Article pubs.acs.org/ac
Simultaneous Nondestructive Analysis of Palladium, Rhodium, Platinum, and Gold Nanoparticles Using Energy Dispersive X‑ray Fluorescence Haidi D. Fiedler,*,† Emma E. Drinkel,† Beatriz Orzechovicz,† Elder C. Leopoldino,† Franciane D. Souza,† Gizelle I. Almerindo,† Cristian Perdona,‡ and Faruk Nome† †
Departamento de Química, Universidade Federal de Santa Catarina, INCT-Catalysis, Florianópolis, SC 88040-900, Brazil Bruker do Brasil, Rodovia D. Pedro I−Km 87.5, Pista Norte−Ponte Alta, Atibaia, SP 12954-260, Brazil
‡
S Supporting Information *
ABSTRACT: A selective method is proposed for the determination of palladium, gold, and sulfur in catalytic systems, by direct liquid analysis using energy dispersive X-ray fluorescence (EDXRF), under an atmosphere of helium or air. This method allows a nondestructive analysis of palladium, rhodium, platinum, and gold nanoparticulate catalysts stabilized by imidazolium propane sulfonate based zwitterionic surfactants, allowing the samples to be reused for catalytic studies. The signals from palladium, rhodium, platinum, and gold samples in the presence of imidazolium propane sulfonate-based zwitterionic surfactants obtained using EDXRF before (Pd2+, Rh2+, Pt2+, and Au3+) and after (Pd0, Rh0, Pt0, and Au0) formation of nanoparticles are essentially identical. The results show that the EDXRF method is nondestructive and allows detection and quantification of the main components of platinum, gold, rhodium, and palladium NPs, including the surfactant concentration, with detection and quantification limits in the range of 0.4− 3 mg L−1. The matrices used in such samples present no problems, even allowing the detection and quantification of interfering elements.
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also similar results can be obtained with both small-angle X-ray scattering (SAXS) and wide-angle X-ray scattering (WAXS). Conversely, powder X-ray diffraction is mostly used to identify the nature of the prepared synthetic samples. Finally, spectroscopic (UV−vis and fluorescence) techniques are sensitive to modification of the structure of the surface, and X-ray photo electron spectroscopy (XPS) allows the examination of the surface and identify elemental composition, oxidation, and nature of the binding ligands.2,14 Despite the advances in the quality and sensitivity of these methods, there is still a strong need for improvement of the quantification of the metallic NPs in aqueous solutions. Based on our experience with analysis of metal ions in the presence of complex matrices,20−24 we decided to explore the use of micelles to stabilize NPs, since the high ionic density of the Stern layer facilitates changes in rate and equilibrium constants which may be applied to analytical studies of anion binding25−28 and/or analytical systems.29,30 Thus, in complex situations and when we are dealing with solutions which are not optically transparent, the use of direct sample analysis of a liquid by energy dispersive X-ray fluorescence (EDXRF) is a particularly interesting possibility.
he quantification of the concentration of nanoparticles in aqueous solutions, using nondestructive (chemical) detection methods for metallic nanoparticles commonly used for catalytical purposes (palladium, platinum, gold, rhodium, and others) is the main purpose of our investigation.1−6 In general, characterization of NPs is centered around methods which focus on thermal stability, characterization of size, elemental composition, and optical properties. Very common approaches for the initial characterization of the samples are thermal gravimetric analysis (TGA) and differential thermal analysis (DTA) which allow the removal of the organic material to be followed and a stability profile as a function of temperature is obtained.7−9 The use of scanning election microscopy (SEM) allows a convenient size and shape analysis and, when the instrumentation is equipped with energydispersive X-ray spectroscopy detectors (EDX), allows the elemental analysis of the NPs,8,10−12 and even more detailed examination can be obtained using scanning transmission election microscopy (STEM) coupled with EDX.2,13 Information on topology of the particles can be obtained with scanning probe microscopy techniques such as atomic force microscopy which can be carried out even in liquid environments.14−16 Scattering analysis techniques, including dynamic light scattering (DLS), allow the determination of the size of NPs in colloidal suspensions16−19 and of the hydrodynamic radius of the NPs either naked or covered with appropriate stabilizers, © 2013 American Chemical Society
Received: May 15, 2013 Accepted: October 3, 2013 Published: October 3, 2013 10142
dx.doi.org/10.1021/ac402419r | Anal. Chem. 2013, 85, 10142−10148
Analytical Chemistry
Article
In an EDXRF analysis, high energy X-rays or γ rays are used to excite the sample and generate X-ray fluorescence radiation, which is detected without being separated first. Since the energies of the radiation represent a fingerprint for each element and the intensities are proportional to the concentration of each element in the sample, EDXRF analysis is ideal to analyze liquid or solid samples, in a nondestructive fashion, with limits of detection in the parts per million range.31 In fact, the range of concentrations detected corresponds well with the concentrations of metal normally used in transition metal catalyzed reactions, i.e., 0.1−1 mol % in 1−2 mL of solvent with 1 mmol of reactants, roughly 50−500 mg/L, which fits well within the limits of the method described herein.32−36 Due to the numerous applications of noble metals in industrial/pharmaceutical processes,37−42 where the metals serve as catalysts, the development of EDXRF metal quantification techniques represents a very useful contribution in many areas of chemistry, such as the analysis of metallic nanoparticles stabilized by ImS3-14 zwitterionic surfactants (Scheme 1) or similar systems.6,31,37−39,43 Zwitterionic
a NANOpure analytical deionization system (type D-4744) or from a Purelab UHQ system were used to prepare standard and reagent solutions. All other reagents were of the best available analytical grade. Analytical Standards and Preparation Procedures. Preparation of Palladium and Gold Standards. Traceable certified reference materials from either TraceCERT or CertiPUR were used for all analytical experiments with palladium, gold, platinum, and rhodium. Standard solutions containing (i) 999 ± 4 mg L−1 of palladium TraceCERT standard for AAS (Fluka-SIGMA-Aldrich; lot BCBD7505), (ii) 1001 ± 3 mg L−1 of gold ICP-OES standard (Au CertiPUR 1.70321.0100; lot HC942080, accredited by the DKD (Deutscher Kalibrierdienst) with control of trace impurities into μg L−1), (iii) 1001 ± 6 mg L−1 of TraceCERT Rhodium Standard for AAS (Fluka-SIGMA-Aldrich; lot BCBG3619 V), and (iv) 1000 ± 4 mg L−1 of TraceCERT platinum standard for AAS (Fluka-SIGMA-Aldrich; lot BCBB4923) were used and diluted as required. Higher concentrations of Pd(II) standard solutions were prepared by weighing palladium(II) oxide (
