Supported vanadium oxide catalysts contain a vanadium oxide phase deposited on a high surface area oxide support (e.g., Al2O3, SiO2, TiO2, etc.) and have found extensive applications as oxidation ...catalysts in the chemical, petroleum and environmental industries. This review of supported vanadium oxide catalysts focuses on the fundamental aspects of this novel class of catalytic materials (molecular structures, electronic structures, surface chemistry and structure-reactivity relationships). The molecular and electronic structures of the supported vanadium oxide phases were determined by the application of modern in situ characterization techniques (Raman, IR, UV-vis, XANES, EXAFS, solid state (51)V NMR and isotopic oxygen exchange). The characterization studies revealed that the supported vanadium oxide phase consists of two-dimensional surface vanadia sites dispersed on the oxide supports. Corresponding surface chemistry and reactivity studies demonstrated that the surface vanadia sites are the catalytic active sites for oxidation reactions by supported vanadia catalysts. Combination of characterization and reactivity studies demonstrate that the oxide support controls the redox properties of the surface vanadia sites that can be varied by as much as a factor of ~10(3).
This article critically reviews the literature on iron-based catalysts for the high-temperature water–gas shift (HT-WGS) reaction. The reaction mechanism, reaction intermediates, rate-determining ...step, kinetics, active site, and promoters are covered. Unlike the low-temperature water–gas shift (LT-WGS) reaction by Cu/ZnO catalysts that has received intensive analysis with modern in situ and operando spectroscopy and DFT studies, the corresponding HT-WGS reaction by Fe-based catalysts still lacks a fundamental understanding because of the absence of modern catalysis studies of this important catalytic system. Given the role of the WGS catalysts on production of H2 for a hydrogen economy, it is imperative that the fundamental molecular-level understanding of the HT-WGS catalyst be advanced.
The selective catalytic reduction (SCR) of NO x with NH3 to harmless N2 and H2O plays a crucial role in reducing highly undesirable NO x acid gas emissions from large utility boilers, industrial ...boilers, municipal waste plants, and incinerators. The supported V2O5–WO3/TiO2 catalysts have become the most widely used industrial catalysts for these SCR applications since introduction of this technology in the early 1970s. This Perspective examines the current fundamental understanding and recent advances of the supported V2O5–WO3/TiO2 catalyst system: (i) catalyst synthesis, (ii) molecular structures of titania-supported vanadium and tungsten oxide species, (iii) surface acidity, (iv) catalytic active sites, (v) surface reaction intermediates, (vi) reaction mechanism, (vii) rate-determining-step, and (viii) reaction kinetics.
The literature of olefin metathesis by heterogeneous supported catalysts, both industrial-type supported metal oxides (ReO x /Al2O3, ReO x /(SiO2–Al2O3), MoO x /SiO2, MoO x /Al2O3, MoO x .../(SiO2–Al2O3), WO x /SiO2, and WO x /(SiO2–Al2O3)) and supported organometallic complexes, is comprehensively reviewed. The focus of this Review is supported metal oxide catalysts, but the well-defined supported organometallic catalyst literature is also covered because such model catalysts have the potential to bridge heterogeneous and homogeneous olefin metathesis catalysis. The recent world shortage of small olefin feedstocks has created renewed interest in olefin metathesis as a route to synthesizing small olefins and is reflected in the recent growth of the patent literature. Despite the extensive application of supported metal oxides in industry for metathesis of small and large olefins, the molecular structures and oxidation states of the catalytic active sites, surface reaction intermediates, and reaction mechanisms of this important catalytic reaction have still not been resolved. The absence of reported in situ and operando spectroscopic studies from the olefin metathesis catalysis literature has hampered progress in this area. It appears from this literature review that the topic of olefin metathesis by heterogeneous supported metal oxide catalysts is still a relatively undeveloped research area and is poised for significant progress in understanding of the fundamental molecular details of these important catalytic systems in the coming years.
Display omitted
•Synthesis method-resulting molecular structure stability/structure/reactivity relationships are discussed.•Diversity of sulfate zirconia preparation methods results in variety of the ...active site structures.•Hydrocarbon reaction mechanisms on sulfate zirconia mostly rely on homogeneous mechanisms.
This article critically reviews the literature on sulfated zirconia catalysts for variety of important hydrocarbon and oxygenate reactions including ethylene dimerization, isomerization, alkylation and esterification. The catalyst synthesis method-resulting molecular structure relationships, stability and reactive transformations of the active site in response to gas phase environment, reaction mechanisms, reaction intermediates, the corresponding kinetics and effect of some popular promoters are covered. Key structural features of the active sites have been extensively investigated but the conclusive structure is yet to be determined due to the diversity of the preparation methods (nature and crystallinity of the support, sulfate deposition method, sulfate coverage) as well as the diversity of the experimental conditions used during the spectroscopic determination. Similarly, the effect of the promoters and the synergistic effects of the promoter/sulfate sites in the presence of hydrocarbons are not well-defined. Finally, multiple theories exist for the reactive mechanisms on sulfated zirconia but most rely on the homogeneous reaction mechanisms, such as those for ethylene dimerization and butane alkylation with butane. Collectively, these show the lack of a fundamental understanding and the need of modern catalysis studies of this important catalytic system.
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 NH3,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.
The literature for the oxidative coupling of methane (OCM) on supported Mn/Na2WO4/SiO2 catalysts is systematically and critically reviewed. The influence of the precursors, starting SiO2 support ...crystallinity, synthesis method, calcination temperature, and OCM reaction conditions on the catalyst structure is examined. The supported Mn/Na2WO4/SiO2 catalyst system is found to be dynamic with the catalyst structure quite dependent on the set of variables. Although almost all of the reported studies have determined the catalyst crystalline structures under ambient conditions (room temperature and air exposed), recent in situ/operando characterization study under OCM reaction conditions revealed that all previously detected crystalline phases of the active Mn–Na–W–O components are not present because the reaction temperature is above the melting points of their oxides. The presence of Na also induces the crystallization of the silica support to SiO2 (cristobalite) at elevated temperatures. The nature of the surface active sites under OCM reaction conditions is still not known because of the absence of in situ/operando surface spectroscopy characterization studies under relevant reaction conditions. Consequently, the proposed structure–activity models in the literature are highly speculative since they are lacking supporting data. The rate-determining-step involves activation of the methane C–H bond by atomic surface O* as demonstrated by a kinetic isotope effect (KIE) between CH4 and CD4. Although the reaction kinetics follow a Langmuir–Hinshelwood type mechanism, r = CH41O21/2, isotopic 18O2–16O2 studies have shown that the catalyst lattice also provides O* for the OCM reaction suggesting involvement of a Mars–van Krevelen mechanism. Recommendations are given regarding the experimental investigations that could establish the fundamental reaction aspects of OCM by supported Mn/Na2WO4/SiO2 catalysts that would allow for the rational design of improved catalysts.
Ethylene oxidation by Ag catalysts has been extensively investigated over the past few decades, but many key fundamental issues about this important catalytic system are still unresolved. This ...overview of the selective oxidation of ethylene to ethylene oxide by Ag catalysts critically examines the experimental and theoretical literature of this complex catalytic system: (i) the surface chemistry of silver catalysts (single crystal, powder/foil, and supported Ag/α-Al2O3), (ii) the role of promoters, (iii) the reaction kinetics, (iv) the reaction mechanism, (v) density functional theory (DFT), and (vi) microkinetic modeling. Only in the past few years have the modern catalysis research tools of in situ/operando spectroscopy and DFT calculations been applied to begin establishing fundamental structure–activity/selectivity relationships. This overview of the ethylene oxidation reaction by Ag catalysts covers what is known and what issues still need to be determined to advance the rational design of this important catalytic system.
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 ...Na2WO4/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 Na2WO4 active phase is unstable and melts under OCM reaction conditions, partially transforming to thermally stable surface Na‐WOx sites. Kinetic analysis via temporal analysis of products (TAP) and steady‐state OCM reaction studies demonstrate that (i) surface Na‐WOx sites are responsible for selectively activating CH4 to C2Hx and over‐oxidizing CHy to CO and (ii) molten Na2WO4 phase is mainly responsible for over‐oxidation of CH4 to CO2 and also assists in oxidative dehydrogenation of C2H6 to C2H4. These new insights reveal the nature of catalytic active sites and resolve the OCM reaction mechanism over supported Na2WO4/SiO2 catalysts.
In the current study, with the aid of state‐of‐the‐art spectroscopic techniques, transient kinetic analysis, and implementation of robust experimental methodologies, we resolve the nature of catalytic active sites and reaction mechanism for oxidative coupling of methane over supported Na2WO4/SiO2 catalysts.
Direct methane conversion into aromatic hydrocarbons over catalysts with molybdenum (Mo) nanostructures supported on shape-selective zeolites is a promising technology for natural gas liquefaction. ...We determined the identity and anchoring sites of the initial Mo structures in such catalysts as isolated oxide species with a single Mo atom on aluminum sites in the zeolite framework and on silicon sites on the zeolite external surface. During the reaction, the initial isolated Mo oxide species agglomerate and convert into carbided Mo nanoparticles. This process is reversible, and the initial isolated Mo oxide species can be restored by a treatment with gas-phase oxygen. Furthermore, the distribution of the Mo nanostructures can be controlled and catalytic performance can be fully restored, even enhanced, by adjusting the oxygen treatment.
PDF
