Stabilizing platinum atoms on CeO2 oxygen vacancies by metal-support interaction induced interface distortion: Mechanism and application
Graphical abstract
Introduction
The dispersion of isolated metal atoms on support surfaces provides a foundation for maximizing the atomic efficiency of precious metals in catalytic reactions [1,2]. For this reason, single atom catalysts (SACs) often possess unprecedented catalytic performance, a trait which in recent years has been capitalized upon and evidenced in several important reactions driven by thermal, electric, and light-energy [[3], [4], [5]]. In light of growing concerns associated with the emission of volatile organic compounds (VOCs) [[6], [7], [8], [9]], several recent publications have demonstrated the potential of using SACs for the low-temperature destruction of VOCs [10,11]. We too, recently demonstrated that a Pt1-Co3O4 SAC was exceptionally active for the destruction of oxygenated VOCs [12]. Despite the progress made, a great challenge remains; to develop thermally and chemically stable SACs, which required to be used in large scale traditional thermal reactions, such as catalytic combustion of hydrocarbons, water-gas shift and methane mineralization [13,14]. The development of thermally robust SACs that can sustain reductive/oxidative processes under high temperature attracts huge interest and challenge for its application [15].
Several strategies have been invoked by researchers to synthesize materials that possess these desirable properties, consisting of (i) reducing the loading quantity of active metal components; (ii) adding the protective species; (iii) exploiting defect vacancies or voids in supporting materials [[16], [17], [18]]. These strategies aim to enhance metal-support interactions and/or prevent the aggregating tendency of isolated atoms. However, as discussed by Xiao and co-workers, the stabilizing mechanism of classical strong metal-support interaction remains elusive in the above mentioned methods [19]. As such, we consider it to be of high importance that a logical approach is implemented, to govern the appropriate selection of metal(s) and support(s) used for the synthesis of SACs, for specific applications [20,21]. Several notable studies have investigated how support-metal strong interactions (SMSIs) influence such catalysts, using both experimental and computational techniques [22]. Li and co-workers recently investigated how on-site Coulomb interactions influence the catalytic performance of Au catalysts supported on several tetravalent-metal dioxides of MO2 (M = Ti, Zr, Ce, Hf, and Th) [23]. The study confirmed that the radial contraction and low orbital energies of the 3d and 4f orbitals in these MO2 oxides are caused by fundamental quantum primogenic effects, which is of great significance for the determination of valence state as well as charge distribution of gold atoms [23]. An additional study by O`Connor et al., predicted the interaction strengths between metal atoms and oxide supports according to the smallest absolute contraction figures and selection operator regression [24]. The investigation concluded a correlation between binding of interfaces and readily available physical properties of supports; such as oxophilicity measured by oxide formation energy, which can be measured by the oxygen vacancy formation energy. These properties can be used to screen interaction strengths between metal-support pairs, which will undoubtedly aid the designation of stable SACs moving forward [25]. Varieties of approaches are available for stabilization of isolated metal atoms; CeO2 for instance as most extensively investigated supports for the SMSI, possesses exclusive redox properties, which have their origin in the effect of quantum primogenic, making it an ideal candidate to support and stabilize single atom sites [26]. In addition, it possesses a diverse electronic structure, which can lead to exposed facets [27].
Regrettably, much of the previous work in this area has predominantly focused on dissecting information on the function of the active sites; nevertheless, the properties of heterogeneous catalysts frequently depends on the synergy between supported phases and support, the extent of which, is often magnified by means of a interface limiting effect [28]. An example of this was recently presented by Bao and co-workers [29], who constructed a series of metal-oxide interfaces, which are considered to be the active sites for CO2 activation and CO reduction, where a synergistic effect between Au and CeOx promotes the stability of key carboxyl mediate (*COOH) and thus facilitates CO2 electroreduction. This has been further evidenced by Murray and co-workers [30], who proposed an evident enhancement for the CO oxidation rate in ceria-based catalysts at the ceria-metal interface, for a range of group VIII metal catalysts, revealing the significance from the support. Shen and co-workers exposed the prospect of stabilizing Au nanoparticles utilizing an interfacial anchoring pattern of gold-oxide [31], which confirmed that the structuring of Au-ceria interfaces facilitates the regeneration of adsorbed molecular CO and energetic oxygen species on ceria. As such, we consider that it is crucial to design a stable mono-dispersed active interface for thermo-catalytic reactions, over which the single atom sites are anchored to the defected sites through SMSIs. Despite several single atom materials having been developed and utilized for the destruction of VOCs, the intrinsic mechanism for the oxidation of VOCs and the relationship between catalytic performance and electronic structure have been somewhat overlooked [[6], [7], [8], [9]].
The work herein, focuses on the construction of a Pt-O-Ce active interface, in which platinum atoms are stabilized on CeO2 oxygen vacancies. According to our experimental and theoretical investigations, this interface was confirmed to distort; a phenomenon which is facilitated by SMSIs. We propose that this SMSI is compounded by chemical bonding and relevant charge transfer at active interfacial regions. The distortion degree at the Pt-O-Ce interfaces was enhanced attributing to the different electron structures of CeO2 support with diverse exposed facets. Compared with Pt1-CeO2{110} and Pt1-CeO2{111} samples, the Pt-O-Ce active interface over Pt1-CeO2{100} exhibits the highest intensity of distortion degree and thus catalyst possesses the exceptional catalytic efficiency and thermal stability for oxygenated hydrocarbon removal. Furthermore, the intrinsic methanol oxidation mechanism over Pt1-CeO2{100} material was revealed, which further assists with the identification of active sites in these materials during oxidation reactions. We hope the present study can provide fundamental assistance in the design of novel SACs with SMSIs; to expand the application of these catalysts in this important area of research.
Section snippets
Preparation of single atom catalysts
The procedures for preparation of CeO2 supports followed the previous reported [32], and the details of which were displayed in Supplementary Material. Immediately, the abundant surface oxygen vacancies over CeO2 supports were constructed by reducing in the hydrogen atmosphere at 250 °C for 2.5 h. The obtained defective supports were mixed with 50 mL of deionized water and stirred for 20 min. After stirring for another 30 min, a urea solution (1 M) was added when the mixture was heated up to
Distortion of Pt-O-Ce interface facilitated by strong metal-support interactions
A series of CeO2 materials were synthesized, each of which consisted predominantly of a specific exposed facet; confirmed by HR-TEM (Fig. S1). Accordingly, CeO2{100} (nanocube), CeO2{111} (nanopolyhedra) and CeO2{110} (nanorod) display clear lattice fringes, with inter-planar spacing equal to 0.27, 0.31 and 0.19 nm, respectively. The FFT (fast Fourier transformation) of exposed facet compares well with the standard CeO2 crystal planes [26,27,32]. From XRD patterns (Fig. S2), all samples exhibit
Conclusions
In summary, we have attempted to develop the stable single atom materials for thermo-catalytic reactions and made clear the stabilizing mechanism induced by classical strong metal-support interaction. Pt-O-Ce interface was constructed by stabilizing Pt atoms on the surface mono-oxygen vacancies of CeO2 with diverse exposed facets. Combination of advanced characterizations and theoretical calculations, the stabilization mechanism was creatively proposed that the formed Pt-O-Ce interface will be
CRediT authorship contribution statement
Zeyu Jiang: Writing - original draft, Investigation, Formal analysis. Meizan Jing: Writing - original draft, Methodology. Xiangbo Feng: Formal analysis. Jingchao Xiong: Resources. Chi He: Supervision, Project administration, Writing - review & editing, Data curation. Mark Douthwaite: Writing - review & editing. Lirong Zheng: Software, Formal analysis. Weiyu Song: Software, Validation. Jian Liu: Supervision, Software, Validation. Zhiguo Qu: Supervision, Methodology.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was financially supported by the National Natural Science Foundation of China (21876139, 21922606, 21677114), the Key R&D Program of Shaanxi Province (2019SF-244, 2019ZDLSF05-05-02), the Shaanxi Natural Science Fundamental Shaanxi Coal Chemical Joint Fund (2019JLM-14), and Basic Science Center Program for Ordered Energy Conversion of the National Natural Science Foundation of China (51888103). The authors gratefully acknowledge the support of K.C. Wong Education Foundation and also
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These authors are contributed equally to this work.