Ligand Effects at Ionic Liquid-Modified Interfaces: Coadsorption of [C2C1Im][OTf] and CO on Pd(111)

Beitrag in einer Fachzeitschrift
(Originalarbeit)


Details zur Publikation

Autor(en): Bauer T, Mehl S, Brummel O, Pohako-Esko K, Wasserscheid P, Libuda J
Zeitschrift: Journal of Physical Chemistry C
Jahr der Veröffentlichung: 2016
Band: 120
Heftnummer: 8
Seitenbereich: 4453-4465
ISSN: 1932-7455


Abstract


Thin films of ionic liquids (ILs) can be used to tune the activity and selectivity of heterogeneous catalysts and electrocatalysts (solid catalysts with IL layer, SCILL). In several cases it has been found that these IL layers have a strong beneficial effect on the selectivity. To explore the molecular origin of this phenomenon, we have performed a model study on ultrahigh-vacuum conditions. We have investigated the coadsorption of CO and the room-temperature IL [CCIm][OTf] (1-ethyl-3-methylimidazolium trifluoromethanesulfonate) on Pd(111) by time-resolved infrared reflection-absorption spectroscopy, temperature-programmed reflection absorption spectroscopy, and temperature-programmed desorption. We find that the [OTf] anion adsorbs specifically to the Pd(111) surface via the SO group, thereby adopting a well-defined orientation with the molecular axis oriented perpendicular to the surface. At higher IL coverage, unspecific but oriented adsorption occurs, before the orientation is successively lost in the multilayer region. Upon coadsorption of [CCIm][OTf] on a CO-saturated Pd(111) surface at 300 K (θ = 0.5) a well-defined coadsorption layer is formed without any loss of adsorbed CO and with very similar CO site occupation. In the coadsorption layer [OTf] is specifically adsorbed between the CO with a molecular orientation perpendicular to the surface. Thus, a dense and homogeneous coadsorption layer is formed in which Pd surface atoms are simultaneously coordinated to both CO and [OTf] ions. From this compressed layer, CO desorbs with peak temperature at 410 K (heating rate, 3.3 K/s). Above this temperature, a low-coverage coadsorption phase of CO and surface-adsorbed IL resides, with little influence of the IL on the CO desorption temperature (peak temperature, 470 K). Coadsorption of the IL gives rise to a pronounced red shift of the CO stretching frequency in the order of 50 cm. The effect originates from the electrostatic interfacial field (Stark effect) generated by the coadsorbed IL and, at high coverage, possibly from additional short-range interactions. The results show that ILs form dense and well-defined mixed phases with strongly adsorbing reactants such as CO, in which a specifically adsorbed carpet of IL anions directly modifies the active surface sites by ligand-like effects.



FAU-Autoren / FAU-Herausgeber

Bauer, Tanja
Professur für Physikalische Chemie
Brummel, Olaf Dr.
Professur für Physikalische Chemie
Libuda, Jörg Prof. Dr.
Professur für Physikalische Chemie
Mehl, Sascha
Professur für Physikalische Chemie
Pohako-Esko, Kaija
Lehrstuhl für Chemische Reaktionstechnik
Wasserscheid, Peter Prof. Dr.
Lehrstuhl für Chemische Reaktionstechnik


Zusätzliche Organisationseinheit(en)
Exzellenz-Cluster Engineering of Advanced Materials


Zitierweisen

APA:
Bauer, T., Mehl, S., Brummel, O., Pohako-Esko, K., Wasserscheid, P., & Libuda, J. (2016). Ligand Effects at Ionic Liquid-Modified Interfaces: Coadsorption of [C2C1Im][OTf] and CO on Pd(111). Journal of Physical Chemistry C, 120(8), 4453-4465. https://dx.doi.org/10.1021/acs.jpcc.6b00351

MLA:
Bauer, Tanja, et al. "Ligand Effects at Ionic Liquid-Modified Interfaces: Coadsorption of [C2C1Im][OTf] and CO on Pd(111)." Journal of Physical Chemistry C 120.8 (2016): 4453-4465.

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