Jittima Meeprasert

Mechanistic insight into the selective catalytic reduction of NO by NH3 over low-valent titanium-porphyrin: a DFT study

In this work, the reaction mechanism of ammonia selective catalytic reduction (NH3-SCR) of nitric oxide over a low-valance Ti-porphyrin catalyst was studied by density functional theory (DFT) calculations for both low- and high-spin states. The reaction proceeds via (i) NH3 complexation with the Ti-porphyrin complex, and its subsequent oxidation to NH2, with a large activation barrier of 32–34 kcal mol−1 because of the difficulty of N–H bond dissociation. (ii) Bonding between NO and the NH2 ligand forms an NH2NO intermediate by an Eley–Rideal-type mechanism. The calculated activation energies for this step are 4.34 and 10.22 kcal mol−1 for the low- and high-spin states, respectively. (iii) Formation of NHNOH by rearrangement of the NH2NO intermediate. Spin crossings in steps (ii) and (iii) play an important role in the overall reaction by providing a mechanism with a smaller activation energy of 17.05 kcal mol−1, compared with 28.02 kcal mol−1 for the un-catalyzed reaction. (iv) In the final step, the decomposition of NHNOH results in the formation of N2 and H2O molecules, with a small energy barrier of approximately 7–9 kcal mol−1. For pairwise pathway comparisons, Ti-porphyrin in the triplet state offers 8.43 kcal mol−1 greater stability than the singlet does, and the reaction is more likely to proceed through a high-spin pathway because of its lower relative energies compared to the low spin. The obtained activation energies for NH3-SCR of NO are comparable with theoretical results for the reduction of NO over V2O5 and Fe-zeolite systems. Thus, Ti-porphyrin has potential as an alternative catalyst for NH3-SCR of nitric oxide.

Organic sensitizers with modified di(thiophen-2-yl)phenylamine donor units for dye-sensitized solar cells: a computational study

A new series of organic donor–π–acceptor (D–π–A) dyes namely B1–6 with modification of donor groups by introducing thiophene (as D) and fluorene-connected carbazole on top of thiophene (as 2D–D) in phenylamine moieties, and with elongation of π-spacer unit of thiophene (1–3 units) in the π-spacer, were molecularly designed by using density functional theory (DFT) and time-dependent DFT. The nature of intramolecular charge transfer of all dyes was elucidated by means of frontier molecular orbital analysis, electronic structures, and absorption spectra to provide their potential use for dye-sensitized solar cells (DSSCs). The structural results show that the 2D–D–π–A dyes have a nonplanar structure on the D–D moiety that may suppress the aggregation of dye and yet maintain the conjugation in the whole D–π–A moiety. The systematically elongating π-spacer of B4–6 dyes with increasing number of thiophene group and the introducing 2D into D–π–A dyes give the redshift on absorption peak and broaden the absorption range, which are in excellent agreement with available experiment. Thus, this redshift improves their overall light-harvesting efficiency (LHE) better than the B1–3 dyes. Among the six dyes, B6 would have the best performance because it has the highest predicted LHE at the maximum absorption wavelength (λmax) and the suitable driving force ΔGinject of the electron injection from the excited state of dyes to the conduction band of TiO2. The prototype of DSSCs performance of selected dyes was further simulated using the chemisorption of dyes onto the (TiO2)38 cluster to reveal the nature of the electron injection mechanism. This current work is expected to assist in the molecular design of new metal-free organic dyes for use in DSSCs yielding highly efficient performance.

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.

Effects of amine organic groups as lattice in ZSM-5 on the hydrolysis of dimethyl ether

The effects of doping amine to ZSM-5 on its catalytic activity for hydrolysis of dimethyl ether (DME) have been studied theoretically using Density Functional Theory with the embedded cluster ONIOM(M06/6-31G(d,p):UFF) model. Doping by amine to ZSM-5 yields two new active centers, namely the protonated Z[NH2] and non-protonated Z[NH] amine sites in addition to the normal Brønsted acid Z[OH] site. The reaction has two possible stepwise and concerted channels. The stepwise channel consists of two elementary steps; (i) the demethylation followed by (ii) the hydrolysis while the concerted channel involves in the demethylation and hydrolysis in a single step. We found that the reaction favors to proceed via the concerted channel at all three active centers. The results predict that the Z[OH] shows the best catalytic performance for the studied reaction. The Z[NH2] is not catalytically active due to the activation barriers are extremely high for both stepwise and concerted pathways. The demethylation step is energetically favorable over the Z[NH] site, however, the product methylamonium surface intermediate is too stable to be further converted to methanol.

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.

Oxotitanium-porphyrin for selective catalytic reduction of NO by NH3: a theoretical mechanism study

The reaction mechanism of the selective catalytic reduction of NO by NH3 (NH3-SCR) on an oxotitanium-porphyrin catalyst was systematically investigated by using density functional theory calculations with the M06L functional. The reaction was proposed to follow the nitrite mechanism over the two forms of active sites; the oxotitanium-porphyrin Lewis acid site (TiO-por) and the Bronsted acid site (TiOH-por). The reaction path consisted of (i) nitrite formation, (ii) NH3 oxidation, (iii) formation of NH2NO and NHNOH intermediates, and (iv) N2 and H2O product formation. The obtained calculations showed that the formation of the NHNOH intermediate was the rate determining step for both active sites with the energy barriers (Ea) of 32.2 and 36.2 kcal mol−1 for the Lewis and Bronsted acid sites, respectively. It is worth noting that the activation energy for NHNOH formation over the oxotitanium-porphyrin active sites was found to be in the same range as that of vanadium oxide cluster models. Furthermore, the product formations of N2 and H2O over the Lewis and Bronsted acid sites of oxotitanium-porphyrin were exothermic processes with reaction energies (Er) of −67.1 and −39.0 kcal mol−1, respectively. Thus, in conclusion, the oxotitanium-porphyrin could theoretically act as an alternative catalyst for NH3-SCR of NO and it would be challenging to test it in experimental studies.

Cyclomatrix Polyphosphazene Porous Networks with J-Aggregated Multiphthalocyanine Arrays for Dual-Modality NIR Photosensitizers

Here, we have developed a kind of cyclomatrix polyphosphazene with excellent photophysical properties and pursued their potential of being organic photosensitizers for dual-modality phototherapy. Briefly, hexachlorocyclophosphazene (HCCP) with D3 h symmetry is adopted as a synthon to attach Zn(II) phthalocyanine (ZnPc) to form dendritic units that are covalently expanded into a soluble porous network through the nucleophilic substitution reaction. Molecular simulation reveals that the multi-ZnPc units around HCCP can be oriented in a side-by-side manner, leading to the remarkably red-shifted and intense absorbance in the near-infrared (NIR) region. To validate the potential in bioapplication, such ZnPc-based polyphosphazenes are assembled by incorporation of polyvinylpyrrolidone (PVP) to produce the uniform nanoparticles with aqueous dispersibility and biocompatibility. From the in vitro results, the PVP-stabilized photosensitizing nanoparticles can undergo the photothermal/photodynamic processes to concurrently generate heat and singlet oxygen for efficiently killing cancer cells upon exposure to a single-bandwidth NIR laser (785 nm). Compared with the known organic photosensitizers, cyclomatrix polyphosphazene would be a promising platform to configure a diversity of reticular arrays with dense and oriented arrangement of dye molecules, leading to their largely enhanced photophysical and photochemical properties.

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.