First Principles Study Of Supported Gold Catalysts: From Nano-catalysis To The Single Activity Of Heterogeneous Catalysis

Since the discovery of Haruta who found that TiO₂ or Fe₂O₃ supported gold catalysts shown exceptionally high catalytic activity for low temperature CO oxidation,it has been found that nano-Au particles when highly dispersed on oxides exhibit excellent catalytic activity and selectivity in a large number of chemical reactions,such as selective hydrogenation of multi-functional compounds, water-gas-shift reaction,epoxidation of propylene.Therefore supported Au catalysts have received great interests in the catalysis field.The research goals are mainly focused on the following two subjects:One is to cover more chemical reactions catalyzed by supported Au catalysts,and the second issue is to address the active site and elucidate the relationship between the activity and the structure in supported Au catalysts,and several contributions have been presented,including coordination effect, quantum size effect,support effect and charge state effect.However,because of the complexity of the system,the inconsistency between different experimental results,and the limitation of the computational methods,many fundamental issues in supported Au catalysts remains elusive.Therefore,Using CO oxidation,selective hydrogenation of 1,3-butadiene and acrolein model reactions, some important questions,including activity,selectivity and stability,in supported Au catalysts have been studied by first-principle calculations based on density functional theory(DFT).The Support Effect in Activity:the catalytic performance of Au/oxide catalysts can vary significantly upon the change of oxide species.Generally,it is known that Au on active oxides(reducible oxides,e.g.TiO₂)is more active than that on inactive oxides(irreducible oxides,e.g.SiO₂).However,ZrO₂ differs from other oxides in that ZrO₂ is irreducible however the valence electron configuration of Zr atom is d²s².It is suspected that Au/ZrO₂ is inactive for CO oxidation in the view of reducibility. However,by different methods,Au/ZrO₂ catalysts shown large different performance have been prepared.Furthermore,the size of the oxide particles was found to be critical to the activity.By extensive density functional theory calculations on a model system,CO oxidation on Au/ZrO₂,it is demonstrated that the oxidation reaction is very sensitive to the oxide structure.The surface structure variation due to the transformation of the oxide phase or the creation of structural defects(e.g.steps)can greatly enhance the activity.We show that CO oxidation off typical Au/ZrO₂ catalysts could be dominated by minority sites,such as monoclinic steps and tetragonal surfaces,the concentration of which is closely related to the size of oxide particle. Importantly,this variation in activity is difficult to understand following the traditional rules based on the O₂ adsorption ability and the oxide reducibility.Instead, electronic structure analyses allow us to rationalize the results,and point towards a general measure for CO+O₂ activity,namely the p-band width of O₂,with important implications for Au/oxide catalysis.The Charge State Effect in Activity:it is well known that the preparation methods,calcinations temperature and reducibility condition significantly affect the charge state of Au at the boundary between Au and oxides.However,the charge state of Au which is active for CO oxidation remains to be resolved.Based on the consideration of the adsorption and activation of O₂,some researchers proposed that anionic Au is the active site for CO oxidation.But more experimental results indicated that cationic or metallic Au is the active site.By first-Principle calculations,the effect of the charge state of Au,that is,the oxidation state of the oxide,on the adsorption behaviors of O₂ and CO+O was studied in rutile-TiO₂ supported Au catalysts.It is found that Au on TiO₂(110)surface oxidized by OH,O₂ and O group adsorbs O₂ more strongly than neutral Au/TiO₂(110)does.The variation is very sensitive to the structure of oxidation species,the degree of oxidation and the position where O₂ adsorbs,but O₂ can also adsorbs on the oxidized Au/TiO₂(110)within certain limits. The adsorption of O₂ on reduced Au/TiO₂-O(2c)H is higher than on neutral Au/TiO₂(110)surface.The variation rule of the adsorption of CO+O is similar to that of O₂ adsorption.Stability:nano-Au particles dispersed on oxides,although can be highly active as catalyst,often lack the long-term stability due to the inherent instability of small Au particles.New thinking for the catalyst design is therefore urgently demanded to enhance the Au sticking on oxides while retain the high activity.By systematically investigating the working mechanism of a ternary system-Au supported on alumina modified anatase titania-using first principles calculations,it is illustrated from the atomic level how a third component(alumina)can be used as the nucleation center to anchor Au particles while itself does not take part in the catalytic oxidation of CO.For Au supported on pure anatase-TiO₂,CO oxidation is facile with the reaction barrier being only 0.22 eV,but the Au adsorption on anatase is rather weak as expected.The step-wise growth of alumina on anatase is shown to produce locally-clustered alumina thin layers on anatase(Al₂O₃/TiO₂).On these Al₂O₃/TiO₂ sites,the binding energy of Au can be more than five times than it on pure anatase.The active site of CO oxidation in the ternary system,however,remains at the boundary of Au/anatase, since it is found that O₂ does not adsorb at the boundary between Au and Al₂O₃ thin films.Electronic structure analyses are utilized to rationalize the results.The synergetic effect revealed here implies that applying non-uniform mixed oxide support could be a promising solution toward practical applications of Au-based catalysts.Recently,a new branch of heterogeneous catalysts,namely single site heterogeneous catalysts(SSHC),emerge in many industrially important reactions,and two merits are concerned:i)the cost of noble metal catalysts can be reduced dramatically,and ii)the gap between homogeneous catalysis and heterogeneous catalysis can be bridged.Single Au atoms when supported on different oxides and zeolites have been tested to be very active in hydrogenation and CO oxidation reactions.By Comparing to the activity of nano-Au supported on oxide catalysts,the investigation of single Au heterogeneous catalysts can be employed to elucidate the nature of the active sites in supported Au catalysts.Selectivity in the hydrogenation of 1,3-butadiene:it is nano-sized Au particles supported on oxides that shown exceptionally high catalytic activity,the activity increases when the Au particle size decreases.But when the size of Au particle decreases to the extent that only one single Au atom is supported on oxides,we will ask whether such single Au atom also exhibits catalytic activity.It is indicated that ZrO₂ supported single Au atom can catalyze the hydrogenation reaction of 1,3-butadiene in the experiment.and it is suspected that Au^{â…¢}is the active site.By first-principle calculations,the geometry structure of Au atom with different charge state supported on tetragonal ZrO₂ surface and the selective hydrogenation mechanism of 1,3-butadiene on Au^{â…Â} were studied.We conclude that the geometry structure of cationic Au on ZrO₂ is very similar to that of Au atom in homogeneous catalysis.It is found that Au^{â…Â} on ZrO₂ is the catalytically active species that is produced from Au^{â…¢}in situ by reduction.The catalytic roles of the oxide defects and the dynamic ligand effect in reactions have been highlighted.It is shown that in single Au heterogeneous catalysis,the oxide not only stabilizes Au monomers as the solution does in homogeneous catalysis but can also act as a catalyst by providing additional reaction sites.Selectivity in the hydrogenation of acrolein:due to the potential value of single Au atom in the single site heterogeneous catalysis for the selective hydrogenation,the geometry structure of Au atom with different charge state supported on monoclinic ZrO₂ surface and the selective hydrogenation mechanism of acrolein on Au^{â…Â} were studied.The calculated results indicate that Au^{â…¢}adsorbs most strongly on m-ZrO₂ surface at low temperature.The stability of Au^{â…Â} increases with the increase of temperature,and finally metallic Au is formed.It is found that the defects in the oxide can stabilize Au monomers,Au^{â…Â} center has a two-coordinate linear structure and Au^{â…¢}center has a four-coordinate planar structure.Our results show that AuOH/m-ZrO₂(212)exhibits high catalytic selectivity for the hydrogenation of acrolein,the energy barrier for the formation allyl alcohol is 0.46 eV,and that for the formation of propionaldehyde is 0.82 eV.Despite C=C bond is more reactive than C=O bond in thermodynamics,C=O bond is more easily attacked in dynamics. Furthermore.single Au^{â…Â} atom on m-ZrO₂ can also prohibit deep hydrogenation.