The Surface Chemistry of 1‐Iodopropane Adsorbed on Pt(111)

Abstract

On Pt(111) at 110 K, 1‐iodopropane, C3H7I, adsorbs molecularly, but for doses below 1.7 × 1014 molecules cm−2, only H2 and I appear in thermal desorption. C–I bond cleavage occurs between 160 and 220 K, forming adsorbed n‐propyl, C(a)H2CH2CH3, and atomic iodine, based on temperature‐programmed desorption (TPD), high‐resolution electron energy loss spectroscopy (HREELS), and X‐ray photoelectron spectroscopy (XPS). n‐Propyl undergoes β‐hydride elimination forming propylene, with desorption peaks at 185 and 240 K. At 240 K, hydrogenation to propane also occurs. Some di‐σ bonded propylene, C(a)H2C(a)HCH3, remains at 240 K and it rearranges to propylidyne near 300 K. Atomic H, bound to Pt, recombines and desorbs at ca. 260 K. Further desorption of H2 is limited by C–H bond breaking and occurs over a broad temperature range with local maxima at ca. 280, 320, and 420 K, typical of propylidyne fragments on Pt. Atomic iodine desorbs in a broad feature at 825 K.

The Thermal Chemistry of 1-Chloro-3-Iodopropane (ClC3H6I) Adsorbed on Pt(111)

“…Finally, we give a more detailed discussion of the H 2 TPD results focusing on the four thermally activated C 3 adsorbates. Figure compares H 2 TPD for monolayer coverages of iodopropane, allyl bromide, 1-chloro, 3-iodopropane, and propylene. , The intensities of the four spectra are normalized to the height of the most intense peak, i.e., 425 K for three of the spectra and 260 K for iodopropane. For propylene, the peak at 425 K is well-established as arising from propylidyne dehydrogenation. , For both C 3 H 5 Br and ClC 3 H 6 I, there is clearly a sharp peak at 425 K, which we also attribute to propylidyne dehydrogenation.…”

“…The pressure behind the pinhole was measured by a MKS absolute pressure transducer and was typically 0.1 Torr. In previous work on C 3 H 7 I, this doser was calibrated to deliver 1.4 (± 0.15) × 10 13 C 3 H 7 I cm -2 s -1 Torr -1 . The flux calibration involved XPS measurements of C(1s) and I(3d) intensities for two standards, CO and atomic I, adsorbed at known absolute coverages of CO per surface Pt atom and I per surface Pt atom. ,, Adjusting for the molecular weight change, the doser will deliver 1.28 × 10 13 ClC 3 H 6 I cm -2 s -1 Torr -1 .…”

“…As for C 3 H 7 I, 25 there is also a strong lowtemperature propane peak indicating that hydrogenation processes compete successfully with dehydrogenation processes even though the H/C ratio in the adsorbate is low. The latter implies that significant concentrations of active hydrogen must be available at relatively low temperatures.…”

“…25 There are two attractive reaction paths leading to these products. First, -hydride elimination of propyl groups to form propylene and atomic hydrogen.…”

“…In the same temperature region, propane desorbs in the TPD of propyl iodide. 25 We propose that, as the temperature exceeds 250 K, the rates of hydrogenation and dehydrogenation accelerate, and there is a narrow temperature interval where the local concentrations of H (a) and C (a) H 2 C (a) HC(a)H 2 are both transiently high enough to form adsorbed C 3 H 6 , which either desorbs, if neighboring Pt atoms are occupied, or forms di-σ-bonded propylene if neighboring Pt atoms are available. In this process, adsorbed C 3 H y<5 species are also formed.…”