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CAGE FORMATION OF N2 UNDER H2O OVERLAYER ON Ru (001)
T. Livneh L. Romm and M. Asscher
Department of Physical Chemistry and the Farkas Center for Light Induced Processes
The Hebrew University, Jerusalem 91904, Israel
Water molecules were found to strongly interact with N2 when
coadsorbed on Ru(001). This system was studied by TPD, AES and work
function change measurements. In addition, collision induced effect
(CI) of the different adsorbates was employed to study the strength of
interaction between H2O and neighbor N2 molecules. It was found that
water molecules repel preadsorbed nitrogen at one monolayer coverage,
up to a coverage which is equivalent to one water monolayer, as
indicated by the shift of the thermal desorption peak from 116K down to
105K. At exposures which result in water coverages higher than 2ML, a
unique new N2 desorption peak gradually emerges near 160K, which
contains a population of 25% of the original 1ML coverage. The
extremely sharp shape of this TPD feature and its peak temperature are
indicative of the formation of a cage of hydrogen bonded water on top
of trapped N2 molecules. Work function change measurements suggest that
the nitrogen molecules remain adsorbed at an upright geometry inside
the cage. Isotope effect due to the adsorption of D2O show stronger
binding of the deuterated water, in qualitative agreement with recent
model calculations on large water cluster with trapped N2 molecules
inside. Bombardment of the adsorbed water molecules with energetic rare
gas atoms in a supersonic beam indicate that the hydrogen bonded
network of adsorbed water molecules is extremely resilient against
collisions. Incident energies of more than an order of magnitude higher
than the hydrogen bonding strength are shown to be insufficient to
break these bonds. The caged nitrogen molecules are also unaffected by
these energetic collisions. The implications of such new cages on the
reactivity of nitrogen on catalytic surfaces is discussed.
REPULSIVE INTERACTIONS OF POTASSIUM ON Re(001)
R.W. Verhoef, W. Zhao and M. Asscher
Department of Physical Chemistry and the Farkas Center for Light Induced Processes,
The Hebrew University, Jerusalem 91904, Israel
Repulsive interactions of potassium on Re(001) were
investigated using temperature programmed desorption (TPD), work
function measurements, and optical Second-harmonic Generation (SHG).
The TPD and work function results were used for the first time to
critically evaluate the validity of available electrostatic models for
explaining simultaneously both sets of data. The activation energy for
desorption in the limit of zero coverage was determined to be 68.4
± 0.1 kcal/mol, decreasing monotonically with increasing
potassium coverage to 22.1 ± 0.1 kcal/mol at a full monolayer
coverage. The TPD data were fit well by a depolarization model which
includes the adsorbate-surface distance. The work function decreases
monotonically upon potassium adsorption until reaching a minimum value
of 4.35 eV below the work function of the bare rhenium surface at a
coverage of approximately 0.5 ML, after which the work function
increased to an ultimate value of 3.00 eV below the work function of
Re(001). Work function of the bare Re(001) surface of 5.26 ± 0.05 eV was thus deduced. The work
function data were fit using a model which considers the change in work
function assuming local contribution by the bare substrate and the
adsorbate, using the same parameters obtained from fitting the TPD
data. SHG measurements were consistent with previous alkali -
transition metal systems, including a characteristic resonant - like
signal enhancement at potassium coverage of 0.3 - 0.4 ML. The SHG data
taken during potassium adsorption and desorption complement information
obtained by TPD and the work function measurements.
WORK FUNCTION STUDY OF THE ADSORPTION, LATERAL INTERACTION AND FRAGMENTATION OF CH3Br ON Ru(001)
T. Livneh and M. Asscher
Department of Physical Chemistry and the Farkas Center for light-Induced Processes,
The Hebrew University, Jerusalem 91904, Israel
The chemistry of methyl bromide on Ru(001) has been studied utilizing work function change (Dj ) measurements and temperature programmed desorption (TPD) at the crystal temperature range of 82K - 1350K. Employing a unique Dj - TPD mode, chemical changes in the adsorbed state could be detected at temperatures below the onset for desorption. A decrease in work function of 2.15 ± 0.02 V has been measured at the completion of a monolayer coverage which has been determined to consist of (3.6± 0.3)× 1014 molecules/cm2, equivalent to CH3Br/Ru = 0.22 ± 0.02. The onset for C-Br bond cleavage near 140K was observed for coverages below 0.1ML . At saturation coverage 50% of the molecules decompose to adsorbed methyl and bromine.
A low temperature increase in work function was found to precede dissociation or desorption as coverage increases. This change in work function is suggested to arise from a thermally activated flipping mechanism in which a fraction of the adsorbate molecules rearrange to adsorb with the methyl group facing the surface, consistent with the bulk molecular crystalline structure. The apparent activation energy for this flipping mechanism decreases with coverage, in agreement with dipolar repulsive interactions.
Sequential dehydrogenation of the methyl fragments, competing with minor methane production at higher coverages, were directly observed by employing the differential work function measurements. The corresponding surface temperature window for each of these reactivity steps has been determined and the detailed reaction mechanism is discussed. Bromine atoms on Ru(001) were found to decrease the work function by 350 mV at a coverage Br/Ru = 0.3, indicating a complex charge redistribution upon adsorption. Deuterium preadsorption, which significantly passivates the surface, has been employed to better understand the various reactivity steps of the hydrocarbon fragments.
Finally, work function measurements indicate the presence of strong interactions of the methyl bromide molecules with the metal surface up to the fourth layer. Alternating contributions to the work function of the first three layers are observed. This is understood in terms of an opposite adsorption geometry in which bromine faces the surface in the first layer, methyl in the second and bromine again in the third. Higher stability of the fourth layer compared with the third has been observed by TPD. This is suggested to arise from the formation of a methyl bromide crystalline bulk-like structure in the fourth layer, accompanied by an increased density.
THE CHEMISTRY OF CH3Br AND CH3Cl ON Ru(001)
T. Livneh and M. Asscher
Department of Physical Chemistry and the Farkas Center for light-Induced Processes,
The Hebrew University, Jerusalem 91904, Israel
The interaction of methyl chloride and methyl bromide on Ru(001) have been studied and compared using work function measurements in a Dj - TPD mode and normal D p - TPD. Both molecules adsorb molecularlat temperatures below 100K with the halide facing down, forming direct bonding to the metal and reducing the work function by 1.9 and 2.1 Volts, for CH3Cl and CH3Br, respectively. Dipole moments of the isolated molecules and their polarizabilities were extracted by using electrostatic models for the work function as a function of coverage. While heating the crystal, methyl chloride desorbs molecularly with no trace of dissociation. This is concluded from the similarity of the differential work function change (DWFC) and the D p - TPD spectra. In contrast, 50% of an initial one monolayer of methyl bromide, dissociates to form adsorbed methyl and bromide. Significant differences between the DWFC and the normal desorption spectra are discussed in terms of the sensitivity of work function to rearrangement of the molecular species at low temperatures and to dissociation of the parent molecule and its methyl fragments at higher temperatures.
THE WORK FUNCTION OF ADSORBED ALKALIS ON
METALS REVISITED: A COVERAGE DEPENDENT POLARIZABILITY APPROACH
R.W. Verhoef and M. Asscher
Department of Physical Chemistry and the Farkas Center for Light Induced Processes,
The Hebrew University, Jerusalem 91904, Israel
A model is presented to explain the change in work function as a function of alkali metal coverage on transition metals based on dipole-dipole depolarization assuming a coverage-dependent polarizability of the adsorbate-substrate complex. The fit of the model is a great improvement over fixed-polarizability depolarization models, and the resultant prediction for the variation of the polarizability with coverage is qualitatively similar for different alkalis on the same surface, and quantitatively similar for the same alkali on different surfaces.
ADSORPTION AND REACTIVITY OF CO2 ON
POTASSIUM COVERED Re(001)
Wei Zhao, Ilan Chacham, Yahel Vackart and Micha Asscher
Department of Physical Chemistry and The Farkas Center for Light Induced Processes
The Hebrew University, Jerusalem 91904, ISRAEL
The interaction of CO2 with potassium covered Re(001) has been investigated. This system has been studied by means of work function (Df ), optical second harmonic generation (SHG) and TPD measurements. Strong electronic interaction between carbon dioxide and potassium is observed upon adsorption at 90K. This is indicated by a rapid quenching of the SHG signal of K following postadsorption of CO2, with a quenching cross section of 70 Å2. Work function change measurements are consistent with such interaction, evidenced by undepolarization effect, namely further decrease of the work function upon CO2 adsorption, below the minimum obtained by pure potassium. In the presence of potassium, the dissociation probability of 0.5 ML adsorbed carbon dioxide increases from 0.5 on the clean metal surface to 0.85 on 1 ML potassium covered Re(001), information obtained from TPD measurements following heating to 1250 K. It is concluded that a K-CO2 surface compound is formed upon adsorption at 90 K on the potassium covered surface.
THE SURFACE CHEMISTRY OF CH3Br AND METHYL
MODIFIED BY COPPER DEPOSITION ON Ru(001).
- Livneh1 and M. Asscher
Processes, The Hebrew University, Jerusalem 91904, Israel
1. Current address: Department of Physical Chemistry, Nuclear Research Center,
Negev, P.O. Box 9001, Beer Sheva, Israel
The chemistry of methyl bromide on Cu/Ru(001) has been studied utilizing work function change
(Dj ) and temperature programmed desorption (TPD) measurements. The remarkable modification in the methyl fragments dehydrogenation at the completion of a single copper layer and the significant difference in reactivity of the Cu(2ML)/Ru(001) or Cu(111) surfaces are the focus of this study.
A decrease in work function at the completion of 1 ML CH3Br of 2.15 ± 0.02 eV and 1.33 ± 0.05 eV was measured for Ru(001) and Cu(2ML)/Ru(001) held at 82K, respectively. Methyl bromide does not dissociate upon adsorption on the clean or the copper covered surfaces and it is bound with the bromine down.
Copper modifies the reactivity of the Ru substrate, gradually decreasing the dissociated fraction of CH3Br from 0.55 of the initial one monolayer on clean Ru(001) to 0.06 on Cu(2ML)/Ru(001), probably due to defects in the copper layer. The methyl fragment dehydrogenation rate slows down as the copper coverage increases. At a narrow copper coverage range between 0.80 - 0.95 ML, adsorbed hydrogen and methyl fragments coexist on the surface in the temperature range of 230K-280K. Sequential decomposition channels of the parent molecules and the methyl fragment, lead to a unique enhancement of methane production rate. This on the account of further hydrocarbon dehydrogenation, as reflected in both Dp and Dj - TPD measurements. Methane is formed on top of copper terraces, as a result of “spill-over” of both methyl and hydrogen atoms, similar to the chemistry over Cu(111) and Cu(110) single crystal surfaces. The dipole moment of adsorbed methyl is reported here for the first time on metal surfaces, being 0.48 D on top of Cu(2ML)/Ru(001).
DIPOLE-DIPOLE INTERACTIONS AMONG CH3Cl
MOLECULES ON Ru(001): CORRELATION BETWEEN WORK FUNCTION CHANGE
MEASUREMENTS AND THERMAL DESORPTION STUDIES.
T.Livneh1, Y.Lilach and M.Asscher
Department of Physical Chemistry and the Farkas Center for Light Induced Processes
The Hebrew University, Jerusalem 91904, Israel
1. Current address: Department of Physical Chemistry, Nuclear Research Center,
Negev, P.O. Box 9001, Beer Sheva, Israel
Work function change measurements (DF) combined with temperature programmed desorption (TPD) were employed to study the layer growth mechanism and the CH3Cl dipole-dipole interactions on Ru(001), over the temperature range of 97K–230K. The activation energy for desorption (Ea) and the molecular dipole moment (m) decrease from 55.9 kJ/mol and 2.44D, at the zero coverage limit, to 38.6 kJ/mol and 1.27D, at one monolayer. This coverage dependence originates from strong dipolar lateral repulsion among neighbor CH3Cl molecules. Using a model introduced by Maschhoff and Cowin (MC) (J. Chem. Phys., 101(9), 8138 (1994)), the isolated adsorbed molecule’s dipole moment, m0 (2.35D) and polarizability a (8.1× 10-24 cm3), were extracted from TPD data. These values agree very well with m0 (2.12D) and
a (9.2× 10-24 cm3) obtained from work function change measurements by employing the same MC model.
The ability to simulate both TPD and work function change data over a wide coverage range within the framework of a single electrostatic model has been demonstrated. It enabled better understanding of fine details of surface dipolar interactions.
Sublimative desorption of Xenon from Ru (100)
Gabriel Kerner, Ori Stein, Yigal Lilach and Micha Asscher*
Dept. of Physical Chemistry, The Farkas Center for Light induced processes,
The Hebrew University of Jerusalem, 91904 Israel.
Sublimative desorption is a process in which desorption of multilayers of an adsorbate precedes melting and surface diffusion. Here we report on the desorption kinetics of Xe atoms from multilayer coverage studied using temperature programmed desorption (TPD) and optical diffraction methods. It is found that decay of the diffraction peak intensities from multilayer coverage grating during surface heating cannot be explained as one dimensional diffusion process. Instead, the diffraction signal follows Xe desorption, as deduced from simultaneous linear diffraction and desorption measurements. This observation suggests that no macroscopic 2D melting and diffusion occur in the case of multilayers of Xe before the onset for desorption. It is concluded that Xe atoms undergo sublimative desorption from the topmost layers. Similar results were obtained in the case of water multilayers on Ru(100). These results suggest that on solid surfaces the desorption of multilayers is thermodynamically favorable over surface melting or diffusion.
