Patent Grant
10898880
The present invention relates to heterogeneous catalytic carbon monoxide (CO) oxidation, and particularly to a method of making a copper oxide-titanium dioxide nanocatalyst for performing catalytic oxidation of carbon monoxide.
Heterogeneous catalytic carbon monoxide (CO) oxidation is an important reaction for the removal of small amounts of potentially poisonous CO from numerous situations, such as in fuel cell applications and environmental remediation processes. Plasmonic nanocatalysts supported on reducible metal oxides, such as CeO2 and TiO2, have been used due to their superior catalytic activity at very low temperatures, but they are relatively expensive and are susceptible to particle agglomeration and sintering at high operating temperatures. Transition metals supported on reducible metal oxides are of considerable interest due to their relatively low costs and versatility, along with activities per unit surface area similar to those of noble metal catalysts. Such catalysts have also been shown to have high oxygen release capacity at a high range of temperatures, and have further been shown to be good candidate materials for oxygen storage, as well as providing oxygen for combustion and oxidation reactions at high temperatures. In particular, supported copper oxide (CuO) nanostructures have received a great deal of attention as inexpensive and non-plasmonic catalysts for oxidation reaction.
Titanium dioxide (TiO2) nanotubes with different microstructures and surface morphologies have been synthesized by various techniques, such as electrochemical anodic oxidation, template-assisted electrodeposition, and the hydrothermal method. In the anodic fabrication of TiO2 nanotubes, highly ordered structures on a substrate can be easily obtained, but the potential for mass production is limited. For the templating method, the fabrication procedure includes many steps and great care must be taken during template removal in order to keep the nanotubes intact. Among them, the hydrothermal treatment method has received great attention owing to a fairly simple synthesis, relatively low cost processing, and chemical versatility and scalability, even though randomly aligned nanotubes are mainly obtained. It would obviously be desirable to be able to easily synthesize a catalyst making use of the desirable properties of copper oxide nanostructures and titanium dioxide nanotube support. Thus, a method of making a copper oxide-titanium dioxide nanocatalyst solving the aforementioned problems is desired.
The method of making a copper oxide-titanium dioxide nanocatalyst is a process for preparing a catalyst for performing catalytic oxidation of carbon monoxide. The copper oxide-titanium dioxide nanocatalyst is in the form of copper oxide (CuO) nanoparticles supported on mesoporous titanium dioxide (TiO2) nanotubes. The copper oxide-titanium dioxide nanocatalyst is prepared by adding an aqueous solution of Cu(NO3)2.3H2O to an aqueous suspension of titanium dioxide nanotubes. Deposition precipitation at constant alkaline pH is used to form the copper oxide nanoparticles supported on mesoporous titanium dioxide nanotubes. Aqueous sodium carbonate is used to adjust the pH. The solid matter (i.e., copper oxide deposited on titanium dioxide nanotubes) is separated from the suspension, washed, dried and calcined, yielding the copper oxide-titanium dioxide nanocatalyst. Carbon monoxide may then flow over a fixed-bed reactor loaded with the copper oxide-titanium dioxide nanocatalyst at a temperature between 80° C. and 200° C. to catalyze oxidation of the carbon monoxide.
These and other features of the present invention will become readily apparent upon further review of the following specification and drawings.
Similar reference characters denote corresponding features consistently throughout the attached drawings.
The method of making a copper oxide-titanium dioxide nanocatalyst is a process for preparing a catalyst for performing catalytic oxidation of carbon monoxide. The copper oxide-titanium dioxide nanocatalyst is in the form of copper oxide (CuO) nanoparticles supported on mesoporous titanium dioxide (TiO2) nanotubes. The copper oxide-titanium dioxide nanocatalyst is prepared by adding an aqueous solution of Cu(NO3)2.3H2O to an aqueous suspension of titanium dioxide nanotubes. Deposition precipitation at constant alkaline pH is used to form the copper oxide nanoparticles supported on mesoporous titanium dioxide nanotubes. Aqueous sodium carbonate is used to adjust the pH. The solid matter (i.e., copper oxide deposited on titanium dioxide nanotubes) is separated from the suspension, washed, dried and calcined, yielding the copper oxide-titanium dioxide nanocatalyst. Carbon monoxide may then flow over a fixed-bed reactor loaded with the copper oxide-titanium dioxide nanocatalyst at a temperature between 80° C. and 200° C. to catalyze oxidation of the carbon monoxide.
The initial TiO2 nanotube support was prepared by the hydrothermal treatment of anatase TiO2 powder in concentrated aqueous NaOH solution, followed by acid treatment in HCl solution. In an exemplary synthesis, 2 g of TiO2 anatase powder was added to 40 mL of 10 M NaOH aqueous solution in a 100 ml Teflon® vessel, specifically a Teflon®-lined stainless steel autoclave. This mixture was stirred for 30 minutes, and then the Teflon® vessel was placed in an electric oven and heated at 140° C. for 48 hours. After this hydrothermal treatment, the products were separated by filtration and rinsed three times with deionized water. The products were then soaked in an aqueous solution for two hours at room temperature after adjusting the pH to 6 with 0.1 M HCl. The products were separated by filtration and dried in an oven at 80° C. overnight. Different samples, both with and without the acid treatment, were calcined at 450° C. for two hours at a ramp rate of 2° C./minute.
The copper oxide-titanium dioxide (CuO/TiO2) nanotube catalyst was synthesized by the deposition precipitation (DP) method at room temperature and constant pH. As noted above, the CuO/TiO2 nanocatalyst was prepared by mixing an aqueous suspension of the TiO2 nanotubes, prepared as described above, and Cu(NO3)2.3H2O with a solution of precipitating agent (Na2CO3) at constant pH and room temperature. Precipitates with different loadings of CuO (2-65 wt %) in mixture with TiO2 were prepared by changing the molar ratio of Cu2+ to TiO2 nanotube support.
In an exemplary synthesis, 100 mg of TiO2 nanotube powder, prepared as described above, was suspended in 100 mL of deionized (DI) water, and the suspension was sonicated for 10 minutes. A pre-determined volume of an aqueous solution containing an appropriate amount of Cu(NO3)2.3H2O was added to the TiO2 suspension, and the mixture was stirred at room temperature for two hours. An appropriate volume of 0.2 mol/L Na2CO3 aqueous solution was then added dropwise to this suspension until the pH of the suspension reached about 9. The mixture with the precipitate was then aged under stirring for another five hours. Upon completion, the resulting precipitate was separated by centrifuge, washed three times with DI water, and then dried at 80° C. in air for 24 hours. The dried powder was calcined in a muffle furnace at 400° C. for two hours in air with a ramp rate of 2° C./minute.
For the analytical results presented below, the experimentally prepared CuO/TiO2 nanocatalysts were characterized with various physical and chemical techniques. Transmission electron microscopy (TEM) images were acquired using a transmission electron microscope with an accelerated voltage of 200 kV. Powder samples for TEM were dispersed in ethanol by ultrasonication, and a droplet of the colloid suspension was dipped onto a carbon-coated 200-mesh copper grid and allowed to dry in air at room temperature. Scanning electron microscopy (SEM) images and energy dispersive X-ray spectroscopy (EDX) measurements were carried out using an electron scanning microscope. The powder was sputter-coated with gold prior to the SEM analysis whenever needed. Powder X-ray diffraction (XRD) measurements were carried out at room temperature using a diffractometer with Cu KαR radiation at 30 kV and 20 mA, between 20 angles of 5° and 80° with a scanning rate of 0.025° per step per second. Elemental analyses were carried out using inductively coupled plasma optical emission spectroscopy (ICP-OES) calibrated with standards. Samples were digested in 5 mL concentrated HNO3 overnight, and the solution was diluted with water prior to measurement. Further, nitrogen adsorption and desorption isotherms of different samples were obtained at 77 K using a pore size analyzer in the range of 0.05 to 1 relative pressure
The specific surface area was derived using the Brunauer-Emmett-Teller (BET) method, and the pore size distribution and average pore size were estimated from the adsorption branch using the Barret-Joyner-Halenda (BJH) method. Prior to analysis measurement, all samples were degassed in an N2—He mixture at 90° C. for one hour and at 150° C. for an additional hour.
Additionally, thermal analysis was conducted using a thermal gravimetric analyzer (TGA) from 50° C. to 600° C. in air at a linear heating rate of 10° C./minute. X-ray photoelectron spectroscopy (XPS) measurements were carried out with a monochromatic Al Kα radiation source (1486.6 eV) in a UHV environment (about 5×10−9 Torr). The operating conditions were kept constant at a resolution pass energy of 20 eV, emission current of 10 mA, and anode HT of 15 kV. In order to subtract the surface charging effect, the Cls peak at 284.8 eV was used for calibration. Surface composition, when applicable, was determined by normalized integration of the resulting peaks.
Catalytic CO oxidation experiments were performed to determine the activity of the present copper oxide-titanium dioxide nanocatalyst. Experiments were performed using a continuous flow fixed-bed catalytic reactor, such as that shown in
The reaction gas feed mixture consisted of 4% CO and 20% O2 in a balance of He and was passed through the catalyst bed at a flow rate of 50 cm3/min. The flow rate was controlled by a set of digital mass flow controllers 22. All experiments were carried out at atmospheric pressure (1 atm) in the temperature range of 30° C. to 400° C. with a ramp rate of 4° C./min. The signal from the thermocouple 20 was acquired using an interface board 24 and a data acquisition unit 25. The effluent gas was fed into an inline multichannel infrared gas analyzer 26 to analyze the exit gas and simultaneously monitor the CO conversion. The volume percent of CO, CO2 and O2 gases were determined simultaneously and logged with the catalyst temperature during the course of the experiment. The catalytic activity was expressed by the conversion of CO in the effluent gas and indicated as CO conversion percentage as CO Conversion (%)=[(COin−COout)/COin]×100.
For kinetic measurements, CO oxidation experiments were performed by varying the catalyst loadings as 15, 30, 50, 75 and 100 mg, respectively, while keeping the catalysts bed length and the gas hourly space velocity of the feed gas mixture fixed at 3600 cm3/h. The CO oxidation reaction rates were calculated as
where XCO is the fractional CO conversion, W is the mass of the catalyst in grams, and FCO is the CO molar flow rate. The apparent activation energies of selected catalysts were determined from the Arrhenius plots using data from the linear region of <20% CO conversion.
The TiO2 nanotubes prepared by the hydrothermal treatment of spherical TiO2 particles described above are shown in
It has been reported that the structure of the nanotubes is affected by the final pH value of the rinse water. After HCl washing at higher pH values, aggregates with low porosities for nanotubes composed of titanate compounds are often obtained. With HCl washing at low pH values of 1-2, high porosity nanotubes aggregates could be obtained. It has been generally accepted that the layered titanate structure identified as A2TiO7 and A2Ti2O4(OH)2/A2Ti2O5 3H2O (A=Na and/or H) are transformed into anatase phase TiO2 nanotubes through the Na/H exchange following the acid treatment. As shown in
Transition metal oxide couples, such as supported CuO—Cu2O—Cu, have been shown to possess high oxygen release capacity at a high range of temperatures. They have been shown as good candidate materials for oxygen storage, and to provide oxygen for combustion during the reversible phase changes between CuO—Cu2O—Cu at high temperature. The reducible metal oxides can release gaseous oxygen at high temperatures, which can be used for the combustion/oxidation of CO, and the metal oxide can subsequently be regenerated in air. The chemical looping in CuO—Cu2O—Cu requires the reversible phase between CuO—Cu2O—Cu, either from reduction of CuO to Cu by the gaseous fuel (CO in this case), or with the release of gaseous oxygen O2 from 2CuO (s)→Cu2O (s)+½ O2 (g), followed by the regeneration in air back to CuO. It should be noted that the equilibrium partial pressure of gaseous O2 release for Cu2O—Cu is too low for practical use. In experiments, highly dispersed CuO nanoparticles supported on TiO2 nanotubes were prepared by DP.
For CuO-modified TiO2 nanotubes, XRD patterns of samples with a CuO content up to 5% do not show reflection peaks of copper oxide phase, and only the phase of anatase TiO2 is observed. This is attributed to the low XRD signal, and a similar behavior was reported for CuO-incorporated TiO2 samples with less than 3% Cu species content. On the other hand, the XRD patterns of CuO-modified TiO2 nanotubes with CuO ratios larger than 5% exhibit reflection peaks at 35.5° (−111) and 38.6° (111), which are characteristic to the CuO phase in correlation with the reference pattern of tenorite CuO (Card JCPDS No. 00-001-1117) and literature values for monoclinic phase CuO. The strong reflection peaks of all samples indicate the purity and high crystalline nature of the TiO2 and CuO phases in the different CuO-modified TiO2 nanostructures.
The weight loss from the thermal gravimetric measurements of the anatase TiO2 and TiO2 nanotubes are shown in
This decrease in the surface area of the TiO2 support upon incorporation of CuO is attributed to the occupying of the porous features with the CuO nanoparticles and is in agreement with trends reported in the literature. While the BET surface area is decreasing with increased amounts of CuO, there is no remarkable change in the corresponding average pore size, ranging from 21 nm to 23 nm (Table 1), which indicates the porous structure did not significantly change, thus making the CuO-modified TiO2 nanotubes suitable for heterogeneous catalytic application, based on their abundant porous channels. The pore size values are in a good agreement with values reported for CuO—TiO2 prepared by a spray drying process.
X-ray photoelectron spectroscopy (XPS) measurements were carried out to investigate the chemical environment and electronic structure of Ti, O and Cu in the TiO2 nanotubes, CuO nanoparticles and CuO—TiO2 nanostructures, as shown in
The formation of oxygen vacancies and the existence of Ti3+ in the CuO-modified TiO2 nanotubes were also confirmed by the upshift of the Eg mode of the CuO-modified TiO2 compared to unmodified TiO2 nanotubes, as presented in the Raman spectra shown in
The structural phases of TiO2 and CuO can be sensitively identified from Raman scattering analysis based on their characteristic Raman bands.
The first Eg mode value associated with the symmetric vibration of O—Ti—O in TiO2 is found to be 140 for the TiO2 nanotubes and 145 for the CuO—TiO2 nanocatalyst, which is in good agreement with the values of 142 and 143, respectively, found in the literature. The second Eg mode value associated with the symmetric vibration of O—Ti—O in TiO2 is found to be 195 for the TiO2 nanotubes, which is in good agreement with values of 194 and 196, found in the literature. The third Eg mode value associated with the symmetric vibration of O—Ti—O in TiO2 is found to be 637 for the TiO2 nanotubes, which is in good agreement with values of 636 and 637 found in the literature.
The B1g value associated with the symmetric bending of O—Ti—O in TiO2 is found to be 394 for the TiO2 nanotubes, which is in good agreement with the values of 393 and 395 found in the literature. The A1g+B1g value associated with the antisymmetric bending vibration of O—Ti—O in TiO2 is found to be 510 for the TiO2 nanotubes, which is in good agreement with the value of 514 found in the literature. The Ag value for the Cuo-TiO2 nanocatalyst is found to be 274.
In the present case, the Raman spectrum exhibits five pronounced peaks at 140 cm−1 (Eg), 194 cm−1 (Eg), 393 cm−1 (B1g), 514 cm−1 (A1g+B1g) and 637 cm−1 (Eg), in addition to a weak combination band/broad feature at 125 cm−1. The intense E2g peak at 140 cm−1, the two low-intensity Eg peaks at 195 cm−1 and 637 cm−1 and other broad peaks at 394 cm−1 and 510 cm−1, corresponding to B1g and A1g+B1g, respectively, confirm the anatase phase of the TiO2 nanotubes and are in good agreement with values reported in the literature. The intense E2g peak associated with the symmetric stretching vibration of O—Ti—O in TiO2 is broadened and shifted to a high wavenumber in the case of CuO-modified TiO2 nanotubes. The Eg band is shifted to a higher wavenumber along with an increase in line width, as indicated by the full width at half maximum (FWHM) when CuO is added, as shown in
The upshift may be attributed to phonon confinement and nonstoichiometric defects in nanosized TiO2 upon modification with the CuO phase, which could have resulted in structural distortion on the crystal lattice of TiO2 nanotubes after the CuO loading, thus indicating a strong interaction between CuO and the TiO2 support. Doping TiO2 with different metallic dopant species induces structural changes, resulting in higher wavenumbers and the broadening due to size decrease, in agreement with the literature for Cu-doped TiO2 and Cr3+ doped TiO2. The ionic size of Cu2+ (0.73 Å) is larger than that of Ti4+ (0.64 Å), thus the incorporation of Cu ions will distort the lattice structure of TiO2.
Additionally, there might be oxygen vacancies in the TiO2 lattice upon Cu incorporation due to the charge difference between Cu2+ and Ti4+ so that the charge neutrality is maintained. This result is confirmed by the XPS analysis of
Further, the quantum size effect has been shown to play a role in broadening and peak shift. This is also possible in the present case, since the CuO is in the nanoscale regime, as indicated by TEM and XRD results, so the phonon confinement will be prominent. Therefore, both phonon confinement and structural defects may result in the shifting and broadening of the Raman Eg peak, primarily due to the oxygen vacancy generation and lattice disruption of CuO—TiO2.
The effect of the contact time in terms of W/FCO on the CO conversion is also presented in
The higher activation energy of the 50 wt % catalyst might suggest that its catalytic activity is lower than the other two catalysts, which contradicts the order of the CO oxidation reaction rates where the 50 wt % catalyst demonstrated the highest reaction rate of 36 μmole s−1 g−1 compared to 30 μmole s−1 g−1 and 4 μmole s−1 g−1 for the 20 wt % and 2 wt % catalysts, respectively. However, the experimentally-determined Arrhenius parameters and activation energies represent, at first, apparent values. The apparent activation energy for a bimolecular catalyzed reaction depends not only on the true surface activation energy and the heat of adsorption of reactants. Thus, the high values of the apparent activation energies may be due to the strong CO adsorption bond energy to the catalyst surface, which increases with increasing Cu loading and not actually due to a hindered surface reaction, as the order of the apparent activation energy may indicate.
It is to be understood that the present invention is not limited to the embodiments described above, but encompasses any and all embodiments within the scope of the following claims.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/US2017/021669 | 3/9/2017 | WO | 00 |
| Publishing Document | Publishing Date | Country | Kind |
|---|---|---|---|
| WO2017/156330 | 9/14/2017 | WO | A |
| Number | Name | Date | Kind |
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| 20100179053 | Lin | Jul 2010 | A1 |
| 20120152336 | Cao | Jun 2012 | A1 |
| 20130017611 | Li | Jan 2013 | A1 |
| 20160045908 | Kumari et al. | Feb 2016 | A1 |
| 20170267542 | Yang | Sep 2017 | A1 |
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| Number | Date | Country | |
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| 20190060877 A1 | Feb 2019 | US |
| Number | Date | Country | |
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| 62306041 | Mar 2016 | US |
