Kamran Qadir

Intrinsic Relation between Catalytic Activity of CO Oxidation on Ru Nanoparticles and Ru Oxides Uncovered with Ambient Pressure XPS

Recent progress in colloidal synthesis of nanoparticles with well-controlled size, shape, and composition, together with development of in situ surface science characterization tools, such as ambient pressure X-ray photoelectron spectroscopy (APXPS), has generated new opportunities to unravel the surface structure of working catalysts. We report an APXPS study of Ru nanoparticles to investigate catalytically active species on Ru nanoparticles under oxidizing, reducing, and CO oxidation reaction conditions. The 2.8 and 6 nm Ru nanoparticle model catalysts were synthesized in the presence of poly(vinyl pyrrolidone) polymer capping agent and deposited onto a flat Si support as two-dimensional arrays using the Langmuir-Blodgett deposition technique. Mild oxidative and reductive characteristics indicate the formation of surface oxide on the Ru nanoparticles, the thickness of which is found to be dependent on nanoparticle size. The larger 6 nm Ru nanoparticles were oxidized to a smaller extent than the smaller Ru 2.8 nm nanoparticles within the temperature range of 50-200 °C under reaction conditions, which appears to be correlated with the higher catalytic activity of the bigger nanoparticles. We found that the smaller Ru nanoparticles form bulk RuO(2) on their surfaces, causing the lower catalytic activity. As the size of the nanoparticle increases, the core-shell type RuO(2) becomes stable. Such in situ observations of Ru nanoparticles are useful in identifying the active state of the catalysts during use and, hence, may allow for rational catalyst designs for practical applications.

Ultrathin titania coating for high-temperature stable SiO2/Pt nanocatalysts

The facile synthesis of silica supported platinum nanoparticles with ultrathin titania coating to enhance metal-support interactions suitable for high temperature reactions is reported, as thermal and structure stability of metal nanoparticles is important for catalytic reactions.

Nature of Rh Oxide on Rh Nanoparticles and Its Effect on the Catalytic Activity of CO Oxidation

Surface oxide layers formed on transition metal catalysts are well known as one of the controlling factors in enhancing or suppressing the catalytic activity of metal catalysts. We investigated the growth of a surface oxide layer on two sizes of Rh metal nanoparticles (NPs) as a function of UV–ozone (UV/O3) dosing as well as how the oxide layer formed on the surface of the Rh NPs affects the catalytic activity for CO oxidation. Monodisperse Rh NPs were synthesized via one-pot polyol reduction using poly(vinylpyrrolidone) as a capping agent. Varying the concentration of the Rh precursors controlled the size of the NPs. The changes that occurred as a function of UV/O3 dosing were characterized using X-ray photoelectron spectroscopy, which showed that the oxidation state increased with increasing surface modification time. The catalytic activity and activation energy of the two-dimensional Rh NPs arrays were measured as the UV/O3 exposure time increased. Our reaction studies indicate that the turnover rate of CO oxidation on the Rh NPs is enhanced as the quantity of the surface oxide layer formed during UV/O3 surface treatment increases, indicating that the oxides grown on the surface of the Rh metal are catalytically active. These results suggest an intriguing way to tune catalytic activity via engineering of the nanoscale surface oxide.Graphical Abstract

Pt/oxide nanocatalysts synthesized via the ultrasonic spray pyrolysis process: engineering metal–oxide interfaces for enhanced catalytic activity

We show that Pt nanoparticles synthesized on oxide nanocatalysts exhibit catalytic activity enhancement depending on the type of the oxide support. To synthesize the Pt/oxide nanocatalysts, we employed a versatile synthesis method using Pt nanoparticles (NPs) supported on various metal oxides (i.e., SiO2, CeO2, Al2O3, and FeAl2O4) utilizing ultrasonic spray pyrolysis. Catalytic CO oxidation was carried out on these catalysts, and it was found that the catalytic activity of the Pt NPs varied depending on the supporting oxide. While Pt/CeO2 exhibited the highest metal dispersion and active surface area, Pt/FeAl2O4 exhibited the lowest active surface area. Among the Pt/oxide nanocatalysts, Pt NPs supported on CeO2 showed the highest catalytic activity. We ascribe the enhancement in turnover frequency of the Pt/CeO2 nanocatalysts to strong metal–support interactions due to charge transport between the metal catalysts and the oxide support. Such Pt/oxide nanocatalysts synthesized via spray pyrolysis offer potential possibilities for large-scale synthesis of tailored catalytic systems for technologically relevant applications.

Role of Surface Oxides on Model Nanocatalysts in Catalytic Activity of CO Oxidation

Rapid advances in the nanosciences and colloidal chemistry have generated new opportunities in the fields of physical and chemical science, including tuning the size, shape, and composition of noble metals at nanoscale, which have revealed many interesting properties. Studies identifying molecular factors that affect catalytic activity provide the means to control catalytic activity, a significant achievement in catalysis. Several molecular factors, including structural and electronic effects, metal–support interactions, and the presence of a surface oxide layer, have been reported as candidates for improving catalytic activity. Among these factors, the oxide layer on the metal surface is considered to play an important role in determining catalytic activity and there are a growing number of studies in this area. Understanding the chemical reactivity of a metal oxide is a rather complicated issue, requiring significant research to date. We outline here recent experimental work on the role of surface oxide on metal nanoparticles (NPs) that determines the catalytic activity of heterogeneous catalysis, including the effect of oxidation states of nanoparticles on the catalytic activity for model catalysts of single crystals and nanoparticles, with several examples, including Pt, Rh, Ru, and Pd.