In-Situ Reaction Experiments for Organic Sulfur Hydrogenation Catalysts: Principles, Methods, and Ap

In-situ reaction experiments for organic sulfur hydrogenation catalysts provide real-time insight into how Co-Mo and Ni-Mo based catalysts convert COS, CS2, mercaptans, and thiophenes into H2S under authentic process conditions. By coupling operando spectroscopy with fixed-bed activity evaluation, these experiments reveal active phase evolution, surface intermediates, and deactivation pathways that conventional ex-situ testing cannot capture. This article outlines the sample types, testing items, experimental methods, applicable standards, and industrial applications of in-situ reaction testing for organic sulfur hydroconversion catalysts.

Technical Overview

Organic sulfur hydrogenation conversion catalysts are the workhorses of deep desulfurization in natural gas processing, coal-based syngas plants, ammonia and methanol synthesis units, and refinery hydrogen streams. Typical formulations are alumina-supported molybdenum sulfide promoted with cobalt or nickel (Co-Mo/Al2O3, Ni-Mo/Al2O3), supplemented by iron-molybdenum, titania-based, and activated-carbon-supported systems. Their function is to convert refractory organic sulfur species, including carbonyl sulfide (COS), carbon disulfide (CS2), mercaptans (RSH), sulfides (RSR), disulfides (RSSR), and thiophene derivatives, into hydrogen sulfide (H2S), which is subsequently captured by zinc oxide sorbents or amine treating downstream.

The conversion network proceeds through parallel hydrogenation and hydrolysis pathways over the sulfided active phase. The principal reactions involved in an in-situ reaction experiment include the following:

• COS + H2 → CO + H2S (hydrogenation route)
• COS + H2O → CO2 + H2S (hydrolysis route)
• CS2 + 4H2 → CH4 + 2H2S
• CS2 + 2H2O → CO2 + 2H2S
• CH3SH + H2 → CH4 + H2S
• C4H4S (thiophene) + 2H2 → C4H6 (butadiene) + H2S, followed by butadiene + 2H2 → C4H10

An in-situ reaction experiment is designed to observe these conversions while the catalyst is actually working, rather than after it has been cooled, exposed to air, or otherwise altered. Operando spectroscopic cells and high-temperature, high-pressure reaction chambers allow the catalyst surface, the MoS2-type active phase, adsorbed intermediates, and gas-phase products to be monitored simultaneously under controlled temperature, pressure, and feed composition. This approach establishes direct structure-performance relationships, identifies true reaction intermediates, and exposes deactivation mechanisms such as sulfation, coking, and MoS2 crystallite sintering.

Testing Samples

Commercial and Industrial Catalysts

Fresh Co-Mo/Al2O3, Ni-Mo/Al2O3, and related hydroconversion catalysts supplied as extrudates, spheres, or tablets can be evaluated either as whole pellets in fixed-bed micro-reactors or crushed and sieved to 20-40 mesh for spectroscopic cells. Sample quantities typically range from 0.2 g to 10 g depending on the reactor configuration.

Laboratory-Prepared and Developmental Catalysts

Powder samples synthesized by impregnation, co-precipitation, sol-gel, or hydrothermal routes, as well as support materials (gamma-Al2O3, TiO2, activated carbon, mixed oxides), are suitable for in-situ DRIFTS, Raman, and XRD studies of phase evolution during sulfidation and reaction.

Spent, Aged, and Regenerated Catalysts

Catalysts retrieved from industrial service are tested to diagnose residual activity, quantify sulfur uptake, and identify poisons or carbon deposits. Regenerated samples can be re-tested in-situ to verify the recovery of active sites and the reformation of the Co(Ni)-Mo-S phase.

Testing Items

The in-situ reaction experiment generates a comprehensive dataset covering activity, selectivity, stability, and mechanistic information. Core testing items include:

• Conversion of individual organic sulfur species: COS, CS2, methyl mercaptan, dimethyl sulfide, and thiophene
• Overall organic sulfur conversion efficiency and residual total sulfur at the reactor outlet
• H2S yield and selectivity, together with by-product distribution (CO, CO2, CH4, light hydrocarbons)
• Breakthrough sulfur capacity and breakthrough curve shape under simulated feed
• In-situ tracking of active phase formation: oxide-to-sulfide (MoO3 to MoS2) transformation, MoS2 stacking and slab length evolution
• Identification of surface intermediates and adsorbed species: thiocarbonate, formate, sulfate, sulfhydryl groups
• Kinetic parameters: apparent activation energy, reaction order with respect to H2, H2O, and organic sulfur partial pressure
• Deactivation behavior: carbon deposition rate, sulfate accumulation, thermal sintering, and resistance to feed impurities
• Operating window validation: activity as a function of temperature (150-450 °C), pressure (ambient to 3 MPa), GHSV (1,000-10,000 h-1), and steam content

Testing Methods

Sample Pretreatment and In-Situ Sulfidation

Because the active phase is molybdenum sulfide rather than the oxide as shipped, every experiment begins with controlled activation inside the reaction cell. The sample is dried under N2 at 120-200 °C, then sulfided in a flowing H2S/H2 (typically 1-5 vol%) or CS2/H2 mixture while ramping at 2-5 °C/min to 300-400 °C. Temperature-programmed sulfidation profiles, monitored by in-situ XRD or Raman, document the stepwise conversion of oxides through oxysulfide intermediates to the final MoS2 phase promoted at the edges by Co or Ni.

In-Situ DRIFTS Reaction Experiments

Diffuse reflectance infrared Fourier transform spectroscopy is performed in a low-dead-volume, high-temperature reaction cell fitted with IR-transparent windows. The sulfided catalyst is exposed to COS/H2, CS2/H2, or thiophene/H2 mixtures at 200-400 °C while spectra are collected continuously. Observed bands assigned to surface thiocarbonates, formates, and SH groups reveal the hydrolysis versus hydrogenation pathways, and time-resolved spectra under feed step-changes quantify the lifetime of intermediates on active sites.

In-Situ XRD and Raman Studies

A high-temperature gas reaction diffractometer permits crystallographic tracking of phase transitions during sulfidation and reaction, confirming MoS2 formation, detecting sulfate or spinel side phases, and measuring crystallite growth during thermal aging. In-situ Raman complements XRD by resolving characteristic MoS2 lattice modes near 380 and 405 cm-1 and sulfate features near 1000 cm-1, enabling detection of amorphous and poorly crystalline species that XRD may miss.

Operando Fixed-Bed Activity Evaluation

Quantitative activity measurement is carried out in a quartz or stainless-steel micro-reactor loaded with 0.5-2 mL of catalyst. After in-situ sulfidation, a simulated feed containing 10-1,000 ppmv of the target organic sulfur compound in H2, H2/N2, or H2/steam is passed over the bed at controlled GHSV and pressure. The reactor effluent is analyzed online by gas chromatography with sulfur chemiluminescence detection (GC-SCD), flame photometric detection (GC-FPD), or mass spectrometry, with total sulfur cross-checked by ultraviolet fluorescence. Species-level quantification follows ISO 19739 and ASTM D5504 for sulfur compounds in gaseous fuels, ASTM D6228 for GC-FPD analysis, ASTM D5453 for total sulfur, and the GB/T 11060 series for natural gas sulfur determination, with iodometric titration available for H2S. Physical characterization of fresh and spent samples, such as BET surface area per ASTM D3663, supports interpretation of deactivation.

Safety and Quality Assurance

All experiments involving H2S and flammable hydrogen mixtures are conducted in ventilated enclosures with continuous gas monitoring, caustic tail-gas scrubbers, and flashback protection. Calibrated mass-flow controllers, certified gas standards, and reference catalyst runs ensure data reproducibility, and studies can be executed under ISO/IEC 17025-aligned quality systems in accredited third-party laboratories or dedicated research facilities.

Applications

In-situ reaction experiments translate directly into improved catalyst design and more reliable plant operation across the gas processing and chemical industries:

• Natural gas purification: validating COS and mercaptan conversion to protect downstream zinc oxide guards in LNG and pipeline gas service
• Coal chemical and syngas plants: optimizing CS2 and COS conversion in rectisol, shift, and methanation feed protection
• Ammonia and methanol synthesis: confirming ultra-deep desulfurization to sub-ppm levels for nickel and copper-based catalyst protection
• Refinery hydrogen and hydrotreating units: evaluating organic sulfur conversion in recycle hydrogen and light naphtha streams
• Claus tail gas and acid gas treatment: assessing hydrogenation activity for SO2 and COS reduction to elemental sulfur recovery
• Fuel cell feed purification: demonstrating total sulfur removal below 10 ppbv for reformer and stack protection
• Catalyst research and development: mechanistic studies of the Co(Ni)-Mo-S active phase, support effects, promoter optimization, and accelerated aging protocols for lifetime prediction
• Quality control and vendor qualification: benchmarking incoming catalyst lots and comparing alternative suppliers on a common experimental basis

FAQ

What is the difference between in-situ and conventional ex-situ catalyst testing?

Conventional testing measures inlet and outlet concentrations after the catalyst reaches steady state, and characterization is done on a separate, air-exposed sample. In-situ experiments keep spectroscopic or diffraction observation running during the actual reaction, capturing transient intermediates, the live state of the active phase, and real-time structural changes that ex-situ analysis can distort or miss entirely.

Why must the catalyst be sulfided before the reaction experiment?

Co-Mo and Ni-Mo catalysts are supplied in oxide form, which has negligible hydrogenation activity. Sulfidation converts the oxides into MoS2 slabs with Co or Ni atoms decorating the edges, forming the Co(Ni)-Mo-S phase that constitutes the true active site. Performing sulfidation inside the measurement cell guarantees that the observed spectra correspond to the working catalyst rather than an oxidized artifact.

What reaction conditions can be simulated?

Typical windows cover 150-450 °C, atmospheric pressure to approximately 3 MPa, GHSV from 1,000 to 10,000 h-1, hydrogen concentrations from dilute H2/N2 mixtures to pure hydrogen, and steam levels up to several percent. Custom feeds containing multiple sulfur species and trace poisons can be blended to replicate plant conditions.

How are trace sulfur compounds quantified reliably?

Species-selective analysis uses GC-SCD or GC-FPD in accordance with ASTM D5504, ASTM D6228, and ISO 19739, providing ppb-level sensitivity and linear response to sulfur. Total sulfur is confirmed by ultraviolet fluorescence per ASTM D5453, and H2S can additionally be determined by iodometric titration following the GB/T 11060 series. Daily calibration with certified gas standards maintains accuracy.

What deliverables are provided at the end of the study?

Standard deliverables include conversion-versus-temperature profiles, breakthrough curves, time-resolved in-situ spectra with band assignments, phase evolution data, kinetic parameters, and a technical report discussing mechanisms, operating windows, and deactivation risks. Recommendations for plant operating conditions or further catalyst development are included when requested.

How much sample is required and how long does testing take?

Spectroscopic in-situ studies typically require 0.2-1 g of crushed catalyst, while fixed-bed activity runs use 1-10 g of pellets. A single operando experiment with sulfidation, steady-state evaluation, and step-change transients generally takes one to three days; long-duration stability tests of 100 hours or more are scheduled as extended programs.