Research / 2020 / Article

Research Article | Open Access

Volume 2020 |Article ID 3049534 | https://doi.org/10.34133/2020/3049534

Asim Riaz, Muhammad Umair Ali, T. Gabriel Enge, Takuya Tsuzuki, Adrian Lowe, Wojciech Lipiński, "Concentration-Dependent Solar Thermochemical CO2/H2O Splitting Performance by Vanadia–Ceria Multiphase Metal Oxide Systems", Research, vol. 2020, Article ID 3049534, 12 pages, 2020. https://doi.org/10.34133/2020/3049534

Concentration-Dependent Solar Thermochemical CO2/H2O Splitting Performance by Vanadia–Ceria Multiphase Metal Oxide Systems

Received30 Jun 2019
Accepted30 Dec 2019
Published29 Jan 2020

Abstract

The effects of V and Ce concentrations (each varying in the 0–100% range) in vanadia–ceria multiphase systems are investigated for synthesis gas production via thermochemical redox cycles of CO2 and H2O splitting coupled to methane partial oxidation reactions. The oxidation of prepared oxygen carriers is performed by separate and sequential CO2 and H2O splitting reactions. Structural and chemical analyses of the mixed-metal oxides revealed important information about the Ce and V interactions affecting their crystal phases and redox characteristics. Pure CeO2 and pure V2O5 are found to offer the lowest and highest oxygen exchange capacities and syngas production performance, respectively. The mixed-oxide systems provide a balanced performance: their oxygen exchange capacity is up to 5 times higher than that of pure CeO2 while decreasing the extent of methane cracking. The addition of 25% V to CeO2 results in an optimum mixture of CeO2 and CeVO4 for enhanced CO2 and H2O splitting. At higher V concentrations, cyclic carbide formation and oxidation result in a syngas yield higher than that for pure CeO2.

1. Introduction

Oxygen-carrier-mediated solar thermochemical reduction–oxidation (redox) cycling is a promising approach to produce renewable fuels utilizing solar energy [18]. A typical solar-thermochemical redox cycle consists of two steps: (1) reduction of the oxygen carrier under an inert or reducing gas atmosphere (Reaction (1)) and (2) reoxidation of the reduced oxygen carrier by CO2 (Reaction (2)) and H2O (Reaction (3)). For methane as the reducing gas, the two-step cycle read:

The process results in the production of synthesis gas (syngas), a mixture of H2 and CO, which can be converted into liquid hydrocarbon fuels via the Fischer–Tropsch (FT) process [911]. In order to achieve high fuel (syngas) yields and high process efficiency in the redox cycling, the oxygen carriers should have a high oxygen exchange capacity and excellent cyclability [12]. Intensive research has been carried out to develop suitable catalyst/oxygen carriers. Some redox-material pairs such as Fe/FeO, CeO2/CeO2-δ, Mn3O4/MnO, and perovskites have been studied in the temperature range of 600–1500°C [9, 1319]. The use of these materials has been demonstrated at laboratory and pilot-scale systems. However, their performance still requires improvements for the successful commercial deployment [20].

Ceria (CeO2) is one of the most efficient oxygen carriers with excellent oxygen ion mobility and redox kinetics. It shows high and stable fuel production rates via a nonstoichiometric oxygen exchange process. The fast redox kinetics and high fuel selectivity are distinct characteristics of CeO2 as compared to other redox materials [10, 11, 15, 2127]. Fuel selectivity indicates the percentage of product gases (CO, H2) produced upon splitting of CO2, H2O, CH4, etc. Doping CeO2 with transition metals has been proposed to further improve the material chemical, thermal, mechanical, and optical characteristics. However, transition metal-doped CeO2 has so far demonstrated inferior redox reaction rates and oxygen ion mobility as compared to pure CeO2 [11, 2628]. In addition, doped CeO2 shows extensive sintering at high temperatures, which decreases gas-phase mass transfer and consequently decreases the overall fuel production performance [10]. Hence, further research is required to develop new combinations of CeO2 and suitable cations to overcome the above-mentioned challenges.

Vanadia (V2O5) is recognized as one of the most efficient catalytic metal oxides. It is utilized for selective redox reactions in batteries [29] and gas-sensing applications [30, 31]. V2O5 supported with SiO2, TiO2, Al2O3, MgO, and CeO2 is also used in the selective oxidation of hydrocarbons for H2 and CO production [3035]. Consequently, vanadia–ceria tertiary metal oxide systems have been widely studied for their structural, chemical, and oxidative properties [36]. Thermochemical redox performance and structural changes of vanadia–ceria multiphase systems were investigated for a sample with a V-to-Ce ratio of 25% [37]. However, the effects of V and Ce concentrations in vanadia–ceria systems have not been systematically studied.

In this work, we investigate the thermochemical performance of vanadia–ceria systems as oxygen carriers via solar thermochemical redox cycles for syngas production. Ultrafine particles of vanadia–ceria systems are produced using a facile liquid-phase precursor thermal-combustion method. V-to-Ce atomic ratios of 0 to 100% are investigated. The changes in the physicochemical characteristics of the oxide systems before and after methane looping reforming are studied. Sequential CO2 splitting (CDS) and water splitting (WS) reactions after methane partial oxidation (MPO) are investigated for syngas yield and purity. This study advances the field of solar thermochemistry towards achieving efficient and low-cost oxygen carriers for enhanced solar fuel production via thermochemical redox cycles.

2. Materials and Methods

2.1. Synthesis of Vanadia–Ceria Metal Oxide Systems

Ultrafine particles of pure CeO2, pure V2O5, and vanadia–ceria systems (V-to-Ce atomic ratios of 25%, 50%, and 75%, denoted as CV25, CV50, and CV75, respectively) are prepared using a liquid-phase precursor thermal-combustion method [38]. This technique allows for a large-scale production of the nanoparticles with controlled particle size and morphology. Briefly, stoichiometric ratios of Ce (III) nitrate hexahydrate (CeN3O9·6H2O, Aldrich) and vanadium oxytripropoxide (VC9H21O4, Aldrich) precursors are dissolved separately in a mixture of 4 mL ethanol and 4 mL deionized (DI) water, respectively. After stirring for 1 hour, both solutions are mixed together and stirred for 3 hours at room temperature. Finally, the precursor solutions are transferred into an alumina crucible and heat treated at 1173 K for 3 hours. During the heat treatment, temperature is raised stepwise: first, the temperature is held at 353 K for 1 hour, then increased to 1173 K at a ramp rate of 3 K min-1.

2.2. Thermochemical Cycling

The cyclic thermochemical redox performance of metal oxide powders is evaluated in a vertical tube reactor (99.98% Al2O3) placed inside an infrared gold image furnace (P4C-VHT, Advance Riko). Highly porous and refractory alumina fiber mats (AIBF-1, 97% Al2O3 and 3% SiO2, ZIRCAR) are used as a sample stage and as an upper protective layer for the powder samples ( mg). To facilitate the solid–gas transfer, a gap of ~2 cm is set between the powder and the upper protective layer mat. The samples and fiber layers are placed in the tube axially in the middle of heating zone to ensure uniform heating. The LabVIEW (National Instruments) software platform is used to operate the flow rate controllers (F201CV, Bronkhorst) and actuated valves (1315R, Swagelok) to achieve desirable gas flow rates in “mL min-1.” A B-type thermocouple sealed in an alumina sheath is located immediately beneath the fiber mat sample stage to monitor the sample temperature. The composition of reactant and product gasses is recorded by a quadrupole mass spectrometer (OmniStar™ GSD 320, Pfeiffer Vacuum). A schematic of the experimental setup used for thermochemical redox cycling is shown in Figure SI 1.

First, Ar (grade 5.0) gas is purged with a flow rate of 500 mL min-1 to remove any gas species (H2, CO, and CO2) from the surface of the tube and gas lines. The reactor is heated under an Ar flow (500 mL min-1) from ambient temperature to an optimized isothermal operating temperature of 1173 K at a ramp rate of 100 K min-1. The reduction (methane partial oxidation, MPO) of the powder samples is carried out under an 8% CH4 flow (20 mL min-1, grade 4.5) diluted with 92% of Ar (230 mL min-1). The reduced samples are reoxidized by a 4% CO2 flow (10 mL min-1) during CO2 splitting (CDS) reactions. For H2O splitting (WS) reactions, steam vapor is generated at 368 K in a water bubbler filled with DI water, then an Ar flow of 30 mL min-1 is passed through the bubbler to carry H2O vapors and further diluted with an Ar flow of 220 mL min-1 before it is delivered to the tubular reactor. Inert gas sweeping with Ar is done with a flow rate of 500 mL min-1 before and after each reduction and oxidation step of the thermochemical cycle. Four different redox sequences are investigated and denoted as MPO–CDS, MPO–WS, MPO–WS–CDS, and MPO–CDS–WS. The details of the sequence and duration of each step and gas flow rates during these redox cycles are as follows:

The redox performance of ultrafine powders is evaluated over 10 cycles. The vanadia–ceria systems are structurally and chemically analyzed before and after 10 cycles, and the results are compared to those obtained with the as-prepared samples.

2.3. Material Characterization

All powder samples are characterized as prepared and after 10 thermochemical cycles. X-ray diffraction (XRD) analysis of powder samples is carried out using a D2 phaser diffractometer (Bruker) with a Cu kα (1.54 Å) radiation source operated at 300 W power (30 kV, 10 mA). XRD patterns are recorded in a diffraction angle range of 10–80° with a step width of 0.02° and a scanning rate of 0.75°min-1. The Scherrer equation is applied onto the most intense peaks of XRD patterns to calculate the crystallite size of the powders.

X-ray photoelectron spectroscopy (XPS) analysis is carried out using a ThermoFisher ESCALAB 250Xi X-ray photoelectron spectrometer equipped with a 180° double focusing hemispherical analyzer. A monochromatic Al kα source with a spot size of 200–900 μm is utilized at 12 kV and 12 mA. A total pressure in the chamber is 10-8 mbar. Samples are scanned at various spots on an area of 25 mm2 and depth of 4 mm with a beam energy of 40–160 eV. XPS spectra are processed using the CasaXPS software version 2.3.18 (Casa software Ltd., Teignmouth, UK). The binding energy of aliphatic carbon peak C 1s at 284.8 eV is used as a reference in the survey spectra.

A Raman imaging microscope (Renishaw Plc, model 2000) equipped with an Olympus BH2 microscope is utilized for the structural analysis of powder samples. Samples are placed on a motorized XYZ stage of the microscope equipped with an air-cooled CCD detector and a CCD camera. The excitation wavelength of the NIR laser is 785 nm. Raman spectra are recorded in a Raman shift range of 100–1200 cm-1. The exposure time is 20 s with an accumulation up to 3, and laser power is adjusted in the range of 0.01–0.5% (<6 MW), depending on the sample response to laser excitation.

Morphological information of the powder before and after the redox cycles is obtained by using a field emission scanning electron microscope (FESEM, Zeiss Ultraplus). A high-resolution transmission electron microscope (HR-TEM, JEOL 2100F) is utilized for the measurements of particle size distribution and lattice spacing. Operating voltage of the microscope is adjusted up to 200 kV according to the resolution and sample response. A lacey carbon-coated 200 mesh copper grid is used as a substrate for samples. The powder is first dispersed in ethanol, and a drop of particle suspension is dried on the copper grid. Information about composition of samples is obtained by energy dispersive X-ray spectroscopy elemental mapping (EDX) analysis carried out by scanning transmission electron microscopy (STEM) mode on JEOL 2100F.

The quantification of Ce and V was carried out using an Agilent 5110 ICP-OES (Agilent Technologies, Australia), operating in Synchronous Vertical Dual View (SVDV) mode, allowing simultaneous detection of axial and radial emission signals. The sample introduction system was made up of a double pass cyclonic spray chamber, a SeaSpray nebuliser, and a 2.4 mm quartz injector. Operating parameters for ICP-OES analysis of oxygen carriers before and after cycling are tabulated in Table SI T1. All dilutions and sample preparation for ICP-OES measurement were performed using ultrapure water (MilliQ, Merck), as well as subboiling distilled HNO3. Calibration solutions for Ce and V measurements were prepared from single element solutions in concentrations ranging from 0.1 to 10 μg mL-1; for analysis, samples were diluted to fall within the calibration curve. Ce and V from a single element standard were diluted to 0.1, 0.5, 1, 5, and 10 ppm concentrations to make a calibration curve for each element.

3. Results and Discussion

Figure 1 shows transmission electron microscopy (TEM) images of as-prepared vanadia–ceria systems, at the same magnification. The particle size increases with increasing V concentration. The average particle size of pure CeO2 is  nm, which increases to  nm with addition of 75% V, while pure V2O5 has the highest average particle size of  nm. Field emission scanning electron microscopy (FESEM) images of as-prepared vanadia–ceria systems confirm these values (Figure SI 2).

The morphological study of vanadia–ceria systems after MPO–CDS cycles is carried out by FESEM. It reveals an extensive sintering in pure V2O5 and CeO2 samples, showing individual particles fused into large microparticles. However, the sintering is prominent in V-rich vanadia–ceria mixed-metal-oxide particles (Figure SI 3). The TEM image of a reduced CV75 sample before MPO–CDS cycles shows sintering of small particles forming a sheet like morphology, while individual particles can also be observed (Figure SI 4a). The CV75 sample after MPO–CDS cycles shows extensive sintering which results in increased particle sizes of >500 nm (Figure SI 4b).

Energy dispersive spectra (EDS) and an overlaid elemental map of as-synthesized as well as reduced CV75 samples before and after MPO–WS–CDS cycles are shown in Figure 2. As-prepared samples show no noticeable presence of carbon. Since V-Kα and Ce-L peaks are situated in the same energy range in EDS spectra, the presence of both elements shows a combined overlay color (Figures 2(a)–2(c)). A nonuniform distribution of V in CV75 is observed after MPO–WS–CDS cycles, indicating a possible V loss during the redox cycles or segregation of V2O5 and CeO2 phases in powder samples.

XRD patterns of as-prepared CeO2, V2O5, and vanadia–ceria ultrafine particles are presented in Figure 3(a). The diffraction patterns of pure CeO2 and V2O5 are in good agreement with that of cubic CeO2 (JCPDS # 72-0076) and orthorhombic V2O5 (JCPDS # 75-0298), respectively. Addition of V in CeO2 promotes the formation of cerium vanadate: (CeVO4, JCPDS # 72-0282), accompanied by a change in the valence of Ce from Ce+4 to Ce+3, while the valence of V remains V5+. With increasing V content to 25%, an increase in the CeVO4 phase with a considerable amount of CeO2 is observed. A maximum conversion of CeO2 into CeVO4 is achieved with 50% V loading, while a small amount of CeO2 remains in the structure. With further increase of V content to 75%, the amount of CeVO4 becomes the smallest (16.9%) with a major portion of V2O3, as listed in Table SI T2. The structural changes caused by the addition of V greatly affect the rates of thermochemical redox reactions, which is discussed in the following sections.

XRD patterns of reduced vanadia–ceria systems are presented in Figure 3(b). Reduced pure CeO2 does not undergo any structural change except shifting of diffraction peaks to lower angles, possibly caused by oxygen depletion. However, reduction of pure V2O5 results in the conversion of V5+ to V4+, which can be seen as the presence of a VO2 phase in the XRD pattern of reduced V2O5 and CV75 [39]. The presence of vanadium carbide (JCPDS#89-1096) and metallic V indicates the catalytic interaction of pure V2O5 with methane to produce carbon during the methane partial oxidation reaction [4042]. A decline in the peak intensities of a CeVO4 phase is observed for CV25, which indicates the conversion of V5+ to V3+ due to the formation of a CeVO3 phase.

The XRD patterns of pure V2O5, CV25, CV50, and CV75 samples after 10 consecutive cycles of MPO–CDS, MPO–WS, MPO–WS–CDS, and MPO–CDS–WS are presented in Figures 3(c)–3(f). In CV25, the diffraction angles of CeVO4 shift to lower angles after 10 MPO–CDS cycles. The presence of peaks associated with CeVO3 indicates the partial oxidation of CeVO3 to CeVO4 due possibly to incomplete recovery of oxygen. Segregated phases of CeVO3, CeVO4, and VO2 are observed in the XRD patterns of CV75. However, oxidation of reduced V2O5 results in the oxidation of vanadium carbide and metallic V to V2O3 and VO2. The XRD analysis of V2O5–CeO2 systems after the redox cycles reveals that the formation of a CeVO4 phase increases with increasing V content, reflecting contribution of V in the redox reactions. Ce3+ sites are stabilized by the valence change (Ce4+ to Ce3+) during the formation of CeVO3/CeVO4 [32, 36].

The surface analysis of as-prepared samples complements the findings of XRD analysis. A typical XPS spectrum of pure CeO2 is composed of two multiplets of 3d 3/2 at 900 and 898 eV, 3d 5/2 at 888 and 882 eV, and neighboring 2 peaks at 907 and 916 eV [43], as presented in Figure 3(a). The locations of these six peaks corresponding to the spin-orbit doublets of 3d 3/2 and 5/2 are in good agreement with the reported XPS analysis of Ce4+/Ce3+ [43]. The binding energy of O 1s is 529.02 eV, corroborating the presence of lattice oxygen species, while the neighboring peak at 531.46 eV refers to the presence of adsorbed oxygen molecules (Figure SI 4a). The binding energy of Ce 3d3/2 peaks changed from 888.5 eV to 885.2 eV with increasing V contents, due to an increase in CeVO4 content. In addition, the disappearance of Ce4+ peak at ~916 eV confirms the change of Ce valence from +4 to +3 due to increasing concentrations of CeVO4 in CV75 sample.

The binding energies of 516.9 eV and 524.66 eV shown in Figure 4(b) correspond to V 2p3/2 and V 2p1/2 spin orbits of pure V2O5, respectively, depicting the +5 valence state of V [44]. In addition, 3d 3/2 to 3d 5/2 ratio decreases with increasing V content in vanadia–ceria systems. An increase in the binding energies of O 1s with V addition also indicates the presence of Ce (III) states and Ce–O–V interactions.

XPS spectra of reduced V2O5, CV25, and CV75 are shown in Figure SI 5. Higher V(V) content can be confirmed from the increasing intensities of V 2P1/2 and V 2P3/2 in Ce–V systems with V content (). The O 1s peaks shift to high binding energies with higher V content, due to higher CeVO4 contents [43]. An additional shoulder peak of V 2P3/2 is observed in reduced V2O5 due to the presence of VO2. An additional O 1s peak is also observed at 531.89 eV for reduced V2O5. After the first MPO–CDS cycle, the additional O 1s peak merged into the main O 1s peak. This may be due to neighboring V species with multiple oxygen states. High concentrations of surface-adsorbed oxygen molecules may also contribute to the presence of the additional O 1s peak.

The information obtained from the XPS spectra provides an insight into the phenomenon of possible V volatilization. By obtaining Ce/V and V/O ratios, the loss of V can be quantified. In as-prepared powders, the V/O ratio increases, and Ce/V ratio decreases with increasing the V content. After the methane partial oxidation reaction, an increase in the V/O and Ce/V ratios is observed due to oxygen and V loss. However, after 10 consecutive cycles, a further decrease of the V/O ratio is observed, which suggests the loss of V and incomplete oxygen recovery. Ce/V segregation is also a possible reason for variable V concentrations at the surface. This can be further investigated by quantifying Ce and V concentrations in the bulk via the ICP-OES technique with a precision of up to parts per billion (ppb) level. The Ce/V ratios before and after thermochemical redox cycles are presented in Figure 4(d). An expected decline in the Ce/V ratio is observed in as-prepared Ce–V oxide samples due to higher V content. However, an irregular trend is observed after redox cycles, which suggests that segregation of Ce/V and V loss both contribute to the variable concentrations of V.

Raman spectra of vanadia–ceria systems are presented in Figure 4(c). Pure CeO2 has a Raman shift at 461 cm-1 [30, 45], while pure V2O5 shows multiple signature peaks at 122, 144, 197, 229, 257, 284, 303, 376, 405, 463, 528, 703, 788, 801, and 863 cm-1 [46, 47]. The V=O interaction seen at 998 cm-1 is dominant in pure V2O5 and CV75, while it diminishes in CV25 and CV50 samples. The intensities of peaks at 144, 197, 284, 303, 405, 528, and 703 cm-1 indicate the presence of CeVO4.

3.1. Thermochemical Redox Performance

The performance of vanadia–ceria systems is evaluated based on the oxygen exchange capacity and the yield of syngas per mole of V during 10 consecutive thermochemical redox reaction cycles. Moles of Ce ions are considered for the calculation of rates and yields of syngas production for pure CeO2 sample. Figure 5 shows the oxygen evolution rates during reduction and oxidation steps of MPO–CDS, MPO–WS, MPO–WS–CDS, and MPO–CDS–WS cycles. The oxygen rates calculated from CO/CO2 evolution rates are referred to as “O1 rates,” while oxygen rates deduced directly from the oxygen signal obtained during gas analysis are referred to as “O2 rates” in the following discussion. This set of data provides an insight into material’s ability to react with reducing and oxidizing atmospheres.

During MPO–CDS cycles, pure CeO2 shows stable average O1 evolution rates of around 0.08 mol mol-1Ce min-1 with a peak of 0.107 mol mol-1Ce min-1. In contrast, pure V2O5 exhibits the highest rates of 0.5 mol mol-1V min-1. An increase in O1 rates from 0.27 mol mol-1V min-1 to 0.43 mol mol-1V min-1 is observed with addition of V from 25% to 75%. In addition, the vanadia–ceria multiphase system demonstrates high and stable oxygen evolution rates during the CO2 splitting reactions with the highest rate of 0.35 mol mol-1V min-1 for 75%V.

During MPO–WS redox cycles, a similar trend of increasing O1 rates with the addition of V (25–75%) content is observed. Pure V2O5 shows the highest oxygen evolution rates up to 0.45 mol mol-1V min-1, followed by CV75 with rates up to 0.38 mol mol-1V min-1, during the reduction step of MPO–WS redox cycles. Interestingly, pure V2O5 shows considerable O1 rates in water splitting reaction, which refers to oxidation of vanadium-carbide carbon species formed during methane partial oxidation reaction, . This phenomenon supports the findings of XRD analysis of reduced and oxidized pure V2O5.

Following this result, in order to investigate the effect of oxidation atmosphere on the reactivity of reduced oxygen carriers with steam and CO2, a combination of WS and CDS reactions following the methane partial oxidation step, i.e., MPO–WS–CDS, is performed. During the methane partial oxidation step of MPO–WS–CDS cycles, pure V2O5 and CV75 demonstrate the highest O1 rates of 3.75 mol mol-1V min-1, closely followed by CV50 at 3.69 mol mol-1V min-1 and CV25 at 2.85 mol mol-1V min-1. During the WS step, the O1 rates are the highest for pure V2O5 followed by the rates for CV75, which confirms the findings of vanadium carbide presence in pure V2O5 and CV75. Lowering the V content minimizes the carbide formation, resulting in pure H2 release during water splitting reaction. Consequently, CV25 and CV50 show moderate to high O1 rates with pure H2 production during MPO–WS–CDS cycles.

A similar trend is observed during the MPO–CDS–WS cycles. During the methane partial oxidation reaction, pure V2O5 shows the highest O1 rates at 3.35 mol mol-1V min-1, closely followed by CV75 and CV50 at 3.25 mol mol-1V min-1. However, the O1 rates significantly increase during the oxidation step, with more than fivefold increase in CO production as compared to MPO–CDS cycles. The high VO2 content observed in the XRD patterns also confirms a higher oxygen recovery in reoxidized pure V2O5 and CV75 samples during MPO–CDS–WS cycles as compared to V2O5 and CV75 reoxidized to V2O3 during MPO–CDS cycles. An increase in the CeVO4 phase also supports efficient reoxidation of CV25 and CV50 during MPO–CDS–WS cycles.

The rates of O2 evolution during all four types of redox cycles are presented in Figure 5(b). The O2 oxygen rates increase with V content (25–100%) during the methane partial oxidation step of MPO–CDS cycles, while the highest O2 rate of 20 mmol mol-1V min-1 is observed for pure V2O5. The evolution rates tend to decrease over multiple cycles due to the oxidation of the deposited carbon and the carbide formation. Interestingly, no considerable O2 rates are observed after the first MPO–WS cycle, while these rates are high in sequential WS and CDS cycles. In addition, O2 rates are higher for high-Ce systems than high-V systems. This is in agreement with the results of XRD study, where V4+ of VO2 is observed in CV75 and pure V2O5 samples, representing better reoxidation capacity of these samples. In contrast, V3+ of CeVO3 is observed in CV25 and CV50 samples, demonstrating incomplete reoxidation during WS reactions in these samples.

The average total yield of syngas during the reduction and oxidation reactions over 10 redox cycles is presented in Figure 6. In MPO–CDS cycles, CV25 shows the highest H2 and CO yields, up to 8.2 mol mol-1V and 3.95 mol mol-1V, respectively, and a H2/CO ratio of 2.21. This is followed by the H2 and CO yields observed for pure V2O5, 3.54 mol mol-1V, and 1.39 mol mol-1V, respectively. As discussed earlier, carbon deposition is observed during MPO–WS cycles, while higher V contents promote the cyclic oxidation of vanadium carbide. This phenomenon leads to a controlled H2/CO ratio with a minimum of 2.6 with 75% V, while all other samples show their H2/CO ratios greater than 4. A combination of WS and CDS cycles significantly improves the H2/CO ratio, where CV25 produces the highest syngas yield with a moderately high H2/CO ratio. During MPO–CDS–WS cycles, the H2/CO ratios tend to increase drastically in Ce-rich samples up to 50%V as compared to pure V2O5 and CV75. Despite the decline in performance, Ce-rich vanadia–ceria systems demonstrate stable reaction rates as compared to the rates obtained with CV75 and pure V2O5. The average total yields of H2 and CO for pure V2O5 are 3.8 mol mol-1V and 1.04 mol mol-1V, respectively. The highest yields of H2 (31.24 mol mol-1V) and CO (6.56 mol mol-1V) are obtained for CV25, with a H2/CO ratio of 4.7.

The average total fuel yield during the oxidation step of MPO–CDS, MPO–WS, MPO–WS–CDS, and MPO–CDS–WS cycles is shown in Figure 6. During MPO–CDS cycles, CV25 showed the highest CO yield of 1.59 mol mol-1V, followed by pure V2O5 with 1.29 mol mol-1V. However, the fuel production rates for CV75 are more stable than those for pure V2O5, as discussed previously. The high yield of CO during oxidation of pure V2O5 also suggests the presence of carbon species deposited in V2O5. During MPO–WS cycles, no considerable CO is observed in Ce-rich vanadia–ceria systems containing up to 50% V, for which a high H2 yield of 3.04 mol mol-1V is observed. Here, the H2 yield decreases with the increasing V content. Sequential WS and CDS cycles result in an improved H2/CO ratio and a high H2 yield during WS reaction. Pure V2O5 shows the highest average total H2 yield of 3.07 mol mol-1V, followed by 1.87 mol mol-1V for CV25. Considerable amounts of H2 are observed during the WS step of MPO–CDS–WS cycles, indicating incomplete reoxidation of reduced samples by CO2. Furthermore, an addition of WS steps to MPO–CDS cycles lowers the CO yield during the CDS step, as compared to MPO–CDS cycles. By analyzing the product yields of the methane partial oxidation reaction during MPO–CDS–WS and MPO–CDS cycles, it is found that water splitting reaction suppresses methane reforming and promotes methane cracking, increasing the H2/CO ratio. The subsequent CDS reaction results in gasification of the deposited carbon and suppresses methane cracking.

4. Conclusions

Synthesis gas production and oxygen exchange capacity were investigated for thermochemical redox cycling of vanadia–ceria multiphase systems with V concentrations in the range 0–100%. The materials were synthesized using a facile method involving combustion of liquid phase Ce and V precursors. Improved structural stability was achieved in mixed vanadia–ceria systems as compared to pure CeO2 and pure V2O5. A phase transformation of CeVO4 to CeVO3 accompanied by the formation of other segregated phases such as VO2 and V2O3 was observed after the thermochemical redox cycling. A mixture of CeO2 and CeVO4 with notable V concentrations showed a synergic effect in syngas yields as compared to CeO2 and CeVO4 alone. High V content facilitated carbide oxidation, which resulted in the H2/CO ratios as low as 2.14 due to low deposited carbon contents. The sequence of H2O and CO2 splitting reactions significantly affected the yields and rates of syngas production. Sequential H2O and CO2 splitting reactions in individual cycles improved the H2 purity and H2/CO ratio (up to 70%) as compared to H2O splitting alone. This study provides important information to advance the experimental investigation of metal-metal and metal-oxygen interactions in oxygen carrier material during thermochemical redox cycles.

Data Availability

Data generated or analyzed during this study are included in this published article and its Supplementary Materials.

Conflicts of Interest

The authors declare no competing financial interests.

Authors’ Contributions

A. Riaz conceived the idea of utilizing vanadia–ceria systems for synthesis gas production and conducted the experiments. A. Riaz wrote the manuscript with the professional guidance of A. Lowe and W. Lipiński. A. Riaz, T. Tsuzuki, A. Lowe, and W. Lipiński contributed to the discussion and revisions of the manuscript. M. Umair performed and processed the XPS analysis of the materials. T. G. Enge performed and interpreted the ICP-OES measurements of the materials.

Acknowledgments

This study used the facilities and the scientific and technical assistance at the Centre for Advanced Microscopy at the Australian National University. We also acknowledge the technical assistance provided by Colin Carvolth and Kevin Carvolth. This work was supported by the Australian Research Council (ARC Future Fellowship FT140101213 by W. Lipiński).

Supplementary Materials

Figure SI 1: schematic of the experimental setup for thermochemical redox cycling. Figure SI 2: effect of V addition on morphology: scanning electron microscopy images of as-prepared (a) pure CeO2, (b) CV25, (c) CV50, (d) CV75, and (e) pure V2O5, representing the change in particle growth and size of pure CeO2 with the addition of V. Figure SI 3: morphological investigation of cycled V2O5–CeO2 systems after MPO–CDS redox cycles: scanning electron microscopy images of after MPO–CDS cycled (a) pure CeO2, (b) CV25, (c) CV50, (d) CV75, and (e) V2O5, showing the change in morphology of the metal oxides due to high temperature sintering and chemical reactions occurring on surface/bulk. Figure SI 4: morphological investigation of after cycled CV75: transmission electron microscopy images of CV75 (a) after reduction and (b) after MPO–WS–CDS cycles, depicting drastic changes in particle size and morphology due to high temperature sintering. Figure SI 5: surface chemical analysis of after cycled samples: X-rays photoelectron spectra of reduced CV25, CV75, and V2O5 and MPO–CDS-cycled V2O5 samples, depicting the binding energy shift and change in intensities. Table SI T1: operating parameters for ICP-OES measurements. Table SI T2: phase percentages present in as-prepared vanadia–ceria systems, quantified by the Rietveld refinement technique on XRD patterns. (Supplementary Materials)

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