Tuning the activities of cuprous oxide nanostructures via the oxide-metal interaction

Metal alloy catalysts are widely used in various applications, often consisting of a precious metal and a cheaper metal component. Under oxidative conditions, the cheaper metal component often segregates to the surface, forming an oxide layer. This oxide layer plays a crucial role in the catalytic properties of the alloy, but a comprehensive atomic-level understanding of this oxide-metal interface is lacking. The interface between cuprous oxide and a metal is particularly interesting, due to the dynamic nature of copper centers whose valence states and coordination numbers vary significantly with the reaction environment. Coordinatively unsaturated (CUS) metal cations are often active sites in catalytic reactions, and stabilizing these CUS centers is crucial for enhancing catalyst performance. The formation of a copper oxide-metal interface has been observed in various metal alloys, such as PtCu, AuCu, and AgCu, and understanding this interface's catalytic properties is essential for designing effective Cu-based alloy catalysts for oxidation reactions. This study aims to address this gap by investigating the atomic structures and catalytic properties of well-defined Cu₂O NSs supported on Pt(111), Au(111), and Ag(111) single crystals, gaining detailed insights into the role of OMI in tuning the activity and stability of these catalysts.

Previous research on metal alloy catalysts has highlighted the importance of the oxide-metal interface in determining catalytic activity. The strong metal-support interaction (SMSI) effect, discovered in the 1970s, demonstrated the profound influence of the support material on the catalytic behavior of supported metals. Recent studies have increasingly focused on the atomic-scale understanding of these interfaces, with emphasis on the unique catalytic properties arising from specific interfacial sites. Studies on interface-confined metal centers, particularly in oxidation reactions, have shown promise in controlling and enhancing catalytic activity. However, the exact nature of the oxide-metal interaction (OMI) and its precise influence on the properties of interfacial sites have remained largely unknown. Existing literature highlights the catalytic interest in the cuprous oxide-metal interface, but difficulties in studying the dynamic nature of copper centers have hindered progress. Research on CUS Cu⁺ centers as active sites for various oxidation reactions, and strategies for stabilizing these sites on precious metal surfaces, underscore the importance of understanding the interfacial chemistry in Cu-based catalysts.

The research employed a combination of experimental and computational techniques to investigate the structure and catalytic activity of Cu₂O NSs supported on different metal substrates. Well-defined Cu₂O NSs were synthesized on Pt(111), Au(111), and Ag(111) single crystals using a method involving Cu evaporation in an oxygen atmosphere followed by annealing. The atomic structures of the Cu₂O NSs and the interfaces were characterized using low-temperature scanning tunneling microscopy (LT-STM), near-ambient-pressure STM (NAP-STM), and element-specific STM (ES-STM). These techniques allowed the researchers to resolve the atomic structures of the Cu₂O NSs and identify the interfacial sites. X-ray photoelectron spectroscopy (XPS) was used to determine the chemical states of the Cu₂O NSs and the metal substrates, while density functional theory (DFT) calculations were used to model the interfacial structures and predict the catalytic activity. The catalytic activity of the Cu₂O/M interfaces for CO oxidation was investigated using both in situ and ex situ STM and NAP-STM, varying CO pressure to determine the reaction onset pressure and observe structural changes during CO oxidation. Furthermore, powder catalysts containing PtCu, AuCu, and AgCu alloys supported on carbon black (CB) were synthesized via co-impregnation, and their catalytic performances in CO oxidation were measured and compared to that of the single metal catalysts and Cu₂O powder. Quasi-in situ XPS after the catalytic experiments characterized the chemical states of the catalysts. Transmission electron microscopy (TEM) was used to analyze the size and structure of the alloy nanoparticles both before and after the reaction. DFT calculations were used to gain insights into the reaction mechanism and the role of OMI in determining the catalytic activity, including calculations of oxygen vacancy formation energy and d-band centers. The adhesion energy, oxygen vacancy formation energy, and the electronic interactions were correlated with the catalytic activities.

The study revealed that despite similar surface and step structures across the three supported Cu₂O NSs, their thermal stability varied significantly. Cu₂O/Pt showed significantly lower thermal stability than Cu₂O/Ag and Cu₂O/Au, decomposing at temperatures lower than the desorption temperature of oxygen atoms on Pt(111). The catalytic activity of the supported Cu₂O NSs for CO oxidation followed the order Cu₂O/Pt > Cu₂O/Au > Cu₂O/Ag. In situ STM and NAP-STM studies showed that CO oxidation occurred via an interfacial dual-site mechanism, where CO adsorbed on Pt sites reacted with neighboring lattice oxygen in Cu₂O. The reaction rate was faster at the step edges of Cu₂O, attributed to the lower coordination numbers of O atoms at these sites. The reduction of Cu₂O by CO resulted in the formation of metallic Cu or triangular Cu₃Oₓ clusters on a PtCu₃ alloy layer in the case of Cu₂O/Pt(111). The CO oxidation reaction on Cu₂O/Pt(111) happened at much lower CO pressure (5 × 10⁻⁸ mbar) compared to Cu₂O/Au(111) (0.5 mbar) and Cu₂O/Ag(111)(>48 mbar). Regeneration of Cu₂O NSs was readily achieved under low-pressure O₂ for Pt, but required near-ambient pressure O₂ for Au and Ag. Powder catalyst tests confirmed the superior activity of PtCu/CB for CO oxidation compared to AuCu/CB, reflecting the model studies' findings. DFT calculations supported the experimental observations, showing that OMI significantly lowers the energy barrier for CO oxidation at the Cu₂O/Pt interface. Oxygen vacancy formation energy (Eovf) was found to be a key descriptor for OMI, correlating strongly with the catalytic activity. The electronic interaction between Cu⁺ and the metal substrate, quantified by the difference in d-band centers, exhibited an excellent scaling relationship with Eovf, and thus with the catalytic activity. Stronger OMI led to easier oxygen removal and CO₂ formation.

The results demonstrate the crucial role of OMI in tuning the catalytic activity and stability of supported Cu₂O NSs. The enhanced activity of Cu₂O/Pt for CO oxidation can be attributed to the strong OMI, which facilitates oxygen vacancy formation and lowers the energy barrier for the reaction. The higher activity of Pt-based catalysts compared to Au- and Ag-based catalysts is consistent with the stronger OMI observed in the DFT calculations. The excellent scaling relationship between the oxygen vacancy formation energy and the difference in d-band centers underscores the importance of electronic effects in determining OMI. The findings highlight that while the presence of an oxide layer on precious metals mitigates the CO-poisoning problem, the strength of the OMI is crucial for sustained catalytic activity at low temperatures. The unique behavior of the Cu₂O/Pt system, where low stability is accompanied by high activity, suggests a dynamic interplay between the catalyst structure and its reactivity.

This study reveals a strong correlation between oxide-metal interaction, oxygen vacancy formation energy, and the catalytic activity of supported Cu₂O nanostructures for CO oxidation. The findings highlight the importance of the electronic interaction between Cu⁺ and the metal substrate in tuning catalyst performance, quantified through the d-band center. The superior activity of Cu₂O/Pt is attributed to a strong OMI that facilitates oxygen vacancy formation and lowers the activation barrier for the reaction, while also mitigating CO poisoning. Future research could explore other metal combinations and reaction systems to further elucidate the general applicability of the proposed OMI descriptors and design more efficient oxidation catalysts.

The study primarily focuses on CO oxidation and may not be directly generalizable to other catalytic reactions. The use of single-crystal substrates for the model studies introduces an inherent limitation on the complexity of the catalyst structure and morphology. While powder catalyst tests validate the findings, the complexity of powder catalysts may mask some detailed interface behaviors. The DFT calculations rely on approximations and may not perfectly capture the complex interfacial interactions. The study primarily considers the effects of the metal substrate and does not explicitly address other factors that might affect the catalytic activity, such as the size and morphology of the Cu₂O NSs.