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Article

Influence of Oxidative Properties of CexZr1−xO2 Catalyst on Partial Oxidation of Dimethyl Ether

by
Zhu Fu
,
Ping Qi
,
Huimin Liu
,
Qijian Zhang
*,
Yonghua Zhao
and
Xiaoqian Feng
School of Chemical & Environmental Engineering, Liaoning University of Technology, Jinzhou 121001, China
*
Author to whom correspondence should be addressed.
Catalysts 2022, 12(12), 1536; https://doi.org/10.3390/catal12121536
Submission received: 2 November 2022 / Revised: 24 November 2022 / Accepted: 28 November 2022 / Published: 29 November 2022
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Abstract

:
Partial oxidation of dimethyl ether (DME) to hydrogen is an efficient route for hydrogen production for application in SOFC. However, quite a large amount of CO2 as a byproduct has been an important obstacle. In this work, cerium–zirconium solid solution (CexZr1−xO2) was applied to determine how the oxidative properties of the catalyst affect the production of CO2 in the partial oxidation of DME. The results show that the catalyst with more adsorbed oxygen (Oads) rather than the lattice oxygen has stronger oxidizability and gives higher DME conversion as well as higher CO2 yield, due to the overoxidation of CO.

1. Introduction

Dimethyl ether (DME) is non-toxic, non-irritating, and non-corrosive [1,2,3]. Because of the high hydrogen content (13 wt%) and easiness to liquefy, DME can be used as a good hydrogen carrier to solve the problem of the explosive risk of hydrogen during storage and transportation [4,5,6,7].
There are usually four methods for hydrogen production from DME, namely: hydrogen production by steam reforming of DME (SRD) [8,9], DME partial oxidation reforming (POX), DME autothermal reforming (ATR) [10,11], and plasma reforming of DME (PR) [12,13]. For plasma reforming, most research revolves around the optimization of reaction devices. The presence of CO in the reaction products of steam reforming of DME can cause electrode poisoning in fuel cells [14,15]. At the same time, adding the water gasification device before the reaction increases the cost. The feeding ratio of raw materials for the DME autothermal reforming reaction is difficult to control, and the hydrogen production rate is low [16].
The products of partial oxidation of DME are mostly H2 and CO, which can be the fuels for solid oxide fuel cells (SOFC) [17,18,19,20]. However, a certain amount of CO2 in the product is harmful to the life of the SOFC. Since the production of carbon dioxide in the partial oxidation reaction is inevitable, it is expected to control its production by catalytic process. It is, therefore, necessary to explore what are the main factors affecting CO2 generation. The oxidative property of the catalyst is considered a key factor in the partial oxidation of hydrocarbons [21,22]. As an example, in the partial oxidation reaction of methane, a certain amount of ZrO2 in the Ce-Fe compound can increase the oxygen vacancy and thereby increase the oxidation performance [21].
CeO2 is an important material for current chemical conversion and energy applications because of its unique redox properties [23,24,25,26]. Doping other metals in CeO2 can adjust the oxidative properties of the catalyst by changing its shape and structure size [27,28,29]. Because the ion radius of Zr is similar to that of Ce, it is often introduced into the CeO2 lattice to form a solid solution. The addition of Zr distorts the CeO2 lattice and greatly affects the oxidative properties [30,31,32]. Therefore, the oxidation performance of the catalyst can be regulated by using cerium–zirconium solid solution catalysts with different components.
In this work, a series of cerium–zirconium solid solutions CexZr1−xO2 were synthesized and the influence of oxidation properties on the performance in partial oxidation of DME was investigated. The structure, composition, and oxidation properties of the catalyst were analyzed by XRD, BET, and XPS.

2. Results and Discussion

The XRD patterns of CexZr1−xO2 oxides are exhibited in Figure 1. For the pure CeO2, the characteristic peaks are located at 2θ = 28.5°, 32.9°, 47.4°, and 56.3°, corresponding to (111), (200), (220), and (311) crystal planes of the cubic fluorite phase of ceria (PDF #78-0694). The diffraction peaks of pure ZrO2 appear at 2θ = 30.2°, 35.1°, 50.5° and 59.9°, respectively, which can be assigned to the (111), (200), (220), and (311) crystal plane of tetragonal phase of ZrO2 (PDF #88-1007). In the case of CexZr1−xO2 oxides, the shape of the diffraction peaks is similar to that of the pure CeO2 component, and no characteristic peak assigned to any phase of ZrO2. There is no phase segregation, indicating the formation of CexZr1−xO2 solid solution, which is consistent with the reports in literatures [33,34]. The diffraction peaks of CexZr1−xO2 oxides locate between the pure components of CeO2 and ZrO2. The diffraction peaks of each catalyst shift to higher angles with the proportion of Zr increase. Compared with the pure CeO2 component, the diffraction peaks of CexZr1−xO2 oxides are broader and weaker, indicating that the average crystal sizes of CexZr1−xO2 oxides are smaller due to the lattice distortion caused by small Zr4+ inserting into the CeO2 lattice [35].
Table 1 shows the specific area for each sample. For CexZr1−xO2, the BET specific surface area varies from 36.9 to 53.5 m2/g. Among them, the largest specific surface area is obtained in the Ce0.4Zr0.6O2 catalyst.
XPS analysis was used to study the surface chemical states of the catalysts and the patterns are shown in Figure 2. The Ce 3d peaks could be decomposed into eight Gaussian–Lorentzian peaks corresponding to two pairs of spin-orbit doubles after subtracting a Shirley background [36,37]. As shown in Figure 2a, the peaks marked with V belong to the ionization peaks of Ce 3d5/2, and the peaks marked with U belong to the ionization peaks of Ce 3d3/2. U’ and V’ are generated by the Ce3+ species, corresponding to the final state 3d94f1Vn. The other peaks (U, V), (U”, V”), and (U‴, V‴) are generated by the Ce4+ species, corresponding to the final states 3d94f2Vn−2, 3d94f1Vn−1, and 3d94f0Vn, respectively. This indicates that Ce3+ and Ce4+ coexist in the samples. At the same time, the addition of Zr reduces the intensity of the Ce 3d peak, indicating that part of Ce in the sample is replaced by Zr [38].
The ratio of Ce3+ in the total amount of Ce3+ and Ce4+ were calculated by the peak area by Equation (n(Ce3+)/n(Ce3+ + Ce4+) = A(Ce3+)/A(Ce3+ + Ce4+)) and the results are listed in Table 2. With the increase in Zr content, the proportion of Ce3+ increases. As the charge compensation effect, the increase in Ce3+ causes the oxygen in the crystal lattice to detach. The oxygen loss forms oxygen vacancies in the catalysts [39,40,41]. Therefore, Adding Zr can improve the amounts of oxygen vacancies. This is in accordance with previous reports [42,43,44].
Figure 2b shows the O 1s spectrum of CexZr1−xO2 catalysts. Two peaks corresponding to two different oxygen species can be clearly seen in the O 1s XPS spectrum of CexZr1−xO2. The peak located at 529.3 eV is attributed to lattice oxygen (Olat) and the peak located at 531.1 eV can be assigned to adsorbed oxygen (Oads) [45]. According to the results reported by Jampaiah D. [46], the active oxygen species in cerium–zirconium solid solutions are Oads with unsaturated chemical bonds brought by surface oxygen vacancies/oxygen defects. Adsorbed oxygen has higher mobility than other oxygen species. In general, the higher the amount of adsorbed oxygen, the higher the oxidizablility of the catalyst.
The ratios of Oads in CexZr1−xO2 are also calculated by the Equation (n(Oads)/n(Oads + Olat) = A(Oads)/A(Oads + Olat)) and listed in Table 2. The changing trend of adsorbed oxygen ratios is basically the same as that of Ce3+. With the increase in Ce3+, the oxygen vacancies increase, and then the amount of adsorbed oxygen increases. Multiple studies have reached the same result [47,48,49,50,51]. This is due to the free electrons produced by the formation of oxygen vacancies to promote the adsorption of oxygen, as reported by Su et al. [52].
However, the adsorbed oxygen ratios decrease for the Ce0.2Zr0.8O2 catalyst. The reason for this is the content of Zr is too much. Although the proportion of oxygen vacancies is high, the total amount is low. So the adsorbed oxygen is also reduced.
The catalytic performance over CexZr1−xO2 in partial oxidation of DME are shown in Figure 3. It can be found in Figure 3a that all CexZr1−xO2 show certain catalytic activity. As the value of x in CexZr1−xO2 varies from 0 to 1, the conversion of DME firstly increases and then decreases. When the value of x is 0.4, the conversion rate of DME is the highest (~40%), while the conversion is relatively stable during the 300 min test. Such a changing trend in the conversion of DME is the same as that of the content of adsorbed oxygen in the catalyst, indicating that the amount of adsorbed oxygen in the catalyst is the key factor for the oxidation reaction of DME.
The H2 yields (Figure 3b) change with different ratios of catalysts in the same trend as the DME conversion. It also reaches the highest value (~20%) when x is 0.4. However, the changing trend of CO selectivities is just the opposite. When the conversion of DME and the yield of H2 reaches the maximum, CO selectivity is the lowest while CO2 selectivity is the highest. This may be due to the DME being over oxidized.
In order to investigate the role of adsorbed oxygen in the reaction, the values of CO/(CO + CO2), calculated by Equation CO/(CO + CO2) = c(CO)/c(CO + CO2), are used to discuss the oxidizability. The values of the adsorbed oxygen ratio Oads/(Oads + Olat) and CO/(CO + CO2) are summarized in Table 3. It can be concluded that the more adsorbed oxygen in the catalyst, the stronger the oxidation. As x increases from 0 to 0.4, the proportion of adsorbed oxygen increases, the oxidation of the catalyst is also enhanced, and more CO is more oxidized to CO2. As the value of x is from 0.4 to 1, the proportion of adsorbed oxygen decreases. When x = 0.4, the proportion of adsorbed oxygen is the largest, the oxidation of the catalyst is also the strongest.
There are generally two mechanisms for CO oxidation reaction. In the L-H mechanism, both reactants are adsorbed on the catalyst surface and then react. In the E-R mechanism, only one reactant is adsorbed and then interacts with another reactant. Among the two mechanisms, the L-H mechanism predominates [53]. Kang et al., conforms the oxidation process of CO in the cerium–zirconium solid solution to the L-H mechanism [54]. During the reaction, DME is partially oxidized to form CO and H2. CO and H2 continue to undergo oxidation reactions with O2 to form CO2 and H2O. CO and O2 are co-adsorbed on the catalyst surface, combined to form CO2, and then removed from the surface. The reaction formula is as follows:
CO + Ce4+-Zr4+ + O2 ↔ CO-Ce4+-Zr4+-O2
CO-Ce4+-Zr4+-O2 ↔ Ce4+-Zr4+ + CO2
The adsorbed O2 can facilitate the oxidation of DME, therefore benefiting the higher conversion of DME. However, CO is also prone to react with adsorbed O2 on the surface of the catalyst to form carbonate species, and finally generate CO2. Therefore, if both high activity and high CO selectivity are to be achieved, the content of adsorbed O2 as well as the oxidizability of the catalyst must be appropriate.

3. Materials and Methods

3.1. Catalyst Preparation

CexZr1−xO2 (x = 0, 0.2, 0.4, 0.6, 0.8, and 1) composites were prepared via the coprecipitation method. In a typical procedure, a total of 0.01 mol of Ce(NO3)3·6H2O (>99.0%, Aladdin, Shanghai, China) and Zr(NO3)4·5H2O (>99.0%, Shandong West Asia Chemical Industries, Ltd., Dezhou, China) were dissolved in 100 mL of the deionized water to form a clear solution. An amount of 300 mL of 0.1 mol/L NH3·H2O (25%, Tianjin yongda chemical company, Tianjin, China) aqueous solution was dropwise added as precipitant under continuous stirring. Then, the suspension was stirred for another 1 h, and then aged for 12 h. Finally, the obtained solid was separated by centrifugation and then washed with deionized water until the pH of the supernatant liquid reached 7.0 ± 0.1. After drying at 110 °C for 12 h, the solid was calcined at 400 °C for 6 h in air with a heating rate of 2 °C/min. The obtained samples were tableted, crushed, and sieved to 40–60 mesh particles for later use.

3.2. Catalyst Characterization

Powder X-ray diffraction (XRD) was performed on an X-ray diffractometer (D/Max2550) (Rigaku, Japan) with a Cu Kα radiation as the X-ray resource (tube voltage 40 kV, tube current 40 mA). All of the samples were scanned from 10° to 80° with a step size of 0.02° and a scan rate of 6°/min.
N2 adsorption/desorption isotherms were measured on a Micromeritics ASAP-2020 (Quantachrome, Boynton Beach, FL, USA) at −196 °C. Before measurement, about 100 mg of the catalyst sample was degassed at 200 °C for 2 h to remove the atmospheric contaminants. The specific surface area of the sample was calculated by the Brunauer–Emmett–Teller (BET) equation.
X-ray photoelectron spectroscopy (XPS) tests were conducted on a K-Alpha X-ray Photoelectron Spectrometer (Thermo Scientific, Waltham, MA, USA). The C 1s spectrum centered at 284.8 eV was applied to calibrate the binding energy (BE).

3.3. Catalytic Oxidation of DME

The catalytic performances of the samples were evaluated in a fixed-bed quartz reactor (i.d. = 10 mm) under the conditions of 500 °C, gas hourly space velocity (GHSV) of 28,800 mL·g−1·h−1, and atmospheric pressure. In each test, 0.1 g of the sample was mixed with the same quality of quartz sand and loaded in the reactor. A mixed gas of H2/Ar (49.9% hydrogen content) was introduced to dynamically reduce the sample at 500 °C for 1 h. After reduction, the reactor was purged with Ar for 10 min and the gas mixture of Ar/DME/O2 was introduced in a molar ratio of 1/4/8 with a total flow rate of 112 mL/min. The composition of the products was analyzed by an online gas chromatograph. To estimate the experimental error, the representative experiments were repeated at least three times.

4. Conclusions

In this study, CexZr1−xO2 catalysts with different ratios of cerium and zirconium were prepared by coprecipitation method and applied in catalysis of partial oxidation of DME. All CexZr1−xO2 catalysts show certain activities. The effect of catalyst oxidizability on the reaction performance was investigated. According to XPS results, the addition of Zr can alternate the ratio of Ce3+, varying the amount of oxygen vacancies as well as the ratio Oads. Oads serves as the main active species in the oxidation of DME. Therefore, the sample with more Oads has stronger oxidizability and shows higher conversion of DME while Ce0.4Zr0.6O2 shows the highest. However, stronger oxidizability also leads to overoxidation of CO, leading to higher CO2 yield. In order to obtain catalysts with both high DME conversion and high CO selectivity, moderate oxidizability is necessary.
This discovery provides a theoretical basis for the design of catalysts for the partial oxidation of DME in the future. At the same time, it provides a reference for the application of catalyst oxidation in other oxidation reactions.

Author Contributions

Conceptualization, H.L. and Q.Z.; methodology, H.L.; validation, Q.Z., H.L., Z.F. and Y.Z.; formal analysis, P.Q.; investigation, Z.F.; resources, Z.F.; data curation, P.Q.; writing—original draft preparation, Z.F.; writing—review and editing, Q.Z. and X.F.; supervision, Q.Z.; project administration, Q.Z.; funding acquisition, Q.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by Xingliao talents plan of Liaoning province (XLYC1902070).

Data Availability Statement

All data are generally available in the manuscript.

Acknowledgments

Authors thank Shiyanjia Lab (www.shiyanjia.com, accessed on 21 February 2019) for the XPS measurements.

Conflicts of Interest

The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

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Catalysts 12 01536 g001 550
Figure 1. XRD pattern of CexZr1−xO2.
Figure 1. XRD pattern of CexZr1−xO2.
Catalysts 12 01536 g001
Catalysts 12 01536 g002 550
Figure 2. Ce 3d (a) and O 1s (b) XPS spectrum of CexZr1−xO2.
Figure 2. Ce 3d (a) and O 1s (b) XPS spectrum of CexZr1−xO2.
Catalysts 12 01536 g002
Catalysts 12 01536 g003 550
Figure 3. Catalytic performance over CexZr1−xO2 in partial oxidation of DME. (a) Conversion of DME, (b) yield of H2, and (c) carbonaceous product selectivity at the reaction time of 300 min. Reaction conditions: 500 °C, DME/O2 = 2/1, GHSV = 28,800 mL/(g·h), 1 atm.
Figure 3. Catalytic performance over CexZr1−xO2 in partial oxidation of DME. (a) Conversion of DME, (b) yield of H2, and (c) carbonaceous product selectivity at the reaction time of 300 min. Reaction conditions: 500 °C, DME/O2 = 2/1, GHSV = 28,800 mL/(g·h), 1 atm.
Catalysts 12 01536 g003
Table
Table 1. The surface of CexZr1−xO2.
Table 1. The surface of CexZr1−xO2.
CatalystSBET (m2/g)
ZrO238.5
Ce0.2Zr0.8O236.9
Ce0.4Zr0.6O253.5
Ce0.6Zr0.4O250.2
Ce0.8Zr0.2O246.2
CeO245.8
Table
Table 2. XPS analysis of CexZr1−xO2.
Table 2. XPS analysis of CexZr1−xO2.
Catalystn(Ce3+)/n(Ce3+ + Ce4+)n(Oads)/n(Oads + Olat)
Ce0.2Zr0.8O232.41%29.84%
Ce0.4Zr0.6O225.48%30.64%
Ce0.6Zr0.4O221.30%28.97%
Ce0.8Zr0.2O216.98%26.31%
CeO212.82%24.30%
Table
Table 3. n(Oads)/n(Oads + Olat) and CO/(CO + CO2) in CexZr1−xO2.
Table 3. n(Oads)/n(Oads + Olat) and CO/(CO + CO2) in CexZr1−xO2.
xn(Oads)/n(Oads + Olat)DME ConversionCO/(CO + CO2)
0.229.84%29.43%82.4%
0.430.64%39%61.8%
0.628.97%33.03%65.3%
0.826.31%23.48%75.1%
124.3%20.32%79.5%
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Fu, Z.; Qi, P.; Liu, H.; Zhang, Q.; Zhao, Y.; Feng, X. Influence of Oxidative Properties of CexZr1−xO2 Catalyst on Partial Oxidation of Dimethyl Ether. Catalysts 2022, 12, 1536. https://doi.org/10.3390/catal12121536

AMA Style

Fu Z, Qi P, Liu H, Zhang Q, Zhao Y, Feng X. Influence of Oxidative Properties of CexZr1−xO2 Catalyst on Partial Oxidation of Dimethyl Ether. Catalysts. 2022; 12(12):1536. https://doi.org/10.3390/catal12121536

Chicago/Turabian Style

Fu, Zhu, Ping Qi, Huimin Liu, Qijian Zhang, Yonghua Zhao, and Xiaoqian Feng. 2022. "Influence of Oxidative Properties of CexZr1−xO2 Catalyst on Partial Oxidation of Dimethyl Ether" Catalysts 12, no. 12: 1536. https://doi.org/10.3390/catal12121536

APA Style

Fu, Z., Qi, P., Liu, H., Zhang, Q., Zhao, Y., & Feng, X. (2022). Influence of Oxidative Properties of CexZr1−xO2 Catalyst on Partial Oxidation of Dimethyl Ether. Catalysts, 12(12), 1536. https://doi.org/10.3390/catal12121536

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