A Novel and Facile Method to Rapidly Synthesize Ce 0.8 Zr 0.2 O 2 Nanoparticles for CO Preferential Oxidation in H 2 -Rich Stream

Abstract

A novel urea grind combustion (UGC) route was reported in this paper to rapidly prepare the ceria-zirconia nanoparticles (Ce_0.8Zr_0.2O₂). For comparison, the conventional surfactant-assisted (SA) and sol-gel (SG) methods were also employed to prepare Ce_0.8Zr_0.2O₂ nanoparticles. CO preferential oxidation in H₂-rich stream (CO-PROX) was chosen as probe reaction to investigate the catalytic performance of these Ce_0.8Zr_0.2O₂ catalysts prepared with different methods to highlight the superiority of UGC. It was found that Ce_0.8Zr_0.2O₂-UGC showed the better reducibility and oxygen mobility than the Ce_0.8Zr_0.2O₂ prepared by SA and SG, because the UGC route favored the more incorporation of zirconia into CeO₂, leading to more serious distortion of the structure, and more defective sites in the Ce_0.8Zr_0.2O₂. As a result, Ce_0.8Zr_0.2O₂-UGC exhibited the higher CO conversion, better O₂ selectivity, and excellent catalytic stability without any deactivation during 72-h reaction on stream. More importantly, the UGC method, as compared to the relatively complex and time-consuming SA and SG method, is simple, facile, low-cost, time-saving (within 30 minutes) and scalable, thereby, might be very promising for the application in many fields.

Keywords

Ceria-zirconia Characterization CO-PROX Nanoparticles Urea grind combustion

Introduction

Proton exchange membrane fuel cells (PEMFCs) have been recognized as one of the most attractive clean alternatives to fossil fuels for energy generation because of their advantages, including high efficiency, large power density together with low operating temperature (1). Hydrogen (H₂) is widely considered as a superior energy carrier for PEMFCs, and can be normally obtained from liquid hydrocarbons and natural gas by steam reforming, partial oxidation, or auto-thermal reforming in alliance with the water gas shift (WGS) reaction (2). However, these resultant H₂-rich gases always contain a small amount of CO (0.5%-2%), which can easily poison the platinum-based anode materials of PEMFCs. Therefore, it is crucial to control the concentration of CO in the H₂-rich gas before entering into PEMFCs. It is generally accepted that CO preferential oxidation (CO-PROX) is one of the most efficient and economical techniques to fulfill this goal (3).

Cerium oxide-based materials have been widely used in CO-PROX owing to their oxygen (O₂) vacancy defects and high oxygen storage capacity (OSC) originating from the facile redox cycle between Ce⁴⁺ and Ce³⁺. However, pure CeO₂ can be easily sintered under higher temperature due to its poor thermal stability (4). Generally, doping of CeO₂ with other elements, such as Zr, Al, Si, Cu, La, Zn, Mn and Ti (4-5-6-7-8-9-10-11), is a promising way to improve its stability. In particular, zirconium oxide is the most studied dopant so far. The addition of Zr to ceria can not only enhance the thermal resistance of CeO₂, but also increase the oxygen storage capacity and redox property, thus promoting the catalytic performance (6, 12, 13). The effects of Ce/Zr ratio on the textural properties and catalytic activities of Ce_xZr_1-xO₂ solid solutions were investigated in detail (6, 12, 14, 15). A perfect example can be found in Trovarelli and coworkers’ report (15) that the Ce_0.8Zr_0.2O₂ was the most texturally stable.

As well known, the preparation method plays a vital role in determining the catalytic behaviors of Ce_xZr_1-xO₂ oxides for many oxidation reactions (16-17-18). Therefore, many researchers have been focusing on the synthesis of ceria-zirconia (Ce_xZr_1-xO₂)-based catalysts via various methods (13, 17, 19-20-21-22-23), such as incipient wetness impregnation, co-precipitation, surfactant-assisted (SA), sol-gel (SG) techniques, and so on. However, all the above methodologies mentioned are relatively complex and time-consuming, and can hardly be scaled up in industry. From the practical point of view, it is highly desirable to develop a rapid and economical method for the preparation of Ce_xZr_1-xO₂.

Therefore, in this paper, a novel route, called urea grind combustion (UGC) route, was conducted to rapidly synthesize ceria-zirconia (Ce_0.8Zr_0.2O₂) nanoparticles, and then compare them to the common SA and SG methods. These resulting Ce_0.8Zr_0.2O₂ samples were studied in the catalytic CO-PROX reaction, and characterized in detail by transmission electron microscopy (TEM), N₂ adsorption-desorption, x-ray diffraction (XRD) and H₂-temperature-programed reduction (H₂-TPR).

Method

Materials

Cetyltrimethylammonium bromide (CTAB), zirconia nitrate (Zr(NO₃)₄·5H₂O), ceria nitrate (Ce(NO₃)₃·6H₂O), urea ((NH₂)₂CO), citric acid (C₆H₈O₇) and sodium hydroxide were purchased from Shanghai chemical regent company of China.

Catalyst preparation

Herein, the ceria-zirconia catalysts prepared with SA, SG and UGC routes were named as Ce_0.8Zr_0.2O₂-SA, Ce_0.8Zr_0.2O₂-SG and Ce_0.8Zr_0.2O₂-UGC, respectively.

The following general procedures were used to prepare Ce_0.8Zr_0.2O₂-UGC. In a typical synthesis batch, 24 mmol of Ce(NO₃)₃·6H₂O, 6 mmol of Zr(NO₃)₄·5H₂O and 25 mmol of (NH₂)₂CO were mixed and grinded in agate mortar until the viscous gel was formed (within a few minutes). Finally, the resulting gel was calcined at 400°C in air for 20 minutes.

For Ce_0.8Zr_0.2O₂-SA, the following synthesis procedures were employed: CTAB (6 mmol) was firstly dissolved in distilled water under vigorous stirring at room temperature (RT) for 1 h, followed by the addition of Ce(NO₃)₃·6H₂O (8 mmol) and Zr(NO₃)₄·5H₂O (2 mmol), with continuous stirring for 0.5 h. Then, 0.2 M NaOH was added into the above mixture solution until a pH value of 10.0 was achieved. After that, the resulting solution was still kept to be stirred at RT for 12 h and then aged at 90°C for 3h. Subsequently, the reaction product was filtered and washed using hot water. After drying at 110°C for 6 h, the calcination was conducted at 400°C for 4h to obtain the target sample.

Ce_0.8Zr_0.2O₂-SG was synthesized by using citric acid, ceria and zirconia nitrate precursors. In a typical synthesis batch, citric acid (15 mmol) was added slowly into the aqueous solution (300 ml) of Ce(NO₃)₃·6H₂O (24 mmol) and Zr(NO₃)₄·5H₂O (6 mmol) at RT with vigorous stirring for 1-2 h. Then, the mixture solution was treated at 90°C without stirring to remove H₂O to obtain the solid product. Similar to Ce_0.8Zr_0.2O₂-SA, the drying and calcinations were conducted finally as above.

Catalyst characterization

A Quantachrome NOVA 2000e sorption analyzer was used to obtain N₂ adsorption-desorption isotherms of the samples at −196°C after outgassing at 200°C for 6 h. TEM image of the sample was obtained using a Leo-1530 microscope equipped with an Energy Dispersive Spectrometer with 200 kV of accelerating voltage. A Rigaku D/max-1200 diffractometer was employed to collect XRD data of the samples, using Cu Ka radiation (λ = 1.5406 Å). H₂-TPR was carried out in an in-house constructed system, which was equipped with a thermal conductivity detector (TCD). Prior to H₂-TPR analysis, approximately 100 mg of catalyst was pretreated in mixture gas (5 vol% O₂/Ar) at 400°C for 60 min. After cooling to 100°C under Ar flow, the pretreated sample was reduced in the flow of mixture gas (10 vol% H₂/Ar) from 100 to 900°C with a ramp rate of 10°C/min.

Catalytic activity measurement

The catalytic activities were tested in a fixed-bed reactor, using 150 mg of catalyst (60-80 mesh). The feed gas mixture contained 1% CO, 1% O₂, 50% H₂ and was balanced with helium. A total flow rate of 80 mL/min was used, corresponding to 32,000 ml∙g⁻¹∙h⁻¹ of weight hourly space velocity (WHSV). After reaction, a gas chromatography, equipped with a TCD detector and a hydrogen flame ionization detector (FID) combined with a methanation reactor, was used to analyze the effluent gas online. The catalytic behaviors of catalysts were expressed by the conversion of CO and selectivity of O₂, which were calculated using the following equations:

CO conversion (%) \frac{{[CO]}_{in} - {[CO]}_{out}}{{[CO]}_{in}} ×100

O_{2} selectivity (%) = \frac{0.5× ({[CO]}_{in} - {[CO]}_{out})}{{[O_{2}]}_{in} - {[O_{2}]}_{out}} ×100

Where [CO]_in (or [O₂]_in) and [CO]_out (or [O₂]_out) were inlet and outlet the concentrations (%(v/v)) of CO (or O₂), respectively.

Specifically, all the catalytic measurements were repeated three times in order to verify their reproducibility.

Results and discussion

Catalytic performance

Ce_0.8Zr_0.2O₂ catalysts, prepared with different methods, were evaluated for CO-PROX in H₂ rich gases and the results were presented in Figure 1. As shown in Figure 1A, CO conversion of these three samples increased with reaction temperature from 300^oC to 400^oC, and achieved the maximum at 400^oC. After that, a distinct decrease in the conversion of CO with reaction temperature was observed for all the three catalysts above 400^oC. For O₂ selectivity, a decrease with reaction temperature from 300^oC to 550^oC was very obvious for all of the three samples (Fig. 1B). Coupling the changes of CO conversion and O₂ selectivity, it could be concluded that the decrease of CO conversion should be closely associated with the fact that oxygen (O₂) in the feed gas mixture was consumed by hydrogen (H₂) to produce water. Noticeably, Ce_0.8Zr_0.2O₂-SA showed the worst catalytic performance in the entire reaction temperature region (300^oC -550^oC), while Ce_0.8Zr_0.2O₂-UGC exhibited the better or comparative CO conversion and O₂ selectivity than Ce_0.8Zr_0.2O₂-SG, especially for the reaction temperature above 450^oC.

Fig. 1

(A) CO conversion and (B) O₂ selectivity over three ceria zirconia catalysts: (a) Ce_0.8Zr_0.2O₂-UGC, (b) Ce_0.8Zr_0.2O₂-SG and (c) Ce_0.8Zr_0.2O₂-SA for CO preferential oxidation in H₂-rich stream (CO-PROX) reaction.

Even more important than the catalytic activity is the stability, which was investigated at 400^oC over Ce_0.8Zr_0.2O₂ catalysts prepared via UGC, SG and SA, and the results were presented in Figure 2. Clearly, all these three catalysts exhibited excellent stabilities, there was no obvious decline in both CO conversion and O₂ selectivity within 72 h on-stream reaction.

Fig. 2

CO conversion and O₂ selectivity with reaction time over (a) Ce_0.8Zr_0.2O₂-UGC, (b) Ce_0.8Zr_0.2O₂-SG and (c) Ce_0.8Zr_0.2O₂-SA. Reaction temperature 400°C.

From the above studies, it was easily concluded that the rapidly prepared Ce_0.8Zr_0.2O₂-UGC showed comparative or better catalytic performance than Ce_0.8Zr_0.2O₂-SG and Ce_0.8Zr_0.2O₂-SA. Importantly, UGC, compared to SG and SA, is much more simple, time-saving and economical for the synthesis of Ce_0.8Zr_0.2O₂. Later in this paper, N₂ adsorption-desorption, XRD and H₂-TPR were employed to explore the possible reasons why UGC is superior to SG and SA for the synthesis of Ce_0.8Zr_0.2O₂.

Catalyst characterization results

Results of N₂ adsorption-desorption isotherms

N₂ adsorption-desorption isotherms of Ce_0.8Zr_0.2O₂-SA, Ce_0.8Zr_0.2O₂-SG and Ce_0.8Zr_0.2O₂-UGC were presented in Figure 3, and the corresponding physicochemical properties were summarized in Table I. As shown in Figure 3, for Ce_0.8Zr_0.2O₂ prepared via SA and UGC, the isotherms were found to be of type IV, and a distinct hysteresis loop was detected for each of them. While the isotherm of Ce_0.8Zr_0.2O₂-SG was found to be of type II. All of the three samples showed a main inflection in the adsorption branch in the relative pressure (P/P₀) range of 0.40-0.85, which was arisen from capillary condensation of nitrogen within the mesopores. Moreover, an adsorption step at P/P₀ >0.9, which is characteristic of interparticle macroporosity (24, 25), could be clearly observed for Ce_0.8Zr_0.2O₂-SG and Ce_0.8Zr_0.2O₂-UGC (especially for SG), but not for Ce_0.8Zr_0.2O₂-SA. For Ce_0.8Zr_0.2O₂-SA, the nitrogen adsorbed volume was the highest, suggesting a maximum in Brunauer-Emmett-Teller (BET) surface areas. As could be seen from Table I, BET surface area of Ce_0.8Zr_0.2O₂-UGC was equivalent to that of Ce_0.8Zr_0.2O₂-SG, which was far lower than that of Ce_0.8Zr_0.2O₂-SA. Combining the results originated from catalytic activity tests (Fig. 1) which showed that both CO conversion and O₂ selectivity of Ce_0.8Zr_0.2O₂-UGC were higher than or comparable to those of Ce_0.8Zr_0.2O₂-SG and Ce_0.8Zr_0.2O₂-SA, it could be concluded that other characteristics (such as oxygen storage capacity and redox property) rather than BET-specific surface area should be responsible for highly catalytic performances.

Table I

Physicochemical properties of ceria-zirconia catalysts prepared different routes

Sample	BET surface area (m²g⁻¹)	Concentration of oxygen vacancies (cm⁻¹)	2θ of CeO₂ (111) (θ)	Lattice parameter (nm)	Crystal size (nm)	H₂ consumption (µmoL/g)
Ce_0.8Zr_0.2O₂-SA	110.9	0.12	28.6	0.53905	7.68	777
Ce_0.8Zr_0.2O₂-SG	71.0	0.15	28.8	0.53831	6.19	730
Ce_0.8Zr_0.2O₂-UGC	70.5	0.17	28.9	0.53465	10.10	731

BET = Brunauer-Emmett-Teller; SA = surfactant assisted; SG = sol-gel; UGC = urea grind combustion.

Fig. 3

N₂ adsorption-desorption isotherms of (a) Ce_0.8Zr_0.2O₂-UGC, (b) Ce_0.8Zr_0.2O₂-SG and (c) Ce_0.8Zr_0.2O₂-SA. The isotherms of (b) and (c) are offset by 20 and 60 cm³g⁻¹ for clarity, respectively.

XRD results

The effect of preparation method on the crystalline structures of Ce_0.8Zr_0.2O₂ was investigated by using XRD, and the corresponding patterns were displayed in Figure 4. Seven well-resolved diffraction peaks were observed for all the three samples at 2θ range of 10-90^o, which could be easily indexed to (111), (200), (220), (311), (222), (400) and (331) reflections of a cubic fluorite type solid solution (26, 27). As could be seen by comparing Figure 4 (inset), there was a slight shift of the (111) reflection toward higher 2θ values over Ce_0.8Zr_0.2O₂-UGC (28.9^o), Ce_0.8Zr_0.2O₂-SG (28.8^o) and Ce_0.8Zr_0.2O₂-SA (28.6^o), corresponding to a decrease in the lattice parameter (shown in Tab. I), which might be caused by the incorporation of ZrO₂ into CeO₂ lattice and the formation of a CeO₂-ZrO₂ solid solution, since the lattice parameter of Zr(IV) ionic radius (0.84Ǻ) is smaller than that of Ce(IV) (0.97 Ǻ) (22, 28-30). Moreover, the largest shift was obtained in Ce_0.8Zr_0.2O₂-UGC, compared to Ce_0.8Zr_0.2O₂-SA and Ce_0.8Zr_0.2O₂-SG, implying that more zirconia was incorporated into the ceria core fcc lattice (18). Generally, the more incorporation of zirconia into CeO₂ leads to more serious distortion of the structure, and then the higher defective sites in the Ce_0.8Zr_0.2O₂-UGC. These might help to increase the reducibility and the mobility of oxygen, which could be further supported by H₂-TPR characterization as following.

Fig. 4

X-ray diffraction (XRD) patterns of (a) Ce_0.8Zr_0.2O₂-UGC, (b) Ce_0.8Zr_0.2O₂-SG and (c) Ce_0.8Zr_0.2O₂-SA.

H₂-TPR results

It has been documented that redox properties of ceria zirconia materials play important roles in determining their catalytic behaviors, which are closely associated with synthesis routes (19-20-21-22-23). Therefore, the redox properties of Ce_0.8Zr_0.2O₂ prepared from different methods were evaluated with H₂-TPR, and the corresponding profiles were provided in Figure 5. For all the samples, three H₂ consumption peaks were detected at 400^oC (α peak), 517^oC (β peak) and 770^oC (γ peak) in the H₂-TPR profile, which had been attributed to the reduction of surface oxygen species, the lattice oxygen in the solid solution and the bulk CeO₂ (31-32-33-34-35), respectively. Obviously, the main peak of Ce_0.8Zr_0.2O₂-SA located in the high temperature region (γ peak), while the reduction of Ce_0.8Zr_0.2O₂-SG and Ce_0.8Zr_0.2O₂-UGC was centralized in the lower temperature region (α and β peak), in particular, the area of α peak of Ce_0.8Zr_0.2O₂-UGC was the largest. Consequently, the Ce_0.8Zr_0.2O₂ prepared by UGC showed the better reducibility as compared to Ce_0.8Zr_0.2O₂-SA, Ce_0.8Zr_0.2O₂-SG, which was likely because that the UGC method was in favor of the incorporation of more Zr⁴⁺ into CeO₂ lattice, thus increasing the number of bulk defects and improving the oxygen mobility (33, 36). This speculation was further supported by the concentration of oxygen vacancies (shown in Table I), obtained from the Raman results (not shown in here). Ce_0.8Zr_0.2O₂ prepared from UGC exhibited the most oxygen vacancies. Usually, the better reducibility of Ce_0.8Zr_0.2O₂ promotes the preferential oxidation of CO, and improves the catalytic performance in CO-PROX. As a result, Ce_0.8Zr_0.2O₂-UGC exhibited the best catalytic performance in the reaction of CO-PROX.

Fig. 5

H₂-temperature-programed reduction (H₂-TPR) profiles of (a) Ce_0.8Zr_0.2O₂-UGC, (b) Ce_0.8Zr_0.2O₂-SG and (c) Ce_0.8Zr_0.2O₂-SA.

TEM results

The shape, size and morphology of Ce_0.8Zr_0.2O₂-UGC nanoparticles were further characterized by TEM, and the corresponding image and particle size histogram obtained from 100 particles of different TEM image were displayed in Figure 6. The sizes of particles were distributed in a range of 3.3-12.8 nm with the average size around 6.8 nm, which was in agreement with the crystal size of Ce_0.8Zr_0.2O₂-UGC calculated from XRD data. Most of them (70%) were smaller than 8.0 nm, indicating that the new UGC was an effective method for the rapid preparation of nano-scale ceria-zirconia particles. This might be the one of the reasons why Ce_0.8Zr_0.2O₂-UGC exhibited high catalytic activities and good stability for CO-PROX. In addition, some larger particles were observed for Ce_0.8Zr_0.2O₂-UGC, which might be associated with the synergistic effects of the overlaps of the smaller particles randomly distributed as well as the agglomeration of them.

Fig. 6

Transmission electron microscopy (TEM) image and related particle size histograms (inset) of Ce_0.8Zr_0.2O₂-UGC.

Conclusions

In this paper, the effect of preparation method on the physicochemical properties and catalytic performance of ceria-zirconia was studied in detail. In comparison to the relatively complex and time-consuming SA and SG methods, the UGC route simplifies producing processes and shortens the corresponding time for synthesizing catalyst on a large scale. Furthermore, the nano-scale ceria-zirconia (Ce_0.8Zr_0.2O₂) particles catalysts via UGC exhibited comparative or better catalytic performance for CO-PROX, than those via the SA and SG methods. In light of the characterization results, the UGC route, compared to SG and SA methods, was in favor of the more incorporation of zirconia into CeO₂ to cause more serious distortion of the structure, and to form the more defective sites in the Ce_0.8Zr_0.2O₂. Due to these, better reducibility and oxygen mobility were achieved on Ce_0.8Zr_0.2O₂-UGC, thus resulting in its best catalytic activity in CO-PROX reaction, together with excellent catalytic stability within 72 h on-stream. In summary, UGC is very simple, facile, low-cost, time-saving (only 30 minutes) and scalable, thus very promising for the application in many fields.

Footnotes

Financial support: The authors gratefully acknowledge the financial support from National Natural Science Foundation of China (Grant No. 21267011, 21367015 and U1402233).

Conflict of interest: The authors do not have any conflict of interest to declare.

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Nakamura

Direct conversion of methane to synthesis gas through gas–solid reaction using CeO₂–ZrO₂ solid solution at moderate temperature.

Appl Catal A Gen. 19991832317–324.

36.

Djinović

Črnive

IGO

Erjavec

Pintar

Details behind the self-regeneration of supported NiCo/Ce_0.8Zr_0.2O₂ bimetallic catalyst in the CH₄-CO₂ reforming reaction.

Chem Cat Chem. 2014 661652–1663.

A Novel and Facile Method to Rapidly Synthesize Ce 0.8 Zr 0.2 O 2 Nanoparticles for CO Preferential Oxidation in H 2 -Rich Stream

Abstract

Keywords

Introduction

Method

Materials

Catalyst preparation

Catalyst characterization

Catalytic activity measurement

Results and discussion

Catalytic performance

Catalyst characterization results

Results of N2 adsorption-desorption isotherms

XRD results

H2-TPR results

TEM results

Conclusions

Footnotes

References

Results of N₂ adsorption-desorption isotherms

H₂-TPR results