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The aim of the research performed since the beginning of this ACS-PRF grant is to study heterogeneous nano-catalysts, more specifically Co-based Fischer-Tropsch catalysts. To achieve this goal, we are using the combination of high-resolution transmission electron microscopy, in particular Z-contrast imaging and electron energy-loss spectroscopy (EELS) in a scanning TEM. Z-contrast imaging provides directly interpretable atomic-resolution images of even the smallest cluster on an oxide support, while atomic-resolution EELS allows local changes in electronic structure to be correlated directly with the size and composition of the nano-catalyst. We combined these two techniques with in-situ heating/reduction experiments, as well as density functional theory calculations to examine the behavior of different nano-catalysts under highly reducing conditions.

Since January 2008, we have concentrated on three different areas involving Fischer-Tropsch heterogeneous nano-catalysts: 1) metallic Co and CoOx nano-clusters; 2) CoPd bimetallic nano-catalysts; and 3) Mn promoted Co-particles on TiO₂ support. The results of each study will briefly described below:

1) Metallic Co and Co oxides nano-particles

It is of great interest to the field of nano-catalysis to develop a method of quantifying the local Co-valence, and be able to differentiate between metallic Co and Co-oxides on the atomic scale. The traditional methods using TEM and EELS have failed so far to distinguish metallic Co from CoO_x.

In this research we have developed such a method for distinguishing the different Co species using the near-edge fine structure of the Co L-edges, electron diffraction and in-situ heating experiments. Metallic Co samples were synthesized by ex-situ reduction of Co₃O₄ to metallic Co under flowing H₂ at 450°C for 10 hours (5°C/min ramp). These powder samples were then immediately loaded into the TEM column, and analyzed after an additional in-situ reduction at 450˚ C for 10 hours  inside the microscope column (O₂ partial-pressure P_O2=5x10^-8Pa) to eliminate any surface oxide that might have formed while loading the sample into the TEM. Using electron diffraction we found that metallic Co and CoO are fcc while Co₃O₄ is spinel structure.

Next, we have used the near-edge fine-structure of the Oxygen K- and the Cobalt L-edges to distinguish between the different Co-oxides as well as the metallic Co. By comparing our experimental results to our first-principles density functional theory calculations, we have been able to identify a feature in the Co L-edge that is unique to metallic Co, and can thus be used to distinguish small amounts of metallic Co from any CoO_x. The results have been submitted to Physical Review B and are currently under review. 

2) CoPd bi-metallic nano-catalyst

In this study, we used Z-contrast imaging in combination with EELS to study the atomic and electronic structures of CoPd catalysts which were synthesized by three different methods, namely the incipient wetness impregnation (IWI), the charge-enhanced incipient wetness impregnation (CEIWI) and Sequential Impregnation. For each method, we divide the analyzed catalyst particles in two groups, one with particle diameter larger than 10 nm and the other with particle diameters smaller than 10 nm. Samples prepared by IWI, CEIWI and Sequential Impregnation are reduced ex-situ at 500˚C, 400˚C, 225˚C in H₂, respectively.

By studying a large number of particles for each synthesis method, we found that:

a) IWI produces a mixture of pure Co, pure Pd and Co-Pd-O alloy. The mixture of Co and Pd particles was expected, while the unexpected occurrence of alloyed Co₂O₃-Pd is due to the high reduction temperature

b) CEIWI results in Co-Pd-O alloy particles after reducing at 400˚C with two distinct particle size distributions: The large particles show a core-shell structure with a CoO-Pd core and a Co₃O₄ shell; the small particles exhibit Co-Pd-O alloy without a distinct Co oxide shell.

c) Sequential Impregnation produces the core-shell structure catalysts with a Co oxide core and Pd shell when reduced at 225˚C for 3 hours. Our results show that using sequential impregnation can be used to synthesize CoPd core-shell catalysts, resulting in a reduced of Pd loading without decreasing the catalysts reactivity.

The results of this study are currently being written up and will be submitted for publication shortly.

3) Mn/Co/TiO₂ Fischer-Tropsch Catalysts

Mn is used as a promoter in Co-based Fischer-Tropsch (FT) catalysts, resulting in an increased activity and selectivity. Co-based catalysts with different Mn loading amount (up to 3.43 wt%) are synthesized by using strong electrostatic adsorption (SEA), followed by calcinations at 400˚C, and a subsequent reduction at 350˚C in H₂. The atomic and electronic structures of catalysts are compared to the changes in the catalysts reactivity and selectivity.

We used Z-contrast imaging in combination with in-situ EELS to analyze the unreduced and reduced Mn/Co/TiO₂ FT catalysts. In order to simulate the elemental re-distribution during the calcination process, a control catalyst sample produced by physical mixture of Co and Ti with Mn promoter mounted by SEA was in-situ heated at 350˚C. We found that the Mn promoter distribute homogeneously on the surface of Co particles after calcination. Co-oxides are being reduced during the in-situ heating while bulk TiO₂ remains unchanged. However, TiO₂ at the interface with Co shows a lower valence compared to the other areas which is due to the strong metal support interaction (SMSI).

The catalyst powder sample exhibits a narrow Co-particle size distribution around 20 nm in diameter. Our results show that Mn is selectively absorbed on the Co particle surface, but not on TiO₂, as expected. After reduction, large diameter (>10nm) and small diameter (<10nm) Co particles are both found. Instead of covering the whole Co surface, Mn accumulates in the interface between Co and TiO₂ after the ex-situ reduction. Some individual Mn-particles are also found on the TiO₂ surface. The presence of Mn on Co surface prevents complete reduction of CoO_x to metallic Co, thereby creating highly selective, active sites.

We have shown that the combination of Z-contrast imaging, EELS and in-situ reduction and first-principles calculations can be used to correlate the catalytic activity of different Co-catalysts to the changes in their atomic and electronic structures. This study is currently being written up and will be submitted for publication very shortly.