Reversible structure transformation of zirconium dioxide on palladium black

Abstract

The structure and behavior of Zr02 on Pd black were studied by means of extended X-ray absorption fine structure (EXAFS). The mmible structure change of ZrO2 with the reduction at 773 K and oxidation at 673 K was observed. The Zr-Zr distances on Zr02/Pd after oxidation at 673 K were determined to be 0.345 and 0.363 nm (named b-Zr02), while Zr02/Pd after reduction at 773 K showed the tetragonal ZrOz structure with a Z r Z r distance at 0.366 nm. The & structure is stable against the H2 reduction at 473 K. The &-Zr02 structure is suggested to be stabilized by the interface Pd oxide. IatrodtlctionThe structures of and catalysis by supported metal particles are affected by the interaction with inorganic oxide supports.'-3 Convcrsely, the surface structures of oxide supports should also vary under the influence of metal particles and undergo physical or chemical change! In many c~8es, however, the change in the surface structures of oxide supports are spectroscopically hidden by the large amounts of unchanged bulk component. Thus little information is available about the structural change of support surfacts. In this work we used Pd black (small Pd particle with a crystallite size of 16.3 nm) on which Zr02 was deposited. The Pd black has a large surface area of 25.5 m2/g and efficiently interacts with Zr02 resulting in a good dispersion of Zr02. In this system one can obtain fundamental information about the structure and behavior of deposited Zr02 layers on Pd surface during the oxidation, " , and thermal treatments. Catalysis by Zr02-supported noble metals has been extensively Unlike Nb20S and Ti02, ZrO2 is hardly reduced and not a socalled SMSI" support. Howwer, the modification of the catalysis of Zr02-supported metals has been attributed to the partially r e d u d Zro, species at the interface by ESR spectroscopy.6 The donation of electrons from Zr02 to Pt in Pt/Zr02 catalyst increased with an increase in the reduction temperature similar to the cases of Ti02 and Nb@S?*lo The surface structural chemistry of Zr02 at the metal-support interface is of interest in relation to the origin of metal-support interaction and catalysis of supported metal.To examine the structure of Zr02 overlayers on Pd black, we employed extended X-ray absorption fine structure (EXAFS)" spectroscopy, which provides direct information on the local structure around an X-ray absorbing atom. In the present study we found a reversible structural change of Zr02 on Pd during the oxidation-reduction cycle. Experimental SectionPd black was purchased from Nippon Engelhalt Co. (crystallite size = 16.3 nm). The surface area of untreated Pd black was 25.5 m2/g by the BET measurement. ZrCll was purchased from the Soekawa Co. The Pd black was impregnated with an ethanol solution of ZrC4. The loading was 7 wt 96, corresponding to the amount of one monolayer of Zr metal atoms. After dried at 373 K, the samples were oxidized at 773 K. Then they were treated under various conditions in U-shaped tubes equipped with air-tight EXAFS cells which were connected t...

Supported Catalysts from Chemical Vapor Deposition and Related Techniques

“…The supported vanadium oxides exhibit a higher activity for the photocatalytic isomerization reactions of 2-butene, as compared with those obtained for the oxides prepared by a conventional impregnation method. Similar reversible structure transformation of a ZrO 2 overlayer has been found with the ZrO 2 overlayer on Pd black by EXAFS [114]. After treatment in H 2 , the ZrO 2 formed non-wetting particles on the Pd, with a sharp Pd/ZrO 2 interface [113].…”

Study of the methane combustion and TPR/TPO properties of Pd/Ce–Zr–M/Al2O3 catalysts with M=Mg, Ca, Sr, Ba

“…According to the previous work in our research group, the reduction of PdO species appears at 60-90 • C for Pd/Ce-Zr-O catalysts [25,26]. So we suggest that ␣ peak may be the PdO dispersed on [28,29]. In addition, we can see that the α peak shifts to lower temperature about decreasing 17 • C after the addition of alkaline earths, indicating that the addition of alkaline earths increases the reducibility of PdO dispersed on alumina-rich grains.…”

Effects of CeO2-ZrO2 present in Pd/Al2O3 catalysts on the redox behavior of PdOx and their combustion activity

“…H 2 -TPR profile of Pd/Ce-Zr/Al 2 O 3 catalyst exhibits two peaks of hydrogen consumption at 35 and 102 8C, respectively, and the corresponding H 2 consumption is 53.7 and 10.6 mmol/g cat , and a negative peak at about 90 8C, however, there is no reduction peak of Ce 4+ to Ce 3+ in the range of 180-800 8C. Therefore, the positive peak at 35 8C should be attributed to consumption of hydrogen on the reduction of highly dispersed PdO species, which indicates variation in the distribution of PdO with support composition, while the new reduction peak between 100 and 150 8C may be attributed to a result of the reduction of stable PdO species and of the strong interaction between the PdO x and the Ce-Zr oxides that would promote the reduction of Ce 4+ to Ce 3+ [9,12,22]. From Fig.…”

“…Many materials, such as noble metals, single transition metal oxides and their binary oxides, and mixed metal oxides with a perovskite-or spinel-type structure have been tested as catalyst for the combustion of methane. Among which the supported PdO catalysts have been thought to perform the best [8][9][10][11], while g-Al 2 O 3 is usually adopted as support of the PdO catalysts in order to maintain favorable dispersion of active PdO x to achieve valid utilization of precious metal. However, alumina-supported PdO catalysts are not stable at high temperature commonly used for the combustion of methane due to a significant decrease in the surface area of alumina and to the transformation of PdO x to Pd 0 , and the addition of a promoter in order to improve the thermal stability of PdO phase in Pd/Al 2 O 3 catalyst is unusually considered as an attractive alternative.…”