Theresa E. Feltes

In situ electron energy loss spectroscopy study of metallic Co and Co oxides

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, Co3O4, 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 L3/L2-ratio equals that of partially reduced Co3O4, 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.

The determination of oxide surface charging parameters for a predictive metal adsorption model

The procurement of oxide surface charging parameters has been a widely researched topic in recent years [1-30]. In this study, a one-site, two-pK surface charging mechanism is used in combination with a diffuse double-layer description of the electric double-layer to fit pH shift data over silica and alumina. From these fits of pH data, with no further adjustment of parameters, metal adsorption can be predicted over both supports to a reasonable degree of accuracy. A multi-dimensional optimization procedure employing a Nelder-Mead simplex algorithm is used to optimize the DeltapK (pK(2)-pK(1)) parameter to obtain a best fit of the pH shift data with fixed PZC and hydroxyl density (N(s)). The resulting set of parameters is then used with no adjustment in a purely electrostatic adsorption model (the Revised Physical Adsorption or RPA model) in order to predict anionic chloroplatinic acid (CPA, [PtCl(6)](-2)) adsorption on alumina and cationic platinum tetraammine (PTA, [Pt(NH(3))(4)](+2)) adsorption on alumina and silica. The optimization procedure developed in this study gives reasonable values of the DeltapK compared to other values reported in the literature, with fits to the pH shift data at various oxide loadings with relative errors below 2.8%.

The Influence of Preparation Method on Mn–Co Interactions in Mn/Co/TiO2 Fischer–Tropsch Catalysts

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 ( OH) 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. For Fischer–Tropsch (FT) synthesis, Mn is often used as a promoter for both supported and unsupported Co systems. In this catalytic conversion of synthesis gas (CO/H2), significant interaction between the Mn and Co species has been demonstrated to enhance the selectivity towards light olefins and C5+ hydrocarbons which is especially apparent in the recent work done on supported Co systems. 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 Co3O4 phase supported on TiO2 for FT synthesis. [10] In the current work, we discuss how differences in the method of promoter addition (Mn/Co/TiO2 and Mn /Co/TiO2) 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 Co3O4 ; [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 Co3O4 occurs around 250 8C for the reduction of Co + to Co and 325 8C for the reduction of Co to Co. The third peak at 450 8C is attributed to be the partial reduction of the TiO2 support in the presence of Co indicating metal–support interactions. 11] As a control experiment (not shown), Mn was supported on TiO2 solely, calcined and exposed 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 Co3O4 makes it difficult to deconvolute the TPR spectra of each species in the mixed Mn–Co catalysts. It is clear that there are four H2 consumption peaks present in the Mn DI sample. The fourth peak has been repeatedly associated with Mn loading in these catalyst systems 10] and the higher reduction temperature than 375 8C could be due to some Mn–Co interactions. The lower temperature peak for Mn (225 8C) has been attributed to the reduction of only Mn oxides, although we caution this association based on our control experiment and the varying degrees of metal oxide interaction this system can employ. We have actually seen very small amounts of Mn slightly decrease the Co reduction temperature when prepared by SEA. However, we can conclude, in comparison with the Mn catalyst, the Co3O4 in the Mn DI sample is more free to reduce at the 350 8C reduction temperature typical for FT catalyst activation, which was employed in our previous work. The Mn sample displays a reduction that is analogous to mixed Mn–Co spinels. 12] Unsupported Mn–Co spinel oxides have been extensively researched as a FT catalytic material. 14] Generally, when reduced at 350–400 8C, the two phases MnO and Co are primarily present, but it has been speculated that a degree of Mn–Co spinel still present has considerable effect on product selectivity. 14] However, our catalyst did not exhibit bulk Mn–Co spinel oxides supported. From X-ray diffraction, the crystalline structure was Co3O4. [10] Nonetheless, the TPR results clearly indicated some kind of complex intimate interaction between the Mn promoter and the Co surface. Scanning tunneling electron microscopy electron energyloss spectroscopy (STEM-EELS) provides a powerful visual representation for elemental mapping and, thus, allows us to Figure 1. . TPR profiles for the Mn and Mn catalyst materials as compared to the unpromoted sample.