ISSN 0018-1439, High Energy Chemistry, 2017, Vol. 51, No. 1, pp. 46–50. © Pleiades Publishing, Ltd., 2017. Original Russian Text © N.N. Vershinin, V.A. Bakaev, V.I. Berestenko, O.N. Efimov, E.N. Kurkin, E.N. Kabachkov, 2017, published in Khimiya Vysokikh Energii, 2017, Vol. 51, No. 1, pp. 50–54.
NANOSTRUCTURED SYSTEMS AND MATERIALS
Synthesis and Properties of a Platinum Catalyst Supported on Plasma Chemical Silicon Carbide N. N. Vershinina, *, V. A. Bakaeva, V. I. Berestenkoa, O. N. Efimova, E. N. Kurkina, b, and E. N. Kabachkova, b, c a
Institute of Problems of Chemical Physics, Russian Academy of Sciences, pr. Akademika Semenova 1, Chernogolovka, 142432 Russia b Scientific Center in Chernogolovka, Russian Academy of Sciences, Chernogolovka, ul. Lesnaya 9, 142432 Russia c Ural Federal Institute, ul. Mira 19, Yekaterinburg, 620002 Russia *е-mail:
[email protected] Received January 15, 2016; in final form, May 19, 2016
Abstract⎯A CO oxidation catalyst based on β–SiC and Pt nanoparticles has been synthesized and studied. The average size of Pt clusters on the surface of the plasma-chemical silicon carbide nanoparticles is close to 4 nm. It has been found that the rate of the CO oxidation reaction at low concentrations (100 mg/m3) in air at room temperature over the catalyst based on platinum and silicon carbide nanoparticles is 60–90 times that over a platinum black-based catalyst with a specific surface area of 30 m2/g. The Pt/SiC catalyst containing 12 wt % Pt has been found to provide the maximum CO oxidation rate. Keywords: carbon monoxide oxidation catalyst, nanoparticles, silicon carbide, platinum DOI: 10.1134/S0018143916060199
CATALYST SYNTHESIS AND PHYSICOCHEMICAL PROPERTIES As a support for platinum, we used β–SiC prepared from tetramethylsilane by arc plasma treatment [9]. The catalyst based on SiC nanoparticles was prepared according to a procedure involving the reduction of clusters of the catalytic metal (Pt) from an aqueous solution of H2PtCl6 ⋅ 6H2O with a reducing agent (lithium formate, LiCOOH) on the surface of support nanoparticles present in the solution, the method that we had patented not long ago [7]. The catalyst preparation procedure comprised the following steps: mixing an aqueous solution of H2PtCl6 ⋅ 6H2O (10–3– 10–2 mol/L) with an aqueous solution of lithium formate (0.02–0.2 mol/L) at 20°C and adding a required amount of the resulting solution of H2PtCl6 ⋅ 6H2O and lithium formate to an aqueous suspension of silicon carbide heated to 60°C with a concentration of solids of 0.4 g/L. After the induction period (8– 15 min), platinum clusters precipitated on the surface of silicon carbide nanoparticles. The resulting solution was allowed to stand for 24 h at room temperature, and the catalyst was washed with distilled water (five to six times) to remove the reaction products. The resulting CO catalyst was dried at 80°C for 24 h. An aqueous suspension (80 mL) containing nanodiamond particles or particles of silicon carbide was prepared by sonication of their initial powders for 60 min at an ultra-
Carbon monoxide CO, which is produced in the combustion of hydrocarbon fuels in motor vehicles and industrial enterprises and by methane combustion in residential appliances, is one of the most hazardous indoor air pollutants (in household and industrial premises, passenger compartment, garages, etc.). Therefore, the creation of catalysts for CO oxidation is crucial for air purification systems. The Au/TiO2 catalyst has a high rate of CO oxidation at room temperature in the case of high CO concentrations [1]. Published data on the properties of CO oxidation catalysts containing clusters of platinum group metals and gold on oxide and carbon supports are briefly surveyed in [2]. In recent studies, nanomaterials have been used for supporting catalytically active metals in the development of CO oxidation catalysts [3–6]. In these studies, the catalytic properties were examined mainly at high CO concentrations. Previously, we found that detonation nanodiamonds (ND) containing platinum clusters hold promise as supports for catalytic systems designed for CO oxidation at room temperature at low concentrations (less than 100 mg/m3) typical of household and industrial premises [7, 8]. In this work, we synthesized a catalyst based on nanoparticles of platinum and plasma-chemical silicon carbide, studied its physicochemical properties, and tested catalytic performance at room temperature over the range of low CO concentrations. 46
SYNTHESIS AND PROPERTIES
47
800
600 35.6 SiC (111)
400
0
60 SiC (220)
40.85 SiC (200)
200
71.65 SiC (311)
20 nm 40
60 2θ, deg
80
100 Fig. 2. TEM image of platinum clusters on the surface of β–SiC with a Pt loading of 10 wt %.
Fig. 1. X-ray powder diffraction pattern of SiC.
sonic power of 50% of the maximum power using an HD 3200 ultrasonic homogenizer. The properties of silicon carbide and the resulting catalyst supported on it were studied by instrumental methods. Nitrogen adsorption/desorption isotherms and the BET specific surface area of silicon carbide samples were measured at the liquid nitrogen temperature (77 K) using a Quantachrome Quadrasorb SI analyzer. The SiC samples were preconditioned at 573 K in a helium atmosphere on a FloVac degasser for 3 h immediately before measurements. X-ray diffraction spectra were recorded using a DRON ADP-2-02 diffractometer using (CuKα-radiation, λ = 0.154056 nm). The surface structure of the silicon carbide-based catalyst was characterized using transmission electron microscopy with a JEOL JEM 2100 microscope at 0.19 nm resolution. The chemicals and samples were weighted on an Acculab ALC-80d4 digital scale. Figure 1 shows the results of X-ray diffraction (XRD) study of an SiC powder with a specific surface area 142 ± 6 m2/g (average SiC particle size is 13 ± 1 nm). It was found from the XRD data that silicon carbide is a single-phase crystalline powder of the β–SiC polymorph with a cubic lattice parameter of a = 0.4366 ± 0.0002 nm. The β-SiC polymorph has Table 1. Calculated value of SPt as a function of Pt cluster thickness Cluster thickness Pt, nm
Calculated value of SPt, m2/g
0.4
163
0.8
105
2.0
70
4.0
58
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the structure of sphalerite and is a diamond-like structure. Figure 2 shows a TEM image of the surface of the catalyst based on silicon carbide and platinum. The dark points are platinum particles, with the maximum of particle size distribution corresponding to the platinum cluster diameter close to 3–4 nm. Previously [8], we obtained similar results in an study of platinum clusters on the surface of detonation nanodiamond particles and showed that flat platinum clusters (of a 3–5 nm diameter) with the average Pt cluster thickness on the order of 0.6 nm (one to two Pt crystal lattice spacings) are formed on the nanodiamond surface. Assuming that the clusters have a cylindrical shape and excluding the cluster surface (cylinder bases) that contacts with the support, we can calculate the specific surface area of Pt clusters by the following equation (1):
S Pt = [(π D 2 4 + π Dh] [(π D 2 h 4)ρ] = (D + 4 h) (Dhρ),
(1)
where SPt is the specific surface area of platinum clusters, m2/g; D is the Pt cluster diameter, m; h is the thickness (height of the cylinder) of the Pt cluster, m; and ρ is the density of platinum, g/m3. Table 1 shows the results of our calculation of SPt depending on the Pt cluster thickness. In the calculations, Pt cluster diameter D and platinum density ρ were assumed to be 4 nm and 21.5 × 10 6 g/m3, respectively. Irregularities on the surface of platinum clusters, which do not exceed half the Pt lattice spacing, can lead to an increase in the specific surface area of platinum clusters by no more than a factor of 1.5 as compared with the calculated value. Therefore, the specific surface area of the Pt cluster at a cluster thickness of 0.8–4 nm and an average cluster diameter of 4 nm will be close to 120 ± 40 m2/g. The number of platinum surface atoms is high (15 ± 30%); therefore, a
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3
CCO, mg/m3 100
4
3
V3 2 5 V1
10
V2
1
1
2
V4
1
0
100
200
300
t, s 6
Fig. 4. Kinetics of CO oxidation at T = 294 K, P = 101.3 kPa, and RH = 30%: (1) Pt on silicon carbide; (2) Pt on nanodiamond; (3) platinum black.
Fig. 3. Schematic of the test bench: (1) pure-air cylinder, (2) cylinder with the calibration gas mixture of CO and air, (3) flow rate regulator of the gas mixture, (4) gas mixture or air humidifier, (5) test chamber, and (6) signal converter box; V1, V2, V3, and V4 are valves.
high catalytic activity in the CO oxidation reaction should be expected. CATALYTIC PROPERTIES OF PLATINUM ON SILICON CARBIDE Figure 3 is a schematic of the test bench. The test chamber of a 300 ± 3 cm3 volume is made of stainless steel and has a jacket connected with a U-4 thermostat through which water circulates. The test chamber was equipped with CO, CO2, and humidity/temperature (RH/T) sensors, the outputs of which were connected to a microprocessor-based converter with a digital display. The following sensors were used: a SO-TE-2100-1 CO sensor (certificate RU.E.31.092.A no. 29335), measurement range 0–125 mg/m3; a MSH— P/CO2 Dynament optical CO2 sensor, measurement range 0–0.5%; and a SHT75 Sensirion humidity (RH) and temperature sensor (relative humidity measurement error ±2%, temperature measurement error ± 0.4 K). A porous glass substrate (30 × 30 × 3 mm3), on
which the catalysts were applied, was placed into the test chamber. The CO oxidation reaction on the catalyst was studied as follows: a calibration CO/air gas mixture with a CO concentration in the air of 125 mg/m3 was blown through the test chamber for 600 s at a flow rate of 50 cm3/s. Then, the inlet (V3) and outlet (V4) valves of the test chamber were closed and a gas flow booster in the test chamber was switched on to maintain the gas mixture circulating through the sample with the catalyst at a flow rate of 30 cm3/s. After the reduction of CO concentration (by the catalytic oxidation) to the level of 100 mg/m3, a digital stopwatch timer (frequency meter) was switched on and sensor readings were recorded. Figure 4 shows the results of the testing of catalytic properties at room temperature for three types of CO catalysts: (1) platinum on silicon carbide, (2) platinum on SDND nanodiamond (from Plasmochem, Germany), and (3) platinum black with a surface area of 30 m2/g. The composition of the test catalysts is shown in Table 2. As the CO concentration in the test chamber decreases, the CO2 concentration simultaneously increases as a result of the irreversible reaction of CO oxidation by atmospheric oxygen on the catalyst surface CО + 1/2О2 = CО2.
Table 2. Composition of test catalysts Support 1, β–SiC 2, C (nanodiamond) 3, unsupported
Size of support particles, nm
Catalyst weight, mg
Pt weight in catalyst, mg
13 ± 1
25
3 ± 0.3
5±1
25
3 ± 0.3
90 (platinum black)
90 ± 0.5
–
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SYNTHESIS AND PROPERTIES Table 3. Dependence of catalytic properties of Pt/SiC on the composition; test conditions: T = 294 K, P = 101.3 kPa, and RH = 30% Sample no.
Vx/V12
Sample weight, g
Pt loading, wt %
1
0.50
0.150
2
2
0.62
0.075
4
3
0.80
0.043
7
4
0.96
0.030
10
5
1.00
0.025
12
6
0.95
0.020
15
7
0.90
0.015
20
8
0.70
0.010
30
9
0.50
0.0075
40
As can be seen from Fig. 4, the dependence of CO concentration on the residence time in the test chamber with the catalyst is described by Eq. (2): CCО(t) = CCO(0)e–kt,
(2)
where CCО(t) is the measured value of the CO concentration in the test chamber; CCO(0) is the CO concentration at the beginning (zero time); k is the reaction rate constant; and t is the time. To study the effect of catalyst composition on the catalytic properties, nine samples of catalyst based on silicon carbide with a platinum loading of 2–40 wt % were synthesized and tested. Table 3 shows experimental data on the influence of the composition of the Pt/SiC catalyst on its catalytic properties. The oxidation of CO over these samples was studied according to the procedure described above. In studying the catalytic properties, the catalyst weight was varied depending on the composition so that the weight of platinum in each sample was the same (3 ± 0.3 mg). The rate of the CO oxidation reaction for each sample was determined after the processing of the experimental data according to Eq. (2). Then, we determined the ratio of reaction rates Vx/V12, where V12 is the rate of CO oxidation for the composition with a Pt loading of 12 wt %, which exhibited the maximum CO oxidation rate, and Vx is the rate of CO oxidation on the catalyst with a Pt loading of x wt %. RESULTS AND DISCUSSION From the experimental data on the CO oxidation kinetics shown in Fig. 4, it was found with allowance for the Pt loading of the catalyst (Table 2) that the rates of the CO oxidation reaction on platinum supported on the nanodiamond or silicon carbide nanoparticles are close in value. The reaction rate of CO oxidation on the Pt/SiC catalyst per unit weight of platinum is a HIGH ENERGY CHEMISTRY
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60–90 times that on platinum black with a specific surface area of 30 m2/g. The increase in the CO oxidation rate can be due to both a 2.8–5.2 times higher specific surface area of platinum clusters on silicon carbide (Table 1) relative to platinum black and a lower activation energy of the CO oxidation reaction on the catalyst surface. It follows from the Arrhenius equation that when the reaction rate increases by a factor of 12–32 as the activation energy decreases to 6–8.5 kJ/mol. A possible reason for the decrease of the activation energy is that a heteroepitaxial platinum layer can be formed when Pt nanoparticles are clustered on the surface of a support that has a structural similarity to Pt. At the same time, distortions of the Pt crystal lattice will occur because of the difference of the lattice spacings of SiC and Pt, which can increase the amount of catalytically active reaction centers on the cluster surface. Similar phenomena are observed in platinum deposited on the surface of Cu–Au nanoparticles [10], and the rate of electrochemical oxidation of hydrogen over platinum supported on the Cu–Au alloy increases four times as compared with pure platinum. An increase in the Pt loading of the catalyst within the range 2–12% (Table 3) increases the catalytic reaction rate as a result of increasing the concentration of active sites by Pt clusters approaching each other. When the platinum content exceeds 12%, the reaction rate decreases because platinum is deposited not only on silicon carbide particles but also separately of the support particles, forming large and less active clusters. A similar behavior we observed in the study of CO oxidation on nanodiamond-supported platinum [7, 8]. CONCLUSIONS The rate of CO oxidation reaction per unit weight of platinum at room temperature and low CO concentrations (100 mg/m3) over platinum supported on SiC nanoparticles is 60–90 times that over platinum black with a specific surface of 30 m2/g. The Pt/SiC catalyst with a Pt loading of 12 wt % shows the maximum rate of CO oxidation at room temperature. REFERENCES 1. Haruta, M., Kobayashi, T., Sano, H., and Yamada, N., Chem. Lett., 1987, vol. 16, p. 405. 2. Vershinin, N.N., Gol’dshleger, N.F., Efimov, O.N., and Gusev, A.L., Al’tern. Energ. Ekol., 2008, no. 8, p. 99. 3. Zhang, L., Li, H., Huang, X., Sun, X., Jiang, Z., Schlogl, R., and Su, D., Angew. Chem., 2015, vol. 127, p. 1. 4. Lin, S., Ye, X., Johnson, R.S., and Guo, H., J. Phys. Chem. C, 2013, vol. 117, p. 17319.
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Translated by V. Avdeeva
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