MnOx–CeO2 mixed oxide catalysts for complete oxidation of formaldehyde: Effect of preparation method and calcination temperature

https://doi.org/10.1016/j.apcatb.2005.08.004Get rights and content

Abstract

MnOx–CeO2 mixed oxides prepared by sol–gel method, coprecipitation method and modified coprecipitation method were investigated for the complete oxidation of formaldehyde. Structure analysis by H2-TPR and XPS revealed that there were more Mn4+ species and richer lattice oxygen on the surface of the catalyst prepared by the modified coprecipitation method than those of the catalysts prepared by sol–gel and coprecipitation methods, resulting in much higher catalytic activity toward complete oxidation of formaldehyde. The effect of calcination temperature on the structural features and catalytic behavior of the MnOx–CeO2 mixed oxides prepared by the modified coprecipitation was further examined, and the catalyst calcined at 773 K showed 100% formaldehyde conversion at a temperature as low as 373 K. For the samples calcined below 773 K, no any diffraction peak corresponding to manganese oxides could be detected by XRD measurement due to the formation of MnOx–CeO2 solid solution. While the diffraction peaks corresponding to MnO2 phase in the samples calcined above 773 K were clearly observed, indicating the occurrence of phase segregation between MnO2 and CeO2. Accordingly, it was supposed that the strong interaction between MnOx and CeO2, which depends on the preparation route and the calcination temperature, played a crucial role in determining the catalytic activity toward the complete oxidation of formaldehyde.

Introduction

Formaldehyde is regarded as the major indoor pollutant emitted from the widely used building and decorative materials in airtight buildings. Long-term exposure to indoor air even containing a few ppm of HCHO may cause adverse effects on human health [1]. Thus, great efforts have been made to reduce the indoor emission of HCHO for satisfying the stringent environmental regulations. Physical absorption with porous materials and combined physical adsorption and chemical reaction by impregnating the chemical reagents, such as potassium permanganate and organic amines, on the adsorption materials were found to be effective for eliminating HCHO emission in a certain short period [2], [3]. However, the overall efficiency of these adsorbent materials was not so promising due to their limited removal capacities.

Complete oxidation of HCHO over heterogeneous catalysts has attracted wide attention in recent years for indoor air purification because of the high effectiveness in achieving total conversion of HCHO into harmless CO2 and H2O even at ambient temperature. Supported noble metals were proved to be promising catalysts for this process at lower temperatures. For example, complete oxidation of HCHO was observed over a Ru/CeO2 catalyst at 473 K [4]. Pd–Mn/Al2O3 catalysts were also reported to possess high activity for complete oxidation of HCHO at about 363 K in air stream [5]. Pt supported by ceramics was developed for indoor air purification and 100% conversion of HCHO was obtained at about 423 K [6]. More recently, complete oxidation of HCHO was achieved over a Pt/TiO2 catalyst even at room temperature [7]. However, the expensive cost of noble metals still limited the widespread application of these catalysts. Transition metal oxides and their combinations, such as Co3O4, MnOx and CeO2, were found to show catalytic activities as high as or slightly higher than those of the supported noble metals in HCHO oxidation reactions [8], [9]. A favorable synergetic effect between the mixed transitional metal oxides was attributed to the improvement of their oxidation abilities.

For indoor air cleaning, the low energy demand and the low concentration of formaldehyde strongly require a catalyst to exhibit high activity for the complete oxidation of HCHO, preferably at ambient temperature. Moreover, HCHO in indoor air is often enriched with water vapor, which frequently leads to severe catalytic deactivation through the strong adsorption on the active sites, especially at low temperatures [10]. Therefore, the development of effective transition metal oxide catalysts possessing high catalytic activity for HCHO complete oxidation at low temperatures and strong resistance to water adsorption is of high relevance. Interestingly, MnOx–CeO2 mixed oxides have been developed as environmental friendly catalysts for the abatement of contaminants in both liquid phase and gas phase, such as oxidation of ammonia [11], pyridine [12], phenol [13] and acrylic acid [14]. It was further showed that the MnOx–CeO2 mixed oxides had much higher catalytic activity than those of pure MnOx and CeO2 owing to the formation of the solid solution between manganese and cerium oxides [14]. The incorporation of manganese ions into ceria lattice greatly improved the oxygen storage capacity of cerium oxides as well as the oxygen mobility on the surface the mixed oxides.

In the present study, we investigated the effect of preparation method and calcination temperature of the MnOx–CeO2 mixed oxides on their structural features and catalytic performance for complete oxidation of HCHO. It was showed that MnOx–CeO2 catalyst prepared by a modified coprecipitation method and calcined at 773 K exhibited extremely high catalytic activity and 100% conversion of HCHO was achieved at a temperature as low as 373 K. The structure properties of the catalysts were characterized by XRD, XPS and TPR measurements, and the reactivity–structure correlation was accordingly established though an oxygen transfer mechanism in the generated solid solution.

Section snippets

Preparation of MnOx–CeO2 mixed oxides

MnOx–CeO2 mixed oxides (Mn/(Mn + Ce) = 0.5, molar ratio) were prepared by three different methods. (1) Sol–gel method: an aqueous solution containing Mn(NO3)2·6H2O, (NH4)2Ce(NO3)6 and citric acid (citric acid/(Mn + Ce) = 1.0, molar ratio) was gradually heated to 323 K and kept at this temperature for 2 h with stirring, resulting in the formation of a yellowish gel. It was then dried at 383 K for 12 h, and calcined at 773 K for 6 h in air. The catalyst thus obtained was designed as SG-773. (2) Coprecipitation

Effect of preparation method

Fig. 1 compares the catalytic activities of the MnOx–CeO2 mixed oxides in terms of HCHO conversion as a function of reaction temperature. Comparatively, pure CeO2 and MnOx and their physical mixture Mn–Ce showed very low HCHO conversions in the temperature range investigated. For the MnOx–CeO2 catalysts, it is obvious that the temperature dependence of HCHO conversion was significantly related to their preparation methods. The catalyst prepared by the modified coprecipitation method showed

Conclusion

Manganese–cerium mixed oxide was highly active and rather stable for the complete oxidation of formaldehyde even in the presence of large amount of moisture. The structure features and catalytic behaviors of MnOx–CeO2 mixed oxides strongly depended on their preparation methods. The catalyst prepared by modified coprecipitation method and calcined at 773 K exhibited much higher catalytic activity toward complete oxidation of formaldehyde than those prepared by sol–gel and coprecipitation methods,

References (31)

  • Y. Sekine

    Atmos. Environ.

    (2002)
  • H. Nakayama et al.

    Solid State Sci.

    (2002)
  • M.C. Álvarez-Galván et al.

    Appl. Catal. B

    (2004)
  • C.B. Zhang et al.

    Catal. Commun.

    (2005)
  • Y. Sekine et al.

    Atmos. Environ.

    (2001)
  • M.Q. Zhang et al.

    Appl. Catal. B

    (1997)
  • H. Chen et al.

    Appl. Catal. B

    (2001)
  • A.M.T. Silva et al.

    Appl. Catal. B

    (2004)
  • F. Kapteijn et al.

    Appl. Catal. B

    (1994)
  • J. Carnö et al.

    Appl. Catal. A

    (1997)
  • A. Bensalem et al.

    Appl. Catal. A

    (1995)
  • S. Hamoudi et al.

    J. Catal.

    (1999)
  • J.P. Holgado et al.

    Appl. Surf. Sci.

    (2000)
  • F. Larachi et al.

    Appl. Surf. Sci.

    (2002)
  • G. Qi et al.

    J. Catal.

    (2003)
  • Cited by (715)

    View all citing articles on Scopus
    View full text