S. Akhter

A static SIMS/TPD study of the kinetics of methoxy formation and decomposition on O/Pt(111)

The kinetics and mechanism of the decomposition of methanol (CH3OD) on oxygen-covered Pt(111) were studied using static secondary ion mass spectrometry (SIMS) and temperature programmed desorption (TPD). The initial sticking coefficient and the saturation first layer coverage of CH3OD are unity and 0.36 ML, respectively. The maximum amounts decomposed in TPD on O/Pt(111) and clean Pt(111) are 0.19 and 0.047 ML, respectively. At low methanol coverages (< 0.05 ML) on O/Pt(111) the only reaction products were CO2, H2O and D2O, whereas at saturation CO, H2O, D2O and H2 were observed. The decomposed amount did not saturate at or before the onset of molecular methanol desorption, but increeased monotonically until saturation of the first layer. No oxygen exchange was observed between CH3OD and preadsorbed 18O. A decomposition mechanism involving methoxy and hydroxyl type species is proposed. Methanol coverages as low as 0.002 ML could be detected with SIMS. The CH3+ ion was the most intense ion in the positive SIMS spectrum of both methanol and methoxy. Larger ion clusters such as (CH3OD)n+ (n = 2, 3) developed successively at specific multilayer coverages. At low coverages on O/Pt(111), methoxy formation occurs above 125 K and its decomposition becomes detectable above 150 K during temperature programming. In the isothermal mode, the SIMS CH3+ ion was used to follow the kinetics. Over the temperature range 120–145 K, the second order Arrhenius rate parameters for methoxy formation are E = 5.5±0.7 kcal/mol and A = 1.5×10−7±0.6 cm2/s·molecule for an initial methanol coverage of 0.05 ML. Methoxy decomposition was studied in the temperature range 150–165 K and at an initial coverage of 0.015 ML. The first order kinetic parameters, E = 11.4±0.5 kcal/mol and A = 5.3×1013±1 s−1 were derived. Advantages and limitations of using SIMS as a tool for kinetic studies are discussed.

The formation and decomposition kinetics of alkylidynes on Pt(111)

Abstract The formation and decomposition kinetics of ethylidyne and propylidyne on Pt(111), were studied using static secondary ion mass spectrometry and temperature programmed desorption. For the maximum amounts of dissociatively adsorbed ethylene and propylene formed during adsorption at 200 K and subsequent temperature programmed desorption, the following activation energies ( E ) and pre-exponential factors ( A ) are determined: (a) for ethylidyne formation: E = 17±1 kcal mol −1 and A = 1×10 12±1 s −1 ; (b) ethylidyne decomposition: E = 27 ±2 kcal mole −1 and A = 6×10 11±1 s −1 ; (c) propylidyne formation: E = 17.5 ± 2 kcal mol −1 and A = 7×10 12±1 s −1 ; and (d) propylidyne decomposition: E = 22.5±2 kcal mol −1 and A = 4×10 11 ± 1 s −1 .

The adsorption of ammonia on Ru(001) and its effect on coadsorbed CO

Abstract The interaction of ammonia with a Ru(001) surface and its effect on coadsorbed CO have been studied by high resolution electron energy loss spectroscopy (HREELS) and temperature programmed desorption (TPD). In HREELS, for coverages below one monolayer we observed two states of ammonia, which differed only in that one showed a Ru-N stretch, while the other did not. No evidence was found for the dissociation of ammonia. CO postdosed on a high coverage (≅ 0.25 ML) of ammonia gave a single but broad C-O stretch band at 1560 cm −1 . With less ammonia, another loss developed at 1960 cm −1 . The effect of ammonia on coadsorbed CO is dominated by a change in the CO adsorption site. Both short- and long-range electronic effects are also involved.

The adsorption and decomposition of benzene on Ni(100) and the effects of pre-adsorbed C, O, H, and Co*

Benzene-d6 adsorption and decomposition on Ni(100) has been studied with temperature programmed desorption and Auger electron spectroscopy. Decomposition in the neighborhood of 472 K produces C(a) and D2(g), and occurs with simultaneous benzene desorption at sufficiently large coverages. Low temperature TPD spectra of benzene from clean Ni(100) and from several precovered Ni(100) substrates were similar. The substrates included are oxygen-, hydrogen-, carbon monoxide-, and carbon-covered Ni(100). All of the coadsorbates except hydrogen completely inhibit benzene decomposition. Hydrogen presaturation of the Ni(100) surface greatly reduces benzene decomposition, and hydrogen either reacts with or is displaced by post-dosed benzene to a small extent. Isotopic labelling experiments on clean Ni(100) showed that low temperature desorption states can freely interconvert. Features of the desorption and decomposition of benzene near 472 K indicate the existence of a variety of adsorption sites with a distribution of benzene adsorption strengths and decomposition facilities. A qualitative model involving benzene adsorption with a variety of bonding energies is presented.

Stabilization of C2Dx fragments by CO on Ni(100)

Abstract The interactions of CO with C2D4, C2Dx (vinyl D2CCD, acetylinic/vinylidene C2D2, acetylide C2D) and CD fragments were investigated on Ni(100) using temperature programmed desorption (TPD), Auger electron spectroscopy (AES), X-ray and UV photoelectron spectroscopies (XPS, UPS) and work function (Δφ) measurements. On clean Ni(100), the C2Dx species decomposed between 190 and 300 K (at low coverage). In the presence of saturation CO, each of these fragments was stable to 375 K ( ∼ 27 kcal mol ) where decomposition produced simultaneous desorption of D2 and CO. About two CO molecules stabilized each C2Dx fragment. No evidence for direct chemical bonding between C2Dx and CO was found. Based on the XPS and TPD data, the stabilization is attributed to (i) blocking of dissociation sites by CO in the immediate neighborhood of the C2Dx fragment and (ii) repulsion between CO and C2Dx which reduced the C2DxNi binding strength. These results are compared with observations during Fisher-Tropsch synthesis on supported metal catalysts.

Water formation on Pt(111): Reaction of an intermediate with H2(g)☆

Abstract The formation of water from an intermediate, I, and H2 on Pt(111) has been studied using static secondary ion mass spectrometry (SSIMS) at temperatures where the product water remains adsorbed (

Methyl formation from methanol decomposition on Pd(111) and Pt(111)

The decomposition of CH3OH adsorbed on Pd{111} and Pt{111} is compared as the surface is heated between 100 and 500 K. Using secondary ion mass spectrometry (SIMS) and thermal programmed desorption (TPD) it is suggested that an anomalous CH3+ ion signal observed previously by Akhter and White on oxygen precovered Pt{111} arises from the formation of a surface CH3 species resulting from activation of the C-O bond of CH3OH. This interpretation stems from a recent observation by Levis, Zhicheng and Winograd that CH3OH decomposes to CH3, OH and OCH3 on clean Pd{111} between 100 and 300 K. The results are discussed in terms of the relative ability of these metals to synthesize CH3OH from CO and H2.

Potassium and its coadsorption with carbon monoxide on Pt(111): A SSIMS/TPD study

Temperature programmed desorption (TPD) and static secondary ion mass spectrometry (SSIMS) have been used to study the chemisorption properties of carbon monoxide on potassium-predosed Pt(111). Nonexponential secondary ion yields of potassium containing ions indicate a local interaction between potassium and nearest-neighbor CO molecules. In TPD, two different CO desorption states are observed in the presence of potassium. The high temperature CO TPD state is accompanied by simultaneous potassium desorption. The magnitude of the coincident potassium and CO TPD peak is related to the amount of CO in the high temperature state. Long-range (beyond nearest-neighbor) as well as short-range (nearest-neighbor) potassium effects on adsorbed CO are supported.

CH bond cleavage for ethylene and acetylene on Ni(100)

Abstract The decomposition of isotopically labeled acetylene and ethylene was studied on Ni(100) using static secondary-ion mass spectrometry (SSIMS) and temperature programmed desorption (TPD). Both acetylene and ethylene adsorb molecularly at 90 K. Only H2 and the parent molecule are found in TPD. There is a strong isotope effect in the molecular ethylene desorption and in its decomposition to form vinyl (CHCH2) species. The vinyl subsequently decomposes to form acetylide (CCH) and there is no isotope effect in the decomposition of the latter. As the coverage of ethylene increases, there is no inhibition of initial vinyl formation, but strong inhibition of its decomposition. For acetylene, there is an isotope effect in its decomposition to form acetylide but, as for ethylene. none in the decomposition of acetylide. The TPD spectra of H2 from surfaces saturated with ethylene and acetylene are very different: there is much more H2 at high temperatures for acetylene. This difference, which disappears for low coverages, is discussed in terms of CH bond breaking in two distinct local environments — the first containing one or more vacant Ni sites and the second containing only carbon-covered Ni sites and requiring higher activation energy.

The adsorption and decomposition of BIS(Benzene)chromium on Ni(100)

Abstract We have studied the bis(benzene) chromium/Ni(100) chenusorption system using TPD, AES, XPS, UPS, and exchange of isotopically labeled molecules. Low coverages of the metal complex are completely decomposed in TPD to produce C (a) , Cr (a) , and H 2(g) . At higher coverages, molecular benzene desorption is observed at ≅ 480 K, and, at still higher coverages, multilayer and monolayer molecular bis(benzene)chromium desorption occur at ≅ 250 and 300 K, respectively. Post-dosed C 6 D 6 exchanges with C 6 H 6 into the monolayer desorption, indicating along with XPS evidence that monolayer bis(benzene)chromium undergoes ligand separation below 300 K. Uncomplexed benzene desorbs at ≅ 160 K from multilayer bis(benzene) chromium; explanations for this are discussed. UPS of monolayer and multilayer benzene and bis(benzene)chromium are compared.