A Monte Carlo approach to describe the reduction profiles of bidimensional MoO_x structures grown on an alumina sustrate

M. C. Abello^1*, A. P. Velasco², O. A. Ferretti³ and J. L. G. Fierro⁴

¹ INTEQUI, Instituto de Investigaciones en Tecnología Química (UNSL- CONICET), Chacabuco y Pedernera, 5700 San Luis, Argentina, e-mail: cabello@unsl.edu.ar
² Departamento de Física, Facultad de Ciencias Físico Matemáticas y Naturales (UNSL), Ejército de los Andes 950, 5700 San Luis, Argentina
³ CINDECA, Centro de Investigación y Desarrollo en Ciencias Aplicadas. Dr. Jorge Ronco (UNLP -CONICET) y Departamento de Ingeniería Química. (UNLP), 47 N° 257, 1900 La Plata, Argentina
⁴ Instituto de Catálisis y Petroleoquímica, CSIC, Cantoblanco, E-28049-Madrid, Spain

Abstract — The reducibility of molybdenum oxide supported on γ-Al₂O₃ was studied by temperature programmed reduction (TPR). Interpretation of the reduction profiles was successfully achieved by applying the Monte Carlo method and taking into account the presence of different molybdenum species on the support surface. Seven reactions were assumed to take place along the reduction process. The first hydrogen consumption peak was assigned to the reduction of Mo⁶⁺ ions present as well-dispersed polymeric species into Mo⁺⁵, while the second one accounts for the reduction not of the Mo⁵⁺ ions generated in the first reduction into Mo⁺⁴ but also for the reduction of monomeric Mo⁶⁺species.

Keywords — Mo Catalysts. Monte Carlo Method. Temperature Programmed Reduction.

I. INTRODUCTION

The interaction of molybdenum oxides on the alumina surface and their reduction processes have been extensively investigated as documented in many literature reports (Massoth, 1978; Wang and Hall,1982; Lewis and Kydd, 1992; Wang et al., 1999; Kim et al., 1992; Lopez Cordero et al., 1991; Del Arco et al., 1993; Spevack and McIntyre,1993; Hu et al., 1995; Suarez et al., 1985; Rajagopal et al., 1994, 1995; Okamoto et al., 1998(a, b); Abello et al., 2001; Thomas et al., 1982). As a general rule, alumina-supported molybdenum oxide catalysts consist of a monolayer of molybdenum oxide built up on the alumina surface. At a high molybdenum oxide content, this simple structure becomes much more complex since a tridimensional MoO₃ and an aluminum molybdate phase also coexist with the monolayer structure.

Although there are several methodologies to investigate catalyst reducibility, temperature program-med reduction remains prominent. TPR profiles of MoO₃/γ-Al₂O₃ catalyst containing variable amounts of molybdenum oxide have been previously reported (Lopez Cordero et al., 1991; Rajagopal et al. 1994; Abello et al., 2001; Damyanova et al., 2002). From these studies, good agreement in the number of peaks, reduction temperatures, final oxidation state of molybdenum and H₂ consumption can be derived from different TPR operating conditions. In addition to these parameters, the coordination of Mo⁶⁺ with O^-2 ions in tetrahedral and/or octahedral coordination and the development of a tridimensional MoO₃ phase are often used to assign the peaks of TPR profiles. However, a detailed examination of all these studies reveals some discrepancies and, in several instances, controversial interpretations. Many authors have assigned the first peak to reduction of weakly bound octahedrally coordinated Mo⁶⁺ species to the alumina surface into Mo⁴⁺ (Lopez Cordero et al., 1991; Rajagopal et al. 1994). In opposition to this interpretation, others have associated the first reduction peak to the reduction of Mo⁶⁺ to Mo⁵⁺ (Abello et al., 2001). The peak placed at high temperature of reduction has usually been attributed to the deep reduction of all molybdenum species, including highly dispersed tetrahedral species, strongly bound to the support. Accordingly to the above, this study was undertaken with the aim of obtaining a better understanding of the experimental temperature programmed reduction profiles of alumina supported molybdenum oxide by applying a Monte Carlo model. In particular, the focus of the discussion is based on the reduction features of the different surface MoO _x species.

II. EXPERIMENTAL

A. Sample preparation

A commercial alumina calcined at 600 °C for 3 h was used as support. This support was impregnated with an excess of an aqueous heptamolybdate (AHM, Baker reagent grade) solution. The concentration of solution was 0.005 M (pH = 5.6) and the volume of solution was chosen in order to obtain a 13 wt% MoO₃ in the final catalyst called 13Mo/γ-Al₂O₃. The excess water was removed under reduced pressure in a rotary evaporator at 60 °C, and then the impregnate was dried at 100 °C overnight. Finally, it was calcined in air at atmospheric pressure according to the following procedure: the temperature was raised linearly for 2.5 h up to 450 °C, kept constant at 450 °C for 3 h, raised linearly up to 600°C and then maintained for 5 h at 600 °C.

B. BET Specific areas

Specific areas were determined by the BET method from the nitrogen adsorption isotherms measured in a Micromeritics Accusorb 2100E instrument. Prior to adsorption, the samples (0.200 g) were degassed at 200 °C under high vacuum for 2 h.

C. Chemical composition

Molybdenum content was determined by atomic absorption spectroscopy. The sample was subjected to alkali fusion with KHSO₄ and then disolved in diluted HCl solution. The measurements were carried out using a Varian AA275 spectrometer previously calibrated with standard solutions.

D. X-ray Diffraction

XRD diffraction patterns were obtained with a Rigaku diffractometer operated at 30 kV and 20 mA using Ni-filtered CuKα radiation (λ = 0.15418 nm). The powder sample was analyzed without previous treatment after deposition on a quartz sample holder. The identification of crystalline phases was made by using references from the JCPDS files.

E. X-ray photoelectron spectroscopy

The XP spectra were recorded with a VG 200 R spectrometer equipped with a Mg Kα X- ray excitation source (hν = 1253.6 eV) operated at 12 kV and 10 mA and a hemispherical electron analyzer. The residual pressure inside the analysis chamber was kept at values below 7·10^-9 mbar. Mo3d, Al2p and C1s emissions were recorded for each catalyst. All spectra were fitted after removal of a sigmoid background. The Mo 3d signal shapes were analyzed as a set of doublets separated by defined binding energy scale, intensity contribution, position and width. All binding energies were determined by fitting the experimental curve to a Gaussian -Lorentzian peak shape with spin orbital pair intervals set at 3.15 eV and a spin-orbit area ratio of 1.5. These constraints ensure that the results make physical sense.

F. Raman spectroscopy

The Raman spectra were obtained from a single monochromator Renishaw System 1000 equipped with a cooled CCD detector (-73 °C) and a holographic super-Notch filter. The holographic Notch filter filters the elastic scattering while the intensity of the signal remains high. The sample was excited with the 514 nm Ar line, the spectrum resolution was better than 2 cm^-1 and the spectrum acquisition consisted of five accumulations of 60 s. The spectrum was acquired under dehydrated conditions at 473 K.

G. Temperature-programmed reduction

TPR experiments were carried out in a conventional apparatus which consists of a gas handling system with mass flow controllers (Matheson), a tubular reactor, a linear temperature programmer (Omega, model CN 2010), a PC for data retrieval, a furnace and various cold traps. In each experiment the sample size was ca. 100 μmol of Mo to assure a good resolution for the experimental conditions used. Before each run, the sample was oxidized in a 30 ml min^-1 flow of 20 vol % O₂ in He at 450 °C for 30 min. After that, helium was admitted to remove oxygen and the system cooled at 50 °C. The sample was subsequently contacted with a 30 ml min^-1 flow of 10 vol % H ₂ in Ar and heated, at a rate of 10 °C min^-1, from 50 °C to a final temperature of 715 °C and held for 2 h. H₂-consumption was monitored by a TCD after removing the water formed. Each TPR run was performed twice and the reproducibility was verified. The characteristic number P proposed by Malet and Caballero (1988) defined as βS_o/V*C_o, where S_o is the initial amount of reducible species in the sample (μmol), V* is the total flow rate (ml min^-1), C_o the initial hydrogen concentration in the feed (μmol ml^-1) and β the heating rate (°C min^-1) was 8.7 °C in order to obtain an unperturbed reduction profile. The area under the curve was integrated to determine the H₂ consumed previous calibration.

III. RESULTS AND DISCUSSION

A. Characterization

The main characteristics of the catalyst are shown in Table 1. It can be seen that the Mo content (12.7 wt % MoO₃), as determined by AAS, is lower than the theoretical amount required to fulfill one monolayer of MoO₃ (16.7 wt % MoO₃, Abello et al., 2001). The specific surface area expressed per gram of sample decreases after Mo impregnation but no changes in the specific surface area per gram of support were found. MoO₃ contributes only to the mass of the catalysts covering partially the support surface. The X-ray diffraction profile of the oxidized sample reveals the typical pattern of γ-Al₂O₃ (JCPDS 10-425); in no case diffraction lines of MoO₃ and Al₂(MoO₄)₃ phases were detected. The H₂-reduced sample showed a dark blue color, however, in spite of this color change, the X-ray diffraction pattern was virtually coincident with that of the oxidized counterpart. The Raman spectrum of the oxidized sample exhibited bands at 1001 and 867 cm^-1 which correspond to the terminal Mo=O bond surface and Mo-O-Mo bridge bond, respectively (Hu et al., 1995). No bands at 817 cm^-1 and 380 cm^-1 corresponding to crystalline α-MoO₃ and Al₂(MoO₄)₃ were observed. This is consistent with the X-ray diffraction results and it suggests that MoO_x exists as two-dimensional domains.

Table 1. Main characteristics of Mo/γ-Al₂O₃ catalyst

The experimental TPR profile of the 13Mo/γ-Al₂O₃ sample is shown in Fig.1.This profile exhibits two well-resolved reduction peaks. The maximum H₂-consumption in the first reduction peak is placed at 452 °C, while the second one appears just at the end temperature of the ramp (>700 °C). No significant effects of H₂ reduction were observed below 300 °C. The extent of reduction has been characterized by the change in the average oxidation number of Mo, ΔON, which depends on the amount and the reducibility of Mo species and on the experimental conditions of TPR. Thus, ΔON = 2 indicates an average reduction of Mo⁶⁺ to Mo⁴⁺. Table 1 shows the corresponding ΔON calculated from the whole TPR profile. Assuming the initial oxidation state to be 6+, which is in agreement with XPS results shown below, the ΔON value suggests that the average molybdenum oxidation state after the TPR run is close to 4+. In Fig.2, the XP spectra of Mo3d core-levels are shown for the oxidized and reduced samples. As judged from the binding energies of the two components of the Mo3d doublet of the oxidized sample, Fig. 2a, it can be inferred that molybdenum is present in one oxidation state (Mo⁶⁺), as identified by the binding energies at 233.2 and 236.4 eV for the Mo 3d_5/2 and Mo 3d_3/2 components, respectively. The binding energies of the Mo 3d doublet, which are summarized in Table 2, agree well with most of the values given in the literature (Peeters et al., 1998; Grunert et al., 1992; Oliveros et al., 1997; De Canio et al., 1989; Noguera et al., 2000; Bañares et al., 1993). Once the initial chemical state of molybdenum was determined, the sample was transferred from the analysis chamber to the pretreatment chamber of the electron spectrometer and then exposed to an atmosphere of pure H₂ for 1 h at 500 °C. The reduction temperature was chosen taking into account the minimum H₂ consumption between the first and the second TPR peaks. Subsequently, the reduced sample was transferred to XPS chamber and the measurement reveals the appearance of Mo⁵⁺ species which are characterized by the binding energies of Mo 3d_5/2 and Mo 3d_3/2 peaks at 232.4 and 235.7 eV, respectively (Fig. 2b). Some authors (Grunert et al., 1992; Oliveros et al., 1997) have reported two binding energy values for Mo⁺⁴: a high binding energy value (Mo 3d_5/2 231.5 - 232 eV) attributed to ionic "isolated" Mo⁺⁴ and a low binding energy value (Mo 3d_5/2 ca. 230 eV) typically associated with Mo⁺⁴ as in MoO₂ and ascribed to paired double-bonded Mo⁺⁴. Then, it could not rule out that Mo3d doublet attributed to Mo⁺⁵ is in fact a convolution of the contribution from Mo⁺⁵ and ionic Mo⁺⁴. The relative abundance of each Mo oxidation state is estimated by dividing the Mo 3d area for a given oxidation state by the total area of the Mo 3d envelope. The fraction of unreduced Mo⁶⁺ is about 54 %. Although the extent of reduction determined under isothermal conditions does not coincide with that obtained under the conditions used in TPR experiments, this observation is relevant because it allows us to assume that the first reduction step consists of the reduction of Mo⁶⁺ to Mo⁵⁺. If an important contribution of ionic Mo⁺⁴ was the responsible of the Mo 3d doublet, the change in the average oxidation number of Mo after TPR experiment should be higher than 2. Then, it is assumed that only Mo⁺⁵ contributes to the Mo3d doublet. From peak intensities and tabulated atomic sensitivity ratios (Wagner et al.,1981), the Mo/Al surface atomic ratios were computed for the oxidized sample and reduced samples.

Figure 1. TPR profile of 13Mo/γAl₂O₃. (° Experimental data; - simulated data)

Table 2. Binding Energies of Mo 3d core-level and surface atomic ratios of oxidized and reduced Mo/γ-Al₂O₃ catalyst

Figure 2. The XP spectra of Mo3d core level of (a) oxidized sample; (b) reduced sample in H₂ at 500°C for 1 h.

These Mo/Al ratios are also included in Table 2. The XPS Mo/Al ratio is higher than the nominal value (0.051), which agrees with the high dispersion of molybdenum species which does not change after hydrogen treatment.

Taking into account the above results, the TPR profile was simulated by a Monte Carlo method. A precise description of the MC methodology has been given elsewhere (Binder, 1997; Rudzinsky et al., 1996; Zhdanov, 1991).

B. Simulation Model

A Monte Carlo approach has been developed to analyze the experimental TPR profiles of the 13Mo/γ-Al₂O₃ sample. Under moderate temperatures of reduction used in this work, two peaks are observed which correspond to the reduction of different Mo species on the γ-Al₂O₃ surface. It is well known that different MoO_x domains are present on alumina from isolated monomolybdate to two-dimensional polymolybdate and three dimensional MoO₃ cluster, scheme I (Fig. 3).

Figure 3: Scheme I

Evidence for the structural evolution has been provided by Raman spectroscopy combined XANES (Hu et al., 1995; Abello ,2002).

Taking into account that on our sample MoO₃ is not detected by Raman, the model only considers two different structures on γ-Al₂O₃. A larger fraction of Mo species is in the polymeric form (which will be represented into the text by scheme II, Fig. 4), and a minor fraction of Mo species forms highly dispersed monomeric structures (which will be represented by scheme III, Fig. 4). A linear distribution of oxygen sites for the Mo-O-Mo bridge is considered for polymeric structures and a bidimensional distribution of oxygen sites for Mo=O in monomeric species.

Scheme II

Scheme III
Figure 4: Schemes I and III

The interaction of the various Mo-O locations with hydrogen is proposed to imply the following reactions. Three different kinetics are considered for polymeric structures assuming a different extent of surface oxygen reducibility.

The first reaction takes place on the surface oxygen shared by two Mo⁶⁺ ions:

where represents an oxygen vacancy. The second reaction occurs from a surface oxygen shared by Mo⁵⁺ and Mo⁶⁺:

The third reaction occurs from a surface oxygen shared by Mo⁵⁺ and Mo⁵⁺:

The three reactions lead to water formation. Reaction (a) reduces Mo⁶⁺ to Mo⁵⁺, the second one generates Mo⁵⁺ and Mo⁴⁺ and the last one only produces Mo⁺⁴.

For reduction of monomeric structures the model assumes four different kinetics. They are schematically depicted as follow:

Reaction (d) reduces Mo⁶⁺ to Mo⁵⁺ and leads to the formation of surface OH. Reactions (e), (f) and (g) lead to the formation of one (reactions e and f) or two (reaction g) water molecules. Reaction (e) reduces two groups from Mo⁺⁶ and Mo⁺⁵ to Mo⁺⁵ and Mo⁺⁴, respectively. Reaction (f) leads to the reduction of Mo=O from Mo⁺⁶ to Mo⁺⁴ and the last one only produces two oxygen vacancies (Mo⁺⁴) from two Mo-OH groups.

In order to perform the simulation, a system consisting of N surface reactive sites (5 x 10⁶) was taken into account (Rudzinsky et al., 1996; Zhdanov, 1991). It was assumed that the polymeric to monomeric concentration ratio was 2.3 (70% of polymeric and 30% of monomeric) (Abello, 2002; Shimada et al, 1992). The initial oxidation state for all molybdenum species was considered to be 6+ in agreement with XPS results.

The simulation scheme was performed according to the following steps:

(i) Definition of the surface of N sites and fixing an initial temperature T₀ = 300 °C.

(ii) A given site "i" in the lattice is chosen at random.

(iii) The reduction process is tested on each site "i" by comparing the probability of each reaction (P_r) with a random number ξ (0 ≤ ξ ≤ 1). The reaction probability is given by the expression

Pr = ν ( r ) . exp (E_a(r, i)/RT) . ΔT/β

where ν(r) is the pre-exponential factor for reaction "r"; E_a(r, i) is the activation energy for reaction "r" on "i" site; β is the heating rate, R is the gas constant and ΔT is the temperature increment.

All the seven (a-g) reactions participate with an equivalent weight according to probability. If the random number (ξ) results greater than the probability (P_r) for the r reaction occurrence, this reaction is not selected and the site reduction is considered as a null event. The simulation follows the steps:

(iv) Before the temperature is increased, the number of times that the reaction is selected, the H₂ consumption, the water production, the amount of surface OH and the reduction state of all sites are estimated.

(v) Increase the temperature by ΔT and repeat from step (ii) until the surface is completely reduced or the temperature reaches 800°C.

The simulation process allows one to determine the activation energy of each reduction kinetics step and the population of both structures. The normal distribution of activation energy and the temperature in the peak maximum are shown in Table 3. The relative intensity of the two peaks after simulation is in very good accordance with that obtained from the experimental TPR profile.

Table 3. Simulation results

In Fig. 5(i), the H₂ consumption and the three peaks corresponding to each reaction for the polymeric structure are shown. The H₂ consumption agrees with the production of water. The maximum temperature of three peaks are 443°, 657° and 734°C. In Fig. 5(ii), the behavior of monomeric species against the reduction is shown. In this case, the H₂ consumption is the sum of the four peaks with maximum temperatures at 511°, 550°, 551°, and 599 °C. It can be noted that the H₂ consumption curve does not coincide with the water formation curve because of the appearance of surface OH in the first step of reduction.

Figure 5. Hydrogen consumption for the reduction process of MoO_x on γ-Al₂O₃ corresponding to (i) polymeric and (ii) monomeric species according to reaccions a, b, c, d, e, f and g.

The presence of Mo-OH groups can explain the increase of Brønsted acidity found on reduced molybdena-alumina samples. Suarez et al. (1985) have reported that reduction of Mo/Al₂O₃ catalysts with hydrogen causes the total acidity to increase to a maximum and then to decrease as the sample is reduced further. The increase in total acidity is caused by a slight increase in Brønsted acidity and a larger increase in the Lewis acidity. The initial increase in Lewis acidity may be due to generation of anion vacancies on molybdenum. These changes studied by infrared spectroscopy and thermo-gravimetric analysis of adsorbed pyridine are in line with the present model (Suarez et al., 1985). Figure 1 shows the global behavior of both species. The agreement between experimental and simulated data is satisfactory. From the evolution of oxidation state of surface Mo, Fig. 6, it is seen that the reduction of Mo species starts at 375 °C.

Figure 6. Evolution of oxidation state of surface Mo on γ-Al₂O₃ under temperature programmed reduction

50% of the Mo⁶⁺ is reduced to Mo⁵⁺ at 460 °C. At 500°C, 30% of Mo species is still in oxidation state 6+. The difference in population of Mo⁶⁺ between XPS and TPR results can be attributed to differences in the reduction regime, sample treatment (use of H₂/Ar mixture instead of pure H₂), different reduction time, different sample forms and flow regimes. The Mo⁶⁺ ions completely disappear at 725 °C where around 60% of Mo⁴⁺ coexists with about 40% of Mo⁵⁺. The maximum fraction of Mo⁵⁺ species can be obtained at 550 °C.

IV. CONCLUSIONS

A Monte Carlo approach to describe the reduction profiles of bidimensional MoO_x structures grown on an alumina sustrate was developed. From the analysis of the experimental and simulated results, the first TPR peak can be assigned to the reduction of polymeric Mo species mainly from 6+ to 5+, while the second peak can be attributed to the reduction of all Mo species to Mo⁴⁺ including the monomeric species from 6+ to 4+. The knowledge of the oxidation state evolution of molybdenum with temperature in the presence of hydrogen allows one to control the initial reduction extent of surface which has a significant effect on the properties of the catalyst.

ACKNOWLEDGMENTS
The authors wish to thank Dr. M.A. Bañares for Raman assistance. Financial support is acknowledged to CONICET, to Universidad Nacional de San Luis and Universidad Nacional de La Plata.

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Received: November 28, 2005
Accepted: June 9, 2006
Recommended by Subject Editor: Ana Lea Cukierman