J. Phys. Chem. 1995,99, 11489- 11493
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Photoreduction Study of Mo Allyl-Based Mo/SiO2 Catalysts Jane M. Aigler, Vadim B. Kazansky, Manvan Houalla,* Andrew Proctor, and David M. Hercules? Department of Chemistry and Materials Research Center, University of Pittsburgh, Pittsburgh, Pennsylvania 15260 Received: August 17, 1994; In Final Form: December IO, 1994@ X-ray photoelectron spectroscopy (XPS or ESCA) measurements were performed on a photoreduced Mo allyl-based Mo/SiO2 catalyst (1.7 wt 3'% Mo). Photoreduction was carried out in CO at room temperature for increasing lengths of time. Factor analysis, deconvolution, and curve fitting were used to determine the number and positions of components present in the series of X P S Mo 3d spectra obtained for the oxidic and photoreduced Mo/Si02 catalyst. The results indicated the formation of a single oxidation state Mo"+ (Mo 3d5/2 binding energy at 232.1 eV) after 2 h of photoreduction and the appearance of Mo4+ as in MoOz (Mo 3d5/2 binding energy at 230.2 eV) with continued photoreduction. Average oxidation state studies indicated that the species designated as Mo"+ should also be assigned to Mo4+but in an environment different than that of M002.
Introduction In addition to their role as hydrotreating catalysts, supported Mo systems are also active for hydrogenation, metathesis, isomerization, and hydrogenolysis reactions. In most instances, activation of these catalysts involves a reduction step. The surface structure of thermally reduced catalysts is often heterogeneous with three of four Mo oxidation states frequently present following reduction at a given temperature. This complicates any attempts to investigate the relationship between the nature and abundance of supported species (oxidation state) and catalytic activity. Clearly, it is best to drive structure/activity relationships from catalysts containing a single well-defined species which on reduction forms a discrete oxidation state. According to Yermakov et al.Ip3 and Iwasawa et al.,4-8 catalysts containing a single Mo species or oxidation state can be obtained by reaction of allyl-based complexes with Si02 followed by controlled reduction and oxidation treatments. Reaction of M O ( T . ? ~ - C ~and H ~ two ) ~ adjacent hydroxyl groups forms a bidentate Mo complex, liberating propene. Reduction of the "anchored" Mo complex at ca. 600 "C reportedly forms a bound Mo2+ species which upon oxidation at 0 and 400 "C yields Mo4+ and Mo6+, respectively. Recent studies9 have confirmed part of the redox cycle proposed by Yermakov et al.IW3 and Iwasawa et al.4-8 X-ray photoelectron spectroscopy ( X P S ) results indicated that reduction of the anchored Mo species at 550 "C primarily led to the formation of Mo2+. However, the reported formation of discrete Mo4+ upon oxidation of the reduced catalyst at room temperature could not be substantiated. Instead, X P S results indicated a mixture of oxidation states ranging from Mo3+ to Mo6+. Kazansky and c o - w ~ r k e r s ~ claimed ~ - ' ~ that photoreduction of impregnated oxidic Mo/Si02 catalysts with CO at room temperature produces catalysts containing a single tetrahedrally coordinated Mo4+ species. The photoreduced catalyst showed greater catalytic activity than the corresponding thermally reduced solid. For instance,I4 a 1 wt % Mo/SiOz catalyst photoreduced in CO showed 6-10 times higher activity for propene metathesis than thermally reduced Mo/Si02 and 2-3 times greater activity than the highly active Mo allyl-based Mo/ Si02 catalysts. + Present address: Department of Chemistry, Vanderbilt University, Box 1822, Station B, Nashville, TN 37235. Abstract published in Advance ACS Absfructs, June 15, 1995. @
0022-3654/95/2099- 11489$09.00/0
The proposed formation of Mo4+ on photoreduction was primarily based on average oxidation state measurements obtained by measuring COz formation on photoreduction or 0 2 uptake on reoxidation of the photoreduced catalyst. The presumed mechanism of formation of Mo4+ is outlined in the scheme shown in Figure 1. According to this scheme, the oxidic Mo6+ species forms a transient Mo5+ state upon irradiation that further reacts with two CO molecules to form Mo4+, C02, and one weakly interacting CO which is removed by evacuation at 150 "C. The findings of Kazansky and c o - w ~ r k e r s ' ~were - ' ~ supported by the the work of GerasimovI6 and Williams and Ekerdt.I7 GerasimovI6 concluded from COz formation measurements that photoreduction in CO of Mo/Si02 catalyst produced Mo4+. Williams and EkerdtI7 used Fourier transform infrared (FTIR),temperature-programmed decomposition (TPDE), and stoichiometry measurements to investigate the Mo species formed by UV photoreduction of Mo6+/Si02 in CO. They determined that the carbonyl-free Mo cation as well as the Mo mono-, di-, and tricarbonyls formed had an oxidation state of +4. The results of Kazansky et al.I0-l3 have been disputed by several authors. Rodrigo et a1.I8 found, using IR spectroscopy, that CO photoreduction of Mo/SiO2 at room temperature leads to the formation of Mo ions in several valence states simultaneously, mainly Mo4+, Mo3+, and Mo2+. Anpo et al.I9 studied Mo/SiOz catalysts photoreduced in CO at room temperature. They found that for an impregnated catalyst the average oxidation state following photoreduction, as determined by the amount of C02 formed, was $4. However, the oxidation state of the anchored Mo/Si02 catalyst following photoreduction showed a dependency on the method of calculating the number of Mo ions involved in the reaction. If the total number of Mo ions present on the catalyst is used, the average valence state increased with increasing Mo loading. However, when the Mo concentration was based on the number of excited state (Mo5+0 - Y formed by photoreduction, as determined by measuring the yield of phosphorescence, then the average valence state was Mo2+ for all loadings of the anchored Mo/Si02. Finally, Ogata et used IR to study MoOs/SiO2 photoreduced in CO at room temperature and assigned the resulting CO bands to Mo metal. 0 1995 American Chemical Society
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Aigler et al. PCA it is possible to determine which abstract components account for signal ( 1 ~and ) which account for noise (n l...c). Statistical methods including the F test,25,26the factor indicator function (IND):4 and the reduced eigenvalue (REV) ratio24were used to help determine the number of abstract components, n, An intuitive leap in logic is then made where it is assumed that the number of real components equals the number of abstract components since it is fairly certain that there are no background problems or other spectral anomalies. Deconvolution. Deconvolution was accomplished by using the point simultaneous overrelaxation Jansson a l g ~ r i t h m .In~ ~ . ~ ~ order to achieve successful deconvolution, each spectrum must first be background subtracted and smoothed. Backgrounds were removed using a Shirley type integral,29 and spectral smoothing was carried out using a cubic ~pline.~O-~* The Jansson algorithm consists of an iterative procedure which is controlled interactively based upon visual evaluation and by monitoring the difference between the original data and the reconstructed data (Le., the convolution of the broadening function and the current deconvoluted spectrum). A detailed description of deconvolution can be found el~ewhere.~' Average Oxidation State Studies. The oxidized catalyst was photoreduced in flowing 4% CO/He (UHP)with a flow rate of 30 mL/min and a trap (at liquid N2 temperature) downstream from the reactor for collection of C02 formed during photoreduction. The catalyst was shaken periodically during photoreduction to achieve homogeneous reduction. Total photoreduction time was 24 h. Following photoreduction, the catalyst was evacuated at room temperature for 20 min and at 175 "C for 20 min. Oxygen uptake studies of the photoreduced catalyst were performed using a conventional BET setup modified to include a glass circulating loop. Measurement of the consumption of oxygen was done manometrically. ESR Analysis. ESR analysis was carried out by transferring some of the photoreduced catalyst, following evacuation at 175 "C, into a quartz ESR tube. The ESR tube was attached to the photoreduction reactor to eliminate contamination during transfer. The sample was analyzed using a Varian E-4 ESR spectrometer. The spectra were recorded at liquid N2 temperature using the X-band with a field set of 3100 G and a range of 1000 G .
+
Figure 1. Scheme for the photoreduction of oxidic Mo/Si02 catalyst.
Clearly, there is a need for an independent method capable of authenticating the proposed formation of a discrete Mo4+ oxidation state following photoreduction of oxidic MoISiOz catalyst. XPS is, in principle, very well suited for this purpose. The eventual presence of a discrete Mo oxidation state will be illustrated by a single XPS Mo 3d doublet. The assignment of this oxidation state can be made from the Mo 3d binding energy value observed and average oxidation state measurements. The purpose of the present paper is to conduct a detailed investigation of the photoreduction process by XPS and to correlate the results with stoichiometric measurements. Experimental Section Catalyst Preparation. The 1.7 wt % MoISiOz catalyst was prepared by sublimation of Mo(q3-C3H5)4 onto Si02 (Davison Grade 62; 340 m2/g; calcined in 0 2 , 550 "C, 2 h) at 40 "C as The prepared catalyst was reduced in described el~ewhere.~ flowing H2 at 550 "C for 12 h, oxidized to Mo6+ in 10% O n e at 300 "C, and then further oxidized at 500 "C for 3 h. Purification of Gases. Carbon monoxide was purified by liquefaction of the gas in a trap with liquid N2. The first third of the liquid was distilled off and evacuated, the second third was distilled off and collected in a glass storage vessel at a pressure exceeding atmospheric pressure, and the last third of the CO was discarded. All other gases used were purified as described el~ewhere.~ Photoreduction Studies. Photoreduction studies were accomplished by adding 20 Torr of CO to a reactor containing a pellet of the oxidized catalyst and then exposing the catalyst to a Hanovia 1000 W W arc lamp equipped with a filter of degassed, distilled, deionized water. The UV arc lamp employed in this study produced radiation wavelengths ranging from 200 to 1400 nm. According to Kazansky and coworkers," photoreduction of Mo6+ to Mo4+ in CO requires U R radiation with wavelengths
