Pd Clusters Research Articles

Charge polarization induced site preferences phenomena of ethylene hydrogenation via electronic metal support interactions

The catalytic activity of supported catalyst is closely related to the electronic metal-support interactions (EMSI). In this paper, first principles density functional theory study was conducted to understand the EMSI between Pd clusters (Pd4, Pd7, Pd10) and CeO2 substrate. When Pd was loaded on CeO2 surface, the electrons of Pd would transfer to the CeO2 substrate via Pd-O bonding interactions. Moreover, the charge redistribution on the Pd cluster leads to significant polarization of electron density. Consequently, the Pd atoms located at the metal-support interface (IF site) are positively charged, while the Pd atoms at the topmost (T site) are negatively charged. The H2 dissociation and ethylene hydrogenation at these sites were calculated with both low and high hydrogen coverage. It was found that the two types of sites should collaborate synergically to promote the hydrogenation of ethylene. In particular, the IF sites exhibit relatively high activity for H2 dissociation, while the T sites show superior catalytic efficiency for ethylene hydrogenation. Therefore, the EMSI induced charge polarization can tune the electronic structure and charge state of supported metal catalysts, which further determines the performance of different region on the catalyst.

Insights into dynamic evolution of surface oxidized Pd clusters for efficient hydrogen peroxide production

The direct synthesis of H2O2 from H2 and O2 involves the structural evolution of nano-sized catalyst during highly exothermic process, ultimately resulting in catalyst deactivation. The mechanism of catalyst structural evolution and its impact on the catalytic performance were elucidated by combining theoretical calculations with experimental characterizations. Through theoretical calculations, the dynamic evolution of surface-oxidized Pd12 clusters loaded on graphene (Pd12@G) during the reaction process was demonstrated. Different oxygen concentrations and reactants can lead to the evolution of the catalyst and consequently to the changes in catalytic activity. The relaxed Pd12@G with a doped O atom exhibits an ultra-low reaction energy of ∼ 0.20 eV for H2O2 production. The experimental findings also confirm the dynamic variation of the palladium oxide catalyst during the reaction. This work provides a microscopic understanding of the complex catalyst-reactant interaction, and casts helpful insights into the catalyst deactivation during the synthesis of hydrogen peroxide.

A highly active Pd clusters hosted by magnesium hydroxide nanosheets promoting hydrogen storage

Liquid organic hydrogen carriers (LOHCs) represent promising candidates for large-scale hydrogen storage, yet the design of high-efficiency and low-cost catalysts on the dehydrogenation of LOHCs remains challenging. Herein, we develop a two-dimensional (2D) magnesium hydroxide nanosheet that hosts Pd clusters (Pd/Mg(OH)2) for the dehydrogenation of dodecahydro-N-ethylcarbazole (12H-NEC), an important reaction for H2 transportation. The catalyst boosts the activity up to a 12H-NEC conversion of 100 % and the hydrogen release of 5.72 wt% with a low metal loading of 0.5 wt%, while the hydrogen release on Pd/MgO is only 2.34 wt% with the conversion rate of 71 % under the same conditions. Coupling various characterization technologies (HAADF-STEM, EXAFS, EPR and In-situ DRIFTS) with DFT calculations, it was found that the work function of magnesium hydroxide nanosheets for the dehydrogenation reaction due to the strong metal-support interaction, which makes Pd species have better dispersion, forms Pdn+-OH stable Pd cluster and reduces the dehydrogenation barrier as well, thus enhancing the dehydrogenation performance and improving the hydrogen production greatly. This work provides a new promising way to design advanced dehydrogenation catalyst with high activity for the hydrogen storage.

The nitro to amine reduction: from millions of tons to single molecule studies.

Palladium nanostructures are interesting heterogeneous catalysts because of their high catalytic activity in a vast range of highly relevant reactions such as cross couplings, dehalogenations, and nitro-to-amine reductions. In the latter case, the catalyst Pd@GW (palladium on glass wool) shows exceptional performance and durability in reducing nitrobenzene to aniline under ambient conditions in aqueous solutions. To enhance our understanding, we use a combination of optical and electron microscopy, in-flow single molecule fluorescence, and bench chemistry combined with a fluorogenic system to develop an intimate understanding of Pd@GW in nitro-to-amine reductions. We fully characterize our catalyst in situ using advanced microscopy techniques, providing deep insights into its catalytic performance. We also explore Pd cluster migration on the surface of the support under flow conditions, providing insights into the mechanism of catalysis. We show that even under flow, Pd migration from anchoring sites seems to be minimal over 4 h, with the catalyst stability assisted by APTES anchoring.

Theoretical insights into the support effect on the NO activation over platinum-group metal catalysts.

We systematically explored NO activation at metal/oxide interfaces by the combination of Sr3Ti2O7, Sr3Fe2O7, CeO2, anatase-TiO2, ZrO2, and γ-Al2O3 supports and the platinum-group metal cluster (Pd4, Pt4, and Rh4) using slab-model density functional theory calculations. These metal clusters can be strongly adsorbed at these metal oxide surfaces. The Pt4 and Rh4 clusters show larger adsorption energies than the Pd4 cluster, yet the γ-Al2O3(100) surface shows smaller adsorption energies than other metal oxide surfaces. One oxygen vacancy close to the metal cluster was constructed to evaluate the NO activation at those metal/oxide interfaces. The O atom of NO refills the oxygen vacancy after NO dissociation, while the N adatom is left on the metal cluster. The exothermic process was identified for the NO activation except for the Sr3Fe2O7 case, indicating the significant role of the interplay between the metal cluster and oxygen vacancy.

The effect of rutile TiO[formula omitted](110) surface on the physicochemical properties of subnanometer Au–Pd clusters: A DFT study

Bimetallic clusters are of particular interest in several applications due to the unique properties they exhibit as a result of the synergistic effect of their metals. In addition, the substrate can alter the structure and stability of these clusters by means of cluster–support interactions. In this work, we study the physicochemical properties of Au–Pd/TiO2(110) supported clusters via density functional theory (DFT) calculations. The effect of surface TiO2(110) on the structural and energetic properties of Au–Pd clusters is relevant. The cohesion energy depends on both metal–metal and metal–support interactions. Au and Au-rich bimetallic clusters change their gas phase configuration upon adsorption. Some clusters do not fit well on the surface, resulting in decreased stability. By contrast, pure Pd and Pd-rich bimetallic clusters have the highest adsorption energies and charge polarization due to the cluster–surface interaction. These clusters exhibit elongation in the metal–metal bonds and redshifts in their vibrations. The Bader charge analysis shows that Au–Pd clusters donate charge to the surface, thus becoming positively charged. Finally, the TiO2(110) surface induces stiffness in the clusters, since the supported clusters do not exhibit low-frequency vibrations like their gas-phase analogs.

Comparative computational study of CO2 hydrogenation and dissociation on metal-doped Pd clusters

Density functional theory calculations are executed to research CO2 adsorption and initial conversion on Pd12M (M = Fe, Co, Ni, Cu, Zn, Ru, Rh, Pd, Ag, Cd) bimetallic clusters, with a focus on finding optimal ancillary metal to Pd element. The stability analysis of the structures demonstrates that the transition metals with higher surface energy (compared to Pd element) doped Pd13 clusters possess better structural stability. For CO2 adsorption, the structural parameters of CO2* have remarkable linear relation with the amount of charges transfer. For CO2 initial conversion, it can be concluded that the Pd12Cu cluster is the best candidate with a lower activation barrier (0.69 eV) for the initial hydrogenation of CO2. According to the electronic structure analysis, the elevated HOMO caused by the doping of Cu atom is the main reason for the enhanced catalytic activity. In addition, the PdCu bimetallic clusters with larger size are constructed and their activity and selectivity towards CO2 initial conversion are also studied. Compared with Pd32Cu6 and Pd42Cu13 bimetallic clusters, Pd12Cu has more advantages in activity and selectivity. Therefore, Pd12Cu is the best choice, that is, it inhibits the generation of by-product and improves the catalytic activity, which makes it become an excellent bimetallic candidate for CO2 initial hydrogenation.

Understanding of Active Sites and Interconversion of Pd and PdO during CH4 Oxidation.

Pd-based catalysts are widely used in the oxidation of CH4 and have a significant impact on global warming. However, understanding their active sites remains controversial, because interconversion between Pd and PdO occurs consecutively during the reaction. Understanding the intrinsic active sites under reaction conditions is critical for developing highly active and selective catalysts. In this study, we demonstrated that partially oxidized palladium (PdOx) on the surface plays an important role for CH4 oxidation. Regardless of whether the initial state of Pd corresponds to oxides or metallic clusters, the topmost surface is PdOx, which is formed during CH4 oxidation. A quantitative analysis using CO titration, diffuse reflectance infrared Fourier-transform spectroscopy, X-ray diffraction, and scanning transmission electron microscopy demonstrated that a surface PdO layer was formed on top of the metallic Pd clusters during the CH4 oxidation reaction. Furthermore, the time-on-stream test of CH4 oxidation revealed that the presence of the PdO layer on top of the metallic Pd clusters improves the catalytic activity. Our periodic density functional theory (DFT) calculations with a PdOx slab and nanoparticle models aided the elucidation of the structure of the experimental PdO particles, as well as the experimental C-O bands. The DFT results also revealed the formation of a PdO layer on the metallic Pd clusters. This study helps achieve a fundamental understanding of the active sites of Pd and PdO for CH4 oxidation and provides insights into the development of active and durable Pd-based catalysts through molecular-level design.

Pd/PdO and hydrous RuO2 difunction-modified SiO2@TaON@Ta3N5 nano-photocatalyst for efficient solar overall water splitting

A novel Pd/PdO and hydrous RuO2 difunction-modified SiO2@TaON@Ta3N5 core-shell structured nano-photocatalyst was synthesized successfully, which displayed excellent photocatalytic activity for overall water splitting into H2 (473.52 μmol−1·g−1·h−1), about 2.86 times higher than unmodified SiO2@TaON@Ta3N5 (165.74 μmol−1·g−1·h−1), under the visible-light irradiation with the wavelength ≥420 nm, without any sacrificial agent, as well as excellent stability against photocorrosion. The apparent quantum yield (AQY) reaches to 0.253% under irradiation intensity of 12 mW cm−2 at 420 nm. The spatially separated Pd, PdO and RuO2 clusters were decorated on the Ta3N5 surface to construct local multi-heterojunctions, which were confirmed to enhance the light absorption capability, drive efficient separation of charge carriers and directional transfer, and promote surface redox reaction kinetics of HER and OER. The trace modification of metallic Pd clusters and TaN could mainly contribute to the significant decrease in the HER overpotential, while PdO exhibited a stronger contribution than RuO2 for OER catalytic activity. The synergetic mechanism of enhanced photocatalytic overall water splitting for hydrogen production was discussed in detail. Thus the combination of core-shell heterojunction construction and surface difunction modification provides a promising strategy for develop efficient all-in-one photocatalysts for solar overall water splitting.

Supported Metal Nanohydrides for Hydrogen Storage

Adsorption of hydrogen on graphdiyne (GDY) and boron-graphdiyne (BGDY) doped with palladium clusters has been investigated by performing density functional calculations. Pd6 fits well on the large holes of those porous layers, preserving its octahedral structure in GDY and changing it to a capped trigonal bipyramid structure in BGDY. Pd6GDY adsorbs up to five H2 molecules with sizable adsorption energies, two dissociated and three nondissociated. The dissociation barrier of H2 on the Pd6GDY cluster is 0.58 eV. Pd6BGDY can adsorb up to six molecules, three dissociated and three nondissociated, and the dissociation barrier of H2 on Pd6BGDY is 0.23 eV. In both cases, the dissociation barriers are substantially smaller than the corresponding dissociation barriers on undoped GDY and BGDY. The Pd clusters saturated with hydrogen can be viewed as nanohydrides. Spilling of the adsorbed hydrogen atoms toward the GDY and BGDY substrates is hindered by large activation barriers. We then propose using BGDY and GDY layers as support platforms for metal nanohydrides. The amount of stored hydrogen using Pd as the dopant is below the target of 6% of hydrogen in weight, but replacing Pd by a lighter metal with similar or higher affinity for hydrogen would substantially enhance the storage.

Pyridinic Nitrogen Sites Dominated Coordinative Engineering of Subnanometric Pd Clusters for Efficient Alkynes' Semihydrogenation.

Supported metal catalysts have played an important role in optimizing selective semihydrogenation of alkynes for fine chemicals. There into, nitrogen-doped carbons, as a type of promising support materials, have attracted extensive attentions. However, due to the general phenomenon of random doping for nitrogen species in the support, it is still atremendous challenge to finely identify which nitrogen configuration dominates the catalytic property of alkynes' semihydrogenation. Herein, it is reported that uniform mesoporous N-doped carbon spheres derived from mesoporous polypyrrole spheres are used as supports to immobilized subnanometric Pd clusters, which provide a particular platform to research the influence of nitrogen configurations on the alkynes' semihydrogenation. Comprehensive experimental results and density functional theory calculation indicate that pyridinic nitrogen configuration dominates the catalytic behavior of Pd clusters. The high contents of pyridinic nitrogen sites offer abundant coordination sites, which greatly reduces the energy barrier of the rate-determining reaction step and makes Pd clusters own high catalytic activity. The electron effect between pyridinic nitrogen sites and Pd clusters makes the reaction highly selective. Additionally, the good mesostructures also promote the fast transport of substrate. Based on the above, catalyst Pd@PPy-600 exhibits high catalytic activity (99%) and selectivity (96%) for phenylacetylene (C8 H6 ) semihydrogenation.

Defect anchoring of atomically dispersed Pd on nitrogen-doped holey carbon nanotube for catalytic hydrogenation of nitroarenes

High-density dispersion and efficient exposure of active metal sites are crucial for enhancing the catalytic activity of metal-based heterogeneous catalysts and improving the utilization of precious metal atoms. Increasing the surface to volume ratio by decreasing the size of metal nanoparticles is considered an ideal and straightforward strategy to increase catalytic activity. However, the generation of stable metal clusters or even single-atom metal species on supports is challenging since metal atoms are prone to agglomerate. In this work, we report a facile and practical method to anchor atomically dispersed Pd on an N-doped holey carbon nanotube (Pd/NHCNT) by efficiently strengthening metal-support interaction. Due to the unique characteristics of defected NHCNT support, highly dispersed Pd clusters and single atoms can be facilely introduced and anchored onto NHCNT. The resultant Pd/NHCNT catalyst exhibits a remarkable activity for nitroarenes hydrogenation with a turnover frequency as high as 1091.49 min−1, which surpasses the reported noble metal-based catalysts. Furthermore, theoretical simulations are applied for the understanding of the defect anchoring of atomically Pd and the interaction between active sites and substrate. The defect-induced anchoring approach provides a new avenue for designing and preparing other highly active atomic metal catalysts with high metal dispersion and can be applied to diverse organic reactions.

Transition from monolayer-thick 2D to 3D nano-clusters on α-Al2O3(0001).

This paper reports on the long-standing puzzle of the atomic structure of the Ag/α-Al2O3(0001) interface by combining X-ray absorption spectroscopy, to determine Ag local environment [i.e. average Ag-Ag (dAg-Ag) and Ag-O (dAg-O) interatomic distances and Ag coordination numbers (CN)], and numerical simulations on nanometric-sized particles. The experimental key was the capability of a structural study of clusters involving only a few atoms. The concomitant decrease of dAg-Ag and CN with decreasing cluster size provides unambiguous fingerprints for the dimensionality of the Ag clusters in the subnanometric regime leading to a series of unexpected results regarding the size-dependent interface structures. At low coverage, Ag atoms sit on surface Al sites to form buckled monolayer-thick islands associated with a Ag-Ag distance (2.75 Å) which fits the alumina lattice. Upon increasing Ag coverage, as 3D clusters appear, the Ag interface atoms tend to leave Al sites to sit atop O atoms as dAg-Ag increases. The then highlighted size-dependent evolution, is built on structural models which seemed so far contradictory in a static vision of the interface. Theory generalizes the case as it predicts the existence of alumina-supported 2D clusters of Pd and Pt at small coverage and a similar 2D-3D transition upon increasing the size. The structural transformation from 2D Ag clusters to macroscopic 3D islands is accompanied by a noticeable reduction of adhesion energy at the Ag/α-Al2O3(0001) interface.

Non-directed C-H arylation of electron-deficient arenes by synergistic silver and Pd3 cluster catalysis.

Transition-metal clusters have attracted great attention in catalysis due to their unique reactivity and electronic properties, especially for novel substrate binding and activation modes at the bridging coordination sites of metal clusters. Although palladium complexes have demonstrated outstanding catalytic performance in various transformations, the catalytic behaviors of polynuclear palladium clusters in many important synthetic methodologies remain much less explored so far. Herein, we disclose the use of an atomically defined tri-nuclear palladium (Pd3Cl) species as a catalyst precursor in Ag(I)-assisted direct C-H arylation with aryl iodides under mild conditions. This catalyst system leads to the formation of synthetically important biaryls in good yields with high site selectivities without the assistance of directing groups.

Organic Heterocyclic Strategy for Precisely Regulating Electronic State of Palladium Interface to Boost Alcohol Oxidation

AbstractExploring highly‐efficient palladium (Pd)‐based electrocatalysts for the alcohol oxidation reaction (AOR) is crucial yet challenging for chemists due to the vague Pd interface with an uncontrollable electronic environment. Herein, an organic heterocyclic strategy is used for the first time to modulate the electronic state of Pd electrocatalysts by anchoring Pd nanoparticles to conjugated microporous polymers (CMPs) with varied S‐, N‐, O‐, or S, N‐heterocycles. Among these CMPs, the S, N‐containing thiazole heterocyclic polymer SNC with Pd catalyst exhibits highly‐efficient current densities of 1575.0 mA mgPd−1 for methanol oxidation and 1071.0 mA mgPd−1 for ethanol oxidation, which are among the highest performance in the heterocyclic modulated Pd systems and surpass the commercial Pd black catalyst. Detailed theory calculations suggest that although the furan (O‐heterocycle) polymer OC has the strongest charge transfer (0.057 |e|) with the Pd cluster, the moderate electron transfer (0.041 |e|) of the Pd/SNC heterojunction with an S···N···Pd noncovalent interaction shows the best catalytic reaction kinetics. Moreover, the d‐band of the Pd/SNC system is closer to the volcano vertex than its counterparts. These results indicate that appropriate electron transfer intensity regulation of Pd electronic state by well‐defined heterocyclic structures may significantly improve AOR activity.

Density Functional Calculations of the Sequential Adsorption of Hydrogen on Single Atom and Small Clusters of Pd and Pt Supported on Au(111)

We have used density functional theory calculations to study the sequential adsorption of hydrogen on Pd and Pt atomic site catalysts such as single-atom alloy catalysts (SAAC), single-atom catalysts (SAC), and single cluster catalysts (SCC) on Au(111). The results show that Pd systems tend to have near-zero free energy of hydrogen adsorption (\(\Delta {G}_{{\mathrm{H}}_{\mathrm{ads}}}\approx 0\)) under various coverage conditions of adsorbed hydrogen. In the case of Pt systems, \(\Delta {G}_{{\mathrm{H}}_{\mathrm{ads}}}\approx 0\) only at high coverage conditions of adsorbed hydrogen. Such differences come from the preference of hydrogen for high-coordination and low-coordination sites on Pd and Pt, respectively. The low coordination of hydrogen results in multiple adsorption sites with \(\Delta {G}_{{\mathrm{H}}_{\mathrm{ads}}}\approx 0\) in SCC of Pt/Au. These results can help to understand the different catalytic properties of Pd/Au and Pt/Au.Graphical Abstract

Generation of adsorbed atomic hydrogen by Pd(II) doped Fe(OH)2 enables ultra-fast reductive dechlorination of trichloroethylene

Trichloroethylene (TCE) is a notorious persistent pollutant in groundwater, while most reductive dechlorination processes result in the formation of toxic less-chlorinated by-products. In this work, adsorbed atomic hydrogen (H*) was produced by Pd(II) doped Fe(OH)2, which was used for ultra-fast reductive dechlorination of TCE without the formation of less-chlorinated by-products. When the Fe(OH)2 dosage is 0.5 mM and the Pd(II) dosage is 20 μM, TCE (42 μM) is completely removed in 60 min with the pseudo first-order reaction rate constant of 7.74 h−1. Radical quenching experiment and electrochemical analysis both confirmed that adsorbed H* rather than absorbed H* contribute to the reduction of TCE. Density function theory calculation demonstrated that small Pd clusters were more beneficial to TCE dechlorination than large Pd clusters. The pseudo first-order rate constant for TCE reduction in real groundwater was as high as 0.018 min–1, demonstrating the potential application of Pd/Fe(OH)2 in groundwater remediation.

Hydrogen generation of single alloy Pd/Pt quantum dots over Co3O4 nanoparticles via the hydrolysis of sodium borohydride at room temperature

To satisfy global energy demands and decrease the level of atmospheric greenhouse gases, alternative clean energy sources are required. Hydrogen is one of the most promising clean energy sources due to its high chemical energy density and near-zero greenhouse gas emissions. A single alloyed phase of Pd/Pt nanoclusters as quantum dots (QDs) was prepared and loaded over Co3O4 nanoparticles with a low loading percentage (1 wt.%) for hydrogen generation from the hydrolysis of NaBH4 at room temperature. L-glutathione (SG) was used as a capping ligand. It was found that the single alloy catalyst (Pd0.5–Pt0.5)n(SG)m/Co3O4 caused a significant enhancement in hydrogen generation in comparison to the monometallic clusters (Pdn(SG)m and Ptn(SG)m). Moreover, the Pd/Pt alloy showed a positive synergistic effect compared to the physical mixture of Pd and Pt clusters (1:1) over Co3O4. The QDs alloy and monometallic Pd and Pt clusters exhibited well-dispersed particle size in ~ 1 nm. The (Pd0.5–Pt0.5)n(SG)m)/Co3O4 catalyst offers a high hydrogen generation rate (HGR) of 8333 mL min− 1 g− 1 at room temperature. The synergistic effect of Pd and Pt atoms in the nanoclusters alloy is the key point beyond this high activity, plus the prepared clusters' unique atomic packing structure and electronic properties. The effect of the NaBH4 concentration, catalyst amount, and reaction temperature (25–60 °C) were investigated, where HGR reaches 50 L min− 1 g− 1 at 60 °C under the same reaction conditions. The prepared catalysts were analyzed by UV–Vis, TGA, HR-TEM, XRD, and N2 adsorption/desorption techniques. The charge state of the Pd and Pt in monometallic and alloy nanoclusters is zero, as confirmed by X-ray photoelectron spectroscopy analysis. The catalysts showed high recyclability efficiency for at least five cycles due to the high leaching resistance of the alloy nanoclusters within the Co3O4 host. The prepared catalysts are highly efficient for energy-based applications.

Insights into Palladium Deactivation during Advanced Oxidation Processes.

A key step in creating efficient and long-lasting catalysts is understanding their deactivation mechanism(s). On this basis, the behavior of a series of Pd/corundum materials during several hydrogen adsorption/desorption cycles was studied using temperature-programmed desorption coupled with mass spectrometry and aberration-corrected transmission electron microscopy. The materials, prepared by impregnation and by sputtering, presented uniform well-dispersed Pd nanoparticles. In addition, single atoms and small clusters of Pd were only detected in the materials prepared by impregnation. Upon exposure to hydrogen, the Pd nanoparticles smaller than 2 nm and the single atoms did not present any change, while the larger ones presented a core-shell morphology, where the core was Pd and the shell was PdH x . The results suggest that the long-term activity of the materials prepared by impregnation can be attributed solely to the presence of small clusters and single atoms of Pd.

Dual Pd2+ and Pd0 sites on CeO2 for benzyl alcohol selective oxidation

Solvent-free aerobic oxidation of benzyl alcohol is a green approach for synthesizing fine chemicals. Pd-based catalysts are extensively investigated due to their excellent catalytic activity compared to other metal catalysts. Most researchers believe that metallic Pd species (Pd0) provide active centers. However, the critical role of active oxygen and the function of Pd2+ is neglected. Herein, supported Pd clusters on CeO2 (Pd/CeO2) with a stable high Pd2+/(Pd0 + Pd2+) ratio are prepared by atomic layer deposition (ALD). The turnover frequency (TOF) and activity on as-prepared Pd/CeO2 catalysts in solvent-free aerobic oxidation of benzyl alcohol reach 3.50 × 105 h−1 and 1.85 × 105 mol/(molPd·h), respectively, which are much superior to previously reported results. As the Pd loading increases, the TOFs exhibit a volcano relationship with the Pd2+/(Pd0 + Pd2+) ratio, pointing to a dual Pd0 and Pd2+ sites catalyzed mechanism. O2 kinetic isotopic effect and further evidence support a dual-site catalyzed oxygen-promoted β-hydride elimination mechanisms, in which benzyl alcohol is selectively adsorbed to Pd2+ sites and reacts with active oxygen species on nearby Pd0 sites during the reaction.