Temperature hydrogen desorption

Iron on alumina as well as iron and iridium supported on the same carrier were suspected of exhibiting hydrogen spillover after treatment with hydrogen at 400 to 500°C. The TPD analysis is described by Paryjczak amd Zielinski (32,33). The high-temperature hydrogen desorption peak was attributed to the spiltover species. 

The sticking coefficient of H2 on a metal has been determined through an adsorption experiment. The metal surface is assumed to have No = 1.5 x 10 sites m and each adsorption site is assumed to be occupied by one hydrogen atom when the surface is saturated. The experiment was performed by exposing the surface to a known pressure of hydrogen over a well-defined period of time (dosis) and then sequentially determining how much was adsorbed by, for example, TPD. All adsorption experiments where performed at such low temperatures that desorption could be neglected. 

The scanning transmission electron microscope (STEM) was used to directly observe nm size crystallites of supported platinum, palladium and first row transition metals. The objective of these studies was to determine the uniformity of size and mass of these crystallites and when feasible structural features. STEM analysis and temperature programmed desorption (TPD) of hydrogen Indicate that the 2 nm platinum crystallites supported on alumina are uniform In size and mass while platinum crystallites 3 to 4 nm in size vary by a factor of three-fold In mass. Analysis by STEM of platinum-palladium dn alumina established the segregation of platinum and palladium for the majority of crystallites analyzed even after exposure to elevated temperatures. Direct observation of nickel, cobalt, or iron crystallites on alumina was very difficult, however, the use of direct elemental analysis of 4-6 nm areas and real time Imaging capabilities of up to 20 Mx enabled direct analyses of these transition metals to be made. Additional analyses by TPD of hydrogen and photoacoustic spectroscopy (PAS) were made to support the STEM observations. 

The dedicated STEM provides a means of obtaining mlcroanalytlcal Information of supported metals not readily obtained by other Instrumentation. The capability to observe and analyze some very highly dispersed metal particles on y-alumlna has been demonstrated and for the noble metals, verified by temperature prograouned hydrogen desorption. 

Figures 3 and 5 give examples of hydrogen TPD profiles observed on both catalysts reduced at 473K. The more intense patterns concern the desorption of the hydrogen fixed on the solids during the reduction step (curve a) while the weaker peaks (curve b) are related to the hydrogen retained after readsorption at room temperature. Hydrogen adsorption is thus an activated process.

This clearly indicates that the support is able to convert a fraction of methane but contributes primarily to coking To investigate the role of acid sites in the conversion of methane to coke and hydrogen, the acid sites of the catalysts were characterized by sorption and temperature programmed desorption (t.p.d) of pyridine T.p.d. of pyridine (see Fig. 4) suggest a higher... 

The Pt dispersion of the fresh samples was measured by dynamic hydrogen chemisorption by using a temperature-programmed desorption (TPD)/R/0 1100 ThermoFisher... 

The simultaneous appearance of ethane with the low temperature ethylene desorption peak suggests that the low temperature ethylene peak may correspond to the desorption of a molecularly adsorbed ethylene species that can also undergo hydrogenation and subsequently desorb as ethane. If this is the case, the effective activation energies for desorption and hydrogenation of the molecular ethylene species are approximately equal. 

The thermal expansion coefficient of bulk silicon is positive at RT (2.6 x 1CT6 K-1), but becomes negative below 120 K. The thermal expansion coefficient of micro PS for heating from 290 to 870 K is found to be negative (-4.3x 10 6 KT1), which can be ascribed to hydrogen desorption and oxidation of the inner surface [Di7]. For meso PS the thermal expansion coefficient was found to increase with porosity in the temperature regime between 90 K and 300 K, from 0.4xl0-6 K 1 to... 

Between 7 and 470 K a is found to increase with temperature. This increase is reversible and corresponds well with that of bulk silicon [Ko4]. If PS is annealed in nitrogen at higher temperatures (600 °C), hydrogen desorption takes place, which changes the condition of the inner surface drastically. At these temperatures an irreversible increase in a is observed for micro PS. A similar increase in a after annealing is found for meso PS. Changes that affect the core of the crystallites, e.g. stress effects [Ko4], as well as surface-related effects like the formation of surface states [Ko5, BalO], are proposed to be responsible for the observed increase of a with T. 

Figure 2. Temperature programmed desorption (TPD) for hydrogenated porous silicon.

Performing a Redhead analysis of the TPD. an estimation of the activation energy (Ea) of hydrogen desorption from npSi can be derived from the following equation [7], where v is the evolution rate, and p is the linear temperature ramp rate. 

A 90% reduction in activation energy, not an unreasonable expectation for catalysts in general, reduces the peak temperature below 0 C. Clearly, only a small amount of catalytic action is required to make dramatic reductions in the release temperature. This implies that, with careful control of the invented process, it should be possible to dial-in the desorption temperature for hydrogen desorption. This allows us to assess how this hydrogen storage media can be applied. 

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