Reaction Pathways of 2-Iodoacetic Acid on Cu(100): Coverage-Dependent Competition between C−I Bond Scission and COOH Deprotonation and Identification of Surface Intermediates

Abstract

The chemistry of 2-iodoacetic acid on Cu(100) has been studied by a combination of reflection-absorption infrared spectroscopy (RAIRS), X-ray photoelectron spectroscopy (XPS), temperature-programmed reaction/desorption (TPR/D), and theoretical calculations based on density functional theory for the optimized intermediate structures. In the thermal decomposition of ICH(2)COOH on Cu(100) with a coverage less than a half monolayer, three surface intermediates, CH(2)COO, CH(3)COO, and CCOH, are generated and characterized spectroscopically. Based on their different thermal stabilities, the reaction pathways of ICH(2)COOH on Cu(100) at temperatures higher than 230 K are established to be ICH(2)COOH --> CH(2)COO + H + I, CH(2)COO + H --> CH(3)COO, and CH(3)COO --> CCOH. Theoretical calculations suggest that the surface CH(2)COO has the skeletal plane, with delocalized pi electrons, approximately parallel to the surface. The calculated Mulliken charges agree with the detected binding energies for the two carbon atoms in CH(2)COO on Cu(100). The CCOH derived from CH(3)COO decomposition has a CC stretching frequency at 2025 cm(-1), reflecting its triple-bond character which is consistent with the calculated CCOH structure on Cu(100). Theoretically, CCOH at the bridge and hollow sites has a similar stability and is adsorbed with the molecular axis approximately perpendicular to the surface. The TPR/D study has shown the evolution of the products of H(2), CH(4), H(2)O, CO, CO(2), CH(2)CO, and CH(3)COOH from CH(3)COO decomposition between 500 and 600 K and the formation of H(2) and CO from CCOH between 600 and 700 K. However, at a coverage near one monolayer, the major species formed at 230 and 320 K are proposed to be ICH(2)COO and CH(3)COO. CH(3)COO becomes the only species present on the surface at 400 K. That is, there are two reaction pathways of ICH(2)COOH --> ICH(2)COO + H and ICH(2)COO + H --> CH(3)COO + I (possibly via CH(2)COO), which are different from those observed at lower coverages. Because the C-I bond dissociation of iodoethane on copper single crystal surfaces occurs at approximately 120 K and that the deprotonation of CH(3)COOH on Cu(100) occurs at approximately 220 K, the preferential COOH dehydrogenation of monolayer ICH(2)COOH is an interesting result, possibly due to electronic and/or steric effects.

Comparative Study on the Reaction Pathways of 2-Chloropropanoic Acid on Cu(100) and O/Cu(100)

“…The O 1s spectrum can be fitted with three peaks located at 531.3, 532.7, and 533.7 eV. Previously, the respective O 1s binding energies have been found at 532.4 eV (broad) and 532.9 eV for the acetic acid on Cu(110) and the iodoacetic acid on Cu(100). , Upon deprotonating the acetic acid on Cu(110) to form acetate, the O 1s peak is shifted to 531.6 eV . Acetate on hydroxylated NiO(111) has an O 1s binding energy of 531.2 eV .…”

“…Previously, the respective O 1s binding energies have been found at 532.4 eV (broad) and 532.9 eV for the acetic acid on Cu(110) and the iodoacetic acid on Cu(100). 11,12 Upon deprotonating the eV has been reported for H 2 O on Cu(100) and Ni(110). 14,15 Four deconvoluted peaks are obtained at 284.9, 286.2, 287.8, and 289.7 eV after curve-fitting to the C 1s spectrum (Figure 2).…”

Adsorption and Reactions of ICH2CN on Cu(100) and O/Cu(100)

“…No ICH 2 CN desorption below 1.2 L indicates that ICH 2 CN dissociates on the surface at 180 K or during the heating course in the TPD experiment. The previous studies regarding the chemistry of alkyl iodides on copper single crystals indicated that C−I bond scission is the primary reaction step, which occurs below 180 K, resulting in the formation of adsorbed iodine atoms and alkyl groups.29−31 Indeed, the separate X-ray photoelectron spectroscopic study of ICH 2 CN/Cu(100), as shown in supporting FigureS1, confirms the C−I breakage at 180 K. FigureS1shows the I 4d XP spectra taken after ICH 2 CN adsorption (2.0 L) on Cu(100) at 180 K, followed by annealing to 210 K. There are four resolved peaks in the 180 K spectrum that can be attributed to ICH 2 CN molecules (50.8 and 52.5 eV) and iodine atoms (49.5 and 51.2 eV) on Cu(100) 32,33. The C−I bonds dissociate completely at 210 K.Spectroscopic Evidence of Coverage-Dependent CH 2 CN Adsorption Structures.…”

Bonding Structure, Dehydrogenation, and Dimerization of 1,3-C6H4 from Decomposition of 1,3-C6H4I2 on Cu(100)

“…In the 370 K spectrum, the peaks of I 4d3/2 at 51.3 eV and I 4d5/2 at 49.6 eV are ascribed to I adsorbed on Cu(100) in accordance with previously reported assignments. 19 This result, together with the absence of other iodine signals, also indicates that both CÀI bonds in 1,3-C 6 H 4 I 2 have dissociated on Cu(100) at 370 K, leaving atomic iodine on the surface. The 230, 290, and 320 K spectra can be nicely fitted with four peaks.…”

“…However, none of hydrocarbons was detected, but H 2 . In Figure 5 showing the TPR/D spectra of hydrogen, it is found that H 2 starts to desorb at approximately 550 K and extends up to ∼900 K. This result suggests that no H atoms are present on the surface before 550 K, i.e., the CÀH bonds remaining intact before 550 K, because it has been known that recombination of H atoms on Cu(100), forming H 2 , occurs around 300 K. 19 After considering that the two CÀI bonds of 1,3-C 6 H 4 I 2 have dissociated at 370 K, as shown in Figure 2, C 6 H 4 is suggested to be the species present on the surface at this temperature. In the inset of Figure 5, the yield of H 2 desorption increases linearly with exposure of 1,3-C 6 H 4 I 2 and levels off as the exposure is approximately above 3.0 L. Figure 6 shows the Auger electron spectra obtained after adsorption of 3 L 1,3-C 6 H 4 I 2 on Cu(100) at 180 K, followed by briefly heating the surface to 450 and 980 K. Both the carbon (∼276 eV) and iodine (∼512 and 521 eV) peaks are detected at 450 K. The carbon is ascribed to the presence of C 6 H 4 from CÀI bond breakage of 1,3-C 6 H 4 I 2 .…”