Ex-situ Iron Catalyst Particle Generator

Background & motivation

Methane pyrolysis offers a route to producing hydrogen and solid carbon without directly generating CO₂. My MPhil research investigated the use of iron nanoparticles as catalysts for carbon-nanotube growth, which required a method of generating and delivering controlled quantities of catalyst particles into a high-temperature reactor. I designed the particle-generation subsystem, developed the supporting numerical models, and designed the experimental campaign used to characterize the resulting particles.

Experimental setup

A carrier gas — argon or hydrogen, optionally carrying sulfur vapour — flows over a boat of iron powder inside a tube furnace. The iron evaporates, cools as it travels downstream, and nucleates into a nanoparticle aerosol that is drawn off through an extraction tube. The metered gas supply feeds the generator, and a second furnace grows carbon nanotubes from the delivered particles.

Schematic of the particle generator: carrier gas flows over a boat of iron powder, which evaporates and nucleates into an aerosol drawn off through an extraction tube 35 cm downstream. — Generator concept — carrier gas (optionally carrying sulfur) evaporates iron off a heated boat; the aerosol is extracted 35 cm downstream.

I designed the particle generator and built the sulfur sublimation subsystem — a heated pack and an insulated delivery line held above the sulfur condensation point along its full length — and integrated it with the reactor's mass-flow-controlled gas panel and counterflow CNT reactor.

Photograph of the physical experimental rig: twin tube furnaces, gas delivery lines, and instrumentation. — The physical rig: twin tube furnaces (generator + CNT reactor), gas delivery, and live particle sizing.

The insulated, heated sulfur delivery line clamped to the furnace flange, with two Omega PID controller boxes. — The sulfur sublimation heater and insulated delivery line I built, on their Omega PID controllers.

Design & analysis

Before building, I modelled the iron aerosol as it travels the reactor, tracking three competing processes — nucleation, condensation/evaporation, and coagulation — along the measured temperature profile. The model let me reason about where particles form and how their size and number evolve, and to pick operating points before committing hardware.

Diagram of the aerosol-dynamics model showing nucleation, condensation/evaporation, and coagulation, with the governing population-balance equation. — What the model tracks: nucleation, condensation/evaporation, and coagulation of the iron aerosol.

A few analyses drove concrete design decisions:

Reactor thermal profile → extraction geometry. The axial temperature field sets where the vapour cools and nucleates, so I positioned the extraction tube where the aerosol is fully formed and chose the furnace setpoint accordingly.
Coagulation timescales → transport strategy. The model showed how quickly particle number is lost downstream, which motivated extracting and transporting the aerosol at low temperature to preserve it.
Vapour-pressure (Antoine) calculations → sulfur delivery. These sized the sulfur-pack temperature range used to target a given sulfur-to-iron ratio, which set the heater's operating window.

Top: the reactor's axial temperature profile, which drives where the vapour cools and nucleates. Bottom: the model's predicted geometric mean diameter (GMD) — the characteristic size of a particle in the population — growing downstream for the argon and hydrogen carrier gases. Right: the measured GMD for each gas, validating the model.

Characterization

To confirm the system produced what it was designed to, I characterised both the catalyst particles and the carbon product. Transmission electron microscopy (TEM) imaged morphology — resolving individual single-walled nanotubes and distinguishing them from amorphous carbon. Raman spectroscopy probed the graphitic structure of the carbon, using the radial-breathing, D and G bands to identify single-walled tubes and gauge quality. Catalyst particle size distributions were measured live with a scanning mobility particle sizer (SMPS) — the primary quantitative readout behind the results below.

Measured particle size distribution from the SMPS — number concentration versus mobility diameter. — A measured SMPS particle size distribution — how the catalyst population was quantified run to run.

Change in geometric mean diameter relative to no sulfur, versus the sulfur-to-iron molar ratio, for argon and hydrogen carriers. — Change in geometric mean diameter versus sulfur-to-iron ratio — in hydrogen, dosing sulfur suppresses coagulation and shrinks the particles.

TEM micrograph showing single-walled carbon nanotubes, some bundled, against a region of amorphous carbon, with a 20 nm scale bar. — TEM of the carbon product — resolving single-walled nanotubes against amorphous carbon.

Representative Raman spectrum with labelled radial-breathing mode, D-peak and G-peak. — A representative Raman spectrum — the RBM, D and G bands used to identify single-walled tubes.

Results

Model-guided diagnosis. Comparing the model against measurements let me trace an unexpected rise in particle number concentration to the carrier gas — hydrogen was chemically reducing impurities in the reactor tube into background particles that seeded nucleation. The discrepancy became a diagnosis rather than an error.
Controlling coagulation with sulfur. Dosing a small amount of sulfur measurably suppressed particle coagulation and reduced the mean particle size — a controllable knob for tuning the catalyst population.
High-value product. The generator grew carbon nanotubes with quality/crystallinity that is consistent with high tensile strength and electrical conductivity.