A. C. F. Kong

The coverage dependence of the pre-exponential factor for desorption

Abstract The pre-exponential factor for desorption υ 0 is seldom measured as other than low surface coverage θ and is typically assumed to be independent of θ. Variations in υ 0 with θ are critically important in describing surface equilibrium and rate processes and in testing theories of desorption. It has often been overlooked that the functional depedence of υ 0 on θ may be readily determined from thermodynamic data used to determine isosteric heats of adsorption if the sticking coefficient S (θ) is known. We present υ 0 (θ) values for approximately 45 adsorption species calculated from or reported in the literature for approximately adsorption systems. It is found that variations in υ 0 by factors of 10 6 are often observed. In approximately half of these examples υ 0 decreases by a factor of 10 3 or more and in the rest of the systems υ 0 is constant or decreases by less than 10 3 ; in very few cases does υ 0 increase significantly with coverage. Current theoretical models for υ 0 are briefly reviewed, and it is shown that none predicts such large coverage variations. However, analysis of low coverage models suggest that adsorbate-induced changes in the surface phonon spectrum may play a critical role.

Surface diffusion of hydrogen and CO on Rh(111): Laser‐induced thermal desorption studies

Surface diffusion of hydrogen, deuterium, and CO on Rh(111) has been investigated by laser‐induced thermal desorption (LITD) and compared with previous results for these species on Pt(111) and on other metals. As the coverage θ of deuterium increases from 0.02 to 0.33, the preexponential factor D0 remains constant at 8×10−2 cm2/s, but the diffusion activation energy Ediff rises from 3.7 to 4.3 kcal/mol. Ediff for hydrogen is 0.6 kcal/mol lower than for deuterium, consistent with the difference in zero‐point energy. For CO, Ediff =7 kcal/mol at all coverages, but D0 rises from 10−3 to 10−2 cm2/s between θ=0.01 and 0.40. Values of Ediff for these adsorbates vary by several orders of magnitude for surfaces on which heats of adsorption are essentially identical. These differences appear to correlate with differences in heats of adsorption in different binding states which form saddle point configurations in surface diffusion. Ediff is found to be nearly identical to the reaction activation energies for the CO...

Adsorption and desorption of NO, CO and H2 on Pt(111): Laser-induced thermal desorption studies

The adsorption, desorption, and equilibrium behavior of NO. CO, and H2 have been investigated using laser-induced thermal desorption (LITD). A pulsed infrared laser beam is focused onto a Pt(111) surface, causing very rapid heating ( ≈ 1010 K/s) and nearly instantaneous desorption of surface species. The desorbed molecules produce a pressure rise in the vacuum system that is proportional to the coverage and is detected with a quadrupole mass spectrometer. For surface temperatures between 160 and 280 K, S0 for D2 increases by ≈ 50% and for H2 by ≈ 70%. Low-coverage sticking coefficients S0 for both NO and CO are found to be independent of surface temperature in the range 160 to 430 K and indicate adsorption through a precursor state. Isosteric heats of adsorption of NO and CO decline as the coverage increases from initial values of 26 ± 2 and 32 ± 2 kcal/mol to 12 ± 2 and 16 ± 2 kcal/mol, respectively. By combining the equilibrium and adsorption data, pre-exponential factors for desorption have been determined as a function of coverage. Laser-induced surface damage does not appear to be significant in these results. However, repeated desorption without high temperature annealing causes a drop in S0 for NO and CO by 30% and a rise in S0 for D2 by a factor of five. Measurements made with LITD agree well with available literature values obtained by other methods. However, LITD has several important advantages, especially the capability for the repeated measurement of surface coverages of a wide variety of adsorbates at high repetition rates in the presence of background pressures up to 10−4 Torr.

Adsorption and desorption of CO and H2 on Rh(111): laser-induced desorption

Abstract Isosteric heats of adsorption Δ H ad of CO and sticking coefficients S for CO and H 2 on Rh(111) are determined by laser-induced thermal desorption (LITD) in which a pulsed laser beam is focused onto the surface, and rapid local heating yields a desorption signal that is proportional to the adsorbate coverage θ . Δ H ad for CO falls from 32.0±2 kcal/mol at low coverage to 14 kcal/mol at saturation, and the desorption pre-exponential factor v d decreases from 10 14±0.5 to 10 10 s -1 . Δ H ad , v d , and S of CO all decline sharply above θ = 0.2, corresponding to the occupation of a second binding state. Sticking coefficients for CO and hydrogen indicate precursor intermediates in adsorption.

CH3NO2 decomposition on Pt(111)

Abstract The adsorption and decomposition of CH 3 NO 2 on Pt(111) have been studied using TPD, XPS and AES. TPD following adsorption at 300 K produces complete decomposition with > 85% of the C and N desorbing as C 2 N 2 ≈10% as CO and NO, 1–2% as HCN, CH 4 and CO 2 and 2 , NH 3 , or NO 2 . The major decomposition path of adsorbed CH 3 NO 2 on Pt(111) is therefore dissociation of CH bonds (which forms H 2 and H 2 O) and NO bonds (which forms H 2 O, CO and NO) to leave adsorbed CN which desorbs as C 2 N 2 between 750 and 1200 K. Adsorption at 100 K produces monolayer and multilayer peaks of CH 3 NO 2 at 165 and 141 K respectively, but decomposition products and product distributions are nearly identical to those following adsorption at 300 K. Hydrogen from CH 3 NO 2 desorbs as several peaks at 520–700 K, while H 2 alone desorbs at ≈ 400 K which suggests that the hydrogen dissociation in CH 3 NO 2 occurs mostly above 500 K. CO 2 and H 2 O also desorb as reaction limited peaks at 400–600 K. No carbon or other residues are left on the surface after heating above 1200 K. Calibration of TPD peak areas, AES C and N peak heights and XPS N(1s) peak area give a saturation monolayer density of (2.2 ± 0.4) × 10 14 molecules/cm 2 . The adsorption and decomposition behavior of CH 3 NO 2 is thus very similar to CH 3 NH 2 on Pt(111) but quite different to these molecules on Ir, Ru and W which give almost complete C−N bond cleavage to yield mostly total dissociation products and carbon residues.

Investigations of adsorption on Pt and Rh by laser‐induced desorption

The adsorption, desorption, equilibrium, and diffusion behavior of NO, CO, and H2 on Pt(111) and Rh(111) have been investigated using laser‐induced thermal desorption. A pulsed infrared laser beam is focused onto a single‐crystal surface, causing very rapid heating (∼1010 K/s) and nearly instantaneous desorption of surface species which are detected with a quadrupole mass spectrometer. Isosteric heats of adsorption of NO and CO are found to decrease as the coverage increases from 26±2 to 12 kcal/mol and from 32±2 to 16 kcal/mol, respectively. By combining the equilibrium and adsorption data, preexponential factors for desorption are found to decrease by a factor of 107 near saturation. Surface diffusion of deuterium and hydrogen on Pt(111) varies strongly with coverage. As the initial surface coverage θ of deuterium is varied from 0.001 to 0.33, the diffusion activation energy Ediff falls from 12 to 7 kcal/mol, and the preexponential factor D0 falls from 3×104 to 0.5 cm2/s. Concentration profiles measured...

Laser-induced desorption of polyatomic molecules with a CO2 laser

Abstract Laser-induced thermal desorption (LITD) has been increasingly employed as a tool for investigating surface processes. In LITD, a pulsed laser beam that is focused onto a surface induces a rapid temperature rise that causes desorption. In spite of the success enjoyed by the CO 2 laser in studies of diatomic molecules, its use with polyatomic molecules is shown to be severely limited by laser-induced dissociation. In desorption experiments with CH 3 OH, HCOOH, CH 3 NH 2 and NH 3 dissociation occurs only when the laser frequency coincides with an infrared absorption band of the molecule. Fragmentation may take place either on the surface or in the dense gas phase present above the surface during the laser pulse.