Density Functional Theory Study Of Adsorption Mechanism Of Gasified Biomass Over Catalyst Pd/γ-Al₂O₃

In the field for catalytic combustion of gasified biomass, the catalyst Pd/γ-Al2O3isreceived much attention from home and abroad due to its high catalytic performance andsuperior lower light-off temperature. However, most of studies are focused on theexperimental research, but there is little information about the adsorption mechanism of gasabsorbed on the catalyst. Density functional theory can essentially reveal the catalyticadsorption mechanism. Therefore, we adopt the DMol3module of Material Studio softwarebased on density functional theory to investigate adsorption mechanism of the maincombustible components CO/H2/CH4of gasified biomass over catalyst Pd/γ-Al2O3.First of all, the catalyst γ-Al2O3and Pd/γ-Al2O3were synthesized by the sol gelmethod. They also were characterized by XRD and SEM techniques. Then the TPR andTPD studies of CO/H2/CH4over two catalysts were carried out, and the DRIFTexperiments of CO and CH4over Pd/γ-Al2O3were added. All the experimental data wereused to provide guidance for adsorption mechanism of gas absorbed over the catalyst.Then detailed research of catalytic adsorption mechanism for Pd deposited overγ-Al2O3was investigated by using DFT method. The region of Pd was surface active site.After the analysis of Mulliken charge and density of states, the calculated result showedthat Pd2prefer to bind with fourfold-coordinated Al (Al4c-1), threefold-coordinated Al(Al3c-1) and threefold-coordinated O (O3c-2) over γ-Al2O3surface.Finally, we investigate the adsorption mechanism of CO/H2/CH4of gasified biomassover catalyst Pd/γ-Al2O3. The adsorption mechanism of CO/H2/CH4is as follows:For CO: After Pd supported on the γ-Al2O3, it became the surface active site. COadsorbed on the Pd. Due to the-π bond formed between Pd and CO, this make Pd bondwith CO in Pd=C=O style, weakening the triple bond of CO and activating CO. This process skipped the CO dissociation barriers, then CO was easier to react with the adsorbedoxygen or lattice oxygen.For H2: H2adsorbed on the Pd, forming the Pd-H, σ bond partially depletes the H-H σbond because electrons that were fully engaged in keeping the twp H atoms together in freeH2are now also delocalized over the Pd. Back bonding into the H-H σ*cause additionalweakening or even breaking of the H-H σ bond because the σ*is anti-bonding with respectto H-H. Eventually the H-H bond breaks and a dihydride are formed. Then catalytichydrogenation on unsaturated atoms was occurred.For CH4: CH4adsorbed on the region of Pd, then CH4is dissociated and Pd-C isformed, breaking the tetrahedral structure of CH4. This make CH4is dissociated into twoparts: methyl-CH3and a free state H. Then CH3can react with adsorbed or lattice oxygen.This process skipped the dissociation of CH4with tetrahedral structure, making the initialreaction: CH4+O2were transformed into the reaction: CH3+O2, it greatly reduced thereaction energy barrier of CH4, the latter was easier.