Structure and properties of molybdenum oxide nitrides as model systems for selective oxidation catalysts
© Kühn et al 2010
Received: 7 April 2011
Accepted: 15 July 2011
Published: 15 July 2011
Molybdenum oxide nitride (denoted as Mo(O,N)3) was obtained by ammonolysis of α-MoO3 with gaseous ammonia. Electronic and geometric structure, reducibility, and conductivity of Mo(O,N)3 were investigated by XRD, XAS, UV-Vis spectroscopy, and impedance measurements. Catalytic performance in selective propene oxidation was determined by online mass spectrometry und gas chromatography. Upon incorporation of nitrogen, Mo(O,N)3 maintained the characteristic layer structure of α-MoO3. XRD analysis showed an increased structural disorder in the layers while nitrogen is removed from the lattice of Mo(O,N)3 at temperatures above ~600 K. Compared to regular α-MoO3, Mo(O,N)3 exhibited a higher electronic and ionic conductivity and an onset of reduction in propene at lower temperatures. Surprisingly, α-MoO3 and Mo(O,N)3 exhibited no detectable differences in onset temperatures of propene oxidation and catalytic selectivity or activity. Apparently, the increased reducibility, oxygen mobility, and conductivity of Mo(O,N)3 compared to α-MoO3 had no effect on the catalytic behavior of the two catalysts. The results presented confirm the suitability of molybdenum oxide nitrides as model systems for studying bulk contributions to selective oxidation.
Molybdenum oxides are active heterogeneous catalysts for selective oxidation of light alkanes and alkenes [1–6]. In addition to catalytic activity and long-term stability, improved selectivity and efficient use of raw materials become of increasing importance in industrial applications. In spite of intensive research, structure-reactivity correlations and knowledge about the catalytically active species remain scarce. Hence, new catalysts are developed by "trial and error" methods. Although this approach has been successful, it becomes increasingly apparent that new ways need to be explored to further advance the design of improved functional materials. Therefore, a deeper understanding of the correlations between structure, activity, and selectivity will serve as the foundation for a knowledge-based development of new and enhanced catalysts.
Selective oxidation of alkenes using molybdenum oxide catalysts is commonly believed to proceed according to a reduction-oxidation-mechanism [1, 4]. In a first step the reactant partially reduces the metal oxide catalyst. Afterwards, the catalyst is re-oxidized by oxygen from the gas phase. Hence, lattice oxygen of the molybdenum oxide catalyst is suggested to selectively oxidize the alkene. Oxidation experiments using 18O have verified that lattice oxygen of α-MoO3 is involved in the catalytic process . Accordingly, mobility of oxygen in the lattice of the oxide catalyst is expected to play an important role for catalyst activity and selectivity.
Conventional molybdenum based oxide catalysts consist of either binary molybdenum oxide system or mixed oxides with additional metal centers. Additional metals such as W, Nb, or V stabilize characteristic crystallographic structures which lead to oxidation catalysts with improved activity and selectivity [8, 9]. However, the influence of structural variety and chemical complexity in the mixed oxide systems on catalytic performance is difficult to distinguish. In order to reduce this inherent complexity of mixed oxide catalysts, suitable model systems are sought. In particular with respect to elucidating the effect of oxygen mobility on catalytic performance it appears counterproductive to vary chemical composition and oxide lattice structure simultaneously. Hence, instead of modifying the cation lattice to affect oxygen mobility it seems more promising to directly modify the anion lattice. Ideally however, this modification should have little influence on the long-range order crystallographic structure of the catalyst.
Along that line, metal oxide nitrides appear to be suitable model systems. In many cases nitrogen can be incorporated in the oxide lattice of a particular metal oxide without changing the crystallographic structure [10, 11]. Alternatively, metastable and previously not available oxide structures can be obtained without changing the cation composition [12, 13]. Both approaches may result in suitable model systems for selective oxidation catalysts. Zirconium oxide nitrides, for instance, have been shown to be active catalysts for ammonia decomposition . It is suggested, however, that in these materials the nitrogen atoms not only affect anion mobility but also directly participate in the catalytic reaction.
In this work we have chosen molybdenum trioxide in the orthorhombic α-MoO3 modification as well-known model system for selective propene oxidation catalysts. α-MoO3 is transformed into the corresponding molybdenum oxide nitride by reaction with gaseous ammonia. Subsequently, the thus obtained model catalyst is used to reveal correlations between lattice oxygen availability, electrical properties, and catalytic activity and selectivity in selective propene oxidation.
Results and Discussion
Structural characterization of Mo(O,N)3
Geometric and electronic structure of Mo(O,N)3 and reference α-MoO3 were investigated by XRD, XAS, and DR-UV-Vis spectroscopy. The aim of these studies was to reveal significant differences in structural and electronic properties of the two materials to be used as model systems for oxidation catalysts. Interestingly, incorporation of nitrogen in α-MoO3 resulted in a pronounced color change of the material. In contrast to the light-grey color of α-MoO3, Mo(O,N)3 exhibited a dark-blue color. This color change, however, cannot be attributed to a significant reduction of MoO3 to MoO2 (< 5%).
Absorption edge energies in the UV-Vis range as determined by DR-UV-Vis spectroscopy (Figure 1) amounted to 3.3 eV for α-MoO3 and 2.8 eV for Mo(O,N)3. Inter-valence transitions in the range from 1 - 3 eV in the UV-Vis spectrum of α-MoO3 corroborated the reduced average valence of 5.96 and the light-gray color of the reference α-MoO3 . The dark-blue color of Mo(O,N)3 is suggested to originate from the formation of color centers accompanying the incorporation of nitrogen in the oxygen lattice. A similar effect has been described for intensely colored zirconium oxide nitrides compared to colorless ZrO2 .
Thermal stability of Mo(O,N)3
Reducibility of α-MoO 3 and Mo(O,N) 3
Electrical properties of Mo(O,N)3
Functional and structural characterization of Mo(O,N)3under catalytic conditions
Functional characterization of Mo(O,N)3 and α-MoO3 in selective propene oxidation was conducted simultaneously with in situ structural UV-Vis and XAS measurements. For a sufficient time-resolution, the gas phase composition was qualitatively analyzed by a non-calibrated mass spectrometer. Time-resolved measurements were required because of the ongoing nitrogen removal from Mo(O,N)3 at temperatures above 600 K. Prolonged experiments under steady-state conditions may have been prone to missing minor differences in the performance of the catalysts. Repeated quantitative measurements of the gas phase composition under selective oxidation reaction conditions were performed by gas chromatography at selected temperatures.
Eventually, structure function correlations of Mo(O,N)3 can be concluded from the described investigations. Characterization of electronic and geometric structure as well as reducibility of Mo(O,N)3 resulted in a pronounced influence of the incorporation of nitrogen in the layer structure of α-MoO3 (Figure 1 - Figure 4). Compared to regular α-MoO3, Mo(O,N)3 exhibited a much increased electronic and ionic conductivity (Figure 5). The latter afforded a lowering of the onset of reduction of Mo(O,N)3 by about 40 K (Figure 4). Structural studies at elevated temperatures showed that nitrogen is removed from the oxide lattice of Mo(O,N)3 above ~600 K. The thus obtained MoO3 exhibited again the same short-range and long-range structural characteristics as the parent α-MoO3 material (Figure 3 and Figure 7). Nevertheless, in spite of these characteristic differences between the α-MoO3 and Mo(O,N)3 used, both materials exhibited no detectable differences in their catalytic behavior. Neither studying onset temperatures by time-resolved methods (Figure 8 and Figure 10) nor measuring conversion or selectivity under steady-state conditions (Figure 11) revealed a pronounced influence of the incorporation of nitrogen in the oxide lattice on catalytic properties.
In the conventional reduction-oxidation mechanism for selective oxidation reactions the reactant is oxidized by nucleophilic lattice oxygen. This way, oxygen vacancies in the oxide lattice are generated at the surface of the catalyst. Subsequently, these vacancies are supposed to be re-filled by diffusion of oxygen from the bulk of the oxide catalyst to its surface. Desorption of products terminates the catalytic cycle and frees the active site for the next reactant molecule. Eventually, the catalyst needs to be re-oxidized by gas phase oxygen to prevent deep-reduction and deactivation. However, if in fact diffusion of oxygen was required to replenish the oxygen reservoir at the catalyst surface, influencing the mobility of oxygen in the catalyst bulk structure should have a distinct influence on its catalytic properties. The results presented here suggest that at least for α-MoO3 in propene oxidation this is not the case. Apparently, varying oxygen mobility has no detectable effect on the catalytic performance of α-MoO3. This implies that if oxygen vacancies are generated at the surface of the catalyst by reaction with propene, these vacancies will be directly filled by gas phase reaction without requiring diffusion of oxygen from the bulk. Hence, the similar catalytic behavior of α-MoO3 and Mo(O,N)3 may be attributed to similar surface structures forming on both materials under reaction conditions. This may underline the importance of the particular structure and composition of the surface rather than optimized bulk properties of selective oxidation catalysts. However, the bulk structural characterization techniques employed here do not permit further conclusions as to the surface structures of the materials. Moreover, whether activation and oxidation of propene, and activation of gas phase oxygen occur simultaneously or in consecutive steps can only be speculated. However, it appears that a conventional Langmuir-Hinshelwood type mechanism may sufficiently describe propene oxidation on α-MoO3 without requiring a particular contribution from the catalyst bulk.
In summary, the results presented here confirm the suitability of molybdenum oxide nitrides as model systems for studying bulk contributions to selective alkene oxidation. With regular α-MoO3, incorporation of nitrogen permits to significantly and reversibly modify electronic properties and bulk oxygen mobility. Nitrogen can be conveniently removed from the oxide lattice by thermal treatment resulting in the original starting material. Hence, catalytic properties of the materials with and without nitrogen in the oxide lattice can readily be probed by consecutive experiments. Future studies on mixed molybdenum oxide nitrides will be employed to confirm both their applicability as model systems and our conclusions regarding correlations between catalytic performance and electrical properties of oxide catalysts.
Preparation of molybdenum oxide nitride
Molybdenum oxide nitride (in the following denoted as Mo(O,N)3) was prepared by reaction of gaseous ammonia with commercial α-MoO3 (Chempur). Ammonolysis was performed in a conventional tube furnace using a silica tube and direct gas supply. After drying at 383 K in air, α-MoO3 was reacted to Mo(O,N)3 at 548 K with an NH3 flow of 10 l/h for 10 h. These particular conditions had been optimized in respect to a maximum concentration of incorporated nitrogen in MoO3 without a detectable amount of rutile-type MoO2 or Mo(O,N)2. Increasing the temperature, the presence of such phases was clearly detected using X-ray powder diffraction. With decreasing temperature the nitrogen content of the MoO3 phase strongly decreased. Quantitative nitrogen/oxygen analysis was carried out using the well-established hot gas extraction method (LECO TC-300/EF-300 N/O analyzer). At this procedure the samples were heated under flowing helium in a graphite crucible up to about 2700 °C. Oxygen was determined as CO2 by IR spectroscopy, the amount of nitrogen as N2 gas by heat conductivity measurements of the nitrogen/helium gas. Such measurements resulted in a nitrogen content of 0.41 (± 0.01) wt-% for the sample used in this work. Hot gas extraction, however, does not permit to study the nitrogen content of the material in situ under catalysis or oxidation-reduction conditions.
Diffuse reflectance UV-Vis spectroscopy
A JASCO V-670 double beam spectrometer was used for DR-UV-Vis measurements. Ex situ spectra were recorded in a BaSO4 coated integration sphere in a wavelength range from 200 to 2000 nm and a scanning speed of 40 nm/min. In situ measurements used a "Praying Mantis" set-up (Harrick) at a scanning speed of 200 nm/min in a range from 220 to 900 nm under catalytic (5% propene and 5% O2 in He) or reducing (5% propene in He) reaction conditions.
On-line gas phase analysis
Quantitative catalysis measurements were performed using an online gas chromatography system (Varian CP-3800) and a non-calibrated mass spectrometer (Pfeiffer Omnistar). Hydrocarbons and oxygenated reaction products were analyzed using a Deans switch consisting of a carbowax capillary column connected to a AL2O3/MAPD column or a fused silica restriction (25 m*0.32 mm each), and flame ionization detectors. Reactant gas flow rates of oxygen, propene, and Helium were adjusted with separate mass flow controllers to a total flow of 40 ml/min. A mixture of 5% propene and 5% oxygen in helium was used for catalytic testing in the temperature range from 300 K to 748 K. All lines and valves were preheated to 473 K.
Powder X-ray diffraction
Ex situ XRD measurements were conducted on an X'Pert PRO MPD diffractometer (Panalytical, θ-θ geometry) using Cu Kα radiation and a solid-state multiple-channel detector. Wide angle XRD scans were collected in reflection mode using a silicon sample holder. In situ XRD experiments were performed in θ-θ geometry on a STOE diffractometer (Cu Kα-radiation) equipped with a PARR reaction chamber. Temperature-programmed experiments were conducted in a range from 473 K to 773 K in steps of 25 K and an effective heating rate of 0.32 K/min (2θ range from 10° to 42°). Measuring time per XRD scan amounted to about 2 hours. The gas-phase composition at the cell outlet was analyzed online with a mass spectrometer (Pfeiffer Omnistar).
X-ray absorption spectroscopy (XAS)
In situ transmission XAS spectra were measured at the Mo K edge (20.0 keV) at beamline X1 at the Hamburg Synchrotron Radiation Laboratory, HASYLAB, using a suitable in situ cell . Samples were pressed to pellets with a diameter of 5 mm (7 mg with 30 mg boron nitride). EXAFS spectra at the Mo K edge in the k space up to 14 Å-1 were collected in ~ 4 min. Temperature-programmed reduction was conducted in 5% propene and He (total flow of 40 ml/min) in a temperature range from 293 K to 773 K at a heating rate of 5 K/min. Reaction tests were performed in 5% propene and 5% O2 in He in the temperature range from 293 K to 723 K (5 K/min, total flow 40 ml/min). The gas atmosphere was analyzed using a non-calibrated mass spectrometer in a multiple ion detection mode (Pfeiffer Omnistar).
X-ray absorption fine structure (XAFS) analysis was performed using the software package WinXAS v3.2 . Linear polynomials and 3rd degree polynomials were fitted to the pre-edge and post-edge region of an absorption spectrum for background subtraction and normalization, respectively. The extended X-ray absorption fine structure (EXAFS) χ(k) was extracted by using cubic splines to obtain a smooth atomic background μ 0 (k). The FT(χ(k)*k3), often referred to as pseudo radial distribution function, was calculated by Fourier transforming the k3 weighted experimental χ(k) function, multiplied by a Bessel window, into the R space.
Impedance of molybdenum oxides and oxide nitrides was obtained by measuring the magnitude |Z| and the phase φ of an alternating current as a response of an applied alternating potential (impedance analyzer N4L: IAI+PSM1735). From that the real part Z' and imaginary part Z'' of the complex impedance was calculated. The impedance was measured as a function of frequency (200 kHz - 0.1 Hz) and temperature (α-MoO3: 527 K - 848 K, Mo(O,N)3: 306 K- 449 K). Oxides and oxide nitride samples were pressed to pellets with a diameter of 15 mm (1.5 g initial weight, 7.5 kPa pressure) and placed between two Pt disc electrodes for impedance measurements.
The Hamburg Synchrotron Radiation Laboratory, HASYLAB, is acknowledged for providing beamtime for this work. A. Stys and A. Walter are acknowledged for contributing to the materials characterization. The authors are grateful to the Deutsche Forschungsgemeinschaft, DFG (Cluster of Excellence "Unifying Concepts in Catalysis") for financial support.
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