Influence of Hydrogen Pressure on the Structure of Platinum–Titania Catalysts

Abstract

Dependent on the application or characterization method catalysts are exposed to different gas pressures, which results in different structures. The quantitative determination of the structure and composition of a catalyst as a function of its gas environment allows the establishment of structure−performance relationships. Herein, we determine the structure of a platinum− titania catalyst under hydrogen during temperature-programmed reduction over 3 orders of magnitude in pressure, from 1 to 950 mbar. The pressure significantly influences the hydrogen uptake kinetics and the consecutive structural transformations of the platinum−titania catalyst. The reduction of the platinum precursor becomes pressure-independent above 30 mbar. Yet, the related spillover and stability of adsorbed hydrogen on the titania are a function of pressure. Higher pressures promote higher hydrogen uptake and prevent desorption of hydrogen from the catalyst. The hydrogen uptake triggers a phase transformation of anatase to rutile which is, as a result, pressure dependent. The presented systematic approach establishes a pressure−structure relation which can be applied for the catalyst treatment and to frame existing results on the catalytic system. Treating the same material at two different pressures will lead to different structures.

Dynamic interplay between metal nanoparticles and oxide support under redox conditions

“…Our work bridges the pressure gap from the low-pressure, quasistatic regime to a regime in which the chemical potential of the involved species (H 2 and O 2 ) is sufficient to induce more substantial redox dynamics. At the applied pressure, activated hydrogen can easily remove oxygen from the underlying TiO 2 support ( 53 ). Overall, this process resembles the Mars van Krevelen mechanism, as oxygen from the catalyst ends up in the product ( 73 ).…”

“…The simultaneous presence of reducing and oxidizing agents induced redox processes that lead to destabilization and overlayer retraction. Hydrogen is easily activated on platinum, but activation on TiO 2 is much more unlikely (53)(54)(55). Its addition to the feed gas triggered oxygen abstraction from the overlayer, thereby destabilizing and finally stripping it from the particle surface.…”

Particle-Size-Dependent Electronic Metal–Support Interaction in Pd/TiO2 Catalysts for Selective Hydrogenation of 3-Nitrostyrene

“…It is worth noting that the mean particle size of 1%Pd/TiO 2 –H800 is much greater than that of 1%Pd/TiO 2 –H400 and 1%Pd/TiO 2 –H600. We found that the surface area of the support TiO 2 treated in hydrogen atmosphere at 800 °C were markedly smaller than that of TiO 2 and TiO 2 treated in hydrogen atmosphere at 400 °C (Figure S4 and Table S1), which led to the formation of larger particles over 1%Pd/TiO 2 –H800 by the aggregation of smaller Pd particles …”

“…We found that the surface area of the support TiO 2 treated in hydrogen atmosphere at 800 °C were markedly smaller than that of TiO 2 and TiO 2 treated in hydrogen atmosphere at 400 °C (Figure S4 and Table S1), which led to the formation of larger particles over 1%Pd/ TiO 2 −H800 by the aggregation of smaller Pd particles. 36 Selective hydrogenation of 3-NS was conducted at 30 °C over the Pd/TiO 2 catalysts to study the particle size effects on the catalytic behaviors. The bare TiO 2 showed no activity.…”

Surface Noble Metal Concentration on Ceria as a Key Descriptor for Efficient Catalytic CO Oxidation

“…During the last decades, the role of the oxide support was reassessed from a broader perspective: the carrier also plays a key role for the catalytic activity, selectivity, and stability of catalysts. − For instance, redox active supports like CeO 2 − have been intensively investigated, and different descriptors are proposed to influence catalytic performance: morphology/exposed facets, − porosity, − reducibility of surface and bulk species, ,− or noble metal particle size. − Furthermore, the strong noble metal−support interaction − is reported to tremendously affect the activity profile. Recent in-depth in situ and operando characterization and theoretical modeling revealed the inherent dynamics of such catalytic systems under reaction conditions, which further increases their complexity. ,,− This includes the evolution of the platinum species themselves but also noble-metal induced reconstruction of the support. ,,,− The dynamics make the catalyst a strongly changing system that responds to the surrounding conditions. ,,− Central to the Pt/CeO 2 catalyst is the formation of isolated Pt species on the CeO 2 surface when the catalyst is exposed to oxygen at high temperatures. This phenomenon leads to the disintegration of the more active Pt nanoparticles. ,,− The reverse process occurs under reducing conditions, when monatomic Pt species reagglomerate to form Pt nanoparticles. ,, Both processes occur in emission control catalysis.…”