Seite - 121 - in Advanced Chemical Kinetics

TheexperimentallyobtainedEA in thiswork, as shown inTable1, is ingoodagreementwith the 38kJmol�1 value reportedbyWörzet al. [27] for glycerol oxidation, although theirwork wasbasedonaPt-Bi/Ccatalyst.Demireletal. [28]alsoreportedactivationenergiesofbetween 40and50kJmol�1 for theAu/Ccatalysedglyceroloxidation,which isaresultnot far fromour own. Finally, Chornaja et al. [10] also reported an apparentEA of about 39� 3 kJmol�1 for glyceroloxidationoverPd/Al2O3,which isalsoconsistentwithour findings. As a control, both the bulk γ-Al2O3 andMoO3/γ-Al2O3 supports did not showany activity towardsglyceroloxidationat90�C. 3.2.4.The effect ofAunanoparticle size The effect of Au particle size on the kinetics of glycerol conversion was also investigated. Figure 5 shows TEM images of three Au/γ-Al2O3 catalysts with different Au particle sizes. When screened for glycerol oxidation under the same reaction conditions, the catalysts sur- prisingly revealed that thekinetics of this reaction stronglydependon themetalparticle size, asshowninFigure6.BigAunanoparticles,ca20nm,exhibit first-orderkineticswhile thedata for smaller Au nanoparticles, ca 4 nm, does not fit this model, instead showing zero-order kinetics as it has alreadybeendiscussed.Moredetails of thisphenomenonhavealreadybeen reportedelsewhere [14]. 3.3.Globalkineticmodelprediction So farwehaveonly concentratedon thedepletionkinetics of the reactant. In this section,we explore theglobalkineticsof thereaction, i.e.weset-upakineticmodel that takes intoaccount the full mass balance of the reaction. At full substrate penetration and surface coverage, conversion isnot limitedbymass transfer; it is a fixedquantity set by thezero-orderkinetics. Accordingly,mass transfer termshavenotbeenexplicitlyexpressed in thekineticmodelused in this study since kinetic data was collected under the kinetic control regime. Therefore, Figure 7presents the reactionnetworkonwhich thekineticmodellingwasbased.The concen- trationsofall thecompounds in the figurewere taken intoaccount in thecalculations.Since the partialpressure (henceconcentration)ofO2wasmaintainedhigh inexcess, its surface coverage Model# Parameter Temperature XA¼ kCA0 � t k 60�C 90�C Value Confidence limits Value Confidence limits 0.0059 0.0058 0.0061 0.018 0.017 0.019 EA¼ T1�T2�RT2�T2 ln k2k1 EA Value Confidence limits 37.4 36.1 38.1 Parametervaluesestimatedat95%confidence level;k=molL�1min�1andEA=kJmol �1. #R=8.314 JK�1mol�1. Table 1. Regression analysis of conversionvs. timedata for the 0.9-wt%Au/γ-Al2O3 catalyst at different temperatures. Oxidation of Glycerol to Lactic Acid by Gold on Acidified Alumina: A Kinetic and DFT Case Study http://dx.doi.org/10.5772/intechopen.70485 121