Design of High-Performance Pd-Based Alloy Nanocatalysts for Direct

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Design of High-Performance Pd-based Alloy Nanocatalysts for Direct Synthesis of H2O2 Haoxiang Xu, Daojian Cheng, and Yi Gao ACS Catal., Just Accepted Manuscript • Publication Date (Web): 09 Feb 2017 Downloaded from http://pubs.acs.org on February 9, 2017

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Design of High-Performance Pd-Based Alloy Nanocatalysts for Direct Synthesis of H2O2 Haoxiang Xua, Daojian Chenga*, Yi Gaob,* a

International Research Center for Soft Matter, Beijing Key Laboratory of Energy Environmental Catalysis, State Key Laboratory of Organic-Inorganic Composites, Beijing University of Chemical Technology, Beijing100029, China E-mail: [email protected] b

Division of Interfacial Water and Key Laboratory of Interfacial Physics and Technology,

Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201800, China E-mail: [email protected]

ACS Paragon Plus Environment

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ABSTRACT The direct synthesis of hydrogen peroxide (H2O2) is a promising alternative to the commercialized indirect process. However, it is still a big challenge for the development of Pdbased catalysts with outstanding activity and selectivity, because the design and optimization of the efficient catalysts cannot be effectively achieved solely based on the well-known Sabatier analysis. In this paper, we proposed a strategy to design more efficient Pd-based nanocatalysts combining the density functional theory (DFT) calculations and Sabatier analysis. The average valence electron of Pd-shell atoms is identified as the intrinsic factor for the activity and selectivity of the Pd-based nanocatalysts, which can be effectively tuned by the dopants. By introducing the dopants with suitable electronegativity, the valence electrons of Pd-shell atoms could be adjusted to the optimal range to enhance the activity and selectivity of the nanocluster simultaneously. With this strategy, Pd-W, Pd-Pb, Au-Pd-W, Au-Pd-Pb, Au-Pd-Mo and Au-PdRu are predicted as the potential candidates with the catalytic performance far exceeding the state-of-the-art experimental systems by scanning the periodic table. This work not only predicts potential Pd-based alloy nanocatalysts for direct synthesis of H2O2 for future experiments, but provides a viable way for the design of highly efficient heterogeneous catalysts in extensive applications.

KEYWORDS : direct synthesis of H2O2 , Pd-based alloys , nanoparticle , selectivity , catalyst design , density functional theory , electronegativity


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1. Introduction Hydrogen peroxide (H2O2) is well known to the general public due to its ubiquitous use as a bleaching agent in the pulp industry, as a disinfectant in the pharmaceutical industry and as a green oxidant in the synthesis of specialty chemicals.1-10 At present, hydrogen peroxide is manufactured industrially by an indirect process in which anthraquinones are sequentially hydrogenated and oxidized in a manner.11 However, considering the high capital and operating costs of energy-intensive separation and concentration in the indirect process,12-14 industry and academia are keen to identify an alternative direct process. To date, the vast majority of researches into the direct synthesis of H2O2 are based on supported Pd nanoparticles.15-23 However, it has not been demonstrated on commercial scale yet, since the addition of vast strong acid24 and halide15,25-26 promoters to the reaction medium promotes metal leaching and requires further purification of the H2O2 products despite suppressing the process of side reaction. Hutchings and co-workers firstly reported that Au-Pd27-32 alloy catalysts can significantly enhance the activity and selectivity in the direct synthesis of H2O2, which stimulates the extensive studies of the promotional effect by the addition of Au33-35, Pt36-38, Ag39, Ni40 to Pd catalysts both in the experiment and theory. Recently, Pd-Sn alloy catalyst is synthesized in experiment and its selectivity is characterized as the current state-of-the-art.41 Edwards et al. reported that the addition of a small amount of Pt to Au-Pd alloy catalyst significantly enhanced the activity and selectivity in the direct synthesis of H2O242. Despite of the above significant progress, the available experimental Pd-based catalysts cannot achieve the desired selectivity and reactivity simultaneously with an absent of acid and halides promoters. On the other hand, a wide range of theoretical studies have been carried out to search for new


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catalysts for the direct synthesis of H2O2 using the Sabatier analysis.43,44 However, Sabatier analysis alone cannot provide guidelines for finding new nanoalloy catalysts, since Sabatier analysis merely provide the correlation between adsorption energy and activation energy. In particular, current studies are mostly focused on the dopants of noble metals. Thus, how to design more efficient Pd-based based on earth-abundant, cost-effective materials to replace noble metals is an urgent and practical task. In this work, we proposed a new strategy to design Pd-based binary and ternary alloy nanocatalysts for direct synthesis of H2O2. Combining with density functional theory (DFT) calculations, Sabatier analysis, and analysis of electronic structures, we identified the average valance electron of Pd-shell atoms as the intrinsic effect for the activity and selectivity of the nanocatalysts, which can be tuned by the doped metals. The doped elements with slightly higher electronegativity (compared to Pd) could withdraw electrons from Pd-shell atoms to the optimal range to enhance the activity and selectivity simultaneously. By the systematic search of the periodic table, Pd-W, Pd-Pb, Au-Pd-W, Au-Pd-Pb, Au-Pd-Mo and Au-Pd-Ru are predicted as good candidates for Pd-based binary and ternary alloy nanocatalysts, which exhibit better catalytic performance potentially than the available experimental systems. Our work serves as a general example to predict new catalysts, which may also find applications in designing new heterogeneous catalysts beyond direct synthesis of H2O2.

2. Computational Details DFT calculations are performed using the PWSCF (Plane-Wave Self-Consistent Field) planewave code in the Quantum ESPRESSO package.45 All the calculations were carried out using Perdew–Burke–Ernzerhof (PBE) xc-functional46 and ultrasoft pseudopotentials.47 The energy cut-offs for the selection of the plane waves for the description of the wave function and the


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electronic density were set to 40 and 400 Ry, respectively. All calculations on cluster models were performed in a 15×15×15 Å3 cubic cell. The first Brillouin zone was sampled at the Gamma-point and the electronic levels were broadened though a Gaussian smearing technique with a smearing parameter of 0.002 Ry. All calculations were performed fully spin-polarized, since it is well-known that magnetic considerations can be significant for certain oxygen-based adsorbates and transitionstates.48-49 Geometric optimizations was performed with a force convergence criterion of PdPt > Pd > AuPd > SnPd > Pt > Au in both the experimental results41,42 and our DFT calculations, indicating the DFT calculations can qualitatively characterize the relative ordering of H2O2 productivity rate by experiments. Meanwhile, the comparison between H2O2 decomposition/hydrogenation rate in experiments42 and side reaction activity by DFT calculations is listed in Table S6 and plotted in Figure 3b, which confirms the accuracy of activity characterization by DFT calculations. High activity of side

reaction

on

Sn25Pd30 in

DFT

calculations

is

in

conflict

with

slow

H2O2

hydrogenation/decomposition rate in experiment because its high selectivity is attributed to tin oxide surface layer that encapsulates small Pd-rich particles (active for decomposing and hydrogenating the H2O2) rather than Sn-Pd alloy.41


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Figure 3: (a) Hydrogen peroxide productivity in experiment and calculated As for main reaction in theory as a function of different model clusters. (b) Hydrogen peroxide hydrogenation/decomposition in experiment and calculated As for side reaction in theoryas a function of different model clusters.

Sabatier analysis provides upper bounds for reaction activity and has successfully described trends in reactivity in terms of simple descriptors, e.g. adsorption energies of certain intermediates. In this paper, Sabatier analysis are performed through a multidimensional activity volcano with ∆GO* and ∆GH*. The correlations between As of main/side reaction and ∆GO*/∆GH* are derived from scaling relations of the binding energies of various adsorbates, and from Brønsted-Evans-Polanyi (BEP) relationships59-61 (see SI including Figure S1 and S2 for full details on the correlations). The activity volcano for main reaction is shown in Figure 4. Similar


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to recent experimental work,42 Pt2Au23Pd30 locates at the top of volcano and shows very high activities in main reaction. In comparison, other clusters lie on hillside of the volcano and show lower activities towards H2O2 than Pt2Au23Pd30. Au25Pd30 shows slightly higher activity than Sn25Pd30, which can be verified by recent experimental work.41 As shown in Figure 4, Sabatier activity volcano is divided into two regions (by blue line) in terms of different predicted ratecontrolling steps. Where hydrogen binds very weakly, H2 dissociation is the rate-limiting step for a range of oxygen adsorption free energies, and the Au55 cluster falls into this region. Accordingly, OOH* formation from H* + O2* becomes the relevant rate-limiting step as the hydrogen binding free energy becomes stronger, and the other five clusters lie in this region.

Figure 4: Activity volcano for direct synthesis of H2O2. ∆GO*and ∆GH* are the adsorption free energies of oxygen and hydrogen adsorption, respectively. The activity, in kJ/mol, is plotted as a function of both ∆GO*and ∆GH*. In the above regions, the predicted rate-controlling steps in each region of descriptor space are shown. The dash area is the selective hydrogen peroxide region of the Sabatier volcano toward H2O2, which satisfy all of the selectivity reaction barrier inequality constraints in the text. The values for different model systems can be taken from their indicated positions.


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To determine whether the catalysts would selectively form hydrogen peroxide rather than water thermodynamically, the intersecting space of inequality constraints (see SI for full details) is delineated by shadow in Figure 4. The dashed area is based on the series of reaction barrier inequalities in Section 5 of Supporting Information, which indicates all activation barriers of elementary steps in main reaction path are lower than the corresponding competing ones in side reaction path. It is clear that Au55, Au25Pd30 and Pt2Au23Pd30 clusters conform to all activation barrier inequality constraints, which are favor for the production H2O2. In addition, Pt25Pd30, Pt55 and Pd55 clusters lie essentially out of the selectivity boundaries. These results are additionally in very good agreement with previous experiments.42,62-63 It is well-known that pure Au catalyst,63 50%Au-50%Pd catalysts62 and Pt-promoted Au-Pd catalysts42 are highly selective to hydrogen peroxide, while pure Pt/Pd catalyst and 50%Pt-50%Pd catalyst can form mixed product phases of H2O/H2O2.

3.2 Intrinsic effect of doped metal In order to get the deep understanding of intrinsic effect of the doped metal, we explored the electronic effects of the nanoclusters. Since Kohn–Sham orbital energy level diagram and d-band center is not a good indicator to explain the adsorption and activity trend of model systems (Figure S3-4), we turn to focus on charge redistribution caused by dopant. The activity of main/side reaction as a function of the average valance electron of Pd atoms in catalyst shell θ(Pd in shell) is plotted in Figure 5a. It is found that Pt2Au23Pd30 is of the highest activity for main reaction and outstanding selectivity. This could be attributed to the optimal θ for Pt2Au23Pd30 compared to Pd55, Pt25Pd30, Sn25Pd30 and Au25Pd30. DOS analysis of adsorbed O2 on Pd55, Au25Pd30, Pt25Pd30 and Pt2Au23Pd30 in Figure S5 shows that moderate electron from Pdbased alloy cluster to O-O bond can facilitate hydrogenation and suppress dissociation (detailed


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explanation in SI). As been reported that there may be particle size effects on Pd particles for direct synthesis of hydrogen peroxide64, additional calculation on larger models, 79-atom cluster, are performed to explore whether the correlation in Figure 5a is robust to the effect of particle size (Figure S6), which shows the same order of the activity and selectivity for studied models as that in Figure 5a. As seen in Figure 5a, θ(Pd in shell) of Pd55 (10.12 |e|) is higher than the optimal range (9.85 |e| – 9.95 |e|), which suggests the performance of Pd55 can be promoted by the dopants which withdraw some electrons from Pd atoms. As is well-known, electronegativity quantifies the electron affinity of certain element in a chemical compound. The atom with higher electronegavitiy could withdraw electrons from the atom with lower electronegativity. For Pdbased alloys, lower electronegativity of Sn (1.96) increases θ(Pd in shell) of Sn25Pd30 (10.24 |e|) compared to Pd55 (10.12 |e|) since a few electrons of Sn atoms flow to Pd atoms. Pt (2.28) is of a slightly higher electronegativity than Pd (2.20), so θ(Pd in shell) of Pt25Pd30 is slightly reduced (10.07 |e|) compared to Pd55 (10.12 |e|) since a few electrons of Pd atoms flow to Pt atoms. Similarly, much higher electronegativity of Au (2.54) decreases more θ(Pd in shell) of Au25Pd30 (9.78 |e|) compared to Pd55 (10.12 |e|). θ(Pd in shell) of Pt2Au23Pd30 is a little more than that of Au25Pd30 since Pt has low electronegativity compared to Au.


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Figure 5: (a) Calculated As for main reaction and side reaction as a function of average valance electron charge of Pd atoms in cluster shell. (b) Average valance electron charge of Pd atoms in shell of Pd-base binary alloy and AuPd-base ternary alloy as a function of descriptor α based on electronegativity of doped element.

Based on the above analysis, metal elements in 4th, 5th, 6th row of the periodic table are examined systematically as the doped elements to Pd clusters and Au-Pd nanoalloys. It is noted that the atom arrangement of binary and ternary clusters are the same as Au25Pd30 and Pt2Au23Pd30, respectively, which exclude the influence of chemical ordering in order to focus on the influence of electronegativity. Here, we introduce α=(EM/EPd) to represent the effect of relative electronegativity of the doped metal (EM and EPd are the electronegativity of doped metal and Pd, respectively, which can be found in Table S7). Figure 5b gives the correlation between


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θ(Pd in shell) and α. It is clear that Pd atoms have fewer valence electrons when doping elements with higher electronegativity into pure Pd cluster or Au-Pd nanoalloys and vice versa. Therefore, the exact intrinsic effect of a doped metal on reactivity of Pd-based alloy catalysts is redistribution of θ(Pd in shell) through the different electronegativity from that of Pd.

3.3 Prediction of New Pd-based Nanocatalysts The dash area in Figure 5b corresponds to the optimal θ(Pd in shell). Thus, W and Pb are predicted to be the promising substituted elements for Pd-based binary alloy catalyst. In addition, Mo, W, Pb, Ir, Os, Ru and Rh are selected to substitute Pt for improving Au-Pd ternary alloy nanocatalysts. In order to verify the prediction, Sabatier analysis is performed to estimate As for main/side reaction on Pb25Pd30, W25Pd30, Mo2Au23Pd30, Ru2Au23Pd30, Os2Au23Pd30, Ir2Au23Pd30, Rh2Au23Pd30,

Pb2Au23Pd30 and W2Au23Pd30 systems. Ag25Pd30, Os25Pd30, Rh25Pd30 and

Ag2Au23Pd30, which are not in the optimal region, are also chosen for comparison. The activity of main/side reaction of candidate Pd-based binary nanocatalysts is listed in Table S8. It illuminates that Pb25Pd30 and W25Pd30 has prior activity and selectivity toward direct synthesis of H2O2 compared with other candidates. Moreover, As for main/side reaction of candidate Pdbased ternary nanocatalysts by Sabatier analysis is summarized in Table S9, indicating that Mo2Au23Pd30,

Ru2Au23Pd30,

Os2Au23Pd30,

Ir2Au23Pd30,

Rh2Au23Pd30,

Pb2Au23Pd30

and

W2Au23Pd30 are superior to Ag2Au23Pd30 in both activity and selectivity. The predicted activity of main/side reaction on predicted Pd-based alloys are consistent with the calculated results by DFT calculation data in Table S10 and Figure S7, indicting the robustness of the rule. The results in Table S5,S6,S8,S9 are visualized in Figure 6a and 6b. We established a volcano relationship between α and the activity of main/side reaction and found that α can serve as the activity descriptor for guideline to choose appropriate doped metal to improve the activity and


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selectivity of Pd-based catalysts. Compared with the previous descriptors such as d-band center65-66 and adsorption energy of reactant or intermedium derived from Sabatier analysis, descriptor α is more relevant to intrinsic effect of dopant and can be conveniently used to predict catalytic activity of Pd-based catalysts. According to Figure 6a and 6b, the best Pd-based binary alloy catalyst is predicted to be the doped metal with electronegativity slightly higher than Pd (α≈1.05-1.1). And the best PdAu-based ternary alloy catalyst is predicted to be the doped metal with electronegativity close to Pd (α≈0.9-1.1). Experimentally, Au-Pd nanocatalysts with a little Pt promoter show better catalytic performance than Pd and Au-Pd,42 fully consistent with our predication. In addition, it is found that all the predicted promising Pd-based catalysts in Figure 6a and 6b can not only improve the activity for direct synthesis of H2O2 (above the red dot line) but also enhance the selectivity (below the blue dot line), compared with Au25Pd30 which has been proved experimentally as an excellent catalyst for direct synthesis of H2O2. Sabatier activity volcano for these tested Pd-based catalysts is shown in Figure S8.


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Figure 6: (a) As for main reaction and side reaction (Sabatier analysis) as a function of descriptor α based on electronegativity of doped element for Pd-base binary alloy clusters. (b) As for main reaction and side reaction (Sabatier analysis) as a function of descriptor α based on electronegativity of doped element for PdAu-base ternary alloy clusters. The red and blue dot lines represent the As for main and side reaction of Au25Pd30, respectively.

Besides, recent experiments reported that the selectivity of Pd-based alloy nanocatalysts would be improved if doped metal can form oxide surface layer and encapsulates small Pd-rich particles which is active for decomposing and hydrogenating the H2O2.41 According to reducibility of metal, Mo, Ru, Pb and W can form oxide after calcinations as same as Sn. Thus, it can be predicted that Pd-W, Pd-Pb, Au-Pd-W, Au-Pd-Pb, Au-Pd-Mo and Au-Pd-Ru are potentially good candidates for Pd-based alloy nanocatalysts, showing better catalytic performance than the


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available experimental systems.

4. Conclusions In summary, combining the density functional theory (DFT) calculations and Sabatier analysis, the average valence electron of Pd-shell atoms is identified as the intrinsic factor for the activity and selectivity of the Pd-based nanocatalysts, which can be effectively tuned by the electronegativity of the dopants. The dopants with suitable electronegativity withdraw electrons from Pd atoms to the optimal range to enhance the activity and selectivity simultaneously. Using descriptor α (relative electronegativity between doped metal and Pd), we reproduce the order of the activity and selectivity of the available experimental Pd-based alloy nanocatalysts, and also predict that Pd-W and Pd-Pb are promising substitution for Au-Pd, and W, Pb, Mo as well as Ru are more superior to Pt as a promoter for Au-Pd. Our calculations indicate these designed new Pd-based alloy nanocatalysts show better catalytic performance than the available experimental systems. By this work, we hope to attract the experimental confirmation of the remarkable performance of these designed new Pd-based alloy nanocatalysts for direct synthesis of H2O2 in near future. Overall, our strategy paves the avenue for the rational design of highly efficient new heterogeneous catalysts, which may also find applications beyond direct synthesis of H2O2.

Supporting Information The supporting information contains additional information regarding details of selection of model nanoparticles, details of adsorption free energy calculations, scaling relationships, reaction barrier inequality constraints, DOS analysis and sabatier analysis for predicted catalysts. This material is available free of charge via the internet at http://pubs.acs.org.


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ACKNOWLEDGMENT This work is supported by the National Natural Science Foundation of China (21576008, 91634116, 91334203, 21273268, 11574340), and "Hundred People Project" from Chinese Academy of Sciences. YG also thanks Special Program for Applied Research on Super Computation of the NSFC-Guangdong Joint Fund (the second phase).

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TOC Figure


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Design of High-Performance Pd-Based Alloy Nanocatalysts for Direct

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