Sodium Promoter Inducing a Phase Change in a Palladium Catalyst

Article pubs.acs.org/JPCC

Sodium Promoter Inducing a Phase Change in a Palladium Catalyst Nicola Seriani* The Abdus Salam International Centre for Theoretical Physics, Strada Costiera 11, 34151 Trieste, Italy S Supporting Information *

ABSTRACT: Alkali metals act as promoters for the catalytic activity of transition metals, but it is unclear whether they only donate electrons or induce the formation of different sodium-containing phases. In the present work, calculations based on density functional theory and ab initio thermodynamics show that, under oxygen-rich conditions, addition of only 6 wt % of sodium to a palladium-based oxidation catalyst can induce a phase change in the catalyst. In presence of sodium, a cubic NaPd3O4 is more stable than tetragonal PdO, which is the stable phase in the absence of sodium. Moreover, the region of stability of the oxides is extended by ∼100 K with respect to the sodium-free case. Wulff construction predicts that (111), (100), and (110) facets should be present at the surface of NaPd3O4 particles at equilibrium. Carbon monoxide and methane adsorb strongest on NaPd3O4(100) and NaPd3O4(110), respectively, with adsorption energies of 3.75 and 3.03 eV, higher than on PdO. In all cases, there is a marked preference for adsorption on surface oxygen rather than on palladium, at variance from what happens on PdO. These results shed new light on the role of sodium as a promoter of the catalytic activity.

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INTRODUCTION Heterogeneous catalysis is a fundamental process in many fields of technological relevance, from fuel production to pollution abatement, and therefore a detailed atomistic understanding of the catalytic processes is highly desirable. Still, the complexity of the technologically relevant systems is a challenge for current experimental and theoretical methods with atomic resolution,1 so that up to a few years ago, simplified model systems were routinely investigated instead.2 These model systems often differed from the industrial catalysts in their composition, structure, and morphology (materials gap). Moreover, the environmental conditions were also different, with investigations at the atomic level often performed under ultrahigh vacuum (pressure gap). In recent years, an ongoing effort to overcome this material- and pressure-gap has been underway; notable achievements have been the inclusion of realistic pressures,3,4 of nanoparticle morphologies,5−8 and of realistic ceramic supports.9−11 Still, in many cases of technological interest, the chemical composition of the active phase is more complex than it was considered in these studies, because often in real (transition-metal) catalysts, the activity is further enhanced through the addition of doping elements. In fact, these recent studies of palladium as a catalyst in oxygen-rich environment deal with pure metallic palladium and with the tetragonal phase PdO. The implicit assumption is that the promoters present in the industrial catalysts will be a small perturbation of the pure system. Promoters are doping elements that increase the catalytic activity of the catalyst. The effects of the promoters, often alkali metals,12 have rarely been investigated at an atomistic level and remain poorly understood. The promotion mechanism is still debated,13 and different effects, from electron donation14 to mixed-phase formation,15 have been postulated. In this paper, I show that in the case of palladium catalysts under oxygen-rich conditions, the addition of a small amount of sodium leads to the © 2012 American Chemical Society

stabilization of an oxide phase different from that present in the sodium-free catalyst. This suggests that the real chemical composition of technologically relevant palladium catalysts is not just a minor perturbation of the pure system, but must be taken into account in the investigation of the atomistic details of the catalytic process. While sodium palladium oxides have been synthesized in the past,16 no systematic investigation of their stability exists. In particular, it is an open question whether they might form under reaction conditions, and might explain the enhanced activity of the catalyst. In this paper, I employ ab initio thermodynamics calculations based on density functional theory to investigate the changes in thermodynamic stability, electronic and chemical properties of palladium oxides induced by small amounts of sodium. In particular, this work is focused on a specific sodium content, namely on the NaPd 3 stoichiometry. This corresponds to 6 wt % sodium in palladium, i.e., very similar to the composition with the strongest promotion effect.25 Alkali doping is known to increase the catalytic activity also in the case of platinum.26 In the present article, it is shown that the addition of only 6 wt % to a palladium catalyst under oxygen-rich conditions leads to the stabilization of a cubic oxide phase with NaPd3O4 stoichiometry, quite different from the well-known PdO phase.27−32 In the next section, the methods employed are presented; then the results are shown and discussed; a summary closes the article.

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COMPUTATIONAL METHODS The first-principles calculations have been performed within density functional theory (DFT), as implemented in the Quantum-Espresso code.17,18 The generalized-gradient approxReceived: August 13, 2012 Revised: October 8, 2012 Published: October 9, 2012 22974

dx.doi.org/10.1021/jp308051v | J. Phys. Chem. C 2012, 116, 22974−22979

The Journal of Physical Chemistry C

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imation (GGA) in the version of Perdew, Burke, and Ernzerhof (PBE)19 has been used for exchange and correlation. Since the approximated exchange and correlation functional constitutes the main source of error of DFT, care has been taken to ensure that the relevant conclusions are independent of the choice of the functional. To his aim, the main results have been checked by repeating the calculations using the local density approximation (LDA) for exchange and correlation. Vanderbilt ultrasoft pseudopotentials20 have been employed to describe the core electrons. For the wave functions, an energy cutoff of 60 Ry has been used, and 720 Ry for the charge density. A cutoff of 40 Ry would have been sufficient for the physical quantities calculated in this work, but the higher cutoff is necessary since in future work we plan to develop a classical force field for these oxides through a force-matching procedure. For this method, an extremely good convergence of forces and stresses is necessary. A mesh of (6 × 6 × 6) mesh of k-points has been used for integration in the first Brillouin zone of NaPd3. Optimizations have been considered converged when forces were smaller than 10−3 a.u. To investigate the effect of oxygen pressure and temperature on the system, the ab initio thermodynamics approach has been employed.21,22 According to this method, Gibbs’ free energies of formation were calculated by adding the chemical potential of the oxygen gas to DFT total energies:

Figure 1. Free energies of formation of sodium−palladium oxides with respect to NaPd3 and the oxygen gas at a pressure of 1 atm, for temperatures between 100 and 1400 K. Brown solid horizontal line: NaPd3; black solid line: cubic NaPd3O4; orange dashed line: tetragonal NaPd3O4; red dot-dashed line: NaPd3O3; blue dotted line: NaPd3O6. For the description of the phases, see text.

Table 1. Gibbs Free Energies of Formation ΔGf (in eV/Pd Atom) of the Oxides Considered with Respect to Bulk NaPd3 and Gaseous Oxygen, For Two Values of the Chemical Potential of Oxygena

f bulk bulk ΔG PdO (T , p) = E PdO (T , p) − E Pd ( T , p) x x

− x·gOgas(T , p)

(1) NaPd3O3 NaPd3O4 (cubic) NaPd3O4 (tetragonal) NaPd3O6

where the E’s are total DFT energies of the solid phases, and ggas O (T,p) is the chemical potential of oxygen in the gas phase: gOgas(T , p) =

1 isol EO + ΔμO(T , p) 2 2

(2)

ΔμO = 0 eV

ΔμO = −1 eV

−1.14 −1.87 −1.79 −1.80

−0.14 −0.54 −0.46 +0.20

ΔμO = 0 eV, corresponding to 0 K, and ΔμO = −1 eV, corresponding, e.g., to a pressure of 1 atm and a temperature of ∼880 K.

a

Eisol O2

Here, is the DFT total energy of an isolated oxygen molecule and zero temperature and pressure, and ΔμO(T,p) is the difference in free energy between the isolated molecule and a molecule in the gas at temperature T and pressure p. For the last term an analytical form fitted to thermodynamic data is used.22−24 In principle, in eq 1 the Gibbs’ free energies of the solid phases should be used. Still, it has been shown that for palladium oxide systems, well below the melting temperature of palladium the phonon contributions can be safely neglected, and the total energies can be employed instead.21,36

Density Approximation (LDA) as well, yielding the same ordering and an energy difference of 0.21 eV/Pd atom between the tetragonal and cubic NaPd3O4, consistent with the value found with PBE. Finally, we have considered a NaPd3O6 structure built from hexagonal PdO2.32,33 This “graphite-like” structure consists of O−Pd−O trilayers33 and therefore offers an obvious interlayer location for doping atoms, in octahedral sites with six surrounding oxygen atoms. Contrary to the case of pure palladium, where a small region of stability for the fully oxidized palladium phase was calculated at very low temperatures,32 for the present system NaPd3O6 is never found to be stable. The cubic NaPd3O4 is the phase with the lowest energy of formation and is stable up to 1190 K, where the reduced NaPd3 becomes stable. The stability of reduced phases at high temperature is due to the increase of the entropy of the oxygen gas with temperature, which stabilizes oxygen in the gas phase. Ab-initio thermodynamics thus not only shows that NaPd3O4 is stable under technologically relevant conditions, but also that the region of stability of the oxides is extended with respect to the sodium-free case. Indeed, for the case of pure palladium the PdO oxide is stable only up to ∼1100 K.32 A similar increase in the reduction temperature was experimentally observed for potassium doping.13 NaPd3O4 is also stable against segregation, lying 0.44 eV/Pd atom lower that the separated phases Na2O + 3PdO + 1/2 O2 at a chemical potential of oxygen ΔμO = 0 eV. Thus, under oxygen-rich conditions, the driving force for mixing of sodium and palladium is much larger than under

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RESULTS AND DISCUSSION Bulk Phases. In Figure 1, the Gibbs’free energies of formation calculated from ab initio thermodynamics for the bulk NaPd3−O system are shown at 1 atm of oxygen pressure. Selected values for various possible structures of a bulk NaPd3 oxide are reported in Table 1. First, structures have been tested where sodium is either an interstitial or a substitutional impurity in tetragonal PdO. The most stable PdO configuration with interstitial sodium, denominated NaPd3O3 in Figure 1 and in Table 1, is obtained when Na is put among four oxygen atoms, albeit this results in a distortion of the original lattice (see the Supporting Information). The phase with substitutional sodium, denominated tetragonal NaPd3O4, lies only ∼80 meV/Pd atom higher than the cubic NaPd3O4, which is the most stable phase we found. To check that this phase ordering is not dependent on the choice of the exchange and correlation functional, the relevant structures have been relaxed with Local 22975



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reductive conditions; the mixing energy for the binary alloy is namely only a few tens of meV.34 More importantly, the stable oxide phase is completely different: for pure palladium the tetragonal PdO phase forms, whereas for the present system NaPd3O4 forms, which is a cubic phase isostructural to NaxPt3O4.33,35 The role of sodium in the stabilization of the cubic phase can be understood in terms of local coordinations. In the sodium-free cubic Pd3O4, each oxygen atom is coordinated to three Pd atoms in a planar geometry. This is a lower coordination than in tetragonal PdO, where each oxygen atom is tetrahedrically coordinated to four Pd atoms. Since Pd−O distances are similar in the two compounds, it is clear that electrostatics is more favorable to the tetragonal PdO in the sodium-free case. Upon sodium addition, the stability order is reversed because in the cubic phase there is a large site among eight O atoms, where the sodium can easily be accommodated. This way, each oxygen atom has an extra neighboring sodium cation with which there is an electrostatic interaction that stabilizes the structure. On the other side, in tetragonal PdO the oxygen atoms are already tetrahedrically coordinated to four Pd cations, and the additional sodium cannot be so easily accommodated in an electrostatically favored site. Thus, the addition of sodium has two consequences: first, to make the oxide stable in regions where pure palladium would be in the FCC metal phase. Second, it is to influence the structure of the catalyst well beyond the simple electron donation often assumed to be the main effect of alkali promoters. It is to be noted that this result contradicts the often unspoken assumption that promoters constitute a small perturbation of the catalyst, that can be neglected in a first approximation. A possible issue is whether there might be a kinetic hindrance to the formation of the sodium palladium oxide. However, the possible pathways to the formation of this oxide depend also on the preparation methods and the history of the sample. In particular, key elements are whether sodium and palladium already form an alloy before exposure to air, e.g., through coprecipitation, and at what temperature the sample has been exposed. For example, in ref 15 Na and Pd salts were added to the sol simultaneously to obtain the catalyst by a sol− gel technique. In this case, one would expect that the formation of a sodium−palladium alloy takes place before exposure of the catalyst to air. As a consequence, the oxidation of a metallic sodium−palladium alloy would be the relevant reaction leading to the formation of the Na−Pd oxide. On the other side, addition of sodium oxide at a later stage of the catalyst’s preparation would imply that the relevant reaction leading to the mixed oxide would be the mixing reaction of sodium oxide with palladium oxide. In Figure 2, the crystal structure of NaPd3O4 is shown. The crystal is cubic with a lattice constant of 5.80 Å (exp. 5.65 Å16). In this phase, each oxygen atom is bound to three Pd atoms in a planar geometry, instead of tetrahedral coordination to four Pd atoms as in PdO. This means that, at surfaces, the oxygen atoms will be normally bound to two Pd atoms only, making them more readily available for oxidation reactions.36 On the contrary, the local coordination of the palladium atoms is similar to that in PdO, i.e., each Pd atom is square-planar coordinated to four oxygen atoms at a distance of 2.05 Å (exp. 2.00 Å16). This is important because X-ray absorption spectroscopy, one of the most used methods to characterize catalysts in situ during the catalytic reaction,37 distinguishes two phases on the basis of oxidation state, local coordinations and

Figure 2. Crystal structure of cubic NaPd3O4; the light blue atoms are sodium, the yellow atoms are palladium and the red atoms are oxygen. Each oxygen is planarly coordinated to three Pd atoms.

interatomic distances; here these quantities are very similar in PdO and NaPd3O4. The atomic charges have been calculated as Löwdin charges, i.e., by projecting the electronic states on atomic orbitals. According to the Löwdin charges (Table 2), Table 2. Atomic Charges in NaPd3, PdO and in NaPd3O4, Calculated with the Löwdin Method NaPd3 Na2O PdO NaPd3O4

Na charge (|e|)

Pd charge (|e|)

+0.50 +0.66

−0.09

+0.70

+0.79 +0.91

O charge (|e|) −1.20 −0.74 −0.80

palladium is only slightly more oxidized in NaPd3O4 than in PdO, as expected from the stoichiometry. To analyze the effect of sodium on the Pd-d states, the changes in the electronic structure from the tetragonal PdO to the cubic NaPd3O4 have been split into two contributions. The first is due to the change of the crystal structure from tetragonal PdO to the cubic Pd3O4, and the second is due to the addition of Na to Pd3O4 to form the final NaPd3O4 compound. In fact, the two contributions yield changes of opposite signs in the occupation of the dstates. The change from PdO to Pd3O4 leads to an increased oxygen content, and expectedly the occupation of the d-states decreases from 8.68 to 8.48, with a total charge of the Pd cation increasing from 0.79 to 1.01 electronic charges. The presence of common peaks between the d-states and the oxygen-p-states suggest that the bonds in Pd3O4 have also a partly covalent character. Addition of Na to Pd3O4, on the contrary, increases the occupation of the d-states to 8.56, with a total charge of the Pd cation reducing to 0.91, as sodium donates its valence electron to oxygen, thus competing with electron donation from palladium. Still, the overall effect is that the d-states of Pd in NaPd3O4 have a lower occupation than in PdO, consistently with expectations from formal oxidation states. Important differences exist in the electronic structure in a neighborhood of the Fermi energy, as shown in Figure 3. A very pronounced peak just below the Fermi energy is present in the density of states (DOS) of NaPd3O4, due to covalent bonding of Pd and O, as revealed by the atom-projected DOS. As we will see in more detail in the next section, the differences in electronic structure reflect in differences in adsorption behavior with respect to PdO; compared to palladium 22976



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Figure 4. Configurations on NaPd3O4 surfaces. (a) The stable sodiumrich termination of the bare (100) surface; (b) palladium-rich termination of the (100) surface; (c) the stable palladium-rich termination of the (111) surface; (d) sodium-rich termination of the (111) surface; (e) the stable sodium-rich termination of the (110) surface; (f) palladium-rich termination of the (110) surface. Gray balls: Pd; blue balls: Na; red balls: O.

Table 3. Surface Energies of Low-Index Surfaces of NaPd3O4 at ΔμO = 0 eV, Corresponding to 0 K Figure 3. Electronic density of states (DOS) of (a) NaPd3O4 and (b) PdO. Black solid line: total DOS; red dotted line: DOS projected on the Pd-d states; blue dashed line: DOS projected on the O-p states; vertical black line: Fermi energy. Each DOS projected on an element has been multiplied by the number of the atoms of that element in the simulation cell.

termination

γ (meV/Å2)

termination

γ (meV/Å2)

Na-rich (100) Na-rich (111) Na-rich (110)

69 72 74

Pd-rich (100) Pd-rich (111) Pd-rich (110)

76 69 82

to be present in substantial amount on the surface of a NaPd3O4 particle. In ref 25, it has been assumed that the main effect of the presence of sodium in the catalyst is to induce a stronger adsorption of carbon monoxide and a weaker adsorption of hydrocarbons on the surface. We have tested this hypothesis by calculating the adsorption of carbon monoxide and the dissociative adsorption of methane on the stable termination of each surface orientation. For CO, the adsorption is much more exothermic than on PdO surfaces, where the highest adsorption energy was calculated to be less than ∼1.5 eV.39 The strongest adsorption is on NaPd3O4(100) on a bridge position between two surface oxygen atoms and amounts to 3.75 eV (4.33 eV in LDA) [Figure 5(a)]. In this configuration, there is a clear difference in bond length between the oxygen pointing upward (C−O distance of 1.22 Å), and the surface oxygen (C− O distance of 1.36 Å). The configuration with CO adsorbed ontop on a surface oxygen has an adsorption energy of 2.00 eV. In this case, the surface oxygen is almost torn away from the surface (C−Oup distance 1.20 Å, C−Osurf 1.23 Å), and the resulting CO2 molecule is bound to the surface mainly through a Pd−C bond (Pd−C distance of 2.10 Å). Adsorption on-top on a Pd atom is endothermic by 0.10 eV. On (111), the most stable adsorption site is on a surface oxygen (1.86 eV) [Figure 5(c)]. The surface Pd atoms are coordinated to three oxygen atoms, similarly to PdO, but still the CO adsorption energy is only 1.13 eV, to be compared to ∼1.4 eV found for similar Pd sites on PdO surfaces.39 On (110), the highest adsorption

monoxide, on NaPd3O4 CO adsorption on surface Pd cations is weaker, and in some cases even endothermic. On the contrary, adsorption on surface oxygen atoms is stronger, thus favoring a direct formation of O−CO bonds. Surfaces of NaPd3O4. The surfaces of cubic NaPd3O4 and their adsorption properties have been investigated. Possible terminations of the low-index surfaces (100), (111), and (110) have been considered. (100) and (111) are the facets with the lowest surface energy (γ = 69 meV/Å2 at ΔμO = 0 eV). For (100), the stable termination is sodium-rich, while the palladium-rich termination has a higher surface energy γ = 76 meV/Å2. The two terminations are shown in Figure 4(a,b). On the contrary, for (111) the stable termination is palladium-rich, with a surface energy of 72 meV/Å2 for the sodium-rich termination [Figure 4(c,d)]. For the (110) surface, the sodiumrich termination (γ = 74 meV/Å2) is more stable than the palladium-rich termination (γ = 82 meV/Å2) (Figure 4(e,f)). In all cases, the key to the stability of a particular termination seems to be the presence of sodium ions in the neighborhood of undercoordinated surface oxygen atoms. The relevant surface energies at ΔμO = 0 eV are reported in Table 3. The surface energies have been used to calculate the equilibrium shape of a particle of NaPd3O4 through the Wulff construction.22,33,38 In the equilibrium shape, (111) facets make up 42% of the surface, (100) 36% and (110) 22%. Thus, these three facets are all likely 22977



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are well established for metals,42−44 but in the case of oxides depend on the type of oxide, the active site, and the dissociating molecule.45 Thus, they cannot simply be applied to the present case. To understand whether the kinetics of the reactions on these surfaces might explain the promotion effect of sodium, it would be necessary to explicitly calculate the reaction barriers. This goes however beyond the scope of the present work.

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CONCLUSIONS Summarizing, we have shown that the addition of 6 wt % of sodium to a palladium catalyst stabilizes a cubic NaPd3O4 phase under oxidizing conditions. At an oxygen pressure of 1 atm, this phase is stable up to 1190 K, where metallic Pd becoms stable. Carbon monoxide and methane adsorb stronger on this phase than on PdO, the stable oxide in absence of sodium. These results give insight into the role of promoters in heterogeneous catalysis, suggesting that they are not only electron donors as previously suggested, but that they also modify the structure of the catalyst.

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Figure 5. Configurations of adsorbed carbon monoxide and dissociated methane on stable terminations of NaPd3O4 surfaces. (a) Carbon monoxide on a bridge position betwen two surface O atoms on the (100) surface; (b) dissociatively adsorbed methane on (100); (c) CO adsorbed on-top on a surface oxygen on (111); (d) dissociatively adsorbed methane on (111); (e) CO adsorbed on-top on a surface oxygen on (110); (f) CH3 and H adsorbed on surface oxygen atoms on (110). Gray balls: Pd; blue balls: Na; yellow balls: C; red balls: O; and small white balls: H.

ASSOCIATED CONTENT

S Supporting Information *

Cell parameters and atomic coordinates for the bulk phases considered; a figure of the free energy of formation for the bulk phases calculated with LDA; the projected density of states (PDOS) of the NaPd3O4(100) surface with dissociatively adsorbed methane; and the electronic state localized on the Osurf−CH3 bond. This material is available free of charge via the Internet at http://pubs.acs.org/.

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energy was found on-top on an oxygen atom, which is itself ontop on a Pd atom (3.45 eV) [Figure 5(e)]. On oxygen atoms in a bridge position between two Pd atoms, the adsorption energy is 1.84 eV. Thus, in comparison to tetragonal PdO, the adsorption of CO is stronger and direct adsorption on surface oxygen is strongly favored. Moreover, adsorption energies on Pd sites are systematically weaker than on Pd sites of PdO. On the stable termination of NaPd3O4(100), the dissociative adsorption of methane is exothermic by 2.12 eV (2.27 eV in LDA), with a preferential adsorption of CH3 and H on the oxygen atoms at the surface [Figure 5(b)]. The electronic structure of the surface has been analyzed to understand the role single states at the surface play in the bonding properties. On the clean surface no surface states have been found, i.e. all states seem to be extended in the slab. On the other side, dissociative adsorption of methane leads to the formation of an electronic state localized on the surface oxygen and the CH3 fragment bound to it. Inspection of the projected density of states and of the spatial extension of this state (shown in the Supporting Information) suggest that it consists mainly of 2p states of oxygen and carbon. On (111) and (110), the configurations are similar, also with a preference for the oxygen atoms as adsorption sites, with adsorption energies of 1.86 and 3.03 eV, respectively [Figure 5(d−f)]. For PdO, the highest value for the dissociative adsorption of methane was found to be 1.78 eV for PdO(100).28,40,41 According to the present calculations, on NaPd3O4 both CO and methane adsorb stronger than on PdO. It should however be stressed that this information is not enough to draw conclusions on the catalytic activity of this compound. Indeed, the activity depends on the energy barriers related to the adsorption, dissociation and reaction of the molecules on the surface. Simple relations between adsorption energies and dissociation barriers, the Brønsted−Evans−Polanyi relations,

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Calculations were performed at CINECA and on the ICTP cluster Argo. The author is greatly indebted to Prof. Wolfgang Pompe for the constant encouragement and the far-sighted suggestions.

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