A contribution of in situ UV/Vis/NIR spectroscopy to characterize molybdenum oxide catalysts

Abstract

Molybdenum oxide based catalysts are suitable candidates for many selective oxidation reactions [1]. Such catalysts containing only Mo and O have been successfully prepared by controlled adjustment of precursor concentration, temperature or nature of counter cation. The obtained materials, namely orthorhombic and hexagonal MoO3, a supramolecular Mo36 and a trimolybdate compound were thoroughly analysed by XRD, TEM, TG, TPRS and Raman spectroscopy [2]. As such materials undergo electronic changes during catalytic reactions we will present an in situ UV/Vis/NIR study on the different families of molybdenum oxide (MoOx) catalysts. Experimental A commercial UV/Vis/NIR spectrometer (Lambda 9, Perkin Elmer) equipped with a BaSO4 coated integrating sphere was supplemented with a new construction (specially formed light conductor in vertical position) to measure in situ diffuse reflectance spectra of different MoOx from room temperature (RT) to 673 K [3]. The spectra were recorded both from 250 to 2500 nm (scan speed of 240 nmmin-1, slit 1 nm) and 250 to 800 nm (scan speed 60 nmmin-1, slit 0.2 nm) with Spectralon (Labsphere) as a white standard in the reference position. Powder samples (ca. 0.6 g) were charged in the home-made microreactor and fed with a flow of air, pure He or 21 % oxygen in helium. The MoOx samples were prepared using precipitation method (0.28 up to 2 mol/L AHM, Na2MoO4, K2MoO4, Li2MoO4 dissolved in bi-distilled water and 1 mol/L up to 5 mol/L HNO3) from 30oC to 70oC. Results and discussion MoOx spectra show NIR bands with different intensities, distinguishable LMCT bands and band gap energies (Eg) at RT. Based on the exact determination of such spectroscopic characteristics the following LMCT bands (nm) (I) and Eg’s (eV) (II) are attributed to the above mentioned MoOx families: (I) 322 (NH4+), 314 (K+); (II) 3.48 (NH4+), 3.44 (K+) to supramolecular Mo36; (I) 313 (NH4+), 319 (K+), 327 (Na+); (II) 3.35 (NH4+); 3.30 (K+), 3.27 (Na+) to hexagonal MoO3; (I) 296 (Li+); (II) 3.44 (Li+) to orthorhombic MoO3 and (I) 284 (K+); (II) 3.77 (K+) to trimolybdate MoOx. From a blue shift of the LMCT band in the series supramolecular/hexagonal orthorhombic trimolybdate and a decreasing broadening of this band it may be concluded that the cluster size decreases. All MoOx samples evolved NIR bands at 1435, 1940, and 2040 nm. They are assignable to an overtone mode of the OH stretching vibration and a combination mode of the OH stretching and bending vibration, respectively. Other NIR bands, e.g., those detected in MoOx samples prepared from AHM at 1570 and 2150 are caused by ammonia. Initial experiments in dependence on temperature show that the bands at 1440, 1940 and 2030 nm initially decrease at higher temperature and then disappear. In addition, by increasing the temperature the band at 2150 nm begins to disappear at 553 K and the band at 1570 nm dimi- nished around 633 K (not shown). In Vis range a new band at 660/670 nm develops at 553 K which decreases in presence of He and increases in presence of O2 in He with increasing temperature (Fig. 1). This band is assigned to a d-d transition. In He this band suffers a blue shift to about 570 nm at 673 K. The appearance of the new Vis band can be correlate with TG/DSC data. At 553 K ammonia as counter ion de- composes to NOx and reduces the Mo matrix; ammonia is completely removed from the sample at around 693 K