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Ind. Eng. Chem. Res. 2004, 43, 30-35
KINETICS, CATALYSIS, AND REACTION ENGINEERING Catalytic Oxidation of Methanol on Nanoscale Copper Oxide and Nickel Oxide Zheng Gu and Keith L. Hohn* Department of Chemical Engineering, 105 Durland Hall, Kansas State University, Manhattan, Kansas 66506-5102
Nanoscale copper oxide and nickel oxide, prepared by a modified sol-gel process, were investigated for their destruction efficiencies for methanol. The activity, selectivity, and stability of nanoscale CuO and NiO were compared with those of commercial microscale CuO and NiO. Nanoscale CuO and NiO were highly active and stable catalysts for complete methanol oxidation, catalyzing methanol oxidation at temperatures ∼100 °C lower than their microscale counterparts. The higher activity per gram of the nanoscale catalysts can be attributed to their higher surface areas, which were roughly 2 orders of magnitude higher than the microscale catalysts. 1. Introduction Reduction of volatile organic chemical (VOC) emissions is becoming an important issue in the chemical industry because emission standards are becoming increasingly stringent. Reduction of alcohol emissions, one example of a VOC, may become increasingly important to automobile makers because the use of alcohols in automotive fuels may become increasingly common to limit smog-forming emissions.1 For these reasons, the development of cheap, efficient technologies to reduce VOC emissions is attractive. Catalytic oxidation is one option for reducing VOC emissions. Compared to thermal oxidation, catalytic oxidation can be done at much lower temperatures. In addition, catalytic oxidation can more efficiently control the distribution of reaction products, for example producing CO2 and water rather than partially oxidized products.2 Both noble metal catalysts and metal oxides have been studied extensively for the destruction of VOCs. Metal oxide catalysts are less active for oxidation reactions, but they can be more tolerant to some catalyst poisons, for example, chlorine.3-5 The development of more active metal oxide catalysts would be advantageous to provide a low-cost alternative to noble metals for oxidative destruction of VOCs. Nanoscale materials offer significant promise as heterogeneous catalysts. Metal oxide nanocrystals can be synthesized with surface areas of at least twice those of traditional metal oxides. In addition, it has been shown that the surface reactivity changes with the particle size. Photocatalytic activity has been found to be dependent on the crystallite size.6 Ying and coworkers have found that nonstoichiometric CeO2 nanoparticles catalyzed SO2 reduction at temperatures 100 °C lower than those of microparticles.7 Klabunde and co-workers have reported increased rates of adsorption * To whom correspondence should be addressed. Tel.: 785532-4315. Fax: 785-532-7372. E-mail:
[email protected].
as well as increased adsorption capacities for MgO and CaO nanocrystals.8,9 They suggested that the large number of edge and corner sites (lower coordination sites) can lead to unique surface properties, for instance, higher basicity.8,10-12 In this paper, we report the complete oxidation of methanol, used as a simple model compound, over nanoscale copper oxide and nickel oxide catalysts. The catalysts were prepared by a modified sol-gel procedure. The nanoscale catalysts were characterized by nitrogen Brunauer-Emmett-Teller (BET) surface area measurements and X-ray diffraction (XRD). Oxidation reactions were performed in a cylindrical quartz reactor. The effects of temperature on the catalytic activity and selectivity were examined. The performance of nanoscale catalysts was compared with that of corresponding commercial catalysts to see whether nanocrystalline materials offer better properties for complete oxidation. 2. Experimental Section 2.1. Catalyst Preparation and Characterization. The nanoscale oxide catalysts were synthesized by a modified sol-gel procedure.13 For aerogel-made nanoscale copper oxide (AM-CuO), copper(II) chloride was dissolved in ethanol to form a clear green solution. Sodium hydroxide was dissolved in ethanol and added dropwise into the first solution to form a copper hydroxide gel. The above steps were performed under an argon atmosphere. The gel was stirred at room temperature for 2 h to achieve complete reaction. Then the solution was filtered and washed with water to remove sodium chloride. The resultant copper hydroxide was air-dried. The dry copper hydroxide powder was then placed into a Schlenk tube under vacuum and heated at 250 °C for 15 min. The resultant nanoscale copper oxide powder was black. The commercially made copper oxide (CM-CuO) was purchased from Aldrich. For nanoscale nickel oxide (AM-NiO), the synthesis steps were similar, except that the starting material was
10.1021/ie030438i CCC: $27.50 © 2004 American Chemical Society Published on Web 12/02/2003
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nickel(II) chloride hexahydrate and the temperature of the heating step was 300 °C. The resultant nanoscale nickel oxide was a dark green powder. The microscale nickel oxide (CM-NiO) was also purchased from Aldrich. All samples were pelletized prior to use. Using a hydraulic press, catalyst powders were pelletized under 1000 lb of force. They were then crushed, and the resultant particles were sieved between 20 and 40 mesh (particle size between 420 and 850 µm). It has been shown that nanocrystalline oxides could retain their high surface areas and large pore volumes after pressure-induced pelletization.12 The catalysts were characterized by nitrogen BET surface area measurements and XRD. Surface area measurements were conducted using a Quantachrome NOVA 1200 instrument. The same instrument also provided information on the pore-size distribution and pore volume. XRD was carried out in a Scintag XDS 2000 spectrometer. Cu KR radiation was the light source with an applied voltage of 40 kV and a current of 40 mA. 2θ angles ranged from 20° to 85° with a speed of 2°/min. The crystallite size was calculated using the Scherrer equation. 2.2. Oxidation Reaction System. A cylindrical quartz reactor (15-mm o.d., 11-mm i.d., and 550-mm length) was used to characterize the catalytic activity, selectivity, and stability. The reactor was heated by a tube furnace. The temperature in the catalyst bed was measured to (1 °C by thermocouples placed at the inlet of the catalyst bed. The gas feed system consisted of compressed gas cylinders of nitrogen and oxygen. All gas cylinders were maintained at a constant delivery pressure of 20 psig, and the flow rates of the feed gases were regulated by mass flow controllers (MFC 7300, Unit Instruments), which were controlled by a digital control system (DX5, Unit Instruments). Liquid methanol was introduced into the feed system by a programmable syringe pump (NE-1000, Syringe Pump Com.). Before entering the reactor, liquid methanol was vaporized in a wall-heated stainless steel line. All of the reactor system lines were heated to prevent condensation. The effluent gases were analyzed by an online gas chromatograph (SRI8610C). The gas chromatograph was equipped with a thermal conductivity detector and a flame ionization detector. Molecular sieve and Hayesep T columns were used for separating the species in the product streams. 2.3. Methanol Oxidation. Oxidation experiments using a feed stream of methanol, nitrogen, and oxygen were performed on AM-CuO, CM-CuO, AM-NiO, and CM-NiO. Before reaction, catalysts were activated by heating them in a flow of nitrogen with 20% oxygen at 300 °C for 4 h. To estimate the contribution of homogeneous oxidation, a blank experimental run in the absence of catalyst was conducted. For all experiments, a feed comprised of 1.6% methanol, 20.5% oxygen, 77.9% nitrogen, and a total catalyst weight of 1.0 g was used. Because both types of catalysts were pelletized prior to use, the bed length was nearly the same (∼1 cm). The total flow rate was 1.73 L/min (space velocity of 1.09 × 105 h-1). Experiments were also conducted to determine whether catalyst deactivation was important at the above reaction conditions. At the reactant composition and space velocity described above, operation was maintained at
Figure 1. XRD patterns for AM-CuO (top) and CM-CuO (bottom).
a temperature sufficient to give greater than 90% conversion for 12 h. The reaction rates were measured on all catalysts at temperatures from 120 to 180 °C. These temperatures were chosen in order to minimize mass-transfer limitations. Conversion was kept low for all trials, except for the AM catalysts at 180 °C. When conversion was low, the differential method of data analysis was used to calculate the reaction rate from experimental data. For the trials for which conversion was very high, the integral method of data analysis was used. The molar flow rate of reactants was varied, and methanol conversion was measured. A plot of conversion versus the weight (W) of the catalyst divided by the methanol molar flow rate (FAo) was generated and was extrapolated to W/FAo equal to zero. The slope at that point gave the catalytic reaction rate at entrance conditions. The catalyst activity was measured in terms of methanol conversion and product selectivity. The methanol conversion was defined as moles of methanol consumed per mole of methanol in the feed. The selectivity to each product was defined as the ratio of moles of product produced to moles of methanol consumed. The carbon balance for all runs closed within 10%, most within 5%. Multiple experimental trials were repeated on each catalyst and on at least two different batches of catalyst samples. For a single catalyst, errors in conversion are estimated at 1% absolute (based on multiple trials conducted at the same reaction temperature). Errors in the 50% conversion temperature are estimated at 2 °C and errors in the 90% conversion temperature are estimated at 4 °C based on multiple trials on different catalyst samples where the temperature was varied to give 50% or 90% conversion. Multiple steady states were noted at some conditions. Unless otherwise specified, all reported results are for the lower steady state, when the temperature was being increased. 3. Results 3.1. Catalyst Characterization. The XRD patterns for AM-CuO and CM-CuO are shown in Figure 1. For CM-CuO, peaks at 34.8°, 38.0°, and 48.1° are evident, while for AM-CuO, the primary peaks are at 35.0°, 38.1°, and 48.2°. These peaks agree well with the published peaks for CuO at 35.273°, 38.504°, and 48.631°, suggesting that only crystalline CuO is present in these samples.14 Peaks are broader for AM-CuO, suggesting a smaller crystal size.
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Figure 3. Methanol oxidation: variation of conversion with temperature for AM-CuO (9) and CM-CuO (2). Figure 2. XRD patterns for AM-NiO (top) and CM-NiO (bottom). Table 1. Physical Properties of All Catalysts
sample
surface area (m2/g)
AM-CuO CM-CuO AM-NiO CM-NiO
63.0-81.0 1.4 124.5-154.1 1.3
average average pore average pore crystal 3 diameter (nm) volume (cm /g) size (nm) 7.5 15.4 3.3 11.7
0.247 13 0.002 56 0.128 25 0.003 74
7-10 24 2-5 33
Figure 2 shows XRD patterns for AM-NiO and CMNiO. For CM-NiO, peaks at 36.8°, 42.9°, and 62.4° are evident, while peaks at 38.0°, 43.3°, and 62.7° are present for AM-NiO. These agree well with the identified NiO peaks at 43.287°, 37.429°, and 62.854°.14 As was the case for CuO, broader peaks are apparent for the AM-NiO sample. Crystallite sizes, based on the broadness of the XRD peaks at 38.504° for CuO and 43.287° for NiO, are reported in Table 1, along with surface area and pore structure information obtained from nitrogen adsorption BET measurements. As seen in this table, AM-CuO and AM-NiO have crystal sizes of less than 10 nm, while CM-CuO and CM-NiO have crystal sizes of greater than 10 nm. The surface areas are approximately 2 orders of magnitude larger for AM-CuO and AM-NiO, as are the pore volumes. From these data, the sol-gelprepared catalysts can be envisioned as agglomerates of nanocrystals with an extensive pore structure. 3.2. Methanol Oxidation. Blank Reactor Runs. Blank reactor experiments were conducted wherein the quartz reactor was filled with quartz wool but no catalyst to estimate the contribution of noncatalytic reactions. Formaldehyde and carbon dioxide were the major products of methanol oxidation. The methanol conversion was less than 10% below a temperature of 200 °C. It was not until the temperature was raised to about 570 °C that the conversion reached 90% or above. Methanol Oxidation over CuO. Methanol conversion as a function of temperature for AM-CuO and CMCuO is plotted in Figure 3. Also shown in this figure is the conversion predicted at each temperature according to a simple pseudo-first-order kinetic model discussed below. This plot shows that, at any given temperature, methanol conversion on AM-CuO was higher than that on CM-CuO. For AM-CuO, conversion is roughly 10% until a temperature of ∼175 °C. From 175 °C, conversion increases dramatically from just over 20% to nearly 90% before leveling off as it approaches 100%. For CM-CuO, conversion slowly increases to 18% by 205 °C and then increases to nearly 90% by 310 °C. The variation of CO2 and formaldehyde selectivities as functions of conversion for AM-CuO and CM-CuO is
Figure 4. Methanol oxidation: variation of selectivity to CO2 and CH2O with methanol conversion for AM-CuO ([ for CO2 and 9 for CH2O) and CM-CuO (2 for CO2 and × for CH2O).
Figure 5. Methanol oxidation: variation of conversion with temperature for AM-NiO (9) and CM-NiO (2).
shown in Figure 4. At low conversions (low temperatures), the formaldehyde selectivity was nearly 100% for either catalyst. Minor amounts of methyl formate (
