## 6 Conclusions  
In this study, macro-porous alumina was applied as a support to improve gas diffusion to the absorbent particles. Macro-porous alumina was prepared by moisture removal and heat treatment of a mixed solution containing PMMA colloidal beads and an alumina precursor solution. A sulfur absorbent was prepared by impregnating CuO with excellent sulfur absorption ability into the macro-porous alumina and commercial alumina. The experimental results suggest that the sulfur capacity of the CuO/macro-porous alumina is larger than the CuO/\(\gamma\)-Al\({}_{2}\)O\({}_{3}\). The enhancement was attributed to the macro-porous alumina support improving the gas diffusion resistance, allowing the inner unreacted active sites to be activated. Such effects become more significant on the CuO absorbents in the pellet-type supports. Interestingly, CO production was also observed during the desulfurization reaction of COS at a low temperature of \(\sim\)473 K.  
DFT calculations were conducted to investigate the surface reaction of COS desulfurization on the CuO(111) surface. The DFT simulation predicts that the surface kinetics would be affected significantly by the local surface environment of oxygen vacancies and surface sulfur. In particular, the surface local environment for the presence of surface oxygen vacancies and surface sulfur, which is potentially formed during the reaction, makes CO-S bond-breaking significantly facile, which would accelerate the desulfurization rate. Moreover, the CO* stability is lower than the CO2 formation energy barrier. This suggests that CO* would preferably undergo CO desorption rather than CO2 formation, which agrees well with the experimental observation.  
Based on the experimental and computational results, a high sulfur capacity was achieved using the macro-porous alumina support, which enhanced the diffusion resistance. Overall, these findings provide a fundamental understanding of the conventional substitution reaction and CO production reaction during the desulfurization reactions on CuO. From the findings of CO production, the Cu-based absorbent can be a catalyst for producing CO(g) from COS(g) dissociation, which would be an interesting topic for CO generation from gasified petroleum coke.  
The following are available online at [[https://www.mdpi.com/article/10](https://www.mdpi.com/article/10)]([https://www.mdpi.com/article/10](https://www.mdpi.com/article/10)) 3390/app11177775/s1, the images of relaxed configurations for NEB calculations.  
Conceptualization, M.K. and N.-K.P.; methodology, N.-K.P., D.B. and Y.J.K.; software, M.K. and D.K.; validation, M.K. and N.-K.P. and D.B.; formal analysis, D.B. and Y.J.K.; investigation, D.K. and D.B.; resources, S.J.L., J.W.L. and Y.Y.; data curation, D.K., D.B. and Y.Y; writing--original draft preparation, D.K. and D.B; writing--review and editing, M.K. and N.-K.P.; supervision, M.K., N.-K.P. and J.W.L.; project administration, S.J.L., J.W.L. and Y.Y.; funding acquisition, M.K. and N.-K.P. All authors have read and agreed to the published version of the manuscript.  
This work was supported by a grant (21 PCHG-C163217-01) from \"Development of Demonstration-scale Hydrogen Production Technology using Petroleum coke\" Program funded by Ministry of Land, Infrastructure and Transport of Korea government, and 2020 Yeungnam University Research Grant.