6 research outputs found
Exceptional Sorption of Heavy Metals from Natural Water by Halloysite Particles: A New Prospect of Highly Efficient Water Remediation
Halloysite particles, with their unique multilayer nanostructure, are demonstrated here as highly efficient and readily available sorbent of heavy metals that can be easily scaled up and used in large-scale water remediation facilities. The various methods of raw material purification were applied, and their effects were verified using techniques such as BET isotherm (determination of specific surface area and size of pores), XRF analysis (composition), and SEM imaging (determination of morphology). A series of adsorption experiments for aqueous solutions of metal ions (i.e., lead, cadmium) were carried out to quantify the sorption capacity of halloysite particles for selected heavy metals. The ability of adequately activated halloysite to efficiently remove heavy metal ions from water solutions was confirmed. The value of the zeta potential of raw and purified halloysite particles in water was determined. This enables us to understand its importance for the sorption of positively charged ions (metal, organics) at various pH values. The adsorption process conducted in the pH range of 6.0–6.5 showed significant improvement compared to the acidic conditions (pH value 3.0–3.5) and resulted in a high sorption capacity of lead ions—above 24.3 mg/g for the sulphuric acid-treated sample. The atomic scale ab initio calculations revealed a significant difference in adsorption energy between the external siloxane surface and cross-sectional interlayer surface, resulting in pronounced adsorption anisotropy. A low energy barrier was calculated for the interlayer migration of heavy metals into the halloysite interior, facilitating access to the active sites in these regions, thus significantly increasing the sorption capacity and kinetics. DFT (density functional theory) calculations supporting this study allowed for predicting the sorption potential of pure halloysite structure towards heavy metals. To confront it with experimental results, it was crucial to determine proper purification conditions to obtain such a developed structure from the mineral ore. The results show a massive increase in the BET area and confirm a high sorption potential of modified halloysite towards heavy metals
Insight on the Interaction of Methanol-Selective Oxidation Intermediates with Au- or/and Pd-Containing Monometallic and Bimetallic Core@Shell Catalysts
Using
density functional theory (DFT), the interaction of crucial
molecules involved in the selective partial oxidation of methanol
to methyl formate (MF) with monometallic Au and Pd and bimetallic
Au/Pd and Pd/Au core@shell catalysts is systematically investigated.
The core@shell structures modeled in this study consist of Au(111)
and Pd(111) cores covered by a monolayer of Pd and Au, respectively.
Our results indicate that the adsorption strength of the molecules
examined as a function of catalytic surface decreases in the order
of Au/Pd(111) > Pd(111) > Au(111) > Pd/Au(111) and correlates
well
with the d-band center model. The preadsorption of oxygen is found
to have a positive impact on the selective partial oxidation reaction
because of the stabilization of CH<sub>3</sub>OH and HCHO on the catalyst
surface and the simultaneous intensification of MF desorption. On
the basis of a dynamical matrix approach combined with statistical
thermodynamics, we propose a simple route for evaluating the Gibbs
free energy of adsorption as a function of temperature. This method
allows us to anticipate the relative temperature stability of molecules
involved in the selective partial oxidation of methanol to MF in terms
of catalytic surface
Toward a Comprehensive Understanding of Enhanced Photocatalytic Activity of the Bimetallic PdAu/TiO<sub>2</sub> Catalyst for Selective Oxidation of Methanol to Methyl Formate
Photocatalytic
selective oxidation of alcohols over titania supported with bimetallic
nanoparticles represents an energy efficient and sustainable route
for the synthesis of esters. Specifically, the bimetallic PdAu/TiO<sub>2</sub> system was found to be highly active and selective toward
photocatalytic production of methyl formate (MF) from gas-phase methanol.
In the current paper, we applied the electronic structure density
functional theory method to understand the mechanistic aspects and
corroborate our recent experimental measurements for the photocatalytic
selective oxidation of methanol to MF over the PdAu/TiO<sub>2</sub> catalyst. Our theoretical results revealed the preferential segregation
of Pd atoms from initially mixed PdAu nanoclusters to the interface
of PdAu/TiO<sub>2</sub> and subsequent formation of a unique structure,
resembling a core@shell architecture in close proximity to the interface.
The analysis of the calculated band gap diagram provides an explanation
of the superior electron–hole separation capability of PdAu
nanoparticles deposited onto the anatase surface and hence the remarkably
enhanced photocatalytic activity, in comparison to their monometallic
counterparts. We demonstrated that facile dissociation of molecular
oxygen at the triple-point boundary site gives rise to in situ oxidation
of Pd. The in situ formed PdO/TiO<sub>2</sub> is responsible for total
oxidation of methanol to CO<sub>2</sub> (no MF formation) in the gas
phase. Our investigation provides theoretical guidance for designing
highly selective and active bimetallic nanoparticlesî—¸TiO<sub>2</sub> catalysts for the photocatalytic selective oxidation of methanol
to MF