ARTICLE pubs.acs.org/JPCC
Density Functional Theory Study on Propane and Propene Adsorption on Pt(111) and PtSn Alloy Surfaces L. Nyk€anen and K. Honkala* Department of Chemistry, Nanoscience Center, P.O. Box 35, University of Jyv€askyl€a, FIN-40014 Jyv€askyl€a, Finland
bS Supporting Information ABSTRACT: Density functional theory calculations were performed to investigate the adsorption of propane, propene, and C and H atoms on Pt and PtSn surfaces employing the revised PerdewBurkeErnzerhof (RPBE) and vdW-DF functionals. Propane adsorption was found to be mediated by van der Waals interactions without significant site preference on any of the studied surfaces. The adsorption characteristics of propene are different: On the Pt(111) and Pt3Sn(111) surfaces, propene adsorption is covalent, and the molecule prefers a di-σ site to a π site. Alloying Pt(111) with Sn leads to weaker adsorption owing to geometric and relaxation effects, whereas electronic effects are found to be small. On the PtSn2(111) surface, propene adsorption is weak and dominated by van der Waals interactions. Our calculations show that addition of Sn leads to unfavorable geometric and electronic effects on the adsorption of carbon and hydrogen atoms. The impact of alloying with Sn on the selective propane dehydrogenation to propene is discussed.
’ INTRODUCTION Light alkenes, such as propene, have been mostly produced as a byproduct of other processes. However, owing to the increased need for propene, on-purpose production technologies have been recently developed, employing methods such as dehydrogenation (DHP) and oxidative dehydrogenation of propane. Propene production via DHP is an equilibrium limited, highly endothermic process favoring high reaction temperatures and low pressures.1 Both Pt and Cr show promising performance for the DHP reaction,2,3 but alloying Pt with Sn has been found to improve the stability and selectivity of catalysts.4 The properties of PtSn catalysts supported on different carriers have been addressed in various experimental studies.2,512 They show that addition of Sn results in higher Pt dispersion, decreases the size of Pt ensembles, and suppresses hydrogenolysis. The role of Sn in coke-forming reactions is controversial: both increases and decreases have been reported in the literature.6 Also, other factors like support, acidity of catalyst, addition of promoters, and interactions between the metal and the support affect coking. In general, PtSn catalysts have been found effective for various reactions involving hydrocarbons.1315 At ultrahigh vacuum, molecular beam techniques and temperature-programmed desorption (TPD) have been applied to study propane trapping on a single-crystal Pt(111) surface.16,17 The results indicate that propane physisorbs on Pt(111), which is compatible with the fact that propane is a saturated compound and has no unpaired electrons. The desorption temperature is found to be 176 K. Recent density functional theory (DFT) calculations applying the PerdewBurkeErnzerhof (PBE)18 functional gave almost thermoneutral propane adsorption with no preference to any particular site on Pt(111) and unaltered CC and CH bond lengths upon adsorption which indicate physisorption.19 r 2011 American Chemical Society
The one double bond in propene makes it an active species, which can chemisorb and react on Pt(111). LEED experiments20 and DFT calculations21 assign propene adsorption to a di-σ configuration. The RAIRS spectra reveal complex adsorption behavior on Pt(111): At low coverage, an asymmetric di-σ adsorption geometry is identified with a tilted methyl group, whereas at higher coverage a symmetric di-σ adsorption configuration develops with two more similar PtC bonds and a methyl group sticking up to vacuum. A π-bonded species is seen only at exposures exceeding the monolayer coverage, which leads to the formation of the second, more weakly held layer.22 From sticking coefficient measurements, propene saturation coverage was determined to be 0.2 ML at 150 K. The TPD studies of Tsai et al. give the desorption temperature 284 K, which corresponds to the desorption energy of 0.75 eV,23 whereas earlier experimental studies reported the value of 0.5 eV.24 The large variety of calculated adsorption energies have been reported for propene,19,21,25,26 and they range from 0.93,19 0.90,25 to 0.5 eV.26 The variation can be assigned to different computational details and surface coverages. We note that special care should be taken when calculated and measured values are compared. This is because propene dehydrogenation and conversion to propylidyne may take place before desorption.22 One should also note that DFT adsorption energies mentioned above are not zero-point corrected, and they do not include entropic corrections. The experimental desorption energies are derived from TPD measurements using the Redhead analysis. This results in considerable uncertainty as the prefactor in the Redhead analysis is commonly unknown. Received: December 22, 2010 Revised: March 18, 2011 Published: April 22, 2011 9578
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√Tsai et√al. have studied propene adsorption on p(2 2) and ( 3 3)R30° surface alloys which correspond to surface compositions Pt3Sn and Pt2Sn, respectively.23 The TPD studies show two effects. First, the desorption temperature of the chemisorbed state shifts down with increasing Sn content. A similar trend has been observed for ethene and cyclobenzene desorption.27 Second, the amount of desorbed propene increases on alloy surfaces compared to Pt(111). This is observed together with a dramatic decrease in H2 desorption on PtSn surface alloys, which is explained to be the consequence of the suppression of propene decomposition. To our knowledge no DFT calculations dealing with propane or propene adsorption on PtSn alloys exist. However, the adsorption of other hydrocarbons, including cyclopentene,27,28 ethene,29,30 aldehyde prenal,31 and butene,32 on PtxSny surface alloys has been addressed by means of DFT calculations. For ethene, the calculations give a significant decrease in adsorption energy on PtSn surface alloys, but the adsorption geometries and CC bond lengths are altered only slightly. A closer look on interaction energies, that is, the adsorbatesurface bond strength, reveals that surface deformations play a significant role in weakening adsorption, whereas the PtC bond strength changes only slightly. As a saturated compound, propane is not able to form a chemical bond to Pt(111), and both experimental findings and DFT calculations indicate that the propanePt(111) interaction is dominated by van der Waals interactions responsible for physisorption.16,19 However, in the previous calculations the role of vdW forces has not been addressed.19 Recently, there has been considerable advances in the implementation of nonlocal correlations, i.e., vdW forces, to DFT. Owing to this, vdW-DFT calculations can now be performed self-consistently with computational cost close to that of general gradient approximation (GGA) calculations.33 The exchange correlation functional incorporating vdW forces, vdW-DF, is of the form ½n ¼ EGGA ½n þ EvdW-DF ½n EvdW-DF xc x c
ð1Þ
where ExGGA[n] is a GGA exchange energy functional and EcvdW-DF[n] is a correlation energy functional splitting into shortand long-range parts. The short-range correlation is treated with the local density approximation (LDA), and the long-range correlation is treated with a nonlocal functional of the form Z 1 drdr0 nðrÞφðr, r0 Þnðr0 Þ ½n ¼ ð2Þ Enl c 2 where φ(r, r0 ) is a densitydensity interaction kernel depending on r r0 , the electronic density n, and the magnitude of its gradient at the points r and r0 .33 The vdW-DF functional has been applied to adsorption of graphene,34 pentacene,35 benzene,36 butane,37 and water.38 In these studies, the vdW-DF results have been compared to those calculated with the PBE and revised PBE (RPBE)39 GGA functionals. For a water bilayer, the PBE functional gives adsorption energy close to the experimental value. However, the energy is right for the wrong reasons, and therefore the adsorption structure is incorrect. The vdW-DF functional provides both the correct overlayer structure and adsorption energy. For the other systems mentioned above, PBE and RPBE functionals predict adsorption energies close to zero in contrast to measured values, and in all cases the vdW-DF functional increases adsorption strength considerably. For example, for butane on Pt(111) the measured desorption energy is 0.53 eV,40
whereas the PBE functional gives 0.06 eV41 and the vdW-DF value is 0.5 eV.37 In this study, we investigated the adsorption of propane, propene, C, and H on the 111-surfaces of Pt, Pt3Sn, PtSn2, and the Pt3Sn/Pt(111) surface alloy using RPBE and vdW-DF functionals self-consistently. Our aim is to shed light on the influence of Sn on the adsorption properties of the abovementioned species and to discuss their relevance for the performance of PtSn catalysts for DHP.
’ COMPUTATIONAL METHODS The calculations were performed with the GPAW code.42 Two different methods were applied to describe exchange and correlation effects: the RPBE and vdW-DF functionals. For the exchange part of the vdW-DF, the revPBE exchange functional was used,43 and the correlation was described according to eqs 1 and 2. The vdW energies arise from electronelectron correlations that depend not only on atomic species but also on their chemical environment, which is taken into account in the vdW-DF functional as it determines correlations from electron densities.44 The GPAW uses a projector-augmented wave method to describe core electrons, and wave functions, electron densities, and potentials are represented on grids. The grid spacing was set to 0.2 Å in all directions of the supercell and applied throughout the work. The lattice constants were determined from bulk calculations, and they are 4.04, 4.10, and 6.63 Å for Pt, Pt3Sn, and PtSn2, respectively. The corresponding experimental values are 3.93,45 4.00,46 and 6.43 Å,47 respectively. The periodic boundary conditions were applied in the surface plane, while the perpendicular direction was kept nonperiodic. To prevent artificial quenching of wave functions, owing to the nonperiodic boundary conditions that force the wave functions to vanish at the boundary, it is important to use thick enough vacuum layers above and below the slab. In this work, the vacuum of 10/6 Å was applied above/ below the slab. For the GGA(vdW) calculations, 2 4 1 (3 5 1) MonkhorstPack k points were employed for Pt and Pt3Sn slabs and 2 2 1(3 4 1) for the PtSn2 slabs. The convergence was tested by calculating adsorption energies with different k point samplings. The atomic positions were optimized self-consistently with both functionals until residual forces for each atom were below 0.05 eV/Å. Decreasing the criterion to 0.01 eV/Å changed adsorption energy less than 0.02 eV. Adsorption energies were calculated according to EAds ¼
EA þ M ðNEA þ EM Þ N
ð3Þ
where EAþM stands for the total energy of the adsorbate and the slab; EA is the energy of gas-phase adsorbate; EM refers to the energy of the clean surface; and N is the number of adsorbates in the supercell. According to this definition, negative EAds corresponds to an exothermic reaction. The gas-phase propane (C3H8) and propene (C3H6) were relaxed in a nonperiodic supercell surrounded by more than 6 Å of vacuum to the cell boundary. The gas-phase species were calculated spin polarized and employing the Hund rule. The inherent accuracy of RPBE DFT calculations is of 0.20.3 eV.39 The RPBE functional does not contain nonlocal correlations, and thus it can not describe vdW interactions. Systematic testing of the vdW-DF functional for covalent adsorption is missing as the functional is mainly 9579
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Figure 1. Top and side views of (a) Pt(111), (b) Pt3Sn/Pt(111), (c) Pt3Sn(111), and (d) PtSn2(111). The light gray circles are Pt atoms, and the dark green circles are Sn atoms. The unit cell is drawn with a black dashed line.
designed to depict vdW interactions for which the error has been reported to be less than 0.15 eV.48 The surface deformations are known to play an important role in adsorption on alloy surfaces, and large relaxation of bonding metal atoms has been found on PtSn surfaces.30,49 To analyze the strength of the adsorbatesurface bond, one can look at the interaction energy, defined as EInt ¼ EAds Edef , surf Edef , mol
Table 1. Layer Separations in Bulk Pt and Pt3Sn and Surface Relaxations Relative to the Bulk Valuesa RPBE
where EAds stands for the adsorption energy and Edef,surf and Edef,mol are the surface and the molecule deformation energies, respectively. These are obtained by calculating the energy difference between the frozen geometry after adsorption and the ideal gas-phase geometry. To evaluate equilibrium adsorption heights of adsorbates, we calculated the difference between the average height of surface atoms and the average height of C atoms in the molecule. A Bader analysis was employed to study electron transfer between Pt and Sn. In the Bader method, atoms are separated by zero flux surfaces, which are surfaces on which the charge density is minimum perpendicular to the surface.50 We found out that charges are independent of calculation parameters, i.e., k-points, grid spacing, slab thickness, and surface relaxations.
’ MODEL SURFACES We selected Pt(111), Pt3Sn(111), PtSn2(111), and Pt3Sn/ Pt(111) surfaces to model various PtSn compositions present in industrial catalysts. The structures are given in Figure 1. Among the stable PtSn alloys, Pt3Sn has the lowest amount of Sn being stable up to 1640 K,51 whereas PtSn2 has a higher Sn concentration and is stable up to 1021 K. There are several computational studies of molecular adsorption on Pt3Sn(111) and Pt3Sn/Pt(111) surface alloys.2729,31,52 Here we compare adsorption on these surfaces, which share the same surface layer but differ in lattice constant and in the composition of the subsurface layers. The PtSn2(111) model surface has a Pt:Sn ratio close to the value that is applied in preparation of supported bimetallic catalysts for experiments, but we note that even higher Sn concentrations can be employed.57,11 Pt(111) and Pt3Sn(111) surfaces were modeled with orthogonal four-layer slabs, each
XC functional
dbulk
d(1,2)
d(2,3)
d(1,2)
d(2,3)
Pt(111)
2.33
0.94
1.25
2.43
0.24
1.65
0.76
5.44
0.98
0.52
0.79
2.83
2.92
Pt3Sn/Pt(111)
ð4Þ
Pt3Sn(111) a
vdW-DF
2.37
The bulk separations are in Å, and the relaxations are in percent.
layer containing 2 4 atoms, and the two lowest layers were fixed to their ideal bulk positions. The PtSn2(111) was modeled with an orthogonal six-layer slab, 2 2 atoms in a layer, with the three lowest layers fixed. has two different terminations, p(2 2) and √Pt3Sn(111) √ ( 3 3)R30°, of which p(2 2) was proved to be the more stable one.53 The Pt3Sn/Pt(111) surface alloy was constructed by replacing every fourth Pt with Sn in the surface layer. On Pt3Sn(111) (Pt3Sn/Pt(111)) surface Sn atoms are 0.19 (0.15) Å higher than the Pt atoms. The measured value for outward buckling is 0.22 Å.54 The surface relaxations for Pt(111), Pt3Sn/ Pt(111), and Pt3Sn(111) are presented in Table 1. With RPBE, the Pt(111) surface contracts the most, the Pt3Sn(111) contracts a little less, and the Pt3Sn/Pt(111) expands as a Sn atom has a larger covalent radius than a Pt. PtSn2(111) is an intermetallic compound with a fluorite crystal structure. No discussion on the surface composition of PtSn2 was found in the literature. However, materials that have the fluorite crystal structure prefer a 111-surface, which has three different possible bulk terminations for which we calculated surfaces energies. The side views of all terminations are presented in Figure 2. The most stable termination is the one labeled A, in agreement with other fluorite crystal materials.55 Termination A has a surface energy of 25 meV Å2, and the terminations B and C have surface energies of about 50 meV Å2. In bulk PtSn2 the separation of Pt and the Sn planes in the 111 direction is 0.96 Å, and the separation of Sn planes is 1.92 Å. In RPBE(vdW-DF) calculations, surface relaxation increases the separation of a surface Sn layer (Pt layer) by 12.8(23.0)%, decreases the separation of 9580
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Table 2. C Adsorption Energies (in eV) on the Pt(111), Pt3Sn/Pt(111), and Pt3Sn(111) Surfaces XC functional site
Figure 2. Three different bulk terminations of PtSn2(111) depicted from the side (A, B, and C) and the most stable structure (A) depicted from the top. In the top view, the dark green circles are surface Sn atoms, gray circles Pt atoms, and the light green circles Sn atoms below the Pt layer.
the Pt layer and the second Sn layer by 17.6(18.8)%, and again increases the separation of the second and the third Sn layers by 1.91(6.54)%. To rank the activities of different surfaces, the d-band centers relative to Fermi energy were determined. We obtained the following values for the surface layer Pt atoms: 1.54, 1.71, 1.87, and 2.13 eV for Pt(111), Pt3Sn/Pt(111), Pt3Sn(111), and PtSn2(111), respectively. In agreement with earlier theoretical studies, alloying Pt with Sn leads to a downshift of the d-band center and depletion of Pt d states at the Fermi level,56 which is observed also in STM experiments.57 The Bader charge analysis indicates increased electron density at Pt atoms induced by charge transfer from Sn to Pt atoms. On Pt3Sn/Pt(111) the Bader charges are 0.94 and 0.26 e for Sn and surface Pt atoms, respectively. The corresponding charges on the Pt3Sn(111) surface atoms are 0.92 and 0.37 e. On PtSn2(111), the atoms in the topmost Sn and Pt layers and in the second Sn layer have charges of 0.51, 1.13, and 0.60 e, respectively.
’ RESULTS AND DISCUSSION Adsorption Energies. Carbon and Hydrogen. We start with a brief discussion on adsorption of carbon and hydrogen atoms. The adsorption properties of hydrogen are important both in dehydrogenation and hydrogenation reactions as in both cases there are hydrogen atoms present at the surface of a catalyst. A major problem with propene dehydrogenation is coke formation. Direct modeling of coke is not feasible, so we selected atomic carbon as our model system. This is justified because the adsorption energy of carbon has been shown to describe well the bonding of carbonaceous species.58 In spectroscopic experiments, propene dehydrogenation on Pt(111) was observed to produce propylidyne, CCH2CH3,59,60 which binds to a hollow site on the surface via a terminal carbon. The observation was confirmed computationally by Valcarcel et al.25 It is reasonable to anticipate that the adsorption properties of propylidyne are also dominated by a carbon atom. Atomic adsorption does not depend on vdW interactions, and hence we can compare how the two functionals, RPBE and vdW-DF, describe covalent bonds. We calculated C and H adsorption energies on various sites over all model surfaces (Tables 2 and 3). On Pt(111) C favors an fcc site instead of hcp and top sites. Our adsorption energies are between those of Yang et al.19 and Ford et al.61 who report somewhat stronger
RPBE fcc
hcp
vdW-DF top
fcc
hcp
top
Pt(111)
6.77
6.63
4.57
6.46
6.30
4.50
Pt3Sn/Pt(111)
5.91
5.83
4.07
5.67
5.60
4.05
Pt3Sn(111)
5.93
5.75
4.13
5.69
5.42
4.12
and weaker adsorption than us. The differences are mainly due to different computational setups like the employed GGA functional, the size of the supercell, and relaxation of surface atoms. Various hollow sites present on Pt3Sn(111) are depicted in Figure 3. All surface atoms forming a hollow site can be Pt, but then the atom beneath is Sn. The second possibility is that one of the atoms forming a hollow site is Sn. A carbon atom clearly prefers hollow sites without Sn: adsorption on a Pt-only fcc site is 0.6 eV stronger than on an fcc site containing Sn. This reduces a number of sites available for carbon and species that adsorb via a single carbon atom, and it is a manifestation of a geometric effect of alloying. The comparison of adsorption energies on Pt(111) and Pt3Sn(111) surfaces shows that the presence of Sn weakens the adsorption by 0.9 eV. This is an electronic effect, and it is consistent with the d-band model which shows lower activity for the alloy surface.62,63 On PtSn2(111), C adsorbs deep between two Sn atoms as shown in Figure 4. The relevant distances are CPt = 2.0 Å and CSn = 2.3 Å. Adsorption energy is 4.33 eV, which is considerably weaker than on Pt3Sn(111), and again in agreement with the d-band model. Our calculations show that the formation of a Pt-terminated surface from a Pt3Sn(111) surface by switching atoms between the first and the second layer costs 1.7 eV per atom—similarly, the formation of a Sn-terminated surface costs 0.6 eV per atom. We investigated the adsorption of C on the Pt-terminated surface to test the possibility of adsorbate-induced segregation of Pt to the surface. The Pt-terminated surface adsorbs C 0.1 eV stronger than the pristine Pt3Sn(111) surface at 0.13 ML. However, the energy gain is negligible compared to the cost due to segregation. Adsorbed H experiences a very flat potential energy surface on Pt(111) as shown in previous calculations.19,61 We obtain slightly weaker adsorption compared to literature values, and this can be assigned to different computational setups. Alloying Pt with Sn has a minor effect on adsorption energies over the Pt3Sn(111) surface indicating a weak electronic effect for hydrogen. We note that H adsorption on hollow sites containing Sn on Pt3Sn(111) is unstable, and upon structure optimization the atom relaxes to a Pt-only hollow site; thus, Sn introduces also a geometric effect for hydrogen. A drastic change is seen on PtSn2(111), where the only stable geometry is at the top of the Pt atom with the adsorption energy of 2.16 eV. For comparison, the adsorption energies were calculated with the vdW-DF functional. The obtained values are compared with the GGA values employing the equation ! EvdW ads 1 100% ð5Þ ΔE ¼ EGGA ads GGA where EvdW ads and Eads are adsorption energies for a particular adsorption site calculated with the vdW-DF and RPBE
9581
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Table 3. Adsorption Energies (in eV) of H on the Pt(111), Pt3Sn/Pt(111), and Pt3Sn(111) Surfaces XC functional site
RPBE
Table 4. Adsorption Energies of Propane on the 111 Surfaces of Pt, Pt3Sn, and PtSn2a
vdW-DF
XC functional
fcc
hcp
top
fcc
hcp
top
coverage
RPBE
vdW-DF
1/4
1/8
1/4
1/8
Pt(111)
2.65
2.60
2.61
2.68
2.62
2.75
Pt(111)
0.09
0.00
0.37
0.34
Pt3Sn/Pt(111)
2.60
2.52
2.51
2.64
2.58
2.67
Pt3Sn/Pt(111)
0.14
0.02
0.34
0.39
Pt3Sn(111)
2.56
2.75
2.63
2.61
2.76
2.72
Pt3Sn(111) PtSn2(111)
0.12 0.02
0.08 0.02
0.33 0.28
0.28 0.24
a
Coverages are given in ML and adsorption energies in eV.
Table 5. Adsorption Heights, h, of Propane in Å hRPBE coverage
Figure 3. fcc and hcp adsorption sites on the Pt3Sn(111) surface. The light gray circles represent Pt atoms and the dark green circles Sn.
hvdW-DF
1/4
1/8
1/4
1/8
Pt(111)
4.59
4.49
4.53
4.29
Pt3Sn/Pt(111)
4.47
4.67
4.26
4.17
Pt3Sn(111)
4.54
4.54
4.34
4.33
PtSn2(111)
4.19
4.20
3.99
4.01
Figure 4. Adsorption structure of C on PtSn2(111) depicted from the top (on left) and from a tilted angle (on right). The light gray circles represent Pt atoms and the dark green circles Sn.
functionals. For both H and C, the maximum variation of ΔE is ∼6% and indicates only a minor impact of the functional employed. This is supported with the C(H)Pt bond lengths, listed in Tables S2 and S3 (in Supporting Information), which show small differences so that the vdW-DF bond lengths are from 0.1% to 1.1% longer than the corresponding RPBE values. Differences between the results obtained with the two functionals are not trivial. The adsorption energies of C (H) calculated with the vdW-DF are less (more) exothermic than the ones calculated with the RPBE. Moreover, the ratio of topsite binding to hollow-site binding is greater with the vdW-DF than with the RPBE. Propane. The adsorption energies of propane on all model surfaces calculated with the RPBE and the vdW-DF functionals at 0.25 and 0.13 ML coverages are summarized in Table 4. The results show that propane does not favor any particular site on the surfaces, which is seen as a small variation in adsorption energy from site to site. In agreement with the previous DFT calculations, we found propane adsorption on Pt(111) to be nearly thermoneutral with the RPBE functional.19 This is also the case on the PtSn alloy surfaces studied. The equilibrium adsorption heights are given in Table 5, and they are rather large varying between 4.2 and 4.7 Å. The propane adsorption geometry is identical to the optimized gas-phase structure (see Table S1 in Supporting Information). Figure 5 displays the local density of states (LDOS)
Figure 5. Local density of states plot for propane adsorption on Pt(111) calculated with the RPBE functional. The states are projected on the d-orbitals of Pt and the s- and p-orbitals of C, and they are relative to Fermi energy. The plots labeled with an asterisk are from the system where propane is adsorbed on the surface, and the other two are from the isolated species. For Pt only the states of the surface layer atoms are presented, and for propane only the states of C atoms are presented.
for propane on the Pt(111) surface. No hybridization between propane and Pt states is seen. Thus, all the analyses suggest that the propanesurface interaction is noncovalent. This is not surprising as in propane all valence electrons are taking part in single bonds, and partial charges are very small due to similar electronegativities. However, adsorption should be exothermic as the TPD experiments show nondissociative adsorption on Pt(111) below 176 K, which corresponds to the desorption energy of 0.44 eV.16 To include dispersion interactions in the calculations we employed the vdW-DF functional. With this functional, the adsorption energies range from 0.24 to 0.39 eV for the studied surfaces. On Pt(111), the adsorption energy is 0.37 eV at 0.25 ML coverage in good agreement with the experimental result. Slightly weaker adsorption is seen at 0.13 ML coverage indicating attractive interactions between propane molecules. This is supported by molecular beam experiments which show increased sticking at higher coverage.16 On average, vdW-DF computed adsorption heights are 0.2 Å smaller than those obtained with RPBE, but only minor modifications are seen in 9582
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Table 6. Propene Adsorption Energies on the Pt(111), Pt3Sn(111), Pt3Sn/Pt(111), and PtSn2(111) Surfaces with Two Different Functionalsa XC functional coverage
vdW-DF
1/4
1/8
1/4
1/8
0.42
0.51
0.75
0.83
0.05
0.07
0.41
0.48
Pt3Sn(111)
0.06
0.24
0.44
0.54
PtSn2(111)
0.03
0.02
0.29
0.23
Pt(111) Pt3Sn/Pt(111)
a
RPBE
Coverages are given in ML and adsorption energies in eV.
Figure 7. Interaction energies of the best adsorption structures of propene on Pt(111), Pt3Sn/Pt(111), and Pt3Sn(111) plotted against the d band centers of the respective surfaces.
Table 7. Interaction and Deformation Energies (in eV) for Propene Adsorption on the 111 Surfaces of Pt, Pt3Sn, and Pt3Sn/Pta RPBE XC functional
a
Figure 6. Propene adsorption geometries on (a) Pt(111), (b) Pt3Sn(111), and (c) PtSn2 at 0.13 ML coverage. Pt atoms are colored light gray and Sn atoms green, whereas dark gray stands for C and white for H.
CC bond lengths and in a bond angle. The LDOS plot (not shown) is similar to the one given in Figure 5. Propene. The adsorption energies of propene on different surfaces and functionals at 0.25 ML and 0.13 ML coverages are collected in Table 6, whereas geometrical details and a schematic picture representing the adsorption system are in Table S4 and Figure S1 (in Supporting Information), respectively. First, propene adsorption on Pt(111) and Pt3Sn(111) surfaces is presented which is followed by adsorption on PtSn2(111), and finally the relevance of the results for the DHP reaction is discussed. On the Pt(111), Pt3Sn/Pt(111), and Pt3Sn(111) surfaces (Figure 6), propene prefers adsorption on a di-σ site to a π configuration, which is 0.2 eV less favorable than the di-σ geometry. The propene adsorption on the PtSn di-σ site is unstable, and the molecule moves to a PtPt di-σ site during the relaxation of the atomic structure. The RPBE-computed adsorption energy on Pt(111) is 0.42 eV, which differs somewhat from the experimentally determined desorption activation energy, 0.75 eV, obtained from the TPD data.23 The covalent nature of the bonding can be seen from a LDOS plot in Figure S2 (in Supporting Information) where the states of propene have hybridized with the states of Pt atoms. Earlier computational studies have reported adsorption energies from 0.93,19 0.90,25 to 0.5 eV.26 The more exothermic adsorption energies were obtained with the PBE functional, whereas the value close to our result is from an RPBE calculation. It is well-known that the PBE functional
Eint
Edef,surf
vdW-DF Edef,mol
Eint
Edef,surf
Edef,mol
Pt(111)
2.42
0.20
1.74
2.60
0.19
1.58
Pt3Sn/Pt(111)
2.29
0.40
1.82
2.52
0.38
1.70
Pt3Sn(111)
2.25
0.2
1.74
2.46
0.26
1.66
The coverage is 0.13 ML.
gives stronger adsorbatesurface interaction than RPBE.39 For propene on Pt3Sn/Pt(111), RPBE gives almost thermoneutral adsorption which is in strong contrast to the activation energy of desorption, 0.6 eV, determined from TPD.23 While the difference in calculated and measured values on Pt(111) is within the accuracy of DFT, this is not the case on Pt3Sn/Pt(111). The possible explanations include the shortcomings of the RPBE functional or the Redhead analysis. It is also possible that obtained energies are correct, and the desorption process is activated. Upon adsorption on the di-σ site on Pt(111), the CdC double bond elongates from the gas-phase value 1.34 Å to 1.50 Å, which is close to the CC bond length in a gas-phase propane. This together with the fact that hydrogens and a methyl radical attached to the CdC skeleton are no more in plane indicate the rehybridization of carbon from sp2 to sp3. The comparison of calculated CC and PtC bond lengths on Pt(111), Pt3Sn/ Pt(111), and Pt3Sn(111) surfaces shows only minor variations. The bond angle, φCCC, decreases considerably from the gasphase value being approximately 117 (115)° for an adsorbed species at 0.13 (0.25) ML coverage. The rehybridization of the C atoms in the double bond and the decreasing bond angle, φCCC, with increasing coverage are also observed in RAIRS measurements.22 The change in the bond angle was assigned to the denser packing of molecules. The geometrical details indicate that the propene adsorption on Pt(111) and Pt3Sn(111) bulk and surface alloy surfaces is covalent despite the fact that RPBE gives almost thermoneutral adsorption on the latter two. Identical bond lengths indicate that PtC interaction has a similar nature on these surfaces. This is also supported by the interaction energies presented in Figure 7 and Table 7. Usually, adsorption energies show a linear dependence on the d-band center. However, the propene on the Pt3Sn/ Pt(111) and Pt3Sn(111) surfaces does not follow the expected trend as the bulk alloy surface binds propene more strongly than the surface alloy. The strong decrease in calculated adsorption 9583
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The Journal of Physical Chemistry C energies contrasts the TPD temperatures, which show only a 40 K decrease when going from a Pt(111) to a (2 2) PtSn/ Pt(111) surface alloy surface. The interaction energy is plotted against the d-band center in Figure 7 showing a linear dependence such that the weakest interaction is on the Pt3Sn(111) surface as expected. We note that interaction energy displays only minor changes with alloying varying from 2.42 to 2.25 eV. A similar trend has been reported for ethene and cyclopentene on Pt(111) and PtSn surface alloys with interaction energies close to 2.5 eV and variation between surfaces less than 0.2 eV.30 For comparison, we calculated the interaction energy for π-site adsorption, which is approximately 1.0 eV reflecting a different bonding mechanism. The corresponding value for ethene is 1.3 eV.49 Adsorption and interaction energies differ in deformation energies, that is, the energetic cost of distortion due to the interaction. Table 7 gives the deformation energies for propene adsorption at 0.13 ML coverage. The values range from 0.2 to 0.4 eV for surfaces and from 1.6 to 1.8 eV for propene. Adsorption on the Pt3Sn/Pt(111) surface alloy induces the largest deformations, whereas adsorption on Pt(111) leads to the smallest ones, which explains why adsorption energies do not depend linearly on the d-band center. A good correlation of d-band centers with interaction energies has been reported earlier for aldehydes on Pt(111) and PtSn surface alloys.64 We also calculated propene adsorption energies on Pt(111) and Pt3Sn(111) surfaces with vdW-DF. They are 0.20.4 eV more exothermic than the corresponding RPBE values. The average PtC bond lengths increase 0.015 Å, and the interaction energies are ∼0.2 eV stronger. It is possible that the interaction contains a dispersion component, but we expect that the increase is mainly due to the different description of short-range correlations in the two functionals employed: the vdW-DF functional describes the short-range correlation with LDA and not with the RPBE correlation functional. As the exchange parts of the functionals are basically identical, they can not explain the observed difference. The comparison of adsorption energies listed in Table 6 indicates that the significant decrease in adsorption strength from Pt(111) to Pt3Sn/Pt(111) does not depend on the functional employed in this study. Non-self-consistent PBE calculations (not reported here) reproduce the trend. However, the change observed in the experimental desorption activation energies is very modest. This could imply that either the prefactor used in the Redhead analysis varies from one surface to another or the desorption process is activated. The difference in RPBE and vdW-DF interaction energies is smaller than the corresponding difference in adsorption energies. This is assigned to the description of the propene molecule: the vdW-DF functional gives smaller deformation energy than RPBE. Propene adsorption on a PtSn2(111) surface is different. Figure 2 shows that the di-σ sites between two PtPt atoms are missing from the most stable termination. The RPBE calculations give endothermic adsorption without any site preference. There are π sites available on the surface, but Pt atoms reside deeper in the surface than Sn atoms. The equilibrium adsorption height is ∼4.2 Å, whereas in the case of Pt and Pt3Sn surfaces it is much smaller being only 2.5 Å. The analysis of the geometrical details shows that the CC bond lengths and the bond angle stay intact upon adsorption, and the average PtC bond length is 5.15 Å. These facts speak for the interpretation that propene adsorption is governed by dispersion forces on PtSn2(111). This is further supported by the LDOS plot (not shown here) which does not show changes in either propene or alloy states. The
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vdW-DF calculations give 0.29 eV for adsorption energy, and the nonlocal correlation pulls the adsorbate closer to the surface decreasing the equilibrium adsorption height by 0.2 Å. The CC bond lengths are 1.34 and 1.51 Å, and the bond angle, φCCC, is 125°, which correspond well with the gas-phase values irrespective of the applied coverage or the functional. For the DHP reaction to be efficient, propene should desorb from the catalyst surface before further dehydrogenation. On PtSn2(111), the question arises whether the propane dehydrogenation to propene takes place at all. Both reactant and product molecules interact only with the surface, and the LDOS analysis shows no changes in the electronic structure. Thus, the impact of the surface on these molecules is negligible. The analysis of TPD spectra reveals strong suppression of propene dehydrogenation on Pt3Sn alloy surfaces: the activity drops from 56% on Pt(111) to below 3% on (2 2) Pt3Sn(111).23 On the basis of adsorption energies we conclude that enhanced reactivity for selective dehydrogenation on PtSn alloy catalysts compared to Pt-only catalysts could be related to the weaker adsorption. To assess if propene desorption and CH bond breaking are competitive reactions, a complete study of the reaction pathways would be needed. This is because one can assume that weaker adsorption also indicates higher activation energy for dehydrogenation65 which could be sufficient to change the branching fraction for desorption and dehydrogenation reactions and lead to suppressing propene dehydrogenation on PtSn catalysts. The other possible factors that can contribute to increased selectivity are a reduced number of certain Pt geometries to accommodate propylidyne, the effect of alloying on a dehydrogenation reaction mechanism, or suppression of dehydrogenation due to hindered hydrogen recombination and desorption.
’ CONCLUSIONS The compositions and properties of PtxSny bimetallic catalysts are complex and depend on Pt:Sn ratio but also on a carrier employed, a preparation process, and reaction conditions. Despite that these aspects are excluded from the present study, they should be taken into account when theoretical results are compared to the experimental ones. In accordance with previous calculations, our DFT results show that alloying Pt with less reactive metal Sn shifts the d-band center further away from the Fermi level. The d-band center decreases with increasing Sn concentration, and this indicates that the activity decreases in the following order Pt(111) > Pt3Sn/Pt(111) > Pt3Sn(111) > PtSn2(111). The Bader charge analysis shows notable electron transfer from Sn to Pt on bulk alloy surfaces as previously seen for surface alloys. The electron transfer indicates that in PtSn alloys Pt atoms become electron rich compared to the plain Pt(111) surface. Applying both RPBE and vdW-DF functionals the adsorption of propane, propene, C, and H atoms was investigated on the above-mentioned model surfaces. The calculations show that alloying Pt with Sn introduces clear geometric and electronic effects for carbon, whereas for hydrogen the geometric effect of Sn is present, but the electronic effect is weak on Pt3Sn surfaces. All our results support the interpretation that propane only physisorbs on these surfaces. The adsorption energy on the Pt(111) obtained with the vdW-DF agrees well with the experimental desorption energy determined using TPD. We also find that Sn concentration has a negligible effect on propane adsorption. Propene adsorption is different, and the pronounced effect of alloying is found in the calculations. On the Pt(111) and 9584
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The Journal of Physical Chemistry C Pt3Sn(111) surfaces, the most favorable adsorption site is a di-σ configuration to which propene binds covalently. On PtSn2(111) propene adsorption is dominated by van der Waals forces. Adsorption energies depend strongly on the Sn concentration, but they do not follow the d-band center model on Pt3Sn/ Pt(111) and Pt3Sn(111) surfaces. However, the analysis of interaction energies reveals a clear dependence on the d-band center as the energy penalty due to structure deformations needed for the bond formation is subtracted. We note that in this study the interaction energy is dominated by the deformation energy of the adsorbate. The electronic effect, that is, the decrease in the interaction energy of the available PtPt di-σ sites, is small. Sn also introduces a geometric effect because it reduces the number of PtPt di-σ sites accessible for propene. On the basis of adsorption energies, we suggest that the increased selectivity toward propane dehydrogenation to propene on the PtSn (model) surface could be due to the weaker propene adsorption which can possibly lead to a lower desorption barrier compared to the dehydrogenation barrier.
’ ASSOCIATED CONTENT
bS
Supporting Information. Tables containing the characteristic bond lengths and angles for gas phase propane and propene (S1) and the adsorption of C (S2), H (S3), and propene (S4). Figures providing a schematic presentation of propene on a di-σ site (S1) and a DOS plot of propene on a di-σ site on Pt(111) (S2). This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail: karoliina.honkala@jyu.fi.
’ ACKNOWLEDGMENT This work was financially supported by the V€ais€al€a Foundation through Finnish Academy of Science and Letters and the Academy of Finland through project 118532. The computational resources were provided by the Nanoscience Center University of Jyv€askyl€a and the Finnish IT Center for Science (CSC) Espoo. ’ REFERENCES (1) Bartholomew, C. H.; Farrauto, R. J. Fundamentals of Industrial Catalytic Processes, 2nd ed.; Wiley-Interscience: NJ, 2006 (2) van Sint Annaland, M.; Kuipers, J. A. M.; van Swaaij, W. P. M. Catal. Today 2001, 66, 427. (3) De Rossi, S.; Ferraris, G.; Fremiotti, S.; Cimino, A.; Indovina, V. Appl. Catal., A 1992, 81, 113. (4) Lieske, H.; Sarkany, A.; V€olter, J. Appl. Catal. 1987, 30, 69. (5) Wang, Y. J.; Wang, Y. M.; Wang, S.; Guo, X.; Zhang, S.-M.; Huang, W.-P.; Wu, S. Catal. Lett. 2009, 132, 472. (6) Kumar, M. S.; Chen, D.; Holmen, A.; Walmsley, J. C. Catal. Today 2009, 142, 17. (7) Nawaz, Z.; Tang, X.; Zhang, Q.; Wang, D.; Fei, W. Catal. Commun. 2009, 10, 1925. (8) Lobera, M. P.; Tellez, C.; Herguido, J.; Menendez, M. Appl. Catal., A 2008, 349, 156. (9) Kumar, M. S.; Holmen, A.; Chen, D. Microporous Mesoporous Mater. 2009, 126, 152. (10) Bai, L.; Zhou, Y.; Zhang, Y.; Liu, H.; Sheng, X.; Duan, Y. Catal. Commun. 2009, 10, 2013.
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