Single-atom Pd Sites Research Articles

In situ photodeposition of ultra-small palladium particles on TiO2.

In situ and operando investigation of photocatalysts plays a fundamental role inunderstanding the processes of active phase formation and the mechanisms ofcatalytic reactions, which is crucial for the rational design of more efficient materials. Using a custom-made operando photocatalytic cell, an in situ procedure to follow the formation steps of Pd/TiO2 photocatalyst by synchrotron-based X-ray absorption spectroscopy (XAS) is proposed. The procedure resulted in the formation of ∼1 nm Pd particles with a much narrower size distribution and homogeneous spreading over TiO2 support compared with the samples generated in a conventional batch reactor. The combination of in situ XAS spectroscopy with high-angle annular dark-field scanning transmission electron microscopy demonstrated the formation of single-atom Pd(0) sites on TiO2 as the initial step of the photodeposition process. Palladium hydride particles were observed for all investigated samples upon exposure to formic acid solutions.

Insight into the Effects of Pd Single-atom Particles in the Activation of the Carbon-Chloride Bond

Palladium (Pd) shows high activity in a variety of catalysis reactions that refer to the activation of the carbon-halogenation bond, and single-atom Pd catalysts have received significant attention as new technology. Nevertheless, there is a huge knowledge gap regarding which Pd sites could form highly reactive centers. It is unclear whether single-atom Pd sites are active in this process or not. In this study, we synthesized Pd/TiO2 nanosheets with Pd loadings of 1%, 5%, and 10% and designed single atom Pd sites and Pd nanoparticles on similar substrates. A scanning transmission electron microscope was used to characterize the synthesized Pd catalysts. As a catalyst, Pd/TiO2 featuring Pd NPs showed high activity in activating the carbon-halogen bond in the activation of carbon-chloride bond, whereas Pd/TiO2 with single Pd atoms showed very low activity in this pros. Accordingly, our results demonstrate that the activation of carbon-halogen bonds cannot be catalyzed by the single-atom Pd sites and emphasize the significance of the ensemble effect in determining the activity of Pd sites.

Asymmetric coordinated single-atom Pd sites for high performance CO2 electroreduction and Zn–CO2 battery

Single-atom catalysts (SACs) have attracted great attention for electrocatalysis. The coordination configurations of heteroatoms surrounding the metal center have a significant influence on the catalytic performance. The construction of asymmetric coordination configurations has recently been found to be an efficient strategy to improve electrocatalytic performance. However, noble-metal-based SACs with an asymmetric coordination environment have seldom been achieved. Herein, we rationally design and construct single-atom Pd catalysts with asymmetric Pd1O3C1 or Pd1N3C1 sites anchored on oxidized mesoporous carbon with optimized porosity and hydrophobicity. The as-prepared catalyst (Pd1-O-CB) selectively produces CO with a high CO Faradaic efficiency (FECO) of 99.6 % at a low potential of –0.6 V versus reversible hydrogen electrode (RHE). Furthermore, the Pd1-O-CB-based gas diffusion layer exhibits a high current density of 280.4 mA cm−2 and long-term stability over 48 h benefitted from the regulated porosity and hydrophobicity of the tri-interface. Finally, a rechargeable Zn–CO2 battery is constructed with the optimal gas diffusion electrode to deliver a maximal power density of 1.72 mW cm−2.

Engineering single-atom Pd sites in ZIF-derived porous Co3O4 for enhanced elementary mercury removal

Engineering metal-based nanoarchitectures and their atomic-scale arrangement is a promising avenue for enhanced mercury adsorbent activities. Interactions between metal single atoms and substrates result in regulated transitions between distinct reaction processes that boost mercury adsorption activity. Herein, a novel sorbent of porous Co3O4 with Palladium atomic dispersed was successfully prepared by one-step pyrolysis of zeolitic imidazolate framework 67 (ZIF-67) precursors. Via engineering single Pd atoms into ZIF-derived Co3O4 (Pd1/Co3O4-ZIF), the strong metal-support interaction (SMSI) activated the reducibility of Co3+ in porous Co3O4 to efficiently adsorb and oxidize Hg0, enhancing its mercury removal efficiency. Specifically, Pd1/Co3O4-ZIF exhibited superior sorption performance for Hg0 over a broad range of temperatures (30–350 °C) and high gaseous hourly space velocity (about 555,500 hr−1) with the average Hg0 removal efficiency higher than 98%. The strategy of constructing atomic active sites on the surface of porous metal-oxide to convert adsorbed Hg0 into HgO is a promising approach to solve the problem that Hg0 in flue gas is challenging to remove. Density functional theory (DFT) calculation further proved that the existence of a single Pd site on the surface of Co3O4 significantly reduced the energy barrier of Hg0 to HgO at the atomic level. This work contributed to a thorough comprehension of the structural and adsorption properties of the ZIF-derived Co3O4 sorbent engineering with a single atom and enabled the efficient capture of elemental mercury in flue gas by such ZIF-derived sorbent.

Unraveling 1-Hexene Hydrogenation over Dilute Pd-in-Au Alloys

Dilute Pd-in-Au alloys are valuable materials for selectively hydrogenating alkynes and isomerizing alkenes. By diluting Pd in a Au host, the selectivity toward semihydrogenated alkene isomers can be significantly enhanced and the unfavorable overhydrogenation to alkanes is suppressed. However, a detailed mechanistic study on the origin of the enhanced alkene selectivity over dilute alloy catalysts is still missing. Here, we combine experiment and theory to unravel the reaction mechanism, identifying rate-limiting and selectivity-controlling steps in 1-hexene hydrogenation over dilute Pd-in-Au catalysts. Using isotope-exchange hydrogenation experiments, we show that 1-hexene and hydrogen over a bimetallic Pd4Au96 in silica catalyst preferentially form 1-hexene isomers, (trans and cis) 2- and 3-hexene and only a small amounts of hexane. The reaction is consistent with a Horiuti–Polanyi mechanism, similar to a monometallic Pd nanoparticle catalyst. Computation of the free-energy profiles for 1-hexene hydrogenation and isomerization over a single Pd atom in a Au surface using first principles calculations indicated that the isomerization of 1-hexene to 2-hexene is energetically favorable due to the relatively large barrier for H2 dissociation preventing hydrogenation to n-hexane. Microkinetic modeling established that H2 dissociation on the single-atom Pd sites and H spillover from these sites onto the Au host are rate-limiting and key in steering the selectivity of dilute Pd-in-Au alloys toward the hexene isomers. The mechanistic insights from this study contribute to the rational design of optimized dilute alloy catalysts for selective alkene isomerization.

Pd single-atom monolithic catalyst: Functional 3D structure and unique chemical selectivity in hydrogenation reaction

Regulating the selectivity of catalysts in selective hydrogenation reactions at the atomic level is highly desirable but remains a grand challenge. Here we report a simple and practical strategy to synthesize a monolithic single-atom catalyst (SAC) with isolated Pd atoms supported on bulk nitrogen-doped carbon foams (Pd-SAs/CNF). Moreover, we demonstrate that the single-atom Pd sites with unique electronic structure endow Pd-SAs/CNF with an isolated site effect, leading to excellent activity and selectivity in 4-nitrophenylacetylene semi-hydrogenation reaction. In addition, benefiting from the great integrity and excellent mechanical strength, monolithic Pd-SAs/CNF catalyst is easy to separate from the reaction system for conducting the subsequent recycling. The cyclic test demonstrates the excellent reusability and stability of monolithic Pd-SAs/CNF catalyst. The discovery of isolated site effect provides a new approach to design highly selective catalysts. And the development of monolithic SACs provides new opportunities to advance the practical applications of single-atom catalysts.

N8 stabilized single-atom Pd for highly selective hydrogenation of acetylene

Single-atom catalysts show a promising future in many reactions even though great challenges still remain such as facile synthesis and long stability. In this work, a single-atom Pd catalyst attached to a designed N8 Lewis base species (Pd1-N8/CNT) is synthesized with cyclic voltammetry (CV) method. The catalyst demonstrates long stability and enhanced C2H4 selectivity in selective hydrogenation of acetylene at 40 °C. CV is carried out in a three-electrode setup with PdO/CNT as the working electrode in NaN3 solution. HAADF-STEM confirms single-atom Pd sites are successfully isolated. XPS measurements and Bader charge calculations indicate N8 is effectively synthesized on CNT substrate after CV treatment while single-atom Pd is stabilized by attaching to the end N of N8. Acetylene-temperature programed desorption (C2H2-TPD) and density functional theory (DFT) calculations suggest C2H2 favors the π bonding on single Pd atom, while H2 dissociates on the N atom (next to Pd) instead of conventionally on Pd. The synergistic effect favors C2H4 formation but prevents full hydrogenation of acetylene to C2H6. This work opens up a new perspective to design and synthesize more selective catalysts with isolated single-atom sites.

Modulating Single-Atom Palladium Sites with Copper for Enhanced Ambient Ammonia Electrosynthesis.

The electrochemical reduction of N2 to NH3 is emerging as a promising alternative for sustainable and distributed production of NH3 . However, the development has been impeded by difficulties in N2 adsorption, protonation of *NN, and inhibition of competing hydrogen evolution. To address the issues, we design a catalyst with diatomic Pd-Cu sites on N-doped carbon by modulation of single-atom Pd sites with Cu. The introduction of Cu not only shifts the partial density of states of Pd toward the Fermi level but also promotes the d-2π* coupling between Pd and adsorbed N2 , leading to enhanced chemisorption and activated protonation of N2 , and suppressed hydrogen evolution. As a result, the catalyst achieves a high Faradaic efficiency of 24.8±0.8 % and a desirable NH3 yield rate of 69.2±2.5 μg h-1 mgcat. -1 , far outperforming the individual single-atom Pd catalyst. This work paves a pathway of engineering single-atom-based electrocatalysts for enhanced ammonia electrosynthesis.

Intermetallic PdxIny/Al2O3 catalysts with isolated single-atom Pd sites for one-pot hydrogenation of diphenylacetylene into trans-stilbene

Novel one-pot synthesis of trans-stilbene comprises hydrogenation of diphenylacetylene into cis-stilbene followed by its hydroisomerization, over the catalyst being intermetallic PdxIny/Al2O3 system with isolated single-atom Pd1 sites. The hydroisomerization rate in the process is comparable or exceeds the over-hydrogenation into 1,2-diphenylethane thus providing 76% yield of trans-stilbene in the case of Pd2In3/Al2O3 catalyst. This result presumably stems from the formation of ordered single-atom Pd1 sites with favorable hydroisomerization ability on the surface of PdxIny catalysts.

Toward Phase and Catalysis Control: Tracking the Formation of Intermetallic Nanoparticles at Atomic Scale

Summary Intermetallic nanoparticles (iNPs) have yielded enormous successes in catalytic applications by the formation of ordered phases. However, atomic-level understanding of the alloying mechanism, which plays a pivotal role in controlling intermetallic phases and tailoring their catalytic properties, is still elusive. In this study, we discovered a consecutive formation of ordered Pt3Sn and PtSn phases during the growth of Pt-Sn iNP inside a well-defined nano-reactor at elevated temperature by using in situ scanning transmission electron microscopy. We found that the surface-mediated diffusion of Sn controls overall dynamics of the reaction, while the unique coherent interfacial structure is determinative for the PtSn transformation. We then further controlled the phase selection of Pt-Sn iNPs and demonstrated their distinguishable catalytic behaviors. Our findings not only provide detailed experimental evidence on the alloying mechanism in intermetallic nanoscale systems but also pave the way for mechanistic control of synthesis and catalytic properties of iNPs.

PdIn intermetallic material with isolated single-atom Pd sites – A promising catalyst for direct formic acid fuel cell

Isolated single-atom Pd sites in intermetallic structures have the higher stability and unique catalytic properties. This study was designed to investigate the properties of isolated single-atom Pd sites in PdIn intermetallic toward formic acid oxidation compared to Pd3In and the pure Pd using density functional theory calculation. Unexpectedly, CO adsorption over PdIn catalyst is significantly lower than those over Pd3In and the pure Pd catalysts, an indication that CO poisoning is remarkably reduced on PdIn catalysts compared to the Pd3In and pure Pd catalysts. More importantly, PdIn catalyst with isolated single-atom Pd sites exhibits higher selectivity for formic acid oxidation to CO2 and inhibits CO formation, whereas Pd3In catalyst with Pd trimer presents better selectivity for formic acid oxidation to CO, which poisons Pd-based catalyst. Moreover, compared to the pure Pd catalyst, PdIn catalyst with isolated single-atom Pd sites not only has the higher activity and stability but also maximize atomic efficiency for noble metal Pd to reduce the catalyst cost. It is expected that the method applied in this research is beneficial to for the design and evaluation of intermetallic catalysts in clean energy technology development.

Highly Stable Single-Atom Catalyst with Ionic Pd Active Sites Supported on N-Doped Carbon Nanotubes for Formic Acid Decomposition.

Single-atom catalysts with ionic Pd active sites supported on nitrogen-doped carbon nanotubes have been synthesized with a palladium content of 0.2-0.5 wt %. The Pd sites exhibited unexpectedly high stability up to 500 °C in a hydrogen atmosphere which was explained by coordination of the Pd ions by nitrogen-containing fragments of graphene layers. The active sites showed a high rate of gas-phase formic acid decomposition yielding hydrogen. An increase in Pd content was accompanied by the formation of metallic nanoparticles with a size of 1.2-1.4 nm and by a decrease in the catalytic activity. The high stability of the single-atom Pd sites opens possibilities for using such catalysts in high-temperature reactions.

Single-atom Pd sites on the surface of Pd–In nanoparticles supported on γ-Al2O3: a CO-DRIFTS study

The CO-DRIFTS investigation of a Pd–In catalyst obtained via the PdIn(AcO)5 complex showed the presence of single palladium atoms isolated by indium on the surface. A short-time exposure to air under ambient conditions induced the decomposition of the single-atom structure, which can be recovered by mild reduction.

Isolated Single-Atom Pd Sites in Intermetallic Nanostructures: High Catalytic Selectivity for Semihydrogenation of Alkynes.

Improving the catalytic selectivity of Pd catalysts is of key importance for various industrial processes and remains a challenge so far. Given the unique properties of single-atom catalysts, isolating contiguous Pd atoms into a single-Pd site with another metal to form intermetallic structures is an effective way to endow Pd with high catalytic selectivity and to stabilize the single site with the intermetallic structures. Based on density functional theory modeling, we demonstrate that the (110) surface of Pm3̅m PdIn with single-atom Pd sites shows high selectivity for semihydrogenation of acetylene, whereas the (111) surface of P4/mmm Pd3In with Pd trimer sites shows low selectivity. This idea has been further validated by experimental results that intermetallic PdIn nanocrystals mainly exposing the (110) surface exhibit much higher selectivity for acetylene hydrogenation than Pd3In nanocrystals mainly exposing the (111) surface (92% vs 21% ethylene selectivity at 90 °C). This work provides insight for rational design of bimetallic metal catalysts with specific catalytic properties.