Understanding the Role of Oxygen Vacancies in the Water Gas Shift Reaction on Ceria-Supported Platinum Catalysts

Nanodispersed Mn3O4/γ-Al2O3 for NO2 Elimination at Room Temperature

Adsorption is an efficient method for atmospheric NO_x abatement under ambient conditions; however, traditional adsorbents suffer from limited adsorption capacity and byproduct formation. Developing a low-cost material with high capacity for atmospheric NO₂ elimination remains a challenge. Here, we synthesized a nanodispersed Mn₃O₄/γ-Al₂O₃ (Mn/Al) material that exhibits excellent ability to remove NO₂. The 10 wt % Mn/Al sample showed the highest removal capacity, with 247.6 mg_NO2/g_Mn/Al, which is superior to that of activated carbon (42.6 mg_NO2/g). There were no byproducts produced when Mn/Al was tested with ppb-level NO₂. The NO₂ abatement mechanism with Mn/Al is different from physisorption or chemisorption. NO₂ removal is mainly a catalytic process in air, during which surface hydroxyls and lattice oxygen are involved in the oxidation of NO₂ to nitrate. In contrast, a chemical reaction between Mn³⁺ and NO₂ is dominant in N₂, where Mn³⁺ is converted into Mn⁴⁺ and NO₂ is reduced to nitrite. Washing with deionized water is an effective and convenient method for the regeneration of saturated Mn/Al, and an 86% adsorption capacity was recovered after one washing. The results suggest that this low-cost Mn/Al material with easy preparation and regeneration is a promising candidate material for atmospheric NO₂ elimination.

Remarkable active-site dependent H2O promoting effect in CO oxidation

The interfacial sites of supported metal catalysts are often critical in determining their performance. Single-atom catalysts (SACs), with every atom contacted to the support, can maximize the number of interfacial sites. However, it is still an open question whether the single-atom sites possess similar catalytic properties to those of the interfacial sites of nanocatalysts. Herein, we report an active-site dependent catalytic performance on supported gold single atoms and nanoparticles (NPs), where CO oxidation on the single-atom sites is dramatically promoted by the presence of H₂O whereas on NPs’ interfacial sites the promoting effect is much weaker. The remarkable H₂O promoting effect makes the Au SAC two orders of magnitude more active than the commercial three-way catalyst. Theoretical studies reveal that the dramatic promoting effect of water on SACs originates from their unique local atomic structure and electronic properties that facilitate an efficient reaction channel of CO + OH.

The issue that whether single-atom sites possess similar catalytic properties to the interfacial sites of nanocatalysts remains unresolved. Here, the authors demonstrate a large H₂O promotional effect on CO oxidation over Au single-atom sites due to their unique local atomic structure and electronic properties.

CO2 Reduction by Hydrogen Pre-Reduced Acceptor-Doped Ceria

The reactivity of H₂ pre-reduced acceptor-doped ceria materials Gd_0.10 Ce_0.90 O_2-δ (GDC10) and Sm_0.15 Ce_0.85 O_2-δ (SDC15) was tested with respect to the reduction of CO₂ to CO in the context of the reverse water-gas shift reaction. It was demonstrated that not only oxygen vacancies, but also dissolved hydrogen is a reactive species for the reduction of CO₂ . Dissolved hydrogen must be considered upon discussion of the mechanism of the reverse water-gas shift reaction on ceria-derived materials apart from oxygen vacancies and formates. The reduction of CO₂ is preceded by the formation of carbonate species of different thermal stability and reactivity. The stability of these carbonates was directly demonstrated by in situ infrared spectroscopy and revealed the largely reversible nature of CO₂ ad- and desorption. In comparison to pre-reduced samples, decreased carbonate coverage is obtained after oxidative treatments of GDC10 and SDC15. No significant effect of the sample treatment (O₂ oxidation or H₂ reduction) on the surface carbonate stability was noticed. Mono-dentate carbonates and carboxylates appear to be more easily formed on pre-reduced (i. e. defective) samples. Ce⁴⁺ reduction to Ce³⁺ (by H₂ ) and re-oxidation to Ce⁴⁺ (by CO₂ ) on GDC10/SDC15 were directly monitored by infrared spectroscopic analysis of a distinct, IR-active electronic transition of Ce³⁺ . These results show the complex interplay of oxygen vacancy/dissolved hydrogen reactivity and surface chemical aspects in acceptor-doped ceria materials.

Direct Identification of Active Surface Species for the Water-Gas Shift Reaction on a Gold-Ceria Catalyst

The crucial role of the metal-oxide interface in the catalysts of the water-gas shift (WGS) reaction has been recognized, while the precise illustration of the intrinsic reaction at the interfacial site has scarcely been presented. Here, two kinds of gold-ceria catalysts with totally distinct gold species, <2 nm clusters and 3 to 4 nm particles, were synthesized as catalysts for the WGS reaction. We found that the gold cluster catalyst exhibited a superiority in reactivity compared to gold nanoparticles. With the aid of comprehensive in situ characterization techniques, the bridged -OH groups that formed on the surface oxygen vacancies of the ceria support are directly determined to be the sole active configuration among various surface hydroxyls in the gold-ceria catalysts. The isotopic tracing results further proved that the reaction between bridged surface -OH groups and CO molecules adsorbed on interfacial Au atoms contributes dominantly to the WGS reactivity. Thus, the abundant interfacial sites in gold clusters on the ceria surface induced superior reactivity compared to that of supported gold nanoparticles in catalyzing the WGS reaction. On the basis of direct and solid experimental evidence, we have obtained a very clear image of the surface reaction for the WGS reaction catalyzed by the gold-ceria catalyst.

The TiO2 topotactic transformation assisted trapping of an atomically dispersed Pt catalyst for low temperature CO oxidation

Atomically dispersed Pt catalysts are deposited on the rough surface of TiO₂, which is synthesized via topotactic transformation from a NH₄TiOF₃ mesocrystal.

Molecular-level understanding of reaction path optimization as a function of shape concerning the metal–support interaction effect of Co/CeO2 on water-gas shift catalysis

In this work, the active sites generated in hydrogen reduction and the reaction pathways for the water gas shift (WGS) reaction over Co/CeO₂ catalysts were studied by in situ XAS and XPS coupled with DFT+U calculations.

Application of modulation excitation-phase sensitive detection-DRIFTS for in situ/operando characterization of heterogeneous catalysts

A new more general method and guidelines for the implementation of modulation excitation-phase sensitive detection-diffuse reflectance Fourier transform spectroscopy (ME-PSD-DRIFTS).

Design, modelling, and application of a low void-volume in situ diffuse reflectance spectroscopic reaction cell for transient catalytic studies

A new low void-volume in situ reaction cell enables application of modulation excitation-phase sensitive detection-diffuse reflectance Fourier transform spectroscopy (ME-PSD-DRIFTS).

Insights into Interfacial Synergistic Catalysis over Ni@TiO2- x Catalyst toward Water-Gas Shift Reaction

The mechanism on interfacial synergistic catalysis for supported metal catalysts has long been explored and investigated in several important heterogeneous catalytic processes (e.g., water-gas shift (WGS) reaction). The modulation of metal-support interactions imposes a substantial influence on activity and selectivity of catalytic reaction, as a result of the geometric/electronic structure of interfacial sites. Although great efforts have validated the key role of interfacial sites in WGS over metal catalysts supported on reducible oxides, direct evidence at the atomic level is lacking and the mechanism of interfacial synergistic catalysis is still ambiguous. Herein, Ni nanoparticles supported on TiO_{2- x} (denoted as Ni@TiO_{2- x}) were fabricated via a structure topotactic transformation of NiTi-layered double hydroxide (NiTi-LDHs) precursor, which showed excellent catalytic performance for WGS reaction. In situ microscopy was carried out to reveal the partially encapsulated structure of Ni@TiO_{2- x} catalyst. A combination study including in situ and operando EXAFS, in situ DRIFTS spectra combined with TPSR measurements substantiates a new redox mechanism based on interfacial synergistic catalysis. Notably, interfacial Ni species (electron-enriched Ni^δ- site) participates in the dissociation of H₂O molecule to generate H₂, accompanied by the oxidation of Ni^δ--O _v-Ti³⁺ (O _v: oxygen vacancy) to Ni^δ+-O-Ti⁴⁺ structure. Density functional theory calculations further verify that the interfacial sites of Ni@TiO_{2- x} catalyst serve as the optimal active site with the lowest activation energy barrier (∼0.35 eV) for water dissociation. This work provides a fundamental understanding on interfacial synergistic catalysis toward WGS reaction, which is constructive for the rational design and fabrication of high activity heterogeneous catalysts.

The electron shuffle: Cerium influences samarium 4f orbital occupancy in heteronuclear Ce-Sm oxide clusters

The anion photoelectron (PE) spectra along with supporting results of density functional theory (DFT) calculations on SmO^-, SmCeO_y^-, and Sm₂O_y^- (y = 1, 2) are reported and compared to previous results on CeO^- [M. Ray et al., J. Chem. Phys. 142, 064305 (2015)] and Ce₂O_y^- (y = 1, 2) [J. O. Kafader et al., J. Chem. Phys. 145, 154306 (2016)]. Similar to the results on Ce_xO_y^- clusters, the PE spectra of SmO^-, SmCeO_y^-, and Sm₂O_y^- (y = 1, 2) all exhibit electronic transitions to the neutral ground state at approximately 1 eV e^-BE. The Sm centers in SmO and Sm₂O₂ neutrals can be described with the 4f⁵6s superconfiguration, which is analogous to CeO and Ce₂O₂ neutrals in which the Ce centers can be described with the 4f 6s superconfiguration (Z_Ce = Z_Sm - 4). The Sm center in CeSmO₂, in contrast, has a 4f⁶ occupancy, while the Ce center maintains the 4f 6s superconfiguration. The less oxidized Sm centers in both Sm₂O and SmCeO have 4f⁶ 6s occupancies. The 4f⁶ subshell occupancy results in relatively weak Sm-O bond strengths. If this extra 4f occupancy also occurs in bulk Sm-doped ceria, it may play a role in the enhanced O^2- ionic conductivity in Sm-doped ceria. Based on the results of DFT calculations, the heteronuclear Ce-Sm oxides have molecular orbitals that are distinctly localized Sm 4f, Sm 6s, Ce 4f, and Ce 6s orbitals. The relative intensity of two electronic bands in the PE spectrum of Sm₂O^- exhibits an unusual photon energy-dependence, and the PE spectrum of Sm₂O₂^- exhibits a photon energy-dependent continuum signal between two electronic transitions. Several explanations, including the high magnetic moment of these suboxide species and the presence of low-lying quasi-bound anion states, are considered.

Role of Support Oxygen Vacancies in the Gas Phase Hydrogenation of Furfural over Gold

Abstract

We have examined the role of support oxygen vacancies in the gas phase hydrogenation of furfural over Au/TiO₂ and Au/CeO₂ prepared by deposition–precipitation. Both catalysts exhibited a similar Au particle size distribution (1–6 nm) and mean (2.8–3.2 nm). Excess H₂ consumption during TPR is indicative of partial support reduction, which was confirmed by O₂ titration. Gold on CeO₂ with a higher redox potential exhibited a greater oxygen vacancy density. A lower furfural turnover frequency (TOF) was recorded over Au/CeO₂ than Au/TiO₂ and is linked to suppressed H₂ chemisorption capacity and strong –C=O interaction at oxygen vacancies that inhibited activity. Gold on non-reducible Al₂O₃ as benchmark exhibited greater H₂ uptake and delivered the highest furfural TOF. Full selectivity to the target furfuryl alcohol was achieved over Au/TiO₂ and Au/Al₂O₃ at 413 K and over Au/CeO₂ at 473 K with hydrogenolysis to 2-methylfuran at higher reaction temperature (523 K). A surface reaction mechanism is proposed to account for the activity/selectivity response.

Graphical Abstract

Selective Production of Toluene from Biomass-Derived Isoprene and Acrolein

Toluene is a basic chemical that is currently produced from petroleum resources. In this paper, we report a new route for the effective synthesis of toluene from isoprene and acrolein, two reactants readily available from biomass, through a simple two-step reaction. The process includes Diels-Alder cycloaddition of isoprene and acrolein in a Zn-containing ionic liquid at room temperature to produce methylcyclohex-3-enecarbaldehydes (MCHCAs) as intermediates, followed by M (M=Pt, Pd, Rh)/Al₂ O₃ -catalyzed consecutive dehydrogenation-decarbonylation of the MCHCAs at 573 K to generate toluene with an overall yield up to 90.7 %. Model reactions indicated that a synergistic inductive effect of the C=C double bond and the aldehyde group in MCHCA plays a key role in initiating the consecutive dehydrogenation-decarbonylation, and that methyl benzaldehydes are the key intermediates in the gas-phase transformation of MCHCAs. Microcalorimetric adsorption of CO on different catalysts showed that decarbonylation of the substrate occurs more likely on the strong adsorption sites. To the best of our knowledge, it is the first report of Pt/Al₂ O₃ -catalyzed consecutive dehydrogenation-decarbonylation of a given compound in one reactor. This work provides a highly efficient and environmental friendly route to toluene by utilizing two compounds that can be prepared from biomass.

Pressure Regulations on the Surface Properties of CeO2 Nanorods and Their Catalytic Activity for CO Oxidation and Nitrile Hydrolysis Reactions

Surface properties of nanoscale CeO2 catalysts in terms of the surface Ce(3+) fraction and concentration of oxygen vacancy can affect their catalytic performance significantly. Continual adjustment on surface properties of CeO2 with the morphological preservation has not been realized by synthetic methods. The revisited studies show that surface properties of CeO2 nanorods can be effectively regulated by synthetic pressures while the rodlike morphology is well-preserved. Such phenomena are ascribed to the contact possibility between Ce(3+) species and dissolved O2, which is balanced by the rapidly increased and gradually saturated dissolution/recrystallization rate of Ce(OH)3 and linearly increased concentration of dissolved O2 with the increase of total air pressure or partial pressure of O2. Surface-property-dependent catalytic activity of CeO2 nanorods synthesized under various pressures was also demonstrated in two benchmark reactions-catalytic oxidation of CO and hydrolysis of nitrile. Such a finding of the pressure regulation on the reducible metal oxides provides an effective approach to rationally design novel catalysts for specific reactions, where ceria are supports, promoters, or actives.

Electrochemical reactivity and proton transport mechanisms in nanostructured ceria

Electrochemical reactivity and ionic transport at the nanoscale are essential in many energy applications. In this study, time-resolved Kelvin probe force microscopy (tr-KPFM) is utilized for surface potential mapping of nanostructured ceria, in both space and time domains. The fundamental mechanisms of proton injection and transport are studied as a function of environmental conditions and the presence or absence of triple phase boundaries. Finite element modeling is used to extract physical parameters from the experimental data, allowing not only quantification of the observed processes, but also decoupling of their contributions to the measured signal. The constructed phase diagrams of the parameters demonstrate a thermally activated proton injection reaction at the triple phase boundary, and two transport processes that are responsible for the low-temperature proton conductivity of nanostructured ceria.

A cooperative pathway for water activation using a bimetallic Pt0–CuI system

Cooperative activation of a water molecule with a bimetallic platinum(0)–copper(i) system results in formation of copper(i) hydroxide and a platinum hydride species. The latter is stable under acidic and neutral conditions but undergoes cyclometalation in the presence of pyridine.

Accelerated synthesis of MnO2 nanocomposites by acid-free hydrothermal route for catalytic soot combustion

MnO₂ nanocomposites with porous structure were successfully synthesized by a facile hydrothermal route from KMnO₄ without the addition of any acid.

Enhancing the catalytic performance of cobalt oxide by doping on ceria in the high temperature water–gas shift reaction

The high temperature water–gas shift (HT-WGS) reaction was performed using a Co–CeO₂ catalyst, prepared through a co-precipitation method.