Kapur N.,Nanostellar |
Hyun J.,Nanostellar |
Shan B.,Nanostellar |
Nicholas J.B.,Nanostellar |
Cho K.,University of Texas at Dallas
Journal of Physical Chemistry C | Year: 2010
Previously reported syngas conversion experiments on silica-supported Rh nanoparticles show that CO conversion and oxygenate selectivity vary as a function of nanoparticle size. Theoretical studies in the literature have examined the effect of steps on CO dissociation, but structure sensitivity for C1 and C2 oxygenates has not been systematically investigated. In this study, density functional theory-based reaction energetics and kinetics for C-H, C-C, C-O, and O-H bond formation on flat Rh(111) and stepped Rh(211) surfaces are reported and compared. Multiple paths for methanol and ethanol formation are considered to ascertain the lowest energy pathways. Nearly an identical methanol formation route via CO → CHO→ CH 2O → CH3O → CH3OH is found to be favored on both Rh terrace and (211) sites. CO insertion into CH2 is deduced to be the precursor for C2 oxygenate formation irrespective of site structure. Ethanol formation pathways, however, are determined to be markedly different on flat and stepped Rh surfaces in terms of barriers and intermediates. Our results show that reaction pathways are typically preferred on Rh step sites irrespective of the bond-breaking and -making (C-H, C-C, and C-O) reactions considered. © 2010 American Chemical Society.
Kapur N.,Nanostellar |
Shan B.,Nanostellar |
Hyun J.,Nanostellar |
Wang L.,Nanostellar |
And 3 more authors.
Molecular Simulation | Year: 2011
Vehicle emission control regulations necessitate the removal of carbon monoxide (CO) from engine exhausts via CO oxidation. Although bismuth (Bi)-promoted platinum (Pt) catalysts show improvement in CO oxidation performance over pure Pt, it is still not known whether Bi acts simply as a site blocker to reduce CO poisoning or whether it is an active participant in the catalytic reactions. In this study, we report density functional theory-based CO oxidation energetics and kinetics in the presence of different Bi dopant configurations. Adsorption energies for both CO and oxygen atoms are found to be reduced by Bi doping. Bi dopant also creates surface areas in its vicinity where CO adsorption is prohibited, whereas molecular oxygen dissociation is promoted. Significant reduction is shown for CO oxidation barriers on Bi-modified Pt(111) surfaces leading to exothermic CO2 formation. Our results elucidate that promoting Pt with Bi affects both the electronic properties of the catalyst and alters the Pt ensemble size available for elementary reactions within CO oxidation mechanism. © 2011 Taylor & Francis.
Chen R.,Huazhong University of Science and Technology |
Chen Z.,Huazhong University of Science and Technology |
Ma B.,Huazhong University of Science and Technology |
Hao X.,Nanostellar |
And 4 more authors.
Computational and Theoretical Chemistry | Year: 2012
Interaction of carbon monoxide (CO) with transition metal surfaces is an essential part of CO oxidation catalysis. In this report, we investigate and compare CO adsorption behavior on Pt (1. 1. 1) and Pd (1. 1. 1) surfaces combining first-principles (FP) calculations and lattice gas Monte-Carlo (LG-MC) simulations. Our results indicate that despite stronger CO binding on Pd (1. 1. 1) at low coverage, more repulsive lateral interactions on Pd surface lead to a more rapid adsorption energy decrease with respect to coverage. This results in lower saturation coverage and weaker CO desorption energies on Pd (1. 1. 1), which could contribute to its excellent reactivity observed under high pressure reaction conditions. © 2011 Elsevier B.V.
Allian A.D.,University of California at Berkeley |
Allian A.D.,Abbott Laboratories |
Takanabe K.,University of California at Berkeley |
Takanabe K.,King Abdullah University of Science and Technology |
And 10 more authors.
Journal of the American Chemical Society | Year: 2011
Kinetic, isotopic, and infrared studies on well-defined dispersed Pt clusters are combined here with first-principle theoretical methods on model cluster surfaces to probe the mechanism and structural requirements for CO oxidation catalysis at conditions typical of its industrial practice. CO oxidation turnover rates and the dynamics and thermodynamics of adsorption-desorption processes on cluster surfaces saturated with chemisorbed CO were measured on 1-20 nm Pt clusters under conditions of strict kinetic control. Turnover rates are proportional to O2 pressure and inversely proportional to CO pressure, consistent with kinetically relevant irreversible O2 activation steps on vacant sites present within saturated CO monolayers. These conclusions are consistent with the lack of isotopic scrambling in C16O-18O2-16O 2 reactions, and with infrared bands for chemisorbed CO that did not change within a CO pressure range that strongly influenced CO oxidation turnover rates. Density functional theory estimates of rate and equilibrium constants show that the kinetically relevant O2 activation steps involve direct O2* (or O2) reactions with CO* to form reactive O*-O-C*=O intermediates that decompose to form CO 2 and chemisorbed O*, instead of unassisted activation steps involving molecular adsorption and subsequent dissociation of O2. These CO-assisted O2 dissociation pathways avoid the higher barriers imposed by the spin-forbidden transitions required for unassisted O2 dissociation on surfaces saturated with chemisorbed CO. Measured rate parameters for CO oxidation were independent of Pt cluster size; these parameters depend on the ratio of rate constants for O2 reactions with CO* and CO adsorption equilibrium constants, which reflect the respective activation barriers and reaction enthalpies for these two steps. Infrared spectra during isotopic displacement and thermal desorption with 12CO- 13CO mixtures showed that the binding, dynamics, and thermodynamics of CO chemisorbed at saturation coverages do not depend on Pt cluster size in a range that strongly affects the coordination of Pt atoms exposed at cluster surfaces. These data and their theoretical and mechanistic interpretations indicate that the remarkable structure insensitivity observed for CO oxidation reactions reflects average CO binding properties that are essentially independent of cluster size. Theoretical estimates of rate and equilibrium constants for surface reactions and CO adsorption show that both parameters increase as the coordination of exposed Pt atoms decreases in Pt201 cluster surfaces; such compensation dampens but does not eliminate coordination and cluster size effects on measured rate constants. The structural features and intrinsic non-uniformity of cluster surfaces weaken when CO forms saturated monolayers on such surfaces, apparently because surfaces and adsorbates restructure to balance CO surface binding and CO-CO interaction energies. © 2011 American Chemical Society.
Nanostellar | Date: 2013-04-01
A monomer is added to a solvent containing metal salt and porous support materials and the solvent is stirred for a period of time to distribute and fix the metal in the pores of the support materials. The solids that are dispersed in the solvent are then separated from the liquid, dried and calcined to form heterogeneous catalysts. The monomer that is added is of a type that can be polymerized in the solvent to form oligomers or polymers, or both. When forming heterogeneous catalysts containing platinum, acrylic acid is selected as the preferred monomer.
Nanostellar | Date: 2010-12-30
An emission control catalyst is doped with bismuth, manganese, or bismuth and manganese. The doped catalyst may be a palladium-gold catalyst or a platinum-based catalyst, or both. The doped palladium-gold catalyst and the doped platinum-based catalyst may be contained in a single washcoat layer or in different washcoat layers of a multi-brick, multi-zoned, or multi-layered emission control system. In all embodiments, zeolite may be added as a hydrocarbon absorbing component.
News Article | April 3, 2007
When stealing copper, two bad things can happen: you can get caught and you can die from electrocution. Two New Hampshire men, presumed to be thieves, were buried this week. The two were found in an unoccupied power plant in Massachusetts next to some cutters and coils of wire, according to InformationWeek. They didn't have permission to be there. Two Arkansas men recently died trying to strip copper wires from utility poles, the magazine said. Chinese authorities are also contemplating relying on fiber rather than wire to connect the rural hinterlands of the sprawling nation. Why? Copper prices are rising. They were $3.14 in March. Other commodities have also become targets for thieves. Rising platinum prices have caused thieves to steal catalytic converters out of cars. Converters contain small amounts of platinum. Platinum pricing is driving companies like Nanostellar to come up with alternatives. Aluminum kegs have also become popular items to steal when aluminum prices climb, according to sources. But a keg or a catalytic converter can only stub your toe. No one has ever been fried.
News Article | October 20, 2006
Guess how much platinum there is in a Volkswagen Passat diesel? If you said $238 worth, you'd be right. Nanostellar is trying to reduce it. The company, which focuses on car emissions, has produced a platinum alloy that can substitute for the pure material inside catalytic converters, according to CEO Pankaj Dhingra. Recently, it began production of 250 kilograms a week. Platinum sprinkled in the catalytic converter captures gases like carbon monoxide and turns them into less dangerous compounds, such as carbon dioxide. But platinum costs a lot. Nanostellar's particles can cut around $56 to $117 out of the platinum budget and cut down on emissions. Crooks have also been stealing catalytic converters for the platinum lately. The stuff sells for $1,100 an ounce, after all. Nanostellar is trying to land deals with automakers, but its materials will end up in aftermarket converters in the relatively near future.
News Article | August 20, 2012
Modern diesel engines are more fuel efficient than gasoline engines. Cleaning up their exhaust is a bit more challenging, though, due to the large amount of oxygen involved in the combustion. In particular, removing the nitrogen oxides (NO ) formed as oxygen and nitrogen in the air reacting at high temperatures requires specialized systems and expensive catalysts like platinum. While everyone would like to get rid of the platinum, no materials have been found that match its catalytic performance in diesel engine exhaust. Until now, apparently. Research published recently in Science describes a new catalyst, a complex mixture of metal oxides including manganese, mullite, and the rare earth metals samarium and gadolinium (Mn-mullite (Sm, Gd)Mn O , to be precise), that actually performs better than platinum. And it’s cheaper. The work was performed by scientists at the nanotechnology startup Nanostellar, with collaborators at the Huazhong University of Science and Technology, University of Kentucky, and the University of Texas at Dallas. Let’s take a step back: why do we need these catalysts in the first place? At the high temperatures inside engines (around 1900K), oxygen and nitrogen in the air begin to dissociate and react to form nitric oxide (NO). Nitrogen dioxide (NO ) can then form by the oxidation of NO, but this is much slower—meaning most of the NO in the exhaust is just NO. There are two common ways to remove NO from diesel exhaust. The first is selective catalytic reduction (SCR), which reacts NO and NO with ammonia or urea in the presence of a (non-platinum) catalyst, forming nitrogen and water. The second is the NO trap, where zeolites absorb the molecules like a sponge. Later, the stored molecules can be reacted with excess fuel to release nitrogen. In both cases, you want to convert some of the NO into NO (the SCR reaction is fastest when you have equal amounts of the two, and the NO trap stores NO more efficiently). This is where the platinum comes into the picture: a catalyst is needed to jumpstart the reaction between oxygen and NO to form NO . Back to the new catalyst: how did it perform? The researchers exposed the material to a gas mixture of 450 parts per million NO and 10 percent oxygen (the rest was inert helium). Over a range of temperatures, the new catalyst performed better than platinum (around 64 percent better at 300 degrees C, and 45 percent better at 120 degrees C). Diesel engines also contain catalytic converters to remove carbon monoxide and unburned hydrocarbons. The authors added their new material to a commercial catalyst (based on platinum and palladium), and found that it oxidized NO without impeding the catalyst's original function. The next step, then, would be to create a catalyst (or modify this new one) that could replace the entire system—removing platinum altogether.