Chemical adsorption is a key surface analysis technique widely used in the field of catalysis to study the surface properties of solid materials. Unlike physical adsorption, which is caused by weak van der Waals forces, chemical adsorption is caused by strong specific interactions, such as covalent or ionic bonds. These effects typically lead to the formation of a single-molecule adsorption layer, which is generally irreversible. Therefore, chemical adsorption has high selectivity and can provide rich surface information.

Physical adsorption is commonly used to determine specific surface area and pore structure, while chemical adsorption technology can provide key information about the number, properties, and strength of active sites on the catalyst surface. These information are crucial for evaluating metal dispersion, adsorption strength, and catalytic reaction activity, and are also core parameters for catalyst design and performance evaluation.

TPR is one of the most commonly used chemical adsorption techniques for characterizing the reducibility of metal oxides. It can provide information on the oxidation state of metal oxides and the strength of metal carrier interactions. Understanding at what temperature metal oxides are completely reduced can help determine the activation conditions for catalysts. In the TPR spectrum, the number of reduction peaks corresponds to different oxidation states, and the peak area can be used to calculate hydrogen consumption.

The working principle of TPR is relatively simple: 10% H₂/Ar mixed gas flows into the sample tube, and during the linear heating process, H₂Reduce metal oxides to metals and generate H₂O. H generated during the reaction₂O is made of liquid nitrogen (LN)₂）Remove using a cold trap composed of isopropanol (IPA). Taking CuO as an example, its reduction reaction is as follows:

CuO + H₂→ Cu + H₂O

Figure 1It is the temperature programmed reduction spectrum of CuO.

Figure 1Programmed temperature reduction spectrum of CuO: active gas concentration vs. temperature

Three analyses were conducted using ChemiSorb Auto, and the results met the specifications specified in the standard material manual.

Table 1The average and standard deviation of three analyses were presented.

The peak shape of TPR spectrum can provide particle size information.Figure 2Display TPR spectra corresponding to two common reduction mechanisms:

• Nucleation model:

For very small and fine particles, H₂Quickly initiate reduction and form the first metal core. As the reaction interface increases, the reduction rate accelerates. The TPR spectrum shows sharp peaks.

• Contracting Sphere Model:

For larger particles, when heated linearly, H₂Firstly, the outer surface of the particles is reduced to form a metal film layer, which then diffuses into the interior and contracts towards the center of the sphere. The reduction process is limited by diffusion, resulting in a slower reduction rate. The TPR spectrum shows wider and higher peaks.

By analyzing the TPR spectrum, qualitative information about particle size can be inferred. In catalytic applications, the ideal situation is to uniformly disperse the metal in the form of small particles on the surface of the carrier, maximizing its accessibility and reactivity in chemical reactions.

TPR can not only provide quantitative data, but also qualitative information about catalyst activation behavior. The reduction peak appearing in the TPR spectrum is usually considered as the activation condition of the catalyst. The reduction temperature itself provides critical information that should be considered in catalyst design.

If a highly dispersed catalyst requires high temperature to activate, there is a risk of sintering. Sintering can lead to a reduction in the active surface area of metals and a decrease in the number of active sites available for reactions, which is a spontaneous process that typically results in a decrease in catalytic performance.

For example,Figure 3The reduction temperature of CuO varies with the loading of Pd. As the Pd loading increases, the reduction temperature decreases, which is advantageous. Low temperature activation helps maintain metal dispersion and reduce sintering risks.

Key conclusion: TPR is an indispensable tool in catalyst characterization, helping researchers evaluate the stability effect of carriers on active components under high temperature and high pressure conditions.

ChemiSorb Auto is equipped with a patented mixing valve that enables automatic gas calibration. The calibration process involves mixing pure argon gas and pure hydrogen gas in 11 steps, H₂The concentration gradually decreased from 10% to 0% to determine the hydrogen consumption.Figure 4It is a typical H₂/Ar gas calibration results.

Gas calibration can be performed before or after analysis. If the analysis conditions (such as gas concentration or flow rate) are different from the previously calibrated conditions, recalibration is required to ensure the accuracy and consistency of the data.

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