METHOD OF MAGNETIC ANALYSIS TO DETERMINE THE CATALYTIC ACTIVITY OF METAL OXIDES INCLUDING NANOCERIA

BACKGROUND
Field of the Disclosure

Transition metal oxides and oxides of the Lanthanide and Actinide metals are of interest as catalysts for numerous chemical reactions. Throughout this discussion unless specified differently, the term “metal oxide” will be understood to generically encompass, transition metal oxides, lanthanide metal oxides and actinide metal oxides. The members of these series may characteristically exist in at least two different oxidation states in the metal oxide lattice structure and redox interconversion of these different oxidation states is conventionally believed to explain or contribute to catalytic activity. Moreover, in the metal oxide matrix or crystal lattice the metal ion may be in coordination with from two to eight oxide (O²⁻) ions and in consideration that the oxygen ions may serve as reducing agents to reduce the metal from a higher oxidation number to a lower oxidation number, the oxidation product from the O²⁻ ion is molecular oxygen (O₂) which may vacate the lattice position occupied by the O²⁻, becoming adsorbed oxygen which may then exit the lattice to an environment external to the lattice. As a result of this series of events a vacancy will be formed in the lattice at the position originally occupied by the O²⁻. The presence of such oxygen vacancies and reduced metal ion may be categorized as lattice defects or simply defects.

A wide variety of methods to explain, understand and ultimately predict the catalytic activity of the metal oxides have been reported. However, to date, a method having universal utility has not been described.

Nanoscale CeO₂(nanoceria) as a metal oxide is a versatile, commercially valuable catalytic material having redox properties which can be tuned by choice of method of preparation, particle size, dopant selection, particle shape and surface chemistry. Cerium in a fluorite crystal lattice may exist as a Ce⁴⁺ oxidation state or a Ce³⁺ oxidation state and this characteristic makes possible the formation of oxygen vacancies in the crystal lattice which may be the basis for a unique oxygen buffering capability as well as a capability to participate in both oxidation and reduction reactions. These properties may serve as the basis for the catalytic reactivity of nanoceria and utilization, for example, in catalytic converters, as diesel oxidation catalysts, intermediate temperature solid oxide fuel cells and sensors.

Oxygen vacancy defects in nanoceria and in metal oxides as described above are of special importance because of their impact on the structure and electronic properties of the surface and additionally, the Ce⁴⁺ to Ce³⁺ or appropriate metal redox cycles through oxygen vacancy formation. One important aspect of nanoceria's catalytic activity is its oxygen partial pressure dependent ability to undergo repeatable Ce⁴⁺ to Ce³⁺ redox cycles through oxygen vacancy formation. Oxygen vacancy transport, i.e. mobility of oxygen in the fluorite lattice, is therefore of fundamental interest due to the reactivity of these vacancies once they reach the crystal surface. Thus, discovery of a method to identify and quantify these vacancies or defects may be of universal utility for metal oxides.

The crystallographic defect structure of nanoceria is predominately oxygen vacancy based, where an oxygen ion is removed from a lattice position and a vacancy is created. The crystal structure of ceria is shown in FIG. 10 where each Ce ion (white sphere) is surrounded by eight O²⁻ ions (black), and each O²⁻ ion is surrounded by four Ce ions. The defect formation may be explained by the reaction:

4Ce⁴⁺+O²⁻→4Ce⁴⁺+2e−/□+1/2O₂→2Ce⁴⁺+2Ce³⁺+□+1/2O₂

where □ is an oxygen vacancy and O²⁻ and O₂represent lattice and molecular oxygen, respectively¹. This transformation is shown in FIG. 1. The Ce⁴⁺ to Ce³⁺ transformation is due to oxygen vacancy formation and the released oxygen in the fluorite lattice is intrinsic oxygen. The valence change of Ce⁴⁺ to Ce³⁺ with concomitant oxygen vacancy formation significantly modifies the reactivity of nanoceria. Esch et al.²proposed that these oxygen vacancies cluster and expose the Ce³⁺ ions to gas phase reactants such as CO and NO. Campbell et al.³specifically reported that reactive Ce³⁺ are formed when three oxygen ions are removed. Much of the interest in ceria materials is because of its high oxygen storage capacity from being able to rapidly form and eliminate these oxygen vacancies. Improvement of the catalytic properties, such as the oxygen storage capacity of the ceria nanoparticles, requires a fundamental understanding of the Ce⁴⁺→Ce³⁺ redox cycle.

Several challenges make difficult a fundamental understanding of the processes described above. Feng et al.⁴reported that high-vacuum spectroscopy studies of CeO₂over-estimate the Ce³⁺ concentration due to enhanced surface reduction of ceria in the sample measurement environment. Furthermore, during temperature programmed reaction (TPR) techniques some artifacts can interfere in the interpretation of measurements, e.g. TPR does not take into account the reduction of impurities (nitrates, carbonates).

Neutron diffraction studies by Mamontov et al.⁶on nanoceria identified that the missing intrinsic oxygen ions occupy interstitial sites and are weakly bound in the fluorite lattice, enabling additional defects in the form of anion-Frenkel pairs. However, there remains the need for a capability to examine interstitial oxygen ions' interactions and dynamics in order to understand and improve the catalytic activity of nanoceria and other metal oxides.

Thus, there is a need for a method to obtain quantitative results concerning the Ce³⁺ ions and other metals of lower oxidation state of a redox couple, arising from surface and bulk reduction as well as oxygen vacancy creation and annihilation mechanisms in nanoceria and other metal oxides in order to develop catalytic materials with specific improved properties.

SUMMARY

The present inventors have determined that as opposed to most measurements that have motivated and driven the current understanding of catalysis of metal oxides and most significantly, of CeO₂, magnetic measurements can provide quantitative results concerning the ions arising from both surface and bulk reduction. The magnetic susceptibility of the metal oxides may be sensitive to the redox couple if one oxidation state is diamagnetic and the other paramagnetic. Thus, ceria is sensitive to the redox transformation of diamagnetic Ce⁴⁺ (4f⁰) to paramagnetic Ce³⁺ (4f¹) with S=1/2 and J=5/2), and the inventors have discovered a method to study the evolution of vacancies and migration of oxygen atoms from inside the ceria structure to its surfaces employing magnetic susceptibility measurement under selected conditions as described herein.

Vincent et al.⁷and Seehra et al.⁸estimated the concentration of Ce³⁺ ions by magnetic susceptibility measurements.

However, the present inventors have discovered that magnetic susceptibility for nanoceria is dependent upon O₂partial pressure and that measurement of the magnetic susceptibility of nanoceria under O₂rich (in air) and deficient (in He) conditions provides a method to determine the intrinsic oxygen storage and release properties of nanoceria, which may be correlated to catalytic activity of that material.

Moreover, the method may be applied to any metal oxide wherein the magnetic susceptibility is sensitive to a redox transformation selected from the group consisting of a diamagnetic oxidation state to a paramagnetic oxidation state, a paramagnetic oxidation state to a ferromagnetic oxidation state and a paramagnetic oxidation state to an antiferromagnetic oxidation state. Such transitions allow the study of the evolution of the metal ions as well as the oxygen vacancies and migration of oxygen atoms from inside the metal oxide structure to its surfaces via magnetic susceptibility measurement according to the embodiments to be described.

Thus, a first embodiment of this application includes a method to predict the catalytic activity of a metal oxide which is subject to one of the listed transformations of formula M_xO_ywhere x is a number from 1 to 3 and y is a number from 1 to 8. The method includes:

measuring the magnetic susceptibility of a metal oxide sample in an oxygen environment at a specified temperature;

correlating the magnetic susceptibility measured to a value of number of (M^d+ or M^p+ or M^f+ or M^a+)/g (O₂rich)

measuring the magnetic susceptibility of the metal oxide sample in an oxygen free environment at the specified temperature;

correlating the magnetic susceptibility measured to a value of number of (M^d+ or M^p+ or M^f+ or M^a+)/g (O₂deficient)

determining the catalytic active (M^d+ or M^p+ or M^f+ or M^a+) concentration according to the equation:

(M^d+ or M^p+ or M^f+ or M^a+)/g (active)=[(M^d+ or M^p+ or M^f+ or M^a+)/g (O₂deficient)−(M^d+ or M^p+ or M^f+ or M^a+)/g (O₂rich)]/(M^d+ or M^p+ or M^f+ or M^a+)/g (O₂deficient);

and

predicting catalytic activity of the metal oxide sample with the (M^d+ or M^p+ or M^f+ or M^a+)/g (active) value;

wherein the metal of the metal oxide has redox coupled oxidation states wherein the redox transformation is between oxidation states selected from the group consisting of a diamagnetic oxidation state (M^d+) and a paramagnetic oxidation state (M^p+), a paramagnetic oxidation state (M^p+) and a ferromagnetic oxidation state (M^f+), and a paramagnetic oxidation state (M^p+) and an antiferromagnetic oxidation state (M^a+) where d, p, f and a are independently numbers from 1 to 6 and one of the oxidation states (M^d+), (M^p+), (M^f+), and (M^a+) is formed by reduction by the O²⁻.

In a special aspect of the first embodiment a method to predict the catalytic activity of a nanoceria sample is provided. The method comprises:

measuring the magnetic susceptibility of a nanoceria sample in an oxygen environment at a specified temperature;

correlating the magnetic susceptibility measured to a value of number of Ce³⁺/g (O₂rich) measuring the magnetic susceptibility of the nanoceria in an oxygen free environment at the specified temperature;

correlating the magnetic susceptibility measured to a value of number of Ce³⁺/g (O₂deficient)

determining the catalytic active Ce³⁺ concentration according to the equation:

Ce³⁺/g (active)=[Ce³⁺/g (O₂deficient)−Ce³⁺/g (O₂rich)]/Ce³⁺/g (O₂deficient); and

predicting catalytic activity of the nanoceria with the Ce³⁺/g (active) value.

In one aspect of this embodiment, the catalytic activity is for the reduction of NO with CO.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the unit cell fluorite structure of CeO₂before and after vacancy formation.

FIG. 2(A) shows a TEM image of nanocubes of nanoceria.

FIG. 2(B) shows a HRTEM image of nanocubes of nanoceria with a 110 plane identified.

FIG. 2(C) shows a HRTEM image of nanocubes of nanoceria with a 100 plane identified.

FIG. 2(D) shows a TEM image of nanorods of nanoceria.

FIG. 2(E) shows a HRTEM image of nanorods of nanoceria with a 111 plane identified.

FIG. 2(F) shows a HRTEM image of nanorods of nanoceria with a 100 plane identified.

FIG. 2(G) shows a TEM image of nanospheres of nanoceria.

FIG. 2(H) shows a HRTEM image of nanospheres of nanoceria with a 111 plane identified.

FIG. 2(I) shows a HRTEM image of nanospheres of nanoceria with a 111 plane identified.

FIG. 3 shows room temperature XRD patterns of CeO₂nanocubes, nanorods and nanospheres.

FIG. 4(A) shows inverse magnetic susceptibility χ(T)⁻¹of O₂rich ceria nanocubes as a function of temperature.

FIG. 4(B) shows inverse magnetic susceptibility χ(T)⁻¹of O₂rich ceria nanorods as a function of temperature.

FIG. 4(C) shows inverse magnetic susceptibility χ(T)⁻¹of O₂rich ceria nanospheres as a function of temperature.

FIG. 4(D) shows the low temperature region of the magnetic susceptibility χ(T) behavior of all O₂rich nanoceria.

FIG. 4(E) shows the M vs μ₀H data at 2 K with Brillouin function fits to calculate the number of Ce³⁺/g for nanocubes.

FIG. 4(F) shows the M vs μ₀H data at 2 K with Brillouin function fits to calculate the number of Ce³⁺/g for nanorods.

FIG. 4(G) shows the M vs μ₀H data at 2 K with Brillouin function fits to calculate the number of Ce³⁺/g for nanospheres.

FIG. 5(A) shows magnetic susceptibility χ(T) of O₂deficient nanoceria shapes.

FIG. 5(B) shows the 40-120 K data with background subtraction for each nanoceria shape.

FIG. 5(C) shows M vs μ₀H data at 2 K of O₂rich nanoceria particles according to shape with Brillouin function fits to calculate the number of Ce³⁺/g.

FIG. 6(A) shows the temperature dependence of field cooled (χ_FC(T)) and zero field cooled (χ_ZFC(T)) magnetic susceptibility for O₂deficient nanospheres.

FIG. 6(B) shows the temperature dependence of the difference in the field cooled and zero field cooled magnetic susceptibility Δχ_FC-ZFCfor O₂deficient ceria nanocubes.

FIG. 6(C) shows the temperature dependence of the difference in the field cooled and zero field cooled magnetic susceptibility Δχ_FC-ZFCfor O₂deficient ceria nanorods.

FIG. 6(D) shows the temperature dependence of the difference in the field cooled and zero field cooled magnetic susceptibility Δχ_FC-ZFCfor O₂deficient ceria nanospheres.

FIG. 7(A) shows NO conversion as a function of time during steady state NO reduction by CO at 300° C. and 350° C. for nanoceria shapes.

FIG. 7(B) shows surface area normalized NO reduction activity as a function of the surface density of Ce³⁺ sites determined by magnetic susceptibility analysis in O₂deficient conditions for the ceria nanoshapes.

FIG. 8(A) shows the magnetic susceptibility χ(T) of a 21.1 wt % Ce on γ-Al₂O₃under O₂rich conditions.

FIG. 8(B) shows the temperature dependence of the inverse magnetic susceptibility χ(T)⁻¹of a 21.1 wt % Ce on γ-Al₂O₃under O₂rich conditions.

FIG. 8(C) shows the M vs μ₀H data at 2 K with Brillouin function fit for the 21.1 wt % Ce on γ-Al₂O₃under O₂rich conditions.

FIG. 8(D) shows the magnetic susceptibility χ(T) of a 21.1 wt % Ce on γ-Al₂O₃under O₂deficient conditions.

FIG. 8(E) shows the temperature dependence of the inverse magnetic susceptibility χ(T)⁻¹of a 21.1 wt % Ce on γ-Al₂O₃under O₂deficient conditions.

FIG. 8(F) shows the M vs μ₀H data at 2 K with Brillouin function fit for the 21.1 wt % Ce on γ-Al₂O₃under O₂deficient conditions.

FIG. 9 shows a comparison between predicted and experimentally obtained surface area normalized NO reduction activity at 350° C. over CeOx/γ-Al₂O₃catalysts with various Ce wt % loadings.

FIG. 10 shows a schematic drawing of the CeO₂unit cell structure.

FIG. 11 shows the CeO₂unit cell with the (111), (110), and (100) surface planes identified.

FIG. 12 shows an image of the sample tube preparation for magnetic susceptibility measurements under a closed (oxygen rich) or open (oxygen deficient) environment.

FIG. 13 shows the schematic diagram showing the oxygen rich and oxygen deficient environment experimental setup for magnetic susceptibility measurements.

FIG. 14(A) shows magnetic susceptibility χ(T) of O₂rich nanoceria nanospheres.

FIG. 14(B) shows magnetic susceptibility χ(T) of O₂rich nanoceria nanorods.

FIG. 14(C) shows magnetic susceptibility χ(T) of O₂rich nanoceria nanocubes.

FIG. 15 shows the temperature dependent first derivative of magnetic susceptibility dχ/dT of the O₂deficient ceria nanoshapes.

FIG. 16(A) shows the linear fits of logarithm of the difference in Ce³⁺ sites between O₂rich and deficient ceria nanospheres log(ΔCe³⁺) as a function of temperature.

FIG. 16(B) shows the linear fits of logarithm of the difference in Ce³⁺ sites between O₂rich and deficient ceria nanocubes log(ΔCe³⁺) as a function of temperature.

FIG. 16(C) shows the linear fits of logarithm of the difference in Ce³⁺ sites between O₂rich and deficient ceria nanorods log(ΔCe³⁺) as a function of temperature.

FIG. 17(A) shows the catalyst mass normalized NO reduction activity at 350° C. for the ceria nanoshapes.

FIG. 17(B) shows the catalyst mass normalized NO reduction activity at 350° C. as a function of the total oxygen vacancies (vacancies and Frenkel defects) for the ceria nanoshapes.

FIG. 17(C) shows the catalyst mass normalized NO reduction activity at 350° C. as a function of the oxygen vacancies only (no Frenkel defects) for the ceria nanoshapes.

FIG. 18 shows room temperature XRD patterns and refinements of the of CeO_x/γ-Al₂O₃catalyst samples with 4.21, 17.2, and 21.1 wt % Ce loading.

FIG. 19 shows room temperature Raman spectra of the of CeO_x/γ-Al₂O₃catalyst samples with 4.21, 17.2, and 21.1 wt % Ce loading.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present inventors have examined the oxygen storage and releasing mechanisms of nanoceria under two different O₂partial pressures (O₂rich-ambient air and deficient-He). The oxygen storage and release properties identified by the magnetic susceptibility are intrinsic to the system; that is, in conventional methods one measures the storage of O₂from gas conversion. In magnetometry, one measures the direct response of the intrinsically present oxygen ions. The main advantages of magnetometry include: (i) the potential to deactivate the active oxygen associated with the high temperatures of gas flow techniques is avoided, (ii) the identified oxygen by magnetometry is due to surface and bulk contributions, and it is dynamic in nature; kinetically transforming O_lattice↔O_2adsis observed (FIG. 1). Neutron diffraction studies by Mamontov et al.⁶on nanoceria identified that the missing intrinsic oxygen ions occupy interstitial sites, and are weakly bound in the fluorite lattice, enabling additional defects in the form of anion-Frenkel pairs. Currently, there is a lack of clarity on a central issue permitting an understanding of nanoceria's unusual reactivity—quantifying the interstitial oxygen ions' interactions and dynamics. Surface termination configurations control the oxygen vacancy creation, and Ce³⁺/Ce⁴⁺ stoichiometry is key to identifying the reduction mechanisms in nanoceria. The modification of the surface termination configurations is permitted by recent developments in the synthesis of shape selected nanoceria of cubes, rods and spheres. While bulk CeO₂exposes preferentially stable (111) planes⁹, at the nanoscale, morphology dependent structures can expose less stable (110) and (100) planes (FIG. 11)¹⁰. Density function theory calculations proposed that surface (100) and (110) planes undergo significant relaxation compared to the (111) planes, and the relative order of surface stability becomes (111)>(110)>(100)¹¹. The (111) surface terminations have a Ce:O coordination of 7:3, whereas (110) and (100) surfaces have Ce:O coordinations of 6:3 and 6:2, respectively. Shape selected nanoceria (cubes, rods, and spheres) were synthesized with well-defined crystallographic surface planes (low index planes (111), (110) and (100)) to vary the Ce:O coordination number¹², thereby changing the stability of the planes and concentration of Ce³⁺/Ce⁴⁺ and □. FIGS. 2A, 2D, and 2G present the transmission electron microscopy (TEM) images of the nanocubes (25±3 nm), nanorods (length 55±14 nm and width 7±2 nm), and nanospheres (21 nm) respectively. FIGS. 2B and 2C show high resolution transmission electron microscopy (HRTEM) images of the cubes bound by (100) and (110) facets with lattice spacings of 0.27 and 0.19 nm, respectively. FIGS. 2E and 2F show the HRTEM images of rods defined by (100) and (111) facets with lattice spacings of 0.27 and 0.31 nm. FIGS. 2H and 21 show the (111) facet dominated spheres with a lattice spacing of 0.34 nm. The observed HRTEM results are in agreement with previous reports on morphology dependent nanoceria^13-15: (100) for nanocubes and (111) for nanospheres. It is still an open question as to what planes should be exposed in nanorod ceria. Earlier reports suggest that nanorods expose (100) and (110) planes¹³. However, the inventors' study results indicate clearly that the (100) and (111) surface planes are exposed, which is in agreement with Agarwal et al.'s recent work¹⁶. Agarwal et al. proposed that the (110) plane is unstable, and the exposure of the (110) or (111) planes is extremely dependent on the synthesis technique.

To further confirm the structure and surface terminations of the different shaped nanoceria, x-ray diffraction (XRD) experiments under ambient conditions were performed followed by full pattern Rietveld refinements using GSAS-II¹⁷to determine the lattice constants, overall crystallite sizes, and preferred orientations. Room temperature XRD patterns are presented in FIG. 3. In FIG. 3 the (hkl) indices of the structure are labeled. In agreement with previous reports, refinements indicated preferred orientations along the (100) plane for nanocubes, and (111) plane for nanospheres. However, nanorods showed an interesting difference in preferred orientation compared with the HRTEM analysis; no (110) plane preferred orientation. It has been shown by atomic imaging techniques that a reversible dynamic shape evolution of ceria nanoparticles can occur when under electron beam bombardment. The differences between the HRTEM and XRD results of the nanorods is likely due to the (reversible) Ce⁴⁺ surface reduction resulting in surface reconstruction under electron beam illumination. The inventors' results (refinements) reveal the fraction of nanocubes exposing the (100) and (110) crystal planes are 0.61 and 0.70, and nanorods expose (110) and (100) planes at 0.61 and 0.65 fraction, respectively. Nanospheres provide a smooth distribution of crystallite alignments (powder average) and expose stable (111) planes.

TABLE 1

Crystalline (nanoparticle) diameter (nm) and lattice constant

(Å) from XRD pattern refinements, and BET measurements for

CeO₂nanocubes, nanorods, and nanospheres.

shape
size (nm)
α (Å)
BET (m²/g)

nanocubes
28.0 ± 1.5
5.426 ± 0.001
35

nanorods
N.A.
5.427 ± 0.001
167

nanospheres
1.6 ± 0.1
5.420 ± 0.001
214

The Brunauer-Emmett-Teller (BET) method was used with N₂physisorption at its normal boiling point to quantify the specific surface areas of the nanoceria shapes (see Table 1). The specific surface areas (m²/g) calculated with the BET method are consistent with the observed nanoparticle surface-to-volume ratios and their bulk density. From the fraction of exposed planes and the Ce:O coordination of these planes, we calculate 31%, 60%, and 63% of surface Ce³⁺ are present at the exposed surfaces of the cubes, rods, and spheres, respectively.

Nanoceria acts as an oxygen buffer by releasing oxygen through a Ce⁴⁺↔Ce³⁺ redox couple. Identifying the structure and exposed crystal planes of CeO₂, the present inventors conducted systematic investigations of the effects of oxygen rich and oxygen deficient environments with magnetic susceptibility measurements. Magnetism is a probe of the Ce⁴⁺ and Ce³⁺ configuration, in addition to the oxygen (through it's antiferromagnetic to paramagnetic transition³²at around 50 K). The environment sensitive Ce⁴⁺↔Ce³⁺ redox property was evaluated by measurement of the temperature dependent magnetic susceptibility of the nanoceria in oxygen rich and oxygen deficient configurations. Nanoceria in the oxygen rich environment was maintained at ambient conditions (approximately 760 Torr), while the nanoceria in the oxygen deficient environment interacts with the 5 Torr helium gas surroundings. Sealing the NMR tubes used to hold the nanoceria during magnetic measurements was done with ECO-BOND (epoxy) under ambient conditions to set the oxygen rich environment. The oxygen deficient sample was allowed to interact with the sample chamber of the magnetic properties measurement system which is at 5 mmHg helium pressure.

In order to investigate the microscopic origin of the redox transformation, temperature dependent magnetic susceptibility measurements were performed. Magnetic susceptibility (χ=M/μ₀H where M is the magnetisation and μ₀H the externally applied field) provides a measure of the valence state of the atom or ion. In the simplest case, χ is described by the following expression.

$χ = \frac{{Ng}_{J}^{2} (J + 1) μ_{B}^{2}}{3 k_{B} T} + \frac{N μ_{B}^{2}}{6 (2 J + 1)} [\frac{F (J + 1)}{E_{J + 1} - E_{J}} - \frac{F (J)}{E_{J} - E_{J + 1}}] = C / T + N α (J)$

where F(J)=(1/J)[(S+L+1)²−J²][J²−(S−L)²)]. C, the Curie constant, gives a direct measure of effective magnetic moment (μ_eff) and the number of unpaired (N) electron spins, and the second term Nα(J)≡χ₀describes the temperature-independent diamagnetism.

χ vs T of oxygen rich environment samples are shown in FIG. 14. The peak in χ(T) identifies an antiferromagnetic-to-paramagnetic transition¹⁸. Fits with χ=C/T+Na(J) at T>10 K were found, and the corresponding C and χ₀values for different nanoceria shapes are presented in Table 2.

TABLE 2

Results of Curie law fits to the O₂rich systems' susceptibility.

Ce³⁺/g from (χ − χ₀)⁻¹vs T fit with χ₀≡

Nα(J), and total number of Ce³⁺/g for the nanocubes,

nanorods, and nanospheres.

C (4π × 10⁻⁸
χ₀(4π × 10⁻¹⁰
Ce³⁺/g

shape
m³K/kg)
m³/kg)
(J = ±5/2)

Nanocubes
2.00 ± 0.03
2.52 ± 0.01
1.40 ± 0.02 × 10¹⁸

Nanorods
1.46 ± 0.02
2.40 ± 0.02
1.02 ± 0.01 × 10¹⁸

Nanospheres
1.20 ± 0.02
4.86 ± 0.04
8.39 ± 0.01 × 10¹⁷

C quantifies the number of Ce₃₊ in the different shapes and indicates that the increasing Ce₃₊ is in the order spheres to rods to cubes (from 8.0×10¹⁷Ce³⁺/g to 1.4×10¹⁸Ce³⁺/g). However, it was determined that while the χ(T) may be described, permitting an approximation of the number of Ce³⁺ ions, further improvement of the description may be possible. The energy level diagram of Ce³⁺ (from theory and experiment^{19, 20}) is crystal field split from its ²F_5/2state into three Krammer doublets with J_z=±1/2, ±5/2, and +3/2. At low temperatures, only the ±1/2 level is populated, and with increasing temperature, ±5/2 (at 10 K) and ±3/2 (at 150 K) levels are also populated¹³. In the cases of J multiplet intervals comparable to kT; that is E*_J−E_Jis approximately kT, it is necessary to use the complete expression of paramagnetic susceptibility. The full formalism for χ(T) for rare-earth ions in a paramagnetic state, such as Ce³⁺, with the J=1/2, 5/2 and 3/2 states become occupied with warming from below 10 K is

$χ = \frac{N \sum_{J} \begin{matrix} {[g_{J}^{2} J (J + 1) μ_{B}^{2} / 3 kT] + α (J)} \\ (2 J + 1) e^{- E_{J}^{0} / kT} \end{matrix}}{\sum_{J} (2 J + 1) e^{- E_{J}^{0} / kT}} .$

Here E_J⁰is the energy of state J in zero field. For the occupation of three J states with 1/2, 5/2, and 3/2, χ(T) becomes

$(χ - χ_{0}) = \frac{N μ_{B}^{2}}{4 kT} [\frac{g_{1 / 2}^{2} + g_{5 / 2}^{2} e^{- Δ_{1} / T} + g_{3 / 2}^{2} e^{- Δ_{2} / T}}{1 + e^{- Δ_{1} / T} + e^{- Δ_{2} / T}}]$

With warming from 10 to approximately 175 K, the J=1/2, 5/2 and 3/2 states become occupied such that:

$(χ - χ_{0}) = \frac{N μ_{B}^{2}}{4 kT} [\frac{g_{1 / 2}^{2} + g_{5 / 2}^{2} e^{- Δ_{1} / T} + g_{3 / 2}^{2} e^{- Δ_{2} / T}}{1 + e^{- Δ_{1} / T} + e^{- Δ_{2} / T}}]$

where N is the number of Ce³⁺ ions. The energy separation of the states is Δ₁=10 K and Δ₂=150 K, and g_1/2=2.07, g_5/2=2.80, and g_3/2=2.30¹³

FIGS. 4A, 4B and 4C show the data and fits to the final expression of the susceptibility, χ⁻¹vs T. The calculated susceptibility expression is shown by the solid lines for the ceria nanocubes, nanorods, and nanospheres. In the high temperature region above Δ₂, an additional term Aexp(−Θ/T) (A is a constant and Θ arises for the thermal excitation of nearby excited levels) is incorporated to describe the effects of thermal expansion of the lattices and its effect on the crystal field splittings. The calculated Ce³⁺ ions/g identified by incorporating crystal field effects are in Table 2 and the Ce³⁺ ions are higher than the simple Curie analysis, where all ions are considered to be in J_5/2state. Ce³⁺ ions are increasingly present from spheres to rods to cubes, respectively, going from approximately 1:5×10¹⁹Ce³⁺/g to approximately 4×10¹⁹Ce³⁺/g. The origin behind the Ce⁴⁺↔Ce³⁺ reduction is due to the removed lattice oxygen (oxygen vacancies) where the excess electrons localize on two cerium atoms turning Ce⁴⁺ to Ce³⁺. Because the sample is in an oxygen rich environment, the oxygen vacancies responsible for turning the Ce⁴⁺-to-Ce³⁺ are intrinsic and formed during sample synthesis (the low temperatures of the measurements further suggests that thermal excitations cannot be responsible for this transformation). With more Ce³⁺ and □ intrinsically available, it is expected that the cubes will offer the highest catalytic activity of the shapes. Indeed, to first-order, this is the case (described below).

Interestingly, in the low temperature region (≤10 K) all samples presented a maximum in their susceptibilities. This (T) behaviour may be ascribed to an antiferromagnetic transition, T_Napproximately 6 K, of the Ce³⁺ ions. It has been shown in Ce-based compounds that 4f localized Ce³⁺ ions have a tendency to show antiferromagnetism^{18, 21, 22}, and to further confirm its presence, the field dependent magnetization (M(μ₀H)) of nanocubes, nanorods, and nanospheres at 2 K, below T_Nand shown in FIGS. 4D-4G, was measured and evidences a meta-magnetic transition. However, in the oxygen rich atmosphere, ceria does not undergo any reduction. To understand the dynamics of the coupled species the susceptibility measurements in an oxygen deficient environment was investigated. The environment selected was a constant 5 Torr helium gas flow sample atmosphere at 2-3 cc/min when the system is at a set-point temperature, and 5-10 cc/min while warming or cooling, depending on the temperature range.

With the intrinsic magnetism of the nanoceria identified, the relationship of the Ce³⁺↔Ce⁴⁺ redox transformation and the dynamics of released oxygen in the lattice were studied. χ(T) measurements of the nanoceria shapes in the O₂deficient environment showed a significantly different magnetic susceptibility compared to the O₂-rich environment (FIG. 5A). No antiferromagnetic transition at low temperatures and new broad maxima of χ(T) between 50 and 120 K are observed for all three shapes. FIG. 5B shows the magnitude of the temperature dependent magnetism due to the released oxygen from the system (background subtracted data) of the O₂deficient environment. This comparison of the magnetism of the oxygen rich and oxygen deficient systems showed that the oxygen storage mechanism may be completely different based on the O₂content of the environment. The oxygen dynamics in O₂deficient nanoceria systems is identified by the absence of antiferromagnetic transition. As previously described, in O₂rich systems the Ce³⁺ ions superexchange via O²⁻ ions; that is O is fixed in its crystallographic position. The absence of the low temperature antiferromagnetic transition in the O₂deficient systems indicates that these O²⁻ ions are no longer in their fixed lattice positions and cannot mediate the superexchange interactions of neighboring Ce³⁺ ions; rather, the O²⁻ ions are diffusing through the ‘bulk’ of the nanoceria to the surface.

Although not wishing to be bound by theory, the inventors believe the occurrence of this broad maximum in χ(T) of the oxygen deficient systems can be understood in the following way. In nanoceria, the carriers responsible for the magnetism are Ce³⁺ and □. At low temperatures, the 4f¹localized Ce³⁺ ions act as non-interacting dipoles and their magnetic susceptibility decreases with increasing temperature. However, above approximately 50 K, the susceptibility rises monotonically with warming to a maximum, and decreases with further warming (≤120 K). This response is due to the γ-β antiferro-to-paramagnetic transition of S=1 molecular oxygen (O₂)²³. Magnetic susceptibility studies of adsorbed oxygen on vycor glass, graphite, and zeolites identified this transition and presented similar χ(T) behaviour to that observed in the nanoceria^24-27. χ(T) from molecular oxygen also broadens depending on the the surface area density and coverage of the oxygen.

The measured χ(T) of nanoceria in a O₂deficient environment clearly indicates that Ce³⁺↔Ce⁴⁺ redox reactions result in the supply of reactive oxygen, and this observed intrinsic oxygen is due to the interconversion of O_lattice↔O_2ads, where “ads” denotes the adsorbed oxygen. In nanoceria each Ce atom ([Xe]4f¹5d¹6s²) can donate four electrons to the bonding orbitals with two O (1s²2s²2p⁴) atoms. When an oxygen vacancy is formed, the two electrons can freely wander in the solid, and these itinerant electrons have a major role in the transformation of O_latticeto O_2adsin the following way:

embedded image

A similar mechanism was described previously for oxidation catalysis of nanoceria²⁸

The χ(T) measurements are direct evidence that nanoceria releases O_latticedue to the gradient in chemical potential with the surroundings. The most plausible explanation for the interaction between released oxygen and nanoceria is adsorption (binding O₂to the lattice). The intrinsic oxygen of ceria which were observed were probably not solely due to the temperature effects, because using the gas flow temperature programmed reaction techniques under ambient conditions identify the lattice oxygen release at approximately 1000 K. Instead, the intrinsic oxygen identified by its magnetism is due to the gradient in concentration with the surroundings. Previously, Mamontov et al.⁶suggested that absorbed oxygen ions in reduced ceria occupy spacious octahedral sites, rather than the vacant tetrahedral sites due to the former being more energetically favourable. The identified intrinsic oxygen is presumably a result of kinetic factors (such as chemical potential gradient) resulting in a dynamic oscillation of O_2adsto O²⁻_lattice. A more detailed quantitative analysis can be performed in the region of the O₂magnetic transition to obtain the transition temperature and amount of adsorbed oxygen. As a metric to identify the transition temperature, dχ(T)/dT was determined and a measure of the oxygen magnetic transitions observed for the different shapes obtained; ceria nanospheres have

$T_{N_{O_{2}}}$

approximately 80 K, nanorods have

$T_{N_{O_{2}}}$

approximately 70 K, and nanocubes have a

$T_{N_{O_{2}}}$

approximately 60 K (FIG. 15). The relationship between shape and

$T_{N_{O_{2}}}$

indicate the overall amount and strengths of the O—O antiferromagnetic exchange that track with the stored O that percolates from the ‘bulk’ of the nanoceria onto its surface to become O₂with warming, and the relative impact of the available surface density of the different shapes (e.g. their preferred surface plane areas). This observation is also consistent with the abundance of O₂on the surfaces in relation to the preferentially exposed planes of the different shapes as indicated by the BET analysis. Also, the shape of the maximum and the temperature range over which it spans is related to the nature of the O₂coverage, again tracking with the BET areal densities. The integrated area of χ(T) over the O₂affected region was ascertained, incorporating the intrinsic (oxygen deficient derived) susceptibility. In the oxygen deficient systems, the χ(T) response due to the dissipated O₂molecules can be described in a way similar to the Curie constant (C). A comparison has been made to identify and quantify the O₂species in the different shapes. Oxygen has a ³Σ ground state with S=1, so the number of O_2ads/g molecules may be obtained by the expression

N_O2ads/g=(3k C)/(μ_B²[4S(S+1)+Λ²]),

where ∧ is the angular momentum quantum of molecular oxygen²⁹. The order in which the amount of O_2ads/g is reacted from the different shapes is rods>cubes>spheres. Quantified intrinsic oxygen storage capacity in mol O₂/g is in the decreasing order of rods (20.1 μmol O₂/g) to cubes (8.3 μmol O₂/g) to spheres (7.1 mol O₂/g). A similar order of the dynamic oxygen storage capacity has been reported by Trovarelli et al.³⁰in Ce_xZr_(1-x)O₂solid solutions. The stoichiometry indicates that for removal of one lattice oxygen (or formation of 1/2O₂), two Ce³⁺ and one E should result. The identified carriers present in the nanoceria cubes, rods, and spheres are given in Table 4. The Mvs μ₀H (2 K) data of oxygen deficient nanoceria was used to ascertain the number of Ce³⁺/g; since the ground state level is populated by J_z=±1/2, the data may be fit to the Brillouin function M=ngS μ_BB_S(x), and the fits further confirm that the order of Ce³⁺/g and O_2adsis rods>cubes>spheres in the O₂deficient systems (FIG. 5C).

TABLE 4

O₂deficient systems: Quantified O₂,

Ce³⁺ and vacancies (□) of CeO₂

Shape
O₂/g
Ce³⁺/g
Vacancies (□)/g

Nanocubes
5.00 ± 0.01 ×
2.00 ± 0.04 ×
1.00 ± 0.02 ×

10¹⁸
10¹⁹
10¹⁹

Nanorods
12.10 ± 0.10 ×
4.85 ± 0.04 ×
2.42 ± 0.02 ×

10¹⁸
10¹⁹
10¹⁹

Nanospheres
4.31 ± 0.01 ×
1.73 ± 0.04 ×
0.87 ± 0.02 ×

10¹⁸
10¹⁹
10¹⁹

Further to the above analysis, the difference between the number of Ce³⁺/g identified by the area under the χ(T) scan and the 2 KM vs μ₀H data indicates that there is an excess of Ce³⁺ present in the nanoceria. To first order, the origin of this excess Ce³⁺ is from the O_latticeto O_adstransformation. To quantify the available excess Ce³⁺ ions and get an idea of the kinetics of the O_latticetransformation, field-cooled (χ_FC(T)) measurements were performed on the O₂deficient nanoshapes. Unlike the O₂rich systems where χ_FC(T) (field cooled) and χ_ZFC(T) (zero field cooled) were identical (not shown), the O₂deficient systems present an irreversibility between χ_FC(T) and χ_ZFC(T) below the respective O₂T_N's, shown in FIGS. 6A-D. The χ_FC(T) measurements identify that T_Nis suppressed, and χ_FC(T) is increased significantly for temperatures below where T_Nwas previously, demarking the increased number of Ce³⁺ ions able to provide a measurable magnetization. The difference between the χ_FX(T) and χ_ZFC(T), Δχ_FC-ZFC(T), is shown in FIGS. 6B, 6C and 6D. An increase in number of Ce³⁺/g as a function of temperature that tracks with the number of O that are released as a function of temperature is consistent with an activated energy process (described most simply by an Arrhenius relation) which identifies the energies necessary to migrate O from the lattice of the nanoceria shapes to their surfaces. According to this analysis the nanoceria cubes require ˜30% more energy to reduce Ce³⁺ and release O than the nanorods and nanospheres, which involve similar energies. The activation energies from linear fits of log_e(ΔCe³⁺) vs T of the cubes, rods, and spheres are ˜8, ˜6, and ˜6 meV, respectively (FIGS. 16A-C). In addition, the total number of Ce³⁺ ions after incorporating the excess Ce³⁺ ions are presented in Table 5 and map correctly on to the Ce³⁺/O₂reduction identified by the χ_ZFC(T) maximum between 50 and 110 K. The total number of Ce³⁺/g identified by the sum of M vs μ₀H (2 K) and Δχ_FC-ZFC(T) is larger than the number of Ce³⁺/g identified (Table 6) by the O_lattice-to-O_2adstransformations (area under susceptibility) gives an estimation of the role of oxygen vacancies in the Ce³⁺ formation. That is, if oxygen vacancy creation is the only responsible factor for the Ce³⁺ ion formation the total number of Ce³⁺/g (Table 5) is simply the area under susceptibility. The difference in Ce³⁺ ions is due to the formation of non-oxygen vacancy defects (e.g. Frenkel pairs), and in order of cubes, rods, and spheres are 29%, 19%, and 3% respectively as presented in Table 6.

TABLE 5

O₂deficient systems' data: Number of Ce³⁺ identified

by the 2 K M vs μ₀H data and excess Ce³⁺ ions from χ

_FC−ZFC(T) along with total Ce³⁺ obtained from

area under susceptibility.

shape
Ce³⁺/g (2 K)
Ce³⁺/g (χ_FC−ZFC)
Ce³⁺/g

Nanocubes
1.62 ± 0.01 ×
1.19 ± 0.05 ×
2.00 ± 0.04 ×

10¹⁹
10¹⁹
10¹⁹

Nanorods
2.59 ± 0.03 ×
3.38 ± 0.05 ×
4.85 ± 0.04 ×

10¹⁹
10¹⁹
10¹⁹

Nanospheres
0.31 ± 0.01 ×
1.48 ± 0.02 ×
1.73 ± 0.04 ×

10¹⁹
10¹⁹
10¹⁹

TABLE 6

O₂deficient systems' data: Total number of Ce³⁺

identified by the sum of 2 K M vs μ₀H data and excess Ce³⁺ ions

from χ_FC−ZFC(T) along with total Ce³⁺ obtained

from area under susceptibility.

M vs μ₀H (2 K) +

%

shape
χ_FC−ZFC(T)
Ce³⁺/g
difference

Nanocubes
2.81 ± 0.05 ×
2.00 ± 0.04 ×
29

10¹⁹
10¹⁹

Nanorods
5.97 ± 0.06 ×
4.85 ± 0.04 ×
19

10¹⁹
10¹⁹

Nanospheres
1.79 ± 0.02 ×
1.73 ± 0.04 ×
3

10¹⁹
10¹⁹

The relationship between the structure and magnetism and catalytic activity of the various nanoceria shapes was examined by performing the steady-state reaction of NO reduction by CO. Differential conversions were achieved at 350° C. for a reactor with a constant loading of 50 mg of nanoceria catalyst. The above magnetic analysis revealed that the surface reduction of Ce⁴⁺-to-Ce³⁺ is favored in the order of cubes (110)+(100), rods (110)+(111), and spheres (111), with crystal planes determined by XRD and HRTEM. The catalytic performance, therefore, is expected to follow the same trend. FIG. 7A shows that the conversion varies as rods (110)+(111)>spheres (111)>cubes (110)+(100). Based on the conversions alone, the trend in catalytic performance does not agree with expected performance. The discrepancy may be attributed to differences in Ce³⁺ concentration-per-mass and specific surface area (SSA) between the various CeO₂nanoshapes. Because the same mass of catalyst was used (50 mg), normalization of activity or Ce³⁺ on a per-mass-of-catalyst basis does not yield any correlation with the trends in Ce³⁺/g (see Examples). Catalysis is a surface phenomenon, thus the appropriate parameters should be normalized in a way that captures the interaction of the NO and/or CO reactants with the catalytic surface sites on the various nanoceria. Therefore, it is most appropriate to use the SSA to normalize the rate and determine the Ce³⁺ surface density (Ce³⁺/nm²).

Ceria is an important catalytic material, especially for automotive emissions control. Therefore, the relationship between the structure, Ce³⁺, and catalytic activity of the various nanoceria shapes was examined by performing steady-state reduction of NO by CO at 350° C. with the different nanoceria. As shown in FIG. 7A), the conversion was negligible below this reaction temperature, but reached a stable steady state within 1 h of achieving the reaction temperature. Conversions at 350° C. were deliberately maintained below ˜15% by using a constant 50 mg of each catalyst, such that accurate reaction rates could be calculated without reactant depletion effects. The important role of the nanoceria shape is first seen in the 40% increase in rate (per g catalyst) comparing the rates (FIG. 17A) of the spherical nanoceria to that of the rod-shaped nanoceria; the cubes presented a much slower rate. Normalization of rates by the SSA yielded a shape-dependent trend (FIG. 7B), which indicates that SSA is not the sole contributing factor to overall activity. If, however, the preferred crystal termination is considered, a trend begins to emerge. It is found that the reaction is 190% faster on the (110) surface than the (111) surface, and ˜90% faster on the (100) surface than the (111) surface. This is in agreement with the theoretical prediction that the relative activity of the surface planes are in the order (110) to (100) to (111).

Although nanoceria was explicitly discussed in much of the preceding text the principle and method described may be applied to other metal oxide catalysts having coupled redox oxidation states.

Thus, based on the analysis and discovery described in the foregoing paragraphs, the inventors have discovered a method to predict the catalytic activity of a nanoceria sample. However, the method is equally applicable to any metal oxide wherein the magnetic susceptibility is sensitive to a redox transformation of a diamagnetic oxidation state and a paramagnetic oxidation state, which allows study of the evolution of oxygen vacancies and migration of oxygen atoms from inside the metal oxide structure to its surfaces.

Thus, a first embodiment of this application includes a method to predict the catalytic activity of a metal oxide of formula M_xO_ywhere x is a number from 1 to 3 and y is a number from 1 to 8. The metal of the metal oxide has redox coupled oxidation states wherein the redox transformation is between oxidation states selected from the group consisting of a diamagnetic oxidation state (M^d+) and a paramagnetic oxidation state (M^p+), a paramagnetic oxidation state (M^p+) and a ferromagnetic oxidation state (M^f+), and a paramagnetic oxidation state (M^p+) and an antiferromagnetic oxidation state (M^a+) where d, p, f and a are independently numbers from 1 to 6 and one of the oxidation states (M^d+), (M^p+), (M^f+), and (M^a+) is formed by reduction by the O²⁻. Any one of the oxidation states (M^d+), (M^p+), (M^f+), and (M^a+) may be the ion specie formed by reduction by the O²⁻. The method comprises:

measuring the magnetic susceptibility of a metal oxide sample in an oxygen environment at a specified temperature;

correlating the magnetic susceptibility measured to a value of number of (M^d+ or M^p+ or M^f+ or M^a+)/g (O₂rich)

measuring the magnetic susceptibility of the metal oxide sample in an oxygen free environment at the specified temperature;

correlating the magnetic susceptibility measured to a value of number of (M^d+ or M^p+ or M^f+ or M^a+)/g (O₂deficient)

determining the catalytic active (M^d+ or M^p+ or M^f+ or M^a+) concentration according to the equation:

(M^d+ or M^p+ or M^f+ or M^a+)/g (active)=[(M^d+ or M^p+ or M^f+ or M^a+)/g (O₂deficient)−(M^d+ or M^p+ or M^f+ or M^a+)/g (O₂rich)]/(M^d+ or M^p+ or M^f+ or M^a+)/g (O₂deficient); and

predicting catalytic activity of the metal oxide sample with the (M^d+ or M^p+ or M^f+ or M^a+)/g (active) value.

In a special aspect of the first embodiment a method to predict the catalytic activity of a nanoceria sample is provided. The method comprises:

measuring the magnetic susceptibility of a nanoceria sample in an oxygen environment at a specified temperature;

correlating the magnetic susceptibility measured to a value of number of Ce³⁺/g (O₂deficient)

determining the catalytic active Ce³⁺ concentration according to the equation:

Ce³⁺/g (active)=[Ce³⁺/g (O₂deficient)−Ce³⁺/g (O₂rich)]/Ce³⁺/g (O₂deficient); and

predicting catalytic activity of the nanoceria with the Ce³⁺/g (active) value.

In one aspect of the above embodiments, the catalytic activity is for the reduction of NO with CO.

Example

The catalytic activity in a generic catalyst system: γ-alumina-supported ceria, CeO_x/γ-Al₂O₃was evaluated according to the method described in the embodiment of this application. In addition to bulk ceria, ceria is often used as a component in mixed oxides or as small domains supported on a nominally inert carrier. Multiple reports^31-33demonstrate that the ceria in these materials remains redox active, but understanding the origins of ceria's activity presents significant challenges. Because the total oxide surface is a mixture of active ceria and inactive support, and because the ceria may not be present as large, crystallographically well-defined phases, reactivity does not easily scale with surface area or a particular surface termination. The method described in the embodiments was applied as follows.

Three CeO_x/γ-Al₂O₃were prepared with Ce loading verified by ICP analysis as 4.21, 17.2, and 21.1 wt % Ce. X-ray diffraction experiments under ambient conditions were performed followed by full refinements using Full-prof³⁴to determine the phase fraction, lattice constants and crystalline sizes (FIG. 18). XRD patterns clearly showed that the 4.21 wt % Ce sample was composed of γ-Al₂O₃(tetragonal I41/amd structure), and the 17.2 and 21.1 wt % Ce are composed of up to 20% γ-Al₂O₃and 80% CeO₂structures, by phase. This structural analysis is consistent with the Raman spectra that clearly identified the F2g vibration of the fluorite CeO₂at 465 cm−1 in the 17.2 wt % and 21.1 wt % CeOx/γ-Al₂O₃systems (FIG. 19). Magnetic susceptibility measurements were performed in O₂rich and deficient environments to estimate the number of Ce³⁺ ions. FIGS. 8A, 8D and 8E a shows typical magnetometry of the 21.1 wt % CeO_x/γ-Al₂O₃catalyst with the low temperature transition in susceptibility and FIGS. 8B and 8E shows the data and fit (solid line) to the χ⁻¹vs T. M(μ₀H) below 2 K was poorly fit by a Brillouin function with S=1/2, and furthermore, was consistent with the observation that the sample undergoes an antiferromagnetic transition. The numbers of Ce³⁺/g in the O₂rich and deficient environments are obtained after subtracting the background contribution of γ-Al₂O₃, and FIG. 8A shows the data and fits to susceptibility to quantify the Ce³⁺/g. Similarly, the O₂deficient system presented no antiferromagnetism and the antiferro-to-paramagnetic O₂transition, as shown by the broad maxima between 50 and 100 K (FIG. 8D) as observed for the ceria nanoshapes. Note the magnitude of the transition is ten times smaller than that of nanoshapes, suggesting that the redox ions of CeO_x/γ-Al₂O₃are considerably fewer than the nanoshapes—suggesting significantly lower NO activities. As with the nanoshape analysis, the available excess Ce³⁺ ions were obtained from the oxygen deficient χ_FC-ZFC(T) scans. Considering the important correlation between the surface localized Ce³⁺ and specific surface area normalized activity, we were able to predict the catalytic activity of the CeOx/γ-Al₂O₃catalysts for the steady-state reduction of CO by NO at 350° C. In order to predict the activities, the surface localized Ce³⁺/nm²were identified by two different methods: Total Ce³⁺ present, and the excess Ce³⁺. These analyses as described later, present bounds that are in excellent agreement to the measured activities. The surface normalized activities of CeOx/γ-Al₂O₃were predicted from a straight line fit of the parameters of the activity vs Ce³⁺/nm²of the ceria shapes (FIG. 7B). The nanoceria shapes intercept that is non-zero essentially identified the bulk ceria base activity. It is important to note that the intercept was taken to be zero for the CeOx/γ-Al₂O₃activity predictions as it identifies the intrinsic activity of the γ-Al₂O₃support (consistent with our measured γ-Al₂O₃activity and results in the literature³⁵. A comparison between the experimental activities and the calculated activity predictions are presented in FIG. 9. Our observation indicated that knowing the surface localized excess Ce³⁺ released under the O₂deficient conditions is the key parameter to design efficient NO reduction ceria based catalysts. Once again, these results proved the potential utility of this relationship to design and screen new materials with improved activity. In addition, the non-obvious finding that a dispersed CeO₂catalyst on a high surface area support has lower activity than the bulk CeO₂nanoshapes delivered previously unknown insight into the role of both the surface and bulk of this catalytically interesting material.

Materials and Methods

Synthesis of CeO₂nanoshapes and CeO_x/γ-Al₂O₃nanoparticles. All reagents were obtained from Sigma-Aldrich and used as received. Water was purified to 18 M O-cm resistivity by a Barnstead NanoPure system.

Ceria nanocubes were synthesized following a previously reported method^{13, 36}. 2.0 mmol of Ce(NO₃)₃6H₂O was dissolved in 5 mL water, mixed with 35 mL of 5 M NaOH, and stirred in a Teflon container for 30 minutes. The Teflon container was put in a stainless steel autoclave and heated for 24 h at 180° C. in an electric oven. The snow-white solids was separated by centrifuge and washed with alternating water and ethanol three times. Nanocubes were dried overnight at 70° C. in air. Their color remained white after drying. Total yield was 0.26 g (76%).

The synthesis of ceria nanorods was based on a previously reported microwave hydrothermal method³⁷. 41.9 mmol of cerium (III) nitrate hexahydrate (Ce(NO₃)₃6H₂O, 99.9% purity) was dissolved in 105 mL water, the solution mixed with 105 mL of 5 M sodium hydroxide (NaOH), stirred for 30 minutes, and split evenly into four Teflon containers. The containers were heated in a Milestone EthosEZ Microwave at 10° C./min to 160° C., held at 160° C. for 1 hour, and cooled to room temperature. A light purple solid was separated by centrifuge and washed with alternating water and ethanol three times. Nanorods were dried overnight at 70° C. in air, and became yellow after drying. Total yield was 7.07 g (98% yield).

The synthesis of ceria nanospheres was based on a previously reported microwave hydrothermal method^{37, 38}5.0 mmol of cerium (IV) ammonium nitrate ((NH₄)₂Ce(NO₃)₆98.5% purity) was dissolved in 25 mL water, mixed with 25 mL of 5 M NaOH in a Teflon container and stirred for 3 hours. The container was heated in a Milestone EthosEZ Microwave at 10° C./min to 100° C., held at 100° C. for 10 minutes, and cooled to room temperature. The creamy yellow solids were separated by centrifuge and washed with alternating DI water and ethanol three times. Nanospheres were dried at 70° C. in air overnight, and stayed light yellow after drying. Total yield was 0.91 g (106% yield, indicating a small amount of retained surface water or ethanol, as expected from the high surface areas).

Three alumina-supported cerium oxide catalysts (CeO_x/γ-Al₂O₃) of differing Ce loading were prepared by incipient wetness impregnation of aqueous solutions of Ce(NO₃)₃.6H₂O of varying concentrations onto a γ-Al₂O₃support (BET SSA=209 m²/g). After impregnation, the samples were dried at 120° C. (1° C./min) for 24 hrs., followed by calcination in air at 500° C. (2° C./min) for 30 min. The final Ce loadings on the γ-Al₂O₃support were determined by ICP to be 4.21, 17.2 and 21.1 Ce wt %.

Transmission Electron Microscopy.

TEM images and electron diffraction data was collected at the EPIC facility of the NUANCE Center at Northwestern University using the JEOL-2100F (0.16 nm resolution and 0.92 eV energy resolution). Catalysts were dispersed in ethanol and sonicated for 30 min. The resulting suspension was pipetted on to a holey carbon grid and dried overnight at room temperature. TEM and electron diffraction data were taken at 200 kV accelerating voltage, 140 A emission and 0.5 angstrom probe size. Electron diffraction parameters were determined using the Digital Micrograph program.

X-Ray Powder Diffraction.

X-ray powder diffraction patterns were collected using a Bruker D8 Davinci with Cu K radiation using Bragg-Brentano geometry under ambient conditions. The diffraction patterns were collected on dried nanoparticle samples with a knife edge above the sample on a zero-background quartz slides.

Brunauer-Emmett-Teller Analysis.

N₂physisorption isotherms were obtained using a Micromeritics ASAP 2010 instrument. Using the BET equation, N₂physisorption isotherms were utilized to determine specific surface areas (58). Before measurements, all materials were degassed 12 h at <5 mTorr and 150° C.

Activity Measurements.

Steady-state catalytic activities were measured using a fixed bed quartz tubular reactor with temperature and gas flow control. The reactor was loaded with 50 mg of catalyst powder that was mixed with quartz sand as a diluent in order to achieve a constant gas hourly space velocity (GHSV) of approximately 15,000 hr⁻¹. The catalyst was first pretreated n 30 sccm of 10% O₂/He at 500° C. for 30 min, and then cooled in flowing UHP He. The reduction of NO by CO was monitored at 300° C. and 350° C. for 45 min each at a total flow rate of 100 sccm with a stoichiometric composition of 4000 ppm NO, 4000 ppm CO, 8000 ppm Ar, and He balance. Mass spectrometry of the gas phase product stream was obtained continuously using a quadrupole mass spectrometer (MKS Instruments, Inc. Cirrus-2). The Ar present in the reactant stream acted as tracer of constant concentration and the Ar signal at m/z=40 was used to normalize each of the mass spectrum traces. The 100% and 0% conversion of NO was defined as the normalized mass spectrum intensity at m/z=30 under inert flow, and under NO+CO reaction mixture flow while bypassing the reactor, respectively. Linear interpolation determined the resulting NO conversions at steadystate temperatures. Rates (mol NO/s) at 350° C. were calculated from differential NO conversions (15%). Mass-normalized activity (mol NO/g/s) and SSA-normalized activity (mol NO/m²/s) were calculated using the mass of catalyst and the BET-determined SSA, respectively.

Magnetic Measurements.

Magnetometry experiments were performed using a Quantum Design magnetic properties measurement system (MPMS XL-5 using the Reciprocating Sample Option (RSO)). The samples were mounted in low background NMR (Norell high resolution S-5-20-8) tubes. DC susceptibility measurements were done using a 10 mT applied field. Experiments under oxygen rich conditions were carried out in sealed tubes at approximately 160 Torr O₂, while oxygen poor experiments were carried out under dynamic pumping to achieve an 5 Torr He atmosphere (see FIGS. 12 and 13). During each magnetic measurement the temperature was held at a fixed set point. NMR tubes are sensitive to thermal gradients, so during the temperature scan between measurements a scan rate of 1 K min⁻¹below 100 K and 2 K min⁻¹above that was used during heating and cooling.

Raman Measurements.

Raman spectra were collected with a Horiba-Jobin Yvon Labram HR high-resolution Raman microscope instrument equipped a 532 nm light laser source (Laser Quantum Ventus 532 Dedicated Raman Laser, diode pumped) and a back-illuminated CCD detector (Horiba-Jobin Yvon Synapse 1024 1 256). The spectrometer was optimized to a spectral resolution of 2 cm Q 1 using a 600 grooves/mm grating. The Rayleigh scattered light was rejected with holographic notch filters.

The above description is presented to enable a person skilled in the art to make and use the invention, and is provided in the context of a particular application and its requirements. Various modifications to the preferred embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments and applications without departing from the spirit and scope of the invention. Thus, this invention is not intended to be limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features disclosed herein. In this regard, certain embodiments within the invention may not show every benefit of the invention, considered broadly.

REFERENCES

1. Liu, X., Zhou, K., Wang, L., Wang, B. & Li, Y. Oxygen vacancy clusters promoting reducibility and activity of ceria nanorods. J. Am. Chem. Soc. 131, 3140-3141 (2009).

2. Esch, F. et al. Electron Localization Determines Defect Formation on Ceria Substrates. Science (80-.). 309, 752-5 (2005).

3. Campbell, C. T. & Peden, C. H. F. Oxygen Vacancies and Catalysis on Ceria Surfaces. Science (80-.). 309, (2005).

4. Zhang, F., Wang, P., Koberstein, J., Khalid, S. & Chan, S. W. Cerium oxidation state in ceria nanoparticles studied with X-ray photoelectron spectroscopy and absorption near edge spectroscopy. Surf Sci. 563, 74-82 (2004).

5. Zotin, F. M. Z., Tournayan, L., Varloud, J., Perrichon, V. & Frety, R. Temperature-programmed reduction: limitation of the technique for determining the extent of reduction of either pure ceria or ceria modified by additiv. Appl. Catal. A Gen. 98, 99-114 (1993).

6. Mamontov, E. & Egami, T. Structural defects in a nano-scale powder of CeO2 studied by pulsed neutron diffraction. J. Phys. Chem. Solids 61, 1345-1356 (2000).

7. Laachir, A. et al. Reduction of CeO 2 by hydrogen. Magnetic susceptibility and Fourier-transform infrared, ultraviolet and X-ray photoelectron spectroscopy measurements. J. Chem. Soc., Faraday Trans. 87, 1601-1609 (1991).

8. Dutta, P. et al. Concentration of Ce³⁺ and Oxygen Vacancies in Cerium Oxide Nanoparticles. Chem. Mater. 18, 5144-5146 (2006).

9. Skorodumova, N., Baudin, M. & Hermansson, K. Surface properties of CeO2 from first principles. Phys. Rev. B 69, 1-8 (2004).

10. Huang, W. & Gao, Y. Morphology-dependent surface chemistry and catalysis of CeO 2 nanocrystals. Catal. Sci. Technol. 4, 3772-3784 (2014).

11. Nolan, M., Grigoleit, S., Sayle, D. C., Parker, S. C. & Watson, G. W. Density functional theory studies of the structure and electronic structure of pure and defective low index surfaces of ceria. Surf Sci. 576, 217-229 (2005).

12. Ganduglia-Pirovano, M. V., Hofmann, A. & Sauer, J. Oxygen vacancies in transition metal and rare earth oxides: Current state of understanding and remaining challenges. Surf Sci. Rep. 62, 219-270 (2007).

13. Mai, H. X. et al. Shape-selective synthesis and oxygen storage behavior of ceria nanopolyhedra, nanorods, and nanocubes. J. Phys. Chem. B 109, 24380-24385 (2005).

14. Yang, S. & and Lian Gao. Controlled Synthesis and Self-Assembly of {CeO_2}Nanocubes. J. Am. Chem. Soc. 128, 9330-9331 (2006).

15. Lin, Y., Wu, Z., Wen, J., Poeppelmeier, K. R. & Marks, L. D. Imaging the Atomic Surface Structures of CeO₂Nanoparticles. Nano Lett. 14, 191-196 (2014).

16. Agarwal, S. et al. Exposed Surfaces on Shape-Controlled Ceria Nanoparticles Revealed through AC-TEM and Water-Gas Shift Reactivity. Chem Sus Chem 6, 1898-1906 (2013).

17. Toby, B. H., IUCr & D., B. I. EXPGUI, a graphical user interface for GSAS. J. Appl. Crystallogr. 34, 210-213 (2001).

18. Lawrence, J. Speculations on the critical behavior of reduced moment antiferromagnetic cerium compounds (invited). J. Appl. Phys. 53, 2117-2121 (1982).

19. Elliott, R. J. & Stevens, K. W. H. A Preliminary Survey of the Paramagnetic Resonance Phenomena observed in Rare Earth Ethyl Sulphates. Proc. Phys. Soc. Sect. A 64, 205-207 (1951).

20. Bogle, G. S., Cooke, A. H. & Whitley, S. Paramagnetism of Cerium Ethylsulphate at Low Temperatures. Proc. Phys. Soc. Sect. A 64, 931-932 (1951).

21. Das, I. & Sampathkumaran, E. V. Magnetic ordering in Ce2RhSi3. J. Magn. Magn. Mater. 137, L239-L242 (1994).

22. Kase, N., Muranaka, T. & Akimitsu, J. Antiferromagnetic Kondo-lattice system with moderate heavy-fermion behavior. J. Magn. Magn. Mater. 321, 3380-3383 (2009).

23. DeFotis, G. C. Magnetism of solid oxygen. Phys. Rev. B 23, 4714-4740 (1981).

24. Gregory, S. Adsorbed Oxygen as an Amorphous Antiferromagnetic System. Phys. Rev. Lett. 39, 1035-1038 (1977).

25. Awschalom, D. D., Lewis, G. N. & Gregory, S. Melting and Wetting Behavior in Oxygen Films. Phys. Rev. Lett. 51, 586-588 (1983).

26. Köbler, U. & Marx, R. Susceptibility study of physisorbed oxygen layers on graphite. Phys. Rev. B 35, 9809-9816 (1987).

27. Murakami, Y. Magnetic and structural phase transitions of physisorbed oxygen layers. J. Phys. Chem. Solids 59, 467-485 (1998).

28. TROVARELLI, A. Catalytic Properties of Ceria and CeO₂-Containing Materials. Catal. Rev. 38, 439-520 (1996).

29. Morrish, A. H. The Physical Principles of Magnetism. Wiley-IEEE Press 1, (1965).

30. Trovarelli, A. et al. Nanophase Fluorite-Structured CeO2-ZrO2 Catalysts Prepared by High-Energy Mechanical Milling. J Catal. 169, 490-502 (1997).

31. Skala, T., Tsud, N., Prince, K. C. & Matolin, V. Formation of alumina-ceria mixed oxide in model systems. Appl. Surf Sci. 257, 3682-3687 (2011).

32. Piras, A. et al. Structural and Morphological Investigation of Ceria-Promoted Al2O3 under Severe Reducing/Oxidizing Conditions. doi:10.1021/jp0440737

33. Shyu, J. Z., Weber, W. H. & Gandhi, H. S. Surface characterization of alumina-supported ceria. J. Phys. Chem. 92, 4964-4970 (1988).

34. Rodriguez-Carvajal, J. Recent advances in magnetic structure determination by neutron powder diffraction. Phys. B Condens. Matter 192, 55-69 (1993).

35. Kim, H., Takashima, K. & Katsura, S. Low-temperature NOx reduction processes using combined systems of pulsed corona discharge and catalysts. J. Phys. D (2001).

36. Wu, Z., Li, M., Howe, J., Meyer, H. M. & Overbury, S. H. Probing defect sites on CeO2 nanocrystals with well-defined surface planes by raman spectroscopy and O2 adsorption. Langmuir 26, 16595-16606 (2010).

37. Arauijo, V. D. et al. CeO₂nanoparticles synthesized by a microwave-assisted hydrothermal method: evolution from nanospheres to nanorods. Cryst Eng Comm 14, 1150-1154 (2012).

38. Bonamartini Corradi, A., Bondioli, F., Ferrari, A. M. & Manfredini, T. Synthesis and characterization of nanosized ceria powders by microwave-hydrothermal method. Mater. Res. Bull. 41, 38-44 (2006).

METHOD OF MAGNETIC ANALYSIS TO DETERMINE THE CATALYTIC ACTIVITY OF METAL OXIDES INCLUDING NANOCERIA

Information

Publication Number

Date Filed

Date Published

Inventors

Original Assignees

CPC

International Classifications

Abstract

Description

Claims