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CO oxidation on subnanometer AlPtn clusters
Computational and Theoretical Chemistry 1036 (2014) 7–15
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CO oxidation on subnanometer AlPtn clusters Ling Guo a,⇑, Ruijun Zhang a, Ling Ling Guo b, Shuangshuo Niu a a b
School of Chemistry and Material Science, Shanxi Normal University, Linfen 041004, China Department of Endocrinology, Beijing Electric Power Hospital, Beijing 10073, China
a r t i c l e
i n f o
Article history: Received 11 December 2013 Received in revised form 23 February 2014 Accepted 4 March 2014 Available online 14 March 2014 Keywords: AlPtn clusters CO oxidation Mechanism Density functional theory
a b s t r a c t Using the CO oxidation as a chemical probe, we perform a comprehensive ab initio study of catalytic activities of subnanometer AlPtn (n = 1–3) clusters. A Langmuir–Hinshelwood (LH) mechanism is proposed, which offers insights into the fundamental mechanism for CO oxidation on nanosized AlPtn clusters. It is shown that mixing two different metals (Al and Pt) can have beneficial effects on the catalytic activity. The alloyed AlPt2 cluster is proposed as the best effective nanocatalyst. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Carbon monoxide oxidation on transition metal and oxide catalysts has attracted much interest in recent years. Tremendous efforts have indeed been devoted to the removal of CO molecule from hydrogen gas fuel aiming at an optimal design of fuel cells. Among the most studied catalysts, pure gold clusters have been a main focus in previous theoretical and experimental studies [1– 3]. However, the reactivity and stability of gold nanocatalysts strongly depend on the size and shape of nanoparticles and the properties of supports [4,5]. Many attempts have been performed to stabilize the gold nanoparticles and enhance chemical reactivity by mixing with an active metal, e.g. Pt, Pd, Cu, or Ir [6–9]. Recently, a number of highly stable mixed gold clusters have been produced in the laboratory [10–13]. So, it would be of both fundamental and practical interest to study the effect of the mixed dopant on the nanogold catalysis. And experimental evidence that real actors of CO oxidation can be subnanometer species containing 10 Au atoms [14]. This finding contribute to trigger studies on heterogeneous catalysis by extremely small (subnanometer) metal clusters, a somewhat novel and fascinating field [15], although older suggestions of its importance have been advanced based on EXAFS data [16]. On the theoretical side, heterogeneous subnanocatalysis offers exciting perspectives, as the small size of the clusters allows for detailed and accurate studies involving all possible products from a specific reaction as well as all possible paths connecting ⇑ Corresponding author. Fax: +86 357 2398380. E-mail address: [email protected] (L. Guo). http://dx.doi.org/10.1016/j.comptc.2014.03.005 2210-271X/Ó 2014 Elsevier B.V. All rights reserved.
them, thus achieving a complete description of the process [17]. Motivated by the above-mentioned studies, we propose the Al-Pt bimetallic clusters as a designable catalyst system, which have been shown to be better CO oxidation catalysts than pure Pt ones. The geometric, energetic, and vibrational characterizations of the transition states on the Al–Pt bimetallic clusters have never been investigated before. In particular, to obtain a ‘‘less expensive and more efficient’’ Al–Pt catalyst for CO oxidation, it is important to know what the desired structure and composition of the bimetallic catalysts are, which can provide a clue to experimenters for improving the catalyst activity. So it is both nature and promising to investigate the effect of Al–Pt alloying at the subnanometer scale. Here we report the potential-energy surfaces for the oxidation of CO via a Langmuir–Hinshelwood (LH) mechanism [1] on AlPtn (n = 1–3) nanoclusters using DFT calculations. The adsorption of CO can provide excess energy that can allow reactions to overcome barriers. Finally, we show that on AlPtn clusters, CO oxidation is easier than on pure Pt clusters. Our results are valuable to understand the reaction mechanism of CO oxidation on Al–Pt bimetallic catalysts, and in particular, the role of ‘‘ensemble effects’’ in heterogeneous catalytic reactions over Pt-based catalysts. Notably, the excellent capability for low-temperature oxidation of CO and comparable low cost will make Al–Pt bimetallic catalysts a widely used alloy in catalytic converters. After description for the computational details in Section 2, we present the results of the adsorption of reactants (e.g., CO, O2, and O), dissociation of O2, and CO oxidation in Section 3. A brief summary is given in Section 4.
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2. Computational details All of the calculations are carried out with the Gaussian03 program [18]. Beckes’s three-parameter hybrid (B3LYP) functional is chosen to describe the reaction systems. The standard 6311 + G(d) basis set, which includes polarization and diffuse functions, is used for aluminum, oxygen and carbon. Considering the strong relativistic effect of Pt, the LANL2DZ pseudopotential is adopted for the valence electrons, and its core electrons are represented by the LANL2DZ effective core potential (ECP). [19,20] This scheme is a good compromise between accuracy and computational effort. And its application has been shown to be effective for many species including transition atom such as PtmAun [21] systems. No symmetric constraints are imposed during geometrical optimizations. The synchronous transit-guided quasi-newton method [22] is adopted for locating the transition states. The nature (minima or first-order saddle points) of optimized structures is identified by the subsequent frequency calculations that also provide zero-point vibrational energy (ZEP) corrections. Intrinsic reaction coordinates (IRC) [23] calculations have been performed to verify that each saddle point links two desired minima. In this work, we calculate the zero-point energy (ZPE) corrected binding energy (BE) of adsorbate A with an AlPtn cluster. It is defined as
clusters, and a gas-phase adsorbate, respectively. A more negative BE corresponds to stronger adsorption. The calculated bond lengths of O2, CO and CO2 at B3LYP/6311 + G(d) level are 1.21, 1.13 and 1.16 Å, and their binding energy values are 5.09, 10.80 and 5.43 eV, respectively. The corresponding experimental values [24] are 1.21, 1.14, 1.16 Å, 5.12, 11.23 and 5.50 eV. This shows the reliability of the basis set and functional used in our calculations. The accuracy and reliability of chosen functional and ECP for describing Al–Pt bimetallic clusters have been confirmed by the calculation of Pt2, Al2 and AlPt dimmers. The results are summarized in Table 1. As illustrated in Table 1, our results are in good agreement with previous experimental and theoretical data. 3. Results and discussion We first examine every distinctive sites of AlPtn (n = 1–3) clusters for CO and O2 adsorption. After the isolated clusters and molecules are optimized, we attach the molecules on the clusters to build initial geometries of cluster-molecule complexes. Various possible binding sites of the molecules on the clusters and the different relative orientations between the cluster and molecule have been taken into account for initially designed geometries.
ZPE ZPE BE ¼ ðEtotal EAlPtn EA Þ þ EZPE total EAlPtn EA
3.1. Adsorption of O2 on AlPtn clusters
In the aforementioned equation, the Etotal, EAlPtn, EA correspond to the energies of adsorbed species on the AlPtn clusters, the bare AlPtn
The Fig. 1 displays optimized structures of the lowest energy isomers of AlPtn clusters corresponding to the adsorption of
Table 1 Calculated bond lengths, averaged binding energies, and vertical ionization potential, experiment results, and this work theoretical study. Pt2
Bond length (Å) Ionization potential (eV) Eb(eV)
AlPt
AlPt2
AlPt3
Al2
This work
Experimental[25–27]
This work
Experimental[28]
2.38 8.44 1.29
2.330 8.800 ± 0.200 1.570 ± 0.020
2.51 6.29 1.17
2.560 6.200 ± 0.200 0.997 ± 0.108
AlPtO2* (0.00 eV)
AlPt2O2* (0.00 eV)
AlPt3O2*(0.00 eV)
AlPtO2(0.95 eV)
AlPt2O2 (1.42 eV)
AlPt3O2 (1.30 eV)
AlPtCO* (0.000 eV)
AlPt2CO*(0.00 eV)
AlPt3CO* (0.00 eV)
AlPtCO (0.95 eV)
AlPt2CO (0.01 eV)
AlPt3CO (0.20 eV)
Fig. 1. Calculated structures for AlPtn, AlPtnO2 and AlPtnCO (n = 1–3). The most stable structure is labeled with a star (*) symbol. The energy differences of the complexes are consigned in parenthesis.
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L. Guo et al. / Computational and Theoretical Chemistry 1036 (2014) 7–15 Table 2 Spin Multiplicity (M), Bing Energy (BE) in eV, Vibrational Frequency in cm1 and NBO Charge of the Adsorbates for AlPtnO2 and AlPtnCO clusters represented in Fig. 1. Complexes AlPtO*2 AlPtO2 AlPt2O*2 AlPt2O2 AlPt3O*2 AlPt3O2
M 2 2 2 2 2 2
BE(O2) 1.788 0.836 1.745 0.323 1.407 0.109
vO2 946.5 1023.1 1117.4 1129.4 1130.4 1123.3
qO2 1.238 0.658 0.644 0.475 0.642 0.486
molecular (AlPtnO2). Table 2 presents more details of the spin multiplicity, binding energies, vibrational frequency and NBO charge of the adsorbates for AlPtnO2 and AlPtnCO clusters represented in Fig. 1. Fig. 1 shows that all the AlPtn clusters prefer the structures with an O2 molecule connecting to the Al atom. For AlPtn (n = 1–3), adsorption of O2 takes side-on adsorption on Al atom. The O–O bonds have stretched from 1.21 Å in isolation to 1.540 Å (AlPt), 1.364 Å (AlPt2), and 1.360 Å (AlPt3), respectively. The spin density on the O atoms shift from 1.0 lb to 0.12 lb (AlPt), 0.51 (AlPt2) and 0.52 (AlPt3) per atom from the molecule to the complex, which
Complexes *
AlPtCO AlPtCO AlPt2CO* AlPt2CO AlPt3CO* AlPt3CO
M
BE(CO)
vco
qco
2 4 2 2 2 2
1.252 1.356 1.274 1.26 2.037 1.834
2072.3 2032.8 2086.9 1910.8 2103.9 2107.3
0.092 0.131 0.06 0.213 0.007 0.007
indicate that the oxygen molecule is activated by the AlPtn. The 3 binding energies of the O2 to the AlPt*n (n = 1–3) are 1.788 eV, 1.745 eV and 1.407 eV, respectively. The substable complexes of AlPtnO2 (n = 1–3) are 0.95 eV, 1.42 eV, and 1.30 eV higher in energies as shown in Fig. 1 and Table 2. We have observed that O–O bond distances are larger in case of adsorbed O2 than in the free molecule (1.21 Å), being maximal for the adsorption of O2 on AlPt cluster. NBO charge analysis shows that charge transfer from AlPtn to adsorbed O2 takes place in the order: AlPt > AlPt2 > AlPt3. For AlPtn, the clear trend of O–O bond distance, O2 binding energy
Fig. 2. The doublet potential energy surfaces for CO oxidation promoted by AlPt along path-1 (black line), path-2 (red line). The corresponding intermediates and transition states related to the two pathways are also presented. The sum of energies of free AlPt, O2, and CO is taken as the zero-point energy, which is in eV. The golden, pink, gray, and red balls denote Pt, Al, C and O atoms, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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and NBO charge is similar, which is due to the fact that metal to Q oxygen back-donation increase the population of * orbital leading to weakening of O–O bonds and thereby this bond gets lengthen. 3.2. Adsorption of CO on AlPtn clusters A few of optimized structures of CO adsorbed AlPtn complexes including the lowest energy isomers are shown in the Fig. 1. It should be noted that the interactions of the O atom of CO with AlPtn clusters, which are referred as the upside-down adsorption of CO on AlPtn clusters, have also been considered. The calculated results indicate that these nonmormal adsorption geometries are much less stable than the corresponding normal adsorptions with the C atom of CO adsorbed on AlPtn clusters. It is observed that in the lowest energy structures of AlPtnCO complexes (Fig. 1), CO binds to Pt atoms through the C atom. Among the AlPtCO complexes, structure with spin of 4 lb and a-top adsorption of CO (AlPtCO in Fig. 1) having Al–C–O of 103.2° is 0.95 eV higher in energy than the lowest energy structure, AlPtCO* in Fig. 1. For AlPt2CO and AlPt3CO complexes, substable structure lies 0.01 and 0.20 eV, respectively, higher in energy than the most stable structure. All the vibrational frequencies of CO adsorbed on AlPtn clusters have got red shifted relative to that of free CO. Binding energy trend among the complexes is in the order: AlPt3CO* > AlPt2CO* > AlPtCO*, which is different from that of AlPtnO*2. CO binding energies of all AlPtn clusters have no correction with
NBO charges on adsorbed CO. This is attributed to the complicated mechanism of CO binding with AlPtn clusters. The increment of C– Q O bond lengths of AlPtnCO are due to the increased metal ? * back-donation seen from the NBO charge analysis (Table 2). From Fig. 1, it is clear that the O2 molecule prefers to bind on Al sites than Pt sites. The adsorption behavior of CO is on the contrary. And it is reasonable to draw out the conclusion from Table 2 that the active sites in AlPt and AlPt2 clusters would be first occupied by the coming O2 rather than CO, in view of its more negative BE than those of CO. In AlPt3, the opposite is true. 3.3. CO oxidation catalyzed by AlPt, AlPt2, and AlPt3 Two different LH pathways for CO oxidation have been considered in the present work. The LH reaction is proposed to be most likely mechanism for CO oxidation by many studies [1,19,20,29– 32]. Path-1 is a conventional LH mechanism. The path-2 is a LH mechanism which has been previously proposed by Liu et al. [32]. It offers new insights into the high activity of nanosized AlPt clusters toward CO oxidation. Specifically, we show that two coadsorbed CO molecules decrease the energy barrier via path-2. To search for the minimum-energy pathway for CO oxidation, we calculate both the doublet and quartet PESs along each pathway for the reaction promoted by AlPtn. And we also calculate both singlet and triplet PESs along each pathway for the reaction promoted by Ptn. Figs. 2–7 only show the energy profiles for CO
Fig. 3. The triplet potential energy surfaces for CO oxidation promoted by Pt2. The corresponding intermediates and transition states related to the pathway are also presented. The sum of energies of free Pt2, O2, and CO is taken as the zero-point energy, which is in eV. The blue, gray, and red balls denote Pt, C and O atoms, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
L. Guo et al. / Computational and Theoretical Chemistry 1036 (2014) 7–15
oxidation on the ground-state surfaces. That is the doublet PESs of CO oxidation catalyzed by AlPtn and the triplet PESs of CO oxidation catalyzed by Ptn, where all of the geometries of minima and transition states along each path are given to understand the reaction processes clearly. The quartet PESs of CO oxidation catalyzed by AlPtn and the singlet PESs of CO oxidation catalyzed by Ptn can be found in Figs. 1S–6S (Supporting Information). 3.3.1. Oxidation of CO promoted by AlPt As shown in Fig. 2, the LH mechanism of path-1 starts from coadsorption of CO + O2 forming the structure of IM1 with adsorption energy of 3.22 eV. After coadsorption, the dissociation of adsorbed O2 has taken place. Then, the adsorbed O2 can approach and bind through its oxygen atom to the adsorbed CO via TS1/2 (Ea = 0.90 eV) forming an adsorbed carbon dioxide (OCO) complex (IM2), which is 5.16 eV lower than the energy of the reactant. In the next desorption path, via TS2 (Ea = 0.92 eV), from IM2 to the product AlPtO + CO2, it involves the scission of one C-O bond, and
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this process is exothermic by 4.63 eV. After first CO2 molecule has desorbed from the nanoparticle, a second CO molecule can react with the remaining O atom via the ER mechanism. CO oxidation proceeds to form OCO by an association of the O atom and CO molecule via transition state TS (with an activation barrier of 0.34 eV). Finally, a complex AlPtCO2 with CO2 is produced. It can disaggregate into AlPt and 2CO2, indicating the accomplishment of the reaction. It requires energy of 0.05 eV. As a result, the over reaction:
AlPt þ O2ðgasÞ þ COðgasÞ ! AlPt O2ðadsÞ COðadsÞ ! AlPtO OCOðadsÞ ! AlPt OðadsÞ þ CO2ðgasÞ ! AlPt CO2ðadsÞ þ CO2 ! AlPt þ 2CO2 is thus calculated to be exothermic by 5.87 eV. From this path, because of forming a very stable carbon dioxide (OCO) adsorbate IM2, the energy gain in the next desorption and the CO2 formation
Fig. 4. The doublet potential energy surfaces for CO oxidation promoted by AlPt2 along path-1 (black line), path-2 (red line). The corresponding intermediates and transition states related to the two pathways are also presented. The sum of energies of free AlPt2, O2, and CO is taken as the zero-point energy, which is in eV. The blue, pink, gray, and red balls denote Pt, Al, C and O atoms, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 5. The triplet potential energy surfaces for CO oxidation promoted by Pt3. The corresponding intermediates and transition states related to the pathway are also presented. The sum of energies of free Pt3, O2, and CO is taken as the zero-point energy, which is in eV. The blue, gray, and red balls denote Pt, C and O atoms, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
is slightly larger (0.53 eV), implying that it is difficult for the first CO2 molecule to desorb from the AlPt cluster because of its large adsorption energy. As for LH mechanism of path-2, we design the cooperation of two CO molecules. From Fig. 2, the free gas CO approaches the Al atom of IM2. Because of the high stability of the carbon dioxide (OCO) complex, there is an energy penalty to break it into AlPtO and CO2. However, IM2 can take an additional CO (but no additional O2). The reaction involves the transition state TS2/3 (Ea = 0.16 eV). In TS2/3, C atom of the free CO is just 2.06 Å away from an O atom of AlPtO–OCO, indicating that a new C–O bond forms. The next step is from IM3 to the final product AlPtCO2 + CO2-2 via TS3 (Ea = 0.35 eV). The barrier along this pathway is only 0.16 eV, which is lower than that of the path-1 (0.92 eV), demonstrating that the cooperation effect of the second CO is very strong and it can prompt CO oxidation. Consequently, the overall reaction
AlPt þ O2ðgasÞ þ 2COðgasÞ ! AlPt O2ðadsÞ COðadsÞ þ COðgasÞ ! AlPtO OCOðadsÞ þ CO ! AlPt OCOðadsÞ OCOðadsÞ ! AlPt CO2ðadsÞ þ CO2 ! AlPt þ 2CO2 is thus calculated to be exothermic by 5.87 eV. To compare the reactivity of doped Al–Pt clusters with corresponding monometallic Pt cluster, we consider Pt2 cluster for CO oxidation along the path-2 (Fig. 3). For Pt2 reaction, a gas CO chemisorbs on a top site of the surface, and then an O2 molecule adsorbs
on a neighbor top site by side on coordination. The O–O peroxide bond length is 1.265 Å. After dissociation of adsorbed O2, the CO(ads) directly extracts one O atom from the top site of Pt atom by passing over an energy barrier of 2.06 eV. Then the OCO complex (IM2) is formed. Because of the high stability of the IM2 complex, there is an energy penalty to break it into Pt2O and CO2. However, IM2 can take an additional CO, which lowers the energy barrier (0.08 eV) to form IM3, and the later removes two CO2. It is found that the calculated energies of all reaction intermediates and transition states along aforementioned path of AlPt is all below the reactants, suggesting that these reactions can take place readily without thermal activation. The energy barriers are much lower with respect to path-2 so that this is the kinetically and thermodynamically preferred route. Comparing the rate-determining step barrier of AlPt along path-2 reaction with that of pure Pt2, it is seen that with the decrease of Pt composition in the cluster, remarkable change is observed. The barrier is only 0.92 eV in AlPt-involved reaction, whereas it is 2.06 eV in the Pt2-involved reaction. This fact implies that AlPt catalyst is more efficient for CO oxidation than the pure Pt2 catalyst. 3.3.2. Oxidation of CO promoted by AlPt2 As shown in Fig. 4, along Path-1, first, an O2 molecule chemisorbs on a top site of Al atoms, and then a gas CO adsorbs on a neighbor top site of Pt atom. This coadsorption process is found to be barrierless. After coadsorption, the dissociation of adsorbed O2 has taken place and one atomic oxygen starts to approach the
L. Guo et al. / Computational and Theoretical Chemistry 1036 (2014) 7–15
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Fig. 6. The doublet potential energy surfaces for CO oxidation promoted by AlPt3 along path-1 (black line), path-2 (red line). The corresponding intermediates and transition states related to the two pathways are also presented. The sum of energies of free AlPt3, O2, and CO is taken as the zero-point energy, which is in eV. The blue, pink, gray, and red balls denote Pt, Al, C and O atoms, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
adsorbed CO by crossing the energy barrier of 0.69 eV, forming an adsorbed carbon dioxide (OCO) complex IM2, which is 4.98 eV lower than the energy of the reactant. IM2 further evolves into a complex of AlPt2O with CO2, which lies below the entrance by 4.70 eV, and serves as the precursor forming CO2. The barrier to be surmounted from IM2 to TS2 is 0.71 eV. And this step is the rate-determining step. The complex of AlPt2O with CO2 then absorbs another CO to produce a complex AlPt2CO2 with CO2 via TS. An energy barrier (Ea = 0.20 eV) is required for this process, indicating that the CO molecule can be readily adsorbed and then react with the atomic O. Finally, the complex AlPt2CO2 with CO2 can disaggregate into AlPt2 and 2CO2 indicating the accomplishment of the reaction. Just like the reaction of AlPt cluster, we design the cooperation of two CO molecules for AlPt2 in the path-2. From Fig. 4, the reaction also starts with the formation of IM1 and converts into IM2. Then, another free gas CO approaches the O atom of IM2, which further evolves into the high stability intermediate IM3 via transition state TS2/3 (with a activation barrier of 0.38 eV). Subsequently, an adsorbed OCO desorbs from the surface and forms a CO2 in gas via a barrier of 0.15 eV.
As shown in Fig. 4, the PES along path-2 is lower than path1.The barrier involved along this path is 0.69 eV, which is lower than those (0.71 eV) along path-1, indicating this reaction channel is viable. Similarly, to compare the reactivity of doped AlPt2 clusters with corresponding Pt3 cluster, we also consider Pt3 cluster for CO oxidation along the path-2 (Fig. 5). The preadsorbed CO molecule on the Pt3 clusters drives the reaction between CO and O2 with a large barrier height of 1.57 eV, which is higher than that along path-2 (0.38 eV) of AlPt2. This fact implies that AlPt2 catalyst is more efficient for CO oxidation than the pure Pt3 catalyst. 3.3.3. Oxidation of CO promoted by AlPt3 Similarly, two paths for the CO oxidation promoted by AlPt3 cluster have been characterized in detail. Fig. 6 shows the calculated PES profiles with the optimized geometries of the stationary points. Along Path-1 and 2, we find the same main reaction pathways as those of AlPt and AlPt2. The oxidation of CO also begins with coadsorption of CO and O2. The coadsorption energy is 3.73 eV, which is a slightly higher than that of AlPt2 (3.90 eV). This coadsorbate might proceed with the dissociated O with the C atom of CO on the TS1/2 with reaction
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Fig. 7. The triplet potential energy surfaces for CO oxidation promoted by Pt4. The corresponding intermediates and transition states related to the pathway are also presented. The sum of energies of free Pt4, O2, and CO is taken as the zero-point energy, which is in eV. The blue, gray, and red balls denote Pt, C and O atoms, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
barrier 1.40 eV. After forming a peroxo-type complex (OOCO) complex IM2, it is further evolves into a complex of AlPt3O with CO2, which lies below the entrance by 4.85 eV and serves as the precursor forming CO2. The barrier to be surmounted from IM2 to TS1 is 2.04 eV, indicating that this step is the rate-determining step. The AlPt3O complex then absorbs another CO to produce a complex AlPt3CO2 with CO2 via TS2. An energy barrier (Ea = 0.31 eV) is required for this process. Finally, the complex AlPt3CO2 with CO2 can disaggregate into AlPt3 and 2CO2 indicating the accomplishment of the reaction. For the reaction along path-2, the reaction also starts with the formation of IM1 and IM2. The calculated results indicate that the reaction involves another elementary step connected by TS3. As shown in Fig. 6, the relative energies of the intermediates and transition states involved in path-2 are all lower than those in path-1. As compared to the activation energies involved along path-1, path-2 is obvious more favorable in energies. To compare the reactivity of doped Al-Pt clusters with corresponding monometallic Pt cluster, we also consider Pt4 cluster for CO oxidation along the path-2 (Fig. 7). For Pt4 reaction, a gas CO chemisorbs on a top site of the surface, and then an O2 molecule adsorbs on a neighbor top site by side on coordination. The O–O peroxide bond length is 1.308 Å. After dissociation of adsorbed O2, the CO(ads) directly extracts one O atom from the top site of Pt atom by passing over an energy barrier of 1.07 eV. Then the OOCO complex (IM2) is formed. Because of the high stability of the IM2 complex, there is an energy penalty to break it into Pt4O and CO2. However, IM2 can take an additional CO, which lowers the energy barrier (1.44 eV) to form IM3, and the later removes two CO2.
4. Conclusion In summary, the catalytic properties of AlPtn (n = 1–3) clusters in CO oxidation are studied via a thorough DFT sampling of the PESs of the cluster systems. Three main conclusions can be drawn from this analysis. First, the CO oxidation on AlPtn clusters can occur through a variety of LH mechanisms. It is led by a strong thermodynamic driving force. Second, for the reaction along path-2, the second CO molecule on the AlPtn (n = 1–3) clusters drives the reaction between CO and O2 with lowest barrier heights of 0.90, 0.69, and 1.40 eV, respectively. The alloyed AlPt2 cluster is proposed as the best effective nanocatalyst. Third, the effect of alloying can be beneficial even though it is difficult to predict because of its highly nonlinear and many body characters. In the present case, all AlPtn (n = 1–3) clusters turn out to be the better CO oxidation catalysts than pure Ptn + 1 clusters. We believe that the present analysis offers interesting perspectives in the understanding and exploitation of heterogeneous subnanocatalysts while pointing to the need of efficient algorithms for structural exploration and sampling to achieve a predictive computational science. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (Grant No. 20603021), the Natural
L. Guo et al. / Computational and Theoretical Chemistry 1036 (2014) 7–15
Science Foundation of Shanxi (Grant No. 2013011009-6), the High School 131 Leading Talent Project of Shanxi, Undergraduate Training Programs for Innovation and Entrepreneurship of Shanxi Province (Grant No. 2013145) and Shanxi Normal University (SD2013CXCY-65) and Teaching Reform Project of Shanxi Normal University (SD2013JGXM-51). Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.comptc.2014. 03.005. References [1] H. Chen, J. Chang, S. Ju, H. Chen, First-principle calculations on CO oxidation catalyzed by a gold nanoparticle, J. Comput. Chem. 31 (2010) 258–265. [2] N. Nikbin, G. Mpourmpakis, D.G. Vlachos, A combined DFT and statistical mechanics study for the CO oxidation on the Au1 10 clusters, J. Phys. Chem. C. 115 (2011) 20192–20200. [3] M. Turner, V.B. Golovko, O.P.H. Vaughan, P. Abdulkin, A. Berenguer-Murcia, M.S. Tikhov, B.F.G. Johnson, R.M. Lambert, Selective oxidation with dioxygen by gold nanoparticle catalysts derived from 55-atom clusters, Nature 454 (2008) 981–983. [4] G. Mpourmpakis, S. Caratzoulas, D.G. Vlachos, What controls Au nanoparticle dispersity during growth?, Nano Lett 10 (2010) 3408–3413. [5] D. Matthey, J.G. Wang, S. Wendt, J. Matthiesen, R. Schaub, E. Lagsgaard, B. Hammer, F. Besenbacher, Enhanced bonding of gold nanoparticles on oxidized TiO2(110), Science 315 (2007) 1692–1695. [6] Y. Lou, H.O. Seo, K.D. Kim, M.J. Kim, W.S. Tai, M. Burkhart, Y.D. Kim, CO oxidation of Au–Pt nanostructures: enhancement of catalytic activity of Pt nanoparticles by Au, Catal. Lett. 134 (2010) 45–50. [7] J. Xu, T. White, P. Li, C. He, J.G. Yu, W.K. Yuan, Y.F. Han, Biphasic Pd–Au alloycatalyst for low-temperature CO oxidation, J. Am. Chem. Soc. 132 (2010) 10398–10406. [8] Y. Gao, N. Shao, Y. Pei, X.C. Zeng, Icosahedral grown gold nanocluster Au43Cu12 with high catalytic activity, Nano Lett. 10 (2010) 1055–1062. [9] X. Bokhimi, R. Zanella, C. Angeles-Chavez, Rutie supported Ir, Au, and Ir–Au catalysts for CO oxidation, J. Phys. Chem. C. 114 (2010) 14101–14109. [10] L.-M. Wang, S. Bulusu, H.-J. Zhai, X.-C. Zeng, L.-S. Wang, Doping golden buckyballs: Cu@Au 16 and Cu@Au17 cluster anions, Angew. Chem. Int. Ed. 46 (2007) 2915–2918. [11] (a) L.-M. Wang, S. Bulusu, W. Huang, R. Pal, L.-S. Wang, X.-C. Zeng, Doping the golden Cage Au 16 with Si, Ge, and Sn, J. Am. Chem. Soc. 129 (2007) 15136– 15137; (b) Q. Sun, Q. Wang, G. Chen, J. Jena, Structure of SiAu16: can a silicon atom be stabilized in a gold cage?, J Chem. Phys. 127 (2007). 214706-1-4. [12] P. Pyykko, N. Runeberg, Icosahedral WAu12: a predicted closed-shell species, stabilized by aurophilic attraction and relativity and in accord with the 18electron rule, Angew. Chem. Int. Ed. 41 (2002) 2174–2176. [13] L.M. Molina, B. Hammer, The activity of the tetrahedral Au20 cluster: charging and impurity effects, J. Catal. 233 (2005) 399–404. [14] A.A. Herzinga, C.J. Kiely, A.F. Carley, P. Landon, G.J. Hutchings, Identification of active gold nanoclusters on Iron oxide supports for CO oxidation, Science 321 (2008) 1331–1335.
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Contents lists available at ScienceDirect
Computational and Theoretical Chemistry journal homepage: www.elsevier.com/locate/comptc
CO oxidation on subnanometer AlPtn clusters Ling Guo a,⇑, Ruijun Zhang a, Ling Ling Guo b, Shuangshuo Niu a a b
School of Chemistry and Material Science, Shanxi Normal University, Linfen 041004, China Department of Endocrinology, Beijing Electric Power Hospital, Beijing 10073, China
a r t i c l e
i n f o
Article history: Received 11 December 2013 Received in revised form 23 February 2014 Accepted 4 March 2014 Available online 14 March 2014 Keywords: AlPtn clusters CO oxidation Mechanism Density functional theory
a b s t r a c t Using the CO oxidation as a chemical probe, we perform a comprehensive ab initio study of catalytic activities of subnanometer AlPtn (n = 1–3) clusters. A Langmuir–Hinshelwood (LH) mechanism is proposed, which offers insights into the fundamental mechanism for CO oxidation on nanosized AlPtn clusters. It is shown that mixing two different metals (Al and Pt) can have beneficial effects on the catalytic activity. The alloyed AlPt2 cluster is proposed as the best effective nanocatalyst. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Carbon monoxide oxidation on transition metal and oxide catalysts has attracted much interest in recent years. Tremendous efforts have indeed been devoted to the removal of CO molecule from hydrogen gas fuel aiming at an optimal design of fuel cells. Among the most studied catalysts, pure gold clusters have been a main focus in previous theoretical and experimental studies [1– 3]. However, the reactivity and stability of gold nanocatalysts strongly depend on the size and shape of nanoparticles and the properties of supports [4,5]. Many attempts have been performed to stabilize the gold nanoparticles and enhance chemical reactivity by mixing with an active metal, e.g. Pt, Pd, Cu, or Ir [6–9]. Recently, a number of highly stable mixed gold clusters have been produced in the laboratory [10–13]. So, it would be of both fundamental and practical interest to study the effect of the mixed dopant on the nanogold catalysis. And experimental evidence that real actors of CO oxidation can be subnanometer species containing 10 Au atoms [14]. This finding contribute to trigger studies on heterogeneous catalysis by extremely small (subnanometer) metal clusters, a somewhat novel and fascinating field [15], although older suggestions of its importance have been advanced based on EXAFS data [16]. On the theoretical side, heterogeneous subnanocatalysis offers exciting perspectives, as the small size of the clusters allows for detailed and accurate studies involving all possible products from a specific reaction as well as all possible paths connecting ⇑ Corresponding author. Fax: +86 357 2398380. E-mail address: [email protected] (L. Guo). http://dx.doi.org/10.1016/j.comptc.2014.03.005 2210-271X/Ó 2014 Elsevier B.V. All rights reserved.
them, thus achieving a complete description of the process [17]. Motivated by the above-mentioned studies, we propose the Al-Pt bimetallic clusters as a designable catalyst system, which have been shown to be better CO oxidation catalysts than pure Pt ones. The geometric, energetic, and vibrational characterizations of the transition states on the Al–Pt bimetallic clusters have never been investigated before. In particular, to obtain a ‘‘less expensive and more efficient’’ Al–Pt catalyst for CO oxidation, it is important to know what the desired structure and composition of the bimetallic catalysts are, which can provide a clue to experimenters for improving the catalyst activity. So it is both nature and promising to investigate the effect of Al–Pt alloying at the subnanometer scale. Here we report the potential-energy surfaces for the oxidation of CO via a Langmuir–Hinshelwood (LH) mechanism [1] on AlPtn (n = 1–3) nanoclusters using DFT calculations. The adsorption of CO can provide excess energy that can allow reactions to overcome barriers. Finally, we show that on AlPtn clusters, CO oxidation is easier than on pure Pt clusters. Our results are valuable to understand the reaction mechanism of CO oxidation on Al–Pt bimetallic catalysts, and in particular, the role of ‘‘ensemble effects’’ in heterogeneous catalytic reactions over Pt-based catalysts. Notably, the excellent capability for low-temperature oxidation of CO and comparable low cost will make Al–Pt bimetallic catalysts a widely used alloy in catalytic converters. After description for the computational details in Section 2, we present the results of the adsorption of reactants (e.g., CO, O2, and O), dissociation of O2, and CO oxidation in Section 3. A brief summary is given in Section 4.
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2. Computational details All of the calculations are carried out with the Gaussian03 program [18]. Beckes’s three-parameter hybrid (B3LYP) functional is chosen to describe the reaction systems. The standard 6311 + G(d) basis set, which includes polarization and diffuse functions, is used for aluminum, oxygen and carbon. Considering the strong relativistic effect of Pt, the LANL2DZ pseudopotential is adopted for the valence electrons, and its core electrons are represented by the LANL2DZ effective core potential (ECP). [19,20] This scheme is a good compromise between accuracy and computational effort. And its application has been shown to be effective for many species including transition atom such as PtmAun [21] systems. No symmetric constraints are imposed during geometrical optimizations. The synchronous transit-guided quasi-newton method [22] is adopted for locating the transition states. The nature (minima or first-order saddle points) of optimized structures is identified by the subsequent frequency calculations that also provide zero-point vibrational energy (ZEP) corrections. Intrinsic reaction coordinates (IRC) [23] calculations have been performed to verify that each saddle point links two desired minima. In this work, we calculate the zero-point energy (ZPE) corrected binding energy (BE) of adsorbate A with an AlPtn cluster. It is defined as
clusters, and a gas-phase adsorbate, respectively. A more negative BE corresponds to stronger adsorption. The calculated bond lengths of O2, CO and CO2 at B3LYP/6311 + G(d) level are 1.21, 1.13 and 1.16 Å, and their binding energy values are 5.09, 10.80 and 5.43 eV, respectively. The corresponding experimental values [24] are 1.21, 1.14, 1.16 Å, 5.12, 11.23 and 5.50 eV. This shows the reliability of the basis set and functional used in our calculations. The accuracy and reliability of chosen functional and ECP for describing Al–Pt bimetallic clusters have been confirmed by the calculation of Pt2, Al2 and AlPt dimmers. The results are summarized in Table 1. As illustrated in Table 1, our results are in good agreement with previous experimental and theoretical data. 3. Results and discussion We first examine every distinctive sites of AlPtn (n = 1–3) clusters for CO and O2 adsorption. After the isolated clusters and molecules are optimized, we attach the molecules on the clusters to build initial geometries of cluster-molecule complexes. Various possible binding sites of the molecules on the clusters and the different relative orientations between the cluster and molecule have been taken into account for initially designed geometries.
ZPE ZPE BE ¼ ðEtotal EAlPtn EA Þ þ EZPE total EAlPtn EA
3.1. Adsorption of O2 on AlPtn clusters
In the aforementioned equation, the Etotal, EAlPtn, EA correspond to the energies of adsorbed species on the AlPtn clusters, the bare AlPtn
The Fig. 1 displays optimized structures of the lowest energy isomers of AlPtn clusters corresponding to the adsorption of
Table 1 Calculated bond lengths, averaged binding energies, and vertical ionization potential, experiment results, and this work theoretical study. Pt2
Bond length (Å) Ionization potential (eV) Eb(eV)
AlPt
AlPt2
AlPt3
Al2
This work
Experimental[25–27]
This work
Experimental[28]
2.38 8.44 1.29
2.330 8.800 ± 0.200 1.570 ± 0.020
2.51 6.29 1.17
2.560 6.200 ± 0.200 0.997 ± 0.108
AlPtO2* (0.00 eV)
AlPt2O2* (0.00 eV)
AlPt3O2*(0.00 eV)
AlPtO2(0.95 eV)
AlPt2O2 (1.42 eV)
AlPt3O2 (1.30 eV)
AlPtCO* (0.000 eV)
AlPt2CO*(0.00 eV)
AlPt3CO* (0.00 eV)
AlPtCO (0.95 eV)
AlPt2CO (0.01 eV)
AlPt3CO (0.20 eV)
Fig. 1. Calculated structures for AlPtn, AlPtnO2 and AlPtnCO (n = 1–3). The most stable structure is labeled with a star (*) symbol. The energy differences of the complexes are consigned in parenthesis.
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L. Guo et al. / Computational and Theoretical Chemistry 1036 (2014) 7–15 Table 2 Spin Multiplicity (M), Bing Energy (BE) in eV, Vibrational Frequency in cm1 and NBO Charge of the Adsorbates for AlPtnO2 and AlPtnCO clusters represented in Fig. 1. Complexes AlPtO*2 AlPtO2 AlPt2O*2 AlPt2O2 AlPt3O*2 AlPt3O2
M 2 2 2 2 2 2
BE(O2) 1.788 0.836 1.745 0.323 1.407 0.109
vO2 946.5 1023.1 1117.4 1129.4 1130.4 1123.3
qO2 1.238 0.658 0.644 0.475 0.642 0.486
molecular (AlPtnO2). Table 2 presents more details of the spin multiplicity, binding energies, vibrational frequency and NBO charge of the adsorbates for AlPtnO2 and AlPtnCO clusters represented in Fig. 1. Fig. 1 shows that all the AlPtn clusters prefer the structures with an O2 molecule connecting to the Al atom. For AlPtn (n = 1–3), adsorption of O2 takes side-on adsorption on Al atom. The O–O bonds have stretched from 1.21 Å in isolation to 1.540 Å (AlPt), 1.364 Å (AlPt2), and 1.360 Å (AlPt3), respectively. The spin density on the O atoms shift from 1.0 lb to 0.12 lb (AlPt), 0.51 (AlPt2) and 0.52 (AlPt3) per atom from the molecule to the complex, which
Complexes *
AlPtCO AlPtCO AlPt2CO* AlPt2CO AlPt3CO* AlPt3CO
M
BE(CO)
vco
qco
2 4 2 2 2 2
1.252 1.356 1.274 1.26 2.037 1.834
2072.3 2032.8 2086.9 1910.8 2103.9 2107.3
0.092 0.131 0.06 0.213 0.007 0.007
indicate that the oxygen molecule is activated by the AlPtn. The 3 binding energies of the O2 to the AlPt*n (n = 1–3) are 1.788 eV, 1.745 eV and 1.407 eV, respectively. The substable complexes of AlPtnO2 (n = 1–3) are 0.95 eV, 1.42 eV, and 1.30 eV higher in energies as shown in Fig. 1 and Table 2. We have observed that O–O bond distances are larger in case of adsorbed O2 than in the free molecule (1.21 Å), being maximal for the adsorption of O2 on AlPt cluster. NBO charge analysis shows that charge transfer from AlPtn to adsorbed O2 takes place in the order: AlPt > AlPt2 > AlPt3. For AlPtn, the clear trend of O–O bond distance, O2 binding energy
Fig. 2. The doublet potential energy surfaces for CO oxidation promoted by AlPt along path-1 (black line), path-2 (red line). The corresponding intermediates and transition states related to the two pathways are also presented. The sum of energies of free AlPt, O2, and CO is taken as the zero-point energy, which is in eV. The golden, pink, gray, and red balls denote Pt, Al, C and O atoms, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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and NBO charge is similar, which is due to the fact that metal to Q oxygen back-donation increase the population of * orbital leading to weakening of O–O bonds and thereby this bond gets lengthen. 3.2. Adsorption of CO on AlPtn clusters A few of optimized structures of CO adsorbed AlPtn complexes including the lowest energy isomers are shown in the Fig. 1. It should be noted that the interactions of the O atom of CO with AlPtn clusters, which are referred as the upside-down adsorption of CO on AlPtn clusters, have also been considered. The calculated results indicate that these nonmormal adsorption geometries are much less stable than the corresponding normal adsorptions with the C atom of CO adsorbed on AlPtn clusters. It is observed that in the lowest energy structures of AlPtnCO complexes (Fig. 1), CO binds to Pt atoms through the C atom. Among the AlPtCO complexes, structure with spin of 4 lb and a-top adsorption of CO (AlPtCO in Fig. 1) having Al–C–O of 103.2° is 0.95 eV higher in energy than the lowest energy structure, AlPtCO* in Fig. 1. For AlPt2CO and AlPt3CO complexes, substable structure lies 0.01 and 0.20 eV, respectively, higher in energy than the most stable structure. All the vibrational frequencies of CO adsorbed on AlPtn clusters have got red shifted relative to that of free CO. Binding energy trend among the complexes is in the order: AlPt3CO* > AlPt2CO* > AlPtCO*, which is different from that of AlPtnO*2. CO binding energies of all AlPtn clusters have no correction with
NBO charges on adsorbed CO. This is attributed to the complicated mechanism of CO binding with AlPtn clusters. The increment of C– Q O bond lengths of AlPtnCO are due to the increased metal ? * back-donation seen from the NBO charge analysis (Table 2). From Fig. 1, it is clear that the O2 molecule prefers to bind on Al sites than Pt sites. The adsorption behavior of CO is on the contrary. And it is reasonable to draw out the conclusion from Table 2 that the active sites in AlPt and AlPt2 clusters would be first occupied by the coming O2 rather than CO, in view of its more negative BE than those of CO. In AlPt3, the opposite is true. 3.3. CO oxidation catalyzed by AlPt, AlPt2, and AlPt3 Two different LH pathways for CO oxidation have been considered in the present work. The LH reaction is proposed to be most likely mechanism for CO oxidation by many studies [1,19,20,29– 32]. Path-1 is a conventional LH mechanism. The path-2 is a LH mechanism which has been previously proposed by Liu et al. [32]. It offers new insights into the high activity of nanosized AlPt clusters toward CO oxidation. Specifically, we show that two coadsorbed CO molecules decrease the energy barrier via path-2. To search for the minimum-energy pathway for CO oxidation, we calculate both the doublet and quartet PESs along each pathway for the reaction promoted by AlPtn. And we also calculate both singlet and triplet PESs along each pathway for the reaction promoted by Ptn. Figs. 2–7 only show the energy profiles for CO
Fig. 3. The triplet potential energy surfaces for CO oxidation promoted by Pt2. The corresponding intermediates and transition states related to the pathway are also presented. The sum of energies of free Pt2, O2, and CO is taken as the zero-point energy, which is in eV. The blue, gray, and red balls denote Pt, C and O atoms, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
L. Guo et al. / Computational and Theoretical Chemistry 1036 (2014) 7–15
oxidation on the ground-state surfaces. That is the doublet PESs of CO oxidation catalyzed by AlPtn and the triplet PESs of CO oxidation catalyzed by Ptn, where all of the geometries of minima and transition states along each path are given to understand the reaction processes clearly. The quartet PESs of CO oxidation catalyzed by AlPtn and the singlet PESs of CO oxidation catalyzed by Ptn can be found in Figs. 1S–6S (Supporting Information). 3.3.1. Oxidation of CO promoted by AlPt As shown in Fig. 2, the LH mechanism of path-1 starts from coadsorption of CO + O2 forming the structure of IM1 with adsorption energy of 3.22 eV. After coadsorption, the dissociation of adsorbed O2 has taken place. Then, the adsorbed O2 can approach and bind through its oxygen atom to the adsorbed CO via TS1/2 (Ea = 0.90 eV) forming an adsorbed carbon dioxide (OCO) complex (IM2), which is 5.16 eV lower than the energy of the reactant. In the next desorption path, via TS2 (Ea = 0.92 eV), from IM2 to the product AlPtO + CO2, it involves the scission of one C-O bond, and
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this process is exothermic by 4.63 eV. After first CO2 molecule has desorbed from the nanoparticle, a second CO molecule can react with the remaining O atom via the ER mechanism. CO oxidation proceeds to form OCO by an association of the O atom and CO molecule via transition state TS (with an activation barrier of 0.34 eV). Finally, a complex AlPtCO2 with CO2 is produced. It can disaggregate into AlPt and 2CO2, indicating the accomplishment of the reaction. It requires energy of 0.05 eV. As a result, the over reaction:
AlPt þ O2ðgasÞ þ COðgasÞ ! AlPt O2ðadsÞ COðadsÞ ! AlPtO OCOðadsÞ ! AlPt OðadsÞ þ CO2ðgasÞ ! AlPt CO2ðadsÞ þ CO2 ! AlPt þ 2CO2 is thus calculated to be exothermic by 5.87 eV. From this path, because of forming a very stable carbon dioxide (OCO) adsorbate IM2, the energy gain in the next desorption and the CO2 formation
Fig. 4. The doublet potential energy surfaces for CO oxidation promoted by AlPt2 along path-1 (black line), path-2 (red line). The corresponding intermediates and transition states related to the two pathways are also presented. The sum of energies of free AlPt2, O2, and CO is taken as the zero-point energy, which is in eV. The blue, pink, gray, and red balls denote Pt, Al, C and O atoms, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 5. The triplet potential energy surfaces for CO oxidation promoted by Pt3. The corresponding intermediates and transition states related to the pathway are also presented. The sum of energies of free Pt3, O2, and CO is taken as the zero-point energy, which is in eV. The blue, gray, and red balls denote Pt, C and O atoms, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
is slightly larger (0.53 eV), implying that it is difficult for the first CO2 molecule to desorb from the AlPt cluster because of its large adsorption energy. As for LH mechanism of path-2, we design the cooperation of two CO molecules. From Fig. 2, the free gas CO approaches the Al atom of IM2. Because of the high stability of the carbon dioxide (OCO) complex, there is an energy penalty to break it into AlPtO and CO2. However, IM2 can take an additional CO (but no additional O2). The reaction involves the transition state TS2/3 (Ea = 0.16 eV). In TS2/3, C atom of the free CO is just 2.06 Å away from an O atom of AlPtO–OCO, indicating that a new C–O bond forms. The next step is from IM3 to the final product AlPtCO2 + CO2-2 via TS3 (Ea = 0.35 eV). The barrier along this pathway is only 0.16 eV, which is lower than that of the path-1 (0.92 eV), demonstrating that the cooperation effect of the second CO is very strong and it can prompt CO oxidation. Consequently, the overall reaction
AlPt þ O2ðgasÞ þ 2COðgasÞ ! AlPt O2ðadsÞ COðadsÞ þ COðgasÞ ! AlPtO OCOðadsÞ þ CO ! AlPt OCOðadsÞ OCOðadsÞ ! AlPt CO2ðadsÞ þ CO2 ! AlPt þ 2CO2 is thus calculated to be exothermic by 5.87 eV. To compare the reactivity of doped Al–Pt clusters with corresponding monometallic Pt cluster, we consider Pt2 cluster for CO oxidation along the path-2 (Fig. 3). For Pt2 reaction, a gas CO chemisorbs on a top site of the surface, and then an O2 molecule adsorbs
on a neighbor top site by side on coordination. The O–O peroxide bond length is 1.265 Å. After dissociation of adsorbed O2, the CO(ads) directly extracts one O atom from the top site of Pt atom by passing over an energy barrier of 2.06 eV. Then the OCO complex (IM2) is formed. Because of the high stability of the IM2 complex, there is an energy penalty to break it into Pt2O and CO2. However, IM2 can take an additional CO, which lowers the energy barrier (0.08 eV) to form IM3, and the later removes two CO2. It is found that the calculated energies of all reaction intermediates and transition states along aforementioned path of AlPt is all below the reactants, suggesting that these reactions can take place readily without thermal activation. The energy barriers are much lower with respect to path-2 so that this is the kinetically and thermodynamically preferred route. Comparing the rate-determining step barrier of AlPt along path-2 reaction with that of pure Pt2, it is seen that with the decrease of Pt composition in the cluster, remarkable change is observed. The barrier is only 0.92 eV in AlPt-involved reaction, whereas it is 2.06 eV in the Pt2-involved reaction. This fact implies that AlPt catalyst is more efficient for CO oxidation than the pure Pt2 catalyst. 3.3.2. Oxidation of CO promoted by AlPt2 As shown in Fig. 4, along Path-1, first, an O2 molecule chemisorbs on a top site of Al atoms, and then a gas CO adsorbs on a neighbor top site of Pt atom. This coadsorption process is found to be barrierless. After coadsorption, the dissociation of adsorbed O2 has taken place and one atomic oxygen starts to approach the
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Fig. 6. The doublet potential energy surfaces for CO oxidation promoted by AlPt3 along path-1 (black line), path-2 (red line). The corresponding intermediates and transition states related to the two pathways are also presented. The sum of energies of free AlPt3, O2, and CO is taken as the zero-point energy, which is in eV. The blue, pink, gray, and red balls denote Pt, Al, C and O atoms, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
adsorbed CO by crossing the energy barrier of 0.69 eV, forming an adsorbed carbon dioxide (OCO) complex IM2, which is 4.98 eV lower than the energy of the reactant. IM2 further evolves into a complex of AlPt2O with CO2, which lies below the entrance by 4.70 eV, and serves as the precursor forming CO2. The barrier to be surmounted from IM2 to TS2 is 0.71 eV. And this step is the rate-determining step. The complex of AlPt2O with CO2 then absorbs another CO to produce a complex AlPt2CO2 with CO2 via TS. An energy barrier (Ea = 0.20 eV) is required for this process, indicating that the CO molecule can be readily adsorbed and then react with the atomic O. Finally, the complex AlPt2CO2 with CO2 can disaggregate into AlPt2 and 2CO2 indicating the accomplishment of the reaction. Just like the reaction of AlPt cluster, we design the cooperation of two CO molecules for AlPt2 in the path-2. From Fig. 4, the reaction also starts with the formation of IM1 and converts into IM2. Then, another free gas CO approaches the O atom of IM2, which further evolves into the high stability intermediate IM3 via transition state TS2/3 (with a activation barrier of 0.38 eV). Subsequently, an adsorbed OCO desorbs from the surface and forms a CO2 in gas via a barrier of 0.15 eV.
As shown in Fig. 4, the PES along path-2 is lower than path1.The barrier involved along this path is 0.69 eV, which is lower than those (0.71 eV) along path-1, indicating this reaction channel is viable. Similarly, to compare the reactivity of doped AlPt2 clusters with corresponding Pt3 cluster, we also consider Pt3 cluster for CO oxidation along the path-2 (Fig. 5). The preadsorbed CO molecule on the Pt3 clusters drives the reaction between CO and O2 with a large barrier height of 1.57 eV, which is higher than that along path-2 (0.38 eV) of AlPt2. This fact implies that AlPt2 catalyst is more efficient for CO oxidation than the pure Pt3 catalyst. 3.3.3. Oxidation of CO promoted by AlPt3 Similarly, two paths for the CO oxidation promoted by AlPt3 cluster have been characterized in detail. Fig. 6 shows the calculated PES profiles with the optimized geometries of the stationary points. Along Path-1 and 2, we find the same main reaction pathways as those of AlPt and AlPt2. The oxidation of CO also begins with coadsorption of CO and O2. The coadsorption energy is 3.73 eV, which is a slightly higher than that of AlPt2 (3.90 eV). This coadsorbate might proceed with the dissociated O with the C atom of CO on the TS1/2 with reaction
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Fig. 7. The triplet potential energy surfaces for CO oxidation promoted by Pt4. The corresponding intermediates and transition states related to the pathway are also presented. The sum of energies of free Pt4, O2, and CO is taken as the zero-point energy, which is in eV. The blue, gray, and red balls denote Pt, C and O atoms, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
barrier 1.40 eV. After forming a peroxo-type complex (OOCO) complex IM2, it is further evolves into a complex of AlPt3O with CO2, which lies below the entrance by 4.85 eV and serves as the precursor forming CO2. The barrier to be surmounted from IM2 to TS1 is 2.04 eV, indicating that this step is the rate-determining step. The AlPt3O complex then absorbs another CO to produce a complex AlPt3CO2 with CO2 via TS2. An energy barrier (Ea = 0.31 eV) is required for this process. Finally, the complex AlPt3CO2 with CO2 can disaggregate into AlPt3 and 2CO2 indicating the accomplishment of the reaction. For the reaction along path-2, the reaction also starts with the formation of IM1 and IM2. The calculated results indicate that the reaction involves another elementary step connected by TS3. As shown in Fig. 6, the relative energies of the intermediates and transition states involved in path-2 are all lower than those in path-1. As compared to the activation energies involved along path-1, path-2 is obvious more favorable in energies. To compare the reactivity of doped Al-Pt clusters with corresponding monometallic Pt cluster, we also consider Pt4 cluster for CO oxidation along the path-2 (Fig. 7). For Pt4 reaction, a gas CO chemisorbs on a top site of the surface, and then an O2 molecule adsorbs on a neighbor top site by side on coordination. The O–O peroxide bond length is 1.308 Å. After dissociation of adsorbed O2, the CO(ads) directly extracts one O atom from the top site of Pt atom by passing over an energy barrier of 1.07 eV. Then the OOCO complex (IM2) is formed. Because of the high stability of the IM2 complex, there is an energy penalty to break it into Pt4O and CO2. However, IM2 can take an additional CO, which lowers the energy barrier (1.44 eV) to form IM3, and the later removes two CO2.
4. Conclusion In summary, the catalytic properties of AlPtn (n = 1–3) clusters in CO oxidation are studied via a thorough DFT sampling of the PESs of the cluster systems. Three main conclusions can be drawn from this analysis. First, the CO oxidation on AlPtn clusters can occur through a variety of LH mechanisms. It is led by a strong thermodynamic driving force. Second, for the reaction along path-2, the second CO molecule on the AlPtn (n = 1–3) clusters drives the reaction between CO and O2 with lowest barrier heights of 0.90, 0.69, and 1.40 eV, respectively. The alloyed AlPt2 cluster is proposed as the best effective nanocatalyst. Third, the effect of alloying can be beneficial even though it is difficult to predict because of its highly nonlinear and many body characters. In the present case, all AlPtn (n = 1–3) clusters turn out to be the better CO oxidation catalysts than pure Ptn + 1 clusters. We believe that the present analysis offers interesting perspectives in the understanding and exploitation of heterogeneous subnanocatalysts while pointing to the need of efficient algorithms for structural exploration and sampling to achieve a predictive computational science. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (Grant No. 20603021), the Natural
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Science Foundation of Shanxi (Grant No. 2013011009-6), the High School 131 Leading Talent Project of Shanxi, Undergraduate Training Programs for Innovation and Entrepreneurship of Shanxi Province (Grant No. 2013145) and Shanxi Normal University (SD2013CXCY-65) and Teaching Reform Project of Shanxi Normal University (SD2013JGXM-51). Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.comptc.2014. 03.005. References [1] H. Chen, J. Chang, S. Ju, H. Chen, First-principle calculations on CO oxidation catalyzed by a gold nanoparticle, J. Comput. Chem. 31 (2010) 258–265. [2] N. Nikbin, G. Mpourmpakis, D.G. Vlachos, A combined DFT and statistical mechanics study for the CO oxidation on the Au1 10 clusters, J. Phys. Chem. C. 115 (2011) 20192–20200. [3] M. Turner, V.B. Golovko, O.P.H. Vaughan, P. Abdulkin, A. Berenguer-Murcia, M.S. Tikhov, B.F.G. Johnson, R.M. Lambert, Selective oxidation with dioxygen by gold nanoparticle catalysts derived from 55-atom clusters, Nature 454 (2008) 981–983. [4] G. Mpourmpakis, S. Caratzoulas, D.G. Vlachos, What controls Au nanoparticle dispersity during growth?, Nano Lett 10 (2010) 3408–3413. [5] D. Matthey, J.G. Wang, S. Wendt, J. Matthiesen, R. Schaub, E. Lagsgaard, B. Hammer, F. Besenbacher, Enhanced bonding of gold nanoparticles on oxidized TiO2(110), Science 315 (2007) 1692–1695. [6] Y. Lou, H.O. Seo, K.D. Kim, M.J. Kim, W.S. Tai, M. Burkhart, Y.D. Kim, CO oxidation of Au–Pt nanostructures: enhancement of catalytic activity of Pt nanoparticles by Au, Catal. Lett. 134 (2010) 45–50. [7] J. Xu, T. White, P. Li, C. He, J.G. Yu, W.K. Yuan, Y.F. Han, Biphasic Pd–Au alloycatalyst for low-temperature CO oxidation, J. Am. Chem. Soc. 132 (2010) 10398–10406. [8] Y. Gao, N. Shao, Y. Pei, X.C. Zeng, Icosahedral grown gold nanocluster Au43Cu12 with high catalytic activity, Nano Lett. 10 (2010) 1055–1062. [9] X. Bokhimi, R. Zanella, C. Angeles-Chavez, Rutie supported Ir, Au, and Ir–Au catalysts for CO oxidation, J. Phys. Chem. C. 114 (2010) 14101–14109. [10] L.-M. Wang, S. Bulusu, H.-J. Zhai, X.-C. Zeng, L.-S. Wang, Doping golden buckyballs: Cu@Au 16 and Cu@Au17 cluster anions, Angew. Chem. Int. Ed. 46 (2007) 2915–2918. [11] (a) L.-M. Wang, S. Bulusu, W. Huang, R. Pal, L.-S. Wang, X.-C. Zeng, Doping the golden Cage Au 16 with Si, Ge, and Sn, J. Am. Chem. Soc. 129 (2007) 15136– 15137; (b) Q. Sun, Q. Wang, G. Chen, J. Jena, Structure of SiAu16: can a silicon atom be stabilized in a gold cage?, J Chem. Phys. 127 (2007). 214706-1-4. [12] P. Pyykko, N. Runeberg, Icosahedral WAu12: a predicted closed-shell species, stabilized by aurophilic attraction and relativity and in accord with the 18electron rule, Angew. Chem. Int. Ed. 41 (2002) 2174–2176. [13] L.M. Molina, B. Hammer, The activity of the tetrahedral Au20 cluster: charging and impurity effects, J. Catal. 233 (2005) 399–404. [14] A.A. Herzinga, C.J. Kiely, A.F. Carley, P. Landon, G.J. Hutchings, Identification of active gold nanoclusters on Iron oxide supports for CO oxidation, Science 321 (2008) 1331–1335.
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