J. Phys. Chem. B 2000, 104, 8145-8152
8145
Studies on the Behavior of Mixed-Metal Oxides: Structural, Electronic, and Chemical Properties of β-FeMoO4 Jose´ A. Rodriguez,* Jonathan C. Hanson, and Sanjay Chaturvedi Department of Chemistry, BrookhaVen National Laboratory, Upton, New York 11973
Amitesh Maiti Molecular Simulations Inc., 9685 Scranton Road, San Diego, California 92121
Joaquı´n L. Brito* Centro de Quı´mica, Instituto Venezolano de InVestigaciones Cientificas (IVIC), Apartado 21827, Caracas 1020-A, Venezuela ReceiVed: May 2, 2000; In Final Form: June 26, 2000
The structural and electronic properties of iron molybdate (FeMoO4) have been studied using synchrotronbased time-resolved X-ray diffraction (TR-XRD), X-ray absorption near-edge spectroscopy (XANES), and first-principles density functional (DFT-GGA) calculations. The β phase of FeMoO4 is stable in a large range of temperatures (25-800 °C). For β-FeMoO4, the results of Reitveld refinement and DFT-GGA calculations give a crystal structure in which Fe is in an almost octahedral environment while Mo is tetracoordinated. The Mo LII-edge XANES spectra of β-FeMoO4 exhibit a line shape that is typical of oxides in which Mo is in a tetrahedral environment. The valence electronic structure of β-FeMoO4 is characterized by an intense peak with Fe 3d character near the top of the valence band. The iron molybdate displays a good chemical affinity for H2, H2S, and SO2. H2-TPR (temperature programmed reduction) spectra for β-FeMoO4 show consumption of hydrogen and evolution of water (β-FeMoO4 + H2,gas f H2Ogas + FeMoOx) between 600 and 850 °C. Upon exposure to H2S at moderate temperatures (50-250 °C), there is formation of sulfides and sulfates on the oxide. H2S exposure at elevated temperatures (350-450 °C) leads to formation of FeMoSx compounds without SOx species (β-FeMoO4 + H2,Sgas f H2Ogas + FeMoSx). β-FeMoO4 is a good sorbent for sulfur dioxide. The adsorption of SO2 on β-FeMoO4 at room temperature produces SO3 and SO4 groups without dissociation of SO2. A correlation is found between changes in the electronic and chemical properties of β-FeMoO4, β-NiMoO4, and β-MgMoO4. β-NiMoO4 exhibits a large density of metal states near the top of its valence band and a substantial reactivity toward H2 and H2S. β-MgMoO4 displays completely opposite trends, and β-FeMoO4 is an intermediate case between the two extremes.
I. Introduction Mixed-metal oxides that combine iron and molybdenum are the main components of catalysts used for the partial oxidation of alcohols and hydrocarbons.1-3 For example, catalysts that contain β-FeMoO4 and Fe2(MoO4)3 have been employed in the synthesis of formaldehyde from methanol on a commercial scale.1,3 Fe-Mo-O catalysts exhibit good activity for the oxidation of toluene to benzaldehyde.2,4,5 During this process, Fe2(MoO4)3 is converted to β-FeMoO4, and the catalytic activity of the system is related to the amount of β-FeMoO4 formed.5 In addition, it has been found that β-FeMoO4 is a very good precursor of catalysts for hydrodesulfurization (HDS) processes.6 Thus, depending on the method of preparation, sulfided β-FeMoO4 catalysts can have an HDS activity that is larger than that of MoS2 and comparable to that of industrial-like CoMo-S catalysts.6 In this article, we examine the structural and electronic properties of β-FeMoO4 using synchrotron-based time-resolved X-ray diffraction (TR-XRD), X-ray absorption near-edge spectroscopy (XANES), and first-principles density functional calculations. The chemical reactivity of the metal * Corresponding authors.
centers in β-FeMoO4 is probed by studying the chemisorption of H2 and H2S, whereas SO2 chemisorption is used to probe the reactivity of the oxygen centers. This study is part of a series of works in which we have been examining, in a systematic way, the behavior of mixed-metal oxides.6-13 Mixed-metal oxides play a relevant role in many areas of chemistry, physics, and materials science.14-17 Recently, much attention has been focused on the preparation and performance of mixed-metal oxide catalysts.18,19 In principle, the combination of two metals in an oxide matrix can produce materials with novel physical and chemical properties that can lead to a superior performance in technological applications. The two metals can behave as “isolated units” that bring their intrinsic properties to the system, or their behavior can be modified by the effects of metal T metal or metal T oxygen T metal interactions. In this respect, it is important to know how to choose the “right” combination of metals. Thus, an atomic-level understanding of the properties of mixed-metal oxides is crucial. Molybdenum is able to form stable oxides in combination with many metals (MeMoO4 compounds; Me ) Mg, Pb, Mn,
10.1021/jp001652c CCC: $19.00 © 2000 American Chemical Society Published on Web 08/01/2000
8146 J. Phys. Chem. B, Vol. 104, No. 34, 2000 Fe, Co, Ni, Zn).15,20-22 These molybdates are ideal for studying the behavior of mixed-metal oxides and constitute an interesting group on their own because of their structural, electronic, and catalytic properties.1-6,8-11,15,20-25 In a previous work, we compared the properties of a molybdate that contains a latetransition metal (NiMoO4) and a molybdate with an alkalineearth metal (MgMoO4).11 The degree of ionicity in MgMoO4 was larger than that in NiMoO4. The nickel molybdate displayed a large density of states near the top of the valence band that was not observed in the case of the magnesium molybdate. This made NiMoO4 more chemically active than MgMoO4.11 It is worthwhile to examine whether the same correlation between electronic and chemical properties is observed in the FeMoO4 system, where Mo is combined with a transition metal located near the center of the Periodic Table (Fe). II. Experimental and Theoretical Methods II.1. Time-Resolved X-ray Powder Diffraction. The β phase of FeMoO4 was prepared by heating a precursor of the FeMoO4‚ nH2O type following the methodology described in ref 6. The time-resolved diffraction data were collected on beam line X7B of the National Synchrotron Light Source (NSLS).26 The sample was kept in a quartz capillary and heated using a small resistance heater placed under the capillary. A chromel-alumel thermocouple was used to measure the temperature of the sample. The accuracy of the thermocouple readings was verified using blank runs with silver powder that were checked against the known changes in the cell parameters of this metal as a function of temperature.26 A translating-image plate (TIP) detector was used in the TR-XRD studies. With this experimental setup, a continuous set of powder patterns can be obtained as a function of time or temperature.8,11,26 Full powder ring data for the samples were collected with a MAR345 detector. The powder rings were integrated using the FIT2D program.27 Rietveld refinements were performed with the program GSAS in a manner similar to that described in a previous work.26b II.2. X-ray Absorption Near-Edge Spectroscopy. The Mo LII-edge and S K-edge spectra were recorded at the NSLS on beam line X19A in the “fluorescence-yield mode” using a Stern-Heald-Lytle detector with helium as the detector gas. The X-ray photons were monochromatized employing a NSLS boomerang-type flat crystal monochromator with Si(111) crystals. The harmonic content was reduced by detuning (85-90%) the monochromator crystals. The energy resolution was ∼0.5 eV. Beam line U7A of the NSLS was used to collect the Mo MIII-edge spectra of the pure and sulfided molybdates. This beam line is equipped with a toroidal-spherical grating monochromator. The Mo MIII-edge data were recorded in the “electron-yield mode” with an energy resolution of 0.3-0.5 eV. No charging of the sample was observed during these measurements. MoS2 and FeSO4 were used as standards for energy calibration in the Mo LII- or MIII- and S K-edge data.8,9 Our interest here is in relative edge positions, not absolute values. II.3. Reduction and Sulfidation of β-FeMoO4. The samples of iron molybdate (BET surface area ≈ 7 m2 g-1 6) were exposed to H2, H2S, and SO2 in a RXM-100 instrument from Advanced Scientific Designs. After treatment with these gases, the samples were sealed and taken to the NSLS for characterization with XANES or XRD. In the H2-TPR experiments, β-FeMoO4 was placed in a flow reactor under a 15% H2/85% N2 mixture (flow rate ) 50 cm3/min), and the temperature was ramped from 40 to 850 °C at a heating rate of 20 °C/min. The consumption of
Rodriguez et al. H2 and evolution of H2O were monitored using a quadrupole mass spectrometer (UTI 100C). Fresh samples of β-FeMoO4 were sulfided by exposure to a 10% H2S/90% He mixture in a U-tube quartz reactor at temperatures ranging from 200 to 450 °C. Pure β-FeMoO4 was exposed to SO2 in a reaction cell (“batch-reactor mode”) at 25 °C for 15 min with a constant SO2 pressure of 10 Torr. After this was completed, the SO2 was pumped out, and the sample was annealed to temperatures up to 300 °C under vacuum. II.4. Density Functional Theory (DFT) Calculations. The equilibrium structure of β-FeMoO4 was obtained from firstprinciples DFT calculations using the CASTEP code28 from Molecular Simulations Inc. In this code, the wave functions of valence electrons are expanded in a basis set of plane waves with kinetic energy smaller than a specified cutoff energy Ecut. The presence of tightly bound core electrons is represented by nonlocal ultrasoft pseudopotentials.29 Reciprocal-space integration over the Brillouin zone is approximated through a careful sampling at a finite number of k points using the MonkhorstPack scheme.30 The exchange-correlation contribution to the total electronic energy is treated in the generalized gradientcorrected (GGA)31 form of the local density approximation (LDA). In all calculations, the kinetic energy cutoff Ecut and the density of the Monkhorst-Pack k-point mesh were chosen high enough in order to ensure convergence of the computed structures and energetics. Because of the delocalized (plane-wave) nature of its basis set, CASTEP yields useful electronic information (levels and band structure) only in the k space. To investigate localized charges and localized electronic density of states around various atoms of interest, we used the DMol3 code.32 In contrast to CASTEP, DMol3 uses localized functions to describe the atomic orbitals. Our calculations employed numerical basis sets of double-ζ quality plus polarization functions to describe the valence orbitals of O, Mo, and Fe. DFT in DMol3 was performed within the GGA approximation using Becke-88 for exchange33 and Perdew-91 for correlation.34 A large body of existing work indicates that CASTEP and DMol3 are very useful for studying the behavior of oxide systems.11,12,32,35-37 III. Results and Discussion III.1. Structure and Thermal Stability of β-FeMoO4. Figure 1 displays the primitive cell (C2/m space group) for the β phase of FeMoO4. The dimensions for this cell, obtained through Rietveld refinement of powder diffraction data (GSAS program,26b T ) 25 °C), are listed in Table 1. The Rietveld cell dimensions are very close to those reported in a previous work,21 in which no atomic coordinates or interatomic distances were mentioned. The structure of β-FeMoO4 contains columns or stacks of four-membered metal-oxygen rings (see Figure 1). There are two types of Fe and Mo atoms. The first type corresponds to metal atoms that occupy mixed Fe1-O-Mo1-O rings. The second type includes metal atoms that are in pure Mo2-O-Mo2-O or Fe2-O-Fe2-O rings. In addition, there are two kinds of oxygens: those that bridge the chains of rings and have only two metal neighbors (Ob) and those that form part of the rings and have three or four metal neighbors (Or). Within the rings, one can have metal-oxygen bonds that are either parallel (Fe-Orc, Mo-Orc) or perpendicular (Fe-Ornc, Mo-Ornc) to the c axis. Table 1 lists a series of metal-oxygen distances calculated by Rietveld refinement. For each metal atom, in general, there are two symmetry equivalent metalOb, metal-Orc, and metal-Ornc distances (i.e., six oxygen neighbors but only three types of metal-O bonds). The
Studies on the Behavior of Mixed-Metal Oxides
J. Phys. Chem. B, Vol. 104, No. 34, 2000 8147
Figure 1. Left: Primitive cell for the β phase of FeMoO4. Right: Partial view of the crystal structure of β-FeMoO4. The dark spheres correspond to O atoms. The metal atoms are represented by the small (Mo) and large (Fe) gray spheres.
TABLE 1: Structure of β-FeMoO4 from Rietveld Refinement and DFT-GGA Calculationsa a ) 6.973 [6.918]a
Cell Dimensions (Å)b b ) 6.973 [6.918]
c ) 7.053 [7.024]
Molybdenum-Oxygen Distances (Å) 1.70(2) [1.76] Mo2-Ob ) 1.78(4) [1.77] 1.93(4) [1.94] Mo2-Orc ) 1.82(4) [1.86] 1.74(5) [1.78] Mo1-Ornc ) 3.45(3) [3.46] Mo2-Ornc ) 3.21(3) [3.16]
Mo1-Ob ) Mo1-Orc )
Fe1-Ob ) Fe1-Orc ) Fe1-Ornc )
Iron-Oxygen Distances (Å) 2.02(4) [1.98] Fe2-Ob ) 2.16(4) [2.10] Fe2-Orc ) 2.01(5) [1.95] 2.17(3) [2.09] Fe2-Ornc )
2.06(3) [1.97] 2.04(3) [2.02] 2.15(3) [2.13]
Metal-Metal Distances (Å) Fe1-Mo1 ) 4.70(2) [4.67] Mo2-Mo2 ) 4.70(2) [4.75] Fe2-Fe2 ) 3.46(2) [3.32] a In brackets are reported the predicted values by CASTEP. b In the c-centered cell: a ) 10.301(1), b ) 9.402(1), c ) 7.053(1), and β)106.28°.
exception to this trend is the two different metal-Orc distances for Fe1 and Mo1. In Table 1, we also show structural parameters calculated with CASTEP at the DFT-GGA level for β-FeMoO4 (in brackets). The density functional results predict the same type of basic crystal structure obtained from Rietveld refinement. The small differences could reflect variations in temperature (298 K for experiment vs 0 K for theory), minor experimental errors (Rietveld refinement), or approximations inherent to DFTGGA level calculations.28,31 An analysis of the metal-oxygen distances in Table 1 indicates that the Fe atoms are in a chemical environment that is close to pure octahedral coordination, whereas the Mo atoms, in practical terms, are tetracoordinated (i.e., very long Mo-Ornc bond distances prevent an effective octahedral coordination for each Mo atom). Previous studies indicate that β-NiMoO4 decomposes at temperatures above 750 °C and transforms into an R phase at temperatures below 250 °C.8 In experiments of time-resolved XRD, we found that β-FeMoO4 has a large thermal stability. It did not decompose upon heating to 850 °C and did not transform into an R phase at room temperature. Figure 2 shows relative variations in the size of the c-centered unit cell of β-FeMoO4 with temperature (Table 1 lists the length for the a, b, and c axes at 25 °C). No significant changes are seen. As the temperature is raised from 25 to 800 °C, there is a small elongation (
