Size effects in electronic and catalytic properties of unsupported palladium nanoparticles in electrooxidation of formic acid

We report a combined X-ray photoelectron spectroscopy (XPS), cyclic voltammetry (CV), and chronoamperometry (CA) study of formic acid electrooxidation on unsupported palladium nanoparticle catalysts in the particle size range from 9 to 40 nm. The CV and CA measurements show that the most active catalyst is made of the smallest (9 and 11 nm) Pd nanoparticles. Besides the high reactivity, XPS data show that such nanoparticles display the highest core-level binding energy (BE) shift and the highest valence band (VB) center downshift with respect to the Fermi level. We believe therefore that we found a correlation between formic acid oxidation current and BE and VB center shifts, which, in turn, can directly be related to the electronic structure of palladium nanoparticles of different particle sizes. Clearly, such a trend using unsupported catalysts has never been reported. According to the density functional theory of heterogeneous catalysis, and mechanistic considerations, the observed shifts are caused by a weakening of the bond strength of the COOH intermediate adsorption on the catalyst surface. This, in turn, results in the increase in the formic acid oxidation rate to CO2 (and in the associated oxidation current). Overall, our measurements demonstrate the particle size effect on the electronic properties of palladium that yields different catalytic activity in the HCOOH oxidation reaction. Our work highlights the significance of the core-level binding energy and center of the d-band shifts in electrocatalysis and underlines the value of the theory that connects the center of the d-band shifts to catalytic reactivity.     

Surface modification of magnetic nanoparticles capped by oleic acids: characterization and colloidal stability in polar solvents

The lyophobic surface of monodisperse magnetic nanoparticles capped by oleic acid was made to be more lyophilic by ozonolysis to increase the stability of the suspension in polar solvents like ethanol. The ozone oxidatively cleaved the double bond of oleic acid to form carbonyl and carboxyl groups on the surface of the nanoparticles. Additionally, interfacial ligand exchange of the capping molecules was applied to make the hydrophobic particle surface more hydrophilic. The magnetic particles showed enhanced miscibility and short-term stability in water after interfacial ligand exchange. The structure changes of the capping molecules on the nanoparticle surfaces were investigated using Fourier transform infrared (FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS). From these spectroscopy studies, the cleavage of the oleic acid and the formations of the carboxyl and carbonyl groups on the particle surface were confirmed. The shape and the magnetic properties of the nanoparticles were maintained after the surface modification. Ozonolysis is an effective method in modifying the lyophobic surface of the magnetic nanoparticles.     

Synthesis and characterization of photoluminescent In-doped CdSe nanoparticles

Indium-doped CdSe nanoparticles have been synthesized and characterized. Their light absorption, photoluminescence, and structure are similar to undoped CdSe nanoparticles. The greater part of the In associated with the nanoparticles is removed when the nanoparticles undergo ligand exchange by pyridine. As observed with undoped nanoparticles, a ZnS capping layer on the indium-doped nanoparticles results in enhanced nanocrystal photoluminescence. Also, the ZnS cap enhances the retention of In by the nanoparticles. Elemental analysis shows ligand exchange causes CdSe to be lost and capping with ZnS results in the loss of Se. We conclude that In-doped nanoparticles have most of the In on their surface, capping helps the nanoparticles retain the In, and they do not have altered electronic properties.     

Surface chemistry: a non-negligible parameter in determining optical properties of small colloidal metal nanoparticles

Surface chemistry can become pronounced in determining the optical properties of colloidal metal nanoparticles as the nanoparticles become so small (diameters <20 nm) that the surface atoms, which can undergo chemical interactions with the environment, represent a significant fraction of the total number of atoms although this effect is often ignored. For instance, formation of chemical bonds between surface atoms of small metal nanoparticles and capping molecules that help stabilize the nanoparticles can reduce the density of conduction band electrons in the surface layer of metal atoms. This reduced electron density consequently influences the frequency-dependent dielectric constant of the metal atoms in the surface layer and, for sufficiently high surface to volume ratios, the overall surface plasmon resonance (SPR) absorption spectrum. The important role of surface chemistry is highlighted here by carefully analyzing the classical Mie theory and a multi-layer model is presented to produce more accurate predictions by considering the chemically reduced density of conduction band electrons in the outer shell of metal atoms in nanoparticles. Calculated absorption spectra of small Ag nanoparticles quantitatively agree with the experimental results for our monodispersed Ag nanoparticles synthesized via a well-defined chemical reduction process, revealing an exceptional size-dependence of absorption peak positions: the peaks first blue-shift followed by a turnover and a dramatic red-shift as the particle size decreases. A comprehensive understanding of the relationship between surface chemistry and optical properties is beneficial to exploit new applications of small colloidal metal nanoparticles, such as colorimetric sensing, electrochromic devices, and surface enhanced spectroscopies.     

Formation and oxidation mechanisms of Pd-Zn nanoparticles on a ZnO supported Pd catalyst studied by in situ time-resolved QXAFS and DXAFS

Formation and oxidation processes of PdZn nanoparticles on ZnO were successfully observed by means of in situ time-resolved X-ray absorption fine structure spectroscopy (XAFS), and the analysis of data on near-edge (XANES) and extended (EXAFS) structures revealed detailed changes in Pd during both processes. PdZn nanoparticles were formed on ZnO through a two-step scheme under a hydrogen atmosphere. The first process was the formation of metallic Pd nanoparticles, which was quickly finished within 1 s. The second process was the formation of PdZn nanoparticles, which took several tens of minutes. Oxidation of the PdZn nanoparticles also consisted of two processes. Zn atoms were oxidized prior to Pd atoms and the metallic Pd nanoparticles surrounded by ZnO were formed afterwards. Oxidation of the metallic Pd nanoparticles was scarce and very slow. According to the results of kinetic analyses, the metallic Pd surrounded by ZnO was a stable species under the oxidative atmosphere.     

Intraparticle Charge Delocalization through Conjugated Metal-Ligand Interfacial Bonds: Effects of Metal d Electrons?

Intraparticle charge delocalization occurs when metal nanoparticles are functionalized with organic capping ligands through conjugated metal-ligand interfacial bonds. In this study, metal nanoparticles of 5d metals (Ir, Pt, and Au) and 4d metals (Ru, Rh, and Pd) were prepared and capped with ethynylphenylacetylene and the impacts of the number of metal d electrons on the nanoparticle optoelectronic properties were examined. Both FTIR and photoluminescence measurements indicate that intraparticle charge delocalization was enhanced with the increase of the number of d electrons in the same period with palladium being an exception.     

Atomic Imaging of Subsurface Interstitial Hydrogen and Insights into Surface Reactivity of Palladium Hydrides

Resolving interstitial hydrogen atoms at the surfaces and interfaces is crucial for understanding the mechanical and physicochemical properties of metal hydrides. Although palladium (Pd) hydrides hold important applications in hydrogen storage and electrocatalysis, the atomic position of interstitial hydrogen at Pd hydride near surfaces still remains undetermined. We report the first direct imaging of subsurface hydrogen atoms absorbed in Pd nanoparticles by using differentiated and integrated differential phase contrast within an aberration-corrected scanning transmission electron microscope. In contrast to the well-established octahedral interstitial sites for hydrogen in the bulk, subsurface hydrogen atoms are directly identified to occupy the tetrahedral interstices. DFT calculations show that the amount and the occupation type of subsurface hydrogen atoms play an indispensable role in fine-tuning the electronic structure and associated chemical reactivity of the Pd surface.     

The interaction of CO with PdAg/Pd(111) surface alloys--a case study of ensemble effects on a bimetallic surface

The interaction of CO with structurally well-defined PdAg/Pd(111) surface alloys was investigated by temperature-programmed desorption (TPD) and high-resolution electron energy loss spectroscopy (HREELS) to unravel and understand contributions from electronic strain, electronic ligand and geometric ensemble effects. TPD measurements indicate that CO adsorption is not possible on the Ag sites of the surface alloys (at 120 K) and that the CO binding strength on Pd sites decreases significantly with increasing Ag concentration. Comparison with previous scanning tunneling microscopy (STM) data on the distribution of Pd and Ag atoms in the surface alloy shows that this modification is mainly due to geometric ensemble effects, since Pd(3) ensembles, which are the preferred ensembles for CO adsorption on non-modified Pd(111), are no longer available on Ag-rich surfaces. Consequently, the preferred CO adsorption site changes with increasing Ag content from a Pd(3) trimer via a Pd(2) dimer to a Pd monomer, going along with a successive weakening of CO adsorption. Additionally, the CO adsorption properties of the surface alloys are also influenced by electronic ligand and strain effects, but on a lower scale. The results are discussed in comparison with previous findings on PdAg bulk alloys, supported PdAg catalysts and PdAu/Pd(111) model systems.     

Oxidation of ethanol on carbon-supported oxide-rich Pd–W bimetallic nanoparticles in alkaline media

Carbon-supported oxide-rich Pd–W bimetallic nanoparticles were prepared by chemical reduction methods. The existence of oxides in the electrocatalysts is confirmed by X-ray photoelectron spectrum (XPS) and high resolution transmission electron microscopy. XPS analysis indicates that the oxygen atoms account for about 50% of all the atoms in Pd–W bimetallic nanoparticles. Compared to Pd/C catalyst, the carbon-supported oxide-rich Pd–W bimetallic nanoparticles exhibit a better catalytic activity for the anode oxidation of ethanol in alkaline media. The onset potential of the as prepared oxide-rich Pd_0.8W_0.2/C catalyst (Pd: W = 8: 2, metal atom ratio) for ethanol oxidation is negative shifted about 90 mV comparing to Pd/C catalyst. The oxide-rich Pd–W/C electrocatalysts provide a new model of noble-metal/promoter system as an extreme case of making the promoter (WO₃) closely adjacent to the noble metal (Pd) by fabricating nanoparticles containing both atom-clusters of oxides and the noble metal atoms.

     

PtPd-nanoparticles supported by new carbon materials

Nanocomposites based on PtPd nanoparticles with chemical ordering like disordered solid solution on surface of multilayer graphene have been prepared through thermal shock of mechanically obtained mixture of double complex salt [Pd(NH₃)₄][PtCl₆] and different carbon materials–exfoliated graphite, graphite oxide and graphite fluoride. An effect of original carbon precursors on formation of PtPd bimetallic nanoparticles was studied using X-ray absorption spectroscopy (XAFS), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). It was shown that the distribution of bimetallic nanoparticles over the multilayer graphene surface as well as the particles size distribution is controlled by the graphene precursors. For all nanocomposites, the surface of the nanoparticles was found to be Pd-enriched. In case when the thermal exfoliated graphite and graphite oxide were used as the graphene precursors a thin graphitized layer covered the nanoparticles surface. Such a graphitized layer was not observed in the nanocomposite, which used the fluorinated graphite as the precursor.     

Nonaqueous synthesis and characterization of capped α-Fe2O3 nanoparticles from iron(III) hydroxy-oleate precursor

The synthesis of capped α-Fe₂O₃ nanoparticles from thermal treatment of iron (III) hydroxy-oleate in boiling organic solvents around 250 °C with retention of the integrity of the oleate units during the reaction process is reported. The formation of capped iron oxide particles is accomplished under aerobic conditions while the solvents used in the synthesis have strong influence on the nature and morphology of nanoparticles. These nanoparticles are studied by means of X-ray powder diffraction, IR and XPS while the morphology and particle size of nanocrystals are evaluated using SEM and TEM analysis suggesting the formation of monocrystalline α-Fe₂O₃ particles having cubical and spherical morphologies with sizes ranging from 20 to 30 nm. This organophilic material with oleate capping around the surfaces can be readily dispersed in organic solvents thus forming organosols. These organosols exhibit band-edge emission photoluminescence band both in toluene as well as in solid state while FT-IR analysis reveals formation oleate capped nanoparticles The XPS data indicate ferric state having doublet from Fe 2p_3/2 and Fe 2p_1/2 core-level electrons.     

Tuning the Surface Chemistry of Pd by Atomic C and H: A Microscopic Picture

Palladium is crucial for industry‐related applications such as heterogeneous catalysis, energy production, and hydrogen technologies. In many processes, atomic H and C species are proposed to be present in the surface/near‐surface area of Pd, thus noticeably affecting its chemical activity. This study provides a detail and unified view on the interactions of the H and C species with Pd nanoparticles (NPs), which is indispensable for insight into their catalytic properties. Density functional calculations of the interplay of C and H atoms at various concentrations and sites on suitable Pd NPs have been performed, accompanied by catalysis‐relevant experiments on oxide‐supported bare and C‐modified Pd NPs. It is shown that on a Pd₇₉ NP a subsurface C atom destabilizes nearby atoms H at low coverage. Our experiments confirm that H atoms bind more weakly on C‐containing Pd NPs than on C‐free NPs. Various factors related to the presence of both H and C atoms on a Pd₇₉ surface, which may influence the penetration of H atoms from the surface into the subsurface area, have been investigated. Carbon atoms facilitate the subsurface penetration of atomic H both thermodynamically and kinetically when the surface is densely covered by H atoms. Moreover, subsurface H atoms are also energetically favored, even in the absence of C atoms, when several facets of the NP are covered by H atoms.     

Pd/Ni异结构纳米催化剂的制备及其对甲酸氧化的电催化

通过两步还原法制备了Pd/Ni双金属催化剂.由于金属Pd原子在先行还原的Ni纳米粒子表面的外延生长以及其在Ni表面及Pd表面生长表现出的吉布斯自由能差异,最终导致了异结构Pd/Ni纳米粒子的形成.高分辨电子透射显微镜结果证实了异结构的存在,然而X射线衍射测量表明Pd/Ni纳米粒子具有类似于Pd的面心立方结构.制备的Pd/Ni纳米粒子与同等条件下合成的Pd纳米粒子相比对甲酸氧化呈现了更高的电催化活性,而且电催化稳定性也要明显优于纯Pd纳米粒子,证明Pd/Ni双金属催化剂是可选的直接甲酸燃料电池阳极催化剂.双金属催化剂对甲酸氧化电催化活性和稳定性增强可能是Ni原子的修饰改变了Pd粒子表面配位不饱和原子的电子结构所致.     

Pd/Pt二元合金电极的制备及氧还原性能

本文利用欠电位沉积亚单层的Cu及Pt置换取代Cu的方法, 制备了具有不同表面元素组成的Pd/Pt二元合金电极(用Pd/Ptx表示, x指欠电位沉积Cu-Pt置换取代Cu过程的次数),并对其表面元素组成、氧还原性能进行了表征. 在控制欠电位沉积Cu的下限电位恒定(0.34 V)的前提下, 表面Pt/Pd的元素组成比通过重复欠电位沉积Cu及Pt置换取代Cu的次数(1~5次)来可控地调变. 光电子能谱(XPS) 以及红外光谱实验表明,Pd/Ptx电极表层区的Pt：Pd元素组成比随着Pt沉积次数增加而增加, 对Pd/Pt4电极, 在电极表层区约2~3 nm内的Pt/Pd的原子比大约是1:4,而最表层裸露Pd原子的比例仍在20%以上。循环伏安结果显示, 随着Pt沉积次数的增加(1-5次), Pd/Ptx电极表面越不易被氧化。氧还原测试结果显示随着Pt沉积次数的增加(1~4次), Pd/Ptx二元金属电极的氧还原活性依次增加, 经过第3次沉积后其氧还原活性已优于纯Pt,而经4次以上沉积,其氧还原活性基本不变。在其它反应条件相同条件的前提下, Pd/Pt4电极上氧还原的半波电位与纯Pt相比右移约25 mV。结合本文与文献的实验结果,我们初步认为Pd/Ptx二元金属体系氧还原性能改善主要源自表层Pd原子导致其邻近的Pt原子上含氧物种吸附能的降低.     

Understanding mercapto ligand exchange on the surface of FePt nanoparticles

Tailoring the surface of nanoparticles is essential for biological applications of magnetic nanoparticles. FePt nanoparticles are interesting candidates owing to their high magnetic moment. Established procedures to make FePt nanoparticles use oleic acid and oleylamine as the surfactants, which make them dispersed in nonpolar solvents such as hexane. As a model study to demonstrate the modification of the surface chemistry, stable aqueous dispersions of FePt nanoparticles were synthesized after ligand exchange with mercaptoalkanoic acids. This report focuses on understanding the surface chemistry of FePt upon ligand exchange with mercapto compounds by conducting X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR) studies. It was found that the mercapto end displaces oleylamine on the Pt atoms and the carboxylic acid end displaces the oleic acid on the Fe atoms, thus exposing carboxylate and thiolate groups on the surface that provide the necessary electrostatic repulsion to form stable aqueous dispersions of FePt nanoparticles.     

Structural investigation of Pd(II) in concentrated nitric and perchloric acid solutions by XAFS

XAFS spectra of palladium(II) in concentrated HNO3/HClO4 acid mixtures have been recorded and analyzed. Structural parameters of the Pd(H2O)4(2+) complex and the mixed nitric Pd(NO3)2(H2O)2 complex, for the first time, were determined by the XAFS method. For pure 5 M HClO4 and for mixtures (0-0.3 M HNO3), the XAFS spectra of the 0.02 M Pd solutions are indeed very similar and originated from four Pd-O(w) equivalent distances. For the Pd(H2O)4(2+) square-planar aqua ion in strong perchloric acid, the use of an FEFF6 theoretical approach led to a first-shell Pd-O(w) distance of 2.00 (1) A and a Debye-Waller (DW) factor of sigma2 = 0.0030 (3) A2. Four water molecules are tightly bound to the Pd2+ ion in the equatorial plane, while two (or one) axial water molecules are weakly bound to the metal ion at 2.5 A with a DW factor of 0.015 (5) A2. For highly concentrated mixtures (4-6 M HNO3) and for pure concentrated (4-6 M) nitric acid as well as for crystalline powder Pd(NO3)2(H2O)2, the XAFS spectra are very similar and are determined by the mixed nitric complex Pd(NO3)2(H2O)2: four Pd-O near-equivalent distances of 2.01 (1) A from two H2O and two NO3 molecules with a total DW factor of sigma2 = 0.0037 (3) A2. Moreover, two Pd---N distances of 2.8-2.9 A were determined in the second coordination shell. Finally, for intermediate mixtures (1-3 M HNO3 in 5 M HClO4), the XAFS spectra are a superposition of the XAFS of Pd(H2O)4(2+) and Pd(NO3)2(H2O)2 complexes. The mean ligand number NO3(-) around Pd2+ has been calculated, and the XAFS results at pH close to zero confirm the spectrophotometric results previously published.     

钯纳米颗粒形貌控制合成及其SPR／SERS性质

在水溶液中分别以十六烷基三甲基溴化铵（CTAB）和CTAB／柠檬酸钠混合剂为包覆剂合成钯纳米颗粒,并研究其形貌演变．钯纳米颗粒在成核阶段会形成具有不同孪晶结构的晶核,在生长阶段又会选择性的放大某一组晶面,这两个因素导致了钯纳米颗粒形貌的多样性．在合成中CTAB既会影响钯纳米颗粒的成核,也会影响颗粒晶面的选择性生长．通过改变CTAB和还原剂的量可以调控钯纳米颗粒的形貌．溶液中CTAB和还原剂浓度的改变,非常明显地影响合成产物中不同形貌钯纳米颗粒的产率．通过向溶液中引入柠檬酸离子调控纳米颗粒的成核与生长过程,首次合成出了星状钯二十面体和截面为五角星形的钯纳米棒．这些不同形貌的钯纳米颗粒有着不同的表面等离子体共振和表面增强拉曼散射性质．     

Lifetime-broadening-suppressed selective XAFS spectroscopy.

Lifetime-broadening-suppressed (LBS) state-selective XAFS spectra can be deduced by analyzing resonant inelastic x-ray scattering (RIXS) in terms of formulae derived from the Kramers-Heisenberg equation. By a combination of third-generation synchrotron sources and spectrometers equipped with a large acceptance as well as high-resolution analyzers, high-quality RIXS data to extract LBS selective XAFS can be collected. In this review, basic aspects of this novel technique are described, and its potentiality as an analytical tool to determine the local electronic and magnetic structures of metal atoms is demonstrated with several recent examples.     

Modulation of the superficial electronic structure via metal–support interaction for H2 evolution over Pd catalysts

Electronic interactions can radically enhance the performance of supported metal catalysts and are critical for fundamentally understanding the nature of catalysts. However, at the microscopic level, the details of such interactions tuning the electronic properties of the sites on the metal particle''s surface and metal–support interface remain obscure. Herein, we found polarized electronic metal–support interaction (pEMSI) in oxide-supported Pd nanoparticles (NPs) describing the enhanced accumulation of electrons at the surface of NPs (superficial Pd^δ−) with positive Pd atoms distributed on the interface (interfacial Pd^δ+). More superficial Pd^δ− species mean stronger pEMSI resulting from the synergistic effect of moderate Pd–oxide interaction, high structural fluxionality and electron transport activity of Pd NPs. The surface Pd^δ− species are responsible for improved catalytic performance for H₂ evolution from metal hydrides and formates. These extensive insights into the nature of supported-metal NPs may open new avenues for regulating a metal particle''s electronic structure precisely and exploiting high-performance catalysts.

A new type of electronic effect, polarized metal-support interaction (pEMSI), in oxide-supported Pd nanoparticles describing the enhanced accumulation of electrons at the superficial surface is responsible for improved catalytic H₂ evolution.     

Effect of Pd nanoparticle size on the catalytic hydrogenation of allyl alcohol

We report a particle size dependence for the rate of hydrogenation of allyl alcohol using 1.3-1.9 nm Pd dendrimer-encapsulated nanoparticle (DEN) catalysts. For particles with diameters of <1.5 nm and containing <147 Pd atoms, the modulation in catalytic activity is due to the electronic properties of the particle. For the larger particles, 1.5-1.9 nm in diameter and containing an average of 147-250 Pd atoms, the size effect is a result of geometrical constraints. Specifically, the hydrogenation reaction is shown to occur preferentially on the face atoms of the larger nanoparticles.