J. Phys. Chem. 1992,96,9029-9035
9029
Thermal Reduction of MOO, P. A. Spevack* and N. S. McIntyre Surface Science Western, University of Western Ontario, London, Ontario N6A 5B7,Canada (Received:August 14, 1991)
X-ray photoelectron spectroscopy (XPS) and laser Raman spectroscopy (LRS) have been used to examine the thermal reduction products of thin-film molybdenum trioxide under flowing hydrogen or nitrogen gases. Some of the LRS measurements were made in situ during the reduction reaction using a custom quartz cell reactor. Thermal reduction of M a 3 was studied over the range 350-730 O C . At most temperatures, M a 2 and MOO3were the only species measured by LRS. Two intermediate molybdenum species, believed to be Mo(IV) and Mo(V), were detected by XPS, along with Moo2 and MOO3. In one set of samples reduced in the 450 O C range, localized reduction could be attributed to the presence of impurities. From these impurities, a chemical "front" is propagated, and at the front, a metastable species intermediate between MOO, and M a 2 is identified.
Introduction Molybdenum oxides are of fundamental importance to the catalytic community. Molybdenum is frequently deposited onto extruded supports, such as y-alumina, using aqueous molybdate solutions to obtain an appropriate concentration of Moo3. Further treatment with aqueous solutions of other transition metal salts (Ni, Co) provides mixed-metal oxide catalysts.'-3 The calcined sample undergoes some form of reduction and/or sulfidation during use or prior to use as a working catalyst. An alternative to the conventional preparation of molybdenum oxides involves recent work on thin-film specimens. This utilizes ion-beam deposition of the precursor metal(s) onto an appropriate substrate of amorphous alumina or graphite followed by calcination to obtain MoO,.'J The specimens may be treated (reduction and/or sulfidation) in a manner analogous to that used in industry. The chemistry of the reduced molybdenum catalysts is likely influenced by the intrinsic properties of the precursor oxide and the intermediates involved during pretreatment. Knowledge of these precursor oxides and intermediates will enable control and tailoring of the chemical, electronic and morphological properties of the subsequent catalyst. Molybdenum is known to exist in a number of oxidation states and as a variety of oxides, sub-oxides, hydroxides, and hydrated complexes.6 The two most stable oxides-molybdenum (VI) oxide represented by MOO3 and molybdenum (IV) oxide represented by Mo02-define the two extremes as far as the catalyst preparation is concerned. The identification and characterization of the intermediate molybdate species have been overlooked due to the complexities involved in measuring these components. Single-techniquestudies of these species suffer from a lack of corroboratingevidence from complementary analytical techniques. This problem is manifest in the literature by the range of photoelectron binding energies reported for molybdenum oxide species.' X-ray photoelectron spe&cmpy (XPS)and laser Raman spectroscopy (LRS)are two well-known techniques used extensively for catalyst characterization research (see refs 8 and 9 for review articles). They are complementary in the information provided. XPS yields nearsurface information on elemental oxidation states, often to a quantitative level. LRS provides structural information on surface components, revealing differences in microstructure and crystallinity, often at a microscopic lateral resolution. Thin planar films are more amenable to surface vibrational (Raman) and chemical (XPS) analysis as compared to their pelletized counterparts. The films are less susceptible to f l m c e effects from the catalyst support that interfere with the LRS and do not undergo surface charging under a X-ray flux in the XPS. This paper explores the nature of species produced in thin films under reducing atmospheres of nitrogen and hydrogen in the temperature region 350-730 OC. The formation of Moo2 and other reduced molybdenum species is discussed.
Experimental Section Specimen Preparation. The molybdenum trioxide used in this work was prepared as a film (>lo0 nm) deposited on an inert graphite substrate or grown directly on a molybdenum metal substrate. No difference in the reduction behavior of either film was found. The oxides on metal were prepared by heating molybdenum foil (Alfa, 99.97% purity) in air at -250 O C for 4 h. The foils had been pre-ion-bombarded in order to clean and activate the surface. The oxides on graphite were prepared by ion beam deposition of a 15Gnm film of molybdenum onto a graphite substrate (Goodfellow Metals, 99.8% purity) using an Ion Tech system,lo followed by calcination (as above). Reduction Appnratus. The hydrogen reduction experiments were performed in a quartz gas flow cell that permitted in situ Raman analysis during reduction (see Figure 1). The cell was constructed of quartz and Macor machinable ceramic and used an R-type platinum vs platinum-13% rhodium thermocouple (hydrogen) or K-type chromelalumel thermocouple (nitrogen) to measure surface temperature (f5 "C).Hydrogen and nitrogen gases used (Matheson, >99.96% purity) were passed through a moisture trap. A flow rate of 50 mL min-' was used for the in situ work, while
