Micron-scale cerium oxide particle having multi-core single-shell structure and preparation method therefor

This application is a 371 of PCT/CN2015/073103 filed Feb. 15, 2015 which is incorporated herein by reference in its entirety.

The present inventing relates to the technical field of advanced inorganic nano- and/or micro-scale materials, particularly to the technical field of catalyst support.

Cerium oxide is an important oxide of rare earth elements, and it has many excellent properties as a catalyst support or an additive to supports, which have been confirmed in catalysis researches. In subsequent researches, it is reported that the cerium oxide can exhibit unique advantages as a catalyst support in oxidative reaction, methanol cleavage and reductive reaction of nitrogen oxides due to good performances in oxygen storage and release and strong redox behaviors between trivalent cerium ions and tetravalent cerium ions.

The physical and chemical properties of cerium oxide greatly depend on the microstructure thereof, e.g., size, morphology, and specific surface area, etc. Cerium oxide materials having micro- and nano-scale porous structure not only can well satisfy requirements of highly effective absorbing materials on the microstructure thereof, but also can increase catalytic active sites of surface by increasing specific surface area, thereby to increase catalytic properties of the cerium oxide materials. Hence, researches for the preparation of such kind of materials have great realistic significance. However, it is found that nano- and/or micro-scale cerium oxide particles and cerium oxide aerogel having a large specific surface area as prepared according to existing methods known in the art, as catalyst supports, have poor mechanical strength and resistance to high temperature, and are easy to be sintered and collapse in hollow structure at a high temperature in use.

When being applied in the field of catalytic after-treatment to exhaust emissions containing CO, NO or volatile organic compounds, supported catalysts as prepared by using cerium oxide having conventional structures as the catalyst support would involve the following defects: 1) Hydrothermal stability is poor. The support per se will easily take sintering and collapsing in hollow structure at a high temperature, which will decreases the surface area and porosity. 2) Active components, e.g., noble metals or transition metals, as supported by the support will be easily embedded during the hydrothermal aging at a high temperature. 3) Sulfur resistance is poor, and if the catalyst is exposed to a sulfur-containing atmosphere for a long period, the catalyst will be deactivated.

The above defects seriously restrict the applications of cerium oxide materials in the field of catalytic after-treatment of exhaust emissions that includes a stationary source and a mobile source. Hence, at now, it is in interest that how to maintain a high specific surface area of a nano-scale cerium oxide support while assuring its mechanical strength and resistance to high temperature.

FIG. 1 is a structural schematic diagram of the micro-scale cerium oxide particles having multi-cores single-shell structure according to the invention.

FIG. 2A - FIG. 2B each are a Scanning Electron Microscope (SEM) photograph for showing the nano-scale cerium oxide aggregate particles C-0 without multi-cores single-shell structure as prepared in the comparative example. As seen from the figures, the particles have rough surfaces and obviously exhibit a porous structure. Furthermore, the surfaces of the micro-scale cerium oxide aggregate particles are formed by compactly aggregating nano-scale cerium oxide particles having lower particle size.

FIG. 3A - FIG. 3B each are a Scanning Electron Microscope (SEM) photograph for showing the micro-scale cerium oxide particles C1-1 having multi-cores single-shell structure as prepared in Example 1 according to the invention. As seen from the figures, the micro-scale cerium oxide particles according to the invention have spherical or sphere-like structure. FIG. 3B is an enlarged view for showing damaged micro-scale cerium oxide particles having multi-cores single-shell structure at the position circled by the dot line in FIG. 3A , and FIG. 3B can demonstrate the presence of the shell.

FIG. 4 is an X-ray diffraction (XRD) pattern for showing the micro-scale cerium oxide particles C1-1 having multi-cores single-shell structure as prepared in Example 1 according to the invention. As seen from the figure, the material is in the Cubic Fluorite structure, and all the diffraction peaks are attributed to the Cubic Fluorite structure of the CeO 2 ; according to the broadening width of diffraction peak of the crystal plane (111), it can be calculated that the grain size of the CeO 2 is about 8.6 nm; it is proved that the cores of micro-scale cerium oxide particles having multi-cores single-shell structure as prepared according to the method in the invention are crystalline.

FIG. 5 is a Scanning Electron Microscope (SEM) photograph for showing the micro-scale cerium oxide particles C2-1 having multi-cores single-shell structure as prepared in Example 5 according to the invention. As seen from FIG. 5 , the micro-scale cerium oxide particles having multi-cores single-shell structure as prepared according to the method in the invention have relatively smooth surfaces.

FIG. 6 is a Scanning Electron Microscope (SEM) photograph for showing the micro-scale cerium oxide particles C2-1 having multi-cores single-shell structure as prepared in Example 5 according to the invention after grinding. As seen from FIG. 6 , after the grinding, the shells of the micro-scale cerium oxide particles having multi-cores single-shell structure are damaged, and thus the nano-scale cerium oxide grain cores aggregates in the interior of the shells are observed to be in a porous structure.

FIG. 7 is a Scanning Electron Microscope (SEM) photograph for showing the micro-scale cerium oxide particles C3-1 having multi-cores single-shell structure as prepared in Example 9 according to the invention. As seen from FIG. 7 , the micro-scale cerium oxide particles having a multi-cores single-shell structure as prepared according to the method in the invention have relatively smooth surfaces.

FIG. 8 is a Scanning Electron Microscope (SEM) photograph for showing the micro-scale cerium oxide particles C3-1 having multi-cores single-shell structure as prepared in Example 9 according to the invention after grinding. As seen from FIG. 8 , after the grinding, the shells of the micro-scale cerium oxide particles having multi-cores single-shell structure are damaged, and thus the nano-scale cerium oxide grain cores aggregates in the interior of the shells are observed to be in a porous structure.

FIG. 9A - FIG. 9B is a curve for showing the catalytic activities for catalyzing a mixed gas containing CO and C 3 H 6 with the catalysts obtainable by respectively supporting Pd with the micro-scale cerium oxide particles C3-1 having multi-cores single-shell structure as prepared in Example 9 and with the cerium oxide aggregates C-0 as prepared in the comparative example and then subjecting the supported catalysts to hydrothermal treatments. FIG. 9-1 is a conversion curve of the CO in the mixed gas, and FIG. 9B is a conversion curve of the C 3 H 6 in the mixed gas, wherein the symbol A represents the supported catalyst Pd/C3-1 after being subjected to the hydrothermal treatments, and the symbol B represents the supported catalyst Pd/C-0 after being subject to the hydrothermal treatments.

FIG. 10A - FIG. 10B is a curve for showing the catalytic activities for catalyzing a mixed gas containing CO and C 3 H 6 with the catalysts obtainable by respectively supporting Pd with the micro-scale cerium oxide particles C3-1 having multi-cores single-shell structure as prepared in Example 9 and with the cerium oxide aggregates C-0 as prepared in the comparative example, and then subjecting the supported catalysts to sulfurization treatments. FIG. 10A is a conversion curve of the CO in the mixed gas, and FIG. 10B is a conversion curve of the C 3 H 6 in the mixed gas, wherein the symbol A represents the supported catalyst Pd/C3-1 after being subject to the sulfurization treatments, and the symbol B represents the supported catalyst Pd/C-0 after being subjected to the sulfurization treatments.

FIG. 11 is a curve for showing the catalytic activities for catalyzing a NO-containing gas with the catalysts obtainable by respectively supporting Mn with the micro-scale cerium oxide particles C3-1 having multi-cores single-shell structure as prepared in Example 9 and with the cerium oxide aggregates C-0 as prepared in the comparative example, and then subjecting the supported catalysts to hydrothermal treatments, wherein the symbol A represents the supported catalyst Mn/C3-1 after being subjected to the hydrothermal treatments, and the symbol B represents the supported catalyst Mn/C-0 after being subjected to the hydrothermal treatments.

FIG. 12 is a curve for showing the catalytic activities for catalyzing a CO-containing gas with the catalysts obtainable by respectively supporting Mn with the micro-scale cerium oxide particles C3-1 having multi-cores single-shell structure as prepared in Example 9 and with the cerium oxide aggregates C-0 as prepared in the comparative example, and then subjecting the supported catalysts to sulfurization treatments, wherein the symbol A represents the supported catalyst Mn/C3-1 after being subjected to the sulfurization treatments, and the symbol B represents the supported catalyst Mn/C-0 after being subjected to the sulfurization treatments.

A first aspect of the invention is directed to a micro-scale cerium oxide particle having multi-cores single-shell structure, comprising: a cerium oxide shell, the shell being composed of crystalline and/or amorphous nano-scale cerium oxide particles; and a plurality of nano-scale cerium oxide grain cores aggregates located in the interior of the shell.

Preferably, the shell is made of crystalline nano-scale cerium oxide particles. There are thousands, ten thousands and even millions of nano-scale cerium oxide grain cores. As for the micro-scale cerium oxide particles having multi-cores single-shell structure according to the invention, the shell therein may protect the nano-scale cerium oxide grain cores aggregates in the interior thereof, so as to increase the properties of the particles in each aspect.

In a preferred embodiment, the micro-scale cerium oxide particles are spherical or sphere-like particles, having an average particle size of 0.5 μm to 50 μm, preferably 1 to 10 μm; a BET specific surface area of 50 to 200 m 2 /g, a pore volume of 0.1 to 0.8 cm 3 /g, and an average pore diameter of 2 to 40 nm; the mass of the plurality of nano-scale cerium oxide grain cores aggregates in the interior of the shell is from 85 to 99% based on the total mass of the micro-scale cerium oxide particles, and the mass of the cerium oxide shell is from 1 to 15% based on the total mass of the micro-scale cerium oxide particles, wherein the mass of the plurality of nano-scale cerium oxide grain cores aggregates in the interior of the shell and the mass of the cerium oxide shell may be controlled by controlling deposition conditions; the cerium oxide shell has a thickness ranging from 10 to 200 nm, preferably from 20 to 100 nm; the nano-scale cerium oxide grains have an average particle size of 2 to 50 nm, preferably from 2 to 40 nm, even preferably 2 to 30 nm, more preferably 2 to 20 nm, and most preferably 2 to 10 nm; the crystalline and/or amorphous nano-scale cerium oxide particles in the cerium oxide shell have an average particle size of 2 to 50 nm.

The nano-scale cerium oxide grain cores aggregates and the cerium oxide shell both are porous, i.e., have many microscopic channels or orifices, so as to facilitate inward and outward diffusions of reactants and reaction products. These microscopic channels or orifices are formed as follows: they are formed by gaps of microparticles when the numerous microparticles are agglomerated together; alternatively, as described below, when an organic adjuvant is used during the precipitation, the organic adjuvant will be entrained in solid particles, and after being calcined, the organic adjuvant is burnt out, then the channels and orifices are formed at the positions as initially occupied by the organic adjuvant.

A second aspect of the invention is directed to a method for preparation of the micron-scale cerium oxide particles having multi-cores single-shell structure, comprising the following steps: (A). Reacting a cerium salt solution with a precipitant to produce a suspension, then air being bubbled into the produced suspension to carry out oxidization reaction to produce a liquid phase dispersion system of nano-scale cerium oxide precursor, wherein the nano-scale cerium oxide precursor comprises hydroxides of cerium and oxides of cerium; (B). Subjecting the liquid phase dispersion system of the nano-scale cerium oxide precursor to spray drying and calcination, to produce an embryo body of nano-scale cerium oxide grain cores aggregates; (C). Dispersing the embryo body of nano-scale cerium oxide grain cores aggregates into a solvent to produce a suspension, then the cerium salt solution and the precipitant being added to the suspension to carry out a deposition shell-forming reaction around the nano-scale cerium oxide grain cores aggregates, to produce a second suspension; (D). Subjecting the second suspension to spray drying and calcination to produce the micro-scale cerium oxide particles having multi-cores single-shell structure, which is named as “C1”.

In a preferred embodiment, between the step A and the step B, the following step is added: an organic adjuvant is added to the liquid phase dispersion system of nano-scale cerium oxide precursor, wherein the organic adjuvant can act the functions of a co-dispersing agent and a pore-forming agent.

In a preferred embodiment, the organic adjuvant includes one or more selected from the group consisting of methylcellulose, starch, aminoacetic acid, 6-aminohexanoic acid, oxalic acid, citric acid, polyethylene glycol having a polymerization degree between 400 and 20000.

In a preferred embodiment, the cerium salt includes cerium(III) nitrate, cerium(III) acetate or cerium(III) sulfate; the cerium salt solution is in a concentration of 0.005 mol/L to 1 mol/L; the precipitant includes ammonia, urea, sodium hydroxide or potassium hydroxide; independently, in the step A, the pH of the suspension, before the oxidization, is adjusted to a range between 7 and 11.5; independently, in the step C, the solvent includes water, anhydrous ethanol, a water-ethanol mixed system or a water-ethylene glycol mixed system; independently, the calcination temperature in the step B and/or step D is from 450 to 750° C.

A third aspect of the invention is directed to a further method for preparation of micro-scale cerium oxide particles having multi-cores single-shell structure, comprising the following steps: (A). Reacting a cerium salt solution with a precipitant to produce a suspension, then air being bubbled into the produced suspension to carry out oxidization reaction to produce a liquid phase dispersion system of nano-scale cerium oxide precursor; (B). Adding an organic adjuvant and the cerium salt solution to the liquid phase dispersion system of nano-scale cerium oxide precursor, to produce a homogenously dispersed liquid phase dispersion system; (C). Subjecting the homogenously dispersed liquid phase dispersion system to spray drying and calcination, to produce the micro-scale cerium oxide particles having multi-cores single-shell structure. Wherein the micro-scale cerium oxide particles having multi-cores single-shell structure as prepared according to this method is named as “C2”.

In a preferred embodiment, the cerium salt includes cerium(III) nitrate, cerium(III) acetate or cerium(III) sulfate; the cerium salt solution is in a concentration of 0.005 mol/L to 1 mol/L; the precipitant includes ammonia, urea, sodium hydroxide or potassium hydroxide; independently, in the step A, the pH of the suspension, before the oxidization, is adjusted to a range between 7 and 11.5; the organic adjuvant includes one or more selected from the group consisting of methylcellulose, starch, aminoacetic acid, 6-aminohexanoic acid, oxalic acid, citric acid, polyethylene glycol having a polymerization degree between 400 and 20000; independently, the calcination temperature in the step C is from 450 to 750° C.

A fourth aspect of the invention is directed to a further method for preparation of micro-scale cerium oxide particles having multi-cores single-shell structure, comprising the following steps: (A). Reacting a cerium salt solution with a precipitant to produce a suspension, wherein the cerium salt solution and/or the precipitant contain an organic adjuvant, and the cerium salt is in the stoichiometric excess relative to the precipitant; then air being bubbled into the produced suspension to carry out oxidization reaction to produce a liquid phase dispersion system of nano-scale cerium oxide precursor; (B). Subjecting the liquid phase dispersion system of nano-scale cerium oxide precursor to spray drying and calcination, to produce the micro-scale cerium oxide particles having multi-cores single-shell structure. Wherein the micro-scale cerium oxide particles having multi-cores single-shell structure as prepared according to this method is named as “C3”.

In the method, cerium ions in the excessive cerium salts will bond to the organic adjuvant, to produce a cerium oxide shell during the spray drying and calcination.

In a preferred embodiment, the cerium salt includes cerium(III) nitrate, cerium(III) acetate or cerium(III) sulfate; the cerium salt solution is in a concentration of 0.005 mol/L to 1 mol/L; the precipitant includes ammonia, urea, sodium hydroxide or potassium hydroxide; the organic adjuvant includes one or more selected from the group consisting of methylcellulose, starch, aminoacetic acid, 6-aminohexanoic acid, oxalic acid, citric acid, polyethylene glycol having a polymerization degree between 400 and 20000; independently, in the step A, the pH of the suspension, before the oxidization, is adjusted between 8 and 11.5; independently, the calcination temperature in the step B is from 450 to 750° C.

A fifth aspect of the invention is directed to a supported catalyst that includes a support and an active component supported on the support, wherein the support is the micro-scale cerium oxide particle having multi-cores single-shell structure according to the first aspect of the invention, and the active component is nanoparticles of noble metals or transition metals, dispersed on the plurality of nano-scale cerium oxide grain cores and/or on the cerium oxide shell.

In a preferred embodiment, the noble metal may be Pt or Pd, wherein the noble metal is in an oxidative state or in an elemental state, and by calculations based on the mass of the noble metals, the loading quantity of the noble metal is 0.02 to 5% based on the mass of the micro-scale cerium oxide particles having multi-cores single-shell structure; independently, the transition metal is Mn, Fe, Co, Ni or Cu, wherein the transition metal is in an oxidative state, and by calculations based on the mass of the transition metals, the loading quantity of the transition metal is 0.1 to 20% based on the mass of the micro-scale cerium oxide particles having multi-cores single-shell structure.

A sixth aspect of the invention is directed to the use of the supported catalyst according to the fifth aspect of the invention, wherein the supported catalyst is used for catalysis of NO oxidation, CO oxidation, three-way catalytic reactions of vehicle off-gases or oxidative elimination reaction of organics.

The present invention has the following beneficial effects: (1). The micro-scale cerium oxide particles having multi-cores single-shell structure according to the invention have large specific surface area, high mechanical strength and good hydrothermal stability. (2). Supported catalysts as prepared by using the micro-scale cerium oxide particles having multi-cores single-shell structure according to the invention have the advantages: good hydrothermal stability, good sulfur resistance, no easiness of supported active components, e.g., noble metals or transition metals, to be embedded during hydrothermal aging at a high temperature.

The following examples are list out for illustrating the invention, while these examples are not limiting.

Comparative Example

Nano-scale cerium oxide grain aggregates without multi-cores single-shell structure are papered according to the following steps: (A). A 0.005 mol/L cerium(III) nitrate solution was reacted with ammonia to produce a suspension, and the pH of the produced suspension was regulated to be 7; air was bubbled into the produced suspension to oxidize it, to produce a liquid phase dispersion system of nano-scale cerium oxide precursor; (B). The produced liquid phase dispersion system of nano-scale cerium oxide precursor was subjected to spray drying, and then was calcined at 450° C. to produce nano-scale cerium oxide grain aggregates; the nano-scale cerium oxide grain aggregates without multi-cores single-shell structure as prepared according to the method was named as “C-0”.

As for the preparation process in the step A, please see the reference document: X.-D. Zhou, W. Huebner, H. U. Anderson, Processing of nanometer-scale CeO 2 particles[J], Chemical Materials, 2003, 15:378-382, which is cited herein for the reference.

Examples 1-4

The micron-scale cerium oxide particles C1 having multi-cores single-shell structure are prepared according to the following steps: (A). Reacting a cerium salt solution with a precipitant to produce a suspension, while regulating the pH of the produced suspension, air being bubbled to carry out oxidization reaction to produce a liquid phase dispersion system of nano-scale cerium oxide precursor; wherein optionally, an organic adjuvant is added to the liquid phase dispersion system of nano-scale cerium oxide precursor; (B). Subjecting the liquid phase dispersion system of the nano-scale cerium oxide precursor to spray drying and calcination, to produce an embryo body of nano-scale cerium oxide grain cores aggregates; (C). Dispersing the embryo body of nano-scale cerium oxide grain cores aggregates into a solvent to produce a suspension, then the cerium salt solution and the precipitant being added to the suspension to carry out a deposition shell-forming reaction around the nano-scale cerium oxide grain cores aggregates, to produce a second suspension; (D). Subjecting the second suspension to spray drying and calcination to produce the micro-scale cerium oxide particles having multi-cores single-shell structure, named as “C1”.

Specific experimental results are shown in Table 1.

Examples 5-8

The micron-scale cerium oxide particles C2 having multi-cores single-shell structure are prepared according to the following steps: (A). Reacting a cerium salt solution with a precipitant to produce a suspension, whiling regulating the pH of the produced suspension, air being bubbled to carry out oxidization reaction to produce a liquid phase dispersion system of nano-scale cerium oxide precursor; (B). Adding an organic adjuvant and the cerium salt solution to the liquid phase dispersion system of nano-scale cerium oxide precursor, to produce a homogenously dispersed liquid phase dispersion system; (C). Subjecting the homogenously dispersed liquid phase dispersion system to spray drying and calcination, to produce micro-scale cerium oxide particles having multi-cores single-shell structure, wherein the micro-scale cerium oxide particles having multi-cores single-shell structure as prepared according to the method is named as “C2”.

Specific experimental results are shown in Table 2.

Examples 9-12

The micron-scale cerium oxide particles C3 having multi-cores single-shell structure are prepared according to the following steps: (A). Reacting a cerium salt solution with a precipitant to produce a suspension, wherein the cerium salt solution and/or the precipitant contain an organic adjuvant, and the cerium salt is in the stoichiometric excess relative to the precipitant; while regulating the pH of the produced suspension, air being bubbled to carry out oxidization reaction to produce a liquid phase dispersion system of nano-scale cerium oxide precursor; (B). Subjecting the liquid phase dispersion system of nano-scale cerium oxide precursor to spray drying and calcination, to produce micro-scale cerium oxide particles having multi-cores single-shell structure, wherein the micro-scale cerium oxide particles having multi-cores single-shell structure as prepared according to the method is named as “C3”.

Specific experimental results are shown in Table 3.