Self-cleaning perovskite type catalysts for the dry reforming of methane

Gas-to-liquid processes are generally used to convert natural gas or other gaseous hydrocarbons into liquid fuels via an intermediate syngas stream. This includes the production of liquid fuels from biomass-derived sources such as biogas. For example, the dry reforming of methane is done by reacting CH4 and CO2, the two main components of natural biogas, into more valuable products, i.e., CO and H2. Nickel containing perovskite type catalysts can promote this reaction, yielding good conversions and selectivities; however, they are prone to coke laydown under certain operating conditions. We investigated the addition of high oxygen mobility dopants such as CeO2, ZrO2, or YSZ to reduce carbon laydown, particularly using reaction conditions that normally result in rapid coking. While doping with YSZ, YDC, GDC, and SDC did not result in any improvement, we show that a Ni perovskite catalyst (Na0.5La0.5Ni0.3Al0.7O2.5) doped with 80.9 ZrO2 15.2 CeO2 gave the lowest amount of carbon formation at 800 °C and activity was maintained over the operating time.

Correlation of Static Ageing Effects on Automotive Catalysts

This study identifies and analyzes the effect that aging time and temperature have on the CO light-off activity of three-way catalyst samples, aged in a static air (oxidizing) atmosphere. The bench aging time (BAT) equation proposed by the Environmental Protection Agency (EPA), which describes aging as dependent upon time at temperature, was used to calculate a range of oven aging times and temperatures based on a RAT-A engine bench aging cycle.

CO light-off tests carried out on cores aged between 800 and 900 °C have shown that it is the aging temperature that has the greatest effect on catalyst deterioration for static aging testing, with aging time having little effect. These results were in contradiction to the BAT equation, an industry norm for the aging of catalysts. This demonstrates that static aging, whilst showing how temperature affects aging, gives little or no time effects. The results have shown that static aging is not representative of actual aging on a vehicle.

Progressive aging conducted at a temperature of 1000 °C was shown to cause a decrease in catalyst activity as the aging time increased. However, even in these extreme conditions, static aging gave a slower rate of aging with time when compared to engine aging as defined by the BAT equation. Overall, static aging in air has been shown to produce a greater increase in aging due to temperature than predicted by the BAT equation, but less aging due to aging time.

Advanced Ceramic Substrate with Ordered and Designed Micro-Structure for Applications in Automotive Catalysts

This study describes an innovative monolith structure designed for applications in automotive catalysis using an advanced manufacturing approach developed at Imperial College London. The production process combines extrusion with phase inversion of a ceramic-polymer-solvent mixture in order to design highly ordered substrate micro-structures that offer improvements in performance, including reduced PGM loading, reduced catalyst ageing and reduced backpressure.

This study compares the performance of the novel substrate for CO oxidation against commercially available 400 cpsi and 900 cpsi catalysts using gas concentrations and a flow rate equivalent to those experienced by a full catalyst brick when attached to a vehicle. Due to the novel micro-structure, no washcoat was required for the initial testing and 13 g/ft3 of Pd was deposited directly throughout the substrate structure in the absence of a washcoat.

Initial results for CO oxidation indicate that the advanced micro-structure leads to enhanced conversion efficiency. Despite an 79% reduction in metal loading and the absence of a washcoat, the novel substrate sample performs well, with a light-off temperature (LOT) only 15 °C higher than the commercial 400 cpsi sample.

To test the effects of catalyst ageing on light-off temperature, each sample was aged statically at a temperature of 1000 °C, based on the Bench Ageing Time (BAT) equation. The novel substrate performed impressively when compared to the commercial samples, with a variation in light-off temperature of only 3% after 80 equivalent hours of ageing, compared to 12% and 25% for the 400 cpsi and 900 cpsi monoliths, respectively.

Synthesis of , of α,β-unsaturated aldehydes and nitriles via cross-metathesis reactions using Grubbs’ catalysts

A series of alpha,beta-unsaturated aldehydes and nitriles of significant interest in the fragrance industry have been prepared using Grubbs' catalysts in cross-metathesis reactions of electron-deficient olefins (i.e., acrolein, crotonaldehyde, methacrolein, and acrylonitrile) with various 1-alkenes, including 1-decene, 1-octene, 1-hexene and 2-allyloxy-6-methylheptane. The latter is of particular interest, as it has not previously being used as a substrate in cross-metathesis reactions and allows access to valuable intermediates for the synthesis of new fragrances. Most reactions gave good selectivity of the desired CM product (>= 90%). Detailed optimisation and mechanistic studies have been performed on the cross-metathesis of acrolein with 1-decene. Recycling of the catalyst has been attempted using ionic liquids. 

Mechanochemical preparation of Ag catalysts for the n-octane-SCR de-NOx reaction: Structural and reactivity effects

Mechanochemical preparation of Ag/Al2O3 catalysts used for the selective catalytic reduction of NOx using hydrocarbons has been shown to substantially increase the activity of the catalyst in comparison with Ag/Al2O3 prepared by wet impregnation. The effect of using different ball-milling experimental parameters on both the structure of the material as well as the catalyst activity has been investigated and the optimum conditions established. A phase transition from γ- to α-alumina was observed milling at high speeds which was found to result in lower catalyst activities. At lower milling speeds both fracturing and agglomeration of the alumina support can be observed depending on the grinding time. However, due to ball-milling, a general enhancement in the NOx reduction activity was observed for all catalysts compared with the conventionally prepared catalysts irrespective of the reductant used. Transient DRIFTS-MS experiments were performed to investigate the effect of H2 in the absence and presence of water on the SCR reaction over catalysts prepared by both ball milling and wet impregnation. In-situ DRIFTS-MS analysis revealed significant differences in both gas phase and surface species. Most notably, isocyanate species were formed significantly more quickly and at higher surface concentration in the case of the mechanochemically prepared catalyst.

Photoelectrochemistry with quinone radical anions: Kinetics of homogeneous redox catalysis

Cyclic voltammograms of quinones were recorded in acetonitrile in the presence of various substrates: carbonyl compounds, halobenzenes, Methyl Viologen and Neutral Red. When illuminated with light of λ >410 nm, catalytic waves were observed. From the ratio of the catalysed to uncatalysed peak current, electron transfer rate constants were calculated using the working curves of Saveant and coworkers. The values of these rate constants were compared with the values obtained by Shukla and Rusling for different systems using a similar method and with quenching rate constants calculated using Rehm-Weller-Marcus theory.

Cationic palladium(II) complexes as catalysts for the oxidation of terminal olefins to methyl ketones using hydrogen peroxide.

Ligated Pd(II) complexes have been studied for the catalytic oxidation of terminal olefins to their corresponding methyl ketones. The method uses aqueous hydrogen peroxide as the terminal oxidant; a sustainable and readily accessible oxidant. The choice of ligand, counterion and solvent all have a significant effect on catalytic performance and we were able to develop systems which perform well for these challenging oxidations.

Metal Redispersion Strategies for Recycling of Supported Metal Catalysts: A Perspective

Catalyst deactivation is ultimately inevitable, and one of the processes known to cause deactivation is sintering of metal particles. Consequently, numerous methods to reverse the sintering process by redispersing metal nanoparticles have been developed. These methods are discussed in this perspective, and the reported mechanisms of redispersion are summarized. Additionally, the longer-term practical use of such treatments and the benefits this can bring are briefly disclosed.

Ethanol Steam Reforming on Rh Catalysts: Theoretical and Experimental Understanding

Through combined theoretical and experimental efforts, the reaction mechanism of ethanol steam reforming on Rh catalysts was studied. The results suggest that acetaldehyde (CH3CHO) is an important reaction intermediate in the reaction on nanosized Rh catalyst. Our theoretical work suggests that the H-bond effect significantly modifies the ethanol decomposition pathway. The possible reaction pathway on Rh (211) surface is suggested as CH3CH2OH -> CH3CH2O -> CH3CHO -> CH3CO -> CH3 + CO -> CH2 + CO -> CH + CO -> C + CO, followed by the water gas shift reaction to yield H-2 and CO2. In addition, we found that the water-gas shift reaction, not the ethanol decomposition, is the bottleneck for the overall ethanol steam reforming process. The CO + OH association is considered the key step, with a sizable energy barrier of 1.31 eV. The present work first discusses the mechanisms and the water effect in ethanol steam reforming reactions on Rh catalyst from both theoretical and experimental standpoints, which may shed light on designing improved catalysts.

A combined transient and computational study of the dissociation of N(2)O on platinum catalysts

The energetics of the low-temperature adsorption and decomposition of nitrous oxide, N(2)O, on flat and stepped platinum surfaces were calculated using density-functional theory (DFT). The results show that the preferred adsorption site for N(2)O is an atop site, bound upright via the terminal nitrogen. The molecule is only weakly chemisorbed to the platinum surface. The decomposition barriers on flat (I 11) surfaces and stepped (211) surfaces are similar. While the barrier for N(2)O dissociation is relatively small, the surface rapidly becomes poisoned by adsorbed oxygen. These findings are supported by experimental results of pulsed N(2)O decomposition with 5% Pt/SiO(2) and bismuth-modified Pt/C catalysts. At low temperature, decomposition occurs but self-poisoning by O((ads)) prevents further decomposition. At higher temperatures some desorption Of O(2) is observed, allowing continued catalytic activity. The study with bismuth-modified Pt/C catalysts showed that, although the activation barriers calculated for both terraces and steps were similar, the actual rate was different for the two surfaces. Steps were found experimentally to be more active than terraces and this is attributed to differences in the preexponential term. (C) 2004 Elsevier Inc. All rights reserved.

Effect of solvent on the hydrogenation of 4-phenyl-2-butanone over Pt based catalysts

The hydrogenation of 4-phenyl-2-butanone over Pt/TiO2 and Pt/SiO2 catalysts has been performed in a range of solvents and it has been observed that the solvent impacted on the selectivity of ketone and aromatic ring hydrogenation as well as the overall TOF of the titania catalyst with no solvent effect on selectivity observed using the silica supported catalyst where ring hydrogenation was favored. For the titania catalyst, alkanes were found to favor ring hydrogenation whereas aromatics and alcohols led to carbonyl hydrogenation. A two-site catalyst model is proposed whereby the aromatic ring hydrogenation occurs over the metal sites while carbonyl hydrogenation is thought to occur predominantly at interfacial sites, with oxygen vacancies in the titania support activating the carbonyl. The effect of the solvent on the hydrogenation reaction over the titania catalyst was related to competition for the active sites between solvent and 4-phenyl-2-butanone.

Periodate – an alternative oxidant for testing potential water oxidation catalysts

The redox catalyst ruthenium dioxide, prepared via the Adams technique, i.e.Ru(Adams), is used as a water oxidation catalyst using the oxidants (i) Ce(IV) in 0.5M H2SO4 and (ii) periodate in 0.5 M H2SO4, water and 0.1 M KOH. Like Ce(IV),periodate is a very strong oxidant that is able to oxidise water to oxygen and can bereadily monitored spectrophotometrically at 280 nm, compared with 430 nm for Ce(IV).More importantly, unlike Ce(IV), which is unstable towards hydrolysis above pH 1,periodate is stable in acid, water and strong alkali. A spectrophotometric study of thekinetics of periodate reduction, and concomitant oxidation of water to O2, reveals thatin the presence of a suitable redox catalyst, Ru(Adams) in this work, periodate is ableto effect the stoichiometric oxidation of water, with a turnover number > 64. In justwater, the kinetics of the latter reaction appear diffusion-controlled, due to the largethermodynamic driving force, a measure of which is the difference in redox potential,i.e. ∆E = 423 mV. As this difference is decreased, ∆E = 396 mV in acid and 290 mVin strong alkali (0.1 M KOH), the kinetics become increasingly activation-controlledand slower. These findings are discussed briefly with regard to the possible use of (i)periodate as an alternative oxidant in the rapid screening of new potential wateroxidation catalyst material powders that are stable only under near neutral and/oralkaline conditions, and (ii) Ru(Adams) as a benchmark catalyst.