(435k) Mechanistic Insights into the Catalytic Elimination of Tar on Nickel and the Promotional Effect of Boron on It: A Combined Theoretical and Experimental Study Using Toluene As a Model Compound

Quang Thang Trinh¹, Anh Vu Nguyen², Dang Chinh Huynh², Thanh Huyen Pham², Samir H. Mushrif ^1,*

¹School of Chemical and Biomedical Engineering, Nanyang Technological University, 62 Nanyang Drive, Singapore 637459, Singapore.

²School of Chemical Engineering, Hanoi University of Science and Technology, 1 Dai Co Viet St., Hanoi, Vietnam

^*Email: SHMushrif@ntu.edu.sg

Abstract: Tar is the undesired viscous black liquid produced during the gasification of biomass. Catalytic elimination of tar, via steam reforming, is an economically promising technique. Nickel (Ni) is the most widely studied catalyst for the process; however, rapid deactivation of Ni catalyst due to coking is a challenge.^1,2 Decorating metals with subsurface Boron was reported to improve the resistance of the catalyst against both, bulk carbide and graphene formation.^3,4 Motivated by that study, we performed first principles calculations to investigate the promotional effect of Boron (B) on Ni catalyst for tar decomposition.⁵ Being the most abundant component in tar, Toluene is chosen as a model compound.¹ On Ni(111) surface, Toluene adsorbs strongly in a bridge configuration and the activation barrier for methyl C-H dissociation is 71.9 kJ/mol. Toluene can further decompose on Ni(111) surface via the stepwise dehydrogenation of the methyl group. The aromatic C-H bond at ortho position could only be activated after complete dehydrogenation of the methyl group and is followed by subsequent ring opening (activation barriers are 112.5 and 83.8 kJ/mol, respectively). Further C-C cleavages are also feasible to generate smaller hydrocarbon compounds. Our calculations reveal that the incorporation of subsurface B into Ni catalysts results in corrugated Ni top surface. Toluene adsorbs 10.3 kJ/mol stronger on Ni-B catalyst than on pure Ni, in the hollow configuration. The activation of Toluene is significantly promoted on Ni-B, with an activation barrier for the first methyl C-H dissociation of 51.0 kJ/mol (vs. 71.9 kJ/mol on Ni). All the subsequent C-H activations are also promoted on Ni-B catalyst, but to a smaller extent, except the aromatic C-H activation (88.7 kJ/mol vs 112.5 kJ/mol on pure Ni). The ring-opening at ortho position occurs with the activation barrier of 93.7 kJ/mol, slightly higher than that on pure Ni (83.8 kJ/mol). Additional calculations on stepped surface models of Ni show that the activation barriers of Toluene decomposition on Ni-B surface are very close to those on B5 and F4 step sites; thus suggesting that the promotional effect of B on the catalytic activity of Ni could be due to the creation of step like corrugations on Ni surface. Experimental study was also performed to validate our predictions. Ni doped with 1% wt of Boron (B-Ni) and pure Ni catalysts supported on bentonite were prepared using the method reported by Xu et al.³ and were tested for toluene decomposition at 600^oC. In excellent agreement with theoretical predictions, initial toluene conversion was higher on B-Ni (68%) than on pure Ni catalyst (62%), demonstrating the role of boron in promoting the activity of the catalyst. SEM images revealed negligible carbon formation on the surface of B-Ni, while the surface of pure Ni catalyst was completely covered with carbon filaments under the same reaction conditions. These experimental evidences confirmed that the presence of boron indeed improved both, the activity and the stability of the catalyst.

 

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