Enhanced Lattice Oxygen Reactivity over Ni-Modified WO3-Based

Apr 11, 2017 - The oxygen carriers were then reoxidized using oxygen (3 mL/min) diluted in .... Significant differences are observed for Ni0.3WOx/Al2O...
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Enhanced Lattice Oxygen Reactivity over Ni-Modified WO3‑Based Redox Catalysts for Chemical Looping Partial Oxidation of Methane Sai Chen, Liang Zeng, Hao Tian, Xinyu Li, and Jinlong Gong* Key Laboratory for Green Chemical Technology of Ministry of Education, School of Chemical Engineering and Technology, Tianjin University; Collaborative Innovation Center of Chemical Science and Engineering, Tianjin 300072, People’s Republic of China S Supporting Information *

ABSTRACT: Partially oxidizing methane into syngas via a twostep chemical looping scheme is a promising option for methane transformation. Providing the optimum lattice oxygen to selectively produce syngas represents the major challenge for the development of oxygen carrier materials in chemical looping processes. This paper describes the design of WO3-based oxygen carriers as the primary source of lattice oxygen with high melting points and attractive syngas selectivity. To further enhance the lattice oxygen availability and methane conversion capacity, NiO nanoclusters are introduced, considering the doping effect on chemical bonding disruption in both bulk and surface regions. For Ni0.5WOx/Al2O3, the nickel cations incorporated into the bulk of WO3 can strongly weaken the tungsten−oxygen bond strength and increase the availability of lattice oxygen. The surface-grafted nickel species can effectively activate methane molecules and catalyze the partial oxidation reaction. Total methane conversion and syngas yield can be substantially increased by about 2.7-fold in comparison with unmodified WO3/Al2O3. This work demonstrates that the bulk and surface modifications are feasible to tailor the active lattice oxygen of oxygen-carrying materials in chemical looping processes. KEYWORDS: chemical looping, partial oxidation, tungsten oxides, nickel particles, lattice oxygen

1. INTRODUCTION Over the past decade, the booming shale gas supply has stimulated renewed interest in efficient and economical approaches to converting methane into valuable chemicals and liquid fuels via an intermediate mixture of hydrogen and carbon monoxide: namely, syngas.1,2 Basic schemes of various syngas production technologies directly determine the product quality and process efficiency. Currently, methane steam reforming (MSR)3 is the most common process for the conversion of methane to syngas. MSR requires an excess amount of steam to attain high methane conversion and suppress coke deposition. Such a high steam to methane ratio results in a hydrogen-rich (e.g., H2:CO > 3) syngas stream, which requires tuning the ratio toward fuel and chemical synthesis.4 Moreover, the strong endothermicity of MSR and the associated shell-tube reactor design cause great energy demands and losses as well as a large carbon footprint. Partial oxidation of methane (POM)5 and autothermal re-forming (ATR)6 have also been commercialized for producing syngas, where pure oxygen is introduced to meet the process heating requirement and adjust the syngas composition. However, the associated high-cost air separation units (ASUs) impede the wide deployment of these technologies.7 As an alternative approach, two-step chemical looping technologies have attracted academic and industrial attention for effective methane conversion.7−12 In particular, two-step autothermal chemical looping partial oxidation (CLPO) of methane uses the lattice oxygen from metal oxides, namely oxygen carriers, for © 2017 American Chemical Society

syngas generation and quality control. The oxygen carriers can then be regenerated by air and recycled back to the previous step with replenished oxygen and released heat.4,13−15 More importantly, the two-step process avoids the capital cost of ASU and the risk of explosion of premixed methane and oxygen mixtures in comparison with POM. In the two-step thermochemical cycles, the redox metal oxides partially oxidize methane into syngas with its active lattice oxygen (O2−) (eq 1). The lattice oxygen consumed is subsequently replenished from air in a separate reactor (eq 2). Therefore, the overall reaction contributes the exothermic partial oxidation of methane to syngas with a H2/CO molar ratio of about 2 (eq 3), which makes it suitable for methanol production or Fischer−Tropsch synthesis.4,16 CH4 + MeO = CO + 2H 2 + Me

(1)

Me + 1/2O2 = MeO

(2)

CH4 + 1/2O2 = CO + 2H 2

ΔH1073K = −23 kJ/mol (3)

Since metal oxides play a decisive role in the CLPO process, substantial research efforts have been made in the design and development of efficient oxygen carriers with high oxygencarrying capacity, excellent redox reactivity, and high selectivity Received: February 9, 2017 Revised: March 23, 2017 Published: April 11, 2017 3548

DOI: 10.1021/acscatal.7b00436 ACS Catal. 2017, 7, 3548−3559

Research Article

ACS Catalysis toward syngas.17 Among various redox metal oxides, nickel oxides,12,18 ferrite oxides,11,14,19,20 and tungsten oxides21−23 show great potential. Nickel-based oxygen carriers have been found to be promising candidates due to their acclaimed reaction reactivity and the subsequently appreciable catalytic activity of metallic nickel toward re-forming reactions.18,24 Nevertheless, the reduced nickel oxides promote methane decomposition, leading to carbon deposition, which deactivates the redox catalysts.12,25 Iron oxides exhibit low cost and natural abundance.26 However, iron oxides, in both pure and supported forms, are significantly less active for methane catalytic reforming. Similar to the case for nickel oxides, Tammann temperatures of iron oxides are within the operation window of chemical looping systems, and thus they suffer from sintering and rapid deactivation.27 Cerium oxide redox catalysts exhibited excellent performance in the partial oxidation of methane to syngas. However, the limited oxygen-carrying capacity of CeO2 may not be suitable for its application as an oxygen carrier in chemical looping processes.28−30 This fact poses significant challenges for maintaining structures and reactivity over hightemperature redox cycles.26,27 Tungsten oxides have attracted ample attention in various applications, including chemical looping technologies, due to their earth abundance, highly tunable composition, and physical stability.31,32 The melting point of metallic tungsten is above 3400 °C, showing the highest anti-sintering potential among all the metals. Thermodynamic computation shows that tungsten oxides have high syngas selectivity and are appropriate for the CLPO process.10 Furthermore, WO3 is made up of perovskite units and presents a stable crystallite structure.32 Tungstenbased oxygen carriers can offer great resistance to sintering while still allowing sufficient redox cycles.22,23,31 However, lower reducibility and methane reactivity limit their application in the CLPO process. Therefore, the further enhancement of lattice oxygen availability and the conversion of methane at the gas−solid interface are two crucial issues in designing effective WO3-based oxygen carriers for the CLPO of methane. Recent research on modified oxygen carriers has been conducted for solving the corresponding issues. Li et al. hydrothermally prepared Ce0.7Fe0.3O2−x oxygen carriers for syngas production from methane. Both the dispersion of surface Fe2O3 and the formation of a Ce-Fe solid solution improved the reactivity and reducibility of solid materials.13,33 Jiang et al. designed IrOx-containing LaFeO3 materials in a twostep thermochemical CO2 reduction reaction. The doping of IrOx in the bulk of LaFeO3 led to the accelerated oxygen diffusion and more available lattice oxygen. Moreover, the surface IrOx catalyzed the chemical reaction of oxygen evolution at the gas−solid interface possibly by promoting O−O bond formation.34 Qin et al. revealed that 1% of lanthanum dopants in Fe2O3 could dramatically enhance redox reactivity while maintaining or improving the recyclability of iron-based oxygen carriers. This could result from the ability of La dopants to lower the barriers of the C−O bond and C−H bond activation during metal oxide redox reactions.35 Perovskite (ABO3) materials have been used as partial oxidation oxygen carriers, where the binding energy and diffusion of lattice oxygen can be tailored by selectively changing A and B cations.36 DFT calculations indicated that both lattice oxygen binding energy in the bulk and methane activation on the catalytic surface can significantly influence the redox performance.11,37 The reaction between methane and oxygen carriers can be divided into three critical steps: (i) methane activation,

(ii) bulk lattice oxygen diffusion, and (iii) surface reaction between methane and oxygen species.38 Specifically, methane activation highly depends on the nature of active sites on the surface of oxygen carriers. Lattice oxygen diffusion from the bulk phase is closely related to the oxygen-carrying capacity and the M−O bond strength. In addition, the reaction between activated methane molecules and oxygen diffused to surface sites determines the terminal product selectivity: total combustion, partial oxidation, and decomposition of methane. Therefore, the reaction kinetics in the CLPO significantly depends on the M−O bonds, which can be tailored by the bulk and surface modification of oxygen carriers. This paper describes the design of a novel Ni-modified WO3based oxygen carrier material for CLPO of methane. Al2O3 is used as a support to increase the dispersion of the tungsten species.39 Nickel is introduced due to the tendency of the formation of Ni-W solid solutions on the basis of their similar ion radii, lower valence of Ni2+ in comparison to W6+,40−46 and the catalytic properties of metallic Ni0 to promote methane reforming.47 We investigated how the nickel species doped in the bulk of WO3 influences the tungsten−oxygen bonds to increase the availability of lattice oxygen and how the nickel species grafted on the surface catalyze and accelerate the surface reaction. We also revealed the potential reaction pathways and mechanism of the bulk and surface-modified WO3-based oxygen carriers for the CLPO of methane.

2. EXPERIMENTAL SECTION 2.1. Preparation of Oxygen Carriers. NiyWOx/Al2O3 oxygen carrier particles were prepared by continuous coprecipitation. WO3 loading was fixed to 40 wt % for all of the samples in the present study. By varying of the amount of nickel addition, Ni and W atomic ratios were tuned to be 0, 0.3, 0.5 and 1, respectively, denoted as y = 0, 0.3, 0.5, 1. The required amounts of WCl6 (99.0%, Aladdin biological technology Co., Ltd.), Al2(NO3)3·9H2O (98.0%, Aladdin biological technology Co., Ltd.), and/or Ni(NO3)3·6H2O (99.0%, Aladdin biological technology Co., Ltd.) were dissolved in ethanol and blended using a magnetic stirrer. The hydroxides were precipitated by dropping of a NaOH solution. The mixtures were stirred for 1 h until the pH value was between 8 and 9. After 2 h, the products were collected by centrifugation and washed thoroughly six times with water and subsequently ethanol. The precipitates were dried at 80 °C for 24 h, grounded to powders, and then calcined in ambient air at 800 °C for 4 h. NiO/Al2O3 with the same addition of nickel as Ni0.5WOx/Al2O3 was prepared by a similar procedure with Ni(NO3)3·6H2O and Al(NO3)3·9H2O. Bulk WO3 was prepared by thermal decomposition of H2WO4 powder at 800 °C for 4 h. 2.2. Characterization. X-ray powder diffraction (XRD) patterns were performed with 2θ values between 20 and 90° by using a Rigaku C/max-2500 diffractometer with graphitefiltered Cu Kα radiation (λ = 1.5406 Å). Raman spectra were recorded under ambient conditions using a Renishaw inVia reflex Raman spectrometer with a 532 nm Ar ion laser beam. Before measurements, the samples were dried at 80 °C for 2 h. UV−vis spectra in the range of 200−800 nm were recorded on a Shimadzu UV-2550 spectrophotometer with BaSO4 as a blank reference. Transmission electron microscopy (TEM) was carried out on a JEM-2100F transmission electron microscope, equipped with a liquid nitrogen cooled energy-dispersive X-ray spectros3549

DOI: 10.1021/acscatal.7b00436 ACS Catal. 2017, 7, 3548−3559

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ACS Catalysis

Figure 1. XRD patterns of (A) fresh oxygen carriers, (B) slow sweep of fresh oxygen carriers, (C) oxygen carriers after reduction during the first cycle, and (D) oxygen carriers after the first cycle. ((a) WO3, (b) WO3/Al2O3, (c) Ni0.3WOx/Al2O3, (d) Ni0.5WOx/Al2O3, and (e) NiWOx/Al2O3).

to 600 °C, and then cooled to the desired temperature to obtain a background spectrum, which was then subtracted from the sample spectrum for each measurement. After a background spectrum was measured from 50 to 600 °C under Ar, samples were held sequentially with the temperature from 50 to 500 °C under a CH4 flow and then up to 600 °C, where the spectra were measured every 1 min for 5 min. 2.3. Redox Tests. Reactivity tests were performed in a fixed bed quartz U-tube reactor (i.d. 8 mm) loaded with 100 mg of the catalyst (20−40 mesh) mixed with 1 mL of quartz particles at atmospheric pressure. Switching between CH4 and O2/He flows was employed during the tests. The bed temperature was typically 800 °C, and the samples were reduced using methane (4 mL/min) diluted in argon (20 mL/min) for 10 min. The oxygen carriers were then reoxidized using oxygen (3 mL/min) diluted in helium (27 mL/min) for 15 min. Between the reduction and reoxidation reactions, an inert period (20 mL/ min of argon) of about 15 min was inserted to prevent mixing between methane and gas-phase oxygen. One redox cycle was completed. The stability test was carried out over Ni0.5WOx/ Al2O3 for ten continuous redox cycles. The time for reduction, reoxidation, and purging was set to 8, 15, and 15 min, respectively. Reaction products for all tests were analyzed with a MS system (Hiden QIC-20). CH4, CO, CO2, and H2 responses were monitored by MS at m/z 16, 28, 44, and 2, respectively. The produced water was not measured. The mass spectra signals were calibrated before each experiment by using a calibration gas of known composition, and these responses were reported as molar fractions. Average CO evolved amounts were calculated as the difference between the inlet and outlet molar flow rates, as measured relative to an internal standard (Ar) using an online mass spectrometer. Control experiments using an inert solid such as silica with the same void volume under identical conditions indicated a low contribution of gasphase reactions ( NiWOx/Al2O3 > Ni0.3WOx/Al2O3 > Ni0.5WOx/ Al2O3 > WO3/Al2O3> WO3. On the basis of TPR results, it can be concluded that, among all the samples, NiO is the easiest for reduction and WO3 is the most difficult. The incorporation of nickel species into the WO3 structure can decrease the reduction temperature and increase the reducibility of WO3, which is essential to efficient syngas generation with methane serving as a reducing agent. To further illustrate the reduction behaviors of oxygen carriers under a flowing CH4 environment, the profiles of CH4TPR mass spectra over WO3/Al2O3 and Ni0.5WOx/Al2O3 are presented in Figure 9. CO2 was not obviously observed due to the trace amount. It can be seen that the formation of CO for WO3/Al2O3 started to occur at 800 °C, suggesting that methane could be partially oxidized at 800 °C. In addition, the formation of H2 was ahead of that of CO, which indicated that methane was activated and dehydrogenized to form carbon intermediates and H2. Then, CO was produced by the reaction of carbon intermediates with the lattice oxygen from WOx.13 In addition, the ratio of H2 to CO was higher than 2 with increasing temperature, which further suggests that the rate of lattice oxygen diffusion should be the rate-determining step over WO3/Al2O3. For Ni0.5WOx/Al2O3, however, there are different reaction behaviors with varying temperature ranges (Figure 9B). A CH4 decrease due to the partial oxidation of methane was occurring from 700 °C, much lower than that over WO3/Al2O3. Furthermore, the formations of CO and H2 were merely simultaneous and kept a higher molar fraction of CO, which indicates that bulk-doped nickel species could efficiently weaken W−O bonds and accelerate lattice oxygen diffusion from the bulk of WOx to the surface for reacting with absorbed carbon intermediates. Then, the fraction of H2 and CO tailed up with a constant ratio close to 2. In a word, bulkdoped nickel can effectively weaken the W−O bond strength, 3556

DOI: 10.1021/acscatal.7b00436 ACS Catal. 2017, 7, 3548−3559

Research Article

ACS Catalysis methane was first fully oxidized to CO2 and then partially oxidized to CO. In comparison to WO3/Al2O3, Ni0.5WOx/ Al2O3 samples exhibited much higher CO mass signals and CH4 conversions. This phenomenon may be ascribed to the catalytic function of reduced metallic nickel, which can promote and accelerate the surface reaction between lattice oxygen and methane.33 Therefore, the surface-grafted Ni species can not only effectively suppress the surface oxygen activity but also accelerate the surface reaction between activated methane molecules and the diffused lattice oxygen for a high yield toward CO. 4.3. Reaction Mechanism between Methane and Oxygen Carriers. On the basis of the above discussions, the reaction mechanism model for CLPO over Ni0.5WOx/Al2O3 can be investigated from the bulk and surface during a gas− solid reaction between methane and oxygen carriers. For solid oxygen carriers, the reduction mechanism under CH4 flow, in particular the structural changes with the reduction time, could be an important mechanism to understand the overall CLPO process. XRD diffraction patterns over fresh and methaneactivated samples at different reaction times are shown in Figure 11. Typical WO3 reflection peaks were observed for the

of CH4 isothermal reactions over Ni0.5WOx/Al2O3 (Figure 5D), we could track the reaction mechanism in a clear way. The first 2 min may be ascribed to region I, when WO3 was reduced to WO2.96 and WO2.72; simultaneously CH4 was fully oxidized into CO2 and H2O. Then, oxygen carriers were further reduced to WO2 and Ni during the following 6 min (from minute 2 to minute 8). When metallic Ni was formed, the enhanced partial oxidation of methane due to the surface catalytic function dominated in region III. With further release of oxygen, metallic W formed. Because of the loss of lattice oxygen and metallic Ni/W catalyzing methane decomposition, carbon deposition occurred with H2/CO being higher than 2 during the last period. To further investigate the reaction mechanism of methane on the surface of oxygen carriers, we show in situ DRIFTS spectra observed over Ni0.5WOx/Al2O3 in Figure 12. In the region of

Figure 12. DRIFTS spectra observed for reaction of CH4 over Ni0.5WOx/Al2O3.

2800−3100 cm−1, one vibrational signature at 3014 cm−1 was identified at 50−600 °C. On the basis of the literature, it could be assigned to the asymmetric and symmetric C−H stretching vibrations of a methyl or/and methoxy group.5 Notably, the vibrational peaks gradually disappeared at 200 °C due to the transformation of C−H species to others. In addition, the band at 1630 cm−1 was attributed to asymmetric and symmetric vibrations of formate-like species, −O−CH(O).5,72 The band decreased with increasing reaction temperature, indicating that they can be transformed to some intermediates or product molecules at a relatively high temperature. Then, two absorption bands at 2357 and 2333 cm −1 ascribed to gaseous CO2 were initially observed, and the intensity increased while the temperature was ramped from 50 to 500 °C. When the temperature was raised to 600 °C, the peaks of CO 2 disappeared and CO formed (2185, 2108 cm−1). With increasing exposure time, the peak intensity of CO decreased and the peaks of carbon deposition appeared and then increased. The results indicated that the surface WO3 was first reduced by methane (accompanied by the reduction of surface oxygen on mixed oxides) to produce CO2 and H2O, followed by the activation of methane on the reduced nickel or tungsten sites to create hydrogen atoms and intermediate carbon species. The hydrogen atoms recombined in pairs to molecules while the carbon was selectively oxidized to CO by the activated lattice oxygen released from solid oxygen carriers. Both the lack of lattice oxygen and the catalyzing effect of

Figure 11. XRD patterns of Ni0.5WOx/Al2O3 at different times on CH4 stream at 800 °C.

fresh sample with no appearance of NiO, NiWO4, and NiAl2O4, which may result from the Ni doping into the lattice of WO3 as discussed above. After 2 min on CH4 stream, the diffraction peaks of WO3 disappeared, while peaks attributed to WO2.96, WO2.72, and WO2 appeared. These diffraction patterns were retained until 6 min, although the intensities of the different phases varied with time on stream. At 4 min, peaks corresponding to Ni appeared, indicating the reduction of Ni species into metallic Ni. At 8 min, diffraction patterns of metallic W appeared. Ten minutes later, in addition to the metallic Ni and metallic W, new phases corresponding to WC and C could be clearly detected.67,71 After 10 min, the reflection patterns did not change, but they were formed in substantial amounts with their intensity continually increasing with time. According to the XRD data, tungsten oxide reduction followed the global mechanism WO3 → WO2.96 → WO2.72 → WO2 → W → WC. The resulting metallic W was assumed to be responsible for the formation of WC and deposited coke. When relating XRD diffraction patterns over fresh and methaneactivated samples at different times on stream with the results 3557

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ACS Catalysis

Figure 13. Proposed reaction processes of CLPO over Ni-doped WO3 oxygen carriers.

catalyze the surface partial oxidation reaction, both of which could be responsible for the enhancement of syngas evolved in comparison with WO3/Al2O3. Furthermore, the WOx species can effectively help nickel disperse and resist coking for better stability in comparison with NiO/Al2O3. In summary, nickel modification improves the performance of tungsten oxides from the aspects of both bulk lattice oxygen diffusion and surface reaction rates. Therefore, Ni0.5WOx/Al2O3 is a promising candidate for the CLPO of methane.

metallic Ni would lead to the deep methane decomposition and carbon deposition accumulation.33 Therefore, the potential reaction paths and mechanism of Ni0.5WOx/Al2O3 in the CLPO process can be proposed as shown in Figure 13. On the basis of these collective findings, it appears that both the bulk-doped nickel species and surface-grafted nickel species can promote the partial oxidation of methane for syngas generation over WO3-based oxygen carriers and the potential reaction paths are also influenced with nickel modification.

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ASSOCIATED CONTENT

S Supporting Information *

5. CONCLUSIONS We have demonstrated that tungsten-based oxygen carriers with nickel modification are effective for the CLPO of methane. The function of bulk-doped and surface-grafted nickel species for enhanced syngas generation were verified, and the potential reaction paths and mechanism of tungsten-based oxygen carriers were also proposed. The results indicated that the oxygen availability, methane conversion, and syngas yield can be significantly increased over Ni0.5WOx/Al2O3 in comparison to WO3/Al2O3. The reaction between CH4 and Ni0.5WOx/ Al2O3 can be divided into four distinct regions: full oxidation, mixed oxidation, partial oxidation, and carbon deposition. Further investigations of these reaction regions revealed that full oxidation in region I could be attributed to reduction of WO3 to WO2.96. In region II, a transition period of full oxidation and partial oxidation dominated, and the oxygen carriers were reduced into Ni and WO2.72. As a result of continuing oxygen consumption, region III was characterized by a further reduction into W and Ni, which could catalyze partial oxidation on the surface of the samples. Carbon deposition and carbide formation in region IV corresponded to depletion of O2− and methane decomposition over metallic Ni/W. Furthermore, Ni0.5WOx/Al2O3 remained nearly intact with slight deactivation due to the structure evolution over 10 redox cycles. There is a strong interaction between nickel species and the WOx species, and the overall activity and selectivity can be directly affected by lattice oxygen diffusion and surface reaction rate. The nickel species incorporated into the bulk of WO3 greatly weaken the intensity of W−O bonds and increase the lattice oxygen availability. The surface-grafted nickel species effectively activate methane molecules and

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.7b00436. Tables S1−S3 and Figures S1−S5 as described in the text (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail for J.G.: [email protected]. ORCID

Jinlong Gong: 0000-0001-7263-318X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the National Key Research and Development Program of China (2016YFB0600901), the National Science Foundation of China (Nos. 21406162, 21525626, and U1663224) and the Program of Introducing Talents of Discipline to Universities (No. B06006) for financial support.

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DOI: 10.1021/acscatal.7b00436 ACS Catal. 2017, 7, 3548−3559