2008 Spring Symposium

 
Charles H.F. Peden, Ja Hun Kwak, Jian Zhi Hu, Do Heui Kim, and János Szanyi
Insti­tute for Inter­fa­cial Catal­y­sis
Pacif­ic North­west Nation­al Lab­o­ra­to­ry
Rich­land, Wash­ing­ton 99352, USA

Abstract — γ-alu­mi­na, one of the metastable ‘tran­si­tion’ alu­mi­na struc­tur­al poly­morphs, is an impor­tant cat­alyt­ic mate­r­i­al both as an active phase and as a sup­port for oth­er cat­alyt­i­cal­ly active phas­es. As such, the bulk and sur­face struc­ture of γ-alu­mi­na, and its for­ma­tion and ther­mal sta­bil­i­ty con­tin­ue to be the sub­ject of a con­sid­er­able amount of research. How­ev­er, due to the low crys­tallini­ty and very fine par­ti­cle size of γ-alu­mi­na, it is very dif­fi­cult to apply well-estab­lished ana­lyt­i­cal tech­niques for deter­min­ing its sur­face struc­tures.

Of par­tic­u­lar impor­tance for under­stand­ing the cat­alyt­ic prop­er­ties of γ-alu­mi­na, relat­ing its sur­face struc­ture to the ori­gin of Lewis acid­i­ty has been of con­sid­er­able inter­est and has been stud­ied by sol­id state NMR and FTIR spec­tro­scopies, and most recent­ly by the­o­ret­i­cal cal­cu­la­tions. In this pre­sen­ta­tion, we report the first use of very high field (21.1T) NMR to iden­ti­fy and quan­ti­fy sur­face Al species thought to be respon­si­ble for impart­ing Lewis acid­i­ty to the γ-Al₂O₃ sur­face. In par­tic­u­lar, a peak in the NMR spec­trum at ~23 ppm with rel­a­tive­ly low inten­si­ty, can be assigned to 5-coor­di­nat­ed Al³⁺ ions, and can be clear­ly dis­tin­guished from the two oth­er peaks rep­re­sent­ing Al³⁺ ions in tetra-, and octa­he­dral coor­di­na­tion sites. Spin-lat­tice ²⁷Al relax­ation time mea­sure­ments clear­ly show that these pen­ta-coor­di­nat­ed Al³⁺ sites are locat­ed on the sur­face of the γ-alu­mi­na sup­port. Fur­ther­more, we report the first obser­va­tion of pref­er­en­tial anchor­ing of an impreg­nat­ed cat­alyt­ic phase onto these pen­ta­co­or­di­nat­ed Al³⁺ sites by not­ing that BaO and Pt depo­si­tion onto this γ-alu­mi­na sam­ple results in the loss of inten­si­ty of the 23 ppm peak lin­ear­ly pro­por­tion­al to the amount of cat­alyt­ic phase deposit­ed. Final­ly, our recent results also sug­gest an impor­tant role for these sites in deter­min­ing the ther­mal sta­bil­i­ty of the γ-Al₂O₃ phase dur­ing high tem­per­a­ture cal­ci­na­tion.

Speaker’s Biog­ra­phy — Dr. Peden is Inter­im Direc­tor of the Insti­tute for Inter­fa­cial Catal­y­sis at Pacif­ic North­west Nation­al Lab­o­ra­to­ry (PNNL). He is also a Lab­o­ra­to­ry Fel­low and man­ages 8 sci­en­tif­ic staff with­in the Chem­i­cal Sci­ences Divi­sion at PNNL. Dr. Peden’s main research inter­ests are in the sur­face and inter­fa­cial chem­istry of inor­gan­ic solids; in par­tic­u­lar, the het­ero­ge­neous cat­alyt­ic chem­istry of met­als and oxides (reac­tion mech­a­nisms, mate­ri­als).