The Catalytic Effect Of Ir-doped And TiO₂Support On Pt Catalysts In The Methane Dehydrogenation: A DFT Study

By using first principles based slab calculations, this paper investigated the methanedehydrogenation catalyzed by PtIr bimetallic catalyst and the resistance of carbon depositionover anatase TiO2supported platinum or nickel catalysts, together with the catalytic activitiestowards methane adsorption and dissociation over anatase TiO2supported platinum catalyst.The induced results will be helpful for the design of novel and highly efficient catalysts aswell as providing theoretical evidence for the reaction mechanism in methane reformingreaction.As compared to the adsorption energy, the most stable configurations of the methane,methane dissociation species and co-adsorption of CHx(x=0-3) with H were obtained. Thekinetic results of the CH4dissociation indicates that the CH4dissociating into CH3and H wasthe rate-determining step on the PtIr(111) and Ir(111) surfaces. CH was the most abundancespecies, indicating the dehydrogenating into C and H was the most difficult. Particularly, theactivation barriers of CH3→CH2+H and CH2→CH+H for PtIr(111) was3.5and1.4times,respectively, higher than that on Pt(111). According to the thermodynamics principles, thesuccessive dehydrogenation of CH4preferred to take place on the PtIr surface.Studies on the adsorption behavior of carbon atom adsorbed by isolated Ni or Pt atomssupported on the anatase TiO2(101) surface can provide a thermodynamics clue forelucidating the carbon deposition. The calculated results by PBE based on generalizedgradient approximation indicated that the adsorption energy of the most stable configurationfor Ni and Pt adsorbed on the TiO2surface were347.16and315.9kJ·mol-1, respectively,corresponding to the bridge site between the two O2catoms. After the adsorption of metalatom, the density of state for TiO2moved to the lower energy, leading to a more stablesystem. The density of state also confirmed that there was a significant overlap betweenp-orbital of carbon atom and d-levels of the metal atom, indicating the effective bonding ofcarbon and metal atoms, which could attenuate the interaction between these two atoms.When carbon was adsorbed on the Ni/TiO2(101) or Pt/TiO2(101) surface, the adsorptionenergy for the preferable structure was474.19and570.08kJ·mol-1, respectively. Our workdemonstrated the Pt supported on TiO2possessed the better ability for inhibiting the carbon deposition.Various adsorption energies of methane and methane dissociation species on thePt/TiO2(101)surface were compared. CH4, CH3and H preferred to adsorb on the top site ofplatinum atom. The most stable site for CH2was B2site. When CH and C were adsorbed onthe surface, the Pt atom tended to be pulled out of the initial site. The adsorption energyfollowed the following order: CH4<CH3<H<CH2<CH<C. CH dissociating into C and H bythe breaking of C-H bond was the most difficult, which indicated that carbon deposition washardly to proceed.