Model Ag/HOPG catalysis for ethylene epoxidation: comparison of methods for the preparation of large Ag particles with high surface coverage

Technical Summary: Model Ag/HOPG Catalysis for Ethylene Epoxidation

Problem Statement
The industrial ethylene epoxidation process relies on Ag/α-Al₂O₃ catalysts, yet optimizing these systems requires a fundamental understanding of the reaction mechanism, specifically the nature of active oxygen species (nucleophilic vs. electrophilic). A significant barrier to this understanding is the difficulty of performing in situ surface-sensitive studies (such as X-ray Photoelectron Spectroscopy, XPS, and Scanning Tunneling Microscopy, STM) on traditional industrial catalysts due to the high oxygen content of the alumina support, which obscures signals from adsorbed oxygen on silver. While silver single crystals and foils serve as model systems, they lack the particle size effects relevant to supported catalysts. Carbon supports like Highly Oriented Pyrolytic Graphite (HOPG) are ideal for these studies due to their lack of intrinsic oxygen. However, a critical challenge remains: depositing large, uniformly distributed silver particles with high surface coverage on pristine HOPG. Traditional thermal evaporation on clean HOPG results in poor adhesion, leading to non-uniform particle distribution where silver aggregates at step edges rather than forming isolated, controlled particles on terraces. Conversely, methods used to induce adhesion, such as Argon ion (Ar⁺) etching, often damage the graphite basal plane, leading to the formation of Ag-C composites or encapsulation, rendering the samples unsuitable for mechanistic studies.

Methodology
The authors compared several preparation methods to create Ag/HOPG model catalysts suitable for in situ XPS and STM.

1. Support Preparation: Three HOPG surface conditions were tested:
   • Freshly peeled and vacuum-annealed (600°C).
   • Ar⁺ etched (500 V, 3s).
   • Ar⁺ etched with a retarding field (500 V, 20s; 580 V retarding) followed by annealing.
2. Silver Deposition:
   • Low-loading tests: Small amounts of Ag were deposited on the various supports to observe initial nucleation.
   • High-loading synthesis: Large Ag particles were prepared using an EFM3 e-beam evaporator (99.99% Ag wire) in an XP-spectrometer preparation chamber (background pressure < 5 × 10⁻⁸ mbar). Deposition times were varied (30–140 min) to control particle size and coverage.
   • Post-treatment: Selected samples were annealed in vacuum (400°C and 500°C) to study stability and ripening.
3. Characterization:
   • STM: Used for low-loading samples to analyze particle distribution and height on terraces.
   • SEM: Used for high-loading samples to visualize surface morphology and particle size distribution.
   • XPS: Used to determine chemical states (Ag3d, O1s, C1s), calculate Ag/C and O/C atomic ratios, and estimate the contribution of surface oxygen on silver versus the support.
   • Data Analysis: Particle size distributions and surface coverage were quantified using deep machine learning analysis (DLgram) on STM and SEM images.

Key Results

• Impact of Support Pretreatment:
  • Freshly peeled HOPG: Silver particles decorated step edges but also formed isolated, evenly distributed particles on terraces. This method preserved the integrity of the graphite basal plane.
  • Ar⁺ Etched HOPG: While this created nucleation sites, it resulted in a high density of small objects (approx. 0.26 nm height) that XPS and STM analysis suggested were Ag-C composites formed by carbon encapsulation of silver. The Ag/C ratio dropped significantly after air exposure, indicating instability and carbon burning. This method was deemed unsuitable for mechanistic studies.
  • Ar⁺ Etched with Retarding Field: This produced a high density of small particles suitable for film formation rather than isolated large particles, and still involved some carbon damage.
• Optimized High-Loading Synthesis:
  • Using e-beam deposition on freshly peeled HOPG successfully produced uniformly distributed, relatively flat Ag islands.
  • By varying deposition time, the authors controlled mean particle sizes in the range of 33 nm to 102 nm with Ag surface coverages up to 0.59 monolayer.
  • XPS confirmed the metallic state of silver (Ag3d5/2 at 368.3 eV).
  • Thermal Stability: Annealing a sample with 102 nm particles at 500°C resulted in Ostwald ripening (mean size increased to 107.4 nm, particle count decreased), while total sublimation occurred at 600°C.
• Suitability for XPS:
  • Quantitative analysis demonstrated that for the high-loading samples, the signal from oxygen adsorbed on silver (OAg) significantly exceeded the background oxygen signal from the HOPG support.
  • Specifically, the estimated fraction of the O1s signal originating from the silver surface ranged from ~25% to ~81% depending on the sample, making these systems viable for distinguishing active oxygen species during epoxidation.

Significance and Claims
The paper claims to have resolved the challenge of preparing Ag/HOPG model catalysts that combine large particle sizes (30–100 nm range), high surface coverage, and uniform distribution while maintaining the chemical integrity of the support.

• The authors assert that their method (e-beam deposition on freshly peeled HOPG) avoids the Ag-C encapsulation issues associated with Ar⁺ etching, thereby providing a stable, "low-oxygen" system.
• The resulting samples are explicitly identified as suitable for in situ instrumental studies (XPS and STM) of ethylene epoxidation mechanisms, particularly for investigating the role of oxygen species and particle size effects in the 30–100 nm range, which is critical for understanding industrial catalyst behavior.
• The study highlights that deep machine learning analysis is an effective tool for quantifying particle size distributions and surface coverage in these complex model systems.