In situ and operando studies of active phase in supported palladium nanocatalysts
Metal nanoparticles play an important role in catalysis. For hydrogenation of hydrocarbons, such as alkynes and alkenes, palladium is recognized as the preferred catalyst. In reaction conditions, palladium nanoparticles may undergo phase changes to hydride and carbide phases, whose natures affect the catalytic properties. Therefore, determining the hydride and carbide formation during a catalytic process becomes an important problem, also relevant to industry. Being a subject of numerous theoretical and experimental studies, palladium hydride is one of the most-studied metal hydrides. In contrast to the hydride, the structure and properties of the carbide phase are still under discussion. The aim of the current work is to present a systematic approach for in situ and operando investigation of the formation of hydride and carbide phases in an industrial palladium-based catalyst under realistic working conditions.
The formation of both hydride and carbide phases is accompanied with an expansion of the palladium lattice, which can be followed by X-ray powder diffraction (XRPD) and extended X-ray absorption fine structure (EXAFS). EXAFS is an element-selective technique and demonstrates high sensitivity to the local structure changes, such as bond distances and coordination numbers of the absorbing atom. In the case of the palladium containing material, whose K- absorption edge is 24357 eV, hard X-rays can be easily applied to perform in situ and operando experiments. One should take into consideration that both EXAFS and XRPD are least sensitive to light atoms, such as carbon and especially hydrogen, due to their low scattering amplitudes compared to palladium. Thus, palladium hydride and carbide phases are observed by these techniques only indirectly, via Pd-Pd distance elongation or lattice expansion.
X-ray absorption near-edge structure (XANES) include a part of the absorption spectrum up to 30 – 50 eV above the absorption edge. It was shown that the formation of palladium hydride directly affects the shape X-ray absorption near edge structure due to mixing of unoccupied states of hydrogen and palladium. This makes XANES a promising tool for in situ investigation of palladium hydride and carbide formation, because unlike EXAFS and XRPD this method is sensitive to the presence of light atoms. For the first time, we have reproduced the changes in XANES by theoretical simulations applying Monte-Carlo approach.
In the presence of hydrogen the first near-edge peak at 24369 eV becomes narrower and is shifted by 1 eV to lower energy. Formation of palladium carbide leads to a broadening of the same peak and shifts it by 1 eV to higher energy. This opposite behavior of XANES spectra during hydride and carbide formation is more pronounced in the difference spectra and provides us a method to discriminate between the two phases under hydrogenation reaction conditions. To test this thesis we investigated the system sending at 100 °C a mixture of 650 mbar of hydrogen and 350 mbar of acetylene. Comparing the XANES spectrum collected under hydrogenation reaction conditions with those obtained sending at 100 °C the pure reagents separately (100 and 600 mbar of H2 or 1000 mbar of C2H2) we can conclude that hydride phase is formed, having a similar composition to that obtained sending 600 mbar of pure hydrogen at the same temperature.
In summary, we have demonstrated a scheme to obtain structural information on supported palladium nanoparticle relevant in catalysts by applying in situ and operando X-ray absorption and powder diffraction techniques. Analysis of XANES spectra allows determining whether hydride or carbide phase is formed in the nanoparticles. Possibility to extract this information from XANES becomes extremely important in the cases, when the catalyst is exposed to a mixture of gasses, and the type of the phase (carbide or hydride) cannot be predicted in advance. In addition to the type of the phase determined from XANES, and interatomic distance obtained from EXAFS, XRPD analysis gives quantitative information on the phase concentrations.
The described scheme of experimental measurements and analysis may be applied to investigate the structural evolution of supported metal nanoparticle catalyst during catalytic processes in operando conditions, such as hydrogenation of hydrocarbons and will allow correlating catalytic properties of the nanoparticles with their structure and phasing content.
List of published papers:
- A. L. Bugaev, A. A. Guda, A. Lazzarini, K. A. Lomachenko, E. Groppo, R. Pellegrini, A. Piovano, H. Emerich, A. V. Soldatov, L. A. Bugaev, V. P. Dmitriev, J. A. van Bokhoven, C. Lamberti "In Situ Formation of Hydrides and Carbides in Palladium Catalyst: When XANES is better than EXAFS and XRD" Catalysis Today 2016 In press (Impact-factor: 4.030) DOI: 10.1016/j.cattod.2016.02.065
- Andrea Piovano, Andrea Lazzarini, Riccardo Pellegrini, Giuseppe Leofanti, Giovanni Agostini, Svemir Rudić, Aram L. Bugaev, Carlo Lamberti, and Elena Groppo "Progress in the Characterization of the Surface Species in Activated Carbons by means of INS Spectroscopy Coupled with Detailed DFT Calculations" Advances in Condensed Matter Physics 2015 2015 (1) 803267 (Impact-factor: 0.862) DOI: 10.1155/2015/803267
- Aram L. Bugaev, Alexander A. Guda, Kirill A. Lomachenko, Lusegen A. Bugaev, Alexander V. Soldatov "Pd hydride and carbide studied by means of Pd K-edge X-ray absorption near-edge structure analysis" Bulletin of the Russian Academy of Sciences. Physics 2015 79 (9) 1180-1185 (Impact-factor: 0.313) DOI:10.3103/s1062873815010098
- Aram L. Bugaev, Alexander A. Guda, Kirill A. Lomachenko, Vasiliy V. Srabionyan, Lusegen A. Bugaev, Alexander V. Soldatov, Carlo Lamberti, Vladimir P. Dmitriev, Jeroen A. van Bokhoven "Temperature-and Pressure-Dependent Hydrogen Concentration in Supported PdH x Nanoparticles by Pd K-Edge X-ray Absorption Spectroscopy" The Journal of Physical Chemistry C 2014 118 (19) 10416-10423 (Impact-factor: 4.772) DOI: 10.1021/jp500734p
- Vasiliy V. Srabionyan, Aram L. Bugaev, Vasiliy V. Pryadchenko, Leon A. Avakyan, Jeroen A. van Bokhoven, Lusegen A. Bugaev "EXAFS study of size dependence of atomic structure in palladium nanoparticles" Journal of Physics and Chemistry of Solids 2014 75 (4) 470-476 (Impact-factor: 1.517) DOI: 10.1016/j.jpcs.2013.12.012
- Aram L. Bugaev, Vasiliy V. Srabionyan, Alexander V. Soldatov, Lusegen A. Bugaev, Jeroen A. van Bokhoven "The role of hydrogen in formation of Pd XANES in Pd-nanoparticles" Journal of Physics: Conference Series 2013 430 (1) 012028 DOI: 10.1088/1742-6596/430/1/012028