Thermal Chemistry of 1,4-Difluoro-2-butenes on Pt(111) Single-Crystal Surfaces

Abstract

The thermal chemistry of cis- and trans-1,4-difluoro-2-butenes on Pt(111) single-crystal surfaces was characterized by temperature programmed desorption (TPD) and reflection−absorption infrared spectroscopy (RAIRS). The study was motivated by the possibility of differentiating the two isomers directly by mass spectrometry, something that is not possible with the nonfluorinated olefins. On the clean surface a preference was identified for the conversion of the cis isomer to its trans counterpart, the opposite to what had been previously reported with regular 2-butenes on hydrogen (or deuterium)-predosed Pt(111). Indeed, the formation of trans-1,4-difluoro-2-butene from the adsorbed cis isomer occurs at around 260 K, as identified directly by RAIRS and in the gas phase by mass spectrometry. Some cis is also formed from the trans, but only in a small yield and at much higher temperatures (∼315 K). The preference for trans-to-cis conversion with the nonfluorinated olefins may therefore be explained by a reversal in adsorption stability induced by coadsorbed hydrogen. It was also determined that significantly more hydrogenation of the fluorinated olefins to their fluorinated alkanes occurs upon coadsorption with hydrogen or deuterium compared with what is seen with the regular olefins. Finally, the inductive effect of the fluorine substitutions leads to some changes in the high-temperature conversion of the adsorbed species.

Ethylene hydrogenation catalysis on Pt(111) single-crystal surfaces studied by using mass spectrometry and in situ infrared absorption spectroscopy

“…The RAIRS data were acquired by using a Bruker Equinox 55 FT-IR spectrometer [58]. In our setup, the IR beam is extracted from the inside of the FT-IR instrument, directed through a polarizer and the NaCl windows on one side of the high-pressure cell, and focused through onto the sample at grazing incidence (∼85°).…”

New advances in the use of infrared absorption spectroscopy for the characterization of heterogeneous catalytic reactions

“…7) have been investigated this way. 182,183,[186][187][188] The surface chemistry of several more complex organics, molecules containing oxygen and/or nitrogen atoms, has also been studied by RAIRS. In terms of alcohols, the first thermal decomposition step on metals typically is the loss of the hydroxo hydrogen and the formation of alkoxide surface intermediates, a conversion that takes place at low temperatures, sometimes immediately upon adsorption, and that is often assisted by coadsorbed atomic oxygen.…”

Shape‐Controlled Nanostructures in Heterogeneous Catalysis

“…In this section, an example from our own laboratory is provided on how this synergy between fundamental studies using modern surface-science techniques [108,[274][275][276][277][278][279][280] together with quantum mechanical calculations [108,281] and specific synthetic nanotechnologies [107][108][109] may work. In general, it is desirable to start with a well-defined hypothesis on the requirements of the specific process to be addressed to select the best synthetic strategy.…”

“…The conversion of olefins with H 2 on transition metals has long been explained by the socalled Horiuti-Polanyi mechanism, which proposes a stepwise hydrogenation of the adsorbed olefins, first to an alkyl surface intermediate and then to the alkane. [278] Upon starting from the 2-butyl surface intermediate (produced by thermal activation of 2-iodobutane) [284] to test the b-hydride elimination step directly, the trans isomer was clearly produced on the surface. The cis-trans isomerization cannot easily be detected directly by TPD because both isomers display almost identical cracking patterns by mass spectrometry (the detection technique used in TPD), but the information can be extracted indirectly by taking advantage of the realization that, in the case of the experiments with D 2 coadsorption, each cis-trans interconversion is accompanied by an exchange of one of the central H atoms by a D atom and that, therefore, the H-D exchange reaction can be used as a proxy to follow the cis-trans transformation.…”

“…[275,276,280] Moreover, in experiments with 1,4-difluoro-2-butenes on clean Pt (111), in which F substitutions were used to facilitate the differentiation between the cis and trans isomers in TPD, a reversed preference for cis-to-trans conversion was seen. [278] Upon starting from the 2-butyl surface intermediate (produced by thermal activation of 2-iodobutane) [284] to test the b-hydride elimination step directly, the trans isomer was clearly produced on the surface. [276,280] To explain the changes in surface chemistry induced by hydrogen co-adsorption, DFT calculations were performed.…”