Received 1 June 2012
An X-ray diffractometer coupled with diffuse reflectance infrared Fourier transform spectroscopy and gas chromatography for in situ and in operando characterization: an innovative analytical laboratory instrument
Laure Braconnier,a,b Isabelle Clémençon,a Christèle Legens,a* Virginie Moizan,a Fabrice Diehl,a Henry Pillière,c P. Echegut,d D. De Sousa Menesesd and Yves Schuurmanb
aIFP Energies Nouvelles, Rond point de l'Echangeur de Solaize, BP3, 69360 Solaize, France,bIrcelyon, UMR 5256, 2 avenue Albert Einstein, 69626 Villeurbanne Cedex, France,cINEL ZA, CD 405, 45410 Artenay, France, and dCEMHTI-CNRS UPR 3079, 1D avenue de la Recherche Scientifique, 45071 Orléans Cedex 2, France
In heterogeneous catalysis, chemical reactions take place at the surface of the material and can be influenced by its structure. To understand better the impact of the material surface and structure on catalytic properties, it is important to characterize them simultaneously. The association of X-ray diffraction and diffuse reflectance infrared Fourier transform spectroscopy, combined in a single dedicated high-temperature and high-pressure reaction cell with an online gas chromatograph, could be the answer to this challenge. For the first time, such an analytical tool has been developed for laboratory applications. The use of this device is illustrated, and it is validated through the in situ study of the thermal decomposition of calcium oxalate.
In order to decrease production costs and human impact on the environment, improving the performance of heterogeneous catalysts has become one of the major challenges in industrial processes. The activity and selectivity of these processes are mainly governed by the catalytic materials. Under working conditions, the catalytic properties are affected by possible variations in the morphology and structure of the active phase. A better understanding of the relationship between structural and catalytic properties would allow us to improve our knowledge of the processes and the materials themselves. This goal can be achieved by using in operando characterization of the catalyst during activation and under working conditions (Banares, 2005; Weckhuysen, 2009; Meunier et al., 2008). In operando characterization can be defined as the characterization of the catalyst structure or surface, while catalytic performance (activity, selectivity and stability) is monitored by an online analyser. When only the catalyst surface and structure properties are characterized, the term `in situ analysis' can be used. Another way of improving our knowledge of the structure-performance relationship of a catalyst is the multitechnique approach at specific and varied length scales (Briois et al., 2005; Kongmark et al., 2010). Combining different analytical techniques is an approach often found at synchrotron facilities. For example, the reduction of cerium species in an ethanol solution has been studied by a multitechnique approach (Briois et al., 2005): synchrotron radiation was associated with simultaneous UV-visible and Raman spectroscopy to illuminate this reduction process. In another study, Raman and X-ray absorption spectroscopies were combined in association with X-ray diffraction (XRD) to follow the formation of bismuth-molybdenum-based crystallites during hydrothermal synthesis (Kongmark et al., 2010). This study revealed that the genesis of the crystal goes through a molybdenum oxide intermediate randomly dispersed on a bismuth oxide matrix. Association of several characterization techniques can also be achieved during catalytic tests (Newton et al., 2010; Wragg et al., 2009). Newton et al. (2010) have developed a special analytical tool to characterize supported palladium nanoparticles during CO/NO cycling by simultaneous XRD, X-ray absorption spectroscopy and diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy. During CO/NO cycling, they managed to show CO dissociation and transient carbon storage. Elsewhere, a methanol-to-olefin process catalyst was characterized under working conditions by simultaneous XRD, Raman spectroscopy and mass spectroscopy (Wragg et al., 2009). During the catalytic test, the formation of carbon species was observed by XRD and their graphitic nature was deduced from Raman spectra. As shown in various examples found in the literature (Briois et al., 2005; Kongmark et al., 2010; Newton et al., 2010; Wragg et al., 2009), the association of several simultaneous characterization techniques applied to the study of catalytic processes has been well developed at synchrotron facilities. However, to our knowledge, in operando and in situ multitechnique analysis has not yet been expanded to laboratory-scale devices.
In the present paper, we report the development and application of an innovative analytical tool associating an XRD system with a DRIFT spectrometer and a gas chromatograph (GC) for in operando studies. XRD analysis reveals information on the crystal structure of the studied material, whereas DRIFT spectroscopy gives information on surface-adsorbed reactants and products, while GC analyses the products of the catalytic reaction. The characterization of materials like catalysts, as well as various other materials, under working conditions is possible thanks to the use of an appropriate reaction cell which was specially designed to support high temperatures and pressures.
To illustrate the use of this new device, the thermal decomposition of calcium oxalate was followed in situ by coupling XRD and DRIFT characterization techniques.
A powder X-ray diffractometer was associated with a DRIFT spectrometer (Fig. 1). Both characterization techniques can be performed simultaneously thanks to the use of a dedicated reaction cell. The analysis chamber also allows high pressures and temperatures and gaseous treatments.
| || Figure 1 |
A photograph of the multitechnique device. (a) The XRD detector, (b) the IR spectrometer and (c) the reaction cell.
The X-ray diffractometer is configured in Debye-Scherrer mode. It is composed of a molybdenum tube (K1 = 0.709 Å, K2 = 0.71359 Å, E = 18 keV) and an INEL CPS 120 curved detector (Evain et al., 1993; Ballon et al., 1983) (Fig. 2). An elliptic mirror from AXO Dresden GmbH (http://www.axo-dresden.de/mainframe_products.htm ), 60.5 mm long, 200 mm wide and 10.5 mm high, suppresses the Mo K radiation and focuses the beam on the detector. The detector has a curvature radius of 250 mm. It contains 6 bar (1 bar = 105 Pa) of an argon-ethane mixture (0.85:0.15). A solid blade anode and a segmented cathode are mounted inside the detector to intercept the X-rays. The detector resolution was measured with a certified LaB6 sample (NIST, Maryland, USA) and is equal to 0.21. The molybdenum tube provides energetic X-rays, allowing the characterization of thick samples under an absorbing atmosphere.
| || Figure 2 |
A schematic view of the X-ray diffraction system.
Diffracted X-rays are collected simultaneously by an INEL curved detector, in order to decrease the analysis time and to retain enough space for both the in situ and in operando cell and the DRIFT spectrometer. Since the centre of the detector is located above the sample, diffraction cones are collected at two points (Fig. 3). Finally, the recorded diffraction patterns are added for signal exploitation.
| || Figure 3 |
A raw diffraction diagram before folding.
The DRIFT spectrometer is an IRcube OEM FT-IR spectrometer provided by Bruker (Bruker Optics, http://www.bruker.com/ ). The mercury cadmium telluride detector allows the analysis of wavenumbers between 12 000 and 600 cm-1 with a resolution between 1 and 4 cm-1. It is cooled by liquid nitrogen and has a running autonomy of 8 h. Two sets of plane and parabolic mirrors made of silica and gold, and nickel-aluminium, respectively, are used to focus the IR beam from the source to the reaction cell and from the reaction cell to the detector (Fig. 4). Both mirrors were specially developed for this system to minimize the loss of signal. Moreover, to avoid signal perturbation by atmospheric gases such as CO2 or water vapour, the optical path is flushed with gaseous nitrogen.
| || Figure 4 |
A schematic view of the DRIFT spectroscopy system.
DRIFT spectroscopic analysis gives information on the surface of the studied material and, more precisely, on the adsorbed species and products during in situ/in operando treatment.
The reflection mode is mainly used to characterize adsorbent species and has not been adapted to quantification.
The reaction cell, which mimics a fixed-bed catalytic reactor, was specially developed to characterize catalysts simultaneously by XRD and DRIFT spectroscopic measurements under working conditions at industrially relevant temperatures and pressures. Thus, the analysis chamber is able to support pressures up to 18 bar and temperatures up to 873 K under atmospheric pressure. Moreover, different gases can be used in the current configuration for in situ analysis, namely H2, CO or N2. The gas enters the analysis chamber flowing up and leaves the cell by passing down through the sample (Fig. 5).
| || Figure 5 |
A schematic view of the reaction cell.
A heating resistor rolled into a cylinder allows thermal treatments. A porous silica fritted disc placed in the cylinder acts as a support for the powder sample (labelled catalyst in Fig. 5), from a few milligrams to 200 mg.
Specific windows allow the X-ray and IR beams to reach the sample. For X-ray measurements, a set of beryllium windows, each 1.5 mm thick, is used, located at the top and bottom of the cell. For IR analysis, ZnSe windows, each 3 mm thick, are installed perpendicular to the X-ray plane.
A gas chromatograph (GC) was installed online after a condenser to allow the online analysis of gases. The gas analyser contains a flame ionization detector (FID) and two thermal conductivity detectors (TCDs). The analytical schema is presented in Fig. 6. The Pona column and FID allow the separation and detection of hydrocarbons such as paraffins, olefins and oxygenates. The first TCD, with Ar as the vector gas and PPQ and MS5A (from Agilent) as chromatographic columns, enables the detection and quantification of hydrogen. The second TCD is used with helium and associated with PPU and MS5A columns (from Agilent) for N2, CO, CO2, light hydrocarbon and O2 analysis.
| || Figure 6 |
A schematic view of the online gas chromatograph. Abbreviations are defined in the text.
The thermal decomposition of calcium oxalate was followed by XRD and DRIFT spectroscopy measurements and compared with thermogravimetric analysis (TGA). This solid constitutes a good reference since the molecular bond vibrations can be observed by IR spectroscopy (Trpkovska et al., 2002) and the starting and intermediate materials have good crystallinity (Kociba & Gallagher, 1996). Moreover, its decomposition temperatures fit with the operating temperature range of our multitechnique device (Kutaish, 1997). During thermal treatment, this material decomposes as described by the reaction scheme presented in Fig. 7.
| || Figure 7 |
The reaction scheme for the thermal decomposition of calcium oxalate hydrate.
In order to validate the reaction cell, the results are compared with TGA.
Thermal decomposition of a calcium oxalate sample (Carlo Erba) was carried out at atmospheric pressure under a flow of 16 ml min-1 of N2. The heating ramp was 5 K min-1 from 293 to 853 K. For XRD and DRIFT spectroscopy analyses, sample characterization was performed every 20 K. About 50 mg of calcium oxalate in powder form was placed on the fritted disc inside the reaction cell. A thermocouple was inserted into the calcium oxalate powder. During the analysis, the temperature was kept constant. 200 DRIFT spectra (resolution 4 cm-1) were recorded and represented on the Kubelka-Munk scale. The background used for the IR measurements was recorded at room temperature on dehydrated KBr (Merck). XRD measurements were carried out for 200 s (resolution of 4096 channels). The XRD pattern of the fritted disc and the beryllium windows was subtracted from the sample patterns.
In parallel, thermal decomposition of calcium oxalate was studied by TGA. The evolution of the sample weight was followed by a Sétaram TGA thermoscale. Calcium oxalate (46.18 mg) in powder form was placed in the basket of the balance. The experiment was carried out under nitrogen (50 ml min-1) with a heating ramp of 10 K min-1 from 298 to 1073 K.
The thermal decomposition of calcium oxalate CaC2O4·H2O powder was followed using TGA (Fig. 8). Two decomposition steps can be observed. The first occurs around 483 K (peak temperature) and represents a loss of 5.68 mg (10.8 wt%), corresponding to the loss of the water (step 1). The second decomposition step occurs around 743 K (peak temperature) and represents a loss of 8.80 mg (21.72 wt%). This weight loss can be explained by the formation of CO and calcium carbonate, CaCO3 (step 2). These experimental weight losses are in good agreement with the theoretical ones.
| || Figure 8 |
Thermogravimetric analysis plot for calcium oxalate (46 mg, 10 K min-1).
After being studied by TGA, the thermal decomposition of calcium oxalate was followed inside the reaction cell by XRD and DRIFT spectroscopy. Fig. 9 shows the evolution of the XRD patterns of calcium oxalate as a function of temperature. The initial product is stable up to 413 K. It presents diffraction lines at 2 = 6.85, 7.01, 11.15, 13.74, 16.34, 17.38 and 19.68° (Fig. 10), characteristic of CaC2O4·H2O (Hocart et al., 1965). At 413 K it starts to decompose into another product. Stable dehydrated CaC2O4 with diffraction lines at 2 = 6.50, 6.85, 7.76, 9.45, 10.83, 13.72 and 16.82° is obtained at 553 K (Hochrein et al., 2008). Finally, this product starts to decompose under nitrogen at 733 K into a new crystallographic structure. From 813 K onwards, the final product exhibits diffraction lines at 2 = 10.55, 13.41, 16.34, 17.86, 19.49 and 21.37°, which correspond to CaCO3 (Pilati et al., 1998).
| || Figure 9 |
X-ray diffraction patterns recorded during the thermal decomposition of calcium oxalate (50 mg, 5 K min-1); blue traces represent stable crystallographic phases and orange diffraction traces indicate intermediate crystallographic structures.
| || Figure 10 |
X-ray diffraction patterns recorded at 293, 513 and 833 K.
The relative heights of the carefully selected diffraction peaks of calcium oxalate hydrate (at 2 = 19.68°), dehydrated calcium oxalate (at 2 = 9.45°) and calcium carbonate (at 2 = 16.34°) were measured as a function of temperature as obtained from the XRD patterns, and they are shown in Fig. 11. The rate of decomposition of calcium oxalate hydrate reaches a maximum between 453 and 513 K, whereas dehydrated calcium oxalate decomposes very quickly between 733 and 793 K. These decomposition temperature ranges are in agreement with those obtained from TGA, despite the small difference in heating rates.
| || Figure 11 |
Relative heights of the calcium oxalate hydrate peak at 2 = 19.68°, the dehydrated calcium oxalate peak at 2 = 9.45° and the calcium carbonate peak at 2 = 16.34°, from the XRD measurements presented in Fig. 9.
In parallel with the XRD measurements, DRIFT spectra of the sample were recorded (Fig. 12). At 293 K the spectrum is characteristic of the hydrated calcium oxalate structure (Bellamy, 1958). The broad band between 3700 and 2600 cm-1 can be attributed to the water. The band at 1606 cm-1 corresponds to C=O vibrations, that at 1330 cm-1 to C-O- vibrations and that at 781 cm-1 to O-(CO) deformations. At 473 K, the IR spectrum can be attributed to calcium oxalate. Vibration and deformation frequencies remain the same, except for the H-O vibrations of the water observable between 3700 and 2600 cm-1: this band disappears during the thermal treatment, as a result of dehydration of the sample. Finally, after thermal treatment at 853 K, the sample exhibits a spectrum representative of calcium carbonate. The band at 1795 cm-1 can be attributed to C=O double-bond vibrations (Bellamy, 1958), and the bands at 882 and 714 cm-1 to O-(CO) deformations (two deformation modes). The absorption band at 1444 cm-1 is characteristic of C-O vibrations.
| || Figure 12 |
DRIFT spectra recorded during thermal decomposition of calcium oxalate (50 mg, 5 K min-1).
Comparing the results obtained by TGA and by XRD and DRIFT spectroscopy measurements, the range of decomposition temperatures is consistent whichever characterization technique is used. This observation demonstrates that the reaction cell is suitable for the thermal treatment of powders. Moreover, the information obtained from DRIFT spectroscopy analysis and XRD measurements is in agreement with the expected results.
It is important to point out that XRD measurements provide a volume analysis of the material, whereas DRIFT spectroscopy is only a surface analysis. Using these two techniques in tandem thus requires extreme care when making comparisons.
For the first time, an XRD system has been combined with a DRIFT spectrometer for laboratory-scale applications. An innovative reaction cell allowing simultaneous XRD and DRIFT spectroscopy characterizations and supporting high temperatures (873 K) and high pressures (18 bar) has also been developed. As different gases are available, heat and gas treatments can be performed on various solid samples. The GC allows the online characterization of gaseous products. This complete device was set up for in operando and in situ characterization of materials under high pressure and at elevated temperatures. To illustrate the use of this innovative analytical tool, the thermal decomposition of calcium oxalate was followed in situ by XRD and DRIFT spectroscopy. This study has showed that there is good correlation between the XRD and DRIFT spectroscopy results, and the data from the analysis are also in good agreement with separate TGA results. This experiment validates the concept of the reaction cell.
The use of this analytical device can be extended to study various materials such as catalysts. The possibility of working under high pressures and high temperatures with various gases gives the opportunity to follow surface and volume properties under working conditions. Online GC analysis allows the monitoring of catalyst activity, selectivity and stability. This device is particularly well adapted to the study of Fischer-Tropsch synthesis on cobalt-based catalysts. The characterization of cobalt catalysts under realistic Fischer-Tropsch synthesis conditions will be detailed in future publications.
IFP Energies Nouvelles is acknowledged for financial support. The authors also thank L. Lemaître from IFPEN for his contribution to the application of DRIFT spectroscopy.
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