High catalytic performances of CeO2–CrOx catalysts for chlorinated VOCs elimination

doi:10.1016/j.ces.2014.12.051

Chemical Engineering Science

Volume 126, 14 April 2015, Pages 361-369

https://doi.org/10.1016/j.ces.2014.12.051 Get rights and content

Highlights

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Porous CeO₂–CrO_x mixed oxide nanoparticles were prepared by coprecipitation method.
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4Ce1Cr mixed oxides were evaluated for deep catalytic oxidation of various Cl-VOCs.
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4Ce1Cr–(NH₄)₂CO₃ shows the best catalytic performances for Cl-VOCs oxidation.
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Accumulation of coke and Cl are the main factors for deactivation at low temperature.
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Cr element can be stabilized due to the strong interaction between CeO₂ and CrO_x.

Abstract

A series of porous CeO₂–CrO_x mixed oxides are synthesized using different precipitants (NaOH, NH₄OH, (NH₄)₂CO₃ and urea) and evaluated for deep oxidation of various chlorinated VOCs (CH₂Cl₂, C₂HCl₃, C₂H₄Cl₂ and C₆H₅Cl). The results show that all these materials represent porous structures and small particle sizes (2–8 nm). Part of Cr element can go into the lattice of CeO₂, resulting in the formation of Ce–Cr–O mixed oxide. Compared with pure CeO₂ and Cr₂O₃, the amount of Cr⁶⁺ species and oxygen vacancies in CeO₂–CrO_x mixed oxides are significantly increased, due to the strong interaction between CeO₂ and CrO_x. Thus, all the CeO₂–CrO_x mixed oxides exhibit high catalytic activity and good durability for deep oxidation of chlorinated VOCs, especially the CeO₂–CrO_x prepared using (NH₄)₂CO₃ as precipitant. Moreover, only slight chlorinated byproducts are detected in the process of chlorinated VOCs oxidation, and the surface accumulation of coke and Cl species as well as the loss of Cr element is slight during the long-term reaction at 265 °C, indicating that these CeO₂–CrO_x materials possess interesting fundamental result, deserving more research before conclusion on practical value.

Graphical abstract

Introduction

Deep catalytic oxidation of volatile organic compounds (VOCs) plays a critical role in the control of air pollutions nowadays. Developing new and more efficient catalysts for reducing the emission of VOCs especially chlorinated VOCs (Cl-VOCs) with very low concentration (100–10,000 ppm) has been urgently demanded (Erlt et al., 2008, Matějová et al., 2012, Gallastegi-Villa et al., 2014).

Some catalytic formulations are being evaluated for deep oxidation of Cl-VOCs, mostly based on noble metals, transition metal oxides and zeolites (Erlt et al., 2008, Matějová et al., 2012, Gallastegi-Villa et al., 2014, Spivey, 1987, Spivby and Butt, 1992, Everaert and Baeyens, 2004, González-Velasco et al., 2000, Paukshtis et al., 2010, Balzhinimaev et al., 2010, Balzhinimaev et al., 2004). Because of the high price of noble metals and their tendency to deactivation due to Cl poisoning, as well as the easiness of coking for zeolites, much more attention has now been concentrated on transition metal oxide catalysts (Matějová et al., 2012, Wang et al., 2014). Compared with other transition metal oxides, Cr₂O₃ is one of the most active catalysts for various Cl-VOCs elimination (Erlt et al., 2008, Hong et al., 1998, Krishnamoorthy et al., 2000, Rotter et al., 2005). However, the commercial application of this catalytic system has been limited by fear about the loss of Cr during the long-term reaction. For example, many studies have reported the irreversible deactivation caused by the loss of active Cr components by the formation of volatile CrO₂Cl₂ on the CrO_x/Al₂O₃ surface (Padilla et al., 1999, Yim et al., 2001). Thus, it is critically demanded for developing new catalysts with higher activity and stability for Cl-VOCs oxidation, and the possible deactivation mechanism as well as the regeneration treatment should also be further investigated. Recently, CeO₂-based materials have generally used as a support, main active component or promoter in many catalyst systems (Harmsen et al., 2001). CeO₂-based materials are also believed to be promising for deep catalytic oxidation of organic pollutants because of the high oxygen storage capacity and rich oxygen vacancy of CeO₂, associated with its strong interaction with other active metal and the easy conversion between Ce⁴⁺ and Ce³⁺ (Si et al., 2004, Gorte, 2010).

In previous work, we have investigated a series of CeO₂-transition metal mixed oxides, and found that CeO₂–CrO_x mixed oxide exhibits excellent catalytic performances for deep catalytic oxidation of various Cl-VOCs (Yang et al., 2015). As is well known, the physicochemical properties of the catalysts are strongly dependent on the synthesis methods, the treatment history as well as the presence of additives (Gorte 2010). Thus, in this paper, four CeO₂–CrO_x mixed oxides are prepared by coprecipitation method using different precipitants and evaluated for deep oxidation of four Cl-VOCs with different kinds of molecule structures. The objective of this work is to investigate the influence of different precipitants on the physicochemical properties of the CeO₂–CrO_x materials, and to get more information about the relationship between structure and catalytic performance. Meanwhile, the durability of 4Ce1Cr mixed oxide catalysts for Cl-VOCs oxidation has also been examined to elucidate the possible deactivation mechanism.

Section snippets

Materials synthesis

CeO₂–CrO_x mixed oxides were synthesized by coprecipitation method adopting different precipitants. These precipitants used were sodium hydroxide (NaOH), ammonia solution (NH₄OH), urea ((NH₂)₂CO) and ammonium carbonate ((NH₄)₂CO₃), respectively. The OH⁻ group contains NaOH and NH₄OH solution, leading to the formation of hydroxides in the coprecipitation process. The carbonate ion group includes ammonium carbonate, while urea can be classified into organic amine, both of which lead to the

Oxidation activities for various Cl-VOCs destruction

In practice, various kinds of Cl-VOCs with quite different molecular structures are always appeared in the waste stream together, and their decomposition mechanisms over the catalysts may also be different (Pinard et al., 2004, Scirè et al., 2003, de Rivas et al., 2007, Aranzabal et al., 2003). Fig. 1(A) displays the results of the catalytic activities for deep oxidation of DCM over the catalysts. In previous blank experiment, the homogeneous reaction for DCM oxidation occurred above 350 °C, and

Conclusions

CeO₂–CrO_x mixed oxide nanoparticles are prepared by coprecipitation method using different precipitants, and then evaluated for deep oxidation of different types of Cl-VOCs. The catalytic performances of these catalysts depend on the different nature of the corresponding structure/texture, since different precipitants have big effect on the physicochemical properties of the 4Ce1Cr mixed oxides. All the materials represent porous structures and small particle sizes (2–8 nm). Part of the Cr

Acknowledgements

We gratefully acknowledge the financial supports from Nature Science Foundation of China (no. 21177110) and Zhejiang Leading Team of Science and Technology Innovation (no. 2009R50020).

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High catalytic performances of CeO2–CrOx catalysts for chlorinated VOCs elimination

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Materials synthesis

Oxidation activities for various Cl-VOCs destruction

Conclusions

Acknowledgements

Appl. Catal., A

J. Catal.

Chem. Eng. Sci.

Catal. Today

Chem. Eng. J.

Appl. Catal., B

Catal. Commun.

J. Catal.

Chem. Eng. Sci.

Microporous Mesoporous Mater.

J. Catal.

Appl. Catal., B

Appl. Catal., A

Appl. Catal., B

Chemosphere

Appl. Catal., B

Appl. Catal., B

Appl. Catal., B

Int. J. Hydrog. Energy

Catal. Today

High catalytic performances of CeO₂–CrO_x catalysts for chlorinated VOCs elimination