Towards highly active Pd/CeO2 for alkene hydrogenation by tuning Pd dispersion and surface properties of the catalysts

Utilisation of in situ formed cyano-bridged coordination polymers as precursors of supported Ir-Ni alloy nanoparticles with precisely controlled compositions and sizes

Ir-Ni alloys supported on SiO2 have been reported to show high catalytic activity for styrene hydrogenation; however, precise control of compositions and sizes of the Ir-Ni alloys is difficult when conventional metal salts are used as precursors. Furthermore, the concomitant formation of unalloyed Ni nanoparticles disturbs quantitative discussion about Ir-Ni alloy compositions. We report herein a preparation method of Ir-Ni alloys with precisely controlled compositions on SiO2 using Ni(NO3)2 and an Ir complex possessing CN- ligands, [Ir(CN)6]3- or [Ir(ppy)2(CN)2]- (ppy = 2-phenylpyridine), as precursors. The in situ formation of cyano-bridged coordination polymers involving Ir and Ni promotes the formation of Ir-Ni alloys, whose compositions are virtually the same as expected from the amounts of Ir and Ni used for the preparation, after heat treatment under H2. The use of [Ir(ppy)2(CN)2]- as the precursor resulted in the formation of smaller Ir-Ni alloy particles than those with [Ir(CN)6]3- related to the structures of the formed coordination polymers.

Inverse ceria-nickel catalyst for enhanced C-O bond hydrogenolysis of biomass and polyether

Regulating interfacial electronic structure of oxide-metal composite catalyst for the selective transformation of biomass or plastic waste into high-value chemicals through specific C-O bond scission is still challenging due to the presence of multiple reducible bonds and low catalytic activity. Herein, we find that the inverse catalyst of 4CeOx/Ni can efficiently transform various lignocellulose derivatives and polyether into the corresponding value-added hydroxyl-containing chemicals with activity enhancement (up to 36.5-fold increase in rate) compared to the conventional metal/oxide supported catalyst. In situ experiments and theoretical calculations reveal the electron-rich interfacial Ce and Ni species are responsible for the selective adsorption of C-O bond and efficient generation of Hδ- species, respectively, which synergistic facilitate cleavage of C-O bond and subsequent hydrogenation. This work advances the fundamental understanding of interfacial electronic interaction over inverse catalyst and provides a promising catalyst design strategy for efficient transformation of C-O bond.

Tuning Particle Sizes and Active Sites of Ni/CeO2 Catalysts and Their Influence on Maleic Anhydride Hydrogenation

Supported metal catalysts are widely used in industrial processes, and the particle size of the active metal plays a key role in determining the catalytic activity. Herein, CeO₂-supported Ni catalysts with different Ni loading and particle size were prepared by the impregnation method, and the hydrogenation performance of maleic anhydride (MA) over the Ni/CeO₂ catalysts was investigated deeply. It was found that changes in Ni loading causes changes in metal particle size and active sites, which significantly affected the conversion and selectivity of MAH reaction. The conversion of MA reached the maximum at about 17.5 Ni loading compared with other contents of Ni loading because of its proper particle size and active sites. In addition, the effects of Ni grain size, surface oxygen vacancy, and Ni–CeO₂ interaction on MAH were investigated in detail, and the possible mechanism for MAH over Ni/CeO₂ catalysts was deduced. This work greatly deepens the fundamental understanding of Ni loading and size regimes over Ni/CeO₂ catalysts for the hydrogenation of MA and provides a theoretical and experimental basis for the preparation of high-activity catalysts for MAH.

Crystal-Plane and Shape Influences of Nanoscale CeO2 on the Activity of Ni/CeO2 Catalysts for Maleic Anhydride Hydrogenation

Through use of the hydrothermal technique, various shaped CeO₂ supports, such as nanocubes (CeO₂-C), nanorods (CeO₂-R), and nanoparticles (CeO₂-P), were synthesized and employed for supporting Ni species as catalysts for a maleic anhydride hydrogenation (MAH) reaction. The achievements of this characterization illustrate that Ni atoms are capable of being incorporated into crystal lattices and can occupy the vacant sites on the CeO₂ surface, which leads to an enhancement of oxygen vacancies. The results of the MAH reaction show that the morphology and shape of CeO₂ play an important role in the catalytic performance of the MAH reaction. The catalyst for the rod-like CeO₂-R obtains a higher catalytic activity than the other two catalysts. It can be concluded that the higher catalytic performances of rod-like CeO₂-R sample should be attributed to the higher dispersion of Ni particles, stronger support-metal interaction, more oxygen vacancies, and the lattice oxygen mobility. The research on the performances of morphology-dependent Ni/CeO₂ catalysts as well as the relative reaction strategy of MAH will be remarkably advantageous for developing novel catalysts for MA hydrogenation.

Reduction-oxidation series coupling degradation of chlorophenols in Pd-Catalytic Electro-Fenton system

Organochlorine pesticides are widespread in soils, sediments and even in groundwater, causing great concern to human health because of its toxicity and carcinogenic effects. The remarkable mineralization and lowered toxicity are particularly important during the removal of organochlorine pesticides. In this study, Pd/CeO₂ was prepared and employed as a bifunctional catalyst, to construct the reduction-oxidation series coupling Electro-Fenton (EF) system. The removal of chlorophenols (CPs) reached over 95% within 10 min at pH 3.0 and a current density of 25 mA/cm² in Pd/CeO₂-EF system. The second-order rate constant of CPs degradation was 10.28 L mmol^-1min^-1 in Pd/CeO₂-EF system, which was 29 times as fast as the sum of electrolysis with Pd/CeO₂ (0.24 L mmol^-1min^-1) and EF (0.11 L mmol^-1min^-1). Dehydrochlorination by Pd [H] contributed to the removal of CPs in Pd/CeO₂-EF system. The generated reactive oxygen species, mainly OH was also confirmed by ESR to contribute to the removal of CPs. The reduction-oxidation series coupling degradation of CPs in Pd/CeO₂-EF system increased the TOC removal to 70% in 360 min. The analysis of intermediate products further revealed the reductive and oxidative products in Pd/CeO₂-EF. Moreover, the system of Pd/CeO₂-EF exhibited an excellent performance treatment for CPs in actual groundwater. This study provides a new stratagem to eliminate organochlorine pesticides in groundwater environments rapidly and thoroughly.

Size-Controlled Synthesis of Pd Nanocatalysts on Defect-Engineered CeO2 for CO2 Hydrogenation

The size effects of metal catalysts have been widely investigated to optimize their catalytic activity and selectivity. However, the size-controllable synthesis of uniform supported metal nanoparticles without surfactants and/or additives remains a great challenge. Herein, we developed a green, surfactant-free, and universal strategy to tailor the sizes of uniform Pd nanoparticles on metal oxides by an electroless chemical deposition method via defect engineering of supports. The nucleation and growth mechanism suggest a strong electrostatic interaction between the Pd precursor and low-defective CeO₂ and a weak reducing capacity for low-defective CeO₂, resulting in small Pd nanoparticles. Conversely, large Pd nanoparticles were formed on a highly defective CeO₂ surface. Combined with various ex situ and in situ characterizations, a higher intrinsic activity of Pd for the CO₂-to-CO hydrogenation was found on large Pd nanoparticles with higher electron density owing to their stronger H₂ dissociation ability and H-spillover effects, as well as the larger number of oxygen vacancies generated in situ for CO₂ activation under hydrogenation conditions.

Tunable bimetallic Au-Pd@CeO2 for semihydrogenation of phenylacetylene by ammonia borane

The fabrication of a bimetallic core and ceria shell nanostructure is considered a promising way to promote catalytic performance and stability. Here, we report an Au-Pd@CeO2 core-shell structure with a tunable Au/Pd ratio through a self-assembly autoredox reaction approach. This process involves the sequence reduction of Au and Pd precursors and then self-assembly of CeO2 nanoparticles to encapsulate the noble metal core. The as-obtained samples exhibit excellent activity and selectivity towards the ammonia borane initiated hydrogenation of phenylacetylene with an enhanced stability owing to the protection from outside CeO2 nanoparticles. Through the construction of an Au-Pd bimetallic structure, an electron modification of Pd due to charge transfer between Au and Pd results in an enhanced catalytic performance. Such a strategy is promising for the synthesis of other bimetallic noble core and ceria shell structures for further applications.

Synergistic catalysis between nano-Ni and nano semiconductor CeO2 of Ni hybrid nanostructured catalysts for highly efficient selective hydrogenation

The existence of strong metal–support interaction between Ni and CeO₂ supported on carrier can enhance the structural and electronic properties.

Stability investigation of a high number density Pt1/Fe2O3 single-atom catalyst under different gas environments by HAADF-STEM

Catalysis by supported single metal atoms has demonstrated tremendous potential for practical applications due to their unique catalytic properties. Unless they are strongly anchored to the support surfaces, supported single atoms, however, are thermodynamically unstable, which poses a major obstacle for broad applications of single-atom catalysts (SACs). In order to develop strategies to improve the stability of SACs, we need to understand the intrinsic nature of the sintering processes of supported single metal atoms, especially under various gas environments that are relevant to important catalytic reactions. We report on the synthesis of high number density Pt₁/Fe₂O₃ SACs using a facial strong adsorption method and the study of the mobility of these supported Pt single atoms at 250 °C under various gas environments that are relevant to CO oxidation, water-gas shift, and hydrogenation reactions. Under the oxidative gas environment, Fe₂O₃ supported Pt single atoms are stable even at high temperatures. The presence of either CO or H₂ molecules in the gas environment, however, facilitates the movement of the Pt atoms. The strong interaction between CO and Pt weakens the binding between the Pt atoms and the support, facilitating the movement of the Pt single atoms. The dissociation of H₂ molecules on the Pt atoms and their subsequent interaction with the oxygen species of the support surfaces dislodge the surface oxygen anchored Pt atoms, resulting in the formation of Pt clusters. The addition of H₂O molecules to the CO or H₂ significantly accelerates the sintering of the Fe₂O₃ supported Pt single atoms. An anchoring-site determined sintering mechanism is further proposed, which is related to the metal-support interaction.

NiCo bimetallic nanoparticles encapsulated in graphite-like carbon layers as efficient and robust hydrogenation catalysts

The highest catalytic performance of Ni_0.5Co_0.5@NC catalysts can be attributed to their optimized electronic structure to facilitate the hydrogen activation.