Interaction of Zr with oxidized and partially reduced ceria thin films

Highlights

• The additive Zr significantly modifies the surface structure and electronic properties of ceria.

• Electron-transfer from Zr to ceria leads to the partial reduction of ceria and oxidation of metallic Zr.

• Zr grows two-dimensionally on CeO2(111) at low coverages.

Abstract

The growth and electronic properties of Zr on the ceria thin films were studied by X-ray photoelectron spectroscopy, low energy electron diffraction (LEED), scanning tunneling microscopy (STM) and work function measurements. Metallic zirconium was vapor-deposited on the well-ordered fully oxidized CeO₂(111) and partially reduced CeO_2-x(111) (0 < x < 0.5) thin films, which were epitaxially grown on a Ru(0001) substrate, under ultrahigh vacuum (UHV) conditions. The results show that the deposition of Zr on both ceria surfaces leads to electron transfer from Zr to ceria, accompanied by partial reduction of Ce from Ce^4 + to Ce^3 + states and oxidation of metallic Zr to Zr^4 +. Moreover, with increasing the Zr coverage, the reduction degree of ceria films increases and eventually only Ce^3 + is observed at a high coverage of Zr. The STM results suggest that Zr grows two-dimensionally (2D) on the CeO₂(111) thin film at low coverages due to the strong interaction between Zr and CeO₂(111).

Introduction

Ceria in stoichiometric (CeO₂) and under stoichiometric (CeO_2-x) (0 < x < 0.5) form are pivotal materials in various sectors of industry due to their interesting mechanical, electrical, optical and catalytic properties. A key feature is its superior oxygen storage capacity [1], as it acts as an oxygen reservoir regulating the partial pressure of oxygen near the catalyst surface, and can provide oxygen to the gas mixture. This property enables ceria to be widely used in important catalyzed chemical reactions such as the conversion of hydrocarbons, oxidation of CO, reduction of NO_x, water gas shift reaction (WGS), automotive exhaust emission-control reactions, and low-temperature methanol synthesis [1], [2], [3], [4], [5], [6], [7]. In order to improve the catalytic performance of ceria, two groups of additional suitable materials are extensively employed as additives in ceria based catalysts. The transition metals, mainly the late transition metals like Ni, Rh, Pd, Ru or Pt, are present as metallic additives that are in the form of nanoparticles supported on the high-surface-area ceria surface [1], [4], [5], [8], [9], [10]. These nanoparticles dispersed on the surface of cerium oxide can promote the activity and selectivity toward the desired chemical reaction to run on the catalyst surface because of the adsorption properties of both clusters and support and spillover effect [11], [12], [13]. Despite widespread application as catalytic supports, one main issue regarding the use of pure ceria is its poor thermal stability at high temperatures. It is well-known that pure CeO₂ nanoparticles undergo seriously sintering at high temperature [14], which brings the loss of its essential oxygen storage capacity [1]. The solution is to dope cerium oxide with easily oxidizable metals or directly with their oxides that strongly interact with CeO₂. The interaction of the additional suitable materials with ceria can result in not only a different surface morphology but also a chemically influenced oxygen storage capacity [15]. In such cases, mixed oxides of both metals, e.g., cerium zirconates, aluminates, tungstates, stannates, gallates, vanadates, or titanates, may be formed depending on the chosen metal, such as Zr, Al, W, Sn, Ga, V, and Ti [7], [15], [16], [17], [18], [19], [20], [21], [22].

More recently, it has been reported that Zr-doped ceria has a great potential for applications in various technological fields. Generally speaking, zirconia-rich compositions, Ce_xZr_1-xO₂ (x < 0.3) find applications as ceramic materials, while ceria-rich oxides (x > 0.3) are used as catalytic materials, such as in cleaning the automotive exhaust-gases [3], [23], [24]. Doping Zr in CeO₂ to form Ce_1-xZr_xO₂ solid solutions, a currently indispensable component in three-way automotive catalysts as an oxygen buffer, dramatically facilitates the formation of the oxygen vacancies, and thus the corresponding oxygen storage–release capacity [25], [26]. It has been suggested that ceria structural modifications mediated by zirconia are responsible for the enhanced OSC properties of ceria-zirconia mixed oxides. To better understand these findings, a wide range of experimental and theoretical work has been carried out [21], [26], [27], [28]. However, the mechanisms for such doping effects remain uncertain and are still under debate. For example, Mamontov et al. reported that doping with smaller Zr^4 + ions into ceria could enhance stability of oxygen defects which might account for the improved OSC compared to pure ceria [29]. However, EXAFS studies showed that the presence of Zr^4 + ions in ceria resulted in the presence of strongly and weakly bound oxygen species in Ce_1 − xZr_xO₂, the latter was believed to be the source of OSC [30]. In addition, bulk fluorite CeO₂ is highly non-reducible with the heat of reduction to Ce₂O₃ being ~ 760 kJ/mol of O₂ liberated. However, the nanostructured forms of ceria that contain a high degree of defects or oxygen vacancies are much more reducible [31]. Thus, the epitaxially grown ceria thin surface is a reasonable model surface to study the interfacial interaction between Zr and CeO₂.

The Zr-doped CeO₂(111) surface was studied with first-principles density functional theory previously [32]. It was found that the Zr doping induces a severe distortion of the surface structure and facilitates the oxygen vacancy formation. Recently, our group has reported the effects of Zr doping on the dispersion and thermal stability of Ag nanoparticles on the CeO₂(111) surface [33]. Predepositing a small amount of Zr on ceria could significantly enhance the dispersity and thermal stability of Ag particles on ceria, which is essential for ceria-supported Ag catalysts [33]. In order to further understand the interaction of Zr with both oxidized and reduced ceria, in this study we reported a detailed study of the interaction between Zr and ceria thin films which were epitaxially grown on the Ru(0001) substrate. The growth and electronic properties of Zr on CeO₂(111) and CeO_2-x(111) (0 < x < 0.5) thin films which are epitaxially grown were systemically investigated by X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), low energy electron diffraction (LEED) and scanning tunneling microscopy (STM). The addition of the metallic dopant Zr affects the structural and electronic properties of both fully oxidized and partially reduced CeO_2-x(111) thin films because of the strong metal-support interaction between Zr and ceria.