Synthesis of Na-Stabilized Nonporous t-ZrO2 Supports and Pt/t-ZrO2 Catalysts and Application to Water-Gas-Shift Reaction

Embedding ruthenium nanoparticles in the shell layer of titanium zirconium oxide hollow spheres to catalyze the degradation of alkali lignin under mild condition

To catalyze the degradation of lignin in refractory wastewater efficiently, a new nanocomposite with Ru nanoparticles embedded on the surface of TiZrO₄ hollow spheres was fabricated with three method a "sol-gel + calcination + vacuum-impregnation" template method, and the unique binary composition of TiZrO₄/Ru prevented the aggregation of Ru and keep its high activity. During 3-h catalytic-oxidation at 160 °C and 2.0 MPa O₂, 98% alkali lignin was degraded and 70% organic carbon was mineralized with the catalysis of TiZrO₄/Ru, while the values were only 50% and 25% without analysts. The catalyst increased the catalytic-oxidation rate constant k₁ (h^-1) of alkali lignin from 0.282 h^-1 to 1.175 h^-1 because of high-efficiency hydroxyl radical production, as determined by EPR. LC-OCD showed that the catalyst decomposed alkali lignin with molecular weight 1-2 kDa to small molecules. Butyl acetate was the main intermediate product, which should be derived from the auto synthesis of butanol and acetic acid. In addition to high conversion efficiency, the catalyst had good stability with 95% capability after five cycles. In real biogas slurry treatment, an increase of biochemical to COD ratio from 0.28 to 0.51, with obvious decoloration, indicated TiZrO₄/Ru enhanced the biodegradability of the refractory wastewater significantly.

Addition of Sodium Additives for Improved Performance of Water-Gas Shift Reaction over Ni-Based Catalysts

The effect of Na loading on water-gas shift reaction (WGSR) activity of Ni@TiO _x -XNa (X = 0, 0.5, 1, 2, and 5 wt %) catalysts has been investigated. Herein, we report sodium-modified Ni@TiO _x catalysts (denoted as Ni@TiO _x -XNa) derived from Ni₃Ti₁-layered double hydroxide (Ni₃Ti₁-LDH) precursor. The optimized Ni@TiO _x -1Na catalyst exhibits enhanced catalytic performance toward WGSR at relatively low temperature and reaches an equilibrium CO conversion at 300 °C, which is much superior to those for most of the reported Ni-based catalysts. The H₂-temperature-programmed reduction (H₂-TPR) result demonstrates that the Ni@TiO _x -1Na catalyst has a stronger metal-support interaction (MSI) than the sodium-free Ni@TiO _x catalyst. The presence of stronger MSI significantly facilitates the electron transfer from TiO _x support to the interfacial Ni atoms to modulate the electronic structure of Ni atoms (a sharp increase in Ni^δ- species), inducing the generation of more surface sites (O_v-Ti³⁺) accompanied by more interfacial sites (Ni^δ--O_v-Ti³⁺), revealed by X-ray photoelectron spectroscopy (XPS). The Ni^δ--O_v-Ti³⁺ interfacial sites serve as dual-active sites for WGSR. The increase in the dual-active sites accounts for improvement in the catalytic performance of WGSR. With the tunable Ni-TiO _x interaction, a feasible strategy in creating active sites by adding low-cost sodium addictive has been developed.

Platinum Based Catalysts in the Water Gas Shift Reaction: Recent Advances

The water gas shift (WGS) is an equilibrium exothermic reaction, whose corresponding industrial process is normally carried out in two adiabatic stages, to overcome the thermodynamic and kinetic limitations. The high temperature stage makes use of iron/chromium-based catalysts, while the low temperature stage employs copper/zinc-based catalysts. Nevertheless, both these systems have several problems, mainly dealing with safety issues and process efficiency. Accordingly, in the last decade abundant researches have been focused on the study of alternative catalytic systems. The best performances have been obtained with noble metal-based catalysts, among which, platinum-based formulations showed a good compromise between performance and ease of preparation. These catalytic systems are extremely attractive, as they have numerous advantages, including the feasibility of intermediate temperature (250–400 °C) applications, the absence of pyrophoricity, and the high activity even at low loadings. The particle size plays a crucial role in determining their catalytic activity, enhancing the performance of the nanometric catalytic systems: the best activity and stability was reported for particle sizes < 1.7 nm. Moreover the optimal Pt loading seems to be located near 1 wt%, as well as the optimal Pt coverage was identified in 0.25 ML. Kinetics and mechanisms studies highlighted the low energy activation of Pt/Mo2C-based catalytic systems (Ea of 38 kJ·mol−1), the associative mechanism is the most encountered on the investigated studies. This review focuses on a selection of recent published articles, related to the preparation and use of unstructured platinum-based catalysts in water gas shift reaction, and is organized in five main sections: comparative studies, kinetics, reaction mechanisms, sour WGS and electrochemical promotion. Each section is divided in paragraphs, at the end of the section a summary and a summary table are provided.

A systematic theoretical study of the water gas shift reaction on the Pt/ZrO2 interface and Pt(111) face: key role of a potassium additive

K can enhance the activity of the WGSR on the Pt₄₀/ZrO₂ model by reducing both the H₂O and COOH dissociation barriers.

Design of catalysts comprising a nickel core and ceria shell for hydrogen production from plastic waste gasification: an integrated test for anti-coking and catalytic performance

A novel core–shell catalyst with high coking resistance ability was applied for hydrogen production from plastic waste.

Morphology-Oriented ZrO2-Supported Vanadium Oxide for the NH3-SCR Process: Importance of Structural and Textural Properties

ZrO₂ supports, with diverse morphologies (hollow sphere, star, rod, mesoporous), were produced using hydrothermal and evaporation-induced self-assembly methods. Zirconia-supported vanadium oxide catalysts were prepared by wet impregnation and used for the low-temperature selective catalytic reduction (SCR) of NO with ammonia. Characterization of catalysts includes N₂ physisorption, elementary analysis, X-ray diffraction, high-resolution transmission electron microscopy, X-ray photoelectron spectroscopy, temperature-programmed reduction by H₂, and temperature-programmed desorption of NH₃. Significant differences in terms of activity are measured. 3 wt % V₂O₅ supported on mesoporous ZrO₂ (V/MZ) presents excellent N₂ yields (>90%, in the 200-400 °C interval), with a wide operating temperature window (NO conversion > 95%, in the 225-425 °C interval), and less interesting performances were obtained when vanadium oxide is supported over stars, hollow spheres, and rods. Surface characterization showed a content of tetravalent vanadium ion, when supported, decreasing in the order of mesoporous > hollow sphere > star > rod. This order is in perfect agreement with the order of performance of the catalyst in the NH₃-SCR reaction. The impact of tetravalent ion's presence on the surface is confirmed by diffuse reflectance infrared Fourier transform spectroscopy analysis, Brønsted acid sites generated on the surface, and the V⁴⁺-OH species involved in the reaction. The production of more important nitrite species over the tetragonal supported vanadium oxide catalyst could be another reason for the excellent NH₃-SCR performance displayed by the V/MZ catalyst. When supported over monoclinic zirconia, like vanadium oxide over star-type morphology, the adsorbed NH₃ species (NH₄⁺ and coordinated NH₃) reacted with NO _x adsorption species (nitrate) to form ammonium nitrate. Ammonium nitrate can be decomposed to N₂ and N₂O (or NO₂). Thus, NO conversion curves and N₂ yield curves over tetragonal zirconia (MZ) at lower temperature were ahead of those over V/star ZrO₂ because of the higher V⁴⁺ surface content and more active B acid sites associated with an easy formation of the nitrito intermediate.

Engineering Stable Surface Oxygen Vacancies on ZrO2 by Hydrogen-Etching Technology: An Efficient Support of Gold Catalysts for Water-Gas Shift Reaction

The surface structure of supports is crucial to fabricate efficient supported catalysts for water-gas shift (WGS). Here, hardly reducible ZrO₂ was etched with hydrogen (H), aiming to modify surface structures with sufficient stable oxygen vacancies. After deposition of gold species, the obtained khaki ZrO₂-H notably improved WGS catalytic activities and stabilities in comparison to the traditional white ZrO₂. The characterization results and quantitative analysis indicate that sufficient surface oxygen vacancies of ZrO₂-H support give rise to more metallic Au⁰ species and higher microstrain, which all boost WGS catalytic activities. Furthermore, optoelectronic properties were successfully used to correlate with their WGS thermocatalytic activities, and then a modified electron flow process was proposed to understand the WGS pathway. For one thing, the introduction of surface oxygen vacancies narrowed the band gap of ZrO₂ and decreased the Ohmic barrier, which facilitated the flow of "hot-electron". For another thing, the conduction band electrons can be easily trapped by oxygen vacancies of ZrO₂ supports, and then these trapped electrons immediately take part in reduction of H₂O to H₂. Thus, the electron recombination was suppressed and the WGS catalytic activity was improved. It is worth extending H₂-etching technology to improve other thermocatalytic reactions.

Ni-Sn-Supported ZrO2 Catalysts Modified by Indium for Selective CO2 Hydrogenation to Methanol

Ni and NiSn supported on zirconia (ZrO₂) and on indium (In)-incorporated zirconia (InZrO₂) catalysts were prepared by a wet chemical reduction route and tested for hydrogenation of CO₂ to methanol in a fixed-bed isothermal flow reactor at 250 °C. The mono-metallic Ni (5%Ni/ZrO₂) catalysts showed a very high selectivity for methane (99%) during CO₂ hydrogenation. Introduction of Sn to this material with the following formulation 5Ni5Sn/ZrO₂ (5% Ni-5% Sn/ZrO₂) showed the rate of methanol formation to be 0.0417 μmol/(g_cat·s) with 54% selectivity. Furthermore, the combination NiSn supported on InZrO₂ (5Ni5Sn/10InZrO₂) exhibited a rate of methanol formation 10 times higher than that on 5Ni/ZrO₂ (0.1043 μmol/(g_cat·s)) with 99% selectivity for methanol. All of these catalysts were characterized by X-ray diffraction, high-resolution transmission electron microscopy (HRTEM), scanning transmission electron microscopy (STEM), X-ray photoelectron spectroscopy, CO₂-temperature-programmed desorption, and density functional theory (DFT) studies. Addition of Sn to Ni catalysts resulted in the formation of a NiSn alloy. The NiSn alloy particle size was kept in the range of 10-15 nm, which was evidenced by HRTEM study. DFT analysis was carried out to identify the surface composition as well as the structural location of each element on the surface in three compositions investigated, namely, Ni₂₈Sn₂₇, Ni₁₈Sn₃₇, and Ni₃₇Sn₁₈ bimetallic nanoclusters, and results were in agreement with the STEM and electron energy-loss spectroscopy results. Also, the introduction of "Sn" and "In" helped improve the reducibility of Ni oxide and the basic strength of catalysts. Considerable details of the catalytic and structural properties of the Ni, NiSn, and NiSnIn catalyst systems were elucidated. These observations were decisive for achieving a highly efficient formation rate of methanol via CO₂ by the H₂ reduction process with high methanol selectivity.

Mesoporous ZrO2 Nanoframes for Biomass Upgrading

The rational design and preparation of a high-performance catalyst for biomass upgrading are of great significance and remain a great challenge. In this work, mesoporous ZrO₂ nanoframe, hollow ring, sphere, and core-shell nanostructures have been developed through a surfactant-free route for upgrading biomass acids into liquid alkane fuels. The obtained ZrO₂ nanostructures possess well-defined hollow features, high surface areas, and mesopores. The diversity of the resultant ZrO₂ nanostructures should arise from the discrepant hydrolysis of two different ligands in zirconocene dichloride (Cp₂ZrCl₂) as the zirconium precursor. The time-dependent experiments indicate that Ostwald ripening and salt-crystal-template formation mechanisms should account for hollow spheres and nanoframes, respectively. Impressively, compared with the hollow sphere, commercial nanoparticle, and the ever-reported typical results, the ZrO₂ nanoframe-promoted Ni catalyst exhibits greatly enhanced catalytic activity in the upgrading of biomass acids to liquid alkane fuels, which should be ascribed to the hollow feature, large active surface area, highly dispersed Ni, and strong metal-support interactions arising from the structural advantages of nanoframes. The nanoframes also possess excellent solvothermal and thermal stability. Our findings here can be expected to offer new perspectives in material chemistry and ZrO₂-based catalytic and other applications.

Recent Advances in Atomic Metal Doping of Carbon-based Nanomaterials for Energy Conversion

Nanostructured metal-contained catalysts are one of the most widely used types of catalysts applied to facilitate some of sluggish electrochemical reactions. However, the high activity of these catalysts cannot be sustained over a variety of pH ranges. In an effort to develop highly active and stable metal-contained catalysts, various approaches have been pursued with an emphasis on metal particle size reduction and doping on carbon-based supports. These techniques enhances the metal-support interactions, originating from the chemical bonding effect between the metal dopants and carbon support and the associated interface, as well as the charge transfer between the atomic metal species and carbon framework. This provides an opportunity to tune the well-defined metal active centers and optimize their activity, selectivity and stability of this type of (electro)catalyst. Herein, recent advances in synthesis strategies, characterization and catalytic performance of single atom metal dopants on carbon-based nanomaterials are highlighted with attempts to understand the electronic structure and spatial arrangement of individual atoms as well as their interaction with the supports. Applications of these new materials in a wide range of potential electrocatalytic processes in renewable energy conversion systems are also discussed with emphasis on future directions in this active field of research.

Controllable design of natural gully-like TiO2–ZrO2 composites and their photocatalytic degradation and hydrogen production by water splitting

Gully-like TiO₂–ZrO₂ composites prepared using an instant centrifugation and one-step hydrolysis method exhibited good photocatalytic degradation and enhanced hydrogen evolution activity.

High Thermal Stability of La2O3- and CeO2-Stabilized Tetragonal ZrO2

Catalyst support materials of tetragonal ZrO2, stabilized by either La2O3 (La2O3-ZrO2) or CeO2 (CeO2-ZrO2), were synthesized under hydrothermal conditions at 200 °C with NH4OH or tetramethylammonium hydroxide as the mineralizer. From in situ synchrotron powder X-ray diffraction and small-angle X-ray scattering measurements, the calcined La2O3-ZrO2 and CeO2-ZrO2 supports were nonporous nanocrystallites that exhibited rectangular shapes with a thermal stability of up to 1000 °C in air. These supports had an average size of ∼ 10 nm and a surface area of 59-97 m(2)/g. The catalysts Pt/La2O3-ZrO2 and Pt/CeO2-ZrO2 were prepared by using atomic layer deposition with varying Pt loadings from 6.3 to 12.4 wt %. Monodispersed Pt nanoparticles of ∼ 3 nm were obtained for these catalysts. The incorporation of La2O3 and CeO2 into the t-ZrO2 structure did not affect the nature of the active sites for the Pt/ZrO2 catalysts for the water-gas shift reaction.

Hierarchically structured tetragonal zirconia as a promising support for robust Ni based catalysts for dry reforming of methane

This work presents a facile approach for preparing nanosheet-accumulating Laminaria japonica-like hierarchically structured ZrO₂ with tetragonal phase, which acts as excellent support for robust supported Ni catalyst towards dry reforming of methane.

SiO2-stabilized Ni/t-ZrO2 catalysts with ordered mesopores: one-pot synthesis and their superior catalytic performance in CO methanation

Ternary SiO₂-stabilized Ni/t-ZrO₂ catalysts with an ordered mesoporous structure were synthesized, which show excellent low temperature activity and thermal stability.

Highly transparent porous ZrO2 thin films: fabrication and optical properties

Porous ZrO₂ thin films that are highly transparent to visible and infrared light were fabricated via a simple sol–gel dip-coating method, and have promising potential applications in solar cells as a high-temperature-resistant insulating layer.