Characterization of uniform, nanodispersed NO x storage catalyst materials synthesized by successive ionic layer deposition Thomas I. Gilbert ⁎, Johannes W. Schwank Department of Chemical Engineering, Transportation Energy Center, University of Michigan, 2300 Hayward, Ann Arbor, MI 48109-2136, United States abstract article info Article history: Received 21 January 2010 Received in revised form 23 March 2010 Accepted 30 March 2010 Available online 8 April 2010 Keywords: NO x storage Barium oxide Dispersion Successive ionic layer deposition (SILD) Nanolayer A dispersed phase of barium oxide (BaO) supported on alumina has been shown to be primarily responsible for NO x storage in practical lean NO x trap catalysts. Conventional impregnation based catalyst loading techniques generate a mixture of dispersed and less active bulk-like BaO phases on alumina, with the bulk- like phase increasing as weight loading increases. Samples of equivalent BaO weight loading on fused alumina were prepared by successive ionic layer deposition (SILD) and wet impregnation. NO 2 temperature programmed desorption experiments demonstrate that SILD is uniquely capable of selectively synthesizing uniform, nanodispersed BaO rafts with high surface coverage. These nanodispersed SILD structures show remarkable thermal stability under high operating temperatures up to 650 °C. © 2010 Elsevier B.V. All rights reserved. 1. Introduction 1.1. Motivation Heterogeneous catalyst design, synthesis, and characterization have been strongly advanced by recent developments in nanoscience [1]. One field that has been extensively investigated is NO x storage and reduction in lean burn engine exhaust [2]. Several studies suggest that γ-alumina supported noble metal catalysts containing a highly dispersed phase of barium oxide for NO x storage are more effective catalysts than those containing a bulk-like phase of barium oxide [3–6]. The Pt/BaO/alumina system is one example of many systems that exhibit interesting catalyst–support interactions, where the extent of dispersion of the active species influences the overall catalytic performance. The inhomogeneity and surface complexity of supported catalyst systems make it difficult to isolate the effects of the catalyst–support interactions. Model catalysts with structural uniformity are often difficult to synthesize in bulk quantities using conventional techni- ques. Bulk scale catalyst synthesis techniques, such as wet impreg- nation, synthesize supported catalyst nanoparticles of which only a fraction of the catalyst mass can directly be influenced by the support. Enhancing the support effects of highly loaded catalysts synthesized by wet impregnation is often difficult because catalyst dispersion tends to decrease as weight loading increases. Furthermore, on many supports post-deposition drying and calcination lead to the agglom- eration of the dispersed catalyst phases into less active and more bulk- like catalyst particles [7,8]. The ability to selectively synthesize a purely dispersed phase on a support would enable a more detailed characterization of the effects of catalyst–support interactions on overall catalytic activity. The purpose of this study was to develop a synthesis procedure that would allow the controlled synthesis of a highly dispersed phase of barium oxide on alumina. There is evidence that the physical distance of noble metal particles from BaO particles plays an important role in overall NO x storage behavior via oxygen spillover during storage or hydrogen spillover or reverse spillover of NO x species from BaO to the noble metal catalyst particles during regeneration [9,10]. Coating the alumina with closely-spaced, highly dispersed rafts of BaO to the extent of near complete coverage would in principle decrease the spacing between noble metal particles and the NO x storage material, thus enhancing the effectiveness of the lean NO x trap system. Because of its ability to control uniform deposition at the nanometer scale and its potential as an industrially scalable synthesis technique, successive ionic layer deposition (SILD) was chosen as the method for producing such a highly dispersed BaO phase on alumina. Although not yet incorporating noble metal catalyst particles, this study focuses on SILD as an enabling synthesis method for depositing highly dispersed BaO on alumina, a prerequisite for enhanced utilization of the noble metal catalysts. The first objective was to gain a fundamental understanding of the appropriate SILD synthesis conditions that produce a dispersed phase of barium oxide on a two-dimensional model support. The second objective was to use the knowledge gained to attempt the extension of the SILD method to three-dimensional support structures by synthesizing a purely dispersed phase of barium oxide on fused alumina powder. To confirm the uniqueness of this highly dispersed phase of BaO, NO 2 temperature programmed desorption (TPD) Catalysis Communications 11 (2010) 896–900 ⁎ Corresponding author. E-mail address: tomcheme@umich.edu (T.I. Gilbert). 1566-7367/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2010.03.019 Contents lists available at ScienceDirect Catalysis Communications journal homepage: www.elsevier.com/locate/catcom