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Preferential Oxidation of Carbon Monoxide in Hydrogen Stream over Au/MgOx-TiO2 Catalysts Li-Hsin Chang, Yu-Wen Chen,* and Natarajan Sasirekha Department of Chemical & Materials Engineering, National Central UniVersity, Chung-Li 320, Taiwan, ROC
A series of Au/MgOx-TiO2 with various Mg/Ti molar ratios was prepared to study its catalytic activity for preferential oxidation of carbon monoxide in hydrogen-rich stream (PROX). An incipient wetness impregnation method was used to prepare the MgOx-TiO2 support, using aqueous solutions of Mg(NO3)2 on TiO2 calcined at 300 °C. A deposition-precipitation (DP) method was utilized to prepare Au/MgOx-TiO2, using HAuCl4 as the starting material. Au/MgOx and Au/TiO2 were also included for comparison. Investigation was carried out to study the effect of Mg/Ti molar ratio on the catalytic properties of gold supported catalysts. The catalysts were characterized by inductively couple plasma-mass spectrometry, X-ray diffraction, transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy. TEM results revealed the presence of nanosized gold particles and narrow particle size distribution on all the catalysts. MgOx was deposited on the TiO2 surface in the form of thin islands without definite shape. MgOx was mainly present as Mg(OH)2. Au particles were deposited on the surface of TiO2 in intimate contact with MgOx. The gold particle size was around 2.5 nm and was smaller than the MgOx particles. Since the surface area of TiO2 was small and the MgOx concentration was comparatively higher than Au concentration, Au should be in intimate contact with MgOx on the TiO2 surface. However, the exact status depends upon the Mg/Ti ratio and pH value during DP process to deposit gold. Au/MgOx-TiO2 with suitable amount of MgOx has higher CO conversion and higher selectivity of O2 to CO oxidation in the PROX reaction than Au/MgO and Au/TiO2. 1. Introduction Metal oxide-supported gold catalysts are active catalysts for a number of catalytic reactions including CO oxidation,1–5 propene epoxidation,6,7 alkene hydrogenation,8 ethyne hydrochlorination,9 selective hydrogenation,10 water-gas shift reaction,11–13 selective oxidation of CO,14–18 and selective reduction of NOx.19 The influence of metal oxides on the catalytic activity of gold nanoparticles has obtained great scientific interest. The metal oxide supports are classified into inert (irreducible oxide) and active (reducible oxide) supports. Even gold supported on inert oxides, such as SiO2,20 MgO,21–24 and Al2O3,22,25 at suitable pretreatment conditions also achieved a higher activity of CO oxidation than as-prepared catalysts. Margitfalvi et al.22,23 reported that Au/MgO modified with ascorbic acid stabilized gold in a positively polarized form, causing the high activity of CO oxidation. The author also mentioned that cooling atmosphere after reduction pretreatment resulted in activity changes in inert and active supports. The catalytic activity of gold supported on an inert support can be attributed to the particle size of gold nanoparticles and the presence of low-coordinated gold atoms, which enhance the feasibility of interaction between CO and O2. Active supports such as TiO2,4,26–29 Fe2O3,14,30–32 CeO2,1,20,33–35 NiO,36 and SnO237 improve the stability of gold particles and furnish oxygen atoms for higher activity. Interface, geometry, and quantum size effects determine the catalytic activity of gold nanoparticles supported on metal oxides. Density functional theory (DFT) calculations for Au supported on reducible and irreducible supports revealed that the catalytic activity mainly depends on the low-coordinated gold corner atoms as a consequence of different interface energy.38 Literature reports39,40 demonstrated that the gold catalysts supported on reducible oxides were more active than those on irreducible oxides due to the ability of reducible oxides * To whom correspondence should be addressed. Tel.: +886 3 422751 ext 34203. Fax: +886 3 4252296. E-mail:
[email protected].
to supply reactive oxygen for CO oxidation at low temperatures, thereby eliminating oxygen dissociation as rate-determining step. Fuel cells are potential battery replacements; however, platinum electrodes used in fuel cells are often poisoned by the presence of CO. Clean hydrogen fuel was necessary to improve the efficiency of the fuel cell. In order to remove CO, highly dispersed gold nanoparticles on metal oxides have been extensively investigated for preferential oxidation (PROX) of the carbon monoxide reaction rather than Pt-group metal catalysts, because of its higher activity for CO oxidation in the hydrogen-rich stream than H2 oxidation and its insensitivity toward the presence of CO2. During PROX reaction, the desired CO oxidation reaction competes with the undesired H2 oxidation in a limited O2 presence, as shown in the following equations. 1 CO + O2 f CO2 ∆H ) -280 kJ/mol 2
(1)
1 H2 + O2 f H2O ∆H ) -240 kJ/mol 2
(2)
In addition, the reaction byproduct, moisture, tends to enhance the activity of gold catalysts.18 Even though numerous reports are available on the effect of metal oxide support on the activity of gold nanoparticles for PROX reaction, there is less attention toward bimetallic oxide supports. Our previous studies41–43 have shown that gold nanoparticles supported on MnO2-CeO2 bimetallic oxide had a higher activity for PROX reaction than that on monometal oxide support. Au/TiO2 has been reported3,4 to have high CO oxidation, but the selectivity of O2 reacting with CO was not high due to the rapid reaction of O2 with H2 at higher reaction temperature. In contrast, Au/MgO had a low CO conversion and high selectivity to CO oxidation,40,44 because the hydrogen oxidation rate was very low. It is expected that adding a suitable amount of MgO on Au/TiO2, one might be able to suppress H2 oxidation and maintain the same CO oxidation as Au/TiO2. Because MgO is a good additive, it can
10.1021/ie071590d CCC: $40.75 2008 American Chemical Society Published on Web 05/23/2008
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stabilize transition metals and avoids them from sintering and evaporation.21 The surface of MgO is positively charged due to its strong basicity and its structural simplicity makes it a suitable support to study the oxidation of CO at MgO supported gold nanoparticles by DFT calculations.21 MgO acts as a structural promoter and creates a cavity to facilitate the interaction of low-coordinated Au atoms and Mg2+ cations with the adsorbate bound to the gold atoms.21 Margitfalvi et al.24 used MgO to modify Au/Al2O3 catalysts. Au/MgO-Al2O3 showed high activity in subambient and ambient temperature range and the authors suggested that most of the gold particles were in contact with MgO. The preparation parameters play a major role in determining the properties of the catalyst and the dispersion of gold nanoparticles. The deposition-precipitation (DP) method has been found to be suitable for uniform and fine dispersions of gold particles on metal oxides, which in turn depend on the isoelectric point of the support.3 In this study, the DP method was used to prepare gold supported catalysts. There is a wide difference in the isoelectric point of MgO and TiO2, and hence, the influence of pH is a vital parameter on the dispersion and the catalytic activity of gold particles. The aim of this study was to investigate the effects of pH during the DP process and Mg/Ti ratio on the catalytic activity of Au/MgOx-TiO2. Au/ TiO2 and Au/MgOx were also included for comparison. 2. Experimental Details 2.1. Catalysts Preparation. TiO2 (Degussa P25) and MgO (Merck) were used as supports for gold catalysts. The MgOx-TiO2 support was prepared by an incipient wetness impregnation method. In a typical synthesis, Mg(NO3)2 was added to distilled water of quantity equal to the pore volume of TiO2. The solution was added into TiO2 powder in drops under vigorous grinding, followed by calcination at 300 °C for 4 h in air. The color of the TiO2 powder remained the same even after the deposition of MgOx. Supported gold catalysts were prepared by the DP method. Dry support was suspended in deionized water and the pH was adjusted to the desired value by dropwise addition of aqueous solution of HNO3 (0.1 mol/L). Diluted HAuCl4 solution was adjusted to pH 9 using NH4OH (0.1 mol/L). The gold solution was poured at a rate of 10 mL/min into the support suspension under vigorous stirring; the pH was readjusted to 9. The resulting suspension was thermostatted at 65 °C for 2 h. Subsequently, the suspension was cooled down to room temperature. The suspension was filtered, and the filtrate was suspended 10 times in hot water (65 °C) until no chloride ions were detected with AgNO3 solution. It was dried overnight at 80 °C and then calcined at 180 °C for 4 h. The Au catalysts in this study were calcined at low temperatures (
