Analysis of Nickel Pincer. Catalyst for Hydrogen. Production

Transcription

1 Analysis of Nickel Pincer Catalyst for Hydrogen Production

2 Abstract: Fuel cells have great potential as an alternative fuel source; however the hydrogen to power the cells is rather costly to produce. A study by Luca et. al were investigating a nickel pincer catalyst instead of platinum because it could be a cheaper alternative for hydrogen production. Density Functional Theory was used to propose a catalytic cycle that was then investigated experimentally with cyclic voltammetry. Cyclic Voltammetry was also used to analyze the effects different ligands had on the catalytic cycle. The nickel pincer catalyst had a 90-95% faradaic yield. The efficiency of the nickel catalyst was high because there was little energy that was lost as heat or byproducts. The researchers concluded that the nickel pincer catalyst had potential to reduce the cost of hydrogen fuel cells. There is a possibility that some hazardous byproducts were made and the byproducts should be studied further along with ways to reduce the byproducts. Introduction: Alternative energy sources are a popular research topic. Hydrogen fuel cells are more attractive than other sources because the main byproduct of the fuel cell is water. The main challenge of making hydrogen fuel cells is the production of hydrogen. There are a few ways to make the hydrogen, but they mainly use an expensive platinum catalyst. The expense is the main limiting factor for the development and implementation of hydrogen fuel cells. Catalysts were used to speed up a reaction by lowering the activation energy of a reaction. Activation energy was the energy necessary for reactants to reach a transition state. The Gibb s Free Energy determines whether a reaction was spontaneous or Page 2 of 16

3 nonspontaneous, or in other words whether the reaction will occur on its own or not. Gibb s Free Energy determines whether a reaction will occur spontaneously. In the research by Luca et. al a catalyst was used to produce hydrogen. The idea of the Luca et. al experiment was to study if the reverse process was also true for a slightly reconfigured nickel catalyst. Luca, et al. focused on the use of a nickel pincer catalyst to produce hydrogen for fuel cells. 1 The particular type of ligand that was examined was in the family known as PCP ligands, or 2,6-C 6 H 3 (CH 2 PR 2 ) 2. 2 An attractive feature of PCP ligands was the variety of R groups that be attached through relatively easy synthesis. 2 The PCP ligand was compared to the POCOP ligand which have metal centers that are expected to be electron deficient. 3 Examples of POCOP and PCP ligands can be found in figure 1. Compound 1 had a POCOP ligand and compound 2 had a PCP ligand. Nickel compounds were studied because they could be a cheaper alternative to the platinum Page 3 of 16

4 compounds that are used. Nickel and platinum have the same amount of electrons in their valence shell. The particular electron configuration favored a square planar structure for both nickel and platinum compounds. The reaction was examined through electrochemistry and use of computer modeling. Density Functional Theory (DFT) is a computational technique used to study the structures of large molecules with many electrons. In smaller systems, the Schroedinger Wave Equation (SWE) can be solved by calculating the wavefunction for each electron in the molecule. Wavefunctions were time consuming to calculate, so Schroedinger s equation cannot be used to study large molecules. The definitions of large and small molecules are based on the number of electrons in the systems. Small systems have few electrons, around one to three. DFT can calculate everything that the SWE can, but DFT makes different approximations that simplify the calculations for large molecules. SWE takes into account all attractions and repulsions of all atoms and electrons in the system. If all of the attractions and repulsions were taken into account for the many bodied molecules calculations take a lot of time and would be complex. DFT approximates the attractions and repulsions of the atoms and electrons to make the problem easier to solve. DFT molecular modeling calculations are simplified by calculating a molecules electron density and not the wavefunction for each electron. Atkins and de Paula showed that the molecule s electron density can be used to calculate the energy and structure of the molecule. 4 Luca et. al used DFT to calculate the energies of the catalysts and intermediates of the reactions in the catalytic cycle. Through the computer calculations an accurate catalytic cycle can be hypothesized. Page 4 of 16

5 The main technique used in the Luca et. al study was cyclic voltammetry. This technique was used calculate the turnover frequency and investigate the catalytic cycle. Cyclic voltammetry is a technique where the voltage going through the system is changed over time while the current is monitored. When there was a change in current that means the catalyst has undergone oxidation or reduction. The reason there was a change in the current when oxidation or reduction happen and that is because the molecules are reaching the potential in which they become oxidized or reduced, Skoog and Leary. 5 The current rapidly increased as the necessary potential to cause the oxidation or reduction occur and the rapid decrease was when there are no more molecules that can react. There was no change in current because there were no species in the range of potentials that would be oxidized or reduced. The cyclic part means that if the voltage was raised to a point and then lowered back to the original voltage. This was also true if the voltage is lowered at first, then it will be raised to the original voltage. In the Luca et. al study, cyclic voltammetry was used to observe the catalytic cycle of the nickel pincer catalyst. Cyclic voltammetry was also used to determine the efficiency of a catalyst by measuring the overpotential. The overpotential is a measure of energy that was lost to heat or that was used to create reaction byproducts. Faradaic yield is the measure of the efficient use of the potential that is used for the reaction. Faradaic yield is a comparison of expected potential of a reaction and the observed potential of the reaction. A standard cell potential can be calculated with the reduction potentials of each part of the reaction. The observed potential is the potential measured with cyclic voltammetry. The difference of the expected potential and the observed Page 5 of 16

6 potential is the Faradaic potential, or overpotential. The larger the overpotential the less efficient the reaction is. The inefficiency of the reaction is caused by the loss of potential as heat or with the production of waste products. As a final qualitative technique to observe whether hydrogen was produced or not, 1 H NMR or proton nuclear magnetic resonance was used. NMR was used to observe chemical shifts of specific atoms in a molecule. Chemical shifts occurred because of shielding of electrons. The more the peaks observed have shifted the more or less shielded the electrons were. A standard was used as the zero for a point of reference. The position of the peaks compared to the standard has been used to determine what the atom being analyzed was bonded to at the time. In proton NMR, protons are the atoms being analyzed and manipulated with nuclear magnetic radiation. Chemical shifts are constants depending on what the atom was bonded to so it was easy to determine what molecules are found in the samples. Results and Discussion: The chemical that was studied was a nickel pincer catalyst. The nickel compound was being used to produce hydrogen gas from protons for use in a fuel cell or for other possible uses for hydrogen. The reaction for the production of hydrogen was being observed with cyclic voltammetry. The ligands were the main focus of the experiment. There were tests done with two different pincer ligands 1 and 2 in figure 1. The different ligands have different donating strength which was found to be a factor in the faradaic yield of the reaction. The stronger the donator the better the observed faradaic yield was. Each of the compounds shown in figure one had similar tridentate ligands, or ligands that bonded to Page 6 of 16

7 three different positions on the nickel atom. Compound 1 had aromatic rings attached to phosphorous atoms that were attached to the nickel atom. The two phosphorus atoms were bonded in a trans configuration, bonded to the opposite side of the nickel atom. At the third connection of the ligand an aromatic ring was bonding to the nickel atom. Compound 1 also had oxygen atoms attached to the phosphorous atoms. The oxygen atoms have a higher electronegativity, higher electronegativity atoms have a stronger pull for electrons, and because of this the electrons around the phosphorus atoms were pulled toward the oxygen atoms and not used to help stabilize the nickel atoms. Compound 2 of the other hand has tertiary-butyls attached to the phosphorus atoms and no oxygen atoms attached to the phosphorus. The lack of oxygen and high electronegative atoms in general meant that more electrons could be donated to help stabilize the nickel atom. Another factor on the reaction is the bulk of the ligand. The bulky ligands like tertbutyl on the phosphorus could affect the reduction potentials of the compound. 6 Bulky ligands cause steric hindrance that affects how and in what conditions an incoming ligand will attach to the metal center. Steric interactions of phosphine ligands have shown an influence on the donor ability of hydride as well as the electronic properties of the phosphine ligands. 7 Phosphine ligands have been known to be either sigma-donors or pi-acceptors and this was examined in the experiment by Luca et. al. Sigma orbitals have direct overlap between orbitals of the ligand and metal and there is more direct electron sharing between the two than when looking at pi orbitals. 8 A sigma-donor is when the ligand donates the electrons it has in the sigma bond to the metal orbitals. Pi orbitals Page 7 of 16

8 are parallel orbitals in the ligand and metal atoms. 8 The sharing of electrons in pi orbitals is weaker than those of the sigma orbitals because of the lower amount of overlap between the orbitals. Pi-acceptors have empty pi orbitals and pull electrons away from the metals. Pi-donors are when the ligand has filled p orbitals. The p orbitals on the ligand can then donate electrons to the d orbitals in the metal atom. The ligand for compound 1 in figure 1 was a pi-acceptor ligand. The ligand was pi-acceptor because the aromatic rings created a back-bonding with the phosphorus atom. Backbonding is when the electrons from the d orbitals are donated to the ligand 8, and this generally happens when there are more than one atom in the ligand. The back-bonding pulled the electrons off of the phosphorus, so the phosphorus had empty pi orbitals which then pulled off the electrons from the nickel atom. The tert-butyl phosphine group was a sigma-donor or pi-donor. The difference between the aromatic ring and the tertbutyl group is the resonance present in the aromatic ring. The bonds between the carbons in the aromatic ring were about 1.5 rather than the 1 in the tert-butyl group. Resonance is when a few different Lewis dot structures can be drawn for a particular molecule. For aromatic rings carbons can have double bonds to the carbons on either side. The transformation of the ring from Lewis dot structure is easy because the energy levels of the two configurations are more or less the same. This similar energy means that the average bond strength is equal to that of about 1.5 bonds instead of single or double bonds. The resonance stabilizes the ring, but it also creates an electron deficiency. The resonance and back-bonding in compound 1 was the reason that it was a pi-acceptor. The reason that the donor ligand produced a more efficient catalyst was because the nickel metal was stabilized. Page 8 of 16

9 In the cyclic experiments were done to observe the effects of the donating strengths of the different ligands present in compound 1 and compound 2. Compound 1 had a larger negative overpotential when observed with cyclic voltammetry when compared compound 2. The difference in the overpotentials, as shown in Table 1, shows that compound 2 had a better yield because the overpotential of the reaction was lower for compound 2 than compound 1. Both compound 1 and compound 2 had fairly good faradaic yields of at least 90%. For this reaction only about 10% of the potential was lost to heat and the creation of byproducts. The higher the faradaic yields the more efficient the reaction was with the energy that was being added to the reaction. The higher the faradaic yield is the more efficient the reaction is with the compound 1 had a lower rate of reaction than compound 2. Compound 1 had a reaction rate of M/s while compound 2 had a reaction rate of M/s. The main difference in the ligands was the donating strength of the phosphorus atoms with and without the oxygen atoms, so the donating strength seems to have a profound effect on the speed of the reactions. Another smaller effect on the donating strength of the ligands was the aromatic rings on Page 9 of 16

10 the phosphorus in compound 1 versus the t-butyls on the phosphorus in compound 2. The t-butyl on the pincer ligand was a stronger electron donor than the pincer ligand with the aromatic rings. T-butyl was a stronger donor because the aromatic rings share electrons between themselves and have resonance structures that can pull electrons from the phosphorous that was bound to the aromatic rings. The pulling of electrons away from the phosphorous means that there were fewer electrons that could be donated to the nickel center while the t-butyl did not pull electrons away from the phosphorous so more electrons could be donated to the nickel center. Redox active ligands are not required for pincer complexes to be active proton reduction. The PCP framework supports the catalytically active compound than what was used in previous experiments for the production of hydrogen. The system used in this experiment gives a better representation of the intermediates involved in the mechanism. Small amounts of 1.0M HCl were incrementally added to the solution that was being studied and the current increased as more and more HCl was added. The properties of the compounds vary based upon several different conditions. 9 The main conditions that affect the redox properties of the ligands are solvent or electrolytes. 9 The use of HCl was a test to see what effects a more acidic condition would have on the reactions redox potential. There was a large current change was caused by the increase of hydrogen that was available for reaction. Studies have been done and proven that ph and variations electrode potentials can affect the rate of the H 2 oxidation and evolution. 10 H 2 evolution is the release of hydrogen gas from a solution or compound. The added hydrogen atoms changed the equilibrium of the reaction. The hydrogen atoms dissociated from the chloride atoms they were bonded to in the HCl. Page 10 of 16

11 Equilibrium is when the forward and reverse reaction occur at similar rates. Catalysts have been used to force reactions further towards the products rather than the reactants. The addition of more reactants forced the reaction to occur quicker than would have been the case otherwise. Part of the large increase that was observed was the reaction of the chloride. Under the same conditions chloride atoms react similarly to Page 11 of 16

12 what was observed which was immediate bubbling after the acid was added to the solution. 1 DFT was used to characterize the structure, spin, and electron densities of compounds 2, 2-H, and 2-MeCN. DFT was also used to calculate the free energy of the compounds at each point in the catalytic cycle. DFT was used to propose a catalytic cycle, shown in figure 3, and the cycle was observed with cyclic voltammetry. The catalyst started with compound 2-MeCN and the nickel atom had an oxidation state of +2. The first step was the reduction of the nickel atom to a +1 oxidation state. The Page 12 of 16

13 second step involved the loss of the acetonitrile ligand and the oxidative addition of hydrogen. Oxidative addition is the addition of the ligand and the increase of oxidation state of the metal atom by 2 8, so in the Luca et. al experiment the nickel atom changed from an oxidation state of +1 to +3. The nickel atom was then reduced to a +2 oxidation state in the next step. Then hydrogen was added to the hydride ligand on the nickel making the ligand a dihydrogen complex. The dihydrogen ligand was thought to be unstable. This could be due to hydrogen only able to make 2 bonds since its valence shell can only hold 2 electrons. The final step was the substitution of the acetonitrile ligand for the dihydrogen ligand. The release of the hydrogen gas was the purpose of the catalytic reaction. 1 H NMR was the final technique done by Luca et. al. The NMR was run to determine whether or not hydrogen gas was indeed found after the reaction occurred. Compound 2-H was found to react cleanly to 2-MeCN by comparing the sample with a separate sample prepared from the abstraction of Cl - from compound 2 to produce compound 2-MeCN. Abstraction was nothing more than removal of the chloride atom and substituting in the acetonitrile ligand. Conclusion: Through the analysis of the cyclic voltammograms it was found that the faradaic yield of the nickel catalyst was around 95%. The catalytic cycle that was proposed from the calculations from the DFT and computer modeling was proven correct by a subsequent cyclic voltammogram. Future experiments will be to look at possibly other metals or compounds that are easy to produce or more efficient. Nickel might not be the most efficient metal that could Page 13 of 16

14 be used. The cost of the nickel could be the cheapest out of the possibilities, but the byproducts should also be tested to find out if there are any hazardous waste byproducts of the reaction. Even though the faradaic yield is high and the amount of byproducts should be small it would really depend on what the byproducts are as to whether the use of the nickel catalyst is worthwhile or if the byproducts are not worth the effect on the environment. The turnover number of the catalysts should be analyzed to give a perspective on how much of the catalyst would be needed to produce some significant amount of hydrogen gas. Ligands would be another area to still test. Another thing that should be looked into is the byproducts and waste caused by this catalyst. Some past research has been done on hydrogenase and the ligand present there contained an amine side group and tests have been done to try and replicate and examine the effects of the amine side group. 11 A future experiment that could be done to compare the use of the PCP ligand and a ligand with a pendant base. The implications of this are that they are one step closer to a cheap and greener fuel source than oil. This would reduce the necessity for oil and would not cause as many international conflicts for oil. There was no data about byproducts or waste that was created from this experiment and the catalytic cycle so there is a question about how green and safe for the environment that the new catalyst would be. This could be an important ethical issue as the byproducts and waste could be fairly nasty to the environment and the workers that would be required to work with the chemicals and waste, if the catalyst is used and the byproducts are more harm to the environment than the move from fossil fuels. Page 14 of 16

15 The experiments on the compounds used by Luca et. al are just a small step to what could be done. Many more experiments will be necessary to discover the best complex to produce the hydrogen gas cleanly and efficiently. References: 1. Luca, O. R.; Blakemore, J. D.; Konezny, S. J.; Praetorius, J. M.; Schmeier, T. J.; Hunsinger, G. B.; Batista, V. S.; Brudvig, G. W.; Hazari, N.; Crabtree, R. H., Organometallic Ni pincer complexes for the electrocatalytic production of hydrogen. Inorganic chemistry 2012, 51 (16), Boro, B. J.; Duesler, E. N.; Goldberg, K. I.; Kemp, R. A., Synthesis, characterization, and reactivity of nickel hydride complexes containing 2,6- C6H3(CH2PR2)2 (R = tbu, chex, and ipr) pincer ligands. Inorganic chemistry 2009, 48 (12), Zhu, K.; Achord, P. D.; Zhang, X.; Krogh-Jespersen, K.; Goldman, A. S., Highly Effective Pincer-Ligated Iridium Catalysts for Alkane Dehydrogenation. DFT Calculations of Relevant Thermodynamic, Kinetic, and Spectroscopic Properties. Journal of the American Chemical Society 2004, 126 (40), Atkins, P.; Paula, J. d., Atkin's Physical Chemistry. 2010, Ninth Edition. 5. Skoog, D. A.; Leary, J. J., Principles ofinstrumental Analysis. 1992, Fourth Edition. 6. Wiedner, E. S.; Yang, J. Y.; Dougherty, W. G.; Kassel, W. S.; Bullock, R. M.; DuBois, M. R.; DuBois, D. L., Comparison of Cobalt and Nickel Complexes with Sterically Demanding Cyclic Diphosphine Ligands: Electrocatalytic H2Production by [Co(PtBu2NPh2)(CH3CN)3](BF4)2. Organometallics 2010, 29 (21), Kilgore, U. J.; Stewart, M. P.; Helm, M. L.; Dougherty, W. G.; Kassel, W. S.; DuBois, M. R.; DuBois, D. L.; Bullock, R. M., Studies of a series of [Ni(P(R)2N(Ph)2)2(CH3CN)]2+ complexes as electrocatalysts for H2 production: substituent variation at the phosphorus atom of the P2N2 ligand. Inorganic chemistry 2011, 50 (21), Miessler, G. L.; Tarr, D. A., Inorganic Chemistry. 2011, Fourth Edition. 9. Konezny, S. J.; Doherty, M. D.; Luca, O. R.; Crabtree, R. H.; Soloveichik, G. L.; Batista, V. S., Reduction of Systematic Uncertainty in DFT Redox Potentials of Transition-Metal Complexes. The Journal of Physical Chemistry C 2012, 116 (10), Page 15 of 16

16 10. Vincent, K. A.; Parkin, A.; Armstrong, F. A., Investigating and Exploiting the Electrocatalytic Properties of Hydrogenases. Chemical Reviews 2007, 107 (10), Wilson, A. D.; Newell, R. H.; McNevin, M. J.; Muckerman, J. T.; Rakowski DuBois, M.; DuBois, D. L., Hydrogen Oxidation and Production Using Nickel-Based Molecular Catalysts with Positioned Proton Relays. Journal of the American Chemical Society 2005, 128 (1), Page 16 of 16