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.

Revealing the Nature of Active Oxygen Species and Reaction Mechanism of Ethylene Epoxidation by Supported Ag/α-Al2O3 Catalysts

The oxygen species on Ag catalysts and reaction mechanisms for ethylene epoxidation and ethylene combustion continue to be debated in the literature despite decades of investigation. Fundamental details of ethylene oxidation by supported Ag/α-Al2O3 catalysts were revealed with the application of high-angle annular dark-field-scanning transmission electron microscopy-energy-dispersive X-ray spectroscopy (HAADF-STEM-EDS), in situ techniques (Raman, UV-vis, X-ray diffraction (XRD), HS-LEIS), chemical probes (C2H4-TPSR and C2H4 + O2-TPSR), and steady-state ethylene oxidation and SSITKA (16O2 → 18O2 switch) studies. The Ag nanoparticles are found to carry a considerable amount of oxygen after the reaction. Density functional theory (DFT) calculations indicate the oxidative reconstructed p(4 × 4)-O-Ag(111) surface is stable relative to metallic Ag(111) under the relevant reaction environment. Multiple configurations of reactive oxygen species are present, and their relevant concentrations depend on treatment conditions. Selective ethylene oxidation to EO proceeds with surface Ag4-O2* species (dioxygen species occupying an oxygen site on a p(4 × 4)-O-Ag(111) surface) only present after strong oxidation of Ag. These experimental findings are strongly supported by the associated DFT calculations. Ethylene epoxidation proceeds via a Langmuir-Hinshelwood mechanism, and ethylene combustion proceeds via combined Langmuir-Hinshelwood (predominant) and Mars-van Krevelen (minor) mechanisms.

Lithium carbonate-promoted mixed rare earth oxides as a generalized strategy for oxidative coupling of methane with exceptional yields

The oxidative coupling of methane to higher hydrocarbons offers a promising autothermal approach for direct methane conversion, but its progress has been hindered by yield limitations, high temperature requirements, and performance penalties at practical methane partial pressures (~1 atm). In this study, we report a class of Li2CO3-coated mixed rare earth oxides as highly effective redox catalysts for oxidative coupling of methane under a chemical looping scheme. This catalyst achieves a single-pass C2+ yield up to 30.6%, demonstrating stable performance at 700 °C and methane partial pressures up to 1.4 atm. In-situ characterizations and quantum chemistry calculations provide insights into the distinct roles of the mixed oxide core and Li2CO3 shell, as well as the interplay between the Pr oxidation state and active peroxide formation upon Li2CO3 coating. Furthermore, we establish a generalized correlation between Pr4+ content in the mixed lanthanide oxide and hydrocarbons yield, offering a valuable optimization strategy for this class of oxidative coupling of methane redox catalysts.

Design of Cr-Free Promoted Copper-Iron Oxide-Based High-Temperature Water-Gas Shift Catalysts

The effect of Ce addition to the Cr-free Al-promoted Cu-Fe oxide-based catalysts is investigated. Catalyst characterization (X-ray diffraction (XRD), in situ Raman spectroscopy, high-sensitivity low-energy ion scattering (HS-LEIS), Brunauer-Emmett-Teller (BET) analysis), CO-temperature-programmed reduction chemical probing, and steady-state WGS activity reveal that (i) in the absence of Al, Ce addition via coprecipitation has a detrimental effect on the catalytic activity related to the poor thermostability and formation of less active Ce-Cu-O NPs, (ii) the addition of Ce via coprecipitation also does not improve the performance of the CuAlFe catalyst because of the formation of a thick CeOx overlayer on the active Cu-FeOx interface, and (iii) impregnation of Ce onto the CuAlFe catalyst exhibits significant improvement in catalytic performance due to the formation of a highly active CeOx-FeOx-Cu interfacial area. In summary, Al does not surface-segregate and serves as a structural promoter, while Ce and Cu surface-segregate and act as functional promoters in Ce/CuAlFe mixed oxide 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.

Redox Dynamics of Active VO x Sites Promoted by TiO x during Oxidative Dehydrogenation of Ethanol Detected by Operando Quick XAS

Titania-supported vanadia (VO _x /TiO₂) catalysts exhibit outstanding catalytic in a number of selective oxidation and reduction processes. In spite of numerous investigations, the nature of redox transformations of vanadium and titanium involved in various catalytic processes remains difficult to detect and correlate to the rate of products formation. In this work, we studied the redox dynamics of active sites in a bilayered 5% V₂O₅/15% TiO₂/SiO₂ catalyst (consisting of submonolayer VO _x species anchored onto a TiO _x monolayer, which in turn is supported on SiO₂) during the oxidative dehydrogenation of ethanol. The VO _x species in 5% V₂O₅/15% TiO₂/SiO₂ show high selectivity to acetaldehyde and an ca. 40 times higher acetaldehyde formation rate in comparison to VO _x species supported on SiO₂ with a similar density. Operando time-resolved V and Ti K-edge X-ray absorption near-edge spectroscopy, coupled with a transient experimental strategy, quantitatively showed that the formation of acetaldehyde over 5% V₂O₅/15% TiO₂/SiO₂ is kinetically coupled to the formation of a V⁴⁺ intermediate, while the formation of V³⁺ is delayed and 10-70 times slower. The low-coordinated nature of various redox states of VO _x species (V⁵⁺, V⁴⁺, and V³⁺) in the 5% V₂O₅/15% TiO₂/SiO₂ catalyst is confirmed using the extensive database of V K-edge XANES spectra of standards and specially synthesized molecular crystals. Much weaker redox activity of the Ti⁴⁺/Ti³⁺ couple was also detected; however, it was found to not be kinetically coupled to the rate-determining step of ethanol oxidation. Thus, the promoter effect of TiO _x is rather complex. TiO _x species might be involved in a fast electron transport between VO _x species and might affect the electronic structure of VO _x , thereby promoting their reducibility. This study demonstrates the high potential of element-specific operando X-ray absorption spectroscopy for uncovering complex catalytic mechanisms involving the redox kinetics of various metal oxides.

Olefin metathesis over supported MoOx catalysts: influence of the oxide support

Supported MoOx catalysts on oxide supports (Al2O3, TiO2, ZrO2, SiO2) were synthesized for propylene metathesis, characterized with in situ spectroscopies (DRIFTS, Raman, UV-vis) and chemically probed with propylene-TPSR, ethylene/2-butene titration.

A combined computational and experimental study of methane activation during oxidative coupling of methane (OCM) by surface metal oxide catalysts

The experimentally validated computational models developed herein, for the first time, show that Mn-promotion does not enhance the activity of the surface Na2WO4 catalytic active sites for CH4 heterolytic dissociation during OCM. Contrary to previous understanding, it is demonstrated that Mn-promotion poisons the surface WO4 catalytic active sites resulting in surface WO5 sites with retarded kinetics for C-H scission. On the other hand, dimeric Mn2O5 surface sites, identified and studied via ab initio molecular dynamics and thermodynamics, were found to be more efficient in activating CH4 than the poisoned surface WO5 sites or the original WO4 sites. However, the surface reaction intermediates formed from CH4 activation over the Mn2O5 surface sites are more stable than those formed over the Na2WO4 surface sites. The higher stability of the surface intermediates makes their desorption unfavorable, increasing the likelihood of over-oxidation to CO x , in agreement with the experimental findings in the literature on Mn-promoted catalysts. Consequently, the Mn-promoter does not appear to have an essential positive role in synergistically tuning the structure of the Na2WO4 surface sites towards CH4 activation but can yield MnO x surface sites that activate CH4 faster than Na2WO4 surface sites, but unselectively.

New Mechanistic and Reaction Pathway Insights for Oxidative Coupling of Methane (OCM) over Supported Na2 WO4 /SiO2 Catalysts

The complex structure of the catalytic active phase, and surface-gas reaction networks have hindered understanding of the oxidative coupling of methane (OCM) reaction mechanism by supported Na₂ WO₄ /SiO₂ catalysts. The present study demonstrates, with the aid of in situ Raman spectroscopy and chemical probe (H₂ -TPR, TAP and steady-state kinetics) experiments, that the long speculated crystalline Na₂ WO₄ active phase is unstable and melts under OCM reaction conditions, partially transforming to thermally stable surface Na-WO_x sites. Kinetic analysis via temporal analysis of products (TAP) and steady-state OCM reaction studies demonstrate that (i) surface Na-WO_x sites are responsible for selectively activating CH₄ to C₂ H_x and over-oxidizing CH_y to CO and (ii) molten Na₂ WO₄ phase is mainly responsible for over-oxidation of CH₄ to CO₂ and also assists in oxidative dehydrogenation of C₂ H₆ to C₂ H₄ . These new insights reveal the nature of catalytic active sites and resolve the OCM reaction mechanism over supported Na₂ WO₄ /SiO₂ catalysts.

Methane activation by ZSM-5-supported transition metal centers

This review focuses on recent fundamental insights about methane dehydroaromatization (MDA) to benzene over ZSM-5-supported transition metal oxide-based catalysts (MO_x/ZSM-5, where M = V, Cr, Mo, W, Re, Fe). Benzene is an important organic intermediate, used for the synthesis of chemicals like ethylbenzene, cumene, cyclohexane, nitrobenzene and alkylbenzene. Current production of benzene is primarily from crude oil processing, but due to the abundant availability of natural gas, there is much recent interest in developing direct processes to convert CH₄ to liquid chemicals. Among the various gas-to-liquid methods, the thermodynamically-limited Methane DehydroAromatization (MDA) to benzene under non-oxidative conditions appears very promising as it circumvents deep oxidation of CH₄ to CO₂ and does not require the use of a co-reactant. The findings from the MDA catalysis literature is critically analyzed with emphasis on in situ and operando spectroscopic characterization to understand the molecular level details regarding the catalytic sites before and during the MDA reaction. Specifically, this review discusses the anchoring sites of the supported MO_x species on the ZSM-5 support, molecular structures of the initial dispersed surface MO_x sites, nature of the active sites during MDA, reaction mechanisms, rate-determining step, kinetics and catalyst activity of the MDA reaction. Finally, suggestions are given regarding future experimental investigations to fill the information gaps currently found in the literature.

Synthesis and molecular structure of model silica-supported tungsten oxide catalysts for oxidative coupling of methane (OCM)

Catalysts with only dispersed phase Na–WO₄ sites where Na/W < 2 are slightly less active but significantly more C₂ selective than the traditional Na₂WO₄/SiO₂ catalysts that contain a crystalline phase where Na/W = 2.

Strong Metal-Support Interactions between Copper and Iron Oxide during the High-Temperature Water-Gas Shift Reaction

The commercial high-temperature water-gas shift (HT-WGS) catalyst consists of CuO-Cr₂ O₃ -Fe₂ O₃ , where Cu functions as a chemical promoter to increase the catalytic activity, but its promotion mechanism is poorly understood. In this work, a series of iron-based model catalysts were investigated with in situ or pseudo in situ characterization, steady-state WGS reaction, and density function theory (DFT) calculations. For the first time, a strong metal-support interaction (SMSI) between Cu and FeO_x was directly observed. During the WGS reaction, a thin FeO_x overlayer migrates onto the metallic Cu particles, creating a hybrid surface structure with Cu-FeO_x interfaces. The synergistic interaction between Cu and FeO_x not only stabilizes the Cu clusters, but also provides new catalytic active sites that facilitate CO adsorption, H₂ O dissociation, and WGS reaction. These new fundamental insights can potentially guide the rational design of improved iron-based HT-WGS catalysts.

Pyrolysis of the Cellulose Fraction of Biomass in the Presence of Solid Acid Catalysts: An Operando Spectroscopy and Theoretical Investigation

Biomass pyrolysis by solid acid catalysts is one of many promising technologies for sustainable production of hydrocarbon liquid fuels and value-added chemicals, but these complex chemical transformations are still poorly understood. A series of well-defined model SiO₂ -supported alumina catalysts were synthesized and molecularly characterized, under dehydrated conditions and during biomass pyrolysis, with the aim of establishing fundamental catalyst structure-activity/selectivity relationships. The nature and corresponding acidity of the supported AlO_x nanostructures on SiO₂ were determined with ²⁷ Al/¹ H NMR and IR spectroscopy of chemisorbed CO, and DFT calculations. Operando time-resolved IR-Raman-MS spectroscopy studies revealed the molecular transformations taking place during biomass pyrolysis. The molecular transformations during biomass pyrolysis depended on both the domain size of the AlO_x cluster and molecular nature of the biomass feedstock. These new insights allowed the establishment of fundamental structure-activity/selectivity relationships during biomass pyrolysis.

Nature of Active Sites and Surface Intermediates during SCR of NO with NH₃ by Supported V₂O₅--WO₃/TiO₂ Catalysts

Time-resolved in situ IR was performed during selective catalytic reduction of NO with NH₃ on supported V₂O₅-WO₃/TiO₂ catalysts to examine the distribution and reactivity of surface ammonia species on Lewis and Brønsted acid sites. While both species were found to participate in the SCR reaction, their relative population depends on the coverage of the surface vanadia and tungsta sites, temperature, and moisture. Although the more abundant surface NH₄⁺_,ads intermediates dominate the overall SCR reaction, especially for hydrothermally aged catalysts, the minority surface NH_3,ads intermediates exhibit a higher specific SCR activity (TOF). The current study serves to resolve the long-standing controversy about the active sites for SCR of NO with NH₃ by supported V₂O₅-WO₃/TiO₂ catalysts.