Anchalee Junkaew

Promotional effect of the TiO2 (001) facet in the selective catalytic reduction of NO with NH3: in situ DRIFTS and DFT studies

Surface engineering of TiO2 is significant in the SCR reaction and worth studying. In a previous study, we found that TiO2 nanosheets (TiO2-NS) with the (001) plane exhibited a much better SCR performance than TiO2 nanoparticles (TiO2-NP) with the (101) plane. In this work, in situ DRIFTS and DFT calculations were applied to address the promotional effect of the (001) plane of TiO2-NS in the SCR reaction. The behavior of adsorption and desorption of NOx and NH3 on the two surfaces was studied. It was found that NH3 adsorbed on both TiO2-NS and TiO2-NP was in the form of NH3(g) on the Lewis acid sites. NOx on TiO2-NP were mainly trans-(NO)2 and N2O4 which were non-reactive. Differently, NO on TiO2-NS was mainly in the form of NO2 which was probably due to the high energy and abundant active oxygen species of the (001) facet. The formed NO2 could trigger the subsequent ‘fast SCR’ reaction thereafter promoting the activity. Therefore, TiO2-NS with more (001) facets had better SCR activity than TiO2-NP with the exposed (101) facet.

Metal cluster-deposited graphene as an adsorptive material for m-xylene

Tetramer clusters of platinum (Pt), palladium (Pd), gold (Au) and silver (Ag) deposited on pristine and defective graphenes were studied as potential adsorptive materials for m-xylene using density functional theory (DFT) calculations including van der Waals contributions to the Hamiltonian. Structural, energetic and electronic (i.e. d-band center, partial density of state and explicit charge) properties have been investigated for understanding the m-xylene adsorption process. The m-xylene adsorption capability of these materials has been compared. The calculation results revealed that Pt4- and Pd4–DG adsorb m-xylene via a chemisorption process, while Au4- and Ag4–DG adsorb m-xylene via physisorption. These insights are valuable for applying and developing carbon-based materials for volatile organic compound (VOC) removal applications, since physisorption-driven materials are suitable as sorbents while their chemisorption counterparts are suitable as catalysts in an oxidation reaction. Those properties in turn can be tuned by modulating metal adsorption on the carbon-based materials.

The complete reaction mechanism of H2S desulfurization on an anatase TiO2 (001) surface: a density functional theory investigation

The complete reaction mechanism of H2S desulfurization on anatase TiO2 (001) surface was elucidated using the plane-wave based density functional theory (DFT) method. The reaction starts from the dissociative adsorption of H2S on the TiO2 surface. Subsequently, two competitive routes, H2O and H2 formation, were investigated. The activation barriers for H2O formation range from 11 to 13 kcal mol−1, whereas those for H2 formation are extremely high in the range of 67–87 kcal mol−1. On the basis of the activation energy barriers, the results indicate that the anatase TiO2 (001) is very active for H2S desulfurization to produce H2O, resulting in S-substitution at the O2c sites on the TiO2 (001) surface. Electronic charge analyses indicate that S-doping onto the TiO2 surface can enhance the photocatalytic activity of TiO2 by reducing its band gap. In addition, by comparison with other metal oxide catalysts, such as TiO2 (101), CeO2 (111), CeO2 (101), ZnO (1010) and α-Fe2O3 (0001), we found that TiO2 (001) is the most promising catalyst for H2S desulfurization.

A Cr-phthalocyanine monolayer as a potential catalyst for NO reduction investigated by DFT calculations

The reaction mechanism of nitric oxide (NO) reduction to nitrous oxide (N2O) and N2 catalyzed by a Cr-phthalocyanine sheet (CrPc) was investigated using periodic density functional theory (DFT). The results show that direct NO dissociation on the catalyst is inhibited by a large energy barrier owing to the difficulty of direct cleavage of the strong NO bond. The dimer mechanism in which two NO molecules meet together is more preferred via three competitive mechanistic pathways consisting of two Langmuir–Hinshelwood (LH1 and LH2) and one Eley–Rideal (ER) mechanism. N2O is produced from LH1 and ER which have activation barriers (Ea) of 0.35 and 1.17 eV, respectively, while N2 is a product from LH2 with an Ea of 0.57 eV. All the three pathways are highly exothermic processes. Based on energetic aspects, LH1 is the kinetically and exothermically most favorable pathway (the Ea of the rate-determining step is 0.35 eV). Therefore, we predict that NO can be easily reduced by CrPc under mild conditions. In an environmental application, CrPc would be a promising catalyst for the abatement of NO at low temperature.

Mechanistic study of NO oxidation on Cr–phthalocyanine: theoretical insight

The reaction mechanisms of NO oxidation on chromium–phthalocyanine (CrPc) were elucidated using density functional theory calculations and compared with NO reduction. The results reveal that the reaction of NO oxidation on CrPc is a two-consecutive step pathway which produces NO2 as a product. The first step can proceed through competitive Langmuir–Hinshelwood (LH) and Eley–Rideal (ER) mechanisms presenting the low activation barriers (Ea) in a range of 0.1 to 0.5 eV with exothermic aspects. Moreover, the ER mechanism is found to be more feasible. In the second step, the reaction requires an Ea of 0.32 eV, which is considered as the rate determining step of the overall reaction. By comparing both NO oxidation and reduction, the results reveal that in the low O2 system, CrPc converts NO to N2 via a dimer (NO)2 mechanism whereas in the excess O2 system, it oxidizes NO to NO2 easily. Both reaction systems required very low Ea values, thus this low cost CrPc catalyst could be a candidate for NO treatment at room temperature.

Silicon-coordinated nitrogen-doped graphene as a promising metal-free catalyst for N2O reduction by CO: a theoretical study

Metal-free catalysts for the transformation of N2O and CO into green products under mild conditions have long been expected. The present work proposes using silicon-coordinated nitrogen-doped graphene (SiN4G) as a catalyst for N2O reduction and CO oxidation based on periodic DFT calculations. The reaction proceeds via two steps, which are N2O reduction at the Si reaction center, producing Si–O*, which subsequently oxidizes CO to CO2. The N2O reduction occurs with an activation energy barrier of 0.34 eV, while the CO oxidation step requires an energy of 0.66 eV. The overall reaction is highly exothermic, with a reaction energy of −3.41 eV, mostly due to the N2 generation step. Compared to other metal-free catalysts, SiN4G shows the higher selectivity because it not only strongly prefers to adsorb N2O over CO, but the produced N2 and CO2 are easily desorbed, which prevents the poisoning of the active catalytic sites. These results demonstrate that SiN4G is a promising metal-free catalyst for N2O reduction and CO oxidation under mild conditions, as the reaction is both thermodynamically and kinetically favorable.

Theoretical guidance and experimental confirmation on catalytic tendency of M-CeO2 (M = Zr, Mn, Ru or Cu) for NH3-SCR of NO

Abstract Herein, we demonstrate that the degrees of catalytic performance of M-CeO2-based catalysts (M=Mn, Cu, Ru or Zr) for an ammonia selective catalytic reduction (NH3-SCR) of nitric-oxide (NO) can be estimated using three theoretical terms; (i) an oxygen vacancy formation energy of a catalyst, (ii) an adsorption energy of NO and (iii) an adsorption energy of NH3. Those terms predict the trend of the catalytic performance as the order; Mn–CeO2 > Cu–CeO2 > Ru–CeO2 > Zr–CeO2 > CeO2. To verify the theoretical prediction, the catalysts were synthesized and tested their performances on the NH3-SCR of NO reaction. The normalized NO conversion rates at low temperatures (100–200 °C) were measured for Mn–CeO2, Cu–CeO2, Ru–CeO2, Zr–CeO2 and CeO2 as 2.61–7.46, 1.30–6.82, 0.73–3.02, 0.81–3.31 and 1.55–2.33 mol s−1 m−2, respectively. In addition, a concept of a structure-activity relationship analysis shows a strong relationship between theoretical and experimental results. Consequently, an application of predicting the catalytic performance of catalysts from theoretical calculations prior the catalyst synthesis is useful in catalyst design and screening that can reduce time and cost.