Co3O4 Clusters Research Articles

Restructuring the interfacial active sites to generalize the volcano curves for platinum-cobalt synergistic catalysis

Computationally derived volcano curve has become the gold standard in catalysis, whose practical application usually relies on empirical interpretations of composition or size effects by the identical active site assumption. Here, we present a proof-of-concept study on disclosing both the support- and adsorbate-induced restructuring of Pt-Co bimetallic catalysts, and the related interplays among different interfacial sites to propose the synergy-dependent volcano curves. Multiple characterizations, isotopic kinetic investigations, and multiscale simulations unravel that the progressive incorporation of Co into Pt catalysts, driven by strong Pt-C bonding (metal-support interfaces) and Co-O bonding (metal-adsorbate interfaces), initiates the formation of Pt-rich alloys accompanied by isolated Co species, then Co segregation to epitaxial CoOx overlayers and adjacent Co3O4 clusters, and ultimately structural collapse into amorphous alloys. Accordingly, three distinct synergies, involving lattice oxygen redox from Pt-Co alloy/Co3O4 clusters, dual-active sites engineering via Pt-rich alloy/CoOx overlayer, and electron coupling within exposed alloy, are identified and quantified for CO oxidation (gas-phase), ammonia borane hydrolysis (liquid-phase), and hydrogen evolution reaction (electrocatalysis), respectively. The resultant synergy-dependent volcano curves represent an advancement over traditional composition-/size-dependent ones, serving as a bridge between theoretical models and experimental observations in bimetallic catalysis.

Encapsulate Co3O4 within ultrathin graphene sheets to enhance peroxymonosulfate activation by tuning surface electronic structures

Heterojunctions composed of cobalt-based materials and carbon materials have been recognized as the efficient catalysts for peroxymonosulfate (PMS) activation to generate reactive oxygen species for the removal of environmental contaminants. However, the role of carbon materials in promoting the heterojunction systems has not been fully understood. This study synthesized a heterojunction material of graphene sheets encapsulating Co3O4 (GCO-500) through the pyrolysis of cobalt MOF and applied it to activate PMS for the removal of lomefloxacin. The results showed a high removal rate of 93.59 % with a degradation rate of k1 = 0.0156 min−1. Co3O4 clusters was encapsulated within ultrathin graphene sheets (<2 nm). DFT calculations revealed that graphene layers improve the electron transfer ability of Co3O4 and increased the d-band center of Co3O4 (−1.61 eV) that promote the adsorption of PMS on GCO-500 (−1.32 eV). In the meanwhile, organic pollutant was enriched in graphene layers with high adsorption energy (−13.08 eV), which greatly enhanced the degradation efficiency of pharmaceuticals. This study provides an effective catalyst for PMS activation and sheds light on the fundamental electronic-level understanding of cobalt-based and carbon heterojunction catalysts in PMS activation.

Strawberry-like Co3O4-Ag bifunctional catalyst for overall water splitting

Water electrolysis has been proven as an effective means to produce clean hydrogen energy, and such a process can be accelerated by active bifunctional electrocatalysts for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Herein, we adopted a laser ablation technique to prepare a strawberry-like catalyst with Co3O4 clusters embedded in Ag nanoparticles, realizing the ingenious mutual regulation of Co3O4 and Ag. The Co3O4 facilitates the water dissociation and optimizes the hydrogen adsorption of Ag by introducing tensile strain and low coordination number, resulting in a low HER overpotential of 51 mV at 10 mA cm−2. Meanwhile, the electron transfer from Co3O4 to Ag facilitates the oxidization of Co3O4 and improves the OER performance, leading to a low OER overpotential of 206 mV at 10 mA cm−2. Ultimately, an ultralow potential of 1.49 V for overall water splitting has been achieved at 10 mA cm−2, outperforming Pt/C+RuO2 couple.

CO2 utilization as a soft oxidant for the synthesis of styrene from ethylbenzene over Co3O4 supported on magnesium aluminate spinel: role of spinel activation temperature

Magnesium aluminate spinel (MgAl2O4) supported Co3O4 catalysts are synthesized and tested for the oxidative dehydrogenation (ODH) of ethylbenzene using CO2 as a soft oxidant. The effect of spinel calcination temperature on the catalytic performance has been systematically investigated. With an increase in the activation temperature from 600 to 900 °C, the active presence of a single-phase MgAl2O4 spinel is observed. A catalyst series consisting of MgAl2O4 spinel with varying Co loadings (10–20 wt%) were prepared and systematically distinguished by ICP, XRD, BET, TPR, NH3-TPD, UV–Vis DRS, FT-IR, XPS, SEM, and TEM. Among the tested cobalt catalysts, 15Co/800MA sample derived by calcination of MgAl2O4 support at 800 °C exhibits the most excellent catalytic performance with the maximum ethylbenzene conversion (≥ 82%). Also, high yields of styrene (≥ 81%) could be consistently achieved on the same active catalyst. Further, the catalyst exhibited almost stable activity during 20 h time-on-stream with a slow decrease in the ethylbenzene conversion from 82 to 59%. However, the selectivity of styrene (98%) stayed almost constant during the reaction. Activation of the MgAl2O4 spinel at 800 °C facilitates a dramatic chemical homogeneity for the alignment of Co3O4 nanoparticles on the surface of the active catalyst. Moreover, the isolated Co3O4 clusters have a strong chemical/electronic interaction with the Mg2+ and Al3+ ions on the support perform a crucial role to achieve the maximum catalytic activity.

Unraveling amazing structural features of a highly efficient “oxo-Co/phosphate” catalyst for water oxidation

The oxo-Co/phosphate based material (CoPi) developed by Kanan and Nocera in 2008 is among the most outstanding catalysts for water oxidation at neutral pH. The remarkable performance and striking properties of CoPi reside on its intriguing and unique structure. In the present contribution, TEM, SAED, UV-vis-NIR absorption and Raman spectroscopy were performed at 77 K, complemented by room temperature experiments, to unprecedentedly reveal that CoPi is composed by short-range ordered β-NaCoPO4 (a solid ionic conductor) and expanded-lattice Co3O4 clusters with electronic structures, vibrational responses and catalytic properties quite deviated from those of their bulk counterparts. Internal morphological features of CoPi are also disclosed. Consequently, some aspects of the previously proposed structure and operating mechanisms are revised. This study derived in a deposition route which yields CoPi layers that outperforms, as water oxidation catalyst, the traditionally electrodeposited CoPi.

Microwave Synthesis of Co Doped ZnO Nanoparticles and Their Electrocatalytic Activity for Water Oxidation

Introduction: Fast and efficient conversion of renewable electricity to storable chemical energy is a subject of utmost importance for realization of sustainable society, and development of electrocatalysts out of non-precious elements is the key. Among various metal oxides previously tested, Co3O4 is known to be one of the best catalyst for oxygen evolution reaction (OER), essential for water splitting. Since water oxidation catalysis the surface phenomenon, nano-particles of Co3O4 and Co oxide clusters exhibit higher performance than bulky crystals. In this study, doping of Co ions into nanocrystals of zinc oxide (ZnO) has been achieved by microwave (MW) assisted hydrothermal reaction to form catalytic surface sites for OER combined with highly conductive and stable ZnO for flexible catalyst design and potential enhancement of the catalytic activity. Experiments: Aqueous precursor solutions containing ZnCl2 and CoCl2 at various ratios in a total concentration of 0.2 M were basified with NaOH to ca. pH 13, and subjected to a 2.45 GHz MW to promote reaction at 160°C for 30 min. The products were characterized by FE-SEM, XRD and UV-Vis. Mesoporous electrodes of Zn1-x Co x O (x = 0, 0.02, 0.05, 0.10, 0.20, 1) were fabricated by doctor blading method onto F-doped SnO2 glass substrate for electrochemical analysis in a 0.1 M KCl aqueous solution (pH 7.4). Evolved O2 has been analyzed by gas chromatography (GC). Results and discussion: Minor addition of CoCl2 in the precursor solution turned product into green, exhibiting characteristic absorption peaks at around 560, 610 and 650 nm due to d-d transition of doped Co(II) ion along with a red shift of the bandgap absorption onset, and associated with morphological change of the nanoparticles and slight enlargement of the lattice constants of Wurtzite ZnO. Doping limit of about 10% has been suggested, as phase separation of Co(OH)2 is observed for x > 0.1. Cyclic voltammograms measured at mesoporous ZnO, Zn0.95Co0.05O and Co3O4 electrodes are shown in Fig. 1. While pure ZnO is totally inactive for water oxidation, the Co-doped electrode exhibits an anodic hump at around +0.8 V associated with the oxidation of Co to be followed by a sharp rise of irreversible anodic current above ca. 1 V. The Co3O4 electrode shows even larger anodic peak at 0.8 V, for which, however, Co is reversibly reduced without showing strong rise of anodic current. No systematic increase of the catalytic current was observed by further increasing the x value. Potentiostatic electrolysis at 1.2 V (corresponding to η = 0.545 V) revealed constant anodic current of about 0.9 mA cm-2 for Zn0.95Co0.05O associated with formation of gas bubbled, which was confirmed as O2 by gas chromatography. Although the currently achieved catalytic activity is not satisfactory, the present result reveals new strategies in designing catalysts for OER. Bulk crystals of transition metal oxides are not needed, but reactive sites can be furnished by small doping of transition metals into chemically stable ZnO. Doping of various transition metals such as Mn, Fe, Ni and their alloys is expected to achieve further increase of catalytic activities. Figure 1

Synthetic strategies for the encapsulation of nanoparticles of Ni, Co, and Fe oxides within crystalline microporous aluminosilicates

A synthetic strategy is reported here for the selective containment of oxide nanoparticles of base metals within zeolitic voids of molecular dimensions. The technique, though generally applicable, is specifically illustrated to encapsulate Ni, Co, and Fe oxides within LTA, MFI, and FAU zeolites through hydrothermal framework crystallization in the presence of ligand-protected metal cations. Such ligands contain bidentate amine groups that preclude the precipitation of metal precursors in alkaline synthesis gels, and alkoxysilane moieties that form covalent linkages with nucleating zeolite precursors to enforce metal uptake into crystallized solids. These ligands are removed by subsequent oxidative treatments that nucleate oxide nanoparticles without structural degradation of the zeolites. The clusters are small (<2.5 nm) and uniformly distributed in size, reflecting their constrained growth within zeolite crystals. In contrast with exchange strategies for encapsulation, which lead to grafted cations and dense metal aluminosilicates, these methods form oxide nanoparticles, evident from infrared spectra of samples exposed to CO. Oxide nanoparticles undergo more facile redox cycles than grafted cations or dense aluminosilicates, thus rendering oxide domains more effective oxidation catalysts. The dynamics and stoichiometry of nanoparticle reduction in H2 confirmed the presence of NiO, Co3O4, and Fe2O3 clusters and their more facile reducibility relative to metal aluminosilicates. Ethanol oxidation rates on these clusters were essentially unaffected by exposure to bulky thiol poisons that titrate metal oxide surfaces, reflecting the selective placement of the oxide nanoparticles within the confines of microporous voids, where they are protected from contact by large molecules. These synthetic strategies and guiding principles circumvent long-standing hurdles in the selective encapsulation of base metals, and provide enabling routes for the synthesis of the many metal-zeolite systems that confront similar hurdles.

Sub-nanometer Co3O4 clusters anchored on TiO2(B) nano-sheets: Pt replaceable Co-catalysts for H2 evolution.

Cobalt-based heterogeneous cocatalysts are important substitutions of noble metal cocatalysts in many important commercial chemical processes, but their efficiency is extremely low on a per metal atom basis, because only the atoms located at surface active-sites participate in the chemical reaction. Thus, cocatalysts with small cluster dispersions are highly desirable to maximize the amount of active-sites and enhance the per atom efficiency. Here, we report the synthesis of sub-nanometer Co3O4 clusters which are anchored to 2D ultrathin TiO2(B) nanosheets, as a cocatalyst for H2 evolution reaction (HER). It was found that the conduction type of Co3O4 clusters turns from P-type to N-type, and the heterojunction band structure between TiO2(B) and Co3O4 clusters changes from type II to type I, when the cluster size is reduced from nanometer scale to the sub-nanometer scale. With a suitable energy band matching between TiO2(B) and sub-nanometer Co3O4 clusters, the electrons generated in TiO2(B) during the photocatalytic process reduce the Co ions into metallic Co atoms, which produce excellent photocatalytic stability and extremely high HER efficiency comparable to that of the noble Pt cocatalyst.

Synthesis of highly dispersed cobalt oxide clusters encapsulated within LTA zeolites

Small Co3O4 nanoparticles uniformly distributed in size were encapsulated within LTA zeolite crystals in a one-step process through hydrothermal self-assembly of crystalline frameworks around ligated Co2+ precursors. The use of bifunctional ligands containing a chelating bidentate amine functionality and an alkoxysilane moiety prevented the precipitation of Co2+ species as colloidal hydroxides in the highly alkaline synthesis gels, while also allowing the formation of linkages between precursors and the framework during the nucleation and growth of LTA crystals. Oxidative treatments of ligated compounds occluded within zeolite crystals removed ligand residues and formed small Co3O4 nanoparticles visible in transmission electron micrographs. These nanoparticles retained their small size (average diameter 1.5 nm) after oxidative treatment at 620–870 K, a reflection of their stabilization by confinement within zeolite voids. The infrared spectra of adsorbed CO on Co-LTA samples confirmed the absence of Co2+ as exchanged cations or aluminosilicates, indicating the presence of Co oxide clusters, with dynamics and stoichiometry of reduction in H2 corresponding to small Co3O4 clusters. Ethanol oxidation rates on Co-LTA samples, exchanged with K+ or Ca2+ cations to vary the diffusive properties of LTA crystals, indicated that more than 97% of the active surfaces on these Co3O4 clusters resided within zeolite crystals, where ethanol and O2 concentrations depend on the diffusive properties of the LTA framework. The Co3O4 clusters prepared by these methods, in contrast with Co2+ in exchanged or aluminosilicate forms, exhibit reactivity in CO and NO oxidation. Their turnover rates (per exposed Co atom), however, were lower than on bulk Co3O4 powders, because of the combined effects of diffusional constraints imposed by the confining framework and the small size of these clusters, which leads to lower intrinsic reactivities as a result of their more difficult reduction during catalytic redox cycles. These clusters would be attractive in catalytic applications requiring stability against sintering during reaction or regeneration, reactant or product shape selectivity, or protection from contact with large molecules that block active surfaces. Such oxide clusters cannot be formed by sequential ion exchange, detachment by reduction to Co0, and re-oxidation because the extremely high temperatures required for reduction destroy the aluminosilicate frameworks. The synthesis protocols and their mechanistic interpretations described herein represent a conceptual and practical platform for the encapsulation of nanoparticles of base elements within a broad range of confining crystalline environments through one-step hydrothermal self-assembly.

Metal Organic Framework-Templated Chemiresistor: Sensing Type Transition from P-to-N Using Hollow Metal Oxide Polyhedron via Galvanic Replacement.

Facile synthesis of porous nanobuilding blocks with high surface area and uniform catalyst functionalization has always been regarded as an essential requirement for the development of highly sensitive and selective chemical sensors. Metal-organic frameworks (MOFs) are considered as one of the most ideal templates due to their ability to encapsulate ultrasmall catalytic nanoparticles (NPs) in microporous MOF structures in addition to easy removal of the sacrificial MOF scaffold by calcination. Here, we introduce a MOFs derived n-type SnO2 (n-SnO2) sensing layer with hollow polyhedron structures, obtained from p-n transition of MOF-templated p-type Co3O4 (p-Co3O4) hollow cubes during galvanic replacement reaction (GRR). In addition, the Pd NPs encapsulated in MOF and residual Co3O4 clusters partially remained after GRR led to uniform functionalization of efficient cocatalysts (PdO NPs and p-Co3O4 islands) on the porous and hollow polyhedron SnO2 structures. Due to high gas accessibility through the meso- and macrosized pores in MOF-templated oxides and effective modulation of electron depletion layer assisted by the creation of numerous p-n junctions, the GRR-treated SnO2 structures exhibited 21.9-fold higher acetone response (Rair/Rgas = 22.8 @ 5 ppm acetone, 90%RH) compared to MOF-templated p-Co3O4 hollow structures. To the best of our knowledge, the selectivity and response amplitudes reported here for the detection of acetone are superior to those MOF derived metal oxide sensing layers reported so far. Our results demonstrate that highly active MOF-derived sensing layers can be achieved via p-n semiconducting phase transition, driven by a simple and versatile GRR process combined with MOF templating route.

Shape, Morphology, and Phase Transitions during Co Oxide Growth on Au(111)

The surface structure and morphology of cobalt oxide thin films grown on Au(111) were studied using scanning tunneling microscopy (STM) and ultraviolet and X-ray photoelectron spectroscopies. Initial growth in O2 led to 2-D CoO clusters exhibiting a superstructure in STM images characteristic of the lattice mismatch between the rocksalt (111) surface and the substrate. The superstructure governed the CoO cluster shape; as a result, the shapes of clusters containing up to thousands of Co atoms were modulated by the most efficient packing of spheres. Continued growth led to a transition to 3-D, fully oxidized, spinel phase Co3O4 clusters. These Co3O4 clusters were embedded with characteristic “Y” shaped grain boundaries that result from high reactivity toward oxygen at specific edges of the CoO clusters, and the doubling of the periodicity when CoO is oxidized to Co3O4. Exposure of the Co3O4 films to O atoms induced oxidation toward Co2O3, which roughened the surface. Reducing these overoxidized films produ...

Co environment and magnetic defects in anatase CoxTi1−xO2 nanopowders

Cobalt environment and magnetic defects nature in hydrothermal synthesized anatase CoxTi1−xO2 nanopowders (0 ≤ x ≤ 0.1) are investigated by x-ray diffraction and a variety of spectroscopic techniques. One shows that cobalt is partially inserted in the anatase lattice, as Co2+ ions located on substitutional and interstitial sites. The fraction of the diluted Co is limited to 3 at. % for x ≥ 0.05, while the rest of the Co atoms gather into Co3O4 clusters. As found by electron paramagnetic resonance, the Co doping brings about hole- and electron-excess defects.

Embedding Co3O4 nanoparticles in SBA-15 supported carbon nanomembrane for advanced supercapacitor materials

A three dimensional porous carbon nanomembrane (CNM), silica-supported CNM (SS-CNM), is developed by formation of a self-assembled monolayer of an enediyne compound on the surface of mesoporous silica (SBA-15) followed by Bergman cyclization and carbonization. The SS-CNM is applied as a conductive support for the electroactive material Co3O4 to fabricate advanced supercapacitors. A large fraction of Co3O4 clusters (66% of total weight) are impregnated in the SS-CNM host to form regularly packed nanorods with diameters of 7 nm. The specific capacitance of the supercapacitor electrode material reaches 1086 F g−1 (1645 F g−1 based on Co3O4) with aqueous electrolyte. This extraordinary high performance of the electrode material is attributed to the unique pore structure of the SS-CNM support that enhances the use of active material and shortens the transport path of ions and electrons.

Effects of titanium impurity on alumina surface for the activity of Co/Ti–Al2O3 Fischer–Tropsch catalyst

The surface impurity of titanium with a low concentration on alumina was investigated to elucidate the effects of titanium to catalytic performance on Co/Ti–Al2O3 catalysts during Fischer–Tropsch (FT) synthesis. Titanium impurity is possibly incorporated as an impurity in Al2O3 preparation process, and its content could alter the surface characters of Al2O3. The variation of surface characteristics also significantly alters the dispersion of Co3O4 clusters, reducibility and it consequently changes the catalytic activity. In the present study, titanium impurity with a composition of 0.2–0.5wt% on γ-Al2O3 was found to be optimal to obtain a high catalytic performance with respect to CO conversion and C5+ selectivity. The enhanced catalytic activity on Ti-modified Co/Al2O3 catalyst was mainly attributed to the increased dispersion of cobalt clusters homogeneously with a suppression of cluster aggregation during FT reaction, and it is mainly resulted from the surface modification of alumina with a low content of titanium oxides by changing the surface acidity of γ-Al2O3.

ChemInform Abstract: Nanostructured Cobalt and Manganese Oxide Clusters as Efficient Water Oxidation Catalysts

AbstractReview: 65 refs.

ChemInform Abstract: Nanocasting of Ordered Mesoporous Co3O4‐Based Polyoxometalate Composite Frameworks.

AbstractHighly ordered mesoporous frameworks consisting of nanocrystalline Co3O4 and HPW12O40 clusters with 6, 11, 15, and 36 wt.% HPW12O40 loading are synthesized by the hard‐templating method from solutions of cubic mesoporous KIT‐6 silica, Co(NO3)2, and H3PW12O40 in EtOH (200 °C, 10 h; calcination in air at 400 °C, 6 h).

Nanostructured cobalt and manganese oxide clusters as efficient water oxidation catalysts

Recent development of new methods of preparing cobalt oxide and manganese oxide clusters has led to oxygen evolving catalysts that operate under mild conditions and modest overpotentials at rates approaching practical utility. Synthesis of nanostructured Co3O4 and Mn oxide clusters in mesoporous silica scaffolds affords catalysts with very high densities of surface metal sites per projected area, with the silica environment providing stability in terms of dispersion of the clusters and prevention of restructuring of catalytic surface sites. Stacking of the nanoclusters of these earth abundant, durable oxide catalysts in the scaffold results in turnover frequencies per projected area that are sufficient for keeping up with the photon flux at high solar intensity. Opportunities for expanding the metal oxide/silica interface approach to heterogeneous water oxidation catalysis to a more general approach for multi-electron catalyst designs based on core/shell constructs are discussed. The results are reviewed in the context of all-inorganic materials for catalytic water oxidation reported recently from other laboratories, in particular electrodeposits generated from Co phosphate solutions, a molecular water oxidation catalyst based on a polyoxotungstate featuring a Co oxide core, and Mn oxide materials with incorporated Ca ions.

Polynuclear Photocatalysts in Nanoporous Silica for Artificial Photosynthesis

In this article, recent progress towards robust photocatalysts for the visible light-driven reduction of CO2 by H2O is presented. All-inorganic polynuclear photocatalysts consisting of an oxo-bridged binuclear charge-transfer chromophore (metal-to-metal charge-transfer) coupled to a multi-electron transfer catalyst anchored in a nanoporous silica scaffold have been developed. Mild synthetic methods afford assembly, on silica nanopore surfaces, of heterobinuclear units with donor and acceptor metal centers selected for optimum solar coverage and photon to chemical energy conversion efficiency. A photocatalyst featuring a TiOCrIII group coupled to an IrO2 nanocluster on MCM-41 silica support is shown to function as an efficient visible light water oxidation unit. Nanostructured Co3O4 clusters in mesoporous silica SBA-15 constitute the first example of a nanometer-sized multi-electron catalyst made of an earth-abundant metal oxide that evolves oxygen from water efficiently. For carbon dioxide reduction, a binuclear unit consisting of a Zr acceptor and a CuI donor (ZrOCuI) acts as light absorber as well as redox site, splitting CO2 to CO and H2O upon excitation of the charge-transfer transition. Structural and mechanistic insights of the photocatalytic units based on static and time-resolved optical, FT-infrared, FT-Raman, EPR, X-ray absorption spectroscopy and transmission electron microscopy are critical for improving designs. The photocatalytic units presented here form the basis for developing robust and efficient artificial photosynthetic systems for the conversion of carbon dioxide and water to a liquid fuel.

DIELECTRIC PROPERTY OF LaAlO3 THIN FILMS DOPED WITH MAGNETIC Co AND Co3O4 CLUSTERS

The thin films of Co -doped and Co3O4-doped dielectric LaAlO3 (LAO) by co-ablation of magnetic metal Co and dielectric LAO on Pt – Ti – SiO2 – Si substrates have been prepared by pulsed laser deposition. A significant enhancement of dielectric constant of LAO upon doping of Co and cobalt oxide clusters is observed. Furthermore, modulation of the dielectric constant of the thin films by applying a magnetic field is verified, obviously due to the ferromagnetism of Co metal and Co oxide clusters embedded in the LAO thin films. A series of microstructural and dielectric characterizations on the as-prepared thin films have been performed and the mechanism underlying the dielectric enhancement upon the doping of Co and Co3O4 clusters is discussed.

Characterization by EXAFS of Co/MFI Catalysts Prepared by Sublimation

Catalysts have been prepared by subliming CoCl2 or CoBr2 vapor onto the H form of the zeolite MFI, followed by replacing the halide ions by OH groups and subsequent heat treatments. The Co sites have been characterized by EXAFS and XANES spectra at liquid nitrogen temperature. The data are correlated with the H2-TPR profiles of the same samples and their catalytic performance in reducing NOx with hydrocarbons. Materials with fairly large Co3O4 particles after calcination, displaying a Co−Co distance of 3.5 A and negligible interaction between Co and T sites (T = Si or Al), have a poor selectivity in NOx reduction, because such oxide particles catalyze the combustion of the hydrocarbon. In contrast, highly selective catalysts show no Co3O4 clusters and distinct interaction between Co and T atoms. Heating in H2 at 400 °C reduces Co−oxo ions, while cobalt oxide clusters are incompletely reduced and isolated Co2+ ions in exchange positions remain unreduced. The water formed in the reduction of oxo species can...