ES2802419A1 - Catalyst for soot combustion (Machine-translation by Google Translate, not legally binding) - Google Patents

Abstract

Soot combustion catalyst. The invention relates to a catalyst for the combustion of soot comprising an oxide of formula CoAl2 O4 in the form of particles with a rough surface, having a spinel phase and defects extended along the following crystal facets (111), (220), (400) and (311), wherein the particles have a size distribution between 20 nm and 220 nm. Furthermore, the invention relates to its production process by ball milling and its use for the combustion of soot. (Machine-translation by Google Translate, not legally binding)

Description

DESCRIPTION

Catalyst for soot combustion

The invention relates to a catalyst for the combustion of soot that comprises an oxide of formula C0AI2O4 in the form of particles with a rough surface having a spinel phase and extended defects along the following crystal facets (111), (220) , (400) and (311), where the particles have a size distribution between 20 nm and 250 nm. Furthermore, the invention relates to its process for obtaining it by attrition grinding and its use for the combustion of soot.

STATE OF THE TECHNIQUE

Diesel engines generally achieve the highest fuel, energy and thermal efficiency. The main problem is that they emit fine black soot particles, which have caused serious problems for human health and the global environment. Today, the catalytic combustion technique combined with Diesel Particulate Filters (DPF) is considered the most efficient after-treatment technology to prevent its release into the atmosphere. The soot is first trapped by the filter and then removed by catalytic combustion in the catalytic coating. Since the maximum temperature of the exhaust gases of diesel engines falls in the range of 260 to 5400C, the catalysts are required to be active enough to oxidize efficiently at these low temperatures. Therefore, there is a strong incentive to develop a catalyst for continuous DPF regeneration that shows good activity at diesel exhaust gas temperature, with good stability and durability, at a relatively low and environmentally friendly cost.

Catalysts based on noble metals such as platinum as the active phase have been reported to promote the oxidation of NO to NO2, which is a stronger oxidant than O2, and thus lower the combustion initiation temperature of soot. However, their high cost and scarcity have led to the study of other non-critical materials that can be competitive in terms of catalytic activity and costs.

Among alternative formulations, cobalt catalysts have been extensively investigated. Harrison et al. [PG Harrison, IK Ball, W. Daniell, P. Lukinskas, M. Céspedes, EE Miró, MA Ulla, Cobalt catalysts for the oxidation of diesel soot particulate, 95 (2003) 47-55] reported a high activity and selectivity of the CoO / Ce02 catalyst system for soot oxidation that was related to the strong redox capacity of CoOx. Some other cobalt-containing catalysts have been proposed as potential candidates for use for soot reduction in diesel exhaust, including nanostructured spinel-type mixed oxides such as AB2O4 (where A = Co or Mn, and B = Cr or Fe ) [D. Fino, N. Russo, G. Saracco, V. Specchia, Catalytic removal of NOx and diesel soot over nanostructured spinel-type oxides, J. Catal. 242 (2006) 38-47], The catalytic activity of a nanocrystalline C0AI2O4 with a high specific area and a mesoporous structure [M. Zawadzki, W. Walerczyk, FE López-Suárez, MJ Illán-Gómez, A. Bueno-López, C0AI2O4 spinel catalyst for soot combustion with NOx / 02, Catal. Commun. 12 (2011) 1238-1241] has been attributed to its high NOx chemisorption capacity, which facilitates the oxidation of NO to NO2; This has been shown to be effective in simultaneously removing soot and NOx, the two prevalent pollutants in diesel exhaust gases in the 350-450 ° C temperature range. However, to date, the proven efficacy of C0AI2O4 is very limited.

For the reasons stated above, it is necessary to develop an efficient Pt-free catalyst for the combustion of soot.

DESCRIPTION OF THE INVENTION

The present invention relates to a C0AI2O4 spinel-type catalyst which does not contain noble metals such as Pt and which is suitable for the application in the treatment of exhaust gases since it reduces the starting temperature of soot oxidation. Said catalyst is obtained by mechanical activation of cobalt aluminate by attriction grinding. The excellent catalytic activity of mechanically activated C0AI2O4 is mainly attributed to the formation of acid sites, as a consequence of the generation of new crystallographic facets and extended defects that promote the formation of reactive NO2 species and their interaction with the surface of the soot, which improves soot combustion. Catalyst particles smaller than 250 nm have a rough surface that maximizes the number of contact points between the catalyst and the soot, thus improving the transfer of the gas phase from O2 to the soot, which is key to direct oxidation. of soot in CO2. Furthermore, the oxygen species could also facilitate the production of the strong oxidant NO2. Therefore, this catalyst could be used in numerous practical applications because it is easy to synthesize, exhibits high catalytic activity, and can be manufactured from cost-effective materials.

A first aspect of the present invention refers to a soot combustion catalyst (here the catalyst of the invention) characterized in that it comprises an oxide of formula C0AI2O4 in the form of particles with a rough surface and sizes less than 250 nm, preferably Particles have a size distribution of between 20 nm and 200 nm, a spinel phase and extended defects along the following crystal facets (111), (220), (400) and (311).

Spinel-type oxides are an important family of compounds with the general formula AB2O4, where A and B represent two different cations of comparable ionic sizes. C0AI2O4 is a normal spinel, exhibiting a cubic structure, with a space group Fd3m (Z = 8). In this structure, the oxygen anions form a compact cubic subnetwork (ccp) surrounded by cations Co2 ^ and Ah ^ occupying tetrahedral and octahedral sites, respectively. Cations occupy only 1/8 of the tetrahedral sites (8th Wyckoff position) and half of the octahedral sites (16d Wyckoff position). The anisotropy of the surface energy causes C0AI2O4 to adopt a face-centered cubic crystal structure, and the general sequence for the surface energies of the spinel structure is y {111} <and {100} <y {110}. During the synthetic heat treatment, the spinel crystal prefers to form an octahedron with eight surfaces (111), due to the lower energy in these crystalline planes.

In the present invention, the characterization of the crystalline phases is carried out by X-ray diffraction (XRD) using Cu Kai radiation, and quantitative information on the structure and microstructure is extracted by the Rietveld analysis of the XRD data. The catalyst of the present invention exhibits a main spinel structure having a unit cell parameter of 8.0983 ± 0.0005 Á and having a crystallite size of 31 ± 1 nm. Furthermore, the catalyst of the present invention exhibits an additional spinel phase with a larger unit cell parameter of 8.2202 ± 0.0005 Á and a shorter crystallite size of 26 ± 1 nm.

In the present invention, the morphology of the catalyst powder is evaluated using field emission scanning electron microscopy (FE-SEM) secondary electron images. The particle size and morphology at the nanoscale is evaluated using a high resolution transmission electron microscope (HRTEM). The The catalyst of the present invention is in the form of particles of sizes less than 250 nm, the size distribution varies from 20 nm to 200 nm with an average particle size of dso «100 nm. Furthermore, the catalyst of the present invention exhibits widespread defects along the following crystal facets (111), (220), (400) and (311). During milling, the particles are broken randomly without a preferential direction and the morphology of the particles evolves to faceted particles; the formation of widespread defects is evidenced by the size of the crystallite, which is only one third of the mean particle size.

The term "extended defects" here refers to the disruption of regular patterns in crystal structures. Extended defects disrupt the atomic arrangement of a crystal and terminate a crystallographically ordered domain of the crystal.

Crystallite size analysis in a material containing extended defects is used to estimate the ordered domain size. The types of widespread defects in crystalline structures are: stacking faults, dislocations or antiphase boundaries, among others. Extended defects do not include point defects that occur at individual network points, i. e., vacancies, interstitial defects, antisite defects, etc.

The term "crystalline facets" refers herein to flat surfaces oriented in specific crystallographic directions in a material. The organization of natural facets was a key parameter for early developments in crystallography, as they reflect the underlying symmetry of the crystal structure. Gemstones commonly have facet cuts to enhance their appearance by allowing them to reflect light. Facets appear on many crystals during their growth because some surfaces grow much more slowly than others.

The number of acid sites in the catalyst of the invention was measured by the desorption method programmed at NH3 temperature (NH3-TPD), thus obtaining a value greater than 10 mmol NH3 / g. Two types of acidic sites were measured in the catalyst of the present invention. Coordinate NH3 molecules attached to Lewis acid sites are thermally more stable than NH4 + ions attached to Bronsted acid sites, and therefore require higher temperatures to desorb.

Acidic sites generated during mechanical activation (milling) are attributed to newly exposed facets and extended defects that break the charge compensation effect and allow charge compensation of the tetrahedral Co2 + sites, which become active acid sites .

The surface area of the catalyst of the present invention S ^bet measured by Brunauer-Emmett-Teller (BET) ranges from 11 m2 / g to 40 m2 / g.

The excellent catalytic activity of the catalyst of the present invention towards the combustion of soot in NOx / 02 is attributed to several factors:

First, the formation of acidic sites as a consequence of the creation of new crystallographic facets and extended defects that increases the concentration of reactive NO2 species during the reaction and their interaction with the soot particles. The quantification of the acidic sites was carried out using the desorption method programmed at NH3 temperature (NH3-TPD). The catalyst of the present invention shows widespread defects on the surface. During ball attriction milling, the balls produce collisions or compression forces in combination with the shear stress forces that promote the creation of reduced size particles and new crystallographic facets as a result of random ball impacts.

Second, particle surfaces (roughness) are characterized by the presence of displaced atoms and steps, as well as by the amortization of the first atomic layer that resulted in a less unstable crystalline surface. The particle size and surface roughness of the C0AI2O4 catalyst of the present invention facilitate the catalyst-soot contact, and also the access of the gaseous species, thus providing more active sites for the oxidation of soot and NO in comparison with the C0AI2O4 microparticles received.

Third, the C0AI2O4 catalyst of the present invention promotes preferential adsorption of acidic NOx species on the surface of soot, which increases the chances that the reaction between soot and NO2 will form highly active surface oxygenated complexes. In this sense, the acidic sites of the C0AI2O4 catalyst of the invention convert soot particles into radical intermediates of active carbon cations in an efficient manner, said radical intermediates of carbon cations Active ingredients are highly active and can be oxidized by molecular oxygen at lower temperatures.

Therefore, the catalyst of the present invention allows low temperature soot combustion in accordance with the large number of active sites and good contact of the active species with the soot particles.

Using the catalyst of the present invention, two catalytic reaction pathways for soot oxidation can be distinguished, namely 1) direct oxidation by oxygen through oxygen vacancies and 2) NO2-assisted oxidation of soot.

In the direct mechanism (1), enabled by the extended defects and the high energy facets, the catalyst of the present invention acts as a redox center, transferring O2 in the gas phase to the soot through an oxygen exchange process. The soot in contact with the catalyst of the present invention or the adsorbed NOx species takes the active oxygen from the surface for oxidation and creates new vacancies, which are quickly filled with the gaseous phase or the subsurface oxygen.

Meanwhile, for NO2 (2) assisted oxidation of soot, the catalyst of the present invention captures NO first and then is oxidized to NO2 by the gaseous O2 or surfactant oxygen species at the acid sites with high capacity. oxidation; then NO is regenerated by reacting NO2 with soot to produce CO2. In addition, in some centers the reduction may proceed to form N2 and therefore a clear removal of NOx takes place.

Both catalytic reaction pathways explain the working behavior of the C0AI2O4 catalyst of the present invention, which has the advantage of avoiding carbonaceous deposits and, therefore, deactivation of the catalyst.

The catalyst acts as a redox center, transferring O2 in the gas phase to the soot through the oxygen exchange process. Soot in contact with the catalyst or adsorbed NOx species takes active oxygen from the surface for oxidation and creates new vacancies, which are quickly filled with the gas phase or subsurface oxygen. Meanwhile, for NO2 assisted soot oxidation, the catalyst first captures NO and then is oxidized by gaseous O2 or carbon species. surfactant oxygen at acid sites with high oxidation capacity; then it is NOT regenerated by reaction with soot to produce CO2. In addition, in some centers the reduction may proceed to form N2 and therefore a clear removal of NOx takes place.

A second aspect of the present invention refers to a process to obtain the catalyst of the invention (in the present document "the process of the invention") characterized in that it comprises a stage of mechanical activation of C0AI2O4 particles in the form of particles of sizes between 1 pm to 3 pm by ball milling, in which the size of the balls used ranges between 0.1 mm and 2 mm and in which said ball milling is carried out at a stirring speed ranging between 50 cpm and 875 cpm (cycles per minute).

During ball milling, the balls produce collision or compression forces in combination with shear forces that promote reduction in particle size. In most ball milling systems, shear forces predominate due to the rotor-stator configuration. The predominant presence of shear forces tends to round the particles, preferably eliminating the edges.

In the present invention, a mixer / mill type grinding equipment is used, in which the grinding jars containing the catalyst suspension and the balls are shaken in a complex figure-8 motion combining oscillations from one side to the other with short lateral movements or by radial oscillations in a horizontal position. The movement of the grinding bowl develops strong G-forces by the grinding balls and causes them to impact with high energy on the sample material. Grinding balls with density between 4.5 g / cm3 and 7 g / cm3 provide higher G-forces, resulting in superior grinding efficiency. In the present invention, the predominance of the collision forces or compression forces of the grinding for the type of mixer / mill used, improves the breaking of the solid particles, reducing their size significantly and the creation of new crystallographic facets as a result of the random hits of the balls. In the present invention, said ball milling is carried out using zirconium stabilized balls with sizes varying between 0.1 mm and 2 mm in size and a stirring speed of between 50 and 875 cpm (cycles per minute) which are equivalent to angular speeds of 34 and 600 rpm (revolution per minute), respectively. The mean particle size is reduced from 2 pm to about 100 nm and the catalyst of the present invention retained its crystalline structure with respect to the previous material.

The balls used in liquid ball milling are made of a ceramic compound with a density between 4.5 g / cm3 and 7 g / cm3, as they provide higher G-forces, resulting in superior grinding efficiency. The balls are preferably selected from cerium stabilized zirconium, yttrium stabilized zirconium, magnesium stabilized zirconium or zirconium.

In a preferred embodiment of the process of the invention, ball milling is carried out in the presence of a solvent. The solvent used here must be inert, this means that it must not react chemically with the catalyst particles. The solvent is selected from water, acetone, acetonitrile, butyl alcohol, diethylene glycol, 1,2-dimethoxyethane, dimethylformamide, 1,2-dioxane, ethanol, ethyl acetate, ethylene glycol, glycerin, methanol, propanol, pyridine, tetrahydrofuran. More preferably, the solvent is an alcohol, preferably ethanol, because the high dielectric constant of ethanol allows dispersion of the particles, and its low boiling temperature facilitates their removal without forming hard agglomerates of the ground particles.

Preferably, the mass / mass ratio during ball milling in the presence of solvent is between 1: 1 and 3: 1 and the volume / solid / volume ratio is between 1: 2 and 1: 5.

The last aspect of the invention relates to the use of the catalyst of the invention for the combustion of soot in the presence of NO. The advantages of the catalyst of the present invention can be summarized as follows:

• 100% soot burning efficiency without CO formation.

• Soot combustion at a lower temperature than the reference Pt-based catalyst.

• The particle size of the C0AI2O4 catalysts of the present invention facilitates the catalyst-soot contact, and also the access of the gaseous species, thus providing more active sites for the oxidation of soot and NO.

• The ball milling of the present invention is an easy route to create defects and acid sites in C0AI2O4. The acidity of the catalyst favors interaction of acidic NO2 species with soot and therefore the indirect oxidation mechanism.

• It is a cost-effective solution to replace catalyst based on critical raw materials such as noble metals and rare earths.

• Allows directing soot combustion mechanisms that will help reduce carbon poisoning of catalytic systems.

Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to whom this invention belongs. Methods and materials similar or equivalent to those described herein can be used in the practice of the present invention. Throughout the description and claims, the word "comprise" and its variations are not intended to exclude other technical characteristics, additives, components or steps. Additional objects, advantages, and features of the invention will be apparent to those skilled in the art upon examining the description or may be learned by practice of the invention. The following examples and drawings are provided by way of illustration and are not intended to be limiting of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a) Collected powder diffraction data for catalysts along with corresponding Rietveld models. The open gray circles show the experimental data. The Bragg positions of the different phases present are represented by the vertical marks below the patterns, in for the spinel C0AI2O4, for the AI2O3 and for the second spinel, respectively, in order of proximity with the diffractogram. b) Enlargement of the main diffraction peak of C0AI2O4.

FIG. 2 FE-SEM micrographs of a-b) 70AI30Co, d-e) C0AI2O4 received and g-h) C0AI2O4 mechanically activated; particle size distribution curves measured by laser diffraction of c) 70AI30Co and e) as received by C0AI2O4 and i) and primary particle size distribution measured from the mechanically activated C0AI2O4 image.

FIG. 3 TEM micrographs of ground C0AI2O4 catalyst, a) Low magnification image, where the agglomeration of the C0AI2O4 nanoparticles is evidenced. b) HRTEM image of a detail of the agglomerated particles, the inserts show the filtered images of the lattice fringes of the area of interest.

FIG. 4 NH3-TPD results from catalyst samples.

FIG. 5 Catalytic oxidation tests: a) Soot conversion profiles, b) CO2 concentration profiles, and c) NO2 profiles as fraction of NO2 to NOx. Reaction conditions: 100 mg of catalyst 20 mg of soot, loose contact, 2500 ppm NO 10% vol. O2 N2, 300 ml / min, 5 ° C / min. The solid lines are experiments with soot and catalyst, the dotted line is the reference uncatalyzed soot burning experiment, and the dotted line in c) is a blank NO oxidation experiment with catalyst but no soot.

FIG. 6 a) Cubic spinel structure centered on the face of C0AI2O4 and magnification of a tetrahedron and an adjacent octahedron that share an oxygen atom. The large spheres marked by Cotet and Aloct represent atoms in tetrahedral and octahedral coordinate sub-lattices, respectively. The lattice parameters a in the (001) plane and c in the direction perpendicular to it are identical under equilibrium conditions, but differ for tetragonal systems. b) Diagram of the lowest energy surfaces of the C0AI2O4 crystal. c) Diagram of the morphology of the triangular shaped C0AI2O4 nanoparticles on the surface of 70AI30Co y a-ALOs. d) Surface atomic configurations of facet (111) for spinel C0AI2O4.

FIG. 7 Concentration profiles during heating under NOx / 02 obtained with the C0AI2O4 catalyst ground with soot (solid lines) and without soot (broken lines). Reaction conditions: 2500 ppm NO 10% vol. O2 N2, 300 ml / min, 5 ° C / min.

FIG. 8 Low NOx / Ü2 NOx removal profile for soot-free ground C0AI2O4 catalyst.

FIG. 9 Combustion of soot with O2 (in the absence of NO) without catalyst and on received and ground C0AI2O4 catalysts under tight and loose contact conditions. The maximum CO2 production temperature is indicated.

FIG. 10 a) Mechanism assisted by NO2, b) Direct mechanism of active oxygen.

EXAMPLES

CATALYST PREPARATION

A commercial micrometric pigment C0AI2O4, from Manuel Riesgo SA, was activated mechanically by means of ball grinding in a liquid medium with zirconia balls stabilized with cerium of 1 mm in size at 50 rpm using a Spex 8000 mixer mill for 3 hours in a type container. polyamide. The mass / mass ratio was 1: 1 and the volume / volume ratio was 1/3 and the solvent was ethanol.

For comparison purposes, a nanostructured catalyst was prepared with 30% by weight of C03O4 nanoparticles on a-Ah03 microparticles (hereinafter referred to as 70AI30Co) by a previously described dry solid state method and heat treated at 1200 ° C for 5 h with a heating rate of 3 ° C / min [CM Álvarez-Docio, J.J. Reinosa, A. del Campo, J.F. Fernández, 2D partiols forming a nanostructured Shell: A step forward cool NIR reflectivityforCoAl204 pigments, Dye. Pigment. 137 (2017) 1-11],

CATALYST CHARACTERIZATION

^• X-RAY DIFFRACTION

The crystalline phases were characterized by X-ray diffraction (XRD, D8, Bruker) using Cu K'i radiation and a LynxEye detector. Quantitative information on the structure and microstructure of the samples was extracted from Rietveld analysis of the XRD data using the FullProf Suite. In the refinements, the spinel phase of the main phase, C0AI2O4, was described in the space group Fd-3m (227), with all tetrahedral sites occupied by Co2 + in and all octahedral sites by Al3 +. The secondary spinel phase present in the commercial samples was also modeled as a Rietveld phase, while the a-AhOs was described using a Le Bail fit. After deconvolution of the instrumental peak broadening contribution, the peak profile was modeled as a Lorentzian-type isotropic broadening, and volume weighted average crystallite sizes were obtained from the Scherrer equation. The background noise was adjusted using an orthogonal Chebyshev polynomial of 3 coefficients.

The XRD patterns of the three samples, shown in Figure 1, show the diffraction peaks related to the spinel structure of C0AI2O4 and with α-ALOs. It is important to note that the commercial C0AI2O4 sample (and consequently the ground C0AI2O4 sample) contains an additional spinel phase with a larger unit cell parameter (8.22 Á). The magnification of Figure 1b shows that after ball milling, the diffraction peaks of C0AI2O4 widened significantly and their intensity decreased. Consequently, the size of the crystallite extracted from the refinements decreases from 57 nm for the commercial C0AI2O4 sample received to 31 nm for the ground C0AI2O4 sample. The quality of the XRD data did not allow discrimination of the size and strain contributions to the spreading of the diffraction peaks. Therefore, deformation effects are not considered in Rietveld models, although presumably a large number of defects are generated in the sample during ball milling. The results of the Rietveld analysis of the XRD data are compiled in Table 1. Figure 1 shows good agreement between the XRD data and the Rietveld models. To study the alteration of cobalt species before and after the mechanical activation processes, an additional study was carried out by XPS (X-ray photoelectron spectrometer, K-Alpha, Thermo Scientific, equipped with a monochromatic Al Ka source, 1486.6 eV, operating at a voltage of 12 KV). However, small changes are observed and the results indicate that all samples had a normal spinel structure and the Co2 + cations were at the tetrahedral site.

Table 1: Refined lattice parameters and volume weighted average crystallite sizes obtained from Rietveld analysis of measured XRD data for sample 70Al30Co, commercial starting powder CoA2O4, and after the ball milling process.

C0AI2O4 Secondary Spinel Phase

Parameter Size of Parameter Size of

C0AI2O4

8.0983 31 8.2202 26 ground

^• FE-SEM CHARACTERIZATION OF CATALYST MORPHOLOGY

The morphology of the powders was evaluated using secondary electron images of field emission scanning electron microscopy (FE-SEM, Hitachi S-4700). An image processing and analysis program (Leica Qwin, Leica Microsystems Ltd, Cambridge, England) was used to determine the microstructural parameters of the FE-SEM micrographs, always considering more than 150 particles in each measurement.

The field emission scanning electron micrographs of the catalysts are shown in Figure 2. Sample 70AI30Co has the characteristic plate-type morphology of a-ALOs support particles, related to crystal growth in the ab plane, with an average particle size dso «7.8 pm (Figure 2a and 2c). C0AI2O4 crystallizations are seen on the surface of a-ALOs microparticles (Figure 2b). These 3D nanocrystals have an irregular shape with an average equivalent diameter of 427 ± 118 nm and have a hole in the middle; they expose the facets of the crystal with the lowest energy (111).

The morphology of the C0AI2O4 powder before ball milling is shown in Figure 2d. The received material is made up of particles that vary from 0.3 to 8 pm (Figure 2f) with an average particle size of ds or «2 pm. A closer inspection of the morphology reveals pseudospherical particles with little presence of surface roughness (Figure 2e).

The micrograph in Figure 2g and the laser diffraction results show that ball milling significantly modified the morphology of C0AI2O4. The primary particle size is reduced to the average particle size d5o «100 nm (Figure 2i), but the state of agglomeration became more marked, leading to a broader bimodal particle size distribution. Furthermore, the morphology of the particles evolves to faceted particles (Figure 2h) due to the impact energy produced during the grinding process; the formation of extended defects was evidenced by the crystallite size of 31 nm, which is only one-third of the average particle size.

^• TEM / HRTEM CHARACTERIZATION OF PARTICLE SIZE AND CATALYST MORPHOLOGY

The particle size distribution of the catalyst powders was determined by laser diffraction (Mastersizer S, Malvern, UK).

Particle size and morphology at the nanoscale were also evaluated using a transmission electron microscope (TEM / HRTEM, JEOL 2100F) operating at 200 kV and equipped with a field emission electronic gun that provides a point resolution of 0, 19 nm. HRTEM filtering and image processing were performed with Digital Micrograph software.

TEM characterization further elucidates the effect of the milling procedure on the morphological structure of the COAI2O4 of the present invention. Figure 3a low magnification image shows the high agglomeration and heterogeneity of the ground C0AI2O4 particles. These consist primarily of nano-sized grain ranges from 20 to 200 nm, and this agrees well with measurements of primary particles on FE-SEM images. The enlarged micrograph of Figure 3b reveals well crystallized nano-regions, with several distinguishable crystalline fringes, that are easier to visualize in the filtered HRTEM images of the inserts. The figure mainly shows a C0AI2O4 particle oriented down the axis of zone [112], exposing (111), whose interplanar distances appear at 4.67 Á (Zone III). However, three more interplanar distances can be observed in other disoriented areas, where the calculated d spaces of 2.02 Á (Zone I), 2.86 Á (Zone II) and 2.44 Á (Zone IV). They are attributable to planes (400), (220) and (311) of C0AI2O4, respectively. These results justify the hypothesis that during the grinding process the particles are broken randomly without a preferential direction.

^• NH ³ DESORPTION PROGRAMMED BY TEMPERATURE

The temperature-programmed desorption of NH3 (NH3-TPD) was performed in a Micromeritics Autochem II 2920 apparatus. Before the NH3-TDP experiments, under He flow, the samples (200 mg) were pre-treated at 2000C for 0.5 h and cooled to 1000C. The samples were then exposed to a flow of NH3 at 1000C for 1 hr, followed by He purging for 1 hr. Finally, the temperature was raised to 8500C in a He flow at a rate of 100C / min.

NH3-TPD was performed to investigate the amount and strength of acidic spinel surface sites; the profiles are shown in Figure 4. NH3 desorption peaks centered at temperatures below 400 ° C are assigned to weak acid sites, while those centered at temperatures above 600 ° C originate from strong acid sites. Coordinate NH3 molecules attached to Lewis acid sites are thermally more stable than NH4 + ions attached to Bronsted acid sites, and therefore require higher temperatures to desorb. Therefore, the observed low-temperature desorption peak for the C0AI2O4 (2820C) sample is assigned to NH4 + ions attached to the Bronsted acid sites, while the high-temperature desorption peak for the 70AI30Co (808 0C) is associated with molecular NH3 adsorbed at Lewis acid sites. Interestingly, the ball milling method has a significant effect on acid properties. The strength of the Lewis acid sites is slightly reduced, consistent with the approximately 500C downward shift of the ammonia desorption peak. However, the amount of the Bronsted and Lewis acid sites in the ground C0AI2O4 sample of the present invention, and therefore the total acidity, is significantly higher than in the 70AI30Co having stable facets (111) or in the samples of cobalt spinel received. This is a consequence of the higher specific surface area of the ground COAI2O4 associated with the reduction in particle size obtained by the milling procedure (Table 2). In addition, an additional peak appears at 401 ° C, indicating that a new type of acid site is generated. These modifications may be related to the generation of less stable crystal facets such as (220), (400) and (311), for example, in addition to surface creases and surface crystalline amortization as the main observed extended defects. The activity of the catalyst C0AI2O4 in deoxygenation has been reported to be practically zero [JJP, MA, RJGH, DJ, PV, BHH, The Surface of Catalytically Active Spinels, J. Catal. 147 (1994) 294-300] according to the common exposed crystal facets of (110) and (111) for crystalline particles. The low activity of such crystallographic planes is attributed to the fact that the Co2 + in tetrahedral coordination is not directly exposed to the molecules of the gas phase. In plane (110) there is an alternative presence of octahedral B3 +, O2 'and tetrahedral A2 + sites for spinel AB2O4 and the surface is more covalent than the interior of the crystal due to a charge compensation effect. Therefore, the acidic sites generated during mechanical activation are attributed to newly exposed facets and extended defects that break the charge compensation effect and allow that the charge compensation of the tetrahedral Co2 + sites become active acid sites. Mechanical activation promotes a wide variety of sites as evidence of the broader range of temperatures where these acidic sites are activated corresponding to different surface energies. It is worth mentioning that commercial C0AI2O4 has a small presence of weak acid sites according to the standard ceramic pigment manufacturing procedure in which dry grinding is used to reduce agglomerates. This dry grinding is a low intensity grinding that causes slight wear of the microparticles but does not increase the widespread defect in the crystal structure. The present invention disclosed an easy way to improve the presence of acidic sites in COAI2O4 by mechanical activation during wet ball milling.

EVALUATION OF CATALYTIC ACTIVITY

^• BET SURFACE AREA OF SOOT CATALYSTS AND COMBUSTION TESTS

The Brunnauer-Emmett-Teller (BET) equation was used to determine the surface areas of catalysts from N2 adsorption-desorption data obtained on a Micrometrics ASAP2020 analyzer.

The soot particles were obtained by burning commercial diesel fuel (British Petroleum, Spain) in a glass container. The soot collected from the walls of the container was dried at 120 ° C for 24 h.

Two types of soot burning tests were tested. Low N0x / 02 activity tests consisted of heating soot-catalyst mixtures from 25 to 750 ° C at 5 ° C / min in a plug flow reactor. The catalyst (100 mg) and the soot (10 mg) were mixed with a spatula in the so-called loose contact mode, which is the most representative contact mode for diesel particles flowing through a catalytic filter. Subsequently, the mixture was diluted in 1000 mg of SiC (particle size 500 pm) and placed in the reactor (= 4 mm; bed length = 70 mm). The catalytic combustion experiments were carried out at atmospheric pressure under a gas flow of 300 ml / min (GHSV 150,000 h'1) containing 2500 ppm of NOx 10% O2 equilibrated with N2. The composition of the effluent gas from the fixed bed reactor was analyzed with a Thermo Nicolet FT-IR spectrometer. Experiments were also conducted in blank with N0 x / 02: no catalyst, to analyze non-catalytic soot combustion; and without soot, to analyze the oxidation capacity of NO to NO2 of the catalysts.

To study the influence of the oxidant and the contact mode, the soot combustion tests with only O2 under loose and tight contact conditions were evaluated on a simultaneous thermal analyzer (STA 6000) connected to a Frontier Fourier transform infrared spectrometer ( FTIR) equipped with a cellular gas, both from PerkinElmer. For the closed contact mode, the catalyst and soot were mixed in a mortar. This method maximizes the number of contact points and, although it does not represent the actual contact conditions that occur in a catalytic trap, it can better discriminate morphologies. About 25 mg of powder were placed in alumina crucibles and subjected to a temperature ramp from 100C min-1 to 9500C in an air atmosphere. IR spectra were obtained in the range of 650 to 4000 cm_1 with a resolution of 2 cm_1 and 2 accumulations.

The catalytic activity of the samples was evaluated in terms of T10, T50 and T90 values, defined as the temperatures for the conversion of soot at 10%, 50% and 90%, respectively. Soot conversion was calculated as the fraction of total carbon released as CO or CO2 by integrating the concentration profiles. Selectivity to CO2 (Sco2) was calculated as the exit concentration of CO2 (Cco2) divided by the sum of the exit concentration of CO2 and CO:

W% _^ ) = io o * _lc ^ _o2 r ₊ : _l r _c ( _or E q -1)

The NO2 concentration at the outlet is expressed as the fraction of NO2 of the total amount of NOx using the following equation:

N02 {%) = 100 x ^ £ 2 (Eq. 2)

N 0 out N 02 out

where NO and NO2 are the concentrations of NO and NO2, respectively, measured at the outlet of the reactor.

The NOx removal profiles were determined with the equation:

NOx removed (%) = 100 x NOx ™ NO ~ xN iOnx ° Ut (Eq. 3)

where NOxin and NOXOut are the concentration of NOx (NO NO2) fed to the reactor and continuously monitored by the specific analyzers during the experiments, respectively.

^• CATALYTIC ACTIVITY OF SOOT COMBUSTION:

Figure 5 shows the NOx / Ü2 soot combustion catalytic activity of cobalt spinels in loose contact mode compared to reactivity in the absence of catalyst. The 50% conversion temperature and selectivity to CO2 are listed in Table 2, along with the specific surface area of the catalysts.

Table 2: Physicochemical parameters and catalytic efficiencies of the soot combustion of the different catalysts. * The Pt ( 1%) / AhO3 values from the literature are included for reference [Y. Gao, W. Yang, X. Wu, S. Liu, D. Weng, R. Ran, Controllable synthesis of supported platinum catalysts: acidic support effect and soot oxidation catalysis, Catal. Sci. Technol. 7 ( 2017) 3268-3274].

S ^bet Total acidity T 50 Sc 02 Catalyst

(m2 / g) (mmol NH3 / g) (° C) (%)

Without catalyst - - 527 67 Pt (1%) / AI203 * 137 0.15 446> 98

70AI30CO 0.3 1.6 511 97

C0 AI2O4 10.2 2.1 478 97

C0 AI2O4 ground 22.9 11.2 411 100

The total number of acid sites in the received C0AI2O4 microparticles increases compared to the nanostructured 70AI30Co and this increase is due to the presence of weak acid sites. Compared to the acid site at the Pt (1%) / AI203 value of 0.15 mmol NH3 / g, its high surface area contributes to soot combustion more than the presence of acid sites. In the soot catalyst of the present invention, the large increase in acid sites reaching 11.2 mmol NH3 / g is evidenced as the key parameter for soot combustion rather than specific surface area when compared to microparticles. received. On the other hand, the present result supports the poor catalytic activity of the C0AI2O4 nanoparticles obtained by synthesis [M. Zawadzki, W. Walerczyk, FE López-Suárez, MJ

lllán-Gómez, A. Bueno-López, C0AI2O4 spinel catalyst for soot combustion with N0x / 02, Catal. Commun. 12 (2011) 1238-1241] in which the heat treatment during synthesis promotes the formation of the surfaces with less energy and, therefore, the low activity despite the high specific surface area. Therefore, these new acidic sites of the catalyst of the present invention convert the soot particles into radical active carbon cation species, which can be easily oxidized.

The soot conversion and CO2 generation profiles represented in Figures 5a and 5b, respectively, show that the nanostructured sample does not significantly reduce the soot combustion temperature due to its low surface area; While the commercial pigment microparticles C0AI2O4 slightly reduced the combustion temperature of the soot. Mechanically activated C0AI2O4 greatly promotes low temperature soot combustion according to the amount of surface acid sites, due to the increase in specific surface area (22.9 m2 / g), and the presence of extended defects and crystalline facets of high energy characterized by a crystallite size of 31 nm. The efficiency of mechanically activated C0AI2O4 soot combustion is comparable to that of Pt (1%) / Al203 based catalysts.

The results obtained with the mechanically activated C0AI2O4 catalyst could have great practical relevance, due to the much lower price of Co-based spinel compared to noble metals, and the simplicity of the activation method by attrition grinding. As Pt catalysts, activated cobalt spinel suppresses the formation of CO seen in the combustion of soot without catalyst. The fact that the C0AI2O4 microparticles are less active and selective is evidence that the mechanical activation process is crucial for promoting catalytic activity. The significant modification of the main crystalline facets of the C0AI2O4 spinel after grinding as shown by XRD, SEM and TEM are responsible for the appearance of weak and strong acid sites that notably changed the peak of CO2 production towards temperatures more low. On the one hand, the smaller particle size, and therefore the higher surface area, of the ball-milled C0AI2O4 catalysts of the present invention facilitates the catalyst-soot contact, and also the access of the gaseous species, providing thus more active sites for both soot and NO oxidation.

For comparison, the nanostructured sample (70AI30Co) reduces the soot burning temperature from the uncatalyzed reaction by only a few degrees, demonstrating that the more stable facets (111) showed poor catalytic properties. C0AI2O4 has a spinel structure (space group Fd3m), the oxygen in the lattice is cubically packed into a unit cell with one eighth of the tetrahedral sites occupied by Co2 + and half of the octahedral sites occupied by Al3 + (Figure 6a). Under equilibrium conditions, C0AI2O4 naturally exists in octahedral form, enclosed by the energetically favorable facets (111) rather than the more reactive facets (110) (Figure 6b-c). The most stable structure of the surface (111) is shown in Figure 6d. On this surface, only three-coordinated Co2 + cations are exposed. CO can be adsorbed at this site exothermically, with an energy gain of 0.92 eV. However, the subsequent reaction of this adsorbed CO with the oxygen of the surface network needs to overcome a very high barrier (> 2.10 eV). This suggests that surface (111) does not exhibit low temperature catalytic oxidation activity and that the Co2 + site is not a key active site.

NO2 plays an important role in the combustion of soot under these conditions, as it is a much more effective oxidant than NO and O2, but NO is the main NOx species in diesel exhaust gases, and NO2 is present only in minor amounts. Therefore, your build on the site can be a speed limiting step. The NOx concentration was monitored during the soot burning experiments, and also during similar experiments performed in the absence of soot, to obtain experimental evidence of the role of NO2 and the soot burning mechanism on catalysts. The evolution of NO2 and CO2, shown in Figure 5b and e, respectively, are closely related. When CO2 formation peaks, the NO2 concentration is minimal and close to zero even for the most active samples. It is clear that NO2 produced from NO oxidation acts as the main oxidant for soot in these systems, because the activity is enhanced in catalysts with a higher NO oxidation capacity. Therefore, in line with the NO2-assisted mechanism reported in the literature, NO must react with the catalyst to generate NO2, and then this can participate in the oxidation of soot to achieve the catalysis cycle. The NO2 to NOx percentage fraction profile obtained in the absence of soot with the most active material, included in Figure 5c, shows the total NO oxidation capacity of the catalyst, which overlaps with that obtained in the presence of soot once that the latter is consumed.

The complete image of the concentration of gaseous species for the mechanically activated catalyst C0AI2O4 is shown in Figure 7. The NO and NO2 profiles clearly show that the ratio of NO2 to NO is minimized in the presence of soot, which can be attributed to the active sites blocked by soot and / or competition of NO and soot for the available oxygen species, but especially to the reduction of NO2 generated back to NO by soot. In addition, since NO2 formation is observed, the evolution of NOx suggests that during the combustion of soot part of the nitrogen oxides is reduced to N2, and this is of utmost importance, since it indicates that NO2 and soot can be removed simultaneously.

The interaction between NOx and the mechanically activated C0AI2O4 sample has been studied in the soot-free blank temperature programmed oxidation experiment, and the results obtained, represented in Figures 5c and 8, are useful to understand the catalytic activity data . NOx removal starts at 2000C and peaks at 2500C. Above 3000C, the NOx concentration is higher at the outlet than at the inlet, producing negative NOx removal values up to 750 ° C. This suggests that between 200 and 300 ° C, nitrogen oxides are reversibly chemisorbed on the surface of new facets of the mechanically activated COAI2O4 sample, and above this temperature the chemisorbed NOx species are released.

The mechanistic arguments have been analyzed to explain the high catalytic activity of the mechanically activated C0AI2O4 catalyst of the present invention and relevant information is obtained from the experiments carried out in the absence of NO, using only O2 as oxidant, under loose and tight contact conditions, such as it is shown in Figure 9. The activity of both received and mechanically activated COAI2O4 decreases dramatically, at T50 values similar to those of the uncatalyzed reaction (5700C). However, for the mechanically activated C0AI2O4 sample a slight catalytic activity is observed. In the narrow contact mode, the soot-catalyst contact points / surfaces, which are key to combustion of soot by the active oxygen mechanism, are maximized and therefore some improvement is observed, with T50 = 5160C, a value 430C lower than in loose contact mode. This result implies that the activity of the received C0AI2O4 sample only occurs in the presence of NOx, the active oxygen mechanism is negligible, but the O2 in the gas phase can be activated by oxygen vacancies and / or by the defects generated during the grinding and converted into surface active oxygen species (O2-, O ") so that soot combustion can take place without continuous NO input. However, the high catalytic activity of the C0AI2O4 sample mechanically activated in the presence of NO must This can be attributed mainly to the reaction between NO2 and soot. This oxygen activation capacity will also contribute to promoting NO oxidation, since the adsorbed NOx species can be oxidized by the generated surface oxygen species (02_, O ") and by the gaseous O2. Furthermore, the increased amount of acidic sites of the mechanically activated C0AI2O4 catalyst can promote preferential adsorption of acidic NOx species on the soot surface, increasing the chances that the NO2 soot reaction will form highly active surface oxygenated complexes. In this sense, the increasing number of acidic sites generated in mechanically activated COAI2O4 converts soot particles more efficiently into radicals of activated carbon cations. These intermediates in the form of cationic radicals are highly active and can be oxidized by molecular oxygen at lower temperatures.

Figure 10 shows two catalytic reaction pathways for soot oxidation using the ground C0AI2O4 catalyst of the present invention, namely direct oxidation by oxygen through oxygen vacancies (Figure 10b) and assisted soot oxidation. by NO2 (Figure 10a). In the direct mechanism, enabled by the extended defects and the new facets of the crystal, the catalyst of the present invention acts as a redox center, transferring O2 in the gas phase to the soot through an oxygen exchange process. The soot in contact with the catalyst of the present invention or with the adsorbed NOx species takes the active oxygen from the surface for oxidation and creates new vacancies, which are quickly filled with the gas phase or the subsurface oxygen. Meanwhile, for NO2 assisted oxidation of soot, the catalyst of the present invention captures NO first and then oxidizes with O2 gas or surfactant oxygen species at the acid sites with high oxidation capacity; then NO is regenerated by reaction with soot to produce CO2. In addition, in some centers the reduction may proceed to form N2 and therefore a clear removal of NOx takes place. These two catalytic reaction routes explain the working behavior of the mechanically activated C0AI2O4 catalyst of the present invention. These reaction pathways prevent carbonaceous deposition and thus deactivation of soot catalysts.

^• CATALYST STABILITY:

Stability is critical to the practical application of catalysts. Therefore, the samples used were recycled for a second test under identical reaction conditions and recharging the soot load. The same procedure was followed to evaluate the stability of a catalyst used as a reference (Pt (1%) / Al203). After two tests at a maximum temperature of 750 ° C, the mechanically activated COAI2O4 sample showed displaced soot conversion at higher temperatures with an AT50 that was> 800 ° C. At the same time, the specific surface area was significantly reduced, indicating that a loss of active sites had occurred during the first catalytic cycle. Paying attention to the Pt (1%) / Al203 catalyst, the results after the two tests at 750 ° C showed a shift at higher temperatures. AT50 was> 300C, which indicates that it has good thermal stability in consecutive tests for soot oxidation; although a slight reduction in catalytic activity is also observed. These differences in stability between both catalysts were attributed to the high temperature reached during the soot burning test (7500C); that generates the deactivation of the C0AI2O4 structure with the consequent loss of surface area due to the growth of its crystallite size. A further experiment was performed for the mechanically activated COAI2O4 sample using 500 ° C as the maximum test temperature. In this case, the catalyst reaction shows that the AT50 increase was 310C, that is from 411 to 4420C. Since the maximum temperature of the exhaust gases of diesel engines is in the range of 260 to 5400C, this fact still makes the mechanically activated catalyst of the present invention C0AI2O4 very

Claims (7)

1. A catalyst for the combustion of soot characterized in that it comprises an oxide of formula C0AI2O4 in the form of particles having spinel phase and defects extended along the following crystalline facets (111), (220), (400) and (311 ), where the particles have a size distribution between 20 nm and 250 nm. 2. A process to obtain the catalyst according to claim 1, characterized in that it comprises a stage of mechanical activation of CoAI204 in the form of particles of sizes between 1 pm and 3 pm by means of ball milling, where the size of the balls used ranges from 0.1mm to 2mm and wherein said ball milling is carried out at a stirring speed ranging from 50 cpm to 875 cpm. The process according to claim 2, wherein the ball milling is performed with balls selected from cerium stabilized zirconium, yttrium stabilized zirconium, magnesium stabilized zirconium or zirconium. 4. The process according to any of claims 2 or 3, wherein the ball milling is carried out in the presence of a solvent. The process according to claim 4, wherein the ball milling in the presence of liquid is carried out in the presence of ethanol. 6. The process according to any of claims 2 to 4, wherein the mass / mass ratio during the attriction milling in the presence of liquid is between 1: 1 and 3: 1 and the ratio volume / mass is solid. volumend¡soivente during ball milling in the presence of liquid is between 1: 2 and 1: 5. 7. Use of the catalyst according to claim 1 for the combustion of soot.