Pd-Al Alloys Research Articles

Pd-Al Alloys Formation by Aluminium Underpotential Deposition on Palladium from Equimolar AlCl3+NaCl Melt

Pd-Al Alloys Formation by Aluminium Underpotential Deposition on Palladium from Equimolar AlCl3+NaCl Melt

The free-standing nanoporous palladium for hydrogen isotope storage

Herein, we present hydrogen isotope storage properties of the free-standing monolithic nanoporous palladium (NP–Pd) with different microstructure feature sizes. The NP-Pd samples fabricated by dealloying from Pd–Al alloy and the samples possess high surface area with all of the open pores and ligaments both at nanoscale, which provide the large quantities of reaction sites. The NP-Pd exhibits efficient hydrogen (H2) separation property from deuterium (D2) with distinguishable absorbing and desorbing plateau pressures. The samples own high and reversible H2 and D2 storage capacities with the absorption values up to 0.61 and 0.595 at room temperature. The hydrogen isotope storage capacities increase with the ligament sizes of the NP-Pd samples, which have been high temperature annealed with the larger ligament size more than 10 nm. While for the fresh dealloyed NP-Pd samples with rough surface and ligament size much smaller than 10 nm, the capacity does not follow this behavior. Therefore, a theoretical model based on the existence of the different storage sites at subsurface and interior region is established, which can be used to predict the hydrogen absorption capacity of nanoporous Pd with different structure size. This work reveals both fundamental insights and practical guidance for nanoporous materials design and fabrication, which can be applied in hydrogen isotope separation and storage applications in green energy fields.

N-Hexane Hydrogenolysis Behavior of Alumina-Supported Palladium\u2013Platinum Alloys

The behavior of alumina-supported palladium-platinum catalysts in the reaction of n-hexane hydrogenolysis was analyzed. The monometallic Pt/Al2O3 catalyst showed much higher activity than the rest of catalysts. The relationship between the catalytic activity of Pd–Pt/Al2O3 catalysts and bulk Pd–Pt composition matched the accepted relation between the surface composition and bulk composition of Pd–Pt, confirming a high surface enrichment in palladium. A linear relation between the apparent activation energy and preexponential factor (compensation effect) suggests that the reaction mechanism is similar for all Pd–Pt catalysts. However, after very high temperature of catalyst reduction (at 600 °C), the behavior of monometallic Pd/Al2O3 catalyst exhibited a considerable departure from the compensation plot, and drastic variations in C1–C5 products distribution. Such big changes are rationalized by assuming the transformation of Pd/Al2O3 into Pd–Al alloy, occurring at very high temperature of reduction. Product distribution of n-hexane hydrogenolysis for Pt/Al2O3 differs from that of Pd/Al2O3, showing more internal bond splitting than a terminal demethylation. However, bimetallic Pd–Pt/Al2O3 catalysts showed even more ‘internal bond splitting’ character than the Pt/Al2O3. Reasons for this synergism and changes in the behavior of Pd/Al2O3 catalysts are analyzed in terms of an electronic interaction between metals and alumina support.Graphic

Alkane isomerization on highly reduced Pd/Al2O3 catalysts. The crucial role of Pd-Al species

Highly reduced (at 500 and 600 °C) 1 wt% Pd/Al2O3 samples showed remarkable isomerization selectivity in n-hexane conversion, >80%, in agreement with previous reports. The catalytic behavior of highly reduced Pd/Al2O3 becomes typical of platinum, the metal regarded as unique in alkane isomerization. Although not detected in low metal loaded catalysts, Pd-Al species evidenced by formation of Pd-Al alloys in 5 wt% metal loaded catalysts appear essential active sites responsible for n-hexane isomerization. Other possible reasons (contribution from alumina acidity and changes in the structure of metal surface) were analyzed and discarded. A mechanistic rationale for the improvement of n-hexane isomerization is outlined.

Hydrogen solubilities and permeabilities in fcc Pd–Al and [formula omitted] alloys (393–523 K)

Hydrogen isotherms have been measured for the Pd0.98Al0.02, Pd0.96Al0.04, and Pd0.0.94Al0.06 alloys in the temperature range from 393 to 523 K. From these equilibrium data, the standard partial thermodynamic parameters at infinite dilution of H, ΔHH∘ and ΔSH∘, have been determined; the former becomes slightly more positive and the latter more negative, with increase of atom fraction Al, XAl.H diffusion parameters have been determined from the permeability of H through Pd–Al alloy membranes (393–523 K). DH* decreases and ED* increases with increase of XAl while the pre-exponential factors, DH∘,*, do not change significantly where the asterisk refers to conditions of infinite dilution of H. The dependence of DH and ED on H content have been determined for several of the Pd–Al alloys and the former decreases and the latter increases with H content. Similar data have been determined for the ternary alloys, (Pd0.77Ag0.23)0.98Al0.02 and (Pd0.77Ag0.23)0.96Al0.04 alloys.

Surface Chemistry of Trimethylaluminum on Pd(111) and Pt(111)

The behavior of trimethylaluminum (TMA) was investigated on the surfaces of Pt(111) and Pd(111) single crystals. TMA was found to dissociatively adsorb on both surfaces between 300–473 K. Surfaces species observed by high-resolution electron energy loss spectroscopy (HREELS) and X-ray photoelectron spectroscopy (XPS) after TMA adsorption at 300 K included Al-CH3 and CHx,ads (x = 1, 2, or 3) on Pt(111), and ethylidyne (CCH3), CHx,ads (x = 1, 2, or 3), and metallic Al on Pd(111). Density functional theory (DFT) calculations predicted methylaluminum (MA, Al-CH3) to be the most kinetically favorable TMA decomposition product on (111) terraces of both surfaces, however, HREELS signatures for Al-CH3 were detected only on Pt(111), whereas ethylidyne was observed on Pd(111). XPS demonstrated higher amounts of carbonaceous species on Pt(111) than on Pd(111). DFT calculations showed that further dissociation of MA to metallic aluminum and methyl groups to be more kinetically favorable on step sites of both metals. In our proposed reaction mechanism, MA migrates to and dissociates at Pd(111) steps at 300 K forming adsorbed methyl groups and metallic Al. Some methyl groups dehydrogenate and recombine forming ethylidyne. Metallic Al or ejected Pd atoms from steps diffuse across Pd(111) terraces until coalescing into irregularly shaped islands on terraces or steps, as observed by scanning tunneling microscopy (STM). Upon heating above 300 K, the Pd–Al alloy diffuses into the Pd bulk. On Pt(111), a high coverage of carbon-containing species following TMA adsorption at 300 K prevented MA diffusion and dissociation at steps, as evidenced by isolated clusters of MA in STM images. Heating above 300 K resulted in MA dissociation, but no Pt–Al alloy formation was observed. We conclude that the differing abilities of Pd and Pt to hydrogenate carbonaceous species plays a key role in MA dissociation and alloy formation, and therefore, the adsorption and dissociation chemistry of TMA depends on properties of the metal substrate surface and determines thin film morphology and composition.

Bulk hierarchical nanoporous palladium prepared by dealloying PdAl alloys and its electrochemical properties

Bulk hierarchical nanoporous palladium (BHNPP) materials composing both bimodal porous structure and monolithic porous structure were fabricated by chemical dealloying of Pd20Al80 alloy with sub-micrometer grain size via mechanical alloying and spark plasma sintering process (MA-SPS). The different nanoporous structures were formed in the regions where the microstructure was heterogeneous. Monolithic porous structure were formed after dealloyed of monolithic grain structure mainly composed of PdAl3 phase, and bimodal porous structure with both larger pore of hundreds nanometers and smaller pore of a few nanometers were obtained after dealloyed of nanocrystalline structure consist of PdAl3 and Al phases, respectively. Effects of dealloying time, concentration of dealloying solutions and chloride ion (Cl−) on the evolution of the microstructure of BHNPP were also studied. Ligament of BHNPP can be facility coarsen by add chloride rather than only raise the concentration of HCl solution. Moreover, the cyclic voltammetry results reveal that the BHNPP materials have superior ethanol oxidation ability.

Reaction of Trimethylaluminum with Water on Pt(111) and Pd(111) from 10–5 to 10–1 Millibar

The reaction between adsorbed trimethylaluminum (TMA) and water was studied on Pt(111) and Pd(111) surfaces. Upon exposure to TMA at approximately 10–5 mbar, C- and Al-containing species appeared on both surfaces, as observed by X-ray photoelectron spectroscopy (XPS). On both surfaces, the adsorbed Al oxidation state observed by XPS was closest to metallic. Density functional theory (DFT) calculations suggest that decomposition to methyl aluminum (Al-CH3; “MMA”) or atomic Al is thermodynamically favorable. The formation of a Pd–Al alloy was observed on Pd(111), but Pt–Al alloy formation was not observed on Pt(111). Following TMA adsorption, each surface was exposed to water vapor at 400 °C either at a pressure of 7 × 10–6 mbar (UHV-XPS) or at 0.1 mbar (in situ XPS). The substrate and water dosing conditions determined the ability of each surface to remove residual carbon: on Pt(111), carbon from the TMA precursor was removed from Pt(111) during 0.1 mbar water exposure at 400 °C, whereas carbon was not rem...

Pd–Al interaction at elevated temperatures: a TEM and SAED study

Epitaxially grown Pd particles partly embedded in amorphous Al2O3 were subjected to annealing and reductive treatments in the temperature range 523–873 K to induce a possible Pd–Al interaction. The structural, morphological and compositional changes were monitored by transmission electron microscopy and selected area electron diffraction. Formation of Pd4Al3 and PdAl alloys has been observed upon annealing in 1 bar He for 1 h at T > 523 K and upon reduction in 1 bar H2 for 1 h at T ≥ 523 K, respectively. Both alloys appear to be stable up to 873 K, although Pd4Al3 shows beginning decomposition at and above 873 K. The stability under oxidative conditions was found to be very similar, a transformation back into metallic Pd sets in for both compounds at around 573–623 K. In agreement with previous studies on Pd/SiO2, the formation of an amorphous hydride phase and/or a heavily distorted Pd lattice has been detected after reduction in hydrogen at 523 K.

H diffusion through partially internally oxidized Pd-alloy membranes

The H flux (J) has been measured through multilayer membranes of Pd and Pd–Al alloy prepared by internal oxidation of the alloy. A plot of 1/J against thickness of the Pd layers, 2dPd, is linear for a series of different 2dPd values. This justifies assumptions of the model used to derive the relation between 1/J and 2dPd. DH for Pd and the alloy can be obtained from intercepts of the plot using known pH2–H content isotherms. Since it has been shown that internal oxidation leads to more reproducible and poison-resistant Pd based membranes, the multilayer technique is an alternate way to obtain DH for Pd alloys.

RHEED study of the growth of Pd–Al/MgO bimetallic system

A Pd–Al bimetallic system was prepared on a MgO (0 0 1) single-crystalline substrate surface by vacuum deposition. The early stages of the growth in dependence of Pd and/or Al deposition sequence were studied. The reflection high-energy electron diffraction method was used for structural investigation during the deposition. The results can be interpreted in terms of formation of stable Pd–Al alloy covered by an aluminum overlayer. Under all deposition conditions the diffraction patterns indicated that the Pd–Al system is composed of three-dimensional epitaxial particles having the following orientation relationship with respect to the substrate surface: Pd–Al (0 0 1) || MgO (0 0 1), Pd–Al [1 1 0] || MgO [1 1 0]. When Pd was deposited at first the evolution of lattice parameter and diffraction spot intesity during the growth showed the formation of a Pd pseudomorphic layer which was spread by a strong Pd–Al bimetallic interaction during subsequent Al deposition. Applying the opposite sequence of deposited materials or simultaneous deposition of both metals three-dimensional growth of clusters was observed from the beginning of the deposition process.

Hydrogen permeation measurements of partially internally oxidized Pd–Al alloys in the presence and absence of CO

Specific permeabilities and diffusion constants for H have been measured for a series of Pd–Al alloy membranes using a novel method for activating the membranes, i.e., partial internal oxidization. If the percent internal oxidation is small relative to the membrane thickness, the diffusion will take place essentially through the unoxidized alloys. The H diffusion constants decrease almost linearly with atom fraction Al and the activation energies increase from 23.2 kJ/mol H ( X Al = 0) to 25.4 kJ/mol H ( X Al = 0.10). It is shown that the partially oxidized alloys are more resistant to CO poisoning than are the corresponding unoxidized alloys and pure Pd. Thus, the partially internally oxidized alloy membranes have two advantages as H 2 purification membranes: (i) they are more reproducible than Pd or the unoxidized Pd–Al membranes; and (ii) they are more resistant to CO(g), which is a common catalytic poison for Pd.

Oxidation of Pd–Al(111) Epitaxial Films in Ultra-High Vacuum

Epitaxial Pd-Al alloy films were grown on α-Al 2 O 3 (0001) substrates by sputter-deposition. The composition of the films was approximately Pd 0 . 8 Al 0 . 2 . The films were completely oriented with (111) plane parallel to the surface without any trace of other intermetallic compounds. The oxidation behavior of the films in ultra-high vacuum at elevated temperature was studied. It was revealed that only aluminum was oxidized and the growth of alumina film stopped quickly at 0.5 nm in thickness. There were some indications that the alumina film was crystalline.

XPS, TDS and static SIMS studies of binary Pd/Al system properties: correlation between Pd–Al bimetallic interaction and CO adsorption

Carbon monoxide (CO) adsorption properties of Pd/Al systems have been studied using X-ray photoelectron spectroscopy (XPS), thermal desorption spectroscopy (TDS) and static secondary ion mass spectroscopy (SSIMS) under UHV conditions. Measured XPS and TDS spectra were compared to that of clean bulk metals (Pd, Al). Pd adlayers, prepared by vacuum evaporation onto a pure polycrystalline Al surface, exhibited a strong interaction with Al atoms resulting in formation of a Pd–Al alloy of a noble metal-like electronic structure. The shifts of the Pd 3d core levels and the Pd 4d-band centroid toward higher binding energies due to bimetallic bond formation were observed. The CO TDS spectra consisted of two desorption peaks, both lying lower than the one typical for a Pd foil, indicating distinct weakening of the Pd–CO chemisorption bond. Partial CO dissociation on the Pd/Al surface during adsorption/desorption cycles, accompanied by CO 2 and H 2O production, was observed. CO dissociation caused creation of Pd–C and Al–O chemical bonds on the Pd/Al surface resulted in weakening of the Pd–Al bimetallic bond.

Characterization and catalytic activity of differently pretreated Pd/Al 2O 3 catalysts: the role of acid sites and of palladium–alumina interactions

The effect of catalyst pretreatment on the performance of chlorine-free γ-alumina-supported palladium catalysts with metal loadings in the range of 0.3 to 2.77 wt% has been investigated in the hydroconversion of n-hexane (nH) and 2,2-dimethylbutane (22DMB) at 290 °C and atmospheric pressure. The catalyst properties were modified by varying the catalyst pretreatment: low-temperature reduction (LTR) at 300 °C; high-temperature reduction (HTR) at 600 °C; and oxidation of highly reduced catalysts followed by LTR (regeneration). The isomerization activity for both nH and 22DMB conversion is enhanced by HTR of the Pd/Al 2O 3 as compared to LTR and regeneration. It is concluded that HTR increases the Lewis acidity of the alumina and the participation of Lewis acid sites improves catalytic performance. The influence of Lewis acid catalyst sites on the macroscopic kinetics overshadows the modest catalytic contribution from palladium. Ammonia effectively blocks the acid centers and limits the overall catalytic performance to metal-only catalysis. The role of support acid sites in the conversion of C 6-hydrocarbons was further demonstrated by isomerization activity associated with 22DMB hydroconversion catalyzed by an alumina-supported Ni catalyst. Based on the results of FTIR studies of adsorbed CO and results of temperature-programmed palladium hydride phase decomposition (TPHD), it was concluded that a part of the alumina-supported Pd transforms into a Pd–Al alloy during the HTR pretreatment. The catalytic performance of the regenerated catalysts in the C 6-alkane conversion is consistent with the suggestion that the Pd–Al alloy formed during HTR decomposes during the regeneration treatment to form Al 2O 3 moieties on the Pd surface. These moieties, most likely, interact strongly with nearby palladium species, creating sites which are active for alkane isomerization and, simultaneously, resistant to coking.

Hydrogen diffusivity and solubility in a Pd–Al alloy under conditions of hydride formation

Samples of Pd0.97Al0.03 and (Pd0.9Pt0.1)0.97Al0.03 alloys, cold-worked and internally oxidized at 1073K for 24 and 72h, were submitted to hydrogen permeation testing at 313K, using different levels of cathodic charging current for hydrogen generation, in the range of 0.1–20mA. It was observed that in the cold-worked Pd0.97Al0.03 alloy, no stable layer of hydride is formed even for high cathodic charging currents. For the Pd0.97Al0.03 alloy which contains nanoprecipitates of Al2O3 the hydride phase formation occurs upon applying a cathodic charging of 5mA. For the (Pd0.9Pt0.1)0.97Al0.03 alloy no hydride is observed for the samples submitted to internal oxidation heat treatment. The hydrogen diffusivity increases with increased cathodic charging, this variation being more expressive for the cold-worked Pd0.97Al0.03 alloy.

Role of Pd–Al bimetallic interaction in CO adsorption and catalytic properties of bulk PdAl alloy: XPS, ISS, TDS, and SIMS study

CO adsorption on the bulk PdAl bimetallic systems has been studied using XPS, ISS, TDS, and SIMS under UHV conditions. Pd exhibits strong interaction with Al atoms resulting in the formation of noble metal-like electronic structure of PdAl alloy. The shifts of both Pd core levels and of Pd valence d-band centroid towards higher binding energies compared to bulk Pd were observed. The surface structure of the alloy changes with temperature (Al surface segregation), ion bombardment (preferential Al sputtering), and is also strongly affected by the presence of ambient CO. The Pd–Al bond is weakened upon the interaction with CO, which tends to dissociate on the surface even at room temperature, with carbon fraction bonding to Pd atoms and oxygen to Al. The TDS of CO spectra consisted of one to three desorption peaks, all lying lower than those from Pd foil, indicating a distinct weakening of the Pd–CO chemisorption bond. Moreover, the partial CO dissociation on PdAl was indicated both as adsorption capacity decay and CO 2 and H 2O production.

The interaction of dissolved H with internally oxidized Pd–Rh alloys

Homogeneous fcc Pd–Rh alloys have been internally oxidized in the atmosphere at several temperatures from 1023 to 1123 K forming oxide precipitates within a Pd matrix. As shown from electron diffraction patterns of the internally oxidized Pd 0.97Rh 0.03 alloy, the oxide that forms is a mixed oxide, PdRhO 2. Internally oxidized Pd–Rh alloys can be reduced with H 2 (573–623 K) forming PdRh precipitates within the Pd matrix. This is a technique for segregating components of a substitutional solid solution binary alloy. The segregated alloy can be returned to the homogeneous Pd–Rh alloy by annealing at an elevated temperature and thus, in contrast to the internal oxidation of, e.g., Pd–Al alloys, the process can be readily reversed. The oxidation and (reduction+H 2O loss) were monitored from weight changes. After internal oxidation/reduction “diagnostic” hydrogen isotherms (323 K) were measured to follow the extent of oxidation and to determine the extent of H trapped at the internal interfaces from the positive intercepts along the H/Pd axis in the dilute phase. The H capacities in the steeply rising hydride region of the H 2 isotherms for the internally oxidized alloys are smaller than for Pd unless the Pd in the PdRh precipitates is considered to be inactive for H 2 absorption for the calculation of H/Pd values. In the two-phase region the absorption plateau pressures were significantly greater than for Pd–H, however, they were much closer to the Pd plateau p H2 than to that of the unoxidized Pd–Rh alloys. The desorption plateaux were very close to those of Pd–H for X Rh=0.01 to 0.05 alloys and therefore it can be concluded that the matrix is pure Pd whose absorption plateaux are affected by the presence of the precipitates. The techniques of TEM, SEM and SANS (small angle neutron scattering) were employed to examine the alloys after internal oxidation both before and after reduction. SANS confirmed directly that internal precipitates form from the internal oxidation and that they are rather large with an appreciable fraction having diameters greater than 100 nm.

On hydrogen solubilities in Pd/alumina composites prepared by internal oxidization of Pd/Al alloys

Internal oxidation of a Pd–Al alloy results in a composite consisting of Al 2O 3 precipitates within a Pd matrix. In accord with earlier studies (Huang et al., 1988. Scripta Met. 22, 1114), H dissolved in the Pd matrix becomes strongly and weakly trapped by the alumina precipitates. It is shown that the extent of strong H trapping is only weakly dependent on the Al content from X Al=0.005 to 0.06. H can be removed from the strong traps by evacuation at 573 K (2 h) but not at 473 K. After evacuation at 973 K, the strong traps themselves disappear but when the alloys are hydrided/dehydrided (cycled) at 323 K, traps reappear along with weak-dislocation traps due to dislocation formation resulting from the cycling. Although the former traps, which appear after cycling, are deep enough so that the corresponding P H 2 ( equil) cannot be readily measured (273 K), they are not as deep as the initial ones produced after internal oxidation since H can be removed from them by evacuation at 323 K. The number of weak-dislocation traps due to cycling the composite formed from internal oxidation (1073 K) is greater than for cycled pure Pd. A higher temperature is needed to anneal out the weak-dislocation traps arising from cycling the composites as compared to Pd; thus the alumina precipitates hinder the movement and annihilation of the dislocations. The enthalpies for H 2 solution in the dilute phase of the internal oxidation Pd–Al alloys have been determined directly by reaction calorimetry and they have a spectrum of trapping enthalpies.

Pd Interaction with Reduced Thin-Film Alumina: XPS and ISS Study

X-ray photoelectron spectroscopy and ion-scattering spectroscopy (ISS) were used to investigate the interaction of palladium with reduced thin-film alumina. The substrate was prepared by Ar+ ion bombardment thinning of the natural oxide layer on polycrystalline aluminum foil. It was found that the constitution of a surface equilibrium is determined by the energy of sputtering ions. Therefore, after the sputtering, the aluminum oxide surface is covered by more or less Al0-rich overlayer. During the first stage of the Pd deposition, the IAl/IO ratio decreases because of Pd atom deposition onto the Al sites, keeping the anionic oxygen sites unaffected. With increasing Pd, the PdAl intermetallic compound is formed. The alloy forms three-dimensional clusters and a bare alumina surface appears, giving the ISS intensity ratio IAl/IO characteristic for γ-Al2O3. At a high amount of deposited Pd, the Pd bulk-like phase is formed, gradually burying the PdAl alloy clusters. To interpret catalytic behavior of Pd/Al2O3 systems, the presence of the PdAl alloy caused by strong metal–substrate interaction should be considered.