View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Digital.CSIC JOURNAL OF CHEMICAL PHYSICS VOLUME 121, NUMBER 11 15 SEPTEMBER 2004 The behavior of mixed-metal oxides: Physical and chemical properties of bulk Ce1ÀxTbxO2 and nanoparticles of Ce1ÀxTbxOy Xianqin Wang, Jonathan C. Hanson, Gang Liu, and Jose´ A. Rodrigueza) Chemistry Department, Brookhaven National Laboratory, Building 555, Upton, New York 11973 Ana Iglesias-Juez and Marcos Ferna´ndez-Garcı´aa) Instituto de Cata´lisis y Petroleoquı´mica, CSIC, Campus Cantoblanco, 28049 Madrid, Spain ͑Received 9 April 2004; accepted 17 June 2004͒ The physical and chemical properties of bulk Ce1ϪxTbxO2 and Ce1ϪxTbxOy nanoparticles (x р0.5) were investigated using synchrotron-based x-ray diffraction ͑XRD͒, x-ray adsorption near edge spectroscopy ͑XANES͒, Raman spectroscopy ͑RS͒, and first-principles density-functional ͑DF͒ calculations. DF results and Raman spectra point to a small tetragonal distortion after introducing terbium in ceria. The results of XRD show a small contraction ͑р 0.08 Å͒ in the cell dimensions. The presence of Tb generates strain in the lattice through the variation of the ionic radii and creation of crystal imperfections and O vacancies. The strain increases with the content of Tb and affects the chemical reactivity of the Ce1ϪxTbxOy nanoparticles towards hydrogen, SO2 , and ϭ ͒ NO2 . DF calculations for bulk Ce1ϪxTbxO2 and Ce8ϪnTbnO16 (n 0, 1, 2, or 4 clusters show oxide systems that are not fully ionic. The theoretical results and XANES spectra indicate that neither a $ Ce Tb exchange nor the introduction of oxygen vacancies in Ce1ϪxTbxOy significantly affect the charge on the Ce cations. In contrast, the O K-edge and Tb LIII-edge XANES spectra for Ce1ϪxTbxOy nanoparticles show substantial changes with respect to the corresponding spectra of Ce 3ϩ 4ϩ and Tb single oxide references. The Ce0.5Tb0.5Oy compounds exhibit a much larger Tb /Tb ratio than TbO1.7 . A comparison with the properties of Ce1ϪxZrxOy and Ce1ϪxCaxOy shows important differences in the charge distribution, the magnitude of the dopant induced strain in the oxide lattice, and a superior behavior in the case of the Ce1ϪxTbxOy systems. The Tb-containing oxides combine stability at high temperature against phase segregation and a reasonable concentration of O vacancies, making them attractive for chemical and catalytic applications. © 2004 American Institute of Physics. ͓DOI: 10.1063/1.1781116͔ I. INTRODUCTION developed.1,3 Ceria or ceria-based materials have been re- ported to be good for improving the performance of catalysts 5–7 Mixed-metal oxides play a very important role in many employed for the destruction of nitrogen oxides (DeNOx) areas of chemistry, physics, and materials science.1,2 In gen- due to their unique ability to combine oxygen storage and eral, there is a need to obtain a fundamental understanding of release by the shift between reduced Ce3ϩ and oxidized the phenomena associated with the behavior of these com- Ce4ϩ states.8–10 The chemical and catalytic behavior of ceria plex systems. In this paper, we investigate the physical and and ceria-based materials in DeNOx and DeSOx processes 10–13 chemical properties of bulk Ce1ϪxTbxO2 and Ce1ϪxTbxOy have attracted a lot of attention. Studies with well- nanoparticles (xр0.5). Rare-earth oxides have been fre- defined single-crystal surfaces have shown the important role quently used as structural and electronic promoters to im- that O vacancies and structural imperfections play in the prove the activity, selectivity, and thermal stability of metal cleavage of N-O and S-O bonds on ceria-based oxides.14 1–4 catalysts. CeO2 , among the rare-earth oxides, is certainly In many cases, the redox properties and chemical activ- 2 the most widely used oxide in industrial catalysis, especially ity of pure ceria can be enhanced by introducing different for the treatment of combustion emissions ͑oxidation of CO, types of metals ͑Zr, Ca, Cu, Tb, Mn, etc.͒ into the oxide destruction of nitrogen and sulfur oxides͒. The emission of lattice.14–17 With respect to single-metal oxides, the chemical nitrogen and sulfur oxides (NOx and SOx) from fuel com- behavior of mixed-metal oxides may be different as a conse- bustion in power stations, industrial heaters, and road trans- quence of several factors.12 At a structural level, a dopant or portation leads to a variety of environmental problems: the second metal can introduce stress into the lattice of an oxide formation of acid rain and the resultant acidification of host, inducing in this way the formation of defects that have ͑ ͒ aquatic ecosystems, the generation of ozone smog in the a high chemical activity. On the other hand, the lattice of the troposphere, and respiratory health hazards for humans. De- oxide host can impose on the dopant element nontypical co- pending on the nature of the combustion process, various ordination modes with a subsequent perturbation in the dop- approaches for NOx and SOx removal have been ant chemical properties. Finally, metal$metal or metal$oxygen$metal interactions in mixed-metal oxides a͒Author to whom correspondence should be addressed. can lead to electronic properties not seen in single-metal 0021-9606/2004/121(11)/5434/11/$22.005434 © 2004 American Institute of Physics Downloaded 24 Feb 2010 to 161.111.180.191. Redistribution subject to AIP license or copyright; see http://jcp.aip.org/jcp/copyright.jsp 5435 J. Chem. Phys., Vol. 121, No. 11, 15 September 2004 Behavior of nanoparticles of Ce1ϪxTbxOy andbulkCe1ϪxTbxO2 oxides.12 Recently, the structural and electronic properties of found no indication of beam induced damage on the oxide nanoparticle that combine Ce and Zr or Ca have been Ce1ϪxTbxOy samples. The temperature programmed sinter- 11͑a͒ studied due to their potential uses in the field of catalysis. ing of the samples in air or reaction under a 5% H2 /He Zr mixing (Ce1ϪxZrxO2) improved the thermal stability of environment was investigated. Each sample was loaded into ceria nanoparticles, but did not introduce a significant num- a sapphire capillary cell which was attached to a flow ber of oxygen vacancies.18 On the other hand, Ca mixing system.18,19,27,28 A small resistance heater was wrapped (Ce1ϪxCaxOy) introduced a lot of oxygen vacancies in the around the capillary, and the temperature was monitored with host structure, which radically modify the chemical proper- a 0.1 mm chromel-alumel thermocouple which was placed ties of the system and facilitate the destruction of N- and straight into the capillary near the sample. In the case of air S-containing pollutants,11͑a͒,12,14 but the mixed-metal oxide environment, both sides of the sapphire cell were open to air, had a relatively low stability and phase segregation into while they were connected to the gas lines for the 5% H2 /He CeO2 and CaO was observed at temperatures close to environment. Diffraction patterns were collected in the tem- 700 °C.19 A priori, it is not clear what type of second metal is perature range of 25–925 °C for sintering in air, and 25– useful to improve both the stability at high temperature and 900 °C in the case of reduction with hydrogen. A MAR345 the chemical activity, which as said above, is directly linked detector was used to record x-ray patterns. The data for oxy- to the level of O vacancies in ceria. It is interesting to inves- gen occupancy determination were collected at the beamline tigate the effects of a lanthanide like Tb on the properties of 1-ID (␭ϭ0.1536 Å) of x-ray Operation and Research in the ceria-based oxides.8,15͑b͒,20,21 Advance Photon Source ͑APS͒ at Argonne National Labora- Ce1ϪxTbxOy compounds have been reported to have a tory with the same setup as that at the NSLS. The data with superior performance in the storage or release of a high-order wave vector Q ͑Ϸ11.5 ÅϪ1͒ from the APS give oxygen.8,20–23 It was proposed that the presence of O vacan- a better determination of the structural parameters. cies associated with Tb3ϩ contributed to enhance the oxygen In this work, a whole profile refinement with the com- mobility.24 In order to fully understand this phenomenon, we mercial Reflex package provided by Accelrys was employed must know well the physical and chemical properties of the to obtain the particle sizes, cell dimension, and lattice strain. 3ϩ Ce1ϪxTbxOy system and verify the existence of Tb and the Here lattice strain was determined from the variation of peak concomitant presence of O vacancies. In this work, first- width, and is an estimation of the stress, produced by imper- principles density-functional ͑DF͒ calculations11,12 are used fections and defects, in the crystal structure.29͑a͒–29͑e͒ The to investigate the structure and bonding in Ce-Tb oxides data analysis consisted of several steps. First, the powder ͑clusters and bulk compounds͒. The physical and chemical rings were integrated with the FIT2D ͑Ref. 30͒ code for each properties of Ce1ϪxTbxOy nanoparticles are systematically slice of the sample. The FIT2D parameters for the integration investigated using synchrotron based time-resolved x-ray of the data were obtained from a standard LaB6 powder pat- diffraction,18,19 x-ray absorption near edge spectroscopy tern, and were ascertained with a silicon powder pattern. ͑XANES͒,11,18,19 and Raman spectroscopy.11,19 At the end, Thereafter, the particle size, cell dimension, and strain in the the results are compared with those obtained with the same lattice of samples were also obtained by a whole profile re- techniques for the Ce1ϪxZrxO2 and Ce1ϪxCaxOy finement with the commercial Reflex package provided by systems.18,19 The Tb-containing oxides are special, showing Accelrys.29͑b͒ The instrumental parameters were derived distinctive structural and electronic properties that make from the fit of a Si reference pattern using a Thompson-Cox- them attractive for chemical and catalytic applications.