Boudouard Reaction

Concept · committed · confidence 0.92

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The reversible gas–solid reaction CO₂ + C ⇌ 2CO, named after the French chemist Octave Boudouard, who studied the equilibrium between carbon monoxide and carbon dioxide over carbon systematically around 1900. The reaction is endothermic (ΔH° ≈ +172 kJ/mol), meaning it is driven forward (toward CO) by increasing temperature and by Le Chatelier’s principle is also favored by decreasing pressure (2 moles of gas produced from 1 mole). At atmospheric pressure the equilibrium crosses from CO₂-dominant to CO-dominant at approximately 700 °C (‘the Boudouard crossover’); above ~900–950 °C the equilibrium strongly favors CO (>95 mol% CO in the gas phase at 1 atm). Below ~400 °C the reverse reaction (2CO → CO₂ + C, the Biot–Stoney reaction or ‘carbon deposition’ reaction) is thermodynamically favored, but is kinetically sluggish at low temperatures. In ironmaking, the Boudouard equilibrium is the central mechanism by which a carbonaceous charge (charcoal or coke) sustains a CO-rich reducing atmosphere: CO₂ produced when CO reduces iron oxides (FeₓOᵧ + CO → Fe + CO₂) is continuously regenerated back to CO by reaction with solid carbon, closing the loop. This self-regenerating atmosphere is what makes carbon-based shaft furnaces (bloomery and blast furnace alike) thermodynamically efficient as iron smelters.

Aliases

• Boudouard equilibrium
• CO2–C equilibrium
• Carbon gasification equilibrium
• CO2 + C ⇌ 2CO

Domain

Physical chemistry / Thermodynamics / Pyrometallurgy

See also

• Direct Reduction of Iron Oxides
• Ellingham diagram
• Bloomery Iron Smelting
• Carbon monoxide
• Carbothermic reduction

Claims

• The Boudouard reaction (CO₂ + C ⇌ 2CO) was systematically characterized by Octave Boudouard in a series of experiments published in 1900 in the Annales de chimie et de physique. (confidence 0.95; sources: CIT-BOU-01)
  • Well-established historical attribution; confirmed by primary source availability and multiple secondary references.
• The forward reaction CO₂ + C → 2CO is endothermic with ΔH° ≈ +172 kJ/mol at standard conditions (298 K). (confidence 0.92; sources: CIT-BOU-05)
  • Value derived from JANAF data for C(graphite) + CO₂(g) → 2CO(g): ΔH°₂₉₈ = 2×(−110.5 kJ/mol CO) − (−393.5 kJ/mol CO₂) = +172.5 kJ/mol. The value applies to graphite; for amorphous carbon (charcoal) it is slightly lower in magnitude. The JANAF value is the standard thermodynamic reference. Confidence reduced slightly from 1.0 because charcoal is not pure graphite and the difference is modest but non-negligible at high precision.
• At atmospheric pressure (1 atm), the Boudouard equilibrium crosses from CO₂-dominant to CO-dominant at approximately 700 °C; above ~950 °C, CO exceeds 95 mol% in the equilibrium gas phase. (confidence 0.92; sources: CIT-BOU-01, CIT-BOU-03, CIT-BOU-05)
  • The ~700 °C crossover temperature is well-established and consistent across Boudouard’s original data and modern thermodynamic calculations (JANAF). The 95 mol% CO figure above ~950 °C is consistent with Turkdogan (1980) Table 2.1 and JANAF equilibrium constants; given as indicative — the exact temperature for a given mole fraction depends on system pressure and carbon activity. Stated at 1 atm throughout.
• Decreasing pressure shifts the equilibrium toward CO (Le Chatelier’s principle: 2 mol gas produced from 1 mol gas); increasing pressure favors CO₂. (confidence 0.98; sources: CIT-BOU-03)
  • Standard application of Le Chatelier’s principle and ideal gas thermodynamics; no uncertainty about the direction, only about the exact magnitude at specific pressures. Pressure effects are discussed explicitly in Turkdogan (1980).
• In a charcoal- or coke-charged iron smelting furnace, the Boudouard reaction continuously regenerates CO from the CO₂ produced by iron oxide reduction (FeₓOᵧ + CO → Fe + CO₂), creating a self-sustaining reducing atmosphere above ~700 °C. (confidence 0.95; sources: CIT-BOU-02, CIT-BOU-03, CIT-BOU-04)
  • Mechanistic claim based on well-established thermodynamics; confirmed across multiple process metallurgy textbooks. The self-sustaining character requires that the furnace temperature in the carbon bed exceeds the Boudouard crossover, which is ensured by the design of both bloomery and blast furnace charges.
• Below approximately 400 °C, the reverse reaction (2CO → CO₂ + C, sometimes called carbon deposition or the Biot reaction) is thermodynamically favored, but the kinetics are slow enough that carbon deposition from CO at low temperatures is a well-known but slow industrial nuisance rather than a rapid equilibration process. (confidence 0.85; sources: CIT-BOU-03)
  • The thermodynamic direction is clearly established; the kinetic qualification (slow at low temperatures) is based on general knowledge in process metallurgy (Turkdogan 1980 discusses carbon deposition catalysis). Confidence is slightly lower on the kinetic aspect because the rate depends heavily on the presence of catalysts (Fe, Ni) and surface area.
• The Boudouard equilibrium line appears on the Richardson–Jeffes Ellingham diagram, intersecting the iron oxide reduction lines and visually demonstrating the temperature windows within which CO is both thermodynamically stable and capable of reducing iron oxides. (confidence 0.95; sources: CIT-BOU-06)
  • Standard feature of the Ellingham diagram as reproduced in Porter, Easterling & Sherif (2009); confirmed across most metallurgical thermodynamics textbooks.

Needs verification

Boudouard's 1900 original publication title and exact journal volume/page range (non-blocking)

The citation CIT-BOU-01 gives commonly cited bibliographic details, but the original Annales de chimie et de physique article has not been directly verified by reading the source. The specific page range 153–241 appears in secondary literature but could be off. The year 1900 is well-attested.

ΔH° = +172 kJ/mol applies to charcoal (amorphous carbon) vs. graphite (non-blocking)

JANAF tables are for graphite as the reference state. The enthalpy of amorphous carbon (charcoal) differs slightly from graphite (~1–3 kJ/mol difference in carbon standard enthalpy). For the purposes of ironmaking thermodynamics this difference is negligible, but for precise thermochemical calculations a correction should be applied.

Connections

Incoming

• Prerequisite knowledge ← Bloomery Iron Smelting — To correctly manage a bloomery smelt — specifically to understand why the CO/CO₂ ratio in the furnace atmosphere matters, why tuyere placement and bellows rate control temperature relative to the ~700 °C Boudouard crossover, and why charcoal acts as a reductant and not merely a fuel — one must understand the Boudouard equilibrium. Without this, operators cannot reason about why excess air re-oxidizes iron or why the furnace atmosphere must be maintained above ~700 °C in the reduction zone.
• Prerequisite knowledge ← Direct Reduction of Iron Oxides — Direct Reduction of Iron Oxides is conceptually inseparable from the Boudouard reaction: the CO reductant that drives the iron oxide reduction sequence (Fe₂O₃ → Fe₃O₄ → FeO → Fe) is generated and maintained by the Boudouard equilibrium above ~700 °C. Understanding direct reduction requires knowing both the iron oxide reduction thermodynamics and the carbon–CO–CO₂ equilibrium that regenerates the reductant.

Sources

• CIT-BOU-01 · Boudouard, O. (1900) Étude de l’équilibre C + CO₂ = 2CO. Annales de chimie et de physique, 7th series, vol. 19, pp. 153–241. — Boudouard’s original systematic experimental study of the CO–CO₂–C equilibrium at various temperatures. Establishes the eponymous equilibrium. The exact crossover temperature reported varies slightly with experimental conditions; later JANAF and NIST data confirm the ~700 °C figure at 1 atm.
• CIT-BOU-02 · Kubaschewski, O.; Alcock, C.B. (1979) Metallurgical Thermodynamics. 5th ed., Pergamon, Oxford. Chapter 5 and pp. 267–271 for iron oxide reduction and Boudouard equilibrium context.. — Standard high-temperature thermodynamics reference for the ironmaking context. Provides free energy data and equilibrium treatment for the C–CO–CO₂ system and its role in iron oxide reduction.
• CIT-BOU-03 · Turkdogan, E.T. (1980) Physical Chemistry of High Temperature Technology. Academic Press, New York. Chapter 2 (Gas-Solid Reactions) and pp. 5–10 for C–O system equilibria.. — Detailed treatment of CO₂/CO equilibria over carbon at steelmaking and ironmaking temperatures. Turkdogan explicitly discusses the Boudouard equilibrium, temperature dependence, and pressure effects. Widely cited in process metallurgy literature.
• CIT-BOU-04 · Fruehan, R.J. (ed.) (1998) The Making, Shaping, and Treating of Steel. 11th ed., AISE Steel Foundation, Pittsburgh. Vol. 1 (Ironmaking), pp. 52–58.. — Applied ironmaking reference covering the Boudouard equilibrium in the context of blast furnace and DRI shaft furnace operation.
• CIT-BOU-05 · Chase, M.W. Jr. (1998) NIST-JANAF Thermochemical Tables. 4th ed., Journal of Physical and Chemical Reference Data, Monograph No. 9, NIST, Gaithersburg, MD.. — Authoritative thermodynamic data source. Provides ΔH°, ΔG°, and equilibrium constants for CO₂ + C → 2CO as a function of temperature. Used here to confirm ΔH° ≈ +172 kJ/mol and the ~700 °C crossover at 1 atm.
• CIT-BOU-06 · Porter, D.A.; Easterling, K.E.; Sherif, M.Y. (2009) Phase Transformations in Metals and Alloys. 3rd ed., CRC Press, pp. 290–295. — Reproduces the Richardson–Jeffes Ellingham diagram with the Boudouard equilibrium line and CO–CO₂ mixing lines overlaid, showing graphically the temperatures at which CO or CO₂ dominates and where iron oxide reduction by CO is thermodynamically favored.