Key indicators

Gravimetric method

Volumetric method

Quantitative method

The adsorption amount is determined by weighing the weight change before and after adsorption, referred to as "gravimetry".

According to the pressure change before and after adsorption in a certain volume, the adsorption amount is calculated according to the "ideal gas state equation", which is referred to as "volumetric method" or "volumetric method" for short.

Core quantitative components

Microbalance

The accuracy of the weight sensor is usually 1-2 orders of magnitude higher than that of the pressure sensor.

Pressure sensor

The reading accuracy of one thousandth is the highest accuracy of the pressure sensor, but the reading accuracy is lower than that of the microbalance。

Types of main adsorbates

Organic vapor, water vapor, gas.

The quantitative method does not depend on the ideal gas equation of state, but only on the weight change, so it can not only test the gas adsorption, but also has a congenital advantage in terms of vapor adsorbates.

Gas

Because the ideal gas equation of state has a narrow quantitative range for steam and a large error, the volumetric method is only suitable for gas quantification. For steam with a large difference from the ideal gas, the quantitative error is large.

Adsorption kinetics analysis

Yes
Isobaric adsorption rate data can be obtained, and adsorption kinetics analysis of gas and steam, water activity analysis, etc. can be carried out.

No

As the quantitative analysis is based on the pressure change before and after adsorption, the isobaric adsorption speed data cannot be obtained, and the adsorption kinetics analysis cannot be carried out. Only the pressure swing adsorption speed curve can be given.

Degassing pretreatment

The "thermogravimetric" curve of the relationship between temperature, weight and time in the degassing pretreatment process can be obtained, which can accurately know whether the sample is constant weight, so as to know whether the treatment is "clean"。

A certain degassing time can only be set according to experience. Whether the specific sample is degassed "clean" is unknown. (Generally, the degassing time shall be increased as much as possible under allowable conditions, and the method of reducing efficiency shall be adopted to ensure the degassing effect.)

Whether the temperature distribution is tested

No

Weigh directly, the quantitative is independent of the temperature area, and the error factor is small.

Yes

Since the amount of "remaining" gas in each temperature zone is needed to know the adsorption amount of the sample, the distribution of temperature zone needs to be tested, which has many error sources.

Key indicators

Vacuum method

Dynamic method

Introduction to the method

Place the adsorbent sample in a vacuum environment, let the adsorbate vapor volatilize into the vacuum system and control it under the specified partial pressure P/P0, and continuously obtain time weight data until the adsorption is balanced;

In this process, the sample is first in a vacuum environment, and the adsorbate vapor is not flowing, but is "static" adsorbed, so it is also called "static method" or "vacuum method" vapor adsorption.

Vacuum gravimetry is an ideal physical adsorption analysis method with strong functions. No carrier gas is required, and there is no factor affecting the adsorption process. It is a research level analytical instrument with high data reliability; This method appeared later than the "dynamic method", with higher technical requirements.

The adsorbent sample is placed in an atmospheric environment with mobile carrier gas balance, so that the mixed gas of carrier gas and adsorbate vapor flows through the sample, and the time weight data is continuously obtained until the adsorption is balanced;

In this process, the adsorbate steam flows "dynamically", so it is called "dynamic method" steam adsorption.

The dynamic gravimetric method, which was applied earlier, was developed from the way of "balance+constant temperature and humidity box" built by early researchers. It can obtain the vapor adsorption data of gravimetric method in a relatively simple way. Because the instrument does not need a vacuum system and the instrument structure is simple, it has become a common method in the early vapor adsorption industry and has been used up to now.

Sample pretreatment

"Vacuum degassing" mode, high efficiency

Remove the water, air and other "impurity" gases on the surface of the sample to be tested by means of "heating and vacuuming"; This pretreatment method is called "degassing";

Because it can be heated for vacuum degassing, the samples with strong adsorption capacity, such as microporous materials, molecular sieves, activated carbon and other large surface samples, have excellent treatment effect. The pretreatment temperature can be as high as 400 ℃, and the treated samples do not have the problem of secondary pollution;

The sample surface treatment is "clean", which is the basis of correct test data.

"Atmospheric purging" mode, low efficiency

The sample is pretreated by "heating and blowing the dry carrier gas through the sample to be tested", which is called "purging";

Pretreatment of samples by carrier gas purging; The highest pretreatment temperature is around 200 ℃, which is difficult to remove gas impurities such as water in micropores;

If the auxiliary method of vacuum oven drying is adopted, the sample is easy to fly because there is no anti pumping method; In addition, when the sample is loaded after treatment, the sample is again exposed to air, and the treatment effect is reduced.

The sample pretreatment is not "clean", and the correct test results are not guaranteed.

Steam inlet mode

   After the sample is heated and vacuum degassed, the sample chamber is in a vacuum environment, and the vapor adsorbate evaporates from the liquid in the reagent tube to the sample chamber and becomes vapor, which is absorbed by the sample; P/P0 partial pressure control is realized by controlling steam pressure.

This mode has high accuracy of partial pressure control (error is less than 0.1%) and wide control range of partial pressure (0~99%);

The sample is in the normal pressure environment, and the carrier gas carries the vapor adsorbate through the sample dynamically and is adsorbed by the sample; P/P0 partial pressure control is realized by controlling the proportion of carrier gas and steam.

This method has relatively low control accuracy (error 1%) and narrow control range (2~90%);