A Highly Active “NiO-on-Au” Surface Architecture for CO Oxidation

Critical role of NiO support morphology for high activity of Au/NiO nanocatalysts in CO oxidation

The effect on catalytic behavior induced by different morphology of NiO supports has been investigated using the example of gold-catalyzed CO oxidation. Three NiO-supported nanogold consisting of nanogold deposited onto NiO nanorods (NiO-R), nanosheet (NiO-S), and nanodiscs (NiO-D) were prepared. Transmission electron microscopy(TEM)/Scanning transmission electron microscopy(STEM) investigations indicated that Au particles dominantly exposed Au(111) facets virtually independent of NiO architectures. Au/NiO-S displayed a normal Arrhenius-type behavior. Au/NiO-R and Au/NiO-D showed an atypical behavior, characterized by a U-shaped curve of activity vs. temperature, which is attributed to the carbonate accumulation on whose catalytically active sites. On Au/NiO-R, a stable CO-conversion rate of 1.78 molCO gAu -1 h-1 at 30°C was achieved, which is among the higher rates reported so far for supported Au-based systems. DRIFTS measurement identified Auδ+ species as crucial CO adsorption sites promoting CO oxidation, and the catalytic CO oxidation should obey Mars-van Krevelen (<200°C) and Eley-Rideal mechanism (>240°C).

Highly Dispersed Nickel Nanoparticles on Hierarchically Ordered Macroporous Al2O3 and Its Catalytic Performance for Steam Reforming of 1-Methyl Naphthalene

In this study, we investigate the effect of a hierarchically ordered macroporous structure of alumina support on the steam reforming of 1-methyl naphthalene with mesoporous alumina-supported nickel and potassium (xK/Ni–MeAl), and macroporous alumina-supported nickel and potassium (xK/Ni–MaAl) catalysts. Hierarchically ordered macroporosity in Al2O3 supports plays an important role in maintaining the high Ni dispersion through multiple interactions in Ni–K over AlO4 tetrahedra in alumina. This, in turn, improves the catalytic performance of steam reforming, including high gas yields, turnover frequency for hydrogen production, and 1-methyl naphthalene conversion. At high K content, the Ni active sites over xK/Ni–MeAl catalysts significantly decrease, resulting in almost zero steam reforming rate in the reaction test. Conversely, the potassium–alumina interaction in xK/Ni–MaAl catalysts not only diminishes the formation of the inactive nickel aluminate phase but also maintains the highly dispersed Ni active sites, resulting in a high steam reforming rate.

Synergy in Au-CuO Janus Structure for Catalytic Isopropanol Oxidative Dehydrogenation to Acetone

The controlled oxidation of alcohols to the corresponding ketones or aldehydes via selective cleavage of the β-C-H bond of alcohols under mild conditions still remains a significant challenge. Although the metal/oxide interface is highly active and selective, the interfacial sites fall far behind the demand, due to the large and thick support. Herein, we successfully develop a unique Au-CuO Janus structure (average particle size=3.8 nm) with an ultrathin CuO layer (0.5 nm thickness) via a bimetal in situ activation and separation strategy. The resulting Au-CuO interfacial sites prominently enhance isopropanol adsorption and decrease the energy barrier of β-C-H bond scission from 1.44 to 0.01 eV due to the strong affinity between the O atom of CuO and the H atom of isopropanol, compared with Au sites alone, thereby achieving ultrahigh acetone selectivity (99.3 %) over 1.1 wt % AuCu_0.75 /Al₂ O₃ at 100 °C and atmospheric pressure with 97.5 % isopropanol conversion. Furthermore, Au-CuO Janus structures supported on SiO₂ , TiO₂ or CeO₂ exhibit remarkable catalytic performance, and great promotion in activity and acetone selectivity is achieved as well for other reducible oxides derived from Fe, Co, Ni and Mn. This study should help to develop strategies for maximized interfacial site construction and structure optimization for efficient β-C-H bond activation.

Nano-Sized NiO Immobilized on Au/CNT for Benzyl Alcohol Oxidation: Influences of Hybrid Structure and Interface

Tiny gold nanoparticles were successfully anchored on carbon nanotubes (CNT) with NiO decoration by a two-step synthesis. Characterizations suggested that Ni species in an oxidative state preferred to be highly dispersed on CNT. During the synthesis, in situ reduction by NaBH₄ and thermal treatment in oxidation atmosphere were consequently carried out, causing the formation of Au-Ni-O_x interfaces and bimetal hybrid structure depending on the Ni/Au atomic ratios. With an appropriate Ni/Au atomic ratio of 8:1, Ni atoms migrated into the sub-layers of Au particles and induced the lattice contraction of Au particles, whilst a higher Ni/Au atomic ratio led to the accumulation of NiO fractions surrounding Au particles. Both contributed to the well-defined Au-Ni-O_x interface and accelerated reaction rates. Nickel species acted as structure promoters with essential Au-Ni-O_x hybrid structure as well as the active oxygen supplier, accounting for the enhanced activity for benzyl alcohol oxidation. However, the over-layer of unsaturated gold sites easily occured under a high Ni/Au ratio, resulting in a lower reaction rate. With an Au/Ni atomic ratio of 8:1, the specific rate of AuNi₈/CNT reached 185 μmol/g/s at only 50 °C in O₂ at ordinary pressure.

Cascade aldol condensation of an aldehyde via the aerobic oxidation of ethanol over an Au/NiO composite

Synthesis of liquid biofuels (C₁₁-C₁₃) from cellulosic ethanol is regarded as a promising and versatile protocol. In this study, oxide-supported nanogold catalysts exhibit good catalytic performance in ethanol conversion with cinnamaldehyde and finally give rise to the C₁₁-C₁₃ hydrocarbon. High selectivity (70%) for C₁₁-C₁₃ hydrocarbons is achieved over Au/NiO via a one-pot cascade reaction, viz. cross-aldol condensations in the presence of oxygen and base (K₂CO₃) and then full hydrodeoxygenation with hydrogen gas. EtOH-TPD and TGA analyses show that the ethanol is activated to acetaldehyde (CH₃CHO*) over the surface oxygen vacancies of the NiO support. The CH₃CHO* then reacts with cinnamaldehyde at the interfacial perimeter of the Au/NiO composite during the cascade reactions, as evidenced by comparison of the catalytic performance with that over another oxide-supported Au NP, chemo-adsorption investigations, and in situ infrared spectroscopy investigations. This work may provide new guidelines for designing efficient catalysts to convert bioethanol into biofuels with high energy density.

Adaptive kinetic Monte Carlo simulations of surface segregation in PdAu nanoparticles

Surface segregation in bimetallic nanoparticles (NPs) is critically important for their catalytic activity because the activity is largely determined by the surface composition. Little, however, is known about the atomic scale mechanisms and kinetics of surface segregation. One reason is that it is hard to resolve atomic rearrangements experimentally. It is also difficult to model surface segregation at the atomic scale because the atomic rearrangements can take place on time scales of seconds or minutes - much longer than can be modeled with molecular dynamics. Here we use the adaptive kinetic Monte Carlo (AKMC) method to model the segregation dynamics in PdAu NPs over experimentally relevant time scales, and reveal the origin of kinetic stability of the core@shell and random alloy NPs at the atomic level. Our focus on PdAu NPs is motivated by experimental work showing that both core@shell and random alloy PdAu NPs with diameters of less than 2 nm are stable, indicating that one of these structures must be metastable and kinetically trapped. Our simulations show that both the Au@Pd and the PdAu random alloy NPs are metastable and kinetically trapped below 400 K over time scales of hours. These AKMC simulations provide insight into the energy landscape of the two NP structures, and the diffusion mechanisms that lead to segregation. In the core-shell NP, surface segregation occurs primarily on the (100) facet through both a vacancy-mediated and a concerted mechanism. The system becomes kinetically trapped when all corner sites in the core of the NP are occupied by Pd atoms. Higher energy barriers are required for further segregation, so that the metastable NP has a partially alloyed shell. In contrast, surface segregation in the random alloy PdAu NP is suppressed because the random alloy NP has reduced strain as compared to the Au@Pd NP, and the segregation mechanisms in the alloy require more elastic energy for exchange of Pd and Au and between the surface and subsurface.

Synthesis of benzimidazolones by immobilized gold nanoparticles on chitosan extracted from shrimp shells supported on fibrous phosphosilicate

We demonstrate the synthesis of benzimidazolones from o-phenylenediamines and CO₂ in the presence of gold nanoparticles supported on a composite material based on microcrystalline chitosan from shrimp shells and fibrous phosphosilicate.

A Rational Solid-State Synthesis of Supported Au-Ni Bimetallic Nanoparticles with Enhanced Activity for Gas-Phase Selective Oxidation of Alcohols

A facile confined solid-state seed-mediated alloying strategy is applied for the rational synthesis of supported Au-Ni bimetallic nanoparticles (BMNPs). The method sequentially deposits nickel salts and AuNP seeds into the ordered array of extra-large mesopores (EP-FDU-12 support) followed by a high-temperature annealing process. The size, structure, and composition of the AuNi BMNPs can be well tuned by varying the AuNP seeds, annealing temperature, and feeding ratio of metal precursors. Kinetic studies and DFT calculations suggest that the introduction of the Ni component can significantly prompt the O₂ activation on AuNPs, which is critical for the selective alcohol oxidation using molecular O₂ as the oxidant. The optimal Au-Ni BMNP catalyst showed the highest turnover frequency (TOF) (59 000 h^-1, 240 °C) and highest space-time yield (STY) of benzyl aldehyde (BAD) productivity (9.23 kg·g_Au^-1·h^-1) in the gas-phase oxidation of benzyl alcohol (BA), which is at least about 5-fold higher than that of other supported Au catalysts.

Crystalline (Ni1-xCox)5TiO7 nanostructures grown in situ on a flexible metal substrate used towards efficient CO oxidation

Severe environmental contamination and the urgent demand for a clean atmosphere require an efficient and low-cost solution to address problems such as CO emission. In this study, we report the in situ integration of non-noble (Ni_1-xCo_x)₅TiO₇ nanostructures with different Co concentrations and tunable size on a flexible metal network support using a conventional PEO method; we further report their utilization for efficient CO oxidation. It was found that the Co/Ni ratios in the original electrolyte precursors directly result in the different size and morphology evolution. The (Ni_1-xCo_x)₅TiO₇ nanowire arrays with x = 0.16 exhibit the best performance towards CO catalytic oxidation along with a good catalytic stability. Further analysis using XPS indicates that the CO catalytic oxidation was mainly determined by the amount of defective oxygen, lattice oxygen and surface area. The single crystal nature, large surface area, excellent CO catalytic capability and strong substrate adhesion of the (Ni_1-xCo_x)₅TiO₇ nanostructures on a flexible metal substrate will open up more applications in CO oxidation ranging from processing autovehicle exhaust to chemical gas emissions in the industry.

Structural transformation of Pd-α-Fe2O3 and Pd-γ-Fe2O3 catalysts and application in the CO oxidation reaction

Pd-α-Fe₂O₃ and Pd-γ-Fe₂O₃ catalysts can be obtained by redox pretreatment and exhibit different reactive performances during CO oxidation.

Room Temperature CO Oxidation over Pt/MgFe2O4: A Stable Inverse Spinel Oxide Support for Preparing Highly Efficient Pt Catalyst

MgFe₂O₄ with inverse spinel structure is demonstrated to be an efficient support for constructing practical potential Pt catalyst (Pt/MgFe₂O₄). The resultant Pt/MgFe₂O₄ exhibits excellent catalytic behavior in CO oxidation under normal temperature and humidity. TOF calculated based on the content of Pt is 0.131 s^-1. The excellent performance of Pt/MgFe₂O₄ attributes to the presence of surface undercoordinated lattice oxygens on MgFe₂O₄ support. These oxygens could participate in the initial CO oxidation and then be recovered under O₂ conditions. Over this Pt/MgFe₂O₄ catalyst, CO catalytic oxidation should mainly follow a redox mechanism.

Multicomponent (Ce, Cu, Ni) oxides with cage and core-shell structures: tunable fabrication and enhanced CO oxidation activity

Solvothermal synthesis of Cu2O cubes from Cu(OAc)2 in ethanol provided templates for tunable formation of novel multicomponent composites: hollow CeO2-Cu2O (), core-shell NiO@Cu2O () and hollow CeO2-NiO-Cu2O (). Composites catalyze the oxidation of CO at a lower temperature than the parent Cu2O cubes.

Strong metal-support interaction in novel core-shell Au-CeO2 nanostructures induced by different pretreatment atmospheres and its influence on CO oxidation

Yolk-shell Au/CeO2 (Y-Au/CeO2) and encapsulated Au/CeO2 (E-Au/CeO2) nanocatalysts were prepared by using silica templates. A strong metal-support interaction (SMSI) in the Au/CeO2 nanostructures induced by different pretreatment atmospheres and its influence on CO oxidation were studied. E-Au/CeO2 pretreated in O2 had the best performance, followed by Y-Au/CeO2 pretreated in O2, Y-Au/CeO2 pretreated in H2, and E-Au/CeO2 pretreated in H2. The reasons for the different activities were discussed. There were two kinds of strong metal-support interactions (SMSI) between Au and CeO2 termed as R-SMSI (pretreated in reductive atmosphere) and O-SMSI (pretreated in oxidation atmosphere). Because of the smaller size of the Au and the larger contact area, both the R-SMSI and O-SMSI of E-Au/CeO2 were larger than those of Y-Au/CeO2. The O-SMSI was accompanied by the formation of cationic Au species that were beneficial to the enhancing of activity. As expected, the activity of E-Au/CeO2 pretreated in O2 with a Au size less than 5 nm was higher than that of Y-Au/CeO2 pretreated in O2 with 25 nm Au. However, it is surprisingly found that the activity of Y-Au/CeO2 pretreated in H2 with 25 nm Au was higher than that of E-Au/CeO2 pretreated in H2 with a Au size less than 5 nm. R-SMSI resulted in the formation of a AuCe alloy that had a negative effect on the activity. Compared with E-Au/CeO2 pretreated in H2, Y-Au/CeO2 pretreated in H2 exhibited a smaller relative content of the AuCe alloy, leading to a better activity of Y-Au/CeO2 pretreated in H2.

A highly efficient potassium-treated Au–Cu/Al2O3 catalyst for the preferential oxidation of carbon monoxide

A novel potassium-treated Au–Cu bimetallic nanoparticles as a highly active and selective catalyst for CO-PROX.

Efficient photocatalytic hydrogen evolution system by assembling earth abundant NixOy nanoclusters in cubic MCM-48 mesoporous materials

A MCM-48 mesoporous support containing NiO and Ni₂O₃ nanoclusters exhibit high activity for photocatalytic hydrogen production in comparison to NiO.

Size-controllable Ni5TiO7 nanowires as promising catalysts for CO oxidation

Ni₅TiO₇ nanowires with controllable sizes are synthesized using PEO method combined with impregnation and annealing at 1050^oC in air, with adjustment of different concentrations of impregnating solution to control the dimension of nanowires. The resulting nanowires are characterized in details using X-ray diffraction, scanning electron microscopy, transmission electron microscopy and energy dispersive X-ray analysis. In addition, the CO catalytic oxidation performance of the Ni₅TiO₇ nanowires is investigated using a fixed-bed quartz tubular reactor and an on-line gas chromatography system, indicating that the activity of this catalytic system for CO oxidation is a strong dependency upon the nanocrystal size.When the size of the Ni₅TiO₇ nanowires is induced from 4 μm to 50 nm, the corresponding maximum conversion temperature is lowered by ~100 ^oC.

Surface Segregation in Bimetallic Nanoparticles: A Critical Issue in Electrocatalyst Engineering

Bimetallic nanoparticles are a class of important electrocatalyst. They exhibit a synergistic effect that critically depends on the surface composition, which determines the surface properties and the adsorption/desorption behavior of the reactants and intermediates during catalysis. The surface composition can be varied, as nanoparticles are exposed to certain environments through surface segregation. Thermodynamically, this is caused by a difference in surface energy between the two metals. It may lead to the enrichment of one metal on the surface and the other in the core. The external conditions that influence the surface energy may lead to the variation of the thermodynamic steady state of the particle surface and, thus, offer a chance to vary the surface composition. In this review, the most recent and important progress in surface segregation of bimetallic nanoparticles and its impact in electrocatalysis are introduced. Typical segregation inducements and surface characterization techniques are discussed in detail. It is concluded that surface segregation is a critical issue when designing bimetallic catalysts. It is necessary to explore methods to control it and utilize it as a way towards producing robust, bimetallic electrocatalysts.

Synthesis of MCF-supported AuCo nanoparticle catalysts and the catalytic performance for the CO oxidation reaction

AuCo/MCF calcined at 500 °C for 1 h was found to produce the most active and stable catalyst for CO oxidation. In situ FTIR results of AuCo/MCF catalysts with different treatments.

Design of a meso-structured Pd/NiO catalyst for highly efficient low temperature CO oxidation under ambient conditions

A meso-structured Pd/NiO catalyst, fabricated through a controlled pyrolysis and in situ reduction protocol, showed much higher catalytic activity and stability for CO oxidation under ambient conditions.

Comparative studies of redox behaviors of Pt–Co/SiO2 and Au–Co/SiO2 catalysts and their activities in CO oxidation

An active CoO_x-on-Pt structure was prepared differently in comparison with a CoO_x-on-Au structure due to the different interaction of Co (CoO_x) with Pt and Au.