Alumina Over-coating on Pd Nanoparticle Catalysts by Atomic Layer Deposition: Enhanced Stability and Reactivity
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Alumina Over-coating on Pd Nanoparticle Catalysts by Atomic Layer Deposition: Enhanced Stability and Reactivity Hao Feng • Junling Lu • Peter C. Stair Jeffrey W. Elam
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Received: 9 December 2010 / Accepted: 7 January 2011 / Published online: 25 January 2011 Ó Springer Science+Business Media, LLC 2011
Abstract ALD Alumina was utilized as a protective layer to inhibit the sintering of supported nano-sized ALD Pd catalysts in the methanol decomposition reaction carried out at elevated temperatures. The protective ALD alumina layers were synthesized on Pd nanoparticles (1–2 nm) supported on high surface area alumina substrates. Up to a certain over-coat thickness, the alumina protective layers preserved or even slightly enhanced the catalytic activity and prevented sintering of the Pd nanoparticles up to 500 °C. Keywords Atomic layer deposition Alumina over-coat Methanol decomposition Pd nanoparticles Sintering
1 Introduction The stability of small particles against sintering has been a serious problem restraining the applications of metal nanoparticles (NPs). This problem is prominent for supported noble metal catalysts in which NP sintering at high temperature is a major contributor to catalyst deactivation H. Feng, J. Lu contributed equally to this work H. Feng P. C. Stair Chemical Science and Engineering Division, Argonne National Laboratory, Argonne, IL 60439, USA J. Lu J. W. Elam (&) Energy Systems Division, Argonne National Laboratory, Argonne, IL 60439, USA e-mail: [email protected] P. C. Stair Department of Chemistry, Northwestern University, Evanston, IL 60208, USA
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[1–4]. Methods have been developed to encapsulate noble metal NPs in various types of porous materials using techniques such as chemical vapor deposition (CVD), grafting, micro-emulsion, dendrimer encapsulation, etc. to form core–shell structures [5–12]. Encapsulated NPs showed good sintering resistance up to 800 °C. However, in most cases there is a decrease in catalytic activity due to the mass transfer resistance associated with the protective shell (usually tens of nm’s thick). Furthermore, many of these NP encapsulation procedures must be carried out in solution making them inconvenient for supported NP catalysts. CVD, while a gas-phase technique, does not provide adequate control over the thickness or composition of the encapsulating shell due to non-self limiting reactions. In contrast to these previous methods, atomic layer deposition (ALD) is a multi-step gas phase chemical process based on self-limiting surface chemistry which provides the possibility for atomically controlled post-modification of supported catalyst particles by applying protective over-coats [13]. The self-limiting, layer-by-layer deposition feature of ALD enables precise control over the thickness of the protective layer; therefore it is a most promising way of solving the mass transfer resistance problem. Ma et al. [14] successfully stabilized gold NPs (*5 nm) on TiO2 surfaces by applying ALD SiO2 over-coats. However, the interaction between the gold NP
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