2 research outputs found
Abiotic Degradation of Glyphosate into Aminomethylphosphonic Acid in the Presence of Metals
Glyphosate
[<i>N</i>-phosphono-methylglycine (PMG)] is
the most used herbicide worldwide, particularly since the development
of transgenic glyphosate-resistant (GR) crops. Aminomethylphosphonic
acid (AMPA) is the main glyphosate metabolite, and it may be responsible
for GR crop damage upon PMG application. PMG degradation into AMPA
has hitherto been reckoned mainly as a biological process, produced
by soil microorganisms (bacteria and fungi) and plants. In this work,
we use density functional calculations to identify the vibrational
bands of PMG and AMPA in surface-enhanced Raman spectroscopy (SERS)
and attenuated total reflectance Fourier transform infrared (ATR–FTIR)
spectra experiments. SERS shows the presence of AMPA after glyphosate
is deposited from aqueous solution on different metallic surfaces.
AMPA is also detected in ATR–FTIR experiments when PMG interacts
with metallic ions in aqueous solution. These results reveal an abiotic
degradation process of glyphosate into AMPA, where metals play a crucial
role
Controlling Carboxyl Deprotonation on Cu(001) by Surface Sn Alloying
We
find that for adsorbed terephthalic acid (TPA) molecules surface Sn
alloying deactivates the Cu(001) surface by decoupling the adsorbed
molecules from the substrate. This effect is investigated for the
case of the 0.5 ML phase of the Sn/Cu(001) surface alloy by applying
fast X-ray photoemission spectroscopy, scanning tunneling microscopy,
near-edge X-ray absorption fine structure spectroscopy, and density
functional theory calculations. The experimental results conclusively
show that the deprotonation reaction of the carboxyl groups occurring
in the clean Cu(001) is fully inhibited on this Sn/Cu(001) surface
alloy, which allows the molecules to form two-dimensional arrays stabilized
by [OH···O] hydrogen bonds. The formed arrays exhibit
a crystal structure that is practically indistinguishable from that
theoretically obtained for unsupported TPA sheets, suggesting an extremely
weak molecule/substrate interaction. This is supported by DFT calculations
of the adsorption energy landscape of the TPA sheets formed on the
Sn/Cu(001) template: the lateral variation of the adsorption energy
(corrugation) is estimated to be less than 0.2 eV, with an adsorption
energy per molecule in the range 1.6–1.8 eV and a contribution
of each double [OH···O] bond of 1 eV. Finally, the
performed thermal desorption experiments show that the TPA sheets
remain stable on the surface alloy until their desorption. From these
experiments, a value of 1.5 eV was determined for the desorption energy
barrier, which is consistent with the important contribution of the
[OH···O] bonds to the stability of the sheets as theoretically
predicted. The results reported in this study suggest that a gradual
activation of the interaction between the TPA molecules and the Cu(001)
surface will also be obtained for decreasing Sn coverage