Thermodynamics of H in disordered Pd-Ag alloys from calorimetric and equilibrium pressure-composition-temperature measurements

In this research, the thermodynamics of H2 solution and hydride formation in a series of disordered Pd-Ag alloys has been determined using both reaction calorimetry and equilibrium PH2-composition-T data. Trends of DeltaHH and DeltaSH with both H and Ag concentration have been determined. For the Pd0.76Ag0.24 alloy, which does not form a hydride phase, DeltaHH and DeltaSH both exhibit minima with H/(Pd0.76Ag0.24) followed by a linear increase of the former. A linear increase of DeltaHH is found for all of the alloys in the high H content region beyond the two-phase region or, if, there is no two-phase region, in the high H content region. DeltaHH degrees at infinite dilution of H decreases with atom fraction Ag, XAg, up to about 0.40 and then increases. Enthalpies for hydride formation/decomposition, 1/2H2(g) + dilute <--> hydride, have been determined calorimetrically for alloys which form two phases (303 K). The enthalpies for hydride formation become more exothermic with XAg while the corresponding entropy magnitudes are nearly constant, 46 +/- 2 J/K mol H.     

Temperature dependence of H permeation through Pd and Pd alloy membranes

H permeabilities (normalized fluxes), have been measured through Pd and some Pd alloy membranes at a series of constant upstream H(2) pressures with the downstream pressure being ~0 in the temperature range from 393 to 573 K. From these data, activation energies for H permeation, E(P), have been determined. Conditions of constant upstream p(H(2)) are of most interest since most determinations of E(P) in the literature have employed this boundary condition. Permeabilities have also been measured at a series of constant upstream H concentrations with the downstream concentration being ~0 and, under these conditions, the slopes of the Arrhenius plots give activation energies equivalent to those for H diffusion. It is shown here that under constant upstream p(H(2)) conditions, nonideality of the H leads to nonlinear Arrhenius plots of P for Pd and especially for some Pd alloy membranes where the H(2) solubilities are significant even at moderate p(H(2)). For example, the permeabilities of a Pd(0.77)Ag(0.23) alloy membrane and a Pd(0.94)Y(0.06) alloy membrane are found to be nearly independent of temperature (423 to 523 K) in the range of constant upstream pressures from 16.1 to 81 kPa.     

Thermodynamics of hydrogen solution and hydride formation in Pd-Mn alloys. 1. Disordered alloys and a correlation effect

The thermodynamics of H(2) solution and hydride formation/decomposition have been determined by reaction calorimetry (303 K) for disordered face centered cubic (fcc) Pd-Mn alloys. This alloy system belongs to the expanded lattice category which predicts that and DeltaH(plat) for H(2) absorption should be more exothermic than those for Pd; the experimental results are that the former is more exothermic, at least at the higher Mn contents, but the latter is not. There is a regular decrease in the H capacity (at p(H)2 = 0.2 MPa) with atom fraction Mn. A linear dependence of log p(H)2 upon H content is found in the single hydride phase for all of these alloys suggesting that DeltaH(H) and DeltaS(H) are also linear functions of r in this region. This is confirmed using the Pd(0.875)Mn(0.125) alloy which has no two-phase region (303 K). It is shown for the Pd(0.875)Mn(0.125) alloy and for Pd that the changes of partial enthalpies and entropies with H content are correlated so as to minimize changes of mu(H).     

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.     

Achieving optimum hydrogen permeability in PdAg and PdAu alloys

The present work investigates both the diffusivity and permeability of hydrogen (H) in palladium-silver (PdAg) and palladium-gold (PdAu) alloys over a 400-1200 K temperature range for Pd(100-X)M(X), M=Ag or Au and X=0%-48% using density functional theory (DFT) and kinetic Monte Carlo simulations (KMC). DFT has been employed to obtain octahedral (O)-, tetrahedral (T)-, and transition state (TS)- site energetics as a function of local alloy composition for several PdAg and PdAu alloys with compositions in supercells of X=14.18%, 25.93%, 37.07%, and 48.15% with the nearest (NNs) and next nearest neighbors (NNNs) varied over the entire range of compositions. The estimates were then used to obtain a model relating the O, T, and TS energies of a given site with NN(X), NNN(X), and the lattice constant. The first passage approach combined with KMC simulations was used for the H diffusion coefficient predictions. It was found that the diffusion coefficient of H in PdAg alloy decreases with increasing Ag and increases with increasing temperature, matching closely with the experimental results reported in the literature. The calculated permeabilities of H in these novel binary alloys obtained from both diffusivity and solubility predictions were found to have a maximum at approximately 20% Ag and approximately 12% Au, which agree well with experimental predictions. Specifically, the permeability of H in PdAg alloy with approximately 20% Ag at 456 K is three to four times that of pure Pd, while the PdAu alloy at 12% Au is four to five times that of pure Pd at 456 K.     

Solubility of hydrogen in PdAg and PdAu binary alloys using density functional theory

The present work deals with the study of palladium-silver (PdAg) and palladium-gold (PdAu) binary alloys over a broad range of temperatures and alloy compositions using density functional theory (DFT) to find possible conditions where the solubility of hydrogen (H) is significantly higher than that of pure palladium (Pd). Several alloy structures, such as Pd(100-x)Ag(x) with x = 14.81, 25.93, 37.04, and 48.51, Pd(100-x)Aux with x = 14.81, 25.93, and 37.04, and Pd(100-x)Cu(x) with x = 25.93 and 48.51 were considered. The lattice constants of these structures were optimized using DFT, and relaxed structures were used for the estimation of binding energy. It was found that the solubility of H in PdAg is higher than pure Pd with a maximum at approximately 30% Ag at 456 K. Also, the solubility of PdAu alloys was higher than pure Pd with a maximum at about 20% Au with a solubility 12 times higher than that of pure Pd. It was found that for a 3.7% H concentration in a PdAg alloy, a cell expansion of 0.15-0.2% occurs, which if ignored may affect the individual binding energy of the O-site by approximately 3.56% and may affect the predicted solubility by approximately 11.8%.     

陶瓷基钯银合金膜制备技术及性能研究

报道采用浆料涂敷与化学镀相结合法在陶瓷支撑上沉积钯银合金膜,采用浆料涂敷法,将含钯银的浆料涂敷在支撑体上,干燥后热分解,使支撑体表面沉积部分钯银。然后,采用化学镀在50℃下沉积钯银膜,热处理后,得到表面较光洁、厚度约为3μm、银含量为23%(原子百分含量)、合金化较完全的钯银合金膜,该膜在压差为0.1MPa,温度为300℃时的渗氢通量达45cm^3(STP)/(cm^2·min),分离因子达500,研究了钯银合金膜中银含量随热处理温度的变化规律,由实验结果发现,银逐渐向表面扩散,并在一定的温度条件达到基本平衡。     

负载型Pd/TiO2和Pd-Ag/TiO2催化剂的乙炔选择性加氢催化性能

利用程序升温还原(TPR)、X-射线衍射(XRD)、CO吸附-红外光谱(CO-IR)、电子顺磁共振(EPR)和微型催化反应评价等手段, 研究了负载Pd/γ-Al2O3, Pd/TiO2和Pd-Ag/TiO2催化剂的结构和乙炔选择性加氢催化性能. 结果表明, Pd/TiO2催化剂具有较Pd/γ-Al2O3催化剂更优良的乙炔选择性加氢催化性能, 这与Pd-TiO2之间的强相互作用密切相关. Pd-TiO2之间的强相互作用不仅使负载型钯金属催化剂具有较高的乙炔加氢催化选择性, 而且具有较高的乙炔加氢催化活性. Pd/TiO2催化剂中添加Ag 组分后, Pd金属可促进Ag+的还原并可能形成Pd-Ag合金, 催化剂的乙烯选择性虽有所增加, 但乙炔转化率和乙烯收率下降.     

甲醇与氧在银钯合金上的吸附和反应

本文运用UPS、超高真空程序升温反应谱(TPRS)研究了氧和甲醇在银钯合金上的吸附和反应。实验结果表明, 合金表面存在两种分别与Ag和Pd原子有关的活性位; 少量钯原子的存在, 一方面提供了甲醇分解反应的活性中心, 另一方面通过与银之间的电子相互作用, 削弱了氧与银的结合, 增强了表面吸附氧的反应活性, 从而改变了甲醇氧化反应的选择性。     

First-principles analysis of the effects of alloying Pd with Ag for the catalytic hydrogenation of acetylene-ethylene mixtures

The effect of alloying Pd with Ag on the hydrogenation of acetylene is examined by analyzing the chemisorption of all potential C(1) (atomic carbon, CH, methylene, and methyl) and C(2) (acetylene, vinyl, ethylene, ethyl, ethane, ethylidene, ethylidyne, and vinylidene) surface intermediates and atomic hydrogen along with the reaction energies for the elementary steps that produce these intermediates over Pd(111), Pd(75%)Ag(25%)/Pd(111), Pd(50%)Ag(50%)/Pd(111), and Ag(111) surfaces by using first-principle density functional theoretical (DFT) calculations. All of the calculations reported herein were performed at 25% surface coverage. The adsorption energies for all of the C(1) and C(2) intermediates decreased upon increasing the composition of Ag in the surface. Both geometric as well as electronic factors are responsible for the decreased adsorption strength. The modes of adsorption as well as the strengths of adsorption over the alloy surfaces in a number of cases were characteristically different than those found over pure Pd (111) and Ag (111). Adsorbates tend to minimize their interaction with the Ag atoms in the alloy surface. An electronic analysis of these surfaces shows that there is, in general, a shift in the occupied d-band states away from the Fermi level when Pd is alloyed with Ag. The s and p states also appear to contribute and may be responsible for small deviations from the Hammer-N?rskov model. The effect of alloying is more pronounced on the calculated reaction energies for different possible surface elementary reactions. Alloying Pd with Ag reduces the exothermicity (increases endothermicity) for bond-breaking reactions. This is consistent with experimental results that show a decrease in the decomposition products in moving from pure Pd to Pd-Ag alloys.(2-5) In addition, alloying increases the exothermicity of bond-forming reactions. Alloying therefore not only helps to suppress the unfavorable decomposition (bond-breaking) reaction rates but also helps to enhance the favorable hydrogenation (bond-forming) reaction rates.     

PHOSPHORUS-31 NMR STUDIES ON BIS(DIPHENYLPHOSPHINO)-METHANE-BRIDGED PALLADIUM-SILVER HETERODINUCLEAR COMPLEXES

<正> Bis (diphenylphosphino) methane (dppm)-bridged Pd-Ag complexes of nitrate,nitrite, acetate and formate have been prepared by treatment of trans- (NC)2Pd (dppm)2 with suitable silver salts respectively and characterized by elemental analysis, infrared spectroscopy,and by proton,carbon-13,phosphorus-31 nmr spectroscopies. Their 31P- {1H} nmr spectra have been analyzed by using the Bruker PANIC program for spectrum simulation to find the correct chemical shifts of the phosphorus atoms and P-P,P-Ag coupling constants of these complexes, each of which is a mixture of a pair of silver-107 and silver-109 isotopic isomers. A comparison of the 31P nmr data for trans-(NC)2Pd-(μ-dppm)2Ag(NO2) and for fo-oBs-(NC)2Pd(>-dppm)2Ag(NO3) shows that the additional oxygen atom in the nitrate complex withdraws p electron density, rather than s, from the silver moiety. Also a comparison of the 31P nmr data for trans- (NC)2Pd(μ-dppm)2Ag (OOCH) and for trans-(NC)2Pd(μ-dppm)2Ag(OOCCH3) shows that the methyl group in the acet     

Metallurgy in a beaker: nanoparticle toolkit for the rapid low-temperature solution synthesis of functional multimetallic solid-state materials

Intermetallic compounds and alloys are traditionally synthesized by heating mixtures of metal powders to high temperatures for long periods of time. A low-temperature solution-based alternative has been developed, and this strategy exploits the enhanced reactivity of nanoparticles and the nanometer diffusion distances afforded by binary nanocomposite precursors. Prereduced metal nanoparticles are combined in known ratios, and they form nanomodulated composites that rapidly transform into intermetallics and alloys upon heating at low temperatures. The approach is general in terms of accessible compositions, structures, and morphologies. Multiple compounds in the same binary system can be readily accessed; e.g., AuCu, AuCu3, Au3Cu, and the AuCu-II superlattice are all accessible in the Au-Cu system. This concept can be extended to other binary systems, including the intermetallics FePt3, CoPt, CuPt, and Cu3Pt and the alloys Ag-Pt, Au-Pd, and Ni-Pt. The ternary intermetallic Ag2Pd3S can also be rapidly synthesized at low temperatures from a nanocomposite precursor comprised of Ag2S and Pd nanoparticles. Using this low-temperature solution-based approach, a variety of morphologically diverse nanomaterials are accessible: surface-confined thin films (planar and nonplanar supports), free-standing monoliths, nanomesh materials, inverse opals, and dense gram-scale nanocrystalline powders of intermetallic AuCu. Importantly, the multimetallic materials synthesized using this approach are functional, yielding a room-temperature Fe-Pt ferromagnet, a superconducting sample of Ag2Pd3S (Tc = 1.10 K), and a AuPd4 alloy that selectively catalyzes the formation of H2O2 from H2 and O2. Such flexibility in the synthesis and processing of functional intermetallic and alloy materials is unprecedented.     

Thermodynamics of hydrogen solution and hydride formation in Pd-Mn alloys. 2. Ordered alloys

There are marked differences in H(2) solubilities between ordered and disordered Pd-Mn alloys with the largest difference found between the L1(2) and the disordered form of the Pd(3)Mn alloy. The thermodynamics of H(2) solution have been determined for the L1(2) form, the long-period superstructure (lps), and the disordered forms of the Pd(0.80)Mn(0.20) and Pd(0.75)Mn(0.25)(Pd(3)Mn) alloys. Relative partial molar enthalpies and entropies were determined mainly by reaction calorimetry over the range of H contents accessible from p(H)()2 approximately 10 Pa to approximately 0.3 MPa (303 K). The enthalpies for absorption of H(2) are more exothermic over most of the range of H contents for the L1(2) forms of the Pd(3)Mn and Pd(0.80)Mn(0.20) alloys than for their other forms. The reaction enthalpies are constant across a relatively wide range of H contents for the L1(2) form of the Pd(0.80)Mn(0.20) and Pd(3)Mn alloys indicating that there are two-phase coexistence regions (303 K). The H-H attractive interaction, which leads to hydride formation, is much greater for the L1(2) than for the other forms of the Pd(3)Mn alloy and for Pd itself. It has been found that the H-H interaction always decreases in magnitude and, accompanying this, the THS (terminal hydrogen solubility) always increases by alloying Pd.(1) The L1(2) ordered Pd(3)Mn alloy is an exception to this, and therefore, the generalization about THS must be restricted to disordered face centered cubic (fcc) Pd alloys.     

Hydrogen-storage properties of solid-solution alloys of immiscible neighboring elements with Pd

Rh and Ag are the elements neighboring Pd, which is well known as a hydrogen-storage metal. Although Rh and Ag do not possess hydrogen-storage properties, can Ag-Rh alloys actually store hydrogen? Ag-Rh solid-solution alloys have not been explored in the past because they do not mix with each other at the atomic level, even in the liquid phase. We have used the chemical reduction method to obtain such Ag-Rh alloys, and XRD and STEM-EDX give clear evidence that the alloys mixed at the atomic level. From the measurements of hydrogen pressure-composition isotherms and solid-state (2)H NMR, we have revealed that Ag-Rh solid-solution alloys absorb hydrogen, and the total amount of hydrogen absorbed reached a maximum at the ratio of Ag:Rh = 50:50, where the electronic structure is expected to be similar to that of Pd.     

醇氧化为醛的银合金催化剂的量子化学研究

本文用EHMO方法计算了氧原子在晶体银及其合金Ag-X(X=Cu,Mg,Cd,Ga,Ge,In,Sn,Se,Te,Al,Bi,Sb)表面上的吸附态;试图通过计算找出这些金属的电子性质与其对甲醇氧化制甲醛反应中催化活性的关系.计算结果表明:银及其合金的的催化活性与体系的电子最高占有轨道HOMO和最低空轨道LUMO的能量差有关,如果HOMO和LUMO的能量差比纯银体系的能量差小,该合金对反应有较好的催化活性,同时吸附在该合金表面上的氧原子有较大的电荷密度分布.     

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.     

碳纳米管负载的纳米Pd-Ag催化剂对乙醇和丙醇氧化的电化学活性

在乙二醇和水混合溶剂中,采用硼氢化钠还原的方法制备了多壁碳纳米管（MWCNT）负载的Pd和Pd-Ag纳米颗粒催化剂;在碱性介质中,用循环伏安法测试了这些催化剂对乙醇、正丙醇和异丙醇的电氧化性能。结果表明,Pd和Pd-Ag纳米颗粒均匀地分散在MWCNT表面;Pd/MWCNT、Pd₄Ag₁/MWCNT、Pd₂Ag₁/MWCNT和Pd₁Ag₁/MWCNT催化剂上金属颗粒的平均粒径分别为7、4、7和11 nm。相比乙醇和异丙醇,所制备的催化剂对正丙醇的氧化表现出较大的电流密度。与Pd/MWCNT催化剂相比,双金属Pd_nAg₁/MWCNT（n=4、2、1）催化剂,尤其是Pd₄Ag₁/MWCNT上的电流密度更大,表明Ag的加入提高了Pd催化剂对醇氧化的电化学活性,其原因是因为醇氧化过程所产生的中间体物种在双金属Pd-Ag/MWCNT催化剂上的吸附力有所减弱。     

Single Atom Alloy Preparation and Applications in Heterogeneous Catalysis

There have been remarkable progresses in manipulating heterogeneous catalysts' nanostructures in the past decade. The concept of single atom alloy (SAA) was firstly proposed in 2012 when researchers successfully stabilized single Pd atoms on the Cu(111) surface. However, earlier work in 2009, which focused on replacing one Au atom with a Pd atom in thiolate protected Au₂₅ nanoclusters, could also be considered as the pioneer work of single atom alloy. Both kinds of single atom alloys exhibited the potential of maximum utilization of scarce elements and attractive catalytic performances. The well‐defined structures of SAA catalysts make accurate modeling possible, which further realizes the rational design of single atom alloy catalysts. In this review, we summarize the research trajectory of single atom alloys as well as recent achievements in this field. We also introduce several commonly adopted characterization methods for SAA catalysts such as scanning tunneling microscopy (STM), temperature programmed reaction (TPR), extended X‐ray absorption fine structure (EXAFS) spectra, matrix assisted laser desorption/ionization mass spectrum (MALDI‐MS) and differential pulse voltammetry (DPV). Through discussing recent progresses in SAA catalysts, we propose that future researches in this filed should be focused on exploring new kinds of metal nanocrystals and controlling the nanostructure of SAA even more precisely.     

Investigations on the adsorption and electrocatalytic oxidation of carbon monoxide on gold + palladium alloys

A bifunctional mechanism of electrocatalysis was found for the CO-oxidation at Au + Pd alloys. The synergetic effect works best at the 20% Au alloy. The different behaviour of An + Pd and Ag + Pd alloys was assigned to the formation of CO-induced ensembles of Pd-atoms at Au + Pd alloys with high gold content, which is possible due to a high mobility of Pd-atoms at the surface. As CO could be shown to react preferably with the Au-oxide species, the course of the energy of activation for the gas phase reaction might be understood on the-basis of geometric effects.     

Preparation and thermal treatment of Pd/Ag composite membrane on a porous α-alumina tube by sequential electroless plating technique for H2 separation

Pd/Ag/α-Al2O3 composite membranes were prepared by sequential electroless plating technique. The prepared membranes were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spec-troscopy, and inductively coupled plasma atomic emission spectroscopy techniques (ICP-AES). Effects of annealing time, Ag content, and air treatment on the hydrogen permeation flux and morphology of the alloys were investigated. The results of the investigation showed that the prepared type of tube had a good potential as substrate for membrane preparation. In addition, a uniform defect-free alloy was prepared by annealing at 550 ℃ in H2 atmosphere. The permeation results showed an increase in H2 permeation flux by increasing the Ag content and the annealing time. In addition, the air treatment of the prepared membranes at 400 ℃ for 1 h changed the morphology of the alloy and substantially enhanced the hydrogen flux.