Activity and Characterization by XPS, HR-TEM, Raman Spectroscopy, and BET Surface Area of CuO/CeO 2 -TiO 2 Catalysts

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Activity and Characterization by XPS, HR-TEM, Raman Spectroscopy, and BET Surface Area of CuO/CeO 2 -TiO 2 Catalysts
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  Activity and Characterization by XPS, HR-TEM, Raman Spectroscopy, and BET SurfaceArea of CuO/CeO 2 -TiO 2  Catalysts Maria Suzana P. Francisco and Valmor R. Mastelaro*  Departamento de Fı ´ sica e Cie ˆ ncia dos Materiais, Instituto de Fı ´ sica de Sa ˜ o Carlos,Uni V  ersidade de Sa ˜ o Paulo, C. P. 369, Sa ˜ o Carlos, SP, Brazil 13560-970 Pedro A. P. Nascente Centro de Caracterizac ¸ a ˜ o e Desen V  ol V  imento de Materiais, Departamento de Engenharia de Materiais,Uni V  ersidade Federal de Sa ˜ o Carlos, Sa ˜ o Carlos, SP, Brazil 13565-905 Ariovaldo O. Florentino  Departamento de Quı ´ mica, Instituto de Biocie ˆ ncias, Uni V  ersidade Estadual Paulista, Botucatu, SP, Brazil 18618-000 Recei V  ed: March 15, 2001; In Final Form: July 14, 2001 Structural and textural studies of a CuO/TiO 2  system modified by cerium oxide were conducted using Ramanspectroscopy, transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and N 2 absorption (BET specific surface area). The introduction of a minor amount of CeO 2  (Ce 0.09 Ti 0.82 O 1.91 Cu 0.09 sample) resulted in a material with the maximum surface area value. The results of Raman spectroscopyrevealed the presence of only two crystalline phases, TiO 2  anatase and CeO 2  cerianite, with well-dispersedcopper species. TEM micrographs showed a trend toward smaller TiO 2  crystallites when the cerium oxidecontent was increased. The XPS analysis indicated the rise of a second peak in Ti 2p spectra with the increasingamount of CeO 2  located at higher binding energies than that due to the Ti 4 + in a tetragonal symmetry. TheCuO/TiO 2  system modified by CeO 2  displayed a superior performance for methanol dehydrogenation thanthe copper catalyst supported only on TiO 2  or CeO 2 . Introduction Previous studies have shown that there are many applicationsfor CuO/TiO 2  material in the field of heterogeneous catalysis,mainly as oxidation catalysts. 1 - 5 However, this system presentsseveral disadvantages such as thermal and mechanical instabili-ties, sintering of both the supports, and the active and titaniaphase transformation. 6,7 Stabilizing a material to avoid theseeffects can usually be achieved by changing its bulk or surfacecomposition. It has been reported that CeO 2  has the propertyof stabilizing not only the active phase in a fine dispersed stateand the resistance to thermal loss of the supported catalystsurface area but also the catalytic activity. 6,8 The dispersing effect of CeO 2  and the synergistic effectbetween CuO and CeO 2  in the combustion of CO and methanehave been observed in the Cu - Ce - O system. 9,10 CeO 2  promotesthe hydrogen reduction of finely dispersed CuO surface speciesat low temperatures. 11 Alves et al. have studied the effect of the addition of CeO 2  in the IrO 2  /TiO 2  system that is employedas an electrocatalyst. 12 They found that ceria did not form asolid solution with either IrO 2  or TiO 2  and that its action wasconfined to that of a dispersing agent, resulting in a higher activesurface area. The effects of the addition of CeO 2  on the Co - SiO 2  catalyst have been investigated, and the results havedemonstrated that surface cobalt is more readily reduced in thepresence of ceria and that ceria was well dispersed on the silicasupport. 13 Oxidative processes in catalysis are most commonly used toobtain products with a high commercial value from moleculeswith low aggregate value. 7,14 Methyl formate is an attractiveintermediate in the production of numerous chemicals, such asformic acid, dimethyl formamide, acetic acid, formamide, andcyanhydric acid. A very well-known process to obtain methylformate is by methanol dehydrogenation on copper-supportedcatalysts, which shows high catalytic activity and selectivity. 2 The present work is an extension of our earlier structuralstudies of the CuO/TiO 2  system modified by cerium oxide. 15 We investigated the catalytic performance of our samples withregard to methanol oxidation. Moreover, structural informationwas obtained through TEM, Raman spectroscopy, and XPStechniques. The specific surface area was obtained with thepurpose of investigating the textural properties, after which thestructural and textural results were compared. The overallfindings were then correlated with the synergistic effect betweencopper and CeO 2 - TiO 2  for methanol dehydrogenation. Experimental ProceduresPreparation Synthesis.  Titania-based mixed oxides wereprepared by the sol - gel method. Commercial (NH 4 ) 2 Ce(NO 3 ) 6 powder (Vetec) was dissolved in an aqueous nitric acid solution(1.5 mol/L). The Cu(NO 3 ) 2 ‚ 3H 2 O powder (Vetec) was thenadded, heated to 353 K, and kept at this temperature for 30min. The resulting solution was dubbed A. Another solution,dubbed B, was prepared with tetraisopropyl orthotitanate(C 12 H 28 O 4 TiO, Merck), which was dissolved in isopropylalcohol (mole ratio  )  1). Solution A was added to solution B * To whom correspondence should be addressed. Phone: 55-16-2739828.Fax: 55-16-2713616. E-mail: valmor@if.sc.usp.br. 10515  J. Phys. Chem. B  2001,  105,  10515 - 1052210.1021/jp0109675 CCC: $20.00 © 2001 American Chemical SocietyPublished on Web 10/09/2001  and subjected to a 50 W ultrasonic vibration for 2 min. Themixture was allowed to rest for 24 h in a saturated atmosphereof isopropyl alcohol. Finally, the resulting gel was dried at 383K for 16 h and then calcined at 723 K for 16 h in air. Theprepared powder CuO - CeO 2  /TiO 2  samples, called Ce 0.91 O 1.91 -Cu 0.09 , Ti 0.91 O 1.91 Cu 0.09 , Ce 0.09 Ti 0.82 O 1.91 Cu 0.09 , Ce 0.27 Ti 0.64 O 1.91 -Cu 0.09 , and Ce 0.45 Ti 0.46 O 1.91 Cu 0.09 , express the amount of eachcomponent as atomic fractions. TiO 2  and CeO 2  were used asstructural references. Characterization.  The specific surface area (BET) wasmeasured with a Micromeritics AccuSorb 2100E instrument,using the adsorption of N 2  at the temperature of liquid nitrogen.Prior to measuring, all of the samples were degassed at 523 Kfor 16 h and finally outgassed to 10 - 3 Torr.Raman spectra were recorded on a triple Jobin-Yvon T64000Raman instrument equipped with a microscope and a CCDdetection system. The spectra were obtained at room temperatureusing the 5145 Å line of an argon ion laser (model SpectraPhysics 2020) excited with an incident power of 50 mW.An analysis was made using HR-TEM (high-resolutiontransmission electron microscopy) at 300 kV on a JEOL JEM-3010 microscope from the Laboratory of Electronic Microscopyat the National Sinchrotron Light Source Laboratory in Campi-nas, Brazil, with a point resolution of 0.17 nm. The powderwas ultrasonically suspended in isopropyl alcohol, and thesuspension was deposited on a copper grid previously coveredwith a thin layer of carbon.An XPS analysis was performed in an ultrahigh vacuum (low10 - 7 Pa range) using a Kratos XSAM HS spectrometer, with aMg K R  ( h ν  )  1253.6 eV) X-ray source operated at 15 kV and15 mA. The powder samples were fixed on a steel holder withdouble-face adhesive tape and analyzed as received. An electronflood gun was used to reduce charge effects. The high-resolutionspectra were obtained with an analyzer pass energy of 20 eV.The binding energies were referenced to an adventitious carbon1s line set at 284.8 eV. Gaussian line shapes were used to fitthe curves for C 1s, O 1s, and N 1s, and a mixed Gaussian/ Lorentzian function was employed for Cu 2p, Ti 2p, and Ce3d. The Shirley background and a least-squares routine wereapplied. The sensitivity factors for quantitative analysis werereferenced to  S  F1s  )  1.0. Catalytic Experiments.  Methanol oxidation was carried outin a fixed bed reactor working under atmospheric pressure andin a temperature range of 443 - 583 K under steady-stateconditions. The temperature was measured by a K-type ther-mocouple located just above the catalyst bed. A catalyst loadof 150 mg was used in each experiment. The methanol was fedinto the reactor by bubbling a flow of nitrogen (50 cm 3  /min)through a saturator - condenser maintained at 305 K, whichsupplied a constant methanol concentration of 11.6% vol. duringthe run. An on-line Shimadzu gas chromatograph GC-14B usingSupel-Q and Carboxen-1010 Plot capillary columns was usedto analyze all of the products. The absence of a homogeneouscombustion of reactants and products at temperatures up to 623K was confirmed by using powdered Pyrex glass. The catalystwas pretreatment-activated with 300 cm 3 h - 1 of flowing 7%H 2  and Ar as inert gas at 653 K for 1 h. Results and DiscussionSpecific Surface Area ( S BET ) - N 2  Adsorption.  The specificsurface area ( S  BET ) for TiO 2  /CuO showed a clear dependenceon added ceria, as shown in Table 1. The TiO 2  anatase phaseand the Ti 0.91 O 1.91 Cu 0.09  catalyst sample presented approximatelythe same value of   S  BET  (79 and 84 m 2  /g, respectively), indicatingno influence of copper on the titania surface area. A 9% additionof CeO 2  to the Ti 0.91 O 1.91 Cu 0.09  sample (Ce 0.09 Ti 0.82 O 1.91 Cu 0.09 catalyst) led to a considerable increase of the surface area, i.e.,124 m 2  /g. The BET area of samples containing greater amountsof CeO 2  (27, 45, and 41%, respectively) decreased (105, 80,and 86 m 2  /g, respectively). In fact, the cerium oxide phasepresented the lowest surface area, 25 m 2  /g.According to Yuan et al., 16 the addition of potassium to theCuO/TiO 2  system prevented titania sintering, increased thespecific surface area of the oxides, and improved the catalytic Figure 1.  FT-Raman spetra of samples: (a) Ce 0.45 Ti 0.46 O 1.91 Cu 0.09 , (b)Ce 0.27 Ti 0.64 O 1.91 Cu 0.09 , (c) Ce 0.09 Ti 0.82 O 1.91 Cu 0.09 , and (d) Ti 0.91 O 1.91 Cu 0.09 . TABLE 1: Specific Surface Area and Titania CrystalliteSizes Obtained through TEM Micrographs samples S  BET (m 2 g - 1 )crystallite sizesof TiO 2 [ t  ( 10% (nm)]CeO 2  25TiO 2  79 17Ti 0.91 O 1.91 Cu 0.09  84 17Ce 0.09 Ti 0.82 O 1.91 Cu 0.09  124 13Ce 0.27 Ti 0.64 O 1.91 Cu 0.09  105 7Ce 0,45 Ti 0.46 O 1.91 Cu 0.09  80 5 10516  J. Phys. Chem. B, Vol. 105, No. 43, 2001  Mastelaro et al.  performance. Titania modified with 3 and 12  µ mol Ce/m 2 surface area of the titania (173 m 2  /g) were prepared and usedas supports for copper oxides. 6 CeO 2  added to the CuO/TiO 2 system stabilized the surface area of the TiO 2  support in thepresence of copper oxide. 6 Raman Spectroscopy.  Figure 1 shows the Raman spectraafter CeO 2  loading in the CuO/TiO 2  catalytic system. The TiO 2 powder phase is known to have well-defined Raman scatteringbands 17,18 which, as shown in our results in Figure 1, is easilydetected by the intense Raman bands at 397, 517, and 637 cm - 1 and is in good agreement with previous manuscripts. 6,17 - 19 Forthe CeO 2  compound, the strong typical band at 460 cm - 1 isdue to the Raman active mode characteristic of fluorite-structured materials. 6,20 The CuO compound presented thestrongest band at 293 cm - 1 , a position similar to that reportedby other authors. 6,21 Figure 1 shows the Raman spectra afterCeO 2  loading in the CuO/TiO 2  catalytic system.The catalyst spectra showed only the TiO 2  and CeO 2  bands.The Raman bands of CeO 2  were only observed in the spectraof samples of the Ce 0.27 Ti 0.64 O 1.91 Cu 0.09  composition (Figure1a,b). The Raman bands of TiO 2  appeared in all of the catalystspectra, although their intensity was lower than that of the ceriaband in samples containing higher amounts of CeO 2 .The fact that the Raman spectra exhibited no detectable CuObands is an indication that CuO was well dispersed in the supportstructure. This state is in agreement with previous XRD analyses,which presented no X-ray reflection relating to the CuO phase,even in the sample with a higher Cu/(Ti  +  Ce) ratio (0.22), theCe 0.41 Ti 0.41 O 1.82 Cu 0.18  sample. 15 According to the literature, thepreparation method and the addition of new components to thesupport have significant effects on the catalytic properties. 6,22 Thus, the introduction of CeO 2  promoted the formation of amaterial with favorable textural properties.The formation of a CuO bulk phase in the Cu - Ce(La) - Ocatalyst system was only observed in samples containing a highamount of copper. 9 From their studies of CuO supported onCeO 2 - Al 2 O 3  and CeO 2 - TiO 2  catalyst systems, Larsson andAndersson 6 concluded that the distribution of copper speciesdepended on both copper oxide and ceria loading. Moreover,they observed the presence of bulk CuO crystallites in bothsystems only in higher copper-loaded samples. 4,6 HR-TEM Micrographs.  Figure 2 shows the micrographs of several samples (TiO 2 , Ti 0.91 O 1.82 Cu 0.09 , Ce 0.09 Ti 0.82 O 1.91 Cu 0.09 ,and Ce 0.45 Ti 0.46 O 1.91 Cu 0.09 ). The fringes appearing in the mi-crographs allow for the identification of the crystallographicspacing of the TiO 2  and CeO 2  nanocrystallites that are identified Figure 2.  TEM micrographs of (a) TiO 2 , (b) Ti 0.91 O 1.91 Cu 0.09 , (c) Ce 0.09 Ti 0.82 O 1.91 Cu 0.09 , and (d) Ce 0.45 Ti 0.46 O 1.91 Cu 0.09 . Activity and Characterization of CuO/CeO 2 -TiO 2  J. Phys. Chem. B, Vol. 105, No. 43, 2001  10517  in the micrographs. The fringes most frequently observedcorrespond respectively to the (101) and (111) crystallographicplanes of TiO 2  anatase and CeO 2  cerianite phases.Only the fringes assigned to TiO 2  anatase and CeO 2  cerianitephases were observed, confirming our previous statement abouta well-dispersed copper species on the support, which wasrevealed by Raman analysis and XRD data. 15 Although the CeO 2  crystallites was also observed in themicrographs, only the TiO 2  average crystallites sizes weremeasured (see Table 1). The HR-TEM micrographs show asignificant increase of titania crystallite sizes ranging from 5 to17 nm. The samples without CeO 2 , (TiO 2  in Figure 2a andTi 0.91 O 1.82 Cu 0.09  in Figure 2b) presented the same crystallitesizes, indicating that copper oxide caused no modification inTiO 2  anatase crystallite size. The TiO 2  particle size decreasedas the amount of CeO 2  on the support increased (Figure 2c,d).Previous XRD measurements also revealed a trend towardsmaller TiO 2  crystallites with the addition of CeO 2  to thesupport. 15 HR-TEM micrographs and textural analyses, therefore,showed that the Ce 0.09 Ti 0.82 O 1.91 Cu 0.09  catalyst presented a highersurface area and a smaller titania crystallite size than the sampleswithout CeO 2  (TiO 2  and Ti 0.91 O 1.91 Cu 0.09 ). Otherwise, there wasa reduction of both surface area and titania particle size in thesamples with higher amounts of CeO 2 . This fact was attributedto the formation of isolated CeO 2  particles, which presented avery small surface area (25 m 2  /g), thus contributing toward thedecrease in the total surface area of the catalysts. XP Spectroscopic Analysis.  The binding energy values of the main peaks in the XPS of reference and catalysts aresummarized in Table 2.Figure 3 shows the Ti 2p XP spectra of the catalysts andTiO 2 - anatase. The spin - orbit components (2p 3/2  and 2p 1/2 ) of each peak of the TiO 2  and Ti 0.91 O 1.91 Cu 0.09  spectra were welldeconvoluted by two curves (at approximately 459.0 and 464.6eV, respectively) corresponding to Ti 4 + in a tetragonal struc-ture. 23 Otherwise, the XP spectra of samples containing CeO 2 were better fitted considering the presence of a second Ti 4 + species. The peak assigned to this second Ti 4 + species hadshifted to a higher energy (2p 3/2  peak at 459.2 eV) andintensified with the addition of CeO 2 .Alves et al. 24 obtained XPS results similar to ours for IrO 2 - TiO 2 - CeO 2  electrocatalysts. The Ti 2p spectra became largerand shifted to higher energy values in CeO 2  rich samples,leading them to suggest that this element is present in morethan one species. The XPS analysis of these authors revealed astrong interaction between TiO 2  and SiO 2 . 25 They also observedan upward shift of the Ti 2p XP-spectra in this system, whichthey ascribed to an increase in the interatomic potentials due toa decrease of the coordination number of Ti and shortening of the Ti - O bond.Our XPS results indicate the presence of a second Ti 4 + specieson the sample surface as the amount of CeO 2  increased, whichis corroborated by our previous Ti K-edge XANES analysis. 15 Figure 4 shows the Ce 3d XP spectra, and Figure 5 displaysa curve fitting example of the Ce 3d XPS peaks (Ce 0.45 Ti 0.46 O 1.91 -Cu 0.09  sample). The ground state of CeO 2  is a mixture of multielectron configurations 4f  0 and 4f  1  L , where  L  denotes ahole in the oxygen 2p orbitals (ligand hole), whereas the groundstate of Ce 2 O 3  is purely 4f  1 . 26 The Ce 3d photoemission givesrise to a rearrange of valance electrons. The XPS Ce 3d 5/2  and3d 3/2  doublets are commonly denoted  u  and  V   and extend in theenergy range of 880 - 920 eV. 6,20,26,27 For CeO 2 , the final statesdue to the photoemission of the Ce 3d give rise to six peaksascribed to the three pairs of the spin - orbita doublets (3d 5/2  e Figure 3.  Ti 2p XP spectra of (a) Ce 0.45 Ti 0.46 O 1.91 Cu 0.09 , (b) Ce 0.27 Ti 0.64 O 1.91 Cu 0.09 , (c) Ce 0.09 Ti 0.82 O 1.91 Cu 0.09 , (d) Ti 0.91 O 1.91 Cu 0.09  catalysis, and (e)TiO 2 - anatase reference. TABLE 2: XPS Binding Energy Values as a Function of CeO 2  Content for the References and Samples B. E. eVO 1 s samplesCu2p 3/2 Ti2p 3/2 Ce3d 5/2  OH - O lattice CeO 2  882.7 531.7 529.4TiO 2  459.0 531.7 530.1Ce 0.91 O 1.91 Cu 0.09  933.7 882.3 531.4 529.4Ti 0.91 O 1.91 Cu 0.09  933.7 459.0 531.4 529.4Ce 0.09 Ti 0.82 O 1.91 Cu 0.09  933.6 459.2/460.8 883.2 532.1 530.5Ce 0.27 Ti 0.64 O 1.91 Cu 0.09  933.7 429.3/460.6 882.6 531.5 530.0Ce 0.45 Ti 0.46 O 1.91 Cu 0.09  933.8 458.7/460.5 882.2 531.6 529.9CuO 933.9 531.8 529.9Cu 2 O 932.1 531.9 530.2 10518  J. Phys. Chem. B, Vol. 105, No. 43, 2001  Mastelaro et al.  3d 3/2 ). These six peaks are due to the different ways of occupancy of the Ce 4f final state strongly hybridized with theoxygen 2p orbital. The  V   and  V  ′′  peaks are attributed to thebonding and antibonding states arising from the multielectronconfiguration 3d 9 4f  2 (O 2p 4 ) and 3d 9 4f  1 (O 2p 5 ) Ce 4 + , and the V  ′′′  peak to a 3d 9 4f  0 (O 2p 6 ) Ce 4 + final state. 6,13,20,26,27 In thecase of Ce 2 O 5 , the Ce 3d photoemission lines correspond totwo pairs of spin - orbital doublets (3d 5/2  e 3d 3/2 ), which arisefrom the hybridization of the multielectron configurations 4f  1 and 4f  2 . However, one of the pairs (denoted  u 0  and V  0 ) is hardlyobserved because it is a shoulder on the Ce 4 + 3d spectrum. The V  ′  peak corresponds to the 3d 9 4f  1 (O 2p 6 ) Ce 3 + final state. 6,13,20,27 The same explanation holds true for the series of   u  structuresof the 3d 3/2  level.After the deconvolution of the Ce 3d spectra, the degree of reduction (Ce 3 + species) can be estimated from the intensity of  V  ′  and  u ′  lines, according to the following equation: 6,13,28,29 where  S  V  ′  and  S  u ′  are the intensities of   V  ′ and  u ′  lines and  S  V   and S  u  are the intensities of   V   and  u  lines.Table 3 shows the amount of Ce 3 + calculated for CeO 2  andthe catalysts. As can be observed, even for the CeO 2  sample,the percentage of Ce reduction is equal to 15%, a similar valuecalculated for the Ce 0.91 O 1.91 Cu 0.09  catalyst. The Ce 0.09 Ti 0.82 O 1.91 -Cu 0.09  catalyst presents the highest percentage of surface Ce 3 + species, 27%. For the others samples, the reduction of ceriaalso took place but in a smaller proportion. The reduction of cerium ions during XPS measurements has been reported inthe literature  6,8,28,30 - 32 and must be taken into account when Figure 4.  Ce 3d XP spectra of (a) Ce 0.45 Ti 0.46 O 1.91 Cu 0.09 , (b) Ce 0.27 Ti 0.64 O 1.91 Cu 0.09 , (c) Ce 0.09 Ti 0.82 O 1.91 Cu 0.09 , (d) Ce 0.91 O 1.91 Cu 0.09  catalysis, and (e)CeO 2  reference. Figure 5.  Fitting of the Ce 3d XP spectrum obtained from the Ce 0.45 Ti 0.46 O 1.91 Cu 0.09  catalysis. Ce 3 + (%) )  S  V  ′ + S  u ′ ∑ ( S  V  + S  u )100 Activity and Characterization of CuO/CeO 2 -TiO 2  J. Phys. Chem. B, Vol. 105, No. 43, 2001  10519
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