Morphology Engineered Nanocatalysts for Sulfur Gases Hydrogenation

  • Malathe Khalil

Student thesis: Master's Thesis

Abstract

Sulfur dioxide (SO2) is a common atmospheric air polluting gas that affects the environment by forming acidic rain that has serious effects on the eco-system. One key sulfur-containing molecules elimination process is hydrodesulfurization (HDS), which rely on the usage of structural-sensitive heterogeneous catalysts. Coking and catalyst poisoning are two crucial factors that affect the stability of the catalysts. Although nickel-supported catalysts have been reported to have the highest activity among the metal phosphides group [1], they suffer from coking problem which effects their stability hence, catalyst deactivation [2]. For this reason, cobalt is chosen as the active metal in metal phosphide catalysts for hydrodesulfurization, since it is more resistant to coking and it has high stability and activity even at high temperatures [3]. Phosphorus on the other hand is a promoter that offers high resistance against sulfur poisoning [4], and high structural stability [5]. To the best of our knowledge there is a gap in understanding the adsorption mechanism of SO2 on CoP catalyst which motivates us to investigate CoP electronic interaction with SO2 molecule. The adsorption energy and the electronic properties of SO2 adsorbed on orthorhombic pristine cobalt phosphide (CoP) catalyst have been determined using the spin-polarized Generalized Gradient Approximation with Perdew-Burke-Ernzerhof functional (GGA-PBE) for the electron exchange and correlation potential as implemented by Quantum-ESPRESSO package. Different facets, surface terminations, and molecular orientations have been considered to find the optimized facet and surface termination for SO2 adsorption. The results suggest that in the most of the cases, the molecule preferred flat-lying position. More specifically, analysis of the surface adsorption energy reveals that the SO2 on Co terminated (110) and (011) surfaces are energetically more favorable for which they achieved an overall adsorption energy of (-2.9 and -2.62 eV respectively) compared to all the considered facets. Usually, the surface (011) is more interesting in catalysis than (110). According to the X-Ray diffraction database, it is found that (110) miller index doesn't appear in X-Ray diffraction pattern of CoP nanocatalysts because the structure factor of (110) is zero which yield to zero intensity in the spectrum. In contrary to (011) which is found to be existing in all CoP nanocatalysts. Hybridization between the cobalt d orbitals and phosphorous p orbitals indicates high stability of the electronically active facet. The high value of adsorption is due to the high charge transfer between the adsorbate and the adsorbent. This indicates that the cobalt atoms can easily transfer electrons to the sulfur and their neighboring phosphorous in the surface layers in order to stabilize the entire system, which in turn enhances the overall adsorption mechanism. Firstly, Charge density difference, Bader charge analysis, density of states and the work function calculations were carried out to understand the electronic interactions with the molecule. Secondly, a set of CoP nanoparticles were synthesized using solution phase arrested precipitation method. XRD, TEM, Raman, and FTIR techniques were used to characterize the nanoparticles. Experimentally, it is found that at 300°C, the conversion of SO2 on CoP-1 is 53.6% which is higher than the commercial catalyst (40.66%) by 31.9% at the same temperature (percent increase). In fact, Ni2P catalyst which is reported in literature to be the best transition metal phosphide catalyst for HDS [6], showed zero conversion in hydrogenating SO2 gas at all measured temperatures. This shows the promising future of CoP nanoparticles as hydrotreading catalysts. Also, it can lead to a reduction in the size of the catalytic reactor in the tail gas treatment unit and it reduces the catalyst mass which in turns reduces the total cost of the system.
Date of AwardMay 2020
Original languageAmerican English

Keywords

  • DFT
  • Adsorption
  • SO2
  • CoP
  • Hydrogenation.

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