Structural Evolution of Iron-Loaded Metal-Organic Framework Catalysts for Continuous Gas-Phase Oxidation of Methane to Methanol

Catalytic partial oxidation of methane presents a promising route to convert the abundant but environmentally undesired methane gas to liquid methanol with applications as an energy carrier and a platform chemical. However, an outstanding challenge for this process remains in developing a catalyst that can oxidize methane selectively to methanol with good activity under continuous flow conditions in the gas phase using O₂ as an oxidant. Here, we report a Fe catalyst supported by a metal-organic framework (MOF), Fe/UiO-66, for the selective and on-stream partial oxidation of methane to methanol. Kinetic studies indicate the continuous production of methanol at a superior reaction rate of 5.9 × 10^-2 μmol_MeOH g_Fe^-1 s^-1 at 180 °C and high selectivity toward methanol, with the catalytic turnover verified by transient methane isotopic measurements. Through an array of spectroscopic characterizations, electron-deficient Fe species rendered by the MOF support is identified as the probable active site for the reaction.

Unveiling the CO Oxidation Mechanism over a Molecularly Defined Copper Single-Atom Catalyst Supported on a Metal-Organic Framework

Elucidating the reaction mechanism in heterogeneous catalysis is critically important for catalyst development, yet remains challenging because of the often unclear nature of the active sites. Using a molecularly defined copper single-atom catalyst supported on a UiO-66 metal-organic framework (Cu/UiO-66), allows a detailed mechanistic elucidation of the CO oxidation reaction. Based on a combination of in situ / operando spectroscopies, kinetic measurements including kinetic isotope effects, and density functional theory-based calculations, we identified the active site, reaction intermediates, and transition states of the dominant reaction cycle as well as the changes in oxidation/spin state during reaction. The reaction involves the continuous reactive dissociation of adsorbed O2, by reaction of O2,ad with COad, leading to the formation of an O atom connecting the Cu center with a neighboring Zr4+ ion as rate limiting step. This is removed in a second activated step.

Photoexcited Anhydrous Proton Conductivity in Coordination Polymer Glass

Optically switchable proton-conductive materials will enable the development of artificial ionic circuits. However, most switchable platforms rely on conformational changes in crystals to alter the connectivity of guest molecules. Guest dependency, low transmittance, and poor processability of polycrystalline materials hinder overall light responsiveness and contrast between on and off states. Here, we optically control anhydrous proton conductivity in a transparent coordination polymer (CP) glass. Photoexcitation of tris(bipyrazine)ruthenium(II) complex in CP glass causes reversible increases in proton conductivity by a factor of 181.9 and a decrease in activation energy barrier from 0.76 eV to 0.30 eV. Modulating light intensity and ambient temperature enables total control of anhydrous protonic conductivity. Spectroscopies and density functional theory studies reveal the relationship between the presence of proton deficiencies and the decreasing activation energy barrier for proton migrations.

Compensation or Aggravation: Pb and SO2 Copoisoning Effects over Ceria-Based Catalysts for NOx Reduction

Severe catalyst deactivation caused by multiple poisons, including heavy metals and SO₂, remains an obstinate issue for the selective catalytic reduction (SCR) of NO_x by NH₃. The copoisoning effects of heavy metals and SO₂ are still unclear and irreconcilable. Herein, the unanticipated differential compensated or aggravated Pb and SO₂ copoisoning effects over ceria-based catalysts for NO_x reduction was originally unraveled. It was demonstrated that Pb and SO₂ exhibited a compensated copoisoning effect over the CeO₂/TiO₂ (CT) catalyst with sole active CeO₂ sites but an aggravated copoisoning effect over the CeO₂-WO₃/TiO₂ (CWT) catalyst with dual active CeO₂ sites and acidic WO₃ sites. Furthermore, it was uniquely revealed that Pb preferred bonding with CeO₂ among CT while further being combined with SO₂ to form PbSO₄ after copoisoning, which released the poisoned active CeO₂ sites and rendered the copoisoned CT catalyst a recovered reactivity. In comparison, Pb and SO₂ would poison acidic WO₃ sites and active CeO₂ sites, respectively, resulting in a seriously degraded reactivity of the copoisoned CWT catalyst. Therefore, this work thoroughly illustrates the internal mechanism of differential compensated or aggravated deactivation effects for Pb and SO₂ copoisoning over CT and CWT catalysts and provides effective solutions to design ceria-based SCR catalysts with remarkable copoisoning resistance for the coexistence of heavy metals and SO₂.

Unique Compensation Effects of Heavy Metals and Phosphorus Copoisoning over NOx Reduction Catalysts

Selective catalytic reduction (SCR) of NO_x from the flue gas is still a grand challenge due to the easy deactivation of catalysts. The copoisoning mechanisms and multipoisoning-resistant strategies for SCR catalysts in the coexistence of heavy metals and phosphorus are barely explored. Herein, we unexpectedly found unique compensation effects of heavy metals and phosphorus copoisoning over NO_x reduction catalysts and the introduction of heavy metals results in a dramatic recovery of NO_x reduction activity for the P-poisoned CeO₂/TiO₂ catalysts. P preferentially combines with Ce as a phosphate species to reduce the redox capacity and inhibit NO adsorption. Heavy metals preferentially reduced the Brønsted acid sites of the catalyst and inhibited NH₃ adsorption. It has been demonstrated that heavy metal phosphate species generated over the copoisoned catalyst, which boosted the activation of NH₃ and NO, subsequently bringing about more active nitrate species to relieve the severe impact by phosphorus and maintain the NO_x reduction over CeO₂/TiO₂ catalysts. The heavy metals and P copoisoned catalysts also possessed more acidic sites, redox sites, and surface adsorbed oxygen species, which thus contributed to the highly efficient NO_x reduction. This work elaborates the unique compensation effects of heavy metals and phosphorus copoisoning over CeO₂/TiO₂ catalysts for NO_x reduction and provides a perspective for further designing multipoisoning-resistant CeO₂-based catalysts to efficiently control NO_x emissions in stationary sources.

Conductive Co-triazole metal-organic framework exploited as an oxygen evolution electrocatalyst

A Co-triazole metal-organic framework (Co-trz) endowed with electrical conductivity was synthesized effortlessly via a microwave-based method. Providing a high density of catalytic centers with electrically conductive features, as suggested by DFT calculations, the framework exhibited a low overpotential for the oxygen evolution reaction (OER) with good kinetics. A mechanistic reaction pathway was proposed based on monitoring alterations in the oxidation state and local coordination environment of Co centers upon the occurrence of the OER. Due to its performance and its chemical and electrochemical robustness, the framework was highlighted as a promising MOF electrocatalyst for the OER.

Alkali and Heavy Metal Copoisoning Resistant Catalytic Reduction of NOx via Liberating Lewis Acid Sites

The catalyst deactivation caused by the coexistence of alkali and heavy metals remains an obstacle for selective catalytic reduction of NO_x with NH₃. Moreover, the copoisoning mechanism of alkali and heavy metals is still unclear. Herein, the copoisoning mechanism of K and Cd was revealed from the adsorption and variation of reaction intermediates at a molecular level through time-resolved in situ spectroscopy combined with theoretical calculations. The alkali metal K mainly decreased the adsorption of NH₃ on Lewis acid sites and altered the reaction more depending on the formation of the NH₄NO₃ intermediate, which is highly related to NO_x adsorption and activation. However, Cd further inhibited the generation of active nitrate intermediates and thus decreased the NO_x abatement about 60% on potassium-poisoned CeTiO_x catalysts. Physically mixing with acid additives for CeTiO_x catalysts could significantly liberate the active Lewis acid sites from the occupation of alkali metals and relieve the high dependence on NO_x adsorption and activation, thus recovering the NO_x removal rate to the initial state. This work revealed the copoisoning mechanism of K and Cd on Ce-based de-NO_x catalysts and developed a facile anti-poisoning strategy, which paves a way for the development of durable catalysts among alkali and heavy metal copoisoning resistant catalytic reduction of NO_x.

Identification of Cooperative Reaction Sites in Metal-Organic Framework Catalysts for High Yielding Lactic Acid Production from d-Xylose

Determining the roles of surface functionality of heterogeneous acid catalysts is important for many industrial catalysts. In this study, the decisive structure of metal-organic frameworks (MOFs) is utilized to identify important features for the effective conversion of d-xylose into lactic acid. Several acidic MOFs are tested and the combination of Lewis acidity and adjacent hydroxy sites is found to be critical to attain high lactic acid yields. This hypothesis is corroborated experimentally by modification of the MOF to increase such sites, which affords an enhanced lactic acid yield of 79 %, and investigation of the acidity by using in situ FTIR spectroscopy. Density functional theory calculations disclose the cooperative behavior of Lewis acid sites and hydroxy groups in promoting the Cannizzaro reaction, a key step in the production of lactic acid.

Bandgap Modulation in Zr-Based Metal-Organic Frameworks by Mixed-Linker Approach

Metal-organic frameworks (MOFs) have been a promising material for many applications, e.g., photocatalysis, luminescence-based sensing, optoelectronics, and electrochemical devices, due to their tunable electronic properties through linker functionalization. In this work, we investigate the effect of mixed organic linkers on the bandgap modulation of polymorphic zirconium-based MOFs, UiO-66 and MIL-140A using density functional theory (DFT) calculations. We show that the electronic properties of both MOFs are in contrast to Vegard's law for semiconductors, that is, mixed-linker systems exhibit bandgaps not intermediate within the range of single-linker systems. Calculations of the total and partial density of states revealed the formation of mid-gap states in mixed-linker MOFs, causing the bandgap reduction. Interestingly, although both MOFs have similar composition, the effect is more significant in MIL-140A than in UiO-66. This is due to the presence of π-π stacking interactions in MIL-140A, which does not occur in UiO-66. The simulation results reveal a direct relationship between the strength of π-π interactions and the bandgap. This illustrates that distinct structural features, particularly the orientation of organic linkers can give rise to different consequences in bandgap modulation. Moreover, this computational work highlights the possibility to engineer the electronic properties of MOFs through a mixed-linker approach.

Self-Protected CeO2-SnO2@SO42-/TiO2 Catalysts with Extraordinary Resistance to Alkali and Heavy Metals for NOx Reduction

Reducing the poisoning effect of alkali and heavy metals over ammonia selective catalytic reduction (NH₃-SCR) catalysts is still an intractable issue, as the presence of K and Pb in fly ash greatly hampers their catalytic activity by impairing the acidity and affecting the redox properties of the catalysts, leading to the reduction in the lifetime of SCR catalysts. To address this issue, we propose a novel self-protected antipoisoning mechanism by designing SO₄^2-/TiO₂ superacid supported CeO₂-SnO₂ catalysts. Owing to the synergistic effect between CeO₂ and SnO₂ and the strong acidity originating from the SO₄^2-/TiO₂ superacid, the catalysts show superior catalytic activity over a wide temperature range (240-510 °C). Moreover, when K or/and Pb are deposited on SO₄^2-/TiO₂ catalysts, the bond effect between SO₄^2- and Ti-O would be broken so that the sulfate in the bulk of SO₄^2-/TiO₂ superacid support would be induced to migrate to the surface to bond with K and Pb, thus prohibiting poisons from attacking the Ce-Sn active sites, and significantly boosting the resistance. Hopefully, this novel self-protection mechanism derived from the migration of sulfate in the SO₄^2-/TiO₂ superacid to resist alkali and heavy metals provides a new avenue for designing novel catalysts with outstanding resistance to alkali and heavy metals.

Boosting Toluene Combustion by Engineering Co-O Strength in Cobalt Oxide Catalysts

Exploring active and low-cost transition metal oxides (TMOs) based catalysts for volatile organic compounds (VOCs) abatement is vital for air pollution control technologies. Since 18 oxygen atoms are required for the complete mineralization of a toluene molecule, the participation of a large amount of active oxygen is a key requirement for the catalytic oxidation of toluene. Here, toluene degradation was improved by weakening the Co-O bond strength on the surface of cobalt oxide, so as to increase the amount of active oxygen species, while maintaining the high stability of the catalyst for toluene combustion. The bond strength of Co-O and the amount of surface active O₂ was regulated by tuning the pyrolysis temperature. The catalyst's redox ability and surface oxygen species activity are improved due to the weakening of the Co-O bond strength. It has been demonstrated that active oxygen plays a crucial role in boosting toluene combustion by engineering Co-O strength in cobalt oxide catalysts. This work provides a new understanding of the exploration and development of high-performance TMO catalysts for VOCs abatement.

Unraveling the Unexpected Offset Effects of Cd and SO2 Deactivation over CeO2-WO3/TiO2 Catalysts for NOx Reduction

It is challenging for selective catalytic reduction (SCR) of NO_x by NH₃ due to the coexistence of heavy metal and SO₂ in the flue gas. A thorough probe into deactivation mechanisms is imperative but still lacking. This study unravels unexpected offset effects of Cd and SO₂ deactivation over CeO₂-WO₃/TiO₂ catalysts, potential candidates for commercial SCR catalysts. Cd- and SO₂-copoisoned catalysts demonstrated higher activity for NO_x reduction than a Cd-poisoned catalyst but lower than that for an SO₂-poisoned catalyst. In comparison to SO₂, Cd had more severe effects on acidic and redox properties, distinctly decreasing the SCR activity. After sulfation of Cd-poisoned catalysts, SO₄^2- preferentially bonded with the surface CdO and released CeO₂ active sites poisoned by CdO, thus reserving the highly active CeO₂-WO₃ sites and maintaining a high activity. The sulfation of Cd-poisoned catalysts also provided more strong acidic sites, and the synergistic effects between the formed cerium sulfate and CeO₂ contributed to the high-temperature SCR performance. This work sheds light on the deactivation mechanism of heavy metals and SO₂ over CeO₂-WO₃/TiO₂ catalysts and provides an innovative pathway for inventing high-performance SCR catalysts, which have great resistance to heavy metals and SO₂ simultaneously. This will be favorable to academic and practical applications in the future.

Efficient photocatalytic reactions of Cr(vi) reduction and ciprofloxacin and RhB oxidation with Sn(ii)-doped BiOBr

Efficient photocatalytic reactions of Cr(VI) reduction, ciprofloxacin and RhB oxidation with Sn(II) doped BiOBr.

Ethane C–H bond activation on the Fe(iv)–oxo species in a Zn-based cluster of metal–organic frameworks: a density functional theory study

The generation of a Fe(iv)–oxo complex and its reactivity for C–H bond activation of ethane have been theoretically unraveled.

Methane activation on Fe- and FeO-embedded graphene and boron nitride sheet: role of atomic defects in catalytic activities

The influence of supporting materials, graphene and boron nitride sheets, on the reactivity of Fe and FeO active species have been unravelled by using a dispersion-corrected DFT (PBE-D2) method.

Direct oxidation of methane to methanol on Fe–O modified graphene

The reaction mechanisms of the partial oxidation of methane to methanol over FeO/graphene are unraveled using an advanced DFT approach.