Adsorption Structure and Electronic Structure of Ethylene ...
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Adsorption Structure and Electronic Structure of Ethylene on Pt 3 Ti(001) and PtTi 3 (001) Surfaces: a DFT Study Kaoru Fujiwara 1,+1 , Yoshiaki Miyawaki 1,+1 , Kazuki Nozawa 2,+2 and Yasushi Ishii 1 1 Department of Physics, Chuo University, Tokyo 112-8551, Japan 2 Department of Physics and Astronomy, Kagoshima University, Kagoshima 890-0065, Japan Adsorption structure and electronic structure of ethylene on Pt 3 Ti(001) and PtTi 3 (001) intermetallic compounds surfaces are studied in terms of density functional calculations. In both intermetallic compounds, adsorption energy of bridge and hollow sites are larger than that of the atop sites. Moreover, obtained adsorption energy and the C-C separations at most sites on both intermetallic compounds are larger than those reported for Pt(111). Analyzing the surface LDOSs, it turns out that the bimodal density of states made by the occupied Pt d-states and unoccupied Ti d-states are effectively interact with the HOMO and LUMO of ethylene, which are bonding and anti-bonding states of the ³-bond between carbon atoms, and then leading the elongated C-C bond and large adsorption energy. Larger adsorption energy at bridge and hollow sites is also understood in the same way. Although we found out the possibility that the bimodal density of states realized in intermetallic compounds composed of early and late transition metals is effective for molecular dissociation reaction generally, no clear evidence indicating the experimentally reported higher catalytic activity of Pt 3 Ti than PtTi 3 and Pt in the ethylene hydrogenation is obtained in this study. [doi:10.2320/matertrans.MF201406] (Received October 2, 2014; Accepted December 2, 2014; Published January 23, 2015) Keywords: density functional method, first-principles calculation, platinum, titanium, intermetallic compound, Pt 3 Ti, PtTi 3 , ethylene, catalysis, chemical reaction on surface 1. Introduction Metals sometimes change their chemical and physical properties when alloying. Utilizing these “alloying effect” positively, in these days many researchers are trying to discover a novel alloy catalyst that has equivalent or superior catalytic performance than pure metals like Pt, Pd and so on. It has long understood that the catalytic activities are dominated by the electronic structure around the Fermi level (EF), because they interact with the highest occupied molecular orbital (HOMO) and/or lowest unoccupied mo- lecular orbital (LUMO) of the adsorbate, and leads dissociation and recombination of molecules via electron donation or back-donation with them. Thus if we can figure out what kind of electronic structure exhibits a high catalytic activity, it would be a guide for the design of new catalysts. Recently, Komatsu et al. reported that Pt 3 Ti and PtTi 3 intermetallic compounds show much higher catalytic activity than pure Pt for H 2 -D 2 equilibration. 1) No intermetallic compound showing higher activity than their component metals has been previously reported. More interestingly, they also reported that PtTi 3 did not catalyze the hydrogenation of ethylene at all, although Pt 3 Ti shows again higher activity than Pt. Mozo et al. investigated hydrogen dissociation on Pt(111) and Pt 3 Ti(111), and reported that Pt 3 Ti does not posses superior catalytic activity compared to Pt. 2) Ethylene hydrogenation on Pt surfaces has been studied extensively both experimentally 3-7) and theoretically. 8-13) But there are no theoretical research for ethylene adsorption on the Pt-Ti intermetallic compounds. Thus we investigated adsorption of ethylene on Pt 3 Ti and PtTi 3 intermetallic compounds in order to find out differences in their adsorption structures and electronic structures by means of density functional calcu- lations. 2. Models and Calculations 2.1 Crystal structure and model surfaces Crystal structure of Pt 3 Ti is L1 2 structure containing 4 atoms (3 Pt atoms and 1 Ti atom) in the unit cell, and the equilibrium lattice constant is 0.3906 nm. 13,14) Because we are interested in and focusing on alloying effects on chemisorption of ethylene here, we study (001) surfaces, where both of Pt and Ti appear, for both compounds. A slab model consists of five atomic layers and a vacuum layer the thickness of which is equivalent to six atomic layers (1.172 nm) is used to represent the surface. 2 ffiffi 2 p 2 ffiffi 2 p periodicity for lateral direction is assumed to realize a low- coverage situation in order to suppress interaction between adsorbate and to analyze unperturbed surface-adsorbate interaction. The edge lengths of the super-cell are then 1.105 nm and 1.953 nm for the lateral and vertical direction to the surface layer, respectively. On the other hand, as shown in Fig. 1(b), PtTi 3 crystalizes in A15 structure with the lattice constant 0.5033 nm. 15) Each atomic layer normal to [001] direction is composed of one Pt atom and two Ti atoms separated by a half of the lattice constant. Surface of PtTi 3 is also represented by a slab with five atomic layers and a vacuum layer of 1.006 nm. For lateral direction of the super cell, 2 © 2 periodicity, meaning the edge length is 1.006 nm, is assumed for the same reason mentioned above. 2.2 Calculations Density functional calculation together with generalized gradient approximation 16) is carried out using Vienna ab- initio simulation package. 17,18) Projector-augmented wave potentials 19,20) are used for describing effective interactions between electrons and ionic cores. Wave-functions are expanded by plane waves up to a kinetic energy of 274eV. The Brillouin zone is sampled at 7 © 7 © 1 Monkhorst-Pack k-point grid for both Pt 3 Ti and PtTi 3 systems. The plane wave +1 Graduate Student, Chuo University +2 Corresponding author, E-mail: nozawa@sci.kagoshima-u.ac.jp Materials Transactions, Vol. 56, No. 4 (2015) pp. 479 to 484 Special Issue on Advanced Metallic Materials for Catalysis © 2015 The Japan Institute of Metals and Materials
Transcript of Adsorption Structure and Electronic Structure of Ethylene ...
Adsorption Structure and Electronic Structure of Ethylene on Pt3Ti(001)and PtTi3(001) Surfaces: a DFT Study
Kaoru Fujiwara1,+1, Yoshiaki Miyawaki1,+1, Kazuki Nozawa2,+2 and Yasushi Ishii1
1Department of Physics, Chuo University, Tokyo 112-8551, Japan2Department of Physics and Astronomy, Kagoshima University, Kagoshima 890-0065, Japan
Adsorption structure and electronic structure of ethylene on Pt3Ti(001) and PtTi3(001) intermetallic compounds surfaces are studied interms of density functional calculations. In both intermetallic compounds, adsorption energy of bridge and hollow sites are larger than that of theatop sites. Moreover, obtained adsorption energy and the C-C separations at most sites on both intermetallic compounds are larger than thosereported for Pt(111). Analyzing the surface LDOSs, it turns out that the bimodal density of states made by the occupied Pt d-states andunoccupied Ti d-states are effectively interact with the HOMO and LUMO of ethylene, which are bonding and anti-bonding states of the ³-bondbetween carbon atoms, and then leading the elongated C-C bond and large adsorption energy. Larger adsorption energy at bridge and hollowsites is also understood in the same way. Although we found out the possibility that the bimodal density of states realized in intermetalliccompounds composed of early and late transition metals is effective for molecular dissociation reaction generally, no clear evidence indicatingthe experimentally reported higher catalytic activity of Pt3Ti than PtTi3 and Pt in the ethylene hydrogenation is obtained in this study.[doi:10.2320/matertrans.MF201406]
(Received October 2, 2014; Accepted December 2, 2014; Published January 23, 2015)
Keywords: density functional method, first-principles calculation, platinum, titanium, intermetallic compound, Pt3Ti, PtTi3, ethylene, catalysis,chemical reaction on surface
1. Introduction
Metals sometimes change their chemical and physicalproperties when alloying. Utilizing these “alloying effect”positively, in these days many researchers are trying todiscover a novel alloy catalyst that has equivalent or superiorcatalytic performance than pure metals like Pt, Pd and soon. It has long understood that the catalytic activities aredominated by the electronic structure around the Fermi level(EF), because they interact with the highest occupiedmolecular orbital (HOMO) and/or lowest unoccupied mo-lecular orbital (LUMO) of the adsorbate, and leadsdissociation and recombination of molecules via electrondonation or back-donation with them. Thus if we can figureout what kind of electronic structure exhibits a high catalyticactivity, it would be a guide for the design of new catalysts.
Recently, Komatsu et al. reported that Pt3Ti and PtTi3intermetallic compounds show much higher catalytic activitythan pure Pt for H2-D2 equilibration.1) No intermetalliccompound showing higher activity than their componentmetals has been previously reported. More interestingly, theyalso reported that PtTi3 did not catalyze the hydrogenation ofethylene at all, although Pt3Ti shows again higher activitythan Pt. Mozo et al. investigated hydrogen dissociation onPt(111) and Pt3Ti(111), and reported that Pt3Ti does notposses superior catalytic activity compared to Pt.2) Ethylenehydrogenation on Pt surfaces has been studied extensivelyboth experimentally37) and theoretically.813) But there areno theoretical research for ethylene adsorption on the Pt-Tiintermetallic compounds. Thus we investigated adsorption ofethylene on Pt3Ti and PtTi3 intermetallic compounds in orderto find out differences in their adsorption structures andelectronic structures by means of density functional calcu-lations.
2. Models and Calculations
2.1 Crystal structure and model surfacesCrystal structure of Pt3Ti is L12 structure containing 4
atoms (3 Pt atoms and 1 Ti atom) in the unit cell, and theequilibrium lattice constant is 0.3906 nm.13,14) Because weare interested in and focusing on alloying effects onchemisorption of ethylene here, we study (001) surfaces,where both of Pt and Ti appear, for both compounds. A slabmodel consists of five atomic layers and a vacuum layerthe thickness of which is equivalent to six atomic layers(1.172 nm) is used to represent the surface. 2
ffiffiffi
2p
� 2ffiffiffi
2p
periodicity for lateral direction is assumed to realize a low-coverage situation in order to suppress interaction betweenadsorbate and to analyze unperturbed surface-adsorbateinteraction. The edge lengths of the super-cell are then1.105 nm and 1.953 nm for the lateral and vertical direction tothe surface layer, respectively.
On the other hand, as shown in Fig. 1(b), PtTi3 crystalizesin A15 structure with the lattice constant 0.5033 nm.15) Eachatomic layer normal to [001] direction is composed of one Ptatom and two Ti atoms separated by a half of the latticeconstant. Surface of PtTi3 is also represented by a slab withfive atomic layers and a vacuum layer of 1.006 nm. Forlateral direction of the super cell, 2 © 2 periodicity, meaningthe edge length is 1.006 nm, is assumed for the same reasonmentioned above.
2.2 CalculationsDensity functional calculation together with generalized
gradient approximation16) is carried out using Vienna ab-initio simulation package.17,18) Projector-augmented wavepotentials19,20) are used for describing effective interactionsbetween electrons and ionic cores. Wave-functions areexpanded by plane waves up to a kinetic energy of 274 eV.The Brillouin zone is sampled at 7 © 7 © 1 Monkhorst-Packk-point grid for both Pt3Ti and PtTi3 systems. The plane wave
+1Graduate Student, Chuo University+2Corresponding author, E-mail: [email protected]
Materials Transactions, Vol. 56, No. 4 (2015) pp. 479 to 484Special Issue on Advanced Metallic Materials for Catalysis©2015 The Japan Institute of Metals and Materials
cut-off energy and k-point mesh were checked by comparingthe adsorption energy of several adsorption configurationswith results calculated using 30% higher cut-off energy ordenser k-point grid (10 © 10 © 1). A typical energy differ-ence with the results using higher cut-off energy is 0.01 eV,and the difference with those using denser k-point grid is0.001 eV. A single ethylene molecule adsorb on one of thetwo exposed surfaces with its C=C bond parallel to thesurface. We tried several initial position and orientation forthe ethylene molecule to find out stable adsorption structure.The atomic positions of both ethylene and the surface exceptfor the topmost layer of the non-adsorbed side are relaxedaccording to calculated Hellman-Feynman force with nosymmetry constraint. The ionic relaxation loop was assumedto reach convergence when the change in the total energies ofthe last two steps is smaller than 1meV.
The adsorption energy is defined in this paper as
Eads ¼ Eethylene+surface � ðEethylene þ EsurfaceÞ:Values from literatures are modified along this definition fordirect comparison.
3. Results and Discussions
We listed obtained adsorption structure of ethylene onPt3Ti(001) and PtTi3(001) in Table 1. The letters A-D standfor the adsorption configurations on Pt3Ti plotted in Fig. 2(a),whereas the letters E-K plotted in Fig. 2(b) show adsorptionconfigurations on PtTi3. In these figures, the blue and grayspheres denote Ti and Pt atoms, respectively. Note that thesefigures are provided only for showing approximate positionand orientational configuration of adsorbed ethylene. Calcu-lated values like interatomic distances and bond angles arenot used to draw the ethylene molecules in the figures. Thenumbers in the parentheses after the letters indicating theadsorption sites (A-K) denote angles of the C-C bond tothe [1-10] direction in Pt3Ti and [100] direction in PtTi3,namely the directions of the right arrows in Fig. 2(a) and (b),respectively. The adsorption energy, length of the C-C bond,average value of C-H bond length and adsorption height arealso shown in the tables. Figures 2(c) and (d) are side viewsof B(0) and F(0) adsorption configurations. As found in thesefigures, surface atoms sometimes protrude from the surfacelayer as a result of interaction with the adsorbate. In mostcases ethylene adsorbed with the C-C bond parallel to thesurface, but in some cases it is inclined. The inclinedmolecules are marked by an asterisk after the C-C bond
angle. In these cases, we adopted the length between thetopmost surface atoms and the center of the C-C bond asthe adsorption height. We also found several adsorption
Table 1 Calculated structures and adsorption energies of ethylene onPt3Ti(001) and PtTi3(001) for different adsorption configurations shown inFig. 2(a) and (b).
Site(angle) E(eV) C-C(nm) C-H(nm) C-surface(nm)
Pt3Ti
A(0) ¹0.92 0.137 0.109 0.245
B(0) ¹1.46 0.144 0.110 0.205
C(166) ¹1.99 0.153 0.111 0.171
C(135) ¹1.46 0.155 0.111 0.187
C(45) ¹0.31 0.151 0.111 0.183
D(90)* ¹1.26 0.150 0.110 0.205
PtTi3
E(90) ¹1.58 0.152 0.111 0.186
F(0) ¹0.82 0.147 0.110 0.206
F(90) ¹0.40 0.143 0.110 0.213
G(0) ¹1.38 0.145 0.110 0.181
G(90) ¹0.58 0.137 0.110 0.251
H(0) ¹1.12 0.147 0.110 0.198
H(90) ¹0.45 0.137 0.110 0.255
I(0) ¹2.11 0.155 0.114 0.113
I(90) ¹0.41 0.152 0.112 0.147
K(0)* ¹1.24 0.150 0.110 0.195
K(13)* ¹1.24 0.150 0.110 0.197
K(161)* ¹1.27 0.150 0.110 0.190
Fig. 2 (a), (b): Adsorption structures of ethylene on (a) Pt3Ti and (b) PtTi3.Numbers in parenthesis denote the C-C bond angle to the [1-10] directionin Pt3Ti and the [100] direction in PtTi3. (c), (d): Side views of (c) B(0)and (d) F(0) configurations.
Fig. 1 Crystal structure of (a) Pt3Ti and (b) PtTi3.
K. Fujiwara, Y. Miyawaki, K. Nozawa and Y. Ishii480
structures, where ethylene is slipped off the high symmetryposition, but we do not discuss details of these structures inthis paper, since they seems to be essentially the same as thestructure listed in the table.
Ge and King studied chemisorption of ethylene on Pt(111)by first-principles calculations, and reported that ethylenefavors to adsorb at the bridge site between neighboring Ptatoms.10) The reported adsorption energy, C-H and C-C bondlength are ¹1.26 eV, 0.109 nm and 0.148 nm respectively forthe bridge site, whereas ¹0.55 eV, 0.109 nm and 0.140 nm forthe atop site. Basaran et al. also reported comparable valuesfor the bridge site adsorption.13) The in-plane atomic densitiesof Pt(111), Pt3Ti(001) and PtTi3(001) are different, but inboth Pt3Ti(001) and PtTi3(001) there are Pt-Ti bridge siteshaving comparable atomic separation with the Pt-Pt bridge onPt(111) surface. The Pt-Pt separation of bridge site of Pt(111)is 0.277 nm, whereas the Pt-Ti separation in Pt3Ti(001) andPtTi3(001) surfaces are 0.276 nm and 0.281 nm, respectively.The D(90) configuration in Table 1 represents the Pt-Tibridge adsorption on Pt3Ti(001). Although ethylene was notstabilized at the center of Pt-Ti bridge on PtTi3(001), weobtained similar adsorption structures K(0), K(13) andK(161) adsorbed near the bridge site. The C-C bond ofethylene is not alignment with Pt-Ti direction though, wesuppose that ethylene at these sites on PtTi3(001) arestabilized by essentially the same mechanism with that atthe D(90) site on Pt3Ti(001), because they have comparableC-C, C-H bond lengths and adsorption energy. Theadsorption energy of D(90), K(0), K(13) and K(161) arecomparable to that of the bridge site on Pt(111), but the C-Cdistance of ethylene on both Pt-Ti intermetallic compoundsare largely elongated than that on Pt(111), and gas phaseethylene (0.134 nm). The C-C bond elongation will bediscussed later.
As for atop adsorption: A(0), B(0), E(90), F(0), F(90),one can find that ethylene earns adsorption energy on bothPt-Ti intermetallic compounds than Pt(111) except for F(90)configuration. Moreover, as well the bridge adsorption cases,the C-C bond length of ethylene on both Pt-Ti intermetalliccompound surfaces are longer than that on Pt(111) surfaceexcept for the A(0) configuration on Pt3Ti(001). Interestingly,the magnitude relationship in the adsorption energy and theC-C separation of Pt-atop and Ti-atop sites is the oppositefor Pt3Ti(001) and PtTi3(001) surfaces, meaning Pt-atop (B)configuration has large adsorption energy and C-C separationthan those of Ti-atop (A) configuration on Pt3Ti, but those ofTi-atop (E) configuration is larger than those of Pt-atop (F)configuration on PtTi3. In order to reveal the underlyingreason, we analyze the orbital-decomposed local density ofstates (LDOSs) of the metallic surfaces and ethylene.Figures 35 show the LDOS for A(0) and B(0) configu-rations on Pt3Ti and F(0) configurations on PtTi3 (a) beforeand (b) after the ethylene adsorption. Blue, green and brownlines denote the LDOS of ethylene, surface Pt and Ti atoms,respectively. Note that we only give the LDOS of surfacemetallic atoms, which are closest to the carbon atom inethylene, since in most cases the interaction between ethyleneand the second closest surface atom is not significant becauseof the larger atomic separation. The atomic distance betweenthe carbon and surface atoms are given in Table 2 with the
adsorption energy and C-C distance again. At the site A(0) onPt3Ti, one can find that hybridization has occurred betweenthe LUMO of ethylene and unoccupied d-states of the surfaceTi atom, but it is not so significant due to the large energyseparation between ethylene LUMO and d-states of Ti.The HOMO of ethylene does not strongly interact withunoccupied d-states of Ti due to inconsistency of thesymmetry, and also it does not interact strongly withoccupied d-states of Pt due to relatively large atomic distance(0.339 nm) between them. Main peaks of ethylene molecularorbitals then remain those positions even after the adsorption.On the other hand, in B(0) configuration on Pt3Ti, it is foundthat the occupied states of ethylene including the HOMO arestrongly hybridized with the occupied d-states of Pt, and thehighest two peaks almost disappear after adsorption. Thehybridization is significant especially with dz2�r2 and dzxorbitals of Pt because of the orbital shapes and symmetriesof the occupied states of ethylene and the Pt d-states. Thehybridization between occupied states of ethylene and d-states of surface Pt causes an upward shift of the molecularorbitals. And then a part of molecular orbitals goes above theFermi level (EF), leading a charge donation from ethyleneto surface. On the other hand, the peak of the LUMO ofethylene also disappears after adsorption, implying hybrid-ization between the LUMO and widely distributed unoccu-pied s-states of Pt. This hybridization in this case causes adownward shift of the LUMO below EF, leading a back-donation from surface to ethylene. Because the HOMO andLUMO of ethylene are a bonding and anti-bonding orbitalsof ³-bond between carbon atoms respectively, the donationfrom the HOMO and back-donation to the LUMO makes the³-bond connecting carbon atoms weak, and it leads the C-Cbond elongation.
The opposite trend found in the atop adsorption on Pt3Tiand PtTi3, namely the trend that ethylene is stabilized at thePt-atop in Pt3Ti, but at the Ti-atop in PtTi3, can be understoodas follows. In Pt3Ti, since Pt d-states are widely distributedbelow EF, those hybridize with occupied states of ethyleneefficiently, whereas interaction between unoccupied orbitalsof ethylene and Ti d-states are relatively small, because Tid-states are localized due to the absence of neighboring Tiatoms. Therefore the Pt-atop can be stable than the Ti-atopon the Pt3Ti surface. Reversely, in PtTi3, Ti d-states formsrelatively wider band contrary to the previous case, and inthis case the Ti-atop site can be a stable adsorption site thanthe Pt-atop site. Namely, the d-bandwidth of Pt and Ti, which
Table 2 Surrounding environment of selected adsorption configurations.
Site(angle) 1st neighbor(nm) 2nd neighbor(nm) E(eV) C-C(nm)
Pt3Ti
A(0) 0.255(C-Ti) 0.339(C-Pt) ¹0.92 0.137
B(0) 0.218(C-Pt) 0.306(C-Ti) ¹1.46 0.144
PtTi3
E(90) 0.222(C-Ti) 0.244(C-Ti) ¹1.58 0.152
F(0) 0.219(C-Pt) 0.303(C-Ti) ¹0.82 0.147
F(90) 0.225(C-Pt) 0.379(C-Ti) ¹0.4 0.143
Adsorption Structure and Electronic Structure of Ethylene on Pt3Ti(001) and PtTi3(001) Surfaces: a DFT Study 481
Fig. 5 Orbital-decomposed local density of states of ethylene and surface Pt atom (a) before and (b) after adsorption of the F(0)configuration on PtTi3.
Fig. 4 Orbital-decomposed local density of states of ethylene and surface Pt atom (a) before and (b) after adsorption of the B(0)configuration on Pt3Ti.
Fig. 3 Orbital-decomposed local density of states of ethylene and surface Ti atom (a) before and (b) after adsorption of the A(0)configuration on Pt3Ti.
K. Fujiwara, Y. Miyawaki, K. Nozawa and Y. Ishii482
is mainly determined by the chemical composition, deter-mines the stable adsorption site for ethylene. A majority atomshowing wide bandwidth provides a stable adsorption site inthese compounds. Here, note that only the wider d-bands ofthe majority atom is not essential for stabilization of ethyleneand the C-C bond elongation. If it was, ethylene should gainlarger adsorption energy on Pt(111) than on Pt-Ti interme-tallic compounds. The essential point is that there are twocompletely different elements (Pt and Ti) that the d-bands ofone of them can hybridize strongly with the occupied statesof ethylene and that of the other one can hybridize withunoccupied states of ethylene. Namely, the bimodal densityof states distributing across the EF is essential. In otherwords, combination of the early and late transition metalsproduces the highly active sites for ethylene adsorption. Aswe pointed out above, ethylene earns larger adsorptionenergy on the atop site on both intermetallic compounds thanon Pt(111) except for the F(90) configuration on PtTi3. Thistrend is not found only in the atop sites, but also in the bridgeand hollow sites. This is because ethylene can interact withboth Pt and Ti on these intermetallic compounds at theseintermediate positions.
The effect of the d-bandwidth originated from the chemicalcomposition may be confirmed when comparing the B(0)configuration on Pt3Ti and F(0) configuration on PtTi3. Pt andTi atoms are located as the first and second neighboringatoms for both adsorption sites, respectively. As shown inTable 2 the separations between the adsorption site and thefirst and second neighboring atoms are comparable for bothsites, but the adsorption energies of these sites are quitedifferent. Because the surrounding geometrical arrangementis similar for both sites, the difference should be explained asa difference derived from the electronic structure. Comparingthe d-band structure of the surface Pt atom shown in Fig. 4(a)and Fig. 5(a), it is clearly found that the Pt d-band of theF(0) configuration on PtTi3 has disadvantages for ethyleneadsorption that the narrow band and the position being awayfrom the HOMO level of ethylene.
Although we have paid attention to the first neighboringatoms so far, the effect of the second neighboring atom is alsofind in the obtained results. Pt is the first neighboring surfaceatom for both F(0) and F(90) configurations on PtTi3. In spiteof the small difference in the separation between the Pt atomand these adsorption sites (0.006 nm), the adsorption energyof the F(0) site is twice as large as that of the F(90) site. Sincethe d-band width and its position of the Pt atom is alsoequivalent for both F(0) and F(90) sites, the difference in theadsorption energies is presumably derived from the secondneighboring Ti atoms, the position of it more precisely.
Komatsu et al. reported that both Pt3Ti and PtTi3intermetallic compounds have higher activities than Pttowards the dissociation of hydrogen. Occupied d-states ofPt and unoccupied d-states of Ti supposedly interact with theHOMO and LUMO of the hydrogen molecule as well. Thus,similar with the above discussion, it is expected that thebonding and anti-bonding character of the molecular orbitalsof the hydrogen would be suppressed through interactionwith the bimodal states produced by the Pt-Ti intermetalliccompounds, and it leads the dissociation of hydrogen.Intermetallic compounds composed of early and late
transition elements may be catalytically active for moleculardissociation generally.
Komatsu et al. also reported that Pt3Ti exhibits a higheractivity than Pt towards the hydrogenation of ethylene, butPtTi3 did not catalyze the hydrogenation of ethylene at all.They mentioned that in this case Ti-Pt pair sites are notable to activate both ethylene and hydrogen. As an overalltendency, the adsorption energy of ethylene on both Pt-Tiintermetallic compounds is larger than Pt, but we did not findout any clear evidence showing superiority of Pt3Ti for thehydrogenation of ethylene than PtTi3. Mozo et al. studiedthe adsorption of hydrogen on Pt3Ti surface using the first-principles calculation.2) For the sake of complete under-standing of the catalytic reactions on these intermetalliccompounds, theoretical study for the hydrogen dissociationon these Pt-Ti intermetallic compounds with pre-adsorbedethylene is required.
4. Summary
Adsorption of ethylene on Pt3Ti(001) and PtTi3(001)intermetallic compounds surfaces is studied in terms ofthe density functional calculation. In most cases, ethyleneadsorb with the C-C bond parallel to the surface. For bothintermetallic compounds, adsorption energy of bridge andhollow sites are larger than that of the atop sites. Moreover,obtained adsorption energy and the C-C separations at mostsites on both intermetallic compounds are larger than thosereported for Pt(111). Analyzing the surface LDOSs, it turnsout that the bimodal density of states made by the occupied Ptd-states and unoccupied Ti d-states are effectively interactwith the HOMO and LUMO of ethylene, and then leadingthe elongated C-C bond and large adsorption energy. Largeradsorption energy at bridge and hollow sites is alsounderstood in the same way. As for atop adsorption, acorrelation between the adsorption energy and chemicalcomposition was found, meaning that larger adsorptionenergy is obtained at the Pt-atop site in Pt3Ti, but at the Ti-atop site in PtTi3. The widely distributed d-states of themajority element seem to contribute to stabilize the adsorbedethylene. We mentioned effectiveness of the bimodal densityof states realized in Pt-Ti intermetallic compounds for thehydrogen dissociation reaction reported by Komatsu et al.Intermetallic compounds made of a combination of earlytransition metal and late transition metal can be a goodcatalyst for molecular dissociation in general. However, noclear evidence indicating the higher catalytic activity of Pt3Tithan PtTi3 and Pt in the ethylene hydrogenation is found outin this study.
Acknowledgments
K.N. thanks S. Kameoka for fruitful discussions andsuggestions. Calculations were carried out using computerfacilities at National Institute for Materials Science (Tsukuba,Japan) and Yukawa Institute, Kyoto University. This workwas partly supported by Grant-in-Aid for Scientific Researchfrom the MEXT, Japan (grant number 24360329 and22540335), and by the Cooperative Research Program of“Network Joint Research Center for Materials and Devices”.
Adsorption Structure and Electronic Structure of Ethylene on Pt3Ti(001) and PtTi3(001) Surfaces: a DFT Study 483
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