Investigation of the Structure and Active Sites of TiO2 Nanorod Supported VOx Catalysts by High-Field and Fast-Spinning 51V MAS NMR

In-Situ Characterization Techniques for Mechanism Studies of CO2 Hydrogenation

The paramount concerns of global warming, fossil fuel depletion, and energy crises have prompted the need of hydrocarbons productions via CO2 conversion. In order to achieve global carbon neutrality, much attention needs to be diverted towards CO2 management. Catalytic hydrogenation of CO2 is an exciting opportunity to curb the increasing CO2 and produce value-added products. However, the comprehensive understanding of CO2 hydrogenation is still a matter of discussion due to its complex reaction mechanism and involvement of various species. This review comprehensively discusses three processes: reverse water gas shift (RWGS) reaction, modified Fischer Tropsch synthesis (MFTS), and methanol-mediated route (MeOH) for CO2 hydrogenation to hydrocarbons. Along with analysing the reaction pathways, it is also very important to understand the real-time evolvement of catalytic process and reaction intermediates by employing in-situ characterization techniques under actual reaction conditions. Subsequently, in second part of this review, we provided a systematic analysis of advancements in in-situ techniques aimed to monitor the evolution of catalysts during CO2 reduction process. The section also highlights the key components of in-situ cells, their working principles, and applications in identifying reaction mechanisms for CO2 hydrogenation. Finally, by reviewing respective achievements in the field, we identify key gaps and present some future directions for CO2 hydrogenation and in-situ studies.

Plasma-assisted manipulation of vanadia nanoclusters for efficient selective catalytic reduction of NOx

Supported nanoclusters (SNCs) with distinct geometric and electronic structures have garnered significant attention in the field of heterogeneous catalysis. However, their directed synthesis remains a challenge due to limited efficient approaches. This study presents a plasma-assisted treatment strategy to achieve supported metal oxide nanoclusters from a rapid transformation of monomeric dispersed metal oxides. As a case study, oligomeric vanadia-dominated surface sites were derived from the classic supported V2O5-WO3/TiO2 (VWT) catalyst and showed nearly an order of magnitude increase in turnover frequency (TOF) value via an H2-plasma treatment for selective catalytic reduction of NO with NH3. Such oligomeric surface VOx sites were not only successfully observed and firstly distinguished from WOx and TiO2 by advanced electron microscopy, but also facilitated the generation of surface amide and nitrates intermediates that enable barrier-less steps in the SCR reaction as observed by modulation excitation spectroscopy technologies and predicted DFT calculations.

Uncovering the Dinuclear Mechanism of NO2-Involved NH3-SCR over Supported V2O5/TiO2 Catalysts

Commercial vanadium oxide catalysts exhibit high efficiency for the selective catalytic reduction (SCR) of NO with NH3, especially in the presence of NO2 (i.e., occurrence of fast NH3-SCR). The high-activity sites and their working principle for the fast NH3-SCR reaction, however, remain elusive. Here, by combining in situ spectroscopy, isotopic labeling experiments, and density functional theory (DFT) calculations, we demonstrate that polymeric vanadyl species act as the main active sites in the fast SCR reaction because the coupling effect of the polymeric structure alters the elementary reaction step and effectively avoids the high energy barrier of the rate-determining step over monomeric vanadyl species. This study unveils the high-activity dinuclear mechanism of the NO2-involved SCR reaction over vanadia-based catalysts and provides a fundamental basis for developing high-efficiency and low V2O5-loading SCR catalysts.

Assignment of Vibrational Bands of Critical Surface Species Containing Nitrogen in the Selective Catalytic Reduction of NO by NH3

The selective catalytic reduction (SCR) of NO by NH₃ on metal oxides plays a key role in minimizing NO_x emissions. Electronic structure calculations at the density functional theory level have been performed to predict the vibrational modes of NH₃/NH₄⁺ bound to validated cluster models of vanadium oxide bound to a TiO₂ surface. Excellent agreement of the scaled calculated values with the observed bands attributed to surface-bound species is found. The presence of NH₃ bound to Lewis acid sites and NH₄⁺ bound to Brønsted acid sites when VOH groups are present is supported by our predictions. NH₄⁺ is expected to dominate the spectra even at low concentrations, with predicted intensities 5 to 30 times greater than those predicted for surface-bound NH₃. This is particularly evident in the lowest-energy N-H stretches of surface NH₄⁺ due to partial proton transfer interactions with the vanadium oxide surface model. The current work is consistent with experimental vibrational spectroscopy results and does not support the presence of a significant amount of NH₂ on the catalyst surface for the SCR reaction on VO_x/TiO₂. The combined experimental and computational results support the presence of both NH₃- and NH₄⁺-type species bound to the surface.

Enhanced Catalytic Performance of a Ce/V Oxo Cluster through Confinement in Mesoporous SBA-15

To increase catalytic efficiency, mesoporous supports have been widely applied to immobilize well-defined metal oxide clusters due to their ability to stabilize highly dispersed clusters. Herein, a redox-active heterometallic Ce₁₂V₆-oxo cluster (CeV) was first presynthesized and then incorporated into mesoporous silica, SBA-15, via a straightforward impregnation method. Scanning transmission electron microscopy (STEM) and Fourier transform infrared spectroscopy (FTIR), in concert with scanning electron microscopy and energy-dispersive X-ray spectroscopy (SEM-EDS), verified the successful introduction of the CeV cluster inside the pore of SBA-15. The ⁵¹V magic angle spinning solid-state nuclear magnetic resonance (⁵¹V MAS NMR) spectroscopy and differential pair distribution function (dPDF) analysis confirmed the structural integrity of the CeV cluster inside the SBA-15. The composite was then benchmarked for liquid-phase oxidation of 2-chloroethyl ethyl sulfide (CEES) under mild conditions and gas-phase oxidative dehydrogenation (ODH) of propane under high temperatures (up to 550 °C). The catalytic reactivity results demonstrated 8- and 14-fold increase in turnover frequency (TOF) values of the composite (CeV@10SBA-2) than the bulk CeV cluster under the same conditions for CEES oxidation and ODH, respectively. These results highlight the improved reactivity of the catalytically active CeV cluster as attributed to the higher dispersion of the discrete cluster upon immobilization within the SBA-15 support.

SO2- and H2O-Tolerant Catalytic Reduction of NOx at a Low Temperature via Engineering Polymeric VOx Species by CeO2

Selective catalytic reduction (SCR) of NO_x over V₂O₅-based oxide catalysts has been widely used, but it is still a challenge to efficiently reduce NO_x at low temperatures under SO₂ and H₂O co-existence. Herein, SO₂- and H₂O-tolerant catalytic reduction of NO_x at a low temperature has been originally demonstrated via engineering polymeric VO_x species by CeO₂. The polymeric VO_x species were tactfully engineered on Ce-V₂O₅ composite active sites via the surface occupation effect of Ce, and the obtained catalysts exhibited remarkable low-temperature activity and strong SO₂ and H₂O tolerance at 250 °C. The strong interaction between Ce and V species induced the electron transfer from V to Ce and tuned the SCR reaction via the E-R pathway between the NH₄⁺/NH₃ species and gaseous NO. In the presence of SO₂ and H₂O, the polymeric VO_x species had not been hardly influenced, while the formation of sulfate species on Ce sites not only promoted the adsorption of NH₄⁺ species and the reaction between gaseous NO and NH₄⁺ but also facilitated the decomposition of ammonium bisulfate through weakening the strong bond between HSO₄^- and NH₄⁺. This work provided a new strategy for SO₂- and H₂O-tolerant catalytic reduction of NO_x at a low temperature.

Efficient NOx Abatement over Alkali-Resistant Catalysts via Constructing Durable Dimeric VOx Species

The presence of alkali metals in flue gas is still an obstacle to the practical application of catalysts for selective catalytic reduction (SCR) of NO_x by NH₃. Polymeric vanadyl species play an essential role in ensuring the effective NO_x abatement for NH₃-SCR. However, polymeric vanadyl would be conventionally deactivated by the poison of alkali metals such as potassium, and it still remains a great challenge to construct robust and stable vanadyl species. Here, it was demonstrated that a more durable dimeric VO_x active site could be constructed with the assistance of triethylamine, thereby achieving alkali-resistant NO_x abatement. Due to the rational construction of polymerization structures, the obtained TiO₂-supported cerium vanadate catalyst featured more stable dimeric VO_x species and the active sites could survive even after the poisoning of alkali metal. Moreover, the depolymerization of VO_x was suppressed endowing the catalysts with more Brønsted and Lewis acid sites after the poisoning of alkali metal, which ensured the efficient NO_x reduction. This work unraveled the effects of alkali metal on the polymerization state of active species and opens up a way to develop low-temperature alkali-resistant catalysts for NO_x abatement.

Molybdenum-decorated V2O5–WO3/TiO2: surface engineering toward boosting the acid cycle and redox cycle of NH3-SCR

Submonolayer Mo-decorated V₂O₅–WO₃/TiO₂ provides abundant vanadia species and unsaturated V⁴⁺ species, accelerating the acid and redox cycling of low-temperature NH₃-SCR.

Significant promotion effect of the rutile phase on V2O5/TiO2 catalysts for NH3-SCR

High rutile content and low specific surface area of the vanadia-based catalysts contribute to an excellent NH₃-SCR activity.

Progress and Perspective for In Situ Studies of CO2 Reduction

CO₂ conversion to chemical fuels through photoreduction, electroreduction, or thermoreduction is considered as one of the most effective methods to solve environmental pollution and energy shortage problems. However, recent studies show that the involved catalysts may undergo continuous reconstruction under realistic working conditions, which unfortunately causes controversial results concerning the active sites and reaction mechanism of CO₂ reduction. Thus, it is necessary, while challenging, to monitor in real time the dynamic evolution of the catalysts and reaction intermediates by in situ techniques under experimental conditions. In this Perspective, we start with the working principle and detection modes of various in situ characterization techniques. Subsequently, we systematically summarize the recent developments of in situ studies on probing the catalyst evolution during the CO₂ reduction process. We further focus on the progress of in situ studies in monitoring the reaction intermediates and catalytic products, in which we also highlight how the theoretical calculations are combined to reveal the reaction mechanism in detail. Finally, based on the achievements in the representative studies, we present some prospects and suggestions for in situ studies of CO₂ reduction in the future.

Effect of treatment atmosphere on the vanadium species of V/TiO2 catalysts for the selective catalytic reduction of NOx with NH3

The decrease of polymeric vanadyl species due to treatment under different atmospheres results in the reduction of NH₃-SCR activity.

Structural characterization of vanadium environments in MCM-41 molecular sieve catalysts by solid state 51V NMR

Advanced analysis of ⁵¹V NMR chemical shift and quadrupolar tensor parameters revealed novel insights into the structure of vanadium species in MCM-41-based catalysts.

Polymeric vanadyl species determine the low-temperature activity of V-based catalysts for the SCR of NO x with NH3

The structure of dispersed vanadyl species plays a crucial role in the selective catalytic reduction (SCR) of NO with NH₃ over vanadia-based catalysts. Here, we demonstrate that the polymeric vanadyl species have a markedly higher NH₃-SCR activity than the monomeric vanadyl species. The coupling effect of the polymeric structure not only shortens the reaction pathway for the regeneration of redox sites but also substantially reduces the overall reaction barrier of the catalytic cycle. Therefore, it is the polymeric vanadyl species, rather than the monomeric vanadyl species, that determine the NH₃-SCR activity of vanadia-based catalysts, especially under low-temperature conditions. The polymeric vanadia-based SCR mechanism reported here advances the understanding of the working principle of vanadia-based catalysts and paves the way toward the development of low vanadium-loading SCR catalysts with excellent low-temperature activity.

Catalytic Applications of Vanadium: A Mechanistic Perspective

The chemistry of vanadium has seen remarkable activity in the past 50 years. In the present review, reactions catalyzed by homogeneous and supported vanadium complexes from 2008 to 2018 are summarized and discussed. Particular attention is given to mechanistic and kinetics studies of vanadium-catalyzed reactions including oxidations of alkanes, alkenes, arenes, alcohols, aldehydes, ketones, and sulfur species, as well as oxidative C-C and C-O bond cleavage, carbon-carbon bond formation, deoxydehydration, haloperoxidase, cyanation, hydrogenation, dehydrogenation, ring-opening metathesis polymerization, and oxo/imido heterometathesis. Additionally, insights into heterogeneous vanadium catalysis are provided when parallels can be drawn from the homogeneous literature.

Immediate hydroxylation of arenes to phenols via V-containing all-silica ZSM-22 zeolite triggered non-radical mechanism

Hydroxylation of arenes via activation of aromatic C_sp2–H bond has attracted great attention for decades but remains a huge challenge. Herein, we achieve the ring hydroxylation of various arenes with stoichiometric hydrogen peroxide (H₂O₂) into the corresponding phenols on a robust heterogeneous catalyst series of V–Si–ZSM-22 (TON type vanadium silicalite zeolites) that is straightforward synthesized from an unusual ionic liquid involved dry-gel-conversion route. For benzene hydroxylation, the phenol yield is 30.8% (selectivity >99%). Ring hydroxylation of mono-/di-alkylbenzenes and halogenated aromatic hydrocarbons cause the yields up to 26.2% and selectivities above 90%. The reaction is completed within 30 s, the fastest occasion so far, resulting in ultra-high turnover frequencies (TOFs). Systematic characterization including ⁵¹V NMR and X-ray absorption fine structure (XAFS) analyses suggest that such high activity associates with the unique non-radical hydroxylation mechanism arising from the in situ created diperoxo V(IV) state.

Hydroxylation of arenes via activation of aromatic C_sp2–H bond remains a challenge. Here, the authors have managed to get various arenes hydroxylated to corresponding phenols using stoichiometric hydrogen peroxide and a series of robust V–Si–ZSM-22 catalysts synthesized via an ionic liquid involved dry-gel-conversion route.

Supported two- and three-dimensional vanadium oxide species on the surface of β-SiC

Dispersing two-dimensional VO_x species on β-SiC offers a new approach to scale up propane ODH.