Hydrogen dissociative chemisorption and desorption on saturated subnano palladium clusters (Pdn, n = 2-9)

H(2) sequential dissociative chemisorption on small palladium clusters was studied using density functional theory. The chosen clusters Pd(n) (n = 2-9) are of the lowest energy structures for each n. H(2) dissociative chemisorption and subsequent H atom migration on the bare Pd clusters were found to be nearly barrierless. The dissociative chemisorption energy of H(2) and the desorption energy of H atom in general decrease with the coverage of H atoms and thus the catalytic efficiency decreases as the H loading increases. These energies at full cluster saturation were identified and found to vary in small energy ranges regardless of cluster size. As H loading increases, the clusters gradually change their bonding from metallic character to covalent character. For the selected Pd clusters, the capacity to adsorb H atoms increases almost proportionally with cluster size; however, it was found that the capacity of Pd clusters to adsorb H atoms is, on average, substantially smaller than that of small Pt clusters, suggesting that the catalytic efficiency of Pt nanoparticles is superior to Pd nanoparticles in catalyzing dissociative chemisorption of H(2) molecules.     

DFT studies of the characteristics of Pd−Pt core-shell clusters

It is reported that Pd?Pt core-shell type nanoclusters in which the inner atoms of the Pd cluster are substituted by Pt significantly enhance the catalytic activity for cycloocatdiene hydrogenation. In order to discuss the electronic states of core-shell clusters, DFT calculations were carried out for Pd₁₃, Pt₁₃, Pt/Pd₁₂, Pd/Pt₁₂ Pd₃₈ and Pd₆/Pt₃₂ clusters. From these calculations, it was found that the charge transfer between the core atoms and the shell atoms played an important role for the modification of the electronic state of the surface atoms in them.     

Nucleation and growth mechanism of Pd/Pt bimetallic clusters in sodium bis(2-ethylhexyl)sulfosuccinate (AOT) reverse micelles as studied by in situ X-ray absorption spectroscopy

We report in situ X-ray absorption spectroscopy (XAS) investigations on the formation of palladium-platinum (Pd/Pt) bimetallic clusters at the early stage within the water-in-oil microemulsion system of water/AOT/n-heptane. The reduction of palladium and platinum ions and the formation of corresponding clusters are monitored as a function of dosage of reducing agent, hydrazine (N(2)H(5)OH). Upon successive addition of the reducing agent, hydrazine (N(2)H(5)OH), five distinguishable steps are observed in the formation process of Pd/Pt clusters at the early stage. Both in situ X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) analysis for both the Pd K-edge and Pt L(III)-edge revealed the formation of Pd/Pt bimetallic clusters. A corresponding structural model is proposed for each step to provide a detailed insight into the nucleation and growth mechanism of Pd/Pt bimetallic clusters. We also discussed the atomic distribution of Pd and Pt atoms in Pd/Pt bimetallic clusters based on the calculated XAS structural parameters.     

Pt，Pd助剂对Ni基催化剂中Ni的分散度及抗积碳性能的影响

研究了添加少量贵金属(Pt,Pd)的Ni/Al2O3催化剂对甲烷水蒸汽重整反应抗积碳能力和催化性能的影响.催化活性实验表明,添加少量Pt的样品显著提高了Ni/Al2O3催化剂的活性,稳定性,抗积碳和抗氧化能力,而添加Pd的样品对Ni/Al2O3催化剂的催化性能提高并不明显.利用氢气程序升温还原(H2-TPR),X射线晶体衍射(XRD),热重-差热分析(TG-DTA)等手段对反应前后的催化剂进行了表征,研究发现在Ni-Pt/Al2O3催化剂中Ni与Pt之间存在较强的相互作用力,在主要由Ni覆盖的表面形成了Ni-Pt双金属簇,提高了Ni的分散度,在催化剂的表面易于形成较小的Ni颗粒,抑制了Ni的烧结,改善了Ni基催化剂的抗积碳能力;贵金属Pt通过H2的溢流效应促进了Ni的还原,抑制了催化剂的氧化.而在Ni-Pd/Al2O3中,Ni和Pd存在着一定的偏析效应,不能有效的形成Ni-Pd双金属簇,在还原过程中分别被还原.     

Structures of small Pd–Pt bimetallic clusters by Monte Carlo simulation

Segregation phenomena of Pd–Pt bimetallic clusters with icosahedral and decahedral structures are investigated by using Monte Carlo method based on the second-moment approximation of the tight-binding (TB-SMA) potentials. The simulation results indicate that the Pd atoms generally lie on the surface of the smaller clusters. The three-shell onion-like structures are observed in 55-atom Pd–Pt bimetallic clusters, in which a single Pd atom is located in the center, and the Pt atoms are in the middle shell, while the Pd atoms are enriched on the surface. With the increase of Pd mole fraction in 55-atom Pd–Pt bimetallic clusters, the Pd atoms occupy the vertices of clusters first, then edge and center sites, and finally the interior shell. It is noticed that some decahedral structures can be transformed into the icosahedron-like structure at 300 and 500 K. Comparisons are made with previous experiments and theoretical studies of Pd–Pt bimetallic clusters.     

Effect of Bimetallic Pd/Pt Clusters on the Sensing Properties of Nanocrystalline SnO<Subscript>2</Subscript> in the Detection of CO

Nanocrystalline tin dioxide modified by Pd and Pt clusters or by bimetallic PdPt nanoparticles was synthesized. Distribution of the modifers on the SnO₂ surface was studied by high-resolution transmission electron microscopy and energy dispersive X-ray microanalysis with element distribution mapping. It was shown that the Pd/Pt ratio in bimetallic particles varies over a broad range and does not depend on the particle diameter. The effect of platinum metals on the reducibility of nanocrystalline SnO₂ by hydrogen was determined. The sensing properties of the resulting materials towards 6.7 ppm CO in air were estimated in situ by electrical conductivity measurements. The sensor response of SnO₂ modified with bimetallic PdPt particles was a superposition of the signals of samples with Pt and Pd clusters.     

Single Pt–Pd Bimetallic Nanoparticle Electrode: Controllable Fabrication and Unique Electrocatalytic Performance for the Methanol Oxidation Reaction

Understanding the electrocatalytic activity at single nanoparticles/nanoclusters level is extremely important. In this work, a method for the electro-deposition of single Pt–Pd nanoparticles (NPs) is described using a single nanopore electrode as a template. The electro-deposition process was investigated carefully and the results show that the process is controlled by diffusion and electro-crystallization process, simultaneously, and the glass sheath property around the nanopore has a large impact on the formation of single Pt–Pd NPs due to the “edge effect”. The prepared single Pt–Pd NPs exhibit excellent electrocatalytic activity in the methanol oxidation reaction, which can be used to screen electrocatalysts with high efficiency for utility in the energy field.     

CO adsorption on pure and binary-alloy gold clusters: a quantum chemical study

We performed density-functional theory analysis of nondissociative CO adsorption on 22 binary Au-alloy (Au(n)M(m)) clusters: n=0-3, m=0-3, and m+n=2 (dimers) or 3 (trimers), M=Cu/Ag/Pd/Pt. We report basis-set superposition error corrections to adsorption energies and include both internal energy of adsorption (DeltaU(ads)) and Gibbs free energy of adsorption (DeltaG(ads)) at standard conditions (298.15 K and 1 atm). We found onefold (atop) CO binding on all the clusters except Pd2 (twofold/bridged), Pt2 (twofold/bridged), and Pd3 (threefold). In agreement with the experimental results, we found that CO adsorption is thermodynamically favorable on pure Au/Cu clusters but not on pure Ag clusters and also observed the following adsorption affinity trend: Pd>Pt>Au>Cu>Ag. For alloy dimers we found the following patterns: Au2>M Au>M2 (M=Ag/Cu) and M2>M Au>Au2 (M=Pd/Pt). Alloying Ag/Cu dimers with (more reactive) Au enhanced adsorption and the opposite effect was observed for PdPt dimers. The Ag-Au, Cu-Au, and Pd-Au trimers followed the trends observed on dimers: Au3>M Au2>M2Au>M3 (M=Ag/Cu) and Pd3>Pd2Au>PdAu2>Au3. Interestingly, Pt-Au trimers reacted differently and alloying with Au systematically increased the adsorption affinity: PtAu2>Pt2Au>Pt3>Au3. A strikingly different behavior of Pt is also manifested by the triplet spin state and onefold (atop) binding in Pt3-CO which is in contradiction with the singlet spin state and threefold binding in Pd3-CO. We found a linear correlation between CO binding energy (BE) and elongation of the CO bond. For Ag-Au and Cu-Au clusters, the increase in CO BE (and elongation of the C-O bond which is probably due to the back donation) is accompanied by the decrease in the cluster-CO distance suggesting that the donation (from 5sigma highest occupied molecular orbital in CO to cluster lowest unoccupied molecular orbital) mechanism also contributes to the BE. For Pd-Au clusters, the cluster-CO distance (and CO bond length) increases with increase in the BE, suggesting that the donation mechanism may not be important for those clusters. No clear trend was observed for Pt-Au clusters.     

The Pb12(2-) and Pb10(2-) zintl ions and the M@Pb12(2-) and M@Pb10(2-) cluster series where M = Ni, Pd, Pt

Ethylenediamine (en) solutions of K4Pb9 react with toluene solutions of ML4 (M = Pt, Pd, L = PPh3; M = Ni, L2= COD) and 2,2,2-crypt to give M@Pb12(2-) cluster anions (M = Pt (1), Pd (2), Ni (3)) as the [K(2,2,2-crypt)]+ salts in low (Ni) to good (Pt) yields. The ions have near perfect Ih point symmetry and have been characterized by X-ray diffraction, 207Pb NMR and LDI-TOF mass spectrometry studies. For M = Ni, the primary product formed is the D4d Ni@Pb10(2-) cluster that has also been structurally characterized. The M@Pb10(2-) clusters (M = Pd, Pt) and the new Zintl ions closo-Pb10(2-) and closo-Pb12(2-) were formed in the gas phase but have not been detected in solution or the solid state. The structural trends of these series of clusters have been investigated through DFT calculations. The Ni@Pb10(2-) cluster is dynamic on the 207Pb NMR time scale at -45 degrees C and 104.7 MHz. The M@Pb12(2-) ions show unusually deshielded 207Pb NMR chemical shifts that presumably arise from sigma-aromatic effects associated with their high symmetries. In the solid state, the salts form superlattices of cations and anions (e.g. the AlB2 lattice of [K(2,2,2-crypt)](2)[Pt@Pb12]) and are prototypes for "assembled cluster materials".     

Nanoclusters Au_(19)Pd and Au_(19)Pt Catalyzing CO Oxidation: a Density Functional Study

The gold atoms on the Au20 cluster had been substituted by the palladium and platinum atoms to obtain the doped clusters with more stable geometries as a function of the bind energy and interaction energy in the previous study. Therefore, we investigated the catalytic activities of the Au_(19)Pd and Au_(19)Pt clusters for CO oxidation along the Langmuir-Hinshelwood mechanism. It is found that the coadsorption of CO and O2 on the doped clusters is obviously stronger than on the Au20 cluster, especially on the doped atom, which makes potential energy of the transition state lower than the total energy of the reactants so that it can promote CO oxidation. The reaction on these doped clusters with the heteroatom on the vertex is more difficult. However, the Au_(19)Pd(S) is more prone to catalyzing the CO oxidation, in which the rate-limiting step has the lower energy barrier of 38.84 kJ/mol for this study. Therefore, the single atom can be modified to change the catalytic activity of the cluster for the CO oxidation. Meanwhile, the different sites on the clusters have different strengths of activity for the reaction.     

A re-examination of platinum-group element concentrations in the environmental certified reference material BCR-723

A detailed statistical examination of replicated data used to certify platinum-group elements (PGEs) in environmental reference material BCR-723 is presented. Certification of Pt, Pd, and Rh concentrations in BCR-723 was based on 16, eight, and nine accepted data sets, respectively. Each accepted data set contained six replicated measurements for each PGE, and the statistical properties of these concentration data were examined, i.e. 96 for Pt, 48 for Pd, and 54 for Rh. This level of investigation has received limited attention but is critical in furthering our understanding of PGE variability and representativeness. Concentrations of Pt, Pd, and Rh were shown to differ significantly between accepted data sets. Palladium and Pt differed in their quantification between detection techniques. Additionally, Pd and Pt concentrations varied significantly between laboratories using a similar definitive method (inductively coupled plasma-isotope dilution mass spectrometry). The distribution of Pd concentrations was found to be bimodal, with a secondary population exhibiting a contamination signal of about 15%. The secondary population, not previously reported in BCR-723, is likely a measurement artifact and not due to a nugget effect. Comparisons of BCR-723 with other environmental media from Europe, i.e. airborne particulate matter, tunnel dust, and road-deposited sediment, indicated that Pd is uncommonly low in BCR-723 (6.0?ng?g^?1) and is generally not representative in terms of its distribution relative to Pt and Rh. Serious consideration should be given to developing a new PGE certified environmental reference material.     

Structure and mobility of metal clusters in MOFs: Au, Pd, and AuPd clusters in MOF-74

Understanding the adsorption and mobility of metal-organic framework (MOF)-supported metal nanoclusters is critical to the development of these catalytic materials. We present the first theoretical investigation of Au-, Pd-, and AuPd-supported clusters in a MOF, namely MOF-74. We combine density functional theory (DFT) calculations with a genetic algorithm (GA) to reliably predict the structure of the adsorbed clusters. This approach allows comparison of hundreds of adsorbed configurations for each cluster. From the investigation of Au(8), Pd(8), and Au(4)Pd(4) we find that the organic part of the MOF is just as important for nanocluster adsorption as open Zn or Mg metal sites. Using the large number of clusters generated by the GA, we developed a systematic method for predicting the mobility of adsorbed clusters. Through the investigation of diffusion paths a relationship between the cluster's adsorption energy and diffusion barrier is established, confirming that Au clusters are highly mobile in the MOF-74 framework and Pd clusters are less mobile.     

Pd‐Pt/modified GO as an efficient and selective heterogeneous catalyst for the reduction of nitroaromatic compounds to amino aromatic compounds by the hydrogen source

In this work, different nitroaromatic compounds were successfully reduced to their corresponding aromatic amines with excellent conversion and selectivity in methanol at 50 °C by using Pd‐Pt nanoparticles immobilized on the modified grapheme oxide (m‐GO) and hydrogen as the reducing source. The catalytic efficiency of Pd and Pd‐Pt loading on the modified GO was investigated for the reduction of various nitroaromatic compounds, and the Pd‐Pt/m‐GO system demonstrated the highest conversion and selectivity. The catalyst was characterized by different techniques including FT‐IR, Raman, UV–Vis, XRD, BET, XPS, FESEM, EDS, and TEM. The metal nanoparticles with the size of less than 10 nm were uniformly distributed on the m‐GO. The catalyst could be reused at least five times without losing activity, showing the stability of the catalyst structure. Finally, the efficiency of the prepared catalyst was compared with Pd‐Pt/AC, and Pd‐Pt/GO catalysts.     

Synthesis and characterisation of nu3-octahedral [Ni36Pd8(CO)48]6- and [Ni35Pt9(CO)48]6- clusters displaying unexpected surface segregation of Pt atoms and molecular and/or crystal substitutional Ni/Pd and Ni/Pt disorder

The synthesis and structure, as well as the chemical and electrochemical characterisation of two new nu(3)-octahedral bimetallic clusters with the general [Ni(44-x)M(x)(CO)(48)](6-) (M = Pd, x = 8; M = Pt, x = 9) formula is reported. The [Ni(35)Pt(9)(CO)(48)](6-) cluster was obtained in reasonable yields (56 % based on Pt) by reaction of [Ni(6)(CO)(12)](2-) with 1.1 equivalents of Pt(II) complexes, in ethyl acetate or THF as the solvent. The [Ni(36)Pd(8)(CO)(48)](6-) cluster was obtained from the related reaction with Pd(II) salts in THF, and was isolated only in low yields (5-10 % based on Pd), mainly because of insufficient differential solubility of its salts. The unit cell of the [NBu(4)](6)[Ni(35)Pt(9)(CO)(48)] salt contains a substitutionally Ni-Pt disordered [Ni(24)(Ni(14-x)Pt(x))Pt(6)(CO)(48)](6-) (x = 3) hexaanion. A combination of crystal and molecular disorder is necessary to explain the disordering observed for the Ni/Pt sites. The unit cell of the corresponding [Ni(36)Pd(8)(CO)(48)](6-) salt contains two independent [Ni(30)(Ni(8-x)Pd(x))Pd(6)(CO)(48)](6-) (x = 2) hexaanions. The two display similar substitutional Ni-Pd disorder, which probably arises only from crystal disorder. The structure of [Ni(36)Pd(8)(CO)(48)](6-) establishes the first similarity between the chemistry of Ni-Pd and Ni-Pt carbonyl clusters. A comparison of the chemical and electrochemical properties of [Ni(35)Pt(9)(CO)(48)](6-) with those of the related [Ni(38)Pt(6)(CO)(48)](6-) cluster shows that surface colouring of the latter with Pt atoms decreases redox as well as protonation propensity of the cluster. In contrast, substitution of all internal Pt and two surface Ni with Pd atoms preserves the protonation behaviour and is only detrimental with respect to its redox aptitude. A qualitative rationalisation of the different surface-site selectivity of Pt and Pd, based on distinctive interplays of M--M and M--CO bond energies, is suggested.     

Effect of elemental composition of PtPd bimetallic nanoparticles containing an average of 180 atoms on the kinetics of the electrochemical oxygen reduction reaction

PtPd bimetallic nanoparticles containing an average of 180 atoms and composed of seven different Pt:Pd ratios have been prepared within sixth-generation, hydroxyl-terminated, poly(amidoamine) dendrimers. Transmission electron microscopy indicates that the sizes of all seven nanoparticle compositions are within +/-0.2 nm of one another and the calculated size. Single-particle energy-dispersive spectroscopy shows that the elemental composition is determined by the ratio of the Pt and Pd precursor salts used to prepare the nanoparticles. Cyclic voltammetry and rotating disk voltammetry measurements show that the Pt:Pd ratio of the nanoparticles determines their efficiency for the oxygen reduction reaction (ORR). The maximum activity for the ORR occurs at a Pt:Pd ratio of 5:1, which corresponds to a relative mass activity enhancement of 2.4 compared to otherwise identical monometallic Pt nanoparticles.     

FTIR study of the effects of water pretreatment on the acid sites and the dispersion of metal particles in Y zeolites and mordenites

FTIR results on zeolite-supported Pt and Pd show that the presence of water during metal reduction modifies both the acid and the metal functions of these catalysts. The water treatment eliminates Lewis acid sites and lowers the concentration of Brønsted acid sites by partial dealumination. At low temperature CO is adsorbed on Brønsted sites; the position of the corresponding FRIR band indicates that for Pt/HMOR, (MOR = mordenite) unlike Pt/HY, the intrinsic acid strength of these sites is increased by the wet reduction procedure. FTIR spectra after CO adsorption at room temperature show that wet reduction markedly improves the dispersion of Pt in HMOR; this effect is weaker for HY and absent for the Na forms of the zeolites. Bands of gem-Pt(CO)₂, which are indicative of very small, possibly electron-deficient Pt clusters, are detected in Pt/HMOR.     

The Critical Role Played by the Catalytic Moiety in the Early‐Time Photodynamics of Hydrogen‐Generating Bimetallic Photocatalysts

The effect of the catalytic moiety on the early‐time photodynamics of Ru/M (M=Pt or Pd) bimetallic photocatalysts is studied by ultrafast transient absorption spectroscopy. In comparison to the Ru/Pd photocatalyst described earlier, the Ru/Pt analogue shows complex excited‐state dynamics with three distinct kinetic components ranging from sub‐ps to 10² ps, requiring a more sophisticated photophysical model than that developed earlier for the Ru/Pd complex. In the Pu/Pt photocatalyst, an additional lower‐lying excited state is proposed to quench the hot higher‐lying triplet metal‐to‐ligand charge‐transfer states. Furthermore, a strong excitation wavelength dependence on the population of excited states is observed for both the Ru/Pt and Ru/Pd complexes, indicating a non‐equilibrated distribution even on the 10² ps timescale. These insights shed light on the significant impact of the catalytic moiety on the fundamental early‐time photophysics of Ru‐based photocatalysts.     

The effect of alloying on the H-atom adsorption on the (100) surfaces of Pd-Ag,Pd-Pt,Pd-Au,Pt-Ag,and Pt-Au. A theoretical study

The effect of alloying on the adsorption of atomic hydrogen was studied using density functional theory (DFT). In the study the (100) surfaces of Pd-Ag, Pd-Pt, Pd-Au, Pt-Ag, and Pt-Au alloys were considered by means of a cluster model. The structural and energetic properties of the H atom adsorbed on the Pd₄Me (Me = Ag, Pt, Au) and Pt₄Me (Me = Pd, Ag, Au) clusters were calculated and compared with the H-atom adsorption on monometallic clusters. The effect of alloying on the H-atom adsorption is evident for all the investigated bimetallic systems. However, it strongly depends on the second metal atom, Me, is placed in the surface layer or in the subsurface one. In general, the H atom adsorbed in a site containing the second metal exhibits different properties from those characteristic of its adsorption on Pd(100) and Pt(100). Hence, the modified interaction between atomic hydrogen and the alloyed surfaces may increase the selectivity of the catalytic hydrogenation reactions on such surfaces.     

Electrochemical Synthesis and Catalytic Properties of Encapsulated Metal Clusters within Zeolitic Imidazolate Frameworks

It is very interesting and also a big challenge to encapsulate metal clusters within microporous solids to expand their application diversity. For this target, herein, we present an electrochemical synthesis strategy for the encapsulation of noble metals (Au, Pd, Pt) within ZIF‐8 cavities. In this method, metal precursors of AuCl₄^2?, PtCl₆^2?, and PdCl₄^2? are introduced into ZIF‐8 crystals during the concurrent crystallization of ZIF‐8 at the anode. As a consequence, very small metal clusters with sizes around 1.2 nm are obtained within ZIF‐8 crystals after hydrogen reduction; these clusters exhibit high thermal stability, as evident from the good maintenance of their original sizes after a high‐temperature test. The catalytic properties of the encapsulated metal clusters within ZIF‐8 are evaluated for CO oxidations. Because of the small pore window of ZIF‐8 (0.34 nm) and the confinement effect of small pores, about 80 % of the metal clusters (fractions of 0.74, 0.77, and 0.75 for Au, Pt, and Pd in ZIF‐8, respectively) retain their catalytic activity after exposure to the organosulfur poison thiophene (0.46 nm), which is in contrast to their counterparts (fractions of 0.22, 0.25, and 0.20 for Au, Pt, and Pd on the SiO₂ support). The excellent performances of metal clusters encapsulated within ZIF‐8 crystals give new opportunities for catalytic reactions.     

Rational Design of Pt−Pd−Ni Trimetallic Nanocatalysts for Room-Temperature Benzaldehyde and Styrene Hydrogenation

Nanostructures of the multimetallic catalysts offer great scope for fine tuning of heterogeneous catalysis, but clear understanding of the surface chemistry and structures is important to enhance their selectivity and efficiency. Focussing on a typical Pt−Pd−Ni trimetallic system, we comparatively examined the Ni/C, Pt/Ni/C, Pd/Ni/C and Pt−Pd/Ni/C catalysts synthesized by impregnation and galvanic replacement reaction. To clarify surface chemical/structural effect, the Pt−Pd/Ni/C catalyst was thermally treated at X=200, 400 or 600 °C in a H₂ reducing atmosphere, respectively termed as Pt−Pd/Ni/C−X. The as-prepared catalysts were characterized complementarily by XRD, XPS, TEM, HRTEM, HS-LEIS and STEM-EDS elemental mapping and line-scanning. All the catalysts were comparatively evaluated for benzaldehyde and styrene hydrogenation. It is shown that the “PtPd alloy nanoclusters on Ni nanoparticles” (PtPd/Ni) and the synergistic effect of the trimetallic Pt−Pd−Ni, lead to much improved catalytic performance, compared with the mono- or bi- metallic counterparts. However, with the increase of the treatment temperature of the Pt−Pd/Ni/C, the catalytic performance was gradually degraded, which was likely due to that the favourable nanostructure of fine “PtPd/Ni” was gradually transformed to relatively large “PtPdNi alloy on Ni” (PtPdNi/Ni) particles, thus decreasing the number of noble metal (Pt and Pd) active sites on the surface of the catalyst. The optimum trimetallic structure is thus the as synthesised Pt−Pd/Ni/C. This work provides a novel strategy for the design and development of highly efficient and low-cost multimetallic catalysts, e. g. for hydrogenation reactions.