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Luo, J.,Gao, F.,Kim, D.H.,Peden, C.H.F. Elsevier Science Publishers 2014 CATALYSIS TODAY - Vol.231 No.-
The effects of K loading and thermal aging on the structural properties and high temperature performance of Pt/K/Al<SUB>2</SUB>O<SUB>3</SUB> lean NO<SUB>x</SUB> trap (LNT) catalysts were investigated using in situ X-ray diffraction (XRD), temperature-programmed decomposition/desorption of NO<SUB>x</SUB> (NO<SUB>x</SUB>-TPD), transmission electron microscopy (TEM), NO oxidation, and NO<SUB>x</SUB> storage tests. In situ XRD results demonstrate that KNO<SUB>3</SUB> becomes extremely mobile on the Al<SUB>2</SUB>O<SUB>3</SUB> surface, and experiences complex transformations between orthorhombic and rhombohedral structures, accompanied by sintering, melting and thermal decomposition upon heating. NO<SUB>x</SUB> storage results show an optimum K loading around 10% for the best performance at high temperatures. At lower K loadings where the majority of KNO<SUB>3</SUB> stays as a surface layer, the strong interaction between KNO<SUB>3</SUB> and Al<SUB>2</SUB>O<SUB>3</SUB> promotes KNO<SUB>3</SUB> decomposition and deteriorates high-temperature performance. At K loadings higher than 10%, the performance drop is not caused by NO<SUB>x</SUB> diffusion limitations as for the case of barium-based LNTs, but rather from the blocking of Pt sites by K species, which adversely affects NO oxidation. Thermal aging at 800<SUP>o</SUP>C severely deactivates the Pt/K/Al<SUB>2</SUB>O<SUB>3</SUB> catalysts due to Pt sintering. However, in the presence of potassium, some Pt remains in a dispersed and oxidized form. These Pt species interact strongly with K and, therefore, do not sinter. After a reduction treatment, these Pt species remain finely dispersed, contributing to a partial recovery of NO<SUB>x</SUB> storage performance.
Kim, Do Heui,Mudiyanselage, Kumudu,Szanyi, Já,nos,Hanson, Jonathan C.,Peden, Charles H. F. American Chemical Society 2014 The Journal of Physical Chemistry Part C Vol.118 No.8
<P>Based on the combined FTIR and XRD studies, we report here that H<SUB>2</SUB>O induces a morphological change of KNO<SUB>3</SUB> species formed on model K<SUB>2</SUB>O/Al<SUB>2</SUB>O<SUB>3</SUB> NOx storage-reduction catalysts. Specifically as evidenced by FTIR, the contact of H<SUB>2</SUB>O with NO<SUB>2</SUB> preadsorbed on K<SUB>2</SUB>O/Al<SUB>2</SUB>O<SUB>3</SUB> promotes the transformation from bidentate (surface-like) KNO<SUB>3</SUB> species to ionic (bulk-like) ones irrespective of K loadings. Once H<SUB>2</SUB>O is removed from the sample, a reversible transformation into bidentate KNO<SUB>3</SUB> is observed, demonstrating a significant dependence of H<SUB>2</SUB>O on such morphological change. TR-XRD results show the formation of two different types of bulk KNO<SUB>3</SUB> phases (orthorhomobic and rhombohedral) in an as-impregnated sample. Once H<SUB>2</SUB>O begins to desorb above 400 K, the former is transformed into the latter, resulting in the existence of rhombohedral KNO<SUB>3</SUB> phase only. On the basis of consistent FTIR and TR-XRD results, we propose a model for the morphological changes of KNO<SUB>3</SUB> species with respect to NO<SUB>2</SUB> adsorption/desorption, H<SUB>2</SUB>O and/or heat treatments. Compared with the BaO/Al<SUB>2</SUB>O<SUB>3</SUB> system, K<SUB>2</SUB>O/Al<SUB>2</SUB>O<SUB>3</SUB> shows some similarities with respect to the formation of bulk nitrates upon H<SUB>2</SUB>O contact. However, there are significant differences that originate from the lower melting temperature of KNO<SUB>3</SUB> relative to Ba(NO<SUB>3</SUB>)<SUB>2</SUB>.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/jpccck/2014/jpccck.2014.118.issue-8/jp410816r/production/images/medium/jp-2013-10816r_0008.gif'></P>
Grafting sulfated zirconia on mesoporous silica
Wang, Yong,Lee, Kwan-Young,Choi, Saemin,Liu, Jun,Wang, Li-Qiong,Peden, Charles H. F. Royal Society of Chemistry 2007 Green chemistry Vol.9 No.6
<P>Recently, sulfated zirconia has received considerable attention as a potential solid acid catalyst to replace problematic homogeneous acid catalysts. In this paper, the preparation and properties of acid catalysts obtained by grafting ziconia with atomic precision on MCM-41 mesoporous silica were studied. TEM and potential titration characterizations revealed that ZrO<SUB>2</SUB>/MCM-41 with monolayer coverage can be obtained using this grafting technique. Sulfated ZrO<SUB>2</SUB>/MCM-41 exhibits improved thermal stability than that of bulk sulfated zirconia, as evidenced by temperature programmed characterizations and XRD analysis. Temperature programmed reaction of isopropanol was used to evaluate the acidity of sulfated ZrO<SUB>2</SUB>/MCM-41. It was found that the acid strength of sulfated ZrO<SUB>2</SUB>/MCM-41 with monolayer coverage is weaker than bulk sulfated zirconia but stronger than SiO<SUB>2</SUB>–Al<SUB>2</SUB>O<SUB>3</SUB>, a common strong acid catalyst.</P> <P>Graphic Abstract</P><P>Sulfated zirconia solid acid catalysts were prepared by grafting zirconium oxide on mesoporous silica with layer-by-layer coverage, followed by depositing sulfate groups on the surface. <IMG SRC='http://pubs.rsc.org/services/images/RSCpubs.ePlatform.Service.FreeContent.ImageService.svc/ImageService/image/GA?id=b614928f'> </P>
Heo, Iljeong,Schmieg, Steven J.,Oh, Se H.,Li, Wei,Peden, Charles H. F.,Kim, Chang Hwan,Szanyi, Já,nos The Royal Society of Chemistry 2018 Catalysis science & technology Vol.8 No.5
<P>Cerium-zirconium mixed oxides (CZ) containing Cu (CZCu) have very high CO oxidation performance at low temperatures under simulated diesel exhaust conditions; however, CZCu is prone to significant deactivation upon hydrothermal aging. In this work, we discovered that Cu supported on commercial CZ (GMR6) is thermally stable, and after aging Cu/GMR6 shows comparable activity for CO oxidation to the fresh CZCu. GMR6 is more thermally stable than homemade CZ, and Cu/GMR6 shows a smaller decrease of BET surface area than Cu/CZ upon hydrothermal aging. The decrease of the BET surface area of the catalyst is accelerated by the presence of Cu. The presence of La and Pr in the GMR6 support provides high structural stability and enhanced oxygen mobility. The formation of both surface and bulk carbonates during CO oxidation results in the activity decrease of Cu/GMR6. Thermal decomposition of these carbonates fully regenerates Cu/GMR6. The presence of low levels of SO2 in the feed gas mixture poisons the CO oxidation activity of Cu/GMR6 forming highly stable sulfates on the CZ support.</P>