Platinum Sintering on H‐ZSM‐5 Followed by Chemometrics of CO Adsorption and 2D Pressure‐Jump IR Spectroscopy of Adsorbed Species

Abstract

Jump to it! 2D IR pressure‐jump spectroscopy of adsorbed CO can be used to assess the accessibility and location of platinum nanoparticles supported on a zeolite and investigate the behavior of Pt atoms after thermal treatment (activation, catalytic reaction, sintering). Different Pt nanoparticle sintering mechanisms were observed in the mesopores and on the external surface of the ZSM‐5 support (see picture; HF=high and LF=low frequency).

Probing zeolites by vibrational spectroscopies

“…The bands decreased with time constants of 113 AE 3 ms (2167 cm À1 ) and 155 AE 15 ms (2105 cm À1 ), which suggests that CO in these sites is additionally trapped by surrounding diphenylacetylene co-product and/or precursor molecules. [408][409][410][411][412][413][414][415][416][417] Subsequently, the 2D-PJAS-IR technique was applied by Rivallan et al 964 to the adsorption of CO on Pt-ZSM-5 catalyst. 957 The finding that CO escapes from the mesopores at room temperature in less than a millisecond shows that the nanoporous silica environment of MCM-41 will not limit photocatalytic turnover rates and is favorable in terms of preventing back reactions.…”

Pt speciation on ZSM-5 by IR of probes co-adsorption and chemometric analysis

“…This is consistent with the fact that catalyst preparation by colloids impregnation leads to spherical particles with a narrow size distribution. As shown recently by the 2D-PJAS-IR technique [9], the Pt nanoparticles on the fresh catalyst are located on the external surface of the ZSM-5 support since the colloids are too large (about 4 nm) to penetrate into the porosity of the zeolite. The thin IR band centred at 2092 cm À1 for the fresh sample is characteristic of linearly adsorbed CO on metallic Pt [20,21].…”

“…For instance, platinum supported on zeolites has been found extremely active in oxidation reactions [3][4][5], where the Pt atoms form the core of the catalytic mechanisms. The activity of these metal centres depends on many parameters, such as the preparation method (metal precursor, zeolite acidity and thermal treatment), the level of Pt unsaturation and the metal location [6][7][8][9]. Indeed, the metal speciation on zeolites can be rather complex, since the metal atoms could be mainly found as grafted cations in exchange position of the crystalline framework [10,11] and/or as supported nanoparticles.…”

“…In such case, CO chemisorption is the most convenient tool, since it allows discriminating the full set of Pt atomic surface species present on the support. Important additional information is then also available such as dispersion, metal coordination and support interactions in the case of Pt nanoparticles [8,9,[12][13][14][15][16][17]. But the spectroscopic data interpretation becomes rather difficult when Pt atoms are also found in exchange positions (mono-atomic species) on the same metal catalyst, simply because the respective IR components are overlapping [10].…”

“…Another possibility consists in a chemometric analysis of the IR spectra (obtained during incremental addition of the desired probe molecule) into reference spectra. This data treatment technique could decompose the overall spectra' evolution into the evolution for the probe molecule adsorbed on each individual Pt sites, since each population reacts differently toward probe molecule concentration increase or thermal desorption [9].…”