WO2005063364A1

WO2005063364A1 - Catalyst for conversion of gases, a method for its production and use of the same

Info

Publication number: WO2005063364A1
Application number: PCT/SG2004/000427
Authority: WO
Inventors: Shoucang Shen; Sibudjing Kawi; Kus Hidajat; Liya E. Yu
Original assignee: National University Of Singapore
Priority date: 2003-12-29
Filing date: 2004-12-27
Publication date: 2005-07-14

Abstract

A method of preparing a catalyst for use in a gas reaction is disclosed. The method comprises the steps of providing a reaction solution containing Al3+ ions and Zn2+ ions; co-precipitating aluminum hydroxide and zinc hydroxide co-precipitate from the reaction solution at a non-neutral pH; and calcining the aluminum hydroxide and zinc hydroxide co-precipitate to form MxZnAl2O4 particles, wherein x is the atom ratio of M to Al2O4 and is within the range of 0 to 0.3.

Description

CATALYST FOR CONVERSION OF GASES, A METHOD FOR ITS PRODUCTION AND USE OF THE SAME

Technical Field The present invention generally relates to a novel method of preparing a catalyst and the catalyst produced thereof. The present invention also relates to a process for use in the reaction of gases such as exhaust gasses, using the catalyst.

Background Diesel and lean-burn gasoline engines provide advantages in fuel economy, but the emission of Nitrogen oxides or NOx (Nitrogen oxide, Nitrogen dioxide and Nitric Oxide) is the main obstacle preventing its commercialization. Legislation is being proposed to limit NO_x emission from diesel engines. The US Environmental Protection Agency has instituted stringent automobile-related environmental regulations. The primary focus of the regulation related to the Corporate Average Fuel Economy (CAFE) standards, which mandate a specified, gradual increase of a corporate fleet' s overall fuel economy by the established target dates. Current and proposed regulations challenge manufacturers to achieve good fuel economy at reduced NO_x emission. In the early 1980' s the automotive industry started adding rhodium to their exhaust catalysts to aid in the reduction of NO_x emissions. Engines were controlled to operate at a stoichiometric air-to-fuel ratio (A/F) of -14.4 to ~14.7, because when the A/F ration is in this range, three-way (Pt-Rh) catalysts simultaneously promote the conversion of three primary exhaust pollutants: CO, hydrocarbons, and N0_X. Recently, automobile manufacturers have developed lean- burn spark-ignition engines, which operate at A/F of 18-23. The use of such lean air/fuel mixtures reduces the consumption of fuel and thus enhances an automobile's fuel economy. The exhaust from lean-burn engines is oxygen-rich and contains lower levels of hydrocarbons and CO. The high concentrations of oxygen in the exhaust render typical exhaust gas catalyst systems, which use noble metals, ineffective for reducing NO_x. It is necessary to develop a lean-NO_x catalyst which can efficiently reduce NO_x emission in the presence of excess oxygen in the exhaust. A lean-NO_x catalyst is defined for this application as a catalyst that can reduce NO_x in exhaust gas under lean-burn conditions. In addition to automotive applications, lean-NO_x catalysts also provides benefit in the controlling of NO_x emissions of stationary power plants that burn fossil fuels. In response to these challenges, the research for selective catalytic reduction (SCR) of NO_x has been conducted on alternate catalyst systems of oxygen-rich engine exhaust. Transition metal-containing zeolites, especially copper- zeolites, have been intensively investigated to reduce NO_x emission for lean mixture. In addition, metal-ion-exchanged zeolites, such as Cu-ZSM-5, Ga-ZSM-5, In-ZSM-5, Co-ZSM-5 and Co-Beta zeolites showed activity for SCR of NO_x at high space velocities. However, a disadvantage with Cu-zeolite catalysts is that they exhibit hydrothermal instability due to de- alumination from the zeolite framework. This hydrothermal instability significantly hinders their use in SCR of NO_x. Additionally, it has been observed that the activity of zeolite-based catalysts decreases irreversibly and rapidly when they are exposed to steam-containing gaseous mixtures, indicating that such catalysts are not suitable for automobile exhaust purification. Applied Catalysis B: Environmental 18 (1998) 163) has reported that mixed oxides of Cu, Ni, Co-Al₂0₃ with Spinel- type structure showed activity for selective catalytic reduction of NO_x. However, the maximum NO_x conversion reported in that literature was only around -60% to -70% and the optimal NO conversion was obviously affected by 0₂ concentration . There is known a multi-component Spinel-type catalyst comprising of Cu, Co, Zn, Al, and 0. Although the optimal reaction temperature was high (~450°C) , the maximum NO conversion reported was found to be less than 50%. On the other hand, catalysts of precious metal supported on various carriers have been investigated. Although catalysts consisting of precious metal (such as Pt) have shown high activity for selectively reducing N0_X at low reaction temperatures with alkene and excess oxygen, the formation of N₂0, a potent greenhouse gas, appears to be inevitable, posing a global environmental threat. In addition, the activity temperature window has been found to be too narrow.

SUMMARY OF INVENTION According to a first aspect of the invention, there is provided a method of preparing a catalyst for use in a gas reaction, the method comprising the steps of: a) providing a reaction solution containing Al³⁺ ions and Zn²⁺ ions; co-precipitating aluminum hydroxide and zinc hydroxide co- precipitates from the reaction solution at a non-neutral pH; and c) calcining the aluminum hydroxide and zinc hydroxide co-precipitates to form M_xZnAl₂0₄, wherein x is the atom ratio of M to A1₂0₄ and is within the range of 0 to 0.3. In one embodiment, there is provided a method of preparing a catalyst for use in reducing nitrogen oxide gas, the method comprising the steps of:

(a) providing a reaction solution containing Al³⁺ ions and Zn²⁺ ions;

(b) co-precipitating aluminum hydroxide and zinc hydroxide co-precipitates from the reaction solution at a non- neutral pH; and (c) calcining the aluminum hydroxide and zinc hydroxide co- precipitates to form M_xZnAl₂0₄, wherein x is the atom ratio of M to A1₂0₄ and is within the range of 0 to 0.3. According to a second aspect of the invention, there is provided a catalyst for use in a gas reaction, the catalyst prepared from a method comprising steps of:

(a) providing a reaction solution containing Al³⁺ ions and Zn²⁺ ions; (b) co-precipitating aluminum hydroxide and zinc hydroxide precipitates from the reaction solution at a non-neutral pH; and (c) calcining the aluminum hydroxide and zinc hydroxide precipitates to form M_xZnAl₂0 , wherein x is the atom ratio of M to A1₂0₄ and is within the range of 0 to 0.3.. According to a third aspect of the invention, there is provided a process for use in a gas reaction, the process comprising contacting a gas with a catalytically effective amount of a ZnAl₂0₄ catalyst, the ZnAl₂0₄ catalyst being prepared by a method comprising the steps of: (a) providing a reaction solution containing Al³⁺ ions and Zn²⁺ ions; (b) co-precipitating aluminum hydroxide and zinc hydroxide precipitates from the reaction solution at a non-neutral pH; and (c) calcining the aluminum hydroxide and zinc hydroxide precipitates to form M_xZnAl₂0₄, wherein x is the atom ratio of M to Al₂0₄ and is within the range of 0 to 0.3. According to a fourth aspect of the invention, there is provided a catalyst for use in a gas reaction, the catalyst comprising _xZnAl₂0₄ particles having a mean pore diameter that is less than about 30 nanometers, wherein x is the atom ratio of M to A1₂0₄ and is within the range of 0 to 0.3. In one embodiment, there is provided a nitrogen oxide gas reducing catalyst, the catalyst comprising M_xZnAl₂0₄ particles having a mean pore diameter that is less than about 30 nanometers, wherein x is the atom ratio of M to A1₂0₄ and is within the range of 0 to 0.3. According to a fifth aspect of the invention, there is provided a process comprising contacting a gas with a catalytically effective amount of a catalyst comprising M_xZnAl₂0 particles having a mean pore diameter that is less than about 30 nanometers, wherein x is the atom ratio of M to Al₂0₄ and is within the range of 0 to 0.3. Brief Description of Drawings The accompanying drawings illustrate the disclosed embodiments and serve to explain the properties of the disclosed embodiments. It is to be understood that the drawings are designed for purposes of illustration only, and not as a definition of the limits of the invention.

FI6.1 graphically depicts the pore-size (pore-diameter) distribution of Catalyst-1 prepared according to the first disclosed embodiment. The pore-size distribution is obtained using the standard BJH N₂-adsorption technique. FIG.2 shows the XRD pattern of a catalyst, prepared in accordance with a disclosed embodiment, having a Spinel structure. FIG.3 shows comparison between the NO_x-conversion in the absence (A) and presence of S0₂ (B) or H₂0 (C) for ZnAl₂0₄ catalyst (Catalyst-1) prepared according to a disclosed embodiment. FIG.4 shows comparison of NO_x-conversion obtained at various Oxygen concentrations using Catalyst-1 prepared in accordance with a disclosed embodiment.

FIG.5 shows a comparison of the NO_x-conversion of Catalyst-1 and Catalyst-2, prepared according to a disclosed embodiment, after aged at 900 °C for 20 hours and comparative example (Catalyst-8) prepared not according to the present invention which is subjected to same treatment . FIG.6 shows comparison of the NOx-conversion of Catalyst-1 and Catalyst-2, prepared according to the disclosed embodiment, after being treated at 800 °C in 50% of H₂0 and 5% 0₂ for 20 hours, with the comparative example (Catalyst-8) .

Detailed Disclosure of Embodiments The embodiments disclosed herein represent an improvement over prior art catalysts for conversion of a gas with regards to higher conversion rates as well as thermal and hydrothermal stability. The catalysts of the disclosed embodiments are cost effective and suitable for use at high oxygen concentrations . Reaction solution The method may comprise the further step of providing a metal "M" in the reaction solution. The metal M may be a metal selected from the group consisting of group lib, Vila, and VIII of the Periodic Table of Elements. In one embodiment, the dopant M may be a metal selected from the group consisting of zinc, manganese, cobalt, nickel, copper. In a disclosed embodiment, the catalysts may be prepared by co-precipitation from a reaction solution at a non-neutral pH. The reaction solution may be an aqueous solution. The reaction solutions may be prepared by dissolving salts of aluminum, zinc and optionally a dopant M in aqueous media. The aqueous media may be water. The dopant M may be a metal selected from the group consisting of group lib, Vila, and VIII of the Periodic Table of Elements. In one of the embodiment the dopant M may be selected from the group consisting of zinc, manganese, cobalt, nickel and copper. The aluminum salt may be selected from the group consisting of aluminum nitrate, aluminum chloride, aluminum bromide, aluminum fluoride, aluminum sulphate, aluminum phosphate and mixtures thereof. The zinc salt may be selected from the group consisting of zinc nitrate, zinc chloride, zinc bromide, zinc fluoride, zinc sulphate, zinc phosphate and mixtures thereof. The salt of M may be suitably selected from, but not limited to, the group consisting of M-nitrate, M-chloride, M-bromide, M-fluoride, M-sulphate, M-phosphate. In one of the embodiment, the reaction solution is prepared by dissolving aluminum nitrate and zinc nitrate in de-ionized water. Co-precipitation During co-precipitation, the reaction solution may be at a temperature in the range selected from the group consisting of about 0°C to about 50°C; 5°C to about 45°C; about 10°C to about 40°C; about 15°C to about 40°C; about 20°C to about

40°C; and about 20°C to about 35°C. Co-precipitation may be carried out at an acidic pH.

Suitably, co-precipitation may be carried out at a pH within the range selected from the group consisting of: 1.5 to 6.5;

2.5 to 6.5; 3 to 6; 4 to 6 and 4.5 to 5.5. In one embodiment, the co-precipitation is carried out at a pH of about 5. Co-precipitation may be carried out at an alkaline pH. Suitably, co-precipitation may be carried out at a pH within the range selected from the group consisting of: 8.5 to 14; 9 to 13; 9.5 to 12; 9.5 to 10.5 and 10.5 to 11.5. In one embodiment, the co-precipitation is carried out at a pH of about 11. The co-precipitation of mixed aqueous solution at a non- neutral pH may be carried out using a precipitating agent. The precipitating agent may be an alkaline solution. Exemplary precipitating agents may be selected from the group consisting of, ammonia solution, alkali or alkaline metal hydroxides such sodium hydroxide, lithium hydroxide, potassium hydroxide, calcium hydroxide, magnesium hydroxide and alkali or alkaline metal carbonates such as sodium carbonate, lithium carbonate, potassium carbonate, calcium carbonate, magnesium carbonate, alkali or alkaline metal phosphates such as sodium phosphate, lithium phosphate, potassium phosphate, calcium phosphate, magnesium phosphate, ammonium carbonate and mixtures thereof. In one of embodiment, 10% ammonia solution is used as a precipitating agent. It is added drop-wise to the reaction solution to reach the non-neutral pH value for co- precipitation. Calcination After co-precipitation, the slurry may be aged at room temperature for a suitable time period. The time period for aging step may be in the range selected from, but not limited to, 1 to 24 hours; 2 to 20 hours, 2 to 10 hours; and 2 to 5 hours. After aging, the co-precipitate may be recovered by filtration and dried. This may be followed by calcination of the co-precipitate, which may be carried out in air at a temperature within the range suitably selected from the group consisting of: 500°C to 1,000°C; 600°C to 950°C; 750°C to 950°C and 800°C to 900 °C. Suitably, the calcination may be carried out for a duration within the range selected from 2 to 20 hours; 5 to 20 hours; 10 to 20 hours; 15 to 20 hours. In one embodiment, the slurry may aged for 3 hours.

Catalyst Composition and Structure The co-precipitate forms zinc hydroxide, aluminum hydroxide and optionally, M hydroxide particles . Calcination of the mixed hydroxide particles may provide a catalyst comprising a crystalline compound having a Spinel type structure. In one embodiment, the reaction concentration of the Zn²⁺, Al³⁺ and optionally M+ ions, is selected to form a compound having a Spinel type structure and having the following general formula: M_xφ_vZnι-_x-_yAl₂0₄ wherein x is the atom ratio of dopant M to A1₂0₄, φ is structure vacancy, y is the molar ratio of vacancy sites (φ) to two moles of Al³⁺ and x+y< 1 and wherein x is within the range selected from the group consisting of, 0 to 0.3; 0 to 0.2; and 0 to 0.1, y is within the range selected from a group of ranges consisting of, 0 to 0.8; 0 to 0.7; and 0 to

0.6, and structure vacancy (φ) is a value within the range 0 to 1. In one embodiment, there is disclosed a catalyst for converting gas. The catalyst comprising ZnAl₂0₄ particles having a mean pore diameter that is less than about 30 nanometers . The mean pore diameter of the ZnAl₂0 particles may be in the range selected from the group consisting of about 1 nm to less than about 30 nm; about 1 nm to about 29.9 nm; about 1 nm to about 25 nm; about 1 nm to about 23 nm; about 1 nm to about 20 nm; about 1 nm to about 18 nm; about 1 nm to about 15 nm; about 1 nm to about 13 nm; about 1 nm to about 10 nm; about 2 nm to about 29 nm; about 3 nm to about 29 nm; about 5 nm to about 29 nm; about 8 nm to about 29 nm; about 10 nm to about 29 nm; about 14 nm to about 29 nm; about 15 nm to about 29 nm; about 18 nm to about 29 nm; about 20 nm to about 29. The disclosed catalyst particles may have a specific surface area within the range selected from the group of consisting of: 7.3 to 180 m²/g; 20 to 160 m²/g and 69 to 116 m/g. The surface area of the said catalyst particles may depend up on the ratio of Zinc to Aluminum in the catalyst compound. The disclosed catalyst may have a total pore volume in the range selected from the group of ranges consisting of:

0.019 to 0.36 cm³/g; 0.05 to 0.3 cm³/g; and 0.13 to 0.18cm³/g. The pore size distribution of the catalyst prepared according to the disclosed embodiments may be in a narrow range, wherein at least 55% of the pores may have a diameter between 18 and 100 A. The maximum mean diameter of pores of the catalyst may be less than about 30 nanometer. Use of catalysts in reaction of gas The catalyst may be used to oxidise some gasses and to reduce other gasses. The gasses which may be reacted by the catalyst may be selected from a the group consisting of nitrogen oxides, hydrocarbons and mixtures thereof. Nitrogen oxides may be reduced by the catalysts to nitrogen. Exemplary nitrogen oxides may be selected from the group consisting of nitrogen monoxide (NO) , nitrogen dioxide

(N0₂) , dinitrogen monoxide (N₂0) , dinitrogen trioxide (N₂0₃) , dinitrogen pentoxide (N₂0₅) , and nitrogen trioxide (N0₃) etc. The hydrocarbons may be oxidised by oxygen over the catalysts. The hydrocarbons may be any hydrocarbon gas and includes low molecular weight aromatic and aliphatic hydrocarbon gasses. Exemplary aromatic gasses are benzene, pyridine, and nitro-benzenes . Exemplary aromatic gasses are benzene, pyridine, and nitro-benzenes. Exemplary aliphatic gasses are alkanes having from 1 to 5 carbon atoms such as methane gas, ethane gas and propane gas, alkenes having from

1 to 4 carbon atoms, and alkynes having from 1 to 4 carbon atoms .

Reduction of nitrogen oxides Also disclosed herein is a process for the conversion of a gas using the disclosed catalyst. In one embodiment, there is provided a process for the hydrocarbon reduction of NO_x utilizing the disclosed catalyst. To reduce nitrogen oxides (NOx) from a gaseous flow- stream, the catalyst may be charged into a stainless steel tube reactor in the form of a stationary bed. The gaseous flow-stream containing NOx may be a gaseous flow-stream comprising: (i) oxygen in the range of 0 to 20 vol.%; (ii) nitrogen oxides NO_x, in an amount typically ranging from 250 to 1000 volume per million (vpm) ; (iii) sulfur oxides SO_x in an amount typically ranging from 1 to 100 vpm; (iv) water in an amount typically ranging 0 to 10 vol.% According to one of the embodiments, the selective reduction of NO_x may require a molecular ratio of C₃H₈ (or

C₃H₆) to NO within the range selected from a group of ranges consisting of: 0.75 to 1.5; 0.78 to 1.25; and 0.8 to 1. The reduction process may be carried out in a temperature range selected from the group of ranges consisting of, 400°C to 1000°C; 420°C to 1000°C; and 450°C to

800°C. The volume velocity per hour (VVH) of the gases to be treated is a function of the temperature of the catalyst, wherein a higher temperature permits a lager VVH for the same result. For the temperature ranges defined above, the VVH may be within a range selected from 10,000 h^"1 to 150,000h^_1; 10,000h^_1 to 100,000h^_1; 10,000h^_1 to 50,000 h^"1; and 10,000h^_1 to 30,000h^_1. Best Mode and Examples A best method of preparing the catalyst presently known to the applicant will now be described with reference to the following non-limiting examples 1 to 7. A comparative example 8 is also disclosed. The surface area and pore-size distribution of samples can be measured in various ways as would be apparent to a person skilled in the art in view of the present disclosure. In this work, it was determined via volumetric adsorption and desorption technique using a BET equation and BJH desorption equation for data analysis. BET and BJH methods are known methods in the art for analysis of surface areas and pore size. A full disclosure of the methods are provides in Brunauer, Emmet,. Teller, Journal of the American Chemical Society, Volume 60, 1938, p 309; and in Adsorption : Theory, Modeling, and Analysis, by Jozsef Toth, publisher, Marcel Dekker, 2002, ISBN: 0 824 70747 8. Example 1 - "^ΛCatalyst 1" Preparation of catalyst-1 was undertaken by co- precipitation at a pH of 11. The catalyst-1, ZnAl₂0₄ (Mxφ_yZnι-_x__yAl₂04 where x=y=0 (ie no dopant M) ) was prepared by co-precipitation of A1(N0₃)₃ and Zn(N0₃)₂ reaction solution. The reaction solution was prepared by dissolving 30.00g of A1(N0₃)₃-9H₂0 and 11.88g of Zn (N0₃) ₂- 6H₂0 in 400 mL de-ionized H₂0 under magnetic stirring. 10% of ammonia solution was added dropwise to the reaction solution until the pH value of precipitating slurry was 11. The pH of the reaction solution was measured by MACHEREY-NAGEL lOOcolor-fixed 0-14 indicator sticks. The reaction solution containing the co-precipitate was aged under stirring for 3 hours. The co-precipitate was recovered by filtration, and followed by drying at 100 °C for 12 hours. The co-precipitate was calcined at 800 °C in air for 20 hours with heating rate of 2°C/min. The catalyst-1 had the following characteristics: Table 1

Specific surface area 69.8 m /g

Total pore volume 14 cm³/100g

Volume of pores with diameters 5.2cm³/100g greater than 100 A

Volume of pores with diameters 12.5cm³/100g greater then 20 A

FIG. 1 shows the narrow pore-size distribution. XRD result indicates the catalyst have a typical Spinel structure, as shown in FIG.2. Example 2 "Catalyst 2" Catalyst-2 was made according to the same method outlined for catalyst 1 above, however co-precipitation was carried out at a pH of 10. Example 3 "Catalyst 3"

Catalyst-3 was made according to the same method outlined for catalyst 1 above, however co-precipitation was carried out at a pH of 8. Example 4 "Catalyst 4"

Catalyst-4 was made according to the same method outlined for catalyst 1 above, however co-precipitation was carried out at a pH of 5. Example 5 "Catalyst 5" The Catalyst-5, Zn₀.₅Al₂O₄ (M_xφ_yZnι__x__yAl₂0₄ where x=0,y=0.5) was prepared by co-precipitation of A1(N0₃)₃ and Zn(N0₃)₂ reaction solution. The reaction solution was prepared by dissolving 30.00g of A1(N0₃)₃-9H₂0 and 5.94g Zn (N0₃) ₂ • 6H₂0 in 400 mL de-ionized H₂0 under magnetic stirring. 10% of ammonia solution^' was added dropwise to the mixture till the pH value of precipitating slurry was 11. pH of the reaction solution was measured by MACHEREY-NAGEL lOOcolor-fixed 0-14 indicator sticks . The reaction solution was aged under stirring for 3 hours. The solid product was recovered by filtration, and followed by drying at 100 °C for 12 hours. The product was calcined at 800 °C in air for 20 hours with heating rate of 2°C/min. Example 6 "Catalyst 6" The Catalyst-6, Cu₀.ιZn₀.₉Al₂O₄ (M_xφ_yZnι__x__yAl₂0₄ where M is Cu, x=0.1,y=0) was prepared by precipitation of A1(N0₃)₃, Zn(N0₃)₂ and Cu(N0₃)₂ reaction solution. The reaction solution was prepared by dissolving 30.000g of Al(N0₃)₃-9H₂0, 10.710g of Zn (N0₃) ₂- 6H₂0 and 0.967g of Cu(N0₃)₂-H₂0 in 400 mL de-ionized H₂0 under magnetic stirring. 10% of ammonia solution was added dropwise to the mixture till the pH value of precipitating slurry was 10. pH of the reaction solution was measured by MACHEREY-NAGEL lOOcolor-fixed 0-14 indicator sticks. The reaction solution was aged under stirring for 3 hours. The solid product was recovered by filtration, and followed by drying at 100°C for 12 hours. The product was calcined at 800 °C in air for 20 hours with heating rate of 2°C/min.

Example 7 "Catalyst 7" The catalyst-7, Co₀.ιZn₀.₉Al₂O₄ (M_xφ_yZn₁__x__yAl₂0₄ where M is

Co, x=0.1,y=0) was prepared by co-precipitation of A1(N0₃)₃,

Zn(N0₃)₂ and Co(N0₃)₂ mixture solution. 30.000g of A1(N0₃)₃-9H₂0, 10.710g of Zn (N0₃) 2 ^• 6H₂0 and 1.164g of Co(N0₃)₂"H₂0 was dissolved in 400 mL de-ionized H₂0 under magnetic stirring. 10% of ammonia solution was added dropwise to the mixture till the pH value of precipitating slurry was 10. pH of the reaction solution was measured by MACHEREY- NAGEL lOOcolor-fixed 0-14 indicator sticks. The reaction solution was aged under stirring for 3 hours. The solid product was recovered by filtration, and followed by drying at 100 °C for 12 hours. The product was calcined at 800 °C in air for 20 hours with heating rate of 2°C/min.

Comparative Example 8 "Catalyst 8" The Catalyst-8 (Cu-ZSM-5 catalyst) was not prepared according to the invention. Cu-ZSM-5 catalyst was prepared by ion exchange of ZSM-5 (Si/Al=25) . 5 g of ZSM-5 was exchanged with 50 mL of 0.1M of Cu(N0₃)₂ solution for 24 h, followed by intensive washing with de-ionized water, drying at 100°C for 12 hours and calcination at a temperatures of 600°C for 5 hours. Example 9 Evaluation of the Catalyst-1~8 The catalysts prepared in the above examples were investigated to determine the efficiency of their ability to reduce N0_X and their resistance to deactivation under various conditions . The gaseous flow resulting from the mixture of gases was employed as a feed to a reactor containing of l.Og (0.65mL) of the catalyst and placed in a temperature controlled vertical tube-furnace from Carbolite of Hope Valley, United Kingdom. The temperature of the furnace was controlled by an electronic digital controller. Each data point reading was taken after the temperature was stabilized for 30 minutes. The exhaust flowstream was then transferred into an apparatus to measure N0_X content by chemiluminescence, and the concentration of N₂0, hydrocarbon and C0₂ was monitored by gas chromatograph. The conversion of N0_X was determined by equation of:

X_NO (% ) = [ (feed NO_x-exhaust NO_x) /feed NO_x]xl00

Wherein: feed NO_x = concentration of NO_x at reactor inlet exhaust NOx = concentration of N0_X at the reactor outlet No N₂0 formation was detected in the results. N0_X was selectively reduced to N₂, and selectivity is close to 100%. Effect of catalyst preparation condition on activity: Table 2 shows the results of catalysts 1-8 activity for NO_x reduction. It can be seen that at higher temperatures

(~>540°C) the activity of catalysts for NOx reduction depended to a great extent on the pH value used for precipitation of the catalyst. Catalyst-2 prepared by using pH of 10 showed highest activity at temperatures of 540°C or more, whereas Catalyst-3 prepared under pH of 8 showed lowest activity at temperatures of 540°C or more. Table 2

Catal pH NO_x-conversion (%) yst used 500°C 520° 540°C 550°C 570° 600°C C C

1 11 20.4 36.1 69.8 78.9 87.4 82.2

2 10 37.8 ■ 61.0 84.3 98.0 96.8 86.8

3 8 44.8 46.8 44.5 42.2 35.1 28.7

4 5 33.1 49.3 67.2 85.0 93.4 81.7 Table 3

Catalyst NO_x conversion (%) 300° 350° 400° 450° 500° 550° 570° 600° 700° C C C C C C C C C ^"~5 ^{~ ~} 571 97^" 16.4 45.2 76.6 74.8 6 — 22.3 33.4 24.2 10.8 — 7 — — 5.9 13.7 37.0 41.0 36.0 31.3 8 52.5 74.1 66.3 62.4 64.6 64.7 64.0 63.5 The results reported in Table 2 and Table 3 were obtained using a mixture of gases containing: 1000 vpm NO_x 1000 vpm C₃H₈ 5% 0₂ The gas mixture was balanced in helium with a volume velocity per hour of 17,500 h^-1. Effect of S0₂ and H₂0: The efficiency of N0_x conversion for Catalyst-1 is shown in FIG. 3. The addition of 100 ppm S0₂ into the reaction gases stream did not cause serious effect on the efficiency of NO_x-conversion. Further, presence of 2.5% water vapor in the gas mixture was found to have little effect on the maximum NO_x-conversion. Effect of 0₂ concentration: The efficiency of NO_x conversion of Catalyst-2 in the presence of different 0₂ concentration is shown in FIG.4. The Catalyst-2 showed high efficiency for NO_x reduction at high concentration of 0₂. The conversion of NO_x was improved by the increase of 0₂ concentration in the reaction stream from 0.4% to 40%. Contrary to conventional catalysts, whose efficiency is significant lowered by S0₂, H₂0 or high oxygen concentration in reaction stream, the catalytic activity of the disclosed catalyst-2 is not affected by S02, H20 or high oxygen concentration.

Durability: The thermal stability of Catalyst-1 was investigated by calcining at 900 °C for 20 hours. The NO_x conversion by catalysts-1 is shown in FIG. 5 and it compared with Catalyst- 8. In addition, FIG.6 shows the NO_x conversion efficiency of Catalyst-1 after exposing to pure water vapor at 800 °C for 20 hour, and compared with Catalyst-8 after similar treatment. Catalyst-1 showed high NO_x conversion efficiency upon severe thermal or hydrothermal treatment. Catalyst-1 thus displayed excellent durability characteristics and was superior to zeolite-based catalyst. Applications An advantage of the embodiments of the present invention is that, higher conversion of Nitrogen oxides (NO_x) can be obtained even at high oxygen concentrations and at high temperatures. Catalysts prepared by the conventional methods can not provide such high conversion of NO_x at high temperatures and oxygen rich environments . Another advantage of the embodiments of the present invention is that presence of water vapor slightly affects the conversion of NO_x. The performance of catalysts of conventional methods is found to deteriorate significantly in presence of water vapor. The present invention provides thermally and hydro- thermally stable catalysts which shows high conversion rates even in oxygen rich environments and in presence of S0₂ and water vapor. Accordingly, the catalysts of present invention are very useful for the reduction of NOx generated by lean- burn engines . Another advantage of the present invention is the simple method of preparation. The catalyst of present invention is prepared by co-precipitation at a selected pH. Present invention does not use any precious metals and does not involve complicated procedures . Accordingly, the method of present invention is more economical as compared to the conventional methods . It has surprisingly been found that using a catalyst having a mean pore diameter less than 30 nanometers provides a higher conversion of NO_x and is thermally and hydro- thermally stable. It should also be realized that the catalysts of present invention can be used for applications other than reduction of N0_X from lean-burn gases. For example, the catalyst of present invention can be used to reduce NOx present in the exhaust gases of a power plant or any engine operated on fossil fuels. It can be used to oxidize hydrocarbons present in any gaseous stream. Accordingly, it will be appreciated that the invention is not limited to the embodiments described herein and additional embodiments or various modifications may be derived from the application of the invention by a person skilled in the art without departing from the scope of the invention.

Claims

1. A method of preparing a catalyst for use in a gas reaction, the method comprising the steps of:

(a) providing a reaction solution containing Al³⁺ ions and Zn²⁺ ions;

(b) co-precipitating aluminum hydroxide and zinc hydroxide co-precipitate from the reaction solution at a non- neutral pH; and

(c) calcining the aluminum hydroxide and zinc hydroxide co-precipitate to form M_xZnAl₂0₄ particles, wherein x is the atom ratio of M to A10₄ and is within the range of 0 to 0.3.

2. A method according to claim 1, wherein the M is one or more metals selected from the group consisting of group lib metals, group Vila metals, and group VIII metals of the Periodic Table of Elements.

3. A method according to claim 2, comprising selecting M from the group consisting of zinc, manganese, cobalt, nickel and copper.

4. A method according to claim 1, wherein the providing step comprises dissolving salts of aluminum and zinc in aqueous media.

5. A method according to claim 1, comprising selecting an acidic pH in step (b) .

6. A method according to claim 5, comprising selecting the acidic pH within the range selected from the group consisting of: 1.5 to 6.5; 2.5 to 6.5; 3 to 6; 4 to 6 and 4.5 to 5.5.

7. A method according to claim 1, comprising selecting an alkaline pH in step (b) .

8. A method according to claim 7, comprising selecting the alkaline pH within the range selected from the group consisting of: 8.5 to 14; 9 to 13; 9.5 to 12; 9.5 to 10.5 and 10.5 to 11.5.

9. A method according to claim 1, comprising setting the temperature of the reaction solution in step (b) at a temperature in the range selected from the group consisting of 0°C to 50°C; 5°C to 45°C; 10°C to 40°C; 15°C to 40°C; 20°C to 40°C; and 20°C to 35°C.

10. A method according to claim 1, wherein step (b) comprises introducing a precipitating agent into the reaction solution.

11. A method according to claim 10, comprising selecting an alkaline solution as the precipitating agent.

12. A method according to claim 1, comprising before step (c) , removing the aluminum hydroxide and zinc hydroxide co-precipitate formed in step (b) from the reaction solution.

13. A method according to claim 12, comprising drying the removed aluminum hydroxide and zinc hydroxide co-precipitate .

14. A method according to claim 1, comprising aging the reaction solution before step (c) .

15. A method according to claim 1, comprising calcining the aluminum hydroxide and zinc hydroxide co- precipitate in step (c) at a temperature within the range selected from the group consisting of: 500°C to 1,000°C; 600°C to 950°C; 650°C to 950°C and 700°C to 900 °C.

16. A process for reacting a gas, the process comprising contacting a gas with a catalytically effective amount of a catalyst, the catalyst being prepared by a method comprising the steps of:

(a) providing a reaction solution containing Al³⁺ ions and Zn²⁺ ions;

(c) calcining the aluminum hydroxide and zinc hydroxide co-precipitate to form M_xZnAl₂0₄, wherein x is the atom ratio of M to A1₂0₄ and is within the range of 0 to 0.3.

17. A process according to claim 16, wherein the gas comprises nitrogen oxide gases (NO_x) .

18. A process according to claim 17, wherein the reacting comprises reducing NO_x to nitrogen.

19. A process according to claim 18, wherein the NO_x gasses are reduced at a temperature in the range selected from the group consisting of: 400°C to 1000°C; 420°C to 1000°C; and 450°C to 800°C.

20. A process according to claim 16, wherein the gas is one or more hydrocarbon gasses.

21. A catalyst for use in a gas reaction, the catalyst prepared from a method comprising steps of: (a) providing a reaction solution containing Al³⁺ ions and Zn²⁺ ions;

22. A catalyst for use in a gas reaction, the catalyst comprising M_xZnAl₂0₄ particles having a mean pore diameter that is less than about 30 nanometers, wherein x is the atom ratio of M to Al₂0₄ and is within the range of 0 to 0.3.particles .

23. A process comprising contacting a gas with a catalytically effective amount of a catalyst comprising M_xZnAl₂0₄ particles having a mean pore diameter that is less than about 30 nanometers, wherein x is the atom ratio of M to A1₂0₄ and is within the range of 0 to 0.3.particles .

24. A method according to claim 1, wherein the formed M_xZnAl₂0₄ particles is a compound having a Spinel type structure defined by the general formula: M_xφ_yZn₁__x__yAl₂0₄ wherein x is within the range selected from the group consisting of 0 to 0.2; and 0 to 0.1; φ is structure vacancy and is a value within the range 0 to 1, and y is the molar ratio of vacancy sites (φ) to two moles of Al³⁺ and x+y< 1, and y is within the range selected from a group of ranges consisting of 0 to 0.8; 0 to 0.7; and 0 to 0.6.