Pdn Clusters Research Articles

Revealing the Hidden Complexity and Reactivity of Palladacyclic Precatalysts: The P(o-tolyl)3 Ligand Enables a Cocktail of Active Species Utilizing the Pd(II)/Pd(IV) and Pd(0)/Pd(II) Pathways for Efficient Catalysis.

The ligand, P(o-tolyl)3, is ubiquitous in applied synthetic chemistry and catalysis, particularly in Pd-catalyzed processes, which typically include Pd(OAc)2 (most commonly used as Pd3(OAc)6) as a precatalyst. The Herrmann-Beller palladacycle [Pd(C^P)(μ2-OAc)]2 (where C^P = monocyclopalladated P(o-tolyl)3) is easily formed from reaction of Pd(OAc)2 with P(o-tolyl)3. The mechanisms by which this precatalyst system operates are inherently complex, with studies previously implicating Pd nanoparticles (PdNPs) as reservoirs for active Pd(0) species in arylative cross-coupling reactions. In this study, we reveal the fascinating, complex, and nontrivial behavior of the palladacyclic group. First, in the presence of hydroxide base, [Pd(C^P)(μ2-OAc)]2 is readily converted into an activated form, [Pd(C^P)(μ2-OH)]2, which serves as a conduit for activation to catalytically relevant species. Second, palladacyclization imparts unique stability for catalytic species under reaction conditions, bringing into play a Pd(II)/Pd(IV) cross-coupling mechanism. For a benchmark Suzuki-Miyaura cross-coupling (SMCC) reaction, there is a shift from a mononuclear Pd catalytic pathway to a PdNP-controlled catalytic pathway during the reaction. The activation pathway of [Pd(C^P)(μ2-OH)]2 has been studied using an arylphosphine-stabilized boronic acid and low-temperature NMR spectroscopic analysis, which sheds light on the preactivation step, with water and/or acid being critical for the formation of active Pd(0) and Pd(II) species. In situ reaction monitoring has demonstrated that there is a sensitivity to the structure of the arylboron species in the presence of pinacol. This work, taken together, highlights the mechanistic complexity accompanying the use of palladacyclic precatalyst systems. It builds on recent findings involving related Pd(OAc)2/PPh3 precatalyst systems which readily form higher order Pdn clusters and PdNPs under cross-coupling reaction conditions. Thus, generally, one needs to be cautious with the assumption that Pd(OAc)2/tertiary phosphine mixtures cleanly deliver mononuclear "Pd(0)Ln" species and that any assessment of individual phosphine ligands may need to be taken on a case-by-case basis.

Adsorption and sensing properties of dissolved gases in transformer oil using Cun and Pdn (n=1–3) cluster doped WSe2 monolayers

When the transformer fails, the transformer oil will decompose and produce fault characteristic gases. Detecting dissolved gases in the oil is significant to maintaining the safety and stability of the transformers. This manuscript mainly studies six modified substrates of Cun and Pdn (n=1–3) clusters modified WSe2, and applies them to detect three target gases (CO, CH4, C2H2). By analyzing key parameters based on density functional theory, TMn clusters can effectively improve the electrical conductivity of WSe2 and improve the detection ability of target gases. The order of adsorption effects of target gases on six modified substrates is as follow: TM1-WSe2 is consistent with Cu3-WSe2 (C2H2 > CO > CH4), and TM2-WSe2 is the same with Pd3-WSe2 (CO > C2H2 > CH4). The optimal recovery time for each gas is ultimately determined and these modifications are found to be effective in distinguishing and detecting the three gases. The research results show that WSe2 can be used as a gas sensor material, which provides a theoretical basis for detecting the operating status of oil-immersed transformers.

Surface reaction mechanisms research of Cun and Pdn (n=1–3) clusters modified MoSTe for fault gases in oil-immersed transformers

Efficient detection and timely resolution of internal faults within synthetic ester oil transformers are imperative for ensuring the stability of power systems. This paper proposes the utilization of MoSTe monolayer modified with Cun and Pdn (n=1–3) clusters for detecting internal fault gases (C2H2, C2H4, and CH4) in transformers. The adsorption mechanism and electronic properties of target gases on the modified substrate surface are systematically studied through density functional theory analysis of adsorption energy, density of states, and deformation charge density. The results indicate that TMn clusters possess the capability to enhance the reactivity of the MoSTe surface, leading to varying degrees of improvement in conductivity and adsorption capacity. Different target gases exhibit distinct adsorption characteristics, with the modified monolayer showing strong adsorption capabilities for C2H2 and C2H4. The calculation of work function and recovery time further reveal the promising potential of Pdn-MoSTe monolayer as resistive gas sensing materials for detecting C2H2 and C2H4, while Cun-MoSTe monolayer are also identified as high-quality candidates for C2H4 sensing materials. This study is expected to facilitate the practical application of MoSTe monolayer in the field of transformer operational state monitoring, providing new insights for exploring novel gas sensing materials.

Adsorption and gas sensing properties of Pdn(n=1–3) cluster-modified PtSe2 on transformer fault characterizing gases under biaxial strain and electric field

In this study, the adsorption behavior of three DGA gases (C2H2, C2H4, and CO) on monolayers of PtSe2 doped with metal clusters Pdn(n=1–3) was investigated using density-functional theory (DFT). The critical influence of the Pd clusters on the sensitivity of PtSe2 gas is revealed by in-depth analysis of the geometrical structure of the adsorption system, DCD, ELF, DOS and band gap. The results show that the detection performance of intrinsic PtSe2 monolayer is poor. However, the doping of Pdn(n=1–3) fundamentally changes the adsorption behavior of PtSe2 on C2H2 and C2H4 from an adsorptive to an exothermic reaction, and endows it with excellent adsorption, desorption, and gas sensitivity properties. In addition, the introduction of external electric field and biaxial strain can significantly change the charge transfer, band gap, and adsorption energy of each adsorption system, which suggests that the Pdn(n=1–3)-PtSe2 monolayer has excellent tunability and can be strain-modulated for gas detection.

Mechanism of adsorption and gas-sensing of hazardous gases by MoS2 monolayer decorated by Pdn (n=1–4) clusters

The adsorption behaviors of hazardous gases (NO2 and SO2) on MoS2 monolayer decorated by Pdn (n=1–4) clusters (Pdn-MoS2) are investigated by first-principles calculations, and the performance of Pdn-MoS2 gas sensors has also been studied in this work. Our results show that Pdn nanocluster can enhance the electrical conductivity of MoS2 monolayer with band gaps of Pdn-MoS2 (n=1–4) sharply decreased, and the adsorption performance of NO2 and SO2 with higher adsorption energy and larger charge transfer. Thus, Pdn-MoS2 show higher sensitive to NO2 and SO2 molecules compared with pristine MoS2. The band gaps of Pdn-MoS2 can be further decreased upon the adsorption of NO2 and SO2 (except for SO2/Pd4-MoS2), indicating Pdn-MoS2 (n=1–3) gas sensor shows improved gas-sensitive response to both NO2 and SO2, and Pd4-MoS2 shows improved gas-sensitive response only to NO2. The recovery time analysis indicates that Pdn-MoS2 (n=1–4) is applicable to be utilized as NO2 and SO2 scavenger at ambient temperature, and the recovery process can be realized simply by heating. Our results indicate that Pdn-MoS2 (n=1–4) monolayer is a favorable candidate as adsorbent for detection and removal of NO2 and SO2, and also provides guidance for further study of the MoS2 monolayer decorated by transition metal clusters.

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

Isolating Pd atoms has been shown to be crucial for the design of a Pd-based electrocatalyst toward 2e- oxygen reduction reaction (ORR). However, there are limited studies focusing on the systematic compositional design that leads to an optimal balance between activity and selectivity. Herein, we design a series of Au@Pd core@shell structures to investigate the influence of the Pd 4d orbital overlapping degree on 2e- ORR performance. Density functional theory (DFT) calculations indicate that enhanced H2O2 selectivity and activity are achieved at Pdn clusters with n ≤ 3, and Pd clusters larger than Pd3 should be active for 4e- ORR. However, experimental results show that Au@Pd nanowires (NWs) with Pd4 as the primary structure exhibit the optimal H2O2 performance in an acidic electrolyte with a high mass activity (7.05 A mg-1 at 0.4 V) and H2O2 selectivity (nearly 95%). Thus, we report that Pd4, instead of Pd3, is the upper threshold of Pd cluster size for an ideal 2e- ORR. It results from the oxygen coverage on the catalyst surface during the ORR process, and such an oxygen coverage phenomenon causes electron redistribution and weakened *OOH binding strength on active sites, leading to enhanced activity of Pd4 with only 0.06 V overpotential in acidic media.

Evidence for Suzuki-Miyaura cross-couplings catalyzed by ligated Pd3-clusters: from cradle to grave.

Pdn clusters offer unique selectivity and exploitable reactivity in catalysis. Understanding the behavior of Pdn clusters is thus critical for catalysis, applied synthetic organic chemistry and greener outcomes for precious Pd. The Pd3 cluster, [Pd3(μ-Cl)(μ-PPh2)2(PPh3)3][Cl] (denoted as Pd3Cl2), which exhibits distinctive reactivity, was synthesized and immobilized on a phosphine-functionalized polystyrene resin (denoted as immob-Pd3Cl2). The resultant material served as a tool to study closely the role of Pd3 clusters in a prototypical Suzuki-Miyaura cross-coupling of 4-fluoro-1-bromobenzene and 4-methoxyphenyl boronic acid at varying low Pd ppm concentrations (24, 45, and 68 ppm). Advanced heterogeneity tests such as Hg poisoning and the three-phase test showed that leached mononuclear or nanoparticulate Pd are unlikely to be the major active catalyst species under the reaction conditions tested. EXAFS/XANES analysis from (pre)catalyst and filtered catalysts during and after catalysis has shown the intactness of the triangular structure of the Pd3X2 cluster, with exchange of chloride (X) by bromide during catalytic turnover of bromoarene substrate. This finding is further corroborated by treatment of immob-Pd3Cl2 after catalyzing the Suzuki-Miyaura reaction with excess PPh3, which releases the cluster from the polymer support and so permits direct observation of [Pd3(μ-Br)(μ-PPh2)2(PPh3)3]+ ions by ESI-MS. No evidence is seen for a proposed intermediate in which the bridging halogen on the Pd3 motif is replaced by an aryl group from the organoboronic acid, i.e. formed by a transmetallation-first process. Our findings taken together indicate that the 'Pd3X2' motif is an active catalyst species, which is stabilized by being immobilized, providing a more robust Pd3 cluster catalyst system. Non-immobilized Pd3Cl2 is less stable, as is followed by stepwise XAFS of the non-immobilized Pd3Cl2, which gradually changes to a species consistent with 'Pdx(PPh3)y' type material. Our findings have far-reaching future implications for Pd3 cluster involvement in catalysis, showing that immobilization of Pd3 cluster species offers advantages for rigorous mechanistic examination and applied chemistries.

Adsorption and gas sensing properties of Cun and Pdn (n = 1–3) clusters modified MoSe2 for lithium battery thermal runaway gases

The manuscript mainly explores the adsorption behavior and related parameters of Cun and Pdn (n = 1–3) clusters modified MoSe2 substrates for thermal runaway gases (CO, CO2, H2) in lithium batteries by DFT. The modified structures are analyzed through Eb, band structure, Qt, and DCD. Compared with the MoSe2 bandgap (1.710 eV), Cun and Pdn (n = 1–3) clusters modifications reduce the bandgap and become more compact. During the modification process, the metal clusters acquire electrons from the substrate and change the DOS of the substrate. The metal clusters improve the conductivity of the substrate. The adsorption and sensing properties between target gases and the substrates through the analysis of the Ed, Qt, DCD, and DOS. The adsorption capacity of target gases on Cu-MoSe2, Pd-MoSe2, Cu2-MoSe2, Pd2-MoSe2, and Pd3-MoSe2 substrates is ranked as follows: CO > H2 > CO2. The adsorption effect of Cu3-MoS2 substrate on target gases is ranked as follows: CO > CO2 > H2. Analysis of molecular orbitals, work functions, and sensitivity can describe the adsorption effect of the substrates on the target gases. The results provide a theoretical basis for the development of gas sensors for detecting the operating status of lithium batteries.

C2H2 semi-hydrogenation over Pdn/TiO2 and PdnCO/TiO2 catalysts: Probing into the roles of Pd cluster size and pre-adsorbed CO in tuning catalytic performance

The size effect of Pd catalyst and CO as a selective accelerator in C2H2 semi-hydrogenation over Pd catalyst are two main factors to affect catalytic performance. In this work, density functional theory calculations were used to unravel the roles of Pd cluster size and introduced CO over the anatase TiO2 supported Pdn (n = 2, 3, 4, 7, 13) clusters in tuning catalytic performance of C2H2 semi-hydrogenation. The results show that as the increasing of Pdn cluster size over Pdn/TiO2 catalysts, C2H4 selectivity generally presents a volcanic-type curve, and the activity of C2H4 and green oil presents an inverted volcanic-type curve. Compared with Pdn/TiO2 catalysts, the introduction of CO greatly improves C2H4 selectivity over Pd2/TiO2 and Pd3/TiO2 catalysts, enhances C2H4 formation activity over Pdn/TiO2 (n = 3, 4, 7, 13), and decreases green oil production over Pd2/TiO2 and Pd13/TiO2 catalysts. Meanwhile, the activation free energy of C2H4 hydrogenation and C2H3 coupling reaction can be used as the evaluate index to quantitatively predict the selectivity of C2H4 and green oil, respectively. The different geometric and electronic effects induced by the introduction of CO could tune catalytic performance. The screened Pd4/TiO2 and Pd2CO/TiO2 catalysts are the most suitable catalysts in the Pdn/TiO2 and PdnCO/TiO2 catalysts, respectively. The relationship of Pdn cluster size and CO introduction with C2H4 selectivity and formation activity would provide valuable structural clue for the construction of C2H2 semi-hydrogenation catalyst with highly catalytic performance.

Insights into the surface catalysis of CeO[formula omitted] supported Pd[formula omitted] clusters ([formula omitted] = 3, 4) for the oxidative addition of bromobenzene

This study reports the activities of different Pdn/CeO2−δ, (n = 3, 4) catalytic systems for the oxidative addition of bromobenzene, which has been reported to be the rate determining step of various C–C coupling reactions. We reported the adsorption energetics of bromobenzene and oxidative addition barriers for a series of Pdn/CeO2−δ catalytic systems differing in the exposed surface planes of ceria. Our investigations highlight the support CeO2 to act as charge donor during the oxidative addition, resulting in a substantial decrease in the activation barriers when compared to those over the corresponding free Pdn clusters. Pristine CeO2(110) and subsurface vacancy defected CeO2(111) surfaces were concluded to be suitable supports which provided stronger anchors for Pdn clusters and offered reduced activation barriers for oxidative addition.

Octahedral Hexapalladium Pseudo-Td Pd6(μ3-CO)4(PEt3)6 with 80 Electron-Deficient Cluster Valence Electrons (CVEs) and a Comparison with Octahedral Hexapalladium Pseudo-C2v Pd6(μ3-CO)4(PMe3)7 with 82 CVEs: Computational Analysis and Resulting Implications.

The elusive octahedral hexapalladium Pd6(μ3-CO)4(PEt3)6 (1) was obtained by the reaction of Pd10(CO)12(PEt3)6 with TlCo(CO)4 in tetrahydrofuran under N2 at 55 °C. Its pseudo-Td octahedral structure, established from a CCD X-ray diffractometry study at 100 K, has the highest ideal symmetry of any of the characterized octahedral-based CO/PR3-ligated homopalladium Pdn clusters (n = 6, 7, 8, 10). Each Pd atom in 1 is coordinated to a PEt3 ligand, and each nonadjacent triangular Pd3 face is capped by a triply bridging μ3-CO ligand. The 31P{1H} NMR and IR spectra of 1 are in accordance with its solid-state molecular structure. Cluster 1 has a total of 80 cluster valence electrons (CVEs), the lowest reported for octahedral-based metal polyhedra that normally conform to the Wade-Mingos bonding rule with an 86 CVE count. Comparative density functional theory calculations involving natural population analysis are presented for trimethylphosphine analogues of the triethylphosphine (1-Me) and the previously reported octahedral hexapalladium trimethylphosphine Pd6(μ3-CO)4(PMe3)7 (2), which has pseudo-C2v symmetry with 82 total CVEs.

A density functional theory study of adsorption and dissociation of H2 molecule on small PdnAgm (n + m ≤ 4) metal clusters

The adsorption and dissociation of hydrogen molecule on small PdnAgm (n + m ≤ 4) clusters is studied employing density functional theory (DFT) calculations. The lowest energy structures of the PdnAgm clusters are initially presented. Based on the obtained structures, various geometries of H2 adsorption and dissociation with these clusters are examined. The cluster size and composition of the PdnAgm clusters are found to control the nature of the adsorption process. The results suggest that molecular adsorption is more favorable on the Pd atom and mixed clusters, while hydrogen dissociation is more favorable on the Pdn (n = 2–4) clusters. The Pd dimer and trimer are found to be the most reactive clusters for hydrogen adsorption, while single Ag atom is the least reactive one. The natural bond orbital (NBO) population analysis indicates that hydrogen atoms tend to draw electrons from the metal clusters in the dissociative form and donate electrons to the metal clusters in the molecular adsorption form. The density of states (DOS) of the PdnAgm trimers are also presented and discussed. The interaction and stabilities of cluster on the graphene support has also been performed.

Well-Defined Pdn Clusters for Cross-Coupling and Hydrogenation Catalysis: New Opportunities for Catalyst Design

In recent studies it has been demonstrated that the privileged reactivity of higher-order metal clusters can be exploited in widely applied catalytic processes, particularly cross-coupling reactions and hydrogenative transformations. Relatively small, well-defined Pdn clusters have been known since the 1960s. Unique reactivity, reaction (product) selectivity, and catalyst behavior have been recently uncovered, from which there is much potential in catalyst design and application. Ligated Pdn clusters of a smaller size (where n is less than 6) may form upon degradation of mononuclear Pd species en route to larger particulate Pd (from <5 nm particles to large moribund forms in the >1 μm range). This review presents the catalytic applications of Pdn clusters. We pay particular attention to the underlying structure of the Pdn clusters, linked to their reactivity. A hypothesis that ligated Pdn clusters may constitute a mechanism by which higher-order Pd species may form (as a bridging point for monoligated Pd species through to PdNPs) is further discussed. Where appropriate, we mention other catalytic reaction processes that complement the discussion focused on cross-coupling and hydrogenation processes.

Small palladium clusters and their adducts with atomic oxygen

A systematic investigation on small Pdn clusters (n = 1 ÷ 8) and their interaction with atomic oxygen was conducted at DFT level. A model to describe cohesive energy for small sized clusters based on a topological approach was proposed and validated. On the basis of the bond length, the stabilisation electronic energy and the second electronic energy difference variation analysis, Pd4 and Pd6 were identified as the most stable clusters. The Natural Bond Orbital (NBO) analysis and the plot of the Molecular Electrostatic Potential (MEP) – both performed for the first time – indicated that the Pd atoms in clusters were not equivalent in terms of atomic charge, population and electron configuration. The plots of the MEP mapped on the electron density also revealed its non-uniform character; areas of positive MEP were found around the Pd atoms, which may undertake nucleophilic attacks. Subsequently, the interaction of atomic oxygen with Pdn clusters was systematically studied, accounting for different possible anchoring positions. The hollow site was revealed to be the most favourable one. The increase in number of Pd atoms favoured higher spin states. Pd6O was identified as the most oxidised and energetically stable cluster. The calculated binding energy value of oxygen (3.5 eV) was in agreement with the Temperature Programmed Desorption (TPD) experimental results conducted on Pd(1 1 0) single-crystal surface. The NBO analysis evidenced that the occupancy of 2 s (2p) orbital of O decreased (increased) as the number of Pd atom increased. The negatively charged Pd atoms had a larger population of 4d orbitals than the neutral or positively charged ones. The interactions between O and Pd atoms were dominated by d-type orbitals.

A Theoretical Study of the In Situ Structural Reconstruction of Pdn (n = 6, 19, 44) Clusters for Catalytic Hydrogen Evolution

How in situ structural reconstructions affect the hydrogen evolution reaction (HER) activity of small Pd clusters is a long-standing problem in the field of heterogeneous catalysis. Herein, we reveal the structural evolution of Pdn (n = 6, 19, 44) clusters under the HER environment via stochastic global potential energy surface searching. We theoretically demonstrated that the HER activity of Pdn clusters first increases and then decreases under long-term working conditions. The intrinsic nature of these phenomenons includes interior H formations and structural reconstructions caused by the supersaturated adsorption of H atoms. This proves that carefully adjusting the hydrogenation degree of Pd clusters is a good strategy for improving the HER’s catalytic performance.

DFT reveals the support effects in Pd nanoclusters over defect-ridden graphene for the oxidative addition of bromobenzene

Oxidative addition of aryl halides is an elementary and the rate limiting step common to several cross-coupling reactions. Metal complex-based homogeneous catalysts are conventionally used for catalysing such reactions. In search for heterogeneous catalysts, graphene has been widely used as a support for immobilising catalytically active Pd nanoparticles or clusters. In this study, we investigated the activities of small Pdn (n = 3, 4) clusters immobilised over pristine and defected graphenes for the oxidative addition of bromobenzene in the presence of water solvation. We compared the adsorption energetics and activation barriers of oxidative addition for a series of graphene supports and contrasted them with homogeneous free Pdn catalysts. Our theoretical investigations reveal the defected graphene supports to act as charge donors thereby drastically reducing the activation barriers of oxidative addition of bromobenzene when compared to free Pdn clusters. Double vacancy defected graphene and B-doped graphene were concluded as the best support materials providing stronger traps for small Pdn (n = 3, 4) clusters and reducing the oxidative addition barriers, making them potential heterogeneous catalysts for cross-coupling reactions.

Developing Efficient Suzuki Cross-Coupling Catalysts by Doping Palladium Clusters with Silver

It is shown that doping of a Pd cluster by Ag atoms can provide an efficient catalyst for the Suzuki–Miyaura cross-coupling reactions. We demonstrate this intriguing possibility by considering a model reaction involving bromobenzene and phenylboronic acid as reagents where the reaction involves oxidation, transmetallation, and reduction steps. We have examined the reaction barriers of all three steps for a conventional ligated Pd catalyst, a nearly icosahedral Pd13 cluster, and a monosilver-doped Pd12Ag cluster using gradient-corrected density functional theory. It is observed that the reaction carried out on the Pd sites adjacent to an Ag atom in a Pd12Ag cluster shows substantially lower barriers for the oxidation and reduction steps compared to the conventional ligated Pd catalyst and the pure Pd13 cluster. A detailed analysis indicates that the Ag site donates charge to the neighboring Pd site. While such a donation may have been expected to reduce the barrier for the oxidative step, the lowering of the barrier for the reduction step indicates that the respective sites not only act as a donor but can also serve as an acceptor for the reduction step. Furthermore, because of the differential donor–acceptor characteristic of the Ag and Pd atoms, it is observed that the barrier heights of the redox steps are primarily dependent on the chosen active site. The calculated results show that by altering the atom (Ag or Pd) at the active site of the reaction, the activation energies of the redox steps can either be reduced or increased. This shows that the active sites of a bimetallic cluster-like Pd12Ag can be utilized to control the barrier heights of suitable chemical reactions. The relative trend of the barrier heights for both clusters is also observed to be predictable by the conceptual density functional theory. Previous studies in our group have indicated that the reaction barriers for Pdn clusters can be lowered by supporting them on reduced graphene. We, therefore, propose that silver-doped Pdn clusters may provide an even better catalyst.

Hyper-Cross-Linked Polystyrene as a Stabilizing Medium for Small Metal Clusters

Among different polymers nanostructured cross-linked aromatics have the greatest potential as catalytic supports due to their exceptional thermal and chemical stability and preservation of the active phase morphology. This work studies the ability of hyper-cross-linked polystyrene (HPS) to stabilize small Pdn and Ptn (n = 4 or 9) clusters. Unrestricted DFT calculations were carried out for benzene (BZ) adsorption at the BP level of theory using triple-zeta basis sets. The adsorption of BZ rings (stepwise from one to four) was found to result in noticeable gain in energy and stabilization of resulting adsorption complexes. Moreover, the interaction of metal clusters with HPS micropores was also addressed. For the first time, the incorporation of small clusters in the HPS structure was shown to influences its geometry resulting in the stabilization of polymer due to its partial relaxation.

Tunable formaldehyde sensing properties of palladium cluster decorated graphene.

The ability to tune the adsorption strength of the targeted gas on sensing materials is crucial for sensing applications. By employing first-principles calculations the adsorption and sensing properties of HCHO on small Pdn (n = 1–6) cluster decorated graphene have been systematically investigated. The adsorption energy is found to depend on the size of the Pdn cluster and can be tuned in a wide range from −0.68 eV on Pd(111) to −1.98 eV on the Pd3/graphene system. We also find that the Pdn/graphene (n = 5 and 6) systems have an appropriate adsorption energy for HCHO gas sensing. The current–voltage curves are calculated by the non-equilibrium Green's function method for the two-probe nano-sensor devices along both the armchair and zigzag directions. The devices constructed with Pdn/graphene (n = 5 and 6), having the highest absolute response over 20% at small voltages, should be applicable for HCHO detection. This work provides a theoretical basis for exploring potential applications of metal cluster decorated graphene for gas sensing.

Ligand-Protected Ultrasmall Pd Nanoclusters Supported on Metal Oxide Surfaces for CO Oxidation: Does the Ligand Activate or Passivate the Pd Nanocatalyst?

Herein, we report on the synthesis of ultrasmall Pd nanoclusters (∼2 nm) protected by L-cysteine [HOCOCH(NH2 )CH2 SH] ligands (Pdn (L-Cys)m ) and supported on the surfaces of CeO2 , TiO2 , Fe3 O4 , and ZnO nanoparticles for CO catalytic oxidation. The Pdn (L-Cys)m nanoclusters supported on the reducible metal oxides CeO2 , TiO2 and Fe3 O4 exhibit a remarkable catalytic activity towards CO oxidation, significantly higher than the reported Pd nanoparticle catalysts. The high catalytic activity of the ligand-protected clusters Pdn (L-Cys)m is observed on the three reducible oxides where 100 % CO conversion occurs at 93-110 °C. The high activity is attributed to the ligand-protected Pd nanoclusters where the L-cysteine ligands aid in achieving monodispersity of the Pd clusters by limiting the cluster size to the active sub-2-nm region and decreasing the tendency of the clusters for agglomeration. In the case of the ceria support, a complete removal of the L-cysteine ligands results in connected agglomerated Pd clusters which are less reactive than the ligand-protected clusters. However, for the TiO2 and Fe3 O4 supports, complete removal of the ligands from the Pdn (L-Cys)m clusters leads to a slight decrease in activity where the T100% CO conversion occurs at 99 °C and 107 °C, respectively. The high porosity of the TiO2 and Fe3 O4 supports appears to aid in efficient encapsulation of the bare Pdn nanoclusters within the mesoporous pores of the support.