Characterization of Pt@Cu core@shell dendrimer-encapsulated nanoparticles synthesized by Cu underpotential deposition

Dendrimer-encapsulated nanoparticles (DENs) containing averages of 55, 147, and 225 Pt atoms immobilized on glassy carbon electrodes served as the electroactive surface for the underpotential deposition (UPD) of a Cu monolayer. This results in formation of core@shell (Pt@Cu) DENs. Evidence for this conclusion comes from cyclic voltammetry, which shows that the Pt core DENs catalyze the hydrogen evolution reaction before Cu UPD, but that after Cu UPD this reaction is inhibited. Results obtained by in situ electrochemical X-ray absorption spectroscopy (XAS) confirm this finding.     

Effect of particle size on the kinetics of the electrocatalytic oxygen reduction reaction catalyzed by Pt dendrimer-encapsulated nanoparticles

Platinum dendrimer-encapsulated nanoparticles (DENs) containing an average of 55, 100, 147, 200, and 240 atoms were prepared within sixth-generation, hydroxyl-terminated, poly(amidoamine) dendrimers. These DENs were immobilized on glassy carbon electrodes, and the effect of particle size on the kinetics of the oxygen reduction reaction (ORR) was quantitatively evaluated using rotating disk voltammetry. The total areas of the Pt DENs were determined by electrochemical CO stripping and hydrogen desorption, and the results were found to be in reasonable agreement with calculated values. The largest particles exhibited the highest specific activities for the ORR.     

Tuning supported catalyst reactivity with dendrimer-templated Pt-Cu nanoparticles

The effects of particle composition on heterogeneous catalysis were studied using dendrimer-encapsulated nanoparticles (DENs) as precursors to supported Pt-Cu catalysts. Bimetallic Pt-Cu DENs with varying Pt/Cu ratios were prepared in an anaerobic aqueous solution and deposited onto a high-purity commercial alumina support. The dendrimer template was then thermally removed to yield supported nanoparticle catalysts, which were studied with toluene hydrogenation and CO oxidation catalysis as well as infrared spectroscopy of adsorbed CO. Incorporating Cu into Pt nanoparticles had opposite effects on the two test reactions. Cu acted as a mild promoter for CO oxidation catalysis, and the promoting effect was independent of the amount of Cu present. Conversely, Cu acted as a strong poison for toluene hydrogenation catalysis, and the normalized rate tracked inversely with Cu content. Infrared spectroscopy of the supported nanoparticles indicated that electronic effects (electron donation from Cu to Pt) were minimal for these materials. Consequently, the catalysis results are interpreted in terms of potential structural differences as a function of Cu incorporation and reaction conditions.     

Platinum submonolayer-monolayer electrocatalysis: An electrochemical and X-ray absorption spectroscopy study

A new Pt monolayer electrocatalyst concept is described and the results of electrochemical and X-ray absorption spectroscopy (XAS) studies are presented. Two new methods that facilitate the application of this concept in obtaining ultra-low-Pt-content electrocatalysts have been developed. One is the electroless (spontaneous) deposition of a Pt submonolayer on Ru nanoparticles, and the other is a deposition of a Pt monolayer on Pd nanoparticles by redox displacement of a Cu adlayer. The Pt submonolayer on Ru (PtRu₂₀) electrocatalyst demonstrated higher CO tolerance than commercial catalysts under conditions of rotating disk experiments. The long-term stability test showed no loss in performance over 870 h using a fuel cell operating under real conditions, even though the Pt loading was approximately 10% of that of the standard Pt loading. In situ XAS indicated an increase in d-band vacancy of deposited Pt, which may facilitate partly the reduced susceptibility to CO poisoning for this catalyst. The kinetics of O₂ reduction on a Pt monolayer on Pd nanoparticles showed a small enhancement in comparison with that from a Pt nanoparticle electrocatalyst. The increase in catalytic activity is partly attributed to decreased formation of PtOH, as shown by XAS experiments.     

CO oxidation on Pt nanoclusters, size and coverage effects: a density functional theory study

CO oxidation on Pt nanoclusters of approximately 1 nm in size was studied using density functional theory (DFT). Reaction barriers on various sites of a cuboctahedral 55-atom cluster and of several two-layer plane clusters representing (111) and (100) facets of the 147-atom cluster have been calculated at various coverage. The effect of atomic structure of various clusters was discussed. It was concluded that the 147-atom cuboctahedral cluster reveals properties of the Pt single crystal surfaces, while a 55-atom cluster cannot be fully described in terms of Pt single crystal surfaces. It was found that CO oxidation may occur faster at higher coverage and that for cluster sizes up to a few nanometers in size, larger platinum clusters can be more efficient in CO oxidation than the smaller clusters. The size effect was found to depend upon coverage.     

Electrodeposition of copper on a Pt(111) electrode in sulfuric acid containing poly(ethylene glycol) and chloride ions as probed by in situ STM

This study employed real-time in situ STM imaging to examine the adsorption of PEG molecules on Pt(111) modified by a monolayer of copper adatoms and the subsequent bulk Cu deposition in 1 M H(2)SO(4) + 1 mM CuSO(4)+ 1 mM KCl + 88 μM PEG. At the end of Cu underpotential deposition (~0.35 V vs Ag/AgCl), a highly ordered Pt(111)-(√3 × √7)-Cu + HSO(4)(-) structure was observed in 1 M H(2)SO(4) + 1 mM CuSO(4). This adlattice restructured upon the introduction of poly(ethylene glycol) (PEG, molecular weight 200) and chloride anions. At the onset potential for bulk Cu deposition (~0 V), a Pt(111)-(√3 × √3)R30°-Cu + Cl(-) structure was imaged with a tunneling current of 0.5 nA and a bias voltage of 100 mV. Lowering the tunneling current to 0.2 nA yielded a (4 × 4) structure, presumably because of adsorbed PEG200 molecules. The subsequent nucleation and deposition processes of Cu in solution containing PEG and Cl(-) were examined, revealing the nucleation of 2- to 3-nm-wide CuCl clusters on an atomically smooth Pt(111) surface at overpotentials of less than 50 mV. With larger overpotential (η > 150 mV), Cu deposition seemed to bypass the production of CuCl species, leading to layered Cu deposition, starting preferentially at step defects, followed by lateral growth to cover the entire Pt electrode surface. These processes were observed with both PEG200 and 4000, although the former tended to produce more CuCl nanoclusters. Raising [H(2)SO(4)] to 1 M substantiates the suppressing effect of PEG on Cu deposition. This STM study provided atomic- or molecular-level insight into the effect of PEG additives on the deposition of Cu.     

Temperature-dependent structuring of Au-Pt bimetallic nanoclusters on a thin film of Al2O3/NiAl(100)

Au-Pt bimetallic nanoclusters on a thin film of Al(2)O(3)/NiAl(100) undergo significant structural evolution on variation of the temperature. Au and Pt deposited sequentially from the vapor onto thin-film Al(2)O(3)/NiAl(100) at 300 K form preferentially bimetallic nanoclusters (diameter ≦ 6.0 nm and height ≦ 0.8 nm) with both Au and Pt coexisting at the cluster surface, despite the order of metal deposition. These bimetallic clusters are structurally ordered, have a fcc phase and grow with their facets either (111) or (001) parallel to the θ-Al(2)O(3)(100) surface. Upon annealing the clusters to 400-500 K, the Au atoms inside the clusters migrate toward the surface, resulting in formation of a structure with a Pt core and an Au shell. Annealing the sample to 500-650 K reorients the bimetallic clusters--all clusters have their (001) facets parallel to the oxide surface--and induces oxidation of Pt. Such annealed bimetallic clusters become encapsulated with the aluminium-oxide materials and a few Au remain on the surface.     

Synthesis of Pd-Pt bimetallic nanocrystals with a concave structure through a bromide-induced galvanic replacement reaction

This article describes a systematic study of the galvanic replacement reaction between PtCl(6)(2-) ions and Pd nanocrystals with different shapes, including cubes, cuboctahedrons, and octahedrons. It was found that Br(-) ions played an important role in initiating, facilitating, and directing the replacement reaction. The presence of Br(-) ions led to the selective initiation of galvanic replacement from the {100} facets of Pd nanocrystals, likely due to the preferential adsorption of Br(-) ions on this crystallographic plane. The site-selective galvanic replacement resulted in the formation of Pd-Pt bimetallic nanocrystals with a concave structure owing to simultaneous dissolution of Pd atoms from the {100} facets and deposition of the resultant Pt atoms on the {111} facets. The Pd-Pt concave nanocubes with different weight percentages of Pt at 3.4, 10.4, 19.9, and 34.4 were also evaluated as electrocatalysts for the oxygen reduction reaction (ORR). Significantly, the sample with a 3.4 wt.% of Pt exhibited the largest specific electrochemical surface area and was found to be four times as active as the commercial Pt/C catalyst for the ORR in terms of equivalent Pt mass.     

Vibrational lifetimes of molecular adsorbates on metal surfaces

We report density functional theory calculations of electron-hole pair induced vibrational lifetimes of diatomic molecules adsorbed on metal surfaces. For CO on Cu(100), Ni(100), Ni(111), Pt(100), and Pt(111), we find that the C-O internal stretch and the bending modes have lifetimes in the 1-6 ps range, and that the CO-surface stretch and the frustrated translational modes relax more slowly, with lifetimes >10 ps for all cases except CO on Ni(111). This strong mode selectivity confirms earlier calculations for CO on Cu(100) and demonstrates that the trends carry over to other metal substrates. In contrast, for NO adsorbed on Pt(111), whereas we still find that the bending mode has the shortest lifetime, about 1.3 ps, we predict the other three modes to have almost equal lifetimes of 8-10 ps. Similarly, for CN adsorbed on Pt(111), we calculate that the internal stretching and molecule-surface stretching modes have approximately equal lifetimes of about 15 ps. Our results are in reasonable agreement with experiment, where available. We discuss some of the underlying factors that may contribute to the observed mode selectivity with adsorbed CO and the altered selectivity with NO and CN.     

Shear instabilities in metallic nanoparticles: hydrogen-stabilized structure of Pt37 on carbon

Using density functional theory calculations, we have studied the morphology of a Pt37 nanoparticle supported on carbon with and without hydrogen (H) passivation that arises with postprocessing of nanoparticles before characterization. Upon heating in an anneal cycle, we find that without H (e.g., in a helium atmosphere or evacuation at high temperature), the morphology change of a truncated cuboctahedral Pt37 is driven by the shearing of (100) to (111) facets to lower the surface energy, a remnant shear instability that drives surface reconstruction in semi-infinite Pt(100). With H passivation from a postprocessing anneal, we show that the sheared structure automatically reverts to the observed truncated cuboctahedral structure and the average first nearest-neighbor Pt-Pt bond length increases by 3%, agreeing well with experiment. We explain the stabilization of the truncated cuboctahedral structure due to H passivation via adsorption energetics of hydrogen on Pt(100) and (111) facets, specifically, the preference for H adsorption at bridge sites on (100) facets, which should be considered in a realistic model for H adsorption on Pt nanoparticles. We find that dramatic morphological change of a nanoparticle can occur even with small changes to first-shell Pt-Pt coordination number. The implications of our findings when comparing to experimental data are discussed.     

Systematic DFT-GGA study of hydrogen adsorption on transition metals

Computational study of hydrogen adsorption on (111) surface of transition metals with face centered cubic (fcc) lattice is reported and the results are compared with available experimental and theoretical data. In addition, dissociative adsorption of hydrogen on Pt(111), Pt(100) and Pt(110) is studied in the range of coverage from 0.25 to 1 monolayer. In the case of Pt(111) preferential adsorption site was found to be three-coordinated fcc-hollow site, while on Pt(100) and Pt(110) surface hydrogen settles on two-coordinated bridge and short bridge site, respectively. Hydrogen adsorption energy was found to decrease with the increasing coverage. Structural changes of studied Pt surfaces upon hydrogen adsorption have been compared with the experimental data existing in the literature and good qualitative agreement has been obtained.     

一种金属单晶电极制备方法的建立和Cu2+在铂单晶上UPD过程的研究

同种材料而表面结构不同的电极往往有完全不同的电化学性能.使用在原子水平上表面结构明确的单晶电极不仅有助于对电极表面吸脱附过程、电场作用下表面结构重组、双电层微观结构、分子水平上的反应机理等基础理论进行深入研究,且对高选择性、高效电催化剂的研制也有指导意义.单晶电化学研究的基础就是制备定向不同的单晶电极.本文建立了金属单晶电极制备方法,并报道了Cu²⁺在Pt单晶电极上UPD(欠电位沉积)过程的研究结果.     

Pt纳米晶上CO电吸附及电氧化的表面增强红外吸收光谱研究

为理解Pt 纳米晶(NCs)表面上吸附与反应的结构效应, 本文利用电化学衰减全反射-表面增强红外吸收光谱(ATR-SEIRAS)初步研究了{100}优先取向的Pt 纳米晶表面CO电吸附和电氧化. 合成并清洗过的Pt 纳米晶在硫酸溶液中的循环伏安图出现了四对氧化还原峰, 其中位于0.26和0.36 V的峰分别对应于短程有序和长程有序Pt{100}上的氢吸脱附. 利用Bi、Ge 不可逆吸附法估算出Pt{100}和Pt{111}纳米晶筹分别占34% 和17%. 在原位红外光谱研究中, 首次分辨出线性吸附的CO (CO_L)物种在Pt 纳米晶的三个基础小晶面上的振动谱峰. 动电位光谱分析结果表明Pt{110}上吸附的CO_L优先电氧化, 其次{111}上的CO_L发生氧化, 而Pt{100}上CO_L氧化过电位最高.     

Experimental and theoretical studies of ammonia decomposition activity on Fe-Pt, Co-Pt, and Cu-Pt bimetallic surfaces

We investigate the decomposition of ammonia on bimetallic surfaces prepared by the deposition of a monolayer of Fe, Co, or Cu on a Pt(111) surface computationally and experimentally. We explore the correlation between predicted activities based on the nitrogen binding energies with experimental decomposition activity on these bimetallic and corresponding monometallic surfaces. Through density functional theory calculations and microkinetic modeling, it is predicted that the Fe-Pt-Pt(111) and Co-Pt-Pt(111) surfaces, with a monolayer of Fe or Co on top of Pt(111), are active toward decomposing ammonia. In contrast, the corresponding subsurface configurations, Pt-Fe-Pt(111) and Pt-Co-Pt(111) are inactive. These predictions were confirmed experimentally through temperature programmed desorption experiments. Decomposition was seen at temperatures below 350 K for the Fe-Pt-Pt(111) and Co-Pt-Pt(111) surfaces. For the Cu∕Pt(111) system, the surface, subsurface and parent metals were each predicted to be inactive, consistent with experiments, further validating the model predictions. The stability of these bimetallic surfaces in the presence of adsorbed nitrogen is also discussed.     

Thermodynamics and kinetics of oxygen-induced segregation of 3d metals in Pt-3d-Pt(111) and Pt-3d-Pt(100) bimetallic structures

The stability of subsurface 3d transition metals (3d represents Ni, Co, Fe, Mn, Cr, V, and Ti) in Pt(111) and Pt(100) was examined in vacuum and with 0.5 ML atomic oxygen by a combined experimental and density functional theory (DFT) approach. DFT was used to predict the trends in the binding energy of oxygen and in the stability of 3d metals to remain in the subsurface layer. DFT calculations predicted that for both (111) and (100) crystal planes the subsurface Pt-3d-Pt configurations were thermodynamically preferred in vacuum and that the surface 3d-Pt-Pt configurations were preferred with the adsorption of 0.5 ML atomic oxygen. Experimentally, the DFT predictions were verified by using Auger electron spectroscopy to monitor the segregation of Ni and Co in Pt-3d-Pt structures on polycrystalline Pt foil, composed of mainly (111) and (100) facets. The activation barrier for the oxygen-induced segregation of Ni was found to be 17+/-1 kcal/mol attributed to the Pt(111) areas and 27+/-1 kcal/mol attributed to the Pt(100) areas of the Pt foil. For Pt-Co-Pt, the activation barrier was found to be 10+/-1 kcal/mol and was attributed to the Pt(111) areas of the Pt foil. The Bronsted-Evans-Polanyi relationship was utilized to predict the activation barriers for segregation of the other Pt-3d-Pt(111) and Pt-3d-Pt(100) systems. These results are further discussed in connection to the activity and stability for cathode bimetallic electrocatalysts for proton exchange membrane fuel cells.     

Chemical speciation of adsorbed glycine on metal surfaces

Experimental studies have reported that glycine is adsorbed on the Cu(110) and Cu(100) surfaces in its deprotonated form at room temperature, but in its zwitterionic form on Pd(111) and Pt(111). In contrast, recent density functional theory (DFT) calculations indicated that the deprotonated molecules are thermodynamically favored on Cu(110), Cu(100), and Pd(111). To explore the source of this disagreement, we have tested three possible hypotheses. Using DFT calculations, we first show that the kinetic barrier for the deprotonation reaction of glycine on Pd(111) is larger than on Cu(110) or Cu(100). We then report that the presence of excess hydrogen would have little influence on the experimentally observed results, especially for Pd(111). Lastly, we perform Monte Carlo simulations to demonstrate that the aggregates of zwitterionic species on Pt(111) are energetically preferred to those of neutral species. Our results strongly suggest that the formation of aggregates with relatively large numbers of adsorbed molecules is favored under experimentally relevant conditions and that the adsorbate-adsorbate interactions in these aggregates stabilize the zwitterionic species.     

Preexponential factors for hydrogen desorption from single crystal metal surfaces

Partition functions for adsorbed hydrogen atoms have been evaluated for Ni(100), Ni(111), Pd(111) and Pt(111) surfaces. These account for the relative order of magnituted of the preexponential factors for associative desorption, ν_d, including the abnormally low value for the Pt(111)/H system. Furthermore application of transition state theory yields numbers for ν_d which are in good agreement with experimental data.     

甲烷部分氧化催化剂抗积碳性能的DFT研究

采用基于密度泛函理论(DFT)的量子力学计算方法,研究比较了甲烷部分氧化反应制合成气过程中Ni(111),Pt(111),NiPt(111),NiIr(111)面上的积碳情况.计算了对积碳过程有影响的三个反应过程:CH的裂解,C的生长,CO的生成.研究发现Pt的掺杂对积碳的形成有一定的抑制作用;Pt的抗积碳性能很好,碳的产生和生长都比较困难;掺杂Ir后NiIr催化剂的抗积碳效果明显,CH容易活化解离,产生的C会及时生成CO,不易累积长大,从而更好地达到了抗积碳的目的.     

Methodical aspects of studying the electroreduction of nitrate on modified single crystal Pt(hkl) + Cu electrodes

Technique of modification of basal faces Pt(hkl) by adatoms and epitaxial copper deposits is developed. Analysis of potentiostatic current transients of copper deposition/dissolution and atomic force microscopy showed that the activity of Pt(hkl) faces regarding the processes of copper nucleation and epitaxial growth increases in the sequence of Pt(111) < Pt(110) < Pt(100). The reaction of nitrate anion reduction is sensitive towards the surface structure, not only in the case of platinum, but also in the case of copper deposits (including a monolayer of adatoms). The highest process rate is observed for the Pt(100) electrode modified by a monolayer of adatoms or islands of bulk copper; nitrate reduction at the lowest rate occurs at Pt(111) + Cu electrodes. Structure-sensitive competitive adsorption of background electrolyte and nitrate anions is the factor that largely determines the kinetics of nitrate reduction on different faces of platinum single crystal and copper deposits.     

Synthesis and Characterization of Dendrimer‐Encapsulated Bimetallic Core‐Shell PdPt Nanoparticles

Using a successive method, PAMAM dendrimer‐encapsulated bimetallic PdPt nanoparticles have been successfully prepared with core‐shell structures (Pd@Pt DENs). Evidenced by UV‐vis spectra, high resolution transmission electron microscopy, and X‐ray energy dispersive spectroscopy (EDS), the obtained Pd@Pt DENs are monodispersed and located inside the cavity of dendrimers, and they show a different structure from monometallic Pt or Pd and alloy PdPt DENs. The core‐shell structure of Pd@Pt DENs is further confirmed by infrared measurements with carbon monoxide (IR‐CO) probe. In order to prepare Pd@Pt DENs, a required Pd/Pt ratio of 1:2 is determined for the Pt shell to cover the Pd core completely. Finally, a mechanism for the formation of Pd@Pt DENs is proposed.