КИНЕТИКА И КАТАЛИЗ, 2013, том 54, № 3, с. 310-319
UDC 541.145:541.128.35:546.824-31
CHARACTERIZATION OF Pd/TiO2 EMBEDDED IN MULTI-WALLED CARBON NANOTUBE CATALYST WITH A HIGH PHOTOCATALYTIC ACTIVITY
© 2013 Fengjun Zhang1, *, Fazhi Xie1, Haiyan Xu1, Jin Liu1, Won Chun Oh2
1 Anhui Key Laboratory of Advanced Building Materials, Anhui Jianzhu University, P.R. China 2 Department of Advanced Materials & Engineering, Hanseo University, Korea *E-mail: zhang-fengjun@hotmail.com Received 30.03.2011 Revision received 14.11.2012
Pd particles loading on TiO2-embedded multi-walled carbon nanotubes (MWCNTs), MWCNTs, and TiO2 particles were prepared via an impregnation method with palladium(II) chlorate solution followed by heat treatment at high temperature. To characterize the catalysts, BET surface area, scanning electron microscopy, transmission electron microscopy, X-ray diffraction, energy dispersive X-ray, Fourier transform infrared spectroscopy and ultraviolet-visible spectroscopy were employed. The prepared catalysts were tested in degradation of methyl orange under visible light. Pd/TiO2-MWCNTs catalyst demonstrates the highest photocatalyt-ic activity, and the phase transformation from PdO to Pd0 phase takes place at heat treatment of embedded TiO2. The nanoparticles size of TiO2 can be decreased by introduction of MWCNTs species. Combining structural characterization with kinetic study results we could conclude that the superior catalytic performance could arise due to the Pd/TiO2-MWCNTs catalyst's structure.
DOI: 10.7868/S0453881113030180
Recent advances in attachment of noble metal nanoparticles to multi-walled carbon nanotubes (MWCNTs) provide a way to obtain novel hybrid materials with useful properties for gas sensor and catalytic application. MWCNTs decorated with gold are proposed as a good candidate material for glucose biosensors [1]. The morphology and size of CNTs make them a suitable catalyst support where catalytically active metal compound nanoparticles may be immobilized on the surfaces of CNTs. It has been reported that Pt, Ru, and Pt/Ru supported on CNTs show excellent catalytic activities for methanol oxidation in fuel cells as well as for hydrogenation reactions [2—5]. These researchers have reported that CNTs-supported noble metal catalysts exhibited good catalytic behavior for various chemical reactions. The enhanced catalytic performance is generally attributed to the metal—CNTs interaction [6]. This interaction induces a peculiar microstructure or modification of the electron density in the metal clusters and enhances the catalytic activity. Therefore, the interactions between noble metal and higher conductivity carriers of CNTs are highly important for supported metal catalysts [7—9]. TiO2 is frequently used to modify the interactions and the catalytic properties of these materials [10, 11]. However, little is known about the role of TiO2 in changing the catalytic properties because of the difficulty in characterizing the metal-support interface.
As we know, titania has been a promising and significant photocatalyst for environment purification
and it has been confirmed that the photocatalytic activity is due to photogenerated electrons and holes. However, there are two main obstacles for the practical application of TiO2 catalyst: low quantum efficiency and restriction to short wavelength excitation. To improve the photocatalytic efficiency in these regards, many studies have undertaken a variety of modifications by introducing doping species like noble metals, metal ions and nonmetals. Among these modifications, noble metal doping has been found to enhance the photocatalytic performance significantly and to change the photocatalytic reaction mechanism under visible light irradiation.
In particular, palladium is used widely as the active component for catalytic reactions [12, 13]. The results indicated that TiO2 helps anchoring PdOx during calcination in air and maintaining it in small ensembles during catalytic reactions. This observation is expected to shed light on how TiO2 as the catalyst support influences the structure and catalytic properties. To facilitate this study, the catalytic system was carefully designed. Palladium(II) chlorate was chosen as the precursor because the Pd precursors readily undergo decomposition on metal oxide carriers offering opportunities to prepare highly dispersed Pd-MWCNTs catalysts [14, 15]. Titanium(IV) n-butoxide (TNB) was chosen as the TiO2 precursor which can be expected to bond to Pd particles on this composite support. Pho-tocatalytic activity was usually evaluated by degradation of organic dyes [16-19], because the excess use of various dyes in the textile, rubber and plastic industries
MCPBA
Oxidation
— COOH COOH
— COOH - COOH PdCl2/HCl
COOH
Heat treatment
Cl >
OH C=O
<O^OH)
—C=O
Si
-C=Or
Yy
+ TNB _r
Hydrothermal
TiO2
I 2 O
—C=O
^Pd
Fig. 1. Scheme of functionalization of MWCNTs and reaction mechanism with PdC^—HCl solution and TNB—benzene solution.
has also led to the severe surface water and groundwater contamination by releasing the toxic and colored effluents, which are important for the sake of increasing amount, its variety and resistance to biological destruction. Methyl orange (MO) was selected as the model compound, which is a water soluble azo dye used in textile industry, printing, paper manufacturing, and pulp processing and pharmaceutical industries and released as a major water pollutant. Like many other dyes of its class MO can enter the body through ingestion and the high content in living systems can prove to be carcinogenic [20, 21]. Here the embedded TiO2 in multi-walled carbon nanotube-supported Pd as a novel catalyst was selected to degrade MO in aqueous solution under visible light irradiation, whereas no information regarding the use of Pd-MWCNTs supports improving TiO2 for degrading MO has been reported.
EXPERIMENTAL
Crystalline MWCNTs powder of 95.9 wt % purity from "Carbon Nano-material Technology Co., Ltd." (Korea, diameter 20 nm, length 5 ^m) was used as a starting material. PdCl2 (99%) was purchased from "Kojima Chemicals Co., Ltd." (Japan). The TNB (Ti(OC4H7)4) as a titanium source and m-chlorper-benzoic acid (MCPBA) as an oxidizing reagent were purchased from "Acros Organics" (USA). Benzene (99.5%) and HCl were used as solvents which were purchased from "Samchun Pure Chemical Co., Ltd." (Korea) and "Duksan Pure Chemical Co., Ltd." (Korea), respectively. The MO was used as an analytical grade reagent which was purchased from "Daejung Chemical & Metals Co., Ltd." (Korea).
The 0.02 g of as-received MWCNTs was refluxed at 353 K for 6 h in MCPBA—benzene solution. PdCl2 was added into 0.1 M HCl solution to obtain PdCl2— HCl solution, which concentration was adjusted to 0.2 mol/L. Then the oxidized MWCNTs was mixed
with PdCl2—HCl solution. After stirring at 343 K for 5 h, the catalysts were dried at 373 K. Embedded TiO2 in Pd/MWCNTs was prepared as follows. 4 mL of TNB was dissolved in 30 mL of benzene to get the sol. The Pd/MWCNTs was put into the sol, agitated at 343 K for 5 h to get Pd/MWCNTs/TiO2 gels. These gels were reacted at 873 K for 1 h with a heating rate of 6°C/min to form the Pd/TiO2-MWCNTs catalyst. By mixing PdCl2—HCl and oxidized MWCNT in an aqueous solution at room temperature in air, PdO or Pd can be produced in situ on surface of CNTs by chemical reactions:
PdCl2 + H2O + MWCNTs — PdO/MWCNTs + 2HCl, PdCl2 + 2H+ + COO- + MWCNTs-—- Pd/MWCNTs + 2HCl + CO2.
(I)
(II)
The functionalization and reaction mechanism of MWCNTs surface with PdCl2 and TNB is schematically illustrated in Fig. 1.
The BET surface area by N2 adsorption method was measured at 77 K using a BET analyzer ("Monosorb", USA). X-ray diffraction (XRD) results (Shimadzu XD-D1, Japan) were used to identify the crystallinity with Cu^a radiation. Scanning electron microscopy (SEM) was used to observe the surface state and structure of Pd/TiO2-MWCNTs catalyst using an electron microscope JSM-5200 ("JOEL", Japan). Transmission electron microscopy (TEM) (JEM-2010, "JEOL", Japan) at an acceleration voltage of 200 kV was used to investigate the size and distribution of the palladium and titanium deposits on the MWCNTs surface. TEM specimens were prepared by placing a few drops of the sample solution on a carbon grid. Energy dispersive X-ray (EDX) spectra were also obtained for determining the elemental information about Pd/TiO2-MWCNTs catalyst. Functional groups formed on the surface of catalysts were examined by a KBr method using Fourier transform infra-
Table 1. BET surface area of initial TiO2, initial MWCNTs, Pd/TiO2, Pd-MWCNTs and Pd/TiO2-MWCNTs catalysts
Catalyst ¿bet, m2/g
Initial TiO2 8.3
Initial MWCNTs 299
Pd/TiO2 18.4
Pd-MWCNTs 192
Pd/TiO2-MWCNTs 84.6
red (FT—IR) spectroscopy. UV-vis absorption parameters for the MO solution decomposed by Pd/TiO2— MWCNTs catalyst were recorded by a UV-vis spectrophotometer ("Optizen Pop Mecasys Co., Ltd.", Korea).
The photocatalytic effect of Pd/TiO2-MWCNTs catalyst was determined using MO degradation in aqueous solution under visible light irradiation (8 W LED lamp, X > 420 nm, "Fawoo Technology", Korea), which was used at the distance of 100 mm from the solution in a dark box. The Pd/TiO2-MWCNTs catalyst (0.05 g) was suspended in 50 mL of MO solution with a concentration of 1.0 x 10-5 mol/L. Then, the mixed solution was placed in the dark for at least 2 h, in order to establish the adsorption—desorption equilibrium, which was hereafter considered as the initial concentration (C0). Then, experiments were carried out under visible light. Solution samples were withdrawn regularly from the reactor after 30, 60, 90 and 120 min; 10 mL samples of solution were immediately centrifuged to separate any suspended solid. The clean transparent solution was analyzed by using a UV-vis spectrophotometer.
RESULTS AND DISCUSSION
Characterization of the Pd/TiO2-MWCNTs Catalys
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