Applied Catalysis A: General 394 (2011) 287–293 Contents lists available at ScienceDirect Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata A study of the redox properties and methanol oxidation rates for molybdenum-based mixed oxides Ivan Baldychev a , Ashay Javadekar b , Douglas J. Buttrey b , John M. Vohs a , Raymond J. Gorte a,∗ a Department of Chemical & Biomolecular Engineering, University of Pennsylvania, Philadelphia, PA 19104, United States b Department of Chemical Engineering, University of Delaware, Newark, DE 19716, United States article info Article history: Received 28 October 2010 Received in revised form 5 January 2011 Accepted 10 January 2011 Available online 14 January 2011 Keywords: Coulometric titration Redox properties MgMoO4 Zr(MoO4)2 Al2(MoO4)3 Cr2(MoO4)3 SrMoO4 Methanol oxidation abstract The equilibrium properties of bulk MgMoO 4 , Zr(MoO 4 ) 2 , Al 2 (MoO 4 ) 3 , SrMoO 4 , and Cr 2 (MoO 4 ) 3 have been characterized by coulometric titration at 873 K in order to understand the effect of the mixed-cation environment on the Mo 6+ –Mo 4+ redox properties and how this in turn affects reactivity for methanol oxidation. The structures of the oxidized and reduced phases were also characterized by XRD. With SrMoO 4 , reduction resulted in the formation of SrMoO 3 ; however, each of the other oxides underwent a reversible decomposition. MgMoO 4 formed a mixture of crystalline MgO and Mg 2 Mo 3 O 8 ; Zr(MoO 4 ) 2 reduced to MoO 2 and a mixture of monoclinic and tetragonal ZrO 2 ; and Cr 2 (MoO 4 ) 3 formed a new crys- talline phase. For MgMoO 4 , Zr(MoO 4 ) 2 , Al 2 (MoO 4 ) 3 , and Cr 2 (MoO 4 ) 3 , removal of one O/Mo occurred at a P(O 2 ) of 10 -6 atm, corresponding to a G of oxidation of -100 kJ/mol-O 2 ; however, the equilibrium between SrMoO 4 and SrMoO 3 occurred at 10 -26 atm O 2 , corresponding to a G of oxidation equal to -375 kJ/mol-O 2 . These thermodynamic properties differ significantly from oxidation of MoO 2 to MoO 3 , for which G is -220 kJ/mol-O 2 at 873 K. All of the mixed oxides were essentially inactive for the selective oxidation of methanol, with specific rates that were much lower than that observed for MoO 3 . © 2011 Elsevier B.V. All rights reserved. 1. Introduction Mo-based catalysts exhibit good catalytic properties for a number of selective oxidation reactions [1–10]. These oxidation reactions are believed to occur through a Mars–van Krevelen mech- anism in which lattice oxygen bound to molybdenum is used to oxidize the hydrocarbon. At least in part because pure MoO 3 is rel- atively volatile, molybdena is almost always used in the presence of a second oxide, either on an oxide support such as alumina, tita- nia, or zirconia [1,6,7,9,11], or in the form of a mixed oxide, such as MgMoO 4 [4,12] or Fe 2 (MoO 4 ) 2 [10,13]. Indeed, Fe 2 (MoO 4 ) 2 is used commercially for conversion of methanol into formaldehyde [10]. The active phase of the iron-molybdate catalyst is reported to be a surface monolayer of MoO x on bulk, crystalline Fe 2 (MoO 4 ) 2 [14]. For both supported catalysts and bulk mixed oxides, the sec- ond metal component in the oxide can significantly alter catalytic properties. For example, molybdena catalysts supported on titania or zirconia have been reported to be six times more active than their alumina- or silica-supported counterparts for methanol oxidation [7]. Even larger differences between various supported catalysts were reported for ethanol oxidation [15]. The fact that turn-over frequencies (TOF) for both of those reactions have been shown to ∗ Corresponding author. Fax: +1 215 573 2093. E-mail address: gorte@seas.upenn.edu (R.J. Gorte). correlate with catalyst reducibility as measured by peak temper- atures in temperature programmed reduction (TPR) suggests that the support affects reactivity by changing the strength of the oxy- gen bonds [7,15]. Indeed, in a study of oxydehydrogenation (ODH) over supported molybdena catalysts, it was suggested that differ- ences in the TOF for that reaction are due to the oxygen site involved in the anchoring Mo–O–support bond [1]. How the support oxide affects the Mo–O bonding is not entirely understood, although a correlation has been reported between catalytic activity and the electronegativity of the support cations, which in turn has been suggested to affect the Mo–O bond strength [1]. Indeed, the importance of the electronegativity of the second cation to oxygen bonding in mixed oxides has been quantitatively demonstrated for vanadium in a thermodynamic study [16]. Con- sidering only those vanadium mixed oxides with V–O–M bonds (where M is the second metal cation), G of oxidation was found to vary by as much as 250 kJ/mol-O 2 , depending on the electroneg- ativity of the M n+ cations. Whether a similar correlation would exist for Mo-based mixed oxides is not known. In order to understand the role of neighboring cations on the reducibility and catalytic activity of molybdena, we have chosen to examine a series of bulk mixed oxides, MgMoO 4 , Zr(MoO 4 ) 2 , Al 2 (MoO 4 ) 3 , SrMoO 4 and Cr 2 (MoO 4 ) 3 . Because these are bulk crys- talline compounds, the Mo–O sites are expected to be relatively homogeneous compared to normal, supported catalysts. The partic- ular mixed oxides that were chosen for study have a second cation 0926-860X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2011.01.016