Combining Time-Resolved Hard X-ray Diffraction and Diffuse Reflectance Infrared Spectroscopy To Illuminate CO Dissociation and Transient Carbon Storage by Supported Pd Nanoparticles during CO/NO Cycling Mark A. Newton,* ,† Marco Di Michiel, † Anna Kubacka, ‡ and Marcos Ferna ´ ndez-Garcı ´a* ,‡ ESRF, 6 Rue Horowitz, BP-220, Grenoble F-38043, France, and Instituto de Cata ´lisis y Petroleoquı ´mica, CSIC, C/Marie-Curie 2, 28049 Madrid, Spain Received December 21, 2009; E-mail: newton@esrf.fr; mfg@icp.csic.es A novel combination of time-resolved hard X-ray diffraction (HXRD) with diffuse reflectance infrared spectroscopy (DRIFTS) and mass spectrometry (MS) that takes advantage of the intrinsic properties of very high energy X-rays has been developed for use in studying gas-solid interactions. The method has allowed us to specify how Al 2 O 3 -supported Pd nanoparticles (∼3 nm diameter) actively participate in prototypical de-NO x chemistry. We show that unresolved issues in the behavior of this system observed using time-resolved extended X-ray absorption fine structure (EXAFS) 1 during CO/NO cycling are due to the dissociation of adsorbed CO species and transient storage of atomic carbon within the Pd nanoparticles, forming PdC x . This bulk PdC x phase is then rapidly removed during the oxidizing (NO) part of the cycle. Furthermore, it is shown that the formation of PdC x promotes the formation of linear CO species on the surface of the changing nanoparticles and has a drastic effect on the linear-to-bridge ratio observed in the IR spectra. Many functional materials, including heterogeneous catalysts, show structural variation on a wide range of length scales, from chemical bonds through nanosized component phases to the micrometer scale. This provides a strong driving force for devising experiments capable of addressing these varied length scales and provide “holistic” structure-function relationships. 2 As such, the sequential combination of X-ray methods (principally diffraction/ scattering and EXAFS) has a long history. 3–6 These previous methods have generally relied upon X-ray energies below 20 keV and/or a restricted number of sample environment types (quartz capillaries and tubes) 4–7 to obtain the angular range necessary for diffraction measurements. Even recent studies that have started to exploit high-energy X-rays for diffraction 7,8 and total scattering analyses 9 of working catalysts have retained this methodology. Here we take a different approach that permits synchronous investigation of the sample using DRIFTS in order to achieve the same net objective within a highly restrictive sample environment. At very high X-ray energies, because of the Bragg diffraction condition (λ/d ) 2 sin θ), a compression of diffraction peaks into smaller solid angles occurs. As a result, at ∼86.8 keV (λ ) 0.143 Å) we can collect a large Q (Å -1 ) range [see the Supporting Information (SI)] using an experimental setup subtending only ca. (10° previously developed 10 for combining transmission EXAFS (a technique that has no solid-angle requirements for data collection) with DRIFTS and MS. Our previous investigations allowed us to start to understand the dynamic structure-reactivity behavior of small supported Pd nanoparticles. 1 However, a number of observa- tions remained to be precisely quantified (e.g., size/shape/disorder change) or even explained. In the latter category, the source of a pronounced fall and then recovery of the first-shell Pd-Pd EXAFS coordination number was left unresolved from structural and reactive points of view. 1 An example of this behavior (as observed using dispersive EXAFS) is shown in Figure 1 (top right, first- shell Pd-Pd central depression) for a 2 wt % Pd/Al 2 O 3 sample during CO/NO cycling at 673 K. Alongside the dispersive EXAFS data, Figure 1 shows a salient portion (see the SI) of the HXRD data that can now be obtained simultaneously with the DRIFTS and MS measurements in the same apparatus. The data for a single CO/NO cycle were extracted from an overall experiment of the type previously described 1 (see the SI). † ESRF. ‡ Instituto de Cata ´lisis y Petroleoquı ´mica. Figure 1. Illustrative data from the four probes DRIFTS, EXAFS, MS, and HXRD (the [311] reflection from Pd, straddled by two Al 2 O 3 reflections) that can be applied in situ using the current methodology. Each of the X-ray techniques may be used in simultaneous time-resolved conjunction with DRIFTS and MS. The data from each technique pertain to the structure- reactivity behavior of a 2 wt % Pd/Al 2 O 3 sample during a single switch from 5% CO/He to 5% NO/He at 673 K (see the SI). Published on Web 03/12/2010 10.1021/ja9107512  2010 American Chemical Society 4540 9 J. AM. CHEM. SOC. 2010, 132, 4540–4541