Elsevier

Journal of Catalysis

Volume 266, Issue 2, 10 September 2009, Pages 182-190
Journal of Catalysis

Reversible changes in the Pt oxidation state and nanostructure on a ceria-based supported Pt

https://doi.org/10.1016/j.jcat.2009.06.005Get rights and content

Abstract

The Pt oxidation state and nanostructure on a ceria-based oxide support were characterized after sequential oxidative, reductive, and re-oxidative treatments using X-ray photoelectron spectroscopy and transmission electron microscopy. The Pt oxidation state and nanostructure depended on the treatment atmosphere: an oxidized Pt monolayer spread along the primary particle surface of the ceria-based oxide support in the oxidized catalyst, whereas metallic Pt existed as particles of 1–2 nm in diameter in the reduced catalyst. Therefore, on the ceria-based oxide surface oxidized Pt formed a nanocomposite oxide with Ce under an oxidative atmosphere and reversibly changed to metallic Pt particles under a reductive atmosphere. The formation of this surface nanocomposite oxide is evidence of the strong interaction between oxidized Pt and CeO2 that involves the formation of a Pt–O–Ce bond, which stabilizes Pt against sintering and redisperses agglomerated Pt particles under an oxidative atmosphere.

Graphical abstract

Depending on the treatment conditions, platinum on a ceria-based oxide support changes reversibly from a small metallic nanoparticle to an oxidized monolayer, providing evidence of Pt–O–Ce bond formation in the latter.

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Introduction

In the mid-1970s, three-way catalysts (TWCs) were first used in the US and Japan to convert harmful carbon monoxide (CO), nitrogen oxides (NOx), and hydrocarbons (HC) in the exhaust from gasoline-fueled vehicles to harmless carbon dioxide (CO2), nitrogen (N2), and water (H2O) [1]. TWC-equipped exhausts are now widely used in gasoline-fueled vehicles to meet stringent emission regulations. The main components in TWCs are a precious metal such as palladium (Pd), platinum (Pt), or rhodium (Rh) as the active catalyst, an inorganic oxide such as γ-alumina (Al2O3) or zirconia (ZrO2) as the support, and an oxygen storage material such as ceria–zirconia (CeO2–ZrO2) to maintain stoichiometry at the catalyst surface and achieve higher purification activity [2], [3], [4]. Because of the scarcity of natural resources and the increasing price of precious metals, there is a focus on improving the durability of TWCs and applying new insights to develop advanced TWCs with lower precious metal content [5], [6], [7], [8], [9], [10], [11], [12], [13], [14].

In principle, when the particle size of a precious metal increases due to sintering during the lifetime of a catalyst, the TWC activity deteriorates [15]. Sintering of precious metal particles has been widely investigated, with some researchers focusing on preventing sintering on aging [2], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29]. CeO2-based inorganic oxides are well known as good supports for stabilization of Pt particles against sintering, whereas Pt easily agglomerates on Al2O3 under an oxidative atmosphere at high temperature [2], [26], [27], [28], [29].

We obtained novel insights into the mechanism by which CeO2-based oxide supports stabilize Pt against sintering in a systematic atomic-level investigation using X-ray absorption fine structure (XAFS) methodology [27]. Curve fitting of the Pt L3-edge extended X-ray absorption fine structure (EXAFS) spectrum of an air-aged catalyst revealed a strong interaction between oxidized Pt and CeO2-based support involving a rigid Pt–O–Ce bond that stabilized Pt atoms under an oxidative atmosphere at high temperature. From the coordination number of Pt–O and Pt–Ce shells estimated from EXAFS analysis, it could be concluded that this Pt–O–Ce bond was formed at the surface of the support and not in the bulk. A similar Pt–O–Ce surface complex on a CeO2-based oxide support was reported by Diwell et al. [28] and Murrell et al. [29]. We also systematically evaluated interaction between Pt and supports after air aging by X-ray photoelectron spectroscopy (XPS), X-ray absorption near edge structure (XANES), and CO adsorption [27]. Our results revealed that a more basic support yielded better Pt stabilization through the formation of rigid Pt–O–M (where M is the support cation) bonds, thus inhibiting Pt sintering.

In another in situ time-resolved XAFS investigation [30], [31], we found that the Pt–O–Ce bond formed under an oxidative atmosphere at high temperature acted as the driving force for redispersion of sintered Pt particles into smaller particles. Previously sintered Pt particles on a CZY support were redispersed as small particles on treatment in an oxidative atmosphere at 873 K. Such Pt redispersion did not occur on an Al2O3 support. In situ TEM video images clearly captured the decrease in Pt particle size with time under an oxidative atmosphere at 1093 K [31].

From these results it can be expected that oxidized Pt will exist as a monolayer on the surface of CeO2-based oxide supports after treatment in an oxidative atmosphere at high temperature. However, complete evidence of this phenomenon is not yet available. In the present study, we focused on the oxidation state and nanostructure of Pt on the surface of a CeO2-based oxide support using XPS and TEM. XPS is more surface-sensitive than XAFS, and advanced high-resolution TEM can capture direct evidence of the Pt nanostructure that forms Pt–O–Ce bonds. We investigated whether reversible changes in the Pt oxidation state and nanostructure occur in sequentially oxidized, reduced, and re-oxidized Pt on a CeO2-based support at high temperature.

Section snippets

Support and catalyst preparation

In previous studies [27], [30], [31], 55 wt.% CeO2 containing CeO2–ZrO2–Y2O3 (CZY) was used as the CeO2-based support; however, the composition of primary particles of this material varies somewhat. Use of such heterogeneous material as a support for XPS Pt oxidation analysis would yield complicated results because the Pt oxidation states on Ce-rich and Zr-rich domains must be different. Thus, a support of simple composition is desirable to avoid ambiguity due to local heterogeneity in the

Pt dispersion following oxidative and reductive treatments

In previous XAFS analyses of sintering inhibition and redispersion of Pt on ceria-based oxide supports [27], [30], [31], CZY and commercial Al2O3 were used as the supports, which differ from the support used in the present investigation. Thus, before evaluating the Pt oxidation state and its nanostructure from XPS or Cs-corrected STEM results, it must be confirmed whether the CA support can inhibit Pt sintering compared to the Al2O3 support and if Pt agglomerates on the CA support can be

Conclusions

We determined changes in the Pt oxidation state and nanostructure on a CA support after oxidative and reductive treatments. From the results obtained in the present study, we can conclude that a monolayer of Pt with a higher oxidation state than metallic Pt forms a complex oxide with Ce on the surface of the CA support, which inhibits Pt sintering and causes redispersion of Pt agglomerates under oxidative conditions. This Pt can be reversibly reduced to the metallic state, and it forms a

Acknowledgments

The authors gratefully acknowledge Mr. Eiji Okunishi of JEOL Ltd. for the JEM-2100F experiments, Mr. Noritomo Suzuki of TOYOTA Central R&D Labs Inc. for the JEM-200CX investigations, and Mr. Kazuhiko Dohmae of TOYOTA Central R&D Labs Inc. for discussions.

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