Issue 45/2014 - Research Progress on Activated Carbon Modification Technology

Research progress of activated carbon modification technology

□ School of Architecture and Environment, Sichuan University Wang Xuejiao Gong Mengdan Guo Jiaxiu Liu Yongjun Yin Huaqiang

Activated carbon is a porous adsorbent mainly composed of carbon, which is widely used in
flue gas desulfurization because of its high stability, large specific surface area and special surface chemical properties.
Compared with other catalysts, activated carbon also has strong adsorption and catalytic performance, and is resistant to acid and alkali, heat, insoluble in water and organic solvents, sufficient raw materials, easy to regenerate, and is an environmentally friendly adsorbent
.
Activated carbon is widely used in environmental protection, chemical industry, food processing, hydrometallurgy, drug refining, military chemical protection and other fields
.

The strong adsorption performance of the modified
activated carbon material of activated carbon is mainly determined
by its special adsorption surface structure characteristics and surface chemical properties.
The microporous structure of its surface can be changed by some physical and chemical treatments, such as pore size, pore size, etc.
, or the surface acidity and alkalinity of activated carbon can be changed, and certain functional groups can be introduced or removed on the surface of carbon to make activated carbon have certain special adsorption properties and catalytic characteristics
.

1.
The adsorption characteristics of activated carbon by chemical modification method
not only depend on its pore structure, but also depend on its surface chemical properties, which determine the chemical adsorption
of activated carbon.
The chemical properties are mainly determined by the type and number of chemical functional groups on the surface, surface heteroatoms and compounds, so it is of great significance
to chemically modify the chemical structure of the surface of activated carbon to make its adsorption have higher selectivity.

Oxidation modification Oxidation modification is mainly to oxidize the surface of activated carbon with a suitable oxidant at an appropriate temperature, so as to improve the content of oxygen-containing functional groups on the surface and enhance the polarity
of the surface.
At present, HNO3, H2O2, HClO and O3 are
mainly used for the oxidation modification of activated carbon.

Morwski et al.
oxidized activated carbon with HNO3 and found that HNO3 treatment could effectively change the internal pore structure and surface physical and chemical properties of activated carbon, thereby improving the adsorption capacity
.
Vinke et al.
found that the acidic groups on the surface of activated carbon increased greatly after modification by strong oxidant HNO3, the microporous structure collapsed, and the specific surface area decreased, while the oxygen-containing groups on the surface of activated carbon increased after HClO modification with mild oxidation, but the microporous structure and specific surface area did not change much
.
Liu Shouxin et al.
treated activated carbon through ozonation and found that the number of oxygen-containing acidic functional groups and surface acidity on the surface of activated carbon increased after ozonation treatment, and the specific surface area decreased
with the increase of O3 concentration.

Reduction modification Reduction modification is mainly through the reduction agent at an appropriate temperature to reduce the surface of activated carbon, improve the content of alkaline groups on the surface of activated carbon, enhance the non-polarity of the surface, so as to improve the adsorption performance
of activated carbon to non-polar substances.
The method of reducing modification is mainly to obtain more basic groups
by high temperature treatment of activated carbon under innoble gas such as H2 or N2 or impregnation in ammonia.

Gao Shangyu et al.
used H2 modified activated carbon, and found that the oxygen-containing functional groups on the surface of activated carbon after H2 modification decreased, especially the oxygen-containing acidic functional groups were significantly reduced, and most of the acidic functional groups and a small number of alkaline functional groups were decomposed into low molecular products such as CO2, CO and water at high temperature at high temperature and separated from the activated carbon, so the total amount of oxygen-containing functional groups was reduced
.
Wanfucheng et al.
modified activated carbon with ammonia and aniline, and found that some surface negative groups were eliminated after modification, and the adsorption capacity
of Au3+ was enhanced.

Acid-base modification Acid-base modification is the use of acid, alkali and other substances to treat activated carbon, according to the actual needs to adjust the number of functional groups on the surface of activated carbon to obtain high-performance activated carbon
.
Acid-base modification of activated carbon can alleviate the problems of few varieties of activated carbon, low technical content, lack of functionalized special activated carbon, and the commonly used acid-base modifiers are HNO3, H2O2, HClO, HCl, citric acid, NaOH, ammonia, etc
.

Hu Hui et al.
used spherical activated carbon as a carrier, using HCl, H2SO4, HNO3, NH3· H2O and NaOH were modified by acid or alkali, and the results showed that the number of surface functional groups and surface chemical properties of activated carbon changed greatly
before and after acid and alkali modification 。 In the acid modification experiment, with the increase of HCl, H2SO4 and HNO3 concentrations, the total amount of acidic functional groups increased, the acidic functional group content on the surface of activated carbon after HNO3 modification was the highest, the specific surface area and pore volume of activated carbon modified by HCl and H2SO4 increased, and the pore structure of activated carbon after HNO3 modification remained basically unchanged.
In the alkali modification experiment, the base content in the sample increased less after ammonia modification, its specific surface area and pore volume increased, the activated carbon base content increased significantly after NaOH modification, and the specific surface area and pore size of the sample decreased slightly after modification
.

Loaded metal modification The principle of loading metal modification is to make metal ions first adsorb on the surface of activated carbon through the reduction and adsorption of activated carbon, and then use the reducibility of activated carbon to reduce metal ions to elemental or low-valence ions, metal or metal ions have a strong binding force to the adsorbent, thereby increasing the adsorption performance
of activated carbon to the adsorb.

Martyniuk et al.
pointed out that the introduction of Ca2+ and Mo6+ into the surface of activated carbon through ion exchange or complexation can greatly promote the absorption and conversion capacity
of activated carbon materials for SO2.
Carabineiro et al.
found that the catalyst prepared by impregnating activated carbon in a solution containing Cu2+ or V5+ has the best desulfurization effect
.
It is also pointed out that when loaded with Ca, Co, Ni, Mn, Fe and V, Mg and other metals on carbon materials, the catalyst prepared has a very good desulfurization ability
.
The reason is that these metal ions form oxides in different valence states on the surface of activated carbon, and the formation of these oxides can effectively improve the desulfurization activity
of carbon-based catalysts.
These studies have shown that the loading of specific metals with activated carbon as a carrier can improve the removal efficiency
of SO2 in flue gas.

Surface plasmon modification The introduction of alkaline functional groups on the surface of traditional activated carbon is mainly through ammonia impregnation and high-temperature deoxygenation
.
In recent years, studies have shown that the introduction of oxygen-containing and fluorine-containing functional groups into the surface of activated carbon modified by oxygen/nitrogen plasma and CF4 plasma has shown good results
in some special fields.
Low-temperature plasma treatment technology can not only change the surface chemical properties of carbon materials, but also control the interfacial properties of materials, showing broad application prospects
in the surface modification of carbon materials.

The experiments show that a large number of oxygen-containing functional groups are introduced after the activated carbon is modified by oxygen plasma, and the activated carbon modified by N2 plasma gradually decreases the surface oxygen-containing acidic functional groups and gradually increases the nitrogen-containing functional groups with the increase of the surface modification strength of activated carbon, and the nitrogen-containing functional groups gradually increase, and activated carbon
rich in nitro, amine and amide groups is obtained.

2.
Physical modification method
microwave radiation modification Microwave modification is to control the surface chemical composition or element content of activated carbon by adjusting the microwave power and radiation time, so as to adjust the surface chemical properties of activated carbon and improve the adsorption performance
。 It is found that adding microwave treatment to the preparation method of traditional supported catalysts can greatly shorten the preparation time of the catalyst, and can also improve the dispersion of the active components on the support to a certain extent and increase the specific surface area of the catalyst, and microwave treatment can also better control the particle size
of the catalyst.
These changes can improve the catalyst activity
to a certain extent.

Jiang Xia et al.
reported the use of microwave modified activated carbon to study the influence of
microwave power irradiation time and sample particles on the adsorption effect of activated carbon.
The results show that microwave heating can effectively improve the adsorption capacity of activated carbon, and microwave power and irradiation time are the key factors determining the
adsorption performance of modified activated carbon.
Li Bing et al.
showed that microwave heating modification reduced the specific surface area and pore volume of activated carbon, reduced the number of acidic functional groups on the surface of activated carbon, and increased the number of basic functional groups on the surface of
activated carbon.

Ultrasonic modification ultrasonic refers to sound waves with frequencies higher than 20000Hz, which have good directionality and strong
penetrating ability.
It can be used for speed measurement, ranging, cleaning, stone crushing, etc.
, and is widely used in medicine, military, industry, agriculture and other aspects
.
Differences in ultrasonic processing time and its physical parameters have different effects
.
Literature studies have shown that ultrasonic action can improve the loading efficiency of active components, and within a certain range, the activity of the catalyst also increases
with the increase of ultrasonic treatment time.

Yu Fengwen et al.
modified the activated carbon with ultrasonic waves, and found that with the extension of ultrasonic treatment time, the specific surface area and micropore volume of activated carbon decreased slightly
.
Proper ultrasonic action can greatly reduce the ash content on the surface of activated carbon and its interior, reduce the unstable oxygen-containing groups on the surface, and keep the pore structure of activated carbon unchanged, thereby facilitating the loading and dispersion of metals, thereby improving the activity
of the catalyst.


The material of activated carbon has a wide range of sources and huge yields, and when used as a catalyst carrier, it generally has to be modified to obtain a large specific surface area and pore size distribution
.
With the continuous development of industry, especially the improvement of environmental awareness, the application field of activated carbon in various countries will be rapidThe rapid expansion, coupled with the regeneration of activated carbon materials, adapts to the pace of China's development of energy-saving society, and makes the wide application of activated carbon inevitable
.
In the next few decades, the development of activated carbon materials with high specific surface area, pore size distribution and stable surface chemical properties will remain a hot spot, which can significantly improve the adsorption efficiency and improve the adsorption selectivity and catalytic performance, and has broad
development prospects.