Activating and Converting CH4 to CH3OH via the CuPdO2/CuO Nanointerface

Article abstract of DOI:10.1021/acscatal.9b00714

The activation and conversion of methane (CH₄) is one of the most challenging processes, because of the high chemical inertness of CH₄ and the subsequent uncontrollable overoxidation. Herein, we report that the CuPdO₂/CuO interface in Pd_xCu_1-xO/C can efficiently activate and convert CH₄ to CH₃OH using H₂O₂ or O₂ as the oxidant under mild conditions, where the CH₃OH yield (4076.5 μmol g^-1) and selectivity (93.9%) of the optimized Pd_0.3Cu_0.7O/C are much higher than those of PdO/C, CuPdO₂/C, and the mixture of CuPdO₂/C and CuO/C. Structural characterizations and mechanism studies reveal that the highly activity of Pd_0.3Cu_0.7O/C is attributed to the strong synergistic effects in Pd_xCu_1-xO/C. The formation of Pd⁴⁺ species at the interfaces of CuPdO₂ and CuO, which is promoted by the electron transfer from Cu to Pd, can selectively oxidize CH₄ to produce CH₃OH. This work highlights the importance of controlled interface in Pd-based catalysts for heterogeneous catalysis and beyond.

Full text of DOI:10.1021/acscatal.9b00714

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Article
Activating and Converting CH4 into CH3OH via CuPdO2/CuO Nanointerface
Shuxing Bai, Yong Xu, Pengtang Wang, Qi Shao, and Xiaoqing Huang
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.9b00714 • Publication Date (Web): 22 May 2019
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Activating and Converting CH into CH OH via CuPdO /CuO
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Nanointerface
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Shuxing Bai, Yong Xu, Pengtang Wang, Qi Shao, and Xiaoqing Huang *
1College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Jiangsu,
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15123, China.
2Institute of Functional Nano&Soft Materials (FUNSOM), Jiangsu Key Laboratory for Carbon-Based
Functional Materials & Devices, Soochow University, Jiangsu, 215123, China.
†These authors contributed equally to this work.
*To whom correspondence should be addressed. E-mail: hxq006@suda.edu.cn
Abstract: The activation and conversion of methane (CH ) is one of the most challenging processes due
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to the highly chemical inertness of CH and the subsequent uncontrollable over-oxidation. Herein, we
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report that the CuPdO /CuO interface in Pd Cu O/C can efficiently activate and convert CH into
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CH OH using H O or O as the oxidant under mild conditions, where the CH OH yield (4076.5 μmol g
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and selectivity (93.9%) of the optimized Pd Cu O/C are much higher than those of PdO/C,
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CuPdO /C and the mixture of CuPdO /C and CuO/C. Structural characterizations and mechanism
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studies reveal that the highly activity of Pd Cu O/C is attributed to the strong synergistic effects in
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Pd Cu O/C. The formation of Pd species at the interfaces of CuPdO and CuO, which is promoted by
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the electron transfer from Cu to Pd, can selectively oxidize CH to produce CH OH. This work
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highlights the importance of controlled interface in Pd-based catalysts for heterogeneous catalysis and
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Keywords: CuPdO , CuO, Interface, Methane conversion, Methanol
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INTRODUCTION
Methane (CH ) has been recognized as one of the most attractive feedstocks for producing high
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value-added products in chemical industry.1
3
Conventionally, CH4 is converted into liquid
hydrocarbons and oxygenates through the formation of CO and H , which is performed under high
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temperature.4,5 The increasing demand for energy requires chemists to develop new strategies for
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converting CH to high value-added products. Recently, the direct conversion of CH to methanol
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(CH OH) has attracted increasing attention. For instance, Kwon et al. demonstrated that CH can be
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converted to CH OH and methylhydroperoxide (CH OOH) on the single-atom Rh catalyst in the
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presence of hydrogen peroxide (H O ). They claim that single-atom Rh can selectively oxidize CH into
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CH intermediates and suppress the over-oxidation to form CO . Huang and co-workers reported the
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Pd O single sites on ZSM-5 with CuO as a cocatalyst can be used as highly selective catalysts for direct
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CH conversion with CH OH yield of 3934 μmol g and CH OH selectivity of 86.4%. Hutchings et al.
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reported that colloidal AuPd nanoparticles can be used as highly selective catalysts to catalyze CH4
towards primary oxygenates (CH OH, CH OOH, and formic acid) with selectivity of 88.0% in the
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presence of H O and oxygen (O ). Despite the tremendous progress, CH direct conversion to CH OH
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is extremely challenging due to the high C-H bond energy (439 kJ mol ) in CH , which results in the
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poor activity and/or low selectivity.10,11 Hence, it is highly desired to develop active and selective
catalysts for the direct conversion of CH to CH OH.
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Composite nanomaterials have recently attracted great attentions in heterogeneous catalysis,
because the synergies between each single component can significantly enhance the catalytic
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performance in terms of activity, stability and selectivity. Among those composite nanomaterials, the
heterostructures are among the most important topic, as the interface in heterostructures may play a vital
role in the activity and selectivity.1316 To date, the heterostructures have been widely used in CH OH
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synthesis, alcohols selective oxidation, CO reduction, etc.
For instance, Graciani reported that the
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metal and oxide interface (Cu/CeO ) is present as the active site for CO hydrogenation to CH OH.
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Huang and co-workers demonstrated that the Ag/Cu interface can selectively catalyze CO to ethylene.
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Our previous work showed that the high density of interfaces between platinum-nickel and nickel sulfide
components significantly enhanced the activity and stability towards hydrogen evolution reaction.22
Despite plenty of reports, CH direct conversion to CH OH on heterostructures has rarely been studied,
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which may open a new avenue for the applications of heterostructures in heterogeneous catalysis.
Considering non-precious metal-based catalysts (Cu, Fe, Mo, et al.) are generally inactive for CH4
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activation (90-150 kJ mol ), while precious metal-based catalysts (PdO, RhO , RuO and IrO ) are
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much more active CH activation (25-40 kJ mol ) but suffering poor selectivity.
We here prepare
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Pd Cu O/C heterostructures with CuPdO /CuO interface for CH direct conversion to CH OH in the
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presence of H O . We expect the synergistic effects between CuPdO and CuO at the interface can
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significantly enhance both the activity and selectivity. Results show that the CH OH yield and
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selectivity reach 4076.5 μmol g and 93.9%, respectively on the optimized catalyst of Pd Cu O/C,
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which are much higher than that from these reference catalysts (PdO/C, CuPdO /C and the mixture of
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CuPdO /C and CuO/C). Structural characterizations and mechanism studies reveal that the highly
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activity of Pd Cu O/C is attributed to the strong synergistic effects in Pd Cu O/C. The formation of
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Pd species at the interfaces of CuPdO and CuO, which is promoted by the electron transfer from Cu
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RESULTS AND DISCUSSION
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Initially, the Pd Cu nanoparticles (NPs) were prepared using a simple wet-chemical method,
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which adopted palladium (II) acetylacetonate (Pd(acac) ) and copper (II) chloride dihydrate
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CuCl ·2H O) as precursors, ascorbic acid as reducing agent and oleylamine as solvent. Taking
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Pd Cu NPs as example, the products are monodisperse with an average diameter of ~6.8 nm, as
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confirmed by the high angle annular dark field scanning transmission electron microscopy (HAADF-
STEM, Figure 1a). The Pd Cu1-x NPs with different molar ratios of Pd to Cu can be obtained by simply
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changing the amount of CuCl ·2H O supplied (Figure S1). Pd NPs and Cu NPs were also synthesized
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as references (Figure S2). The X-ray diffraction patterns (XRD, Figure 1b&Figure S3) of Pd Cu
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NPs match well with the face-centered cubic (fcc) phase PdCu (PDF No. 481551).26 The (111)
diffraction peaks of Pd Cu NPs shift from 41.4° (Pd Cu NPs) to 42.0° (Pd0.25Cu0.75 NPs) with the
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increase of Cu content (Figure S3). The elemental compositions of these Pd Cu NPs are determined
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by the scanning electron microscopy energy-dispersive X-ray spectroscopy (SEM-EDX, Figure S4),
where the atom ratios of Pd/Cu in Pd Cu NPs, Pd Cu NPs, Pd Cu NPs and Pd0.25Cu0.75 NPs
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are 51.5/48.5, 40.4/59.6, 31.3/68.7 and 25.7/74.3. The lattice space of 0.219 nm corresponding to the
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HRTEM, Figure S5) of Pd Cu NPs.
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Figure 1. Morphological and structural characterizations. (a) HAADF-STEM image of Pd Cu NPs. (b) XRD
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patterns of Pd NPs, Pd Cu NPs, PdO/C and Pd Cu O/C. (c) HRTEM image, (d) line-scan analysis, (e) elemental
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To study the catalytic performance on CH direct conversion, the as-prepared Pd Cu NPs were
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then loaded on carbon (C, Vulcan XC72R) with the loading amount of Pd at 1 wt%, where C as the
support for nanoparticles to prevent them from agglomeration and exclude the influence of the
complicated interface originating from inorganic oxides (Al O , SiO , TiO , etc). Afterwards, these
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samples were calcined at 350 °C in air for 1 h to form heterostructures (Figure S6). The XRD patterns
(Figure 1b red curve & Figure S7) of Pd Cu O/C show obvious peak at 34.6°, 35.5°, and 38.9°,
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corresponding to (101) of CuPdO (PDF No. 440185) and (111) and (200) of CuO (PDF No.
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450937). With the decrease of x in Pd Cu O/C, the diffractions peaks of CuO become more and
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more obvious, while the diffractions peaks of CuPdO are maintained (Figure S7). The slight preserved
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within the NPs, because the oxidation of metal NPs usually starts from the surface. The lattice fringes of
0.164 nm and 0.253 nm correspond to the (112) facet of CuPdO and the (002) facet of CuO (Figure 1c).
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diffraction spots, CuPdO and CuO. This segregation structure of Pd Cu O/C has been also confirmed
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by line-scan analysis (Figure 1d) and elemental mapping (Figure 1e), in which Pd mainly distribute
unilaterally of the NPs, while Cu distribute evenly in the whole NPs. These above results implied that
CuPdO /CuO heterostructures with CuPdO and CuO interfaces were formed in Pd Cu O/C after
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Pd Cu O/C, (c) Pd Cu O/C and (d) Pd Cu O/C, CuPdO /C and CuPdO /C+CuO/C. Reaction conditions: 10 mg
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catalyst, 0.5 MPa CH , 20 mL 1 mol L H O , 50 °C, 1 h.
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S9S13), where CH OH, CH OOH, CO, and CO were detected. Since the chemical state of the active
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component is vital for catalytic performances, we first explored the catalytic performances of active
component with metallic state (Pd/C, Cu/C, and Pd Cu /C, Figure 2a) and oxidation state (PdO/C,
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CuO/C, and Pd Cu O/C, Figure 2b). It was found that the Pd/C, Cu/C, and Pd Cu /C exhibit poor
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activity and low selectivity of oxygenates for direct CH conversion, where the overall yields are 85.1,
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89.0, and 702.5 μmol g and the oxygenates’ selectivities are 33.0%, 42.2%, and 56.0%, respectively.
Interestingly, the CuO/C, PdO/C, and Pd Cu O/C display improved activity and oxygenate selectivity,
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79.9%, and 88.3%, indicating that the catalytic performance was strongly enhanced over the
CuPdO /CuO heterostructures. To further reveal the contribution of interface, we have systematically
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decrease of Pd/Cu ratio in Pd Cu O/C leads to a volcano-shape of oxygenates’ yield and selectivity,
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indicating that the interface of CuPdO and CuO plays a vital role in CH direct conversion. To further
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quasi-spherical CuPdO NPs. CuO/C is inactive for CH direct conversion, giving a poor oxygenates’
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yield of 93.4 μmol g (Figure 2b). When CuPdO /C was used as catalyst for CH direct conversion, the
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yield of oxygenates strongly decreases to 970.6 μmol g , while the selectivity decreases to ~ 82.1%
(Figure 2d). When the mixture of CuPdO /C and CuO/C were used as catalyst, the yield of oxygenates
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slightly increases to 1133.0 μmol g , while the selectivity of oxygenates is kept identical at 82.2%. It
should be noted that all the catalysts above exhibit much poorer catalytic performance than
Pd Cu O/C, indicating that the interface of CuPdO and CuO can significantly enhance the CH direct
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conversion in terms of activity and selectivity. In addition, the effects of reaction temperature, reaction
time, H O concentration and CH partial pressure on CH direct conversion have also been investigated
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(
Figure S16). Pd Cu O/C exhibits the oxygenates’ yield of 5466.3 μmol g at a selectivity of 93.9%
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under the optimized conditions, which outperforms most previously reported catalysts (Table S1).
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Figure 3. Products yields and oxygenates selectivity for Pd Cu O/C with (a) O (0.5 MPa O , 50 °C) and (b) H O
2
0.3
0.7
2
2
2
-1
and molecular oxygen (0.5 MPa O , 20 mL 0.05 mol L H O , 50 °C) as oxidant. Reaction conditions: 10 mg catalyst,
2
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2
3
MPa CH , 20 mL H O, 50 °C. (c) Products yields and oxygenates selectivity for Pd Cu O/C over ten rounds of
4 2 0.3 0.7
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successive reactions. Reaction conditions: 10 mg catalyst, 3 MPa CH , 20 mL 1 mol L H O , 50 °C, 1h. (d) TEM
4
2
2
image of Pd Cu O/C after ten rounds of successive reactions.
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In addition to H O , O can also be used as an oxidant for CH direct conversion to CH OH. We
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2
2
4
3
further investigated the catalytic performance of CH direct conversion over Pd Cu O/C in the
4
0.3
0.7
presence of O . Figure 3a shows that the Pd Cu O/C displays the oxygenates’ yield and selectivity of
2
0.3
0.7
-1
3
56.3 μmol g and 18.9%, which are 4.1-fold and 2.7-fold higher than those of PdO/C, respectively.
Both the yield and selectivity of oxygenates are much lower than those when H O is used as the oxidant.
2
2
These phenomena are similar with Hutchings and coworkers’ work, which suggests that the activation of
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CH is triggered by radicals. Therefore, we added additional H O as radical initiating agent (0.05 mol
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L ) in the presence of O (0.5 MPa). As shown in Figure 3b, both the yield and selectivity of
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oxygenates significantly increase on PdO/C and Pd Cu O/C catalyst. In particular, for Pd Cu O/C
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.3
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catalyst, the oxygenates’ yield and selectivity increase to 3161.1 μmol g and 91.9%, respectively. Even
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though the enhancement in catalytic performance by adding H O , the yield and selectivity of
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oxygenates from Pd Cu O/C are 3.3-fold and 2.7-fold higher than those of the PdO/C catalyst. In
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.3
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addition, no obvious decays in the yield and selectivity of oxygenates were observed in ten consecutive
rounds, indicating that the Pd Cu O/C could be used as highly active, selective and stable catalyst for
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CH direct conversion (Figure 3cd).
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spectra and (c) Pd 3d XPS curves of PdO/C and Pd Cu O/C. (d) Schematic showing the proposed mechanism for
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CH activation and direct conversion.
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CH temperature-programmed reaction spectroscopy (TPRS) was performed to further study the
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1
mechanism of CH direct conversion on PdCuO/CuO heterostructures. As shown in Figure 4a and
4
Figure S17, to the contrast of free desorption on CuO/C, obvious features of CH , CO and H O are
4
2
2
observed in TPRS patterns, suggesting that CH molecules can strongly adsorb on PdO/C and Pd Cu
1-
4
x
O/C. The TPRS pattern of PdO/C exhibits the feature of CO at 185250 °C, indicating that PdO/C
x
2
favors to oxidize CH to CO (Figure 4a). Interestingly, in addition to CO , the feature of CH appears
4
2
2
4
*
in the TPRS patterns of Pd Cu O/C at 200450 °C, which is formed via the hydrogenation of CH on
x
1-x
3
the catalyst (Figure S17a). These results imply that Pd Cu O/C may tend to selective activate CH to
x
1-x
4
*
CH on Pd Cu O/C, leading to the formation of oxygenates in the presence of H O and/or O .
3
x
1-x
2
2
2
Compared to CuO/C and PdO/C, the peak intensities and areas are obviously different, indicating that
the chemisorption and activation are different among different catalysts. Furthermore, we used diffuse
reflectance infrared Fourier transform spectroscopy (DRIFTS) to investigate the intermediate species
after exposing these catalysts in CH . When CH was introduced, two groups of peaks are observed at
4
4
-1
3
018 and 1304 cm , which can be ascribed to the physical absorption of CH on catalysts (Figure S18).
4
After flushing with Ar to remove the physical absorption of CH , no obvious peaks are observed in the
4
spectrum of CuO/C, suggesting that CuO/C is inactive for CH activation under the indicated conditions
4
-1
(
Figure S19). However, the bands at 11001700 cm appeared on PdO/C and Pd Cu O/C (Figure
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.3
0.7
3
2,33
4b), illustrating the CH dissociation of CH on Pd oxide species.
On PdO/C, various intermediate
4
-1
*
2-
-1
*
-1
*
-1 *
species were detected at 1590 cm ( CO ), 1538 cm ( CO), 1453 cm ( CH O), 1378 cm ( CH ),
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2
3
-1
*
-1
*
*
1240 cm ( CO) and 1108 cm ( CH O), and the extremely weak peak of CH O is consistent with the
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result of low CH OH yield (77.1 μmol g ) using O as oxidant (Figure 3a). Notably, the strong bands
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*
appeared on Pd Cu O/C at 1538 cm and 1108 cm corresponding to the CH O species, indicating
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3
that Pd Cu O/C can selectively oxidize the strongly adsorbed CH to CH OH.
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Figure 5. (a) Pd K-edge XANES spectra of Pd foil, PdO, and Pd Cu O/C. (b) Cu K-edge XANES spectra of Cu foil,
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.3
0.7
CuO, and Pd Cu O/C. (c) Pd K-edge Fourier transform EXAFS spectra of Pd foil, PdO, and Pd Cu O/C. (d) Cu
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.3
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K-edge Fourier transform EXAFS spectra of Cu foil, CuO, and Pd Cu O/C.
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.3
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Afterwards, X-ray photoelectron spectroscopy (XPS) measurements were performed on different
catalysts. As shown in Figure 4c, two peaks at ~337.5 eV and ~342.7 eV were observed in the XPS
2+
2+
34
spectrum of PdO/C, which could be assigned to Pd 3d and Pd 3d , respectively. For the Pd Cu
1-
5
/2
3/2
x
2
+
O/C, in addition to the Pd , two peaks appeared at ~338.6 eV and ~343.7 eV in the XPS spectrum,
x
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which could be indexed as Pd 3d and Pd 3d , respectively.
Similar results were observed in
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other Pd Cu O/C catalysts with different compositions (Figure S20). No peaks corresponding to N
x
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indicate that the surface ligands have been removed (Figure S21). In order to exclude the influence of
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the support, the Pd4+ species and Pd2+ species were also observed in the Pd 3d XPS of Pd Cu O
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without support (Figure S22). More interestingly, when Pd Cu O/C heterostructures were etched by
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0.7
2
+
2+
acetic acid to remove CuO, only two peaks corresponding to Pd 3d and Pd 3d were observed in
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/2
3/2
4
+
the XPS spectrum (Figure S22). We thus speculated that the Pd species might play a vital role in the
4
+
enhancement of catalytic performance. In addition, we have further studied the effects of Pd ratio on
the TCH4 and CH OH yield over Pd Cu O/C catalysts. As depicted in Figure S23, it is shown that both
3
x
1-x
4
+
2+
4+
the TCH4 and CH OH yield were strongly relevant to the Pd /(Pd + Pd ). The optimized catalyst of
3
4
+
Pd Cu O/C, who gives the best DMC perforamcne, has the highest ratio of Pd (Figure 4d). X-ray
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.3
0.7
absorption near edge structure (XANES) measurement has further been performed at Pd K-edge and Cu
K-edge to study the synergistic effects in Pd Cu O/C. As shown in Figure 5a, compared to the
0
.3
0.7
references of Pd foil and PdO, the XANES spectrum from Pd Cu O/C gives similar features with PdO
0
.3
0.7
foil. However, the Pd K-edge obviously shifts to higher energy (inset of Figure 5a), indicating that the
Pd2+ and Pd4+ species exist in Pd Cu O/C. The XANES spectrum of Pd Cu O/C at Cu K-edeg
0.3
0.7
0.3
0.7
indicates that Cu in the catalyst has been oxidized into Cu2+ after calcination, which is in good
agreement with XPS results (Figure 5b). Interestingly, Cu K-edeg in Pd Cu O/C shifts to higher
0
.3
0.7
energy position in comparison to CuO reference, giving a higher intensity of white line (inset of Figure
3
8
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b). These results imply that electrons may transfer from Cu to Pd. The coordinations of Cu and Pd in
Pd Cu O/C are analyzed by extended X-ray absorption fine structure (EXAFS) spectra. As depicted in
0.3
0.7
Figure 5c5d, Pd foil and Cu foil gives a strong feature of PdPd and CuCu coordination at ~2.49 and
.23 Å, respectively. For PdO and CuO, PdO and CuO feature is observed at ~1.56 and 1.57 Å, while
2
the features of PdPd and CuCu appear at 2.293.56 and 2.243.18 Å, respectively. Despite the very
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similar PdO and CuO cooridination in Pd Cu O/C comparing to the PdO and CuO reference, it
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.3
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should be noted that the radial distance of PdO (1.58 Å) in Pd Cu O/C is slightly longer than that of
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PdO, while the the radial distance of PdO in Pd Cu O/C is slightly shorter than that of CuO (1.52 Å).
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This can be interpreted as electrons transfer from Cu to Pd leads to electron-rich Pd environment, as a
result of longer PdO bond. On the contrary, the electron-deficient Cu will lead to a shrunken CuO
bond. Therefore, the electron transfer from Cu to Pd in the Pd Cu O/C catalyst, which can
0
.3
0.7
significantly enhance catalytic performance of CH selective conversion into CH OH and CH OOH, has
4
3
3
been well revealed by XANES and EXAFS analysis. These strong synergies can not only promote the
*
selective activation of CH to CH species at the interface, but also suppress the dehydrogenation of
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*
39
CH OH to CHO, leading to a higher yield and selectivity of CH OH.
3
3
CONCLUSIONS
To summarize, a class of Pd Cu O/C serving as highly active, selective and stable catalysts for
x
1-x
direct CH conversion to CH OH have been demonstrated. These Pd Cu O/C catalysts possess hetero-
4
3
x
1-x
junction structure with the interfaces between CuPdO and CuO, and exhibit much higher activity and
2
selectivity for direct CH conversion to CH OH than those of PdO/C, CuPdO /C and the mixture of
4
3
2
CuPdO /C and CuO/C. Especially, the high activity and oxygenates’ selectivity of the Pd Cu O/C
2
0.3
0.7
outperform those of most previously reported catalysts. CH -TPRS and DRIFTS results demonstrate that
4
Pd Cu O/C can selectively oxidize CH to CH OH. Detailed characterizations show that the electron
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.3
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4
3
4
+
transfer from Cu to Pd will result in a high ratio of Pd species at the interface. This work highlights the
importance of designing Pd-based catalysts with precisely modulated interfaces for heterogeneous
catalysis and beyond.
ASSOCIATED CONTENT
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Supporting Information. Figure S1Figure S23&Table S1. This material is available free of charge via
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the Internet at http://pubs.acs.org.
Corresponding Author
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hxq006@suda.edu.cn
ACKNOWLEDGMENT
This work was financially supported by the Ministry of Science and Technology (2016YFA0204100,
2
017YFA0208200), the National Natural Science Foundation of China (21571135, 51802206), Young
Thousand Talented Program, Jiangsu Province Natural Science Fund for Distinguished Young Scholars
(BK20170003), the Priority Academic Program Development of Jiangsu Higher Education Institutions
(
PAPD), and the start-up supports from Soochow University.
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