Determination of Carbon Deposition (Coke Content) in Catalysts

Determination of carbon deposition (coke content) in catalysts is a core analytical service for refining, petrochemical, and catalysis research laboratories that need to quantify catalyst deactivation and evaluate regeneration performance. Using validated combustion-infrared, thermogravimetric (TGA), and temperature-programmed oxidation (TPO) techniques in accordance with ASTM, ISO, and GB standards, this testing delivers accurate total carbon, hydrogen-to-carbon ratio, and coke characterization data for spent, regenerated, and laboratory-aged catalysts.

Technical Overview

Carbon deposition, commonly referred to as coking, is one of the most important deactivation mechanisms in heterogeneous catalysis. During reactions such as catalytic cracking, reforming, dehydrogenation, oligomerization, and methanol-to-olefins conversion, carbonaceous species accumulate on the catalyst surface, covering active sites, blocking micropores and mesopores, and progressively reducing activity and selectivity. Quantifying the coke deposited on a catalyst is therefore essential for diagnosing deactivation, designing regeneration protocols, validating kinetic models, and controlling the quality of regenerated catalysts.

The coke level on industrial catalysts varies widely depending on the process: hydrotreating catalysts typically carry less than 5 wt% carbon at end-of-run, fluid catalytic cracking (FCC) catalysts often hold 1–3 wt% carbon on equilibrium catalysts, continuous catalytic reforming (CCR) catalysts operate near 3–6 wt% carbon, and methanol-to-olefins or dehydrogenation catalysts may accumulate deposits exceeding 10 wt% before regeneration. Accurate determination of carbon deposition requires controlled sample handling, calibrated instrumentation, and in some cases the ability to distinguish between reactive, hydrogen-rich "soft" coke and refractory, graphitic "hard" coke, because these species burn off at different temperatures and respond differently to regeneration.

Testing Samples

Spent Catalysts from Refining and Petrochemical Units

Representative spent catalyst samples collected from commercial units, including FCC equilibrium catalysts, CCR platforming catalysts, hydrotreating and hydrocracking catalysts, steam reforming catalysts, propane dehydrogenation (PDH) catalysts, and methanol-to-olefins (MTO) catalysts. Samples may be supplied as pellets, extrudates, spheres, beads, or powders, and are typically handled under inert conditions to prevent oxidation of the deposits prior to analysis.

Regenerated Catalysts

Catalysts that have undergone coke burn-off in commercial regenerators or laboratory furnaces. Residual carbon measurement verifies regeneration completeness against acceptance specifications, supports mass-balance closure over the regeneration cycle, and helps identify uneven burn or over-oxidation damage.

Laboratory-Deactivated and Aged Samples

Catalysts deactivated under accelerated aging protocols, thermal cycling, cyclic dehydrogenation-regeneration testing, or bench-scale and pilot-plant reactor campaigns. These samples support research and development programs, lifetime studies, and deactivation kinetics investigations.

Zeolites, Supported Metals, and Structured Catalysts

Coked zeolites (ZSM-5, SAPO-34, Y-type, beta), supported metal catalysts (Pt, Pd, Ni, Cr-based), washcoated monoliths, and crushed or milled fractions. Powdered samples are sieved to a defined particle size range to ensure representative combustion behavior.

Testing Items

Typical measurands and deliverables for catalyst carbon deposition determination include:

• Total carbon content (wt% C) on the as-received, dry, or ash-free basis
• Hydrogen content of the deposits and the calculated H/C molar ratio of the coke
• Residual carbon on regenerated catalysts versus the specified acceptance limit
• Distribution of carbon species by TPO: low-temperature reactive coke versus high-temperature graphitic or refractory carbon
• Coke oxidation onset temperature, peak burn-off temperature, and burn-off profile
• Sulfur and nitrogen associated with the deposits (relevant to hydrotreating and FCC catalysts)
• Ash or residual inorganic mass after complete combustion
• Graphitization degree of the deposit evaluated from Raman D-band/G-band intensity ratios
• Moisture and volatile hydrocarbon content of the spent sample (pre-drying loss)

Testing Methods

Combustion–Infrared Elemental Analysis

A weighed catalyst sample is combusted at approximately 950–1,100 °C in a stream of purified oxygen, often with a combustion accelerator such as tungsten oxide or vanadium pentoxide. The evolved CO2 (and CO converted to CO2) is quantified by non-dispersive infrared detection and converted to carbon content using calibration curves built from certified reference materials such as coal, coke, or calcium carbonate standards. The method is consistent with the principles of ASTM D5373, ASTM D5291, ISO 29541, and GB/T 30733, and provides total carbon down to levels of roughly 0.01–0.1 wt% depending on sample mass and matrix. Simultaneous CHN detection also yields the hydrogen content and H/C ratio of the deposit.

Thermogravimetric Analysis (TGA/DTG)

The sample is heated under nitrogen or argon to remove moisture and volatile hydrocarbons, then switched to air or oxygen for the coke burn-off step. The mass loss attributable to combustion of the carbonaceous deposit, corrected for moisture and volatiles, gives the coke content, while the derivative (DTG) curve identifies the oxidation temperature range of the deposit. TGA is particularly useful for distinguishing adsorbed hydrocarbons from true coke and for comparative screening of regeneration behavior.

Temperature-Programmed Oxidation (TPO)

The catalyst is heated at a controlled ramp rate in a dilute oxygen stream, and the CO2/CO evolution is monitored continuously by mass spectrometry (TPO-MS) or infrared detection (TPO-IR). The resulting oxidation profile resolves multiple carbon species: hydrogen-rich, aliphatic deposits burn below approximately 400 °C, while polyaromatic and graphitic carbons oxidize at 500–700 °C or higher. TPO provides both quantitative carbon totals and qualitative information on coke nature, aging severity, and metal-catalyzed oxidation effects.

Complementary Characterization

Where a fuller picture of the deposit is required, accredited third-party laboratories and research facilities may combine the quantitative methods above with Raman spectroscopy for carbon ordering (ID/IG ratio), X-ray photoelectron spectroscopy for surface carbon speciation, temperature-programmed hydrogenation (TPH) to quantify hydrogenable carbon, and elemental CHNS/O analysis for associated heteroatoms. These techniques support mechanistic studies and advanced regeneration process development.

Applications

Carbon deposition determination is applied throughout the catalyst life cycle and across all major catalytic processes:

• Fluid catalytic cracking (FCC): monitoring equilibrium catalyst carbon, evaluating stripper performance, and optimizing regenerator operation
• Continuous catalytic reforming (CCR): controlling coke build-up within the design window for continuous regeneration
• Propane and butane dehydrogenation: tracking coke accumulation over reaction-regeneration cycles and assessing catalyst stability
• Methanol-to-olefins (MTO) and methanol-to-gasoline (MTG): quantifying coke on SAPO-34 and ZSM-5 catalysts for cycle design
• Hydrotreating and hydrocracking: end-of-run coke measurement supporting decisions on regeneration, reactivation, or disposal
• Steam reforming and hydrogen production: diagnosing carbon formation and carbon whisker growth on nickel-based catalysts
• Catalyst manufacture and regeneration services: quality control and certification of residual carbon content
• Academic and industrial R&D: deactivation kinetics, coke structure studies, and validation of kinetic and reactor models
• Environmental compliance and spent catalyst management: characterization for classification, valuation, and recycling decisions

FAQ

What sample size and detection limit can be expected?

Combustion-infrared analysis typically requires 50–200 mg of representative sample and can quantify total carbon from approximately 0.01 wt% up to over 20 wt%. TGA and TPO methods generally use 10–100 mg, depending on the expected coke level and instrument configuration.

How should spent catalyst samples be submitted?

To prevent oxidation or loss of volatile deposits, spent catalysts should be cooled under inert gas, stored in sealed containers (ideally under nitrogen or argon), and shipped promptly. Samples should be clearly labeled with unit origin, sampling date and location, and any known process history.

Why do TGA and combustion-IR results sometimes differ slightly?

Combustion-IR measures evolved carbon directly and is largely unaffected by concurrent mass changes such as moisture loss, sulfate decomposition, or metal oxidation. TGA infers coke from mass loss and can be biased by overlapping phenomena such as volatile hydrocarbon release or mass gain from oxidation of reduced metals. Cross-checking both methods improves confidence in the reported value.

Can the test distinguish reactive coke from graphitic carbon?

Yes. TPO profiles and Raman spectroscopy differentiate hydrogen-rich, easily burned deposits from refractory, graphitic carbon, which is valuable when evaluating regeneration difficulty or comparing deactivation severity across catalyst samples.

What standards govern the determination?

Quantification is typically performed in accordance with the principles of ASTM D5373, ASTM D5291, ISO 29541, and GB/T 30733, adapted and validated for catalyst matrices by accredited third-party laboratories and catalysis research laboratories.