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Notes: We have employed molecular beam techniques to investigate the molecular trapping and trapping-mediated dissociative chemisorption of C3H8 and C3D8 on Ir(110) at low beam translational energies, Ei≤5 kcal/mol, and surface temperatures, Ts, from 85 to 1200 K. For Ts=85 K, C3H8 is molecularly adsorbed on Ir(110) with a trapping probability, ξ, equal to 0.94 at Ei=1.6 kcal/mol and ξ=0.86 at Ei=5 kcal/mol. At Ei=1.9 kcal/mol and Ts=85 K, ξ of C3D8 is equal to 0.93. From 150 K to approximately 700 K, the initial probabilities of dissociative chemisorption of propane decrease with increasing Ts. For Ts from 700 to 1200 K, however, the initial probability of dissociative chemisorption maintains the essentially constant value of 0.16. These observations are explained within the context of a kinetic model which includes both C–H (C–D) and C–C bond cleavage. Below 450 K propane chemisorption on Ir(110) arises essentially solely from C–H (C–D) bond cleavage, an unactivated mechanism (with respect to a gas-phase energy zero) for this system, which accounts for the decrease in initial probabilities of chemisorption with increasing Ts. With increasing Ts, however, C–C bond cleavage, the activation energy of which is greater than the desorption energy of physically adsorbed propane, increasingly contributes to the measured probability of dissociative chemisorption. The activation energies, referenced to the bottom of the physically adsorbed molecular well, for C–H and C–C bond cleavage for C3H8 on Ir(110) are found to be Er,CH=5.3±0.3 kcal/mol and Er,CC=9.9±0.6 kcal/mol, respectively. The activation energies for C–D and C–C bond cleavage for C3D8 on Ir(110) are 6.3±0.3 kcal/mol and 10.5±0.6 kcal/mol, respectively. The desorption activation energy of propane from Ir(110) is approximately 9.5 kcal/mol. These activation energies are compared to activation energies determined recently for ethane and propane adsorption on Ir(111), Ru(001), and Pt(110)–(1×2), and ethane activation on Ir(110). © 1996 American Institute of Physics.

Notes: The deuteration of oxygen adatoms on the Ru(001) surface has been investigated by means of temperature programmed desorption and high-resolution electron energy loss spectroscopy. Exposure of gas-phase atomic deuterium to the p(1×2) oxygen overlayer with a fractional adatom coverage of oxygen of 0.5 leads to the production of water at a surface temperature as low as 90 K. After exposure to molecular deuterium, no reaction is observed, suggesting that a direct Eley–Rideal (ER) reaction occurs between the impinging deuterium atoms and the preadsorbed oxygen. Only after a very low exposure of deuterium was it possible to isolate chemisorbed OD groups on the surface, implying that OD formation is the rate-limiting step in the formation of water via ER kinetics on Ru(001). Estimates of the ER reaction cross sections were made, and for the deuteration of adsorbed oxygen and hydroxyls, the cross sections were found to be (7.0±0.3)×10−17 cm2 and (2.2±0.1)×10−15 cm2, respectively. In addition to the ER mechanism, the chemisorbed OD groups could also react with coadsorbed deuterium adatoms via Langmuir–Hinshelwood (LH) kinetics at surface temperatures near 170 K, suggesting an activation barrier that is less than 9 kcal/mol. This implies that OD formation is also the rate-limiting step in the formation of water via LH kinetics on Ru(001). © 1996 American Institute of Physics.

Notes: A method is introduced for evaluating the adsorption probability as a function of surface coverage within the context of a lattice gas model. We delineate the methodology by considering dissociative adsorption for which nearest-neighbor empty surface sites are required. For direct, dissociative adsorption a dynamical Monte Carlo simulation algorithm is used to evaluate the spatial correlation between adsorbates as surface coverage increases over time. The influence on the probability caused by these spatial correlations between adsorbates due to lateral interactions between adsorbates and mobility of the adsorbate are evaluated exactly from Monte Carlo simulations. For precursor-mediated adsorption, Monte Carlo simulations combined with an approximate continuum equation have been used to describe the coverage-dependent adsorption probability. The effects of lateral interactions between adsorbates, lattice geometry, and precursor states on the scaling of the coverage-dependent adsorption probability are quantified using various representative parameters. © 1995 American Institute of Physics.

Notes: We have employed molecular beam techniques to investigate the initial probability of direct dissociative chemisorption, Pd, and the intrinsic trapping probability, ξ, of C3H8, C3D8, and (CH3)2CD2 on Ir(110) as a function of beam translational energy, Ei, from 1.5 to 59 kcal/mol. For C3H8 and (CH3)2CD2, a measurable (≥ 0.02) initial probability of direct dissociative chemisorption is observed above a beam energy of approximately 7 kcal/mol. For C3D8 this energy is roughly 10 kcal/mol. Above these energies the initial probability of direct chemisorption of each of the isotopomers of propane increases nearly linearly with Ei, approaching a value of approximately Pd=0.48 at Ei=52 kcal/mol for C3H8 and (CH3)2CD2, and Pd=0.44 at Ei=59 kcal/mol for C3D8. This kinetic isotope effect for the direct chemisorption of C3D8 relative to C3H8 is smaller than that expected for a mechanism of H (or D) abstraction by tunneling through an Eckart barrier, suggesting a contribution of C–C bond cleavage to direct chemisorption. The lack of a kinetic isotope effect for the direct chemisorption of (CH3)2CD2 relative to C3H8 indicates that 1° C–H bond cleavage dominates over 2° C–H bond cleavage during the direct chemisorption of propane on Ir(110). The trapping behavior of each of these isotopomers of propane is approximately identical as a function of Ei, with ξ 〉0.9 at Ei=1.5 kcal/mol, ξ = 0.3 at Ei=20 kcal/mol, and ξ 〈 0.1 above Ei= 40 kcal/mol. © 1996 American Institute of Physics.

Notes: We have employed molecular beam techniques to investigate the dissociative chemisorption of cyclopropane on Ir(110) as a function of beam translational energy, Ei, from 1.5 to 48 kcal/mol, and surface temperature, Ts, from 85 to 1200 K. For Ts=85 K, c-C3H6 is molecularly adsorbed on Ir(110) with a trapping probability, ξ, of 0.97 at Ei=1.5 kcal/mol and ξ=0.90 at Ei=5 kcal/mol. For Ei≤5 kcal/mol, c-C3H6 is dissociatively adsorbed through a mechanism of trapping-mediated chemisorption, with initial probabilities of chemisorption, Pa, decreasing with increasing surface temperature from the intrinsic trapping probability at Ts=150 K, to Pa〈0.05 above Ts=1000 K. The activation energy for trapping-mediated chemisorption of c-C3H6, referenced to the bottom of the physically adsorbed well and attributed to C–C bond cleavage, is 3.6±0.2 kcal/mol. For Ei≥10 kcal/mol, direct dissociative chemisorption increasingly contributes to the overall measured initial probability of chemisorption of cyclopropane. The initial probability of direct dissociative chemisorption of c-C3H6 increases approximately linearly from Pa=0.1 at Ei=10 kcal/mol, to Pa=0.5 at Ei=45 kcal/mol. No isotope effect is observed for the direct dissociative chemisorption of c-C3D6 for beam translational energies of 17 to 48 kcal/mol, indicating that C–C bond cleavage is the initial reaction coordinate for direct chemisorption of cyclopropane on Ir(110). © 1996 American Institute of Physics.

Notes: A two-dimensional ultrahigh vacuum compatible positioner is presented. The positioner uses two piezoelectric inchworms which allow for motions of up to 1 cm with a precision of 4 nm mounted at right angles to each other in order to give two dimensions of motion. Images of three-dimensional In0.3Ga0.7As islands in cross section are presented to demonstrate the functionality of the positioner. It is found that motion towards the tip is smooth, while motion in the perpendicular direction is less smooth. © 1998 American Institute of Physics.

Notes: Vibrational electron energy loss spectroscopy combined with thermal desorption mass spectrometry have been used to investigate the adsorption and decomposition of NO2 on the Ru(001) surface. The results indicate that the initial NO2 adsorption is dissociative at 80 K. The reaction products, molecularly adsorbed NO and atomically adsorbed oxygen, passivate the surface, and subsequent (submonolayer) adsorption is molecular. The molecularly adsorbed NO2 is bound weakly (∼9 kcal/mol) through the nitrogen atom with C2v symmetry. With increasing exposure, the formation of N2O4 dimers in the condensed multilayer is observed.

Notes: High resolution electron energy loss spectroscopy and thermal desorption mass spectrometry have been employed to investigate the molecular chemisorption of N2 on both disordered and ordered overlayers of atomic oxygen on the Ru(001) surface, as well as the chemisorption of CO on overlayers of N2 on Ru(001). Pertinent results obtained for the adsorption of N2 on the clean Ru(001) surface are also presented for comparison. Disordered oxygen poisons a fraction of the surface to the subsequent adsorption of N2 whereas the N2 that does adsorb is indistinguishable from N2 on clean Ru(001). The fraction of the surface that is poisoned to the adsorption of N2 is approximately twice the fractional surface coverage of disordered oxygen. The p(2×2) overlayer of ordered oxygen adatoms, which is formed at a fractional surface coverage of 0.25, stabilizes the chemisorption of N2 into a new binding state with a heat of adsorption that is approximately 1.5 kcal/mol greater than any one observed for the adsorption of N2 on the clean surface. Coverage measurements indicate that this state results from the stoichiometric addition of one N2 molecule to each unit cell of the p(2×2)–O overlayer. Electron energy loss spectroscopic results suggest that this N2 binding state results from stabilization of the dominant σ donor contribution to the Ru–N2 bond, due to the presence of the electronegative oxygen adatoms of the p(2×2) overlayer. Measurements of the adsorption of CO on saturated overlayers of N2 show that N2 is displaced from the surface by increasing coverages of subsequently adsorbed CO. For low coverages of CO in the presence of N2, the observed value of ν(CO) is lower than observed under any conditions for the adsorption of CO alone on the Ru(001) surface. The N2 admolecules enhance the ability of the surface ruthenium atoms to backdonate electron density into the 2π orbital of coadsorbed CO under these conditions. At coverages of CO in excess of 0.10 monolayer, the results are consistent with CO island formation and segregation of N2 and CO admolecules into different local regions on the surface.

Notes: The formation and decomposition of formate species on the clean and on potassium-modified Ru(001) surfaces have been investigated with time-resolved vibrational spectroscopy and thermal desorption mass spectrometry (TDMS). Utilizing Fourier transform-infrared reflection absorption spectroscopy (FT-IRAS) we have characterized chemisorbed formate produced by the decomposition of formic acid on clean Ru(001), Ru(001)–((square root of)3×(square root of)3)R30° K and on a K-multilayer adsorbed on Ru(001). The vibrational spectra show that formate is adsorbed on both clean Ru(001) and Ru(001)–((square root of)3×(square root of)3)R30° K with C2v symmetry indicative of a bridged or bidentate species. There are, however, characteristic differences in the vibrational spectra, which indicate that for the Ru(001)–((square root of)3×(square root of)3)R30° K surface the formate is directly bound to potassium. The vibrational spectrum of the latter species is found to be in good agreement with that of bulk potassium formate adsorbed on Ru(001).Based on the agreement with literature data for bulk formate, we propose a bonding model for the potassium formate monolayer, which also accounts for the observed contraction of the potassium monolayer resulting from the compound formation. The thermal decomposition of the various formate overlayers has been monitored by simultaneous thermal desorption mass spectrometry and time-resolved FT-IRAS. This combination allows us to correlate the desorbing gas-phase products with the appearance and disappearance of surface intermediates. In the case of formate adsorbed on the clean Ru(001), the C–H and C–O bond cleavage reactions occur simultaneously, leading to the production of equal amounts of CO and CO2. The simultaneous observation of desorbing CO2 (TDMS) and of adsorbed CO (IR) confirms earlier work, which postulated a mechanism involving a coupling of the C–H and C–O bond cleavage reaction channels of two neighboring formates. The presence of potassium changes dramatically the reaction pathway of the formate as it suppresses the C–H bond cleavage channel, leaving CO and OH as the main decomposition products. Compound formation with potassium also leads to thermal stabilization of the formate in comparison to formate adsorbed on the clean surface. However, formate adsorbed on the potassium-modified ruthenium substrate is found to be thermally less stable than formate adsorbed on clean Ru(001).