Perovskite-Based Catalysts as Efficient, Durable, and Economical NOx Storage and Reduction Systems

Syngas production and gas-N evolution over heterogeneously doped La-Fe-O perovskite-type oxygen carriers in chemical looping gasification of microalgae

Chemical looping gasification (CLG) is a promising technology for syngas production with low pollutant emission. In this study, doped La-Fe-O perovskites including LaFeO₃ (LF), LaFe_0.5Ni_0.5O₃ (LN5F5) and La_0.3Ba_0.7FeO₃ (L3B7F) were developed for microalgae CLG. The as-prepared perovskites exhibited an outstanding performance in syngas production with accumulative syngas yield > 33 mol/kg. For gas-N evolution, perovskites were beneficial to the formation of NH₃ and HCN, while the iron ore may convert precursors to NO. Below 400 °C, NO_x can be stored on the perovskite surface in the form of nitrite/nitrate species. When the temperature was above 700 °C, NO_x can be selectively reduced by reducing components in tar or syngas under the catalysis of L3B7F, resulting in the final reduction of NO_x emission. Thus, CLG over L3B7F may be a promising way for efficient utilization of microalgae to overcome the intractable nitrogen-related obstacles in the commercial application of biomass gasification technologies.

Trends in Surface Oxygen Formation Energy in Perovskite Oxides

Perovskite oxides comprise an important class of materials, and some of their applications depend on the surface reactivity characteristics. We calculated, using density functional theory, the surface O vacancy formation energy (E _Ovac) for perovskite-structure oxides, with a transition metal (Ti-Fe) as the B-site cation, to estimate the catalytic reactivity of perovskite oxides. The E _Ovac value correlated well with the band gap and bulk formation energy, which is a trend also found in other oxides. A low E _Ovac value, which is expected to result in higher catalytic activity via the Mars-van Krevelen mechanism, was found in metallic perovskites such as CaCoO₃, BaFeO₃, and SrFeO₃. On the other hand, titanates had high E _Ovac values, typically exceeding 4 eV/atom, suggesting that these materials are less reactive when O vacancy formation is involved in the reaction mechanism.

Advances in Designing Efficient La-Based Perovskites for the NOx Storage and Reduction Process

To overcome the inherent challenge of NOx reduction in the net oxidizing environment of diesel engine exhaust, the NOx storage and reduction (NSR) concept was proposed in 1995, soon developed and commercialized as a promising DeNOx technique over the past two decades. Years of practice suggest that it is a tailor-made technique for light-duty diesel vehicles, with the advantage of being space saving, cost effective, and efficient in NOx abatement; however, the over-reliance of NSR catalysts on high loadings of Pt has always been the bottleneck for its wide application. There remains fervent interest in searching for efficient, economical, and durable alternatives. To date, La-based perovskites are the most explored promising candidate, showing prominent structural and thermal stability and redox property. The perovskite-type oxide structure enables the coupling of redox and storage centers with homogeneous distribution, which maximizes the contact area for NOx spillover and contributes to efficient NOx storage and reduction. Moreover, the wide range of possible cationic substitutions in perovskite generates great flexibility, yielding various formulations with interesting features desirable for the NSR process. Herein, this review provides an overview of the features and performances of La-based perovskite in NO oxidation, NOx storage, and NOx reduction, and in this way comprehensively evaluates its potential to substitute Pt and further improve the DeNOx efficiency of the current NSR catalyst. The fundamental structure–property relationships are summarized and highlighted to instruct rational catalyst design. The critical research needs and essential aspects in catalyst design, including poisoner resistance and catalyst sustainability, are finally addressed to inspire the future development of perovskite material for practical application.

Perovskite-Based Nanocomposite Electrocatalysts: An Alternative to Platinum ORR Catalyst in Microbial Fuel Cell Cathodes

Microbial fuel cells (MFCs) are biochemical systems having the benefit of producing green energy through the microbial degradation of organic contaminants in wastewater. The efficiency of MFCs largely depends on the cathode oxygen reduction reaction (ORR). A preferable ORR catalyst must have good oxygen reduction kinetics, high conductivity and durability, together with cost-effectiveness. Platinum-based electrodes are considered a state-of-the-art ORR catalyst. However, the scarcity and higher cost of Pt are the main challenges for the commercialization of MFCs; therefore, in search of alternative, cost-effective catalysts, those such as doped carbons and transition-metal-based electrocatalysts have been researched for more than a decade. Recently, perovskite-oxide-based nanocomposites have emerged as a potential ORR catalyst due to their versatile elemental composition, molecular mechanism and the scope of nanoengineering for further developments. In this article, we discuss various studies conducted and opportunities associated with perovskite-based catalysts for ORR in MFCs. Special focus is given to a basic understanding of the ORR reaction mechanism through oxygen vacancy, modification of its microstructure by introducing alkaline earth metals, electron transfer pathways and the synergistic effect of perovskite and carbon. At the end, we also propose various challenges and prospects to further improve the ORR activity of perovskite-based catalysts.

Oxidation and Storage Mechanisms for Nitrogen Oxides on Variously Terminated (001) Surfaces of SrFeO3-δ and Sr3Fe2O7-δ Perovskites

The Ruddlesden-Popper (RP)-type layered perovskite is a candidate material for a new nitrogen oxide (NO_x) storage catalyst. Here, we investigate the adsorption and oxidation of NO_x on the (001) surfaces of RP-type oxide Sr₃Fe₂O_7-δ for all of the terminations by comparing to those of simple perovskite SrFeO_3-δ by the density functional theory (DFT) calculations. The possible (001) cleavages of Sr₃Fe₂O₇ generate two FeO₂- and three SrO-terminated surfaces, and the calculated surface energies indicated that the SrO-terminated surface generated by the cleavage at the rock salt layer is the most stable one. The oxygen of the FeO₂-terminated surfaces could be removed with significantly low energy because the process involves the favorable reduction of the Fe⁴⁺ site. Consequently, the surface oxygen at the FeO₂ site could easily oxidize adsorbed NO to NO₂ by the Mars-van Krevelen mechanism. The resulting oxygen vacancy in the surface would be filled easily with lattice oxygen in bulk. The oxidation of NO with adsorbed molecular O₂ was unfavorable by both the Langmuir-Hinshelwood and Eley-Rideal mechanisms because this process does not involve the reduction of the Fe⁴⁺ site. The oxygen of the SrO-terminated surfaces was tightly bound and acted as the adsorption site of NO and NO₂. An electron transfer strengthened the NO_x binding to the surface by forming nitrite (NO₂^-) or nitrate (NO₃^-) species. The DFT calculations revealed that the RP-type structure promoted NO_x oxidation and storage properties by forming active oxygen due to the Jahn-Teller distortion and by exposing SrO-terminated surfaces due to the cleavage at the rock salt layer.

Aftertreatment DeNOx Systems for Future Light Duty Lean-Burned Emission Regulations

Future light duty vehicles in Europe and the United States are required to be certified, owing to progressively more and more stringent regulations [...]