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The Key Role of Support Surface Hydrogenation in the CH-toCHOH Selective Oxidation by ZrO-Supported Single-Atom Catalyst 3

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KARIM HARRATH, Xiaohu Yu, Hai Xiao, and Jun Li ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.9b02093 • Publication Date (Web): 19 Aug 2019 Downloaded from pubs.acs.org on August 19, 2019

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The Key Role of Support Surface Hydrogenation in the CH4-to-CH3OH Selective Oxidation by ZrO2Supported Single-Atom Catalyst Karim Harrath†, Xiaohu Yu‡, Hai Xiao*†, and Jun Li*†,§ †Department

of Chemistry and Key Laboratory of Organic Optoelectronics & Molecular

Engineering of Ministry of Education, Tsinghua University, Beijing 100084, China ‡Shaanxi

Key Laboratory of Catalysis and School of Chemical & Environment Sciences,

Shaanxi University of Technology, Hanzhong 723000, China §Department

of Chemistry, Southern University of Science and Technology, Shenzhen 518055,

China

ABSTRACT. Direct conversion of methane to methanol has attracted much interest and yet remains a challenge. Here, we investigate the catalytic mechanisms for methane oxidation on the Rh single-atom catalyst (SAC) dispersed on zirconia support Rh1/ZrO2 by first principles calculations. We find that, by comparing with other metal SACs dispersed on ZrO2, the spontaneous dissociative adsorption of H2O2 on Rh1/ZrO2 surface is a key factor that initiates the active site and hydrogenates the surrounding oxide support surface. We further reveal that the hydrogenation of oxide support surface plays a key role in suppressing the further production of

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CH3OOH and CO2. Additionally, we propose a non-noble-metal Fe1/ZrO2 SAC as an active and selective catalyst for direct CH4-to-CH3OH conversion, potentially performing better than the current best Rh1/ZrO2 SAC. Our findings provide insights for design of highly selective and efficient catalysts for direct CH4-to-CH3OH conversion.

KEYWORDS. methane to methanol, selectivity, single-atom catalyst, support surface hydrogenation, density functional theory calculations.

INTRODUCTION Converting methane into methanol unleashes its great potential as a chemical feedstock.1-6 The current industrial practice is to firstly reform methane to syngas, followed by conversion of syngas to methanol. This process requires high temperatures (up to 1300 K) and high pressures (up to 30 bar), and thus it is an energy-inefficient route.7-12 Alternatively, methane can be directly converted to methanol via selective activation of C-H bond, which is potentially more economical and environmentally friendly.13-18 However, the direct conversion of methane to methanol suffers from low productivity and low selectivity. Thus, great efforts have been devoted to designing highly efficient and selective catalysts. Inspired by the biocatalytic enzyme systems,19 various metal-exchanged zeolite catalysts were reported to be capable of converting methane to methanol via a stepwise activation method.20-29 For example, the Cu-exchanged ZSM-5 was shown to yield methanol with 98% selectivity.30 Nonetheless, they are not viable for industrial applications because of low


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productivity. This is due to both the low concentration of active sites in zeolite catalysts and the high desorption energy of methanol. Additionally, Cu-oxo and bis(μ-oxo) dicopper clusters stabilized in the metal-organic framework (MOF) were reported to catalyze selective oxidation of CH4 to CH3OH. Similar to metalexchanged zeolite catalysts, these MOF-based catalysts suffer from low productivity as well.31,32 Methane activation has also been achieved using metal clusters and metal oxide catalysts, but unfortunately these systems lead to complete oxidation of methane to CO2. Recently, single-atom catalysts (SACs) have aroused considerably interest in heterogeneous catalysis due to its great potential for high activity and selectivity.33-35 For CH4 activation, coking resistance has been reported for the Pt/Cu single-atom alloy (SAA),36 and it has been shown that the single-atom Pd and Pt-loaded ceria (Pd1/CeO2 and Pt1/CeO2) deliver improved activity.37,38 Besides, a few SACs have been theoretically proposed as candidate catalysts for selective oxidation of methane to methanol.39-41 Kwon et al.42 demonstrated that the SAC with Rh dispersed on zirconia support Rh1/ZrO2 can activate methane at mild conditions, and convert methane to methanol using H2O2 as the oxidant. The yield can reach 1.07 µmol of methanol per 1µmol Rh site in 30 min, which is the current record. Thus, this work represents an important step forward. However, besides methanol, the Rh1/ZrO2 SAC delivers methyl hydroperoxide (CH3OOH) and CO2 as by-products. Also, the Rh SAC renders a drastically different product spectrum from those by Pd, Ir and Pt counterparts. These results point to one key question: what are the atomistic mechanisms relating the activity and selectivity to the local structure of Rh SAC active site? Answering this would provide a better understanding of the nature of active site in Rh1/ZrO2 and more importantly the guidelines for design of highly efficient and selective SACs for CH4-to-CH3OH conversion under mild conditions.


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Herein, we explore the pivotal features in the selective oxidation of methane to methanol catalyzed by Rh1/ZrO2 using density functional theory (DFT) calculations. By carrying out a comprehensive analysis of the mechanisms for methane oxidation to methanol, CH3OOH and CO2 on various metal SACs anchored on ZrO2, we reveal that the initial dissociation of H2O2 on catalyst surface is crucial for preparing the active site configuration, and that the hydrogenated oxide support surface plays a decisive role in the selectivity of methane oxidation. We further predict that the Fe1/ZrO2 SAC is a potentially more efficient and selective candidate catalyst than the current record Rh1/ZrO2 SAC.

COMPUTATIONAL DETAILS The spin-polarized DFT calculations were performed using the Vienna ab initio simulation package (VASP),43-45 with the Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional46 and the +U correction with a value of 4 eV for the Hubbard correction on the d orbitals of Rh, Zr, Pt, Pd, Ir, Fe, Ru, Ni, Cu and Os.47 The projector augmented wave method (PAW)48,49 with a plane-wave kinetic energy cutoff of 400 eV was used. The Brillouin zone was sampled by only the Γ-point. We have tested a finer k-point mesh of 3×3×1 for sampling the Brillouin zone (Table S1) and found no significant difference (~0.1 eV or smaller) from the Γ-point sampling. The support ZrO2(101) surface was modeled by a (2 × 2) supercell with a size of 12.605 Å × 10.775 Å and a vacuum layer of 15 Å, and the surface is O-terminated. Such setup of supercell size is sufficient to eliminate interactions between periodic images for modeling surface chemistry involving only small molecules. The support slab contains 9 atomic layers (3 O-Zr-O tri-layers), with 72 O atoms and 36 Zr atoms. We have tested a thicker slab model with


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12 atomic layers (4 O-Zr-O tri-layers) and found no significant difference (< 0.1 eV) from the slab model employed here. The experimental XPS and EXAFS results showed that the Rh single atom substitutes the Zr site of support surface.36 Thus, the ZrO2-supported SAC models here were constructed with the transition metal single atom substituting a Zr atom on the surface (see Figure 1). All atoms were allowed to relax during geometry optimization, and the atomic positions were optimized till the forces were less than 0.02 eV/Å. The transition states (TS) were searched by the Dimer method50 and further confirmed by vibrational frequency analysis. Only one imaginary frequency was found for each of the TS structures reported in this work. The calculated relative Gibbs free energy (G), either the reaction energy or the barrier, was evaluated as G = E + ZPE – TS, where E is the DFT-calculated relative energy, ZPE is the change of zero-point energy (ZPE), and S is the change of entropy. The ZPE and entropic contributions were calculated from the vibrational frequencies, which were obtained using the finite displacement method with only the degrees of freedom of adsorbates and the transition metal single atom included. The adsorption energy is defined as Eads = E(adsorbate/slab) − E(adsorbate) − E(bare slab), where E(adsorbate/slab), E(adsorbate), and E(bare slab) represent the total energies of surface slab with the adsorbate, the isolated adsorbate molecule, and the optimized bare slab, respectively.

RESULTS AND DISCUSSION We first examine the adsorption of H2O2 on M1/ZrO2 (M = Rh, Pd, Ir, Pt) catalyst surfaces. Previous studies investigated the reactivity of H2O2 on oxide surfaces, including zirconia, and it was shown that the decomposition of H2O2 proceeds via the homolytic dissociation of H2O2 to


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form surface-bound hydroxyl radicals.51 Here we find that the adsorption of H2O2 on M1/ZrO2 surfaces occurs by two types, i.e., the dissociative adsorption in the Rh1/ZrO2 and Pd1/ZrO2 cases, and the molecular adsorption in the Ir1/ZrO2 and Pt1/ZrO2 cases, as shown in Figure 1.

Figure 1. The preferred H2O2 adsorption configurations on (a) Rh1/ZrO2, (b) Pd1/ZrO2, (c) Ir1/ZrO2, and (d) Pt1/ZrO2.

Table 1. The adsorption energies (Eads, in eV) of H2O2 and O2 molecule resulted from dissociative adsorption of H2O2 on Rh1/ZrO2, Pd1/ZrO2, Ir1/ZrO2, and Pt1/ZrO2. The O-O bond lengths (d, in Å) of O2 are also listed.

SACs

Eads(H2O2) Eads(O2)

Rh1/ZrO2

-2.87

-0.82

d(O-O) 1.32


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Pd1/ZrO2

-3.28

Ir1/ZrO2

-1.49

Pt1/ZrO2

-1.32

-0.22

1.25

Especially noteworthy is the finding that the dissociation of H2O2 on Rh1/ZrO2 and Pd1/ZrO2 surfaces leads to the formation of adsorbed O2 molecule (*O2) and hydrogenation of surface with two *H. Table 1 shows that the binding energy of *O2 on Rh1/ZrO2 is much larger than that on Pd1/ZrO2.This results in the formation of O2Rh surface site with chemisorbed O2 in the Rh1/ZrO2 case, while the O2 molecule is released from the Pd1/ZrO2 surface spontaneously with the entropic contribution. Thus, among the four M1/ZrO2 cases, the Rh1/ZrO2 catalyst presents a unique starting configuration (O2Rh/ZrO2-2H, see Figure 1a) initiated by the adsorption of H2O2, and it is most likely this particular configuration that delivers the selective oxidation of methane, different from on the other three M1/ZrO2 catalysts.


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Figure 2. The reaction network for conversion of methane on the Rh1/ZrO2 SAC (see Fig. S10 for the detailed configurations of numbered species).

Either the released O2 on Pd1/ZrO2 or the intact H2O2 on Ir1/ZrO2 and Pt1/ZrO2 renders the scenario similar to direct oxidation of methane with O2 or H2O2, which is neither efficient nor selective. Thus, we focus on the methane oxidation on Rh1/ZrO2. Figure 2 shows the elementary steps composing the reaction network we predict for the conversion of methane on the Rh1/ZrO2 catalyst (see Figures S1-S4 for more details). After the adsorption of methane on O2Rh/ZrO2-2H, the C-H bond activation is started by H abstraction with a barrier of 1.23 eV, resulting in a methyl radical and the hydrogenated HOO-Rh site. The methyl radical can further react with the HOO-Rh site to produce CH3OH or CH3OOH with a barrier of 0.30 or 0.67 eV, which is in line with the experimental results that CH3OOH is a minor product.


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In the alternative non-radical pathway, the methyl radical is quenched by adsorbing to an O atom on the catalyst surface, and the resulted *CH3 has to overcome a large barrier of 2.66 or 2.16 eV to further react with the HOO-Rh site to deliver CH3OH or CH3OOH, which cannot be driven under mild conditions. Thus, the methane activation and conversion on the Rh1/ZrO2 catalyst proceeds via the radical mechanism, which is in line with the results by Nørskovet al.52 Following the desorption of the first CH3OH, a second methane molecule can be adsorbed on the ORh/ZrO2-2H site, and is easily activated to generate a methyl radical with a barrier of 0.60 eV, much lower than that for the first methane activation by the O2Rh/ZrO2-2H site. The second CH3OH is then produced with a barrier of 0.39 eV also via the radical mechanism, as the nonradical mechanism presents a much larger barrier of 2.38 eV. To complete the catalytic cycle, the ORh/ZrO2-2H active site is regenerated by a second H2O2 molecule that is reduced to a water molecule. It is worth noting here that the Rh site is reoxidized by H2O2 to the ORh site, instead of the O2Rh site in the initialization step, and thus the pathway toward production of CH3OOH is then eliminated due to the absence of HOO-Rh site. This difference is attributed to the presence of hydrogenated support ZrO2-2H with hydroxyl groups surrounding the SAC metal site on the surface. Thus, the hydrogenation of support ZrO2 around the SAC site to block the formation of O2M site plays a key role in ruling out the byproduct CH3OOH. To understand the product spectrum, it is equally important to investigate the pathways toward CO2 production, and we examine three different channels to CO2, which start from the methyl moiety, CH3OH, and CH3OOH, respectively. The first channel can proceed only via the reaction between *CH3 and HOO-Rh to deliver *CH2 and H2O, which has a barrier of 1.48 eV (see


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Figure S5). This is much higher than the barrier for CH3OH formation, indicating that it is kinetically unfavorable to produce CO2 from further activating the C-H bonds of *CH3. The second channel starts with the dissociation of CH3OH to form *OCH3 and *H (Figure S7) with a barrier of 0.55 eV, which is already higher than the desorption energy of CH3OH (0.35 eV). If the dissociation ever takes place, the resulted *OCH3 group can be further dehydrogenated to form *OCH2, which requires a low barrier of 0.36 eV and is thus plausible. This result is not surprising, as a major obstacle for selective oxidation of methane to CH3OH is the fact that CH3OH is more reactive than methane and thus it can be easily further oxidized to CO or CO2.The experiment indeed showed that a small amount of CH3OH is converted to CO2 on Rh1/ZrO2. However, the *OCH3 group can also be converted back to CH3OH by the hydrogenated oxide support surface with a barrier of 0.49 eV. This implies that the hydrogenation of oxide support surface might also serve as a key factor to diminish the production of CO2. The third channel proceeds via also the dissociation of CH3OOH with a negligible barrier of 0.11 eV to generate *OCH3 and *OH (Figure S6). Afterwards, this channel just follows the pathways similar to the CH3OH case, and *OCH3 can be either hydrogenated by the hydrogenated oxide support surface to produce CH3OH with a barrier of 0.53 eV, or further dehydrogenated toward CO2 production with a barrier of 0.40 eV. It is therefore clear that the hydrogenation of oxide support surface plays a crucial role in selective oxidation of methane to CH3OH by suppressing the conversion to the by-products CH3OOH and CO2. Thus, we propose tuning the degree of hydrogenation of oxide support


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surface as a potential strategy to significantly improve the selectivity for methane oxidation to CH3OH. A recent study by Rodriguez et al.53 demonstrated, by both experimental and computational investigations, that the adsorption of water on Ni/CeO2 catalyst surface blocks the sites for the production of CO and CO2, and thus promotes the CH3OH formation. This simple site-blocking mechanism is intrinsically different from our finding for the Rh1/ZrO2 catalyst, on which the hydrogenation of oxide support surface inhibits the formation of O2Rh site for CH3OOH production, and also provides the reducing environment to suppress further oxidation to CO2.


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Figure 3. The optimized H2O2 adsorption structures on (a) Au1/ZrO2, (b) Ag1/ZrO2, (c) Ni1/ZrO2, (d) Cu1/ZrO2, (e) Fe1/ZrO2, (f) Os1/ZrO2, and (g) Ru1/ZrO2.

With an atomistic understanding of mechanisms and key factors for the performance of selective oxidation of methane on Rh1/ZrO2 (compared to on Pd1/ZrO2, Ir1/ZrO2, and Pt1/ZrO2), we further performed a comprehensive investigation on the adsorption of H2O2 for the crucial initial hydrogenation on other M1/ZrO2 SACs with M = Fe, Co, Ni, Cu, Ru, Ag, Os, and Au, to explore for possible alternative catalysts with high selectivity and activity. Figure 3 and Table S2 show that the dissociative adsorption of H2O2 to prepare the key initial hydrogenation of oxide support surface occurs spontaneously only on Ru1/ZrO2 and Fe1/ZrO2 surfaces. Thus, these two SACs may serve as alternative candidate catalysts for converting methane to methanol. We then investigate the full reaction mechanisms of methane conversion on Ru1/ZrO2 and Fe1/ZrO2. The results (Figures 4 and S8) show that both SACs can deliver CH3OH from methane at mild conditions via the radical pathway, similar to on the Rh1/ZrO2 SAC. However, we find that the Ru1/ZrO2 SAC can produce CH3OOH as well with a favorable barrier of 0.57 eV, while the pathway to produce CH3OOH on the Fe1/ZrO2 SAC is kinetically blocked by a high barrier of 2.77 eV. In addition, we find that the initial step for CO2 production, i.e., the dehydrogenation of CH3OH to form *OCH3 group, on the Fe1/ZrO2 SAC can hardly occur, owing to the hydrogenated support surface (Figure S9). This again highlights the key role of hydrogenation of catalyst surface in preventing further oxidation of the CH3OH product. Thus, we propose the Fe1/ZrO2 SAC as a selective candidate catalyst for converting methane to methanol.


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Further comparing the proposed Fe1/ZrO2 SAC with the experimentally established Rh1/ZrO2 SAC, our results (Figure 4 and Figure 2) show that the methanol production on the OFe/ZrO2 site is more favorable than on the ORh/ZrO2 site, with the barriers for methyl radical formation and methanol formation on OFe/ZrO2 lowered by 0.49 and 0.13 eV, respectively, than those on ORh/ZrO2. Thus, the Fe1/ZrO2 SAC is potentially a more active catalyst than the current best Rh1/ZrO2 SAC.

Figure 4. The reaction network for conversion of methane on the Fe1/ZrO2 SAC (see Fig. S12 for the detailed configurations of numbered species).

CONCLUSION


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In summary, we investigate the complete reaction mechanisms for the oxidation of methane to methanol and other by-products on different M1/ZrO2 SAC models. Thus, we reveal that the spontaneous dissociative adsorption of H2O2 on the current best-performing Rh1/ZrO2 SAC, distinguishing it from the other experimentally explored M1/ZrO2 SACs (M = Pd, Ir, Pt), is a key factor for selective oxidation to methanol that initiates an active O2Rh site and hydrogenates the surrounding oxide support surface. The hydrogenation of oxide support surface is shown to play a key role in suppressing the further production of CH3OOH and CO2. Furthermore, we propose the Fe1/ZrO2 SAC as a highly selective and active catalyst for converting methane to methanol, with potentially better performance than the current record Rh1/ZrO2 SAC. Our findings provide guidelines for further advancing the novel design of non-noble-metal catalysts for direct CH4-toCH3OH conversion.

ASSOCIATED CONTENT Supporting Information This material is available free of charge via the Internet at http://pubs.acs.org. Supplementary figures and tables are provided.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] *E-mail: [email protected]


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ACKNOWLEDGMENT This work is financially supported by the National Natural Science Foundation of China (Grant Nos. 21590792, 91426302, and 21433005) to J.L., the Thousand Talents Plan for Young Scholars to H.X., and the Natural Science Basic Research Program of Shaanxi Province (Grant No. 2019JM-226) to X.Y. The calculations were performed using supercomputers at Tsinghua National Laboratory for Information Science and Technology.

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