The emission of gaseous pollutants in the atmosphere

The emission of gaseous pollutants in the atmosphere, industrial and house
environments are of a great concern due to the risk that these pollutants exert1. Although
the natural production of most atmospheric pollutants is much higher than artificial and
industrial production, the problem of the latter is that it usually occurs in a much localized
way, so that in places close to the emission source concentrations can be very high2.
Carbon dioxide (CO2) capture is of interest due to its environmental and economic
relevance. Likewise, carbon monoxide (CO) and hydrogen sulfide (H2S) are considered as
4
suffocating agents for humans, even in reduced concentrations; as well as sulfur oxides
(SOX), which has a negative contribution to the environment and human health1, 2.
These gases are also recognized to contribute to the global warming through the greenhouse
effect, cause acid rain and photochemical smog, and several respiratory diseases due to its
chronic exposure3-8. Therefore the sensing of these gases to avoid chronic exposure is of
a great interest, in addition to the development of materials allowing its collection and
capture before the release of burning or industrial gases to the environment.
An alternative of environmental monitoring to this problem is the emerging
application of graphene as gas sensing or capture material due to its low cost, low power
consumption and high surface area9, 10. Graphene is highly stable, causes low
contamination and bind gas molecules by intermolecular interactions to its surface,
reaching high molecular adsorption and storage9-13. For instance, the adsorption abilities
of graphene towards toxic gaseous species (such as CO2, CO, NO2, NH3) have been well
described9, 11, 14-19. In this regard, most of the recent developments are focused on
materials enhancing the adsorption stability of adsorbates onto graphene. In this sense, the
local reactivity of graphene can be tailored via doping, which creates more reactive
adsorption sites20-26. Metals such as Cu, Ag, Au, Ti, Cr, Mn and Pd have been
theoretically considered as dopants in graphene to enhance its adsorption, storage capacity
and sensing properties towards CO, CO2, NO2, NO, H2S and other harmful molecules27-
31. Moreover, doping of graphene oxide has been also reported to form stable and
excellent sorbents for gas collection and filtration, even with low interference of O2
molecules32. Thus, the application of metal-doped graphene for gas collection/sensing
applications is expected to emerge as these materials will be experimentally available.
Theoretical studies based on the Density Functional Theory (DFT) framework have
shown the excellent sorption properties of Fe-embedded graphene (FeG) for a wide class of
air and water pollutants, significantly enhancing the adsorption with respect to intrinsic
graphene through strong Lewis-acid-base interactions25, 26, 33-36. Among the
advantages of iron as dopant are its low cost, low environmental toxicity compared to noble
metals, and its high acceptor character, making it an excellent candidate in terms of
improving the sensitivity of graphene at environmental levels. In addition, iron is bonded to
5
graphene with high binding energies (?7.0 eV) and high diffusion barriers (?6.8 eV)37,
38, then forming high stable adsorbents. In this sense, synthesized FeG through aberrationcorrected
transmission electron microscopy technique shows high stability at the air and
resistance to oxidants and corrosive species, where the dopants are able to disperse and
bind to defective graphene with low cluster formation39. Additionally, the bandgap of
graphene is opened by Fe-doping, which turns it useful for sensing applications25; for
example, the CO2 detection onto FeG nanoribbons has been proved40, 41. Furthermore,
we recently have theoretically studied the adsorption and sensing properties of FeG toward
nitrogen oxides and formaldehyde25, 26, indicating that FeG is highly sensitive to these
gas molecules even in the presence of oxygen.
Taking into consideration that FeG emerges as a promising material for adsorption,
filtration, collection and/or sensing of harmful gas molecules, a DFT study was performed
to study the gas adsorption of harmful gas molecules (CO, CO2, SO2 and H2S) as target
gases onto FeG nanosheets, characterizing also the role of O2 interference in the adsorption
mechanism. The FeG-Gas systems were characterized from its geometrical, energetic,
electronic and binding properties. Molecular dynamics studies were performed to analyze
the FeG-Gas interaction stability at ambient conditions, and the adsorption stability was
also characterized in aerobic conditions. As a reference, the gas adsorption was also studied
onto pristine graphene. Through this study, FeG is suggested to enhance the gas adsorption
process of toxic gaseous pollutants with negative effects on human health and on the
environment.