption energies, i.e., the lessthe electron transfer for the surface to CO2, the weaker thebinding of CO2.f. CO2 Adsorption on Fe bcc (100) and Co hcp (1010) Surfaces.

According to the calculated thermodynamics for CO2chemisorption on late 3d metal (100) surfaces, it is clear thatthe metal plays a significant role in the activation of CO2.Hence, questions arise: is the metal the most important factorin CO2 adsorption? How much would other factors such asstructures of the surface impact the reaction thermodynamics?To address these questions, calculations were also performedon CO2 adsorption on the Fe bcc(100) and Co hcp(1010).Body centered cubic is the ground state of Fe, as is hexagonalclose packed (hcp) for elemental Co. The optimized lowestenergy structures for these CO2/M models are shown in Figure3 (for structural parameters see Supporting Information). For

the bcc Fe(100) system, CO2 preferentially binds in long-bridge 4-fold conformation, which differs from the CO2/Fe(100)-fcc system discussed above. Moreover, the adsorptionenergy for the Fe bcc system is calculated to be −14.8 kcal/mol(in agreement with −16.7 kcal/mol20), which is much lessnegative than that of the Fe fcc system (−33.4 kcal/mol)(Table 2). Figure 4 shows the energy relationship between Fe

bcc(100) and fcc(100) systems that were modeled. The energydifference between Fe (fcc) and (bcc) system is as high as 31.8kcal/mol; however, the energy difference between CO2/Fe(fcc) and CO2/Fe(bcc) decreased to only 13.2 kcal/mol. ForFe(fcc), after CO2 binding, the bond lengths between some Featoms on the top two layer decreased about 0.1−0.2 Å, and themagnetic moment decreased from 2.723/atom to 2.689/atom.Fe(fcc) is nonmagnetic near equilibrium but switches toferromagnetic at larger distances. These results suggested that

Figure 2. (a) Valence charge density plot of Fe′ surface, CO2′ moiety,and CO2/Fe. (b) Charge density difference plot of CO2/Fe system:Δρ(z) = ρCO2/Fe(z) − [ρFe′(z) + ρCO2′(z)]. Fe′ denotes the Femorphology from the CO2/Fe(100) system, and CO2′ is the bent CO2moiety of the same CO2/Fe(100) system.

Table 3. Partial Charges for Selected Systems Calculated byBader Analysis (|e|) (Partial Charged for the Other Systems;See Supporting information)

O1 C O2 CO2 moiety

CO2 −1.061 2.122 −1.061 0CO2/Fe −1.119 1.018 −1.054 −1.155CO2/Co −1.037 1.086 −1.035 −0.986CO2/Ni −1.020 1.188 −0.971 −0.802CO2/Cu −1.095 1.470 −1.004 −0.629

Figure 3. The most stable calculated structure of CO2/Fe bcc(100)and CO2/Co hcp(1010).

Figure 4. Energy diagram of Fe bcc and fcc systems (kcal/mol).



after CO2 binding the Fe(fcc) system becomes morecondensed and therefore less magnetic, which agrees with thestatement above that the spin-polarized calculations for Fe(fcc)point to a higher state (at the asymptotic limit) which becomesless pronounced once the CO2 binds the surface. For Cosurfaces, on the other hand, CO2 adsorption on hcp(1010) isnot as favorable as on the fcc(100) surface. The calculatedadsorption energy is 2.0 kcal/mol for the hcp surface, comparedto −9.6 kcal/mol for the fcc surface. This indicates thatspontaneous adsorption of CO2 is unlikely to occur on aCo(1010)-hcp surface. In summary, not only the intrinsicelectronic structure of the 3d metal but also the morphology ofthe surfaces plays an important role in CO2 adsorption.2. Decomposition of CO2: CO2/M = (CO + O)/M. a.

Reaction Energies. Binding of CO2 is followed by breaking of thecoordinated C−O1 bond. Thus, the resulting structure will be asurface-bound oxygen and a surface-bound CO. The moststable conformation of (CO + O)/M isolated for each TM isshown in Figure 5. In the (CO + O)/Fe system, the oxygen

atom binds to four Fe atoms (4-fold site), whereas the carbonof the CO moiety binds to one Fe atom. In all of the othermetal systems, the oxygen also binds to four metal atoms (4-fold site), but the carbon of CO binds to two metal atoms(short-bridge). The reaction energies are shown in Table 4.

The Fe(100) system shows the lowest reaction energy (−24.1kcal/mol), followed by Co (−22.3 kcal/mol), and Ni (−20.4kcal/mol). The CO2/Cu → (CO + O)/Cu reaction energy,however, is only slightly downhill (−0.6 kcal/mol). It is notablethat for Fe, Co, and Ni(100) surfaces the reaction energies arevery close to each other, although in the first step (CO2chemisorption) Fe has a much more negative binding energy

than Co, which in turn has a much more negative CO2 bindingenergy than Ni. In other words, Fe does not show muchthermodynamic favorability in CO2 decomposition, possiblydue to the stronger interaction of O2 and the surface for Fe inthe first step, compared to Co, Ni, and Cu surfaces. Therefore,even though Fe still has the lowest reaction energy in CO2decomposition, this may not translate into a lower reactionbarrier, since additional energy may need to be consumed forO2 to “escape” from the surface.b. Reaction Barriers. Reaction barriers for the decomposition

of CO2 on the surfaces were calculated using the CI-NEBmethod. Four images were shown for each metal system(Figure 6). Vibrational frequencies were calculated for eachtransition state (see Supporting Information). For the Fe(100)surface, two transition states were found; a major transitionstate TS1 is obtained in which the C−O1 bond is breaking, anda insignificant barrier TS2 was found when breaking one of theC−Fe bonds resulting in CO forming a top conformation. Thetotal reaction barrier (relative to the initial state gas phaseenergy) for CO2 reduction on the Fe surface is −6.3 kcal/mol,suggesting a high activity on the Fe surface. For Co, Ni, andCu(100) surfaces, the structures of the transition states foundwere similar to TS1 for the Fe surface, and the total barrierswere calculated to be 3.0, 11.1, and 39.5 kcal/mol, respectively.In general, the total reaction barriers still keep the same trendwith the reaction energies: Fe < Co < Ni < Cu.Moreover, the reaction barriers (ΔE‡) and the reaction

energies (ΔE = Eads + ΔEdec) follow a linear relationship for theC−O bond scission of CO2, and the BEP relationship wasdeveloped: ΔE‡ = 0.63ΔE + 26.7 (kcal/mol) (Figure 7). ThisBEP relationship could be important to estimate reactionbarriers on other metal surfaces for the C−O bond scission ofCO2. It is also notable that even Fe has the lowest total barrier,its system falls into a thermodynamics sink, due to the“overbinding” of CO2 on the Fe surface, and it requires 27.1kcal/mol energy for CO2/Fe to climb over the barrier, which ismuch higher than that for Co (12.6 kcal/mol) and Ni (12.2kcal/mol). Therefore, the “overbinding” of O2 of CO2 greatlyimpacts the reaction coordinate on the Fe surface as weexpected earlier, but not as remarkably for the Co surface.Thus, Co and Ni are more favorable in terms of a smallerfluctuation on reaction energies/barriers of the elementarycatalytic steps.

3. Desorption of CO: (CO + O)/M = O/M + CO. Thereaction energy of CO desorption from the surface was alsocalculated. The desorption of CO from Cu(100) is expected tobe less endergonic than the other surfaces due to the weakinteraction between Cu and CO; the calculated reaction energyis 19.5 kcal/mol, which is consistent with Wang’s calculation,19.1 kcal/mol19a). On the other hand, CO dissociation on Fe,Co, and Ni surfaces is highly endergonic, and the calculatedreaction energies are 38.3 kcal/mol, 38.4 kcal/mol (cf. Ge’s37.1 kcal/mol21), and 41.3 kcal/mol, respectively.

4. Comparison of Metal Surfaces and Correlation withHomogeneous Models. Previously, we investigated CO2activation and conversion mediated by first-row TM β-diketiminato complexes L′M (L′ = β-diketiminate, M = TM),using B3LYP/aug-cc-pVTZ.28,29 The mechanism of CO2reduction using these homogeneous catalysts is quite similarto that using the metal surfaces (shown in Figure 8). For Feand Co, the homogeneous catalysis starts with CO2 binding tothe metal center and forms L′M(CO2) (Figure 9, correspond-ing to CO2/M in this paper). This CO2 complex then goes

Figure 5. The most stable calculated structures of (CO + O)/M(100)(M = Fe, Co, Ni, Cu).

Table 4. Geometrical Parameters and Reaction Energies ofCO + O System on fcc (100) Surfaces

parameter Fe Co Ni Cu

dC−O2 (Å) 1.17 1.18 1.23 1.17dC−M1 (Å) 1.91 1.88 2.00dC−M2 (Å) 1.77 1.92 1.88 1.99dO1−M(3,4,5,6) (Å) 1.98 1.98 1.97 2.01ΔEdec (kcal/mol) −24.1 −22.3 −20.4 −0.6



though a TS and forms a carbonyl oxo-complex L′M(CO)(O),in which the coordinated C−O bond is broken. AndL′M(CO)(O) then reacts with another catalyst molecule andforms L′M(O) and L′M(CO) (corresponding to (CO + O)/Min this paper). L′M(CO) eventually releases CO. For Ni andCu complexes, on the other hand, the mechanism is similar tothat of Fe and Co, except that the TSs for C−O bond breakingwere not obtained. Potential energy surface scanning suggeststhat for Ni and Cu systems L′M(CO2) reacts with anothercatalyst molecule without first breaking the coordinated C−O

bond; since the reversible reaction is highly exergonic, thereaction barrier should be very close to the reaction energy.Comparing the calculated the thermodynamics between the

heterogeneous (fcc(100) surfaces) and the homogeneouscatalyst models, an interesting trend was observed for both,as a function of metal (Figure 10); from the earlier metal Fe tothe later metal Cu, reactions tend to be more endergonic. Towit, earlier metals tend be more favorable in terms of CO2adsorption/binding. However, for Fe system, CO2/Fe falls intoa remarkable thermodynamic sink on the reaction coordinate,due to the O2 “overbinding” to the surface in the first step.

Figure 6. Calculated reaction coordinates of CO2 reduction on Fe, Co, Ni, and Cu fcc(100) surfaces. Total reaction barriers (relative to the energy ofCO2 + M) are −6.3, 3.0, 11.1, and 39.5 kcal/mol. In addition, for Fe, the reaction pathway involves a C−Fe bond breaking to form a topconformation for CO from bridge, and this process has a small barrier about 0.2 kcal/mol. CO1 bond distance in TS1 (Fe) is 1.78 Å; C−Fe distancein TS2 for Fe is 2.29 Å; C−O1 in TS(Co) is 1.76 Å; C−O1 in TS(Ni) is 1.82 Å; C−O1 in TS(Cu) is 2.41 Å.

Figure 7. Brønsted−Evans−Polanyi (BEP) relationship for C−Obond scission of CO2 on TM surfaces. Reaction barriers and reactionenergies are relative to initial state gas phase energies.

Figure 8. Reaction mechanism of homogeneous catalysis of CO2reduction.



“Overbinding” does not occur for the homogeneous catalysis,since no extra metal centers react with the uncoordinated O.The dissociation of CO in the homogeneous models is moreendergonic for earlier metals, as expected; the reaction energiesfor Fe, Co, Ni and Cu β-diketiminate complexes are 27.0, 27.6,15.7, and 11.8 kcal/mol, respectively. In addition, in ourprevious study,28,29 it was shown that the homogeneous systemcould have a reaction to form very stable bimetallic compounds,such as L’M(CO)OML’, which would lead the reaction to fallinto a thermodynamic sink, and as a result terminate thecatalysis. Nevertheless, such dimerizations can be avoided in theheterogeneous catalysis. In summary, the comparison showsthat metal plays a very important role in the catalysis, indicatingthat the reactions for earlier metals are more exergonic than

those of later metals. Earlier metal surfaces (Fe) tend to overbind CO2, which causes a thermodynamic sink on the reactioncoordinate. However, among the four metal fcc(100) surfaces,Co and Ni show favorable thermodynamics and low reactionbarriers on CO2 reduction.

■ SUMMARY AND CONCLUSIONSA DFT study was carried out to examine the CO2 adsorptionand decomposition on Fe, Co, Ni, and Cu fcc(100) surfaces.Calculations show spontaneous chemisorption of CO2 andfavorable thermodynamic properties for Fe, Co, and Nisurfaces, whereas the Cu surface has only a very weakinteraction with CO2. Valence electron density calculationsshow that the activation of CO2 involves a charge transfer fromthe metal surface to the CO2 moiety. Reaction energies andtotal reaction barriers show an interesting trend as a function ofmetal: reactions of earlier metals are more exergonic and havelower total barriers than those of later metals. The Brønsted−Evans−Polanyi (BEP) relationship was developed for thereaction energies and the total barriers of C−O bondingscission of CO2. The Fe surface is the most favorable surface forCO2 adsorption. However, the “overbinding” of CO2 on Fesurface engenders a thermodynamic sink on the reactioncoordinate. Studies of CO2 adsorption on Fe bcc(100) and Cohcp(1010) surfaces indicate that not only metals but also thesurface structures significantly affect the thermodynamics of thecatalysis.Comparison between the present heterogeneous catalyst

models with our previous studies of homogeneous CO2catalysis28,29 using TM β-diketiminato complexes suggeststhat a thermodynamic trend occurs for both heterogeneousand homogeneous catalysis: from earlier metals to later metals,the reaction tends to be more endergonic. And the metal playsan important role in CO2 activation and reduction. Thehomogeneous catalysis is able to avoid the “overbinding” ofCO2 that happened on the surfaces, while the heterogeneouscatalysis does not have to concern about the unfavorabledimerization reaction that could happen in the homogeneouscatalysis. Overall, of the four fcc(100) surfaces, both Co and Nishow favorable thermodynamics and low CO2 decompositionbarriers for CO2 reduction.

■ ASSOCIATED CONTENT*S Supporting InformationTables of reaction energies and figures of all conformations.This material is available free of charge via the Internet athttp://pubs.acs.org.

■ AUTHOR INFORMATIONNotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThe authors acknowledge the support of the United StatesDepartment of Energy (BER-08ER64603) and NSF-CRIF(CHE-0741936) for equipment support. The authors alsoacknowledge Dr. Corneliu Buda (U. Virginia) for help with theNEB calculations, Mr. Bhaskar Chilukuri (UNT) for help withthe electron density difference calculations, and Mr. SivabalanManivasagam for working on the convergence tests.

Figure 9. Optimized structures of L′M(CO2) using B3LYP/aug-cc-pVTZ.

Figure 10. (a) Calculated relative reaction energies of CO2 adsorptionand decomposition on Fe, Co, Ni, and Cu fcc(100) catalysis,indicating that the reactions for earlier metals are surfaces. (b).L′M(CO) Calculated reaction energies of CO2 activation by L′M (L =β-diketiminate, M = Fe, Co, Ni, and Cu).



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Reduction on Transition Metal (Fe, Co, Ni, and Cu .../67531/metadc...homogeneous catalysis of CO 2 reduction display the same energetic trend as a function of metal. INTRODUCTION Adsorption,