A novel and facile method to rapidly synthesize Ce0.8Zr0.2O2 nanoparticles for CO preferential oxidation in H2-rich stream

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 (%)  [ CO ] in - [ CO ] out [ CO ] in ×100          [1]

O 2  selectivity (%) = 0.5× ( [ CO ] in - [ CO ] out ) [ O 2 ] in - [ O 2 ] out ×100          [2]

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.

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.

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.

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