Catalyst compositions comprising zirconium, cerium and yttrium oxides having a high reducibility

Abstract

Catalyst compositions useful for the conversion of vehicular exhaust gases contain zirconium, cerium and yttrium oxides with a cerium oxide proportion of from 3% to 15%, and yttrium oxide proportions corresponding to the following conditions: 6% at most if the cerium oxide proportion is from 12% excluded and 15% included; 10% at most if the cerium oxide proportion is from 7% excluded and 12% included; 30% at most if the cerium oxide proportion is from 3% to 7% included, the balance being zirconium oxide; such compositions may optionally include an oxide of a rare earth selected from among lanthanum, neodymium and praseodymium.

1 .- 15 . (canceled) 
     
     
         16 . A catalyst composition based on oxides of zirconium, of cerium and of yttrium, comprising the following proportions by weight:
 a proportion of cerium oxide ranging from 3 to 15%; proportions yttrium oxide which meet the following conditions:   proportion of yttrium oxide of at most 6% if the proportion of cerium oxide ranges from 12% exclusive to 15% inclusive;   a proportion of yttrium oxide of at most 10% if the proportion of cerium oxide ranges from 7% exclusive to 12% inclusive;   a proportion of yttrium oxide of at most 30% if the proportion of cerium oxide ranges from 3 to 7% inclusive; and   
       the remainder being zirconium oxide. 
     
     
         17 . The catalyst composition as defined by  claim 16 , further comprising at least one oxide of a rare earth metal selected from among lanthanum, neodymium and praseodymium in the following proportions by weight:
 a proportion of oxide of said rare earth metal of at most 10% if the proportion of cerium oxide ranges from 12% exclusive to 15% inclusive;   a proportion of oxide of said rare earth metal of at most 18% if the proportion of cerium oxide ranges from 7% exclusive to 12% inclusive; and   a total proportion of yttrium oxide and of oxide of said rare earth metal of at most 30% if the proportion of cerium oxide ranges from 3 to 7% inclusive.   
     
     
         18 . The catalyst composition as defined by  claim 16 , comprising a content of yttrium of at least 3%. 
     
     
         19 . The catalyst composition as defined by  claim 16 , having a degree of reducibility, measured on the composition calcined at 1,000° C. for 4 hours, of at least 90%. 
     
     
         20 . The catalyst composition as defined by  claim 16 , having a degree of reducibility, measured on the composition calcined at 1,000° C. for 4 hours, of at least 94%. 
     
     
         21 . The catalyst composition as defined by  claim 16 , having a maximum reducibility temperature of at most 550° C. 
     
     
         22 . The catalyst composition as defined by  claim 17 , in the form of particles which exhibit a concentration gradient for the yttrium and for the rare earth metal. 
     
     
         23 . A process for the preparation of a catalyst composition as defined by  claim 16 , comprising the following stages:
 (a) contacting a basic compound and compounds of zirconium, of cerium, of yttrium and optionally of another rare earth metal in liquid medium, whereby a precipitate is obtained;   (b) heating said precipitate in a liquid medium;   (c) adding an additive, selected from among anionic surfactants, nonionic surfactants, polyethylene glycols, carboxylic acids and salts thereof, and surfactants of the carboxymethylated ethoxylates of fatty alcohols type, to the precipitate obtained in the preceding stage; and   (d) calcining the precipitate thus obtained.   
     
     
         24 . The process as defined by  claim 23 , wherein stage (a) is carried out in two steps, in a first step, together contacting the compounds of zirconium and of cerium and a portion of the basic compound or all of the latter and, optionally, at least one of the compounds of yttrium and of the rare earth metal, in the case of the preparation of a composition based on oxides of zirconium, of cerium, of yttrium and of the other rare earth metal, and subsequently, in a second step, together contacting the other portion of said basic compound, if appropriate, and the compound of yttrium and optionally, in the case of the preparation of a composition based on oxides of zirconium, of cerium, of yttrium and of the other rare earth metal, the compound of yttrium and/or of the other rare earth metal which has not yet been together contacted. 
     
     
         25 . A process for the preparation of a catalyst composition as defined by  claim 16 , comprising the following stages:
 (a′) together contacting at least a portion of the basic compound and of the compounds of zirconium and of cerium and a portion of the yttrium compound, in the case of the preparation of a composition based only on oxides of zirconium, of cerium and of yttrium, a portion of at least one of the compounds of yttrium and of the rare earth metal, in the case of the preparation of a composition based on oxides of zirconium, of cerium, of yttrium and of a rare earth metal, in a liquid medium, whereby a precipitate is obtained;   (b′) heating said precipitate in a liquid medium;   (c′) on conclusion of stage (b), adding the remaining portion of the basic compound, if appropriate, and the remaining portion of the yttrium compound or of at least one of the compounds of yttrium and of the rare earth metal to the liquid medium;   (d′) adding an additive, selected from among anionic surfactants, nonionic surfactants, polyethylene glycols, carboxylic acids and salts thereof, and surfactants of the carboxymethylated ethoxylates of fatty alcohols type, to the precipitate obtained in the preceding stage; and   (e′) calcining the precipitate thus obtained.   
     
     
         26 . The process as defined by  claim 23 , wherein the compounds of zirconium, of cerium, of yttrium and of any rare earth metal comprise the nitrates, sulfates, acetates and/or chlorides thereof. 
     
     
         27 . The process as defined by  claim 23 , wherein the heating of the precipitate is carried out at a temperature of at least 100° C. 
     
     
         28 . The process as defined by  claim 23 , wherein the precipitate is washed before the calcination. 
     
     
         29 . A catalytic system, comprising a catalyst composition as defined by  claim 16 . 
     
     
         30 . A process for the treatment of exhaust gases from internal combustion engines, comprising contacting same with a catalyst composition as defined by  claim 16 .