The present invention relates to a nanocrystal cerium zirconium composite oxide, and the preparation and application thereof.  More specifically, the present invention relates to a nanocrystal cerium zirconium composite oxide comprising 4-98% by weight of CeO2 and 1-95% by weight of ZrO2. 
Cerium oxide, zirconium oxide and cerium-zirconium composition have been used as post-combustion catalyst to treat the exhaust from the internal combustion engine.  The using of cerium oxide, zirconium oxide and cerium-zirconium composition as multiple functional catalyst has been disclosed in the Chinese Patent Applications 96196505.3 and 94194552.9 (both by Rhone-Poulenc Chimie, France), 95119164.0, 95119160.8 and 95119163.2 (by Nippon Santoku Kinzoku Kogyou Kabushiki Kaisha), and 89103518.4 (by Nippon Shokubai Kagaku Kogyou Kabushiki Kaisha). 
The commercially available catalysts comprising cerium-zirconium oxide composition can not only oxidize carbon monoxide and hydrocarbon present in the exhaust, but also reduce the nitrogen oxide therein, so they are referred to as xe2x80x9ctri-functional catalystsxe2x80x9d.  To obtain a composition containing mixed cerium and zirconium oxides as its main components, the conventional preparation method is:
(1) preparing a liquid mixture containing cerium compound and zirconium compound;
(2) contacting the above liquid mixture with carbonate or bicarbonate to form a reacting medium, the pH of which is neutral or basic (generally the pH being controlled to 8-11);
(3) separating and recovering the resulting precipitates; and
(4) calcining the precipitates. 
A mixed cerium-zirconium oxide having a relatively large specific surface area can be obtained by this method, however, as disclosed in the above references, the products usually exist in the form of non-compounds.  In other words, in the resulting catalyst, it can be found that the two components exist as a simple physical mixture of well separated cerium oxide and zirconium oxide particles, as also confirmed by the X-ray diffraction analysis thereof. 
It is an object of the present invention to provide a nanocrystal cerium zirconium composite oxide which has a large specific surface area, high chemical reactivity and low ignition loss, the ignition loss thereof after igniting at 900xc2x0 C. for one hour being less than 5%. 
It is another object of the present invention to provide a nanocrystal cerium zirconium composite oxide with excellent thermal stability, the cerium zirconium composite oxide of the present invention remaining as a single phase after igniting at high temperature for a prolonged period of time. 
In the present invention, a nanocrystal cerium zirconium composite oxide and its preparation and application are disclosed.  The nanocrystal cerium zirconium composite oxide of the present invention comprises 4-98% by weight of CeO2 and 1-95% by weight of ZrO2, the crystalline particle size thereof is 100 nm or less, and the ignition loss after igniting at 900xc2x0 C. for one hour is smaller than 5%. 
The nanocrystal cerium zirconium composite oxide with the above characteristics are obtained by using the novel preparation method of the present invention. 
Thermal decomposing the corresponding cerium zirconium precursor compound containing hydrazine (or hydrazine salt) at 200-1000xc2x0 C. can obtain the nanocrystal cerium zirconium composite oxide of the present invention, said precursor compound is selected from the nitrate, chloride, oxalate, carbonate and hydroxide of cerium and zirconium or the mixture thereof. 
The above precursor compound containing hydrazine or hydrazine salt may be prepared by co-precipitating, mixing or other methods:
(1) The co-precipitating method comprises first adding hydrazine (or hydrazine salt) into cerium compound or zirconium compound and then co-precipitating; or adding hydrazine (or hydrazine salt) into the mixed solution of cerium-zirconium compound and then co-precipitating.  
(2) The mixing method comprises adding hydrazine or hydrazine compound into the mixture of cerium compound and zirconium compound and then agitating homogeneously.  
The nanocrystal cerium zirconium composite oxide can be produced by heating the above cerium-zirconium precursor compound to decompose it, cooling to room temperature, deagglomerating, and sieving. 
The advantages of the method of the present invention are:
The hydrazine (or hydrazine salt) decomposes at 180-300xc2x0 C. and simultaneously releases a large amount of gas and heat, the instantaneous releasing of heat causes the precursor compound to decompose more completely, and the large amount of gas produced at the same time makes the crystalline particles of the thermal decomposition product difficult to agglomerate and grow, so that the crystalline particles of the thermal decomposition product thus obtained are smaller than those of the product obtained by the thermal decomposition of the same precursors which do not contain hydrazine (or hydrazine salt), thus the product of the present invention has higher activity and thermal stability, as well as a lower ignition loss when igniting. 
The nanocrystal cerium zirconium composite oxide obtained by the method of the present invention is a novel multiple component composite oxide of the solid solution type, it comprises 4-98% by weight of CeO2 and 1-95% by weight of ZrO2.  The crystalline particle size of the Nanocrystal cerium zirconium composite oxide of the present invention is 100 nm or less, the specific surface area thereof as measured by BET method is 5-120 m2/g, it has a relatively high chemical activity and thermal stability, and the ignition loss after ignition at 900xc2x0 C. for one hour is smaller than 5%.  The nanocrystal cerium zirconium composite oxide of the present invention can remain as a single phase after igniting at high temperature for a prolonged period of time.  Particularly, when the composite oxide has cerium as its major component, i.e. the content of CeO2 is 70-98% by weight, it remains as a single phase after igniting at 1100xc2x0 C. for 12 hours. 
The nanocrystal cerium zirconium composite oxide of the present invention may also contain 0.01-10% by weight of other elements, said other elements are selected from the group comprising rare earth elements, hafnium, silicon, aluminum and molybdenum.  The performance of the Nanocrystal cerium zirconium composite oxide of the present invention is even better when incorporating these elements. 
The nanocrystal cerium zirconium composite oxide of the present invention has excellent performances when used as an abrasive, catalyst or in environmental protection industry, it has a bright prospect of wide application, especially, nanocrystal oxide is an superior abrasive for the present-day high quality grinding. 
The excellent performances of the nanocrystal cerium zirconium composite oxide of the present invention are further illustrated with reference to the following figures and examples.