Letter pubs.acs.org/JPCL
The Golden Crown: A Single Au Atom that Boosts the CO Oxidation Catalyzed by a Palladium Cluster on Titania Surfaces Jin Zhang† and Anastassia N. Alexandrova*,†,‡ †
Department of Chemistry and Biochemistry, University of California, Los Angeles, Los Angeles, California 90095-1569, United States ‡ California NanoSystems Institute, 570 Westwood Plaza, Building 114, Los Angeles, California 90095, United States ABSTRACT: We show that at the subnano scale, the catalytic properties of surface-supported clusters can be majorly impacted by strategic doping and the choice for the supporting surface. This is a first-principles investigation of CO oxidation catalyzed by two subnanoclusters, Pd4Au and Pd5, deposited on rutile TiO2(110) surfaces. The titania surface was found to participate in the reaction directly via providing additional reaction pathways. The bimetallic cluster Pd4Au shows enhanced catalytic activity, whereas the monometallic Pd5 is poisoned and deactivated in the presence of CO and oxygen, and this trend is reversed from that in the gas phase.
SECTION: Surfaces, Interfaces, Porous Materials, and Catalysis
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choosing Au as a dopant, based on the known chemistry of subnanoclusters of Pd and Au. It is fundamentally important that in the subnanoregime, accurate atomistic and electronic insight into the catalyst behavior can be gained because the number of opportunities for the catalytic process is finite.37 Therefore, catalyst design can be done with the precision of a jeweler. In recent years, the understanding started emerging that on titania, subnanoclusters of Pd and Au behave very differently in terms of geometrical configurations and the preferred nucleation site.1,3 Pd, being a d10 element, avoids electron-rich O vacancies, and therefore, its clusters bind to the stoichiometric part of titania.8,9 Pd binds to surface oxygen atoms and wets the surface at small cluster sizes.1,8,9 Clusters of Au, on the other hand, preferentially bind to surface oxygen vacancies. Being more electronegative, Au clusters can uptake the electrons from the vacancy and transmit them to the antibonding orbitals of the bound molecules, thus activating them. Hence, the two elements can be considered “orthogonal” when it comes to their behavior on the support surface and catalytic mechanism. In this work, we choose to dope a Pd cluster with Au. We anticipate that Pd, having a high affinity to the surface, would lay in between titania and the Au dopant, and Au, being more electronegative than Pd, would still uptake a partially negative charge. We therefore hope to predict a potentially bifunctional catalyst that does both the jobs of Pd and Au and thus is more efficient than either of its constituents.
ubnano transition-metal clusters have been considered a promising avenue to develop future advanced catalysts. These materials are highly tunable and can indeed be highly potent.1−9 On the scale of several atoms, the size as well as the atomic configuration of metal clusters can cause a dramatic change in the catalytic activity. For example, the rate of CO oxidation catalyzed by subnano Pdn clusters deposited on TiO2(110) surfaces in vacuum shows an erratic oscillating behavior for n smaller than 20.1 Though a number of studies have been performed on subnanoclusters, most of these studies concentrate on monometallic clusters. Doping of subnano surface-supported clusters has not been explored much,10 whereas it is known to be an excellent general strategy for catalyst design and tuning in the context of extended surfaces and nanoparticles. It is wellknown that bimetallic heterogeneous catalysts often outperform their monometallic counterparts. In particular, the bimetallic Pd−Au system, prepared either in a thin film form or as nanoparticles, has shown greatly enhanced catalytic activity, selectivity, and resistance to catalyst poisoning in a series of reactions.11−36 In the nanoregime, mixing Pd and Au is also known to help catalysis. Pd−Au nanoparticles show core−shell structures mostly with Pd atoms coating a Au core.32−36 The effect of catalyst enhancement via adding Au to the Pd nanoparticle mainly comes from two sources, varying the geometry on the particle surface and changing the electronic structure of the particle. How would doping with Au change the catalytic properties of surface-supported subnanoclusters of Pd? Can we also anticipate an improvement? Here, we address these questions. Below, we further lay out the reason behind © XXXX American Chemical Society
Received: May 11, 2013 Accepted: June 25, 2013
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Using density functional theory (DFT) with Hubbard U correction, we perform a comparative study on the CO oxidation catalyzed by Pd5 and Pd4Au in the gas phase and on TiO2(110) surfaces.38,39 We show that indeed doping with Au leads to dramatically enhanced catalytic properties of the Pd cluster on titania. Because the structures of the two systems are found to be nearly identical, the enhanced catalytic activity of Pd4Au is due solely to the modification of electronic structures of replacing one Pd atom with Au. This work shows that strategic doping of subnano surface-supported clusters is a viable catalyst design strategy. The CO oxidation assisted with a catalyst M, 2CO + O2 + M → 2CO2 + M, often involves one or more reaction pathways, which can be further decomposed into several subreactions with intermediate products. To make the number of reaction pathways tractable, we define the reaction space (the initial reactants) as exactly 1 mol of M, 2 mol of CO, and 1 mol of O2. The final products are 2 mol of CO2 and 1 mol of M. The possible intermediate products are MCO, MO2, MCOO, MCOO2, and MO. Because we focus on the reaction pathways, only the relative energy change is of importance. In the following, if not explicitly stated, the energy of the system is computed according to ΔE = Er + Ep − E i
(1)
where ΔE is the total energy change of the system in the reaction space. The reference energy Ei is the total energy of the initial reactants, Ei = 2E(CO) + E(O2) + E(M). Er and Ep represent the total energy of the partial reactants and products in the middle of the reaction pathway. The total energy release during the process was computed to be 146 kcal/mol (6.33 eV). The experimental value is 135 kcal/mol. 40 This discrepancy comes from the famous PBE failure in describing the oxygen molecule accurately.41 We did not attempt to correct this error in the current work because it does not alter the relative energy change ΔE along most of the reaction pathways. We first consider the CO oxidation catalyzed by Pd4Au and Pd5 in the gas phase. To the best of our knowledge, the Pd4Au cluster in the gas phase has not been studied previously, though some bimetallic Pd−Au clusters with Au and Pd mixed in equal proportions have been considered.42,43 For each minimum on the reaction profile, including the initial isolated cluster, initial reactant complex, intermediates, and products, we identify the global minimum structure by computing a large number of different trial configurations and selecting the energy minimum as well as other geometries with energy differences smaller than 0.3 eV. The global minimum of the Pd5 cluster is a spin triplet, having a Jahn−Teller distorted pyramidal structure. The energy difference between the distorted global minimum and the undistorted C4v structure is small (0.05 eV). The HOMO− LUMO energy gap of Pd5 is 0.12 eV. The global minimum of Pd4Au (doublet) has a similar pyramidal structure with Au atoms sitting on the top (see Figure 1). Despite the geometrical similarity, the Jahn−Teller effect is much stronger in Pd4Au; the undistorted structure is higher in energy than the ground state by 0.17 eV. In addition, replacing the top Pd atom with Au widens the HOMO−LUMO gap to 0.3 eV. In terms of charge distribution, the Au atom is partially negatively charged with 0.12 additional electrons. This is in line with its electronegativity being higher than that of Pd.
Figure 1. The reaction pathways of CO oxidation catalyzed by the Pd4Au (upper panel) and Pd5 (lower panel) clusters. Pd/O/Au/C are shown in sky blue/dark red/golden/yellow, respectively. The starting reactants are 1 mol of the catalyst M, 1 mol of O2, and 2 mol of CO. During each stage, the reactants involving the catalyst M are plotted together with the total energy change of the system ΔE defined in eq 1. Molecules in the gas phase are not shown; however, their total energies are accounted for when ΔE is computed. The energies of the transition states are shown in parentheses, whereas others are energies of intermediates. The spin state (S) of each intermediate is also shown.
In the study of the pathways of the catalyzed reaction, one needs to take into account the possible scenario in which the gas molecules directly attack adducts already attached to the catalyst, instead of attaching to the catalyst prior to interacting. In the case of CO oxidation with the intermediate product MO or MO2, the CO molecule may attack the oxygen adatom without reacting with the catalyst M first. In these cases, CO2 was found to form barrierlessly. The net result of this process is shifting the system to the following subreaction stage. Because these types of events occur with low probability, they are neglected in the following discussion. The CO oxidation reaction pathways are shown in Figure 1. Due to the simplicity of the reaction, one can quickly establish the sequence of intermediates as follows: M → (MCO/MO2) → MCOO2 → MO → MCOO → M. Before the intermediate MCOO2, there are two possible pathways, involving the catalyst M being first attacked by either an oxygen molecule or a carbon monoxide. For both considered clusters, the overall reaction proceeds barrierlessly. For subreactions along the pathway, the highest barrier observed is at the stage of MCOO2 → MO + 2251
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bond length of 1.34 Å (the equilibrium O2 bond length is 1.23 Å). The HOMOs shown in Figure 2b,d illustrate that the p−d hybridization is stronger for the dissociated oxygen atom with Pd4Au. Now, let us turn our attention to the surface-supported cluster catalytic systems. The rutile TiO2(110) surface has fiveand six-fold-coordinated Ti atoms, and the five-fold coordinated Ti may interact with the adsorbate. The other available surface site is the protruding two-fold coordinated surface oxygen atom in the bridging row. Unlike the Pt clusters, which prefer to occupy oxygen vacancies and Ti interstitials,47−52 both Pd5 and Pd4Au clusters preferentially bind to the stoichiometric part of TiO2(110) and majorly retain their nearly pyramidal gas-phase structures. Pd4 wets the surface, and Au sits on top. This configuration is 0.4 eV lower in energy than the isomers obtained by switching Au with one of the Pd atoms. Each Pd atom on average loses about 0.13 electrons to nearby bridging oxygen atoms. The Au atom retains its negative charge of −0.17 e. The adsorption energies for Pd4Au and Pd5 clusters on titania are 3.5 and 3.3 eV, respectively. In our calculation setup, the Hubbard U term on Ti atoms introduces the possibility of electron localization near the surface Ti ions,8 which may affect catalysis significantly. However, for all obtained intermediates deposited on stoichiometric surfaces, no such strongly reduced Ti3+ ions were observed, even though the five-fold Ti atom does participate in catalysis in a direct manner. Now, it remains to be investigated whether or not the cluster is also a better catalyst than Pd5. For subnanosized clusters, it is expected that their catalytic characteristics may be modified by the surface−cluster interaction upon deposition. Below, we demonstrate that both Pd4Au and Pd5 show additional CO oxidation pathways on the TiO2 surfaces, and therefore, multiple pathways become possible. For the convenience of representation, the pathways are divided into two halves by the intermediate product MO. The reaction pathways from the initial reactants until MO are shown in Figure 3. The first difference between the reaction in the gas phase and that on the support surface is that the oxygen molecule no longer dissociates but attaches to two Pd atoms of the Pd4Au cluster in the latter case (upper reaction pathway in Figure 3). This indicates that upon deposition, the cluster is further stabilized by the interaction between the Pd atom and the surface oxygens in the bridging row. Pd4Au back-donates fewer electrons to the oxygen molecule compared to the gas-phase situation. The attached oxygen molecule further depletes the neighboring Pd atom’s electron by the amount of 0.24 each. On the contrary, Pd5 now is able to barrierlessly dissociate the O2 molecule. The key ingredient making a difference here is the top Pd atom. Charge analysis shows that it is slightly negatively charged (−0.07 e). This additional charge makes the top Pd atom an active site in oxygen dissociation. The substantial modification of the characteristics of both clusters when deposited on titania is due to the fact that the five-fold surface Ti atom near the cluster directly participates in the catalytic reaction. In the lower-energy reaction pathway for Pd4Au, the oxygen molecule dissociates via binding one O atom to the surface Ti atom. The situation is even clearer in the case of the intermediate, MO. The surface Ti atom is obviously more attractive than the top Au atom. The energy difference between the O atom binding to Au and to Ti is 0.51 eV. When the O atom binds to the three-fold hollow site, the Au atom is positively charged (0.11 e), whereas it has a negative charge of
CO2. The barrier is about 1.0 eV for Pd4Au, whereas it increases to 1.4 eV in the case of Pd5. Despite the apparent similarity, the reaction pathways of Pd4Au and Pd5 have several noticeable differences. The first one is that when the Pd4Au adsorbs a CO molecule, CO attaches to the Au atom on top. The adsorption energy is 0.1 eV higher than the case where CO attaches to Pd atoms. The new complex, Pd4AuCO, has a high multiplicity of 4, while the energy change ΔE is only half of that of the Pd5CO case. Figure 2a,c shows the HOMO state of the CO complexes with Pd4Au
Figure 2. The HOMO state for (a) Pd4AuCO, (b) Pd4AuO2, (c) Pd5CO, and (d) Pd5O2. The coloring of the different elements is the same as that of Figure 1.
and Pd5, respectively. The bonding between the CO molecule and Pd4Au is mostly of the s−p σ-type, with a large concentration of electron density on the CO molecule. In the case of Pd5CO, a mixed s−p−d character is observed, clearly indicating strong back-donation to the π* orbital of CO. The electron density between the carbon atom and the cluster is also higher. Charge analysis shows that in the case Pd5, the bound CO molecule accumulates ∼0.23 additional electrons, in line with the observed back-donation. The CO bond length increased to 1.20 Å (1.14 Å in equilibrium), suggesting that CO is highly activated. In contrast, there is little charge transfer to the bound CO from Pd4Au, with the CO bond length remaining at 1.15 Å. The weaker CO adsorption in the presence of Au indicates that the insertion of Au reduces the reactivity of the cluster, which is in line with the fact that Pd− Au has better resistance to the sulfur poisoning, for example.16,19 At the same time, gold on the nanoscale has been shown to exhibit strong activity in catalyzing reactions such as CO oxidation.3,44−46 Indeed, if the catalyst binds the molecule too strongly, it is unlikely to be catalytically active, and therefore, the weaker binding of CO to Pd4Au may be a good quality. The structures and energetics of the O2 adsorption to the two clusters also differ. For Pd4Au, we found that the oxygen molecule naturally dissociates into two oxygen adatoms. One may expect that the autodissociation of O2 should also occur in the case of Pd5, but it was not observed. The oxygen molecule instead attaches to the side of the Pd5 cluster with an activated 2252
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Figure 4. The reaction pathways of CO oxidation (second half) catalyzed by deposited Pd4Au (upper panel) and Pd5 (lower panel). No identifiable transition states were found in this stage. The thermodynamically unfavored intermediates are connected with dotted lines. Gray spheres represent Ti atoms. The labels and coloring of other atoms are the same as those in Figure 1.
Figure 3. The reaction pathways of CO oxidation (first half) catalyzed by deposited Pd4Au (upper panel) and Pd5 (lower panel). Gray spheres represent Ti atoms. The labels and coloring of other atoms are the same as those in Figure 1.
drives the total energy of the system lower than the final product, effectively leading to poisoning of the catalyst during the last stage of the reaction. This is never observed for Pd4Au on titania, which exhibits a flawless reaction profile. In summary, using DFT, we addressed the effect of doping and deposition on titania on the catalytic behavior of a subnano Pd cluster toward the process of CO oxidation. We compared the catalytic efficiency of Pd5 to that of Pd4Au, both in the gas phase and on the titania surfaces. There are two key findings: (i) upon surface deposition, the catalytic behavior of both clusters changes dramatically, owing to the direct participation of the surface in binding reactants and the intermediate; (ii) doping turns the noncatalytic surface-supported cluster into a catalyst. Specifically, in the gas phase, Pd5 and Pd4Au are comparable catalysts for CO oxidation. However, on the stoichiometric TiO2(110) surface, Pd5 is not a catalyst at all because it gets poisoned by strongly bound and dissociated oxygen, whose dissociation is facilitated by the TiO2(110) surface. However, surface-supported Pd4Au exhibits a smooth catalytic reaction profile and favorable energetics. This result echoes the experimental observation by the Toshima group that the “crown jewel” Pd−Au structure on the surface of Pd nanoparticles boosts the catalyzed glucose oxidation rate several fold.11 The role of Au dopant is to alter the charge distribution in the system, leading to its decreased ability to dissociate O2 as compared to Pd5. It is a purely electronic effect.
−0.20 e when the O atom binds to the Ti atom. In the Pd5 case, the oxygen binding to the Ti three-fold hollow site of the cluster is almost equally favored in energy, suggesting that the surface Ti atom serves as an additional and competitive active site for catalysis. Also notice the role of the Au dopant in modifying the electronic structure of the system driving it toward different reactivity. The maximal barrier heights for the subreactions during the first stage (Figure 3) for the Pd4Au and Pd5 clusters are 0.69 and 0.65 eV, respectively (we chose the pathway with the most favorable energetics overall). These values are significantly lower than the unsupported cluster values. We attribute these lowered barrier heights to the surface/cluster/reactant interactions, which enhance the ability of the complexes to dissociate the oxygen molecule. The second half of the reaction pathways shows a clear difference between the two systems. The surface can both enable and disable a given catalyst via surface−cluster interaction. Though both clusters can catalyze the reaction to the intermediate product MO, the weaker binding between the oxygen adatom and the surface/cluster complex in the case of Pd4Au is critical in the last step of CO2 release. Figure 4 shows that only the Pd4AuO complex reacts with CO and easily transforms it, via a series of metastable states, to the final product (CO2). The series of metastable configurations with similar energies involves CO adsorbed to the Pd4AuO complex. Transforming one state to another only involves a small barrier (
