An X-Ray and Neutron Diffraction Study of the Structure of -Bi2Mo3O12 as a Catalyst for Partial Oxidation of Propylene to Acrolein

H. Fansuri.1, D.K. Zhang1, D. French2, M. Elcombe3 and A. Studer3

1Centre for Fuels and Energy, Curtin University of Technology, 1 Turner Avenue, Technology Park, Bentley WA 6102, Australia

2CSIRO Division of Energy Technology, Lucas Heights Science and Technology Centre, Private Mail Bag 7, Bangor NSW 2234, Australia

3Bragg Institute, Lucas Heights Science and Technology Centre, Private Mail Bag 7, Bangor NSW2234, Australia

Abstract

A series of X-ray and neutron diffraction experiments of a selective oxidation catalyst of alpha phase bismuth molybdates have been performed to study the crystalline structure of the catalyst. A Siemens D500 with a Cu source and a Philips PW1050 with a Co source were used for the x-ray diffraction analysis, while a High Resolution Powder Neutron Diffraction (HRPD) was used for the neutron diffraction experiments. All diffraction data collected were refined using the RIETICA refinement software to extract the crystallographic parameters including unit cell parameters, atomic coordinates, temperature factors, and site occupancy. Two structure models from International Crystal Structure Database (ICSD) collections (ICSD No. 2650 and 63640) were used as the input files.

All the refinements gave very good fit with both models. The best refinement fit was achieved from HRPD data with fitting parameters: Rp, Rwp, RBragg and GoF being 5.42, 6.36, 1.84 and 1.36, respectively. The unit cell parameters and atomic positions of the catalyst are closer to the parameters in the second model.

The very good fitting of the structure with the model shows that the catalyst that was synthesised in this work is an -Bi2Mo3O12 with a little modification in its unit cell parameters and atomic positions. The information gained in this study is very important as the starting point in investigating the real time structure dynamic of the catalyst in order to understand the mechanisms of catalytic oxidation on the catalyst.

Introduction

Bismuth molybdates are well known catalysts for partial oxidation of propylene to acrolein. There are three active phases of bismuth molybdates denoted by , , and , each of which, has different structure and composition.

The crystal structure of the alpha phase (Bi2Mo3O12 or Bi2(MoO4)3) was established before the structure of the other phases. The structure was first elucidated by A. F. van den Elzen and G. D. Rieck ([van den Elzen, 1973 #131] using x-ray diffraction technique on a single crystalline of synthetic Bi2Mo3O12.

Van den Elzen and Rieck derived the structure from mineral calcite (CaWO3). The scheelite has fluorite-type geometry, normally written as ABO4. The A cation is eight-coordinated by oxygen and the B cation is hexavalent and present in discrete BO4 tetrahedra. In the alpha phase, the structure is considered as scheelite structure with ordered cation vacancies. Due to the vacancies, the structure can be regarded as Bi2/31/3MoO4 [Buttrey, 1986 #199]. The model structure of alpha bismuth molybdate established by van den Elzen and Rieck now become the standard model collected by International Collection on Structure Database (ICSD) and given the collection number 2650. Later, Francois Theobald and Ahmed Laarif [Theobald, 1984 #200] use a neutron diffraction technique in order to find more precise position of oxygen atoms in the crystal structure (lattice oxygens). The results of Theobald and Laarif works were collected by ICSD and given a collection number 63640.

Many researchers believe that lattice oxygen plays the main role in determining the activities of bismuth molybdates in catalysing partial oxidation of olefin [references]. It has been proven that the oxidation reaction using lattice oxygen and following the Mars-van Krevelen (Redox) mechanisms [1, 2]. Several studies have also shown that the lattice oxygen anions are involved in the oxidation process [1, 3, 6-8]. In a more specific term, Haber [9] mentioned that the easiness of oxygen movement in bismuth molybdates, by the formation of shear plane and rearrangement of corner-linked metal oxide into edge-linked octahedra of molybdenum as well as tungstate oxide, favour their activities and selectivity towards acrolein formation.

The availability of the suitable crystal model of -Bi2Mo3O12 and the importance of its lattice oxygens was the driven force for us to study the structure of alpha bismuth molybdate under reaction condition using in-situ neutron diffraction technique (Hamzah et al, 2003) and at temperature variance (Hamzah et al, 2003) using x-ray diffraction technique. The studies were aimed to investigate the structural dynamics of the bismuth molybdate with reference to the structure of its’ room temperature and ambient atmosphere condition.

This paper will describe the investigation of the room temperature structure of -Bi2Mo3O12 using diffraction technique. In order to extract the structural information on unit cell unit, the diffractograms from Cu- and Co-xrd and neutron diffraction were refined by method developed by Hugo Rietveld (Rietveld, 1965) using software called RIETICA. The method is actually fitting the experimental data to calculated values. The criteria of fit was represented by several R values namely:

Rp = R-pattern

Rwp = R-weight pattern

RBragg = R-Bragg

Re = R-expected, and

GOF = (Rwp/Re)

The refined structure as the result of this works will be used as the starting model at room temperature structure for study of the real time structural dynamics.

EXPERIMENTAL

Catalyst Preparation

The catalysts were prepared using the so-called co-precipitation method [12, 14]. Bismuth nitrate, Bi(NO3)3.5H2O and ammonium hepta-molybdate, (NH4)6Mo7O24.4H2O were dissolved separately in hot water (70oC). The bismuth nitrate solution was then dropped slowly into the vigorously stirred an ammonium hepta-molybdate solution, yielding a yellowish suspension. The suspension was kept in the water bath at 70oC and stirred well to evaporate the liquid slowly until it became a paste. The paste was then put into an oven at 120oC for 20 hours in air. The dried paste was crushed and calcined at 250o for 2 hours in an air oven. The catalyst was then grinded into a powder and calcined for 20 hours at 480oC.

Diffraction Analyses

X-ray diffraction (XRD) analyses were carried out using two x-ray instruments with different x-ray source. A Siemens D500 diffractometers (XRD) available at Applied Physics Department, curtin University of Technology was used as a Cu-source XRD. A Philips PW 1050 available at CSIRO Division of Energy and Technology was used as a Co-source XRD. The wavelengths of Cu and Co XRD are 1.54184 and 1.7889 A, respectively. Both machine were operated at 30 mA and 40 kV. Experiment set up for 2range, step size and speed of the instruments are tabulated in Table 1.

Table 1 Experimental set up of Siemens D500 and Philips PW1050 diffractometers

Experiment parameters / Siemens D500 / Philips PW1050
2range / 5o – 120o / 3o – 90o
Step size / 0.02o / 0.04o
Speed / 0.5o min-1 / 0.24o min-1

Neutron diffraction analysis was carried out at Bragg Institute-ANSTO in Lucas Height Technology Park, New South Wales. The experiment used High Resolution Powder Diffraction (HRPD) instrument. The HRPD experiment was run for at the neutron beam wavelength of 1.495 Å

Data Analysis

Difractograms from XRD and HRPD were analysed by using the LHPM Rietica to extract the lattice parameters (, , , a, b, c and volume of unit cells), interatomic distances, and thermal parameters. Input file in the refinement works was taken from International Crystal Structure Database (ICSD). The collection code of ICSD file is 2650, originated from the work by van den Elzen and G. D. Rieck [11]. An ICSD collection code 63640 from the neutron diffraction work by Tehobald and Laarief [Theobald, 1984 #200] was also used as a reference.

The steps of the structure refinement follow the sequence in Table 2. Atomic coordinates and thermal parameters were refined the last. Only coordinate of heavy atoms (Bi and Mo) were refined on x-ray diffactograms and no refinement was made for thermal parameters. On the other hand, all parameters mentioned in Table 2 plus atomic coordinate and thermal parameters were refined on neutron diffractogram.

Table 2. Sequence in the structure refinements of all diffractograms

Step / Parameters
1 / Sample displacement
2. / Background (B-1 through B2)
3. / Phase scale factor
4 / Unit cell parameters (a, b, c and )
5 / Peak profile (size and gamma-0)
6 / Peak shape (U, V, W)

Results and Discussion

The diffractograms of Cu- and Co-xrd and HRPD are depicted in Figure 1. Phase analysis on the diffractograms using JADE™ x-ray software and database shows only -Bi2Mo3O12 (Fansuri, 2003).


Figure 2. Unit cell structure of -Bi2Mo3O12

unit cell structure information of the sample was extracted by refining the diffractogram using the Rietveld method. Figure 1 shows the Rietveld refinement results of the room temperature HRPD pattern and the refined parameters are tabulated in Table 1. The refinement shows a very good fit between the sample and the model diffractograms. The absence of residual peaks, apart from the noise on the lower curve in Figure 4, indicates that no other phases were detected and shows that the sample contained only -Bi2Mo3O12.

Figure 1. Diffractograms of -Bi2Mo3O12 from a) Cu-XRD, b) Co-XRD, and c) HRPD




Figure 3. The refinement results of: a) Cu-XRD, b) Co-XRD, and c) HRPD diffractograms. The experimental data is represented by plus ("+") signs, while the model (ICSD no. 2650) is represented by solid lines. The lower curve is the difference between the experimental and the model patterns.

The refined cell parameters, as shown in Table 1, are in good agreement with the XRD results previously obtained [12]. Furthermore, the ND pattern refinement provides more refined parameters such as temperature factors and atomic coordinate in the unit cell. However, it gives less accurate cell parameters (a, b, c and ) than those produced by XRD. The room temperature refinement was carried out simultaneously for the two instruments, in which, each provided more accurate refinement parameters to the other. The unit cell parameters and atomic coordinates, as a result of RT-HRPD refinement, are used as the room temperature model for in-situ MRPD pattern refinement.

Table 2. Unit cell parameters(Å)

Parameters / ICSD 2650 / ICSD 63640 / Cu-XRD / Co-XRD / HRPD
a / 7.685(6) / 7.7104(3) / 7.7138(2) / 7.7117(2) / 7.7155(6)
b / 11.491(16) / 11.5313(4) / 11.5270(3) / 11.5220(4) / 11.5267(8)
c / 11.929(10) / 11.9720(5) / 11.9762(3) / 11.9730(4) / 11.9768(10)
 / 115.4(2) / 115.276(3) / 115.2810(15) / 115.2840(19) / 115.2760(42)

Table 3. Atomic coordinates of heavy atoms

Conclusions

References