WO2013115613A1

WO2013115613A1 - Durability-improved catalyst for reductive elimination of nitrogen oxides

Info

Publication number: WO2013115613A1
Application number: PCT/KR2013/000859
Authority: WO
Inventors: 한현식; 김은석; 서곤; 박세민
Original assignee: 희성촉매 주식회사
Priority date: 2012-02-02
Filing date: 2013-02-04
Publication date: 2013-08-08
Also published as: KR20130089348A

Abstract

The present invention relates to a catalyst for the reductive elimination of nitrogen oxides using hydrogen and, more specifically, to a catalyst for the reductive elimination of nitrogen oxides, the catalyst having high nitrogen selectivity and improved durability and, more particularly, to a complex oxide catalyst for the reductive elimination of nitrogen oxides, wherein reducing metal oxides are fixed to one or both of the basic oxides consisting of SiO₂ and Al₂O₃, and noble metals are supported thereon.

Description

Nitrogen oxide reduction removal catalyst with improved durability

The present invention relates to a catalyst for the reduction and removal of nitrogen oxides using hydrogen, and more particularly, to a catalyst for the reduction and removal of nitrogen oxides having high nitrogen selectivity and improved durability.

Precious metals can activate hydrogen and thus have catalytic activity in the H ₂ -SCR reaction of NOx, but under oxygen-containing conditions, the noble metal proceeds simultaneously in the H ₂ -SCR reaction and hydrogen-oxygen reaction, thus providing a selectivity for the removal of nitrogen oxides. low. In addition, in the oxygen-containing conditions, N ₂ O at low temperature and NO ₂ is mainly generated at low temperature, rather than nitrogen being mainly generated by H ₂ -SCR reaction, so the selectivity for nitrogen is low. Therefore, rather than using the noble metal alone as an active material, the noble metal is supported on oxides such as MnOx, MgO-CeO ₂ and TiO ₂ -ZrO ₂ to enhance nitrogen selectivity in H ₂ -SCR reaction of nitrogen oxides. Adsorption of noble metals to these oxides by their interaction with these oxides is important for determining the extent of H ₂ -SCR reaction and product selectivity.

Alumina is widely used as a support for catalysts for automobile exhaust gas purification as well as petrochemical industry. It is suitable as a precious metal support because it has good dispersibility and thermal stability because of proper interaction with metal. However, in the purification of diesel engine exhaust gas containing sulfur compounds, sulfur oxides and aluminum react to form aluminum sulfate, which accumulates in the catalyst, thereby lowering the activity of the catalyst. In addition, the noble metal and aluminum reacts to produce an inert material such as PtAl ₃ to lower the activity of the noble metal. In order to replace alumina in this regard, studies have been conducted to increase the dispersion of precious metals supported on silica and to not react with the precious metals.

Unlike alumina, silica has no reactivity with sulfur oxides or noble metals, and thus can inhibit the activity deterioration due to the reaction with them. In addition, since the surface area is large and the pore size can be adjusted according to the use, it is very preferable as a support. However, due to the weak interaction with noble metals, prolonged exposure to high temperatures, catalysts containing platinum on silica have a problem that N ₂ O is mainly generated in the H ₂ -SCR reaction of nitrogen oxides, resulting in low selectivity for nitrogen as a reduction and removal product. there was.

According to Korean Patent Publication No. 2010-0013534 disclosed by the present applicant according to the catalyst for the reduction and removal of nitrogen oxides using hydrogen and a method for reducing and removing nitrogen oxides using the same, instead of directly reducing and removing nitrogen oxides with hydrogen, A catalyst composition for reducing a nitrogen oxide in two steps by reacting the prepared ammonia with nitrogen oxide by reacting the ammonia, and a method for reducing nitrogen oxide using the catalyst composition are disclosed. Specifically, one or two or more metal oxides of Group A metal oxides consisting of Fe ₂ O ₃ , Co ₂ O ₃ , NiO and CuO; And a complex oxide catalyst for reducing and removing nitrogen oxide using hydrogen as a reducing agent, in which one or two or more metal oxides of group B metal oxides composed of V ₂ O ₅ , Cr ₂ O ₃ , MnO ₂ , and MoO ₃ are co-precipitated and mixed. Is initiated.

However, there is still a need for catalysts with excellent durability and nitrogen selectivity. In order to solve the above problems, the present invention provides a catalyst supporting a precious metal after fixing a transition metal oxide such as TiO ₂ , ZrO ₂ , CeO ₂ to silica and / or zirconia, so that the performance of the precious metal in the H ₂ -SCR reaction is maximized. It is to propose a catalyst for nitrogen oxide reduction removal that can be expressed as.

In order to achieve the above object, the present invention comprises a composite oxide catalyst as follows.

In the present invention, one or two or more metal oxides of the metal oxides consisting of TiO ₂ , ZrO ₂ and CeO ₂ are fixed to one or both of the base oxides of SiO ₂ and Al ₂ O ₃ , and the precious metal is supported. And a composite oxide catalyst for reducing and removing nitrogen oxides. Without limitation, the present invention has the following features. The metal oxide may be contained in an amount of 0.5 to 50% by weight based on the total weight of the composite oxide catalyst, and the precious metal may be supported by 0.1 to 5% by weight of one precious metal component selected from the group consisting of platinum and palladium.

Nitrogen oxide reduction and removal catalyst carrying a precious metal after fixing a transition metal oxide such as TiO ₂ , ZrO ₂ , CeO ₂ to silica and / or zirconia according to the present invention will exhibit the maximum performance of the precious metal in the H ₂ -SCR reaction. It is excellent in durability and selective reduction.

1 shows an XRD diffraction pattern of a Pt / SiO ₂ catalyst and a Pt / MOx-SiO ₂ catalyst according to the present invention.

2 shows the Pt 4f XPS spectrum of the platinum catalysts according to the present invention.

Figure 3 shows a TEM picture of the catalyst according to the present invention.

Figure 4 shows the IR spectrum of the CO adsorbed on the platinum catalyst according to the present invention. The measurement conditions were measured in N ₂ air stream exposed to CO mixed gas at 50 ° C. for 30 minutes.

FIG. 5 shows FT-IR spectra (A) obtained at a constant temperature of 50 ° C. while flowing NO / H ₂ / O ₂ gas in Pt / SiO ₂ catalyst and Pt / Zr-SiO ₂ catalyst, and 250 ° C. temperature increase process. The obtained FT-IR spectrum (B) is shown.

FIG. 6 shows FT-IR spectra obtained at a constant temperature of 50 ° C. while flowing NO / H ₂ / O ₂ gas in a Pt (1.0) / Ti-SiO ₂ catalyst and a Pt (1.0) / Ce-SiO ₂ catalyst. A) and the FT-IR spectrum (B) obtained during the 250 ° C. temperature increase process.

FIG. 7 shows Pt (1.0) / SiO ₂ and Pt (1.0) / Zr-SiO ₂ while flowing oxygen-free NO / H ₂ / He (NO 480 ppmv / H ₂ 0.8% / He balance) mixed gas. The investigated NO conversion and product yield are shown.

8 shows H of Pt (1.0) / SiO ₂ and Pt (1.0) / Zr-SiO ₂ catalysts at NO / O ₂ / H ₂ reactant (NO 480 ppmv / O ₂ 5% / H ₂ 0.8% / He balance). _It exhibits behavior in the ₂ -SCR reaction.

9 shows Pt / MOx-SiO ₂ (M = Ti, Ce) and Pt (1.0) / Al ₂ at NO / O ₂ / H ₂ reactant (NO 480 ppmv / O ₂ 5% / H ₂ 0.8% / He balance). It shows behavior in H ₂ -SCR reaction of O ₃ catalyst.

10 shows NO conversion and product yield in Pt / Zr-SiO ₂ catalysts with different platinum loading contents.

FIG. 11 shows NO conversion and product yield in Pt, Pd, Rh, Ru (5.0) / Zr—SiO ₂ catalysts.

12 is hydrothermal treatment of Pt (1.0) / SiO ₂ , Pt (1.0) / Zr-SiO ₂ , Pt (1.0) / Al ₂ O ₃ catalyst for 4 hours at 850 ° C. while flowing nitrogen containing 10% by volume of water. The XRD pattern of one catalyst is shown.

FIG. 13 shows the results of H ₂ -SCR reaction (NO 480 ppmv / O ₂ 5% / H ₂ 0.8% / He balance) of NO in (A) and after (B) catalysts before hydrothermal treatment.

The present invention will be described in more detail with reference to the following examples. However, these examples are intended to illustrate this invention in detail, and the present invention is not limited thereto.

Example 1 Preparation of Composite Oxide Catalyst ( Preparation of Pt / MOx-SiO ₂ (M = Ti, Zr, Ce) with Metal Oxide Fixed)

Titania, zirconia on amorphous silica (Si635, Aldrich) using titanium butoxide (99%, Johnson Matthey Co.), zirconium propoxide (99.99%, Aldrich) and cerium nitrate hexahydrate (99.99%, Aldrich) as precursors Ceria was fixed. Tiania and zirconia were fixed to silica by sol-gel method, and ceria was fixed by impregnation method. After dissolving the titanium precursor in anhydrous ethanol was added to silica and reacted at 80 ℃ for 4 hours. The filter was washed, dried for 12 hours at 100 ° C., and calcined at 450 ° C. for 4 hours. Zirconia-fixed silica was also prepared in the same manner. Silica was added to a solution of cerium precursor dissolved in distilled water, stirred for 4 hours, put in a rotary evaporator, and evaporated to dryness. The ceria was fixed and calcined at 450 ° C. for 4 hours to prepare. On the other hand, platinum, a precious metal, was supported by impregnation method. Platinum catalyst was formed by supporting tetraamine platinum (II) nitrate (99%, Aldrich) on silica fixed with metal oxides so that the platinum loading was 0.1, 1.0, 4.0 wt%. The platinum-supported catalyst was dried in a 100 ° C. drier, and the catalyst was reduced in 100 ml / min hydrogen (H ₂ / N ₂ = 50/50, volume%, Korea special gas) stream at 400 ° C. for 2 hours. In this embodiment, titania, zirconia, and ceria oxide-fixed silica are denoted as Ti-SiO ₂ , Zr-SiO ₂ , and Ce-SiO ₂ , respectively, and the platinum supported catalyst is preceded by Pt, for example, Pt / Ti. Named in the form -SiO2.

Experimental Example 1 X-Ray Diffraction Analysis (XRD)

The X-ray diffraction pattern of the catalyst was obtained with an X-ray diffractometer (Rigaku, HR-XRD, Ultima III) using Cu Kα X-rays passed through a Ni filter. In the 40 kV and 40 mA conditions, a range of 5 to 90 degrees was injected at a rate of 2 degrees / minute.

Experimental Example 2 Measurement of Nitrogen Adsorption Isotherm

The adsorption isotherm of nitrogen at liquid nitrogen temperature was obtained by an automatic adsorption device (Mirae SI nanoPorosity-XG) to measure the surface area of the catalyst. 0.1 g of catalyst was placed in a sample tube and evacuated while heating at 250 ° C. for 2 hours before measurement. The surface area was calculated by applying the BET equation to the adsorption isotherm measured at 77 K.

Experimental Example 3 Measurement of Platinum Dispersion

The platinum dispersion supported on the catalyst was observed by transmission electron microscope (TEM, JEM 2000 FXII, JEOL). The catalyst was placed in acetone, vibrated by ultrasonic wave, dispersed, and a part was taken and placed on a carbon grid to prepare a specimen.

Experimental Example 4 Surface Analysis

The surface composition and chemical state of the catalyst were analyzed by X-ray photoelectron spectroscopy (XPS, VG MultiLab 2000) using a monochromated MgKα X-ray source (300 W). The binding energy of each atom was corrected for C 1s (285 eV).

Experimental Example 5 X-ray Absorption Spectroscopy (XANES and EXAFS) Experiment

X-ray absorption spectra of iron and manganese were obtained from the 3C1 beamline of the Pohang Accelerator Center. By adjusting the absorbance of the monochromatic X-rays to 70 ~ 80% range through a double monochromator composed of Si (111) and Si (311) crystals, it is possible to adjust the absorbance of Fe K-edge (7122 eV) and Mn K-edge (6539 eV). Absorbance was measured by fluorescence mode.

Experimental Example 6 Investigation of NO Adsorption Status and Reduction Removal Using FT-IR

The occlusion state of nitrogen oxides was investigated with a BIO-RAD FT-IR spectrophotometer (FTS-175C) attached to in-situ cells (GRASEBY, SPECAC). That is, 10 mg of catalyst was compressed to make a wafer specimen, placed on a specimen support in a cell, and activated at 400 ° C. for 1 hour while flowing helium gas at a rate of 100 ml / min. After cooling to 50 ℃, flowing IR and hydrogen mixture gas (Korea Special Gas, NO 480 ppmv / O ₂ 5% / H ₂ 0.8% / He balance) at 100 ml / min. Painted in the range of -700 cm ^-1 . The signal / noise ratio (S / N) was increased by drawing 16 IR spectra per minute with a resolution of 4 cm ⁻¹ . After maintaining for 30 minutes, the temperature was raised to 400 degrees and the adsorption and reduction behavior of NO were investigated.

Experimental Example 7 Investigation of Reduction Removal Behavior of Nitrogen Oxide in a Flow-type Reactor

The reduction reduction reaction of NO by hydrogen was investigated in an atmospheric pressure type reactor. Roughly, 0.1 g of platinum-supported catalyst was charged into a quartz tube with an outer diameter of 6 mm, and activated at 500 ° C. for 1 hour. A flow-through reactor equipped with an electronic flow regulator was fabricated and the H2-SCR reaction activity was investigated under continuous operating conditions. NO mixture gas (NO 480 ppmv / O ₂ 5% / He balance) was flowed at 100 ml / min at 50˜300 ° C. to reach a steady state, and then hydrogen was supplied. The hydrogen injection amount was adjusted to 0.8%. NO and N ₂ O concentrations were measured by installing a NDIR gas analyzer (Siemens, ULTRAMAT 6) at the end of the reactor, and mass spectrometers (Balzers, QMS200) were used to determine the N ₂ , NO, N ₂ O, and NH ₃ The yield was investigated.

Experimental Example 8 Hydrothermal Treatment Method

The platinum-supported catalyst prepared in Example 1 was placed in an alumina crucible and placed in a quartz tube of a circular kiln. Nitrogen was flowed into a steam vaporizer contained in a circulating precision constant temperature water bath (Jeico Tech, WBC-1506W) to create a mixed gas containing 10% water vapor by volume. The catalyst was hydrothermally treated at 850 ° C. for 4 hours while flowing nitrogen gas containing steam at 100 ml / min.

Experimental Example 9 Sulfur Treatment Method

The platinum-supported catalyst prepared in Example 1 was placed in an alumina crucible and placed in a quartz tube of a circular kiln. Nitrogen was flowed into a steam vaporizer contained in a circulating precision constant temperature water bath (Jeico Tech, WBC-1506W) to create a mixed gas containing 10% water vapor by volume. Nitrogen gas containing water vapor was added to 80 ml / min and SO ₂ (97%, Dong-A Co.) was added to 20 ml / min, and the catalyst was sulfurized at 400 ° C. for 2 hours while flowing at a flow rate of 100 ml / min.

Examine the physicochemical properties of the composite oxide catalyst of the present invention according to the above experiments. First, the composition and surface area of the catalyst carrying platinum are summarized in Table 1.

[Revisions under Rule 91 20.02.2013]
Table 1

The amount of zirconia was around 10 wt%, but the amount of titania was 5.2 wt%. The ceria fixation amount was 8.8 wt%, and all transition metal oxides were fixed in silica. The amount of platinum increased according to the added amount, and most of the added amount was fixed regardless of the type of the transition metal oxide or the type of the support. The surface concentration of the fixed transition metal oxide was considerably high, but the surface concentration of ceria was 0.5 atom%, which was significantly lower than that of titania or zirconia. The surface concentration of platinum was similar (0.1 ~ 0.2 atom%) regardless of fixation of transition metal oxide.

The surface area differed slightly depending on whether or not the transition metal oxide was fixed. As the platinum loading of Pt / SiO ₂ catalyst increases, the surface area decreases from 495 m ² / g to 435 m ² /g.However, in catalysts with fixed transition metal oxides, the surface area is near 400 m ² / g. The results were similar regardless of the type or platinum loading. The surface area of the comparative catalyst Pt (1.0) / Al ₂ O ₃ was 185 m ² / g, which was considerably smaller than that of the platinum catalyst fixed on silica due to the small surface area of the alumina itself.

Agglomeration of platinum or transition metal oxides results in their XRD diffraction peaks. 1 shows the XRD diffraction pattern of the Pt / SiO ₂ catalyst and the Pt / MOx-SiO ₂ catalyst to which the transition metal oxide is fixed. The Pt (4.0) / SiO ₂ catalyst having 4 wt% platinum showed large diffraction peaks (2θ = 39.9, 46.2, 67.4 °) of platinum. In Pt (1.0) / Zr-SiO ₂ with 1 wt% platinum, the diffraction peak of platinum was small, but the diffraction peak was similar in Pt (4.0) / Zr-SiO _{2 with} 4 wt% loading. Pt (1.0) / Ti-SiO ₂ , Pt (1.0) / Ce-SiO ₂ , and Pt (1.0) / Al ₂ O ₃ catalysts showed no diffraction peaks of platinum. Pt / Zr-SiO ₂ and Pt / Ti-SiO ₂ catalysts showed no diffraction peaks of transition metal oxides. However, Pt (1.0) / Ce-SiO ₂ catalyst showed a large diffraction peak of ceria at 2θ = 28.4, 32.9, and 47.4 °. Only Pt (1.0) / Al ₂ O ₃ catalyst showed diffraction peaks of γ-alumina.

Except for ceria, no diffraction peaks of transition metal oxides were observed, so they were considered to be evenly dispersed on the silica surface. Platinum, however, differed in size depending on the type of fixed transition metal oxide. The Pt (4.0) / SiO ₂ catalyst, which had a large amount of platinum directly fixed to silica, showed a large diffraction peak, indicating that platinum was agglomerated smaller than silica. Pt (2.0) / Zr-SiO ₂ and Pt (4.0) / Zr-SiO ₂ catalysts, in which platinum is supported on zirconia-fixed silica, also show small diffraction peaks of platinum, which clumps platinum. Platinum was not severe in the catalyst supported on silica having ceria and titania or directly supported on alumina due to no diffraction peaks of platinum.

The Pt 4f XPS spectrum of the platinum catalyst is shown in FIG. 2. In a catalyst having a platinum loading of 1.0 wt% or less, the platinum loading was small and the peak of platinum was not clear. However, as the amount of platinum supported increased, the peak increased considerably. In the Pt (4.0) / SiO ₂ catalyst in Pt4f7 / 2 peak at 70.8 eV, Pt (4.0) / Zr-SiO2 catalyst appears at 71.5 eV. In the zirconia-fixed catalyst, the binding energy of platinum is slightly high, but all are in the metallic Pt 4f7 / 2 peak range, and thus all platinum is present in the metallic state [1-3]. Pt (4.0) / Zr-SiO the peak intensity of the _second catalyst Pt (4.0) / SiO greater than the _second catalyst Pt (0.1) / Zr-SiO platinum peak of _divalent Pt (0.1) / SiO fixing the cursor zirconia than ₂ to The dispersion state of platinum was improved.

3 shows a TEM image of the platinum catalyst supported on the silica to which the transition metal oxide is fixed. In the Pt (1.0) / SiO ₂ catalyst supported on silica without immobilized transition metal oxide, the size of platinum particles was not large, such as 2-5 nm, but they were gathered together. In contrast, the Pt (1.0) / Zr-SiO ₂ catalyst had a very uniform dispersion of platinum particles of about 5 nm. In the Pt (1.0) / Ti-SiO ₂ catalyst, the particle size of platinum was smaller and uniform, below 5 nm. On the other hand, Pt (1.0) / Ce-SiO ₂ catalysts have platinum particles smaller than 2 nm, but they are agglomerated and non-uniform in size, resulting in low dispersion. The particle size of the platinum supported on the Pt (1.0) / Al ₂ O ₃ catalyst was uneven to 10-20 nm.

In addition, the IR spectrum of CO adsorbed on the platinum catalyst is shown in FIG. 4. In Pt / SiO ₂ catalysts without immobilized transition metal oxides, the absorption bands of CO adsorbed on platinum appear in the range of 2070 to 2080 cm ⁻¹ . As the platinum loading increases, it moves to a slightly higher wavenumber, and the absorption band becomes considerably larger. The absorption band of CO adsorbed at 2070 ~ 2090 cm ^-1 also appears in the platinum catalyst supported on the silica to which the transition metal oxide is fixed. Regardless of whether the transition metal oxide is fixed or not, all absorption bands of adsorbed CO appear in this region, and it can be determined that platinum is in a metal state (Pt0) [4]. However, the absorption band of CO adsorbed on the platinum catalyst supported on the silica fixed with transition metal oxide was considerably smaller than the absorption band of CO adsorbed on Pt / SiO ₂ . In titania or zirconia-fixed catalysts capable of strong metal support interactions (SMSI) with platinum and transition metal oxides, the adsorption band is small due to the small adsorption of CO [5]. However, the CO absorption band was large in the platinum catalyst supported on ceria fixed silica in which SMSI did not appear.

The oxidation state of platinum in Pt / MOx-SiO ₂ catalyst supported on silica fixed with transition metal oxide was not significantly different from that of platinum in Pt / SiO ₂ catalyst, but the particle size and dispersion of platinum were fixed The difference was large depending on the type of metal oxide. In Pt (1.0) / Ti-SiO ₂ and Pt (1.0) / Zr-SiO ₂ catalysts in which titania and zirconia with SMSI are fixed on silica, platinum is immobilized on transition metal oxides evenly dispersed on the surface of platinum particles. It was distributed. On the other hand, in the Pt (1.0) / Ce-SiO ₂ catalyst without SMSI, ceria itself is largely agglomerated, and platinum particles are also unevenly partially agglomerated. In platinum catalysts supported on titania or zirconia-fixed silica, platinum is in the Pt0 state, but due to SMSI, the absorption band of CO is significantly smaller than that of Pt / SiO ₂ catalysts.

Hereinafter, the H2-SCR reaction behavior of NO in a Pt / MOx-SiO ₂ (M = Ti, Zr, Ce) catalyst having a transition metal oxide fixed according to the present invention is analyzed.

The H2-SCR reaction behavior of NO was investigated by FT-IR on Pt / MOx-SiO ₂ catalysts loaded with platinum on silica fixed with transition metal oxides. Spectrum (A) obtained by flowing NO / H ₂ / O ₂ gas in Pt / SiO ₂ catalyst and Pt / Zr-SiO ₂ catalyst and kept constant at 50 ° C and spectrum (B) obtained at elevated temperature at 250 ° C 5 is shown. When the NO / H ₂ / O ₂ mixed gas flowed into the Pt (1.0) / SiO ₂ catalyst at 50 ° C., an absorption band appeared at 1610 cm ⁻¹ . Increasing the temperature quickly disappeared and was not visible at 110 ° C., and no other absorption bands appeared.

The absorption band at 1610 cm ⁻¹ is the absorption band of nitrate (NO ₃ ⁻ ) adsorbed on platinum [6]. In the Pt (1.0) / Zr-SiO ₂ catalyst, on the other hand, the NO / H ₂ / O ₂ mixed gas flows initially at an absorption band of 1627 cm ^-1 , and over time, at 1610, 1589, 1436 cm ^-1 An absorption band appears. 1627 cm ^-1 absorption band of water, 1610cm ^-1 and 1589 cm ^-1 absorption band adsorbed on the surface is NO ₃ adsorbed on platinum and zirconia ^- an absorption band of [4]. The absorption band at 1436 cm ^-1 is due to the NH ₄ ⁺ adsorbed on the Bronsted acid point [7-9]. As the temperature increased, the absorption bands due to NO ₃ ⁻ adsorbed on platinum rapidly decreased around 100 ° C., but the absorption bands of NO ₃ ⁻ adsorbed on zirconia and NH ₄ ⁺ adsorbed on acid sites were the largest at around 120 ° C. As the temperature increased, the temperature gradually decreased, but was not visible at 230 ° C.

In the catalyst directly loaded with platinum on silica, NO was adsorbed to platinum only in the NO ₃ ⁻ state. As the temperature increased, the absorption bands decreased, but no other absorption bands appeared. Therefore, it was not possible to confirm the progress of the H ₂ -SCR reaction of NO. In contrast, in a platinum catalyst supported on zirconia-fixed silica, NO is adsorbed to the NO ₃ ⁻ state and ammonia is produced. Ammonia is adsorbed on the acid sites formed at the interface between silica and immobilized zirconia, resulting in NH ₄ ⁺ absorption bands, showing the possibility of reaction between NO and hydrogen and NO and ammonia.

An IR spectrum of the platinum catalyst supported on silica fixed with titania and ceria instead of zirconia is shown in FIG. 6. In the Pt (1.0) / Ti-SiO ₂ catalyst with titania immobilized on silica, the NO / O ₂ / H ₂ mixed gas flows directly at 1627 cm ^-1 , resulting in a large absorption band due to water, and NH at 1435 cm ^-1 . _The absorption band due to ₄ ⁺ gradually increases. Over time, the absorption band due to water decreases rapidly, and the absorption band of NO ₃ ⁻ adsorbed to platinum and titania continues to grow at 1610 and 1589 cm ⁻¹ . As with the Pt (1.0) / Zr-SiO ₂ catalyst, the absorption bands were all larger at 70 ° C but rapidly decreased at higher temperatures, all disappearing above 170 ° C. On the other hand, when NO / O ₂ / H ₂ mixed gas flows through a Pt (1.0) / Ce-SiO ₂ catalyst with ceria fixed on silica, the absorption band due to NO and water adsorbed on platinum at 1764 and 1627 cm ^-1 Appeared and did not change over time. As the temperature rises to around 210 ° C, the absorption bands of Pt-NO appear and then decrease again [4].

The IR spectrum of the NO / O ₂ / H ₂ mixed gas flowed through a Pt (1.0) / Ti-SiO ₂ catalyst supported on silica with titania is similar to that of the Pt (1.0) / Zr-SiO ₂ catalyst. The formation and dissipation behaviors of NO ₃ ^- and NH ₄ ⁺ are almost identical. However, the IR spectrum when the Pt (1.0) / Ce-SiO2 catalyst is exposed to the NO / O2 / H2 mixture is similar to that found in the Pt (1.0) / SiO ₂ catalyst directly loaded with platinum on silica.

The activity and selectivity of the composite oxide catalyst according to the present invention are analyzed.

The activity and selectivity of Pt / SiO ₂ and Pt / MOx-SiO ₂ catalysts in the H ₂ -SCR reaction of NO investigated by the flow reactor were significantly different depending on the presence or absence of oxygen. FIG. 7 shows NO conversion and product yields investigated for Pt (1.0) / SiO ₂ and Pt (1.0) / Zr-SiO ₂ while flowing NO / H ₂ / He gas containing no oxygen. The NO conversions for Pt (1.0) / SiO ₂ and Pt (1.0) / Zr-SiO ₂ catalysts were nearly equal to 100% in the range of 50-300 ° C. In the H ₂ -SCR reaction, N ₂ , N ₂ O, and NH ₃ were produced, but the difference in product yield was not clear. The N ₂ yield remained constant at 50% over the temperature range. The yield of N ₂ O was as high as 60% at 50 ° C., but rapidly decreased as the temperature increased, and hardly formed above 200 ° C. At elevated temperatures, NH ₃ was produced and yields reached nearly 50% at 300 ° C.

This analysis can be summarized as follows. In the absence of oxygen, in the range of 50 to 300 ° C, NO is N ₂ , N ₂ O, NH in both Pt (1.0) / SiO ₂ or Pt (1.0) / Zr-SiO ₂ regardless of fixation of transition metal oxides. Switch to ₃ The yield of N ₂ is constant at around 50%, N ₂ O is produced at low temperatures, and NH ₃ is produced at elevated temperatures.

Surprisingly, however, the presence of oxygen in the reactants significantly altered the catalytic behavior in the H ₂ -SCR reaction. The behavior in the H ₂ -SCR reaction of these catalysts in the NO / O ₂ / H ₂ reactants is shown in FIG. 8. In the Pt (1.0) / SiO ₂ catalyst, the NO conversion was very low at 50 ° C (18%), but increased to 100% at 100 ° C. The higher the temperature, the faster the conversion. On the other hand, in the Pt (1.0) / Zr-SiO ₂ catalyst, the NO conversion was high as 100% even at 50 ° C., and the temperature was lowered as the temperature was increased, but the conversion was maintained as high as 70% even at 300 ° C.

In the reactant containing oxygen, N ₂ , N ₂ O, NO ₂ were produced, and NH ₃ was not produced. The yield of N ₂ in Pt (1.0) / SiO ₂ catalyst was as high as 43% at 100 ° C., but lower or higher yields were very low. Yields of N ₂ O also show a similar trend, and are produced around 100 ° C. NO ₂ began to form at 150 ° C. and gradually increased with increasing temperature. In Pt (1.0) / Zr-SiO ₂ catalysts, the N ₂ yield was quite high, reaching 70% at 100-125 ° C, and gradually lowered at higher temperatures. At 50 ° C., the N ₂ O yield was as high as 60%, but quickly dropped as the temperature increased. N ₂ O began to form at 150 ° C. and the yield increased steadily as the temperature increased.

In H ₂ -SCR reactions of oxygen-free NO and hydrogen in Pt (1.0) / SiO ₂ or Pt (1.0) / Zr-SiO ₂ catalysts, the conversion with temperature and the yield change of the product are independent of zirconia fixation It was all the same. NO was all converted to N ₂ , N ₂ O, NH ₃ in the range of 50-300 ° C. The yield of N ₂ is constant regardless of the temperature at around 50%, N ₂ O is mainly produced at low temperatures and NH ₃ yield increases with increasing temperature.

In contrast, the behavior of NO ₂ H ₂ -SCR in oxygen-containing reactants differed significantly depending on whether zirconia was immobilized. In Pt (1.0) / SiO ₂ , the conversion was high only at 100 ° C., and at lower or higher temperatures, the conversion was lower. Both N ₂ and N ₂ O yields were high near 100 ° C. and low outside this temperature. NH ₃ was not produced, and when the temperature was increased, NO ₂ was produced at 150 ° C. or higher. However, the Pt (1.0) / Zr-SiO ₂ catalyst supported on zirconia-fixed silica has a very different H ₂ -SCR reaction behavior of NO. The N ₂ yield is also high in the range of 50-200 ° C., and is excellent at 70% in the range of 100-125 ° C. On the other hand, the N ₂ O yield is high at very low temperatures, and the NO ₂ yield is high at 150 ° C or higher.

Consider the theoretical aspect. In oxygen-free NO / H ₂ gas mixtures, H ₂ -SCR mainly occurs in platinum, so there is no fixed effect of zirconia. The dissociated adsorbed N_ * and O_ * also react with the dissociated adsorbed H_ * on the platinum surface. 2H_ * + O_ * → H ₂ O reaction proceeds to remove oxygen from the surface to increase the concentration of N_ * and they react to form N ₂ . Some O_ * reacts with N_ * to N ₂ O and N_ * reacts with H_ * to NH ₃ . In other words, the zirconia fixation affects the oxidation state and the dispersibility of platinum. However, the zirconia fixation does not show an effect of zirconia fixation because of its greater function as a catalyst of platinum. However, the presence of oxygen in the reactants greatly changes the reaction pathway. This is because NO may be activated on the zirconia surface and the acid viscosity generated at the zirconia and silica interface may be involved in the reaction. The H ₂ -SCR reaction behavior of nitrogen oxides differs between a platinum catalyst supported on zirconia-fixed silica and a platinum catalyst directly supported on silica. Oxygen is also adsorbed on the platinum surface and activated, thus competing with dissociative adsorption of NO, thereby inhibiting dissociative adsorption of NO, thereby lowering the conversion rate.

In Pt (1.0) / SiO ₂ catalyst, the lower conversion rate when the temperature is lower or higher than 100 ° C. may be explained by the competitive adsorption of oxygen and NO. At low temperatures, oxygen is mainly adsorbed, so that dissociative adsorption of NO and hydrogen is suppressed, so that the surface N_ * concentration is lowered, so that both N ₂ yield and N ₂ O yield are low. As the reaction temperature increases, the oxidation reaction proceeds due to O_ * on the surface, thereby increasing the NO ₂ yield. It is assumed that the increase in NO conversion and N ₂ and N ₂ O yields at 100 ° C in Pt (1.0) / SiO ₂ catalysts may be related to the dissociation adsorption of NO, but there is no experimental basis to clarify this.

Pt (1.0) / Zr-SiO ₂ catalysts supported on zirconia-fixed silica have high conversion rates because zirconia and acid sites participate in H ₂ -SCR reactions. In the IR spectrum, the formation of ammonia and storage on the surface were confirmed. Thus, the activated NO and spillover H- * reacted in platinum reacted with NH to form NH _3, which formed at the silica and zirconia interface. It is stored in the form of NH ₄ ^{+ at the} acid point. Therefore, in addition to SCR by the direct reaction of hydrogen and NO, the conversion of NO is increased due to the SCR of NO by NH ₃ and the N ₂ yield is high. The low yield of N ₂ O can be explained by the fact that the reaction proceeds in platinum in addition to the zirconia surface.

On the other hand, conversion and product yields in the H ₂ -SCR reaction of NO in a catalyst loaded with platinum on silica fixed with other transition metal oxides besides zirconia are shown in FIG. 9. The NO conversion of Pt (1.0) / Ti-SiO ₂ catalyst was 80% at 50 ℃ and 100% at 75 ~ 200 ℃. The higher the temperature, the lower the temperature, but the conversion rate is high as 78% even at 300 ° C. The results of H ₂ -SCR reaction on Pt (1.0) / Ti-SiO ₂ catalysts are very similar to those on Pt (1.0) / Zr-SiO ₂ catalysts, except that the N ₂ O yield is slightly higher and the N ₂ yield is slightly lower. Titania, like zirconia, is a transition metal oxide that can be reduced, and its catalytic activity is similar because platinum and SMSI are possible.

The platinum-supported Pt (1.0) / Ce-SiO ₂ catalyst after fixing ceria on silica had a high NO conversion of 100% only at 100 ° C and a significantly low conversion rate at other temperature ranges. The low yield of N ₂ and N ₂ O and the generation of NO ₂ above 150 ° C. are almost the same as for Pt (1.0) / SiO ₂ catalysts. Since ceria is an oxygen storage material, NO does not dissociate and adsorb to ceria, and the electronegativity is small so that no acid point is generated at the interface with silica. That is, since most of the H ₂ -SCR reaction of NO in Pt (1.0) / Ce-SiO ₂ catalyst proceeds in platinum, it is similar to the reaction result in Pt (1.0) / SiO ₂ catalyst.

In the Pt (1.0) / Al ₂ O ₃ catalyst as a comparative catalyst, the NO conversion reached 100% in the range of 100 to 150 ° C., but the conversion was lower than 50% in the other temperature range. The yield of N ₂ and N ₂ O was about halfway between Pt (1.0) / Ti-SiO ₂ and Pt (1.0) / Ce-SiO ₂ , and the NO ₂ yield was very low. Alumina does not directly participate in the H ₂ -SCR reaction of NO, such as a support or a ceria with high dispersion of platinum.

In the H ₂ -SCR reaction, platinum is an important active substance that activates hydrogen and NO. FIG. 10 shows NO conversion and product yield in Pt / Zr-SiO ₂ catalysts having different platinum loadings. In the Pt (0.1) / Zr-SiO ₂ catalyst having a platinum loading of 0.1 wt%, the NO conversion was lower than 20% at 50 ° C. However, the catalysts with high platinum loadings had a high NO conversion rate of 100% even at 50 ° C. The catalysts with high platinum loadings had a relatively small decrease in conversion rate with increasing temperature. N ₂ yield was higher with less platinum loading except 50 ℃, while N ₂ O and NO ₂ yields were higher with higher platinum loading, as with NO conversion. The more increased platinum loading can be explained as due to a lot of the reaction that active oxygen increases in the degree of N ₂ is the N ₂ O and NO ₂ produced more reaction progress is generated.

The catalytic behavior also depends on the H ₂ -SCR reaction of NO depending on the type of precious metal supported on the zirconia-fixed silica. FIG. 11 shows the NO conversion and the yield of the product irradiated with Pt, Pd, Rh, Ru (5.0) / Zr-SiO ₂ catalysts having different precious metals. Depending on the type of precious metal, the activity and product yield differed significantly by temperature. The NO conversion was the highest in the Pt (1.0) / Zr-SiO ₂ catalyst with platinum. In the Pd (1.0) / Zr-SiO ₂ catalyst, the reaction proceeds only at 100 ° C. or higher, and the conversion rate increases rapidly with temperature, but drops sharply as it rises above 200 ° C. On the other hand, the Rh (1.0) / Zr-SiO ₂ and Ru (1.0) / Zr-SiO ₂ catalysts had very low conversions at temperatures below 250 ° C., resulting in no catalytic activity. In the Pd-supported Pd (1.0) / Zr-SiO ₂ catalyst, the N ₂ yield was high as 60% at 150-200 ° C. N ₂ O was produced only on catalysts carrying platinum and palladium, and NO ₂ was only generated on catalysts carrying platinum and ruthenium.

Depending on the noble metal different in kind from the H ₂ -SCR of NO activity of the catalyst significantly, Pd, Rh, Ru was not appropriate as an active material in the H ₂ -SCR reaction of NO. It is clear that these differences depending on the types of precious metals may be related to the adsorption properties of NO, NO ₂ , NH ₃ , hydrogen and oxygen, but the precious metals other than platinum have too low catalytic activity and no specific studies have been conducted.

Exhaust gas of diesel engine contains sulfur compounds such as water and sulfur dioxide and needs to be used for a long time, so the catalyst used for H ₂ -SCR reaction of NOx should have excellent hydrothermal stability and endothelial toxicity against sulfur compounds. Therefore, the H2-SCR reaction behavior of NO of Pt / Zr-SiO ₂ catalysts by hydrothermal treatment and sulfur treatment was examined.

Pt (1.0) / SiO ₂ , Pt (1.0) / Zr-SiO ₂ , Pt (1.0) / Al ₂ O ₃ catalysts were treated at 850 ° C. for 4 hours with nitrogen containing 10% by volume of water. The stability was examined. XRD patterns of these catalysts obtained after hydrothermal treatment are shown in FIG. 12. All of the hydrothermally treated catalysts showed the diffraction peaks of platinum, but the behavior of peak changes due to hydrothermal treatment was different. The Pt (1.0) / Zr-SiO ₂ catalyst had the same XRD pattern regardless of hydrothermal treatment. On the other hand, in the Pt (1.0) / SiO ₂ or Pt (1.0) / Al ₂ O ₃ catalysts, the diffraction peak of platinum was significantly increased. That is, the hydrothermal treatment sinters the platinum of the Pt (1.0) / SiO ₂ or Pt (1.0) / Al ₂ O ₃ catalyst so that the diffraction peak is increased, but the platinum is stable in the platinum catalyst supported on the zirconia-fixed silica. It is supported so that it is not sintered, so the diffraction peak of platinum does not increase.

The H ₂ -SCR reaction result of NO in the hydrothermally treated catalyst is shown in FIG. 13. In the Pt (1.0) / Zr-SiO ₂ catalyst, there was no difference in NO conversion or N ₂ yield between the catalysts before and after hydrothermal treatment. However, the Pt (1.0) / SiO ₂ catalyst had a 10% lower NO conversion and a lower N ₂ yield. In the Pt (1.0) / Al ₂ O ₃ catalysts, the overall NO conversion was higher, but the N ₂ yield was lower, increasing the N ₂ O and NO ₂ yields. The Pt (1.0) / Zr-SiO ₂ catalyst, in which zirconia-fixed silica was supported on platinum, maintained its catalytic activity in the H ₂ -SCR reaction even when subjected to hydrothermal treatment under severe conditions. This is because zirconia itself is very stable under hydrothermal conditions and has a strong SMSI effect with platinum, so that platinum, zirconia, and acid sites, which are involved in H ₂ -SCR reaction, are well dispersed without dissipation or disappearing. The activity of Pt (1.0) / SiO ₂ or Pt (1.0) / Al ₂ O ₃ catalysts was reduced by hydrothermal treatment, and the degradation was presumably due to the sintering of platinum, as confirmed by the XRD pattern.

Pt (1.0) / SiO ₂ , Pt (1.0) / Zr-SiO ₂ , Pt (1.0) / Al ₂ O at 400 ° C. for 2 hours while flowing a mixture gas in which sulfur dioxide gas was added to nitrogen gas containing 10% by volume of water. ₃ catalysts were treated to examine their durability to sulfur compounds. Pt (1.0) / SiO ₂ , Pt (1.0) / Zr-SiO ₂ , and Pt (1.0) / Al ₂ O ₃ catalysts were investigated for H ₂ -SCR reaction at 100 ° C with the highest NO conversion and N ₂ yield. Table 2 shows the results of NO conversion and product yield of the sulfur treated catalyst. The NO conversion and product yields were slightly decreased for Pt (1.0) / SiO ₂ and Pt (1.0) / Zr-SiO ₂ catalysts, but there was no significant difference. However, in Pt (1.0) / Al ₂ O ₃ catalysts, NO conversion was reduced by 40% after sulfur treatment, and N ₂ O yield was also reduced. Pt (1.0) / SiO ₂ and Pt (1.0) / Zr-SiO ₂ catalysts using silica as a support had little effect on SO ₂ treatment. This is because SO ₂ is adsorbed on the support rather than platinum to lower the activity of the catalyst. Since alumina reacts with sulfur to form aluminum sulfate to cover the catalyst, the activity is lowered in the H ₂ -SCR reaction, but silica does not react with the sulfur compound, so the durability of the sulfur compound is excellent.

TABLE 2

Claims

A composite oxide catalyst for reducing and removing nitrogen oxides, wherein a reducing metal oxide is fixed to one or both base oxides of a base oxide consisting of SiO 2 and Al 2 O 3, and on which a precious metal is supported.
The composite oxide catalyst of claim 1, wherein the reducing metal oxide is one or two or more metal oxides of a metal oxide consisting of TiO 2 , ZrO 2, and CeO 2 .
The complex oxide catalyst of claim 1 or 2, wherein the metal oxide is contained in an amount of 0.5 to 50 wt% based on the total weight of the composite oxide catalyst.
The composite of claim 1 or 2, wherein the precious metal is supported by 0.1 to 5 wt% of one precious metal component selected from the group consisting of platinum, palladium and rhodium. Oxide catalyst.