Chemical adsorption is an analytical method commonly used to study the surface properties of solid materials, especially catalysts. Unlike physical adsorption, which is caused by weak van der Waals forces, chemical adsorption involves strong interactions such as covalent or ionic bonds. This interaction is highly specific, usually irreversible, and only forms monolayer adsorption. The chemical adsorption interaction mainly depends on the chemical properties of the solid surface and the adsorbate.

Chemical adsorption technology is crucial in the field of heterogeneous catalysis, providing information on the number, properties, and strength of active sites on the catalyst surface. This information can be used to optimize catalyst performance, determine metal dispersion, evaluate catalyst adsorption strength, activity, and reactivity, and is a core parameter for catalyst design and performance evaluation.

Various chemical adsorption techniques are widely used for catalyst characterization, including pulse chemical adsorption and programmed temperature analysis (such as TPR, TPO, TPD, and TPSR). In this article, Micromeritics Pt/Al will be tested on a ChemiSorb Auto compact fully automatic chemical adsorption instrument₂O₃Perform pulse chemical adsorption characterization on standard samples.

In pulse chemical adsorption technology, the first step is to remove H₂/Ar mixed gas flows into the sample tube and reduces the sample at high temperature. Maintain constant temperature and switch to inert gas to blow away residual H₂Subsequently, cool the sample to ambient temperature (such as 35 ℃). Finally, select the appropriate adsorbate (such as H) based on the type of active metal₂、CO、O₂Or N₂Inject a known amount of adsorbate from the pulse ring into the sample tube in a pulsed manner until the sample adsorption reaches saturation. After removing the adsorbed gas, the unreacted gas injected by the pulse will enter the thermal conductivity detector (TCD) and form a pulse peak in the signal.

Pulse chemical adsorption is a surface characterization technique widely used to quantify the number of active sites and metal dispersion available for chemical reactions on solid materials, as well as to study the surface area of active metals in certain applications.Choosing the appropriate adsorbent is crucial, with two key selection principles: stoichiometry and binding affinity.

For metals such as Cu and Ag, they have an impact on H₂The adsorption affinity with CO is very low, and almost no adsorption occurs. And adsorbate N₂The strong binding affinity between O and Cu, Ag, etc. makes it a more suitable adsorbate for the chemical adsorption characterization of metals such as Cu and Ag.

O₂Commonly used for hydrogen oxygen titration characterization in pulse chemical adsorption technology. For metals such as Pd, H₂It is easy to form hydrides with CO, so CO is usually more suitable for the chemical adsorption characterization of metals such as Pd. When the catalyst is loaded on a carbon support, H₂The adsorbate may significantly adsorb onto the carbon carrier, resulting in inaccurate results.

Although CO is more suitable for metals such as Pd, it is not applicable to all pulse chemical adsorption experiments. For metals such as Ni and Rh, CO may form carbonyl complexes with them, thereby poisoning the active sites and reducing catalytic activity. Therefore, caution should be exercised when selecting CO as the adsorbent. H₂Both CO adsorbates can be used for pulse chemical adsorption of Pt, as they can both adsorb on the Pt surface. The choice of adsorbate will affect the stoichiometry used in the calculation of metal dispersion. H₂Dissociation adsorption occurs on the Pt surface, with a stoichiometric number of 2; And CO may be adsorbed in linear, bridge or even multiple ways, each corresponding to a different stoichiometric number. For Pt/Al₂O₃The standard sample adsorbs CO in a linear manner, with a corresponding stoichiometric number of 1.

In this article, ChemiSorb Auto was used to analyze 0.5% Pt/Al₂O₃Chemical adsorption characterization of standard samples using H₂The specification range for metal dispersion with CO as adsorbent is 31.2% ± 5%.Figure 1AandFigure 1BCorresponding to the use of 10% H₂/Pulse chemical adsorption spectra of Ar and 10% CO/He as adsorbates. Here we use H₂/Ar mixed gas is due to H₂The thermal conductivity of Ar relative to air is 7.07 and 0.68, respectively, H₂The significant difference between TCD and Ar enables TCD to effectively distinguish unreacted H₂The same principle applies to CO/He mixtures.

In some cases, if 10% H cannot be obtained₂/Ar mixed gas, N can be used₂As a carrier gas. Due to N₂The thermal conductivity relative to air is 1.00, therefore 10% H₂/N₂Mixed gases can also be used for pulse chemical adsorption.

asFigure 1AUsing 10% H₂/Ar serves as the adsorbate, and the first pulse peak displays the injected H₂The adsorbate is completely adsorbed by the sample; The second pulse peak shows that most of the adsorbate is not adsorbed and detected by TCD; When the difference in peak area between the last three pulse peaks is less than the set threshold (5%), it indicates that the sample is saturated with adsorption and there is no need to continue injecting gas.

asFigure 1BUsing 10% CO/He as the adsorbate, the first pulse peak shows that the injected CO adsorbate is almost completely adsorbed by the sample; The second and third pulse peaks indicate that the adsorbate is not completely adsorbed and detected by TCD. When the difference in peak area between the last three pulse peaks is less than the set threshold (5%), it indicates that the sample is saturated with adsorption and there is no need to continue injecting gas.

Integrating the pulse peak area and calculating the cumulative amount of adsorbed gas can obtain information such as metal dispersion, metal surface area, and grain size.

For 0.5% Pt/Al₂O₃The standard sample was analyzed six times on ChemiSorb Auto, and the average metal dispersion and standard deviation were as followsTable 1.

0.5% Pt/Al₂O₃Not all Pt can participate in catalytic reactions with a metal loading of 0.5%. Measuring metal dispersion is crucial for evaluating catalyst activity. For example, H₂/The Ar pulse chemical adsorption results showed a dispersion of 31.39%, indicating that only 31.39% of Pt can be adsorbed by H₂Contact and participate in surface reactions. The remaining Pt may be embedded inside the carrier or encapsulated by the carrier structure, making it unable to participate in catalytic reactions. The preparation method of catalysts has a significant impact on their accessibility. In some cases, active metal particles may be embedded in the carrier, thereby hindering the exposure of some active sites.

ChemiSorb Auto compact fully automatic chemical adsorption instrument can provide valuable data such as the percentage of active species on the catalyst surface. The higher the metal dispersion, the higher the catalytic activity. Accurately testing the activity of catalysts can help users make precise and efficient decisions when expanding product scale or redesigning catalyst performance.

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