Environmental catalysts are broadly defined as all catalysts that can improve environmental pollution. In recent years, environmental protection has become more and more popular, and the research and application of environmental catalysts have become more and more in-depth. The environmental catalysts for processing different reactants have corresponding performance requirements, among which the specific surface area and pore size are one of the important indexes for characterizing the properties of environmental catalysts. It is of great significance to use gas adsorption technology to accurately characterize the physical parameters such as the specific surface area, the pore volume and the pore size distribution of the environmental catalysts for the research and optimization of their performance.
01Environmental protection catalyst
Currently, oil refining, chemical and environmental protection industries are the main application fields of catalysts. Environmental catalysts generally refer to the catalysts used to protect and improve the surrounding environment by directly or indirectly treating toxic and hazardous substances, making them harmless or reducing them, broadly speaking, catalysts capable of improving environmental pollution can be attributed to the category of environmental catalysts. Environmental catalysts can be divided into exhaust gas treatment catalysts, wastewater treatment catalysts and other catalysts according to the direction of application, such as molecular sieve catalysts that can be used for the treatment of exhaust gases such as SO2, NOX, CO2, and N2O, activated carbon that can be used as a typical adsorbent for the adsorption of liquid/gas-phase pollutants, as well as semiconductor photocatalysts that can degrade organic pollutants, and so on.
02 Specific surface and pore size analysis and characterization of environmental catalysts
Catalyst surface area is one of the important indexes to characterize catalyst properties. The surface area of catalyst can be divided into outer surface area and inner surface area. Since the majority of the surface area of environmental catalyst is inner surface area and the active center is often distributed on the inner surface, generally, the larger the specific surface area of environmental catalyst is, the more activation centers are on the surface, and the catalyst has a strong adsorption capacity for reactants, which are all favorable to the catalytic activity. In addition, the type of pore structure has a great influence on the activity, selectivity and strength of the catalyst. Before the reactant molecules are adsorbed, they must diffuse through the pores of the catalyst to reach the active center on the inner surface of the catalyst, and this diffusion process is closely related to the pore structure of the catalyst, and different pore structures show different diffusion laws and apparent reaction kinetics, for example, the strong selectivity of molecular sieve catalysts is due to the fact that the pore size of its pore can only allow a certain kind of molecule to enter the pore to the surface of the catalyst and be catalyzed.
Therefore, it is necessary to characterize the specific surface area, pore size distribution and other performance parameters of environmental catalysts. At present, gas adsorption technology is one of the important methods to characterize the physical properties of materials. Based on the adsorption analysis, the specific surface area, pore volume and pore size distribution of environmental catalysts can be accurately characterized, which can further analyze the catalytic activity, selectivity, diffusion rate of reactive substances, reaction efficiency, etc., and provide accurate characterization of environmental catalysts with better performance.
03 Application Cases of Specific Surface Area and Pore Size Distribution in Characterization of Environmental Catalysts
(1) Characterization of specific surface area and pore size distribution of molecular sieves
As a representative of green catalyst, molecular sieve catalyst has regular and uniform pore structure, strong acid center and oxidation-reduction activity center, large specific surface area and adjustable functional groups, due to the pore size of different orders of magnitude, which only allows molecules with a diameter smaller than the pore size to enter, so the molecular sieve is a high-performance catalyst and catalyst carrier, and it is widely used as adsorption material, ion exchange material They are widely used as adsorption materials, ion exchange materials and catalytic materials. High performance is the most basic requirement and goal of molecular sieve catalyst for catalysis. Catalytic activity requires molecular sieve to have large specific surface area, uniform pore distribution and adjustable pore size. Molecular sieve catalysts can be classified into microporous molecular sieve, mesoporous molecular sieve, macroporous molecular sieve and multistage porous molecular sieve according to the classification of pore size, under certain conditions, multistage porous molecular sieve It is advantageous to play the microporous molecular sieve with good thermal stability and mesoporous molecular sieve with stronger molecular diffusivity at the same time.
CIQTEK EASY-V high-performance microporous analyzer was used to characterize the specific surface and pore size distribution of the molecular sieves. Before testing, the samples were degassed by heating under vacuum at 300°C for 6 hours. As shown in Fig. 1, the specific surface area of the sample was calculated to be 776.53 m2/g by the multi-point BET equation, and the microporous area of the sample was further obtained to be 672.04 m2/g, the outer surface area to be 161.35 m2/g, and the microporous volume to be 0.108 ml/g by t-plot method, which showed that the microporous area of this molecular sieve accounted for about 86% of the total specific surface area. In addition, the analysis of the N2 adsorption-desorption isotherm curve of this molecular sieve (Fig. 2, left) reveals that the adsorption isotherm occurs in the microporous adsorption when the relative pressure is small, and the adsorption amount increases sharply with the increase of the relative pressure, and then the curve is relatively flat after reaching a certain value, which indicates that the sample has abundant micropores. The characterization of the microporous structure of the molecular sieve is the main research direction when the molecular sieve contains abundant micropores. Through the analysis of the microporous pore size in the right panel of Fig. 2, the molecular sieve has a concentrated distribution of microporous pore sizes at 0.47 nm[h1] , and the distribution of molecular sieve's pore size affects the adsorption characteristics, and the characterization of the pore sizes provides a reference basis for the direction of its application in the field of catalysis.
Fig. 1 Specific surface area test results (top) and t-Plot results (bottom) of molecular sieve samples
Fig.2 N2-sorption and desorption isotherm profiles (top) and SF-pore size distribution profiles (bottom) of molecular sieve samples
CIQTEK EASY-V Series Specific Surface and Pore Size Analyzer adopts the principle of static volumetric testing, with fully automated operation, user-friendly operation interface, easy to learn.
