Structural evolution of NiAu nanoparticles under ambient conditions directly revealed by atom-resolved imaging combined with DFT simulation.

From an economic point of view, the structural stability of noble-transition bimetallic catalysts is as significant as their well-studied catalytic efficiency. The structural evolution and corresponding dynamics of NiAu bimetallic nanoparticles under ambient conditions are investigated using in situ Cs-corrected STEM and DFT calculations. During oxidization, the Au component promotes dissociation of oxygen and initiates Ni oxidization, which simultaneously drives the migration of Au atoms, thus yielding multi-shell structures (denoted by Ni@Au@NiO). The subsequent hydrogen reduction induces surface reconstruction, forming fcc-NiAu clusters. After several cycles of catalyzing CO oxidization, both inverse Au segregation and Ni recrystallization occur, which are ascribed to exothermic excitation. The results of this study can help researchers understand the evolutionary behaviors of the bimetallic nanoparticles under ambient conditions as well as optimize the structural design of bimetallic catalysts.

In situ tracing of atom migration in Pt/NiPt hollow spheres during catalysis of CO oxidation.

NiPt hollow spheres decorated by Pt nanoparticles were synthesized by a facile wet chemical route through galvanic replacement. In situ STEM imaging and 3D reconstruction were performed to evidence the migration of Pt atoms during catalysis of CO oxidation, providing a practical insight into the structural stability of bimetallic catalysts.

Improvement of activity and SO2 tolerance of Sn-modified MnOx-CeO2 catalysts for NH3-SCR at low temperatures.

The performances of fresh and sulfated MnOx-CeO2 catalysts for selective catalytic reduction of NOx by NH3 (NH3-SCR) in a low-temperature range (T < 300 °C) were investigated. Characterization of these catalysts aimed at elucidating the role of additive and the effect of sulfation. The catalyst having a Sn:Mn:Ce = 1:4:5 molar ratio showed the widest SCR activity improvement with near 100% NOx conversion at 110-230 °C. Raman and X-ray photoelectron spectroscopy (XPS) indicated that Sn modification significantly increases the concentration of oxygen vacancies that may facilitate NO oxidation to NO2. NH3-TPD characterization showed that the low-temperature NH3-SCR activity is well correlated with surface acidity for NH3 adsorption, which is also enhanced by Sn modification. Furthermore, as compared to MnOx-CeO2, Sn-modified MnOx-CeO2 showed remarkably improved tolerance to SO2 sulfation and to the combined effect of SO2 and H2O. In the presence of SO2 and H2O, the Sn-modified MnOx-CeO2 catalyst gave 62% and 94% NOx conversions as compared to 18% and 56% over MnOx-CeO2 at temperatures of 110 and 220 °C, respectively. Sulfation of SnO2-modified MnOx-CeO2 may form Ce(III) sulfate that could enhance the Lewis acidity and improve NO oxidation to NO2 during NH3-SCR at T > 200 °C.

Measuring and relating the electronic structures of nonmodel supported catalytic materials to their performance.

Identifying structure-performance relationships is critical for the discovery and optimization of heterogeneous catalysts. Recent theoretical contributions have led to the development of d-band theory, which relates the calculated electronic structure of a metal to its chemical and catalytic activity. While there are many contributions where quantum-chemical calculations have been utilized to validate the d-band theory, experimental examples relating the electronic structures of commercially relevant nonmodel catalysts to their performance are lacking. We show that even small changes in the near-Fermi-level electronic structures of nonmodel supported catalysts, induced by the formation of surface alloys, can be measured and related to the chemical and catalytic performance of these materials. We demonstrate that critical shifts in the d-band center in alloys are related to the formation of new electronic states in response to alloying rather than to charge redistribution among constitutive alloy elements, i.e., the number of d holes and d electrons localized on the constitutive alloy elements is constant. On the basis of the presented results, we provide a simple, physically transparent framework for predicting shifts in the d-band center in response to alloying and relating these shifts to the chemical characteristics of the alloys.

Controlling carbon surface chemistry by alloying: carbon tolerant reforming catalyst.

Steam reforming is a process where a hydrocarbon is converted into hydrogen and oxygenated carbon species. Ni is often used as catalyst for the reaction. Long term stability of steam reforming catalysts is governed by their ability to selectively oxidize C atoms while preventing C-C bond formation. In this communication we demonstrate that C atom chemistry over Ni surfaces can be controlled by surface alloying. We show that bimetallic Sn/Ni catalyst is much more carbon-tolerant that monometallic Ni. The main reason for this is that Sn alloying results in dramatically lower rates of C-C bond formation as compared to C-oxidation. The bimetallic catalyst was identified in quantum computational studies of the underlying atomic-scale phenomena that govern C atom surface chemistry. The catalysts were also characterized with various electron- and X-ray-based microscopies and spectroscopies.

Polypyrrole/poly(methylmethacrylate) blend as selective sensor for acetone in lacquer.

A film of alpha-naphthalene sulfonate-doped polypyrrole/poly(methylmethacrylate), PPy/alpha-NS(-)/PMMA, obtained from solution mixing was successfully used as sensing material for acetone vapor in lacquer with a high degree of selectivity based on electrical conductivity over acetic acid and a high degree of stability over the humidity change. Compared with pure PPy/alpha-NS(-), the selectivity ratio of acetone/acetic acid response of PPy/alpha-NS(-)/PMMA blend with a PMMA/PPy weight ratio of 3.0 was ca. 3.9 times higher. The film was found to be insensitive to moisture unless the relative humidity (RH) was lower than 20% RH in which the selectivity ratio of acetone/acetic acid response was enhanced. The time required to reach the equilibrium for acetone exposure was found to increase slightly with increasing humidity.