Articles you may be interested inBand gap and defect states of MgO thin films investigated using reflection electron energy loss spectroscopy AIP Advances 5, 077167 (2015); 10.1063/1.4927547Probing optical band gaps at the nanoscale in NiFe2O4 and CoFe2O4 epitaxial films by high resolution electron energy loss spectroscopy Determining the Co valence, particularly in Co-based nanocatalysts is a longstanding experimental challenge. In this paper, we utilize in situ electron energy-loss spectroscopy and first-principles density functional theory calculations to distinguish between metallic Co, Co 3 O 4 , as well as CoO. More specifically, differences in the O K-and Co L-edges are utilized to determine the Co valence in different Co-oxide particles. We will further demonstrate that while the metallic Co L 3 / L 2 -ratio equals that of partially reduced Co 3 O 4 , the near-edge fine-structure of the metallic Co L-edge exhibits additional features not present in any Co-oxide. The origin of these features will be discussed. Based on our experimental and theoretical results, we will propose a fitting method to distinguish metallic Co from Co-oxides.

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The utilization of metal nanoparticles traverses across disciplines and we continue to explore the intrinsic size-dependent properties that make them so unique. Ideal nanoparticle formulation to improve a process's efficiency is classically presented as exposing a greater surface area to volume ratio through decreasing the nanoparticle size. Although, the physiochemical characteristics of the nanoparticles, such as phase, structure, or behavior, may be influenced by the nature of the environment in which the nanoparticles are subjected1, 2 and, in some cases, could potentially lead to unwanted side effects. The degree of this influence on the particle properties can be size-dependent, which is seldom highlighted in research. Herein we reveal such an effect in an industrially valuable cobalt Fischer-Tropsch synthesis (FTS) catalyst using novel in situ characterization. We expose a direct correlation that exists between the cobalt nanoparticle's size and a phase transformation, which ultimately leads to catalyst deactivation.

Using well defined supported cobalt nano-crystallites in a novel in-situ sample presentation device for laboratory X-ray diffractometers, we could investigate the extensively studied structure sensitivity of Fischer-Tropsch (FT) catalysts under simulated high conversion conditions, i.e. at high water to synthesis gas ratios. This study has to be regarded as further small step towards a full understanding to the various processes governing FT activity and selectivity. We were able to show, for two different crystallite sizes, that water has an overall enhancing effect on carbon monoxide conversion and surface specific turn over frequency on metallic surfaces as well as improving the overall product selectivity with decrease of methane selectivity. For small crystallites oxidation was observed at elevated water partial pressures which caused a decrease of activity. The selectivity to the undesired product methane is suppressed in favor of chain growth. This influence on the selectivity might originate from water induced changes on the active sites responsible for chain growth or by an inhibiting effect of water on methanation sites. Due to a stronger effect of water on smaller crystallites, the impact on the methane selectivity reverses previously described trends of increasing methane selectivity with decreasing crystallite size. Secondary olefin reactions, clearly more pronounced on smaller crystallites and under "dry" conditions, are severely suppressed via the addition of water, resulting in a pseudo structure insensitivity of this class of reactions.

Promotion of supported metal catalysts is a ubiquitous but poorly understood phenomenon in heterogeneous catalysis. Being a local effect, close association of the promoter and the metal is highly desired. However, promoters are typically added by dry impregnation (DI), where the promoter salt is dissolved in enough water to fill the pore volume of the mixed oxide support material prior to impregnation. In this case, the probability of metal-promoter interaction is determined largely by their respective concentrations and the surface area of the support, which is a highly inefficient process. By simply adjusting the solution pH with regard to the surface charging hydroxyl groups (ÀOH) of a mixed oxide support material and choosing an appropriate precursor complex for the promoter, selective adsorption of the promoter onto the supported catalyst's oxide phase can be achieved and thus, the promoter material is more effectively utilized. [1] For Fischer-Tropsch (FT) synthesis, Mn is often used as a promoter for both supported and unsupported Co systems. [2] In this catalytic conversion of synthesis gas (CO/H 2 ), significant interaction between the Mn and Co species has been demonstrated to enhance the selectivity towards light olefins and C 5 + hydrocarbons which is especially apparent in the recent work done on supported Co systems. [2][3][4][5][6][7][8][9] Recently, we reported on the use of this technique, referred to as strong electrostatic adsorption (SEA), to drive the initial placement of a Mn promoter onto the precursor Co 3 O 4 phase supported on TiO 2 for FT synthesis. [10] In the current work, we discuss how differences in the method of promoter addition (Mn DI /Co/TiO 2 and Mn SEA /Co/TiO 2 ) affect the interaction between the Mn and Co prior to and following reduction procedures. In both catalyst systems, the Mn/Co molar ratio was approximately 0.3.When strongly interacting, Mn has been known to hinder the reduction of Co 3 O 4 ; [3,5,8] therefore, temperature-programmed reduction (TPR) may give insight to the level of interaction between these two approaches of promotion. Based on these results (Figure 1), when left unpromoted, the twostep reduction of Co 3 O 4 occurs around 250 8C for the reduction of Co 3 + to Co 2 + and 325 8C for the reduction of Co 2 + to Co 0 . The third peak at 450 8C is attributed to be the partial reduction of the TiO 2 support in the presence of Co indicating metal-support interactions. [6,11] As a control experiment (not shown), Mn was supported on TiO 2 solely, calcined and ex-posed to the same TPR conditions. Its reduction temperatures followed a two-step process, as well, with reduction peaks at 300 8C and 375 8C. The overlay of similar reduction peaks with Co 3 O 4 makes it difficult to deconvolute the TPR spectra of each species in the mixed Mn-Co catalysts. It is clear that there are four H 2 consumption peaks present in the Mn DI sample. The fourth peak has been repeatedly associated with Mn loading in these catalyst systems [5,10] and the higher reduction temperature...