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From theory to practice: TG/DTA as a tool in catalysis research
Catalysts are at the heart of many industrial processes – whether in chemical synthesis, exhaust gas aftertreatment or energy storage. Their ability to accelerate chemical reactions without being consumed makes them a central object of investigation in materials and process research. Particularly relevant for laboratory practice is the question of how the thermal stability, redox behaviour and structural changes of a catalyst can be characterized – because these have a significant influence on its activity in later use.
This is where thermal analysis methods such as thermogravimetry (TG) and the differential thermal analysis (DTA) come into play. Although catalytic reactions themselves cannot be analyzed directly with an STA it is possible to precisely analyze process-related changes in the catalyst material – such as dehydration, decomposition, oxygen release or changes in the oxidation state. This information provides valuable information on thermal stability, activation potential and ageing behavior.
Modern simultaneous thermal analysis devices that combine TG and DTA enable detailed investigation of such effects in a wide temperature range and under defined atmospheric conditions. For example, reaction peak shifts, onset temperatures or mass losses can be measured, which allow conclusions to be drawn about changes in the material – e.g. during thermal ageing, oxidation or reduction.
Studies by Bouba et al [2] and Duan et al [1] have shown that TG/DTA can even provide indirect indications of the reactivity and deactivation tendency of a catalyst – especially by comparing fresh and aged samples or by combining them with other methods such as mass spectrometry.
This article therefore focuses on the extent to which TG/DTA can contribute to the evaluation of thermally relevant properties of catalysts – with a particular focus on materials that must remain stable during thermal or oxidative use.
Thermogravimetry (TG): Principle, structure and use in catalysis research
Thermogravimetry (TG) is an established method for analyzing mass changes under controlled thermal conditions. In catalysis research, it is primarily used to track thermally induced processes such as decomposition, desorption or oxidation/reduction – before and after the catalyst is used.
Measuring principle and system architecture
TG systems combine a temperature-stable microbalance with an oven system and defined atmosphere control. The sample is continuously weighed during a specified temperature program. The change in mass – usually in the submilligram range – results from:
- Degassing of adsorbates (e.g. H₂O, CO₂),
- Decomposition of organic or inorganic components,
- Oxygen release or absorption in redox materials.
The balance is thermally decoupled to minimize drift with increasing temperature. Optionally, gas developments can be analyzed using coupling techniques such as TG-MS or TG-FTIR, which is particularly relevant for reaction mechanisms.
Application for catalytic converters
For catalytically active materials, TG allows, among other things:
- the distinction between reversible and irreversible mass change,
- the evaluation of coke formation after catalytic use,
- the detection of material loss due to volatilization of active components,
- the characterization of ageing or thermal deactivation.
A comparison between fresh and aged samples under identical conditions is essential for differentiating statements. In this context, the TG curve provides more than just thermal stability data – it shows the extent to which the material has changed as a result of the process.
Differential thermal analysis (DTA)
Differential thermal analysis (DTA) measures the temperature difference between a sample and a reference with the same temperature. As long as no reaction takes place, the signal remains at zero – both crucibles heat up synchronously. Only when the sample absorbs or releases more or less heat than the reference as a result of a reaction does a measurable difference arise: the sample becomes either warmer or colder, causing the system to register a positive or negative signal – depending on the convention of the device.
Reactions that release heat (e.g. oxidations) generate exothermic peaks, while endothermic processes (e.g. melting or dehydration) lead to inverse peaks. The shape, position and area of these peaks allow conclusions to be drawn:
- the reaction enthalpy (during calibration),
- Phase transitions and their reversibility,
- Indications of reaction rate and material distribution.
In combination with TG, thermal events can be clearly assigned to specific mass losses – for example in the oxidation of adsorbates or the decomposition of precursors. In catalysis research, DTA thus provides specific energetic signatures that allow conclusions to be drawn about material changes or activity processes.
Simultaneous measurement with STA devices
Simultaneous thermal analyzers combine TG and DTA in one system. The simultaneous measurement of mass and heat flow allows complex reaction processes to be resolved precisely. Features of the device:
- High resolution and measuring accuracy up to 1600 °C,
- Temperature change rates up to 50 K/min,
- Various possible measuring atmospheres (e.g. air, oxygen, nitrogen, argon),
- High sensitivity with small sample quantities.
Especially in thermally activated processes , DTA can be used to indirectly infer reaction dynamics: parameters such as onset temperature, peak shape and peak area can provide information on energetic activation, reaction rate and the extent of conversion. For example, Bouba et al [2] show that differences in the reaction behavior of hydrocracking catalysts can be identified via the shift and shape of the DTA peaks – especially when comparing fresh and aged samples.
Reaction rate and reaction enthalpy: thermal analysis as an indicator
In practical applications, researchers therefore often use characteristic parameters from the DTA curve – such as the onset temperature or the peak area – to compare different sample states with each other. Although these parameters do not allow a direct determination of the reaction rate in the kinetic sense, they do provide information on whether thermally activatable processes have been weakened or postponed by ageing, poisoning or structural changes. An example of this is provided by the work of Bouba et al [2], in which differences in the thermal behavior of fresh and aged hydrocracking catalysts were demonstrated by shifts in the exothermic DTA peaks.
Recognize reaction enthalpy
Exothermic reactions – such as oxidations on the catalyst surface – generate characteristic peak structures in the DTA or DSC curve. Depending on the device configuration and convention, an exothermic event appears either as an upward or downward signal. The decisive factor is that the area under the peak is proportional to the reaction enthalpy released (ΔHr), provided that calibration with a suitable standard (e.g. indium or zinc) has been carried out.
In a study on methane oxidation on Co₃O₄ catalysts, Duan et al [1] demonstrate how the thermal signature of this reaction can be captured using DTA and DSC. It was particularly striking that the peak area – and thus the measured enthalpy – depended strongly on the amount of catalyst. This is interpreted as an indication that an energetic saturation behavior of the active centers occurs at certain loadings. The reaction enthalpy per gram of catalyst decreases when active sites are exhausted or blocked by side reactions – a phenomenon that can also be interpreted as the onset of deactivation.
In addition, the oxidation occurred in the same temperature range as a mass loss in the TG curve, which allows the thermal event to be assigned to a specific material conversion – presumably through the release of CO₂ and water.
Derive reaction rate
Even though TG/DTA are not direct kinetic measurements, they allow an approximation of the reaction speed by evaluating the peak position (Tmax), peak width and onset temperature (tone). A typical indication of a fast reaction is a sharp, intense DTA peak with a low onset temperature. If peaks shift to higher temperatures, this may indicate reduced reactivity – for example due to catalyst deactivation.
These relationships are also used by the classic Kissinger method, in which the activation energy (EA) – a key kinetic parameter – is determined by varying the heating rate and analyzing the Tmax shift. In combination with mass loss data from the TG, a robust picture of the reaction dynamics is obtained.
An example from practice can be found in the work of Bouba et al. (2015), who use TG/DSC to investigate hydrocracking catalysts. By evaluating the DTA curves and TG mass losses, they determine clear differences in the reaction rate at different activation states of the catalyst. Theclay temperature in particular proves to be a sensitive indicator of incipient deactivation [2].
Recognize loss of activity: Deactivation of catalysts in TG/DTA curves
A key aspect in the evaluation of catalysts in the laboratory is not only their initial activity, but also their stability behavior over time. Thermal analysis – especially in the form of TG/DTA – is an extremely sensitive tool for detecting and quantifying activity losses at an early stage.
Thermal signatures of deactivation
Typical characteristics of an incipient loss of activity can be seen in both the TG and DTA curves. The conspicuous signs include
- Shift of the DTA peaks to higher temperatures: Indication of higher activation barriers and reduced reactivity.
- Reduction of the peak area: Indicates a decreasing reaction enthalpy – often the result of a lower conversion rate.
- Changed TG curves: e.g. flatter mass losses or the occurrence of new stages, for example due to coke formation or structural changes.
A typical scenario is the gradual passivation of active centers by adsorbate overlay or thermal degradation, visible by the gradual decrease in reactivity with repeated temperature cycles. In practice, differences between fresh and aged catalysts can be detected without the need for complex reaction apparatus [1].
Example: Hydrocracking catalysts
In their TG/DSC study on hydrocracking catalysis, Bouba et al [2] demonstrate how deactivation tendencies of catalysts can be tracked on the basis of thermal characteristics – not by measuring the reaction itself, however, but by analyzing the catalyst samples before and after use. Samples are systematically taken during a catalytic process, dried and analyzed in the TG/DSC. This method makes it possible to quantify thermally relevant changes such as coke formation, structural decay or changes in surface chemistry.
In the evaluation, the peak position (Tmax) and the peak area proved to be the most significant parameters:
- A shift of the reaction maximum to higher temperatures indicated an energetically more difficult reaction – for example as a result of blocked active centers.
- A reduced peak area indicated a reduced heat release – possibly due to lower reaction intensity or surface changes.
In addition, new mass losses occurred in the TG curve that were not observed in fresh catalysts – interpreted as indications of pore structure losses, oxidation of residual materials or coke formation. These parameters provide valuable information on the stability of a catalyst under operating conditions – without the catalytic reaction itself having to be measured in the STA.
Reproducibility and cyclical measurements
One advantage of thermal analysis is the ease with which cyclical measurements can be carried out, e.g. via heating/cooling cycles or repeated treatments with reaction gases. These provide reliable information on the stability of the material under realistic conditions. In contrast to purely spectroscopic methods, functional properties can be directly observed here.
Practical example: Thermoanalytical investigation of methane oxidation on Co₃O₄ catalysts (according to Duan et al. [1])
In their study, Duan et al. investigated the oxidative decomposition of methane on cobalt(III) oxide catalysts (Co₃O₄) using TG/DSC analyses. The aim was to quantify the thermal activity and ageing resistance of the catalyst – with particular consideration of the heat of reaction released and the changes in the temperature profile.
Methodology
- Catalyst: Co₃O₄ powder, partially modified with various additives
- Reaction atmosphere: synthetic air (80 % N₂, 20 % O₂)
- Temperature program: linear heating up to 700 °C
- Objective: Comparison of reaction enthalpies and activity curves via TG/DSC
Results
- DSC: Exothermic peaks at approx. 300-350 °C, corresponding to the oxidation of CH₄ → CO₂ + H₂O
→ The peak area increased with increasing methane quantity and catalyst modification – an indication of higher activity or specific heat release - TG: No significant mass loss, as the reaction was gas-phase driven – nevertheless important as a reference signal for parallel processes
- Influence of catalyst modification: Certain dopants shifted the reaction maximum downwards – indicating a lower activation barrier and thus increased activity
Laboratory relevance
This study shows that TG/DSC is also suitable for the qualitative and quantitative evaluation of catalytic gas reactions – provided that the catalyst system generates thermally detectable reaction heat. For laboratory users, this means
- Comparison of catalyst formulations based on reaction enthalpy
- Assessment of changes in activity due to thermal or chemical stress
- Determining the optimum operating temperature ranges
Conclusion and outlook
Thermoanalytical methods such as TG/DTA do not provide direct information about catalytic reaction mechanisms – but they are indispensable tools when it comes to understanding the state and changes in catalysts under the influence of temperature. Especially when it comes to evaluating thermal stability, residues, deactivation and structural changes, they make a contribution that reactor-based measuring systems alone cannot offer.
The analyzed studies show that comparative measurements – e.g. between fresh and aged samples – allow clear statements to be made about performance degradation, coke formation or material degradation. The strength lies in the combination of simple implementation, high sensitivity and the ability to indirectly visualize complex processes.
In future, the focus will be less on the technology itself and more on its targeted application in practice-relevant scenarios: for example, in the development of regenerative catalysts, the characterization of new types of carrier materials or the systematic analysis of ageing processes in long-term operation. Those who use TG/DTA strategically can make valuable contributions to process optimization beyond pure material evaluation – not as a substitute, but as a methodological supplement to reactor testing.
References
[1] Duan, W. et al. Differential thermal analysis techniques as a tool for preliminary examination of catalyst for combustion. Scientific Reports, 13, 11010 (2023). DOI: 10.1038/s41598-023-36912-5
[2] Bouba, L. et al. TGA-DSC: A Screening Tool for the Evaluation of Hydrocracking Catalysts. Open Journal of Applied Sciences, 5, 103-112 (2015). DOI: 10.4236/ojapps.2015.52008
[3] Bhargava, S. K. et al. Additive Manufacturing for Chemical Sciences and Chemical Engineering. Springer Nature Singapore (2024). DOI: 10.1007/978-981-97-0978-6
