Study On The Reaction Mechanism Of Sulfur-and Nitrogen-containing Compounds On Mo₂C And Rh Catalysts

Sulfur-and nitrogen-containing compounds in crude oil are the source of air pollutants SO₂ and NO_x after its combustion,and also could lead to catalyst poisoning in chemical industries.Recent studies have indicated that transition metal carbides show high activity in hydrodesulfurization（HDS）and hydrodenitrogenation（HDN）,among which the molybdenum carbide（Mo₂C）is considered to be one of the most promising hydrofining catalysts due to its appropriate activity and high stability.However,current studies mainly focused on the catalyst preparation,activity measurement and surface property characterization.There are few studies on the reaction mechanisms of sulfur-and nitrogen-containing compounds over Mo₂C and single metal catalysts.In this dissertation,the adsorption and reaction mechanisms are investigated for hydrogen sulfide,pyridine,quinoline and piperidine on Mo₂C（001）as well as sulfur dioxide on Rh（111）using self-consistent periodic density functional theory（DFT）.The present theoretical works reach the points as follows.（1）H₂S adsorbs stably at top and bridge sites on Mo₂C（001）.The adsorption is mainly through the interaction between 3s orbital of S atom and 4d orbital of Mo atom,involving charge donation from the S lone pair electrons of HOMO orbital to surface Mo atoms and back-donation of the Mo 4d electrons.The most stable adsorption sites for SH group are threefold hcp and fcc sites.Base on the analysis of energy barriers and rate constants of related reactions,the optimal pathway for H₂S dissociation on Mo₂C（001）is determined to be H₂S_top→SH_fcc+H_fcc→S_fcc+H_fcc+H_fcc,in which the first one is the rate-determining step because of the highest energy barrier involved.The S atoms produced form H₂S decomposition will strongly adsorbs on Mo₂C（001）,leading to the poisoning of the catalyst surface.（2）Pyridine has two stable adsorption configurations on Mo₂C（001）:one is parallel mode in which pyridine adsorbs through N,C₂ and C₄ atoms bonding with three surface Mo atoms formingη³（N,C₂,C₄）or through C₁,C₃ and C₅ atoms bonding with three surface Mo atoms formingη³（C₁,C₃,C₅）;the other is vertical mode in which pyridine adsorbs through N-Mo interaction.In parallel adsorption configurations,the interaction of pyridine molecule with surface Mo atoms is through the overlap ofπbonds of pyridine and 4d orbital of Mo atom with the relatively large adsorption energies.The vertical adsorption of pyridine is due to the interaction of lone pair electrons of N atom with surface Mo atom,giving a relatively low adsorption energy.Thus,pyridine tends to adsorbs in parallel mode on Mo₂C（001）.When H atom is the hydrogen source for the hydrogenation of pyridine on Mo₂C（001）,the hydrogenation position in the first step is the C₂ atom forming 2-monohydropyridine,and in the second step it is the C₄ atom forming 2,4-dihydropyridine.（3）The stable adsorption configurations of quinoline on Mo₂C（001）is the parallel mode,in which it adsorbs through its C atoms binding with surface Mo atoms formingη⁵（C₁,C₃,C₅,C₇,C₉）.The structural variations in quinoline after adsorption destroy theπbond system of the molecule,weakening the aromaticity of quinoline and thus promoting the hydrogenation reactions.For the benzene rings of quinoline,the hydrogenation position in the first step is the C₄ atom forming 4-monohydroquinoline;for the pyridine rings of quinoline,the hydrogenation to the C₈ atom is easy to proceed,and the hydrogenation to the pyridine ring of quinoline is preferred as the first step.（4）Piperidine weakly adsorbs on Mo₂C（001）with the adsorption energy of 15.69kcal/mol.The weak adsorption facilitates its protonation.The C-N bond cleavage of piperidine can only occur by E₁ elimination mechanism when atomic H is the hydrogen source.In the presence of H₂S,the C-N bond cleavage of piperidine could proceed through both the E_l elimination and SH nucleophilic substitution mechanisms.（5）On Rh（111）,there are two stable adsorption configurations for SO₂,i.e.,η²（S）-η¹（O）andη¹（S）-η¹（O）-η¹（O）;SO,the intermediate of SO₂ deoxidation,also has two stable adsorption configurations,which areη²（S）-η¹（O）andη³（S）;SO₃,the product of SO₂ oxidation,adsorbs through either the"crown"mode ofη¹（O）-η¹（O）-η¹（O）or the"armchair"mode ofη¹（S）-η¹（O）-η¹（O）;Both S and O atoms adsorb at hcp and fcc sites givingη³（S）andη³（O）configurations,respectively.SO₂ dissociation includes two elementary steps:SO₂→SO+O and SO→S+O.The optimal path for SO₂ dissociation is SO₂（η¹（S）-η¹（O）-η¹（O））→SO（η²（S）-η¹（O））+O（η³（O））→S（η³（S））+2O（η³（O））,in which the first one is the rate determination step.SO₂ disproportionation reactions on Rh（111）includes SO₂+SO→SO₃+S,2SO₂→SO+SO₃ and SO₂+O→SO₃,which are impossible to occur due to higher energy barriers involved.The S atoms generated from SO₂ dissociation strongly adsorb on Rh surfaces and could not be removed through the reverse reactions of dissociation（association reactions）,leading to the poisoning of Rh surfaces.