Adsorption Sites on Oxide Surfaces

Example of the use of small molecules as chemical probes for the titration of specific surface sites.  Shown in this figure are CO TPD spectra from a Ni(110) crystal covered with different coverages of oxygen atoms.  Discrete temperature changes on the order of 100 K are seen as the oxygen coverage is increased, in a way that correlates with the number of oxygen atoms coordinated to the nickel atom being probed, and, more indirectly, to its oxidation state.

Ammonia, although it displays far more complex surface chemistry, can also be used as a probe molecule, in this case to identify hydrogen-bonding and acidic surface sites.  Initial studies on the reactivity of ammonia on clean Ni(110) surfaces by TPD indicated limited decomposition to NH2(ads) at about 300 K, further dehydrogenation to NH(ads) and N(ads) at around 380 K, and some NH2(ads) + H(ads) recombination to NH3(g) at ~360 K [D. Chrysostomou, J. Flowers, and F. Zaera, Surf. Sci. 439 (1999) 34].  More to the point of this project, it was shown that the dissociative pathway can be enhanced by surface oxygen and/or hydroxo groups, as indicated by the TPD and XPS data shown below [H. Guo, D. Chrysostomou, J. Flowers and F. Zaera, J. Phys. Chem. B 107(2003) 502; H. Guo and F. Zaera, Surf. Sci. 524 (2003) 1].  Of particular interest in this case is the new high-temperature (~400 K) desorption state observed at intermediate oxygen coverages.  Our work, as well as that of others, strongly suggest that ammonia dissociates easily below 400 K, and that the chemistry observed in that high-temperature state may involve NH2 (and perhaps NH) surface intermediates.  We propose a facile interchange along the NH3(ads) + O(ads) -> NH2(ads) + OH(ads) -> NH(ads) + H2O(ads) reaction coordinate, similar to that seen with water [H. Guo and F. Zaera,Catal. Lett. 88 (2003) 95].

Evidence for the high reactivity of some surface oxygen atoms towards ammonia activation on partially oxidized Ni(110) surfaces.  Left panel: sequence of N 1s X-ray photoelectron spectroscopy (XPS) data for ammonia coadsorbed with oxygen as a function of temperature illustrating the facile interchange of hydrogen atoms between the two species.  Rapid interconversion between ammonia and water is attained via the formation of NH2, NH, and OH surface intermediates.  Right panel: sequence of TPD traces for ammonia desorption from Ni(110) surfaces treated with different amounts of oxygen.  A new high-temperature state is seen above 400 K at the intermediate oxygen doses associated with O atoms at the end of O–Ni–O surface chains (see diagram).  These oxygen atoms are believed to be particularly active towards the abstraction of hydrogen atoms from adsorbed ammonia, the step responsible for the reactions indicated in the left panel.  These experiments exemplify the importance of specific local ensembles of surface atoms in defining sites for selective catalysis.

Illustration of the use of ion scattering spectroscopy (ISS) for the identification of adsorption sites in complex systems.  The data in the main frame correspond to the ISS spectra obtained after different uptakes of 2-propyl iodide on O/Ni(100) surfaces prepared by adsorption of 3.0 L of O2 at 300 K.  The inset, which shows the ISS peak areas normalized to the signal of the Ni and O peaks for the surface with 0.0 L 2-C3H7I, highlights the fact that the alkyl iodides prefer to adsorb on the metal sites.

More recently, we have coupled the use of Xe with other adsorbates as probes for minority surface sites.  Xe is quite inert, but, thanks to his high polarizability, highly sensitive to its local electronic environment.  This can be detected by changes in the binding energy of the Xe photoelectrons.  The Figure below illustrates how this so-called PAX spectroscopy works. The left panel displays typical Xe 3d5/7 XPS data obtained for xenon adsorbed on Ni(110) in equilibrium with increasing gas-phase Xe pressures, and the right panel summarizes the data recorded for clean, sputtered, and oxygen-treated surfaces.  Red shifts of up to 1.0 eV in binding energy are seen here in all three cases, a direct manifestation of the sequential adsorption of xenon on sites with increasing local electrostatic potentials.  Moreover, each sample starts at a different initial value, pointing to the different nature of the initial adsorption sites in each case.  For instance, it is seen from the data that sputtering of the surface leads to the production of more unsaturated surface nickel atoms with a higher local work function.  Finally, the different Xe uptakes observed as a function of pressure in each of the reported cases indicates clear differences in the distribution of sites.

The high polarizability of xenon also leads to important changes in its adsorption energy by the local surface surroundings, and that energy can be easily measured by TPD.  The power of Xe TPD for the identification of unique surface sites is illustrated by the data below for atomic oxygen adsorbed on Ni(110).  Notice in particular the increase in Xe desorption temperature seen at low oxygen coverages (left panel), which we believe is associated with a highly mobile oxygen atomic species at the end of incomplete –Ni–O surface rows.  That high-temperature Xe peak disappears and is replaced by a weaker state after oxygen exposures above 0.5 L, and increasing xenon doses on the low-coverage surfaces also brings up that second adsorption mode.

Xe TPD spectra used for the characterization of O-Ni(110) single-crystal surfaces.  Left:  Xe TPD traces obtained after 0.6 L Xe exposures on Ni(111) treated with various O2 predoses at 400 K.  The formation of a new adsorption site after 0.1 - 0.2 L O2 treatments is signified by the distinct new high-temperature peak about 100 K.  Right: Xe TPD data versus Xe exposure for a 0.2 L O2 predose.  In this case saturation of peak at 100 K, presumably from Xe adjacent to oxygen atoms in an active –Ni–O environment, is followed by the growth of a second feature around 75 K, perhaps the result of a structural change in the remaining of the surface.

Correlation between the physical characteristics of O-Ni(110) surfaces with different oxygen coverages and their reactivity toward methyl hydrogenation and ammonia hydrogenation-dehydrogenation.  Plotted in the main frame are the TPD yields for methane production from methyl iodide (the precursor for surface methyl groups) and for ammonia recombination.  In both cases the data show particularly high activity in the 0.1 - 0.3 L O2 exposure range corresponding to the formation of uniquely row-end –Ni–O unsaturated oxygen atoms.  Those sites are identified directly by the high-temperature Xe TPD peak shown in the inset.

 Last modified on July 28, 2005