Catalytic Purification of Exhaust Gases Over Pd–Rh Alloy Catalysts

Adsorption Forms of NO on Iridium-Doped Rhodium Clusters in the Gas Phase Revealed by Infrared Multiple Photon Dissociation Spectroscopy

The adsorption of an NO molecule on a cationic iridium-doped rhodium cluster, Rh₅Ir⁺, was investigated by infrared multiple photon dissociation spectroscopy (IRMPD) of Rh₅IrNO⁺·Ar_p complexes in the 300-2000 cm^-1 spectral range, where the Ar atoms acted as a messenger signaling IR absorption. Complementary density functional theory (DFT) calculations predicted two near-isoenergetic structures as the putative global minimum: one with NO adsorbed in molecular form in the on-top configuration on the Ir atom in Rh₅Ir⁺, and one where NO is dissociated with the O atom bound to the Ir atom in the on-top configuration and the N atom on a hollow site formed by three Rh atoms. A comparison between the experimental IRMPD spectrum of Rh₅IrNO⁺ and calculated spectra indicated that NO mainly adsorbs molecularly on Rh₅Ir⁺, but evidence was also found for structures with dissociatively adsorbed NO. The estimated fraction of Rh₅IrNO⁺ structures with dissociatively adsorbed NO is approximately 10%, which was higher than that found for Rh₆⁺, but lower than that for Ir₆⁺. The DFT calculations indicated the existence of an energy barrier in the NO dissociation pathway that is exothermic with respect to the reactants, which was considered to prevent NO from dissociating readily on Rh₅Ir⁺. The height of the barrier is lower than that for NO dissociation over Rh₆⁺, which is attributed to the higher binding energy of atomic O to the Ir atom in Rh₅Ir⁺ than to a Rh atom in Rh₆⁺.

The Attractiveness of the Ternary Rh-Pd-Pt Alloys for CO Oxidation Process

Ternary alloys of platinum group metals attract a growing interest due to their unique catalytic properties. The present research is aimed to synthesize a series of Rh-Pd-Pt alloys with varied ratios of metals using a single-source precursor approach. Rhodium and palladium are partly miscible metals, while each of these metals is unlimitedly miscible with platinum. Thermolysis of complex salts used as a precursor results in the formation of metastable systems. The 3D nanostructure alloys are being formed after the complete decomposition of the single-source precursor. High-resolution transmission electron microscopic studies have shown that the nanoalloys are composed of interconnected polycrystalline ligaments with a mean diameter of 50 nm. The single-phase composition is confirmed by an X-ray diffraction analysis. The ratio of metals plays an important role in determining the catalytic activity of alumina-supported alloys and their thermal stability. According to UV-vis spectroscopy data, the higher palladium portion corresponds to worse dispersion of initially prepared, fresh catalysts. Treatment of the catalysts under prompt thermal aging conditions (up to 800 °C) causes redispersion of palladium-rich alloyed nanoparticles, thus leading to improved catalytic activity and thermal stability.

Perovskite lattice oxygen contributes to low-temperature catalysis for exhaust gas cleaning

A Pd catalyst supported on Ba-substituted LaAlO₃ perovskite (Pd/La_0.9Ba_0.1AlO_3−δ) was investigated for NO reduction at low temperature by propylene, which revealed that Pd/La_0.9Ba_0.1AlO_3−δ has remarkably higher activity than other Pd catalysts at low temperatures (≤573 K) for NO reduction by propylene. To elucidate the surface reaction pathway, transient response tests were conducted using ¹⁸O₂. Also, X-ray photoelectron spectroscopy (XPS) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) measurements were conducted. Comparison with a Ba-impregnated catalyst (Pd/Ba/LaAlO₃) demonstrated that Pd/La_0.9Ba_0.1AlO_3−δ shows higher activity for the formation of oxygenated species (C_xH_yO_z) as an intermediate for NO reduction because the surface lattice oxygen has improved mobility via Ba²⁺ substitution in LaAlO₃. Therefore, Pd/La_0.9Ba_0.1AlO_3−δ have high activity for NO reduction, even at low temperatures in a humid condition.

A Pd catalyst supported on Ba-substituted LaAlO₃ perovskite (Pd/La_0.9Ba_0.1AlO_3−δ) has remarkably higher activity than other Pd catalysts at low temperatures (≤573 K) for NO reduction by propylene.

Porous Nanocrystalline Silicon Supported Bimetallic Pd-Au Catalysts: Preparation, Characterization, and Direct Hydrogen Peroxide Synthesis

Bimetallic Pd-Au catalysts were prepared on the porous nanocrystalline silicon (PSi) for the first time. The catalysts were tested in the reaction of direct hydrogen peroxide synthesis and characterized by standard structural and chemical techniques. It was shown that the Pd-Au/PSi catalyst prepared from conventional H₂[PdCl₄] and H[AuCl₄] precursors contains monometallic Pd and a range of different Pd-Au alloy nanoparticles over the oxidized PSi surface. The PdAu₂/PSi catalyst prepared from the [Pd(NH₃)₄][AuCl₄]₂ double complex salt (DCS) single-source precursor predominantly contains bimetallic Pd-Au alloy nanoparticles. For both catalysts the surface of bimetallic nanoparticles is Pd-enriched and contains palladium in Pd⁰ and Pd²⁺ states. Among the catalysts studied, the PdAu₂/PSi catalyst was the most active and selective in the direct H₂O₂ synthesis with H₂O₂ productivity of 0.5 mol gPd-1 h-1 at selectivity of 50% and H₂O₂ concentration of 0.023 M in 0.03 M H₂SO₄-methanol solution after 5 h on stream at −10°C and atmospheric pressure. This performance is due to high activity in the H₂O₂ synthesis reaction and low activities in the undesirable H₂O₂ decomposition and hydrogenation reactions. Good performance of the PdAu₂/PSi catalyst was associated with the major part of Pd in the catalyst being in the form of the bimetallic Pd-Au nanoparticles. Porous silicon was concluded to be a promising catalytic support for direct hydrogen peroxide synthesis due to its inertness with respect to undesirable side reactions, high thermal stability, and conductivity, possibility of safe operation at high temperatures and pressures and a well-established manufacturing process.

CO Oxidation over Pd/ZrO₂ Catalysts: Role of Support's Donor Sites

A series of supported Pd/ZrO₂ catalysts with Pd loading from 0.2 to 2 wt % was synthesized. The ZrO₂ material prepared by a similar technique was used as a reference sample. The samples have been characterized by means of transmission electron microscopy (TEM), X-ray diffraction analysis (XRD), X-ray photoelectron spectroscopy (XPS), temperature-programmed reduction (TPR), testing reaction of ethane hydrogenolysis (HGE), N₂ adsorption, and electron paramagnetic resonance (EPR) spectroscopy. 1,3,5-trinitrobenzene was used as a probe molecule for the EPR spin probe method. The catalytic performance of samples was tested in the model reaction of CO oxidation. It was shown that the concentration of donor sites of support measured by EPR spin probe correlates with catalytic behavior during light-off tests. Low concentration of donor sites on a support’s surface was found to be caused by the presence of the specific surface defects that are related to existence of coordinately unsaturated structures.

Improved three-way catalytic activity of bimetallic Ir–Rh catalysts supported on CeO2–ZrO2

The addition of a small amount of Ir caused a significant increase in the TWC performance of Rh/CeO₂–ZrO₂.