Molecular catalysis science: Perspective on unifying the fields of catalysis

Abstract

Colloidal chemistry is used to control the size, shape, morphology, and composition of metal nanoparticles. Model catalysts as such are applied to catalytic transformations in the three types of catalysts: heterogeneous, homogeneous, and enzymatic. Real-time dynamics of oxidation state, coordination, and bonding of nanoparticle catalysts are put under the microscope using surface techniques such as sumfrequency generation vibrational spectroscopy and ambient pressure X-ray photoelectron spectroscopy under catalytically relevant conditions. It was demonstrated that catalytic behavior and trends are strongly tied to oxidation state, the coordination number and crystallographic orientation of metal sites, and bonding and orientation of surface adsorbates. It was also found that catalytic performance can be tuned by carefully designing and fabricating catalysts from the bottom up. Homogeneous and heterogeneous catalysts, and likely enzymes, behave similarly at the molecular level. Unifying the fields of catalysis is the key to achieving the goal of 100% selectivity in catalysis.Two major breakthroughs have revolutionized molecular catalysis science over the last 20 y. The first is in the development of nanomaterials science (1-4), which has made it possible to synthesize metallic (5-7), bimetallic, and core-shell nanoparticles (8, 9), mesoporous metal oxides (10, 11), and enzymes (12-16) in the nanocatalytic range between 0.8 and 10 nm. The second innovation is in the advancement of spectroscopy and microscopy instruments (17-20)-including nonlinear laser optics (21), sum-frequency generation vibrational spectroscopy (22-24), and synchrotronbased instruments, such as ambient pressure X-ray photoelectron spectroscopy (8,(25)(26)(27), X-ray absorption near-edge structure, extended X-ray absorption fine structure (28-30), infrared (IR) and X-ray microspectroscopies (31), and high-pressure scanning tunneling microscopies (32, 33)-that characterize catalysts at the atomic and molecular levels under reaction conditions (34). Most of the studies that use these techniques focus on nanoscale technologies, such as catalytic energy conversion and information storage, which have reduced the size of transistors to below 25 nm (35).Catalysts are classified into three types-heterogeneous, homogeneous, and enzymatic-and, in most cases, range in size from 1 to 10 nm, which is even smaller than the transistors being developed by the latest size-fabrication technologies. Heterogeneous catalysts work in reaction systems with multiple phases (e.g., solid-gas or solid-liquid phase); homogeneous catalysts reside in the same phase as the reactants, almost always in the liquid phase; and enzymatic catalysts, which are most active in an aqueous solution, make use of active sites in proteins. The catalysis of chemical energy conversion provides ever-increasing selectivity in producing combustible hydrocarbons, gasoline, and diesel.The tenets that direct our catalysis research involve nanoparticle synthesis, characterization under reaction con...

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