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

Abstract

CH 3 CHCOOH and CH 3 CHCOO have been theoretically predicted to be important in the decarboxylation or decarbonylation of propanoic acid on Pd(111). In the present study, we explore the possibility to prepare these two intermediates on Cu(100) and oxygen-predosed Cu(100) (O/Cu(100)), with CH 3 CHClCOOH as the precursor, and to investigate their adsorption geometries and reactions on the surfaces using X-ray photoelectron spectroscopy, reflection− absorption infrared spectroscopy, temperature-programmed reaction/desorption, and calculations of density functional theory. CH 3 CHClCOO and CH 3 CHCOO are suggested to be formed on Cu(100) at 250 K by heating the CH 3 CHClCOOH adsorption layers. However, CH 3 CHCOO can promptly recombine with the H from the deprotonation of CH 3 CHClCOOH to generate CH 3 CH 2 COO. The adsorbed propanoate decomposes mainly into H 2 , CO, and CO 2 at a temperature higher than ∼380 K. On O/Cu(100), CH 3 CHClCOO is the predominant species found in the dissociation of CH 3 CHClCOOH at 250 K. The preadsorbed O disables the CH 3 CHCOO hydrogenation to form CH 3 CH 2 COO due to the lack of adsorbed H. This intermediate is not stable above ∼400 K and decomposes into H 2 , H 2 O, CO, and CO 2 , probably with a small amount of acrolein or methylketene. The CH 3 CHCOO is shown theoretically to be bonded on Cu(100) via the unsaturated carbon and one of the oxygen atoms.

Surface Reaction Mechanisms: 3-Bromopropanoic and 2-Bromopropanoic Acids on Cu(100) and O/Cu(100)

“…Careful examination of the spectrum (210 K) further reveals the infrared peaks of 1292, 1363, 1417, and 1467 cm –1 that are likely in part originated from CH 3 CH 2 COO (propanoate), which is responsible for the infrared peaks observed in the spectra of 320–550 K. Note that the peaks at 1205 and 1255 cm –1 , from CH 2 CH 2 COO, are no longer detected at 320 K or higher temperatures. The infrared absorptions of CH 3 CH 2 COO on Cu(100) have been reported previously and are consistent with the infrared spectra (320–550 K) in Figure . The CH 3 CH 2 COO peaks can be assigned as follows: 1303 cm –1 to CH 2 wagging, 1361 cm –1 to CH 3 symmetric bending, 1417 cm –1 to COO symmetric stretching, and 1465 cm –1 to CH 3 antisymmetric bending modes.…”

“…However, a completely different infrared absorption behavior occurs upon heating to 320 K, with peaks at 1301, 1363, 1407, 1429, 1467, and 1633 cm –1 . These peaks are from CH 3 CH 2 COO and CH 2 CHCOO (propenoate), with evidence of consistent frequencies reported previously. , The CH 3 CH 2 COO contributes to the absorptions at ∼1301, 1363, 1429, and 1467 cm –1 and the CH 2 CHCOO to those at ∼1363, 1407, 1429, and 1633 cm –1 , with vertical adsorption geometries. The first three peaks of CH 2 CHCOO are assigned to CH 2 scissoring and/or COO symmetric stretching modes and the 1633 cm –1 to CC stretching vibration. , Similar absorption features are observed in the 400 K spectrum.…”

“…In the previous study of CH 3 CH 2 Br/Cu­(100), the C–Br bond is found to dissociate at ∼180 K . However, the C–Br breakage of BrCH 2 CH 2 COOH already occurs on Cu(100) at 110 K. In the O 1s spectrum, the peaks at 531.7 and 533.1 eV can be clearly ascribed to the −COO (carboxylate) and −COOH groups, respectively, which are consistent with the reported systems of CH 3 COOH/Cu­(110) and CH 3 CHClCOOH/Cu­(100). , The ratio of −COO/–COOH is found to be ∼1.3. For propanoic acid on Pd(111) and acetic acid on Cu(110), loss of the acidic H to form the corresponding carboxylates occurs near 235 K. , The result of Br 3d and O 1s emissions shows the mutual effect of the two functional groups of BrCH 2 CH 2 COOH on Cu(100), moving the dissociation of the C–Br and O–H bonds toward lower temperatures.…”

“…The CH 3 CH 2 COO peaks can be assigned as follows: 1303 cm –1 to CH 2 wagging, 1361 cm –1 to CH 3 symmetric bending, 1417 cm –1 to COO symmetric stretching, and 1465 cm –1 to CH 3 antisymmetric bending modes. Because no COO antisymmetric stretching peak near 1600 cm –1 is detected, the propanoate is suggested to be adsorbed at upright geometry . Above 500 K, the decrease of CH 3 CH 2 COO agrees with the product evolution shown in Figure .…”

“…The relatively strong, broad peak at 286.2 eV has contributions from the BrCH 2 CH 2 – of the BrCH 2 CH 2 COOH and BrCH 2 CH 2 COO and from the 2 CH 2 of 3 CH 2 2 CH 2 1 COOH and 3 CH 2 2 CH 2 1 COO. The 288.8 and 290.4 eV peaks are from the C 1s emission of −COO and −COOH. , Note that the 290.4 eV peak may have a contribution of CO 2(ad) from background adsorption . The relatively small feature at 283.6 eV suggests that CH 2 CH 2 COOH and CH 2 CH 2 COO are minor at 110 K. In addition, the ratio of BrCH 2 –/Br (ad) , 1.6, corresponds to the ratio of (BrCH 2 CH 2 COOH + BrCH 2 CH 2 COO)/(CH 2 CH 2 COOH + CH 2 CH 2 COO).…”