Synthesis of Catalysts and Its Application for Low-Temperature CO Oxidation
A series of Au/-TiO2 with various Co/Ti ratios prepared. /TiO2 was prepared by incipient wetness impregnation with aqueous solution of cobalt nitrate. Au catalysts were prepared by deposition-precipitation (DP) method at pH 7 and 338 K. The catalysts were characterized by inductively coupled plasma-mass spectrometry, temperature programming reduction, X-ray diffraction, transmission electron microscopy, high-resolution transmission electron microscopy, and X-ray photoelectron spectroscopy. The reaction was carried out in a fixed bed reactor with a feed containing 1% CO in air at weight hourly space velocities of 120,000 mL/h g and 180,000 mL/h g. High gold dispersion and narrow particle size distribution were obtained by DP method. The addition of into Au/TiO2 enhanced the activity of CO oxidation significantly. Au/5% -TiO2 had the highest catalyst among all the catalysts. was mainly in the form of nanosize Co3O4 which could stabilize the Au nanoparticles. donated partial electrons to Au. The interactions among Au, , and TiO2 account for the high catalytic activity for CO oxidation.
Carbon monoxide is a toxic, colorless, and tasteless gas. It can cause human being to die in short time. Low-temperature CO oxidation has been extensively studied because it plays an important role in gas purification in CO2 lasers, CO gas sensors, air purification devices, and removing trace quantity of CO from ambient air in enclosed atmospheres such as submarines and aircrafts [1–3]. When gold is deposited as nanoparticles on metal oxides, it exhibits surprisingly high catalytic activity for CO oxidation at a temperature as low as 100 K. The activity of gold catalysts also depends on support, preparation method and condition. Haruta and coworkers [1–4] found the high activity of supported gold catalysts for low-temperature CO oxidation. It is believed to occur on the metal-support interface. Cobalt oxides are good in removing the CO, , and VOCs in the air. The cobalt oxides have different oxidation states, such as CoO2, Co2O3, Co3O4, and CoO. The Co3O4 and CoO are more stable than the others. For the activity of CO oxidation reaction, the Co3O4 has a higher activity than CoO [5–8]. The Co3O4 has been reported to be an effective catalyst in the oxidation reaction. Furthermore, it has also been used as a support for gold. was reported to be active for CO oxidation, but not very active at room temperature. Although CO oxidation on Au catalysts has been extensively studied [9–13], none the of researchers has used -TiO2 binary oxide as the support.
In this study, the effect of the content of in Au/-TiO2 on the activity of CO oxidation was elucidated. The catalysts were characterized by various techniques.
2.1. Catalysts Preparation
Reagents used here were analytical grade. P25 TiO2 was obtained from Evonik Degussa Company. -TiO2 was prepared by incipient wetness impregnation method. Various amounts of aqueous Co(NO3)2 solution were added into TiO2 powder under stirring. The traditional incipient wetness impregnation was used in the preparation. The pore volume of TiO2 was 0.6 cm3/g. It was calcined in air at 473 K for 4 h. The temperature was not too high to have Crystalline phase of Co3O4. Au was then added by deposition-precipitation (DP) technique. An aqueous solution of HAuCl4 was added into the solution containing suspended -TiO2 support at a rate of 10 mL/min. The temperature of the solution was maintained at 338 K. 1 M NH4OH solution was used to adjust the pH value to 7. After aging for 2 h, the precipitate was filtered and washed with hot water until no chloride ions were detected. Finally, the sample was dried overnight in air at 353 K and then calcined at 453 K for 4 h. This temperature was high enough to reduce cationic gold to metallic gold, but not too high to cause the sintering of gold particles.
The catalysts were characterized by inductively coupled plasma-mass spectrometry (ICP-MS), X-ray diffraction (XRD), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), and X-ray photoelectron spectroscopy (XPS).
The exact gold content was analyzed by ICP-MS (PE-SCIEX ELAN 6100 DRC). The cross flow pneumatic nebulizer and double-pass-scott-type spray chamber were used to nebulize the samples. The solution was transferred by peristaltic pump, and the nebulizer used to nebulize the samples into spray chamber detected by DRC-ICP-MS. A CEM MDS-2000 (CEM, Matthews, NC, USA) microwave apparatus equipped with Teflon vessels was used to digest the powder samples.
XRD analysis was performed on a Siemens D500 powder diffractometer using CuKα1 radiation (0.15405 nm) at a voltage and current of 40 kV and 40 mA, respectively. The sample was scanned over the range of ° at a rate of 0.05°/min to identify the crystalline structure. The sample was prepared as a thin layer on a sample holder.
The morphologies and particle sizes of the samples were determined by TEM on a JEM-2000 EX II operated at 160 kV and HRTEM on a JEOL JEM-2010 operated at 160 kV. Initially, a small amount of the sample was placed into a sample tube filled with a 95% methanol solution, and after agitating under ultrasonic environment for 10 min, one drop of the dispersed slurry was dipped onto a carbon-coated copper mesh (no. 300) (Ted Pella Inc., CA, USA), and dried in an oven for 1 h. Images were recorded digitally with a Gatan slow scan camera (GIF). Based on the several images of TEM or HRTEM, more than 100 particles were counted and the size distribution graph was obtained.
The XPS spectra were recorded with a Thermo VG Scientific Sigma Probe spectrometer. The XPS spectra were collected using AlKα radiation at a voltage and current of 20 kV and 30 mA, respectively. The base pressure in the analyzing chamber was maintained in the order of 10−7 Pa. The spectrometer was operated at 23.5 eV pass energy and the binding energy was corrected by contaminant carbon ( eV) in order to facilitate the comparisons of the values among the catalysts and the standard compounds. Peak fitting was done using XPSPEAK 4.1 with Shirley background and 30 : 70 Lorentzian/Gaussian convolution product shapes. The full width at half maximum (FWHM) in the entire spectra was 1.3 eV.
2.3. CO Oxidation
The catalytic activities of CO oxidation in air were carried out in a downward, fixed-bed continuous-flow, Pyrex glass tubular reactor loaded with 0.05 g of catalyst. The reactant gas containing 1% CO in air was fed into a reactor with a flow rate of 100 mL/min (WHSV = 120,000 mL/h·g) or 150 mL/min (WHSV = 180,000 mL/h·g). The outlet gas was analyzed by a gas chromatograph (China Chromatography 8700T) equipped with a MS-5A column and a thermal conductivity detector. Calibration of the gases was done with a standard gas containing the known concentration of the components. The CO conversion was calculated as follows: where [CO] is the concentration of CO.
3. Results and Discussion
The elemental analysis results in Table 1 show the real gold and cobalt loadings in the catalysts. In this study, the original amount of gold in the solution was 1 wt.%, and the content of cobalt was between 1 and 10 wt.%. Only about 70% of Au was deposited by DP process, in agreement with the literature data. Au/10 wt.% -TiO2 catalyst had the highest Au loading among all catalysts, inferring that adding could change the isoelectric point of TiO2. It is known  that the amount of Au deposit in DP process was influenced by the isoelectric point of the support.
Figure 1 shows the XRD patterns of Au/-TiO2. All catalysts containing TiO2 support showed intense XRD peaks for anatase and rutile, as expected. It is known that TiO2 P-25 has both anatase and rutile structures. Adding cobalt and Au and pretreatment did not change its crystallinity, because the condition was not harsh. The peaks at ° (311), 59.35° (511), and 65.27° (440) corresponding to Co3O4 were observed in the XRD patterns, but the peaks were very weak even in the sample with 10 wt.% , indicating that was a very small particle of Co3O4. No distinct gold peaks at ° and 44.5° were observed, because the particle size of gold was too small to detect.
Figure 2 shows the TEM images and the gold particle size distributions of various Au/-TiO2 catalysts. The gold particles in images were observed as dark spots. The TEM images clearly show that the average particle size of Au in these catalysts was around 3 nm. The results are in accordance with the XRD results, which showed that Au particle sizes were less than 4 nm. The electron diffraction pattern of Au/3% -TiO2 catalyst is shown in Figure 3. It shows many diffraction rings corresponding to the crystalline lattices of TiO2, Co3O4, and Au, in agreement with XRD results.
The HRTEM images on the sample Au/3% -TiO2 in different spots are shown in Figure 4. Figure 4(a) shows that aggregated on TiO2 surface. The gold particles were well dispersed on both and TiO2. Figure 4(b) shows that and TiO2 had a strong interaction with each other. Figure 4(c) shows that the gold particle deposited on , and had strong interaction with -TiO2. Figure 4(d) shows that the gold particle deposited on TiO2. It can be observed that the particle size of Au was very small. The Au particle size was about 3 nm. Both TEM and HRTEM diffraction images show the diffraction ring of Co3O4 (220), confirming the formation of Co3O4 in these samples.
The XPS analysis was carried out on all the samples, and the binding energies and compositions of all the samples were tabulated in Tables 2 and 3. Au 4f is characterized by the doublet of two spin-orbit components, namely, Au 4f7/2 and Au 4f5/2 as shown in Figure 5. The binding energies of Au0 and Au+ in Au 4f7/2 are 84.0 and 85.5 eV, respectively. The binding energy of Au 4f shifted to a lower one by adding in Au/TiO2, but there was no regular trend with the content of . The composition of Au0 and Au+ did not have the relationship with content. The binding energy of Au 4f7/2 on all of the catalysts shifted to a higher binding energy after reaction. The composition of Au0 and Au+ was changed after reaction as shown in Figure 6. The presence of Au+ species seems to be effective in promoting activity for CO oxidation [10, 13]. The Au+ species in Au/3% -TiO2 was abundant compared to other catalysts.
The XPS spectra of Co 2p of various samples are shown in Figure 7. Co 2p is characterized by the doublet of two spin-orbit components, namely, Co 2p3/2 and Co 2p1/2. The binding energies of Co 2p3/2 and Co 2p1/2 were 778 and 793 eV . The binding energy shifted to a lower energy when gold was on the support. The binding energy decreased with increasing the content of . The XRD results and TEM diffraction images showed that the was in the form of Co3O4. The XPS spectra of Co 2p of various samples after reaction are shown in Figure 8.
As shown in Figure 9, the binding energy of lattice oxygen in TiO2 was 529 eV [12–15], and the binding energy of OH− group in TiO2 was 531.8 eV [12–15]. The O 1s spectra showed no distinction between Au/TiO2 and Au/-TiO2. Figure 9(a) shows that O 1s spectrum of 1% -TiO2 shifted to higher a position, due to the SMSI effect on Co and TiO2, and the binding energy was ~533 eV. The binding energy of O 1s did not change with the content of . There is no distinction between the samples before and after reaction, as shown in Figure 10.
Ti 2p is characterized by the doublet of two spin-orbit components, namely, Ti 2p3/2 and Ti 2p1/2. The gap between Ti 2p3/2 and Ti 2p1/2 is 5-6 eV, it is ascribed to Ti4+ (458.9 eV) . As shown in Figure 11, the binding energies of Ti 2p3/2 and Ti 2p1/2 of Au/-TiO2 shifted to lower ones than those of Au/TiO2, indicating the interaction of with TiO2. The Ti 2p spectra of the samples did not change after reaction as shown in Figure 12.
3.6. CO Oxidation
All of the Au/-TiO2 catalysts were very active for CO oxidation. Figure 13 shows that the addition of to Au/TiO2 could enhance the activity of the catalyst significantly. At weight hourly space velocity (WHSV) of 120,000 mL/g h, CO conversions on all Au/-TiO2 catalysts reach 100% even at 308 K. In contrast, Au/TiO2 was not active at room temperature. It should be noted that only 0.7 wt.% Au was used in this study. In order to differentiate the activities of these catalysts, a higher WHSV of 180,000 mL/h g was used. Figure 14 shows that the best catalyst was Au/3% -TiO2. Au/3% -TiO2 could oxidize CO completely at low temperature, and the result was similar with Cunningham et al. . It contained the highest gold loading and the highest content of active sites (Au+). Our results were different from that reported by Haruta  for Au/TiO2, because they used a different amount of gold. 1 wt.% Au/TiO2 was used in this study, and 10 wt.% Au/TiO2 was used in their study . It should be noted that Co3O4 is an active catalyst for CO oxidation [18, 19], but not at ambient condition. By adding Au on the Co3O4, the catalyst became very active at ambient condition. The results clearly demonstrated that Au/-TiO2 catalyst is a promising catalyst for CO oxidation at ambient condition. The small Au particle size, narrow Au particle size distribution, and well Au dispersions on -TiO2 support play the key roles. The presence of could stabilize the nano Au particles, resulting in high activity of the catalyst [20–25].
-TiO2 supports were prepared by incipient wetness impregnation method. Au catalysts were prepared by DP method to load Au on these supports. The results showed small Au particle size (2.4–3.3 nm), narrow Au particle size distribution, and well Au dispersions on these catalysts. Au/10% -TiO2 had the maximum gold loading. CoO and TiO2 had interactions between each other. Adding Co into Au/TiO2 could increase the activity of CO oxidation. CO chemisorbed on octahedrally coordinated cobalt sites, while tetrahedrally coordinated cobalt sites were inactive for CO adsorption. It could enhance the activity of CO oxidation. Comparing the other catalysts in this study, the Au/3% -TiO2 catalyst had the highest CO conversion at a low temperature. It contained the highest gold loading and the highest content of active sites (Au+).
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