CO oxidation catalyzed by RuO₂ nanoparticles supported on mesoporous Al₂O₃ prepared via atomic layer deposition

Highlights

• We prepared RuO₂ nanoparticle on mesoporous Al₂O₃ by atomic layer deposition (ALD).

• Ru decorated the core part of mesoporous Al₂O₃, without limit of diffusion.

• RuO₂ nanoparticle maintained the size of ∼10 nm after annealing at 700 °C.

• Deactivated RuO₂ annealed at 700 °C was fully recovered after annealing at 700 °C.

Abstract

Using atomic layer deposition (ALD), RuO₂ was deposited on mesoporous Al₂O₃ with a mean pore size of ∼12 nm and particle diameter of ∼1 mm. The entire internal structure of mesoporous Al₂O₃ was coated by RuO₂ nanostructures using ALD, forming a mesoporous bead of RuO₂/Al₂O₃. Substantially high CO oxidation activity at reaction temperatures below 100 °C is also shown. There was a decrease in CO oxidation activity with time at 100 °C, and post-annealing at 700 °C fully recovered the catalytic activity of fresh RuO₂/Al₂O₃. The RuO₂/Al₂O₃ can be useful as an oxidation catalyst for eliminating harmful molecules.

Introduction

CO oxidation is one of the most widely studied reactions in surface science and heterogeneous catalysis. The simplicity of this reaction allows molecular-level understanding of its catalytic reaction path, and this reaction is of technological importance in the automobile industry [1], [2], [3], [4]. Pt-group metals including Pt, Pd, and Rh have been commercially used as catalysts for emission gas control in automobiles [4], [5], [6]. There has been significant discussion regarding the structures of the catalytically active surfaces of these catalysts; they are generally considered either metallic surfaces covered by chemisorbed species or thin layers of metal oxide [7], [8], [9], [10], [11], [12].

For Ru, oxygen-rich surface phases have been suggested as highly reactive for CO oxidation. Many studies reported that catalytically active oxygen-rich phases correspond to the RuO₂ layers on metallic Ru [13], [14]. The relationship between atomic structure and catalytic activity of RuO₂ thin films on single crystal surfaces of Ru has been widely studied [7], [15], [16], [17], [18], [19]. Nanoparticles consisting of Ru-core and RuO₂-shell have been studied in detail using various tools, including ambient-pressure photoemission spectroscopy [20], [21], [22].

Nanoparticles with high catalytic activity in heterogeneous catalysis often have low stability and undergo significant agglomeration under reaction conditions to form more thermodynamically stable larger particles [23], [24], [25], [26]. Mesoporous substrates can be used as supporting materials to stabilize small nanoparticles by confining them inside pores to limit nanoparticle diffusion (or atoms from nanoparticles). Here, experimental methods for efficient incorporation of transition metal/metal-oxide nanoparticles into mesoroporous media are useful. Among various methods, atomic layer deposition (ALD) has been shown to be effective for inserting nanoparticles into mesoporous templates [27], [28]. Transition metal and metal-oxide can be incorporated into mesoporous media with a typical diffusion depth of several tens of micrometers [27], [28], [29].

In the present work, RuO₂ was incorporated into mesoporous Al₂O₃ gel with a particle diameter of 1 mm. We show that the RuO₂ nanostructure can be distributed inside the entire structure of the 1 mm mesoporous Al₂O₃ particle. The CO oxidation activity of the RuO₂/Al₂O₃ structure is presented.