Shape-controlled synthesis of Pd nanocrystals with exposed {110} facets and their catalytic applications

Recent progress in defect-engineered metal oxides for photocatalytic environmental remediation

Rapid industrial advancement over the last few decades has led to an alarming increase in pollution levels in the ecosystem. Among the primary pollutants, harmful organic dyes and pharmaceutical drugs are directly released by industries into the water bodies which serves as a major cause of environmental deterioration. This warns of a severe need to find some sustainable strategies to overcome these increasing levels of water pollution and eliminate the pollutants before being exposed to the environment. Photocatalysis is a well-established strategy in the field of pollutant degradation and various metal oxides have been proven to exhibit excellent physicochemical properties which makes them a potential candidate for environmental remediation. Further, with the aim of rapid industrialization of photocatalytic pollutant degradation technology, constant efforts have been made to increase the photocatalytic activity of various metal oxides. One such strategy is the introduction of defects into the lattice of the parent catalyst through doping or vacancy which plays a major role in enhancing the catalytic activity and achieving excellent degradation rates. This review provides a comprehensive analysis of defects and their role in altering the photocatalytic activity of the material. Various defect-rich metal oxides like binary oxides, perovskite oxides, and spinel oxides have been summarized for their application in pollutant degradation. Finally, a summary of existing research, followed by the existing challenges along with the potential countermeasures has been provided to pave a path for the future studies and industrialization of this promising field.

Hydrothermal-treated Pt/Al2O3 as an excellent catalyst for toluene total oxidation

The preparation of highly active supported noble metal catalysts with a low noble metal loading has always been the ultimate goal of researchers working on catalysis. Hydrothermally treated Pt/Al₂O₃ (Pt/Al₂O₃-H) exhibits better catalytic activity than that (Pt/Al₂O₃-C) treated via the conventional calcination approach. At the high space velocity of 100,000 mL/(g∙hr), the temperature that correspond to 50% toluene conversion (T₅₀) of Pt/Al₂O₃-H is 115°C lower than that of Pt/Al₂O₃-C, and the turnover frequency (TOF) value can reach 0.0756 sec^-1. The mechanism by which the hydrothermal approach enhances Pt/Al₂O₃ activity has been investigated. The structure associated with the high catalytic activity of Pt nanoparticles (NPs) can be retained via hydrothermal treatment. Furthermore, the support is transformed to AlO(OH) with numerous surface hydroxyl groups, which in turn can facilitate the adsorption of toluene. And the synergistic effects of Pt NPs and AlO(OH) increases the contents of Pt in oxidation state and active oxygen, which are beneficial for toluene oxidation.

Challenges and prospects in the selective photoreduction of CO2 to C1 and C2 products with nanostructured materials: a review

Solar fuel generation through CO₂ hydrogenation is the ultimate strategy to produce sustainable energy sources and alleviate global warming. The photocatalytic CO₂ conversion process resembles natural photosynthesis, which regulates the ecological systems of the earth. Currently, most of the work in this field has been focused on boosting efficiency rather than controlling the distribution of products. The structural architecture of the semiconductor photocatalyst, CO₂ photoreduction process, product analysis, and elucidating the CO₂ photoreduction mechanism are the key features of the photoreduction of CO₂ to generate C1 and C2 based hydrocarbon fuels. The selectivity of C1 and C2 products during the photocatalytic CO₂ reduction have been ameliorated by suitable photocatalyst design, co-catalyst, defect states, and the impacts of the surface polarisation state, etc. Monitoring product selectivity allows the establishment of an appropriate strategy to generate a more reduced state of a hydrocarbon, such as CH₄ or higher carbon (C2) products. This article concentrates on studies that demonstrate the production of C1 and C2 products during CO₂ photoreduction using H₂O or H₂ as an electron and proton source. Finally, it highlights unresolved difficulties in achieving high selectivity and photoconversion efficiency of CO₂ in C1 and C2 products over various nanostructured materials.

Compatibility between Activity and Selectivity in Catalytic Oxidation of Benzyl Alcohol with Au-Pd Nanoparticles through Redox Switching of SnOx

A balance between catalytic activity and product selectivity remains a dilemma for the partial oxidation processes because the products are prone to be overoxidized. In this work, we report on the partial oxidation of benzyl alcohol using a modified catalyst consisting of nanosized Au-Pd particles (NPs) with tin oxide (SnO_x) deposited on a mesoporous silica support. We found that the SnO_x promotes the autogenous reduction of PdO to active Pd⁰ species on the Au-Pd NP catalyst (SnO_x@AP-ox) before H₂ reduction, which is due to the high oxophilicity of Sn. The presence of active Pd⁰ species and the enhancement of oxygen transfer by SnO_x led to high catalytic activity. The benzaldehyde selectivity was enhanced with the increase of SnO_x content on catalyst SnO_x@AP-ox, which is ascribed to the modulated affinity of reactants and products on the catalyst surface through the redox switching of Sn species. After H₂ reduction, SnO_x was partially reduced and Au-Pd-Sn alloy was formed. The formation of Au-Pd-Sn alloy weakened both the catalytic synergy of Au-Pd alloy NPs and the adsorption of benzyl alcohol on the reduced catalyst, thus leading to low catalytic activity.

Pd nanoparticles deposited on Co(OH)2 nanoplatelets as a bifunctional electrocatalyst and their application in Zn-air and Li-O2 batteries

The development of affordable electrocatalysts for both oxygen reduction and evolution reactions (ORR/OER) has received great interest due to their importance in metal-air batteries and regenerative fuel cells. We developed a high-performance bifunctional oxygen electrocatalyst based on Pd nanoparticles supported on cobalt hydroxide nanoplatelets (Pd/Co(OH)₂) as an air cathode for metal-air batteries. The Pd/Co(OH)₂ shows remarkably higher electrocatalytic activity in comparison with commercial catalysts (Pt/C, IrO₂), including an ORR half-wave potential (E_1/2) of 0.87 V vs. RHE and an OER overpotential of 0.39 V at 10 mA cm^-2 in aqueous alkaline medium. The Zn-air battery constructed with Pd/Co(OH)₂ presents stable charge/discharge voltage (ΔE_OER-ORR = 0.69 V), along with durable cycling stability for over 30 h. Also, this cathode exhibits a maximum discharge capacity of 17 698 mA h g^-1, and stable battery operation over 50 cycles at a fixed capacity of 1000 mA h g^-1, as an efficient air electrode for Li-O₂ batteries, indicating that Pd/Co(OH)₂ can be a potential candidate for both aqueous and non-aqueous metal-air batteries.

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.

Platinum and Platinum Group Metal-Free Catalysts for Anion Exchange Membrane Fuel Cells

The development of active hydrogen oxidation reaction (HOR) and oxygen reduction reaction (ORR) catalysts for use in anion exchange membrane fuel cells (AEMFCs), which are free from platinum group metals (PGMs), is expected to bring this technology one step closer to commercial applications. This paper reports our recent progress developing HOR Pt-free and PGM-free catalysts (Pd/CeO2 and NiCo/C, respectively), and ORR PGM-free Co3O4 for AEMFCs. The catalysts were prepared by different synthesis techniques and characterized by both physical-chemical and electrochemical methods. A hydrothermally synthesized Co3O4 + C composite ORR catalyst used in combination with Pt/C as HOR catalyst shows good H2/O2 AEMFC performance (peak power density of ~388 mW cm−2), while the same catalyst coupled with our flame spray pyrolysis synthesised Pd/CeO2 anode catalysts reaches peak power densities of ~309 mW cm−2. Changing the anode to nanostructured NiCo/C catalyst, the performance is significantly reduced. This study confirms previous conclusions, that is indeed possible to develop high performing AEMFCs free from Pt; however, the challenge to achieve completely PGM-free AEMFCs still remains.

Single atomic site catalysts: synthesis, characterization, and applications

Single atomic site catalysts (SASCs) have attracted great attention in heterogenous catalysis due to their maximized atomic utilization and unique electronic structure. This feature article summarizes the recent contributions of the authors in the synthesis, characterization, and applications of SASCs. Firstly, we disclose the tricks of our recent progress in the synthesis of SASCs, including impregnation of metal precursors on defect-rich supports, pyrolysis of polymer-encapsulated metals and isolation of contiguous atoms by alloying. Then, we show several key characterization technologies to identify the geometric and electronic structure of SASCs, and reveal the advantages and disadvantages of these characterization technologies. Finally, the applications of the SASCs in heterogenous catalysis are presented, which are classified into electrocatalysis and thermocatalysis, and the structure-function relationships are disclosed.