Novel, 3D hierarchical Co 3 O 4 twin-spheres with an urchin-like structure are produced successfully on the large scale for the fi rst time by a solvothermal synthesis of cobalt carbonate hydroxide hydrate, Co(CO 3 ) 0.5 (OH) · 0.11H 2 O, and its subsequent calcination. The morphology of the precursor, which dominates the structure of the fi nal product, evolves from nanorods to sheaf-like bundles, to fl ower-like structures, to dumbbell-like particles, and eventually to twin-spheres, accompanying a prolonged reaction time. A multistepsplitting growth mechanism is proposed to understand the formation of the 3D hierarchical twin-spheres of the precursor, based on the time effect on the morphologies of the precursor. The 3D hierarchical Co 3 O 4 twin-spheres are further used as electrode materials to fabricate supercapacitors with high specifi c capacitances of 781, 754, 700, 670, and 611 F g − 1 at current densities of 0.5, 1, 2, 4, and 8 A g − 1 , respectively. The devices also show high chargedischarge reversibility with an effi ciency of 97.8% after cycling 1000 times at a current density of 4 A g − 1 .

We develop an experimental protocol to quantify the speciation of Cu ions (ZCu II OH and Z 2 Cu II ) in Cu-CHA catalysts for the NH 3 Selective Catalytic Reduction of NOx (NH 3 -SCR). Toward this end, we performed four transient tests, namely H 2 -TPR, NO + NH 3 TPR, NO 2 adsorption + TPD, and NH 3 adsorption + TPD, over two sets of Cu-CHA research catalysts characterized by different Cu contents (0−2.1% w/w) and SiO 2 /Al 2 O 3 (SAR) ratios (10−25). Preliminary H 2 -TPR tests on the samples with the extreme SAR and Cu loading values were used to identify the variability range of the fractions of ZCu II OH and Z 2 Cu II species in these catalysts. The ZCu II OH fraction was found to vary between 0.55 (at Cu/Al = 0.11) and 0.79 (at Cu/Al = 0.29). NO+NH 3 TPR runs demonstrated that the NO + NH 3 mixture is a much stronger reducing agent than H 2 : full reduction of all the Cu was obtained already at lower temperature, and differences in the reducibility of ZCu II OH and Z 2 Cu II were strongly attenuated. Both the integral NO consumption and the integral N 2 release were found to be effective estimators of the reducible Cu in Cu-CHA, matching the total Cu from ICP measurements. NO 2 adsorption + TPD tests pointed out that NO 2 is adsorbed in the form of nitrates on ZCu II OH ions only, the nitrates storage capacity being therefore dependent on SAR and Cu loading: on increasing both parameters, the amount of stored NO x increased, as well as their stability. Both the NO released during isothermal NO 2 adsorption and the NO 2 released during the following TPD can be used to directly estimate the number of ZCu II OH ions in Cu-CHA. Finally, NH 3 -TPD provided information on the acid sites in the Cu-CHA samples. From the NH 3 stored on Lewis sites, it was possible to evaluate the number of NH 3 molecules coordinated to each Cu atom: a decrease of the NH 3 /Cu ratio on increasing both SAR and Cu content was observed. This behavior is explained by the changes in the distribution of ZCu II OH and Z 2 Cu II sites in Cu-CHA, as a result of varying the Cu/Al ratio. In accordance with literature results, we found that Cu ions are able to ligate either 3 (ZCu II OH) or 4 (Z 2 Cu II ) NH 3 molecules, when gaseous NH 3 is present, the NH 3 /Cu ratios estimated from our experiments falling close to this range. When only preadsorbed NH 3 was present, however, (no gaseous ammonia), the NH 3 /Cu ratio dropped to either 1 (ZCu II OH) or 2 (Z 2 Cu II ). On the basis of these elements, NH 3 TPD can also be used to quantify the two Cu species in Cu-CHA. We recommend however the more straightforward approach based on (i) NO + NH 3 TPR (for direct quantification of the overall reducible Cu) and (ii) NO 2 TPD (for direct quantification of the ZCu II OH species).

Cu-CHA is the state-of-the-art catalyst for the Selective Catalytic Reduction (SCR) of NOx in vehicle applications.Although extensively studied, diverse mechanistic proposals still stand in terms of the nature of active Cu-ions and reaction pathwaysinSCR working conditions.Herein we address the redoxm echanism underlying Low-Temperature (LT) SCR on Cu-CHA by an integration of chemical-trapping techniques,transient-response methods,operando UV/Vis-NIR spectroscopyw ith modelling tools based on transient kinetic analysis and density functional theory calculations.W es how that the rates of the Reduction Half-Cycle (RHC) of LT-SCR displayaquadratic dependence on Cu II ,t hus questioning mechanisms based on isolated Cu II-ions.W ep ropose,i nstead, aCu II-pair mediated LT-RHC pathway,inwhichNOoxidative activation to mobile nitrite-precursor intermediates accounts for Cu II reduction. These results highlight the role of dinuclear Cu complexes not only in the oxidation part of LT-SCR, but also in the RHC reaction cascade.

We combine gas phase Transient Response Methods with Transient Kinetic Analysis to investigate the reduction half-cycle (RHC: CuII → CuI) of the Standard SCR redox mechanism over Cu-CHA (chabazite) catalysts. The results confirm that NO + NH3 can readily reduce CuII at low temperatures (150–220 °C) according to a Cu:NO:NH3:N2 = 1:1:1:1 stoichiometry. The observed CuII reduction dynamics are invariant with the CuII speciation. Unexpectedly, the CuII reduction rates show a quadratic dependence on CuII, which is hardly compatible with the so far proposed single-site RHC mechanisms. The second order kinetics are found to apply under both dry and wet conditions (0% and 2% H2O v/v in the feed gas, respectively) across different temperatures, space velocities, and NO feed concentrations over two powdered Cu-CHA catalysts with different Cu loadings as well as over a commercial Cu-CHA washcoated honeycomb monolith catalyst. Another unprecedented finding is that H2O significantly inhibits the CuII reduction rate and lowers the RHC apparent activation energy. These findings provide for the first time a complete kinetic description of the low-temperature RHC reaction cascade and, from a mechanistic perspective, strongly suggest a dinuclear-CuII mediated RHC pathway, which may renew interrogations on the current mechanistic understanding of the CuII reduction pathway in the low-temperature NH3-SCR redox chemistry over Cu-CHA.

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