Morphology and chemical states of Ni supported on Ti-modified CeO_x(111) interfaces

Highlights

• Deposition of Ti onto CeO₂ and reduced CeO_1.92(111) can produce titania-CeO_x interfaces.

• Ni forms small particles upon deposition on titania-ceria at 300 K and after heating to 700 K.

• Doping ceria with Ti can affect the morphology, size, and sintering of the Ni particles.

• Both Ni⁰ and Ni²⁺ species are present upon Ni deposition on Ti-CeO₂ and CeO_1.92 at 300 K.

• The extent of Ni oxidation to NiO depends on the nature of Ti-doped ceria interfaces.

Abstract

Low coverage of Ni was deposited onto well-ordered Ti-doped CeO_x(111) (1.5 < x < 2) thin films at 300 K under ultrahigh vacuum conditions. The morphology and interaction of Ni with Ti-ceria interfaces were investigated with X-ray photoelectron spectroscopy and scanning tunneling microscopy. Ti doped-CeO_x(111) mixed oxide interfaces can be prepared by physical vapor deposition of submonolayer coverages of Ti onto both fully oxidized CeO₂(111) and partially reduced thin films (e.g. CeO_1.92) at room temperature followed by heating to 700 K. Ti is oxidized to Ti⁴⁺ at the cost of ceria reduction. Titania features are uniformly distributed on CeO_x with an average height of 0.31 nm. Upon Ni deposition over both Ti-CeO₂(111) and Ti-CeO_1.92 surfaces at 300 K, oxidation of Ni to NiO occurs and thus both metallic Ni and Ni²⁺ species are present in the particles formed over Ti-CeO_x surfaces. The extent of Ni oxidation depends on the nature of Ti-CeO_x. 28% of deposited Ni is in the +2 state over Ti-CeO₂. With heating to 700 K, the percent of the Ni²⁺ species increases to 37% over Ti-CeO₂. However, metallic Ni is the predominant species over Ti-CeO_1.92. The difference in the Ni species present in the particles results in a different particle sintering behavior with heating. Compared to pure ceria, modifications of ceria with addition of Ti metal dopants can significantly stabilize Ni as smaller nanoparticles upon heating.

Introduction

Ni has been widely studied for catalytic reactions including ethanol reforming and dry reforming of methane [1], [2], [3], [4], [5], [6], [7], [8]. Biomass-derived materials or bio-fuels can be used for the production of hydrogen via steam reforming of ethanol [9,10]. In recent years, hydrogen has been considered as a clean energy source. Dry reforming of methane utilizes two abundantly available greenhouse gases to produce industrially important syngas, which can be used further to produce synthetic petroleum as fuels or chemicals. Noble metals are active toward these reforming reactions [11], [12], [13]. Ni has been studied as a suitable catalyst [14,15]. It can effectively break the C-C and C-H bonds and it is cheaper as a practical reforming catalyst than noble metals [16,17]. However, Ni can rapidly deactivate due to the particle sintering and coke formation during the reforming process [18,19].

Ceria supports can improve the stability and catalytic performance of Ni. Unique redox properties and oxygen storage capacity of ceria can potentially help reduce the coke formation. Dispersing Ni on ceria forming smaller particles can increase the surface area of Ni and minimize the carbon deposit [20], [21], [22]. The strong metal−support interaction between Ni and ceria can improve the performance of Ni [23,24]. Doped ceria can provide a better catalytic support for metal catalysts for practical applications compared to pure ceria. Doping ceria with additional metal elements can enhance its thermal stability [25], [26], [27]. The interaction of metal dopant with ceria can also lower the activation energy needed for the release of oxygen, which result in the improvement of its redox properties and oxygen storage capacity and consequently the enhancement of its catalytic activity [28], [29], [30], [31]. Our group has been interested in the study of ceria with metal dopants including Ti and Mn [32], [33], [34], [35]. Previously, we have examined the interfacial structures of Ti-doped ceria as well as the interactions between Ti dopant and ceria. Our data has shown that deposition of Ti onto well-ordered CeO_x(1.5 ≤ x ≤ 2) can form titania-ceria mixed oxide interfaces and modify both electronic and structural properties of ceria [34]. The purpose of this study is to understand the effect of Ti as metal dopant in ceria on the electronic and morphological properties of supported Ni nanoparticles at room temperature. The structure and chemical state of Ni over Ti-doped ceria were investigated using scanning tunneling spectroscopy (STM) and X-ray photoelectron spectroscopy (XPS). The interaction between Ni and Ti-doped ceria was further studied with respect to heating. To examine the role of the degree of Ce reduction, both partially reduced and fully oxidized ceria were grown to prepare Ti-doped ceria supports for Ni.