Modulating the Electrocatalytic Performance of Palladium with the Electronic Metal–Support Interaction: A Case Study on Oxygen Evolution Reaction

Tailoring Zirconia Supported Intermetallic Platinum Alloy via Reactive Metal-Support Interactions for High-Performing Fuel Cells

Developing efficient and anti-corrosive oxygen reduction reaction (ORR) catalysts is of great importance for the applications of proton exchange membrane fuel cells (PEMFCs). Herein, we report a novel approach to prepare metal oxides supported intermetallic Pt alloy nanoparticles (NPs) via the reactive metal-support interaction (RMSI) as ORR catalysts, using Ni-doped cubic ZrO2 (Ni/ZrO2) supported L10-PtNi NPs as a proof of concept. Benefiting from the Ni migration during RMSI, the oxygen vacancy concentrations in the support are increased, leading to an electron enrichment of Pt. The optimal L10-PtNi-Ni/ZrO2-RMSI catalyst achieves remarkably low mass activity (MA) loss (17.8 %) after 400,000 accelerated durability test cycles in a half-cell and exceptional PEMFC performance (MA=0.76 A mgPt -1 at 0.9 V, peak power density=1.52/0.92 W cm-2 in H2-O2/-air, and 18.4 % MA decay after 30,000 cycles), representing the best reported Pt-based ORR catalysts without carbon supports. Density functional theory (DFT) calculations reveal that L10-PtNi-Ni/ZrO2-RMSI requires a lower energetic barrier for ORR than L10-PtNi-Ni/ZrO2 (direct loading), which is ascribed to a decreased Bader charge transfer between Pt and *OH, and the improved stability of L10-PtNi-Ni/ZrO2-RMSI compared to L10-PtNi-C can be contributed to the increased adhesion energy and Ni vacancy formation energy within the PtNi alloy.

Nanoengineered, Pd-doped Co@C nanoparticles as an effective electrocatalyst for OER in alkaline seawater electrolysis

Water electrolysis is considered one of the major sources of green hydrogen as the fuel of the future. However, due to limited freshwater resources, more interest has been geared toward seawater electrolysis for hydrogen production. The development of effective and selective electrocatalysts from earth-abundant elements for oxygen evolution reaction (OER) as the bottleneck for seawater electrolysis is highly desirable. This work introduces novel Pd-doped Co nanoparticles encapsulated in graphite carbon shell electrode (Pd-doped CoNPs@C shell) as a highly active OER electrocatalyst towards alkaline seawater oxidation, which outperforms the state-of-the-art catalyst, RuO2. Significantly, Pd-doped CoNPs@C shell electrode exhibiting low OER overpotential of ≈213, ≈372, and ≈ 429 mV at 10, 50, and 100 mA/cm2, respectively together with a small Tafel slope of ≈ 120 mV/dec than pure Co@C and Pd@C electrode in alkaline seawater media. The high catalytic activity at the aforementioned current density reveals decent selectivity, thus obviating the evolution of chloride reaction (CER), i.e., ∼490 mV, as competitive to the OER. Results indicated that Pd-doped Co nanoparticles encapsulated in graphite carbon shell (Pd-doped CoNPs@C electrode) could be a very promising candidate for seawater electrolysis.

AuFe3@Pd/γ-Fe2O3 Nanosheets as an In Situ Regenerable and Highly Efficient Hydrogenation Catalyst

Heterogenous Pd catalysts play a pivotal role in the chemical industry; however, it is plagued by S^2- or other strong adsorbates inducing surface poisoning long term. Herein, we report the development of AuFe₃@Pd/γ-Fe₂O₃ nanosheets (NSs) as an in situ regenerable and highly active hydrogenation catalyst. Upon poisoning, the Pd monolayer sites could be fully and oxidatively regenerated under ambient conditions, which is initiated by •OH radicals from surface defect/Fe_Tetra vacancy-rich γ-Fe₂O₃ NSs via the Fenton-like pathway. Both experimental and theoretical analyses demonstrate that for the electronic and geometric effect, the 2-3 nm AuFe₃ intermetallic nanocluster core promotes the adsorption of reactant onto Pd sites; in addition, it lowers Pd's affinity for •OH radicals to enhance their stability during oxidative regeneration. When packed into a quartz sand fixed-bed catalyst column, the AuFe₃@Pd/γ-Fe₂O₃ NSs are highly active in hydrogenating the carbon-halogen bond, which comprises a crucial step for the removal of micropollutants in drinking water and recovery of resources from heavily polluted wastewater, and withstand ten rounds of regeneration. By maximizing the use of ultrathin metal oxide NSs and intermetallic nanocluster and monolayer Pd, the current study demonstrates a comprehensive strategy for developing sustainable Pd catalysts for liquid catalysis.

Electron Delocalization of Au Nanoclusters Triggered by Fe Single Atoms Boosts Alkaline Overall Water Splitting

The rational design and in-depth understanding of the structure-activity relationship (SAR) of hydrogen and oxygen evolution reaction (HER and OER) bifunctional electrocatalysts are vital to decreasing the energy consumption of hydrogen production by electrochemical water splitting. Herein, we report an inducing electron delocalization method where Fe single atoms as inducers are used to regulate the electron structure of Au nanoclusters by the M-N_x-C substrate to acquire satisfactory intrinsic HER activity. Meanwhile, Fe single atoms also serve as efficient OER active sites to construct bifunctional electrocatalysts. On account of the strong synergistic effect between Au nanoclusters and Fe single atoms, the hybrid catalyst Au-Fe₁NC/NF performs an outstanding alkaline HER and OER activity. Only 35.6 mV, 246 mV, and 1.52 V are needed to reach 10 mA cm^-2 for alkaline HER, OER, and two-electrode electrolytic cells, respectively. In addition, the bifunctional electrocatalysts also display excellent electrochemical stability. DFT calculations demonstrate that the strong synergistic effect can enhance the O-H bond activation ability of Au nanoclusters and upshift the d-band center of the Fe single atom to promote alkaline electrocatalytic water splitting. The strong synergistic effect is proven to arise from the electron delocalization of Au nanoclusters triggered by Fe single atoms.

Recent progresses on single-atom catalysts for the removal of air pollutants

The booming industrialization has aggravated emission of air pollutants, inflicting serious harm on environment and human health. Supported noble-metals are one of the most popular catalysts for the oxidation removal of air pollutants. Unfortunately, the high price and large consumption restrict their development and practical application. Single-atom catalysts (SACs) emerge and offer an optimizing approach to address this issue. Due to maximal atom utilization, tunable coordination and electron environment and strong metal-support interaction, SACs have shown remarkable catalytic performance on many reactions. Over the last decade, great potential of SACs has been witnessed in the elimination of air pollutants. In this review, we first briefly summarize the synthesis methods and modulation strategies together with the characterization techniques of SACs. Next, we highlight the application of SACs in the abatement of air pollutants including CO, volatile organic compounds (VOCs) and NOx, unveiling the related catalytic mechanism of SACs. Finally, we propose the remaining challenges and future perspectives of SACs in fundamental research and practical application in the field of air pollutant removal.

Surface Design Strategy of Catalysts for Water Electrolysis

Hydrogen, a new energy carrier that can replace traditional fossil fuels, is seen as one of the most promising clean energy sources. The use of renewable electricity to drive hydrogen production has very broad prospects for addressing energy and environmental problems. Therefore, many researchers favor electrolytic water due to its green and low-cost advantages. The electrolytic water reaction comprises the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER). Understanding the OER and HER mechanisms in acidic and alkaline processes contributes to further studying the design of surface regulation of electrolytic water catalysts. The OER and HER catalysts are mainly reviewed for defects, doping, alloying, surface reconstruction, crystal surface structure, and heterostructures. Besides, recent catalysts for overall water splitting are also reviewed. Finally, this review paves the way to the rational design and synthesis of new materials for highly efficient electrocatalysis.

Green synthesis of hybrid papain/Ni3(PO4)2 rods electrocatalyst for enhanced oxygen evolution reaction

An eco-friendly approach was used to produce the binary papain/Ni3(PO4)2 in the presence of papain, which is derived from green papaya fruits. Rod shape Papain/Ni3(PO4)2 showed excellent OER activities in alkaline, neutral and acidic media.

Oxygen-Evolution Reaction by a Palladium Foil in the Presence of Iron

Herein, we investigate the oxygen-evolution reaction (OER) and electrochemistry of a Pd foil in the presence of iron under alkaline conditions (pH ≈ 13). As a source of iron, K₂FeO₄ is employed, which is soluble under alkaline conditions in contrast to many other Fe salts. Immediately after reacting with the Pd foil, [FeO₄]^2- causes a significant increase in OER and changes in the electrochemistry of Pd. In the absence of this Fe source and under OER, Pd(IV) is stable, and hole accumulation occurs, while in the presence of Fe this accumulation of stored charges can be used for OER. A Density Functional Theory (DFT) based thermodynamic model suggests an oxygen bridge vacancy as an active site on the surface of PdO₂ and an OER overpotential of 0.42 V. A substitution of Pd with Fe at this active site reduces the calculated OER overpotential to 0.35 V. The 70 mV decrease in overpotential is in good agreement with the experimentally measured decrease of 60 mV in the onset potential. In the presence of small amounts of Fe salt, our results point toward the Fe doping of PdO₂ rather than extra framework FeO_x (Fe(OH)₃, FeO(OH), and KFeO₂) species on top of PdO₂ as the active OER sites.

The effect of nanoparticulate PdO co-catalysts on the faradaic and light conversion efficiency of WO3 photoanodes for water oxidation

WO3 photoanodes offer rare stability in acidic media, but are limited by their selectivity for oxygen evolution over parasitic side reactions, when employed in photoelectrochemical (PEC) water splitting. Herein, this is remedied via the modification of nanostructured WO3 photoanodes with surface decorated PdO as an oxygen evolution co-catalyst (OEC). The photoanodes and co-catalyst particles are grown using an up-scalable aerosol assisted chemical vapour deposition (AA-CVD) route, and their physical properties characterised by X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HR-TEM) and UV-vis absorption spectroscopy. Subsequent PEC and transient photocurrent (TPC) measurements showed that the use of a PdO co-catalyst dramatically increases the faradaic efficiency (FE) of water oxidation from 52% to 92%, whilst simultaneously enhancing the photocurrent generation and charge extraction rate. The Pd oxidation state was found to be critical in achieving these notable improvements to the photoanode performance, which is primarily attributed to the higher selectivity towards oxygen evolution when PdO is used as an OEC and the formation of a favourable junction between WO3 and PdO, that drives band bending and charge separation.

Defect-assisted electronic metal-support interactions: tuning the interplay between Ru nanoparticles and CuO supports for pH-neutral oxygen evolution

Electronic metal-support interactions (EMSIs) comprise an area of intense study, the manipulation of which is of paramount importance in the improvement of heterogeneous metal nanoparticle (NP) supported catalysts. EMSI is the transfer of charge from the support to NP, enabling more effective adsorption and interaction of reactants during catalysis. Ru NPs on CuO supports show different levels of EMSI (via charge transfer) depending on their crystal structure, with multiple twinned NPs showing greater potential for EMSI. We use magnetron-assisted gas phase aggregation for the synthesis of batches of Ru NPs with different populations of single crystal and multiple twinned nanoparticles, which were deposited on CuO nanowires (NWs). The surface charging of the Ru-CuO catalysts was investigated by Kelvin probe force microscopy (KPFM) and X-ray photoelectron spectroscopy (XPS). By doubling the population of multiple twinned NPs, the surface potential of the Ru-CuO catalysts increases roughly 4 times, coinciding with a similar increase in the amount of Ru⁴⁺. Therefore, tuning the amount of EMSI in a catalyst is possible through changing the population of multiple twinned Ru NPs in the catalyst. Increasing the amount of multiple twin NPs resulted in improved activity in the oxygen evolution reaction (a roughly 2.5 times decrease in the overpotentials when the population of multiple twinned NPs is increased) and better catalyst stability. This improvement is attributed to the fact that the multiple twin NPs maintained a metallic character under oxidation conditions (unlike single crystal NPs) due to the EMSI between the NP and support.

Climbing with support: scaling the volcano relationship through support–electrocatalyst interactions in electrodeposited RuO2 for the oxygen evolution reaction

The interfacial charge transfer and support-induced electrocatalyst faceting in thin catalysts enable ‘climbing up’ the volcano map for OER electrocatalysts. The conductivity of the support determines the OER activity of thick catalysts.

Utilizing ballistic nanoparticle impact to reconfigure the metal support interaction in Pt-TiN electrocatalysts

Tuning the metal support interaction (MSI) in heterogeneous catalysts is of utmost importance for various applications in different catalysis reactions. Pt-TiN systems are strong contenders for commercial catalysts, although the charge screening of Pt and non-involvement of N reduces their effective MSI and limits it to the Pt-Ti interface. Here, the bias driven landing of gas phase synthesized Pt nanoparticles (NPs) is used to change the nature of the MSI and enhance the charge transfer phenomenon. Bias driven landing of the Pt NPs translates their impact energies to the TiN surface, resulting in a weakening of the Ti-N bonds. This facilitates a new interaction between the Pt and N atoms, resulting in an electronic equilibration in the N-Pt-Ti triumvirate, nullifying the charge screening of Pt. This change in the nature of the MSI enables long range charge transfer throughout the catalyst surface and an increase in the electrocatalytic hydrogen evolution reaction (HER) activity of the Pt-TiN system.

Phyto-inspired and scalable approach for the synthesis of PdO-2Mn2O3: a nano-material for application in water splitting electro-catalysis

A modified co-precipitation method has been used for the synthesis of a PdO-2Mn₂O₃ nanocomposite as an efficient electrode material for the electro-catalytic oxygen evolution (OER) and hydrogen evolution reaction (HER). Palladium acetate and manganese acetate in molar ratio 1 : 4 were dissolved in water, and 10 ml of an aqueous solution of phyto-compounds was slowly added until completion of precipitation. The filtered and dried precipitates were then calcined at 450 °C to obtain a blackish brown colored mixture of PdO-2Mn₂O₃ nanocomposite. These particles were analyzed by ultra violet visible spectrophotometry (UV-vis), infrared spectroscopy (FTIR), powder X-ray diffractometry (XRD), scanning electron microscopy (FE-SEM), energy dispersive X-ray spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS) for crystallinity, optical properties, and compositional and morphological makeup. Using Tauc's plot, the direct band gap (3.18 eV) was calculated from the absorption spectra. The average crystallite sizes, as calculated from the XRD, were found to be 15 and 14.55 nm for PdO and Mn₂O₃, respectively. A slurry of the phyto-fabricated PdO-2Mn₂O₃ powder was deposited on Ni-foam and tested for electro-catalytic water splitting studies in 1 M KOH solution. The electrode showed excellent OER and HER performance with low over-potential (0.35 V and 121 mV) and Tafel slopes of 115 mV dec^-1 and 219 mV dec^-1, respectively. The outcomes obtained from this study provide a direction for the fabrication of a cost-effective mixed metal oxide based electro-catalyst via an environmentally benign synthesis approach for the generation of clean energy.

Lateral silicon oxide/gold interfaces enhance the rate of electrochemical hydrogen evolution reaction in alkaline media

The production of solar hydrogen with a silicon based water splitting device is a promising future technology, and silicon-based metal-insulator-semiconductor (MIS) electrodes have been proposed as suitable architectures for efficient photocathodes based on the electronic properties of the MIS structures and the catalytic properties of the metals. In this paper, we demonstrate that the interfaces between the metal and oxide of laterally patterned MIS electrodes may strongly enhance the catalytic activity of the electrode compared to bulk metal surfaces. The employed electrodes consist of well-defined, large-area arrays of gold structures of various mesoscopic sizes embedded in a silicon oxide support on silicon. We demonstrate that the activity of these electrodes for hydrogen evolution reaction (HER) increases with an increase in gold/silicon oxide boundary length in both acidic and alkaline media, although the enhancement of the HER rate in alkaline electrolytes is considerably larger than in acidic electrolytes. Electrodes with the largest interfacial length of gold/silicon oxide exhibited a 10-times larger HER rate in alkaline electrolytes than those with the smallest interfacial length. The data suggest that at the metal/silicon oxide boundaries, alkaline HER is enhanced through a bifunctional mechanism, which we tentatively relate to the laterally structured electrode geometry and to positive charges present in silicon oxide: Both properties change locally the interfacial electric field at the gold/silicon oxide boundary, which, in turn, facilitates a faster transport of hydroxide ions away from the electrode/electrolyte interface in alkaline solution. This mechanism boosts the alkaline HER activity of p-type silicon based photoelectrodes close to their HER activity in acidic electrolytes.

Size-dependent electrocatalytic activity of ORR/OER on palladium nanoclusters anchored on defective MoS2monolayers

The catalytic performance of MoS₂monolayer for oxygen reduction/evolution can be effectively tuned by carefully controlling the sizes of the deposited Pd clusters.

Complete dehalogenation of bromochloroacetic acid by liquid phase catalytic hydrogenation over Pd/CeO2 catalysts

Bromochloroacetic acid is classified as one of the typical disinfection byproducts (DBPs). In this work, supported palladium catalysts on different supports (CeO_2, Al₂O₃, SiO₂ and activated carbon (AC)) (labelled as Pd/support) were synthesized via the deposition-precipitation method (D-P method) and their activities for the complete dehalogenation of bromochloroacetic acid by liquid phase catalytic hydrogenation were evaluated. Comprehensive characterizations of the catalysts were conducted by powder X-ray diffraction (XRD), transmission electron microscopy (TEM), point of zero charge (PZC), X-ray photoelectron spectroscopy (XPS) and CO chemisorption. Results indicated that the PZCs of the supports varied with each other. The stronger Pd-support interaction and higher Pd dispersion of Pd/CeO₂ and Pd/Al₂O₃ than those of Pd/AC and Pd/SiO₂ were confirmed by X-ray photoelectron spectroscopy and CO chemisorption. Pd/CeO₂ had a higher ratio of positively charged Pd to metallic Pd (Pdⁿ⁺/Pd⁰) than Pd/Al₂O₃ and Pd/AC due to a stronger metal-support interaction. Accordingly, a negligible bromochloroacetic acid conversion was observed on Pd/SiO₂, whereas bromochloroacetic acid was found to be readily decomposed on Pd/CeO₂, Pd/Al₂O₃ and Pd/AC. However, the dechlorination reaction could not further proceed on Pd/Al₂O₃ and Pd/AC catalysts after the bromine functionality was removed from bromochloroacetic acid. A complete dehalogenation of bromochloroacetic acid occurred only on Pd/CeO₂. Furthermore, the dechlorination rate constants of monochloroacetic acid and bromochloroacetic acid over Pd(1.40)/CeO₂ were 0.018 and 0.031 min^-1 respectively, confirming an induced synergistic effect due to the existence of bromine atoms. It was worth noting that a stepwise-concerted pathway was verified during the liquid phase catalytic hydrodehalogenation of bromochloroacetic acid.

The role of catalyst–support interactions in oxygen evolution anodes based on Co(OH)2 nanoparticles and carbon microfibers

A set of OER electrodes based on Co(OH)₂ nanoparticles and carbon microfibers of tailored composition is reported, which allows extracting valuable insights on the influence of the metal-support interface in their electrocatalytic performance.

Engineering the Metal/Oxide Interface of Pd Nanowire@CuOx Electrocatalysts for Efficient Alcohol Oxidation Reaction

The development of new type electrocatalysts with promising activity and antipoisoning ability is of great importance for electrocatalysis on alcohol oxidation. In this work, Pd nanowire (PdNW)/CuO_x heterogeneous catalysts with different types of PdOCu interfaces (Pd/amorphous or crystalline CuO_x ) are prepared via a two-step hydrothermal strategy followed by an air plasma treatment. Their interface-dependent performance on methanol and ethanol oxidation reaction (MOR and EOR) is clearly observed. The as-prepared PdNW/crystalline CuO_x catalyst with 17.2 at% of Cu on the PdNW surface exhibits better MOR and EOR activity and stability, compared with that of PdNW/amorphous CuO_x and pristine PdNW catalysts. Significantly, both the cycling tests and the chronoamperometric measurements reveal that the PdNW/crystalline CuO_x catalyst yields excellent tolerance toward the possible intermediates including formaldehyde, formic acid, potassium carbonate, and carbon monoxide generated during the MOR process. The detailed analysis of their chemical state reveals that the enhanced activity and antipoison ability of the PdNW/crystalline CuO_x catalyst originates from the electron-deficient Pd^δ+ active sites which gradually turn into Pd₅ O₄ species during the MOR catalysis. The Pd₅ O₄ species can likely be stabilized by moderate crystalline CuO_x decorated on the surface of PdNW due to the strong PdOCu interaction.

Highly effective bromate reduction by liquid phase catalytic hydrogenation over Pd catalysts supported on core-shell structured magnetites: Impact of shell properties

Liquid phase catalytic reduction of bromate with supported noble metals as the catalysts is a promising method to remove bromate in water. Magnetic supports provide a feasible way to recover catalysts whose surface properties also strongly influence the catalytic efficiency. In this study, Pd nanoparticles supported on core-shell structured magnetites with varied shells (e.g., carbon, SiO₂, polypyrrole, polyaniline, polydopamine and chitosan) were prepared and catalytic reduction of bromate on the catalysts was investigated. The results showed that in comparison with other catalysts Pd/(Fe₃O₄@polyaniline) exhibited a higher catalytic efficiency due to its higher point of zero charge and surface hydrophilicity. In parallel, bromate reduction on Pd/(Fe₃O₄@polyaniline) followed the Langmuir-Hinshelwood model, confirming the crucial role of bromate adsorption. At pH 5.6 and a catalyst dosage of 0.05 g/L, 0.4 mM bromate could be completely reduced into bromide within 120 min. Furthermore, the magnetic catalysts could be effectively separated and recovered under an external magnetic field within 3 min. The results of catalyst reuse showed that after five consecutive catalytic reduction cycles Pd/(Fe₃O₄@polyaniline) retained 87% of its fresh catalyst activity. The present findings indicate that Pd/(Fe₃O₄@polyaniline) with polyaniline as the shell is a highly active, stable and recyclable catalyst for liquid phase catalytic hydrogenation of pollutants in water.

Environmentally-Friendly Exfoliate and Active Site Self-Assembly: Thin 2D/2D Heterostructure Amorphous Nickel-Iron Alloy on 2D Materials for Efficient Oxygen Evolution Reaction

A class of 2D layered materials exhibits substantial potential for high-performance electrocatalysts due to high specific surface area, tunable electronic properties, and open 2D channels for fast ion transport. However, liquid-phase exfoliation always utilizes organic solvents that are harmful to the environment, and the active sites are limited to edge sites. Here, an environmentally friendly exfoliator in aqueous solution is presented without utilizing any toxic or hazardous substance and active site self-assembly on the inert base of 2D materials. Benefiting from thin 2D/2D heterostructure and strong interfacial coupling, the resultant highly disordered amorphous NiFe/2D materials (Ti3C2 MXene, graphene and MoS₂ ) thin nanosheets exhibit extraordinary electrocatalytic performance toward oxygen evolution reaction (OER) in alkaline media. DFT results further verify the experimental results. The study emphasizes a viable idea to probe efficient electrocatalysts by means of the synergistic effect of environmentally friendly exfoliator in aqueous solution and active site self-assembly on the inert base of 2D materials which forms the unique thin 2D/2D heterostructure in-suit. This new type of heterostructure opens up a novel avenue for the rational design of highly efficient 2D materials for electrocatalysis.