Short CommunicationPt structured catalysts prepared using a novel competitive impregnation method for the catalytic combustion of propionic acid
Graphical abstract
Introduction
The dramatically increasing amounts of volatile organic compounds (VOCs) have become a serious problem [1]. Among these pollutants, the emission of malodorous gases has given rise to widespread concern because they pollute the environment and are harmful to humans [2]. Approximately 20% of the two million kinds of organic compounds have a malodorous smell [3] and are emitted by the chemical engineering, pharmacological, service industries, among others. For example, in the service industry, at least 100 components of malodorous gases come from restaurants, including propionic acid, n-butyric acid, and pentanoic acid [4]. The elimination of malodorous gases is difficult because of their low concentrations and large treatment flows [5], [6]. Among the available technologies, such as adsorption, oxidation, and biological deodorization, the catalytic combustion of pollutants to CO2 and H2O has been regarded as the most efficient. Catalytic combustion technology is applied due to its low reaction temperature and energy consumption [7], [8].
Reactors with packed beds can be easily constructed, but will cause a high pressure drop [9]. Hence, structured catalysts are used because of their low pressure drop and satisfactory forming property [10]. Kameyama et al. [11], [12] used anodic alumina support to prepare structured catalysts for the catalytic combustion of VOCs. Besides, the anodic alumina support contains unbranched and regular pores, which are useful for supporting active components. Two categories of catalysts are generally used for the catalytic combustion of VOCs: (i) noble metals (Pt, Pd, and Au); and (ii) transition metal (Cu, Ce, and Mn) oxides. Supported noble metal catalysts have been proven to be promising because of their high activities and low initiation temperatures. Particularly, platinum on alumina is widely used for the catalytic combustion of VOCs [13], [14], [15]. However, the high cost of Pt limits its widespread use. Moreover, the catalytic performance of catalysts strongly depends on their preparation method, which determines the metallic particle size and dispersion on the support [16], [17]. The most commonly used technology is to introduce a metal precursor on the support by impregnation or ion-exchange, such as the ordinary impregnation method and the co-impregnation impregnation method [18]. However, metal particles usually aggregate on the external surface of the support in these methods, which leads to poor Pt distribution. Therefore, to prepare a catalyst with a low Pt amount and high dispersion, a novel competitive impregnation method was used by adding a competitive adsorbent, such as lactic acid, acetic acid, and oxalic acid [19]. In this process, the competitive adsorbent plays a positive role that enables uniform distribution of the active component on the support. Shi et al. [20] reported that metal dispersion can be significantly improved by this method, compared with the ordinary method. This method provides a possibility for the preparation of catalysts with low Pt amounts and high dispersion. However, the explicit preparation and analysis methods of such catalysts have not yet been reported. In addition, the mechanism of the competitive impregnation has not been elucidated in detail.
In the present study, a structured anodic alumina support was prepared through the anodic oxidation and a catalyst with a low Pt amount and high dispersion on anodic alumina was prepared by a novel competitive impregnation method. The catalytic performances of these catalysts were then compared with those prepared by the co-impregnation method. Electronic probe microanalysis analyzer (EPMA), field emission scanning electron microscopy (FESEM), energy dispersive spectrometer (EDS), and CO-pulse analysis were used to characterize the structure properties of the prepared catalysts. Based on the results, the mechanism of the competitive impregnation process was proposed. A 180 h stability test was also performed.
Section snippets
Catalyst preparation
Catalyst supports were prepared through the anodic oxidation process. The plate was anodized in a 0.4 wt.% oxalic acid solution for 16 h at 25 °C with the current density of 50 A/m2. Subsequently, the plate was calcined to decompose the residual oxalic acid at 350 °C for 1 h. The support was then immersed in a mixed solution of H2PtCl6·6H2O and lactic acid at 25 °C for 30 min. Finally, the Pt structured catalyst was obtained by drying and calcining the support at 500 °C for 3 h.
The catalysts prepared by
The catalytic activities of the catalysts prepared by different impregnation methods
Fig. 1 shows the catalytic activities of the anodic alumina support and the catalysts prepared using different impregnation methods. It can be seen that the Pt catalyst supported on the anodic alumina exhibits higher activity than the anodic alumina support, indicating that the beneficial effect of Pt on the improvement of the catalytic activity. Meanwhile, for Pt structured catalysts, the difference of the activities is also significant with the changing impregnation method. Catcom-0.05
Conclusion
Pt structured catalysts were prepared using a novel competitive impregnation method and used for the catalytic combustion of propionic acid. Meanwhile, these Pt catalysts were compared with those prepared by the co-impregnation method. Results show that the competitive impregnation method increases Pt dispersion and effectively decreases the Pt amount and particle size. Catcom-0.06 with 0.06 wt.% Pt and 35.1% Pt dispersion prepared by the competitive impregnation method exhibits high activity
Acknowledgments
This work was financially supported by the Shanghai Rising-Star Program (B type) (Grant No.13QB1401300) and the Fundamental Research Funds for ECUST (Grant No. WA1214032).
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