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A good heterogeneous catalyst is required to have high activity, selectivity, and stability.
1. Activity is measured as moles of product formed per unit time and mass of catalyst.
2.Selectivity is most generally defined as the amount of desired product divided by the total amount of products, both desired and undesired, produced in a certain amount of time.
3. Catalyst stability describes a wide variety of properties, such as resistance to poisoning or sintering, mechanical strength, and thermal stability, or the ability to be regenerated after deactivation occurs (typically between 1 s and 5 years). All of these fundamental characteristics are affected by the preparation technique and by the physical and chemical properties of the chosen system.
Industrial large scale use of heterogeneous catalysts are summarized below:
CATALYST TERMINOLOGY - ATOM EFFICIENCY, E FACTORS AND ENVIRONMENTAL FRIENDLINESS The atom efficiency is the molecular weight of the desired product divided by the total molecular weight of all products. For example the conventional oxidation of a secondary alcohol 3C6H5–CHOH–CH3 + 2CrO3 + 3H2SO4→ 3C6H5–CO–CH3 + Cr2(SO4)3 + 6H2O has an atom efficiency of 360/860 = 42%. By contrast, the catalytic route C6H5–CHOH–CH3 + 1/2O2→ C6H5–CO–CH3 + H2O offers an atom efficiency of 120/138 = 87%, with water as the only byproduct.
Another indicator of environmental acceptability is the E factor – the weight of waste or undesirable byproduct divided by the weight of the desired product. The production of fine chemicals and pharmaceuticals generate the highest amounts of waste per unit weight of product. Atom efficiencies and E factors can be calculated from each other, but in practice E factors can be higher due to yields being less than optimum and reagents that are used in excess. Also, loss of solvents should be included, and perhaps even the energy consumption with the associated generation of waste CO2.
To express that it is not just the amount of waste but rather its environmental impact, the environmental quotient EQ as the E factor multiplied by an unfriendliness quotient, Q, which can be assigned a value to indicate how undesirable a byproduct is. For example, Q = 0 for clean water, 1 for a benign salt, NaCl, and 100–1000 for toxic compounds. Evidently, catalytic routes that avoid waste formation are highly desirable, and the more economic value that is placed on, for example, the unfriendliness quotient, the higher the motivation to work on catalytic alternatives.
Skeletal catalyst consists of two metals (an alloy) in roughly equal proportions, where one metal is the desired catalytic material, and the other is dissolvable in hydroxide, is first made. This alloy is crushed and leached in concentrated hydroxide solution. The soluble metal selectively dissolves, leaving behind a highly porous sponge like structure of the desired catalytic metal. Catalysts formed by this technique show high activity and selectivity, and have found wide use in industry, particularly for hydrogenation and dehydrogenation reactions.
Advantages and Disadvantages of Skeletal Catalyst
The main advantages of skeletal catalysts are high activity at low temperatures, high selectivity, and strength against attrition. Compared with precious metal catalysts, skeletal catalysts are relatively cheap owing to high activity with base metals. Selectivity allows skeletal catalysts to be employed in reactions on complex substrates such as pharmaceuticals. Selectivity also allows high yields of intermediate products, such as methanol from syngas over skeletal copper, where skeletal nickel would give methane.
Strength against attrition is particularly important for catalysts in slurry-bed reactors, where physical breakage of the catalyst particles, ultimately to fines, can prevent their use for those reactions. The strength of the high surface area skeletal structures can be contrasted against activated carbon, which readily breaks down due to attrition in these types of environments. For the few environments where attrition is still a problem (e.g., oxidative dehydrogenation of alcohols), the skeletal catalytic material technique can find use in fuel cell electrodes or as surface-catalyzed reactors.
The primary disadvantages of skeletal catalysts include deactivation and waste disposal.
CATALYST SPILLOVER in supported catalyst
The spillover of reactant is a well-known phenomenon in heterogeneous catalysis. It is due to the diffusion of atomic or molecular intermediates, formed, e.g. by dissociation, from the catalyst to the support material. The reverse spillover is the opposite process and it corresponds to the
diffusion of molecular species adsorbed on the support towards the catalyst's particles
Reason for using support materials is that in many cases the active catalytic material, i.e., the precursor, is an expensive metal (e.g., Pt, Pd, Rh, Au), which needs to be used sparingly. This can be achieved by using supported catalysts, where the active metal or the metal oxide is dispersed on a porous solid support with a high surface area with a predefined pore structure and particle shape. Hence, the weight fraction of the active catalyst can be low. The support particle not only offers a high surface area, but it also acts as a thermal stabilizer, and it provides easy access of the reactive gas. It may also have bifunctional activity. The preparation of supported catalysts clearly poses a significant challenge for the manufacturing step; i.e., the challenge is how
Figure: Supported catalyst, consisting of small particles on a high surface area carrier such as silica or alumina, along with two simplified model catalyst systems, which in general offer much better opportunities for characterization at the molecular level.
to distribute a microscopic amount of metal such that a large surface area is maintained, and a stable catalyst with high activity and
selectivity is obtained. The performance of supported catalysts critically depends on the distribution of the metal within the porous support.
Depending on the environment under which the catalyst will operate, different types of solid support materials are used. The most common supports in industry are solid oxide supports. Typically, these materials include alumina (Al 2 O 3 ), silica (SiO 2 ), and titania (TiO 2 ). Oxide surfaces are generally covered with hydroxyl groups, which are usually represented as S-OH, where S stands for aluminum, silica, or titanium. When contacted with water (during the
impregnation step), these groups can behave as Brønsted acids or bases by exchanging a proton with the liquid solution, which gives them the ability to interact with several catalytic precursors. However, under aggressive conditions, oxide supports may dissolve (high pH) or be chemically attacked (low pH). Thus, as an alternative, carbon-supported catalysts, which are more stable under these conditions, can be used. In addition, the use of carbon supports allows straightforward recovery of the active metal, as the support can be easily burned off.
Silica (SiO2)
Silica is the support of choice for catalysts used in processes operated at relatively low temperatures (below about 300 C), such as hydrogenations, polymerizations or some oxidations. Its properties, such as pore size, particle size and surface area are easy to adjust to meet the specific requirements of particular applications. Compared with alumina, silica possesses lower thermal stability, and its propensity to form volatile hydroxides in steam at elevated temperatures
also limits its applicability as a support. Most silica supports are made by one of two different preparation routes: sol–gel precipitation to produce silica xerogels and flame hydrolysis to give so-called fumed silica. In the sol–gel route, an alkaline (pH = 12) sodium silicate solution, called water glass, is mixed with sulfuric acid. At low pH, the silicate ions convert into Si(OH)4 monomer units which polymerize to colloidal silica particles and agglomerate into a hydrogel, a three-dimensional network of spheres. Sodium and sulfate are removed by washing; Drying at moderate temperatures (150–200 _C) produces a so called silica xerogel, a highly porous network of interconnected silica spheres, which will absorb water from the ambient atmosphere equivalent to 5–10% of its own weight. This xerogel is insufficiently strong and lacks the macropores needed to prevent excessive pressure drops in a reactor. Therefore the material is milled to powders with the desired particle size, and mixed with binders and shaped into pellets or extrudates.
The pore structure is primarily determined by the size of the colloidal silica spheres and the way in which these agglomerate in the hydrogel, but can still be subsequently affected by adding acids or bases in the washing stage. The second preparation route uses flame hydrolysis, a versatile way to produce all kinds of oxides with high specific surface areas. The advantages of fumed silica over xerogels are the better mechanical properties and higher purity of the former. The starting material to produce fumed silica is SiCl4, which is volatilized in air, mixed with hydrogen and fed into a flame reactor, where the mixture reacts in a complicated scheme to give an aerosol of nanometre-sized SiO2 particles and HCl. The latter is removed from the surface of the silica by steam and air in a fluid bed. The thus-formed silica has a low density and needs to be compacted before it is pelleted or extruded into larger shapes. The original size and aggregation of the silica spheres can be controlled by several parameters such as the temperature of the flame, the H2/O2 ratio and the SiCl4 content of the feed and the residence time in the flame reactor.
Owing to its excellent thermal and mechanical stability and its rich chemistry, alumina is the most widely used support in catalysis. Although aluminium oxide exists in various structures, only three phases are of interest, namely the nonporous, crystallographically ordered a-Al2O3, and the porous amorphous g- and c-Al2O3. The latter is also used as a catalyst by itself, for example in the production of elemental sulfur from H2S (the Claus process), the alkylation of
phenol or the dehydration of formic acid. Alumina supports are made by thermal dehydration of Al(OH)3 (Gibbsite or Bayerite) or AlOOH (Boehmite).
Aluminium is found in nature in the form of bauxite (named after Les Baux, France), an ore consisting of aluminium hydroxides, silica and other oxides. Depending on origin, the bauxite contains mainly gibbsite (Surinam, Ural), boehmite (France) or diaspore (Balkan). Aluminium is extracted from the ore by treating it with NaOH, yielding a Na2Al2O4 solution from which Gibbsite is obtained by crystallization. As a consequence, Gibbsite always contains alkali, which may be undesirable for some catalytic applications. Alkali-free aluminas are therefore prepared by hydrolysis of aluminium alcoholate, e.g. Al(OC2H5)3, which yields a gelatinous form of boehmite, also referred to as pseudo-boehmite. Yet another preparation route starts from bayerite, formed by precipitation of Al(OH)3 from aluminium salts to which ammonia is added in solution. Bayerite forms at high pH (about 12), boehmite at more neutral pH values. Alumina exists in several forms, among which only the a-form is crystalline. The others, called transitional aluminas, have spinel-like structures with different orders in which layers are stacked. In the spinel structure of, for example, MgAl2O4, oxygen ions are packed in a fcc manner, while Mg occupies tetrahedral and Al octahedral sites. As the stoichiometry of Al2O3 does not quite correspond to that of a true spinel, the transitional alumina structures are cation- deficient and contain many defects. Nevertheless, the X-ray diffractograms are characteristic of spinels. As a support, c-Al2O3 offers high surface areas (50–300 m 2 g –1 ), mesopores of between 5 and 15 nm, pore volumes of about 0.6 cm3 g–1, high thermal stability and the ability to be shaped into mechanically stable extrudates and pellets. The thermal stability of c-Al2O3 can be significantly improved by lanthanum oxide additives, which reduce the rate of sintering and retard the conversion into other phases of alumina. Its surface contains several hydroxyls, between 10 and 15 OH per nm 2 , the linear ones being Brønsted bases (H+ acceptors), and the bridged ones Brønsted acids (H+ donor). After dehydroxylation, the surface develops Lewis acidity (electron acceptor) on the uncoordinated Al δ+ sites. Whereas cristoballite presented a realistic model for the surface of a silica support, such a convenient structure is not available for the alumina surface. The hydroxyl population is varied, exhibiting linear as well as two- and three-fold bridged species, which can be identified by vibrational spectroscopy. In solution, alumina is a polyanion of positive charge at pH values below approximately 7 and negative at higher values, offering many possibilities to bind many ionic catalyst precursors.
The highly stable a-Al2O3 is used in high temperature applications, such as in steam reforming, or in cases where low surface areas are desired.
Porous carbons are used as a support for noble metal catalysts in hydrogenation reactions of organic compounds, particularly in liquid media. Active carbons, containing functional groups at their surface, are usually prepared by the pyrolysis of wood, coal, coconut shells, etc. in an inert gas, CO2 or steam at temperatures of between 800 and 1500C. Activation of the surface is achieved by subsequent treatment in oxygen. Surface areas may be as high as 1500 m 2 g –1 with micropores smaller than 1 nm. Treatment at higher temperatures leads to graphitic carbons of low surface area. The advantages of carbon are relative chemical stability and the easy recoverability of the expensive noble metals from spent catalysts. Carbon is the support of choice for noble metals in low temperature hydrogenation catalysts, and also for liquid phase reactions.
Preparation of Supported Catalysts
There are in principle two ways to make supported catalysts:
(1) By coprecipitating the catalytically active component and the support to give a mixture that is subsequently dried, calcined (heated in air), and reduced to yield a porous material with a high
surface area. This procedure is followed when materials are cheap and obtaining the optimum catalytic activity per unit volume of catalyst is the main consideration. (2) By loading pre-existing support materials in the form of shaped bodies with the catalytically active phase by means of impregnation or precipitation from solution. This is the preferred method when catalyst precursors are expensive and the aim is to deposit the catalytically active phase in the form of nanometre-sized particles on the support. All noble metal catalysts are manufactured in this way.
The term is often applied to those sites for adsorption which are the effective sites for a particular heterogeneous catalytic reaction. The overall chemical transformation in a catalytic reaction is known to depend on the nature of the active site, its surroundings, and the path to access it. Therefore, the important properties of a catalyst include an active site with the correct ensemble of metal atoms, metal ions, or other active components such as oxides, a cavity around the active site that may change configuration to facilitate binding of a specific reactant to the active site and expulsion of the product, and a cavity wall that facilitates passage of the desired reactants and products from the ambient to the active site.
Lewis concept of acid and base
Acid accepts pair of electrons and base donates a pair of electron
(acid) (base)
Bronsted concept of acid and base
Acid is able to donate proton and base has the ability to gain proton.
Determination of active sites:
1. Active site determination by Infrared spectra
The best identification of the potential active sites by IR spectroscopy is the direct detection of
their IR fingerprint. Eg., zeolites, for which the strong Brønsted-acid sites are often identified
through the ν(OH) band of hydroxyl groups. If this is not possible, specific methods must be
employed to obtain a spectroscopic response for the sites. The most common is the adsorption of probe molecules, which provides IR spectra specific to the interaction with a single site. The direct detection of adsorption sites by IR spectroscopy is often not possible, therefore
specific methods must be employed to obtain a spectroscopic response of the sites. The most common is the adsorption of probe molecules which gives IR spectra specific to the interaction with the site.
Selection of the appropriate probe molecule
The choice of the adapted probe will depend on many parameters: the chemical function providing the interaction with the site under study; the size of the molecule, depending on the site accessibility; the optimum of the interaction (sufficient to furnish valuable information, not excessive so as to limit surface modifications); the spectral response, producing a signal intense enough, with band positions sensitive to the interaction; stability on the surface catalyst to avoid decomposition; and sufficient vapour pressure to be easily introduced in an IR cell.
The investigated sites are very often cations, acting as Lewis centres, but infrared spectroscopy has also been widely used in order to characterize the metallic centres in oxides and deposited metal complexes also.
Acidic sites …………. Basic probes (NH3, CO, NO, Pyridine)
Basic sites …………… Acid probes (acetonitrile)
It is difficult to directly examine the nature of surface acid sites
Typically, probe molecules are used to evaluate the nature of surface sites – The IR spectrum of pyridine (a base) shows frequency shifts depending on whether it is interacting with a Bronsted site or Lewis acid site
This can be used to distinguish a quantify the sites that are present on a catalyst surface.
2. Temperature-programmed desorption (TPD)
Temperature-programmed desorption by the dynamic method involves placing a sample in a sample cell and pre-treating it to remove any adsorbed species from the active surface. Next, a selected gas or vapor is chemisorbed onto the active sites until saturation is achieved, after which the remaining adsorptive molecules are flushed out with an inert gas. Temperature (energy) is increased at a controlled rate while a constant flow of inert gas is maintained over the sample. The inert gas and any desorbed molecules are monitored by a thermal conductivity detector (TCD). The TCD signal is proportional to the quantity of molecules desorbed as thermal energy, monitored by a thermocouple, overcomes the binding energy. Quantities desorbed at specific temperatures provide information about the number, strength and heterogeneity of the adsorption sites. Analysis data are usually plotted as quantity desorbed versus temperature, or as both temperature and quantity desorbed plotted over time. TPD setup is shown below:
Output Ex., is given in figure
TPD chromatogram of Hydrogen desorbing from alumina supported platinum. Three distinct adsorption bond strengths are evident. The first peak is obtained at a temperature of 393 K. This peak corresponds to the weak adsorption of hydrogen and may relate to desorption from the support or from weak chemisorption. The second peak obtained at the next higher temperature (493 K) probably corresponds to hydrogen spillover due to the presence of Pt on alumina. The third and final peak obtained at 570 K corresponds to the hydrogen chemisorbed by Pt. This peak is quantified to estimate the quantity of Pt available for catalytic activity in a reactor.
Morphology effects appear for reactions that are structure sensitive. A good example is the reduction of NO by CO on Pd, which shows the highest activity on (111) planes and lower activity on open surfaces. However, in the case of supported particles it is difficult to control the particle shape independently of particle size. However, by using a careful TEM characterization it is possible to disentangle size and morphology effects. An example is given by the NO reduction of CO on Pd/MgO(100) model catalysts. Figure shows the reaction probability as a function of temperature for three different samples having particle
Steady-state NO-reaction probability for three different samples with different particle sizes.
sizes of 2.8, 6.9 and 15.6 nm. In the low-temperature regime, the reaction is limited by the dissociation of NO, it can be seen that the largest particles are the least reactive ones, while the smallest ones are less active than the middle sized ones. The same evolution for the three different samples is also observed for the NO dissociation rate measured on the same samples Here the shape of the particles play a vital role along with the size. The largest particles have coalesced and present mainly (100) facets, which are the least active for the reaction. The medium sized particles, which have a truncated octahedron shape that
mainly presents (111) facets, are the most active ones. Finally, the smallest particles, which reveal also a majority of (111) facets, are a little less active because for that size (2.8 nm) the proportion of edge sites is no longer negligible. These sites are, however, low-coordinated atoms, which reveal low activity. The origin of the low activity on open surfaces (and low-coordinated sites) emanates from the second type of adsorbed nitrogen atom, resulting from the dissociation of NO, and which are strongly bound and thus do not desorb at the reaction temperature. In other words, they poison the catalyst.
If other gases than the reactants and products of the catalytic reaction are present and if these foreign atoms are chemisorbed much more weakly than the intermediates of the catalytic reaction, the presence of the foreign gas will be of no consequence for the kinetic of the catalytic reaction. In this case the foreign gas behaves as an inert. If the foreign gas is chemisorbed much more strongly than the intermediates, the surface will to a large extent become covered by adsorbed foreign atoms. In this case the foreign gas behaves as a poison and the consequences for the kinetics will mainly be a large decrease in the reaction rate. The most complex situation occur when the binding energy for the foreign atoms is about equal to the binding energy for the reaction intermediates. The coverage of foreign atoms on the surface will then depend on the reaction conditions. The kinetics of the reaction under dynamic poisoning may be quite complicated. Catalysts lose their catalytic activity as a function of time. Some of them lose their activity in
minutes and some may last for up to ten years, but ultimately all catalysts undergo deactivation.
Causes of Deactivation:
Though mechanical, thermal and chemical factors contribute to deactivation, it is impossible to separate and single out one reason. Temperature of operation influences the sensitivity to poison as well as sintering. Both chemical and thermal forces can weaken a particle to cause mechanical
failure. The causes for deactivation are listed in Table 1.
Table 1. Causes for Catalyst Deactivation
Following processes primarily contribute to the loss of catalytic activity:
1 Sintering
The catalytic reaction takes place at the surface of the catalyst. For this reason the catalyst must have a large area and the active component is present in the catalyst in the form of small particles, which are much less stable than large particles. A catalyst will slowly loose catalytic
activity due to growth of the particle and loss of surface area.
2 Fouling
Catalytic activity may be lost due to the formation of carbon or due to the deposition of impurities or of dust in the catalyst. The formation of carbon is in some cases reversible. The catalyst can be taken of stream and the carbon removed by oxidation. The reactant gas may contain impurities, a few examples are metalorganic compounds in hydrodesulphurization or H2S in steam reforming. These impurities react with the catalyst and reduces the catalysts activity. Fortunately, the reaction between catalyst and impurity is often very strong and the impurity is completely absorbed in the first few percent of the catalyst bed. The catalyst bed is then designed to be a little larger than initially necessary, and the catalyst temperature is initially slightly lower than the nominal operating temperature. Over the lifetime of the catalyst, the temperature of the catalyst is slowly increased to compensate for the loss of catalytic activity due to fouling. Fouling by dust carried into the reactor can occur, a few examples are removal of NOx from flue gases or removal of CO from exhaust from car exhaust. In this case the catalyst must be manufactured in a shape, which will allow the dust to pass through the reactor.
3. Dynamic poisoning
The more interesting situation occurs when the catalyst is partially and reversibly poisoned by impurities in the reactant gas. The degree of loss of catalyst activity then depends on the
operating conditions.
Catalytic poisoning
Poisoning is a chemical effect, in which a poison (a chemical agent) reacts permanently with an
active site thereby deactivating the catalyst.
Solutions to the deactivation problem:
1. Harmful process conditions that can lead to deactivation which should be avoided or changed.
2. Decreasing the temperature of operation and increasing the reactive gas pressure can lower the
catalyst deactivation. For constant conversion, increasing the temperature gradually as the
catalyst activity decays.
3. Poisons in the feed stock should be avoided.
4. Optimization of support, in order to avoid catalyst sintering.
5. Addition of promoters to resist deactivation. For eg., addition of small amounts of SiO 2 . ZrO2
to Al 2 O 3 stability (to enhance the support stability).
6. Addition of promoters to neutralize the effect of poisons, eg., Sulphur poisoning of Ni is
reduced in the presence of copper chromite, as Cu and Cr preferentially form sulphides.
Application of Catalyst in Environment
The emission of carbon monoxide, nitrogen oxides (NOx), sulfur oxides (SOx), and all sorts of organic compounds are responsible of environmental pollution. Catalysis has become indispensable in converting these environmentally harmful molecules into more benign species such as N2, H2O and CO2, although the latter is now a suspected greenhouse gas. Environmental catalysts are now used in various applications, e.g. large-scale processes such as cleaning flue gases from power plants, in the transport sector, to catalysis in charcoal broilers in restaurants to decompose aromatic molecules formed during grilling of steaks and hamburgers, in toilets to reduce unpleasant odors, to remove volatile organic compounds (VOCs) in industry and to decompose ammonia in waste water streams. 1. Automotive Exhaust Catalysis The exhaust consists of hydrocarbons, NOx, CO, H 2 and O 2 . Unburnt gasoline and cracked hydrocarbons such as ethylene and propylene are also substantial constituents of exhaust. Gasoline contains additives such as benzene, toluene and branched hydrocarbons to achieve the necessary octane numbers. The direct emission of these volatile compounds, e.g. at gas stations, is a significant source of air pollution. Leaded fuels, containing antiknock additions such as tetraethyl-lead, have been abandoned because lead poisons both human beings and the three- way exhaust catalyst, especially for the removal of NO by rhodium. The three-way catalyst, consisting of Pt and Rh particles supported on a ceramic monolith, represents a remarkably successful piece of catalytic technology. It enables the removal of the three pollutants CO, NO and hydrocarbons by the following overall reactions
Additionally, NO is reduced by H 2 and by hydrocarbons. To enable the three reactions to proceed simultaneously – notice that the two first are oxidation reactions while the last is a reduction – the composition of the exhaust gas needs to be properly adjusted to an air-to-fuel ratio of 14.7. At higher oxygen content, the CO oxidation reaction consumes too much CO and hence NO conversion fails. If, however, the oxygen content is too low, the entire NO is converted, but hydrocarbons and CO are not completely oxidized. An oxygen sensor is mounted in front of the catalyst to ensure the proper balance of fuel and air via a microprocessor- controlled injection system.
Catalytic converters used to convert the harmful exhaust gases to less toxic ones. The major supporting structure is the monolith, covered by a 30–50 mm layer of porous 'washcoat.' The latter is the actual support and consists largely of c-Al2O3 (70–85%) and other oxides such as ceriumoxide (10–30%), lanthanumoxide or alkaline earth oxides (BaO). Some formulations use NiO as a sulfur getter in the outer layers of the washcoat. Denser oxides such as a-Al2O3 or ZrO2 are sometimes used as a diffusion barrier, to prevent incorporation of rhodium in the support at high temperatures. Only 1–2% of the washcoat's weight corresponds to
noble metals (Pt, Pd, Rh). Some manufacturers use all three, but most converters contain Rh together with Pt. Recently, all-palladium converters have been also introduced.
Alumina, present in the gamma modification, is the most suitable high surface area support
for noble metals. The δ-Al2O3 in washcoats typically has a surface area of 150–175 m 2 g –1 .
However, at high temperatures δ -alumina transforms into the alpha phase, and stabilization to
prevent this is essential. Another concern is the diffusion of rhodium into alumina, which calls for the application of diffusion barriers.
Ceria is a partially reducible oxide (its reduction is promoted by the noble metals). When the
air/fuel oscillation swings to the lean side it takes up its maximum capacity of oxygen; this oxygen is available for CO2 formation when the composition swings to the rich side. Ceria thus counteracts the effect of oscillating feed gas compositions. It further stabilizes the high surface area of the δ -Al2O3 by inhibiting its phase change into the a-phase and it impedes the agglomeration of the noble metals and the loss of noble metal surface area by acting as a diffusion barrier. Oxygen vacancies, conceivably also in regions where CeOx is in contact with
noble metal, are active sites for CO oxidation by O2, or by H2O through the water-gas shift reaction.
Lanthanum oxide is valence invariant, and does not exhibit any oxygen storage capacity, but it
effectively stabilizes δ -Al2O3. It spreads over the alumina surface and provides a barrier against
dissolution of rhodium in the support.
Platinum serves as the catalyst for the oxidation of CO and hydrocarbons. It is relatively
insensitive to contamination by lead or sulfur. At high temperatures it is not known to dissolve in the washcoat, but sintering into larger particles may lead to a substantial loss of platinum surface area with dramatic consequences for the overall oxidation activity.
Rhodium is the crucial ingredient of the three-way catalyst. The metal, a byproduct of Pt
extraction, although expensive, rhodium is difficult to replace owing to its unique properties with
respect to NO surface chemistry. Loss of rhodium activity is due to particle growth under reducing conditions (>900 K), and to diffusion into the alumina support under oxidizing
conditions (>900
Palladium can be present in addition to Rh and Pt but may also replace
them. Palladium is as good for oxidation as platinum (even for the oxidation of saturated hydrocarbons) but it is somewhat less active for NO reduction. Hence, noble metal loadings of a Pd-only catalyst are 5–10 times higher than for a Pt- Rh catalyst. Palladium is less resistant to residual lead in gasoline than Pt. Palladium catalysts also require higher ceria loadings to help prevent high-temperature deactivation.
The ideal operating temperatures for the three-way catalyst lie between 350 and 650 C. After a cold start it takes at least a minute to reach this temperature, implying that most CO and hydrocarbons emission takes place directly after the start. Temperatures above 800C should be avoided to prevent sintering of the noble metals and dissolution of rhodium in the support.
Techniques normally used:
1. Infrared Spectroscopy
3. Scanning Tunneling Microscopy and Atomic Force Microscopy
4. Raman Spectroscopy
In infrared spectroscopy a molecule absorbs photons with the same frequency as its vibrations. In contrast, Raman spectroscopy is based on the inelastic scattering of photons, which lose energy by exciting vibrations in the sample. Figure illustrates the scattering process. Monochromatic light of frequency v, falls on a sample, where the majority of the photons undergo Rayleigh scattering, i.e.scattering without energy exchange. In a quantum mechanical picture it is as if the molecule is excited to an unstable state with energy hv 0 , above the ground state, from which it decays back to the ground state. No energy is exchanged between the molecule and the photon. When, however, the excited molecule decays to the first vibrational level with frequency v vib , it effectively takes an amount of energy equal to h v vib , away from the photon. Hence the scattered light exhibits intensity at the frequency v 0 - v vib . This Raman peak is called the Stokes band. The reverse process may also take place. If the collision with a photon brings a vibrationally excited molecule to the unstable state of energy hv 0 +hv vib , it may decay to the ground state, transferring a net amount of energy h v v i b to the photon, which leaves the sample with a higher frequency equal to v 0 +v vib . This peak, called the anti-Stokes band, has much lower intensity than the Stokes band, because the fraction of vibrationally excited molecules is usually small.
Fig. The Raman effect
A strong point of Raman spectroscopy for research in catalysis is that the technique is highly suitable for
in situ studies. The spectra of adsorbed species interfere weakly with signals from the gas phase, enabling studies under reaction conditions to be performed. A second advantage is that typical supports such as silica and alumina are weak Raman scatterers, with the consequence that adsorbed species can be measured at frequencies as low as 50 cm -1 . This makes Raman spectroscopy a powerful tool to study catalytically active phases on a support. A disadvantage of the technique is the small
cross sections for Raman scattering; most of the scattered intensity goes into the Rayleigh band, which is typically about three orders of magnitude stronger than the Stokes bands. Of course, the use of an intense laser increases all intensities, but this has the disadvantage that the sample may heat up during measurement and surface species may decompose or desorb.
Experimental set-up for in situ or dynamic X-ray diffraction studies of catalysts
Finally, fluorescence of the sample, giving rise to spectral backgrounds, may seriously limit the detectability of weak signals.
5. Transmission Electron Microscopy
6. Temperature programmed desorption
7. In situ XRD (for following solid state catalysis reactions)
Since X-rays have considerable penetrating power, such that XRD can be used to study catalysts under in situ conditions. Figure shows a set-up for in situ XRD studies which consists of a cell with windows which are largely transparent to X-rays and a heating stage covered by a platinum foil, which serves as a thermally conducting sample holder for the powdered catalyst. This cell
was used for eg., to follow the reduction of alumina-supported iron oxide at 675 K as a function of time.


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A general view of the Swedish Academy's annual meeting at the Old Stock Exchange building in Stockholm, Sweden December 20, 2017. TT News Agency/Jonas Ekstromer via REUTERS/File Photo
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The Swedish Academy, plunged into crisis over allegations of rape and sexual assault by the husband of one its members, may postpone the 2018 Nobel Literature Prize, Swedish Radio reported Wednesday.According to Swedish Radio, several members of the Academy have suggested reserving this year's prize, and awarding it alongside the 2019 prize next year.Englund is one of five Academy members who have stepped down in recent weeks over the scandal, including permanent secretary Sara Danius.Technically, the Academy's 18 members are appointed for life and cannot resign, but they can choose not to participate in its meetings and decisions.Arnault denies the allegations.

subset: commoncrawl


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Look past the disjointed rambling in President Trump's Rose Garden statement and press conference Friday.
Look past the lies, such as when he insisted again that undocumented immigrants, who are less likely to commit crimes than native-born Americans, threaten the nation's safety.
And when he said most drugs come through unwalled areas, not ports of entry, contradicting his own Drug Enforcement Administration.
And when he said more people are trying to immigrate illegally than ever before, when border apprehensions have fallen by 80% since 2000.
What matters most right now is that Trump just declared a national emergency to override a determination that Congress just made — namely, not to spend billions of public dollars on construction of a border wall.
He now seeks to unilaterally move billions allocated for other purposes to respond to a crisis that by any objective measure is not a crisis.
Congress, especially Republicans who spent years railing against Barack Obama for executive overreaches that look positively shy in comparison, must fight this hard. And the courts — including the five judges who constitute the Supreme Court's conservative majority — must reject this patent perversion of the balance of power.


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Executive Bedroom Design
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text: Here are some of the most controversial scams exposed by Indian media. Bismee Taskin @MainaBismee 1/8 Started by the UN General Assembly, World Press Freedom Day is celebrated on May 3 to mark the importance of press in a democratic nation. According to the Press Freedom index 2019, India ranked 140 out of the 180 countries. In spite of this statistic, some journalists have broken the shackles and exposed major scams. Here are some of the most controversial scams exposed by Indian media. (Image: Reuters) 2/8 The Bofors scam | The kickbacks behind a major weapons deal during the 1980s and 1990s, between India and Swedish artillery manufacture Bofors AB was brought to the public eye by N.Ram and Chitra Subramanian's report in The Hindu. As many as 350 documents relating to the weapons deal, the flouting kickbacks and illicit bypassing pertaining to the deal were exposed. (Image: Wikimedia Commons) 3/8 The Cement scam | The Indian Express team under Arun Shourie exposed the irregularities in the cement quota granted by the government. Shourie compiled evidence which exposed the then CM of Maharashtra, A.R Antulay's chronicle of foul funds. Eventually, the scam came to be known as the Antulay's Trustgate scandal. 4/8 2G spectrum scam | Vinod Mehta stood his ground even when corporate groups terminated all editorial and commercial engagement with the Outlook group over the coverage of Nira Wadia tapes. Outlook exposed the 2G spectrum scam and the transcripts of Nira Wadia's conversations which highlighted the manipulations involved in this high profile scam 5/8 The Fodder scam | This corruption scandal that ended the long reign of Lalu Prasad Yadav was brought to light through an investigative scoop by journalist Ravi S Jha, who was working with the Asian Age. Jha drew public and legal attention into the fodder scam that involved an apparent foul play in funds worth 950 crores. 6/8 1992 Securities scam | Sucheta Dalal had exposed Harshad Mehta's scam amounting upto Rs 1000 crore. Mehta was considered superstar of the Indian stock market until the journalist shone light on his scams and was eventually banished from the stock market. Mehta and few of his associates had reportedly managed to siphon a walloping amount through interbank transactions. (Image: Reuters) 7/8 AgustaWestland chopper scam | The Indian express team led by Shekhar Gupta and pursued by journalist Manu Pubby exposed the VVIP chopper scam in 2012-2013. This was the first instance where irregularities in the chopper deal were brought to the public domain. 8/8 The Human trafficking racket | Ashwini Sarin broke a few laws to expose flesh trade. Sarin purchased a woman from the borders of Rajasthan, UP and Madhya Pradesh, to penetrate through the rugged network of human trading. The Indian Express reporter exposed the atrocities inflicted on this marginalised part of the society. (Image: Reuters) First Published on May 3, 2019 05:54 pm


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Australia knew it had something special in Ash Barty and now the rest of the world does too.
The 23-year-old Queenslander produced a brilliant performance in the French Open final to win her maiden grand slam singles title, taking down Marketa Vondrousova 6-1 6-4.
Just as she did in her semi-final, Barty started the match on fire, racing to a 4-0 lead. But unlike her previous match, she didn't suffer from any nerves and she closed out the set without any trouble.
She faced greater resistance from Vondrousova in the second set but stood firm and iced the match.
For more stories like this head to news.com.au
Commentator Jim Courier said that Barty's life had changed. "Ash Barty is now a certified star in the game," he said.
After hitting a smash winner on match point to become the first person in 46 years from Down Under to win the singles crown at Roland Garros, Barty had just enough breath left to deliver an all-Aussie response, dropping an f-bomb before sinking to her knees in disbelief.
"It's unbelievable, I'm speechless," she said later. "I played the perfect match today.
"To my team, thank you guys for sticking with me. It's been the most amazing journey we've been on the past three years, and I feel like it's just the start. Let's go celebrate tonight."
Depending on the outcome of the final, one image was waiting in the wings to be unleashed and when match point came and went, the internet duly delivered.
Barty held up her end of the bargain and it meant a well known photo of the Aussie superstar was given a new life online.
The happy snap — taken of Barty in 2002 showing her holding up a trophy as a junior — lit up the Twittersphere after her historic victory, which saw her become the first Australian in 46 years to win a singles title on the Paris clay.
The speed at which the photo went viral meant Barty was already being asked about it soon after receiving the most previous silverware of her career during an interview with British broadcaster ITV.
"That photo has gone far and wide across the world," Barty said of the image that shows her smiling gleefully at her junior club, the West Brisbane Tennis Centre.
Everywhere you looked in the aftermath of Barty's win, you had the 2002 version of her staring back at you.
Barty claims French Open!
Tennis: Ashleigh Barty ended Australia's 46-year wait for a French Open singles title by thrashing nervous Czech teenager Marketa Vondrousova in a one-sided women's final.
Barty will rise to No. 2 in the world after her breakthrough major victory, which will be the highest ranking for an Aussie woman since Evonne Goolagong was No. 1 in 1976.
By winning in Paris, Barty became Australia's first indigenous major champion since Goolagong triumphed at Wimbledon in 1980.
"Evonne sent me a text a couple days ago ... I spotted her name on the trophy," Barty said. "It's amazing what she's done for Indigenous tennis ... hopefully we can continue to create those opportunities for kids to know this (can be) a career."
Barty will be hoping to hit top spot in the rankings soon given the grass court season — her favourite surface — is just around the corner.
Saturday was the first time Vondrousova had ever stepped foot on Court Philippe Chatrier and the occasion clearly got to the Czech teenager. She took too long to get into the match and by the time she found her rhythm, it was too late.
Vondrousova was shattered after the match as tears flowed but she was all class at the post-match presentation, paying tribute to Barty not just as a player but as a person too.
"First of all, congrats Ash and your team, you gave me a lesson today. You're an amazing player and you're such a nice person, so you deserve this," Vondrousova said on court before heaping more praise on Barty in her post-match press conference.
"She was just too good today. I think she plays amazing match. I didn't have too many chances today.
"She was just better at everything today.
"She's playing too good. She's mixing things up. And she has a huge serve.
"So it's all, like, very tough to play against."
Tennis legends and players, both from Australia and overseas, were quick to congratulate Barty.
Aussie legend Rod Laver was watching on in the stands and was quick to pose for a photo with Barty, while a gif of American tennis great Chris Evert giving the Aussie a thumbs up spread like wildfire across the internet.
Madison Keys, who Barty defeated in the quarter-finals, tweeted: "Congrats @ashbar96! Well done on your first grand slam. So well deserved."
Nick Kyrgios also chimed in on Twitter, saying: "Congrats @ashbar96 nothing changed, you were dominating juniors and now winning slams."
Barty's win becomes even more significant when you consider she almost gave tennis away for good. Homesick and struggling to deal with the burdens placed on professional players, she left the sport aged 18 in 2014 to try her hand at cricket.
She was part of the Brisbane Heat squad in the Women's Big Bash League (WBBL) and representing Australia in an Ashes Test rather than the Fed Cup looked the more likely prospect.
But tennis was in Barty's blood and she returned to her true love in 2016. Since then, she's risen from world No. 623 to grand slam champion. It's a journey that's had to be seen to be believed.


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00 Hardcover PR19 Containing encyclopedic coverage of all aspects of English literature from Homer to hypertext, The Oxford Companion to English Literature has been a standard source for students, scholars and general readers since the 1930s.
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Among publishers of reference titles OXFORD UNIVERSITY PRESS stands as a leader and among new releases is the sixth edition of The Oxford Companion to English Literature ([pound]25.


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The Minyan: A Small Cadre of Men Who Turned the World Upside-Down
NOOK Book(eBook)
Available on Compatible NOOK Devices and the free NOOK Apps.
THE MINYAN by Dr. C. David Jones is an account of the lives, ministry and legends of each of the thirteen Apostles who were members of the Minyan of Jesus Christ. The book provides new materials based upon historical research, the principles of biblical redaction criticism/analysis and an examination of the sociological, inter-personal and group dynamics of this amazing group of Jesus' Apostles. The Appendices include a unique Minyan sociogram and maps that offer additional resources to the reader. Dr. Maxie D. Dunnam, President of Asbury Theological Seminary, Wilmore, KY is author of the "Introduction."
CAN YOU name the Thirteen Apostles of Jesus Christ?
When asked that question one fellow said, "Sure!" Then he confidently began to rattle off their names:
"Matthew, Mark, Luke, John . . ."
He paused for a moment, puzzled and bewildered, and then continued:
"Happy, Dopey, Grumpy, Doc . . .
Donder, Vixen, Blitzen . . . and Rudolph!"
He really didn't know who the Apostles were but, undaunted, he rattled off the first names that came to his mind. There were, however, actually thirteen men who made up the "Minyan."
A MINYAN is the number of adult Jews required to form a synagogue or a Hebrew congregation. The necessary quorum that must be present for the daily prayers or for any special prayers or services of Hebrew worship is ten. Originally the number was twelve -– the same number of the original twelve tribes of Israel.
The Rabbi, Jesus of Nazareth, chose twelve men to form his own itinerant "Minyan" to travel with him wherever he went throughout his earthly ministry.
THE HISTORIC ACCOUNT of the Apostolic Minyan begins with the enigmatic figure of St. John the Baptist. His story is the Prologue to the chapters that follow in The MINYAN.
THE CENTRAL FIGURE around whom all else revolves is the dynamic and mystical person of JESUS CHRIST. What made him choose these particular twelve men for his Minyan? Explore with the author the powerful sociological dynamics and religious ferment that impacted the lives of these men.
DISCOVER the identity of the "first" apostle; learn the secret of the fisherman who walked on water; then meet the first apostolic martyr, the Greek nobleman and the noble Egyptian rabbi, the boy with loaves and fishes who became the teen-age apostle, the skeptical shipwright, the revenuer, the guerrilla revolutionaries and terrorists among the Minyan, the embezzling treasurer, and the man who came out on "the short end of the stick!"
EXAMINE the Sociogram which charts the inter-relationships within the Minyan and shows how many of the men were relatives of the Rabbi Jesus. See the interlocking forces that knit business partners and tradesmen together. Trace the travels of the Minyan on the maps and follow them as they spread the Good News of God's love to the far corners of the Roman Empire and beyond.
FOLLOW in the EPILOGUE the influence of the Minyan and the development of Christianity through the centuries to modern times. You will be amazed and astounded at the lives and ministries of these thirteen men who had such a profound impact in turning our world "Upside-down" or, more exactly, in helping to turn it "Right-side UP!"
"Dr. C. David Jones has given us a marvelous and expansive piece of research work and writing in this book in which he tells the fascinating story of the original twelve men and then the thirteenth man who joined them to form a very special Minyan: their lives changed the world!"
--from the "Introduction" by Dr. Maxie D. Dunnam, President of Asbury Theological Seminary
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