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Undercoordinated Site-Abundant and TensileStrained Nickel for Low-Temperature COx Methanation Hao Wang, Ke Xu, Xuanyu Yao, Danhong Ye, Yan Pei, Huarong Hu, Minghua Qiao, Zhen-Hua Li, Xiaoxin Zhang, and Baoning Zong ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b02944 • Publication Date (Web): 02 Jan 2018 Downloaded from http://pubs.acs.org on January 2, 2018
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ACS Catalysis
Undercoordinated Site-Abundant and Tensile-Strained Nickel for Low-Temperature COx Methanation Hao Wang,†, Ke Xu,†, ‡ Xuanyu Yao,† Danhong Ye,§ Yan Pei,† Huarong Hu,§ Minghua Qiao,*,† Zhen Hua Li,*,† Xiaoxin Zhang,‡ Baoning Zong*,‡ †
Collaborative Innovation Center of Chemistry for Energy Materials, Department of Chemistry and Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Fudan University, Shanghai 200433, P. R. China ‡ State Key Laboratory of Catalytic Materials and Chemical Engineering, Research Institute of Petroleum Processing, SINOPEC, Beijing 100083, P. R. China § Shanghai Institute of Space Power-Sources, Shanghai 200245, P. R. China
Supporting Information Placeholder ABSTRACT: By means of the rapid quenching (RQ) technique, we fabricate RQ Ni with peculiar undercoordinated sites (UCSs)abundant and tensile-strained structural characteristics. In liquidphase CO methanation at 473 K, RQ Ni displays markedly higher specific activity and CH4 selectivity than Raney Ni, supported Ni, and Al2O3-supported Pd and Pt. RQ Ni shows comparable activity but higher CH4 selectivity as compared to Ru/Al2O3, with Ru being documented as the most active metal for CO methanation. Density functional theory (DFT) calculations confirm that the UCSs are the active centers and reveal that the tensile-stain effect can further accelerate the rate-limiting CO dissociation step. Attractively, RQ Ni is also powerful in converting the greenhouse gas CO2 to CH4 at 473 K with unprecedentedly high TOF of CO2 of 86.9 × 10–3 s–1 and impressively high selectivity of >99%. KEYWORDS: nickel, rapid quenching, undercoordinated site, tensile strain effect, methanation
ventional synthetic strategies.7 Recently, we reported that for RQ Fe dealloyed from the RQ Fe50Al50 alloy casted by the rapid quenching (RQ) technique capable of solidifying the melt at a speed of ca. 107 K s–1, its Fe–Fe coordination number (CN) is 4.0, only one half of the standard Fe–Fe CN of body centered cubic (bcc) Fe. Moreover, the Fe–Fe distance (R) is expanded to 2.50 Å as compared to 2.48 Å for the bcc Fe standard.8 In this contribution, we report that the same approach can fabricate the Ni analogue, RQ Ni, with the desired structural characteristics for low-temperature CO methanation. At 473 K and using the slurry-phase reactor to dissipate the reaction heat, RQ Ni displays excellent activity, selectivity, and robustness in CO methanation.
M
ethane serves as a clean energy carrier and a potential feedstock for high-valued chemicals.1 Due to the finiteness of natural gas, producing synthetic natural gas (SNG) from the more abundant coal- or renewable biomass-derived syngas (a mixture of CO and H2) becomes a current research focus.2 Thermodynamically, CO methanation is favored at low temperature.3 However, the existing Ni-based catalysts are kinetically inefficient below 500 K (Table S1). It has been identified that the dissociation of CO is the rate-limiting step (RLS) in CO methanation.4 By DFT calculations, ultra-high vacuum experiments on Ni single crystals, and catalytic measurements on supported Ni catalysts, Chorkendorff and coworkers elucidated that the UCSs serve as the active centers for hydrogen-assisted CO dissociation through COH in the presence of H2 and direct CO dissociation in the absence of H2.5 Furthermore, by DFT calculations of CO dissociation on tensile-strained and unstrained Ru(0001) surfaces, Mavrikakis et al. uncovered that the CO dissociation barrier decreases proportionally with the expansion of crystal lattice.6 These enlightening fundamental insights indicate that it is feasible to obtain an active catalyst in low-temperature CO methanation as far as one can impart abundant UCSs and distinct tensile strain to Ni. Unfortunately, fabricating such a catalytic material remains a great challenge for con-
Figure 1. (a) The scheme for the fabrication of the RQ Ni50Al50 alloy (Ni/Al, w/w) and RQ Ni. (b) XRD patterns, (c) and (d) HRTEM images with two-dimensional lattice fringes, and (e) the RDFs after Fourier transformation of the Ni K-edge k3-weighted χ(k) data of RQ Ni, Raney Ni, and the Ni foil. Inset in (e) compares the position of the first Ni–Ni coordination shell of RQ Ni with those of Raney Ni and the Ni foil. The scheme for fabricating the RQ Ni50Al50 alloy (Ni/Al, w/w) and RQ Ni is illustrated as Figure 1a. The preparation details are described in the Supporting Information. Similar to Raney Ni dealloyed from a commercial Ni50Al50 alloy (Fluka), RQ Ni deal-
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Table 1. The Structural Parameters Derived from the k3Weighted Curve Fittings of the Ni K-Edge EXAFS Data of RQ Ni and Raney Ni ∆E0 (eV)
2.48
6.23
2.73
6.78
2.49
10.56
3.18
Ni–Al
1.25
2.44
2.65
1.84
Ni–Ni
5.13
2.51
11.54
6.04
Ni-Al
1.13
2.46
3.28
3.62
CNa
Rb (Å)
Ni foil
Ni–Ni
12.0
Raney Ni
Ni–Ni
RQ Ni
a
∆σ2 (10−3 Å2)
pair
sample
b
Error range: ±10%. Error range: ±0.02 Å.
For alloys fabricated by the RQ technique, the metal atoms are frozen before reaching their equilibrium positions, as the solidification speed surpasses the diffusive speed of the metal atoms in the melt.11 On the other hand, Hashimoto found that the higher the solidification speed is, the more the defects are formed in the alloy.12 These structural features are anticipated to exist in the RQ Ni50Al50 alloy and carried over by RQ Ni. Moreover, the degree of undercoordination is further enhanced after the leaching of Al,
thus rendering RQ Ni with abundant UCSs and distinct tensile strain. In CO methanation at 473 K, the highly undercoordinated and tensile strained RQ Ni exhibits high specific activity (5.49 molCO molNi–1 h–1) and CH4 selectivity (95.7%), as illustrated in Figure 2. On Raney Ni, the corresponding values are only 1.23 molCO molNi–1 h–1 and 90.8% under the same reaction conditions. Throughout the reaction course, the CO conversion over RQ Ni is higher than that over Raney Ni (Figure S4a), and the product distributions are essentially invariable (Figures S4b and S4c). The CH4 selectivities are also insensitive to the reaction temperature of 443–473 K (Figures S5a and 5b) and the H2/CO volume ratio of 1–4 (Figures S6a and 6b), but increase with the pressure first, and then level off at above 30 bar (Figures S7a and 7b). C2H6
C3 H8
C4H10
CO2
100
6 80
5
60
4 3
40
2
Selectivity (%)
CH4
7
20
1 0 3
3
l2 O
u/ A R
3
l2 O
/A
N
/A Pd
Pt
i
l2 O
i N
Q R
M C
ey
T-
an R
H
2
N
i/M
gO
2
2
O
iO i/S
N
i/T i
i/Z r N
O
0
N
loyed from the RQ Ni50Al50 alloy bears the skeletal texture (Table S2). X-ray diffraction (XRD) patterns in Figure 1b show that both RQ Ni and Raney Ni are mainly constituted by the face centered cubic (fcc) Ni phase (JCPDS 04-0850). X-ray photoelectron spectroscopic (XPS) spectra of the Ni 2p3/2 level (Figure S1) confirm the metallic nature of Ni. An additional small peak from the Ni2Al3 phase (JCPDS 14-0648) is resolved for RQ Ni (Figure 1b). Linear fittings of the X-ray absorption near-edge structures (XANES) at the Ni K-edge (Figure S2) also verify the presence of more Ni2Al3 in RQ Ni than in Raney Ni, which is in agreement with previous finding that the Ni2Al3 phase in the RQ Ni–Al alloys is more resistant to dealloying plausibly due to its nonstoichiometric composition induced by rapid quenching.9 By excluding the contribution from Ni2Al3, we obtain the 2θ of 44.2o for the Ni(111) peak of RQ Ni, which corresponds to the spacing of the (111) planes (d111) of 2.051 Å. This spacing is substantially larger than the standard value of 2.034 Å for fcc Ni (JCPDS 040850) and that of 2.035 Å for Raney Ni derived from its 2θ of 44.4o. The difference in d111 is immediately visualized by highresolution transmission electron microscopy (HRTEM), which reveals that the distance between two adjacent (111) planes is 2.06 Å for RQ Ni, while it is 2.03 Å for Raney Ni (Figure 1c vs. Figure 1d). Figure 1e presents the non-phase corrected radial distribution functions (RDF) transformed from the k3χ(k) data of the Ni Kedge of RQ Ni, Raney Ni, and the Ni foil as a reference. For RQ Ni, the main Ni–Ni coordination peak appears at a distance longer than those for Raney Ni and the Ni foil (inset in Figure 1e). Meanwhile, the peak amplitudes of both RQ Ni and Raney Ni are much weaker than that of the Ni foil, reflecting the presence of fewer Ni neighbors. Table 1 summarizes the structural parameters by simulating these extended X-ray absorption fine structure (EXAFS) data (Figure S3). For the Ni foil, the CN and R of Ni–Ni are 12.0 and 2.48 Å, respectively.10 The R of Ni–Ni is 2.49 Å for Raney Ni, which is substantially shorter than 2.52 Å for RQ Ni. Furthermore, both RQ Ni and Raney Ni are highly populated with the UCSs, as manifested by small CNs of Ni–Ni of 5.30 and 6.78, respectively; and RQ Ni is more defective than Raney Ni. The CN of Ni–Ni of Raney Ni derived here is in-between the values of 6.610a and 6.910b reported previously for Raney Ni.
Activity (molCO molM−1 h−1)
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Figure 2. Comparison of the specific activity and product distribution of RQ Ni with those of Raney Ni and the suported Ni, Ru, Pd, and Pt catalysts in low-temperature CO methanation. Reaction conditions: T = 473 K, p = 30 bar at RT, H2/CO/N2 = 72/24/4 by volume, 1.0 g supported catalysts or 0.50 g RQ Ni and Raney Ni, 40 mL of ethylene glycol, and stirring rate of 1000 rpm. The CO conversions are controlled at around 5% to obtain the kinetically meaningful activity and to compare the product selectivity at similar conversion level. Activity = (moles of CO converted)/(moles of metal × reaction time). With the exception of RQ Ni, Raney Ni outperforms the Ni/MgO, Ni/SiO2, Ni/TiO2, and Ni/ZrO2 catalysts prepared by the deposition–precipitation method in both activity and selectivity in CO methanation at 473 K (Figure 2). The supports chosen are commonly used in the Ni-based HT CO methanation catalysts.13 Figure 2 also shows that a commercial Ni/Al2O3 methanation catalyst dedicated to the removal of trace amount of CO in H2 at 438–473 K (HT-MC, Haitai Sci-Tech) is only about 70% as active as Raney Ni, though more active than other supported Ni catalysts. Aside from the supported Ni catalysts, the Al2O3-supported Ru, Pd, and Pt precious metal catalysts (Alfa Aesar) are evaluated under the same reaction conditions. The specific activities rank as Raney Ni < Pd/Al2O3 < Pt/Al2O3 < RQ Ni < Ru/Al2O3 (Figure 2), which agrees with the consensus that Ru represents the most active metal for CO methanation.14 However, the specific activity of Ru/Al2O3 is not significantly superior to RQ Ni (5.92 molCO mol–1 –1 h vs. 5.49 molCO molNi–1 h–1), and its CH4 selectivity is far Ru inferior to RQ Ni (57.6% vs. 95.7%) owing to the high chaingrowth probability over Ru.15 Furthermore, the precious Ru is not economic in low-temperature CO methanation, as it is about two orders of magnitude more expensive than the earth-abundant Ni.14b,16 On the basis of the reaction courses in Figure S4a and the active surface areas of Ni (SNi) in Table S2, the TOFs of CO over RQ Ni and Raney Ni at 473 K are derived as 10.3 × 10–2 s–1 and
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ACS Catalysis 2.11 × 10–2 s–1, respectively. Although the compositions of RQ Ni and Raney Ni prepared by typical alkali leaching procedures are not the same (Table S2), Raney Ni with the composition identical to that of RQ Ni by modifying the alkali leaching conditions does not show significant improvement in the activity (Table S1), signifying that the composition difference is not crucial for the difference between the activities of RQ Ni and Raney Ni. In sharp contrast, the TOFs of CO over the Ni(100) single crytal and supported Ni catalysts at comparable temperatures (463–493 K) are one- to two-order of magnitude lower (Table S1). At 573 K, Li et al. reported a TOF of CO of 9.2 × 10–2 s–1 over Ni/Al2O3,17 which is still lower than the value over RQ Ni recorded at 100 Klower temperature. The Arrhenius plots in Figure 3a afford the apparent activation energies (Ea) of 83.1 kJ mol–1 (0.85 eV) over RQ Ni and 105.1 kJ mol–1 (1.09 eV) over Raney Ni, confirming that RQ Ni is indeed intrinsically more active for CO methanation. Note that the Ea over Raney Ni complies with the Ea values of 1.0–1.1 eV reported for CO methanation over both Ni single crystals and supported Ni catalysts.18 Chorkendorff and co-workers identified by DFT calculations that the saturated terrace sites on the Ni(111) surface are catalytically inactive in CO methanation. For comparison, the effective activation energies are only 1.08–1.30 eV over the stepped sites on surfaces such as Ni(311) and (211), which match well with the experimentally determined Ea values within the error range of the DFT method. They concluded that only the UCSs function as the active centers.5 Hence, we interpret the identical Ea but higher TOF of CO over Raney Ni relative to the literature Ni catalysts (Table S1) as the existence of more UCSs on Raney Ni, as manifested by its low CN of Ni–Ni (Figure 1d and Table 1). In contrast, the UCSs only occupy a small fraction (99%. It is probable that the additional O* species dissociated from CO2 function as the partition between adjacent CHx species, thus impeding their polymerization to longer hydrocarbons. CO2 conversion (%)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
0
Raney Ni
RQ Ni
Figure 4. The CO2 conversion and CH4 selectivity over RQ Ni and Raney Ni in low-temperature CO2 methanation. Reaction conditions: T = 473 K, p = 30 bar at RT, H2/CO2/N2 = 76/19/5 by volume, 0.50 g catalyst, 40 mL of ethylene glycol, stirring rate of 1000 rpm, and reaction time of 1.0 h. In summary, we demonstrate that RQ Ni derived from the rapid quenching technique is highly active, selective, and robust in lowtemperature CO methanation. Highly abundant UCSs and distinct tensile strain are responsible for the outstanding catalytic performance, which jointly enable the dissociation of CO at low temperature. RQ Ni also displays unprecedentedly high activity in low-temperature CO2 methanation with >99% selectivity to CH4. This work not only shows promise for the development of a novel low-temperature route for COx utilization, but also opens a new avenue to upgrade the catalytic activity of the inexpensive earthabundant metals to approaching or surpassing those of the expensive precious metals in other important reactions.
ASSOCIATED CONTENT Supporting Information Catalyst preparation procedures; characterization techniques; COx methanation conditions and product analysis; theoretical calculation details; Figures S1–10; Tables S1–3. This material is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION Corresponding Authors
[email protected];
[email protected] [email protected];
zongbn-
Notes The authors declare no competing financial interests.
ACKNOWLEDGMENT This work was supported by the National Key R&D program of China (2017YFB0602200, 2016YFB0301600), the National Natural Science Foundation of China (21373055), the Science & Technology Commission of Shanghai Municipality (08DZ2270500), the International Joint Laboratory on Resource
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Chemistry (IJLRC), and the Beijing Synchrotron Radiation Facility (BSRF).
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Table of Contents
Undercoordinated Site-Abundant and Tensile-Strained Nickel for Low-Temperature COx Methanation Hao Wang, Ke Xu, Xuanyu Yao, Danhong Ye, Yan Pei, Huarong Hu, Minghua Qiao, Zhen Hua Li, Xiaoxin Zhang, Baoning Zong
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