In-situ regeneration of Au nanocatalysts by atmospheric-pressure air plasma: Significant contribution of water vapor

https://doi.org/10.1016/j.apcatb.2015.05.020Get rights and content

Highlights

  • In-situ and rapid regeneration of Au catalysts is achieved by humid air plasma.

  • Regeneration degree depends strongly on water vapor content in air plasma.

  • Water vapor speeds up carbonate species decomposition on the deactivated catalysts.

  • Water vapor inhibits nitrogen oxides formation during air plasma regeneration.

Abstract

In-situ regeneration of deactivated Au nanocatalysts during CO oxidation, was conducted effectively by pure oxygen plasma, but poisoned by dry air plasma in our previous work (Appl. Catal. B 2012, 119120, 49–55). With extension of previous study, a simple and effective technique of atmospheric-pressure cold plasma of humid air is explored for in-situ regeneration of Au nanocatalysts. In comparison with ineffective regeneration by dry plasma, humid plasma using synthetic air (20% O2 balance N2) as discharge gas surprisingly exhibited effective regeneration performance over Au catalyst due to significant contribution of water vapor. After plasma regeneration for 5 min, the regeneration degree of Au catalysts significantly increased up to 98% under humid plasma in presence of 2.77 vol.% water, while decreased down to negative 29% under dry plasma. To disclose the mechanism of water vapor contribution to greatly improved regeneration degree, the characterizations of regenerated catalysts, and the analyses of electric discharge characteristics and gaseous products during the plasma regeneration were conducted. The significant contribution of water vapor embodies in that it speeds up the decomposition of carbonate species and simultaneously inhibits the formation of poisoning species of nitrogen oxides. Furthermore, normal air instead of synthetic air in humid plasma regeneration was implemented on the evaluations of the deactivated Au catalysts after a long-term reaction and during ten deactivation-regeneration cycles, which ensured the feasibility and reliability of in-situ plasma regeneration of Au nanocatalysts as a simple, effective and promising technique.

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).

References (62)

  • M. Kipnis

    Appl. Catal. B

    (2014)
  • S. Scirè et al.

    Appl. Catal. B

    (2012)
  • S.A. Nikolaev et al.

    Appl. Catal. B

    (2015)
  • P. Sudarsanam et al.

    Appl. Catal. B

    (2014)
  • A. Sandoval et al.

    Appl. Catal. B

    (2013)
  • W. Li et al.

    J. Catal.

    (2006)
  • Y. Denkwitz et al.

    Appl. Catal. B

    (2009)
  • A. Karpenko et al.

    J. Catal.

    (2007)
  • Y. Denkwitz et al.

    J. Catal.

    (2009)
  • P. Konova et al.

    Catal. Commu.

    (2004)
  • P. Konova et al.

    J. Mol. Catal. A: Chem.

    (2004)
  • H.H. Kim et al.

    Appl. Catal. A

    (2007)
  • H.S. Oh et al.

    Stud. Surf. Sci. Catal.

    (2001)
  • J. Van Durme et al.

    Appl. Catal. B

    (2008)
  • X. Liu et al.

    J. Catal.

    (2012)
  • Z.H. Wei et al.

    Mater. Lett.

    (2011)
  • H.Y. Fan et al.

    Appl. Catal. B

    (2012)
  • B. Schumacher et al.

    J. Catal.

    (2004)
  • M. Daté et al.

    J. Catal.

    (2001)
  • J. Saavedra et al.

    J. Catal.

    (2013)
  • M.M. Schubert et al.

    J. Catal.

    (2004)
  • J.T. Calla et al.

    J. Catal.

    (2006)
  • M. Ojeda et al.

    J. Catal.

    (2012)
  • R. Zanella et al.

    J. Catal.

    (2004)
  • A.C. Gluhoi et al.

    J. Catal.

    (2005)
  • J. Huang et al.

    J. Catal.

    (2007)
  • J. Pouilleau et al.

    Mater. Sci. Eng. B

    (1997)
  • J. Mizera et al.

    Catal. Today

    (2012)
  • H. Liu et al.

    J. Catal.

    (1999)
  • M.M. Schubert et al.

    J. Catal.

    (2001)
  • C.K. Costello et al.

    Appl. Catal. A

    (2003)
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