Resolving the Reaction Mechanism for H₂ Formation from High-Temperature Water–Gas Shift by Chromium–Iron Oxide Catalysts

The reaction mechanism of the high-temperature water–gas shift (HT-WGS) reaction catalyzed by chromium–iron oxide catalysts for H₂ production has been studied for 100 years with two reaction mechanisms proposed: redox and associative (involving surface HCOO*). Direct experimental support for either mechanism, however, is still lacking, which hinders a thorough understanding of catalytic roles of each elements and the rational design of Cr-free catalysts. The current study demonstrates, with temperature-programmed surface reaction (TPSR) spectroscopy (CO-TPSR, CO+H₂O-TPSR, and HCOOH-TPSR), for the first time that the HT-WGS reaction follows the redox mechanism and that the associative mechanism does not take place

Determination of Number of Activated Sites Present during Olefin Metathesis by Supported ReO_<i>x</i>/Al₂O₃ Catalysts

The number of surface rhenia sites on alumina participating in olefin metathesis, usually counted by chemical titration of surface reaction intermediates with olefins, has been debated for several decades. The current olefin titration measurements, however, demonstrate that the number of reactive surface intermediates on supported rhenia/alumina catalysts is strongly dependent on the experimental variables. The new findings reveal that the accepted titration procedures significantly undercount the number of activated surface rhenia sites upon exposure to olefins by ∼10¹–10² because of invalid assumptions

Reaction Mechanism and Kinetics of Olefin Metathesis by Supported ReO_<i>x</i>/Al₂O₃ Catalysts

The self-metathesis of propylene by heterogeneous supported ReO_<i>x</i>/Al₂O₃ catalysts was investigated with in situ Raman spectroscopy, isotopic switch (D–C₃⁼ → H–C₃⁼), temperature-programmed surface reaction (TPSR) spectroscopy, and steady-state kinetic studies. The in situ Raman studies showed that two distinct surface ReO₄ sites are present on alumina and that the olefins preferentially interact with surface ReO₄ sites anchored at acidic surface sites of alumina (olefin adsorption: C₄⁼ > C₃⁼ > C₂⁼). The isotopic switch experiments demonstrate that surface Re*CH₃ and Re*CHCH₃ are present during propylene metathesis, with Re* representing activated surface rhenia sites. At low temperatures (<100 °C), the rate-determining step is adsorption of propylene on two adjacent surface sites (rate ≈ [C₃⁼]­[Re*]². At high temperatures (>100 °C), the rate-determining step is the recombination of two surface propylene molecules (rate ≈ [C₃⁼]²[Re*]). To a lesser extent, the recombination of surface Re*CH₃ and Re*CHCH₃ intermediates also contributes to self-metathesis of propylene at elevated reaction temperatures

Catalyst Activation and Kinetics for Propylene Metathesis by Supported WO_<i>x</i>/SiO₂ Catalysts

The activation and kinetics of propylene metathesis by well-defined supported WO_<i>x</i>/SiO₂ catalysts were investigated with transient temperature-programmed surface reaction (TPSR) spectroscopy and steady-state olefin metathesis. The TPSR measurements revealed for the first time that catalyst activation with olefins creates three distinct activated sites [highly active (<i>T</i>_p ∼ 160 °C), modestly active (<i>T</i>_p ∼ 450 °C), and sluggishly active (<i>T</i>_p ∼ 600–750 °C)]. The number and reactivity of the activated surface WO_<i>x</i> sites increase with activation temperature, olefin partial pressure, and olefin size (C₂⁼ ≪ C₃⁼ ∼ C₄⁼). Only propylene and 2-butene are able to generate the highly active sites because of the presence of CHCH₃ groups in these olefins, while ethylene is able to create only the sluggish active sites. The specific activity (TOF) of the surface WO_<i>x</i> sites increases with surface coverage because of the increasingly strained configuration (larger bridging O–W–O bond angles) at higher levels of surface WO_<i>x</i> coverage on the SiO₂ support. Steady-state propylene metathesis exhibits a first-order dependence on propylene partial pressure, and the rate-determining step is propylene adsorption on activated sites. Propylene metathesis by supported WO_<i>x</i>/SiO₂ catalysts can be represented by the simple Langmuir-type kinetics of rate = <i>kK</i>_ads[C₃⁼]/(1 + <i>K</i>_ads[C₃⁼]), where <i>k</i> is an Arrhenius rate constant and <i>K</i>_ads is the propylene equilibrium adsorption constant, that during typical high-temperature metathesis reduces to rate = <i>kK</i>_ads[C₃⁼] because the <i>K</i>_ads is a small value

Determining Number of Active Sites and TOF for the High-Temperature Water Gas Shift Reaction by Iron Oxide-Based Catalysts

This study demonstrates, with C¹⁶O₂/C¹⁸O₂ isotope switch and H₂-TPR experiments, for the first time that (<i>i</i>) the high-temperature water–gas shift (HT-WGS) reaction by copper–chromium-iron oxide catalysts follows a redox mechanism dominated by the surface layer, (<i>ii</i>) the number of catalytic active sites can be quantified by the isotopic switch, and (<i>iii</i>) the turnover frequency (TOF) can be determined from knowledge of the number of sites. The quantitative TOF values reveal that chromium is only a textural promoter, whereas copper is a chemical promoter

Activation of Surface ReO_<i>x</i> Sites on Al₂O₃ Catalysts for Olefin Metathesis

The nature of activated surface ReO_<i>x</i> sites and surface reaction intermediates for supported ReO_<i>x</i>/Al₂O₃ catalysts during propylene self-metathesis were systematically investigated for the first time using in situ spectroscopy (Raman, UV–vis, XAS (XANES/EXAFS) and IR). In situ Raman spectroscopy reveals that olefins selectively interact with the surface dioxo ReO₄ sites anchored at acidic alumina hydroxyls. In situ UV–vis indicates that surface Re⁵⁺ and some Re⁶⁺ sites form, and in situ XAS indicates a reduction in the number of ReO bond character in the propylene self-metathesis reaction environment, especially as the temperature is increased. The appearance of oxygenated products during propylene activation supports the conclusion that catalyst activation involves removal of oxygen from the surface rhenia sites (pseudo-Wittig mechanism). Isotopic CD₃CDCD₂ → CH₃CHCH₂ switch experiments demonstrate the presence of surface ReCD₂ and ReCDCD₃ reaction intermediates, with the surface ReCD₂ species being the most abundant reaction intermediate. In situ IR spectroscopy indicates the presence of significant surface propylene π complexes on alumina and rhenia sites of the catalyst, which complicates analysis of surface reaction intermediates during propylene self-metathesis

Nature of WO_<i>x</i> Sites on SiO₂ and Their Molecular Structure–Reactivity/Selectivity Relationships for Propylene Metathesis

Supported WO_<i>x</i>/SiO₂ catalysts were investigated for propylene metathesis as a function of tungsten oxide loading and temperature. The catalysts were synthesized by incipient-wetness impregnation of an aqueous ammonium metatungstate solution onto the silica support and calcined at elevated temperatures to form the supported tungsten oxide phase. <i>In situ</i> Raman spectroscopy under dehydrated conditions revealed that below 8% WO_<i>x</i>/SiO₂, only surface WO_<i>x</i> sites are present on the silica support: dioxo (O)₂WO₂ and mono-oxo OWO₄. The <i>in situ</i> XANES analysis showed that dioxo surface WO₄ sites were the dominant surface WO_<i>x</i> sites on SiO₂ (>90%). The isolated nature of the surface WO_<i>x</i> sites was confirmed with <i>in situ</i> UV–vis spectroscopy. The surface WO_<i>x</i> sites are activated by exposure to propylene at elevated temperature that removes oxygen from these sites. The activation process produces a highly active surface WO_<i>x</i> site that can perform olefin metathesis at ∼150–250 °C. For 8% WO_<i>x</i>/SiO₂ and higher tungsten oxide loading, crystalline WO₃ nanoparticles (NPs) are also present, and their amount increases with greater tungsten oxide loading. WO₃ NPs, however, are not active for propylene metathesis. The acid character of the surface WO_<i>x</i> sites (Lewis) and WO₃ NPs (Brønsted) is responsible for formation of undesirable reaction products (C₄–C₆ alkanes and dimerization of C₂⁼ to C₄⁼). This study represents the <i>first time</i> that molecular level structure–activity/selectivity relationships have been established for propylene metathesis by conventionally impregnated supported WO_<i>x</i>/SiO₂ catalysts

Nature of Catalytic Active Sites Present on the Surface of Advanced Bulk Tantalum Mixed Oxide Photocatalysts

The most active photocatalyst system for water splitting under ultraviolet (UV) irradiation (270 nm) is the promoted 0.2% NiO/NaTaO₃:2% La photocatalyst with an optimized photonic efficiency of 56%, but fundamental issues about the nature of the surface catalytic active sites and their involvement in the photocatalytic process still need to be clarified. This is the first study to apply cutting-edge surface spectroscopic analyses to determine the surface nature of tantalum mixed oxide photocatalysts. Surface analysis with high-resolution X-ray photoelectron spectroscopy (1–3 nm) and high-sensitivity low-energy ion scattering spectroscopy (0.3 nm) indicates that the NiO and La₂O₃ promoters are concentrated in the surface region of the bulk NaTaO₃ phase. The NiO is concentrated on the NaTaO₃ outermost surface layers, while La₂O₃ is distributed throughout the NaTaO₃ surface region (1–3 nm). Raman and UV–vis spectroscopy revealed that the bulk molecular and electronic structures, respectively, of NaTaO₃ were not modified by the addition of the La₂O₃ and NiO promoters, with La₂O₃ resulting in a slightly more ordered structure. Photoluminescence spectroscopy reveals that the addition of La₂O₃ and NiO produces a greater number of electron traps resulting in the suppression of the recombination of excited electrons and holes. In contrast to earlier reports, La₂O₃ is only a textural promoter (increasing the BET surface area by ∼7-fold by stabilizing smaller NaTaO₃ particles) and causes an ∼3-fold decrease in the specific photocatalytic TOR_s (micromoles of H₂ per square meter per hour) rate because surface La₂O₃ blocks exposed catalytic active NaTaO₃ sites. The NiO promoter was found to be a potent electronic promoter that enhances the NaTaO₃ surface-normalized TOR_s by a factor of ∼10–50 and turnover frequency by a factor of ∼10. The level of NiO promotion is the same in the absence and presence of La₂O₃, demonstrating that there is no promotional synergistic interaction between the NiO and La₂O₃ promoters. This study demonstrates the important contributions of the photocatalyst surface properties to the fundamental molecular/electronic structure–photoactivity relationships of promoted NaTaO₃ photocatalysts that were previously not appreciated in the literature

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

Fundamental Bulk/Surface Structure–Photoactivity Relationships of Supported (Rh_2–<i>y</i>Cr_<i>y</i>O₃)/GaN Photocatalysts

The supported (Rh_2–<i>y</i>Cr_<i>y</i>O₃)/GaN photocatalyst was examined as a model nitride photocatalyst system to assist in the development of fundamental structure–photoactivity relationships for UV activated water splitting. Surface characterization of the outermost surface layers by high-sensitivity low energy ion scattering (HS-LEIS) and high-resolution X-ray photoelectron spectroscopy (HR-XPS) revealed that the GaN support consists of a GaO_<i>x</i> outermost surface layer on a thin film of GaO_<i>x</i>N_<i>y</i>. HR-XPS demonstrates that the supported (Rh_2–<i>y</i>Cr_<i>y</i>O₃) mixed oxide nanoparticles (NPs) consist of Cr³⁺ and Rh³⁺ cations that are surface enriched for (Rh_2–<i>y</i>Cr_<i>y</i>O₃)/GaN. Raman and UV–vis spectroscopy show that the bulk molecular and electronic structures, respectively, of the GaN support are not perturbed by the deposition of the (Rh_2–<i>y</i>Cr_<i>y</i>O₃) NPs. The function of the GaN bulk lattice is to generate photoexcited electrons/holes, with the electrons harnessed by the surface Rh³⁺ sites for evolution of H₂ and the holes trapped at the Ga oxide/oxynitride surface sites for splitting of water and evolving O₂