60th Annual Report on Research 2015

1. Introduction We proposed to investigate nitrogen oligomer to improve acetylene selective hydrogenation activity or selectivity. Within the 1st year, we synthesized nitrogen oligomer using CV method and achieved different nitrogen oligomer loading amounts by varying azide concentrations in buffer solution, characterized the samples using FTIR, and TPD and proved nitrogen oligomer can be a Lewis base chemisorption site for acetylene. Based on the 1st year results, we proposed to work on the following tasks within the 2nd year: 1) Test acetylene reaction on nitrogen oligomer and explore other petrochemical reactions; 2) Increase hydrogen chemisorption by adding Pd on nitrogen oligomer-CNT composite (Pd-N3-activated carbon); 3) Modify conventional Pd/Al2O3 catalyst by introducing N-N-N sites onto the support (Pd-N3-Al2O3). Due to the graduation of the student who worked on the project, the project was delayed a bit. We partially finished task 1 and task 2. The project was extended and the tasks will be continued within the 3rd year.

2. Results 2.1 Pd-PN/CNT preparation A series of Pd-PN/CNT were synthesized with different PN loading with impregnation method. 2.2 Characterization The samples were characterized using Inductively Coupled Plasma Mass Spectrometry ICP-MS (Agilent, 7700X), Temperature programmed decomposition (TPD), Temperature program oxidation (TPO). Fig. 1. N14 signals normalized by sample weight from the temperature programmed decomposition results for dipped Pd/CNT and Pd-PN/CNT sheets with different azide concentrations for electrochemical synthesis. Table 1 Surface and physical properties of different samples. Sample NaN3 conc. a (mol/L) N element loading b (wt. %) Pd element loading c (wt. %) Pd loss percentage (%) CO uptake (?mol /g cat) 0 M 0 - 0.65 3 17.71 0.5 M 0.5 3.9 0.033 95.1 3.33 1 M 1 4.7 0.0092 98.6 3.67 2 M 2 7.1 0.00018 99.97 6.89 1 M* 1 1.5 0.41 38.8 8.93 Pd/CNT - - 0.67 - 15.08 a Sodium azide concentration in the electrolyte for CV. b Calculated by integration of the TPD results and comparing the peak areas with those from injection of pure nitrogen under the same experiment conditions. c Determined by ICP-MS. As shown in Fig. 1, nitrogen desorbed at 346 °C on dipped Pd/CNT (1 M*) sample which was attributed to azide decomposition. However, much high desorption temperature was observed on samples with electrochemical treatment, which indicated that the PN species formed on the Pd/CNT substrate and was thermally stable. The overall desorption temperature in present study for Pd-PN/CNT is slightly lower than that in our previous results while the PN formed here was much higher than before which may be attributed to Pd species loading on MWNTs surface. The ICP-MS results of different samples are shown in Table 1. The theoretical palladium loading of Pd/CNT sheet was 1 wt. %. The ICP result showed 0.67 wt. % palladium was detected for Pd/CNT which would be attributed to incomplete loading due to the ethanol vaporization during preparation process. After electrochemical treatment in the electrolyte without azide, the Pd loading of 0 M changed little. However, the Pd loading of sample 1 M* dipped in electrolyte with azide but without CV decreased by more than one third. This comparison result indicated that azide would facilitate Pd dissolving into solution as a result of hydrazoic acid formation during sodium azide dissolution. Interesting results were found on samples with electrochemical treatment in azide including electrolyte. The Pd loading of these three samples were dramatically decreased. Indeed, most of Pd loading outer layers of the nanotubes would be taken away during CV while few amount Pd inter layers would stay. To investigate the chemisorption properties of samples after electrochemical treatment, CO-chemisorption was carried out and results are showed on Table 1. The fact that CO uptake of 0 M and Pd/CNT are very close indicated that simple electrochemical treatment on Pd/CNT without azide in electrolyte showed very limited effect on chemisorption properties. Meanwhile, CO uptake of the sample 1 M* (dipped in azide including solution without CV) decreased by more than 40% which suggested that some Pd dissolved into solution or were covered by azide. Based on above ICP results, it could be expected that much lower CO uptake would be observed on samples with CV in azide including electrolyte since most of Pd lost during electrochemical treatment. However, relatively high CO uptake was achieved on 0.5, 1 and 2 M respectively. The only reasonable explanation for this would be the synergetic effect between Pd and PN which was induced by electrochemical treating with azide including solution as electrolyte simultaneously. In summary, distinct from conventional annealing methods to establish covalent interactions in intermetallics or make alloy with harsh condition, we have presented an electrochemical treatment method to synthesize active site isolated catalyst with high performance under ambient condition.

3. Future work a. Continue with hydrogen chemisorption by adding Pd on nitrogen oligomer-CNT composite (Pd-N3-activated carbon) and measure the activity of these materials b. Modify conventional Pd/Al2O3 catalyst by introducing N-N-N sites onto the support (Pd-N3-Al2O3) c. Explore other petrochemical reactions by tune Lewis or Base site using PN or N atoms.

4. Impact of the project Two PhD students, two PhD students worked on the project. Thanks to ACS PRF grant, Zhiyi Wu were supported and completed his PhD degree. Maocong Hu is supported by the grant and continue the project in the 3rd year. The grants allow two graduates to be able to do their PhD work. As the PI, I also personally benefited greatly for my academic career. Maocong Hu attended ACS Boston meeting (Aug. 2015) and gave an oral presentation based on the results from this project. Within 2nd year, 2nd paper was accepted and in press in Catalysis Today. 3rd paper is in preparation. In the coming year, a couple of more papers will be published through the support of this grant.