Cobalt oxide with flake-like morphology as efficient passive NOx adsorber

Designing Efficient Nitrate Reduction Electrocatalysts by Identifying and Optimizing Active Sites of Co-Based Spinels

Cobalt-based spinel oxides (i.e., Co3O4) are emerging as low-cost and selective electrocatalysts for the electrochemical nitrate reduction reaction (NO3-RR) to ammonia (NH3), although their activity is still unsatisfactory and the genuine active site is unclear. Here, we discover that the NO3-RR activity of Co3O4 is highly dependent on the geometric location of the Co site, and the NO3-RR prefers to occur at octahedral Co (CoOh) rather than tetrahedral Co (CoTd) sites. Moreover, CoOhO6 is electrochemically transformed to CoOhO5 along with the formation of O vacancies (Ov) during the process of NO3-RR. Both experimental and theoretic results reveal that in situ generated CoOhO5-Ov configuration is the genuine active site for the NO3-RR. To further enhance the activity of CoOh sites, we replace inert CoTd with different contents of Cu2+ cations, and a volcano-shape correlation between NO3-RR activity and electronic structures of CoOh is observed. Impressively, in 1.0 M KOH, (Cu0.6Co0.4)Co2O4 with optimized CoOh sites achieves a maximum NH3 Faradaic efficiency of 96.5% with an ultrahigh NH3 rate of 1.09 mmol h-1 cm-2 at -0.45 V vs reversible hydrogen electrode, outperforming most of other reported nonprecious metal-based electrocatalysts. Clearly, this work paves new pathways for boosting the NO3-RR activity of Co-based spinels by tuning local electronic structures of CoOh sites.

Improvement of NOx uptake/release over Pd/Beta by propylene: shielding effect of intermediates on adsorbed NOx species

Passive NOx adsorbers (PNAs) are capable of trapping NOx at low temperature and releasing the trapped NOx into the gas circuit at higher temperatures, where downstream NOx reduction catalysts are activated. Hydrocarbons have a significant effect on the performance of PNAs, nonetheless research in this area has been overlooked. Here the chemistry of NOx adsorption and desorption in the presence of C3H6 was studied. For different pore-size zeolites (BEA, MFI and CHA), the addition of C3H6 always increased the NOx adsorption capacity at a low temperature and raised the NOx desorption temperature. Spectroscopic and computational investigations were performed using the model Pd/Beta to unravel the relevant mechanism. Fourier transform infrared (FTIR) spectra indicated that more Pd+ was formed in the presence of C3H6, which contributed to higher NOx storage capacity. An intermediate Pd-NC3H6O was probed and its evolution procedure was modeled by density functional theory (DFT) calculations. The results showed that a shielding effect of Pd-NC3H6O on Pd+-NO improved the NOx desorption temperature. This investigation has important implications for how short-chain olefins and even more complex gas mixtures affect the NOx adsorption and desorption performance of Pd/zeolite.