Origin of the Normal and Inverse Hysteresis Behavior during CO Oxidation over Pt/Al₂O₃

Abstract

The hysteresis in the CO oxidation profile over Pt/Al2O3 catalysts has been intensively studied, but its origin is still controversial. In this work, the influence of the Pt particle size and size distribution was systematically investigated. By the application of conventional and advanced preparation methods, such as flame spray pyrolysis and supercritical fluid reactive deposition, a series of catalysts containing homogeneous distributions of Pt particles were obtained. An optimal Pt particle size of 2–3 nm was identified for the CO oxidation light-off on Pt/Al2O3 catalysts. The CO oxidation results show a clear correlation between the Pt nanoparticle size and the ignition/extinction profile, including the switch of the hysteresis loop. On the basis of high-angle annular dark-field scanning transmission electron microscopy characterization combined with in situ diffuse reflectance infrared Fourier transform spectroscopy and operando X-ray absorption spectroscopy measurements, the appearance of inverse hysteresis for catalysts containing very small Pt nanoparticles (<2 nm) and typical hysteresis for larger Pt nanoparticles could be related to the different CO adsorption strengthd, to surface/bulk oxidation of Pt particles, and to the ability of the catalyst to regenerate the active sites, considering also the exothermicity of the CO oxidation reaction.

Large‐Pore Mesoporous CeO2–ZrO2 Solid Solutions with In‐Pore Confined Pt Nanoparticles for Enhanced CO Oxidation

“…In order to test and verify this mechanism, an additional ignition/extinction experiment was conducted to compare the catalysis hysteresis behavior of various catalysts in CO oxidation. The different catalytic activity during the light‐off and light‐out procedures was defined as the catalysis hysteresis behavior, which was usually found on Pt/Al 2 O 3 catalysts with a narrow hysteresis loop (Δ T ) in previous reports . Interestingly, our mesoporous Pt/Ce 0.8 Zr 0.2 O 2 shows more visible catalysis hysteresis phenomenon in CO oxidation, presenting a much lower T 100 value of 90 °C and a broader hysteresis loop (Δ T = 40 °C) during the extinction process ( Figure A; Table S5, Supporting Information).…”

“…However, the mesoporous Pt/Al 2 O 3 , Pt/ZrO 2 , and Pt/TiO 2 catalysts show higher light‐off and light‐out temperatures as well as narrower hysteresis loops than mesoporous Pt/CeO 2 and Pt/Ce 0.8 Zr 0.2 O 2 , indicating their superior catalytic activity and enhanced catalysis hysteresis (ECH) behavior for CO oxidation. To the best of our knowledge, few reports have been focused on this ECH phenomenon; even the normal or inverse hysteresis in CO oxidation was studied on Pt/Al 2 O 3 catalysts, which was usually explained as a combination of three possible reasons, such as inherent kinetic bistability, interaction between reaction kinetics and diffusion phenomena, and locally high temperatures on the catalyst surface. The intrinsic surface properties of mesoporous Ce 0.8 Zr 0.2 O 2 solid solutions and interfacial interactions with Pt nanoparticles are quite different from the matrices of mesoporous Al 2 O 3 , ZrO 2 , TiO 2 , and CeO 2 .…”

Impact of Preparation Method and Hydrothermal Aging on Particle Size Distribution of Pt/γ-Al2O3 and Its Performance in CO and NO Oxidation

“…surface has been previously discussed in detail 14 and is only briefly described here. In general, the different preparation methods led to a good distribution of the noble metal component but but has not been carried out here in order to allow better comparability with Pt particles generated in earlier PLAL studies.…”

“…This is due to pronounced changes in the redox behavior and binding energies of the reactants and products [9][10][11] . For Pt/Al2O3 catalysts significant differences in the CO, NO and C3H6 oxidation activity have been reported if the Pt particle size or shape is varied [4][5][12][13][14][15] . An optimal particle size in the 2-3 nm range was proposed to obtain highly active catalysts for CO oxidation 14,16 whereas slightly larger particles are necessary to efficiently convert NO to NO2 5 .…”

“…For Pt/Al2O3 catalysts significant differences in the CO, NO and C3H6 oxidation activity have been reported if the Pt particle size or shape is varied [4][5][12][13][14][15] . An optimal particle size in the 2-3 nm range was proposed to obtain highly active catalysts for CO oxidation 14,16 whereas slightly larger particles are necessary to efficiently convert NO to NO2 5 . Additionally, reduced Pt sites seem to represent the active species for such oxidation reactions 11,16 .…”