Page - 116 - in Advanced Chemical Kinetics

BIOVIAMaterials Studio 2016 environment using the generalised gradient approximation (GGA), with a double numerical basis set (DNP) and the Perdew-Becker-Ernzerhof (PBE) exchange-correlation functional. All electrons were included in the calculations with unrestricted spin-polarization. A fine integration grid was used together with a Fermi smearing of 0.005Hartree (Ha). The energy convergence tolerancewas set to 1.0 10 5Ha; themaximumforcewas0.002Ha/Åandmaximumdisplacementwas set at 0.005Å.The self- consistent field (SCF)densityconvergencewasset1.0e 6. The complete linear synchronous transit and quadratic synchronous transit (LST/QST) method [19] was used to locate the transition state structures according to the optimised structures of reactants and products. The nudged elastic band (NEB) method [20], as implemented in DMol3, was used to confirm that the transition state structures lead to the expected reactant andproductmolecular structures. Frequency calculationswere performed toconfirmthenatureofall stationarypointsaseitherminimaor transitionstates (TSs). When investigating a reaction that takes place on the surface of a heterogeneous catalyst by quantum chemical methods, one of the main difficulties is modelling an infinite catalyst system as highlighted by Handzlik and Ogonowski [21], and a viable approach with the capacityof solving this problem is by the applicationof clustermodels. Songet al. [22] claim that, ideally, clustermodels are appropriate if a suitable boundary condition is obtained such that chargesare in reasonabledistributionon thesurface. In this study,wechoseaAl3MoO7H cluster to represent anactive siteofMoonMoO3/γ-Al2O3.Assuming that theoxidationstates of theelements inthemodelaredistributedasfollows:Al=3+,Mo=4+,H=1+andO=2 ; then theoverall chargeof theclusterwouldbezero.Fromthispointofviewit canbeassumedthat whenthesubstrateadsorbsonAl3MoO7H, itoccupies thevacant sitesonMoto formacluster model inwhichtheMois inanapproximateoctahedralenvironmentwitha6+oxidationstate. This adsorptionmode is consistentwith theworkofKonget al. [23]who indicatedhowLAC couldbe formedfromadsorbed intermediates suchaspyruvaldehyde (PA). 3.Results 3.1.Catalyst characterisation Thesampleof theNH3-TPDprofiles isdisplayed inFigure1 shows that theAu-MoO3/γ-Al2O3 catalystwasmore acidic than the originalγ-Al2O3 support.All thematerials displayednotice- ableLewisacidity (that is, electronacceptingsitesasopposed to theBrønstedacidity,whichare regardedasprotondonatingsites) since theammoniadesorptionoccurredat the lower temper- atures (below 300 C). Theγ-Al2O3 support indicated Lewis acidity having different strengths with theweaker sites desorbingNH3at about 180 Cwhile the stronger sites desorbedNH3at about300 C. Ontheadditionofgold, theAu/γ-Al2O3catalyst showedashift in the twopeaks to thehigher temperatures, that is, 220and450 C, respectively.However, theadditionofMoO3onγ-Al2O3 support exhibitedonly the (weaker) Lewis acid sites thatdesorbedNH3at the lower temper- atures,peakingat220 C.This finding is inagreementwithanumberof researcherswhohave Advanced Chemical Kinetics116