Crucial impact of exchange between layers on temperature programmed desorption

Kinetic modelling shows that layer exchange between the 1st and 2nd adsorbate layer on a surface alters the appearance of desorption spectra considerably. Especially, a rapid layer exchange causes a broader desorption peak and a flatter leading edge.

Effects of Ball Milling and TiF3 Addition on the Dehydrogenation Temperature of Ca(BH4)2 Polymorphs

The changes introduced by both ball milling and the addition of small amounts of TiF3 in the kinetics of the hydrogen desorption of three different Ca(BH4)2 polymorphs (α, β and γ) have been systematically investigated. The samples with different polymorphic contents, before and after the addition of TiF3, were characterized by powder X-ray diffraction and vibrational spectroscopy. The hydrogen desorption reaction pathways were monitored by differential scanning calorimetry. The hydrogen desorption of Ca(BH4)2 depends strongly on the amount of coexistent α, β and γ polymorphs as well as additional ball milling and added TiF3 to the sample. The addition of TiF3 increased the hydrogen desorption rate without significant dissociation of the fluoride. The combination of an α-Ca(BH4)2 rich sample with 10 mol% of TiF3 and 8 h of milling led to up to 27 °C decrease of the hydrogen desorption peak temperature.

A Simple Mechanical Method to Modulate the Electrochemical Electrosorption Processes at Metal Surfaces

The coupling of electrochemical processes and surface strain has been widely investigated in the past. The present work briefly introduces a simple method to modulate the electrochemical process at metal surfaces by mechanical bending. In this way, the static strain at the metal layer can reach the order of 1%. The cyclic voltammogram was used to study the electrosorption process of oxygen species at sputtered metal surfaces under different strain states. The experimental results show that the desorption peak potential of oxygen at the Au surface shifted positively by tensile strain, whereas the desorption peak potential at the Pt surface shifted negatively. This phenomenon indicates that tensile strain has an opposite effect on the electrosorption process for Au and Pt surfaces. Our results agree with the previous reports on the potential variation induced by dynamic strain. This work thus offers a simple method to modulate the electrosorption process at metal surfaces and then to enhance the reactivity of metal electrodes.

Hydrogen Trapping at Defects in Pd and Thermally Activated Desorption

Hydrogen interaction with vacancies and dislocations in Pd were investigated in the present work. Well annealed and plastically deformed Pd samples were electrochemically doped with hydrogen up to various hydrogen concentrations. Subsequently the samples were subjected to linear annealing (10 K/min) and hydrogen desorption was studied by differential scanning calorimetry (DSC). An endothermic peak caused by hydrogen desorption was observed in the DSC curve of well annealed sample at ∼ 178 °C. In plastically deformed samples this peak is shifted to higher temperatures since hydrogen is trapped at dislocations and its diffusivity is suppressed. Moreover, it was found that if the atmosphere surrounding the heated sample contains oxygen the endothermic hydrogen desorption peak is followed by a strong exothermal peak caused by fusion of desorbed hydrogen with oxygen into water vapour molecules. To avoid this undesired effect DSC measurements have to be done in a protective atmosphere which does not contain oxygen.

Effect of Cr and Zr Dopes on Hydrogen Behaviour in Rapidly Solidified Aluminium Foils

Hydrogen (H) behaviour in materials was investigated in rapidly solidified (RS) foils of pure aluminium (Al), Al-0.4 Cr and Al-0.25 Zr alloys (at %) by means of thermal desorption spectroscopy (TDS). In addition, Al-0.25; 0.3 Zr alloys were examined with respect to microstructure and its instability during the thermal process using SEM and microhardness measurements. The effect of dopes and heating rate on H desorption was summarized. The lowest energy desorption is attributed with significant thermal desorption peak which temperature was found is correlated with sample composition.

Chemisorption of CO on Au/TiOx/Pt(111) Model Catalysts with Different Stoichiometry and Defectivity

Au/TiOx/Pt(111) model catalysts were prepared starting from well characterized TiOx/Pt(111) ultrathin films, according to an established procedure consisting in a reactive evaporation of Ti, subsequent thermal treatment in O2 or in UHV, and final deposition of submonolayer quantities of Au. Temperature Programmed Desorption measurements were performed to compare the interaction of CO in the case of two reduced TiOx/Pt(111) substrates (indicated as w-TiOx and w′-TiOx, being the former characterized by an ordered array of defects that can act as template for the deposition of a stable array of Au nanoparticles), with the case of a stoichiometric rect′-TiO2/Pt(111) substrate. It was found that in all cases CO is molecularly adsorbed and two different desorption peaks are detected: one at ≈140 K corresponding to CO desorption from less active adsorption sites (terraces) of the Au nanoparticles and one at ≈200 K corresponding to CO desorption from Au nanoparticles step sites. After annealing at 770 K, the high temperature CO desorption peak is still present in the case of the defective reduced w-TiOx phase, supporting the good templating and stabilizing effect of such phase. On the rect′-TiO2 stoichiometric phase, the CO uptake decreases after annealing but only to a minor extent.

Hydrogen desorption from nanostructured magnesium hydride composites

The influence of 3d transition metal addition (Fe, Co and Ni) on the desorption properties of magnesium hydride were studied. The ball milling of MgH2-3d metal blends was performed under Ar. Microstructural and morphological characterization were performed by XRD and SEM analysis, while the hydrogen desorption properties were investigated by DSC. The results show a strong correlation between the morphology and thermal stability of the composites. The complex desorption behavior (the existence of more than one desorption peak) was correlated with the dispersion of the metal additive particles that appear to play the main role in the desorption. The desorption temperature can be reduced by more than 100 degrees if Fe is added as additive. The activation energy for H2 desorption from the MgH2-Fe composite is 120 kJ/mol, implying that diffusion controls the dehydration process.