Methane pyrolysis produces hydrogen and carbon black without CO₂ emissions. The particle physics should be straightforward: higher temperature means more thermal energy, faster reactions, larger particles. The naive prediction is that primary carbon particle size increases with temperature.
Clark, Adib, and colleagues find the opposite. In shock tube experiments spanning 1850-2450 K, primary particle size decreases with increasing temperature. Hotter produces smaller.
The mechanism is nucleation kinetics. At higher temperatures, the supersaturation of carbon-forming precursors is so extreme that nucleation rates explode — far more nuclei form per unit time, each consuming a smaller share of the available carbon. The total carbon mass is fixed by the methane input; spreading it across more nuclei produces smaller particles. The competition between nucleation rate (which increases with temperature) and growth rate (which also increases but is starved by the nucleation burst) is won by nucleation.
Simultaneously, graphitic nanostructure increases with temperature — the individual particles are smaller but better ordered internally. More thermal energy doesn't make bigger particles; it makes more particles that are individually more crystalline.
The simulations capture the gas-phase chemistry accurately but fail to predict the particle behavior, revealing that current soot formation models handle molecular-scale kinetics well while missing the nucleation-growth competition that determines particle size distributions.
The counterintuitive result: turning up the heat makes the particles smaller and more perfect. The system inverts the naive expectation because nucleation and growth compete for the same feedstock.