ARTICLE pubs.acs.org/JPCC Combining X-ray Absorption and X-ray Diffraction Techniques for in Situ Studies of Chemical Transformations in Heterogeneous Catalysis: Advantages and Limitations ,† ‡ ‡ ‡ §,|| § || § A. I. Frenkel,* Q.§ Wang, N. Marinkovic,§ J. G. Chen, L. Barrio, R. Si, A. Lopez Camara, A. M. Estrella, J. A. Rodriguez, and J. C. Hanson*, † Department of Physics, Yeshiva University, New York, New York 10016, United States ‡ Department of Chemical Engineering, University of Delaware, Newark, Delaware 19716, United States § Department of Chemistry, Brookhaven National Laboratory, Upton, New York 11973, United States Instituto) de Catalisis y Petroleoquímica, CSIC, Marie Curie 2, 28049 Madrid, Spain bS Supporting Information ABSTRACT: Recent advances in catalysis instrumentations include synchrotron- based facilities where time-resolved X-ray scattering and absorption techniques are combined in the same in situ or operando experiment to study catalysts at work. To evaluate the advances and limitations of this method, we performed a series of experiments at the new XAFS/XRD instrument in the National Synchrotron Light Source. Nearly simultaneous X-ray diffraction (XRD) and X-ray absorption fine- structure (XAFS) measurements of structure and kinetics of several catalysts under reducing or oxidizing conditions have been performed and carefully analyzed. For CuFe2O4 under reducing conditions, the combined use of the two techniques allowed us to obtain accurate data on kinetics of nucleation and growth of metallic Cu. For the inverse catalyst CuO/CeO2 that underwent isothermal reduction (with CO) and oxidation (with O2), the XAFS data measured in the same experiment with XRD revealed strongly disordered Cu species that went undetected by diffraction. These and other examples emphasize the unique sensitivity of these two complementary methods to follow catalytic processes in the broad ranges of length and time scales. I. INTRODUCTION data-collection devices. A recent in situ study of the active phase À of CuO CeO2 catalysts during the water gas shift (WGS) Investigation of the structure, dynamics, and electronic prop- 11 erties of catalysts in real time and under reaction conditions is reaction illustrates both the power, and the limitations, of essential for elucidating reaction pathways and kinetics. Among time-resolved XRD and time-resolved XAFS at separate beam- other techniques, synchrotron X-ray absorption fine-structure lines available for the characterization of catalysts at the National (XAFS) spectroscopy and diffraction (XRD) are the ones most Synchrotron Light Source (NSLS). In that work, the partial reduction of cerium in the ceria was determined from very commonly used for in situ and operando research. Using time- ff resolved XRD, one can get information concerning the following di erent measurements. Ce L3 edge XANES spectra measured fi at X18B beamline as a function of temperature under WGS properties: phase identi cation, fractional composition, crystal- 3+ line properties, and molecular structure of catalysts under reaction conditions showed the variation of the relative amounts of Ce and Ce4+ cations. The XRD data, obtained at the X7B beamline, conditions, as well as kinetics of crystallization of nanoparticles 3+ and bulk solids. also indicated, albeit in a less direct way, that the amount of Ce Time-resolved XAFS allows one to follow the structural in the catalyst varies with the CO/H2O ratio. That result was changes in the reactions with characteristic times ranging from inferred from the change in the cerium oxide lattice parameter minutes to tens of milliseconds. Most applications of time- at constant temperature. The lattice is sensitive to the presence À 3+ resolved XAS use either energy dispersive1 5 or energy scanning of Ce and expands (or contracts) when the concentration of 6À9 this cation increases (or decreases), in accordance with the mode. ’ For more than 10 years researchers have been conducting Vegard slaw. subminute, time-resolved in situ XRD experiments under a wide variety of temperature and pressure conditions (80 K < T < Received: June 2, 2011 1200 K; P < 50 atm).10 These advances result from combining Revised: August 4, 2011 the high intensity of synchrotron radiation with new parallel Published: August 05, 2011 r 2011 American Chemical Society 17884 dx.doi.org/10.1021/jp205204e | J. Phys. Chem. C 2011, 115, 17884–17890 The Journal of Physical Chemistry C ARTICLE That work (ref 11) demonstrates that the both sets of data XAFS.17 Quick EXAFS (QEXAFS) data can be collected in 0.5 s provide information about vacancies in the catalysts, but there are (for concentrated samples), and in 5À15 s for dilute (∼1wt% important details about the system that are missing in each of catalyst loading) ones. them. XAFS is sensitive to the local structure only, within a few Three X-ray detectors were used in the XRD measurements coordination shells around the absorbing atom. In systems with described in this work. They include an INEL linear curved small to moderate disorder, XAFS can characterize local structure detector mounted on the 2θ arm, a silicon linear detector accurately even if the long-range order is absent. However, (SiLD), and a Perkin Elmer (PE) amorphous silicon detector. because of its local nature, XAFS is not capable of characterizing SiLD is a microstrip array of silicon diodes (800 elements, 0.125 the volume of each structurally/compositionally different region  4mm2 in area per element), designed and made at BNL.18 It is in the sample, nor can it tell whether these regions are ordered or mounted on the 2θ arm and can be used in stationary mode. It disordered. On the other hand, Bragg diffraction originates from has ca. 10° acceptance angle, the readout time (low limit) is 0.03 coherent scattering and thus requires long-range periodicity s, and the typical exposure time in the experiments described within a region at least a few unit cells in size. Thus, it will detect below is 30 s. A PE detector is the one most commonly used. It ordered phases only but will underestimate the contribution of has 2048  2048 pixels and a 200  200 μm2 pixel size. The strongly disordered and/or low dimensional phases, as well as detector records patterns with an area of 40  40 cm2. The metastable reaction intermediates. Such contributions are a very detector is mounted on the 2θ arm, the typical 2θ range is 5 to common phenomenon in catalytic processes. 48°, and the typical exposure in the experiments described below A recent example is the work by Piovano et al.,12 studying was 30 s. fl electrochemical oxygen intercalation in SrFeO2.5 by separately The XAFS data were usually collected in the uorescence using Fe K-edge extended XAFS (EXAFS) and neutron diffraction mode with a passivated implanted planar silicon (PIPS) detector. in the same system. No disordered contribution was observed in For concentrated samples, where fluorescence signal suffers from the diffraction experiment in this system during its electrochemical thickness effect, a steady state measurement is taken as well, in oxidation. However, principal component analysis of XANES and fluorescence and transmission modes. Such measurement is then EXAFS data has shown the presence of an intermediate phase. By used to recover the constant amplitude factor that, to a good comparing the results of these two experiments, the authors of ref approximation, corrects for the thickness effect.19 12 concluded that the unknown phase is strongly disordered, The reaction products of the in situ experiments described which explains why it was observed by XAFS and not XRD.13 To below are measured with a 0À100 amu quadruple mass spectro- avoid such factors as irreproducibility of reaction conditions (due meter (QMS, Stanford Research Systems). More details about to, e.g., thermal/compositional gradients in the reactor and/or this setup and additional detectors available at X18A beamline for radiation-induced changes of the electronic state or structure of the combined QEXAFS/XRD can be found in ref 17. sample) and to obtain internally consistent structural and kinetic information about catalytic reaction, one needs to combine fast XAFS and fast XRD in a single experiment under in situ condi- III. STRUCTURAL TRANSFORMATIONS OF CUFE2O4 tions, coupled with online product analysis.14 UNDER CO FLOW AT ELEVATED TEMPERATURES 15 8 The pioneering work of Thomas and Clausen on combin- Cuprospinel CuFe2O4 is a well-known precursor for copper ing XRD and XAFS in one single experiment with enough time catalysts with high thermal stability and activity. It is used as a resolution to follow the kinetics of structural changes of solids catalyst in hydrogen production and thermal decomposition of under in situ conditions opened the door to the employment of a ammonium perchlorate and is potentially useful for the WGS powerful characterization tool for unraveling complex structural reaction.20 The copper spinel has an inverse structure in which transformations and its application to mixed oxides systems. the Cu2+ cations occupy the octahedral positions of the tetra- Further developments in the technique increased the resolution gonal unit cell whereas Fe2+ and Fe3+ cations split between of both the XRD and XAFS measurements.16 In this article we octahedral and tetrahedral positions of the crystalline cell. The introduce an experimental setup (section II) for such investiga- reduction of this copper spinel with hydrogen and carbon tions built at beamline X18A of the NSLS and demonstrate its monoxide has been previously studied by separate XRD and applicability for in situ studies of catalytic reactions. Examples XAFS measurements.20 Copper cations segregate out of the shown below will emphasize complementary qualities of each structure under reducing conditions at high enough tempera- technique by pointing out what information revealed by one tures and, again under oxidizing conditions, copper readily technique is not accessible by the other. This complementarity reincorporates back into the spinel structure.