In SituFourier Transform Infrared Measurements of Sulfate Adsorption on Hematite in Aqueous Solutions

The oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) are core steps of various energy conversion and storage systems. However, their sluggish reaction kinetics, i.e., the demanding multielectron transfer processes, still render OER/ORR catalysts less efficient for practical applications. Moreover, the complexity of the catalyst-electrolyte interface makes a comprehensive understanding of the intrinsic OER/ORR mechanisms challenging. Fortunately, recent advances of in situ/operando characterization techniques have facilitated the kinetic monitoring of catalysts under reaction conditions. Here we provide selected highlights of recent in situ/operando mechanistic studies of OER/ORR catalysts with the main emphasis placed on heterogeneous systems (primarily discussing first-row transition metals which operate under basic conditions), followed by a brief outlook on molecular catalysts. Key sections in this review are focused on determination of the true active species, identification of the active sites, and monitoring of the reactive intermediates. For in-depth insights into the above factors, a short overview of the metrics for accurate characterizations of OER/ORR catalysts is provided. A combination of the obtained time-resolved reaction information and reliable activity data will then guide the rational design of new catalysts. Strategies such as optimizing the restructuring process as well as overcoming the adsorption-energy scaling relations will be discussed. Finally, pending current challenges and prospects toward the understanding and development of efficient heterogeneous catalysts and selected homogeneous catalysts are presented.

Oxygen Evolution/Reduction Reaction Catalysts: From In Situ Monitoring and Reaction Mechanisms to Rational Design.

Recent works on preparation of solid superacid catalysts which are active for reactions of butanes and pentanes are reviewed. Sulfated metal oxides are obtained by adsorbing sulfate ion onto amorphous oxides of Fe, Ti, Zr, Hf, Sn, and Si followed by calcination in air; a superacid of Al2O3 is prepared from the crystallized oxide. Superacids by metal oxides are synthesized in the same manner as those of the sulfate superacids by supporting ZrO2, SnO2, TiO2, and Fe2O3 with WO3 and ZrO2 with MoO3 and B2O3. Metal-promoted superacids which are highly active for the reaction of butane to isobutane are prepared by addition of salts of Pt, Ir, Rh, Ru, Os, Pd, Fe, and Mn (equivalent to Pt of 7.5 wt%) to sulfated zirconia gel followed by calcination. Similarly active catalysts are obtained by treatment of zirconia-supported Fe2O3 with sulfuric acid and calcination for long period.

Preparation of superacids by metal oxides for reactions of butanes and pentanes

Nonprecious metal heterogeneous catalysts composed of first-row transition metals incorporated into nitrogen-doped carbon matrices (M-N-Cs) have been studied for decades as leading alternatives to Pt for the electrocatalytic O2 reduction reaction (ORR). More recently, similar M-N-C catalysts have been shown to catalyze the aerobic oxidation of organic molecules. This Focus Review highlights mechanistic similarities and distinctions between these two reaction classes and then surveys the aerobic oxidation reactions catalyzed by M-N-Cs. As the active-site structures and kinetic properties of M-N-C aerobic oxidation catalysts have not been extensively studied, the array of tools and methods used to characterize ORR catalysts are presented with the goal of supporting further advances in the field of aerobic oxidation.

Heterogeneous M-N-C Catalysts for Aerobic Oxidation Reactions: Lessons from Oxygen Reduction Electrocatalysts.

Adsorption complexes that are formed by exposing Cu/ZSM-5 to nitric oxides have been identified by FTIR. Some of them are thermally decomposed below 200°C, but others, though much more stable thermally, are reduced at low temperature by chemical interaction with gaseous propane. The FTIR spectra suggest that the latter complexes are nitro and/or nitrate groups associated with Cu ions. Their formation and reduction when Cu/ZSM-5 is exposed to NO + O2 + C3H8 seems consistent with the assumption that they are intermediates in catalytic NOx reduction. Mass spectrometric analysis of the gases that are released when these complexes react with propane reveals formation of N2, whereas thermal decomposition at higher temperatures only yields NO2, NO, H2O, traces of O2, but no N2. The results suggest that adsorbed NOy, groups (y ⩾ 2) are able to abstract H atoms from hydrocarbon molecules.

Potential reaction intermediates of NOx reduction with propane over Cu/ZSM-5

To advance the widespread implementation of electrochemical energy storage and conversion technologies, the development of inexpensive electrocatalysts is imperative. In this context, Fe/N/C‐materials represent a promising alternative to the costly noble metals currently used to catalyze the oxygen reduction reaction (ORR), and also display encouraging activities for the reduction of CO2. Nevertheless, the application of these materials in commercial devices requires further improvements in their performance and stability that are currently hindered by a lack of understanding of the nature of their active sites and the associated catalytic mechanisms. With this motivation, herein the authors exploit the high sensitivity of modulation excitation X‐ray absorption spectroscopy toward species undergoing potential‐induced changes to elucidate the operando local geometry of the active sites in two sorts of Fe/N/C‐catalysts. While the ligand environment of a part of both materials’ sites appears to change from six‐/five‐ to fourfold coordination upon potential decrease, they differ substantially when it comes to the geometry of the coordination sphere, with the more ORR‐active material undergoing more pronounced restructuring. Furthermore, these time‐resolved spectroscopic measurements yield unprecedented insights into the kinetics of Fe‐based molecular sites’ structural reorganization, identifying the oxidation of iron as a rate‐limiting process for the less ORR‐active catalyst.

/pdf/time-resolved-potential-induced-changes-in-fe-n-c-catalysts-3tyfc365.pdf

Time‐Resolved Potential‐Induced Changes in Fe/N/C‐Catalysts Studied by In Situ Modulation Excitation X‐Ray Absorption Spectroscopy

The ammonium sulfate-iron oxide system treated at different temperatures has been characterized by ESR, IR and Mossbauer spectroscopy. Results show that during the heat treatment some interaction between ammonium sulfate and iron oxide takes place. As a result of this interaction a surface sulfato complex of iron is formed. The complex is involved in the sites responsible for activity for acid catalytic esterification. Above 573 K, the sulfato complex will gradually decompose with a further increase in temperature. A new structure scheme of the sulfato complex is proposed.

Interaction between ammonium sulfate-iron oxide shown by ESR and Mössbauer spectroscopy

A new experimental result of thermally treating ammonium sulfate-titanium oxide (AS-TiO2) is reported. The results strongly suggest the reversibility of the transformation between the isolated and the bidentately bound sulfate ion involved in the sites responsible for the acidity and the catalytic activity of sulfate promoted metal oxides. On the basis of the results a scheme for the reversible transformation is suggested.

Reversible transformation between the isolated and the bidentately bound sulfate ion in sulfate promoted metal oxides

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Time‐Resolved Potential‐Induced Changes in Fe/N/C‐Catalysts Studied by In Situ Modulation Excitation X‐Ray Absorption Spectroscopy

Interaction between ammonium sulfate-iron oxide shown by ESR and Mössbauer spectroscopy

Reversible transformation between the isolated and the bidentately bound sulfate ion in sulfate promoted metal oxides