Coexistence of reactive functional groups at the interface of a powdered activated amorphous carbon: a molecular view

Abstract

Titration experiments using ten reactive probe gases interacting with a Powdered Activated Carbon (PAC) sample have been performed in a two-chamber Knudsen flow reactor equipped with molecular-beam modulated mass spectrometric detection. The goal of these uptake and desorption experiments performed at ambient temperature was to establish a map of reactive sites at the PAC interface of 1580 m²/g BET surface area. The extent of the net uptake of the probe gases in molecule cm⁻² was in the order O₃ >> NO₂ > TFA (trifluoroacetic acid, CF₃COOH) >HA (hydroxylamine, NH₂OH) > C₄H₄S (thiophene) > TMA (trimethylamine) > HCl > H₂O > H₂S > SO₂. PAC may, therefore, be characterised as a reduced and weakly basic type of amorphous carbon. The sum of individual coverages amounts to 62.5% and corresponds to a highly decorated PAC interface. Six out of ten probe gases (H₂O, NO₂, HCl, TMA, TFA and thiophene (C₄H₄S)) spontaneously desorb to variable extents at ambient temperature under flow reactor conditions from a previously saturated PAC interface. The adsorption/desorption rate coefficients were fitted to the experimental desorption rates using an analytical solution. For HCl, NO₂ and H₂O a rather large discrepancy of a factor of 7.3, 12.7 and 71.4, respectively, between the initial uptake rate constant k₀ on a virgin non-polar PAC surface and a gas-saturated PAC surface upon readsorption occurring in spontaneous desorption experiments (k_a) has been detected.

KEYWORDS:

• Interfacial kinetics
• surface functional groups
• Knudsen flow reactor
• amorphous carbon/soot
• surface titration using gas probes

Acknowledgements

We thank the Isfahan University of Technology for supporting the summer visit of N. Mirghaffari, as well as M. Soleimani and N. Mojoudi for their help in preparing the activated carbon material. We also thank Dr Mohammed Tarik for the BET measurement of PAC, then at Paul Scherrer Institute (PSI) and Dr Cristian Contescu of Oak Ridge Laboratory for pointing us to refs 38, 39 and 40.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This work was supported by Isfahan University of Technology fund for summer visit (NM), IIE/ENAC/EPFL internal funds for scholarly activities within ENAC faculty (MJR).