Subject Number : S-16-MS-0045
Support Type : Collaborative research
Proposal Title (English) : Photoassisted Nitrous Oxide Decomposition over Water Interface Oxotitanium Porphyrin: A Theoretical Study
Username (English) : Phornphimon Maitarad
Affiliation (English) : Research Center for Nano Science and technology, Shanghai University, Shanghai China 200444

1. Summary
Reaction mechanism of nitrous oxide (N2O) decomposition to N2 and O2 catalyzed by oxotitanium (IV) porphyrin (TiO-por) under photoassisted condition in the presence of water interface was investigated using density functional theory. The entire reaction is the exothermic and mechanism was divided into four elementary steps. (i) water dissociation to produce Ti(OH)2-por active site, it requires 23.17 kcal/mol for activation barrier. (ii) the first N2O decomposes over the Ti(OH)2-por, the dissociation of N-O bond is the rate determining step by requiring activation barrier of 27.29 kcal/mol. (iii) there are two alternative routes; Path A: the second N2O decomposition followed by H2O desorption or Path B: the H2O desorption followed by the second N2O decomposition. On the basis of the activation energies, Path B is more favorable. (iv) O2 desorption which is the key reaction step for catalyst regeneration; its activation barrier is less than 1 kcal/mol and its desorption energy is only 3.40 kcal/mol. In summary, the TiO-por is predicted as a candidate catalyst for photocatalytic N2O decomposition due to the low activation barriers, the easy process of catalyst regeneration, and exothermic reaction processes. On contradictory to other catalysts, the presence of water doesn’t inhibit the TiO-por’s activity for the N2O decomposition.

2. Computational Details
The model of TiO-por was constructed and then optimized by using the DFT based on the M06L functional without any restrictions. The 6-31G(d,p) basis set was assigned to carbon, oxygen, nitrogen and hydrogen atoms and Dunning-Hay-Wadt with LANL2DZ was assigned to Ti transition metal. All calculations were carried out by using the Gaussian09 program package revision B01.
For the reaction mechanism investigations, the neutral form of H2O and N2O gases were used as the reactants. Thus, the spin state of the complex models was assigned according to the catalysts’ spin state. In the calculations, we considered both singlet and triplet spin states to simulate the reactions under thermal and photoassised conditions, respectively. Thus, the spin crossing was also considered along the potential energy surface of ground (singlet state) and excited states (triplet state). During the geometry optimizations, all atoms in the system were fully relaxed to simulate the real reaction phenomena. The transition states were carefully optimized and their frequencies were then calculated in the same level of theory as the optimization. Only one imaginary frequency is found for each transition state structure which confirms the real saddle point. The reaction energy profiles of each step were presented in the relative energy which is defined as the following:
∆E = Ecomplex – ( Ecatalyst + Eadsorbate) (1)
where Ecomplex, Ecatalyst, and Eadsorbate are the total energy of the given mechanism geometry containing catalyst and small adsorbate molecules, the total energy of intermediate catalyst at each step, and the energy of small gas molecules, e.g., N2O, O2 and N2, respectively.

3. Results and Discussion
As mentioned above the presence of water would produce some steric hindrance around the active site and then would increase the activation barriers of the reaction. Alternatively, the Consideration on the water interface, the N2O decomposition over the TiO-por can be divided into four reaction steps as follows:
Step 1: water interface formation on TiO-por
TiO-por + H2O → Ti(OH)2-por
Step 2: First N2O decomposition
Ti(OH)2-por + N2O → TiO(OH)2-por +N2
Step 3-1: Second N2O decomposition followed by water desorption (Path A)
TiO(OH)2-por + N2O → Ti(OOH)2-por +N2
Ti(OOH)2-por → TiO3-por +H2O
Step 3-2: water desorption followed by second N2O decomposition (Path B)
TiO(OH)2-por → TiO2-por + H2O
TiO2-por + N2O → TiO3-por + N2
Step 4: Catalyst regeneration: oxygen formation and desorption
TiO3-por → TiO-por + O2
Along the reaction mechanism investigation, TiO-por was computed in neutral form both ground and excited states to represent the reaction routes under thermal and photoassisted conditions. The obtained activation energies of all steps on ground and excited states are listed in Table 1. Based on the pairwise comparison, it is clearly shown that the overall reaction processes of N2O direct decomposition with the water interface are favorable to proceed over the excited state TiO-por catalyst due to much lower activation barriers. Therefore, the details of each reaction step based on the excited state geometries are deliberately discussed.

3.1 Summary of N2O Decomposition over Ti(OH)2-por catalysts
Over all reaction mechanism of N2O decomposition over the TiO-por with water interface is summarized in Figure 1. Firstly, water molecule strongly adsorbed on the TiO-por catalyst at -9.7 kcal/mol (AD1), then, the active site becomes Ti(OH)2 (IN1). Consideration on the first N2O decomposition, N2O molecule is slightly adsorbed over Ti(OH)2-por active sites (AD2), and it requires for 27.3 kcal/mol (TS2) for dissociation of the N-O bond to produce N2 molecule, and then, its active site becomes the TiO(OH)2-por (IN2-1). As mentioned above the IN2-1 intermediate can possible proceed through two pathways. Path A starting with the second N2O decomposition and then H2O formation, their activation energies are 28.7 (TS3A) and -36.5 (TS4A) kcal/mol, respectively. It is worth to mention that the process of H2O formations occurs the spin change from triplet to singlet states; therefore, its activation barrier becomes extremely exothermic process. For Path B, the H2O is firstly released from TiO(OH)2-por catalyst intermediates, its activation energy is -4.8 kcal/mol (TS3B) which is an exothermic process. Then the second N2O decomposition over the TiO2-por requires activation energy for 11.5 kcal/mol (TS4B). Therefore, based on the activation barriers, the reaction would favor Path B determined by a spontaneous process of the H2O formation over the TiO(OH)2-por intermediate. In addition, the activation barrier for the second N2O decomposition over the TiO2-por of Path B is significantly less than it’s over the TiO(OH)2-por of Path A due to the steric hindrance around an Ti active center. Furthermore, at the final step of the catalyst regeneration by oxygen molecule formation is presented by a very low activation barrier, less than 1 kcal/mol for Path B while Path A requires 9.7 kcal/mol because of the structural configuration and spin state differences. Thus, overall reaction the dissociation of N-O bond of the first N2O molecule in the step 2 is the rate determining step which corresponds well with the previous reports.

3.2 Effect of water interface to the N2O decomposition barriers
As focusing on the pairwise comparison of theoretical reaction mechanism for N2O decomposition with and without introducing H2O, there is only few works as been addressed that issue. As well known that the Fe-ZSM-5 zeolite is one of the potential catalysts for the N2O direct decomposition; importantly, it had been completely theoretically reported on the activation barriers for the first- and second-N2O decompositions and oxygen molecule formation. Thus, the present oxotitanium porphyrin catalyst for deN2O with/without water interface could be compared with the Fe-ZSM5 zeolite to elucidate the catalytic potential and water effect. Therefore, the above mentioned activation barriers over the oxotitanium porphyrin and Fe-ZSM-5 zeolite clusters are listed in Table 1. Firstly, consideration on the Fe-ZSM-5 zeolite catalyst, for the absence and presence waters during the N2O decomposition represent by Z-[FeO]+ and Z-[Fe(OH)2]+ active sites, respectively. It is clearly that the Z-[Fe(OH)2]+ active sites resulted in the higher activation barriers to decompose the N2O and oxygen formation. Next, consideration on the thermal condition of with and without water over the oxotitanium porphyrin, represented by 1Ti(OH)2-por and 1TiO-por, respectively. The first N2O decomposition over the 1Ti(OH)2-por seems to be easier than 1TiO-por while the second N2O decomposition and oxygen molecule formation over them are similar activation barrier. These imply that the hydroxyl site Ti(OH)2-por which produced by water dissociation doesn’t increase the activation energy barrier for deN2O whereas the Z-[Fe(OH)2]+ catalyst showed the significantly increase of activation energies in three transition steps of the first- and second-N2O decompositions and oxygen molecule formation, as in compare with the bare Z-[FeO]+ catalyst.
For further details on the photoassisted condition which is the main present on this work, the effect of water interfaced active site (3Ti(OH)2-por) on the catalytic reactivity for N2O decomposition is intensively discussed. At the absence water, the 3TiO-por catalyst, the first N2O can be directly decomposed to N2 product and 3TiO2-por catalyst intermediate (Scheme 1). The first N2O adsorbed on the 3TiO-por active site is -5.4 kcal/mol, which is about two fold weaker than the water molecule adsorbed on the 3TiO-por site (-9.7 kcal/mol). One can say that water molecule would cover the catalyst surface and become the 3Ti(OH)2-por form. A pairwise comparison for the first N2O decomposition over the bare 3TiO-por and over the hydroxyl 3Ti(OH)2-por active sites, their N2O adsorption energies are -5.4 and -5.3 kcal/mol, respectively. And then the activation barriers to decompose N2O molecule are calculated to be 29.9 and 27.3 kcal/mol over the 3TiO-por and 3Ti(OH)2-por active sites, respectively. These evidences mean that the N2O decomposition does not inhibit by the water interface over the active site 3Ti(OH)2-por. On the other hand, the water molecule seems to be an assisting agent for N2O decomposition as it can easily desorb after the N2O decomposition process. Therefore, based on the theoretical investigations, the TiO-por or oxotitanium porphyrin is purposed is a candidate catalyst for photoassisted N2O decomposition.

4. Others
N/A

5. Publication/Presentation
N/A
6. Patent
N/A