A. Brad Anton

The mechanism and kinetics of water formation on Pt(111)

Abstract We use modulated molecular beam relaxation spectroscopy (MMBRS) to identify the sequence of elementary reaction steps and to quantify the rate parameters for catalytic water (D 2 O) formation on Pt(111). This investigation is restricted to surface temperatures in the range 373 ⩽ T S ⩽ 723 K and oxygen and deuterium pressures on the order of 10 −5 mbar, where coverages of oxygen-containing species are low ( ϑ ⩽ 0.04 monolayer), and rate parameters can be assumed coverage-independent. Under these conditions, we find that adsorbed hydroxyl (OD a ) formation is rapid and (nearly) irreversible; the rate-limiting, majority pathway for water production is D a + OD a → D 2 O ( E a = 16 kcal/mol); and water production by OD a disproportionation. i.e. 2 OD a → D 2 O + O a , contributes as a minority pathway ( E a = 18 kcal/mol). From the results we construct potential energy diagrams that account for the energetics of nearly all elementary steps in these reactions.

A molecular beam study of the interaction between hydrogen and the Pt(111) surface

Hydrogen adsorption on and desorption from a Pt(111) surface is investigated in the temperature range 570 45°). The results can be described by a desorption function which is proportional to the square of the perpendicular energy: Sd = S1E⊥2. According to the principle of detailed balance the sticking probability Sa should show a cos4ϑi dependence in that case. However a marked deviation from this behaviour is observed which shows that a second interaction mechanism is involved which is not (or hardly) dependent on E⊥: Sa = S1E⊥2 + S2. It is found that S2 increases strongly with temperature whereas S1 decreases by about 35% when heating the surface from 670 to 1070 K. The present results are in good agreement with adsorption experiments performed at low temperature (160 K) which will be presented elsewhere. Both interaction mechanisms are discussed in terms of atomistic models. The temperature dependence of S1 seems to be in conflict with the activation barrier model, which has been proposed previously, suggesting that another process is responsible for the observed behaviour. The second mechanism (S2) cannot be explained by surface defects. We attribute this mechanism to adsorption in a molecular precursor state which becomes more efficient with increasing surface temperature.

A molecular beam study of the interaction of CO molecules with a Pt(111) surface using pulse shape analysis

Abstract CO adsorption/desorption on a clean Pt(111) surface has been studied using molecular beam relaxation spectroscopy (MBRS). In contrast to conventional MBRS experiments, lock-in tecniques (or Fourier analysis) have been used here only for a qualitative survey. Pulse shape analysis, which allows the deduction of more detailed information from the experimental data, is discussed in detail, compared to conventional Fourier analysis and used for the results presented here. Detailed analysis of the shape of the MBRS pulse waveform has been used to determine the rate constant for CO desorption, the fraction of scattered signal attributable to chemisorption and the time-of-flight distribution of the non-chemisorbed fraction. The rate constant for CO desorption was measured with MBRS in the temperature range 530–650 K. Complementary measurements under quasi-equilibrium conditions using thermal energy atom (helium) scattering (TEAS) were also performed to extend the desorption rate constant determination down to 430 K, allowing accurate determinations of k over six orders of magnitude. On the clean surface (coverage k = 1.5 × 10 5 T 3 s exp (−28.8/ RT s ) s −1 (or in Arrhenius form, k = 4.3 × 10 14 exp (−32.0/ RT s ) s −1 ), with R in kcal mol . The MBRS measurements have also afforded identification of three distinct interactions of CO molecules with the clean Pt(111) surface: chemisorption, direct scattering and a third interaction showing all characteristics which are expected for physisorption. Approximately 3% of the molecules incident at zero coverage desorb from this state independently of temperature in the range 530–650 K. The angular distribution of the directly scattered molecules shows a peak with its maximum shifted away from the specular direction toward the surface normal. The time-of-flight distribution of the directly scattered molecules is similar to that of the incident beam, though somewhat broadened. Both the shift away from the specular direction and the broadening are ascribed to inelastic effects. Measurements of the relative intensities of the chemisorbed and physisorbed signals as a function of surface temperature provide no support to the assignment of the clean surface physisorption state as a precursor to chemisorption.

Kinetics of water formation on Pt(111)

Measurements using modulated molecular beam relaxation spectroscopy identify ODa consumption as the rate‐limiting step for D2O formation on Pt(111). In the limit of low surface coverage, D2O is produced via two parallel pathways, Da+ODa→D2Oa with activation energy Ea≊16 kcal/mol, and 2ODa→D2Oa+Oa with Ea≊18 kcal/mol. From these results the reactions Da+Oa →ODa, Da+ODa→D2Oa, and D2Oa+Oa→2ODa are found to be exothermic with enthalpy changes ΔH≊−16, −9, and −8 kcal/mol, respectively, and ΔHOHa≊−61 kcal/mol is obtained for the heat of adsorption of hydroxyl on platinum. The general features of the reaction mechanism cause complex kinetic behavior, manifest as changes in the apparent order of the rate function and the effective activation energy as surface temperature and reactant pressures are varied. The implications for transient and steady‐state rate measurements are discussed.

An alternative for gas dosing in ultrahigh vacuum adsorption studies

We describe the design, construction, calibration, and operation of a dosing system for controlled exposures of solid surfaces to gases in ultrahigh vacuum. The doser includes seven parallel tubes in a hexagonal pattern, sized to give nearly uniform flux over a 6‐mm‐diam circle on the sample, and arranged to compensate for an oblique incidence direction. This arrangement gives an enhancement in the ratio of total flux to diffuse flux from the background in excess of 10:1, in spite of a 15.5‐mm distance between the doser and the sample.

Temperature programming for isothermal desorption rate measurements

We describe a digital feedforward/feedback controller for the implementation of rapid changes in the temperature of a small, resistively heated metal adsorbent in ultrahigh vacuum. The feedforward loop is based on a second‐order model of the system’s thermal response, modified by inclusion of a time delay to account for the characteristic time for heat conduction in the adsorbent, and the feedback loop includes proportional and derivative actions. These features allow the temperature of a nickel single crystal, 10 mm diameter and 1 mm thick, to be increased 80 K within approximately 4 s without overshoot, after which desorption rates can be measured at constant surface temperature with a mass spectrometer. The performance of the temperature controller and the advantages of isothermal data collection and analysis are demonstrated with CO desorption from Ni(110), for which the functional dependence of the rate on surface coverage at constant temperature identifies clearly the participation of a mobile precu...

Precursor and overlayer structural effects in the interaction of CO with Ni(110)

We present isothermal measurements of CO adsorption and desorption rates on Ni(110) that reveal clearly the participation of a precursor state in both processes at all coverages. Accounting for its effect allows activation energies and preexponential factors to be determined that are specific to the coverage regimes where c(8×2), c(4×2), and (2×1) ordered structures are known to form at low temperatures. The preexponential factors include a significant contribution attributable to relaxation of the adsorbed layer from one structure to another as desorption proceeds, and the relaxation process is also manifested in the relationship between measured activation energies and integral heats of adsorption. The results have general implications for adsorption systems involving series of ordered overlayers.

Magnitude and effects of adsorbent temperature nonuniformities in temperature‐programmed desorption

In a typical apparatus for temperature‐programmed desorption (TPD) measurements, heat is not added uniformly to the entire adsorbent volume, but rather is generated in support wires and conducts in at the contact points, forming a temperature gradient in the adsorbent during heating. Based on a simplified, one‐dimensional treatment, we determine the approximate magnitude of the temperature profile and investigate its deleterious effects in TPD simulations for first‐order kinetics. Analysis of the coupled equations governing surface temperature and coverage leads to a dimensionless group of parameters involving the heating rate, the desorption rate parameters, and the size and physical properties of the adsorbent that determines the magnitudes of systematic errors introduced by the temperature nonuniformity. The simulations allow a general expression to be developed for the maximum heating rate in TPD that will give estimates of E and ln(ν) reliable to within a few percent. The predictions of the model are...

Effects of the hydrogen-induced (1 × 2) surface reconstruction on the kinetics of ethylene hydrogenation on Ni(110)

Abstract We show that deuterium in the (1 × 2)-reconstructed phase on Ni(110) is highly active for ethylene hydrogenation, whereas the (1 × 1) surface is active only for decomposition of adsorbed ethylene in UHV. When ethylene-d4is adsorbed on the deuteriumsaturated, (1 × 2)-reconstructed Ni(110) surface, decomposition and hydrogenation proceed concurrently as ethane-d6 is evolved near 180 K in TPD. A detailed analysis of the competing rates of these processes at constant temperature gives an effective activation energy of 16 ± 2 kcal/mol and a preexponential factor of 5 × 1018 ± 2 ML−1 s− for hydrogenation, as compar 12 ± 1 kcal/mol and 3 × 1013 ± 1 ML−1 s−1 for decomposition. The relative magnitudes of the preexponential factors su significant entropy increase accompanies the conversion of surface sites from (1 × 2) to (1 × 1) symmetry, and this entropic contribution provides a driving force for hydrogenation that outweighs the unfavorable activation energy and ultimately determines the selectivity between the competing reaction pathways.

Variances of parameters obtained from nonlinear curve fits

An analytical expression is derived that quantifies the variances of parameters obtained from curve fits in terms of the variances of the individual data. In contrast to solutions of this problem offered previously, this solution accounts rigorously for nonlinear dependence of the fitting function on the fit parameters.