CeO2 Catalysts for

Article pubs.acs.org/JPCC

Effect of Sodium on the Catalytic Properties of VOx/CeO2 Catalysts for Oxidative Dehydrogenation of Methanol Yan Li,† Zhehao Wei,† Junming Sun,† Feng Gao,‡ Charles H. F. Peden,‡ and Yong Wang*,†,‡ †

Gene and Linda Voiland School of Chemical Engineering and Bioengineering, Washington State University, Pullman, Washington 99164, United States ‡ Institute for Integrated Catalysis, Pacific Northwest National Laboratory, Richland, Washington 99354, United States ABSTRACT: A series of VOx/CeO2 catalysts with various sodium loadings (Na/V ratio from 0 to 1) have been studied for oxidative dehydrogenation (ODH) of methanol. The effect of sodium on the surface structure, redox properties, and surface acidity/basicity of VOx/CeO2 was investigated using hydrogen temperature-programmed reduction (H2-TPR), Raman spectroscopy, and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The experimental results indicate that the effect of sodium on VOx/CeO2 is highly dependent on the Na/V ratio. At a low Na/V ratio (Na/V < 0.25), sodium addition only slightly decreases the redox ability of VOx/CeO2 and has little effect on its activity and selectivity to formaldehyde, even though the Brönsted acidity is almost completely eliminated at a Na/V ratio of 0.25. At a high Na/V ratio (Na/V > 0.25), sodium addition greatly alters the nature of the active sites by V−O−Ce bond cleavage and V−O−Na bond formation, leading to significantly reduced activity of the VOx/ CeO2 catalysts. At Na/V > 0.25, the selectivity to formaldehyde also decreases with increasing Na/V ratio due to (1) the suppressed reducibility of VOx and (2) increased basicity leading to increased CO2. exposed facets on the activity of VOx/CeO2, we first report the sodium effect in this study. Alkali addition to supported vanadium oxide catalysts has been found to influence activity and selectivity in ODH reactions by alternating the catalyst redox properties as well as surface acidity/basicity.22,23 For instance, the addition of potassium to alumina-supported vanadium oxide catalysts results in decreased activity in ODH of light alkanes and increased selectivity to alkenes.24,25 Potassium and sodium have been reported to decrease the activity of VOx/TiO2 catalysts in selective catalytic reduction (SCR) of NOx to N2 by blocking the Brönsted and/or Lewis acid sites responsible for ammonia adsorption.26−28 In general, the influence of alkali on supported VOx catalysts is rather complex and can be attributed to multiple factors such as the type and amount of alkali applied, the impregnation sequence, and the identity of support.29−31 Thus far, most relevant studies have focused on potassium addition to vanadium oxide catalysts supported on TiO2, Al2O3, and SiO224,31−33 while the effects of sodium addition are much less studied.23,27,34 To the best of our knowledge, the focus of the present study, i.e., the effect of sodium addition on methanol partial oxidation over ceria-supported vanadium oxide, has not been reported previously.

1. INTRODUCTION Supported vanadium oxide (VOx) catalysts have received great attention in catalytic processes including oxidative dehydrogenation (ODH) of alkanes and alcohols1−4 and selective catalytic reduction of NOx.5 This is mainly attributed to their high activity and controllable selectivity toward the desired products. Catalyst supports are known to have a strong influence on the catalytic activities of supported vanadium oxides.6−8 For example, VOx catalysts supported on reducible supports typically show a much higher activity (orders of magnitude higher in turnover frequency (TOF)) in ODH reactions than those supported on irreducible supports, such as silica and alumina.9−11 Ceria has been extensively studied as a reducible support in the past and is known to have unique redox properties, high oxygen mobility, and high oxygen storage capacity.12−14 However, the origin of its promotional effect in VOx/CeO2 catalysts is still less clear.15−17 Recently, the successful synthesis of nanostructured ceria with well-defined morphologies and plane orientations, such as nanopolyhedra,13 nanorod,13,18,19 nanocube13 and nano-octahedron,19 provides new opportunities in studying support effects for both metal and metal oxide catalysts.20,21 However, during the synthesis of shape-controlled CeO2 nanoparticles, a sodium-containing solution (e.g., sodium hydroxide or trisodium phosphate) is unavoidably used as the structure-directing agent, resulting in residual sodium in the final CeO2 supports. To better understand the effect of CeO2 supports with controllable © 2013 American Chemical Society

Received: October 23, 2012 Revised: February 22, 2013 Published: February 22, 2013 5722

dx.doi.org/10.1021/jp310512m | J. Phys. Chem. C 2013, 117, 5722−5729

The Journal of Physical Chemistry C

Article

using a Bruker Tensor 27 FTIR spectrometer equipped with a Praying Mantis accessory and a high-pressure reaction cell (Harrick Scientific Products Inc.). For DRIFTS characterizations of surface hydroxyl group, samples were purged in He (30 mL/min) for 1 h at 50 °C, then heated to 200 °C at 10 °C/ min, and held for 30 min. The spectrum at the target temperature was acquired using the spectrum of potassium bromide (KBr) at the same temperature as the background. Catalyst acidities were monitored with DRIFTS measurements of pyridine chemisorption. Prior to pyridine adsorption, samples were pretreated at 400 °C in 10% O2/He for 30 min, cooled down to 50 °C, and purged with He for another 30 min. Pyridine vapor was introduced using a bubble generator with He as the carrier gas. Physisorbed pyridine was removed by He purging. Measurements were carried out using the spectra of the bare samples (without pyridine chemisorption) at 50 °C as backgrounds. 2.3. ODH Reaction of Methanol. ODH reactions of methanol were conducted in an isothermal fixed-bed reactor with a quartz tube (7 mm i.d.). The feed gas consisted of 6% methanol, 19% O2, and 75% N2. The flow rates of O2 and N2 were controlled with mass flow controllers (Brooks 5850E). A syringe pump was used to inject methanol (Sigma-Aldrich, HPLC grade) into an evaporator held at 100 °C to ensure complete evaporation before mixing with the gas stream (O2 and N2) and entering the reactor. Reaction temperatures were monitored by a K-type thermal couple inserted into the catalyst bed. Reaction products were analyzed online with an Agilent 490 Micro GC equipped with four columns (a Molsieve column, a CP-PoraPLOT U column, an aluminum oxide column, and a CP-Sil 5CB column) and TCD detectors. 10 mg of catalyst diluted in 0.5 g of SiC with 100−120 mesh size was used for activity measurements at 230 °C and ambient pressure with a total gas feed rate of 100 mL/min. Product selectivities were obtained at a fixed methanol conversion of 10 ± 1%. To compare the methanol ODH rates, methanol TOF was calculated as the number of methanol molecules converted per active site per second. The number of vanadium atoms added to the samples was used as the number of active sites since the coverage of VOx is below monolayer coverage in this study.

Here, we report a detailed study on the effect of sodium addition on the surface structure, redox properties, and surface acidity/basicity of a VOx/CeO2 catalyst. Using methanol partial oxidation as a probe reaction, the sodium effect on the catalytic properties of VOx/CeO2 was further studied. The present study also explores a possible structure of the active center that provides structure−activity relationships quantitatively consistent with the experimental results.

2. EXPERIMENTAL METHODS 2.1. Catalyst Preparation. VOx/CeO2 catalysts with various Na/V atomic ratios were prepared via a two-step incipient wetness impregnation method. In the first step, a commercial CeO2 support (Sigma-Aldrich, 99.95%) was calcined in air at 450 °C for 4 h followed by impregnation with NaNO3 (Sigma-Aldrich, 99.999%) aqueous solutions of various concentrations to achieve different sodium loadings. The impregnated samples were dried at room temperature for 12 h followed by calcination at 450 °C for 4 h in air. In the second step, the sodium-loaded CeO 2 supports were impregnated with an aqueous ammonium metavanadate solution which was prepared by dissolving ammonium metavanadate (NH4VO3, Sigma-Aldrich, 99.999%) in an aqueous solution of oxalic acid (H2C2O4, Sigma-Aldrich, 99.999%) with a NH4VO3/H2C2O4 molar ratio of 1:2. The samples were then dried at room temperature for 12 h and calcined at 450 °C for 4 h in air. The prepared catalyst without sodium addition is denoted as VOx/CeO2 while those with sodium are denoted as y Na-VOx/CeO2 with y (y = 0.25, 0.5, 0.75, 1) being the Na/V atomic ratio. The total vanadium oxide content in the y Na-VOx/CeO2 catalysts was held constant at 5 V atoms/nm2 based on the measured surface area of the support, which is close to monolayer coverage.15 Ceria supports modified with sodium alone are denoted as y Na-CeO2, which have the same sodium loadings as y Na-VOx/CeO2. NaVO3/ CeO2 samples were also prepared using the same molar amount of NaVO3 as the precursor instead of NH4VO3. The preparation process was identical to the second step described above. 2.2. Catalyst Characterizations. The specific surface areas (SSAs) of the samples were measured using N2 adsorption− desorption isotherms recorded at −196 °C on a TriStar II 3020 automatic physisorption analyzer (Micromeritics). Before the measurements, samples were degassed under vacuum at 150 °C for 2 h. The Brunauer−Emmett−Teller (BET) method was used to calculate SSA. H2 temperature-programmed reduction (TPR) was performed on an AutoChem II 2920 chemisorption analyzer (Micromeritics) equipped with a thermal conductivity detector (TCD). Pretreatment of the samples was carried out with 10% O2/He at 400 °C for 30 min. The sample temperature was ramped from ambient to 800 °C in 10% H2/Ar at 10 °C/min. Catalyst surface structures were characterized using a Horiba LabRAM HR Raman/FTIR microscope equipped with a 532 nm (Ventus LP 532) laser source and a Synapse CCD (charge coupled device) detector. Samples were dehydrated in an in situ sample cell (Linkam CCR1000) at 400 °C (ramp rate 10 °C/ min) under 10% O2/He (30 mL/min) for 30 min. After cooling down to ambient temperature, Raman spectra of the dehydrated samples were recorded. The stretching vibrational modes of surface hydroxyl groups and catalyst acidities were further investigated with diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS)

3. RESULTS 3.1. Catalyst Characterization. The BET specific surface areas (SSAs) of the VOx/CeO2 and y Na-VOx/CeO2 samples are displayed in Table 1. CeO2 and VOx/CeO2 have essentially Table 1. Composition and Physical Characteristics of the CeO2 Support, VOx/CeO2, and y Na-VOx/CeO2 Catalysts (y = 0.25, 0.5, 0.75, 1)

sSample CeO2 VOx/CeO2 0.25 NaVOx/CeO2 0.5 Na-VOx/ CeO2 0.75 NaVOx/CeO2 1 Na-VOx/ CeO2 5723

Surface V density (V/nm2)

V2O5 loading (wt %)

Na/V molar ratio

Na2O loading (wt %)

SSA (m2/g)

5 5

1.2 1.2

0 0.25

0.1

16.5 16.8 17.0

5

1.2

0.5

0.2

16.8

5

1.2

0.75

0.3

15.8

5

1.2

1

0.4

15.9



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Figure 1. H2-TPR profiles of (A) VOx/CeO2 and y Na-VOx/CeO2 (y = 0.25, 0.5, 0.75, and 1) and (B) y Na-CeO2 (y = 0.5 and 1) and NaVO3/ CeO2. For the purpose of direct comparison, the TPR curve of pure CeO2 is also included in both (A) and (B).

Raman spectroscopy was used to better understand the structure of the surface vanadium oxide species affected by sodium addition. Figure 2 shows the Raman spectra of

the same SSA, suggesting a high dispersion of VOx on CeO2. The addition of sodium only slightly affects the SSA. Even at high sodium loadings (Na/V atomic ratio of 0.75 and 1), the SSA decreased by merely ∼4%. For VOx supported on other supports such as Al2O3 and TiO2, the addition of sodium has been reported to have led to a more dramatic reduction of the SSA.31,35 This implies a very good dispersion of VOx and sodium on CeO2 prepared in this study as well as a strong interaction between VOx and CeO2. To investigate the effect of sodium addition on the reducibility of CeO2 and VOx,36 H2-TPR was performed on VOx/CeO2, y Na-VOx/CeO2, y Na-CeO2, NaVO3/CeO2, and CeO2; the results are presented in Figure 1. A high-temperature H2 consumption peak at ∼733 °C observable on all the samples can be assigned to the reduction of bulk CeO2 (Ce4+ → Ce3+).37 It is worth noting that the reduction of bulk CeO2 is not affected by the addition of VOx or sodium, suggesting that these moieties are located primarily on the CeO2 surfaces and, therefore, do not influence the bulk properties of CeO2. For the CeO2 sample, a much weaker reduction peak was also observed at ∼478 °C, which corresponds to the reduction of surface CeO2.38 This feature is weak due to the relatively low surface area of CeO2 used in this study. After dispersion of VOx onto CeO2 (VOx/CeO2), a new reduction peak appears at 455 °C, which is predominately associated with the reduction of VOx species.39 This reduction peak shifts gradually to higher temperatures with increasing sodium loading. Furthermore, it can also be seen from Figure 1A that a weak reduction peak centered at ∼400 °C appears, and its intensity increases as the Na/V atomic ratio increases from 0 to 0.5. We note from Figure 1B that y Na-CeO2 samples (y = 0.5 and 1) also have reduction features at a similar temperature, albeit much stronger. Considering the fact that the redox potential for the Na0/Na+ pair is −2.71 eV, Na2O/NaOH reduction by H2 is extremely unlikely. In the meanwhile, many dopants such as Ca, Nd, and Pb have been found to promote oxygen storage and redox properties of CeO2.40 Alkali addition has also been found to facilitate TiO2 reduction.41 This reduction state, therefore, can be assigned to the reduction of surface CeO2 modified by sodium. The weakness of this reduction peak on Na-VOx/CeO2 samples (Figure 1A) indicates that, in the presence of near monolayer of VOx, the amount of “free” surface CeO2 is rather minimal. Finally, the similarity in line shapes of 0.75 and 1 NaVOx/CeO2 (Figure 1A) and that of NaVO3/CeO2 (Figure 1B) strongly suggests rather similar surface V-containing moieties in these samples.

Figure 2. Raman spectra of VOx/CeO2, y Na-VOx/CeO2, and NaVO3/CeO2.

dehydrated VOx/CeO2, y Na-VOx/CeO2, and NaVO3/CeO2 samples. For the VOx/CeO2 sample, four bands are present at 1034, 1021, 925, and 851 cm−1, in addition to a broad band at ∼1167 cm−1 attributed to the 2LO band of CeO2.42 The sharp band at 1021 cm−1 along with the shoulder peak at 1034 cm−1 are assigned to the stretching mode (ν) of VO in dimeric and trimeric VOx species tetrahedrally coordinated to the CeO2 surface, respectively.21,43 No VO stretching mode of VOx monomers, which is reported to display a Raman band at 1008 cm−1,44 was detected on VOx/CeO2 sample primarily due to the high surface vanadium density studied here. This result is consistent with the scanning tunneling microscopy (STM) and infrared reflection−absorption spectroscopy (IRAS) studies by Abbott et al., who found the disappearance of VOx monomers at vanadium densities higher than 2 V/nm2 in IRA spectra.15 Similarly, a Raman study by Wu et al. on VOx/CeO2 also shows the disappearance of VO stretching band of VOx monomers when vanadium density is higher than 2.7 V/nm2.44 The band at 925 cm−1 is attributed to V−O−V bond of polymeric surface vanadate species, and the broad band at 851 cm−1 corresponds to the stretching mode of V−O−Ce bond.44,45 No Raman bands attributed to crystalline V2O5 or CeVO4 were observed, indicating that the VOx species are highly dispersed on CeO2, which is consistent with the results from BET specific surface area measurement as aforementioned. 5724



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two bands as compared to OH vibrations on CeO2 is consistent with the greater electronegativity of V5+ than Ce4+.52−54 For the Na-VOx/CeO2 samples, the isolated OH vibrational bands shift to higher wavenumbers with increasing Na/V ratios. This suggests that the hydroxyl groups become more basic compared to sodium-free VOx species, as expected. The broad OH vibrational features for H-bonded OH below 3500 cm−1, in contrast, are less well-resolved and less informative. Pyridine DRIFTS was used to further investigate the effect of sodium on surface acidity of the VOx/CeO2 catalysts. Figure 4

The VO stretching mode undergoes rather complex changes with the addition of sodium. First, the main VO peak gradually red-shifts from 1021 to ∼985 cm−1 as the Na/V ratio increases from 0 to 1. This red-shift indicates a gradual increase of VO bond length due to the addition of sodium, which may be caused by an electrostatic interaction or direct bonding between sodium and VOx species.35 Second, the shoulder peak at 1034 cm−1 red-shifts and diminishes with increasing Na/V ratio, suggesting that sodium addition depolymerizes the trimeric VOx species. Similar phenomena have also been reported in a UV−vis study on VOx upon alkali addition which showed a blue-shift of the energy gap of alkalimodified catalysts.29,46,47 Third, a new VO feature at ∼970 cm−1 develops and its intensity increases with the Na/V ratio (from 0.25 to 0.75). For 1 Na-VOx/CeO2, only one VO mode is observed at ∼985 cm−1. The assignment of these two peaks will be discussed later. In addition, with increasing Na/V ratio, the V−O−V band at 925 cm−1 is first weakened and becomes less well-resolved, due at least partially to the loss of VOx trimers. Then this band gains intensity again and shifts slightly to 933 cm−1 at high sodium loadings (especially at a Na/V ratio of 1). The V−O−Ce band at 851 cm−1 essentially does not change at Na/V ratios below 0.75 and then slightly shifts to a higher wavenumber when the Na/V ratio reaches 1. To better understand the Raman spectra of the Na-VOx/CeO2 samples, the spectrum of NaVO3/CeO2 was collected as a reference. As shown in Figure 2, 1 Na-VOx/CeO2 shows an almost identical Raman spectrum as NaVO3/CeO2, indicating the presence of the same surface V-containing species as that of NaVO3/CeO2 although generated from different precursors. Metal oxide surfaces are generally covered with hydroxyl groups under ambient conditions. The investigation of surface hydroxyl groups can therefore provide useful surface information on metal oxide catalysts. Figure 3 displays DRIFTS

Figure 4. DRIFTS spectra of absorbed pyridine on VOx/CeO2 and y Na-VOx/CeO2 (y = 0.25, 0.5, 0.75) at 50 °C.

displays pyridine vibrational bands at 50 °C on VOx/CeO2 and y Na-VOx/CeO2 catalysts. The sodium-free catalyst (VOx/ CeO2) exhibits both Lewis and Brönsted acidity, as evidenced by the coexistence of the 1440 cm−1 band assigned to pyridine chemisorbed on surface Lewis acid sites and the 1540 cm−1 band assigned to pyridium cation generated on surface Brönsted acid sites.55 Upon sodium addition, Brönsted acidity greatly diminishes. Specifically, the 1540 cm−1 band becomes barely detectable on 0.25 Na-VOx/CeO2 while on 0.5 Na-VOx/ CeO2, this band becomes completely absent. The same conclusion can be made by examining the 1488 cm−1 band which is attributed to the combination of Brönsted and Lewis acidities.55 Again, the intensity of this feature decreases dramatically as the Na/V ratio increases from 0 to 0.25 and becomes unchanged for 0.5 and 0.75 Na-VOx/CeO2 samples, indicating complete elimination of Brönsted acidity. Since there is almost no Brönsted acidity on the CeO2 support,56 it is suggested that Na preferentially coordinates to Brönsted acid sites of vanadium oxides. This result is consistent with Kustov et al.’s report that the potassium modified VOx/TiO2 catalyst with a K/V ratio of 0.47 was inactive in NO SCR with ammonia, which is due to the fact that the Brönsted acid sites essential for the reaction are completely poisoned at this potassium loading.57 3.2. ODH of Methanol. In this study, ODH of methanol (at 230 °C) was used as a probe reaction to evaluate the effect of sodium on the activity of VOx/CeO2 catalysts. The CeO2 support exhibited negligible activity under the conditions studied. Figure 5A displays the rates of methanol oxidation (TOFs) over VOx/CeO2 catalysts as a function of Na/V ratio, and Figure 5B shows the product selectivities at a methanol conversion of about 10%. Interestingly, sodium addition has little effect on methanol TOF below a Na/V ratio of 0.25 (only decreases by 4%), whereas methanol TOF decreased almost

Figure 3. DRIFTS spectra of the OH stretching vibration region of CeO2, VOx/CeO2, and Na-VOx/CeO2.

spectra of the OH stretching vibrational region (4000−3000 cm−1) of the selected catalysts. Vibrations at >3500 cm−1 are typically assigned to isolated OH species while those at 90% at Na/V ratio of 1 (Figure 5A). As shown in Figure 5B, formaldehyde (FA) constitutes the major product with small amounts of CO2, dimethoxymethane (DMM), and methyl formate (MF). FA selectivity decreases and CO2 selectivity increases as the Na/V ratio increases. The decreased FA selectivity is caused by the suppressed redox properties as evidenced by TPR. The increased CO2 selectivity with Na/V ratio is most likely due to the increased surface basicity introduced by sodium addition, which results in a stronger adsorption of surface methoxyl groups which promote CO2 formation. This result is consistent with the report that methoxyl groups adsorbed on alkali ions (K) predominantly produce CO2 with a small amount of FA.31 In addition, DMM only formed on sodium-free VOx/CeO2. Since DMM formation requires the presence of both redox and Brönsted acid sites,58 the absence of DMM on Na-VOx/CeO2 sample is consistent with the pyridine DRIFTS observation that surface Brönsted acidic sites were neutralized by sodium at a Na/V ratio of 0.25.

4. DISCUSSION Selective oxidation of methanol to formaldehyde has received extensive attention and is often used as a probe reaction in 5726



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neutralizes the Brönsted acidic sites but has little effect on the redox properties and the activity of VOx/CeO2 catalyst for ODH of methanol, implying that the nature of active site is minimally affected with sodium addition. With further increasing Na/V ratios from 0.25 to 1, the reducibility of VOx species significantly diminishes and the activity of VOx/ CeO2 catalyst decreases linearly with the Na/V ratio, likely due to the cleavage of the active V−O−Ce bonds to form the inactive V−O−Na bonds. Moreover, the selectivity to formaldehyde is reduced while CO2 formation is promoted due to the suppressed redox properties and increased surface basicity by sodium addition.

Scheme 1. Surface Reaction between Sodium and VOx at Low Sodium Loadings (Na/V < 0.25)

It is also noteworthy that despite the proposed modification of sodium on the structure of VOx/CeO2 at low sodium loading (Na/V < 0.25) in Scheme 1, the potential active centers (V− O−Ce) for ODH of methanol are not significantly altered as evidenced by the slight change in methanol TOFs measured at Na/V < 0.25 (Figure 5A). At Na/V > 0.25, methanol ODH TOFs decrease linearly with Na/V ratios as shown in Figure 5A, suggesting that the nature of the active sites is strongly affected at such high sodium loadings. To better understand the nature of active sites at high sodium loadings, the structure of the 1 Na-VOx/CeO2 surface is discussed as this represents a nearly “inactive” catalyst. We note first that, from both TPR (Figure 1) and Raman (Figure 2) analyses, the 1 Na-VOx/CeO2 and the NaVO3/CeO2 surfaces are essentially identical. Importantly, the strong V−O−V band at 933 cm−1 clearly shows that on both surfaces VOx moieties are present as dimers rather than monomers. Also, unlike Na-VOx/CeO2 surfaces at lower sodium loadings where 2 (or 3) VO stretching bands coexist (Figure 2), there is only one VO vibration mode centered at 985 cm−1 for 1 Na-VOx/CeO2 and NaVO3/CeO2. This means that within the vanadia dimers the two VO bonds are in chemically equivalent positions. This could only be achieved by V−O−Ce bond cleavage to form the V−O−Na bond as proposed in Scheme 2 for the surface reaction between sodium and VOx at high Na loadings (Na/V > 0.25).

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AUTHOR INFORMATION

Corresponding Author

*Tel (509) 371-6273; Fax (509) 371-6242; e-mail yongwang@ pnl.gov. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the financial support from the U.S. Department of Energy (DOE), Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences. Pacific Northwest National Laboratory (PNNL) is a multiprogram national laboratory operated for DOE by Battelle.

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REFERENCES

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Scheme 2. Surface Reaction between Sodium and VOx at High Sodium Loadings (Na/V > 0.25)

We also note in this surface model that, as the two VO bonds are in identical positions, dipolar coupling becomes more efficient, allowing a blue-shift from ∼970 to 985 cm−1 (Figure 2). In this case, the number of possible active centers for ODH of methanol, i.e., V−O−Ce, decreases as the sodium loading increases. In addition, the chemical nature of these centers might also be modified by sodium; i.e., the tetrahedral coordination of VOx with the surface is likely broken. Both factors appear to contribute to the reduction of VOx/CeO2 activity with sodium addition at high sodium loadings (Na/V > 0.25). As expected, a nearly complete reduction in methanol TOF (>80%) was observed at a Na/V ratio of 1 (Figure 5A).

5. CONCLUSION Experimental results reported here show that the effect of sodium on VOx/CeO2 is complex and highly dependent on the Na/V ratios. At a Na/V ratio of

CeO2 Catalysts for

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