DFT+U study on the oxygen adsorption and dissociation on CeO2-supported platinum cluster
Introduction
Air pollution from automobile exhaust is one of the major environmental problems in the modern civilization. In order to reduce harmful emissions from diesel engines, the diesel oxidation catalyst (DOC) has been widely investigated. In DOC systems, carbon monoxide (CO) and hydrocarbons (HC) are oxidized to harmless chemical substances such as H2O and CO2, while nitrogen oxide (NO) is converted to NO2 for easy treating in selective catalytic reaction (SCR) – another stage of diesel catalytic converter [1], [2], [3], [4]. DOCs have a honeycomb-like, monolithic structure. The monolithic support is made either from metallic (stainless steel) or ceramic material and is coated with high porous oxides, such as γ-Al2O3 (alumina), CeO2 (ceria) and precious metals (on top), such as Pd, Pt, and Rh to increase the catalytic activity or to stabilize the structure of the catalyst. In DOCs, ceria provides multiple functions, one of which is to store excess oxygen under oxidizing (fuel lean) conditions and release it in reducing (fuel rich) conditions to oxidize CO and HC, where the transformation between Ce4+ and Ce3+ occurs. This process allows the catalyst to operate over wide air-to-fuel ratio [5], [6], [7]. On the other hand, CeO2 is also widely used for stabilizing precious metal particles [8]. Experimentally, it has been reported that Pt particles in PtxCeO2 catalyst do not sinter during high temperature aging in the presence of oxidative environment [9], [10], [11], [12].
Furthermore, oxidation process plays a very important role in DOC systems (e.g. HC conversion: HC + O2 → CO2 + H2O) and so, an understanding of the interaction of the metal/metal oxide system with O2 is imperative. To date, there are numerous researches on the adsorption and dissociation of O2 molecules on the organic systems and the bimetallic surfaces [13], [14], [15] but very few have been done on noble metal clusters or noble metal cluster/metal oxide, for instance, Pt cluster on CeO2. Yoon et al. [16] investigated the molecular and dissociative adsorption of O2 on Au clusters without the presence of metal oxide support using density functional theory (DFT). Halachev et al. [17] studied the dissociative adsorption of O2 on the transition metal clusters in the presence of general subsurface oxygen using extended Huckel method, leaving the effect of subsurface on the electronic structure of the metal clusters unexplored. This lack of understanding of reactivity of metal clusters with the support motivates this work to tackle the reactivity of Pt cluster supported on ceria in relation to its wide application in DOC systems as described above as well as the supported cluster's geometry and electronic features. Specifically, in the reactivity, the energetics of the adsorption and dissociation of O2 on Pt4/CeO2(1 1 1)p3 × 3 surface is studied. We chose CeO2(1 1 1) because of its stability over other surfaces like (1 1 0) or (1 0 0) [18], [19], [20]. It is an important factor for stabilizing the metal cluster on the support surface and hence, preventing the small cluster from sintering effect. Furthermore, we draw the effects of CeO2 support. In terms of the geometry/electronic properties of the catalyst, the adsorption of Pt cluster on CeO2 support is investigated to verify the cluster's stabilization as observed in experiment [21]. In this work, the tetrahedron structure of the Pt4 cluster was chosen because of its relative stability over the planar rhombus cluster [22], [23]. This characteristic is also retained when the cluster is placed on the ceria support. Our calculations show that, the tetrahedral cluster is about 0.68 eV more stable on the CeO2(1 1 1) surface as compared to the planar rhombus one. In addition, the Pt4 cluster size is used since it is large enough to capture both the “contact” and “non-contact” parts with the CeO2 surface and, at the same time, is small enough for reasonable computational cost. Indeed, Pozdnyakova et al. [24] prepared Pt–ceria catalysts containing 0.5–0.6 nm small Pt particles by using impregnation (IMP) method; making it possible for fabricating such a small clusters on surface.
Section snippets
Computational methods
First-principles calculations are performed using the spin-polarized version of the Vienna ab initio Simulation Package (VASP) [25], [26]. The Perdew–Burke–Ernzerhof (PBE) version of the generalized gradient approximation (GGA) is used to describe the exchange-correlation [27]. The interaction between the core and valence electrons is treated by the projector augmented wave (PAW) method [28], [29]. The valence electron configurations of cerium, oxygen, and platinum are represented in a plane
Pt4 cluster adsorption on CeO2(1 1 1)
We investigated the adsorption of Pt4 on CeO2(1 1 1) by setting up all possible initial adsorption sites of the cluster on the CeO2 surface. In these sites, the Pt4 cluster is either on top of the three-fold Ce, on top of Ce site or on top of O site. After optimizing the Pt4/CeO2(1 1 1) systems using the conjugate gradient method [49], we found that the Pt4 cluster always tends to shift to the most stable position on CeO2 surface, that is, where the center of Pt4 stays on the three-fold Ce atoms
Conclusion
The reactivity of Pt4/CeO2(1 1 1) toward oxygen as well as the geometry and the electronic property of the Pt4/CeO2(1 1 1) system is investigated using density functional theory with inclusion of the Hubbard parameter, U (DFT+U). We found that, Pt4 cluster strongly binds and thus can act as “anchor” to prevent sintering effect in catalyst confirming experiments. The adsorption of Pt4 on CeO2(1 1 1) involves a direction of electron charge transfer from Ptb atoms to CeO2 surface as well as Ptt atom.
Acknowledgements
This work is supported by MEXT (Ministry of Education, Culture, Sports, Science and Technology of Japan) through QED (Quantum Engineering Design) program. M.C.S. Escaño extends gratitude to Tenure Track Program for Innovative Research, Ministry of Education, Culture, Sports, Science and Technology, Japan (MEXT) and Japan Science and Technology Agency (JST). Some of the calculations presented here were done using the computer facilities at the following institutes: Cybermedia Center (Osaka
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Present address: Graduate School of Engineering, University of Fukui, 3-9-1 Bunkyo, Fukui 910-8507, Japan.