Martyn V. Twigg

Deactivation of supported copper metal catalysts for hydrogenation reactions

Abstract Laboratory and industrial results are used to elucidate the general features of the deactivation of supported copper metal catalysts in hydrogenation reactions. Hydrogenations with copper catalysts are milder than with their nickel or platinum counterparts, and they have selectivities that are exploited commercially. They are used in single stream plants for production of hydrogen via the low-temperature water shift gas reaction, and for methanol manufacture from synthesis gas, and also in hydrogenation of speciality organic compounds. Common catalyst types are based on Cu/Cr 2 O 3 (copper chromite) or Cu/ZnO formulations that contain stabilisers and promoters such as alkaline earth oxides and Al 2 O 3 . These have several roles, including inhibition of sintering, and poison traps that prevent poisoning of the active metal surface. The best understood are Cu/ZnO formulations that have improved sulphur resistance due to formation of thermodynamically stable ZnS. Copper catalysts are susceptible to thermal sintering via a surface migration process and this is markedly accelerated by the presence of even traces of chloride. Care must be, therefore, taken to eliminate halides from copper catalysts during manufacture, and from the reactants during use. Operating temperatures must be restricted, usually to below 300°C when catalyst longevity is important with large catalyst volumes. Water can soften some Cu/ZnO formulations during use, and cause particle breakage that leads to high-pressure drop and maldistribution of flow through large catalyst beds and impaired performance. Commercial copper catalysts are not acidic, and since they operate under mild conditions, carbon deposition (coking) is uncommon. However, conventional site blocking poisoning with sulphur compounds, and particularly by H 2 S, is common. The initial phase involves interaction with surface hydroxyl groups and elimination of water. Sulphur is retained strongly on the catalyst, and when partially sulphided they can exhibit selectivity in hydrogenation of organic hydrogenations. A variety of other sulphur compounds, and some chlorinated organic compounds, can cause complete deactivation or enhanced selectivity.

Deactivation of copper metal catalysts for methanol decomposition, methanol steam reforming and methanol synthesis

Laboratory and industrial results are reviewed to elucidate the general features of the deactivation of supported copper metal catalysts in various reactions involving methanol as reactant or product. Most catalyst types are based on Cu/ZnO formulations that contain stabilisers and promoters such as alumina, alkaline earth oxides and other oxides. These additional materials have several roles, including the inhibition of sintering and absorption of catalyst poisons. All copper catalysts are susceptible to thermal sintering via a surface migration process, and this is markedly accelerated by the presence of even traces of chloride. Care must be taken, therefore, to eliminate halides from copper catalysts during manufacture, and from reactants during use. Operating temperatures must be restricted, usually to below 300°C.In methanol synthesis involving modern promoted Cu/ZnO/Al2 O3 catalysts neither poisoning nor coking is normally a significant source of deactivation; thermal sintering is the main cause of deactivation. In contrast, catalyst poisoning and coking have been observed in methanol decomposition and methanol steam reforming reactions.

Dry reforming of methane over a Ni/Al2O3 catalyst in a coaxial dielectric barrier discharge reactor

A coaxial double dielectric barrier discharge (DBD) reactor is developed for plasma-catalytic conversion of CH4 and CO2 into syngas and other valuable products. A supported metal catalyst (Ni/Al2O3) reduced in a methane discharge is fully packed into the discharge region. The influence of the Ni/Al2O3 catalyst packed into the gas gap on the electrical characteristics of the discharge is investigated. The introduction of the catalyst pellets leads to a transition in discharge behaviour from a typical filamentary microdischarge to a combination of spatially limited microdischarges and a predominant surface discharge on the catalyst surface. It is also found that the breakdown voltage of the CH4/CO2 discharge significantly decreases when the reduced catalyst is fully packed in the discharge area. Conductive Ni active sites dispersed on the catalyst surface contribute to the expansion of the discharge and enhancement of charge transfer. In addition, plasma-catalytic dry reforming of CH4 is carried out with the reduced Ni/Al2O3 catalyst using a mixing ratio of CH4/CO2 = 1 and a total flow rate of 50 ml min−1. An increase in H2 selectivity is observed compared with dry CH4 reforming with no catalyst, while the H2/CO molar ratio significantly increases from 0.84 to 2.53 when the catalyst is present.

Reduction of model steam reforming catalysts : NiO/α-Al2O3

Abstract The effect of NiO loading (5–21 wt.-%) on the reduction of NiO/α-A 2 O 3 catalysts, prepared by multiple impregnation of nickel nitrate solution followed by calcination at 650°C has been characterized usingtemperature-programmed reduction, isothermal hydrogen consumption, magnetization, X-raydiffraction, and electron microscopy. X-ray diffraction analysis of fresh catalysts indicated normal NiO crystallites about 30 nm in size. Studies from 270°C to 350°C show hydrogen consumption at lower temperatures is faster than the subsequent growth of nucleated clusters of nickel atoms into crystallites, with the rates of the two processes approaching each other at higher temperatures. As NiO loading increases, chemical reduction becomes more difficult but nickel crystallite growth is not affected. This decreased reducibility is believed due to Al 3+ ion incorporation into NiO surface layers during impregnation. Isothermal hydrogen consumption from 270°C to 450°C follows a shrinking core model with NiO crystallites decreasing progressively in size. Magnetic measurements show crystallite growth is dependent on diffusion-controlled nucleation. Decreasing hydrogen flow has a profound effect on chemical reduction and nucleation but not on growth. Similar results are found with added water vapor. X-ray diffraction and transmission electron microscope measurements reveal 23 nm nickel crystallites with some evidence for Al-Ni alloy formation. A mechanism is proposed in whichadsorbed water inhibits chemical reduction and nucleation, and foreign ions such as A1 3+ increase this effect.

Urea as a hydrogen carrier: a perspective on its potential for safe, sustainable and long-term energy supply

Recently, there have been publications reporting the use of urea, as a source of hydrogen/fuel cell power. There have however been no reports that singularly assess the suitability of urea for this purpose. This article provides not only a perspective on the attributes of urea ((NH2)2CO) as a hydrogen carrier for fuel cells but also presents the findings of a review on the feasibility of utilising the enormous natural resource of urea that exists. Urea is a cheap and widely available commodity with well developed manufacturing infrastructure and a rapidly increasing volume of production. This offers rapid implementation of urea for application as a hydrogen carrier either directly or as a source of ammonia. Compared with other industrial chemicals previously considered, urea has the advantages of being non-toxic, stable, and therefore easy to transport and store. This report reveals that the natural resource of urea could be a solution to long-term future sustainable hydrogen supply and that the present status of scientific knowledge necessary to extract this natural resource is in the most part understood. It is considered realistic that these sustainable routes could be exploited if they are given sufficient focus of research attention.

The Use of NO x Adsorber Catalysts on Diesel Engines

NOx adsorber catalysts (NACs) can be applied in lean-burn gasoline and diesel engines to reduce NOx emissions. Typically NACs are formulated using platinum as an oxidation catalyst, barium as a storage component and rhodium for NOx reduction. There has been intense research to optimise these catalysts for use on diesel engines aimed at increasing their efficiency, durability, sulfur tolerance and operating temperature window. This paper describes these developments and outlines the current level of knowledge of NOx adsorber catalyst systems using results from small-scale laboratory tests as well as engine-bench work.

Cleaning the Air We Breathe - Controlling Diesel Particulate Emissions from Passenger Cars

The mechanism of formation of particulate matter (PM) in the diesel engine combustion process is outlined, and the increasingly stringent PM emissions limits in current and projected environmental legislation are noted in the context of the increasing use of fuel-efficient high-performance diesel engines in passenger cars. The types of filter systems for abating diesel particulates are described, as are the principles of filter regeneration – the controlled oxidation of PM retained in the filter, to prevent an accumulation which would ultimately block the filter and degrade engine performance. PM is characterised in terms of both particle size (coarse, accumulation mode, and nucleation mode nanoparticles) and chemical composition, and the filtration issues specific to the various PM types are outlined. Likely future trends in filter design are projected, including multifunctional systems combining PM filtration with NOx control catalysts to meet yet more stringent legislative requirements, including European Stage 5 and 6, and the so called ‘Bin 5’ levels in the U.S.A.

Photochemical reactions of Ru3(CO)12 involving metalmetal bond fission

Abstract Photolysis of Ru3(CO)12 in the presence of donor ligands rapidly produces monomeric ruthenium species.

Effect of organometallic fuel additives on nanoparticle emissions from a gasoline passenger car.

Particle size measurements were performed on the exhaust of a car operating on a chassis dynamometer fueled with standard gasoline and gasoline containing low levels of Pb, Fe, and Mn organometallic additives. When additives were present there was a distinct nucleation mode consisting primarily of sub-10 nm nanoparticles. At equal molar dosing Mn and Fe gave similar nanoparticle concentrations at the tailpipe, whereas Pb gave a considerably lower concentration. A catalytic stripper was used to remove the organic component of these particles and revealed that they were mainly solid and, because of their association with inorganic additives, presumably inorganic. Solid nucleation mode nanoparticles of similar size and concentration to those observed here from a gasoline engine with Mn and Fe additives have also been observed from modern heavy-duty diesel engines without aftertreatment at idle, but these solid particles are a small fraction of the primarily volatile nucleation mode particles emitted. The solid nucleation mode particles emitted by the diesel engines are likely derived from metal compounds in the lubrication oil, although carbonaceous particles cannot be ruled out. Significantly, most of these solid nanoparticles emitted by both engine types fall below the 23 nm cutoff of the PMP number regulation.

Driving down on-highway particulate emissions

It has been reported that particulate emissions from diesel vehicles could be associated with damaging human health, global warming and a reduction in air quality. These particles cover a very large size range, typically 3 to 10 000 nm. Filters in the vehicle exhaust systems can substantially reduce particulate emissions but until very recently it was not possible to directly characterise actual on-road emissions from a vehicle. This paper presents the first study of the effect of filter systems on the particulate emissions of a heavy-duty diesel vehicle during real-world driving. The presence of sulfur in the fuel and in the engine lubricant can lead to significant emissions of sulfate particles < 30 nm in size (nanoparticles). We have demonstrated that when using low sulfur fuel in combination with a uniquely formulated low sulfur lubricant and a suitable filter system that the particulate emissions of a heavy-duty vehicle were reduced to the levels already present in the ambient environment. PM EMISSIONS AND DIESEL PARTICULATE FILTER (DPF) TECHNOLOGY Diesel Particulate Matter (PM) consists primarily of carbonaceous soot and a Volatile Organic Fraction (VOF) composed mainly of hydrocarbons with lesser amounts of nitrate and sulfate species. It is becoming increasingly recognised that PM emissions from dieselpowered vehicles may have adverse environmental effects. For example, it was proposed that the elemental carbon fraction can increase global warming effects. In addition, the medical community is closely examining the effects of PM on human health as a function of particle size. Reports in scientific literature suggest that there is a link between environmental exposure to fine particles less than 2.5 m in size to adverse health effects. These studies elucidated a range of causal mechanisms but have not developed a quantitative understanding of their relative importance. Studies that are more recent investigated the hypothesis that ultrafine particles <100 nm in size are detrimental to human health. It has also been reported that the relationship between ultrafine particles and health may be at least partially due to the high efficiency of particle deposition in the respiratory tract for the very small particles (Alveolar deposition is highest for particles approximately 20 nm in size). Regulatory agencies such as the U.S. Environmental Protection Agency (EPA) have adopted mass-based air pollution regulations for particulate matter. Other metrics, such as particle number or surface area, may be more important in characterising the physical properties of aerosol related to health effects. Figure 1 illustrates relationships between combustion aerosol number, surface area and mass weighted size distributions. In this case the distribution typifies a diesel aerosol distribution. The shape of the aerosol size distribution from a spark ignition engine would be similar but with relatively less material in the accumulation mode region. 2006-01-0916 Driving Down On-Highway Particulate Emissions D. B. Kittelson, W. F. Watts and J. P. Johnson University of Minnesota, Department of Mechanical Engineering