CHAMBER EXPLOSION SYNTHESIS OF TIO2-TIC HYBRIDS

Abstract

A one-step process to synthesize narrow band gap TiO.sub.2TiC core-shell particles is described. A mixture of a fuel source, particularly a hydrocarbon, and a titanium precursor is detonated with a source of oxygen in a constant volume reaction vessel to produce TiO.sub.2TiC core-shell particles. This process can synthesize TiO.sub.2TiC core-shell structures with tailored morphology, size, phase, absorption behavior, and other hybrid morphologies with different properties depending on the Ti/C ratio used in the feed.

Claims

1. A method of producing a TiO.sub.2TiC hybrid material comprising: forming a gaseous reaction mixture of a fuel and a titanium precursor material within a reaction vessel; supplying energy to the reaction mixture in the presence of oxygen and initiating an exothermic reaction and forming the TiO.sub.2TiC hybrid material.

2. The method of claim 1, wherein the fuel comprises a C2 to C12 hydrocarbon compound.

3. The method of claim 2, wherein the hydrocarbon compound comprises an aromatic hydrocarbon compound.

4. The method of claim 4, wherein the aromatic hydrocarbon compound comprises xylene, toluene, and/or benzene.

5. The method of claim 1, wherein the titanium precursor material has a boiling point of from about 80 C. to about 300 C.

6. The method of claim 5, wherein the titanium precursor comprises a halogenated titanium compound and/or a titanium alkoxide compound.

7. The method of claim 6, wherein the titanium precursor comprises titanium tetrachloride and/or titanium isopropoxide.

8. The method of claim 1, wherein the exothermic reaction comprises a detonation reaction.

9. The method of claim 1, wherein the step of supplying energy to the reaction mixture comprises supplying an electric spark to the reaction mixture.

10. The method of claim 1, wherein the molar ratio of the titanium precursor to the fuel in the reaction mixture is from about 0.05:1 to 2:1.

11. The method of claim 1, wherein the TiO.sub.2TiC hybrid material further comprises graphene.

12. The method of claim 11, further comprising the step of calcining the TiO.sub.2TiC hybrid material to remove graphene therefrom.

13. The method of claim 12, wherein the calcining step occurs at a temperature of at least 400 C.

14. The method of claim 1, wherein the TiO.sub.2 of the hybrid material comprises rutile and/or anatase.

15. The method of claim 1, wherein the TiO.sub.2TiC hybrid material comprises a plurality of core-shell particles having particle sizes of 1 m or less.

16. The method of claim 1, wherein the TiO.sub.2TiC hybrid material comprises a plurality of core-shell particles having particle sizes of from about 50 nm to about 1 m.

17. The method of claim 1, wherein the TiO.sub.2TiC hybrid material comprises a plurality of core-shell particles, wherein the core comprises TiC and the shell comprises TiO.sub.2.

18. A TiO.sub.2TiC hybrid material formed by the method of claim 1.

19. The hybrid material of claim 18, wherein the hybrid material exhibits a band gap of less than 3.2 eV.

20. The hybrid material of claim 19, wherein the hybrid material exhibits a band gap of from about 2.93 to 3.06 eV.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] FIG. 1 is a schematic depiction of an exemplary apparatus for producing the TiO.sub.2TiC hybrid materials;

[0018] FIG. 2 provides SEM images of TiO.sub.2TiC hybrids from explosion synthesis: (a) TiO.sub.2TiC-0.16, (b) TiO.sub.2TiC-0.38, (c) TiO.sub.2TiC-0.73, and (d) TiO.sub.2TiC-1;

[0019] FIG. 3 depicts: (a) a TEM image of the core-shell structure; (b) a high-resolution TEM image, (c) an SAED pattern; (d) a dark field TEM image and the corresponding EDS mapping; and (e) another TEM image and the EDS scan line along the arrow in the image;

[0020] FIG. 4 depicts: (a) NO conversion and (b) DeNOx index and NOx storage selectivity of TiO.sub.2TiC-0.73 under different humidity levels (high and low humidity), (c) NO conversion and (d) DeNOx index and NOx storage selectivity of P25 and TiO.sub.2TiC-0.73 under high humidity; and

[0021] FIG. 5 depicts charts of NOx oxidation over TiO.sub.2TiC hybrids and commercial TiO.sub.2 (P25) under blue light exposure and 50% relative humidity with (a) illustrating NO conversion and (b) illustrating DeNOx index.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0022] In one or more embodiments, a gaseous reaction mixture comprising a fuel and a titanium precursor undergoes a rapid, exothermic reaction in the presence of an oxidizing agent, preferably molecular oxygen, to yield a TiO.sub.2TiC hybrid material.

[0023] Preferably, the fuel comprises a material that is typically a liquid at 25 C. In further embodiments, the fuel comprises a C2 to C12 hydrocarbon compound. In still further embodiments, the hydrocarbon compound comprises an aromatic hydrocarbon compound such as xylene, toluene, benzene, or a mixture thereof.

[0024] In one or more embodiments, the titanium precursor material has a boiling point of from about 80 C. to about 300 C., or from about 100 C. to about 250 C., or from about 125 C. to about 200 C., thus enabling the liquid precursor to be volatilized under relatively mild heating conditions. In particular embodiments, the titanium precursor comprises a halogenated titanium compound (e.g., a titanium halide) and/or a titanium alkoxide compound. In certain embodiments, the titanium precursor comprises titanium tetrachloride and/or titanium isopropoxide.

[0025] In one or more embodiments, the exothermic reaction comprises a detonation reaction. As used herein, detonation is distinguished from mere deflagration or burning of the carbon-containing material. Detonation typically involves a supersonic exothermic front that accelerates through a medium that eventually drives a shock front propagating directly in front of it. Deflagration is typically described as subsonic combustion propagating through heat transfer. Detonation reactions are also generally characterized by the production of higher temperatures in the reactants and reaction products. The exothermic reaction can be initiated via any conventional means such as an electrical spark generator.

[0026] In one or more embodiments, the exothermic reaction achieves temperatures within the reaction vessel of at least 2000K, at least 2250K, or at least 2500K. Preferably, the temperature within the reaction vessel during the exothermic reaction is from about 2000K to about 4000K, about 2500K to about 3500K, or about 2750K to about 3000K, thus permitting the formation of graphene along with the TiO.sub.2TiC hybrid material.

[0027] In particular embodiments, as depicted in FIG. 1, the exothermic reaction takes place within a constant volume reaction vessel 10. The use of a constant volume reaction vessel is preferred as the exothermic reaction produces no work. Thus, all energy released by the exothermic reaction is used to raise the temperature within the reaction vessel leading to the formation of the graphene and TiO.sub.2TiC hybrid material.

[0028] Fuel from a fuel source 12 and the titanium precursor form a titanium precursor source 14 are fed to the reaction vessel 10. The fuel 12 and titanium precursor 14 can be fed directly to the reaction vessel 10 separately, or they can be combined upstream of the reaction vessel and introduced as a combined stream. In addition, the fuel 12 and titanium precursor 14 can be introduced into the reaction vessel 10 as an aerosol in which droplets of the liquid fuel 12 and liquid titanium precursor 14 are suspended within an oxygen-containing gas. Alternatively, the liquid fuel 12 and liquid titanium precursor 14 can be heated and vaporized and introduced into the reaction vessel 10 in gaseous form. A source of oxygen, such as molecular oxygen, air, or a NOx compound, can be separately introduced into the reaction vessel 10, or it can be included within one of streams 12 or 14.

[0029] The physical characteristics of the TiO.sub.2TiC hybrid material formed via the exothermic reaction can be controlled by controlling the ratio of the titanium precursor material to fuel present within the reaction mixture. In one or more embodiments, the molar ratio of the titanium precursor to the fuel in the reaction mixture is from about 0.05:1 to 2:1, from about 0.075:1 to 1.5:1, from about 0.1:1 to 1.25:1, or from about 0.16:1 to 1:1.

[0030] The reaction mixture within the reaction vessel 10 is ignited, such as with a spark-generating device. Once initiated, the reaction proceeds exothermically to produce reaction products 16 and a waste stream 18 comprising unreacted fuel or titanium precursor and/or gaseous byproducts. As mentioned above, the exothermic reaction may be conducted at temperatures sufficiently high in which graphene is formed. Thus, the reaction product 16 formed not only comprises the TiO.sub.2TiC hybrid material, but quantities of graphene as well. In certain embodiments, it is desirable to remove the graphene from the TiO.sub.2TiC hybrid material. Therefore, methods according to the present invention may further comprise a step of calcining the TiO.sub.2TiC hybrid material to remove graphene therefrom. In particular embodiments, the calcination step occurs at a temperature of at least 400 C., at least 450 C., or at least 500 C.

[0031] In one or more embodiments, the TiO.sub.2TiC hybrid material comprises a plurality of core-shell particles. Particularly, the core of the core-shell particles comprises TiC and the shell comprises TiO.sub.2. In certain embodiments, the hybrid material comprises a plurality of core-shell particles having particle sizes of 1 m or less, 500 nm or less, or 300 nm or less. In alternate embodiments, the hybrid material comprises a plurality of core-shell particles having particle sizes of from about 50 nm to about 1 m, from about 100 nm to about 500 nm, or from about 200 nm to about 300 nm.

[0032] In one or more embodiments, the TiO.sub.2TiC hybrid material exhibits a band gap of less than 3.2 eV. In particular embodiments, the hybrid material exhibits a band gap of from about 2.93 to about 3.06 eV.

EXAMPLES

[0033] The following examples set forth preferred compositions and methods according to one or more embodiments of the present invention. It is understood, however, that these examples are provided by way of illustration and nothing therein should be taken as a limitation upon the overall scope of the invention.

[0034] In these examples a mixture of hydrocarbon (such as toluene (C.sub.7H.sub.8 (l), xylene (l), or benzene (l)) and titanium precursor (such as titanium tetrachloride (TiCl.sub.4 (l)) or titanium isopropoxide (C.sub.12H.sub.28O.sub.4Ti (l)) are exploded in a batch reactor (see, FIG. 1) via an electric spark in the presence of oxygen (O.sub.2) to form TiO.sub.2TiC hybrids. In this example, various concentrations of C.sub.7H.sub.8 and TiCl.sub.4 are used, while maintaining a constant O.sub.2 concentration. This process produces large amounts of TiO.sub.2TiC hybrids per second, 5 grams per explosion. XRD analysis confirmed the formation of TiO.sub.2TiC hybrids, evident by the presence of peaks associated with TiC and TiO.sub.2. The samples obtained are labeled as TiO.sub.2TiC-(ratio of moles of TiCl.sub.4 fed into the reactor to moles of C.sub.7H.sub.8 fed into the reactor).

Experimental

[0035] To fabricate the hybrid structures, a mixture of C.sub.7H.sub.8 (l) and TiCl.sub.4 (l) with different ratios was injected into the reactor, which is preheated to 80 C. The different ratios, as shown in Equations 1-4, were 0.16, 0.38, 0.73, and 1.0. The mixture is detonated in the presence of oxygen by a spark (10,000 V) generated from an industrial step-up transformer. Various concentrations of C.sub.7H.sub.8 and TiCl.sub.4 are used, while O.sub.2 concentration was maintained at 0.36 mole, as shown in equations (1-4). TiO.sub.2TiC core-shell was obtained for a Ti/C feed mole ratio of 0.73. The produced structures are denoted as TiO.sub.2TiC-x, where x=mole of C.sub.7H.sub.8/mole of TiCl.sub.4. The resulting products were calcined at 550 C. and characterized using a plethora of material characterization techniques, including X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), transmission electron microscopy (TEM), Raman spectroscopy, and UV-Visible spectroscopy.

[00001]$\begin{array}{cc}0.027\left(\mathrm{mole}\right){\mathrm{TiCl}}_4+0.160\left(\mathrm{mole}\right)C_7{H}_8+0.36\left(\mathrm{mole}\right)O_2={\mathrm{TiO}}_2-\mathrm{TiC}-0.16& \left(1\right)\end{array}$$\begin{array}{cc}0.054\left(\mathrm{mole}\right){\mathrm{TiCl}}_4+0.14\left(\mathrm{mole}\right)C_7{H}_8+0.36\left(\mathrm{mole}\right)O_2={\mathrm{TiO}}_2-\mathrm{TiC}-0.38& \left(2\right)\end{array}$$\begin{array}{cc}0.082\left(\mathrm{mole}\right){\mathrm{TiCl}}_4+0.112\left(\mathrm{mole}\right)C_7{H}_8+0.36\left(\mathrm{mole}\right)O_2={\mathrm{TiO}}_2-\mathrm{TiC}-0.73& \left(3\right)\end{array}$$\begin{array}{cc}0.095\left(\mathrm{mole}\right){\mathrm{TiCl}}_4+0.095\left(\mathrm{mole}\right)C_7{H}_8+0.36\left(\mathrm{mole}\right)O_2={\mathrm{TiO}}_2-\mathrm{TiC}-1& \left(4\right)\end{array}$

[0036] To evaluate the photocatalytic activity, the NOx oxidation experiment was performed in a continuous-flow reactor built following ISO standards. Two gas cylinders were connected to the reactor containing 100 ppm NO in N.sub.2 and breathing air. Mass flow controllers monitored the flow rate of gases to the reactor. The humidity on the inlet gas mixture was measured by a sensor-push HT1 humidity sensor and used to adjust for high and low humidity levels. The reactor was made from steel with a quartz glass window to allow light penetration to the surface of the photocatalyst. The emission wavelength ranges of light used were UV-A (370-405 nm), violet (395-425 nm), royal (410-445 nm), blue (425-465 nm), and green (455-515). A chemiluminescent 42C Low Source analyzer (Thermo Fisher Scientific) was used to measure NO and NO.sub.2 concentrations. A substrate coated with a photocatalyst was loaded into the reactor, and NOx was introduced to the reactor for adequate adsorption of gaseous molecules on the photocatalyst surface. The light was turned on, and the reaction was continued at 1.0 ppm NOx and a total airflow of 1000 sccm at 10 or 50% relative humidity (RH). The light was turned off after the experiment, and the NOx was allowed to re-equilibrate. Average NO, NO.sub.2, and NOx concentrations were determined by averaging all data obtained during the oxidation reaction.

Results and Discussion

[0037] XRD analysis of the detonation structures established the presence of TiC and rutile TiO.sub.2, evident by the peaks at 27.4, 44, 54.4, 56.6, 62.7, 64, and 69 attributed to (110), (210), (211), (220), (002), (310), and (301) facets of rutile, respectively, while peaks at 36.1 and 41.3 are associated with (111) and (200) planes of TiC, respectively. The presence of anatase TiO.sub.2 is only observed at C.sub.7H.sub.8/TiCl.sub.4<1, evident by the presence of peaks at 25, 37.9 and 48.2 ascribed to (101), (004), and (200) planes of anatase TiO.sub.2. The results suggest that the ratio of TiCl.sub.4 to C.sub.7H.sub.8 is a crucial factor in regulating TiO.sub.2 phases in the structure. In addition, SEM images of synthesized structures and the corresponding size histograms indicate that this ratio is also essential to control the size and morphology of the samples. XPS analysis of the samples does not detect the presence of peaks associated with TiC, suggesting that the surface of the samples mainly comprised TiO.sub.2, likely due to the formation of a core-shell structure.

[0038] The process offers precise control over the size and geometry of the produced hybrids. The SEM images in FIG. 2 reveal the possibility of forming TiCTiO.sub.2 with spherical-like structures, as in the case of TiO.sub.2TiC-0.16 and TiO.sub.2TiC-0.73, and sheet-like structures, as in the case of TiO.sub.2TiC-0.38 and TiO.sub.2TiC-1 (see FIG. 2). Consequently, this process empowers the customization of TiO.sub.2TiC hybrids, allowing for the tailoring of their geometry and structure to match the specific requirements of the intended application.

[0039] Compared with P25, the samples exhibited noticeable absorption in the visible light region. Based on the modified Tauc plot approach, the band gaps of TiO.sub.2 in TiO.sub.2TiC-0.16, TiO.sub.2TiC-0.38, TiO.sub.2TiC-0.73 and TiO.sub.2TiC-1 are estimated to be 3.00, 3.14, 2.93 and 3.06 eV, respectively, which are smaller than that of commercial TiO.sub.2 (P25, 3.25 eV). Since the band gap of TiO.sub.2TiC-0.73 is the lowest among the other hybrids, this is the structure of focus hereafter.

[0040] TEM image (a) of FIG. 3 of TiO.sub.2TiC-0.73 confirmed the formation of core-shell structure, and the high-resolution TEM image of (b) of FIG. 3 taken on the shell asserts that the shell is made of TiO.sub.2, evident by the presence of dominant lattice fringe corresponding to (011) rutile TiO.sub.2. EDS mapping (d) of FIG. 3 and EDS line scanning (e) of FIG. 3 detect the presence of a high amount of carbon in the core, suggesting that the TiC is located in the core. While the peaks in the XPS spectrum of TiO.sub.2TiC-0.73 are mainly attributed to TiO.sub.2, as mentioned earlier, peaks associated with TiC appear upon etching the sample by argon beam, as observed in XPS depth profile analysis. This further indicates that the TiC is located in the core. Therefore, TEM, EDS, and XPS data confirm the formation of a TiC core sandwiched by a TiO.sub.2 shell by the detonation process.

[0041] TiO.sub.2TiC-0.73 is used in NOx oxidation in a continuous flow reactor. TiO.sub.2TiC-0.73 performance under visible light represented by NO conversion, NOx storage selectivity, and DeNOx index are shown in (a) and (b) of FIG. 4. TiO.sub.2TiC-0.73 shows noticeable NO conversion with a purification effect under low and high humidity levels, confirming the potential of utilizing this structure in visible light-driven photocatalysis. The NO conversion of TiO.sub.2TiC-0.72 (ca. 30%) is three times higher than that of commercial TiO.sub.2 (ca. 10%), as depicted in (c) of FIG. 5, further confirming the activity of TiO.sub.2TiC-0.73 under visible light. It is noted that under visible light and high humidity, commercial TiO.sub.2 did not generate NO.sub.2, evident by its purification effect ((d) of FIG. 4). This observation contradicts the characteristic performance of P25, which is featured by its toxification effect. Therefore, considering the relatively large band gap of P25 (3.25 eV), it is reasonable to assume that the 10% conversion achieved under visible is due to non-photocatalytic routes. Interestingly, TiO.sub.2TiC-0.73 showed higher NO conversion than P25 under UV light. Both P25 and TiO.sub.2TiC-0.72 have a more or less identical surface area, 47.89 m.sup.2/g for TiO.sub.2TiC-0.73 and 58.96 m.sup.2/g for P25. Therefore, the difference in the photocatalytic activity of P25 and TiO.sub.2TiC-0.73 under UV light is attributed to the difference in the concentration of charge carriers on their surfaces, with the likelihood of the presence of higher concentration of carriers on TiO.sub.2TiC-0.73.

[0042] The process produced photocatalyst hybrids that can be utilized in NOx oxidation (abatement) under conditions relevant to practical applications, including visible light illumination and humidity levels representative of most urban areas. As depicted in FIG. 5, TiO.sub.2TiC-0.73 showed 35% NO conversion under blue light while demonstrating a positive DeNOx index, indicating its purification effect. Upon further modification of TiO.sub.2TiC-0.73 with nickel (Ni), the NO conversion further increased to 45%, and the catalyst demonstrated an exceptional purification effect, evidenced by its high positive DeNOx index of 0.23. It is important to note that the commercial TiO.sub.2 (P25), the gold standard in photocatalysis, demonstrated a lower conversion of 10%, confirming the superior activity of TiO.sub.2TiC hybrids and their potential as efficient photocatalysts to combat NO.sub.x pollution.

[0043] In TiO.sub.2TiC-0.73, TiC is expected to accept the electrons from TiO.sub.2, leaving holes in the shell, which are then tarped by H.sub.2O adsorbed on the surface to form hydroxyl radical, enabling the hole-mediated NOx oxidation. It is noted that TiC is characterized by strong localized surface plasmon resonance (LSPR) absorption in visible light; hence, it can form hot electrons. These energetic electrons may get ejected from TiC to TiO.sub.2 and then trapped by the oxygen adsorbed at the surface to form superoxide radicals, enabling electron-mediated NOx oxidation. The impact of this expected spatial transfer of photoexcited electrons from TiO.sub.2 to TiC and hot electrons from TiC to TiO.sub.2 may favor electron-hole separation.

[0044] In summary, a facile, economical, and scalable approach to synthesize a narrow band gap TiO.sub.2TiC core-shell structure has been demonstrated. This approach is based on detonating a mixture of TiCl.sub.4 and C.sub.7H.sub.8 in the milli-liter reactor to produce 7 grams of the core-shell structure per second. TiO.sub.2TiC core-shell structure absorbs longer wavelengths than commercial TiO.sub.2 (P25) and performs effective NOx oxidation under visible light.

[0045] The process also enables control of the optical absorption behavior of the TiO.sub.2TiC hybrids. The UV-Vis absorption spectra of the samples reveal that the absorption of TiO.sub.2TiC can be tuned depending on the concentration of TiCl.sub.4 and C.sub.7H.sub.8 used in the explosion. Tauc plots showed that the hybrids possess band gaps ranging from 2.93 to 3.06 eV. It is noteworthy that the band gaps of explosion hybrids are smaller than that of commercial TiO.sub.2 (P25, 3.2 eV); therefore, they are capable of absorbing longer wavelengths than P25, making them suitable for the efficient harvesting of renewable energy from the sun.

[0046] The process enables the formation of a TiO.sub.2TiC core-shell structure. For example, TiO.sub.2TiC-0.73 is a core-shell structure, represented by a TiO.sub.2 shell surrounding a TiC core as depicted by TEM images and corresponding X-ray energy dispersive spectra (EDS) and selected area electron diffraction (SAED) analysis (FIG. 3). The TiO.sub.2TiC core-shell structure is highly desired in various applications, including catalysis and energy storage.