In-situ regeneration of Au nanocatalysts by atmospheric-pressure air plasma: Significant contribution of water vapor
Graphical abstract
Introduction
Supported gold (Au) catalysts have been intensively investigated for catalyzing a wide variety of reactions at relatively mild conditions in the past decades [1], [2], [3], [4], [5]. Due to its superior activity on CO oxidation at low temperature [6], [7], [8], Au catalyst is expected as an excellent candidate for environmental protection [9], [10], [11], [12], such as indoor air purification and canister respirators. Unfortunately, the practical applications of Au catalyst still remain a big problem because it shows a gradual deactivation with time on stream (TOS) [13], [14], [15]. Currently, agglomeration of Au particles and accumulation of carbonate species on catalyst surface are generally considered as the two major reasons for the deactivation phenomenon [16], [17]. Meanwhile, for CO oxidation over Au catalysts at low temperature, the deactivation is mainly caused by the reversible surface carbonate species accumulating rather than the irreversible Au nanoparticle aggregating. The surface carbonate species on the deactivated Au catalysts can be decomposed or removed by heat-treatment [15], [16], [18]. However, the conventional heat-treatment method is prone to bring a negative effect of Au nanoparticles aggregation.
A promising alternative technique is the cold plasma (abbreviated as plasma below) [19], [20], [21], in which the interaction of active species (e.g. electrons, ions and radicals) with Au catalyst could effectively remove the accumulated carbonate species from catalyst surface at low temperature. Although various plasma techniques have been employed to prepare or modify Au catalysts [22], [23], few works on plasma regeneration of Au catalysts were reported [17], [24]. Our previous work demonstrated that pure oxygen plasma is a fast and effective approach to in-situ regenerate Au/TiO2 nanocatalyst [24]. Undoubtedly, as a discharge gas of plasma regeneration, air is far easier accessible and much cheaper than pure oxygen. However, the presence of nitrogen in air causes a side effect of extra poisoning towards Au catalyst due to nitrogen oxides produced in air plasma [24], which suppresses the technique of air plasma regeneration of Au catalyst. Inspired by the literatures [25], [26], [27], the presence of water vapor in air plasma can effectively inhibit the formation of nitrogen oxides, which might be a simple and feasible solution to avoid the poison effect of nitrogen oxides on Au catalysts during air plasma regeneration. Meanwhile, regeneration efficiency of the air plasma could also be improved by water vapor, due to that water vapor favors the decomposition of surface carbonate species [18], [28], [29], [30], [31], [32], [33], [34].
In this paper, with extension of previous work [24], we demonstrate a simple and effective technique of atmospheric-pressure cold plasma of humid air instead of pure oxygen or dry air, for in-situ regeneration of deactivated Au/TiO2 nanocatalysts, which features a rapid regeneration of Au catalyst and the elimination of extra poisoning of nitrogen oxides produced in dry air plasma on Au catalyst. Moreover, the significant contribution of water vapor to the regeneration of Au catalysts in air plasma was disclosed, for which the mechanism was further discussed.
Section snippets
Catalyst preparation, reactor and catalytic activity evaluation
A nominal 1 wt.% Au catalyst was prepared by a modified impregnation method [35]. A 4.3 mL aqueous HAuCl4 solution (2.43 × 10−2 mol/L) was slowly added to 2 g TiO2 powder (Degussa P25) under manual stirring. The slurry was aged at room temperature for 18 h, and rinsed twice with an aqueous ammonia solution (pH 8) and another twice with deionized water to remove residual chloride ions. After rinsing, the filtered cake was dried in air at 80 °C for 6 h and calcined at 300 °C for 2 h to obtain the Au/TiO2
Plasma regeneration of Au/TiO2 catalyst and effect of humidity
Under circumstances of humid or dry synthetic air (abbreviated as humid plasma or dry plasma below) as discharge gas for plasma regeneration of Au/TiO2 catalysts, CO oxidation reaction was conducted firstly over the fresh Au/TiO2 catalysts for 70 min to allow the catalysts deactivated at the same level. The deactivated catalysts were in-situ regenerated for 5 min by the humid plasma (in presence of 2.77% H2O) and dry plasma at Pin of 3 W and total flow rate (F) of 200 mL/min, respectively, over
Deactivation of Au catalyst
Results shown in Fig. 2 clearly demonstrated that the deactivation phenomenon of Au/TiO2 catalysts existed during CO oxidation, which has also been reported in literatures [13], [14], [15], [16], [17]. It is generally accepted that the Au particle growth and the accumulation of carbonate species on Au catalyst surface are two major reasons for the deactivation phenomenon [16], [17]. The Au particle sizes change induced by the reaction heat during CO oxidation at room temperature is negligible
Conclusions
In-situ regeneration of the deactivated Au/TiO2 nanocatalysts during CO oxidation reaction by atmospheric-pressure cold plasma of air was explored. A significant contribution of water vapor in enhanced regeneration performance of Au catalysts by air plasma was surprisingly observed. After plasma regeneration of the catalysts for 5 min, the regeneration degree significantly increased up to 98% under humid plasma in presence of 2.77 vol.% H2O, but decreased down to negative 29% under dry plasma.
As
Acknowledgments
This work is supported by National Natural Science Foundation of China (11175036, U1201231) and the Fundamental Research Funds for the Central Universities (DUT14RC(3)012).
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