Adsorption Of D2 Research Articles

Isotope effects in hydrogen adsorption on Ni(111): Direct observation of a molecular precursor state

The nature of the interaction of hydrogen with Ni(111) is probed by comparing the adsorption and desorption kinetic behavior of H2 and D2. Pure H2 and pure D2 adsorbed on Ni(111) at 140 K exhibit identical desorption behavior. The adsorption rate law depends upon (1-θ) rather than (1-θ)2, suggesting that the rate determining step involves the interaction of an H2 molecule with a single Ni site. The temperature programmed desorption spectra show two desorption features, β1 and β2, which have maxima in desorption rate at 290 and 370 K. When either pure H2 and pure D2 or a mixture of H2 and D2 interact with the Ni(111) surface at 87 K, evidence for kinetic retardation of D2 adsorption is observed. When the H2:D2 mixture is exposed to the Ni(111) surface, held at 87 K, an additional desorption feature, α, is uniquely observed at 100 K for D2. α-D2 desorbs with first order kinetics exhibiting Ed =11.1 kJ/mol and a preexponential factor of 1.2×105 s−1. It may be due to desorption from an intrinsic molecular precursor state. The observed differences between hydrogen and deuterium adsorption indicate the presence of an activation energy barrier for dissociative chemisorption, and a sensitivity of the adsorption process to differences in zero point vibrational energies. A molecular precursor state is proposed to describe the H2+Ni(111) interaction. Dissociative adsorption of hydrogen on intentionally produced surface defects has been shown to destabilize α-D2.

Adsorption of hydrogen isotopes at high pressures

1. Precise data were obtained for the first time for the equilibrium adsorption of H2 and D2 on zeolite NaX in a broad range of occupancies (up to 10 mmoles/g), pressures (0.7-7 MPa), and temperatures (77–300°K). 2. All the experimental data for the adsorption of hydrogen isotopes on zeolite were approximate by a thermal equation with five constants using a computer. The constants are virtually independent of temperature and have concrete physical significance. The approximation errors were determined. 3. A simple equation was obtained which permits the rapid and highly precise calculation of the compressibility coefficient of H2 and D2 in the temperature and pressure range studied.

An apparatus for measuring the equilibrium adsorption of pure H2 and D2

1. An apparatus was designed and constructed for studying the equilibrium adsorption of hydrogen isotopes by a volumetric method at pressures up to 6 MPa in the temperature range from 100 to 300°K. 2. Data on the adsorption of hydrogen isotopes on zeolite NaX were obtained 1) by the direct measurements of the adsorption isotherms and 2) measurement of the adsorption along constant gas quantity curves.

Dynamics of interaction of H 2 and D 2 with Ni(110) and Ni(111) surfaces

Abstract Elastic and direct-inelastic scattering as well as dissociative adsorption and associative desorption of H2 and D2 on Ni(110) and Ni(111) surfaces were studied by molecular beam techniques. Inelastic scattering at the molecular potential is dominated by phonon interactions. With Ni(110), dissociative adsorption occurs with nearly unity sticking probability s0, irrespective of surface temperature Ts and mean kinetic energy normal to the surface 〈 E⊥ 〉. The desorbing molecules exhibit a cos θe angular distribution indicating full thermal accommodation of their translation energy. With Ni(111), on the other hand, s0 is only about 0.05 if both the gas and the surface are at room temperature. s0 is again independent of Ts, but increases continuously with 〈 E⊥ 〉 up to a value of ∼0.4 for 〈 E⊥ 〉 = 0.12 eV. The cos5θe angular distribution of desorbing molecules indicates that in this case they carry off excess translational energy. The results are qualitatively rationalized in terms of a two-dimensional potential diagram with an activation barrier in the entrance channel. While the height of this barrier seems to be negligible for Ni(110), it is about 0.1 eV for Ni(111) and can be overcome through high enough translational energy by direct collision. The results show no evidence for intermediate trapping in a molecular “precursor” state on the clean surfaces, but this effect may play a role at finite coverages.

Dynamics of interaction of H2 and D2 with Ni(110) and Ni(111) surfaces

Elastic and direct-inelastic scattering as well as dissociative adsorption and associative desorption of H2 and D2 on Ni(110) and Ni(111) surfaces were studied by molecular beam techniques. Inelastic scattering at the molecular potential is dominated by phonon interactions. With Ni(110), dissociative adsorption occurs with nearly unity sticking probability s0, irrespective of surface temperature Ts and mean kinetic energy normal to the surface 〈 E⊥ 〉. The desorbing molecules exhibit a cos θe angular distribution indicating full thermal accommodation of their translation energy. With Ni(111), on the other hand, s0 is only about 0.05 if both the gas and the surface are at room temperature. s0 is again independent of Ts, but increases continuously with 〈 E⊥ 〉 up to a value of ∼0.4 for 〈 E⊥ 〉 = 0.12 eV. The cos5θe angular distribution of desorbing molecules indicates that in this case they carry off excess translational energy. The results are qualitatively rationalized in terms of a two-dimensional potential diagram with an activation barrier in the entrance channel. While the height of this barrier seems to be negligible for Ni(110), it is about 0.1 eV for Ni(111) and can be overcome through high enough translational energy by direct collision. The results show no evidence for intermediate trapping in a molecular “precursor” state on the clean surfaces, but this effect may play a role at finite coverages.

Adsorption studies of H2 isotopes on ZnO: Coverage-induced IR frequency shifts and adsorbate geometry

The coverage dependence of the IR stretching frequencies for dissociative type I adsorption of H2 and D2 on ZnO powders has been measured using transmission IR spectroscopy. By comparing the frequency shifts observed when the isotopic composition of the adsorbate is varied at constant total coverage with the shifts observed when the total coverage of either pure component is varied, we can separate the dynamic and static contributions to the coverage-induced frequency shifts. The ZnH and ZnD shifts are due primarily to electrodynamic interactions. The observed shifts are in good agreement with the Hammaker model for dynamic dipole–dipole interactions, if adsorption is assumed to occur on (2×2) reconstructed ZnO(0001) surface planes. In contrast, the OH and OD shifts are due to electrostatic and inductive interactions. The electrostatic contribution can be estimated using a modification of Buckingham’s treatment of local environment effects. A qualitative model based on electron localization effects is presented to explain the observed inductive contribution.

The Adsorption, Desorption, and Exchange Reactions of Oxygen, Hydrogen, and Water on Platinum Surfaces. II. Hydrogen Adsorption, Exchange, and Equilibration

The adsorption, desorption, exchange, and equilibration reactions of hydrogen and deuterium on a platinum filament have been investigated by thermal desorption mass spectrometry. A surface saturated with hydrogen at 120 °K has a coverage 4.2 × 1014 molecules cm−2 and gives desorption spectra with four distinct peaks: β1,(165 °K), β2(220 °K), β3(280 °K), and β4(350 °K). Apparent activation energies and pre-exponential factors were determined for the β2-, β3-, and β4-peaks. For both co-adsorption and sequential adsorption of H2 and D2 the mass 2, 3, and 4 desorption spectra have identical shapes and the gas desorbs at equilibrium throughout. It is concluded that hydrogen adsorbs dissociatively. Exchange and equilibration were studied at 120, 210, and 285 °K by determining the surface composition and isotope distribution after varying fractions of preadsorbed H had been replaced. Following exchange at 120 °K the desorption spectra show a higher D content and a lack of equilibrium in the desorbing gas at low temperature. In most other experiments the mass 2,3, and 4 desorption spectra had identical shapes and the gas desorbed at equilibrium. The results are interpreted by a model which requires that the polycrystalline platinum surface is intrinsically heterogeneous. It appears that different mechanisms are unnecessary to interpret the differences in kinetics observed for exchange and equilibration at low temperatures.

Sequential adsorption of D2 and C2H4 on single-crystal planes of tungsten

The sequential adsorption of deuterium and ethylene has been studied on the (110), (100), (112), and (111) planes of tungsten at [inverted lazy s] 135°K using thermal desorption mass spectroscopy. Adsorption of D2 followed by C2H4 on both the (110) and (112) planes, respectively, leads to 0.24 and 0.37 monolayers of ethylene adsorbed with no displacement of deuterium for an ethylene exposure which would just be sufficient to saturate a bare surface. Since the resulting spectra are apparently superpositions of ethylene decomposition and deuterium desorption, it appears that ethylene and deuterium in part occupy separate sites. On both the (100) and (111) planes adsorption of ethylene occurs through displacement of 1.3 molecules of deuterium for each molecule of ethylene adsorbed. This displacement is more extensive on the (111) plane: θ C2H4=0.86 and θ D2=0.36 on the (111) plane vs θC2H4=0.52 and θD2=0.67 on the (100) plane. Displacement on the (100) plane occurs through adsorption of ethylene on sites separate from deuterium with the ethylene perturbing deuterium sufficiently to lead to desorption. Arguments are given for this interaction being relatively short range. On the (111) plane hydrogen displacement is a thermal process with both D2 displaced by C2H4 and H2 by D2. The D2 is able to displace an amount of H2 corresponding to all the β1 and β2 states and [inverted lazy s] 3/4 of the β3 state. The extent of this displacement indicates that there is extensive interchange between the β states on W(111). Previous data for the sequential adsorption of D2 and C2H4 on polycrystalline tungsten were interpreted as a composite of contributions from individual planes with specifically the (100) plane contributing a high C2H4:D2 ratio and the (110) plane a low C2H4:D2 ratio. The single-crystal results are shown to be in agreement with this. If we assume the polycrystalline surface was partially carbon contaminated, both the spectra and the coverages resulting from sequential adsorption of D2 and C2H4 and the ethylene decomposition spectrum, can be reconstructed using a reasonable linear combination of the results from these four planes.

Manganese Ions as Paramagnetic Probes in Magnesium Oxide. I. Surface Reactivity

AbstractThe reversibility of the Mn2+ → Mn3+ → Mn4+ reaction, in MgO samples containing initially near 1 at.‐ppm of bulk Mn2+ ions is examined as a function of evacuation pretreatments at nearly 1200 K eventually followed by adsorption of H2, D2, or H2O vapor at 298 K. Two bulk e. s. r. signals are observed which correspond respectively to Mn2+ and Mn4+ ions.The Mn lines are strongly affected by adsorption at room temperature or 77 K of various gases and vapors.Mn3+ ions, not detected by e. s. r., must also be present to account for the relative intensity changes of the Mn2+ and Mn4+ species as detected by e. s. r. Electronic conduction paths, must be available between the surface and the bulk.

Mixed Adsorption of D2 and C2H4 on Polycrystalline Tungsten

The adsorption and reaction of hydrogen and ethylene on polycrystalline tungsten were studied by means of flash filament spectroscopy. For an ethylene exposure just sufficient to form a monolayer on a bare surface, a D2 covered surface adsorbs 12 monolayer of ethylene without loss of D2; for the inverse process, preadsorption of C2H4, no D2 is adsorbed. In flash filament spectra taken subsequent to a dose of hydrogen followed by ethylene, the major gas phase products are hydrogen and small quantities of ethane. The hydrogen spectra contain peaks at 300°K (ethylene decomposition), 450°K (both ethylene decomposition and hydrogen desorption), and 650°K (hydrogen desorption) with H2 predominant in the 300° peak and D2 predominant in the high temperature peaks. Since coverage studies show that ethylene decomposition occurs via transfer of hydrogen to the surface where H2 and D2 are known to equilibrate, this concentration of isotopes in specific peaks indicates that C2H4 adsorption does not occur uniformly over the D2-covered surface. These results are interpreted in terms of a surface composed of two regions: region A where D2 and C2H4 coadsorb on independent sites, and region B where D2 and C2H4 occupy the same sites. Arguments are presented for associating W(100) with region A and W(110) with region B. The production of ethane during the flash is consistent with a Langmuir-Hinshelwood mechanism with migration of hydrogen as the rate limiting step.

Quantum Treatment of the Physical Adsorption of Isotopic Species

The results of isotopic substitution in the adsorption of H2, D2, CH4, and CD4 on a graphitized carbon black observed by Constabaris, Sams, and Halsey are explained in terms of two competing effects: the quantum statistical mass effect on the vibrational energy levels normal to the surface and the quantum mechanical effect of isotopic substitution on the dispersion energy.

Measurements on the adsorption of light and heavy hydrogen on charcoal between 90°K and 17°K

Summary Measurements have been made on the adsorption on charcoal of D2 and H2 in the temperature regions 90-55°K and 20-17°K. Between 90-55°K the adsorption of D2 is found to be about 8% greater than that of H2. Between 20-17°K this difference has increased to about 15%. It seems to us that this difference must be explained by a difference in potential and kinetic properties between the two isotopes (a difference in diameter being out of the question) and that the way to treat this problem is analogeous to that developed by K. Schafer, when dealing with the van der Waals' attraction. (Z. phys. Chem., B. 36, 98, 1937). Between 90-55°K the adsorption isotherms can be represented by a Freundlich equation, between 20-17°K by a Williams equation. The results obtained cannot be brought into agreement with the potential theory of Polanyi. From the data obtained for the adsorption isotherms the heat of adsorption was calculated at various temperatures as a function of the adsorbed quantity. From this it can be concluded that there is a constant difference between the heat of adsorption of D2 and of H2 the adsorption heat of D2 being higher than that of H2 (about 180 cal.). The measurements of the adsorption were extended as far as capillary condensation, enabling us to derive the structure curve of the charcoal from which, the surface tension of D2 was, finally, computed. In this way we found: 20,36°K, 0.81 dyn/cm; 19.05°, 1.11; 17.48°, 1.70.