Biomimetic CO oxidation below -100 °C by a nitrate-containing metal-free microporous system

MOF-derived high oxygen vacancies CuO/CeO2 catalysts for low-temperature CO preferential oxidation

The CO preferential oxidation reaction (CO-PROX) is an effective strategy to remove residual poisonous CO in proton exchange membrane fuel cells, in which oxygen vacancies play a critical role in CO adsorption and activation. Herein, a series of CuO/CeO2 catalysts derived from Ce-MOFs precursors were synthesized using different organic ligands via the hydrothermal method and the CO-PROX performance was investigated. The CuO/CeO2-135 catalyst derived from homophthalic tricarboxylic acid (1,3,5-H3BTC) exhibited superior catalytic performance with 100 % CO conversion at a relatively low temperature (T100% = 100 °C), with a wide reaction temperature range and excellent stability. The superior catalytic properties were attributed to the structural improvements provided by the 1,3,5-H3BTC precursors and the promotional effects of oxygen vacancies. Additionally, in-situ Raman spectroscopy was performed to verify the dynamic roles of oxygen vacancies for CO adsorption and activation, while in-situ DRIFTS analysis revealed key intermediates in the CO-PROX reaction, shedding light on the mechanistic aspects of the catalytic process. This work not only demonstrates insights into the effective CuO/CeO2 catalysts for CO preferential oxidation, but also provides a feasible way to synthesize MOF-derived catalysts.

On the Nature of Extra-Framework Aluminum Species and Improved Catalytic Properties in Steamed Zeolites

Steamed zeolites exhibit improved catalytic properties for hydrocarbon activation (alkane cracking and dehydrogenation). The nature of this practically important phenomenon has remained a mystery for the last six decades and was suggested to be related to the increased strength of zeolitic Bronsted acid sites after dealumination. We now utilize state-of-the-art infrared spectroscopy measurements and prove that during steaming, aluminum oxide clusters evolve (due to hydrolysis of Al out of framework positions with the following clustering) in the zeolitic micropores with properties very similar to (nano) facets of hydroxylated transition alumina surfaces. The Bronsted acidity of the zeolite does not increase and the total number of Bronsted acid sites decreases during steaming. O5Al(VI)-OH surface sites of alumina clusters dehydroxylate at elevated temperatures to form penta-coordinate Al1O5 sites that are capable of initiating alkane cracking by breaking the first C-H bond very effectively with much lower barriers (at lower temperatures) than for protolytic C-H bond activation, with the following reaction steps catalyzed by nearby zeolitic Bronsted acid sites. This explains the underlying mechanism behind the improved alkane cracking and alkane dehydrogenation activity of steamed zeolites: heterolytic C-H bond breaking occurs on Al-O sites of aluminum oxide clusters confined in zeolitic pores. Our findings explain the origin of enhanced activity of steamed zeolites at the molecular level and provide the missing understanding of the nature of extra-framework Al species formed in steamed/dealuminated zeolites.

Identification of the mechanism of NO reduction with ammonia (SCR) on zeolite catalysts

Nitrosyl ions in zeolites that form via the NO + O2 reaction react with NH3 to form N2 and H2O (SCR reaction) via formation of transient diazo N2H+ cation.