Heats of interaction of hydrogen with gold and platinum powders and its effect on the subsequent adsorptions of oxygen and noble gases

Exposure of pure gold powders to hydrogen flow at 125 °C and atmospheric pressure causes heat evolution accompanied by hydrogen adsorption. The exposure takes place in a flow-through microcalorimeter, in which the metal powders are purged by nitrogen flow used as an inert carrier gas. The adsorbed hydrogen is slowly desorbed by nitrogen flow. The heats of hydrogen adsorption and its uptake by the gold powder are greatly increased by its sequential treatments with micromole quantities of oxygen and noble gases, such as helium and argon. This increase does not take place if the gold treatment is confined only to oxygen, or only to pure noble gases. The radically increased hydrogen adsorption by gold is caused by a combination of its treatments with oxygen and the noble gases. Similar results were obtained with pure platinum powder exposed to hydrogen at room temperatures. Gold powder containing adsorbed hydrogen reacts very strongly with molecular oxygen/argon mixtures, generating heats of adsorption several times higher than the heat of formation of water. The heat evolution is very rapid and is not accompanied by the formation of water. These intense interactions are not observed after complete desorption of hydrogen from the gold surfaces.     

Hydrogen and noble gas interactions with iron nano-flakes

Exposure of pure iron nano-flakes to hydrogen generates a high heat evolution associated with hydrogen uptakes shown by flow-through microcalorimetry. A large part of the hydrogen was found to be irreversibly absorbed by the iron flakes at 220 °C and atmospheric pressure, but an increased desorption of hydrogen was achieved by noble gases, such as helium and argon. Thus the iron surfaces displayed strong affinity for hydrogen, but also, surprisingly, for the noble gases, which were found to be able to displace hydrogen from the iron surfaces.The uptake of hydrogen by the iron flakes was observed to reach 9 wt.% after exposure for 5 h, which may be of interest in hydrogen storage applications. Desorption with the help of argon may provide an acceptable method of hydrogen recovery.     

Effect of oxygen on the production of abnormally high heats of interaction with hydrogen chemisorbed on gold

Abnormally high heats, exceeding 1600 kJ/mol (16 eV) per molecular oxygen, are generated by interaction of the oxygen with the hydrogen adsorbed on gold surfaces at 125 °C. The highest heats were observed during the interactions of fine gold particles supported on titanium oxide, approaching 1700 kJ/mol for three consecutive 100 nmol pulses of O₂ interacting with the adsorbed hydrogen atoms. The heats rapidly decrease after the hydrogen is consumed. It was also observed that the interactions of the gold particles with pure oxygen in the presence of noble gases, such as argon and helium, produced the heats markedly higher than those observed in the absence of noble gases. The abnormally high heats revealed by this work reach values from 3.5 to 6.1 times higher than the heats of formation of gaseous water from molecular hydrogen and oxygen.     

Anomalous effects in charging of Pd powders with high density hydrogen isotopes

A twin system for hydrogen absorption experiments has been constructed to replicate the phenomenon of heat and ⁴He generation by D₂ gas absorption in nano-sized Pd powders reported by Arata and Zhang, and to investigate the underlying physics. For Pd⋅Zr oxide nano-powders, anomalously large energies of hydrogen isotope absorption, 2.4±0.2 eV/D-atom and 1.8±0.4 eV/H-atom, as well as large loading ratio of D/Pd=1.1±0.0 and H/Pd=1.1±0.3, respectively, were observed in the phase of deuteride/hydride formation. The sample charged with D₂ also showed significantly positive output energy in the second phase after the deuteride formation.     

Specific heat,electrical resistance,and other properties of superconducting Pd-H alloys

Specific heat and electrical resistance measurements have been carried out in the ～1.2 to 4.2 K range for a series of Pd-H alloys having H/Pd atomic ratios in the ～0.82 to 0.88 range. Superconductive transitions in the resistance were observed; the specific heat data exhibited broad, yet very pronounced peaks with characteristics that depended strongly upon hydrogen concentration. A fairly successful theoretical fitting of the specific heat data was carried out on the basis of the assumption that Pd-H alloys are simply inhomogeneous BCS superconductors. Data for the dependence of the superconducting transition temperature upon hydrogen concentration were derived from both the specific heat and electrical resistance measurements. An interesting scaling feature of the specific heat data is discussed. A search for a possible interrelationship between the superconductive properties and an exothermic process which occurs in Pd-H alloys yielded negative results. A marked suppression of superconductive effects was observed in Pd-H alloys based on one particular Pd ingot; this was probably due to Fe impurities.     

The plasma oxidation of Pd(1 0 0)

The oxidation of Pd(1 0 0) by an oxygen plasma was characterized using X-ray photoelectron spectroscopy (XPS), low energy ion scattering spectroscopy (ISS), temperature programmed desorption (TPD), and low energy electron diffraction (LEED). The oxygen uptake followed a typical parabolic profile with oxygen coverages reaching 32 ML after 1 h in the plasma; a factor of 40 higher than could be achieved by dosing molecular oxidants in ultra high vacuum. Even after adsorbing 32 ML of oxygen, XPS revealed both metallic Pd and PdO in the surface region. The R27^o LEED pattern previously attributed to a surface oxide monolayer, slowly attenuated with oxygen coverage indicating that the PdO formed poorly ordered three dimensional clusters that slowly covered the ordered surface oxide. While XPS revealed the formation of bulk PdO, only small changes in the ISS spectra were observed once the surface oxide layer was completed. The leading edges of the O₂ TPD curves showed only small shifts with increasing oxygen coverage that could be explained in terms of the lower thermodynamic stability of small oxide clusters. The desorption curves, however, could not be adequately described as simple zero order decomposition of PdO. There has been an ongoing debate in the literature about the relative catalytic activities of PdO and oxygen phases on Pd, the results indicate that any differences in the reactivity between bulk PdO and surface oxides are not associated with differences in the density of exposed Pd atoms or the decomposition kinetics of these two phases.     

Ultra-thin films and surface alloying of Pd on Cu(1 1 1) investigated by medium energy ion scattering

The structure of Pd films on Cu(1 1 1) and the alloying between the films and the substrate have been investigated by medium energy ion scattering (MEIS) using 100 keV H⁺ ions. Data are presented for the and alignments (nominal one- and three-layer alignments, respectively). It is found that beyond 1 ML the Pd grows in a twinned fcc structure, the incommensurate nature of which increases the visibility of the Cu(1 1 1) substrate to MEIS. Deposition of 0.2 ML of Pd produces a structure in which Pd mostly occupies the top two layers which have interlayer distances d₁₂ = 208 ± 4 pm and d₂₃ = 211 ± 4 pm. Some twinning is also present in this structure. Upon annealing 1.6 ML of Pd to 600 K for 1 min, the copper and palladium interdiffuse leaving around 0.4 ML of visible palladium. Energy plots show that there are several layers with an altered structure present over at least part of the surface. This may be due to large scale interdiffusion or alloy island formation. Incremental annealing to successively higher temperatures shows that the structural transformation begins around 500 K.     

Sum frequency generation and density functional studies of CO-H interaction and hydrogen bulk dissolution on Pd(1 1 1)

CO-H interaction and H bulk dissolution on Pd(1 1 1) were studied by sum frequency generation (SFG) vibrational spectroscopy and density functional theory (DFT). The theoretical findings are particularly important to rationalize the experimentally observed mutual site blocking of CO and H and the effect of H dissolution on coadsorbate structures. Dissociative hydrogen adsorption on CO-precovered Pd(1 1 1) is impeded due to an activation barrier of ∼2.5 eV for a CO coverage of 0.75 ML, an effect which is maintained down to 0.33 ML CO. Preadsorbed hydrogen prevented CO adsorption at 100 K, while hydrogen was replaced from the surface by CO above 125 K. The temperature-dependent site blocking of hydrogen originates from the onset of hydrogen diffusion into the Pd bulk around 125 K, as shown by SFG and theoretical calculations using various approaches. When Pd(1 1 1) was exposed to 1:1 CO/H₂ mixtures at 100 K, on-top CO was absent in the SFG spectra although hydrogen occupies only threefold hollow sites on Pd(1 1 1). DFT attributes the absence of on-top CO to H atoms diffusing between hollow sites via bridge sites, thereby destabilizing neighboring on-top CO molecules. According to the calculations, the stretching frequency of bridge-bonded CO with a neighboring bridge-bonded hydrogen atom is redshifted by 16 cm⁻¹ when compared to bridging CO on the clean surface. Implications of the observed effects on hydrogenation reactions are discussed and compared to the C₂H₄-H coadsorption system.     

Optical Properties of Amorphous Films of an a-Si<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Ge<Subscript>x</Subscript>:H Solid Solution with Different Concentrations of Hydrogen

The IR absorption spectra of hydrogenated amorphous films of an a-Si_0.60Ge_0.40:H solid solution that were obtained by plasmachemical deposition at different partial pressures of hydrogen have been investigated. The oscillator strengths Γ that depend substantially on have been determined. It is shown that hydrogen is contained in the films mainly in the GeH and SiH forms. Using integral absorptions I_w, the concentrations of hydrogen were determined. The highest value of is observed at Γ = 0.51, P = 4.16, N_H = 9.7·10²¹ cm⁻³, and C^H = 23.7 at.%. It has been established that the oscillator strengths depend on the hydrogen concentration: they decrease on hydrogen effusion and increase with the hydrogen concentration. It is shown that the hydrogen concentration in the films of the a-Si_0.60Ge_0.40:H solid solution can be controlled by changing its pressure. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 72, No. 3, pp. 371–376, May–June, 2005.     

A molecular-beam investigation of the reaction H2 + 12O2 → H2O on Pd(111)

Using molecular-beam relaxation techniques and isotopic exchange experiments, the water-formation reaction on Pd(111) has been shown to proceed via a Langmuir-Hinshelwood mechanism. The reaction product H₂O is emitted from the surface with a cosine distribution. The rate-determining step is the formation of OH_ad in the reaction O_ad + H_ad → OH_ad. The activation energy for this step is 7 kcal/mole with a pre-exponential factor, v, of 4 × 10^?8 cm² atom^?1 sec^?1. This value for v lies well below that observed for simple second-order desorption of dissociatively adsorbed diatomic gases, but is roughly of the order of that obtained for the oxidation of CO on Pd(111). The formation of H₂O proceeds differently under conditions of excess O₂ or H₂. In an excess of H₂, the kinetics is dominated by the transport of atomic hydrogen between the bulk and the surface as was found for the H?D exchange reaction on Pd(111). In an excess of O₂, diffusion of hydrogen into the bulk is blocked by adsorbed oxygen and the hydrogen reservoir available for reaction at the surface is decreased by several orders of magnitude. This results in a drastic reduction of the reaction rate which can be reversed by increasing the partial pressure of H₂.     

Investigations of the selective population of hydrogen subsurface sites on Pd(110) using He diffraction and thermal-desorption spectroscopy

Surface-structure models for the 2×1 and 1×2 hydrogen chemisorption phases formed on Pd(110) at 100 K have been derived from He-diffraction data. The respective coverages correspond to 1 and 1.5 monolayers (ML). Upon heating to 200 K, the 1×2 saturation phase transforms back into the 2×1, and 0.5 ML hydrogen moves subsurface. Based on structural arguments, we suggest that only the first available subsurface sites, i.e., the octahedral interstitials between topmost and second layers are populated by thermal activation. The subsurface movement is eased since H-chemisorption sites on top of the second Pd layer can be occupied in the 1×2 owing to substrate reconstruction. Structural considerations also explain that exactly 1 ML H can be accommodated subsurface by thermal cycling. New TDS measurements corroborate these notions: only the ₂ desorption state, probably associated with Hin subsurface sites between first and second Pdlayers, is selectively filled by the thermal-cycling processes. The ₁ state remains empty upon thermal cycling, and is very likely connected with hydrogen deeper in the bulk.     

Theory of inelastic atom-surface scattering: Examples of energy distributions

The theory of the energy distribution of atoms scattered inelastically by solid surfaces which was developed previously is applied to various examples. The dependence of the results on a number of parameters is studied in detail. The importance of many phonon contributions as compared to the validity of first order distorted wave Born approximation is considered in particular. It turns out that low energy He atoms scattered by heavy transition metals provide a good example for which one phonon emission (or absorption) dominates. All other noble gases show appreciable many phonon contributions increasing, of course, with increasing mass of the noble gas and temperature of the solid. For heavy noble gases such as Kr and Xe the energy distribution approaches a gaussian, the width of which is due to the thermal and zero-point motion of the lattice. This width is quite large and thus probably masks most of the fine structure of the energy distribution occuring in classical trajectory calculations. We have also tried to apply the theory to light diatomic molecules. Although the results are less certain, partly because of the neglect of the internal motion of the molecules and partly because of uncertainties in the interaction parameters, one probably can expect appreciable many phonon effects already for H₂ and, of course, more so for N₂ and O₂. Recent experimental results on the Debye-Waller factor of Ne/Cu can be reproduced with reasonable potential parameters.     

The Effect of Temperature on Lattice Mechanical Properties of Noble and Transition Metals

A simple pseudopotential model is used for the calculation of the temperature dependence of lattice mechanical properties which also depend on the phonon density of states such as lattice heat capacity C _V , Debye temperature _D, harmonic contribution to free energy, thermal pressure, isothermal bulk modulus corrected to the fourth order, volume thermal expansion coefficient , Debye-Waller factor, mean-square displacement, Debye-Waller temperature parameter, and X-ray characteristic temperature _M of Cu, Ag, Au, Ni, Pd, Pt, Rh, and Ir. The contribution of d-like electrons is taken into account by introducing repulsive short-range Born-Mayer-like term. Very recently proposed screening function due to Sarkar et al. has been used to obtain the screened form factor. The theoretical results are compared with experimental findings wherever possible. A good agreement between theoretical investigations and experimental findings show the ability of our model potential to reproduce wide class of properties in noble and transition metals.     

Intermolecular Vibrational Relaxation in Mixtures of Excited Benzophenone and Anthraquinone Molecules with Water Vapor

The intermolecular vibrational energy transfer from triplet molecules of benzophenone and anthraquinone to H₂O molecules has been investigated. To determine the rates of establishment of vibrational (V-V) and thermal (V-T) equilibrium in a vibrational quasi-continuum of mixed singlet-triplet levels, the dependences of the decay rates and intensities of the fast and slow components of delayed fluorescence on the H₂O vapor pressure have been investigated. For V-V relaxation, the efficiencies ₁ and the mean energies E transferred per collision in mixtures with H₂O and other polyatomic foreign gases have been compared. It has been established that the efficiencies ₁ for quasi-resonant vibrational energy transfer (V-V) from benzophenone and anthraquinone to H₂O are an order of magnitude lower than the gas-kinetic ones and lower than those obtained under the same experimental conditions for such foreign gases as C₅H₁₂, SF₆, and CCl₄, and decrease with increasing temperature in the 433–513 K range. It has been concluded that the mechanism of V-V relaxation in mixtures with H₂O are determined by long-range attractive forces. In mixtures with H₂O, no acceleration of V-T relaxation due to the formation of hydrogen bonds has been revealed. The low-efficiency thermalization process (V-T relaxation) is controlled by short-range repulsive forces, and the differences between the intensities ₂ for mixtures of benzophenone and anthraquinone with H₂O and other polyatomic gases are determined by the change in the reduced mass of interacting molecules.     

Structure and reaction pathways of methyl lactate on Pd(1 1 1)

The adsorption and reaction of methyl lactate (CH₃CH(OH)COOCH₃) is studied in ultrahigh vacuum on a Pd(1 1 1) surface using temperature-programmed desorption (TPD) and reflection–absorption infrared spectroscopy (RAIRS). Methyl lactate reacts at relatively low temperatures (220 K) by O–H bond scission. This intermediate can either react with hydrogen to reform methyl lactate at 280–300 K or undergo β-hydride elimination to form flat-lying methyl pyruvate. This decomposes to form acetyl and methoxy carbonyl species as found previously following methyl pyruvate adsorption on Pd(1 1 1). These species predominantly react to form carbon monoxide, methane and hydrogen.     

Surface X-ray scattering studies of the growth of Pd thin films on the Pt(0 0 1) electrode surface and the effects of the adsorption of CO

The morphology of electrochemically deposited Pd films on the Pt(0 0 1) electrode surface has been examined through the combination of cyclic voltammetry (CV) and in situ surface X-ray scattering (SXS). Analysis of SXS measurements has indicated that the Pd grows via pseudomorphic island formation, with the partial occupation of successive layers occurring at a first layer occupation of 0.8 ML. Further Pd deposition sees the formation of larger islands built onto the now complete monolayer, characteristic of pseudomorphic Stranski–Krastanov (SK) growth. In the H_UPD potential region the effect of CO on the surface expansion of the multilayer Pd film is negligible. In the hydrogen evolution region, however, the effect of the adsorption of CO has been shown to produce surface normal expansion and in-plane disorder of the Pd film. It is suggested that hydrogen permeation into the Pd film is enhanced on the CO-poisoned surface.     

Munucleon atoms

The paper presents the consideration of problems concerning formation and observation of munucleon atoms produced by the capture of negative muon by atoms of noble gases by means of theSR-method.     

The cleavage of the methane CH bond over PdO/H-BEA: A density functional theory study

Cluster models were used to represent the β-type cationic sites of the protonated beta zeolite (H-BEA) and the loading of PdO on these sites. The properties of these clusters and the cleavage of methane CH bond over these clusters were studied using density functional theory (DFT) method. The stability of H-BEA was enhanced due to the formation of hydrogen bonds. After PdO loading, the Pd atom bonds to four oxygen atoms among which three H-BEA framework oxygen atoms are included to form an approximate planar structure with Pd in the centre. This structure is very similar to that of bulk PdO. The acidic proton of H-BEA and the oxygen atom of PdO participate in the cleavage of methane CH bond, indicating that PdO is the active species for the activation of methane. Over the clusters constructed in the present work, the calculated energy barriers for the cleavage of methane CH bond are in the region between 17.54 and 21.02 kcal mol⁻¹.     

X-ray photoelectron spectroscopy study of Pd oxidation by RF discharge in oxygen

The low-temperature oxidation of polycrystalline palladium by RF oxygen plasma was studied via X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). Detailed information about the electronic states of palladium and oxygen was obtained based on the XPS curve fitting analysis of Pd3d and Pd3p + O1s lines. The results showed that Pd oxidation by oxygen plasma was different from Pd oxidation in pure O₂ at high temperature. SEM shows well-structured submicron PdO particles result from oxidation in pure O₂, whereas plasma oxidation results in the predominant formation of two-dimensional PdO structures covering the initial crystallites of the Pd foil. Further oxidation to a three-dimensional PdO phase occurs under prolonged treatment with oxygen plasma. The formation of a PdO_x (x > 1) species, characterized by a E_b(Pd3d_5/2) = 338.0–338.2 eV value that is close to the Pd⁴⁺ oxidation state, was also observed. This PdO_x species was found to have low thermal stability (T < 400 K). It is proposed that the PdO_x species can be localized within the boundaries of crystallites.     

Modification of Pd-CeO2 catalyst by different treatments: Effect on the structure and CO oxidation activity

To investigate the interaction between noble metal and CeO₂, a Pd-doped CeO₂ catalyst was prepared by sol-gel method, and the catalyst was then treated in static air and a H₂/O₂ alternating flow at 800 °C, respectively. It is found by step-scanning XRD that Pd ions migrate out of the ceria lattice during the redox treatment, while the exudation of Pd is not so obvious after the oxidative treatment. For the CO oxidation activity, the redox treated catalyst is seriously weakened compared with the oxidative-treated one. This difference is ascribed to the encapsulation of Pd crystallites by the CeO₂ support during the redox treatment, which is confirmed by XPS and CO-TPR. Based on the activity and FTIR results, it is proposed that, CO oxidation at low temperatures proceeds mainly via the reaction between the adsorbed CO on Pd sites and the lattice oxygen of surface CeO₂ at the Pd-Ce interface.