Investigating the Mechanism of Alkyne Hydrogenation through an Open-Ended, Inquiry-Based Undergraduate Research Project Exploring Heterogeneous Catalysis

The catalytic hydrogenation of alkenes and alkynes is an important part of the undergraduate chemistry curriculum and is a fundamental process in chemical industry. Inquiry-based laboratory activities are presented that investigate the hydrogenation of alkynes on a nanoparticle palladium surface to form alkenes, which go on to form alkanes. Alkyne hydrogenation using H2 and/or D2 proceeds via a vinyl-palladium intermediate to form a π-bonded alkene-Pd species that can desorb or remain on the palladium surface and undergo further hydrogenation via the Horiuti-Polanyi mechanism, associated with extensive deuterium-hydrogen exchange. Central to the experiments is an inexpensive, easy-to-build glass tube containing palladium nanoparticles on alumina beads that can be used indefinitely. A total of seven inquiry-based questions are discussed regarding hydrogenation of alkynes. A similar number of open questions are discussed for further investigations by interested persons. These activities are suitable as guided research projects for science majors. Each experiment is performed by groups of two or three students in about an hour including analysis by mass spectrometry. An additional hour is allowed for student analysis and discussion of the mass spectral results, writeup, and future planning followed by about 30 min with the mentor for group presentation and discussion of the results. Results often lead to additional questions, either for clarification or for new exploration and form the basis for inquiry-based learning and problem-solving.