Surface Optimization and Redox Behavior of Vanadium Oxides

Jun 21, 2011 - Pr)3, OV(OEt)3 and OV(OPr)3. It was observed that the actual loading of the metal oxide is controlled by a number of factors, including...
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Surface Optimization and Redox Behavior of Vanadium Oxides Supported on γ-Al2O3 Arindom Saha† and Darrell P. Eyman* Department of Chemistry, University of Iowa, Iowa City, Iowa 52242-1294, United States

bS Supporting Information ABSTRACT: This report describes optimization of the synthesis of γ-Al2O3 supported vanadium oxides using three different vanadium precursors: OV(OiPr)3, OV(OEt)3 and OV(OPr)3. It was observed that the actual loading of the metal oxide is controlled by a number of factors, including the kinetics and thermodynamics of the grafting process, the time allowed for the grafting reaction to proceed, the amount of available precursor, the concentration of the grafting solution, and temperature at which grafting is pursued. From TPR (temperature programmed reduction) and XPS (X-ray photoelectron spectroscopy) studies it was observed that V4+ and V5+ were the predominant oxidation states present under both normal and rigorous reducing conditions simulating those prevalent in the steam reforming process.

’ INTRODUCTION Supported vanadium oxides are used in a range of heterogeneous catalytic processes. Historically, these catalysts have been prepared by supporting the oxide on a suitable support using sol gel and impregnation techniques. These techniques, though easy to use, have led to microcrystallite formation and nonuniformity on the surface of the support. Through grafting techniques, as used in the work reported herein, a consistent surface, uniform in composition and distribution, can be obtained. During the last thirty years much attention has been given to the synthesis, characterization and catalytic behavior of vanadium oxide monolayered catalysts immobilized on inorganic oxide supports.1 Various precursors like OVCl3,2 4 OV(acac)2,5,6 polyvanadates, gaseous vanadic acids,7 and vanadium alkoxides,1 8 have been used to react with the surface hydroxyl groups to generate immobilized surface species. A wide range of studies on characterization and catalytic activities have also been performed on vanadium oxide monolayer catalysts.9 Supported vanadium oxide catalysts form a two-dimensional surface overlayer on high-surface area oxide supports like γ-Al2O3 and have widespread industrial applications, including methanol oxidation and reduction of NOx emissions.10,11 Researchers have also been able to successfully characterize and study the presence of isolated and polymeric surface vanadate (VO43 ) species on the oxide supports in dehydrated supported vanadia catalysts using techniques such as Raman spectroscopy,12 X-ray absorption spectroscopy (EXAFS and XANES)13,14 solid-state 51V NMR spectroscopy15 and X-ray photoelectron spectroscopy (XPS).16 SiO2-supported vanadium catalysts have proved to be very effective for the selective oxidation of low alkanes to oxygenates due to the special physicochemical properties of the SiO2 carrier and the role of vanadyl species.17 More recently, supported vanadium oxide catalysts have also received attention as modelsupported metal oxide catalyst systems.18 This report discusses grafting reactions on γ-Al2O3 spheres with three different vanadium alkoxide precursors, namely OV(OiPr)3, OV(OEt)3 and OV(OPr)3. The purpose of this study is r 2011 American Chemical Society

to understand the influence of different reaction conditions on metal oxide loading. Optimization of the surface density of the vanadium oxides under these conditions is thought to be a way to optimize the catalytic activity. The higher the surface density of the oxide, the better is the anticipated catalytic performance in steam reforming processes. An additional objective of this study is to identify the oxidation state(s) of vanadium present, using TPR and XPS during normal and rigorous reduction conditions. These reduction conditions are intended to simulate those prevalent in the steam reforming process for hydrogen generation. The studies pursued here to understand the role of vanadium in catalysis was inspired by previous research in this group on a V Zr mixed oxide catalyst grafted on γ-Al2O3. This catalyst was observed to give very impressive conversion rates in production of hydrogen in steam reformation of alkanes and low molecular weight alcohols.19

’ EXPERIMENTAL SECTION All gases used in this research were purchased from Air Products. These gases were used for TPR studies and for maintaining an inert atmosphere in Schlenk lines and glove boxes. Drisolv@ Tetrahydrofuran (THF), with special mesh caps on the bottles purchased through VWR to prevent entry of moisture, was used as a solvent in all grafting reactions. Distilled water, nitric acid (50 70%, Fischer Scientific) and sulfuric acid (98%, Fischer scientific) were used to dissolve samples for elemental analysis. The three vanadium precursors used for grafting were OV(OiPr)3 (95 98%, Alfa Aesar), OV(OEt)3 (95%, Aldrich Chemicals) and OV(OPr)3 (98%, Aldrich Chemicals). They were all stored and used in an inert glovebox atmosphere. The vanadium-ICP standard used for calibration of vanadium in inductively coupled plasma (ICP) studies was purchased from Received: February 8, 2011 Accepted: June 21, 2011 Revised: June 1, 2011 Published: June 21, 2011 9027

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Table 1. Various Categories of Grafted Samples Synthesized and Studied precursor i

type of study

name of sample

OV(O Pr)3

time(12,24,36,48,72)h

t-4.0(iPr)VOx/Al2O3

OV(OEt)3 OV(OPr)3

time(12,24,36,48,72)h time(12,24,36,48,72)h

t-4.0(Et)VOx/Al2O3 t-4.0(Pr)VOx/Al2O3

OV(OiPr)3

feed (1 8 V/nm2)

f-(iPr)VOx /Al2O3

2

OV(OEt)3

feed (1 8 V/nm )

OV(OPr)3

feed (1 8 V/nm2)

f-(Et)VOx /Al2O3 f-(Pr)VOx /Al2O3

OV(OiPr)3

conc. (5,10,15,20,25)mL

c-4.0(iPr)VOx/Al2O3

OV(OEt)3

conc. (5,10,15,20,25)mL

c-4.0(Et)VOx/Al2O3

OV(OPr)3

conc. (5,10,15,20,25)mL

c-4.0(Pr)VOx/Al2O3

OV(OiPr)3 OV(OEt)3

temp(R.T,40, 60, 80) C temp(R.T, 40, 60, 80) C

T-4.0(iPr)VOx/Al2O3 T-4.0(Et)VOx/Al2O3

OV(OPr)3

temp(R.T, 40, 60, 80) C

T-4.0(Pr)VOx/Al2O3

Aldrich Chemicals and stored ambiently. All reagents and solvents were used as received without further purification or processing. The γ-Al2O3 spheres, ∼1 mm in diameter, used for the grafting studies were generously donated by Saint-Gobain Norpro Corporation. The samples were dried at 110 °C for at least 24 h before use. A Micromeritics ASAP 2000 was used to measure surface area by adsorption and desorption of nitrogen at liquid nitrogen temperature. The surface area was determined from the acquired data using the BET (Brunaeur-Emmett-Teller) equation. Thermogravimetric analysis (TGA) was used to determine the surface hydroxyl density of the γ-Al2O3 pellets. The physical properties of the γ-Al2O3 used were as follows: surface area ∼219 m2/g, pore volume ∼0.59 cm3/g, and average pore diameter 6.7 nm. From TGA the surface hydroxyl density was observed to be ∼13.02 ( 0.31 OH/nm2. The grafted vanadium alkoxide samples were synthesized on a Schlenk-line under an inert nitrogen atmosphere. After synthesis the samples were calcined at 500 °C for 24 h to obtain the final grafted vanadium materials (see the Supporting Information for detailed grafting procedures). Categories of Grafted Materials. The vanadium oxides grafted on γ-Al2O3, using different precursors and reaction conditions, are listed in Table 1. Four categories of grafted vanadium oxides were synthesized to study the effects of different reaction conditions and the precursor ligand sizes on the grafted loading. The categories were t-4.0VOx/Al2O3, f-VOx/Al2O3, c-4.0VOx/ Al2O3 and T-4.0VOx/Al2O3. Each category of sample was synthesized independently with the three vanadium precursors mentioned above. In this nomenclature “t” stands for time of grafting, “f” for metal alkoxide feed in grafting solution, “c” for concentration of grafting solution and “T” for temperature of grafting reaction. For instance, t-4.0 (iPr) VOx /Al2O3 indicates a grafted material synthesized from the precursor OV(OiPr)3 when the available amount of the precursor in the feed corresponds to a theoretical loading of 4.0 V/nm2.

’ MATERIAL CHARACTERIZATION Elemental Analysis. The surface density of vanadium on the grafted samples was calculated from the amount of vanadium determined through quantitative elemental analysis and the surface area of the γ-Al2O3 substrate. This analysis was performed using a Varian 720-ES inductively coupled plasma optical

Figure 1. Schematic diagram of the TPR apparatus.

emission spectrophotometer (details in the Supporting Information).

’ H2/He TPR AND XPS STUDIES Reduction studies of grafted vanadium samples were performed in a specially constructed apparatus as depicted in Figure1 (for details see the Supporting Information). These reductions were performed with two distinct gas compositions: a) 10% H2/ He mixture and b) 20% H2/He mixture. Samples were reduced for 6 h at temperatures of 500 °C, 600 °C, 700 °C, and 750 °C respectively using both these above gas compositions. The temperature range from 500 to 750 °C was chosen as it is the range typically used for steam reforming of hydrocarbons, other than methane. After reduction, the samples were stored under inert conditions and then taken to a custom-designed Kratos Axis Ultra X-ray photoelectron spectroscopy system located at the Central Microscopy Facility at the University of Iowa. There, with minimum exposure to atmosphere the sample pellets were cut into halves and mounted on the sample analyzer using carbon sticky tape. After the samples were inserted into the analysis chamber, the evacuation process was initiated under UHV (ultra high vacuum) conditions. After this the XPS spectra were collected to determine the metal oxidation states. Quantitative processing of the acquired data using CasaXPS software was performed to determine the relative percentages of different oxidation states of vanadium. X-ray Diffraction Studies. The sample spheres were first powdered using a mortar and pestle. The X-ray diffraction (XRD) patterns were then measured on these powdered samples using a Siemens D5000 powder X-ray diffractometer equipped with Cu KR radiation. The diffractograms were obtained using Cu KR radiation at a wavelength of 0.1542 nm. ’ RESULTS AND DISCUSSION A. Effect of Reaction Conditions on Surface Loading Optimization of Vanadium. These studies were pursued by

monitoring various reaction conditions including time of grafting, loading precursor feed, grafting solution concentration, and temperature to understand the effects of different reaction conditions and precursor ligand sizes on the vanadium surface density. All the precursor loading values reported here were taken as an average of three measurements. The errors in the loadings of 9028

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Figure 2. Change in loading as a function of time for samples synthesized using the three precursors: (a) OV(OiPr)3, (b) OV(OEt)3, and (c) OV(OPr)3.

Figure 3. Change in loading as a function of precursor feed for samples synthesized using the three precursors: (a) OV(OiPr)3, (b) OV(OEt)3, and (c) OV(OPr)3.

vanadium obtained from all the time studies discussed were observed to be in a range of 0.8 3.7%, in the precursor loading studies were observed to be in a range of 0.4 5.9% and in the concentration studies were observed to be in a range of 0.3 3.2%. Variation of Loading with Time. All the time series samples, t-4.0VOx/Al2O3, were synthesized at room temperature using vanadium feed corresponding to a loading of 4.0 V/nm2. A fixed solution concentration of 0.324 M (corresponding to 25 mL of solvent) was maintained in all sample syntheses. The reaction time was varied from 12 to 72 h with an increment of 12 h between consecutive samples. These studies established that for each of the three vanadium precursors the grafting reaction proceeded slowly. From these studies it was observed that the loading increased with increase in grafting time until a maximum was reached, presumably when a grafting-degrafting equilibrium was established. With each of the precursors, equilibrium was reached after approximately 36 h (Figure 2). It was observed that the loading achieved at equilibrium with the precursor OV(OiPr)3, was 3.73 ( 0.03 V/nm2. Under the same conditions for the precursors OV(OEt)3 and OV(OPr)3, slightly higher loadings of 3.95 ( 0.06 V/nm2 and 3.86 ( 0.03 V/nm2 were obtained, respectively, at equilibrium . Previous work20 on grafting studies using various zirconium alkoxide precursors established that the steric effects influencing maximum loading by grafting were the result of the total volume of the initially grafted species -OZr(OR)3. This grafted species, and its associated van der Waals domain of repulsion, hereafter referred to as the grafted canopy, influences the distance of approach of subsequent reacting Zr(OR)4 precursor molecules. From these studies it was also observed that the volume of this grafted canopy, covering a sector of the support surface, decreased in size with these three precursor alkoxides in the order OPr > -OiPr ∼ -OEt. The concept involving grafted canopy volume is applicable only when comparing the loading attained at higher metal feeds when maximum loading is reached indicating saturation on the surface. At an intermediate feed of 4.0 V/nm2 the vanadium loading is governed by the grafting-degrafting equilibrium and is lower than that found on the saturated surface. The observed loading values suggest that the precursor with a secondary carbon at the VOR alkyl, reacts with a degrafting-grafting equilibrium favoring degrafting more than the precursors having primary alkyls on the VO moiety.

Variation of Loading with Precursor Feed. The next series of samples synthesized were from the group f-VOx/Al2O3. All the samples in this group were grafted at room temperature using 25 mL of THF. Materials were synthesized with sample feeds ranging from 1.0 V/nm2 to 8.0 V/nm2 with an increment of 1.0 V/nm2 feed between consecutive samples. With OV(OiPr)3 and OV(OEt)3 similar maximum loadings of ∼4.45 V/nm2 were observed (Figure 3). This loading offers some insight into the hapticity of the vanadyl “single sites” which are known to be tetrahedral in nature and thus are expected to have a hapticity of 3 with the surface. A loading of 4.45 V/nm2 multiplied by the hapticity, which is 3 assuming “single sites”, indicates that there are a total of 13.35/nm2 sites of attachment. This can be interpreted as an indication that the grafting process had consumed 13.35 OH/nm2 on the surface. From TGA it was determined that the γ-Al2O3 surface had a surface hydroxyl density of 13.02 ( 0.31 OH/nm2. This value, which is very close to that obtained from the measured loading, suggests that all available OH surface sites were involved in the grafting reaction. In the OV(OiPr)3 studies, the maximum loading was attained only at a feed of 8.0 V/nm2. With OV(OEt)3 the maximum loading was attained with a precursor feed of 5.0 V/nm2. No additional loading was observed on increasing the precursor feed to as high as 8.0 V/nm2. This observation is consistent with OV(OEt)3 reacting with surface hydroxyl groups more extensively at lower feeds. This is perhaps attributable to steric factors associated with the -OR groups resulting in an equilibrium, involving grafting and degrafting, that favors grafting more for OV(OEt)3 than for OV(OiPr)3. With the third precursor, OV(OPr)3, equilibrium was achieved on the surface with a precursor feed of 5.0 V/nm2 and a maximum loading of 3.88 ( 0.11 V/nm2 was obtained on the surface (Figure 3). A lower maximum loading with this precursor is attributable to the higher volume of the monohapto -OV(O)(OPr)2 grafted canopy. Variation of Loading with Precursor Concentration and Temperature. This study involved the two series c-4.0VOx/ Al2O3 and T-4.0VOx/Al2O3 in combination with each other. The concentration (“c”) samples were synthesized by varying the volume of the grafting solvent, THF, from 5 to 25 mL with an increment of 5 mL between consecutive samples. Even though in all cases the precursor feed was taken as 4.0 V/nm2, changing the volume of the solvent changed the concentrations of the grafting solutions in a range from 0.324 M to 1.62 M. Initially all the samples in this concentration series were synthesized at room 9029

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Figure 4. Change in loading as a function of concentration of the grafting solution for samples synthesized using the precursor OV(OiPr)3 at (a) R. T., (b) 40 °C, (c) 60 °C, and (d) 80 °C.

Figure 5. Change in loading as a function of concentration of the grafting solution for samples synthesized using the precursor OV(OEt)3 at (a) R. T., (b) 40 °C, (c) 60 °C, (d) 80 °C.

temperature. Subsequently, samples with different concentrations were synthesized at 40 °C, 60 °C, and 80 °C, respectively. This is the origin of the temperature dependent series, T-4.0VOx/Al2O3. In studies with OV(OiPr)3 the metal oxide loading trends at four different temperatures are shown in Figure 4. At the intermediate feed of 4.0 V/nm2, at all temperatures, the loading increased with increase in concentration of the grafting solution. At room temperature and at higher temperatures (60 and 80 °C) the system reached grafting-degrafting equilibrium at concentrations of 0.82 M (10 mL THF) and 0.54 M (15 mL THF), respectively. At room temperature the highest loading observed was 3.70 ( 0.08 V/nm2, whereas at the elevated temperature of 80 °C a loading of 3.95 ( 0.03 V/nm2 was observed. This observed increase in equilibrium loading for this precursor with increase in temperature (3.70 ( 0.08 V/nm2 to 3.95 ( 0.03 V/ nm2) suggests that the grafting reaction is endothermic in nature. In previous grafting studies with Zr(OnBu)4 it was observed that the loading decreased with increase in temperature consistent with a process that was exothermic in nature.20 With OV(OEt)3 at a feed of 4.0 V/nm2 it can be seen that the equilibrium loading remained nearly constant with variation of the grafting temperature (Figure 5). It was observed to be in a range from 3.91 ( 0.02 V/nm2 at room temperature to 3.97 ( 0.01 V/nm2 at 80 °C. This small difference suggests a very low

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Figure 6. Change in loading as a function of concentration of the grafting solution for samples synthesized using the precursor OV(OPr)3 at (a) R. T., (b) 40 °C, (c) 60 °C, and (d) 80 °C.

ΔH value for the grafting reaction. This observation also indicates that the maximum surface loading using a feed of 4.0 V/nm2 was reached in 24 h at room temperature and the system was at equilibrium. Studies with the precursor OV(OPr)3 display a number of similarities with studies performed with OV(OEt)3. The data indicate that the equilibrium loading remained nearly constant with variation of the temperature of grafting. It was observed to be in a range from 3.94 ( 0.03 V/nm2 at room temperature to 3.98 ( 0.01 V/nm2 at 80 °C (Figure 6). These similar loadings again suggest a very low ΔH value for the grafting reaction as was observed in the study with the OV(OEt)3 precursor. B. H2/He TPR and XPS Studies. TPR and XPS were used together to identify the vanadium oxidation states present under simulated conditions comparable to those used for catalytic production of hydrogen from steam reforming of hydrocarbons and alcohols. The experimental procedures were devised to determine the oxidation states of vanadium present following oxidation and reduction procedures. All the prepared vanadium oxide materials were yellow in appearance following calcination. On performing TPR it was observed that the yellow color changed to brown. Samples with higher loading resulted in more intense color before and after reduction. The intensity of color is a very helpful optical indicator of the loading and agrees qualitatively with the data obtained through elemental analysis. The reduced samples did not change color upon exposure to the atmosphere during mounting into the transfer chamber of the XPS. Even after a period of two months following the characterization studies, the color of the reduced samples did not change significantly on being exposed to air. These observations indicate that the grafted vanadium oxides do not oxidize rapidly on exposure to ambient conditions. This aspect was helpful in identifying the oxidation state(s) of the vanadium present in the reduced samples using XPS. There are instances of other transition metals including manganese, for which the reduced states are not stable and oxidize rapidly on the slightest exposure to air.21 A previous report on vanadium grafting indicated that calcinations of the samples led to formation of tetrahedral “single sites” on the surface.18 TPR studies were not necessary to identify the oxidation state of the calcined samples because they were expected to be entirely in the highest oxidized state, V5+. They were subjected to XPS studies directly. Three calcined samples 9030

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Figure 7. XPS Spectra of three calcined samples synthesized using OV(OiPr)3: (a) time sample [t-4.0(iPr)VOx/Al2O3-36 h (ox)], (b) precursor feed sample [f-(iPr)VOx/Al2O3-3.0(ox)], and (c) concentration sample [c-4.0(iPr)VOx/Al2O3-25 mL- R.T.(ox)].

Figure 8. XPS Spectra of three reduced samples synthesized using OV(OiPr)3: (a) time sample [t-4.0(iPr)VOx/Al2O3-36 h (red), (b) precursor feed sample [f-(iPr)VOx /Al2O3-3.0(red)], and (c) concentration sample [c-4.0(iPr)VOx/Al2O3-25 mL-R.T. (red)].

chosen for these studies were t-4.0(iPr)VOx/Al2O3-36 h (ox), f-(iPr)VOx/Al2O3-3.0(ox) and c-4.0(iPr)VOx/Al2O3-25 mL-R. T.(ox), respectively. CasaXPS was used for peak fitting and all the peaks were found to be very symmetrical in nature and hence, could be fitted with only one Shirley fit. The standard deviations in the peak positions were generated using Monte Carlo simulations. The positions of the peaks observed were at 518.00 ( 0.01 eV, 517.87 ( 0.01 eV and 517.64 ( 0.01 eV for the time sample, precursor feed sample and the concentration sample, respectively (Figure 7). The binding energy values for both supported and unsupported V2O5 were obtained from the National Institute of Standard and Technology (NIST) Web site. For V2p3/2 the values were observed in the range of 517.00 518.80 eV.22,23 All the sample peaks in this study lay in this

binding energy range and could be peak-fitted entirely under one single peak. This confirmed the presence of only the V5+ species in all three samples. Reduction studies using different gas compositions and temperatures were performed on three representative samples synthesized from each of the vanadium precursors OV(OiPr)3, OV(OEt)3 and OV(OPr)3.24 For the OV(OiPr)3 precursor the three representative samples chosen were t-4.0(iPr)VOx/Al2O336 h (red), f-(iPr)VOx/Al2O3-3.0 (red), and c-4.0(iPr)VOx/ Al2O3-25 mL-R.T (red). These samples were all reduced in the TPR setup for 6 h at a temperature of 700 °C using 10% H2/He gas mixture. After the XPS data were collected, asymmetry was observed in the data peaks indicating the presence of multiple oxidation states. When these peaks were fit using CasaXPS 9031

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increased in favor of V4+ over V5+ with increase in temperature and percentage of H2 gas.24 C. Powder XRD Studies. Powder X-ray diffractograms were recorded for the sample t-4.0VOx/Al2O3-36 h synthesized using the OV(OiPr)3 precursor. If it is assumed that the grafting technique produces homogeneously dispersed single sites on the support surface, the surface vanadium oxide species should appear amorphous in nature. The diffractograms observed for powdered γ-Al2O3 and this sample were identical suggesting that the material produced by the grafting process displayed an amorphous nature (Figure 10). However, due to the detection limit of XRD, these observations do not exclude the possibility of crystallites with sizes smaller than 4.0 nm being present on the surface.

Figure 9. XPS spectrum of time sample synthesized from OV(OiPr)3 and reduced under 20% H2/He gas mixture at 1000 °C for 24 h.

Figure 10. Powder XRD spectra of bare γ-Al2O3 sample and a grafted time sample, t-4.0VOx/Al2O3-36 h synthesized from OV(OiPr)3.

software, a mixture of the oxidation states V5+ and V4+ was observed (Figure 8). Upon combining these peak-fits with the color change observed from yellow to brown and the nature of the reducing environment imposed during TPR, it can be concluded that V5+ and V4+ were the only oxidation states present on the surface. Similar TPR and XPS studies were also pursued on oxidized and reduced samples grafted using OV(OEt)3 and OV(OPr)3 precursor on γ-alumina (see the Supporting Information; Figures S1 S4). To confirm peak assignments for the oxidation states V5+ and 4+ V and to observe if other reduced species could be detected under more rigorous conditions, the time sample, t-(iPr) 4.0VOx / Al2O3-36 h was reduced further at 1000 °C for 24 h using a 20% H2/He gas mixture. However, even under these circumstances the sample displayed XPS peak positions with binding energies similar to those previously observed for the V5+ and V4+ oxidation states. No other oxidation states at energies lower than V4+ could be detected. The V4+ and V5+peak positions in this study were observed at 517.36 ( 0.01 eV and 518.87 ( 0.01 eV, respectively (Figure 9). From the entire set of studies performed with samples synthesized from OV(OiPr)3, OV(OEt)3, and OV(OPr)3 it was also observed that the ratio of oxidation states

’ CONCLUSIONS In the time studies it was observed that with all the precursors grafting-degrafting equilibrium was reached on the surface after approximately 36 h. The loading achieved at equilibrium with the precursor OV(OiPr)3, was observed to be lower than that obtained with the precursors OV(OEt)3 and OV(OPr)3 using the same precursor feed of 4.0 V/nm2. In the precursor loading studies with OV(OiPr)3 and OV(OEt)3 precursors, similar maximum loadings of 4.45 ( 0.11 V/nm2 and 4.45 ( 0.08 V/nm2 were obtained at higher feeds of 8.0 V/nm2 and 5.0 V/nm2, respectively. With the OV(OPr)3 precursor, on the other hand, a maximum loading of ∼3.88 ( 0.11 V/nm2 was obtained only after using a precursor feed of 8.0 V/nm2. The lower maximum loading in this case can be attributed to the higher volume of the OV(O)(OPr)2 canopy, generated after the first condensation step, compared to those of the other two precursors. In concentration and temperature studies with the precursor OV(OiPr)3 an increase in loading values is observed with increase in temperature (3.71 ( 0.08 V/nm2 at room temperature vs 3.95 ( 0.03 V/nm2 at 80 °C) which indicates that the grafting reaction with this precursor had a larger ΔH value which is endothermic in nature. On the other hand with the precursors, OV(OPr)3 and OV(OEt)3 at a feed of 4.0 V/nm2, the loading remained more or less constant (∼3.90 V/nm2) with increase in temperature. This suggests that these systems reached grafting-degrafting equilibrium at room temperature and above. These observations are consistent with the hypothesis that precursors having primary alkyl groups undergo grafting with a very low ΔH. TPR studies combined with XPS permitted identification of the vanadium oxidation states in different samples. It was observed that in calcined samples V5+ was the observed species as anticipated. Reduction studies on the same samples under different reducing conditions indicated that a mixture of V5+ and V4+ species were present on the sample surface. The identity of the species in the reduced samples was further confirmed by performing reduction studies under more rigorous conditions (1000 °C, 24 h, 20% H2/He gas mixture). Powder XRD studies confirmed that the grafting process was successful as the diffractogram of the time sample, t-4.0VOx/Al2O3-36 h was observed to be amorphous in nature and very similar to the bare γ-Al2O3 spheres. ’ ASSOCIATED CONTENT

bS

Supporting Information. Additional discussion and XPS spectra of TPR studies. This material is available free of charge via the Internet at http://pubs.acs.org.

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’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Present Addresses †

6E Fenske Laboratories, Department of Chemical Engineering, Pennsylvania State University, State College, Pennsylvania 16801, United States

’ ACKNOWLEDGMENT We gratefully acknowledge the support of the Department of Chemistry, University of Iowa. The assistance of Dr. Dale Swenson, Dr. Sarah Larsen, and the staff at the Central Microscopic Facility at University of Iowa for equipment use is much appreciated. ’ NOMENCLATURE Grafting = A catalyst loading technique, which utilizes a direct chemical reaction between a suitable graft precursor and the surface hydroxyl groups of a support material Impregnation = A catalyst loading technique, which involves electrostatics and capillary diffusion to deposit a metal salt precursor onto the surface of a support material Feed = It represents the theoretical maximum loading possible with the available quantity of grafting precursor Loading = The actual amount of metal precursor deposited on the support surface ’ REFERENCES (1) Glinski, M. Monolayer vanadia catalysts from vanadium alkoxide precursors. Study of reactivity of various R3VO4. Appl. Catal. A-Gen 1997, 164 (1 2), 205–209. (2) Bond, G. C.; K€onig, P. The vanadium pentoxide-titanium dioxide system: Part 2. Oxidation of o-xylene on a monolayer catalyst. J. Catal. 1982, 77 (2), 309–322. (3) Fierro, J. L. G.; Gambaro, L. A.; Cooper, T. A.; Kremenic, G. Structure and activity of silica-supported vanadia catalysts for the oxidation of propylene. Appl. Catal. 1983, 6 (3), 363–378. (4) Hanke, W.; Bienert, R.; Jerschkewitz, H. G. Untersuchungen an katalytisch aktiven oberfl€achenverbindungen. Herstellung und untersuchung von Vanadinoxid-Phasen auf SiO2. Z. anorg. allg.Chem. 1975, 414 (2), 109–129. (5) Van Hengstum, A. J.; Pranger, J.; van Ommen, J. G.; Gellings, P. J. Influence of phosphorus and potassium impurities on the properties of vanadium oxide supported on TiO2. Appl. Catal. 1984, 11 (2), 317–330. (6) Van Hengstum, A. J.; Van Ommen, J. G.; Boseh, H.; Gellings, P. J. Selective gas phase oxidation of toluene by vanadium oxide/TiO2 catalysts. Appl. Catal. 1983, 8 (3), 369–382. (7) Roozeboom, F.; Fransen, T.; Mars, P.; Gellings, P. J. Vanadium oxide monolayer catalysts. I. Preparation, characterization, and thermal stability. Z. Anorg. Allg. Chem. 1979, 449 (1), 25–40. (8) Glinski, M.; Kijenski, J. Monolayer vanadia on titania systems from alkoxide precursors. Part I. Physicochemical properties. React. Kinet. Catal. Lett. 1992, 46 (2), 379–386. (9) Bond, G. C.; Tahir, S. F. Vanadium oxide monolayer catalysts Preparation, characterization and catalytic activity. Appl. Catal. 1991, 71 (1), 1–31. (10) Deo, G.; Wachs, I. E. Reactivity of supported vanadium oxide catalysts: The partial oxidation of methanol. J. Catal. 1994, 146 (2), 323–334. (11) Miller, J. M.; Lakshmi, L. J. V2O5 catalysts supported on Al2O3-SiO2 mixed oxide: 51V, 1H MAS solid-state NMR, DRIFTS and

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methanol oxidation studies. Appl. Catal. A-Gen. 2000, 190 (1 2), 197–206. (12) Chan, S. S.; Wachs, I. E.; Murrell, L. L.; Wang, L.; Hall, W. K. In situ laser Raman spectroscopy of supported metal oxides. J. Phys. Chem. 1984, 88 (24), 5831–5835. (13) Jehng, J.-M.; Turek, A. M.; Wachs, I. E. Surface modified niobium oxide catalyst:synthesis, characterization, and catalysis. Appl. Catal., A 1992, 83 (2), 179–200. (14) Kozlowski, R.; Pettifer, R. F.; Thomas, J. M. X-ray absorption fine structure investigation of vanadium(V) oxide-titanium(IV) oxide catalysts. 1. The titania support. J. Phys. Chem. 1983, 87 (25), 5172– 5176. (15) Eckert, H.; Wachs, I. E. Solid-state vanadium-51 NMR structural studies on supported vanadium(V) oxide catalysts: vanadium oxide surface layers on alumina and titania supports. J. Phys. Chem. 1989, 93 (18), 6796–6805. (16) Vuurman, M. A.; Wachs, I. E.; Hirt, A. M. Structural determination of supported vanadium pentoxide-tungsten trioxide-titania catalysts by in situ Raman spectroscopy and x-ray photoelectron spectroscopy. J. Phys. Chem. 1991, 95 (24), 9928–9937. (17) Zhao, Z.; Liu, J.; Duan, A.; Xu, C.; Kobayashi, T.; Wachs, I. Effects of alkali metal cations on the structures, physico-chemical properties and catalytic behaviors of silica-supported vanadium oxide catalysts for the selective oxidation of ethane and the complete oxidation of diesel soot. Top. Catal. 2006, 38 (4), 309–325. (18) Kim, T.; Wachs, I. E. CH3OH oxidation over well-defined supported V2O5/Al2O3 catalysts: Influence of vanadium oxide loading and surface vanadium-oxygen functionalities. J. Catal. 2008, 255 (2), 197–205. (19) Eyman, D. P. Int. Patent. Appl. 04/004827. 2004. (20) Xu, L., Synthesis, characterization, and catalytic applications of grafted metal oxide submonolayers on γ-Al2O3. PhD Thesis. Department of Chemistry, University of Iowa: Iowa City, IA. 2003. (21) Schoenfeldt, N. J., Synthesis and characterization of γ-alumina supported manganese oxides prepared by grafting and impregnation for applications in heterogeneous catalysis. PhD Thesis. Department of Chemistry, University of Iowa: Iowa City, IA. 2008. (22) Nag, N. K.; Massoth, F. E. ESCA and gravimetric reduction studies on V/Al2O3 and V/SiO2 catalysts. J. Catal. 1990, 124 (1), 127–132. (23) Barbaray, B.; Contour, J. P.; Mouvier, G. Sulfur dioxide oxidation over atmospheric aerosol—X-ray photoelectron spectra of sulfur dioxide adsorbed on V2O5 and carbon. Atmos. Environ. 1977, 11 (4), 351–356. (24) Saha, A. Synthesis and characterization of grafted vanadium and co-grafted vanadium/zirconium mixed oxides on γ-alumina surface. Phd Thesis. Department of chemistry; University of Iowa: Iowa City, IA. 2008.

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