RU2367595C2 - Porous carbon nanomaterial and method thereof - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: porous carbon nanomaterial with specific surface area up to 750 m²/g which main structure element are interleaved porous nanosized graphene spheres with inner hollow having size 0.5÷2.0 nm is obtained by oxidation of the inner shells of carbon onion structure at temperature 550-750°C in the flow of the dried air duluted with inert gas or in the mixture of carbon dioxide and inert gas.

EFFECT: method improvement of porous carbon nanomaterial preparation.

2 cl, 6 dwg, 6 ex

Description

The invention relates to the field of production of nanomaterials that can be used as sorbents with high selectivity to sorbed molecules of a certain size; for the transport of drugs in living organisms; for incorporating inorganic nanosized particles and crystals into carbon shells (including metals, oxides, semiconductors, etc.) to obtain composite materials for various purposes.

An analogue of this material is sibunite, developed at the Institute of Catalysis of the SB RAS, obtained by burning the amorphous component of carbonized soot (US 4978649, СВВ 31/10, B01J 20/20, 12/18/1990) and the natural mineral shungite, which contains fullerene-like hemispheres as one of the components sizes from 15 to 100 A (US 7239261, H05K 9/00, 02.17.2005).

Shungites, as materials of natural origin, have an uncontrolled particle size and morphology of graphene formations, with pore sizes varying widely. In addition, they contain a large number of mineral components, which leads to a significant change in the quantitative and structural characteristics from field to field and ultimately also affects the selectivity of adsorption of various compounds.

The closest in its characteristics is a carbon-carbon composite material - sibunit (US 4978649, СВВ 31/10, B01J 20/20, 12/18/1990; Pat. RU 1706690, СВВ 31/10, 01/23/92). This material has a mesoporous structure with a specific surface area of 80-600 m ² / g, a pore volume of 0.2-1.7 cm ³ / g, and a diameter in the range of 20-200 nm. This is the reason that sibunit is not applicable for the selective adsorption of molecules having a small size.

The invention solves the problem of developing a method for producing a porous carbon nanomaterial, the main structural element of which is porous nanoscale graphene spheres embedded in each other, with an internal cavity of 0.5 ÷ 2.0 nm in size

Drawing captions

Figure 1. The scheme for obtaining porous carbon nanomaterial by selective burning of the inner carbon shells of the onion structure - ULS.

Figure 2. High-resolution electron microscopic image of the initial carbon sample of the onion-shaped structure of the ULS obtained by thermal annealing of nanodiamonds in vacuum at a temperature of 1900 K. Dark contrast lines correspond to the projections of graphene planes oriented perpendicular to the picture plane and parallel to the electron beam of the transmission microscope. The distances between graphene planes of 0.34–0.36 nm slightly exceed the interlayer distance characteristic of hexagonal graphite (d ₀₀₀₂ = 0.3354 nm).

Figure 3. The data of differential thermal analysis (heating in air flow, DTA and DTG curves) for a ULS sample obtained at an annealing temperature of 1900 K. The burning onset temperature of the sample is 605 ° C. This temperature is used for the controlled burning of the inner shells of a given sample.

Figure 4. Comparative high-resolution electron microscopy images of the ULS samples before (a) and after (b) the oxidation. Dark contrast lines correspond to the projections of graphene planes oriented perpendicular to the plane of the figure and parallel to the electron beam of the transmission microscope. After the oxidative treatment of the ULS, one can see the formation of an increased number of shell defects, as well as the absence of several internal shells.

Figure 5. Porometry data (pore size distribution — dV (cm ³ / A / g)) obtained by the low-temperature nitrogen adsorption method for ULS samples before and after oxidation at 605 ° С.

6. Nitrogen adsorption isotherm obtained by the method of low-temperature nitrogen adsorption for a ULS sample after oxidation at 605 ° C.

The essence of the invention (see Figure 1) is the selective oxidation of the internal spheres in the primary carbon particles of the onion structure (ULS) obtained by the method protected by US Pat. RU 2094370, СВВ 31/00, 10.27.97, and having an average size of primary particles formed by closed graphene shells embedded in each other with a size of about 0.5-2.0 nm.

When oxidation of an ULS at a temperature of 550-750 ° C in a stream of dried air diluted with an inert gas with a ratio of air: inert gas - nitrogen or argon in the range of ratios 1: 1-1: 5, or in a mixture of carbon dioxide gas with a volume ratio of carbon dioxide : inert gas - nitrogen or argon in the range of ratios 1: 5-1: 10 carbon is removed in the form of volatile products (burning of internal spheres).

Figure 2 presents a characteristic electron microscopic image (TEM) of the initial carbon sample of the onion-shaped structure of the ULF obtained by thermal annealing of nanodiamonds in vacuum at a temperature of 1620 ° C.

The predominant oxidation of the inner shells of ULS is achieved due to the higher reactivity of the inner shells compared to the outer ones, due to their greater surface curvature, greater defectiveness, and greater oxidation reactivity.

To determine the optimum temperature for the oxidation of the inner shells, the method of differential thermal analysis in the atmosphere of air is used, which gives information about the temperatures of the onset of burning out of various samples (550-750 ° C) and the temperature stability of the ULS to air oxygen (Figure 3). When carrying out burning in another oxidizing atmosphere (for example, a mixture of CO ₂ - argon), it is necessary to determine the temperature of the onset of combustion of the ULS in this atmosphere. To prevent uncontrolled combustion of SFM, reaction temperatures are used that are slightly higher than the temperature at which the initial sample burned out and diluted with a mixture of air with an inert gas (for example, nitrogen or argon) or a mixture of carbon dioxide and inert gas. Samples of ULS are kept in a stream of diluted air at a reaction temperature for 1-10 hours. The amount of carbon burned can be analyzed analytically by the amount of carbon oxides emitted or by weight method and can vary from 20 to 80% by weight of the initial ULS.

The obtained samples are characterized using transmission electron microscopy and the method of low-temperature nitrogen adsorption.

As a result of this procedure, ULS particles hollow inside and having a porous shell are obtained. According to the method of low-temperature nitrogen adsorption, which characterizes the pore size distribution, as well as transmission electron microscopy, the formation of internal cavities with a size of 0.5-2 nm, which correspond to the size of the molecules of biologically active substances, is observed. The primary particles are combined into agglomerates with a size comparable to the sizes of the agglomerates of the initial nanodiamond used for the synthesis of ULS due to the formation of common curved graphene sheets simultaneously associated with several ULS particles.

The invention is illustrated by the following examples and illustrations.

Example 1

A portion of the ULS obtained by thermal annealing of nanodiamonds in vacuum at a temperature of 1620 ° С (RU 2094370, СВВ 31/00, 10.27.97) is placed in a flow reactor providing uniform heating and mixing of the treated sample, the reactor is heated in a stream of air diluted with an inert gas (air: inert gas (nitrogen, argon) = 1-1 ÷ 5) to the temperature at which the oxidation of this sample begins, ULS 605 ° C and kept at the reaction temperature for 1 h in order to ensure the necessary degree of burning of the sample (56%).

After that, the obtained material is studied by electron microscopy and low-temperature nitrogen adsorption. Figure 4 shows the electron microscopic images of the original (a) and oxidized (b) particles of the ULS. It can be seen that ULS particles processed at an elevated temperature in an air stream are characterized by an increased number of shell defects, while a larger proportion of particles lack some inner shells.

Analysis using the method of porosimetry for nitrogen shows that after oxidation, the micropore volume increases especially strongly in the diameter range of 0.5-2.0 nm (Figure 5). A 1.5-fold change in pore volume in the region of 0.5-2.0 nm indicates an increase in the number of pores with a corresponding diameter by 3-5 times. Thus, the selective oxidation of the ULS leads to modification of the sample due to partial burning of the carbon shells. The nature of the nitrogen adsorption isotherm (Fig.6) shows the presence of bottle-shaped pores in the region of diameters of 0.5-2.0 nm, which corresponds to the assumption of predominant burning of internal highly reactive shells and the formation of channels with a diameter of 0.5-1.0 nm of such cavities.

Using the BET method, the specific surface area of the ULS samples is determined before and after burning.

The data obtained indicate an increase in the specific surface area of the samples after burning by 1.5 times in comparison with the initial ones (from 425 m ² / g to 722 m ² / g).

Example 2

A portion of the ULS obtained by thermal annealing of nanodiamonds in vacuum at a temperature of 1620 ° С (RU 2094370, СВВ 31/00, 10.27.97) is placed in a flow reactor providing uniform heating and mixing of the treated sample, the reactor is heated in a mixture of carbon dioxide and inert gas with a ratio of the volumes of carbon dioxide: inert gas - argon = 1: 5 to the temperature of the onset of oxidation of this ULS sample of 700 ° C and kept at this temperature for 2 hours in order to ensure the necessary degree of burning of the sample (50%).

Example 3

Analogously to example 1, it differs in that ULS are obtained by thermal annealing of nanodiamonds in vacuum at a temperature of 1450 ° C for 5 hours (RU 2094370, СВВ 31/00, 10.27.97). The processing temperature of the sample in a stream of air diluted with an inert gas - argon, with a ratio of air: inert gas - argon = 1: 5 corresponds to the temperature of the onset of oxidation of this ULS sample according to DTA and is 550 ° C, the oxidation treatment time is 2 hours. Received the data indicate an increase in the specific surface area of the samples after burning by ~ 1.7 times in comparison with the initial ones (from 415 m ² / g to 704 m ² / g).

Example 4

Analogously to example 1, it differs in that ULS are obtained by thermal annealing of nanodiamonds in vacuum at a temperature of 1950 ° C for 3 hours (RU 2094370, С01В 31/00, 10.27.97). The processing temperature of the sample in a stream of air diluted with an inert gas - argon, in the ratio of air: inert gas - argon = 1: 5, corresponds to the temperature of the onset of oxidation of this ULS according to DTA and is 650 ° C, the time of oxidative treatment is 1 hour. The data obtained indicate an increase in the specific surface area of the samples after burning ~ 1.5 times in comparison with the initial ones (from 430 m ² / g to 655 m ² / g).

Example 5

Analogously to example 2, it is distinguished by the fact that ULS are obtained by thermal annealing of nanodiamonds in vacuum at a temperature of 1450 ° C for 5 hours (RU 2094370, С01В 31/00, 10.27.97). The processing temperature of the sample in a stream of carbon dioxide diluted with an inert gas - argon, with the ratio of the volumes of carbon dioxide: inert gas - argon = 1: 5 corresponds to the temperature of the onset of oxidation of this ULS sample according to DTA and is 685 ° C, the time of oxidative treatment is 2, 5 hours. The data obtained indicate an increase in the specific surface area of the samples after burning by ~ 1.8 times in comparison with the initial ones (from 415 m ² / g to 745 m ² / g).

Example 6

Analogously to example 2, it differs in that the ULS is obtained by thermal annealing of nanodiamonds in vacuum at a temperature of 1950 ° C for 3 hours (RU 2094370, С01В 31/00, October 27, 1997). The processing temperature of the sample in a stream of carbon dioxide diluted with an inert gas - argon, with the ratio of the volumes of carbon dioxide: inert gas - nitrogen = 1:10 corresponds to the temperature of the onset of oxidation of this ULS according to DTA and is 750 ° C, the time of oxidative treatment is 1 h The data obtained indicate an increase in the specific surface area of the samples after burning ~ 1.5 times in comparison with the initial ones (from 430 m ² / g to 630 m ² / g).

The technical result is to obtain a material with a given number of internal cavities with a diameter of 0.5-2.0 nm and an increase in the specific surface of the material up to 750 m ² / g.

Claims (2)

1. Porous carbon nanomaterial, characterized in that its main structural element is porous nanoscale graphene spheres embedded in each other with an internal cavity of 0.5 ÷ 2.0 nm in size. 2. A method of preparing a porous carbon nanomaterial, the main structural element of which is porous nanoscale graphene spheres embedded in each other with an internal cavity of 0.5 ÷ 2.0 nm in size, which consists in oxidizing the inner carbon shells of the onion structure at a temperature of 550-750 ° C flow of dried inert gas, dried air or in a mixture of carbon dioxide and inert gas.

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