59th Annual Report on Research 2014

1.     Introduction

This proposal involves pioneering work in the development of a game-changing polymeric material which can have applications in areas ranging from hydrogenation catalysis to multipurpose applications in petroleum chemistry. The synthesis of nitrogen oligomer with well-controlled and stable properties for performance testing is a challenging task that requires first phase demonstration. Within the first year, we completed the following tasks: 1) Synthesized nitrogen oligomer using CV method and achieved different nitrogen oligomer loading amounts by varying azide concentrations in buffer solution; 2) Characterized the samples using FTIR, and TPD; 3) Proved nitrogen oligomer can be a Lewis base chemisorption site for acetylene.

2.     Results

2.1  Synthesis

A series of nitrogen oligomer was synthesized on MWNT sheet using CV method. NaN₃ (Aldrich) dissolved in a buffer solution (pH=4.0) was used as the electrolyte. As references, MWNT sheets were dipped in the same kind of electrolyte without CV scanning. The samples were labelled as PN-MWNT and dipped MWNT with or without CV scanning, respectively. The concentrations of sodium azide for synthesis of nitrogen oligomer were varied with 0.5, 1, 2, 3 and 4M.   

2.2  Characterization

TPD was carried out to determine the thermal stability of the samples after synthesis using a Micromeritics^® AutoChem II 2920 system. The released species during TPD are monitored with a mass spectrometer (SRS QMS200).  As shown in Figure 1, with the increasing of azide concentration, both the nitrogen desorption temperature for dipped MWNT sheet and PN-MWNT sheet shifted to lower temperatures, which indicates an increasing amount of weaker attached nitrogen compounds as the azide concentration increases. Indeed, nitrogen compounds tend to occupy the sites that will form stronger interactions with MWNT for a lower overall thermodynamics free energy. For dipped MWNT sheets, initially the nitrogen desorption amount increased with the increasing of azide concentration, from 2M to 4M the desorption amount did not increase much which suggests saturation of the sites within the MWNT. However, the nitrogen desorption amount for PN-MWNT continuously increased with the increase of azide concentration, suggesting more nitrogen oligomers formed with higher azide concentrations from 0.5M to 4M. The quantities of nitrogen oligomer (Table 1) on MWNT are calculated based on the TPD results from Figure 1. This is further proved by FTIR results in Figure 2, which showed that the intensity of the peak at around 2050 cm^-1 continuously increased with azide concentration.  

Figure 1. TPD results for dipped MWNT sheets and PN-MWNT sheets with 0.5~4M azide concentrations for electrochemical synthesis. The results have been normalized by sample weight.

Table 1. Corresponding nitrogen desorption amount (mmol/grams of sample) calculated by integration of the TPD results and comparing the peak areas from Figure 1 with those from injection of pure nitrogen under the same experiment conditions.

 Figure 2. FTIR results for PN-MWNT sheets with different azide concentrations for electrochemical synthesis. MWNT background have been subtracted for the PN-MWNT curves.

2.3  Acetylene chemisorption and hydrogenation To approve nitrogen oligomer can potentially serve as a Lewis base active site, we investigated chemisorption properties using acetylene as a probe molecule.  After the sample was saturated with acetylene at room temperature, we did TPD. Figure 3 shows TPD patterns of the hydrocarbons that were desorbed from 2M PN-MWNT sample.  C2 signals are too low to be distinguished as a peak while there is an obvious peak in C1 signal. The result indicated that nitrogen oligomer can chemisorb acetylene due to its Lewis base properties.    
Figure 3. TPD desorption of the C1 and C2 hydrocarbons obtained from 2M PN-MWNT catalyst.    Figure 4 showed the results of selective hydrogenation of acetylene over 2M PN-MWNT at different temperatures. Basically, 2M-MWNT showed some activity but a bit low. We then investigated hydrogen chemisorption on sample, we found that nitrogen oligomer cannot dissociatively chemisorb hydrogen molecules. This might be the reason that the activity for the selective hydrogenation of acetylene is low.