Abstract:
A supported noble metal catalyst and a process for preparing the same in situ are provided. Hexamethylenetetramine, a soluble divalent metal salt solution, a Al 2 O 3  carrier and a soluble noble metal salt solution, are mixed and crystallized, in which the hexamethylenetetramine acts as both a precipitating agent for producing hydrotalcite and a reducing agent for the noble metal precursor, and a supported catalyst Me-LDHs-Al 2 O 3  containing an elementary substance of a noble metal is prepared by a one-step reaction. During the growth of the hydrotalcite, Al 3+  on the surface layer of the Al 2 O 3  carrier is directly used as the trivalent metal ion to form the slab structure of the hydrotalcite, and the hydrotalcite is grown in situ on the surface of the alumina carrier. The noble metal element particle in the catalyst has a particle size of 10 to 60 nm, and has an even and stable dispersion on or between slabs of the hydrotalcite. The calcination and reduction processes in the traditional production method are no longer required, and the reduced noble metal catalyst is obtained by a single step of a crystallization process, while the utilization ratio of the noble metal is high.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to a supported noble metal catalyst and a process for preparing the same in situ, in particular to a noble metal catalyst with alumina as a carrier produced by an in situ process, and its preparation process. The catalyst may be applied in various reactions in the fields of petrochemical industry, organic chemistry or the like. 
       BACKGROUND OF THE INVENTION 
       [0002]    Platinum group elements include noble metal elements such as ruthenium, rhodium, palladium, osmium, iridium and platinum, in which the elements ruthenium (Ru), rhodium (Rh), palladium (Pd) and platinum (Pt) have an excellent catalytic performance, and have been widely used in the petrochemical industry and various reactions in organic chemistry and are excellent catalysts or one of the important components of the catalysts. However, when the noble metal catalyst is prepared using a conventional impregnation method, under the influence of the surface tension of the impregnating solution and the solvation effect in the drying stage, the precursor of the noble metal active component is deposited on the surface of the carrier in the form of aggregates; such a high degree of aggregation is difficult to break down in the subsequent calcination process, which seriously affects the dispersion degree and catalytic activity of the active component. Additionally, when the supported metal catalyst is prepared using a conventional impregnation method, calcination and reduction processes must be carried out to obtain a zero-valent noble metal catalyst having catalytic activity, and the preparation process is therefore complicated. 
         [0003]    Hydrotalcite is a type of anionic layered compound, a member of a class of materials also known as layered double hydroxides, abbreviated as LDHs. It has characteristics such as a regular arrangement of the interlayer anions and the chemical composition of slabs, a weak alkalinity, and good thermal stability, and can be used as a carrier for a noble metal catalyst. The composite metal oxide produced by calcination of a hydrotalcite precursor has a high specific surface area and good thermal stability, and a noble metal catalyst having a high degree of dispersion with the composite metal oxide as the carrier can be obtained by subjecting the hydrotalcite-supported active component to calcination and reduction. However, the aforementioned catalysts prepared by conventional methods such as impregnation must go through the steps of calcination and reduction before a form of the noble metal element having catalytic activity can be obtained. 
         [0004]    Alumina has characteristics such as a large specific surface area, good thermal stability, high mechanical strength, an adjustable surface acidity and a low price, and has been widely used in the fields of petrochemical industry and catalysis. Currently, the catalyst carrier used in hydrogenation processes is generally alumina in both China and elsewhere. In terms of the geometric shape, alumina carriers can be divided into spherical, bars, cylindrical, trefoil, honeycomb shape and the like. Compared with the others, spherical carrier particles, which are in contact with each other at points in a fixed bed, bear stress evenly, easily give a filling of a dense phase, and are not prone to channeling, offer improved mass transfer and catalytic performance, and have become a focus of research in recent years. 
         [0005]    In Document [1] Effects of Pd precursors on the catalytic activity and deactivation of silica-supported Pd catalysts in liquid Phase hydrogenation,  Applied Catalysis A,  2005, 292:322-327, catalysts with SiO 2 /MCM-41 as the carrier are prepared using different Pd precursors, Pd(NO 3 ) 2 , PdCl 2  and Pd(OOCCH 3 ) 2  by the impregnation method, and the experimental results show that the Pd particles in the prepared catalysts have a small size, a high degree of dispersion, and a high catalytic activity in liquid-phase hydrogenation reactions. However, after five cycles of reactions, the active ingredient—Pd particles—become severely sintered, and their catalytic activity is decreased significantly. 
         [0006]    In Document [2] Hydrogenation of 2-butyne-1,4-diol on supported Pd catalysts obtained from LDH precursors,  Microporous and Mesoporous Materials,  2007, 99:118-125, Pd catalysts are prepared using hydrotalcite as the precursor by the impregnation method, coprecipitation method, and ion-exchange method, and the test results show that the catalysts prepared by these three methods have a large specific surface area, a high degree of dispersion of the active ingredient, and a good catalytic activity and a good selectivity. However, similarly to the case for other supported metal catalysts, calcination and reduction steps must be carried out to obtain Pd 0  having catalytic activity, and the preparation process is complicated. 
         [0007]    In Document [3] Selective hydrogenation of acetylene over Pd catalysts supported on nanocrystalline α-Al 2 O 3  and Zn-modified α-Al 2 O 3   , Catalysis Communications,  2008, 9:2297-2302, Pd—Al 2 O 3  catalysts are prepared using Al 2 O 3  as the carrier by the sol-gel method and the solvothermal method respectively, and the characterization results demonstrate that the metallic Pd in the catalysts has a large particle size and a low degree of dispersion. 
         [0008]    In summary, the preparation methods currently used for supported noble metal catalysts are complicated, and calcination and reduction steps are required after the noble metal has been supported; during the calcination, the particles of the active noble metal component easily become sintered, which has a serious adverse effect on the catalytic activity. Therefore, the development a noble metal catalyst produced by a simple preparation method, having a large specific surface area of the carrier, a high degree of dispersion of the metal and a high utilization ratio of the active component, a good catalytic activity and a good selectivity has become an important target. 
       SUMMARY OF THE INVENTION 
       [0009]    The object of the invention is to provide a noble metal catalyst supported on alumina as the carrier. 
         [0010]    Another object of the invention is to provide a process for preparing a noble metal catalyst supported on alumina as the carrier. 
         [0011]    The invention provides a noble metal catalyst supported on alumina as the carrier, which may be represented by: 
         [0000]      Me-LDHs-Al 2 O 3   (I)
 
         [0012]    In the formula (I), Me is a noble metal as the active component of the catalyst, and Me is Ru, Rh, Pd or Pt; the noble metal element particle in the catalyst has a particle size of 10 to 60 nm, and has an even and stable dispersion on or between slabs of hydrotalcite; and the hydrotalcite is grown in situ on the surface of the alumina carrier by taking advantage of Al element in the alumina. 
         [0013]    In the formula (I), the LDHs is hydrotalcite having a chemical composition formula of [M 2+   1-x Al 3+   x (OH) 2 ] x+ (A n− ) x/n .mH 2 O, wherein M 2+  is a divalent metal ion positioned on the main slabs, and M 2+  is one or more of Mg 2+ , Zn 2+ , Ni 2+ , Co 2+ , Cu 2+  and Mn 2+ , and preferably Mg 2+ , Zn 2+  or Ni 2+ ; A n−  is one or more of Cl − , NO 3   − , SO 4   2−  and CO 3   2− ; x is a value of the molar ratio of Al 3+ /(M 2+ +Al 3+ ), and 0≦x≦1; m is the number of moles of interlayer water molecules, and 0.5≦m≦4.5; the trivalent metal ion Al 3+  is supplied from the Al 2 O 3  carrier, and the LDHs is grown in situ on the alumina carrier. 
         [0014]    In the formula (I), Al 2 O 3  is used as the carrier, and suitable performance indicators thereof such as crystalline form, pore diameter, size, and specific surface area can be selected according to the actual needs of the target reaction. Generally, the crystalline form thereof is one or more of α, β, θ, γ and δ, and the carrier may have a shape which is spherical, bar-like, cylindrical, trefoil, honeycomb, or powder. 
         [0015]    The invention provides a process for preparing a noble metal catalyst supported on alumina as the carrier, in which hexamethylenetetramine, a soluble divalent metal salt solution, a Al 2 O 3  carrier and a soluble noble metal salt solution are mixed and crystallized. The hexamethylenetetramine is used as both the precipitating agent for producing hydrotalcite and the reducing agent for the noble metal precursor due to its the property of releasing ammonia and formaldehyde upon decomposition, and the supported catalyst Me-LDHs-Al 2 O 3  containing an elementary substance of noble metal is prepared by one-step reaction. During the growth of the hydrotalcite, Al 3+  on the surface layer of the Al 2 O 3  carrier is directly used as the trivalent metal ion in the formation of the slab structure, and the hydrotalcite is grown in situ on the surface of the alumina carrier. 
         [0016]    Specifically, the invention provides a process for preparing in situ the supported noble metal catalyst comprising the steps of:
       A) dissolving a soluble noble metal salt precursor in a solvent to prepare a noble metal salt solution with a concentration of 0.004 to 0.01 mol/L, wherein the anion in the solution is NO 3   − , Cl − , CH 3 COO −  or C 5 H 7 O 2   −  and the solvent may be deionized water, ethanol, or toluene;   preferably, in this step, the soluble noble metal salt precursor is Ru(NO)(NO 3 ) 3 , Ru(NO 3 ) 3 .2H 2 O, RuCl 3 .3H 2 O, RhCl 3 .3H 2 O, Rh(CH 3 COO) 3 , Rh(NO 3 ) 3 , Pd(NH 3 ) 2 Cl 2 , Pd(NO 3 ) 2 , Pd(C 5 H 7 O 2 ) 2 , Na 2 PdCl 4 , Pd(CH 3 COO) 2 , H 2 PtCl 6 , Pt(NO 3 ) 2 , [Pt(NH 3 ) 4 ]Cl 2  or Pt(C 5 H 7 O 2 ) 2 ;   B) dissolving a soluble divalent metal salt in deionized water to prepare a divalent metal salt solution with a concentration of 0.02 to 0.08 mol/L, wherein the divalent metal M 2+  is one or more of Mg 2+ , Zn 2+ , Ni 2+ , Co 2+ , Cu 2+  and Mn 2+ , and preferably Mg 2+ , Zn 2+  or Ni 2+ , and the anion in the salt solution is one or more of Cl − , NO 3   − , SO 4   2−  and CO 3   2− ;   C) adding hexamethylenetetramine to the divalent metal salt solution prepared in the step B), and stirring to reach a sufficient dissolution while the temperature of the solution is maintained at 0 to 10° C., followed by adding thereto the noble metal salt solution prepared in the step A) and stirring well to obtain a mixed solution, wherein the molar ratio of M 2+  to hexamethylenetetramine is 1:3 to 28, and preferably 1:3 to 10, and the molar ratio of the noble metal to M 2+  is 0.15 to 7.5:100, and preferably 0.15 to 1.5:100;   D) adding a Al 2 O 3  carrier to the mixed solution as above according to a theoretical supported amount of 0.01 to 5% of the noble metal Me on the Al 2 O 3  carrier, subjecting the resultant mixture to crystallization for 1 to 6 hours at 130° C. to 180° C., discharging, washing the solid product with deionized water until the pH value of the washings becomes 7 to 8, and subsequently performing a drying step for 8 to 24 hours at 30° C. to 90° C., to obtain Me-LDHs-Al 2 O 3 .       
 
         [0022]    In the process for preparing in situ the supported noble metal catalyst of the invention, the crystalline form of the Al 2 O 3  carrier may be one or more of α, β, θ, γ and δ; and the shape thereof may be spherical, bar-like, cylindrical, trefoil, honeycomb, powder or the like, and suitable performance indicators thereof such as crystalline form, size, pore structure, and specific surface area can be selected according to the actual needs of the target reaction. 
         [0023]    The supported noble metal catalyst obtained as the product according to the invention is gray or black, and is characterized by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and the like. The characteristic peaks of the hydrotalcite and the alumina carrier can be found in the XRD pattern. It is shown in the SEM micrographs that the hydrotalcite has a regular hexagonal layered structure and an intact crystalline form, and is evenly grown in situ on the surface of the alumina carrier; the noble metal particles have a small size and are in the shape of triangle, rhombus, pentagon and several irregular shapes and evenly dispersed on the slabs of the hydrotalcite. It is known from the results of XPS analysis that zero-valent noble metal particles having a catalytic activity are present in the catalyst. 
         [0024]    The advantage of the invention is that it is the first time where hexamethylenetetramine is used as both the precipitating agent and the reducing agent to successfully prepare Me-LDHs-Al 2 O 3  catalyst; during the crystallization, the noble metal is reduced from the noble metal in the precursor to an elementary substance Me 0  by formaldehyde released from the decomposition of hexamethylenetetramine; the steps of calcination and reduction in the conventional preparation method are no longer required and a reduced noble metal catalyst can be obtained by a single step, namely a crystallization process; the preparation method is simple and convenient, with almost no loss of the active ingredient in the raw materials and an improved utilization ratio of the noble metal. Based on the characteristics of the hydrotalcite such as a large specific surface area and a high degree of dispersion of the active component, the invention uses the alumina carrier as the aluminum source such that the hydrotalcite is grown in situ on the surface of the alumina, and the noble metal particles can be uniformly distributed with a small particle size on the surface of the hydrotalcite, thereby effectively improving the dispersion degree of the noble metal as the active component. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0025]      FIG. 1  is a XRD pattern of the Pd-LDHs-Al 2 O 3  catalyst in Example 1; 
           [0026]      FIG. 2  is a SEM micrograph of the Pd-LDHs-Al 2 O 3  catalyst in Example 1; 
           [0027]      FIG. 3  is a XPS spectrum of the Pd-LDHs-Al 2 O 3  catalyst in Example 1; 
           [0028]      FIG. 4  is a SEM micrograph of the Pd-LDHs-Al 2 O 3  catalyst in Example 4; and 
           [0029]      FIG. 5  is a XPS spectrum of the Pd-LDHs-Al 2 O 3  catalyst in Example 4. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0030]    Hereinafter, the techniques and characteristics of the invention will be described in detail by referring to specific examples, but it is not intended that the scope of the invention is limited to these examples. 
       Example 1 
       [0031]    In Example 1, a supported noble metal catalyst was prepared in accordance with the following steps:
       Step A) 0.0114 g of Pd(NO 3 ) 2  was weighed and dissolved in 10 mL of deionized water to prepare a precursor solution of Pd metal;   Step B) 1.67 g of Mg(NO 3 ) 2 .6H 2 O was weighed and dissolved in 90 mL of deionized water to prepare a metal salt solution;   Step C) 6.41 g of hexamethylenetetramine was weighed, and dissolved in the metal salt solution from the step B) under conditions of an ice bath such that the molar ratio of Mg to hexamethylenetetramine was 1/7; the solution was stirred under conditions of the ice bath until hexamethylenetetramine was completely dissolved, and then the Pd(NO 3 ) 2  solution prepared in the step A) was added thereto and stirred well;   Step D) 5 g of a spherical α-Al 2 O 3  carrier having an average diameter of 2.5 mm was added to the solution from the step C); the above mixture was transferred to a hydrothermal reactor, and crystallized for 6 hours at 130° C.; after being cooled to room temperature, the solid product was washed with deionized water until the pH value of the washings became 7, and was dried for 12 hours at 80° C., to obtain a noble metal catalyst Pd-LDHs-Al 2 O 3  supported on alumina as the carrier of the present Example.       
 
         [0036]    In the catalyst of the Example, the LDHs had a chemical formula which could be written as [Mg 2+   0.67 Al 3+   0.33 (OH) 2 ] 0.33+ (CO 3   2− ) 0.165 .H 2 O; the original spherical α-Al 2 O 3  carrier had a specific surface area of about 19 m 2 /g, and the specific surface area after in situ growth of the hydrotalcite was about 25 m 2 /g; the loss ratio of Pd metal was less than 5%, and the dispersion degree thereof was 72%. 
         [0037]    The XRD pattern, SEM micrograph and XPS spectrum of the catalyst of the Example are shown in  FIGS. 1 ,  2  and  3 , respectively. The characteristic peaks of the hydrotalcite crystal (003, 006) and the characteristic peaks of the alumina (α-Al 2 O 3  in the Example) can be found from  FIG. 1 . As shown in  FIG. 2 , the hydrotalcite having an intact crystal is grown in situ on the surface of the alumina carrier, and has a hexagonal shape and a size of about 1 micron; the Pd metal particles are in the shape of a triangle, rhombus, pentagon, hexagon and other irregular polygons, and have a size of about 30 to 60 nm. The results of XPS analysis in  FIG. 3  show that in the Pd catalyst supported on alumina, the 3d 2/5  state of Pd has a peak at 335.7 eV, indicating that the valence state of the noble metal particles is zero. 
       Example 2 
       [0038]    In Example 2, a supported noble metal catalyst was prepared in accordance with the following steps: 
         [0039]    Steps A) and B) were the same as those in Example 1; Step C) 2.75 g of hexamethylenetetramine was weighed, and dissolved in the metal salt solution under conditions of an ice bath such that the molar ratio of Mg to hexamethylenetetramine was 1/3; the solution was stirred under conditions of the ice bath until hexamethylenetetramine was completely dissolved, and then the Pd(NO 3 ) 2  solution was added thereto and stirred well;
       Step D) 5 g of a spherical α-Al 2 O 3  carrier having an average diameter of 5.0 mm was added to the solution from the step C); the above mixture was transferred to a hydrothermal reactor, and crystallized for 6 hours at 150° C.; after being cooled to room temperature, the solid product was washed with deionized water until the pH value of the washings became 7, and was dried for 12 hours at 80° C., to obtain a noble metal catalyst Pd-LDHs-Al 2 O 3  supported on alumina as the carrier of the present Example.       
 
         [0041]    In the catalyst of the Example, the LDHs had a chemical formula which could be written as [Mg 2+   0.67 Al 3+   0.33 (OH) 2 ] 0.33+ (CO 3   2− ) 0.165 .H 2 O; the original spherical α-Al 2 O 3  carrier had a specific surface area of about 12 m 2 /g, and the specific surface area after in situ growth of the hydrotalcite was about 17 m 2 /g; the loss ratio of Pd metal was less than 5%, and the dispersion degree thereof was 67%. 
       Example 3 
       [0042]    In Example 3, a supported noble metal catalyst was prepared in accordance with the following steps:
       Steps A) and B) were the same as those in Example 1;   Step C) 12.82 g of hexamethylenetetramine was weighed, and dissolved in the metal salt solution under conditions of an ice bath such that the molar ratio of Mg to hexamethylenetetramine was 1/14; the solution was stirred under conditions of the ice bath until hexamethylenetetramine was completely dissolved, and then the Pd(NO 3 ) 2  solution was added thereto and stirred well;   Step D) 5 g of a spherical γ-Al 2 O 3  carrier having an average diameter of 5.0 mm was added to the solution from the step C); the above mixture was transferred to a hydrothermal reactor, and crystallized for 6 hours at 180° C.; after being cooled to room temperature, the solid product was washed with deionized water until the pH value of the washings became 7, and was dried for 12 hours at 80° C., to obtain a noble metal catalyst Pd-LDHs-Al 2 O 3  supported on alumina as the carrier of the present Example.       
 
         [0046]    In the catalyst of the Example, the LDHs had a chemical formula which could be written as [Mg 2+   0.67 Al 3+   0.33 (OH) 2 ] 0.33+ (CO 3   2− ) 0.165 .H 2 O; the spherical γ-Al 2 O 3  carrier had an original specific surface area of about 182 m 2 /g, and the specific surface area after in situ growth of the hydrotalcite was about 211 m 2 /g; the loss ratio of Pd metal was less than 5%, and the dispersion degree thereof was 85%. 
       Example 4 
       [0047]    In Example 4, a supported noble metal catalyst was prepared in accordance with the following steps:
       Steps A) and B) were the same as those in Example 1;   Step C) 9.16 g of hexamethylenetetramine was weighed, and dissolved in the metal salt solution under conditions of an ice bath such that the molar ratio of Mg to hexamethylenetetramine was 1/10; the solution was stirred under conditions of the ice bath until hexamethylenetetramine was completely dissolved, and then the Pd(NO 3 ) 2  solution was added thereto and stirred well;   Step D) 5 g of a bar-shaped α-Al 2 O 3  carrier having an average length of 10 mm was added to the solution from the step C); the above mixture was transferred to a hydrothermal reactor, and crystallized for 4 hours at 130° C.; after being cooled to room temperature, the solid product was washed with deionized water until the pH value of the washings became 7, and was dried for 12 hours at 80° C., to obtain a noble metal catalyst Pd-LDHs-Al 2 O 3  supported on alumina as the carrier of the present Example.       
 
         [0051]    In the catalyst of the Example, the LDHs had a chemical formula which could be written as [Mg 2+   0.67 Al 3+   0.33 (OH) 2 ] 0.33+ (CO 3   2− ) 0.165 .H 2 O; the bar-shaped α-Al 2 O 3  carrier had an original specific surface area of about 9 m 2 /g, and the specific surface area after in situ growth of the hydrotalcite was about 13 m 2 /g; the loss ratio of Pd metal was less than 5%, and the dispersion degree thereof was 62%. 
         [0052]    The SEM micrograph and XPS pattern of the catalyst of the Example are shown in  FIGS. 4 and 5 , respectively. As shown in  FIG. 4 , the hydrotalcite having an intact crystal is grown in situ on the surface of the alumina carrier, and has a hexagonal shape and a size of about 1 micron; the Pd metal particles are in the shape of triangles and several other shapes, and have a size of about 30 to 60 nm. The results of XPS analysis in  FIG. 5  show that in the Pd catalyst supported on alumina, the 3d 2/5  state of Pd has a peak at 335.7 eV, indicating that the valence state of the noble metal particles is zero. 
       Example 5 
       [0053]    In Example 5, a supported noble metal catalyst was prepared in accordance with the following steps:
       Step A) 0.0228 g of Pd(NO 3 ) 2  was weighed and dissolved in 10 mL of deionized water to prepare a precursor solution of Pd metal;   Step B) 1.32 g of MgCl 2 .6H 2 O was weighed and dissolved in 90 mL of deionized water to prepare a metal salt solution;   Step C) and D) were the same as those in Example 1.       
 
         [0057]    In the catalyst of the Example, the LDHs had a chemical formula which could be written as [Mg 2+   0.67 Al 3+   0.33 (OH) 2 ] 0.33+ (CO 3   2− ) 0.165 .H 2 O; the original spherical α-Al 2 O 3  carrier had a specific surface area of about 19 m 2 /g, and the specific surface area after in situ growth of the hydrotalcite was about 25 m 2 /g; the loss ratio of Pd metal was less than 5%, and the dispersion degree thereof was 65%. 
       Example 6 
       [0058]    In Example 6, a supported noble metal catalyst was prepared in accordance with the following steps:
       Step A) 0.0114 g of Pd(NO 3 ) 2  was weighed and dissolved in 10 mL of deionized water to prepare a precursor solution of Pd metal;   Step B) 1.89 g of Ni(NO 3 ) 2 .6H 2 O was weighed and dissolved in 90 mL of deionized water to prepare a metal salt solution;   Step C) and D) were the same as those in Example 1.       
 
         [0062]    In the catalyst of the Example, the LDHs had a chemical formula which could be written as [Ni 2+   0.75 Al 3+   0.25 (OH) 2 ] 0.25+ (CO 3   2− ) 0.125 H 2 O; the original spherical α-Al 2 O 3  carrier had a specific surface area of about 19 m 2 /g, and the specific surface area after in situ growth of the hydrotalcite was about 25 m 2 /g; the loss ratio of Pd metal was less than 5%, and the dispersion degree thereof was 69%. 
       Example 7 
       [0063]    In Example 7, a supported noble metal catalyst was prepared in accordance with the following steps:
       Step A) 0.0058 g of NaCl and 0.0088 g of PdCl 2  were weighed in a molar ratio of 2:1, and dissolved in 10 mL of deionized water to prepare a solution of Na 2 PdCl 4  as the Pd metal precursor;   Step B) 1.32 g of MgCl 2 .6H 2 O was weighed and dissolved in 90 mL of deionized water to prepare a metal salt solution;   Step C) 9.16 g of hexamethylenetetramine was weighed, and dissolved in the metal salt solution under conditions of an ice bath such that the molar ratio of Mg to hexamethylenetetramine was 1/10; the solution was stirred under conditions of the ice bath until hexamethylenetetramine was completely dissolved, and then the Na 2 PdCl 4  solution was added thereto and stirred well;   Step D) 5 g of a bar-shaped γ-Al 2 O 3  carrier having an average length of 10 mm was added to the solution from the step C); the above mixture was transferred to a hydrothermal reactor, and crystallized for 2 hours at 180° C.; after being cooled to room temperature, the solid product was washed with deionized water until the pH value of the washings became 7, and was dried for 12 hours at 80° C., to obtain a noble metal catalyst Pd-LDHs-Al 2 O 3  supported on alumina as the carrier of the present Example.       
 
         [0068]    In the catalyst of the Example, the LDHs had a chemical formula which could be written as [Mg 2+   0.67 Al 3+   0.33 (OH) 2 ] 0.33+ (CO 3   2− ) 0.165 .H 2 O; the bar-shaped γ-Al 2 O 3  carrier had an original specific surface area of about 159 m 2 /g, and the specific surface area after in situ growth of the hydrotalcite was about 187 m 2 /g; the loss ratio of Pd metal was less than 5%, and the dispersion degree thereof was 80%. 
       Example 8 
       [0069]    In Example 8, a supported noble metal catalyst was prepared in accordance with the following steps:
       Step A) 0.0159 g of Pt(C 5 H 7 O 2 ) 2  was weighed and dissolved in 10 mL of ethanol to prepare a precursor solution of Pt metal;   Step B) 1.87 g of ZnSO 4 .7H 2 O was weighed and dissolved in 90 mL of deionized water to prepare a metal salt solution;   Step C) 9.16 g of hexamethylenetetramine was weighed, and dissolved in the metal salt solution under conditions of an ice bath such that the molar ratio of Zn to hexamethylenetetramine was 1/10; the solution was stirred under conditions of the ice bath until hexamethylenetetramine was completely dissolved, and then the Pt metal precursor solution prepared in the step A) was added thereto and stirred well;   Step D) 5 g of Al 2 O 3  carrier having α,γ-mixed crystalline form and an average diameter of 5 mm was added to the solution from the step C); the above mixture was transferred to a hydrothermal reactor, and crystallized for 4 hours at 150° C.; after being cooled to room temperature, the solid product was washed with deionized water until the pH value of the washings became 7, and was dried for 12 hours at 80° C., to obtain a noble metal catalyst Pt-LDHs-Al 2 O 3  supported on alumina as the carrier of the present Example.       
 
         [0074]    In the catalyst of the Example, the LDHs had a chemical formula which could be written as [Zn 2+   0.67 Al 3+   0.33 (OH) 2 ] 0.33+ (CO 3   2− ) 0.165 .H 2 O; the original carrier had a specific surface area of about 109 m 2 /g, and the specific surface area after in situ growth of the hydrotalcite was about 125 m 2 /g; the loss ratio of Pt metal was less than 5%, and the dispersion degree thereof was 78%. 
       Example 9 
       [0075]    In Example 9, a supported noble metal catalyst was prepared in accordance with the following steps:
       Step A) 0.0130 g of RuCl 3 .3H 2 O was weighed and dissolved in 10 mL of deionized water to prepare a Ru precursor solution;   Step B) 1.93 g of Zn(NO 3 ) 2 .6H 2 O was weighed and dissolved in 90 mL of deionized water to prepare a metal salt solution;   Step C) 6.41 g of hexamethylenetetramine was weighed, and dissolved in the metal salt solution under conditions of an ice bath such that the molar ratio of Zn to hexamethylenetetramine was 1/7; the solution was stirred under conditions of the ice bath until hexamethylenetetramine was completely dissolved, and then the precursor solution of Ru metal prepared in the step A) was added thereto and stirred well;   Step D) was the same as that in Example 1.       
 
         [0080]    In the catalyst of the Example, the LDHs had a chemical formula which could be written as [Zn 2+   0.67 Al 3+   0.33 (OH) 2 ] 0.33+ (CO 3   2− ) 0.165 .H 2 O; the original spherical α-Al 2 O 3  carrier had a specific surface area of about 19 m 2 /g, and the specific surface area after in situ growth of the hydrotalcite was about 25 m 2 /g; the loss ratio of Ru metal was less than 5%, and the dispersion degree thereof was 72%. 
       Example 10 
       [0081]    In Example 10, a supported noble metal catalyst was prepared in accordance with the following steps:
       Step A) 0.0143 g of Rh(NO 3 ) 3  was weighed and dissolved in 10 mL of deionized water to prepare a Rh precursor solution;   Step B) 0.84 g of Mg(NO 3 ) 2 .6H 2 O and 0.97 g of Zn(NO 3 ) 2 .6H 2 O were weighed in a molar ratio of 1:1, and dissolved in 90 mL of deionized water to prepare a mixed metal salt solution;   Step C) 9.16 g of hexamethylenetetramine was weighed, and dissolved in the mixed metal salt solution prepared in the step B) under conditions of an ice bath such that the molar ratio of Mg, Zn and hexamethylenetetramine was 1/1/20; the solution was stirred under conditions of the ice bath until hexamethylenetetramine was completely dissolved, and then the precursor solution of Rh metal prepared in the step A) was added thereto and stirred well;   Step D) was the same as that in Example 1.       
 
         [0086]    In the catalyst of the Example, the LDHs had a chemical formula which could be written as [Mg 2+   0.33 Zn 2+   0.33 Al 3+   0.33 (OH) 2 ] 0.33+ (CO 3   2− ) 0.165 .H 2 O; the original spherical α-Al 2 O 3  carrier had a specific surface area of about 19 m 2 /g, and the specific surface area after in situ growth of the hydrotalcite was about 25 m 2 /g; the loss ratio of Rh metal was less than 5%, and the dispersion degree thereof was 73%.