No abstract

For the first time, the standard and fast selective catalytic reduction (SCR) of NO by NH3 are described in a complete catalytic cycle that is able to produce the correct stoichiometry while allowing adsorption and desorption of stable molecules only. The standard SCR reaction is a coupling of the activation of NO by O2 with the fast SCR reaction, enabled by the release of NO2. According to the scheme, the SCR reaction can be divided into an oxidation of the catalyst by NO + O2 and a reduction by NO + NH3; these steps together constitute a complete catalytic cycle. Furthermore, both NO and NH3 are required in the reduction, and finally, oxidation by NO + O2 or NO2 leads to the same state of the catalyst. These points are shown experimentally for a Cu-CHA catalyst by combining in situ X-ray absorption spectroscopy (XAS), electron paramagnetic resonance (EPR), and Fourier transform infrared spectroscopy (FTIR). A consequence of the reaction scheme is that all intermediates in fast SCR are also part of the standard SCR cycle. The activation energy calculated by density functional theory (DFT) indicates that the oxidation of an NO molecule by O2 to a bidentate nitrate ligand is rate-determining for standard SCR. Finally, the role of a nitrate/nitrite equilibrium and the possible influence of Cu dimers and Brønsted sites are discussed, and an explanation is offered as to how a catalyst can be effective for SCR while being a poor catalyst for NO oxidation to NO2.

Promoter elements enhance the activity and selectivity of heterogeneous catalysts. Here, we show how methanol synthesis from synthesis gas over copper (Cu) nanoparticles is boosted by zinc oxide (ZnO) nanoparticles. By combining surface area titration, electron microscopy, activity measurement, density functional theory calculations, and modeling, we show that the promotion is related to Zn atoms migrating in the Cu surface. The Zn coverage is quantitatively described as a function of the methanol synthesis conditions and of the size-dependent thermodynamic activities of the Cu and ZnO nanoparticles. Moreover, experimental data reveal a strong interdependency of the methanol synthesis activity and the Zn coverage. These results demonstrate the size-dependent activities of nanoparticles as a general means to design synergetic functionality in binary nanoparticle systems.

X-ray absorption and emission spectroscopy, FTIR and DFT unravel the major Cu species in the activated Cu-SSZ-13 catalyst for NH3-SCR.

Although extended gold surfaces are generally considered chemically inert [1,2] nanosized (< 5 nm) gold particles can be very effective catalysts for a number of oxidation reactions. [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17] There are reports of similar size effects for silver catalysts. [18,19] The origin of the nanoeffects in the catalytic properties of these metals is widely debated, [15] and no consensus has been reached. Based on a set of density functional theory calculations of the full reaction pathway for CO oxidation over extended surfaces as well as over small nanoparticles of a number of metals, we show that although platinum and palladium are the most active catalysts for extended surfaces at high temperatures, gold is the most active for very small particles at low temperature. The calculations capture the special catalytic properties of nanosized particles observed experimentally, which allows the origin of the effect to be analyzed.Herein, we focus on intrinsic metal effects; that is, we do not include additional possible effects that involve the support. It is not that such effects may not be important, [5,20,21] but it is useful to first establish the intrinsic metal effects, [15] in particular as it has been shown experimentally that nanostructured gold with no support is also catalytically active. [22,23] The key feature of our analysis is that we compare catalytic activities of different transition and noble metals for one specific reaction, the CO oxidation.The CO oxidation reaction on close-packed fcc(111) surfaces was considered initially, which will give a dominant contribution to the total catalytic rate over large metal particles. We consider the following elementary reactions:For the metals we consider herein, Reactions (R1) and (R2) are unactivated and fast, and we assume that these two reactions are in equilibrium. This means that we are limited to temperatures high enough that desorption is also fast. The possible formation of an oxide layer on the more reactive metals is neglected.The forward and reverse rate constants of the Reactions (R3) and (R4) are given by, where n i is a prefactor, E ai is the activation energy, k is the Boltzmann constant, and T is the absolute temperature. The activation energies are E a = max(E TS ÀE IS , 0) where E IS is the initial state energy and E TS is the transition-state energy. DS ai is the entropy difference between the transition state and the initial state. The entropy of adsorbed species are assumed to be zero, and the gas-phase entropies are taken from Ref. [24]. The adsorption energies of the different species E CO , E O 2 , and E O and the transition state energies are given with respect to the gas-phase molecules.Assuming the prefactors and adsorption entropies are independent of the metal, there are five metal-dependent parameters determining the kinetics: E CO , E O 2 , E O , E TS3 , and E TS4 . The transition-state energies are, however, found to scale linearly with the adsorption energies, as shown for E TS3 and E TS4 in F...

We analyse the transition state energies for 249 hydrogenation/dehydrogenation reactions of atoms and simple molecules over close-packed and stepped surfaces and nanoparticles of transition metals using Density Functional Theory. Linear energy scaling relations are observed for the transition state structures leading to transition state scaling relations for all the investigated reactions. With a suitable choice of reference systems the transition state scaling relations form a universality class that can be approximated with one single linear relation describing the entire range of reactions over all types of surfaces and nanoclusters.

Many catalytic reactions under fixed conditions exhibit oscillatory behaviour. The oscillations are often attributed to dynamic changes in the catalyst surface. So far, however, such relationships were difficult to determine for catalysts consisting of supported nanoparticles. Here, we employ a nanoreactor to study the oscillatory CO oxidation catalysed by Pt nanoparticles using time-resolved high-resolution transmission electron microscopy, mass spectrometry and calorimetry. The observations reveal that periodic changes in the CO oxidation are synchronous with a periodic refacetting of the Pt nanoparticles. The oscillatory reaction is modelled using density functional theory and mass transport calculations, considering the CO adsorption energy and the oxidation rate as site-dependent. We find that to successfully explain the oscillations, the model must contain the phenomenon of refacetting. The nanoreactor approach can thus provide atomic-scale information that is specific to surface sites. This will improve the understanding of dynamic properties in catalysis and related fields.

The origin of the extraordinary catalytic activity of gold nanoparticles is discussed on the basis of density-functional calculations, adsorption studies on single crystal surfaces, and activity measurements on well characterized supported gold particles. A number of factors are identified contributing to the activity, and it is suggested that it is useful to consider lowcoordinated Au atoms as the active sites, for example, CO oxidation and that the effect of the support can be viewed as structural and electronic promotion. We identify the adsorption energy of oxygen and the Au-support interface energy as important parameters determining the catalytic activity.