Rubidium Carbonate (Rb₂CO₃) in Catalysis & Materials Science: A Promoter for Microwave-Assisted Carbon-Supported Single-Atom Materials

1) Overview

Carbon-supported single-atom materials are widely studied in catalysis and optoelectronic-related materials research due to their high atom utilization, strong stability, and abundant anchoring sites on carbon. A key bottleneck in fast microwave routes is that many organic precursors (including some MOFs/COFs and metal–organic complexes) absorb microwaves poorly, which can lead to incomplete pyrolysis and low carbonization.

A practical solution is to combine ZnCl₂ (as a microwave absorber) with a promoter to amplify microwave heating and raise the effective pyrolysis temperature rapidly. In this workflow, Rubidium Carbonate (Rb₂CO₃) can be selected as the promoter to enhance the heating efficiency of the ZnCl₂-assisted system, enabling short, air-atmosphere microwave treatment to produce highly carbonized carbon-supported single-atom materials, followed by simple washing to remove residual salts.

2) Detailed Experimental Procedure (Microwave-Assisted Route)

Step-by-step workflow

1. Select the organic precursor (metal-containing preferred).
   • Options include MOFs, COFs, metal–organic complexes, metal–organic coordination polymers, ionic polymers, or other metal-containing organic precursors.
   • Example families: Ni-ZIF type precursors, Ni-MET type precursors, metal phthalocyanines, metal porphyrins, acetylacetonate complexes (Fe/Co/Ni/Cu), etc.
2. Decide whether to add an auxiliary nitrogen source and/or carbon source (optional).
   • N sources: melamine, dicyandiamide, urea, NH₄F, NH₄Cl, NH₄Br, NH₄HCO₃ (one or more).
   • C sources: ascorbic acid, sucrose, cyclodextrin, glucose, carotene, starch, PVP (one or more).
   • Typical ranges: precursor:N source = 1:(2–10); precursor:C source = 1:(2–30).
3. Weigh reagents and dry-mix thoroughly.
   • Combine the organic precursor, ZnCl₂ (microwave absorber), and Rb₂CO₃ (promoter).
   • Use an agate mortar (or equivalent) to grind until the mixture becomes homogeneous.
4. Load for microwave heating (air atmosphere).
   • Place the mixed powder into an open container or microwave tube suited for solids.
   • Keep the sample stable and upright (a simple conical flask holder setup is commonly used).
   • Because this is rapid high-power heating, use microwave-compatible labware and shielding practices appropriate for corrosive salts.
5. Microwave pyrolysis.

   Parameter	Working window	Typical screening point
   Power	400–3000 W	~800 W
   Band	300–6000 MHz	Common lab microwave bands; tune to equipment
   Time	1.5–10 min	~3 min
   Atmosphere	Air	Air

   Rb₂CO₃ is used here to strengthen the ZnCl₂-assisted microwave heating so the organic precursor can undergo more complete thermal decomposition and carbonization within minutes.

6. Cool, recover, and (optionally) re-grind.
   • Allow the solid to cool to a safe handling temperature.
   • Grind gently to break up agglomerates and expose trapped salts for efficient removal.
7. Ultrasonic washing to remove Zn residues and the promoter.
   • Use one or more of: water, HCl solution, HNO₃ solution, H₂SO₄ solution (or combinations).
   • pH window: 1–7; ultrasonication time: 0.5–5 min per wash; number of washes: 1–5.
   • After each wash: centrifuge/decant, then repeat as needed until conductivity/ion tests stabilize.
8. Drying.
   • Dry the cleaned solid (typical lab range 60–100 °C). Common practice is ~90 °C for ~12 h, adjusted for batch size and vacuum capability.
9. Verify single-atom anchoring and carbon structure (recommended characterization).
   • XRD: confirm absence of bulk metal crystalline peaks.
   • Raman: D/G features to gauge carbonization/graphitization changes with promoter loading.
   • N₂ adsorption: BET surface area and pore size distribution.
   • XAS/EXAFS (or equivalent): validate M–N coordination and absence of M–M bonding for single-atom signatures.

3) Comparison Summary: This Microwave Route vs Traditional Thermal Pyrolysis

In short, the ZnCl₂ + promoter strategy is designed to overcome insufficient microwave absorption of organic precursors. This directly addresses incomplete pyrolysis under microwave-only conditions and supports faster production of highly carbonized carbon frameworks suitable for stabilizing single atoms.

4) Why Rubidium Carbonate (Rb₂CO₃) Is Advantageous as the Promoter in This Application

• Boosts effective microwave heating with ZnCl₂: Rb₂CO₃ acts as a promoter to strengthen the microwave-heating performance of the ZnCl₂-assisted system, helping the precursor reach higher effective pyrolysis intensity within a short time window.
• Improves completeness of pyrolysis for poor microwave-absorbing organics: This is critical when your precursor is not inherently microwave-active; improved heating translates to higher carbonization and better-developed carbon structures.
• Supports higher carbonization/graphitization indicators: Increased carbonization typically improves electrical conductivity and structural robustness of the carbon host, which is beneficial for catalytic electron transport and stability.
• Easy removability after heating: A core practical requirement is that the promoter should be readily washed out after microwave treatment. Rb₂CO₃ fits this “post-process removable salt” role, aligning with fast purification workflows.
• Process-friendly and scalable: As a solid inorganic additive, Rb₂CO₃ integrates cleanly into dry-mixing and can be implemented with standard washing/centrifugation steps—unit operations that scale well.
• R&D tuning lever: By adjusting the precursor:Rb₂CO₃ and ZnCl₂:Rb₂CO₃ ratios within the defined windows, teams can tune heating strength, carbonization degree, and (often) textural properties without changing the core precursor chemistry.