Chapter 15
Structure and Reactivity of Tin OxideSupported Vanadium Oxide Catalysts Downloaded by UNIV OF CALIFORNIA SAN DIEGO on January 7, 2017 | http://pubs.acs.org Publication Date: May 5, 1993 | doi: 10.1021/bk-1993-0523.ch015
B. Mahipal Reddy Catalysis Section, Indian Institute of Chemical Technology, Hyderabad 500 007, India
The surface structure and reactivity of vanadium oxide monolayer catalysts supported on tin oxide were investigated by various physico-chemical characterization techniques. In this study a series of tin oxide supported vanadium oxide catalysts with various vanadia loadings ranging from 0.5 to 6.4 wt.% have been prepared and were characterized by means of X-ray diffraction, oxygen chemisorption at -78°C, solid state V and H nuclear magnetic resonance and electron spin resonance techniques. Reactivities of these catalysts were also evaluated for partial oxidation of methanol to formaldehyde at atmospheric pressure. Oxygen uptake results suggest the formation of V-oxide 'monolayer' at about 3.2 wt.% vanadia loading; where a maximum activity for methanol oxidation was also observed. Solid-state V NMR results show the presence of two types of vanadium oxide species, one due to a dispersed phase and the other due to the crystalline vanadia phase, supporting oxygen chemisorption results. ESR results also reveal V-oxide in highly dispersed state at the same 3.2 wt.% loading. A direct relationship was also noted between the oxygen uptake and MeOH oxidation activity of the catalysts. 51
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Supported vanadium oxides represent one of the technologically most important class of solid catalysts. These catalysts are useful for partial oxidation of various hydrocarbons (1), ammoxidation of alkyl substituted N-heteroaromatic compounds (2) and most recently for ΝΟ reduction (3). For a catalyst to be a successful one in industry, it should exhibit high activity with maximum selectivity, thermal and mechanical stability and long life etc. For getting some of these functionalities, the active component has to be dispersed uniformly on a support material. The V2O -SnO2 mixed oxide combination is often used for the oxida tion of benzene, naphthalene and various other organic compounds. x
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0097-6156/93/0523-0204$06.00/0 © 1993 American Chemical Society
Oyama and Hightower; Catalytic Selective Oxidation ACS Symposium Series; American Chemical Society: Washington, DC, 1993.
15. REDDY
Tin Oxide-Supported Vanadium Oxide Catalysts
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A good number of patents are reported in literature (4). However, very little attention has been paid towards investigation of the role of S n 0 as promoter in vanadia-tin oxide catalysts. These catalysts are normally prepared by coprecipitation or mixing of individual metal oxides together. Therefore, it is hard to understand the active sites and their relevance to the catalytic reaction. The best method to overcome this difficulty is perhaps to study a vanadia monolayer on tin oxide support, because only then vanadium oxide interacting directly with the SnÛ2 basal plane is exposed on the surface. Ready formation of vanadium oxide monolayers on various supports such as A I 2 O 3 , T 1 O 2 , ZvOi and C e 0 2 has already been established in previous investigations (5). Hence the primary purpose of this investigation was to prepare a vanadia monolayer on tin oxide support and additionally to identify the active sites available for methanol oxidation.
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Experimental Catalyst Preparation. Tin oxide support was prepared from stannic chloride by hydrolysis with dilute ammonia solution. The resulting stannic hydroxide precipitate, washed several times with deionized water till it was free from chloride ions, was dried at 120°C for 16 h and calcined at 600°C for 6 h in air. The tin oxide support thus obtained had a N2 BET surface area of 30 m g " \ The V20s/Sn02 catalysts with various vanadia loadings ranging from 0.5 to 6Ά wt.% were prepared by the standard wet impregnation method. Tin oxide support (0.5 mm average particle size) was added to a stoichiometric aqueous ammonium metavanadate solution and excess water was evaporated on a hot plate with continuous stirring. The impregnated samples were further oven dried at 120°C for 12 h and calcined at 500°C for 5 h in an air circula tion furnace. 2
Oxygen Uptake Measurements. Oxygen chemisorption measurements were made at -78°C on a standard static volumetric all-glass, high-vacuum system equipped with a mercury diffusion pump and an in-line liquid nitrogen cold trap (6). The standard procedure employed for oxygen uptake measurement was reduction of catalyst sample for 5 h at 500°C followed by evacuation for 2 h (1 χ 10" torr) at the same temperature. Before admitting oxygen the system was further evacuated for 1 h at the temperature of chemisorption (-78°C) and then purified oxygen was let in from a storage bulb into the catalyst chamber. The first adsorption isotherm was obtained representing the sum of physisorbed and chemisorbed oxygen. The physisorbed oxygen was then removed by evacuating (1 χ 10" torr) for 1 h at the same temperature and soon after, a second isotherm representing only the physisorbed oxygen was generated in an identical manner. From these two isotherms, which are parallel in the pressure range studied (100-300 torr), the volume of the chemisorbed oxygen was determined (7). The BET surface area of the catalyst was obtained by the Ν 2 physisorption at -196°C using 0.162 nm as the area of cross-section of the nitrogen molecule. 6
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X-Ray Diffraction. X-ray powder diffraction patterns were recorded on a Philips JPW 1051 diffractometer with nickel-filtered CuK radiation ( λ = 1.54187 A).
Oyama and Hightower; Catalytic Selective Oxidation ACS Symposium Series; American Chemical Society: Washington, DC, 1993.
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CATALYTIC SELECTIVE OXIDATION
Solid State NMR Measurements. The solid-state V NMR spectra have been recorded on a Bruker CXP-300 NMR spectrometer at a frequency of 78-86 MHz in the frequency range of 150 kHz, using 1 μ s radio fre quency pulses with repetition rate of 10 Hz. Chemical shifts were mea sured relative to V O C l as an external reference. The H NMR spectra with magic angle spinning technique (MAS) were obtained on the same instrument at a frequency of 300 MHz. The frequency range was 50 kHz, (Π/2) pulse duration was 5 μ s, and the pulse repetition frequency was 1 Hz. The chemical shifts were measured relative to tetramethylsilane as an external standard. Surface OH groups were quantitatively estimated by measuring the area under the peak with reference to a known standard sample (8). 5 1
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Electron Spin Resonance. The ESR spectra of reduced and unreduced catalysts were recorded on a Bruker ER 200D-SRC X-band spectrometer with 100 kHz modulation at ambient temperature. Reduced catalysts for ESR study were prepared according to the procedure described else where (9). After hydrogen reduction at 500°C for 4 h the sample was evacuated at the same temperature for 2 h and sealed off under vacuum. Activity Measurements. To test catalytic properties of various samples partial oxidation of methanol to formaldehyde was studied in a flow micro-reactor operating under normal atmospheric pressure (10). For each run about 0.2 g of catalyst sample was used and the activities were measured at 175°C in the absence of any diffusional effects. The feed gas consisted of 72, 2k and 4% by volume of nitrogen, oxygen and methanol vapor respectively. Reaction products were analysed with a 10% Carbowax 20 M column (2m long) maintained at 60°C oven temperature. Results and Discussion X-ray powder diffraction patterns of S n 0 , V O and V 0 / S n 0 samples are presented in Figure 1. In all the samples studied only the lines assignable to S n 0 support (ASTM 21-1250) were obtained and no lines due to the crystalline V 0 or V 0 - S n 0 intermediate compounds were detected. The absence of characteristic V2O5 X R D lines may be either due to the absence of crystalline vanadia phase or the crystallites formed are less than the detection capability of the X R D technique (i.e., < ^ nm size). In the case of bulk V 0 s - S n 0 catalysts prepared by a precipita tion method only S n 0 phase at Sn/(V+Sn) > 0.7 and V O at Sn/(V+Sn) < 0.3 were reported (11). Nevertheless, the conventional X R D technique fails to provide any further information regarding the nature of V-oxide phase on S n 0 support. Oxygen uptakes obtained at -78°C on various V 0 / S n 0 catalysts are shown in Table 1. The tin oxide support was also found to chemisorb some small amount of oxygen under the experimental conditions employed in this study. Therefore, the contribution of pure support was substracted from the uptake results. The amount of chemisorbed oxygen on V O s catalyst increased linearly as a function of reduction temperature in the range 300 to 500°C and then levelled off. Therefore, 500°C was chosen as the standard temperature of reduction by hydrogen. The factor for the conversion of unit volume of chemisorbed oxygen to the corresponding active vanadia area was determined by the method applied 2
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Oyama and Hightower; Catalytic Selective Oxidation ACS Symposium Series; American Chemical Society: Washington, DC, 1993.
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Tin Oxide-Supported Vanadium Oxide Catalysts
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Downloaded by UNIV OF CALIFORNIA SAN DIEGO on January 7, 2017 | http://pubs.acs.org Publication Date: May 5, 1993 | doi: 10.1021/bk-1993-0523.ch015
15. REDDY
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