Small Platinum Clusters Research Articles

Ethane and Propane Dehydrogenation on Small Platinum Clusters Supported on Silica: An Ab Initio Molecular Dynamics and DFT Study.

The size-dependent activity of catalysts has been researched for a long time in the field of catalysis. Positively charged small Pt clusters enhance catalytic activity than bigger clusters and bulk for propane dehydrogenation. We performed DFT calculations on small Pt clusters adsorbed on silica support. The planar structure of Pt clusters is present till 4 Pt atoms, after which three-dimensional structures are observed. AIMD and DFT calculations for silica showed that it has a high surface area and thermal stability suitable to conduct dehydrogenation reactions. The adsorption of Pt cluster on silica results in the formation of directional bonds which affects the properties of the adsorbed Pt catalysts by changing the redox properties. In the bulk phase, ethane and propane molecules undergo dehydrogenation reactions with 0.133 eV atom-1 and 0.244 eV atom-1 energies, respectively. NEB calculations showed that except for Pt-2/SiO2 , all the even Pt clusters require less activation energy than the neighboring odd Pt clusters. Ethane molecule interacting with Pt-4/SiO2 , Pt-5/SiO2 , Pt-6/SiO2 , and propane with Pt-3/SiO2, Pt-4/SiO2 , Pt-5/SiO2 , Pt-6/SiO2 , follows the reverse Horiuti-Polanyi mechanism during dehydrogenation, whereas non-reverse Horiuti-Polanyi mechanism (which requires comparatively lower activation energy) is followed for smaller Pt clusters.

Dual-site catalysts featuring platinum-group-metal atoms on copper shapes boost hydrocarbon formations in electrocatalytic CO2 reduction

Copper-based catalyst is uniquely positioned to catalyze the hydrocarbon formations through electrochemical CO2 reduction. The catalyst design freedom is limited for alloying copper with H-affinitive elements represented by platinum group metals because the latter would easily drive the hydrogen evolution reaction to override CO2 reduction. We report an adept design of anchoring atomically dispersed platinum group metal species on both polycrystalline and shape-controlled Cu catalysts, which now promote targeted CO2 reduction reaction while frustrating the undesired hydrogen evolution reaction. Notably, alloys with similar metal formulations but comprising small platinum or palladium clusters would fail this objective. With an appreciable amount of CO-Pd1 moieties on copper surfaces, facile CO* hydrogenation to CHO* or CO-CHO* coupling is now viable as one of the main pathways on Cu(111) or Cu(100) to selectively produce CH4 or C2H4 through Pd-Cu dual-site pathways. The work broadens copper alloying choices for CO2 reduction in aqueous phases.

Theoretical study on the mechanism of the hydrogen evolution reaction catalyzed by platinum subnanoclusters.

The smallest subnanocluster models of platinum colloid (Ptn) are supposed to diffuse in aqueous media in order to examine their behaviors when they are subjected to the electrocatalytic hydrogen evolution reaction under zero overpotential conditions at pH 0. The DFT approach allows us to clarify the nature of individual proton transfer (PT) and electron transfer (ET) processes together with the importance of relying on concerted proton-electron transfer (CPET) pathways to promote the majority of H* adsorption processes by Ptn subnanoclusters. Although the CPET processes are closely correlated with the Volmer steps (Pt + H+ + e- → Pt-H*) described so far in electrochemistry, our study for the first time points out the essential capability of the Ptn clusters to promote the multiple PT steps without the need to transfer any electrons, revealing the fundamentally high basicity of the naked Ptn clusters (pKa = 27-28 for Pt4, Pt5, and Pt6). The discrete cluster models adopted herein avoid the structural constraints forced by the standard slab models and enable us to discuss the drastic alterations in the geometric and electronic structures of the intermediates given by the consecutive promotion of multiple CPET steps. The weakening of the Pt-H* bond strength with the increasing number of CPET steps is well rationalized by carefully examining the changes in the ν(Pt-H*) vibrational frequencies, the hydricity, and the H2 desorption energy. The behaviors are also correlated with the underpotential and overpotential deposited hydrogen atoms (HUPD and HOPD) discussed in electrochemical studies for many years.

Influence atoms of Co, Ni, Cu on the catalytic activity of small Pt clasters: First principles calculations

Based on the calculations from the first principles, we obtained the distributions of valence electron densities and electronic energy spectra for small Ptn clusters (where n = 1-5 atoms). According to the results of calculations, it is determined that the inclusion of oxygen atoms or atoms of other kinds in small Ptn clusters, as a rule, affect the catalytic activity of research systems. It is established that during doping of small platinum clusters by atoms of 3-d transition metals (Cu, Ni, Co), the electronic structure of the cluster and the band gap change. This in turn helps to increase the catalytic activity of platinum.

The growth pattern of Ptn (n = 1–6) clusters on pentagonal B2C monolayer support: A computational study

Using dispersion-corrected density functional theory (DFT-D), we systematically investigated the growth pattern of small platinum (Pt) clusters on pentagonal B2C (penta-B2C). Firstly we calculated the bonding energy of the single Pt atoms on (2 × 2), (3 × 3) and (4 × 4) penta-B2C supercells, and chose (3 × 3) penta-B2C for the subsequent simulations. Secondly we confirmed that the single Pt atom on penta-B2C is stable. And then we stepwise investigated the growth pattern of Pt clusters. The result indicates that the Pt1, Pt2 and Pt4 clusters on penta-B2C are stable, and the single Pt atom on penta-B2C is the most stable structure. Based on the analysis of the electronic structure and the frontier molecular orbitals, the Pt1, Pt2 and Pt4 clusters on penta-B2C have the potential to be used as the efficient catalysts.

Germanium as key dopant to boost the catalytic performance of small platinum clusters for alkane dehydrogenation

Pt is the most active pure metal for the dehydrogenation of light alkanes, to produce light alkenes, like ethylene and propylene. These compounds are exceedingly important products in the chemical industry. Pt, however, suffers from a low selectivity, tending to fully dehydrogenate hydrocarbons to pure carbon and giving rise to coking that blocks the active sites, deactivating the catalyst. Different dopants or co-alloying agents have been proposed to tune its selectivity and produce catalysts with longer lifetimes, such as Sn, B and Si. However, often, the improved selectivity is achieved at the expense of catalytic activity. In this work we study MgO-supported Pt clusters and show that nanoalloying Pt with Ge can lead to an improved selectivity by halting deeper dehydrogenation that leads to coke, without harming the activity towards alkane dehydrogenation as compared to pure Pt clusters. Moreover, Ge reduces the propensity of Pt nanoclusters to deactivate via Ostwald ripening, and reduces the binding energy to carbon. Therefore, Ge-doped Pt nanocatalyst are more selective and resistant to deactivation. The effect of alloying Pt with Ge has an electronic origin: Ge quenches the unpaired electrons in the metal clusters, which are needed to activate alkenes for dehydrogenation.

Effect of single metal dopant (Rh, Ru and Sn) on Ptn+ (n = 3 and 4) clusters for controlled CO tolerance

The basic unsettled problem of CO poisoning of platinum catalyst is addressed with the introduction of single dopant atoms on small cationic platinum clusters. Here, ‘Rh’, ‘Ru’ and ‘Sn’ doped on Ptn+ (n = 3 and 4) clusters are investigated under three DFT functional (MN12-SX, M06-D3 and TPSSh) using def2TZVPP basis set. Our findings reveal a notable amount of reduction of adsorption energy equal to 0.3 eV for dopant Sn over dopant Ru and Rh in Pt3+ cluster with MN12-SX functional. Thus, the promotional effect of Sn on Pt catalyst could be a suitable system for improved CO tolerance.

Direct Electrochemical Observation of Single Platinum Cluster Electrocatalysis on Ultramicroelectrodes.

We demonstrate a method to electrodeposit and observe the electrocatalysis of small platinum clusters and nanoparticles (NPs) in real time as they form on an ultramicroelectrode (UME). Water droplets ( rdrop ∼ 700 nm), stabilized by sodium dodecyl sulfate (SDS), were suspended in a solution of dichloromethane (DCM) and tetrabutylammonium perchlorate ([TBA][ClO4]), which was used to mitigate charge balance during droplet electrolysis. When droplets collided with an UME biased sufficiently negative to drive water reduction, large blips of current were observed. Droplets were synthesized with varying concentrations of H2PtCl6 (from 24.4 mM to 32 nM), which can be reduced to Pt0 at 0.8 V more positive than water reduction on a Au or C UME. The observation of current blips synthesized with mM amounts of H2PtCl6 indicated water droplets deliver H2PtCl6 to the electrode surface, where a cathodic potential caused Pt NPs to form. The formation of clusters was observed by biasing the electrode potential more negative than water reduction on Pt but more positive than water reduction on Au, giving current blips for droplets containing μM to nM amounts of H2PtCl6. These blips corresponded to the electrocatalysis of thousands to tens of atoms (clusters). Droplet electrolysis allows for a large amplification such that the electrocatalysis of clusters can be observed in real time. Single, isolated clusters were further characterized voltammetrically on carbon fiber UMEs. The isolation step used the amperometric method, which unambiguously depicted cluster formation in real time. Carbon was chosen due to its large potential window and slow kinetics toward many inner-sphere reactions, such as proton reduction, used in this study. Voltammetric characterization of proton reduction in HClO4 and NaClO4 allowed for cluster size analysis using the limiting current. The reduction of proton on the clusters ( E1/2 ∼ -0.6 V vs Ag/AgCl) occurred at ca. 400 mV more negative than bulk, polycrystalline platinum.

Hydrogen storage in N- and B-doped graphene decorated by small platinum clusters: A computational study

In this work, we perform density functional theory (DFT) calculations to investigate the hydrogen adsorption on Pt4 cluster supported on pristine, B-, and N-doped graphene sheets. It is found that the doping B or N atom in the graphene could enhance the interaction between the Pt4 cluster and the supporting substrate. The first H2 molecule is found to be dissociative chemisorption on the three substrates. Further, dissociative and molecular adsorption of multiple H2 molecules are co-adsorbed on the three substrates. In addition, the interaction between Pt4(H2)x and the substrate is illustrated for the stability of Pt4(H2)x on the substrate. AIMD simulation is also performed to verify the stability and hydrogen storage. Accordingly, the B-graphene is predicted to be the most potential materials for hydrogen storage among these three materials.

Stability of small cationic platinum clusters

The relative stability of small cationic platinum clusters is investigated by photofragmentation experiments. Mass spectra show a smooth intensity distribution except for a local intensity minimum at Pt5+, revealing enhanced stability of the platinum tetramer Pt4+. The possibility that radiative cooling competes with statistical fragmentation after photoexcitation is examined and it is shown that clusters in the N = 3-8 size range do not radiate on the time scale of the experiment. In the absence of radiative cooling, the mass spectra of photofragmented clusters can be well explained by dissociation energies computed using density functional theory. The large calculated HOMO-LUMO gap for Pt4+ (∼1.2 eV) is attributed to its highly symmetric structure and provides an explanation for the surprisingly low reactivity of this cluster in different gas-phase reactions.

Substrate induced reconstruction and activation of platinum clusters: A systematic DFT study

The fundamental understanding of the electronic and geometric structures of small platinum clusters on metal oxide support is important to design the futuristic Pt-based novel materials for heterogeneous catalysis. Here we report a systematic theoretical study on the trend in the structural and electronic properties of alumina supported Ptn (n=1–7 and 10) clusters with a focus to highlight the effect on the substrate. All calculations were carried out using the plane wave based pseudo-potential approach. The model for the support has been designed by using Al-terminated α-Al2O3 (0001) surface which is the most stable surface termination under ultrahigh vacuum conditions. The results show that the binding of Pt atom with the Al2O3 surface releases 1.84eV energy which is significantly higher than atomic adsorption energy of other noble metal atoms (Ag, Au, and Pd). As a consequence, the equilibrium geometries of free Ptn clusters (n=3–7) are significantly altered on the alumina surface. Whilst Pt10 cluster favors tetracapped prism structure in the gas phase, on alumina support it prefers a layered structure. The geometrical changes of Pt clusters on the alumina surface have been attributed to the energy balance between the Pt-Pt and Pt-substrate interactions. The nature of interaction between the Ptn clusters and surface has been verified using the electronic density of states analysis. Surface induced electronic charge on the deposited cluster results in red shift in its energy levels, indicating electron rich activation of platinum clusters. The inclusion of spin-orbit coupling(SOC) significantly changes the electronic structure of gas phase platinum cluster; however, the extent of SOC influence reduces due to interfacial bonding with alumina support.

The influence of the small platinum clusters on hydrogen sorption properties

Hydrogen sorption abilities of Pt-B 2 O 3 /Al 2 O 3 systems with different molar ratio of oxides obtained by sol-gel method were examined. Platinum was introduced by surface impregnation. Main goal of the research was to check an influence of metallic component (platinum) on sorption properties of B 2 O 3 /Al 2 O 3 binary oxides. The oxide systems were characterized using XRD and TEM. Hydrogen adsorption was tested in the volumetric system and TPD measurements were taken. Results show that the amount of adsorbed hydrogen depends not only on the amount of platinum in the system but also on the type of oxide support and mainly on the content of boria.

Controlling the Adsorption of Carbon Monoxide on Platinum Clusters by Dopant‐Induced Electronic Structure Modification

AbstractA major drawback of state‐of‐the‐art proton exchange membrane fuel cells is the CO poisoning of platinum catalysts. It is known that CO poisoning is reduced if platinum alloys are used, but the underlying mechanism therefore is still under debate. We study the influence of dopant atoms on the CO adsorption on small platinum clusters using mass spectrometry experiments and density functional calculations. A significant reduction in the reactivity for Nb‐ and Mo‐doped clusters is attributed to electron transfer from those highly coordinated dopants to the Pt atoms and the concomitant lower CO binding energies. On the other hand Sn and Ag dopants have a lower Pt coordination and have a limited effect on the CO adsorption. Analysis of the density of states demonstrates a correlation of dopant‐induced changes in the electronic structure with the enhanced tolerance to CO poisoning.

Controlling the Adsorption of Carbon Monoxide on Platinum Clusters by Dopant-Induced Electronic Structure Modification.

A major drawback of state-of-the-art proton exchange membrane fuel cells is the CO poisoning of platinum catalysts. It is known that CO poisoning is reduced if platinum alloys are used, but the underlying mechanism therefore is still under debate. We study the influence of dopant atoms on the CO adsorption on small platinum clusters using mass spectrometry experiments and density functional calculations. A significant reduction in the reactivity for Nb- and Mo-doped clusters is attributed to electron transfer from those highly coordinated dopants to the Pt atoms and the concomitant lower CO binding energies. On the other hand Sn and Ag dopants have a lower Pt coordination and have a limited effect on the CO adsorption. Analysis of the density of states demonstrates a correlation of dopant-induced changes in the electronic structure with the enhanced tolerance to CO poisoning.

Hydrazine decomposition on a small platinum cluster: the role of N2H5 intermediate

This work presents a comprehensive DFT study on the interaction between hydrazine derivatives with a platinum catalyst surface, which is represented by a tetrahedral Pt4 cluster model. Three separate reaction pathways were investigated; two of which are related to possible pathways of NH3 formation. The first pathway describes the intramolecular transfer of one hydrogen atom in the hydrazine molecule forming the NHNH3 intermediate, then dissociating into NH and NH3. The second describes the addition of one external hydrogen atom to hydrazine forming N2H5, followed by its dissociation to NH2 and NH3. The third reaction pathway involves the formation of N2H3 by means of hydrogen abstraction by an external hydrogen. The reactions were studied in both the absence and the presence of a Pt4 cluster. We find that the assistance of the Pt4 cluster lacks a systematic effect on the reactions barrier heights. It is also shown that the ammonia formation can possibly proceed through the formation of the N2H5 intermediate, leading to more exothermic intermediate steps in the presence of the Pt4 cluster.

Interactions of small platinum clusters with the TiC(001) surface

Density functional theory calculations are used to elucidate the interactions of small platinum clusters (Ptn, n = 1–5) with the TiC(001) surface. The results are analyzed in terms of geometric, energetic, and electronic properties. It is found that a single Pt atom prefers to be adsorbed at the C-top site, while a Pt2 cluster prefers dimerization and a Pt3 cluster forms a linear structure on the TiC(001). As for the Pt4 cluster, the three-dimensional distorted tetrahedral structure and the two-dimensional square structure almost have equal stability. In contrast with the two-dimensional isolated Pt5 cluster, the adsorbed Pt5 cluster prefers a three-dimensional structure on TiC(001). Substantial charge transfer takes place from TiC(001) surface to the adsorbed Ptn clusters, resulting in the negatively charged Ptn clusters. At last, the d-band centers of the absorbed Pt atoms and their implications in the catalytic activity are discussed.

Recognizing Single Collisions of PtCl6(2-) at Femtomolar Concentrations on Ultramicroelectrodes by Nucleating Electrocatalytic Clusters.

We report the electrodeposition of electrocatalytic clusters of platinum from femtomolar platinate solutions. An inert carbon fiber ultramicroelectrode (UME) was held at a potential where proton reduction was unfavorable on carbon but favorable on platinum in a 1 M sulfuric acid solution. Upon addition of femtomolar amounts of hexachloroplatinic acid, which will also reduce to Pt(0) at the applied potential on the carbon fiber UME, cathodic blips were observed in the amperometric i-t response. These blips correspond to the reduction of protons to molecular hydrogen at the small platinum clusters followed by a rapid deactivation likely due to hydrogen bubble formation. On average, these current spikes occur when five platinum atoms have been formed on the electrode, as determined by a comparative analysis of experimental cathodic blips and calculated hexachloroplatinate molecule collision frequencies.

Structure, vibrational, and optical properties of platinum cluster: a density functional theory approach

Using density functional theory, stability, chemical, and optical properties of small platinum clusters, Ptn (n = 2 to 10) have been investigated. An attempt has been made to establish a correlation between stability and chemical reactivity parameters. The calculated geometries are in agreement with the available experimental and theoretical results. The atom addition energy change (ΔE1) and stability function (ΔE2) reveal that Pt7 is more stable than its neighboring clusters. Very good agreement of the calculated electron affinity with the available experimental results has been observed. The polarizability of the Ptn clusters depends almost linearly on the number of atoms. A correlation between the static polarizability and ionization potential is found, paving a way to calculate polarizabilty of larger clusters from their ionization potential. The calculated vibrational frequencies are compared with available experimental and theoretical results and good agreement between them has been established. In general, the prominent peak of molar absorption coefficient is shifting toward the lower energy side when cluster size grows. Our DOS calculation suggests that d orbital is primarily responsible for HOMO position and s orbital is responsible for LUMO position.

Spin–orbit effects on the structural, homotop, and magnetic configurations of small pure and Fe-doped Pt clusters

We report ab initio calculations of the atomic and electronic structures, and related magnetic properties of platinum clusters of seven atoms both pure and substitutionally doped with an iron impurity. A relativistic Hamiltonian including spin–orbit coupling effects is self-consistently solved in the noncollinear framework within the density functional theory as implemented in the VASP code. We show that spin–orbit coupling is crucial for determining the energetic order of structural isomers of small platinum clusters, but not for the low-lying iron-doped platinum clusters. We analyze the influence of Fe doping on the overall improvement of the magnetic efficiency of the nanoparticle, i.e., large total moment and high stability against remagnetization. Results are reported for the spin–orbit energy, the magnetic anisotropy energy corresponding to the remagnetization barrier, and the orbital contribution to the total magnetic moment, which can be experimentally measured in free-standing clusters by means of X-ray magnetic circular dichroism spectroscopy.

Small Platinum Cluster Hydrides in the Gas Phase

The reactions of small cationic platinum clusters (Pt2(+)-Pt7(+)) with molecular hydrogen were investigated, and the structures of the hydride complexes were analyzed using IR spectroscopy. We determined the relative reaction rates for the addition of the first H2 molecule to the platinum clusters, and we report the hydrogen saturation coverages observed at high H2 concentration. High H atom per Pt atom ratios were observed, similar to earlier measurements on other group-10 transition metals. The structures of the fully saturated complexes of Pt2(+)-Pt5(+) were investigated using a combination of infrared multiple-photon dissociation (IR-MPD) spectroscopy in the frequency range of 550-2050 cm(-1) and density functional theory-based calculations. We found molecularly bound hydrogen alongside bridge and often atop binding of hydrogen atoms for all of the low-energy structures, in contrast to earlier theoretical predictions.