Section 2: Experimental Data
5.3 Results and Discussion
Of particular note was the CO yields on Ni2Al3 catalysts. These films showed ~30%
FE for CO compared to ~ 0% for the Ni control, Figure 5-5. These findings are
unexpected as Ni alone binds CO very tightly. By alloying Ni with Al, it is possible the binding energy of CO to Ni is decreased sufficiently that once CO2 is reduced to CO it desorbs from the surface and is not further reduced. This suggests that going from Ni to NixGay the CO binding energy is weakened sufficiently to produce hydrocarbons, and moving up group 13 from Ni5Ga3 to Ni2Al3 weakens the CO binding energy even more such that CO is desorbed in significant quantities. An additional possibility is the
generation of CO at the Al sites which is not known to generate CO from CO2 in its pure metallic phase.It is, however, possible that by alloying Al with Ni the CO2R activity is increased sufficiently to reduce CO2 to CO. Water soluble salts of In were not available,
Figure 5-5. Product distributions for CO2Rin MeCN are shown for the 5 different bimetallics tested detected using GC-TCD. H2 and CO were not detectable using GCMS due to the low mass of H2 and the overlap of CO with N2 on the GCMS chromatogram.
All potentials are vs. Fc/Fc+.
however alloys of NixIny would be a logical next step and could perhaps improve on NixGay.
Electrolyses under CO were also conducted to test the bimetallics activity towards COR. The Fe bimetallics showed a significant increase in CH4 production under CO compared to CO2, Figure 5-6. These results are significant given the Fe control produced almost no hydrocarbon products through CO2R or COR. FE of the Ni bimetallics
decreased as compared to CO2R. Overall FE for HER decreased significantly when moving from CO2 to CO, Figure 5-7. This result is not surprising as CO is known to cover Fe and Ni surfaces which would prevent HER from occurring at the same rates as under CO2.
After product analysis in nonaqueous solvents, the three most active catalysts, Fe2Co3, Ni2Al3, and FeAl2.8, were tested for their activity towards CO2Rand COR in H2O, Figure 5-8. As expected, due to the decreased solubility of CO2 and CO in H2O, and large increase in available H+, FE toward hydrocarbons decreased significantly, but
Figure 5-6. Product distributions for CORin MeCN shown for the 5 different bimetallics tested detected using GCMS. All potentials are vs. Fc/Fc+.
Figure 5-7. Product distributions for CO2R and HERin MeCN are shown for the 5
different bimetallics tested detected using GC-TCD. H2 and CO were not detectable using GCMS due to the low mass of H2 and the overlap of CO with N2 on the GCMS
chromatogram. All potentials are vs. Fc/Fc+.
the full range of C1-C3 products was still observed. Generally, hydrocarbon FE was <
0.5% for all materials tested in H2O. Although FE is low, Fe2Co3 and Ni2Al3 both produced > 0.1 % C3 products propylene and propane. These products are significant because of the formation of 2 C-C bonds from 3 molecules of CO2. These reactions between > 2 molecules of CO2 become increasingly difficult as the number of C-C bonds is increased. It is important to note that direct comparisons of potentials is difficult between H2O and MeCN. Because of the different polarities of the solvents, the different supporting electrolytes, and the different ionic strengths of the solvents, conversion between reference electrodes is difficult. As the potential is made more negative in H2O the FEhydrocarbons is increased. It is possible that the electrode in H2O cannot be polarized as negatively compared to in nonaqueous conditions because the amount of HER that occurs in H2O would cause the potentiostat to reach its compliance voltage.
COR was also performed in H2O using a phosphate buffer as the supporting electrolyte to eliminate the presence of CO2 in solution that would be generated from the equilibrium reaction between water and a carbonate electrolyte. COR was performed at -1.7V vs.
Ag/AgCl. The same decrease in HER was noted between CO2R and COR, indicating the surfaces are covered with a substantial amount of CO, which decreases the available sites for HER. Additionally, an increase in CH4production was noted for COR in H2O
compared to CO2R in H2O. However, due to the relatively low FE of all catalysts, this increase is relatively small and could be due to fluctuations which can occur from experiment to experiment.
The general trends and product distributions for CO2Rand COR are similar in H2O and MeCN, indicating that the reaction proceeds by the same mechanism in both solvents. Through this screening process in nonaqueous solvents, many bimetallic combinations were tested and three interesting catalysts were identified. These catalysts
Figure 5-8. Product distributions for CO2Rin 0.1 M NaHCO3 H2O with 1 atm CO2
acidified to pH 7 shown for the 5 different bimetallics tested detected using GCMS. All potentials are vs. Fc/Fc+.
Figure 5-9. Product distributions for CORin H2O shown for the 5 different bimetallics tested detected using GCMS. All potentials are vs. Fc/Fc+.
exhibited behavior different than that of their pure component metals and catalyzed the reduction of CO2 in aqueous solutions as well.