Research Interests and Equipment
Catalytic Performance, Steady-state and Transient Kinetic Studies
In situ investigation of gas/solid interactions using step-gas concentration switches and temperature-programmed techniques to study kinetics and mechanistic aspects of heterogeneous catalytic reactions and adsorption/desorption processes are conducted in this home-made apparatus. Techniques used in our laboratory are as follows:
• SSITKA-Mass Spectrometry (Balzers, Quadrupole 1-300 amu) methodology for the quantification of the active reaction intermediates and inactive (spectator) species.
• H₂ or CO-TPD and gas-pulse chemisorption for the determination of the particle size and dispersion of metal in supported metal catalysts.
• NH₃- and CO₂-TPD determination of surface acidity and basicity of solids
• Temperature-programmed (TPR) and Transient Isothermal Reduction (TIR) to investigate redox properties/kinetics of solids.
• ¹⁶O/¹⁸O Transient Isothermal Isotopic Exchange (TIIE) for the estimation of transient kinetic rates of surface and bulk oxygen exchange/diffusion along with the estimation of important kinetic parameters of the exchange process (e.g. Deff (cm²/s), Eₐₚₚ).
• Steady state kinetic experiments free of mass/heat transport effects allow detailed micro-kinetic modeling or kinetic rates coupled with apparent activation energies and orders of reaction.
In situ DRIFTS and Operando (DRIFTS-Mass Spectrometry) Methodology
In situ DRIFTS-SSITKA methodology is used to probe the chemical nature/composition of active reaction intermediates and inactive/spectator species formed during the reaction on the catalytic surface. Also, in situ DRIFTS (Perkin Elmer, Frontier model) coupled with step-gas concentration switches allows to perform transient isothermal reaction studies, where modeling of the time resolved IR spectra allows obtaining important kinetic information about the reactivity of adsorbed species related to a given reaction path. Furthermore, combination of DRIFTS and MS (Operando methodology) in the transient mode allows the recording of both gas-phase and surface dynamics for better description of reaction kinetics and mechanism. Use of Gas/Liquid feeds streams is possible (e.g., WGS, Steam Reforming Reactions).
Catalytic Measurements and Transient Kinetic Studies Using Gas/Liquid Feed Streams
This home-made experimental set-up allows to perform catalytic measurements and transient kinetic studies with online Mass Spectrometry using Gas/Liquid feed streams.
In situ UV–vis/DRS Methodology
In situ UV–vis/DRS (Perkin Elmer, Lambda 950 model) coupled with Diffuse Reflectance cell (Harrick Praying Mantis) allows obtaining information on the coordination environment, metal oxidation states and energy band gap of metal oxides semiconductors. It allows also obtaining oxidation states of a metal-oxide - supported metal. UV–vis/DRS can give structural information for solids of very small primary particle size (< 4 nm) and low content (
Powder X-ray Diffraction
Powder XRD (Shimadzu 6000 Series diffractometer) is used for the determination of crystallinity (amorphous or crystal phase(s)) and microstructural parameters (e.g., mean primary crystallite size, lattice strain) of powdered solids. In situ XRD is also performed using commercial environmental chamber (up to 900 ºC) and a home-made gas flow-system.
Textural Analysis of Porous Solid Catalysts
Determination of specific surface area, pore size distribution, and pore volume of the solids by N₂ physical adsorption/desorption isotherms and using the BET and BJH methods.
Preparation of Gas Mixtures
This apparatus is used to prepare with accuracy better than 2% gas mixtures from pure gas components or other rich gas mixtures in the given components of interest. Vacuum system and Baratron absolute pressure transducers are used for this purpose.
Transient Techniques
Steady-State Isotopic Transient Kinetic Analysis (SSITKA)
This technique aims at the use of stable isotopes in heterogeneous gas/solid catalytic reactions that permits in a direct way the in situ evaluation of forward and reverse reaction rates of individual elementary reaction steps and the surface concentration, mean life-time of truly active reaction intermediates. A more accurate site activity determination, TOFITK (s⁻¹) based on the concentration of these active intermediates is then possible. A step-gas concentration switch from the non-isotopic to the equivalent isotopic feed gas or vice versa is made at constant T, P and flow rate. Based on the recorded reactants/products gas responses by mass spectrometry and appropriate mass balance relationships, the above-mentioned kinetic quantities can be estimated.
In the graph shown, a SSITKA switch ¹²CO₂/¹²CH₄ → ¹³CO₂/¹²CH₄ during Dry Reforming of Methane over a supported Ni catalyst allows to follow the CO₂ activation path towards CO formation, where active carbon (CO₂ + C ↔ 2 CO) and inactive adsorbed CO₂ can be estimated.
¹⁶O/¹⁸O Transient Isothermal Isotopic Exchange (TIIE)
This technique considers the step-gas switch x vol% ¹⁶O₂/Ar → x vol% ¹⁸O₂/Kr/Ar performed over a metal oxide or supported metal catalyst (OSC materials) aiming to measure the dynamics of bulk oxygen diffusion and surface diffusion/exchange under equilibrium conditions between gas-phase and lattice oxygen in the solid phase. A novel analysis of the measured ¹⁶O₂, ¹⁶O¹⁸O and ¹⁸O₂ gas-phase transient responses recently reported (Topics in Catalysis 62 (2019) 219-226) allows via quantitative descriptors the assessment of differences in the kinetics of bulk oxygen diffusion over a series of commercial and lab-synthesized mixed metal oxides without the need of performing any mathematical and kinetic modelling of bulk/surface oxygen diffusion processes.
Transient Operando Methodology (Mass Spectrometry – DRIFTS)
The investigation of kinetics of surface catalytic reactions is followed under dynamic conditions using an operando methodology (Mass Spectrometry – DRIFTS), where both the transient responses of adsorbed species and those of gas phase reaction products are followed. As an example, the kinetics of methanation reaction over various types of adsorbed CO-s formed on an alumina-supported Co surface can be investigated and rival mechanisms can be examined. Following catalyst treatment with 5%CO/10%H₂/Ar (230 °C, 1 h) and Ar purge, the catalyst (placed in a DRIFTS reactor cell) is exposed to 50% H₂/Ar (230 °C), where the IR band of CO-s (a) and CH₄ formation response (b) are followed simultaneously by the DRIFTS and Mass Spectrometer, respectively. Deconvolution of the IR band of CO-s (c) allows to obtain time resolved IR spectra regarding specific kinds of adsorbed CO-s (d). Micto-kinetic simulations can lead to the estimation of site activity of hyrogenation of CO-s (Journal of Catalysis 379 (2019) 60-77).
Transient Isothermal Hydrogenation (TIH) Technique
The reactivity of adsorbed reaction intermediates towards hydrogen over a solid catalytic surface is studied under isothermal dynamic conditions based on a sequence of step-gas concentration switches. Such a technique is applied to hydrogenation reactions of compounds in gaseous feed streams (e.g. CO/H₂, N₂/H₂, olefin/H₂) and to various NOx control catalytic reactions (e.g. NO/H₂/O₂ (H₂-SCR), NSR).
In the graph shown, the transient kinetics of hydrogenation of various N-containing adsorbed species formed under NO/O₂ gas treatment of a catalytic surface is investigated by the sequence of step-gas switches He → NO/O₂/He → x vol% H₂/He. During the first gas switch, the transient kinetics of NO adsorption and reaction (e.g. NO₂/N₂O formation) can be probed, whereas in the second gas switch the reactivity of adsorbed NOx and N-containing intermediates (formed during the first gas switch) towards hydrogen is probed.
Temperature-Programmed Desorption (TPD) Technique
TPD technique allows to study the adsorption site distribution on a solid surface for a given adsorbate gas (e.g., H₂, CO, CO₂, NO, O₂, NH₃) and the associated energetics considering the use of a gradientless micro-reactor and under dynamic gas desorption conditions far from equilibrium (small rate of readsorption within the porous solid). Appropriate design of gas-flow and quartz micro-reactor of CSTR behavior systems are used (see Journal of Catalysis 219 (2003) 259-272).
In the graph shown, different hydrogen adsorption states over a supported metal catalyst are probed.
Temperature-Programmed Surface Reaction (TPSR) Technique
The reactivity of adsorbed reaction intermediates towards a given reactant is studied under conditions of increasing (linear mode) catalyst’s temperature (temperature-programmed surface reaction) in a reactive gas atmosphere (e.g. H₂-TPSR). In the example given in the graph, the CH₄ transient response vs T was recorded after a given time in CO/H₂ reaction over a supported Co catalyst following the sequence of step gas switches: CO/H₂ (230 ⁰C, 2 h) → He (30 s) → cool down in He flow to 100 ⁰C → 5%H₂/He, T is increased to 600 ⁰C (β=30 ⁰C/min).
The various peaks recorded reflect different kinds of carbon formed under the CO/H₂ reaction.
