Pd Surface Research Articles

Enhanced Electrochemical CO2 Reduction Using Nonthermal Plasma: Insights into Pd Catalyst Reactivation and Precise Control of H2O2 for Improved CO2 Reduction Reaction Activity

This study investigates the electrochemical reduction of CO2 on Pd/C with in situ‐generated H2O2 through low‐temperature nonthermal plasma. Catalyst deactivation, a common challenge in CO2 conversion, is addressed by leveraging the oxidizing environment created by H2O2. Experimental studies using linear sweep voltammetry and cyclic voltammetry demonstrate significantly improved CO2 reduction activity during plasma discharge, correlated with an enlarged hydrogen desorption peak. Multicomponent physics‐based computational simulation highlights the role of H2O2, a long‐lived species, in enhancing CO2 reduction. Formic acid is identified as a major liquid product, validated by nuclear magnetic resonance. The presence of H2O2 prevents CO poisoning on Pd surfaces, and H2O2 electroreduction alters hydrogen sorption, potentially creating an active PdHx phase for effective CO2 reduction. The study demonstrates the precise control of H2O2 concentration through nonthermal plasma, offering insights into Pd catalyst reactivation and improved CO2 reduction activity. These findings contribute to the understanding of electrochemical CO2 reduction mechanisms and provide a basis for optimizing catalytic processes in the presence of H2O2.

Weakening Pd─O Bonds by an Amorphous Pd Layer to Promote Electrocatalysis.

Construction of core-shell structured electrocatalysts with a thin noble metal shell is an effective strategy for lowering the usage of the noble metal and improving electrocatalytic properties because of the structure-induced geometric and electronic effects. Here, the synthesis of a novel core-shell structured nanocatalyst consisting of a thin amorphous Pd shell and a crystalline PdCu core and its significantly improved electrocatalytic properties for both formic acid oxidation and oxygen reduction reactions are shown. The electrocatalyst exhibits 4.1 times higher catalytic peak current density and better stability in the formic acid oxidation compared to both a PdCu nanoalloy catalyst and a Commercial Pd-C catalyst. An excellent electrocatalytic performance of the core-shell nanocatalyst is also observed in the oxygen reduction reaction. Computational calculation results reveal that tuning of the electronic state of Pd by the amorphous shell and the Cu in the PdCu core weaken the binding strength of surface Pd─O bonds, leading to a bond elongation to facilitate bond breaking. As a result, the electrocatalytic activity in both formic acid oxidation and oxygen reduction reactions is enhanced.

Enhanced resistance to poisoning of Pd in alkynes semi‐hydrogenation by metal–ligand electronic interactions

AbstractGiven that the retention of nitrogen readily renders active site poisoning, designing versatile catalysts characterized by notable selectivity and even resistance to poisoning for alkyne semi‐hydrogenation under nitrogen‐containing conditions is considerably challenging. In this article, oxanilide‐decorated Pd/C (Pd/C‐oxa) catalyst is facilely synthesized by leveraging impregnation‐coordination, which exhibit remarkable performance in the semi‐hydrogenation of nitrogen‐containing alkynes, with ultrahigh turnover frequency (TOF) of 15,831 h−1 and selectivity of 97.2%. Strikingly, it still sustains TOF of 12,137 h−1 in a sulfur‐containing system, demonstrating distinguished tolerance to sulfur. Comprehensive studies corroborate that oxanilide tunes the electron density of Pd by constructing metal–ligand electronic interactions, facilitating hydrogen activation. Simultaneously, the reaction microenvironment is optimized, which effectively promotes the desorption of nitrogen‐containing olefins and attenuates the aggregation of nitrogen on the Pd surface. This strategy is universal and holds promising industrial applications, making it appropriate for use in commercial Pd/C catalysts as well.

Hydride-Induced Reconstruction of Pd Electrode Surfaces: A Combined Computational and Experimental Study.

Designing electrocatalysts with optimal activity and selectivity relies on a thorough understanding of the surface structure under reaction conditions. In this study, experimental and computational approaches are combined to elucidate reconstruction processes on low-index Pd surfaces during H-insertion following proton electroreduction. While electrochemical scanning tunneling microscopy clearly reveals pronounced surface roughening and morphological changes on Pd(111), Pd(110), and Pd(100) surfaces during cyclic voltammetry, a complementary analysis using inductively coupled plasma mass spectrometry excludes Pd dissolution as the primary cause of the observed restructuring. Large-scale molecular dynamics simulations further show that these surface alterations are related to the creation and propagation of structural defects as well as phase transformations that take place during hydride formation.

Predictive evaluation of hydrogen diffusion coefficient on Pd(111) surface by path integral simulations using neural network potential

Quantum diffusion of hydrogen (H) on palladium (Pd) (111) was studied using two types of path integral simulations: quantum transition state theory (QTST) and ring polymer molecular dynamics (RPMD). The use of an artificial neural network potential trained by density functional theory calculations has made it feasible to perform path integral simulations considering nuclear quantum effects (NQEs) of a many-body Pd-H system with accuracy. The QTST result has shown a clear non-Arrhenius behavior in the diffusion coefficient (D) of H below the temperature of 150–200 K due to the NQEs. Comparing the D on Pd surface and bulk Pd, it was found that surface and bulk diffusion are competitive at high temperature. Consistent with this, it was observed in the RPMD simulation at 800 K that a part of the quantum trajectories of H on the surface bifurcates to the Pd subsurface. As the temperature decreases, the surface diffusion becomes much faster than the bulk diffusion. Furthermore, distinct quantum behaviors of H were identified at the surface and within the bulk. The D values obtained from this study using a “flexible” Pd surface model differed considerably from those previously reported using “frozen” Pd surface models, indicating an important contribution from Pd vibrations coupled with H diffusion. Published by the American Physical Society 2024

Effect of para-substituents on NC bonding of aryl isocyanide molecules adsorbed on metal surfaces studied by sum frequency generation (SFG) spectroscopy.

The effect of para-substituents on the NC bond of aryl isocyanide molecules adsorbed on Au, Ag, Pt, and Pd surfaces was examined by vibrational sum frequency generation (VSFG) spectroscopy. The frequency of NC stretching vibration, υNC, depended on the electronic property of the substituents. When the substituents were more electron-withdrawing and more electron-donating, υNC shifted to lower and higher frequencies, respectively. However, υNC shifted to lower frequency again when the substituents were too electron-donating. The strong dependence of υNC on metal substrates was also observed, reflecting the difference in metal d-band energies, which leads to the difference in the contributions of σ-donation and π back-donation of the NC-metal coupling.

Highly active cobalt oxide-palladium nanohybrid catalyst enables selective electrocatalytic synthesis of glycolic acid

Glycolic acid is a unique value-added chemical, while its large-scale production is limited by harsh synthesis conditions and poor selectivity. Herein, we have ingeniously constructed a highly active cobalt oxide-supported palladium electrocatalyst on nickel foam (Pd-Co3O4/NF) for ethylene glycol oxidation to glycolic acid with a high Faradaic efficiency (93.1 %) at ultra-low potential (0.9 V vs. RHE). In situ Fourier transform infrared spectroscopy revealed the pathway of ethylene glycol oxidation reaction (EGOR). The theoretical results reveal that Co3O4/NF can effectively promote the formation of *OH and weaken the adsorption of toxic *CO intermediates on Pd-Co3O4/NF. And the downshifted d-band center of Pd-Co3O4/NF is inducive to facilitate the desorption of GA from the Pd surface, prevents the excessive oxidation thus improving the selectivity. Moreover, Pd-Co3O4/NF exhibits considerable anode current density with respect to electro-reforming polyethylene terephthalate (PET)-derived EG oxidation, accompanied by the generation of green hydrogen at the cathode. This work puts forward an innovative approach for catalyst construction toward EGOR and highlights the strategy of electro-upcycling of PET.

MnOx and Pd Surface Functionalization of TiO2 Thin Films via Photodeposition UV Dose Control

This study investigated the influence of the ultraviolet (UV) dose (DUV) on the photodeposition of MnOx and Pd cocatalysts on 300-nm-thick anatase TiO2 thin films, which were prepared via sol–gel dip-coating on a glass substrate. MnOx and Pd were photodeposited using increasing UV doses ranging from 5 to 20 J cm−2, from 5 mM aqueous electrolytes based on Mn2+/IO3− or Pd2+, respectively. The effect of the DUV on the MnOx photodeposition resulted in an increase in Mn2+ surface content, from 2.7 to 5.2 at.%, as determined using X-ray photoelectron spectroscopy (XPS). For Pd, increasing the UV dose led to a reduction in the oxidation state, transitioning from Pd2+ to Pd0, while the overall Pd surface content range remained relatively steady at 2.2–2.4 at.%. Both MnOx/TiO2 and Pd/TiO2 exhibited proportional enhancements in photocatalytic activity towards the degradation of methylene blue. Notably, Pd/TiO2 demonstrated a significant improvement in photocatalytic performance, surpassing that of pristine TiO2. In contrast, TiO2 samples functionalized through wet impregnation and thermal treatment in the same electrolytes showed overall lower photocatalytic activity compared to those functionalized via photodeposition.

Pd 4d Orbital Overlapping Modulation on Au@Pd Nanowires for Efficient H2O2 Production

Electrochemical synthesis of hydrogen peroxide has been emerging as an appealing process for the onsite production of this chemically valuable oxidant, while the catalyst design for H2O2 production is conducted mainly using trial-and-error approaches. It results in any knowledge acquired of active sites and catalytic mechanisms remains opaque and empirical. Herein, we design a series of Au@Pd core-shell structure nanowire to reveal the quantitative structure-performance correlation in atomic level. The structure of Pd shell was controlled by the amount of PdCl2 precursor, to achieve the epitaxial-growth of several atom layer, one layer, and surface site isolation. Electrochemical evaluation shows the selectivity is relevant to the surface structure and composition. With the reducing of surface Pd deposition, the predominant ORR mechanism on catalyst changes from the 4e- to 2e- pathway to produce H2O2. Moreover, a high kinetics mass activity (5.71 A mg-1 at 0.4V) of Au@Pd(15:1) NWs is achieved, which is almost three times larger than the state-of-the-art Pd-Hg alloy catalyst. The excellent performance can be attributed to the unique structure of Pd shell with low Pd-Pd coordination number, active sites isolation, and high Pd utilization efficiency. Structural and electrochemical analysis reveal that Pd4 structure is crucial for the ensuring high selectivity. Particle size beyond Pd4 will be more beneficial for driving the 4-electron pathway to produce H2O. Interestingly, our initial theoretical prediction by Density Functional Theory (DFT) calculations demonstrates Pd4 is a 4-electron catalyst that binds *OOH intermediate too strongly. Such discrepancy is from the oxygen coverage phenomenon under operando conditions. The oxygen coverage on Pd4 surface causes electron redistribution and weakened *OOH binding strength. It pushes Pd4-O to the peak of the volcano plot with only 0.06 eV difference from the ideal value.

Mesoporous silica nanospheres-encaged metal-metalloid alloy nanoparticles as efficient alkyne semi-hydrogenation catalysts

In this work, we report metal-metalloid Pd2B alloy nanoparticles as highly efficient and selective catalysts for semi-hydrogenations of alkynes. The mentioned materials feature the residence of small Pd2B nanoparticles in the cavities of hollow mesoporous silica nanospheres (Pd2B@HMSNs), and were synthesized by in situ transformation of Pd@HMSNs with B sources. At near room temperatures and atmospheric H2 pressure, Pd2B@HMSNs exhibited high alkene selectivity for hydrogenations of a series of alkynes. Especially, such high alkene selectivity can be maintained with a prolonged reaction time, and nearly 100 % selectivity of cis to trans alkene is obtained for those internal alkynes. The excellent performance of Pd2B@HMSNs is originated from the residence of catalytic functionality inside hollow nanostructures and the change of electronic/geometric properties of surface Pd species by alloying with B, where the former improves their reusability and the latter suppresses the deep-hydrogenation of alkenes.

Revealing Dynamics and Competitive Mechanism of Gas-Induced Surface Segregation of PdFe0.08 Dilute Alloy by Multi-Dimensional Imaging.

The restructuring of dilute alloys under gas environments has shown a great impact on their catalytic performance due to intriguing structural sensitivity, but the structural dynamics and underlying mechanism remains elusive. Herein, we directly resolved the distinct dynamic behaviors of PdFe0.08 dilute alloys under CO or O2 environment by multidimensional imaging. The stronger binding of gaseous CO with Fe atoms stimulates Fe segregation out of the PdFe0.08, resulting in 3D growth of Fe islands, whereas the dissociative adsorption of O2 results in 2D layer-by-layer growth of segregated FeO as encapsulation overlayers that bind strongly with the Pd surface underneath. Such varied structures remarkably tune the catalytic activity for CO oxidation, showing a considerably high activity for a CO-treated sample. Our results reveal the competitive mechanism between adsorbate-metal and metal-metal interaction for gas-induced surface segregation, which should be highly considered for the rational design of dilute alloys with dynamically tuned structure and reactivity.

Tuning the selectivity of Pd-based catalysts for CO oxidative esterification: Regulating Pd's electronic effect

CO oxidative esterification is crucial in converting inorganic CO into organic oxygenated compounds within C1 chemistry and technology. However, understanding the structure-activity relationship underlying this reaction remains limited, primarily due to the entanglement of the Pd's geometric and electronic structures, which complicates selectivity control in practical applications. In this work, we design four Pd-based catalysts using the Al(111) motif to investigate their catalytic performance in CO oxidative esterification through density functional theory (DFT) calculations. Firstly, the larger lattice parameter of Al weakens horizontal Pd–Pd interactions while enhancing the vertical stability of Pd. Secondly, the electronic structure of Pd is locally regulated via charge transfer between subsurface and surface Pd, driven by their differences in work function. Our results indicate that the relative activity between CC coupling for dimethyl oxalate (DMO) and CO coupling for dimethyl carbonate (DMC) is primarily influenced by Pd's electronic effects, as revealed by energy barrier decomposition analysis. Thus, focusing on the regulation of Pd's electronic effects may offer a new strategy for controlling the selective oxidation esterification of CO.

Mechanistic Insights on Coverage-Dependent Selectivity Limitations in Vinyl Acetate Synthesis.

Developing improved catalysts for sustainable chemical processes often involves understanding atomistic origins of catalytic activity, selectivity, and stability. Using density functional theory and steady-state kinetic analyses, we probe the elementary steps that form decomposition products that limit selectivity in vinyl acetate (VA) synthesis on Pd surfaces covered with acetate species. Acetate formation and coupling with ethylene control the VA formation catalytic cycle and steady-state coverage, but acetate and ethylene can separately decompose to form CO2. Both decompositions involve initial C-H activations at acetate vacancies, followed by additional C-H activations and eventual C-O formations and C-C cleavages involving reactions with molecular oxygen. Acetate decomposition paths with non-oxidative kinetically-relevant steps exhibit similar free energy barriers to oxidative paths. In contrast, the non-oxidative ethylene path involving an ethylidyne intermediate exhibits a much lower barrier than paths with oxidative kinetically-relevant steps. Ethylene decomposition is very facile at low coverages but is more coverage-sensitive, leading to similar decomposition and VA formation barriers at coverages accessible at steady state, which is consistent with moderate VA selectivity in measurements and ethylene vs. acetate decomposition contributions assessed from regressed kinetic parameters. These insights provide a detailed framework for describing VA synthesis rates and selectivity on metallic catalyst surfaces.

A Comprehensive Investigation on Catalytic Behavior of Anaerobic Jar Gassing Systems and Design of an Enhanced Cultivation System.

The rapid and reliable diagnosis of anaerobic bacteria constitutes one of the key procedures in clinical microbiology. Automatic jar gassing systems are commonly used laboratory instruments for this purpose. The most critical factors affecting the cultivation performance of these systems are the level of residual oxygen remaining in the anaerobic jar and the reaction rate determined by the Pd/Al2O3 catalyst. The main objective of the presented study is to design and manufacture an enhanced jar gassing system equipped with an extremum seeking-based estimation algorithm that combines real-time data and a reaction model of the Pd/Al2O3 catalyst. The microkinetic behavior of the palladium catalyst was modeled through a learning-from-experiment methodology. The majority of microkinetic model parameters were derived from material characterization analysis. A comparative validation test of the designed cultivation system was conducted using conventional gas pouches via six different bacterial strains. The results demonstrated high cell viability, with colony counts ranging from 1.26 × 105 to 2.17 × 105 CFU mL-1. The favorable catalyst facets for water formation on Pd surfaces and the crystal structure of Pd/Al2O3 pellets were identified by X-Ray diffraction analysis (XRD). The doping ratio of the noble metal (Pd) and the support material (Al2O3) was validated via energy-dispersive spectroscopy (EDS) measurements as 0.68% and 99.32%, respectively. The porous structure of the catalyst was also analyzed by scanning electron microscopy (SEM). During the reference clinical trial, the estimation algorithm was terminated after 878 iterations, having reached its predetermined termination value. The measured and modelled reaction rates were found to converge with a root-mean-squared error (RMSE) of less than 10-4, and the Arrhenius parameters of ongoing catalytic reaction were obtained. Additionally, our research offers a comprehensive analysis of anaerobic jar gassing systems from an engineering perspective, providing novel insights that are absent from the existing literature.

Proton Ionic Liquid Modulates Hydrogen Coverage and Subsurface Absorbed Hydrogen to Enhance Pd Metallene Electrocatalytic Semi-hydrogenation of Alkynols.

Electrochemical semi-hydrogenation of alkynols to produce high-value alkenols is a green and sustainable approach. Although Pd can exhibit excellent semi-hydrogenation properties, its intrinsic mechanism still lacks in-depth study. Herein, a proton ionic liquid (PIL)-modified Pd metallene (Pdene@PIL) is synthesized for the electrocatalytic semi-hydrogenation of 2-methyl-3-butyn-2-ol (MBY) to 2-methyl-3-buten-2-ol (MBE). The PIL modification of Pdene@PIL resulted in an MBY conversion of 96.1% and MBE selectivity of 97.2%, respectively. Theoretical calculations indicate the electron transfer between Pdene and PIL, leading to easier adsorption of MBY on the Pd surface. The d-band center of Pdene@PIL shifts away from the Fermi level, which weakens the adsorption of over-hydrogenated intermediates. At the same time, the PIL modification facilitates the adsorption of surface-adsorbed hydrogen (H*ads) and inhibits the formation of subsurface-absorbed hydrogen (H*abs). In particular, the PIL modification optimizes Hads* coverage, reduces the reaction energy of the rate-determining step (C5H8O*-C5H9O*), and inhibits HER. The reduction of H*abs formation inhibits the transfer of Pd to PdHx and suppresses the over-hydrogenation. This work provides new insights into the modulation of H* to enhance the alkynol electrocatalytic semi-hydrogenation reaction (ESHR) process from the perspective of surface modification.

Inhibiting Emulative Oxygen Adsorption via Introducing Pt-Segregated Sites into the Pd Surface for Enhanced H2 Sensing in Air.

Pd-modified metal sulfide gas sensors exhibit excellent hydrogen (H2) sensing activity through spillover effects. However, the emulative oxygen adsorption often occupies an exposed Pd surface and thus limits the effective Pd-H interaction, impeding the H2 sensing performance in air. Herein, we develop an edge-rich Pt-shell/Pd-core structure to adjust the selective adsorption between oxygen and hydrogen for effective H2 sensing in an air atmosphere. Detailedly, through accurately regulating the rate of Pt deposition onto the icosahedron Pd surface, an edge-rich Pt-shell/Pd-core structure can be first achieved. It has been found that marginal Pt aggregations can segregate the oxygen molecules around the Pt species and induce easier Pt-O bonding, further guiding accessible Pd surfaces for effective Pd-H interactions, which can be verified by 1H ssNMR, in-situ Raman, ex-situ XPS, and density functional theory analyses. The final ZnS/PdPt sensor exhibits an ultrasensitive response (8608 to 4% H2) and a wide detected range (0.5 ppm-4%) in air, exceeding most reported hydrogen sensors.

On the Design of the Metal-Support Interface in Methanol Electrocatalytic Oxidation.

In this work, we present a theoretical investigation of the SrTiO3 perovskite-supported Pd catalyst in the methanol electro-oxidation reaction. In order to determine the metal-support interactions, we designed a system consisting of a Pd (100) double layer supported on one of the two possible terminations of the (100) perovskite surface. These terminations are characterized by different reducibilities of the layers directly interacting with the Pd bilayer and result in the difference in the stability of the surface-bound intermediates. Despite the fact that the Pd surface is identical in terms of geometry, we observed significant differences in the overpotential required for the reaction; in the case of TiO2 termination, the overpotential has been determined to be 0.68 V, while in the case of SrO termination, it amounts to as much as 1.35 V. We further investigate the charge transfers within the components of the system and the geometries of the intermediates to unravel the role of the electron structure on the overall efficiency of the process.

Long-term reliable wireless H2 gas sensor via repeatable thermal refreshing of palladium nanowire

The increasing significance of hydrogen (H2) gas as a clean energy source has prompted the development of high-performance H2 gas sensors. Palladium (Pd)-based sensors, with their advantages of selectivity, scalability, and cost-effectiveness, have shown promise in this regard. However, the long-term stability and reliability of Pd-based sensors remain a challenge. This study not only identifies the exact cause for performance degradation in palladium (Pd) nanowire H2 sensors, but also implements and optimizes a cost-effective recovery method. The results from density functional theory (DFT) calculations and material analysis confirm the presence of C = O bonds, indicating performance degradation due to carbon dioxide (CO2) accumulation on the Pd surface. Based on the molecular behavior calculation in high temperatures, we optimized the thermal treatment method of 200 °C for 10 minutes to remove the C = O contaminants, resulting in nearly 100% recovery of the sensor’s initial performance even after 2 months of contamination.

Interaction of H2S with perfect and O-covered Pd(100) surface: A first-principles study

Using density functional theory (DFT) calculations, we investigated the dissociative mechanism of H2S adsorption and its dissociation on both clean and oxygen covered Pd (100) surface. For adsorption mechanism, the site preference was considered for H2S and HS monomers on both surfaces. The results suggest that H2S is energetically adsorbed on high symmetry sites, particularly the hollow site, on clean surface and top site on O-covered Pd (100) surface. Chemisorption of HS is stronger than H2S at the favorable hollow site on clean surface and O-covered surface. The energy barriers for S–H bond-breaking during the first H2S dehydrogenation are 0.35, 0.07, 0.32, and 0.16 eV, while for second dehydrogenation are 0.32 and 0.30 eV, respectively. On the oxygen-covered surface, the H2S adsorption site transitions from bridge to the top site and their energy barrier for first-order decomposition is 0.05 eV, significantly lesser than that observed on a clean surface. To examine more, the electronic densities of states as well as d-band center model were used to characterize the interaction of adsorbed H2S with the surfaces. Overall, our findings show that H2S decomposition on these surfaces is exothermic and relatively easy, but the presence of surface oxygen hinders the breaking process of H–S bond due to kinetic and thermodynamic factors.

Study of argon plasma modified Pd/AlGaN/GaN high electron mobility transistor for superior hydrogen gas detection

The surface and microstructure of the sensitive material play an important role in the gas adsorption and detection. However, few report of the surface modification affection on the work function-based gas sensor is carried out. Herein, facile Ar+ modification is proposed and utilized to modify the surface of the Pd sensitive layer, which acts as the gate of a AlGaN/GaN high electron mobility transistor (HEMT) based hydrogen (H2) gas sensor. Results show that the Pd surface is significantly changed to smoother microstructure by 5 s Ar+ modification. Compared with the device without Ar+ modification, the response values increase by ∼ twice at all operating temperatures. Interestingly, the response time is pronouncedly shortened from 10 to 2.3 s at 500 ppm, suggesting the significant effect of Ar+ modification in the enhancement of gas detection. At the optimized operating temperature of 120 °C, a high response of 739% and an ultrafast response time of 1.4 s are observed for the 500 ppm H2 gas. In addition, this sensor exhibits a low limit of detection ∼200 ppb, along with excellent repeatability, consistency, and selectivity. Our study indicates that the facile Ar+ modification of the gate surface is a highly effective technology in the enhancement of gas detection and may provide a novel approach for the design and development of all the work function type gas sensors.