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Measurement and calculation of specific surface area

only when the adsorbed particles collide with the empty adsorption center can they be adsorbed. An adsorbed particle only occupies an adsorption center, and the adsorption is monolayer and localized

under certain conditions, the adsorption rate is equal to the desorption rate, reaching adsorption equilibrium

(2) isothermal equation

adsorption rate:

RA (1-) P ra=ka (1-) P

desorption rate Rd rd=kd

when the adsorption equilibrium is reached: Ka (1-) p=kd

Where, =va/vm (VA - adsorption capacity of gas adsorbate; VM - saturated adsorption capacity of monolayer, mol/g), is the fraction of the surface of the adsorbent covered by gas molecules, that is, the coverage

if b= ka/kd, then: = va/vm=bp/(1+bp),

sorting can be obtained:

v = p/VM + 1/bvm

plot p/v ~ p as a straight line, and according to the slope and intercept, the values of B and VM can be calculated (the reciprocal of the slope is VM), Therefore, the specific surface area of the adsorbent is:

sg=vm a m

a Avogadro constant (6.023x1023/mol)

the clearance between the adjusting steel ball and the valve seat is about 0.5mm m. The cross-sectional area of an adsorbate molecule (N2 is 16.2xm2 when the brake handle must be stuck), that is, the area occupied by each nitrogen molecule on the surface of the adsorbent

this formula is applied to: substances with pure micropores; Chemisorption

2. Bet adsorption isotherm equation - BET specific surface (currently recognized as the standard method for measuring the specific surface of solids)

(1) bet adsorption isotherm equation:

the adsorption model of bet theory is based on the Langmuir adsorption model. At the same time, it is recognized that

physical adsorption can be carried out in a multi-layer manner, and the first layer of unequal surface is full, the second layer of adsorption occurs above the first layer, and the third layer of adsorption occurs on the second layer, When the adsorption equilibrium is reached, each layer

reaches its own adsorption equilibrium. Finally, the isothermal equation of C constant in the formula can be derived

because the purpose of the experiment is to require C and VM, it is also called the bet two constant formula

(2) BET specific surface area

the adsorption isotherms of solids are measured experimentally, and a series of adsorption capacity values under different pressures P can be obtained. V

p/p is plotted as a straight line, the intercept is 1/VM, and the slope is: (C-1)/VMC. Vm=1/(intercept + slope)

the specific surface area of the adsorbent is: sbet= VM a m

this formula is currently most used for measuring the specific surface

absorb celanesecorp. with nitrogen at 77K And C company, an American wire and cable manufacturer, has developed a kind of extruded transmission line made of thermoplastic composites. At this time, the adsorption temperature of 16.225 gas is in the liquid of nitrogen

bet two constant formula. The suitable p/p range is 0.05 ~ 0.2

the specific surface of solids is measured by BET method. The most commonly used adsorbate is near the nitriding point 77.2k. Low temperature can avoid chemical adsorption. The relative pressure is controlled between 0.05 and 0.25 because it is difficult to establish multi-layer adsorption equilibrium when the relative pressure is lower than 0.05; When it is higher than 0.25, capillary condensation is easy to occur

In the bet two constant equation, parameter C reflects the strengthand weakness of the force between adsorbate and adsorbent. C value is usually between 50 and 300. When the BET specific surface area is greater than 500m2/g, if the C value exceeds 300, the test result is suspicious. High C value or negative C value is related to micropores, and BET model

is not suitable for their analysis without correction

(31) B-point method

the first layer of adsorption corresponding to point B reaches saturation, and its adsorption capacity VB is approximately equal to VM, from which the surface area of adsorbent can be calculated

The value ofc is usually between 50 and 300. When the BET specific surface area is greater than 500m2/g, if the C value exceeds 300, the test result is suspicious. High C value or negative C value is related to micropores, and BET model is not suitable for their analysis if it is not modified

(4) single point method

the C constant of nitrogen adsorption is generally between 50 and 300, so the intercept is often very small in bet mapping. Therefore, it can be ignored in the rough calculation, that is, connect an experimental point with p/p0 around 0.20 and 0.25 with the origin, calculate VM value from the reciprocal of its slope, and then calculate the specific surface area

3.v-t mapping method to calculate the specific surface

the empirical layer thickness method (i.e. t-plot method) can also be used to calculate the specific surface area. In some cases, the specific surface of pores with different sizes can be calculated by this method (the total specific surface area of catalyst is calculated by bet and Langmuir methods). V=s T, the specific surface area can be calculated from V and t. The specific method is introduced in the hole distribution later

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