16 Jun 2009

Energy Nuggets: Wise Energy Use – The Challenge of Nitrogen Fixation

Literature Discussion

Submitted by Maggie Geselbracht, Reed College
Categories
Description: 
This literature discussion activity is one of a series of “Energy Nuggets,” small curricular units designed to illustrate: The Role of Inorganic Chemistry in the Global Challenge for Clean Energy Production, Storage, and Use.

The chemistry of nitrogen fixation, converting N2 into NH3, serves as the gateway into the nitrogen cycle and requires a catalyst.  Industrially, this process is carried out at high temperatures and pressures and consumes a large amount of energy.  Inorganic chemists have long been interested in how nature accomplishes this same reaction under ambient conditions with the enzyme nitrogenase.  This learning object examines two key papers related to this field through a series of discussion questions.  In the first paper by Doug Rees’s group, a high resolution X-ray crystal structure reveals the details of the active site of nitrogenase.  The second paper from Richard Schrock’s group reports a synthetic molybdenum complex that catalytically converts N2 into NH3.  

Learning Goals: 

After reading these papers and working through the discussion questions, a student will be able to:

  • Discuss the industrial production of ammonia including the chemistry, the production scale, and the energy requirements.
  • Describe our current level of understanding of the enzyme nitrogenase and the structure of the active site of dinitrogen reduction in the Fe-Mo cofactor.
  • Discuss the structure and characterization of the first dinitrogen complex as well as later complexes capable of reducing N2 to NH3 at a metal center.
  • Describe the unique characteristics of the Schrock catalyst, present the proposed mechanism for the reduction of N2 to NH3, and the clever conditions used to achieve catalytic turnover.
  • Use Web of Science to conduct a citation search and discuss the impact of a landmark paper on related research.
  • Gain confidence and experience in oral presentation skills including the use of PowerPoint.
Implementation Notes: 
I divided my class into two teams, one team assigned to each of the two papers.  I let the students choose which team they were on and sign up for a discussion question based on their interest.  The team was responsible for teaching the rest of us what the paper was all about during a class meeting (80 minutes).  The students used PowerPoint to present the answers to the discussion questions in some unified fashion.  The assumption was that the other team had not read the paper that was being presented, but they were asked to record their answers to the discussion questions based on what they learned in class.
Time Required: 
2 class sessions
Evaluation
Evaluation Methods: 
Adam Johnson from Harvey Mudd College and I collaborated to develop several peer evaluation rubrics for this learning object.  A peer evaluation form was handed out to the “audience” that included a copy of the discussion questions assigned to the presenting team.  The students in the audience were asked to answer the questions based on what they learned from the presenting team as well as evaluate the presenters both individually and as a team in their effectiveness.  An intragroup peer evaluation was handed out to the presenting team in which the team members were asked to numerically rate themselves (on a scale of 1 to 10) and the other members on content, reliability, preparation, and style.
Evaluation Results: 
The PowerPoint presentations were generally quite effective, although most students agreed that this format did not work very well to stimulate discussion.  Both teams were focused on getting through the presentations.  While they would answer my questions, there were not a lot of questions from the other students.  However, content was effectively conveyed as measured by the observation that nearly all audience members were able to answer the questions based on the presentations.  In the intragroup evaluations, some students were particularly hard on themselves.  I ended up averaging their scores, and adding in my own score to reach a final assessment for this project.
Creative Commons License: 
Creative Commons Licence

Comments

Do you think this could work within a General Chemistry course?

 

I think the topic is certainly well suited for a General Chemistry course.  In my opinion, the papers are a bit too sophisticated for Gen Chem.  However you might be able to make it work if you provided some guidance for the students in their reading.  The Perspective article from Science on Schrock's catalyst, for example, would probably work better than the actual paper.  Perhaps you could find something similar on the structure of nitrogenase?  Does anyone else have any other ideas about how to adapt it to the Gen Chem level?  

I used this activity in my class during spring term 2009.  I divided my class in half;  one half presented the Schrock paper on one week, while the other half presented the Rees paper the following week. 

The presentations were ok, but not great.  I would encourage groups to meet with the instructor in advance to plan their presentation.  Groups should use handouts instead of relying on powerpoint presentations.  The students in teh class were frantically trying to copy everything down on the summary sheets (I changed the instructions for week 2 so that they don't need complete sentences but just were to summarize the main points.)  All in all, they tended to hit the main points of the article, and answered most of the questions from the other half of the class well.

Adam

Thanks so much for this activity - I'm coupling it with a discussion of the new Fe catalyst from the Peters group and will post the discussion questions after we have class.  

I do have a question though related to the oxidation states/magnetism of the intermediate complexes they presented.  Complex 2 is Mo(IV) and diamagnetic.  Complex 6 is Mo(IV) and paramagentic.  I understand the oxidation states, but how is it diamagnetic in one case and paramagnetic in the other?  

The complexes are not well defined in that paper, but complex 2 is a Mo=N (putative triple bond) and the 2 d-electrons are likely paired up in a non-bonding orbital. Complex 6, also Mo(IV) has no pi ligands and the 2 d-electrons can end up in one of 2 non-bonding degenerate orbitals.

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