Novel [Ce_1−xLa_xO₂, La_2−yCe_yO₃]/Bi₂Mo_0.9W_0.1O₆ catalysts for CO oxidation at low temperature

Abstract

Novel Ce_1−xLa_xO₂ and La_2−yCe_yO₃ solid solutions supported on Bi₂Mo_0.9W_0.1O₆ were prepared by the citric acid route and tested for CO oxidation at low temperature. The main purpose was to investigate whether those mixed oxide systems could exhibit high efficiency for an oxidation process. The structure, morphology and surface area were determined by means of XRD, SEM and BET methods; also the catalytic efficiency of samples was evaluated for CO oxidation as a function of doping content. It was found that high cerium or lanthanum contents (>0.9) in mixed oxides favour higher CO conversion and lower activation temperature than that shown either by CeO₂ or Bi₂Mo_0.9W_0.1O₆ systems, separately.

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1. Introduction

Low temperature catalytic oxidation of CO has been a constant topic of study, because of its applications in gaseous emission control. A platinum-based catalyst that reduces hydrocarbon and carbon monoxide emissions was the first system under study.¹ The search for inexpensive and higher conversion catalysts drives to introduce ceria compounds to replace high-priced metals. Ceria and its derivatives have been used as catalytic materials for oxidation of volatile organic compounds (VOC).² In addition, ceria is used frequently in miniature fuel cells and as luminescent material for production of blue-violet light.³ Recently ceria has been used on sensor devices for NO and CO detection.⁴ A subject of growing interest is the increase of oxygen storage capacity (OSC) in ceria introducing other active metals that have shown importance in oxidation processes to form Ce_1−xM_xO₂ solid solutions, for instance ruthenium and lanthanum.^2,5,6 Deganello and Martorana⁷ reported that the OSC enhancement is based on the increase of lanthanum in Ce_1−xLa_xO₂ catalysts. They found that this behavior depends on the number of vacancies, responsible for oxygen mobility within the CeO₂ network. Similar performance was reported in different works by Sayle et al.⁸ and Bueno-López et al.⁹ as well. Other improvement has been the study of platinum supported on Ce_1−xM_xO₂ solid solutions, where M = Pb, Bi, Zr, considering CO oxidation. Conversions close to 90% have been reported; however at high temperatures the CO decreases, concluding that the rate-determining step is the velocity to release oxygen from the Ce_1−xM_xO₂ surface to platinum.¹⁰ Also, ceria-based systems are usually supported on transition aluminas, with the aim of achieving better dispersion of the active phase and improvement of the oxygen exchange rate finding fairly good results in terms of stability and CO oxidation.¹¹

In addition, other materials that have demonstrated efficiency for CO oxidation are the Bi–Mo compounds.¹² These catalysts are effective also for propylene or alcohol oxidation.¹³ It has been proved that both selectivity and activity of the Bi–Mo catalysts are influenced by the Bi/Mo ratio. This relationship leads to the formation of three catalytic active phases: α-Bi₂Mo₃O₁₂, β-Bi₂Mo₂O₉, and γ-Bi₂MoO₆. It has been shown that the oxidation promoted by these catalysts follows a redox mechanism and that the oxygen in the crystalline network plays a key role as the main source of oxygen for the reaction and controls its development; this characteristic is common to CeO₂.^13–15

On that perspective it is reasonable to assume that the synergistic effect favourable to improve the CO oxidation could be achieved, by supporting Ce_1−xM_xO₂ on Bi–Mo–W-based compounds. Our hypothesis is that the Bi–Mo–W chemical support serves as an effective way to increase the oxygen transfer rate to the surface. This allows a more efficient energy path for the reaction to be carried out. This is the main objective of the present study.

2. Materials and methods

Ce–La and Bi–Mo–W mixed oxides were prepared separately. Stoichiometric amounts of La(NO₃)₃·6H₂O, C₆H₉O₆Ce·H₂O and citric acid were dissolved in water or isopropyl alcohol and stirred for 4 h at 80 °C; later the mixture slowly evaporated to obtain the gel. The gel received a thermal treatment at 110 °C for 12 h and finally the solid obtained was heated at 600 °C for 4 h with a ramp of 5 °C min⁻¹.⁶ The support was prepared by chemical co-precipitation starting with (NH₄)₆Mo₇O₂₄·4H₂O, (NH₄)₆W₁₂O₆·H₂O, and Bi(NO₃)₂·5H₂O compounds and using NH₄OH (0.1 M) to adjust pH = 5. Following precipitation the compounds were oven-dried and calcined at 500 °C. Finally, the Ce_1−xLa_xO₂/Bi₂Mo_0.9W_0.1O₆ or La_2−yCe_yO₃/Bi₂Mo_0.9W_0.1O₆ catalyst systems were made by mixing the compounds (10% active phase and 90% of support), using isopropyl alcohol as mixing media and stirring for 2 h, dried at 120 °C and slightly ground for subsequent oxidation studies and characterization.

3. Characterization

The systems under study were characterized by X-ray diffraction (Siemens, D-5000 model), operating at 30 keV and 20 mA, with step size 0.02° min⁻¹ from 10° to 70° (2θ). Surface images were obtained by an SEM JSM-6400 JEOL Noran Instrument, at 20 KeV and 10⁻⁶ Torr. The surface area was measured by a BET method using a Micrometrics Gemini 2060 RIG-100 model at 77 K. Catalytic activity was evaluated in the temperature range of 100 °C to 400 °C. Catalysts were pre-treated for 1 h using dried air 30 mL min⁻¹ at 200 °C and tested for CO oxidation studies. Experimental conditions were 1% CO, 0.5% O₂, 98.5% He; 80 mL min⁻¹; 0.1 g. CO conversion was evaluated indirectly by measuring the thermal conductivity of the gas phase.

The catalytic activity was evaluated in the range of 100 to 400 °C. Catalysts were pretreated for 1 h by using dried air 30 mL min⁻¹ at 200 °C and tested for CO oxidation. Experimental conditions were 1% CO, 0.5% O₂, 98.5% He; 80 mL min⁻¹; 0.1 g. CO conversion was followed measuring thermal conductivity of the gas phase.

4. Results and discussion

4.1 XRD

Fig. 1 and 2 show the results of X-ray diffraction analyses. With respect to Fig. 1, on the top and in the bottom, are displayed the CeO₂ and La₂O₃ compounds, respectively, for the purpose of comparison with the Ce_1−xLa_xO₂ or La_2−yCe_yO₃ solid solutions. Some displacement can be noticed and the guidelines are placed to observe the shifting of the main reflections as a result of doping in La₂O₃ or CeO₂ compounds. La₂O₃ shows its main reflection at 29.96° (2θ). Increasing the cerium content leads to shifting toward a lower angle from 29.96° to 29.40° (2θ). Also an increase in the broadening of the mean peak is observed. For the opposite case, in which CeO₂ is settled as the host compound, a shift to lower diffraction angles and peak broadening are observed, when La is added. Their main reflection (JCPDS 34-0394 card) is shifted from 28.74 to 28.56° (2θ). However, interestingly for the La_2−yCe_yO₃ compound in the level of x = 0.1, the appearance of one reflection relative to the La₂O₃ (2θ = 26.00°) compound is appreciated. This is indicative that the solubility limit of cerium has been reached as a host atom into the La₂O₃ structure. On the other hand, the Ce_1−xLa_xO₂ compound did not show other crystalline phases. In general our ceria and ceria doped samples match well with the fluorite structure type cubic phase of CeO₂. Based on these results it was decided to establish y = 0.1, as the upper limit of doping. An additional reason for establishing this limit was based on previous reports given by Bueno-López et al.⁹ and Suda et al.¹⁶ who found that optimal values of surface area and catalytic oxidation were for x = 0.1 in Ce_1−xLa_xO₂ solid solutions.

Fig. 1 Powder X-ray diffraction pattern of La_2−xCe_xO₃ and Ce_yLa_1−yO₂ systems.

Fig. 2 Powder X-ray diffraction pattern of La_2−xCe_xO₃/Bi₂Mo_0.9W_0.1O₆ and Ce_yLa_1−yO₂/Bi₂Mo_0.9W_0.1O₆ systems.

Because the peak broadening can be indicative of the decrease in the crystallite size, calculations were performed using the Scherrer equation.¹⁷ The (111) reflection index was used to carry out the calculations. The results were 482, 253 and 205 Å, for CeO₂, Ce_0.95La_0.05O₂ and Ce_0.9La_0.1O₂, respectively. While for La_1.95Ce_0.05O₃ and La_1.9Ce_0.1O₃ the values were 375 and 161 Å, respectively. Therefore, a decrease in the crystallite average size was noticed in comparison to CeO₂ due to doping.

The X-ray diffraction patterns for Ce_1−xLa_xO₂/Bi₂Mo_0.9W_0.1O₆ and La_2−yCe_yO₃/Bi₂Mo_0.9W_0.1O₆ systems are shown in Fig. 2. It is noted that the active phase has an influence on the type of structure of the Bi₂Mo_0.9W_0.1O₆ compound. Which means that when supporting the Ce–La catalyst on the Bi–Mo–W–O compound, the orthorhombic Bi₂MoO₆ phase prevails (see their main reflection at 29.31° (2θ) according to the 21-0102 JCPDS card) while for the La–Ce/Bi₂Mo_0.9W_0.1O₆ catalyst the tetragonal Bi₂MoO₆ phase subsists and their main reflection (103) is located at 27.43° (2θ), 18-0243 JCPDS card.

4.2 SEM

Through SEM microscopy the catalysts were analysed for their morphology and the representative images are provided as ESI† (see Fig. S1). In general adequate active phase/support dispersion is observed and the grain size is found to be less than 1 μm.

In addition to the purpose of comparing real and stoichiometric (desired) composition, EDS microanalysis was carried out finding good agreement. For example it was established the chemical composition Ce_0.95La_0.06O_2.4 by means of EDS microanalysis, when we wanted to obtain the Ce_0.95La_0.05O₂ composition. In general, it follows from the EDS microanalysis of an oxygen-rich surface.

4.3 Specific surface area

Regarding the surface area determination (see Table 1) cerium oxide exhibited 17.0 m² g⁻¹, La₂O₃ showed 10.1 m² g⁻¹ and Bi₂Mo_0.90W_0.1O₆ exhibited 6.5 m² g⁻¹. It was noticed that the increments in the surface area were proportional to the lanthanum content in the Ce_1−xLa_xO₂/Bi₂Mo_0.9W_0.1O₆ system. In the case of La_2−yCe_yO₃/Bi₂Mo_0.9W_0.1O₆ systems the surface area decreases when the cerium content is increased. The increase or decrease in the surface area for all the systems was not significant; however its variation is attributable mainly to the differences in atomic radius sizes of lanthanum (1.1 Å) and cerium Ce (0.97 Å).

Table 1 Surface area for Ceria or Lanthana doped compounds

Catalyst	Surface area/m^2 g^−1
La1.95Ce0.05O3/Bi2Mo0.90W0.1O6	4.7
La1.9Ce0.1O3/Bi2Mo0.90W0.1O6	4.5
La1.5Ce0.5O3/Bi2Mo0.90W0.1O6	2.2
Ce0.95La0.05O2/Bi2Mo0.90W0.1O6	4.7
Ce0.9La0.1O2/Bi2Mo0.90W0.1O6	6.0
CeO2	17.0
La2O3	10.1
Bi2MoxW(1−x)O6	6.5

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4.4 Catalytic activity

All materials were tested in the CO oxidation reaction, following the effect of Bi₂Mo_0.9W_0.1O₆ and CeO₂ catalyst activity. Comparison of the supported systems (see Fig. 3a) reveals that the Bi₂Mo_0.9W_0.1O₆ compound was activated at above 250 °C and reached a maximum of 100% in the CO oxidation at 450 °C while the CeO₂ catalyst activity started at 200 °C and reached 100% CO conversion at 350 °C. With the purpose of observing conclusively the tendency of supported compounds, experiments were performed considering Ce_0.95La_0.05O₂ and La_1.9Ce_0.1O₂ phases. These images have been provided as ESI† (see Fig. S2). The experiments were carried out in the range of 25 to 500 °C. As seen in Fig. S2 (ESI†), conversion to CO₂ using the Ce_0.95La_0.05O₂ compound shows activity near to 175 °C and shows 90% CO conversion at 500 °C. By comparing the behavior shown by the compound La_1.9Ce_0.1O₂ it is possible to establish a weaker performance of this catalyst. Since its activation occurs at 300 °C, maximum CO conversion is lesser than 60% at 500 °C. Our values vary in a non-significant way when compared with results reported by Deganello and Martorana and Benjaram et al.^7,18

Fig. 3 (a) Catalytic activity of Bi₂Mo_0.9W_0.1O₆ and CeO₂ compounds, (b) catalytic activity of Ce_0.95La_0.05O₂/Bi₂MoW, La_1.5Ce_0.5O₂/Bi₂MoW, and La_1.9Ce_0.1O₂/Bi₂MoW systems. Experimental conditions for both experiments: 1% CO, 0.5% O₂, 98.5% He; 80 mL min⁻¹; 0.1 g.

Regarding the measurements of catalytic activity(see Fig. 3b), the Ce–La/Bi₂Mo_0.9W_0.1O₆ or the La–Ce/Bi₂Mo_0.9W_0.1O₆ systems showed that low dopant concentrations exhibit the best CO conversion. By comparing the Ce_0.9La_0.1O₂/Bi₂Mo_0.9W_0.1O₆ and La_1.9Ce_0.1O₃/ Bi₂Mo_0.9W_0.1O₆ compounds, it was appreciated that both materials display an activation temperature close to 125 °C. This value is much lower than that achieved by CeO₂ or Bi₂Mo_0.9W_0.1O₆ discussed above. Unexpectedly 95% CO conversion for Ce_0.9La_0.1O₂/Bi₂Mo_0.9W_0.1O₆ and La_1.9Ce_0.1O₃/Bi₂Mo_0.9W_0.1O₆ was reached at the same value of 350 °C. Differently for the La_1.5Ce_0.5O₃ compound their activity started at 175 °C to reach 100% conversion at 390 °C.

Analysing the results of catalytic activity for the different solid solutions, it is relevant to note that there is synergism by supporting La–Ce or Ce–La systems on Bi₂Mo_0.9W_0.1O₆. Based on the results previously discussed, it is possible to ascertain that the proposed Bi₂Mo_0.9W_0.1O₆ support increases the number of active sites and in turn facilitates the transport of oxygen to the [La–Ce] or [Ce–La] catalyst surface, which is reflected in the lowering of activation temperature to 125 °C. Additionally, it was observed that increasing the atomic content of a substituent also increases the activation temperature.

Comparing supported and unsupported systems, it was found that the supported ones have the activation temperature decreased to at least 75 °C. These results are in excellent agreement with those reported recently by Benjaram et al.¹⁸ who stated that lanthanum substitution in CeO₂ increases their surface area and shifting of the X-ray diffraction peaks. Also they studied the CO oxidation with unsupported Ce–La solid solutions and reported 100% conversion at 427 °C. Recently Katta et al. performed studies on Ce–La/Al₂O₃ catalysts for CO oxidation and established that 100% CO oxidation is reached at 427 °C while for Ce–Zr/Al₂O₃ the 100% conversion is found at 500 °C.¹¹ This temperature is higher than the value reported in this work and in previous studies.⁶ From the CO oxidation tests it is concluded that the O₂ uptake ability is essentially determined by the ceria/lanthana ratio in the solid solution, while it is independent of the surface area and CeO₂ particle size.

5. Conclusions

In summary, in comparison to CeO₂ and Bi₂Mo_0.9W_0.1O₆ it is noted that the proposed systems take advantage of the Ce–La/Bi₂Mo_0.9W_0.1O₆ synergism, promoting oxygen mobility and outstanding behavior during CO oxidation. Ce_0.9La_0.1O₂/Bi₂Mo_0.9W_0.1O₆ and La_1.9Ce_0.1O₃/ Bi₂Mo_0.9W_0.1O₆ catalytic systems exhibited activation temperatures of 125 °C, reaching 95% CO conversion at 350 °C. This temperature value is lower than that reported for compounds supported on Bi₂MoO₆. In regard to the X-ray diffraction analysis of the Ce_1−xLa_xO₂ or La_2−yCe_yO₂ materials, the crystal size in doped-compounds was found to be smaller in comparison to CeO₂. The shift of the main reflections can be considered as an indirect indicator of lanthanum or cerium atomic substitution. We have also shown, with respect to published results of Ce–La and Ce–La/Al₂O₃ systems, an improvement in terms of the temperature at which 100% CO conversion is achieved. As a main conclusion the [Ce–La] and [La–Ce] catalysts exhibited a high CO oxidation activity and thermal stability over Bi₂Mo_0.9W_0.1O₆.

Acknowledgements

P. Bartolo-Pérez acknowledges a grant from Conacyt-59998. Technical help was provided by W. Cauich from Cinvestav-IPN. R. Rangel acknowledges CIC-UMSNH, project 2011, E. Aparicio and I. Gradilla from CNyN-UNAM.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c2cy00368f

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