Combinatorial screening of electrochemical catalysts by current-voltage methods can be unwieldy for large sample sizes. By converting the ions generated in an electrochemical half-cell reaction to a fluorescence signal, the most active compositions in a large electrode array have been identified. A fluorescent acid-base indicator was used to image high concentrations of hydrogen ions, which were generated in the electrooxidation of methanol. A 645-member electrode array containing five elements (platinum, ruthenium, osmium, iridium, and rhodium), 80 binary, 280 ternary, and 280 quaternary combinations was screened to identify the most active regions of phase space. Subsequent “zoom” screens pinpointed several very active compositions, some in ternary and quaternary regions that were bounded by rather inactive binaries. The best catalyst, platinum(44)/ruthenium(41)/osmium(10)/iridium(5) (numbers in parentheses are atomic percent), was significantly more active than platinum(50)/ruthenium(50) in a direct methanol fuel cell operating at 60°C, even though the latter catalyst had about twice the surface area of the former.

https://science.sciencemag.org/content/sci/280/5370/1735.full.pdf

Combinatorial Electrochemistry: A Highly Parallel, Optical Screening Method for Discovery of Better Electrocatalysts

The fate of many atmospheric trace species, including pollutants such as nitrogen oxides and some volatile organic compounds, is controlled by oxidation reactions In the daytime troposphere, these reactions are dominated by photochemically produced OH radicals; at night and in polluted environments, NO3 radicals are an important oxidant1 Ozone can contribute to the oxidation of atmospheric species during both day and night1 In recent years, laboratory investigations2,3,4, modelling studies5,6,7, measured Cl deficits in marine aerosols8 and species-nonspecific observations9,10,11 of gaseous inorganic chlorine compounds other than HCl have suggested that reactive halogen species may contribute significantly to—or even locally dominate—the oxidative capacity of the lower marine troposphere Here we report night-time observations of molecular chlorine concentrations at a North American coastal site during onshore wind flow conditions that cannot be explained using known chlorine chemistry The measured Cl2 mixing ratios range from <10 to 150 parts per 1012 (ppt), exceeding those predicted5 for marine air by more than an order of magnitude Using the observed chlorine concentrations and a simple photochemical box model, we estimate that a hitherto unrecognized chlorine source must exist that produces up to 330 ppt Cl2 per day The model also indicates that early-morning photolysis of molecular chlorine can yield sufficiently high concentrations of chlorine atoms to render the oxidation of common gaseous compounds by this species 100 times faster than the analogous oxidation reactions involving the OH radical, thus emphasizing the locally significant effect of chlorine atoms on the concentrations and lifetimes of atmospheric trace species in both the remote marine boundary layer and coastal urban areas

Unexpectedly high concentrations of molecular chlorine in coastal air

Combinatorial methods in the development of enantioselective homogeneous catalysts constitute a new branch of catalysis research. The goal is to prepare libraries of potential asymmetric catalysts, rather than choosing the traditional one-catalyst-at-a-time approach. Several conceptional advancements have been reported in the parallel preparation of chiral ligands. Currently the most meaningful systems constitute modularly constructed ligands on solid supports, which allow high degrees of structural diversity and thus the maximum probability of finding enantioselective catalysts or even new types of ligands for asymmetric catalysis. Search strategies have been developed which amongst other things, lead to catalysts not likely to have been discovered by traditional methods. Genuine application of such strategies involve thousands of catalysts and require high-throughput screening systems capable of assaying enantioselectivity. The first high-throughput ee-screening systems were in fact developed for use in the directed evolution of enantioselective enzymes, a process based on "evolution in the test tube" in which the appropriate methods of random mutagenesis, gene expression, and ee assays are combined. Since no screening system is likely to be universal, different approaches are necessary. Thus far these include assays based on UV/Vis, fluorescence, circular dichroism, mass spectrometry, and even modified gas chromatography as well as special forms of capillary electrophoresis. One of the most efficient systems involves the concept of the mass-spectrometric detection of deuterium-labeled pseudo-enantiomers and pseudo-prochiral compounds with which about 1000 exact ee determinations can be achieved per day, although the assay is restricted to kinetic resolution and/or reactions of prochiral compounds bearing enantiotopic groups. Super-high-throughput screening for enantioselectivity is possible in many cases by making use of chirally modified capillary array electrophoresis in a parallel step. Accordingly, 7000 to 30 000 ee determinations can be carried out per day. These and other analytical developments are expected to stimulate further research in the combinatorial search for asymmetric homogeneous catalysts and in the directed evolution of enantioselective enzymes for use in organic chemistry.

Combinatorial and Evolution-Based Methods in the Creation of Enantioselective Catalysts.

Methods and apparatus for the preparation and use of a substrate having an array of diverse materials in predefined regions thereon. A substrate having an array of diverse materials thereon is generally prepared by delivering components of materials to predefined regions on a substrate, and simultaneously reacting the components to form at least two materials. Materials which can be prepared using the methods and apparatus of the present invention include, for example, covalent network solids, ionic solids and molecular solids. More particularly, materials which can be prepared using the methods and apparatus of the present invention include, for example, inorganic materials, intermetallic materials, metal alloys, ceramic materials, organic materials, organometallic materials, non-biological organic polymers, composite materials (e.g., inorganic composites, organic composites, or combinations thereof), etc. Once prepared, these materials can be screened for useful properties including, for example, electrical, thermal, mechanical, morphological, optical, magnetic, chemical, or other properties. Thus, the present invention provides methods for the parallel synthesis and analysis of novel materials having useful properties.

Combinatorial synthesis of novel materials

Combinatorial synthesis methods allow the rapid preparation and processing of large libraries of solid-state materials. The use of these methods, together with the appropriate screening techniques, has recently led to the discovery of materials with promising superconducting1, magnetoresistive2, luminescent3,4,5 and dielectric6 properties. Solid-state catalysts, which play an increasingly important role in the chemical and oil industries7, represent another class of material amenable to combinatorial synthesis. Yet typically, catalyst discovery still involves inefficient trial-and-error processes8,9,10, because catalytic activity is inherently difficult to screen. In contrast to superconductivity, magnetoresistivity and dielectric properties, which can be tested by contact probes, or luminescence, which can be observed directly, the assessment of catalytic activity requires the unambiguous detection of a specific product molecule above a small catalyst site on a large library. Screening by in situ infrared thermography11 and microprobe sampling mass spectrometry12,13 have been suggested, but the first method, while probing activity, provides no information on reaction products, whereas the second is difficult to implement because it requires the transport of minute gas samples from each library site to the detection system. Here I describe the use of laser-induced resonance-enhanced multiphoton ionization for sensitive, selective and high-throughput screening of a library of solid-state catalysts that activate the dehydrogenation of cyclohexane to benzene. I show that benzene, the product molecule, can be selectively photoionized in the vicinity of the catalytic sites, and that the detection of the resultant photoions by an array of microelectrodes provides information on the activity of individual sites. Adaptation of this technique for the screening of other catalytic reactions and larger libraries with smaller site size seems feasible, thus opening up the possibility of exploiting combinatorial approaches as an efficient method for the discovery and optimization of solid-state catalysts.

High-throughput screening of solid-state catalyst libraries

A combinatorial library of catalyst candidates, each consisting of a different metal element supported on γ-alumina, is screened for hydrogen oxidation catalytic activity Heat liberated on the surface of active catalysts by the catalyzed reaction is detected by noninvasive IR thermography A 16-candidate library identifies four distinctly active pellets, which correspond to active formulations known from the literature A higher density library shows similar results, but heat and mass transport effects influence the pellet temperatures This method may be used to screen and optimize catalyst formulations more efficiently and quickly than current methods and may also be useful for study of operational lifetime, resistance to poisons, and regenerability

Infrared Thermographic Screening of Combinatorial Libraries of Heterogeneous Catalysts

Hot zone formation during carbon monoxide oxidation in a radial flow reactor

Back-and-forth-moving temperature fronts separating regions with high and low temperatures (amplitudes up to 100 K) formed on a Pd on alumina ring during the atmospheric oxidation of carbon monoxide. The back-and-forth front movement reflects the intrinsic nonuniformity of the catalytic system and the impact of global coupling. The ignition and extinction fronts usually moved at different velocities, in agreement with theoretical predictions. Complex spatiotemporal patterns were observed due to small-amplitude temperature fronts that moved on the high-temperature region at a much higher velocity than the high-amplitude fronts. The qualitative motion features were affected by changes in the excess oxygen concentration. The nonuniform states existed mainly for feed conditions for which the only uniform state was an extinguished one.

Oscillating temperature pulses during CO oxidation on a Pd/Al2O3 ring

The atmospheric oxidation of a mixture containing 6 vol % carbon monoxide was carried out on a hollow cylindrical catalytic pellet. The catalyst was held in a conical reactor which enabled simultaneous measurement of the temperature patterns on the top and side of the pellet by an IR imager. Upon a decrease in the reactor temperature the fully ignited, high temperature state of the pellet is transformed to a nonuniform one with temperature fronts separating high and low temperature regions. The transition and the resulting states are rather intricate and are strongly influenced by the nonuniformity of the catalyst and the transport to and from it, as well as the global coupling, which stabilizes temperature fronts and patterns, which would not exist in its absence. Intricate pulse splitting and extinction were observed both on the top and the side of the pellet. Highly irregular motions and conversions were obtained following a decrease in the reactor temperature.

Temperature patterns on a hollow cylindrical catalytic pellet.

Stationary and spatially oscillating temperature fronts separating regions with high and low temperatures (amplitudes up to 120 K) formed during the atmospheric oxidation of carbon monoxide on a Pd on alumina ring kept in a mixed reactor. The sharp fronts bounded either one or two low temperature regions. The transition from stationary to spatially oscillating fronts occurred at a feed CO/O2 concentration ratio close to 2.0 and was independent of the reactor temperature. The net rate of local heat generation on the ring was nonuniform leading to angular variation of the temperature of the ignited state. Front motions were affected by both the nonuniformity of the system and the interaction between the catalyst and the ambient gas. The two fronts bounding a low temperature region usually moved at different velocities and their velocities increased with increasing oxygen concentration. In most cases the two fronts did not move in phase, but sometimes they moved in tandem generating a back-and-forth pulse motion. The spatial amplitude of the oscillations decreased as the slope of the net heat generation activity became steeper. The spatial variation of the standard deviation of the temperature was helpful in assessing the dynamics of the oscillating fronts.Stationary and spatially oscillating temperature fronts separating regions with high and low temperatures (amplitudes up to 120 K) formed during the atmospheric oxidation of carbon monoxide on a Pd on alumina ring kept in a mixed reactor. The sharp fronts bounded either one or two low temperature regions. The transition from stationary to spatially oscillating fronts occurred at a feed CO/O2 concentration ratio close to 2.0 and was independent of the reactor temperature. The net rate of local heat generation on the ring was nonuniform leading to angular variation of the temperature of the ignited state. Front motions were affected by both the nonuniformity of the system and the interaction between the catalyst and the ambient gas. The two fronts bounding a low temperature region usually moved at different velocities and their velocities increased with increasing oxygen concentration. In most cases the two fronts did not move in phase, but sometimes they moved in tandem generating a back-and-forth pulse mo...

J. Annamalai

Papers

Infrared Thermographic Screening of Combinatorial Libraries of Heterogeneous Catalysts

Hot zone formation during carbon monoxide oxidation in a radial flow reactor

Oscillating temperature pulses during CO oxidation on a Pd/Al2O3 ring

Temperature patterns on a hollow cylindrical catalytic pellet.

Effects of reactant composition and nonuniformities on temperature fronts