Role of Ag, Pd cocatalysts on layered SrBi₂Ta₂O₉ in enhancing the activity and selectivity of photocatalytic CO₂ reaction

Highlights

• Ag and Pd are anchored on SrBi₂Ta₂O₉ to construct CO and CH₄ evolution sites.

• Ag and Pd attract and accumulate the electrons originating from SrBi₂Ta₂O₉.

• The intermediate species CO* desorb and form CO directly on the surface of Ag.

• The CO* is further hydrogenated to form CH₄ on the surface of Pd.

Abstract

Photocatalytic CO₂ reduction in presence of H₂O provides an ideal way to alleviate the greenhouse effect and obtain valuable chemicals. Constructing active sites on the surface of photocatalysts significantly affects the activity and selectivity of photocatalytic CO₂ reduction, which involves a multi-electron reduction process. Herein, we anchor Ag and Pd particles on the surface of layered perovskite SrBi₂Ta₂O₉ to build CO and CH₄ evolution sites, respectively, via a photodeposition method. Ag and Pd not only attract and accumulate the photoinduced electrons originating from SrBi₂Ta₂O₉ due to the formed Schottky junction between them, but also activate CO₂ molecules, thereby resulting in higher CO₂ reduction activity. On the surface of Ag, intermediate species CO* desorb and form CO directly, whereas, on the surface of Pd, intermediate species CO* is further hydrogenated to form CH₄, thus resulting in different selectivity. This work offers a new avenue for developing photocatalysts with high activity and selectivity for CO₂ reduction.

Introduction

The use of fossil energy sources causes not only an energy crisis, but also ecological problems (greenhouse effect) due to the massive emission of CO₂, which seriously threaten the survival and development of human beings [1], [2], [3]. Using clean solar energy with H₂O as a reducing agent to reduce CO₂ to carbon fuel or high value-added chemicals can not only alleviate the energy crisis, but also reduce the concentration of CO₂ in the atmosphere, thus solving the corresponding environmental problems, which is considered as one of the most promising CO₂ conversion solutions [4], [5], [6]. Although photocatalytic materials such as TiO₂ [7], [8], [9], [10], CdS [11], [12], [13], and g-C₃N₄ [14], [15], [16], [17] have been reported to achieve CO₂ reduction, their reduction efficiency is still relatively low. This is due to the high thermodynamic stability and kinetic inertness of CO₂, which requires high energy input to achieve the bending (activation) of CO₂ molecules and the breaking of CO₂ double bonds [18]. In addition, most photocatalytic materials have low photogenerated carrier separation efficiency and lack sufficient electrons to participate in the CO₂ reduction reaction [19], [20]. Therefore, it is still challenging to achieve efficient photocatalytic CO₂ reduction. In addition, CO₂ reduction involves the transfer processes of multiple electrons and protons, resulting in a variety of reaction products and complex reaction paths [21], [22], [23]. The relationship between the physical structure and selectivity of photocatalytic materials is still ambiguous.

It has been widely reported that the modification of co-catalysts not only enhances the separation efficiency of photogenerated carriers but also activates CO₂ to make it easier to be reduced. In addition, co-catalysts with different atomic structures will result in different charge transfer pathways, thus generating different reduction products and changing the selectivity of CO₂ reduction [24], [25]. Considering that the value of different reduction products is different, it is of great significance to precisely regulate the reduction products through co-catalyst modification. As reported, Ag and Cu cocatalysts could occupy the active sites of hydrogen evolution and low the overpotential of CO evolution, thereby resulting in a high selectivity of CO [26], [27], [28], [29]. Also, it is found that Ni is beneficial for the activation of CO, thus improving the selectivity of CO [30], [31]. Pt and Pd cocatalysts usually promote the CH₄ selectivity, which probably originates from their intensive H adsorption performance, thus supplying sufficient protons for CH₄ [32], [33].

SrBi₂Ta₂O₉ is a typical Aurivillius phase oxide, where Sr cation fills in the gap formed by 8 connected TaO₆ octahedrons to form a perovskite layer, and Bi₂O₂ fluorite layer fills in perovskite interlayers to form a Bi layered Ta-based perovskite oxide [34]. The conduction band of SrBi₂Ta₂O₉ composed of Ta 5d orbitals has enough negative conduction potential to achieve a CO₂ reduction reaction [35]. Because of its unique layered structure, SrBi₂Ta₂O₉ is not only favorable for photogenerated charge separation, but also for ion exchange or interlayer reaction. Importantly, in-situ growth of cocatalysts through ion exchange can not only obtain closer interfacial contact but also more easily receive from the interlayer carrier, thus acting as the active centers of CO₂ reduction. Therefore, the construction of suitable cocatalysts on SrBi₂Ta₂O₉ is expected to obtain high CO₂ reduction activity and high selectivity. Herein, SrBi₂Ta₂O₉ was prepared by a citric acid polymerizable method for CO₂ reduction. Pd and Ag as cocatalysts were loaded on the surface of SrBi₂Ta₂O₉ by a photodeposition method. The effect of Pd and Ag on promoting photocatalytic activity and selectivity of CO₂ reduction is investigated. Additionally, the mechanism of Pd and Ag in determining CO₂ reduction is also proposed.