Influence of Ce3+/Ce4+ Ratio on the Oxidative Catalytic Activity of Cerium Oxide Nanostructures. Cerium oxide (Ce2O3 and CeO2) nanostructures have attracted considerable attention for their high oxidative catalytic activities and relatively large abundance in the earth's crust.1, 2 Cerium oxides have been applied in a wide range of important industrial applications including oxygen sensors, three-way catalytic converters, solid oxide fuel cells, the water gas shift reaction catalysts, and other oxidative reaction catalysis. Literature often attributes the catalytic activity of cerium oxide to its high oxygen storage capacity (OSC) which is largely due to the multi-valence nature of cerium. The shift between the cerium(III) (Ce3+) to cerium(IV) (Ce4+)states leads to a high oxygen mobility in ceria lattice that in turn leads to a strong catalytic potential.3-6 Recently, nanosized cerium oxide has been extensively studied for their enhanced oxidative and reductive catalytic activity because they possess large surface areas and increased surface energy when compared to the bulk materials. Many methods such as hydrothermal synthesis, precipitation, spray pyrolysis, and electrochemical methods to produce nanoscale cerium oxide have been explored to yield a wide variety of other materials morphologies such as nanowires, nanocubes, nanorods, and nanotubes.
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Surface and sub-surface lattice defects of catalysts also play important roles in influencing their catalytic activity. Much of the catalytic ability of the cerium oxide is attributed to high mobility of oxygen in the lattice and to its large oxygen storage capacity.7, 8 The high mobility of oxygen within the lattice combined with the ability of cerium to readily oxidize and reduce between Ce3+ and Ce4+ leads to the formation of oxygen vacancy defect (OVD) sites. OVDs have been studied as a major contributor to the catalytic activity of the cerium oxide because oxidation and reduction can be achieved easily at these sites. Specifically, the ratios of Ce3+ to Ce4+ in cerium oxides have been found to indirectly correlate to the number of oxygen vacancy defect sites. This provides a convenient means to evaluate the OVD density in cerium oxide catalyst. Since the size, concentration, and type of OVDs can critically affect the enhanced catalytic activity of cerium oxide, these features are considered as possible catalyst design criteria. Nonetheless, most of the catalyst design research conducted for cerium oxides so far has revolved around the naturally occurring OVDs but not the intentional introduction of the defects.
Synthetic strategy to introduce oxygen lattice defects in cerium oxide catalysts essentially can be modulated to the control of Ce3+/Ce4+ ratio in a reproducible and energetically favorable manner. Cerium oxide naturally occurs as a mixture of Ce3+ and Ce4+, in the bulk form because â€¦.. The ratio Ce3+ /Ce4+ at the bulk material surface is typically determined between 2 and 15 % primarily by x-ray photoelectron spectroscopy (XPS) analysis. Thus a significant increase in Ce3+ /Ce4+ ratio over 20 % can be applied an indirect indication of increased OVD in the cerium oxide catalyst.
Here we report our study of the oxidative catalytic activity of nanostructured cerium oxide catalyst with intentionally introduced of oxygen lattice defects. We found that the number of stable OVDs on cerium oxide nanoparticles and nanotubes could be increased through the use of elevated temperature annealing under vacuum in the post production activation process. The oxidative catalytic activities of these nanostructured catalysts were evaluated using the standard carbon monoxide (CO) oxidation reaction and were found to have much lower light-off temperatures when compared to the bulk counterpart. The chemical equilibrium reactions on the catalysts surface in vacuum were hypothesized to explain the unusual increase in the OVD density of the reported cerium oxide nanostructured catalysts.
All water used in this experiment was Ultrapure water of >18 Mâ„¦ resistivity and filtered through 0.22 nm pore-sized filters. All chemicals were used as purchased unless otherwise noted. Bulk cerium oxide powder with 300 mesh size (Sigma-Aldrich, St. Louis, MO) and cerium oxide nanopartices with 7-nm average diameter (Nanoscale, Manhattan, KS) were used for catalytic activity evaluations and comparisons with cerium oxide nanotubes. The cerium oxide nanoparticle samples have aggregate particle size â‰¤ 9 Âµm.
Synthesis of Cerium Oxide Nanotubes
Cerium oxide nanotubes were synthesized using a modified method developed by Zhou et al.3 (Scheme 1) Briefly, a sample of 0.5 g cerium(III) sulfate hydrate (Ce2(SO4)3Â· X H2O, Sigma-Aldrich, St. Louis, MO) was first dissolved into 40 mL of 10 M sodium hydroxide aqueous solution (NaOH (aq), Sigma-Aldrich, St. Louis, MO) solution. The solution was transferred to a 45 mL total volume Parr autoclave, and was allowed to react at 120 Â°C for 15 h. The cooled sample was filtered using 0.8 Âµm membranes (Millipore, Billerica, MA) and rinsed with 3 aliquots of 50 mL water. After rinsing, the sample was placed in a convection oven at 50 Â°C for 1 h. The samples were then gently powdered using a spatula and heated at 50 Â°C for an additional hour for this partial oxidation step. The resulting samples were mixed with 50 mL of water, and 50 mL of â‰ˆ15% hydrogen peroxide (H2O2, Sigma-Aldrich, St. Louis, MO), immediately followed by sonication for 30 min following sonication the samples were left in the H2O2 solution for an additional 60 minutes for their oxidative transformation into tubular structures. Lastly, the material was filtered using a 0.8 Âµm membrane rinsed with three aliquots of water and dried in a convection oven at 50 Â°C.
Activation treatment of cerium oxide samples
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Typically, 100 mg of cerium oxide sample (nanoparticles, nanotubes or bulk materials) was activated by heating the sample in a 1" quartz tube furnace with a 100 SCCM flow of a nitrogen-oxygen mixture (80% N2 and 20% O2) for 1 hour at 350 Â°C under vacuum with an operating pressure of 0.1 Torr. Control samples were activated using similar experimental condition but with 1 atm operation pressure.
CO oxidation catalysis
The CO oxidation catalysis was carried out in a U-shaped quartz micro reaction chamber fitted with a coarse quartz frit sample platform. Typically, in each catalysis test, a mixture of 79% helium, 20 % oxygen and 1 % carbon monoxide was flowed continuously throughout the reaction at a rate of 30 SCCM through a 100-mg sample which completely covered the frit. The reaction chamber was warmed in 5 â°C increments from room temperature to 250 â°C. A 1 mL sample of process gas obtained at each reaction temperature was analyzed using a gas chromatography instrument (Gow-Mac, manufacturer information) equipped with an eight foot porapak Q column (Manufacturer information). The percentage of CO conversion was determined by quantifying the carbon dioxide concentration in the processed gas.
The structural morphology of the cerium oxide samples was examined with a field emission scanning electron microscope (FE-SEM, Hitachi S4700, Hitachi High Technologies America, Inc. Pleasanton, CA) operated at 15keV Detailed structures of nanomaterials were investigated by high resolution transmission electron microscopy (HRTEM) with a Tecnai G2 F20 S-Twin operated at 200keV (FEI, Hillsboro OR). Selected area electron diffraction (SAED) was used to determine the local structures of the materials. Each TEM sample was prepared by drop-casting a solution of sample sonnicated in methanol onto a holey carbon film on a copper grid support. X-ray diffraction (XRD) of the samples was performed with a Bruker AXS D8 Discover equipped with a GADDS area (Bruker AXS Inc. Madison, WI) to examine the crystallinity and crystal structure of the samples in bulk form. The weighted average wavelength of the Cu KÎ± x-ray source is 1.5417 Å.
The surface areas of the three types of cerium oxide catalysts (bulk powder, nanoparticles, and nanotubes) determined via the Braunauer Emmet Teller (BET) extension of the Langmuir isotherm with a Micromeritics ASAP 2010 (Micromeritics, City, ST). Nitrogen was used as the adsorbent gas in these experiments.
Chemical composition characterization
The elemental compositions of the cerium oxide samples were determined by energy dispersive x-ray spectroscopy (EDS) using an EDX detector (EDAX Inc, Mahwah, NJ) equipped with the HRTEM. The Ce3+/Ce4+ ratio for each sample was quantified by XPS (VersaProbeâ„¢ Scanning XPS Microprobe, Physical Electronics, Inc, Chanhassen, MN) according to recent literature. Briefly, for each experimental spectrum, the XPSpeak program (publisher) was first applied to subtract a fitted base-line using the Shirley algorithm from the data. Semi-Voigt functions (convolved Gaussian-Lorentzian lineshapes) were then fitted to the resulting spectrum to determine the ten peak areas corresponding to the signals from Ce3+ and Ce4+. . Since it has been widely reported that no single peak is linearly proportional to the ratio of Ce3+ to Ce4+, it was necessary to deconvolute and to calculate the areas of each of the ten peaks. The deconvolution of the XPS data to yield the Ce3+/Ce4+ ratios was accomplished by using the program XPSpeak (PUBLISHER) to fit the baseline using the Shirley algorithm and the relevant XPS peaks corresponding to the cerium signals. Reported peak locations from the liturature were used; at least 80% Gaussian character was assumed. The pPeaks were fitted in a series of steps which allowed the areas and their full width at half maximum to vary throughout all steps. , tThe percentage of gaussian Gaussian contribution for each lineshape was allowed to vary between 80 and 100% after the initial fit. and tThe peak location was allowed to vary up to 0.2 eV due to the nanoscale nature of the materials only during the last step. The ratio of the integrated peak area was then calculated as shown indicated in eqn and by comparing the peak areas for Ce3+area to compared to the total area calculated for both Ce3+ and Ce4+ according to the reported literature..
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