Single Silver Adatoms on Nanostructured Manganese Oxide Surfaces: Boosting Oxygen Activation for Benzene Abatement

The involvement of a great amount of active oxygen species is a crucial requirement for catalytic oxidation of benzene, because complete mineralization of one benzene molecule needs 15 oxygen atoms. Here, we disperse single silver adatoms on nanostructured hollandite manganese oxide (HMO) surfaces by using a thermal diffusion method. The single-atom silver catalyst (Ag₁/HMO) shows high catalytic activity in benzene oxidation, and 100% conversion is achieved at 220 °C at a high space velocity of 23 000 h^–1. The Mars-van Krevelen mechanism is valid in our case as the reaction orders for both benzene and O₂ approach one, according to reaction kinetics data. Data from H₂ temperature-programmed reduction and O core-level X-ray photoelectron spectra (XPS) reveal that Ag₁/HMO possesses a great amount of active surface lattice oxygen available for benzene oxidation. Valence-band XPS and density functional theoretical calculations demonstrate that the single Ag adatoms have the upshifted 4<i>d</i> orbitals, thus facilitating the activation of gaseous oxygen. Therefore, the excellent activation abilities of Ag₁/HMO toward both surface lattice oxygen and gaseous oxygen account for its high catalytic activity in benzene oxidation. This work may assist with the rational design of efficient metal-oxide catalysts for the abatement of volatile organic compounds such as benzene

Surface-Confined Atomic Silver Centers Catalyzing Formaldehyde Oxidation

Formaldehyde (HCHO) is a prior pollutant in both indoor and outdoor air, and catalytic oxidation proves the most promising technology for HCHO abatement. For this purpose, supported metal catalysts with single silver atoms confined at 4-fold O₄-terminated surface hollow sites of a hollandite manganese oxide (HMO) as catalytic centers were synthesized and investigated in the complete oxidation of HCHO. Synchrotron X-ray diffraction patterns, X-ray absorption spectra, and electron diffraction tomography revealed that geometric structures and electronic states of the catalytic centers were tuned by the changes of HMO structures via controllable metal–support interactions. The catalytic tests demonstrated that the catalytically active centers with high electronic density of states and strong redox ability are favorable for enhancement of the catalytic efficiency in the HCHO oxidation. This work provides a strategy for designing efficient oxidation catalysts for controlling air pollution

Surface-Confined Atomic Silver Centers Catalyzing Formaldehyde Oxidation

Formaldehyde (HCHO) is a prior pollutant in both indoor and outdoor air, and catalytic oxidation proves the most promising technology for HCHO abatement. For this purpose, supported metal catalysts with single silver atoms confined at 4-fold O₄-terminated surface hollow sites of a hollandite manganese oxide (HMO) as catalytic centers were synthesized and investigated in the complete oxidation of HCHO. Synchrotron X-ray diffraction patterns, X-ray absorption spectra, and electron diffraction tomography revealed that geometric structures and electronic states of the catalytic centers were tuned by the changes of HMO structures via controllable metal–support interactions. The catalytic tests demonstrated that the catalytically active centers with high electronic density of states and strong redox ability are favorable for enhancement of the catalytic efficiency in the HCHO oxidation. This work provides a strategy for designing efficient oxidation catalysts for controlling air pollution

Enhanced Performance of Ceria-Based NO_<i>x</i> Reduction Catalysts by Optimal Support Effect

CeO₂-based catalysts have attracted widespread attention in environmental-protection applications, including selective catalytic reduction (SCR) of NO by NH₃, and their catalytic performance is often intimately associated with the supports used. However, the issue of how to choose the supports of such catalysts still remains unresolved. Herein, we systematically study the support effect in SCR over CeO₂-based catalysts by using three representative supports, Al₂O₃, TiO₂, and hexagonal WO₃ (HWO), with different acidic and redox properties. HWO, with both acidic and reducible properties, achieves an optimal support effect; that is, CeO₂/HWO exhibits higher catalytic activity than CeO₂ supported on acidic Al₂O₃ or reducible TiO₂. Transmission electron microscopy and X-ray diffraction techniques demonstrate that acidic supports (HWO and Al₂O₃) are favorable for the dispersion of CeO₂ on their surfaces. X-ray photoelectron spectroscopy coupled with theoretical calculations reveals that reducible supports (HWO and TiO₂) facilitate strong electronic CeO₂–support interactions. Hence, the excellent catalytic performance of CeO₂/HWO is mainly ascribed to the high dispersion of CeO₂ and the optimal electronic CeO₂–support interactions. This work shows that abundant Brønsted acid sites and excellent redox ability of supports are two critical requirements for the design of efficient CeO₂-based catalysts

Self-Protection Mechanism of Hexagonal WO₃–Based DeNO_<i>x</i> Catalysts against Alkali Poisoning

A good catalyst for efficiently controlling NO_<i>x</i> emissions often demands strong resistance against alkali poisoning. Although the traditional ion-exchange model, based on acid–base reactions of alkalis with Brønsted acid sites, has been established over the past two decades, it is difficult to be used as a guideline to develop such an alkali-resistant catalyst. Here we establish a self-protection mechanism of deNO_<i>x</i> catalysts against alkali poisoning by systematically studying the intrinsic nature of alkali resistance of V₂O₅/HWO (HWO = hexagonal WO₃) that shows excellent resistance to alkali poisoning in selective catalytic reduction of NO_<i>x</i> with NH₃ (SCR). Synchrotron X-ray diffraction and absorption spectroscopies demonstrate that V₂O₅/HWO has spatially separated catalytically active sites (CASs) and alkali-trapping sites (ATSs). During the SCR process, ATSs spontaneously trap alkali ions such as K⁺, even if alkali ions initially block CASs, thus releasing CASs to realize the self-protection against alkali poisoning. X-ray photoelectron spectra coupled with theoretical calculations indicate that the electronic interaction between the alkali ions and ATSs with an energy saving is the driving force of the self-protection. This work provides a strategy to design alkali-resistant deNO_<i>x</i> catalysts

Sodium Rivals Silver as Single-Atom Active Centers for Catalyzing Abatement of Formaldehyde

The development of efficient alkali-based catalysts for the abatement of formaldehyde (HCHO), a ubiquitous air pollutant, is economically desirable. Here we comparatively study the catalytic performance of two single-atom catalysts, Na₁/HMO and Ag₁/HMO (HMO = Hollandite manganese oxide), in the complete oxidation of HCHO at low temperatures, in which the products are only CO₂ and H₂O. These catalysts are synthesized by anchoring single sodium ions or silver atoms on HMO(001) surfaces. Synchrotron X-ray diffraction patterns with structural refinement together with transmission electron microscopy images demonstrate that single sodium ions on the HMO(001) surfaces of Na₁/HMO have the same local structures as silver atoms of Ag₁/HMO. Catalytic tests reveal that Na₁/HMO has higher catalytic activity in low-temperature oxidation of HCHO than Ag₁/HMO. X-ray photoelectron spectra and soft X-ray absorption spectra show that the surface lattice oxygen of Na₁/HMO has a higher electronic density than that of Ag₁/HMO, which is responsible for its higher catalytic efficiency in the oxidation of HCHO. This work could assist the rational design of cheap alkali metal catalysts for controlling the emissions of volatile organic compounds such as HCHO

Alkali-Resistant Mechanism of a Hollandite DeNO_<i>x</i> Catalyst

A thorough understanding of the deactivation mechanism by alkalis is of great importance for rationally designing improved alkali-resistant deNO_<i>x</i> catalysts, but a traditional ion-exchange mechanism cannot often accurately describe the nature of the deactivation, thus hampering the development of superior catalysts. Here, we establish a new exchange-coordination mechanism on the basis of the exhaustive study on the strong alkali resistance of a hollandite manganese oxide (HMO) catalyst. A combination of isothermal adsorption measurements of ammonia with X-ray absorption near-edge structure spectra and X-ray photoelectron spectra reveals that alkali metal ions first react with protons from Brønsted acid sites of HMO via the ion exchange. Synchrotron X-ray diffraction patterns and extended X-ray absorption fine structure spectra coupled with theoretical calculations demonstrate that the exchanged alkali metal ions are subsequently stabilized at size-suitable cavities in the HMO pores via a coordination model with an energy savings. This exchange-coordination mechanism not only gives a wholly convincing explanation for the intrinsic nature of the deactivation of the reported catalysts by alkalis but also provides a strategy for rationally designing improved alkali-resistant deNO_<i>x</i> catalysts in general

Crystallization-Induced Morphological Tuning Toward Denim-like Graphene Nanosheets in a KCl-Copolymer Solution

Although nucleation and crystallization in solution-processed materials synthesis is a natural phenomenon, the morphology design of graphene nanosheets by controlling the dual crystallization has not been established. In this work, we systematically demonstrate how the dual crystallization of ice and potassium chloride induces the morphological variation of the freeze-dried scaffold from fractal structure toward stepped sheet-like structure. A denim-like graphene nanosheet (DGNS) has been fabricated by annealing the F127-coated stepped sheet-like scaffold in nitrogen. DGNS shows parallel and straight stripes with an average stripe spacing of 10 nm. When used as a lithium-ion battery anode, DGNS possesses a superhigh reversible capacity of 1020 mAh g^–1 at the current density of 1 A g^–1 after 600 cycles. This work reports the control of dual crystallization of ice and salt crystals and provides an efficient way to design the morphology of two-dimensional materials by adjusting the crystallization

Catalytic Control of Typical Particulate Matters and Volatile Organic Compounds Emissions from Simulated Biomass Burning

Emissions of particulate matters (PMs) and volatile organic compounds (VOCs) from open burning of biomass often cause severe air pollution; a viable approach is to allow biomass to burn in a furnace to collectively control these emissions, but practical control technologies for this purpose are lacking. Here, we report a hollandite manganese oxide (HMO) catalyst that can efficiently control both typical PMs and VOCs emissions from biomass burning. The results reveal that typical alkali-rich PMs such as KCl particles are disintegrated and the K⁺ ions are trapped in the HMO “single-walled” tunnels with a great trapping capacity. The K⁺-trapping HMO increases the electron density of the lattice oxygen and the redox ability, thus promoting the combustion of soot PMs and the oxidation of typical VOCs such as aldehydes and acetylates. This could pave a way to control emissions from biomass burning concomitant with its utilization for energy or heat generation