Ceria-based mixed-metal oxide structure, including method of making and use

Abstract

A homogeneous ceria-based mixed-metal oxide, useful as a catalyst support, a co-catalyst and/or a getter, is described. The mixed-metal oxide has a relatively large surface area per weight, typically exceeding 150 m 2 /g, a structure of nanocrystallites having diameters of less than 4 nm, and including pores larger than the nanocrystallites and having diameters in the range of 4 to about 9 nm. The ratio of the pore volumes, V P , to skeletal structure volumes, V S , is typically less than about 2.5, and the surface area per unit volume of the oxide material is greater than 320 m 2 /cm 3 , such that the structural morphology supports both a relatively low internal mass transfer resistance and large effective surface area for reaction activity of interest. The mixed metal oxide is made by co-precipitating a dilute metal salt solution containing the respective metals, which may include Zr, Hf, and/or other metal constituents in addition to Ce, replacing water in the co-precipitate with a water-miscible low surface-tension solvent, and relatively quickly drying and calcining the co-precipitate at moderate temperatures. A highly dispersive catalyst metal, such as Pt, may be loaded on the mixed metal oxide support from a catalyst-containing solution following a selected acid surface treatment of the oxide support. The mixed metal oxide, as catalyst support, co-catalyst or getter, is applied in various reactions, and particularly water gas shift and/or preferential oxidation reactions as associated with fuel processing systems, as for fuel cells and the like.

What is claimed is:  
     
         1 . A homogeneous, nanocrystalline, mixed metal oxide of cerium and at least one other metal constituent from the group consisting of Zr, Hf, Nb, Ta, La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Mo, W, Re, Rh, Sb, Bi, Ti, V, Mn, Co, Cu, Ga, Ca, Sr and Ba, said mixed metal oxide having a surface area of at least about 150 m 2 /g, an average crystallite size less than 4 nm and agglomerated to form a skeletal structure with pores, average pore diameters being greater than 4 nm and normally being greater than the average crystallite size, and wherein the surface area of the skeletal structure per volume of the material of the structure is greater than about 320 m 2 /cm 3 .  
     
     
         2 . The mixed metal oxide according to  claim 1  wherein the skeletal structure with pores has a pore volume per unit mass,V P , and a structural volume per unit mass, V S , and wherein the ratio of V P  to V S , (V P /V S ), is less than about 2.5.  
     
     
         3 . The mixed metal oxide according to  claim 1  wherein, based on metals-only atomic percent, the sum of Ce and one or more optional constituents selected from the group consisting of La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Mo, W, Re, Rh, Sb, Bi, V, Mn, Co, Cu, Mo, Ca, Sr, Ba, and Ga is at least 60 percent and the sum of constituents selected from the group consisting of Zr, Hf, Nb, Ta, and Ti is 40 percent or less.  
     
     
         4 . The mixed metal oxide according to  claim 3  wherein, within the group of constituents consisting of Zr, Hf, Nb, Ta, and Ti, a sub-group consisting of Zr and Hf comprises 30% or greater and a sub-group consisting of Nb, Ta, and Ti comprises 10% or less.  
     
     
         5 . The mixed metal oxide according to  claim 1  wherein the at least one other metal constituent comprises up to 20 atomic percent, on a metals basis, of Bi.  
     
     
         6 . The mixed metal oxide according to  claim 1  wherein the at least one other ingredient is zirconium.  
     
     
         7 . The mixed metal oxide according to  claim 1  wherein the at least one other ingredient is hafnium.  
     
     
         8 . The mixed metal oxide according to  claim 1  wherein the surface area of the oxide is at least about 180 m 2 /g, the average crystallite size is equal to or less than 3.6 nm, and the average pore diameters are at least about 4.5 nm.  
     
     
         9 . The mixed metal oxide according to  claim 8  wherein the at least one other metal constituent is selected from the group consisting of Zr, Hf, Nb, Ta, Mo, W, Re, Rh, Ti, V, Mn, and Co, the average crystallite size is less than 3.5 nm, and the average pore diameters are at least about 5.0 nm.  
     
     
         10 . The mixed metal oxide according to  claim 9  wherein, by metals-only atomic percent, at least 10 percent is from the group consisting of Zr and Hf.  
     
     
         11 . A process for the preparation of the mixed metal oxide as defined in  claim 1 , including: 
 a. dissolving urea and salts of the Ce and the at least one other constituent in water to form a dilute metal salt solution;    b. heating the solution to near boiling and coprecipitating homogeneously an oxide of the Ce and the at least one other constituent as a nanocrystalline coprecipitate;    c. replacing water existing in the coprecipitate with a water miscible, low surface-tension solvent that displaces water;    d. drying the coprecipitate to remove substantially all of any remaining water and the solvent; and    e. calcining the dried coprecipitate at a moderate temperature for an interval sufficient to remove adsorbed impurities.    
     
     
         12 . The process according to  claim 11  including, following step b, the further step of maturing the coprecipitate in accordance with a thermal schedule.  
     
     
         13 . The process according to  claim 11  wherein the step d of drying the coprecipitate includes the step of refluxing the coprecipitate in a dry solvent in the presence of heat.  
     
     
         14 . The process according to  claim 11  wherein the water miscible, low surface tension solvent of step c is selected from the group consisting of an alcohol, a ketone, and an ester, each with 4 carbons or less.  
     
     
         15 . The process according to  claim 14  wherein the water miscible, low surface tension solvent is selected from the group consisting of dried 2-propanol, acetone, methyl ethyl ketone, and 1-propanol.  
     
     
         16 . The process according to  claim 15  wherein the water miscible, low surface tension solvent is dried 2-propanol.  
     
     
         17 . The process according to  claim 11  including, following the step of coprecipitating the coprecipitate, the further step of isolating the coprecipitate from the solution.  
     
     
         18 . The process according to  claim 11  wherein the metal concentration in the dilute metal salt solution is less than about 0.16 mol/L.  
     
     
         19 . The process according to  claim 18  wherein the metal concentration in the dilute metal salt solution is less than about 0.02 mol/L.  
     
     
         20 . The process according to  claim 19  wherein the metal concentration in the dilute metal salt solution is less than about 0.016 mol/L.  
     
     
         21 . The process according to  claim 11  wherein the concentration of urea in the dilute metal salt solution is relatively high, being greater than 0.5 mol/L.  
     
     
         22 . The process according to  claim 21  wherein the concentration of urea in the dilute metal salt solution is at least about 2.0 mol/L.  
     
     
         23 . The process according to  claim 12  wherein the maturing of the coprecipitated solution includes boiling the solution for a first interval and allowing the solution to cool to ambient during a second interval, the sum of the first and second intervals being less than about 72 hours.  
     
     
         24 . The process according to  claim 23  wherein the sum of the first and the second intervals is less than about 24 hours.  
     
     
         25 . The process according to  claim 24  wherein the first interval is in the range of six to ten hours and the second interval is less than about sixteen hours, and including the step of stirring the solution throughout both the heating/coprecipitating step and the maturing step.  
     
     
         26 . The process according to  claim 12  wherein, following the step of maturing the coprecipitate, the coprecipitate is isolated from the solution and washed with water, and water remaining in the water-washed coprecipitate is replaced by washing the coprecipitate with the water miscible, low surface-tension solvent.  
     
     
         27 . The process according to  claim 26  wherein the water miscible, low surface tension solvent is selected from the group consisting of an alcohol, a ketone, and an ester, each with 4 carbons or less.  
     
     
         28 . The process according to  claim 27  wherein the water miscible, low surface tension solvent is selected from the group consisting of dried 2-propanol, acetone, methyl ethyl ketone, and 1-propanol.  
     
     
         29 . The process according to  claim 28  wherein the water miscible, low surface tension solvent is dried 2-propanol.  
     
     
         30 . The process according to  claim 26  wherein the step of washing the coprecipitate with the water miscible, low surface tension solvent comprises one or more washings initially at room temperature and the subsequent drying step includes refluxing the coprecipitate in a dry solvent in the presence of heat for an interval of less than about one hour.  
     
     
         31 . The process according to  claim 11  wherein the step of calcining the dried coprecipitate comprises heating the dried coprecipitate to a calcining temperature in the range of about 250°-60° C. over a period of one to about six hours.  
     
     
         32 . The process according to  claim 31  wherein the dried coprecipitate is heated to a calcining temperature in the range of about 350°-500° C. over a period of about two to about four hours.  
     
     
         33 . A process for the preparation of a mixed metal oxide of cerium, Ce, and at least one other metal constituent, comprising: 
 a. dissolving urea and salts of the Ce and the at least one other constituent in water to form a dilute metal salt solution;    b. heating the solution to near boiling and coprecipitating homogeneously an oxide of the Ce and the at least one other constituent as a nanocrystalline coprecipitate;    c. replacing water existing in the coprecipitate with a water miscible, low surface-tension solvent that displaces water;    d. drying the coprecipitate to remove substantially all of any remaining water and the solvent; and    e. calcining the dried coprecipitate at a moderate temperature for an interval sufficient to remove adsorbed impurities.    
     
     
         34 . The process according to  claim 33  wherein the step of calcining the dried coprecipitate comprises heating the dried coprecipitate to a calcining temperature in the range of about 350°-500° C. over a period of about two to four hours.  
     
     
         35 . In a water gas shift reaction, a catalyst containing a noble metal on a mixed metal oxide in accordance with  claim 1 .  
     
     
         36 . The catalyst of  claim 35  wherein the noble metal is platinum.  
     
     
         37 . For use in a water gas shift reaction, a catalyst containing a noble metal on a support prepared in accordance with  claim 11 .  
     
     
         38 . The catalyst of  claim 37  wherein the noble metal is platinum.  
     
     
         39 . A catalyst on a mixed metal oxide in accordance with  claim 1 , the catalyst consisting of a highly dispersed metal, including at least a noble metal, loaded on the mixed metal oxide and having a concentration in the range of about 0.1 to about 5 percent by weight.  
     
     
         40 . The catalyst of  claim 39  wherein the noble metal has crystallites that are predominantly less than 2.5 nm.  
     
     
         41 . The catalyst of  claim 40  wherein the noble metal is platinum.  
     
     
         42 . A process for the preparation of the catalyst defined in accordance with  claim 39 , comprising the steps of: 
 a) contacting the mixed metal oxide with a solution containing an acid from the group consisting of amino acids, hydroxy dicarboxylic acids, hydroxy polycarboxylic acids, and keto polycarboxcylic acids to treat the surface of the mixed metal oxide; and    b) loading the surface-treated, mixed metal oxide with the noble metal by contacting the surface-treated mixed metal oxide with a solution containing the noble metal.    
     
     
         43 . The process according to  claim 42  wherein the acid selected for surface treating the mixed metal oxide is selected from the group consisting of malic acid and citric acid.  
     
     
         44 . The process according to  claim 42  wherein the noble metal is platinum and the solution is tetraamineplatinum nitrate.  
     
     
         45 . The process according to  claim 42  wherein the step a) of surface treating the mixed metal oxide comprises: 
 i. heating the mixed metal oxide in an acid-containing solution of ethanol and the acid at a mild temperature of about 50° C. for about 2 hour, and  
 ii. rinsing the mixed metal oxide with ethanol until the pH is greater than 4; the step b) of loading the noble metal on the surface-treated mixed metal oxide comprises: 
 i. submerging the mixed metal oxide in a solution of tetraamineplatinum nitrate having about 1 weight percent platinum, 1 weight percent ammonia hydroxide and 15 weight percent 2-propanol for about 2 hour at room temperature to metal load the mixed metal oxide and  
 ii. filtering and drying the metal loaded mixed metal oxide; and including the further step of: 
 c) calcining the metal loaded mixed metal oxide for up to about four hours at a heating rate of about 2° C./hr to a calcining temperature in the range of about 250°-600° C.  
 
 
 
     
     
         46 . The process according to  claim 45  wherein the calcining temperature to which the metal-loaded mixed metal oxide is heated is in the range of about 400°-500° C.  
     
     
         47 . A catalyst comprising a metal-loaded, ceria-based mixed metal oxide containing at least one other metal constituent, the catalyst made according to the process of: 
 a. dissolving urea and salts of the Ce and the at least one other constituent in water to form a dilute metal salt solution;    b. heating the solution to near boiling and coprecipitating homogeneously an oxide of the Ce and the at least one other constituent as a nanocrystalline coprecipitate;    c. replacing water existing in the coprecipitate with a water miscible, low surface-tension solvent that displaces water;    d. drying the coprecipitate to remove substantially all of any remaining water and the solvent;    e. calcining the dried coprecipitate at a moderate temperature for an interval sufficient to remove adsorbed impurities to provide the mixed metal oxide;    f. contacting the mixed metal oxide with a solution containing an acid from the group consisting of amino acids, hydroxy dicarboxylic acids, hydroxy polycarboxylic acids, and keto polycarboxcylic acids to treat the surface of the mixed metal oxide; and    g. loading the surface-treated, mixed metal oxide with noble metal by contacting the surface-treated mixed metal oxide with a solution containing the noble metal.    
     
     
         48 . The process according to  claim 47  wherein the step f) of surface treating the mixed metal oxide comprises: 
 i. heating the mixed metal oxide in an acid-containing solution of ethanol and the acid at a mild temperature of about 50° C. for about 2 hour, and  
 ii. rinsing the mixed metal oxide with ethanol until the pH is greater than 4; the step g) of loading the noble metal on the surface-treated mixed metal oxide comprises: 
 i. submerging the mixed metal oxide in a solution of tetraamineplatinum nitrate having about 1 weight percent platinum, 1 weight percent ammonia hydroxide and 15 weight percent 2-propanol for about 2 hour at room temperature to metal load the mixed metal oxide; and including the further step of: 
 h) calcining the metal loaded mixed metal oxide for up to about four hours at a heating rate of about 2° C./hr to a calcining temperature in the range of about 250°-60 ° C.