RU2557229C1 - Catalyst for low-temperature oxidation of carbon monoxide and preparation method thereof - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: disclosed catalyst for low-temperature oxidation of carbon monoxide, which is silver deposited on a silicon dioxide surface in amount of 1-16% of the weight of the catalyst. The catalyst contains silver in the form of nanoparticles with a size smaller than 6 nm, which are uniformly distributed on the surface of mesoporous silica gel with specific surface area of 50-200 m²/g and pore size of 3-60 nm, which is used as a support. The invention also relates to a method of using the catalyst to remove carbon monoxide from air, which is carried out using the catalyst by passing a stream of moist air containing up to 100-115 mg/m³ CO through the catalyst bed at room temperature.

EFFECT: improved catalyst properties.

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Description

The invention relates to the field of heterogeneous catalysis, namely to low-temperature oxidation of CO (carbon monoxide, carbon monoxide), and can be used for air purification systems in closed rooms (for example, a motor vehicle), industrial, office and residential premises.

The catalytic oxidation of atmospheric air pollutants (carbon monoxide, formaldehyde and other volatile organic compounds) to harmless CO ₂ and water has recently attracted special attention as an effective and promising way to purify air. Carbon monoxide is one of the most dangerous and common pollutants. It is formed during the ignition of various materials, forest fires, the operation of internal combustion engines or the untimely closing of the chimney of a heating furnace. The maximum permissible concentration of CO in the air of the working area is not more than 20 mg / m ³ (GN 2.2.5.1313-03), and for residential premises the permissible average daily concentration is 3 mg / m ³ (GN 2.1.6.1338-03). The main difficulty in purifying air from CO impurities is that the catalytic oxidation of CO must occur at room temperature and at a humidity of at least 50%.

The most active in the low-temperature oxidation of CO are supported catalysts containing gold, palladium and platinum as the active component [RF No. 2506988 from 02.20.2014, RF 2339446 from 11.27.2008, RF 2464086 from 10.20.2012]. In literary sources, two main groups of catalysts can be distinguished. The first group consists of catalytic compositions, which are a noble metal deposited on inert carriers, such as Al ₂ O ₃ [X. Zou, S. Qi et al. Activity and deactivation of Au / Al ₂ O ₃ catalyst for low temperature CO oxidation // Catalysis Communications. - 2007. - V. 8. - P. 784-788; A. Satsuma, K. Osaki, M. Yanagihara et al. Activity controlling factors for low temperature oxidation of CO over supported Pd catalysts // Applied Catalysis B: Environmental. - 2013 .-- V. 132-133. - P. 511-518; K. Arnby, A. Törncrona et al. Investigation of Pt / γ-Al ₂ O ₃ catalysts with locally high Pt concentrations for oxidation of CO at low temperatures // Journal of Catalysis. - 2004. - V. 221. - No. 1. - P. 252-261] or SiO ₂ [H. Zhu, Z. Ma, Jason C. Clark et al. Low temperature CO oxidation on Au / fumed SiO ₂ -based catalysts prepared from Au (en) ₂ Cl ₃ precursor // Applied Catalysis A: General. - 2007. - V. 326. - P. 89-99; JL Margitfalvi, I. Borbáth, M. Hegedűs. Low temperature oxidation of CO over tin-modified Pt / SiO ₂ catalysts // Catalysis Today. - 2002. - V. 73. - No. 3-4. - P. 343-353].

The second group is represented by catalysts in which transition metal oxides, for example, cerium oxide, act as a carrier [A. Satsuma, K. Osaki, M. Yanagihara et al. Activity controlling factors for low temperature oxidation of CO over supported Pd catalysts // Applied Catalysis B: Environmental. - 2013 .-- V. 132-133. - P. 511-518; X. Huang, H. Sun, L. Wang et al. Morphology effects of nanoscale ceria on the activity of Au / CeO ₂ catalysts for low temperature CO oxidation // Applied Catalysis B: Environmental. - 2009. - V. 90. - P. 224-232], iron [S. Kudo, T. Maki et al. A new preparation method of Au / ferric oxide catalyst for low temperature CO oxidation // Chemical Engineering Science. - 2010. - V. 65. - No. 1. - P. 214-219; L. Liu, F. Zhou, L. Wang et al. Low temperature CO oxidation over supported Pt, Pd catalysts: Particular role of FeO _x support for oxygen supply during reactions // Journal of Catalysis. - 2010. - V. 274. - No. 1. - P. 1-10], manganese [Q. Ye, J. Zhao, F. Huo et al. Nanosized Au supported on three-dimensionally ordered mesoporous b-MnO ₂ : Highly active catalysts for the low temperature oxidation of carbon monoxide, benzene, and toluene // Microporous and Mesoporous Materials. - 2013. - V. 172. - P. 20-29].

However, such catalyst systems are not sufficiently stable in the presence of CO ₂ and water vapor. The cause of deactivation may be water adsorption, the formation of adsorbed carbonates and hydroxocarbonates, which block the active sites of the catalyst surface. To improve the stability of the catalyst, additional air purification is necessary, which complicates the design of the entire air purification plant. So, in [H. Zhu, Z. Ma, Jason C. Clark et al. Low temperature CO oxidation on Au / fumed SiO ₂ -based catalysts prepared from Au (en) ₂ Cl ₃ precursor // Applied Catalysis A: General. - 2007. - V. 326. - P. 89-99] used a dry mixture based on air with a water vapor content of less than 4 ppm. However, even under these conditions, the Au / SiO ₂ catalyst underwent deactivation (after 20 h of operation, the conversion decreased from 85 to 70%). The authors of another work [Y. Shen, G. Lu, Y. Guo. An excellent support of Pd-Fe-O _x catalyst for low temperature CO oxidation: CeO ₂ with rich (200) facets // Catalysis Communications. - 2012. - V. 18. - P. 26-31] a study was conducted of the stability of the catalyst Pd-FeO _x / CeO ₂ at 25 ° C. It was found that when using a “CO + air” mixture with a water vapor content of less than 10 ppm, the catalyst begins to lose activity after 2 hours of operation.

Due to the above disadvantages and the high cost of catalysts based on Au, Pd and Pt (even with a sufficiently low content), more stable and cheaper silver-containing systems can potentially be used in indoor air purification systems.

The patent [EP 2191884 dated 06/26/2013] describes an Ag / Al ₂ O ₃ catalyst obtained by impregnating boehmite with an aqueous solution of silver nitrate, followed by drying at 100 ° C, annealing the catalyst at 900-1000 ° C in air, and reducing it at 100- 500 ° C in a stream of a mixture of 1% H ₂ / He. However, this catalyst does not have activity in the oxidation of CO at room temperature.

A silver-containing catalyst is known in which active oxide supports, for example CeO ₂ , are used instead of an inert support [US 8360073 from 01.29.2013]. This catalyst was developed as an additive to a cigarette filter to reduce the concentration of carbon monoxide in inhaled smoke, but could potentially be used to purify air. However, the low-temperature activity of this catalyst with a relatively low silver content (10 wt.% In terms of Ag ₂ O) is not high enough. With an increase in silver content up to 40 wt. % (in terms of Ag ₂ O), the low-temperature activity increases, but significantly decreases within a few minutes. To return activity to the initial level, heating of the catalyst layer to 110 ^° C is necessary. It should also be noted that an increase in the silver content in the catalyst composition is economically unprofitable.

Closest to the claimed technical essence is a CO oxidation catalyst based on silver nanoparticles stabilized on mesoporous silica gel, described in [CN 101890349 from 11.24.2010] and selected as a prototype.

The catalyst containing 1-16 wt. % Ag / SiO ₂ , obtained by one-step synthesis using silver nitrate (AgNO ₃ ) as a precursor of silver particles, tetraethoxysilane (Si (OC ₂ H ₅ ) ₄ , TEOS) as a source of silicon, formaldehyde (HCHO) as a reducing agent and dodecylamine (C ₁₂ H ₂₅ NH ₂ ) as a template. The molar ratio of TEOS reagents: C ₁₂ H ₂₅ NH ₂ : C ₂ H ₅ OH: AgNO ₃ : HCHO: H ₂ O = 1: 0.2965: 7.593: 0.0055-0.088: 0.131: 24.58. After completion of the processes of silver reduction and gelation (24-48 h), the resulting product is washed with deionized water, dried at 80-120 ° C and calcined at 400-600 ° C. The catalytic properties of the catalyst (200 mg load) in the CO oxidation reaction were evaluated by the degree of CO conversion at various temperatures using a gas mixture containing 1% CO, 0.5-20% O ₂ , 98.5-79 He (volumetric feed rate 30 ml / min). The catalyst provides 100% CO conversion at 60 ° C for 12 hours of continuous operation.

A feature of the prototype catalyst is a high specific surface area (878-1142 m ² / g) with a pore size of 2.2-2.8 nm. Materials with similar texture characteristics have low strength characteristics and are highly dispersed powders, poorly pressed or difficult to form to obtain granules of a given size and shape [AP Karnaukhov Adsorption. The texture of dispersed and porous materials. Novosibirsk: Nauka, 1999. - 470 p.]. The catalyst in this form seems inconvenient for operation in real systems of air purification from CO.

Given that the pore size of the prototype catalyst is 2.2-2.8 nm, low stability of the catalyst in the presence of water vapor should be expected, i.e. in humid conditions. The adsorption of water vapor followed by condensation in high humidity conditions will block the active surface (the inner part of the granule or block part of the porous space) and deactivate the catalyst.

It is also important to note that the study of the activity and stability of the catalyst was carried out using a He / O ₂ / CO gas mixture free of water vapor, therefore, nothing is known about the activity of the prototype catalyst in the presence of water vapor and the change in catalyst activity during continuous operation over 12 hours.

The technical problem to which the present invention is directed is the development of a catalyst that provides high efficiency for the oxidation of carbon monoxide to carbon dioxide (CO ₂ , carbon dioxide) for a long period of time at temperatures close to room temperature in atmospheric air containing water vapor (humidity not less than 50%).

To solve this problem, a catalyst for the oxidation of carbon monoxide containing silver (1-16 wt.%) Deposited on silicon dioxide (silica gel) with a specific surface area of 50-200 m ² / g and a pore size of 3-60 nm, and may also contain oxides of cerium, zirconium, manganese, or a mixture of these oxides in an amount up to 10% by weight of the catalyst.

The proposed silver-containing catalyst is obtained by impregnation from an aqueous solution of silver nitrate (the most affordable and cheapest precursor of the active component), followed by heat treatment to (400-700) ° C. The proposed method is easily implemented in laboratory conditions and can be used for industrial (semi-industrial) catalyst preparation.

The technical result of the method of using the catalyst is to increase the stability of the catalyst due to preliminary heat treatment of the carrier and additives of the oxide modifier, as well as due to the uniformity of deposition and small particle size of the active component is silver.

The problem is solved in that the method of using the catalyst for air purification from carbon monoxide is carried out using the catalyst according to claims. 1-3 formulas by passing a stream of moist air containing CO (up to 100-115 mg / m ³ ) through the catalyst bed at room temperature.

Silica gels of various grades can be used as a carrier, including KSKG technical silica gel available on the Russian market. The technical problem is solved due to the special conditions of pretreatment of the carrier, consisting in hydrothermal processing of silica gel in the presence of an aqueous solution of ammonia to change the porous structure and provide additional strength to the granules, followed by high-temperature processing at temperatures of 500-1000 ° C.

The catalyst has the shape of spherical granules, the size of which is determined by the size of the granules of the original carrier. Also, the catalyst can be obtained in the form of granules of a different shape and size depending on the requirements of the air purifying device in which it will be used.

The essence of the invention is as follows.

The low-temperature oxidation catalyst CO contains silver as an active component in an amount of (1-16)% by weight of the catalyst, silicon dioxide as a support and may contain cerium, zirconium and manganese oxides in an amount of up to 10% by weight of the catalyst. The catalyst is characterized in that it has a mesoporous structure with a developed system of transport pores (3-60 nm), providing efficient transfer of reagents, including water vapor. Nitrogen adsorption-desorption isotherms and pore size distributions for the catalysts described in Examples 1-3 are shown in FIG. 1 and FIG. 2, respectively. It is seen that for all samples, the main pore distribution lies in the region of 15–40 nm.

Another feature of the catalyst is that silver is in the form of particles less than 6 nm in size. In FIG. Figure 3 shows the TEM image, as well as the size distribution of silver particles for the catalyst described in Example 2. It can be seen that silver is uniformly distributed along the surface of the support, and the particle size of silver is 1-6 nm with an average particle size of 2.8 nm.

Air purification from CO is carried out by passing a stream of moist air containing 100-115 mg / m ³ CO through the catalyst bed at temperatures close to room temperature. The technical effect observed in this case is the conversion of more than 80% CO to CO ₂ and the preservation of this conversion for a long time (at least 20 hours).

The proposed catalyst was obtained by the following method.

The silica gel preliminarily calcined at 500–900 ° С was impregnated with an aqueous solution containing a given amount of silver nitrate or the ammonia complex of silver nitrate, then the obtained samples were dried at 70 ° С without access of light for 24 hours and calcined to 500 ° С in air. Catalysts containing transition metal oxides were prepared by impregnating silica gel with an aqueous solution of nitrates of the corresponding metals and silver nitrate, followed by heat treatment under similar conditions.

The essence of the invention is illustrated by the description, drawings and table, where:

in FIG. 1 shows nitrogen adsorption-desorption isotherms for the catalysts described in examples 1-3;

in FIG. 2 shows pore size distributions for the catalysts described in Examples 1-3;

in FIG. 3 shows the TEM image and the distribution of silver particles by the size of the catalyst described in example 2;

in FIG. 4 shows the dependence of the degree of conversion of CO on time for catalysts, the compositions of which are described in examples 1-4;

table 1 shows the porous structure of the catalysts and compares the oxidation of CO under real conditions of air purification with a moisture content of at least 50% on the catalysts described in examples 1-4.

The efficiency of the catalyst is characterized by the following values: the degree of conversion of CO at 29 ° C, expressed as a percentage, the lifetime of the catalyst in an air stream with a humidity of at least 50%.

The invention is illustrated by examples of specific performance.

Example 1. The catalyst composition is 8 wt. % Ag, SiO ₂ - the rest.

Commercial silica gel brand KSKG (GOST 3956-76, S _beats. = 268 m ² / g, V _then = 0.95 cm ³ / g, D _then = 10 nm) was used as a carrier. Silica gel was subjected to hydrothermal treatment in an aqueous solution of ammonia at 120 ° C for 3 hours. Then the silica gel is dried in air at a temperature of 120 ° C for 3 hours. The resulting carrier has a specific surface area of 98 m ² / g Silica gel (2.76 g) is soaked in moisture capacity (water volume 2.0 ml) with an aqueous solution of silver nitrate (0.378 g), then the sample is dried at 70 ° C for one day, calcined in air at 500 ° C.

It differs in that technical silica gel of the KSKG brand (GOST 3956-76, S _beats = 268 m ² / g, V _por = 0.95 cm ³ / g, D _por = 10 nm) is used as a carrier, in the form of spherical granules 3-6 mm in diameter or in the form of granules of irregular shape of a smaller size.

Example 2. The catalyst composition is 8 wt. % Ag, SiO ₂ - the rest.

Similar to example 1. The difference is that the silica gel used as a carrier was subjected to an additional high-temperature treatment at 700 ° C before applying the active component.

Example 3. The catalyst composition of 8 wt. % Ag, 10% wt. MnO _x (based on MnO ₂ ), SiO ₂ - the rest. Similar to example 2, the difference is that in its composition contains manganese oxides in an amount of 10% wt. based on MnO ₂ .

Example 4. The catalyst composition is 8 wt. % Ag, 10 wt. % Ce _0.5 Zr _0.5 O ₂ , SiO ₂ - the rest.

Similar to example 2. The difference is that in its composition contains oxides of zirconium and cerium in an amount of 10% wt.

Example 5. A method of purifying air from CO consists in passing moist air containing 100-115 ppm CO through the catalyst bed described in paragraphs. 1-4.

In table 1 and in FIG. Figure 4 shows the results of catalytic tests of the proposed catalysts under conditions close to the actual operating conditions of air purifying devices, as well as their comparison with the prototype catalyst (CN 101890349 from 11.24.2010). Air with a humidity of not less than 50%, containing 100-115 ppm CO, was passed through a quartz reactor with a catalyst from PP. 1-4 (catalyst volume 0.5 cm ³ ) at a space velocity of 100 ml / min at a temperature of 29 ° C. The residual concentration of CO was measured by an electrochemical sensor.

Analysis of the results of the experimental determination of the effectiveness of the catalysts, the composition of which is shown in Table 1, in the process of purifying moist air from impurities of carbon monoxide, showed that the proposed catalysts provide effective removal of 100-115 mg / m ³ CO (conversion of at least 80%) over a long period time period (20 hours).

All the catalysts described in the examples contain no more than 8.0 wt. % silver, and the method of obtaining the proposed catalyst is simpler in comparison with the prototype.

Using the preliminary calcination of silica gel and applying a composition of transition metal oxides makes the catalyst more resistant to adsorption of water vapor and CO ₂ , which allows to increase activity and extend its service life.

Thus, the above information indicates the implementation when using the claimed invention of the following combination:

- the claimed method and the catalyst are intended for use in the catalytic purification of moist air from impurities of carbon monoxide;

- for the claimed invention in the form as described in the independent claims, the possibility of its implementation using the means and methods described in the application is confirmed.

Claims (4)

1. The catalyst for low-temperature oxidation of carbon monoxide, which is silver deposited on the surface of silicon dioxide in an amount of 1-16% by weight of the catalyst, characterized in that it contains silver in the form of nanoparticles <6 nm in size, which are uniformly distributed on the surface of mesoporous silica gel with a value specific surface area of 50-200 m ² / g and a pore size of 3-60 nm, used as a carrier. 2. The catalyst according to claim 1, characterized in that it contains manganese oxides in an amount up to 10 wt.% Calculated on MnO ₂ . 3. The catalyst according to claim 1, characterized in that it contains zirconium and cerium oxides in an amount up to 10 wt.%. 4. The method of using the catalyst for air purification from carbon monoxide, characterized in that it is carried out using the catalyst according to claims. 1-3 by passing a stream of moist air containing CO (up to 100-115 mg / m ³ ) through the catalyst bed at room temperature.

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