title: Methionine bound to Pd/γ-Al2O3 catalysts studied by solid-state (13)C NMR.
abstract: The chemisorption and breakdown of methionine (Met) adsorbed on Pd/γ-Al2O3 catalysts were investigated by solid-state NMR. (13)C-enriched Met (ca. 0.4mg) impregnated onto γ-Al2O3 or Pd/γ-Al2O3 gives NMR spectra with characteristic features of binding to γ-Al2O3, to Pd nanoparticles, and oxidative or reductive breakdown of Met. The SCH3 groups of Met showed characteristic changes in chemical shift on γ-Al2O3 (13ppm) vs. Pd (19ppm), providing strong evidence for preferential binding to Pd, while the NC carbon generates a small resonance at 96ppm assigned to a distinct nonprotonated species bound to O or Pd. Additionally, NMR shows that the SCH3 groups of Met are mobile on γ-Al2O3 but immobilized by binding to Pd particles; on small Pd particles (ca. 4nm), the NCH groups undergo large-amplitude motions. In a reducing environment, Met breaks down by C-S bond cleavage followed by formation of C2-C4 organic acids. The SCH3 signal shifts to 22ppm, which is likely the signature of the principal species responsible for strong catalyst inhibition. These experiments demonstrate that solid-state magic-angle spinning NMR of (13)C-enriched Met can be a sensitive probe to investigate catalyst surfaces and characterize catalyst inhibition both before reaction and postmortem.
authors.affiliation: Department of Chemistry, Iowa State University, Ames, IA 50011, United States.
authors.forename: Robert L
authors.initials: RL
authors.lastname: Johnson
authors.affiliation: Department of Chemical and Biological Engineering, University of Wisconsin, Madison, WI 53706, United States.
authors.forename: Thomas J
authors.initials: TJ
authors.lastname: Schwartz
authors.affiliation: Department of Chemical and Biological Engineering, University of Wisconsin, Madison, WI 53706, United States.
authors.forename: James A
authors.initials: JA
authors.lastname: Dumesic
authors.affiliation: Department of Chemistry, Iowa State University, Ames, IA 50011, United States; Department of Chemistry, Brandeis University, Waltham, MA 02453, United States. Electronic address: srohr@brandeis.edu.
authors.forename: Klaus
authors.initials: K
authors.lastname: Schmidt-Rohr
mesh_terms: D000537:Aluminum Oxide / Q000737:chemistry*; D002384:Catalysis; D009682:Magnetic Resonance Spectroscopy; D053768:Metal Nanoparticles / Q000737:chemistry*; D008715:Methionine / Q000737:chemistry*; D010084:Oxidation-Reduction; D010165:Palladium / Q000737:chemistry*; D011753:Pyrones / Q000737:chemistry; D013455:Sulfur / Q000737:chemistry; D013499:Surface Properties
chemical_list: D011753:Pyrones; D010165:Palladium; C030720:triacetic acid lactone; D013455:Sulfur; D008715:Methionine; D000537:Aluminum Oxide
keywords: Catalyst characterization postmortem; Multicp/MAS; Surface NMR