CO-Induced Morphology Changes in Zn-Modified Ceria: A FTIR

Article pubs.acs.org/JPCC

CO-Induced Morphology Changes in Zn-Modified Ceria: A FTIR Spectroscopic Study A. Penkova,* O. H. Laguna, M. A. Centeno, and J. A. Odriozola Departamento de Química Inorgánica e Instituto de Ciencia de Materiales de Sevilla, Centro Mixto Universidad de SevillaCSIC, Avda Americo Vespucio 49, 41092 Sevilla, Spain ABSTRACT: A FTIR study of the CO adsorption on a Znmodified ceria is presented. The results indicate that at lower activation temperatures only Ce4+ carbonyls were detected, which were reduced with the increase of the activation temperature. At higher activation temperatures, up to three Zn2+ carbonyls were also identified according to the arrangement of the Zn2+ cations. The consecutive CO adsorptions demonstrated an irreversible modification of the surface, resulting in an agglomeration of the zinc cations. A stepwise conversion of the isolated Zn2+ carbonyls into carbonyls of the closely situated zinc cations followed by formation of bigger zinc oxide clusters was observed. The carbon monoxide coordinated on the isolated Zn2+ cations at the interface with ceria reacts with the lattice oxygen leading to formation of oxygen vacancies. An insight into the origin of the activation during the CO oxidation process is proposed. molecule to cationic sites shifts the ν(CO) frequency with respect to that of the gaseous CO molecule (2143 cm−1) allowing evaluation of the strength of Lewis acid sites from the ν(CO). Moreover, CO is also very useful in evaluating the acidity of the surface hydroxyl groups. Upon CO adsorption at low temperatures, the O−H stretching modes shift toward lower frequencies. The magnitude of this shift is a measure of the acid strength of the hydroxyl groups: the bigger the shift, the higher the acidity of the corresponding hydroxyl group. Besides this, CO molecules can react with strongly basic sites like O2− that, in the presence of CO, may result in the formation of carbonite CO22− ions, which in excess of CO disproportionate, leading to the formation of carbonate ions CO32− and polymeric structures like (CO)n2− as earlier proposed by Lavalley.15 Ceria and zinc oxides are largely studied by IR spectroscopy of probe molecules.14,15,17,18 Nevertheless, there are no IR studies concerning the doping effect of zinc oxide on ceria. In previous studies we have shown that Au supported on ZnOmodified ceria is highly active in the CO oxidation reaction.19,20 For these systems, XRD data demonstrate the coexistence of the CeO2 and ZnO phases, depending on the ZnO dispersion on the actual reaction conditions. Therefore, it might be expected that the Zn addition would not affect the concentration of oxygen vacancies in the ceria lattice, as there is not evidence for the incorporation of Zn cations in the ceria matrix. However, the Raman spectra of these catalysts show the existence of oxygen vacancies.19 Thus, taking into account the advantages of CO as probe molecule, we intend to clarify the nature of the surface sites, to find out their role in CO oxidation reactions and the nature of

1. INTRODUCTION Cerium oxide has been considered and largely used as a very attractive promoter in environmental catalysis due to its redox properties (Ce4+/Ce3+) and oxygen storage/release capacity (OSC) that results in oxygen mobility in the ceria lattice.1 Oxygen vacancies in CeO2 are formed at high temperature after heating in vacuum or moderate temperature reduction in H2 or CO atmospheres, and these treatments easily lead to the formation of nonstoichiometric oxides CeO2−x (0 < x < 0.5).2−4 Theoretical calculations have shown that the formation of oxygen vacancies is more favorable on the surface than in the bulk.5,6 The concentration of oxygen vacancies can be modified by the addition of dopants, which has resulted in considerable interest in studying ceria modification by metal oxides.7 Appropriate modifiers of the structural and chemical properties of ceria are cations possessing ionic radius and electronegativity close to those of cerium cation.8−11 The incorporation of these cations in the ceria causes structural distortions leading to lattice strains that favor the formation of oxygen vacancies. Thus, it has been proposed that small amounts of Sm and Zn oxides improve the performance of modified ceria, while further addition of these oxides causes a decrease in the catalytic activity.8,12 However, the interaction mechanism is still controversial, and it was proposed that the improved activity is due to the formation of Zn−Ce solid solution12 or surface segregation of the zinc oxide.8 Spectral studies of adsorbed molecules are not only of great importance for understanding the mechanism of reactions but also provide unique information about the structure and properties of the surface. Coordinatively unsaturated metal cations and oxygen vacancies present in oxide surfaces are likely participants in the activation of reactants in catalytic reactions.1,13 The adsorption of probe molecules is a frequently used approach to characterize donor−acceptor properties of surface sites in metal oxides.14−16 The coordination of the CO © 2012 American Chemical Society

Received: November 15, 2011 Revised: January 27, 2012 Published: February 6, 2012 5747

dx.doi.org/10.1021/jp210996b | J. Phys. Chem. C 2012, 116, 5747−5756

The Journal of Physical Chemistry C

Article

3686 cm−1 with shoulders at 3719 and 3660 cm−1 were also observed in the ν(OH) region. According to conventional assignments,16,17 the bands detected at wavenumbers above 3600 cm−1 are ascribed to free hydroxyl groups. Oxygen atoms are tetrahedrally coordinated in both CeO2 and ZnO that crystallize in fluorite and wurzite type structures, respectively. That is why hydroxyl groups bonded to one, two, or three metal atoms may exist on the surface. Surface hydroxyl groups on metal oxide surfaces are characterized by a set of bands associated with three different surface sites named type I (terminal, Mn+−OH), II (bridging, (Mn+)2OH), and III (multicentered, (Mn+)3OH) where the Roman numerals indicate the coordination number of the group. Isolated surface hydroxyls on the oxidized ceria surface present bands at 3710 and 3640 cm−1 that were ascribed to type I and III hydroxyls, respectively.23 Type II hydroxyls are characterized in oxidized ceria by bands at 3655 and 3634 cm−1.18 Zaki et al.24 observed, for the three hydroxyl types, bands at 3684, 3652, and 3621 cm−1, respectively, assigning them to hydroxyls of Ce4+ sites on different facets of the fluorite structure. Other authors have registered a band at 3690 cm−1 arising from Ce−OH terminals hydroxyls and another one at 3646 cm−1 due to Ce2(OH) doubly bridging groups interacting with oxygen vacancies.25 Zaki and Knozinger26 have reported that Ce4+−OH bands at 3664 and 3640 cm−1 were not shifted upon CO adsorption due to the basic character of ceria. Isolated hydroxyls of Zn2+ cations have been observed in the 3675−3625 cm−1 region and classified as hydroxyls linearly bonded to single Zn ions27 and/or as bridging hydroxyls, type II and III.17,18,24,28 Zecchina at al.27 reported that upon CO adsorption the bands at the highest frequencies decrease partially, while the lowest frequency band is fully eroded and shifted. As the shift is directly related to the acidity of the involved OH groups, they concluded that the hydroxyls with the highest frequency bands possess the lowest acidity. A further heat treatment at the same temperature results in an intensity decrease of the bands associated with carbonates while the band at 3660 cm−1 hardly decreased, accompanied by insignificant decrease of the carbonate bands (Figure 1, spectrum b). In the following sample heating at 573 K, a change of color was observed. Before the treatment the sample was bright yellow and after activation in vacuum at 573 K became pale blue. This color change is indicative of the changes in the electronic properties of the solid. Taking into account that the heating of CeO2 was carried out in vacuum, it is supposed that the color change is due to a surface reduction process. The intensity of the bands due to carbonate species hardly decreases, and the band at 3660 cm−1 is not noticeable (Figure 1, spectrum c). The band peaking at 3686 cm−1 was observed, but the intensity was reduced. It has to be noted that Binet et al.29 have detected a similar band on a reduced ceria sample. A new band peaking at 3670 cm−1 appears upon activation at 573 K; this band can be assigned to hydroxyl species bonded to Zn2+ cations.23,28 The subsequent activation of the sample at 673 K (Figure 1, spectrum d) led to a considerable decrease in the intensity of the carbonate bands (1600−1300 cm−1), to a decrease of the band at 3670 cm−1, in the ν(OH) region, the band at 3686 cm−1 that appears as a shoulder becoming almost undistinguishable. Obviously, the evolution with the activation temperature of the background spectrum evidences the removal of surface hydroxyl groups and the elimination of part of the

the modification of ZnO dispersion as a function of the reaction conditions.

2. EXPERIMENTAL SECTION The ZnO−CeO2 mixed oxide with 25 mol % Zn loading was synthesized by a pseudo-sol−gel method involving thermal decomposition of the corresponding propionates. These propionates were produced after dissolution of adequate amounts of Ce(III) acetate and Zn(II) acetyl acetonate in propionic acid (0.12 M). The propionic acid excess was retired from the mixture through simple distillation obtaining a resinlike substance that was further calcined at 773 K for 2 h in air. FTIR spectra were recorded with THERMO NICOLET Avatar 380 FT-IR Spectrophotometer, equipped with a DTGS/ KBr detector, and accumulating 64 scans at a spectral resolution of 2 cm−1. The experiments were performed in situ using a purpose-made IR cell connected to a conventional vacuumadsorption apparatus with a residual pressure lower than 10−3 Pa. The sample powder was pressed into self-supporting pellets (density ∼30 mg/cm2) under a pressure of 107 Pa. The sample was activated in situ in the IR cell by heating in dynamic vacuum at a rate of 10 °C/min up to 473, 573, and 673 K and maintained at the same temperature for 1 h. Then the pellet was transferred to the area of the infrared beam and allowed to cool down in vacuum. Carbon monoxide (>99.97 purity) was supplied by Air Liquid. CO adsorptions were carried out at 100 K. All spectra were background subtracted, except those in Figure 1, where the background spectra were shown after the different treatments.

Figure 1. Background spectra of ZnO/CeO2 activated at 473 K (a), 473 K (once again) (b), 573 K (c), and 673 K (d). Panel A, OH region; panel B, carbonate region.

3. RESULTS 3.1. Background IR Spectra. The background spectrum of the sample was acquired at the temperature of liquid nitrogen (100 K) after activation at 473 K (Figure 1, spectrum a). The spectrum is dominated by intense bands at 1478, 1465, 1390, and 1360 cm−1 and a weak band at 1065 cm−1, typical of asymmetric and symmetric stretching vibrations of carbonate species,21,22 and the low activation temperature of the sample accounts for the presence of these surface species. Bands at 5748



Article

ν(OH) bands. The band at 2158 cm−1, which was detected here, appeared at the same wavenumber and was characterized by low stability. The low stability of the band and the slight decrease of the OH bands at 3686 and 3660 cm−1 allow us to assign the 2158 cm−1 band to CO hydrogen bonded to the hydroxyls of Ce4+ cations. The band at 2173 cm−1 is more stable toward evacuation suggesting formation of a stronger bond. As observed in the Figure 2A, the band displays a complex contour. With the decrease of the CO coverage, the band decreases in intensity and another band at 2180 cm−1 becomes distinguishable. Obviously there are carbonyls formed with participation of two different coordinatively unsaturated sites (cus). The electron configuration of Ce4+ is [Xe]4f05d0. Therefore, CO should be bonded to Ce4+ cations by an electrostatic interaction and/or a σ-bond. Surface carbonyls on Ce4+ cations are observed in the region 2180−2141 cm−1.14 As expected, the species assigned to Ce4+−CO complexes are unstable and decompose easily upon evacuation. Carbonyl complexes of Zn2+ cations on ZnO are characterized by an intense band at 2168 cm−1 and a shoulder at 2178 cm−1.14 The carbonyls observed are assessed with low stability that supposes electrostatic interactions, whereas the filled third shell of Zn2+ suggests formation of more stable bond (most probably enhanced sigma bonding) in comparison with the Ce4+ cations. However, the stabilities of the bands observed here are comparable which implies that the carbonyls under consideration are due to CO adsorbed on Ce4+ cations with different degree of coordinative insaturation.31−33 All bands registered after CO adsorption disappeared after evacuation at liquid nitrogen temperature. In order to check the surface after CO adsorption, the sample was submitted to a subsequent CO adsorption at the same conditions (Figure 3A, spectrum b). When the carbon monoxide was adsorbed, the development of the same bands was observed with the same behavior during the evacuation. The complexes formed during the adsorption are characterized by low stability and decomposed at low temperature. It should be noted that the band assigned to carbonyls of Ce4+ has shown lower intensity after the second adsorption, and this tendency was kept in further CO adsorption (Figure 3A, spectrum c). It can be concluded that the consecutive adsorptions of CO after activation of the sample at 473 K caused changes on the surface of the sample affecting Ce4+ sites. Then, the sample was evacuated once again at 473 K at the same conditions. As a result, the hydroxyl and carbonate bands observed in the background spectrum (Figure 1, spectrum b) became less intense after this treatment. Obviously, a consecutive heating of the sample, even at the same temperature, leads to additional liberation of adsorption centers. When 100 Pa CO was introduced at liquid nitrogen temperature, development of bands identical to those detected after the first activation of the sample at 473 K (2173 and 2158 cm−1) was observed (Figure 2B, spectrum a). Whereas after the first activation of the sample (Figure 2A) the band of CO hydrogen bonded to hydroxyl groups (2158 cm−1) was more intense than those of the Ce4+ carbonyls (2173 cm−1), after this adsorption the relative intensities of the two bands were inverted. In addition, the band characterizing Ce4+ carbonyls was detected with enhanced intensity (Figure 3A and B, spectrum a). The smaller amount of Ce4+−OH−CO complexes (lower intensity of the band at 2158 cm−1) is in accordance with the less intense hydroxyl band (3660 cm−1) registered in the background spectrum after this treatment. Evidently, dehydroxylation of the surface sites occurs, and more cus

residual carbonate species resulting in an increase of the concentration of surface sites able to react with probe molecules. We can tentatively suggest taking into account the results obtained after CO adsorption (see below), that the band at 3660 cm−1 is due to bridged hydroxyls (II-B) of ceria interacting with oxygen vacancies, while the 3686 cm−1 band corresponds to another kind of bridged hydroxyls of Cen+ (II-A) and/or Zn2+ cations. In accordance with the literature data, the band with a maximum at 3670 cm−1, observed after activation of the sample at 573 K, could be attributed to hydroxyls of another type of Zn2+ ions. 3.2. Activation at 473 K. The introduction of 100 Pa CO, at 100 K on the sample activated at 473 K, results in the appearance of two carbonyl bands with maxima at 2173 and 2158 cm−1 (Figure 2A, spectrum a). At the same time, the

Figure 2. FTIR spectra of CO (100 Pa) adsorbed at 100 K on ZnO/ CeO2 (a) and evolution of the spectra under dynamic vacuum at 100 K (b-z). Panel A, I adsorption after activation of the sample at 473 K; panel B, I adsorption after second activation at 473 K. The spectra are background corrected. Inset A: FTIR spectra of ZnO/CeO2 activated at 473 K (a′), after CO (100 Pa) adsorbed at 100 K (a), and the difference spectrum (a − a′).

bands in the OH region slightly decreased (Figure 2, inset). Changes in the carbonate region have not been observed. The low-frequency band at 2158 cm−1 is very sensitive to the CO coverage and disappears quickly upon evacuation at low temperature, which suggests weak interaction (van der Waals type). Bands due to CO bonded to hydroxyl groups sitting on Zn2+ cations appear at 2148−2145 cm−1.28,30 On the other hand, it is considered that ceria hydroxyls do not interact with CO even at low temperature.14,26,31 However, at high CO coverage, Binet et al.31 have observed an intense carbonyl band at 2151 cm−1 and another weak one at 2162 cm−1 on activated CeO2. Upon decreasing the coverage these bands disappear independently and shift to 2157 and 2168 cm−1, respectively. The high-frequency band at 2168 cm−1 has been attributed to carbonyls bonded to Ce4+ cations. The band at 2157 cm−1 is assigned to a weak CO interaction with surface OH groups in conformity with the observed ill-defined downshift of the 5749



Article

attributed to Ce4+ carbonyls, was not observed after this treatment, while the band at 2158 cm−1 here was observed as a shoulder. In the OH region the band at 3686 cm−1 was hardly affected (Figure 4A inset, spectrum a′ − a).

Figure 3. FTIR spectra of CO (100 Pa) adsorbed at 100 K on ZnO/ CeO2: sample activated at 473, 473 (once again), and 573 K (plots A, B, and C, respectively). Spectra a, b, and c correspond to I, II, and III adsorption, respectively. The spectra are background corrected.

Ce4+ are detected. The careful analysis of the spectra allows us to detect a shoulder around 2150 cm−1 which is clearly observable at low CO coverages (Figure 2B). As observed in the Figure 2B, the shoulder around 2150 cm−1 was characterized by a greater stability and persisted when the band at 2158 cm−1 disappeared. This supposes formation of complexes with stronger interaction than the hydrogen-bonded CO. One possibility is that the band at 2152 cm−1 is due to Zn2+−CO complexes. As mentioned above, the carbonyls of cus Zn2+ ions were detected at higher frequency in the 2200− 2165 cm−1 region.28,34,35 Another alternative that cannot be excluded is that this band is associated with carbonyls formed with participation of other type cus cerium cations. The shoulder is detected at lower frequency than the band assigned to Ce4+ carbonyls (2173 cm−1) which suggests lower oxidation state. Normally, carbonyl complexes of Ce3+ appear at 2127 cm−1.32,36,37 On the other hand, depending on the conditions of reduction, some authors propose carbonyls of Ce3+ at 2161 and 2157 cm−1.32,38,39 The heating of the sample in vacuum and the performance of three consecutive CO adsorptions suggest autoreduction of some of the Ce4+ cations (although the adsorption takes place at low temperature, CO acts as a reducing agent). This reveals two types of interaction: (i) CeOH−CO at 2158 cm−1 (unstable) and (ii) Ce3+−CO at 2152 cm−1 (more stable). In addition, a shoulder around 2190 cm−1 was distinguished. Since the carbonyl complexes of Ce4+ were detected at lower wavenumbers, the latter could be attributed to carbonyls of cus Zn2+ cations.28,34 The subsequent adsorptions of CO, carried out after this activation procedure (Figure 3B, spectra b and c), resulted in the appearance of the same bands. Here also a further decline in the intensity of the carbonyl band at 2173 cm−1 was observed. This can be explained by autoreduction of some of the Ce4+ cations. The detection of the band at 2152 cm−1, attributed to Ce3+ carbonyls, corroborates this suggestion. 3.3. Activation at 573 K. The introduction of 100 Pa CO on the sample activated at 573 K (Figure A, spectrum a) resulted in the appearance of carbonyl bands with maxima at 2169 and 2152 cm−1 and a not well-resolved shoulder at higher wavenumbers. It should be noted that the band at 2173 cm−1,

Figure 4. FTIR spectra of CO (100 Pa) adsorbed at 100 K on ZnO/ CeO2 activated at 573 K (a) and evolution of the spectra under dynamic vacuum at 100K (b−z). Plots A and B correspond to I and III adsorption, respectively. The spectra are background corrected. Inset A: FTIR spectra of ZnO/CeO2 activated at 573 K (a′), after CO (100 Pa) adsorbed at 100 K (a), and the difference spectrum (a − a′).

Generally, the CO adsorption on ZnO leads to the appearance of the intense band at 2169 cm−1 and the highfrequency low-intensity band(s).17,28,30,34,35,40 It is established that the ZnO power surface presents almost exclusively 1010 planes.35 That is why the intense band has been assigned to the CO stretching mode of CO molecules adsorbed on zinc cations of this planes, while the low-intensity band(s) is as a CO adsorbed on defects sites or at the border of the predominated faces or other (less abundant) faces.28,35 During the evacuation, the intense band at 2169 cm−1 and the high-frequency shoulder decreased simultaneously and were characterized with higher stability than the band attributed to the Ce3+ carbonyls, 2152 cm−1 (Figure 4A). However, their behaviors seem different. The band at 2169 cm−1 disappeared almost without changing its position, while the shoulder at about 2187 cm−1 was blue-shifted up to 2194 cm−1 with the decrease of the CO coverage. Such behavior is well-known as a static shift and is due to a weakening of the electron-accepting properties of the cations after occupation of the adjacent sites with a CO molecule.14,33 This causes downward shift of the band with increasing coverage and is an indication that the adsorption sites are not isolated. The observed blue shift of 7 cm−1 suggests that coordination of CO molecules occurs on nonisolated coordinatively unsaturated sites. The band at 2169 cm −1 is typical for zinc oxide samples.17,28,30,34,35,40 Lavalley et al.30 established by CO and H2 coadsorption experiments on ZnO that the band is due to CO molecules coordinated on zinc cations on 1010 planes that 5750



Article

have at least one ZnH group on the nearest neighbor cation site. They observed the same band after CO adsorption on the clear surface without hydrogen and concluded that the hydrogen was coordinated on zinc sites between two CO molecules. Thus, on the surface there are coordinatively unsaturated zinc cations that cannot coordinate CO molecules, and at least one coordinated unsaturated zinc cation exists as a neighbor to zinc carbonyls. Besides, it should be taken into account that even though it exists as a separated ZnO phase, the presence of cerium cations near additionally affects the distribution of zinc cations. However, it seems the band is more complex. Assuming the band at 2169 cm−1 is due to Ce4+ carbonyls (slightly shifted as a consequence of removal of the same carbonate structures, Figure 1, spectrum c), they should disappear quickly or with the band of Ce3+ carbonyls (2152 cm−1) because the CO interaction with cerium cations is essentially electrostatic and lower than Zn2+−CO. We observed (Figure 4A, spectrum j) that when the band at 2152 cm−1 completely disappears, the band at 2169 cm−1 still persists. The observed shoulder at 2158 cm−1, which was attributed to Ce4+OH−CO species, suggests that not all cerium cations were reduced and maybe part of the band is due to a unreduced Ce4+ cations where it overlaps with the intense band of zinc carbonyls (2169 cm−1). Therefore, the observed band after this sample activation at 573 K can be assigned to isolated Zn2+ and Ce4+ carbonyls. Further, the sample was subjected to subsequent CO adsorptions on the same conditions (Figure 3C, spectra b, c). When the carbon monoxide was added, formation of the same carbonyls with bands at 2169 and 2152 cm−1 was observed. It should be noted that the carbonyl bands became less intense. Additionally, a band with maximum at 2183 cm−1 appeared. Let us see in detail the behavior of the bands after the third adsorption of CO (Figure 4B). The bands at 2169 and 2152 cm−1 showed the same behavior as their performance after the first adsorption. With the decrease of the CO coverage, the new band detected at 2183 cm−1 disappeared along with the band at 2169 cm−1 and the highfrequency shoulder that were attributed to Zn2+ carbonyls. Moreover, the carbonyl complexes of Cen+ cations have been detected at lower wavenumbers. Therefore, the new band can be assigned to carbonyls of another type of Zn2+ sites. The comparison of the spectra obtained after the first and the third adsorptions (Figure 3C, spectra a, c) demonstrates that this band developed with the subsequent CO adsorptions. After the third adsorption of CO on the sample activated at 573 K, three different Zn2+ sites (labeled A, 2169 cm−1; B, 2183 cm−1; C, high-frequency shoulder) were detected. As observed in the Figure 3C, the band at 2169 cm−1, which was attributed mainly to carbonyls of isolated Zn2+ sites, decreased with subsequent adsorption of CO and the band at 2183 cm−1 became more intense. In addition, the carbonyl band of Ce3+ was hardly decreased. That reveals that part of the isolated Zn2+ sites associate and cover part of the Ce3+ sites. 3.4. Activation at 673 K. In continuation, the sample was activated at 673 K. The subsequent adsorption of CO led to development of the bands at 2189 and 2171 cm−1 and the lowintensity band at 2152 cm−1 (Figure 5, spectrum a). In addition, carbonate bands at 1610, 1490, 1380, and 1278 cm−1 were developed. These bands have been attributed to tridentate carbonates in the vicinity of Ce3+ cations or of oxygen vacancies.22 The formation of carbonate species is an indication of CO oxidation and concomitant reduction of cerium cations.

Figure 5. FTIR spectra of CO (100 Pa) adsorbed at 100 K on ZnO/ CeO2 activated at 673 K (a) and evolution of the spectra under dynamic vacuum at 100 K (b−w) and at increasing temperatures (x−z). Spectrum a′ corresponds to 100 Pa CO adsorbed at 100 K on ZnO/CeO2 activated at 573 K. The spectra are background corrected. Inset: FTIR spectra (a and z) in the carbonate region.

After this treatment the band (shoulder) at 2158 cm−1 was not observed, nor were changes in the OH region. The observed band at 2171 cm−1 appeared much less intense compared to its intensity after activating the sample at 573 K (Figure 5, spectrum a′). We can conclude, then, that the reduction of cerium cations is complete and that the band is due only to isolated Zn2+ cations. In addition, the high-frequency shoulder at ∼2187 cm−1, detected after the activation of the sample at lower temperatures, here appears as a well-defined band with maximum at 2189 cm−1. Under evacuation the band shifts upward, to 2199 cm−1. The observed blue shift of 10 cm−1 is bigger than those detected after activation of the sample at 573 K, which suggests that the CO molecules are coordinated to more agglomerated Zn2+ cations. After this adsorption we found that on the surface Ce3+ carbonyls (2152 cm−1), two kinds of Zn2+ carbonyls (2171 and 2189 cm−1) and carbonates in the vicinity of Ce3+ cations or oxygen vacancies (1610, 1490, 1380, and 1278 cm−1) exist. During evacuation in the carbonate region, perturbation in some of the bands occurred. The bands at 1610, 1490, 1380, and 1278 cm−1 diminished without completely disappearing, and new bands at 1566 and 1284 cm−1 developed, accompanied by CO2 formation. The new bands have been found on activated ceria and assigned to tridentate carbonates.22 These results show that some of oxygen vacancies were replenished and part of the cerium cations were oxidized. The incomplete disappearance of the carbonate bands at 1610, 1490, 1380, and 1278 cm−1 suggests that on the surface Ce3+ cations and oxygen vacancies are still present. The second adsorption of CO led to formation of the same carbonyl bands with maxima at 2189 and 2171 cm−1 (Figure 6, spectrum a). In addition, the band at 2182 cm−1 was also distinguished. After this adsorption, three types of zinc carbonyls were detected: carbonyls formed on isolated cationsA type (2171 cm−1), carbonyls formed with participation of zinc cations that are surrounded by other zinc cationsB type (2182 cm−1), and carbonyls formed on zinc oxide clustersC type (2189 cm−1). After reaching its maximum intensity (Figure 6, spectrum b), the carbonyl band at 2171 cm−1 5751



Article

began to decline and the other band at 2182 cm−1 developed. The observed conversion between the two carbonyl bands shows that some of the isolated cations regroup in the presence of CO. When, in turn, the band at 2182 cm−1 reached a maximum intensity (Figure 6, spectrum m), a further decrease

Figure 7. FTIR difference spectra of the species formed on Zn/CeO2 during 2 h in 100 Pa CO. The spectra are identical with the spectra presented in plots A and B of Figure 6.

bands, along with the decrease of the band characterizing isolated zinc carbonyls, can be explained by the redistribution of the zinc cations on the surface, which provokes perturbation of the carbonate species of coordinatively saturated neighboring cations. Interestingly, a band at 1127 cm−1 also developed. This band seems to behave differently from those of the carbonate structures. Undoubtedly, in the region characteristic for ν(C−H) vibrations, no bands were observed. Therefore we discard the probability of existence of formate and hydrogen carbonate species. Besides, the stretching modes of the carbonate and carboxylate species were displayed in the 1700−1200 cm−1 region as the corresponding symmetric vibrations were detected at frequencies lower than 1100 cm−1.16,21 The frequency of the Zn−O bond has been registered below 800 cm−1.21 A possible candidate for the band at 1127 cm−1 could be some dioxygen adducts. It is known that the vibrational frequency of the oxygen molecule in the ground state is 1580 cm−1. When the oxygen molecule is coordinated, the bond order decreases which causes an increase of the O−O distance and a decrease of the ν(O−O). Dioxygen compounds are generally classified into superoxide (O2−) with bands displayed between 1200 and 1100 cm−1 and peroxide species (O22−) in the 900−800 cm−1 region.21 Superoxide and peroxide species have been identified after oxygen adsorption on reduced ceria with characteristic bands in the 1135−1127 cm−1 range and overtones detected at 2237 cm−1 and in the 877−831 cm−1 region. These bands were assigned to the O−O stretching vibration of dioxygen species bound to one- and two-electron defects on the ceria surface, respectively.49,50 It seems the detected band at 1127 cm−1 characterizes the formation of superoxide species as the corresponding overtone frequency was observed at 2236 cm−1. The superoxide species formed during CO and CO2 adsorptions on ceria have not been observed. The last one is detected by oxygen adsorption and is an indication for the availability of oxygen vacancies. Corma et al. observed superoxides with a band at 1123 cm−1 and on goldsupported nanocrystalline ceria and was not observed when the gold was deposited on conventionally precipitated ceria.51 Furthermore, in the nanocrystalline sample, CO2 formation has been identified after CO adsorption. Our results showed that the zinc addition led to diminution of the crystalline size of the ceria phase and shifted the reducibility to lower temperatures.19

Figure 6. FTIR spectra of second CO adsorption (100 Pa) on Zn/ CeO2 activated at 673 K (a) and evolution of the spectra with time during the first hour (plot A, spectra b−m) and the second hour (plot B, spectra m−z). Plot D: Difference spectra. The spectra are background corrected.

of the carbonyl band due to isolated Zn2+ cations was observed. Besides, the band at 2189 cm−1 was blue-shifted and broadened after allowing the sample to stay in contact with CO for 2 h (Figure 6, spectrum z). All this suggests that another part of the isolated cations were rearranged to form larger clusters. Meanwhile, a band associated with CO2 formation at 2335 cm−1 is grown up (Figure 7). It should be noted that the last one was not observed in the previous adsorptions. It is wellknown that CO2 can be coordinated to Lewis acid sites. Usually the value of its vibrational frequency is shifted upward in comparison with the frequency of the unperturbed molecule.41 The frequency observed here is very close to the frequency of the solid CO2 (2344 cm−1) which reveals the presence of linear CO2 species. A band at 2330 cm−1 has been detected after introduction of oxygen on Au/ZnO sample with preadsorbed CO and assigned to CO2 linearly coordinated to Zn−CO near gold particles.42 It was found that CO2 coordinated to ZnO appeared at 2353 cm−140 as the same frequency was detected for unreduced and reduced ceria.31 In addition, two new sets of bands at 1475 and 1375 cm−1 and at 1560 and 1286 cm−1 were registered due to carbonate structures. The first couple of bands were found to be carbonates on zinc cations, and the frequencies coincide very well with those reported in the literature.21,43,44 It should be noted that the same carbonate bands have been detected on reduced ceria.22,45,46 The corresponding symmetric stretching modes appear at 1070 cm−1. The other couple of bands are the same observed in the previous CO adsorption. Now, the band of gas phase CO2 was observed again (Figure 7, left panel), which supposes a partial oxidation of some of the cerium cations. This was confirmed by a band observed at 3660 cm−1, which was detected after sample activations at 473 and 573 K. Meanwhile, negative bands at 1443 and 1392 cm−1 due to residual surface zinc carbonate structure were observed.47,48 The carbonate on the ceria at these frequencies has not been reported. The decrease of these 5752



Article

Besides, doping ceria, for example with Zr, improves significantly the superoxide formation in comparison with the ceria.52 The availability of more oxygen vacancies has been explained by Zr substitution of Ce which provokes stress in the unit cell and strongly favors defects formation. The band at 1127 cm−1 that we detected shows the presence of superoxide formed during CO adsorption. This evidences generation of an additional number of oxygen vacancy sites as compared to ceria where a band at 1127 cm−1 has not been observed. Most probably this takes place on the ceria/zinc oxide interface. The observed regrouping of zinc cations distorted ceria lattice, leading to the weakening of the Ce−O bond and simultaneous liberating of oxygen followed by formation of oxygen vacancies.

Table 1. Assignment of the Carbonyl Bands Observed in Zn−Ceria

4. DISCUSSION Figure 8 summarizes the changes observed on the surface after CO adsorption on the sample pretreated at different temperatures. Generally, on increasing the activation temperature gradual liberation of more cus sites occurs (Figure 1). Thus, after activation of the sample at 473 K, only Ce4+ carbonyls were registered (Figure 8, spectrum a). The decline of the intensity of Ce4+ carbonyl band after the consecutive CO adsorptions shows availability of less Ce4+ cations on the surface. At the next CO adsorption carried out after reactivation of the sample at the same temperature, the number of detected Ce4+ cations increases (Figure 8, spectrum b), as the trend for a

ptions each and one more hour activation at 573 K were sufficient for reduction of almost all Ce4+ cations. Moreover, as a result of this heat treatment of the sample, a color change was observed that is a clear indication of the reduction of the surface cerium sites.29 As mentioned above, the reduction of ceria is accompanied by the removal of oxygen from the ceria lattice and formation of oxygen vacancies (VO·· ). In addition, CO adsorption performed after this activation showed formation of two types of Zn2+ carbonyls: isolated with a band at 2169 cm−1 that were labeled type A and carbonyls of type C that are characterized by a high-frequency shoulder. The next two consecutive CO adsorptions revealed some perturbation of zinc carbonyls (Figure 3C). The carbonyl band characterizing the isolated sites appeared with lower intensity (2169 cm−1), and a new band, attributed to closely situated zinc cations (type B) at 2183 cm−1, is grown up. It seems that these surface transformations are the result of the CO interaction with the zinc cations. Some of the isolated Zn2+ cations were attracted and formed a new phase of closely situated Zn2+ cations (2183 cm−1). This new phase covered part of the Ce3+ sites as evidenced by the lower intensity of the Ce3+ carbonyl band. In the next activation of the sample at 673 K, the major part of the residual carbonate structures was decomposed and the CO adsorption revealed a significant decrease of the carbonyl band due to isolated Zn2+ cations at the expense of the more intense band of the zinc carbonyls of type C (Figure 8, spectrum d). This indicates that an important part of isolated sites rearranged into larger ZnO clusters. Here, as the first CO adsorption on the sample activated at 573 K, carbonyls of types A and C were detected. At the same time, fewer Ce3+ carbonyls were detected which clearly shows that the larger ZnO clusters cover more of the Ce3+ sites. In the following CO adsorption, in addition to the carbonyls observed during the previous adsorption cycle (types A and C), Zn2+ carbonyls of type B were distinguished (Figure 8, spectrum e). These carbonyls were found in the third CO adsorption on the sample activated at 573 K and were formed as a result of the consecutive CO adsorptions. Evidently, after the activation of the sample at higher temperature, the rearrangement of the cations occurs faster. When the bands gained maximum intensities, conversion between the three types of zinc carbonyls was observed (Figure 6). The stepwise reaction takes place through transformation of the isolated carbonyls into carbonyls of the adjacent zinc sites (type B) and the further rearrangement in larger clusters (type C) (Figure 8, spectrum f and Figure 6). This conversion, for the first time, reveals the mechanism of the surface modification through a redistribution of the cationic density and the enlargement of the

band position, cm−1 2158 2173 2152

2169−2171 2183 2187 (2189) → 2194 (2199)

Figure 8. FTIR spectra of CO (100 Pa) adsorbed at 100 K on ZnO/ CeO2 after different temperatures of activation (a, 473 K; b, 473 K (once again); c, 573 K; d, 673 K; e, II adsorption after activation at 673 K) and after 2 h in CO (f).

diminution of the band after successive CO adsorptions is repeated (Figure 3B). The appearance of a bigger fraction of Ce4+ cations can be explained by a partial decomposition of the residual carbonates. In addition, some zinc carbonyls were detected (Figure 8, spectrum b, high-frequency shoulder). The decrease of the intensity of Ce4+ carbonyl band (2173 cm−1) and the appearance of a clearly observable band at 2152 cm−1 attributed to Ce3+ carbonyls show a partial reduction of the surface Ce4+ cations (Figure 2B). The band at 2173 cm−1 attributed to Ce4+ carbonyls was not observed after CO adsorption on the sample activated at 573 K where only Ce3+ carbonyls were found (2152 cm−1) (Figure 8, spectrum c). It seems that the twice activation in vacuum at 473 K with three CO adsor5753

assignment

comments

Ce4+−OH−CO not observed after activation at 573 K not observed after activation at 573 K Ce4+−CO Ce3+−CO formed after subsequent CO adsorption and after activation at higher temperature; more stable than Ce4+−CO 2+ 2+ Zn −CO type A isolated Zn sites, observed after evacuation at 573 and 673 K Zn2+−CO type B closely located Zn2+ cations Zn2+−CO type C biggest clusters; the band is blueshifted at decreasing coverage



Article

ZnO clusters. In addition, during this process, coordinated CO2 and superoxide species formation were observed. Further, we intend to explain the phenomenon observed and to propose a possible interaction mechanism. As shown by the XRD analysis,19 two phasesCeO2 and ZnOcoexist in the sample. The presence of the oxygen vacancies, found by Raman spectroscopy,19 also showed that the Zn addition did not generate complementary oxygen vacancies. Therefore, these vacancies belong to CeO2 and were formed during the calcination of the sample. When ceria was exposed to the air, the surface Ce3+ cations were easily oxidized, which was confirmed by the CO adsorptions performed after the sample activation at 473 K. Although the activation was carried out in vacuum, this temperature was too low to reduce the oxidized Ce4+ cations. At higher activation temperature (573 K) and after repetitive CO adsorptions, almost all Ce4+ cations were reduced. At the same time, some of the residual carbonates were decomposed and also begin to detect the Zn2+ cations. After CO adsorption was performed, Zn2+ cations were activated. It was found that when the zinc oxide was exposed to CO, irreversible modification of the surface occurred.53 CO interacts with the lattice oxygen and forms oxygen vacancy and adsorbed CO2. When the adsorbed CO2 is released as CO2,53 an electron deficit in the zinc oxide is created. This would cause a redistribution of the electron density in the solid. The oxygen vacancy formed in the zinc oxide is filled by oxide ions of ceria generating oxide vacancies in the ceria matrix. As explained above, the creation of oxygen vacancies was accompanied by oxygen diffusion from the bulk toward the surface and release as O2. The released oxygen was coordinated to the oxygen vacancy, where it forms a superoxide. The observed increase of the band at 1127 cm−1 corroborates this hypothesis. When there are carbonyls of adjacent isolated Zn2+ sites with oxygen vacancy between them, the electron density will be distributed. Then, the carbonyls (cations) will begin to attract each other and attach. Thus, rearrangement occurs initially in small clusters of type B, followed by formation of larger clusters where some of the oxygen vacancies of ceria will be covered. A summary of the reactions can be written as follows: 1 x 4 + CO 3+ OO + V ··OCeO + O2 CeO2 + 2Ce ⎯⎯⎯→ 2Ce 2 2

Figure 10. Reaction mechanism of the surface modification observed after CO adsorption on ZnCe sample after treatment in different conditions.

x 2+ OO ZnO + CO(g) + Zn

→ V ··OZnO + Zn 2 +−CO2 (ads)

(2)

Zn 2 +−CO2 (ads) → Zn 2 + + CO2(g)

(3)

x x ·· V ··OZnO + OO CeO2 → V OCeO2 + OOZnO

(4)

V ··OCeO + Ce3 + → V′OCeO + Ce4 + 2 2

(5)

x Ce4 +−CO + 2OO

→ Ce3 +−CO32 − + V ·OCe O + V ··OCeO 2 3 2

O2 + V′OCe O → O− 2 (ads) 2 3

(6) (7)

Evidently, the limiting reaction is the formation of oxygen vacancies in the ZnO. Then, when the supported phase is distributed as isolated zinc cations they will act as the generators of the additional oxygen vacancies in ceria, which are the active sites in the catalytic reaction. Unambiguously, it is necessary to find the optimum amount of the supported phase because less zinc oxide does not create enough oxygen vacancies, while the larger amount of supported ZnO would lead to formation of bigger clusters. Unsupported ZnO is not active at temperatures below 473 K. At this temperature some oxygen vacancies, situated only on the surface, were formed.54 Thus, the bigger

(1)

Figure 9. Schematic picture of the state of the surface upon CO adsorption on Zn-modified ceria subjected to different activation procedures. 5754



Article

(10) Rossignol, S.; Micheaud-Especel, C.; Duprez, D. Stud. Surf. Sci. Catal. 2000, 130, 3327−3332. (11) Bao, H.; Chen, X.; Fang, J.; Jiang, Z.; Huang, W. Catal. Lett. 2008, 125, 160−167. (12) Avgouropoulos, G.; Manzoli, M.; Boccuzzi, F.; Tabakova, T.; Papavasiliou, J.; Ioannides, T.; Idakiev, V. J. Catal. 2008, 256, 237−247. (13) Gellings, P. J.; Bouwmeester, H. J. M. Catal. Today 2000, 58, 1−53. (14) Hadjiivanov, K.; Vayssilov, G. Adv. Catal. 2002, 47, 307−511. (15) Lavalley, J. C. Catal. Today 1996, 27, 377−101. (16) Davydov, A. Infrared Spectroscopy of Adsorbed Species on the Surface of Transition Metal Oxide; John Wiley & Sons: New York, 1990. (17) Knözinger, H. Adv. Catal. 1976, 25, 184−271. (18) Sahibed-Dine, A.; Aboulayt, A.; Bensitel, M.; Mohammed Saad, A. B.; Daturi, M.; Lavalley, J. C. J. Mol. Catal. A: Chem. 2000, 162, 125−134. (19) Laguna, O. H.; Centeno, M. A.; Romero-Sarria, F.; Odriozola, J. A. Catal. Today 2011, 172, 118−123. (20) Laguna, O. H.; Romero Sarria, F.; Centeno, M. A.; Odriozola, J. A. J. Catal. 2010, 276, 360−370. (21) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination Compounds. Part B: Applications in Coordination, Organometallic, and Bioinorganic Chemistry; John Wiley & Sons: New York, 2009. (22) Vayssilov, G N.; Mihaylov, M.; Petkov, P. St; Hadjiivanov, K. I.; Neyman, K. M. J. Phys. Chem. C 2011, 115, 23435−23454. (23) Tsyganenko, A. A.; Filimonov, V. N. J. Mol. Struct. 1973, 19, 579−589. (24) Zaki, M. I.; Hasan, M. A.; Al-Sagheer, F. A.; Pasupulety, L. Colloids Surf., A 2001, 190, 261−274. (25) Azambre, B.; Idriss, I.; Bueno-López, A.; García-García, A. J. Phys. Chem. C 2010, 114, 13300−13312. (26) Zaki, M. I.; Knozinger, H. Mater. Chem. Phys. 1987, 17, 201−215. (27) Scarano, D.; Spoto, G.; Bordiga, S.; Zecchina, A.; Lamberti, C. Surf. Sci. 1992, 276, 281−298. (28) Ghiotti, G.; Boccuzzi, F.; Scala, R. J. Catal. 1985, 92, 79−97. (29) Binet, C.; Badri, A.; Lavalley, J. C. J. Phys. Chem. 1994, 98, 6392−6398. (30) Lavalley, J. C.; Saussey, J.; Tsyganenko, A. A. Surf. Sci. 1994, 315, 112−118. (31) Binet, C.; Daturi, M.; Lavalley, J. C. Catal. Today 1999, 50, 207−225. (32) Centeno, M. A.; Hadjiivanov, K.; Venkov, Tz.; Klimev, Hr.; Odriozola, J. A. J. Mol. Catal. A: Chem. 2006, 252, 142−149. (33) Bensalem, A.; Bozon-Verduraz, F.; Delamar, M.; Bugli, G. Appl. Catal., A 1995, 121, 81−93. (34) Tsyganenko, A. A.; Denisenko, L. A.; Zverev, S. M.; Filimonov, V. N. J. Catal. 1985, 94, 10−15. (35) Scarano, D.; Spoto, G.; Zecchina, A.; Reller, A. Surf. Sci. 1989, 211/212, 1012−1017. (36) Romero-Sarria, F.; Penkova, A.; Martinez, T., L.M.; Centeno, M. A.; Hadjiivanov, K.; Odriozola, J. A. Appl. Catal., B 2008, 84, 119−124. (37) Zaki, M.; Vielhaber, B.; Knozinger, H. J. Phys. Chem. 1986, 90, 3176−3183. (38) Tabakova, T.; Boccuzzi, F.; Manzoli, M.; Andreeva, D. Appl. Catal., A 2003, 252, 385−397. (39) Ghiotti, G.; Boccuzzi, F.; Chiorino, A. Adsorp. Catal. Oxide Surf. Stud. Surf. Sci. Catal. 1985, 21, 235. (40) Chauvin, C.; Saussey, J.; Lavalley, J. C.; Djega-Mariadassou, G. Appl. Catal. 1986, 25, 59−68. (41) Ramis, G.; Busca, G.; Lorenzelli, V. Mater. Chem. Phys. 1991, 29, 425−435. (42) Manzoli, M.; Chiorino, A.; Boccuzzi, F. Appl. Catal., B 2004, 52, 259−266. (43) Boccuzzi, F.; Ghiotti, G.; Chiorino, A. Surf. Sci. 1985, 162, 361−367.

ZnO clusters block the oxygen vacancies of ceria without generating new ones. Figure 9 summarizes the modification observed after CO adsorptions on the surface of our sample after different treatments. A possible mechanism describing the surface modification is presented (Figure 10). Actually, the catalytic tests performed19 demonstrated that the catalyst with the highest ZnO loading (50 mol %) possessed a poor catalytic activity even lower than those of the unmodified ceria.

5. CONCLUSIONS CO adsorption on the sample activated at 473 K leads to formation of Ce4+ carbonyls only. Upon further activation at the same temperature and after consecutive CO adsorption, reduction of some of the Ce4+ cations was observed. When the CO adsorption was performed on the sample activated at 573 K, Ce3+ and Zn2+ carbonyls were detected. The performed three consecutive CO adsorptions revealed decrease in intensity of the isolated Zn2+ carbonyls, and another type of Zn2+ carbonyls was detected. CO adsorption on Zn-modified ceria activated at 673 K showed existence of carbonyls of isolated Zn2+ sites and bigger clusters. Upon further CO adsorption, a surface reaction was observed. A conversion of the isolated Zn2+ carbonyls into closely located Zn2+ cations was followed by formation of bigger clusters. The process was accompanied by a superoxide formation. A reaction mechanism for the activation during CO oxidation reaction was proposed.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; phone +34 954489500/9218; fax +34 954460665. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The financial support for this work has been obtained from the Spanish Ministerio de Ciencia e Innovación (ENE2009-14522C05-01) cofunded by FEDER funds from the European Union and from Junta de Andaluciá (TEP-106). A.P. and O.H.L. kindly thank the Spanish Ministerio de Educación y Ciencia for the “Juan de la Cierva” contract and the FPI fellowship awarded respectively.

■

REFERENCES

(1) Trovarelli, A. Catal. Rev. Sci. Eng. 1996, 38, 439−520 and references therein. (2) Laachir, A.; Perrichon, V.; Badri, A.; Lamotte, J.; Catherine, E.; Lavalley, J. C.; E1-Fallah, J.; Hilaire, L.; le Normand, F.; Quéméré, E.; Savion, G. N.; Touret, O. J. Chem. Soc., Faraday Trans. 1991, 87, 1601−1609. (3) Rao, G. R. Bull. Mater. Sci. 1999, 22, 89−94. (4) Körner, R.; Ricken, M.; Nölting, J.; Riess, I. J. Solid State Chem. 1989, 78, 136−149. (5) Conesa, J. C. Surf. Sci. 1995, 339, 337. (6) Nolan, M.; Fearon, J. E.; Watson, G. W. Solid State Ionics 2006, 177, 3069−3074. (7) Choudhary, C. B.; Maiti, H. S.; Subbarao, E. C. Solid Electrolytes and Their Applications; Plenum Press: New York, 1980. (8) Papavasiliou, J.; Avgouropoulos, G.; Ioannides, T. Appl. Catal., B 2007, 69, 226−234. (9) Gamarra, D.; Martínez-Arias, A. J. Catal. 2009, 263, 189−195. 5755



Article

(44) Edwards, J. F.; Schrader, G. L. J. Phys. Chem. 1984, 88, 5620− 5624. (45) Holmgren, A.; Andersson, B.; Duprez, D. Appl. Catal., B 1999, 22, 215−230. (46) Meunier, F. C.; Tibiletti, D.; Goguet, A.; Reid, D.; Burch, R. Appl. Catal., A 2005, 289, 104−112. (47) Farmer, V. C. Mineralogical Society Monograph 4: The Infrared Spectra of Minerals; Mineralogical Society of Great Britain & Ireland: Middlesex, U.K., 1977. (48) Frost, R. L.; Dickfos, M. J. Spectrochim. Acta, Part A 2008, 71, 143−146. (49) Li, C.; Domen, K.; Maruya, K. −I.; Onishi, T. J. Catal. 1990, 123, 436−442. (50) Pushkarev, V.; Kovalchuk, V.; d’Itri, J. J. Phys. Chem. B 2004, 108, 5341−5348. (51) Guzman, J.; Carrettin, S.; Corma, A. J. Am. Chem. Soc. 2005, 127, 3286−3287. (52) Descorme, C.; Madier, Y.; Duprez, D. J. Catal. 2000, 196, 167−173. (53) Hotan, W.; Gapel, W.; Haul, R. Surf. Sci. 1979, 83, 162−180. (54) Hao, Z.; Fen, L.; Lu, G. Q.; Liu, J.; An, L.; Wang, H. Appl. Catal., A 2001, 213, 173−177.

5756


CO-Induced Morphology Changes in Zn-Modified Ceria: A FTIR

Recommend Documents