CuO catalysts supported on activated red mud for efficient catalytic carbon monoxide oxidation
Introduction
As a common component of automotive exhaust and industrial waste gas, carbon monoxide (CO) is largely harmful to human health and environment, and the catalytic oxidation of CO is considered to be one of the most efficient routes for controlling the CO emission [1], [2]. Over the last several decades, many kinds of noble metals (Pt, Au, Pd, etc.) exhibit superior catalytic activity for CO oxidation [3], [4], [5], [6], [7], [8]. However, the high cost, low availability, and poor stability at elevated temperature seriously impede their wide application. Recently, CuO-based catalysts have been demonstrated to be the very promising alternatives to substitute for the noble metal catalysts, due to their low cost, high activity, and good stability [9], [10], [11], [12]. And many kinds of supports, such as ceria, alumina, titania, mesoporous silica and so on, have been widely used for preparation of supported CuO catalysts [13], [14], [15]. And the nature of these supports can significantly affect their catalytic performance, as well as the reaction mechanism. Schubert et al. [16] clarified the influence of the support materials (SiO2, Al2O3, MgO, Fe2O3, TiO2, NiOx, CoOx) on the activity of CO oxidation and divided them into “inert” and “active” support materials. Fu et al. [17] also demonstrated that the interface-confined coordinatively unsaturated ferrous sites together with the metal supports were active for oxygen molecule activation, thus creating highly reactive oxygen atoms, which was highly efficient for catalytic CO oxidation. To date, many supports have been demonstrated to be beneficial for CO oxidation, but the exploitation of new catalyst support with much low cost and rich porous structure is still a challenge.
Red mud (RM) is a large scale waste product from alumina production, which is a hazardous material with high alkalinity [18]. While, RM contains a relatively large amount of aluminum and iron, which can be used as a potential alternative catalyst for various catalytic reactions [19], [20]. Before using it, activation procedures are usually used to increase its surface area, as well as minimize the effect of its composition. Up to now, the activated red mud (ARM) has been utilized for many different kinds of catalytic reactions, for example, hydrogenation [21], hydrodechlorination [22], catalytic combustion [23] and hydrogen production [24], [25]. Recently, Sushil et al. [26] observed that the hydroxylated phases and the high surface area of the acid-digested red mud catalysts were beneficial to the enhancement of catalytic activity for CO oxidation. Indeed, Cao et al. [27] investigated the removal of CO on mesoporous CuO–Fe2O3 composite catalysts with high activity and thermal stability. Qiao et al. [28] prepared ferric hydroxide supported CuO catalysts for preferential oxidation of CO in the presence of H2, and the CuO/Fe(OH)x catalyst exhibited superior catalytic performance. RM contains a large amount of iron, which can be used as a promising catalyst carrier for CuO in catalytic oxidation of CO. Very recently, Cao et al. [29] prepared CuO/modified red mud by an impregnation method, however, this catalyst exhibited relatively low catalytic activity. So, it is urgent to exploit a more efficient CuO-red mud catalyst system by a simple method.
In this work, the as-received RM was activated through a modified Pratt and Christoverson method [30], and employed as support of CuO catalysts for CO oxidation. The catalytic performances of the CuO/ARM catalysts with different loading amount were tested, and the effects of the precalcination temperature of the catalysts were investigated. The resultant catalysts with much low cost exhibit high activity in the oxidation of CO, suggesting a practical potential in the fields of the environmental catalyst system.
Section snippets
Catalysts preparation
RM was supplied by Henan Zhongmei Aluminum Co. (China). Its main constituents are Al2O3 (23.25%), SiO2 (16.94%), CaO (16.94%), Fe2O3 (15.05%), TiO2 (4.27%), Na2O (3.72%), MgO (1.93%) and K2O (1.82%). Commercial CuO, HCl and NH4OH were purchased from Tianjin Guangfu Fine Chemical Research Institute.
For RM activation, 20 g of RM was added into 100 ml of distilled water, followed by adding 100 ml of 6 M HCl solution. The mixture was digested at 90 °C for 2 h. Then, ammonium hydroxide was added dropwise
Results and discussion
The RM is a complex mixture, mainly comprised of Al2O3, SiO2, CaO, Fe2O3 and Na2O [25]. During the process of activation, sodium and calcium oxides were almost removed. The XRD patterns of RM, ARM and 200 °C-calcined CuO/ARM-x% catalysts are shown in Fig. 1. It is shown that RM is a complex mixture of many minerals, which are identified as katoite (Ca2.93Al1.97(Si0.64O2.56)(OH)9.44), tilleyite (Ca5Si2O7(CO3)2), bayerite (Al(OH)3), hematite (Fe2O3), lepidocrocite (FeO(OH)) and rutile (TiO2).
Conclusions
The activation of red mud was performed by the modified Pratt and Christoverson method, yielding the significantly enlarged surface area and porosity. After being loaded with CuO nanoparticles, the resulting composite catalysts are highly active for CO oxidation. The CuO content and the precalcination temperature can influence the catalytic performance of the CuO/ARM catalysts. And the differences between these prepared catalysts are attributed to the aggregation nature of copper species and
Acknowledgments
This work was supported by the National Natural Science Foundation of China (21421001, 21573115), the Natural Science Foundation of Tianjin (15JCZDJC37100), and the 111 Project (B12015).
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