Improved activity and stability of Pd@CeO2 core–shell catalysts hybridized with multi-walled carbon nanotubes in the water gas shift reaction

What Is to Be Expected from Heterogeneous Catalysis in the Pipeline to Circular Economy?

Modern society requires a change in the philosophy of doing science, which faces the enormous challenge of being compatible with the new sustainability principles. Inorganic chemistry holds the keys to accelerate the transition given that most chemical processes or technology devices rely on the use or integration of inorganic materials. In particular, heterogeneous catalysis has a central role in promoting the transition from a linear economy to a circular one. To accomplish this, it is imperative that the modern schemes for catalysis will adopt a holistic approach based on sensible choice of raw materials, reliance on clean energy inputs and establishment of a robust framework of resource use and recovery. Some of these concepts are analysed here and discussed in Ref. [to a few selected examples.

Biomass-Derived Carbon Materials in Heterogeneous Catalysis: A Step towards Sustainable Future

Biomass-derived carbons are emerging materials with a wide range of catalytic properties, such as large surface area and porosity, which make them ideal candidates to be used as heterogeneous catalysts and catalytic supports. Their unique physical and chemical properties, such as their tunable surface, chemical inertness, and hydrophobicity, along with being environmentally friendly and cost effective, give them an edge over other catalysts. The biomass-derived carbon materials are compatible with a wide range of reactions including organic transformations, electrocatalytic reactions, and photocatalytic reactions. This review discusses the uses of materials produced from biomass in the realm of heterogeneous catalysis, highlighting the different types of carbon materials derived from biomass that are potential catalysts, and the importance and unique properties of heterogeneous catalysts with different preparation methods are summarized. Furthermore, this review article presents the relevant work carried out in recent years where unique biomass-derived materials are used as heterogeneous catalysts and their contribution to the field of catalysis. The challenges and potential prospects of heterogeneous catalysis are also discussed.

Comparative Study of Moisture-Treated Pd@CeO2/Al2O3 and Pd/CeO2/Al2O3 Catalysts for Automobile Exhaust Emission Reactions: Effect of Core-Shell Interface

In this article, moisture-treated Pd@CeO₂/Al₂O₃ and Pd/CeO₂/Al₂O₃ catalysts were synthesized and applied in automotive three-way catalytic (TWC) reactions. Compared to the Pd/CeO₂/Al₂O₃ catalyst, the Pd@CeO₂/Al₂O₃ core-shell catalyst had better TWC activities. Transmission electron microscopy (TEM) images and X-ray photoelectron spectra (XPS) showed excess PdO₂ on the Pd and CeO₂ interface of Pd@CeO₂ nanoparticles. Fourier transform infrared (FT-IR) spectra analysis demonstrated the generation of the hydroperoxyl (*OOH) groups on the surface of the Pd@CeO₂ nanoparticle. CO-diffuse reflectance Fourier transform (DRIFT) measurement suggested that the CO adsorbed on *OOH species contributed to the formation of CO₂ and intermediate *COOH. NO-DRIFT results showed that more *NO₂ species appeared on the moisture-treated Pd@CeO₂ nanoparticle, which was the main active site in the automobile TWC reaction. These were the main factors contributing to the moisture-treated Pd@CeO₂/Al₂O₃ catalyst's high catalytic activities. The collected data revealed the crucial role of the co-promoting effect of moisture and core-shell interface on TWC reactions over the Pd@CeO₂/Al₂O₃ catalyst, which could be applied to other catalytic reactions.

Cerium Oxide Nanoparticles Inside Carbon Nanoreactors for Selective Allylic Oxidation of Cyclohexene

The confinement of cerium oxide (CeO₂) nanoparticles within hollow carbon nanostructures has been achieved and harnessed to control the oxidation of cyclohexene. Graphitized carbon nanofibers (GNF) have been used as the nanoscale tubular host and filled by sublimation of the Ce(tmhd)₄ complex (where tmhd = tetrakis(2,2,6,6-tetramethyl-3,5-heptanedionato)) into the internal cavity, followed by a subsequent thermal decomposition to yield the hybrid nanostructure CeO₂@GNF, where nanoparticles are preferentially immobilized at the internal graphitic step-edges of the GNF. Control over the size of the CeO₂ nanoparticles has been demonstrated within the range of about 4-9 nm by varying the mass ratio of the Ce(tmhd)₄ precursor to GNF during the synthesis. CeO₂@GNF was effective in promoting the allylic oxidation of cyclohexene in high yield with time-dependent control of product selectivity at a comparatively low loading of CeO₂ of 0.13 mol %. Unlike many of the reports to date where ceria catalyzes such organic transformations, we found the encapsulated CeO₂ to play the key role of radical initiator due to the presence of Ce³⁺ included in the structure, with the nanotube acting as both a host, preserving the high performance of the CeO₂ nanoparticles anchored at the GNF step-edges over multiple uses, and an electron reservoir, maintaining the balance of Ce³⁺ and Ce⁴⁺ centers. Spatial confinement effects ensure excellent stability and recyclability of CeO₂@GNF nanoreactors.

Cu/CeO2 Catalyst for Water-Gas Shift Reaction: Effect of CeO2 Pretreatment

CuO/CeO₂ is a kind of promising catalysts for the water-gas shift (WGS) reaction. Efforts were put in to improve its performance through modification of CeO₂ support. In this study, portions of CeO₂ prepared by a co-precipitation method were separately annealed at 300 °C in air, under vacuum and with H₂ , and were used as supports for the fabrication of CuO/CeO₂ catalysts. The physicochemical properties of the catalysts were characterized by X-ray diffraction, N₂ -physisorption, inductively coupled plasma, Raman spectroscopy, CO₂ temperature-programmed desorption, and H₂ temperature-programmed reduction techniques. The relation between catalytic performances and physicochemical properties of the CuO/CeO₂ catalysts were discussed. Among the three catalysts, the one with CuO supported on H₂ -reduced CeO₂ shows the highest catalytic activity, mainly due to strong CuO-CeO₂ synergetic interaction and high concentration of Frenkel-type oxygen vacancies. The superior catalytic activities can also be attributed to the Cu⁰ crystals of small size and the oxygen vacancies in non-stoichiometric CeO_2-x .

Versatile hollow fluorescent metal-silica nanohybrids through a modified microemulsion synthesis route

Silica-metal nanohybrids are common materials for applications in biomedicine, catalysis or sensing. Also, hollow structures are of interest as they provide additional useful features. However, in these materials the control of the size and accessibility to the inner regions of the structure usually requires complex synthesis procedures. Here we report a simple colloidal procedure for synthesizing hollow silica-metal nanohybrids, driven by the diffusion of metal precursors through the porous silica shell and subsequent reduction in aqueous solutions. The formation of hollow nanoparticles is controlled by the colloidal conditions during synthesis, which affect the ripening of hollow nanoparticles in presence of organosilanes. The modification of the conditions during synthesis affected the growth of silica precursors in presence of fluorescein isothiocyanate (FITC). The limited access to water molecules during the hydrolysis of silica precursors is attributed to the hydrophobicity of organic fluorescent molecules linked to the condensing silica clusters at the initial stages of nanoparticle formation and to the limitation of water content in the microemulsion method used. Finally, the growth of metal nanoseeds at the core of hollow nanoparticles can be easily achieved though a simple method in aqueous environment. The pH and thermal conditions during the reduction process affect the formation of metal-silica nanohybrids and their structural features.

Multi-shelled ceria hollow spheres with a tunable shell number and thickness and their superior catalytic activity

Multi-shelled ceria hollow spheres with tunable shell number and thickness have been prepared via a coordination polymer precursor method. Besides, this method was extended to the preparation of other rare earth oxides. Au and AuPd loaded ceria hollow spheres composites display superior catalytic activity.

Carbon nanotubes and catalysis: the many facets of a successful marriage

Carbon nanotubes have emerged as unique carbon allotropes that bear very interesting prospects in catalysis.

A highly active and stable Ni–Mg phyllosilicate nanotubular catalyst for ultrahigh temperature water-gas shift reaction

Ni–Mg binary metal PSNTs have been developed by introducing Mg into Ni phyllosilicate nanotubes and shown to exhibit superb activity and stability for WGS reaction at 650 °C.