Catalysis by Ceria and Related Materials: 12 (Catalytic Science Series)
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A few decades later, a catalytic converter was patented by Eugene Houdry , a French mechanical engineer and expert in catalytic oil refining,  who moved to the United States in When the results of early studies of smog in Los Angeles were published, Houdry became concerned about the role of smokestack exhaust and automobile exhaust in air pollution and founded a company called Oxy-Catalyst. Houdry first developed catalytic converters for smokestacks called "cats" for short, and later developed catalytic converters for warehouse forklifts that used low grade, unleaded gasoline.
He was awarded United States Patent 2,, for his work. Widespread adoption of catalytic converters did not occur until more stringent emission control regulations forced the removal of the antiknock agent tetraethyl lead from most types of gasoline. Lead is a "catalyst poison" and would effectively disable a catalytic converter by forming a coating on the catalyst's surface. Catalytic converters were further developed by a series of engineers including Carl D.
Keith , John J. Mooney , Antonio Eleazar, and Phillip Messina at Engelhard Corporation,  creating the first production catalytic converter in William C. Pfefferle developed a catalytic combustor for gas turbines in the early s, allowing combustion without significant formation of nitrogen oxides and carbon monoxide.
Upon failure, a catalytic converter can be recycled into scrap. The precious metals inside the converter, including platinum , palladium , and rhodium , are extracted. Therefore, the first catalytic converters were placed close to the engine, to ensure fast heating. However, such placement can cause several problems. One of these is vapor lock.
As an alternative, catalytic converters were moved to a third of the way back from the engine, and were then placed underneath the vehicle. A 2-way or "oxidation", sometimes called an "oxi-cat" catalytic converter has two simultaneous tasks:. This type of catalytic converter is widely used on diesel engines to reduce hydrocarbon and carbon monoxide emissions. They were also used on gasoline engines in American- and Canadian-market automobiles until Because of their inability to control oxides of nitrogen , they were superseded by three-way converters.
Three-way catalytic converters TWC have the additional advantage of controlling the emission of nitric oxide NO and nitrogen dioxide NO 2 both together abbreviated with NO x and not to be confused with nitrous oxide N 2 O , which are precursors to acid rain and smog. Since , "three-way" oxidation-reduction catalytic converters have been used in vehicle emission control systems in the United States and Canada; many other countries have also adopted stringent vehicle emission regulations that in effect require three-way converters on gasoline-powered vehicles.
The reduction and oxidation catalysts are typically contained in a common housing; however, in some instances, they may be housed separately. A three-way catalytic converter has three simultaneous tasks: . Reduction of nitrogen oxides to nitrogen N 2. Oxidation of unburnt hydrocarbons HC to carbon dioxide and water , in addition to the above NO reaction. These three reactions occur most efficiently when the catalytic converter receives exhaust from an engine running slightly above the stoichiometric point. For gasoline combustion, this ratio is between In general, engines fitted with 3-way catalytic converters are equipped with a computerized closed-loop feedback fuel injection system using one or more oxygen sensors , [ citation needed ] though early in the deployment of three-way converters, carburetors equipped with feedback mixture control were used.
Three-way converters are effective when the engine is operated within a narrow band of air-fuel ratios near the stoichiometric point, such that the exhaust gas composition oscillates between rich excess fuel and lean excess oxygen. Conversion efficiency falls very rapidly when the engine is operated outside of this band. Under lean engine operation, the exhaust contains excess oxygen, and the reduction of NO x is not favored. Under rich conditions, the excess fuel consumes all of the available oxygen prior to the catalyst, leaving only oxygen stored in the catalyst available for the oxidation function.
Closed-loop engine control systems are necessary for effective operation of three-way catalytic converters because of the continuous balancing required for effective NO x reduction and HC oxidation. The control system must prevent the NO x reduction catalyst from becoming fully oxidized, yet replenish the oxygen storage material so that its function as an oxidation catalyst is maintained. Three-way catalytic converters can store oxygen from the exhaust gas stream, usually when the air—fuel ratio goes lean. A lack of sufficient oxygen occurs either when oxygen derived from NO x reduction is unavailable or when certain maneuvers such as hard acceleration enrich the mixture beyond the ability of the converter to supply oxygen.
Unwanted reactions can occur in the three-way catalyst, such as the formation of odoriferous hydrogen sulfide and ammonia. Formation of each can be limited by modifications to the washcoat and precious metals used. It is difficult to eliminate these byproducts entirely. Sulfur-free or low-sulfur fuels eliminate or reduce hydrogen sulfide. For example, when control of hydrogen-sulfide emissions is desired, nickel or manganese is added to the washcoat. Both substances act to block the absorption of sulfur by the washcoat.
Hydrogen sulfide forms when the washcoat has absorbed sulfur during a low-temperature part of the operating cycle, which is then released during the high-temperature part of the cycle and the sulfur combines with HC. For compression-ignition i. DOCs contain palladium , platinum , and aluminium oxide , all of which catalytically oxidize the hydrocarbons and carbon monoxide with oxygen to form carbon dioxide and water. These converters often operate at 90 percent efficiency, virtually eliminating diesel odor and helping reduce visible particulates soot.
These catalysts do not reduce NO x because any reductant present would react first with the high concentration of O 2 in diesel exhaust gas. Reduction in NO x emissions from compression-ignition engines has previously been addressed by the addition of exhaust gas to incoming air charge, known as exhaust gas recirculation EGR.
In , most light-duty diesel manufacturers in the U. There are two techniques that have been developed for the catalytic reduction of NO x emissions under lean exhaust conditions: selective catalytic reduction SCR and the lean NO x trap or NO x adsorber. Instead of precious metal-containing NO x absorbers, most manufacturers selected base-metal SCR systems that use a reagent such as ammonia to reduce the NO x into nitrogen. Ammonia is supplied to the catalyst system by the injection of urea into the exhaust, which then undergoes thermal decomposition and hydrolysis into ammonia.
Diesel exhaust contains relatively high levels of particulate matter PM. Catalytic converters do not remove PM so particulates are cleaned up by a soot trap or diesel particulate filter DPF. In the U. Note that this applies only to the diesel engine used in the vehicle. As long as the engine was manufactured before January 1, , the vehicle is not required to have the DPF system. This led to an inventory runup by engine manufacturers in late so they could continue selling pre-DPF vehicles well into For lean-burn spark-ignition engines, an oxidation catalyst is used in the same manner as in a diesel engine.
Emissions from lean burn spark ignition engines are very similar to emissions from a diesel compression ignition engine. Many vehicles have a close-coupled catalytic converter located near the engine's exhaust manifold. The converter heats up quickly, due to its exposure to the very hot exhaust gases, enabling it to reduce undesirable emissions during the engine warm-up period. This is achieved by burning off the excess hydrocarbons which result from the extra-rich mixture required for a cold start.
When catalytic converters were first introduced, most vehicles used carburetors that provided a relatively rich air-fuel ratio. Oxygen O 2 levels in the exhaust stream were therefore generally insufficient for the catalytic reaction to occur efficiently. Most designs of the time therefore included secondary air injection , which injected air into the exhaust stream. This increased the available oxygen, allowing the catalyst to function as intended.
Some three-way catalytic converter systems have air injection systems with the air injected between the first NO x reduction and second HC and CO oxidation stages of the converter. As in two-way converters, this injected air provides oxygen for the oxidation reactions. An upstream air injection point, ahead of the catalytic converter, is also sometimes present to provide additional oxygen only during the engine warm up period. This causes unburned fuel to ignite in the exhaust tract, thereby preventing it reaching the catalytic converter at all.
This technique reduces the engine runtime needed for the catalytic converter to reach its "light-off" or operating temperature. Most newer vehicles have electronic fuel injection systems, and do not require air injection systems in their exhausts. Instead, they provide a precisely controlled air-fuel mixture that quickly and continually cycles between lean and rich combustion.
Oxygen sensors monitor the exhaust oxygen content before and after the catalytic converter, and the engine control unit uses this information to adjust the fuel injection so as to prevent the first NO x reduction catalyst from becoming oxygen-loaded, while simultaneously ensuring the second HC and CO oxidation catalyst is sufficiently oxygen-saturated. Catalyst poisoning occurs when the catalytic converter is exposed to exhaust containing substances that coat the working surfaces, so that they cannot contact and react with the exhaust.
The most notable contaminant is lead , so vehicles equipped with catalytic converters can run only on unleaded fuel. Other common catalyst poisons include sulfur , manganese originating primarily from the gasoline additive MMT , and silicon , which can enter the exhaust stream if the engine has a leak that allows coolant into the combustion chamber. Phosphorus is another catalyst contaminant. Although phosphorus is no longer used in gasoline, it and zinc , another low-level catalyst contaminant was until recently widely used in engine oil antiwear additives such as zinc dithiophosphate ZDDP.
Depending on the contaminant, catalyst poisoning can sometimes be reversed by running the engine under a very heavy load for an extended period of time. The increased exhaust temperature can sometimes vaporize or sublime the contaminant, removing it from the catalytic surface. However, removal of lead deposits in this manner is usually not possible because of lead's high boiling point.
Any condition that causes abnormally high levels of unburned hydrocarbons—raw or partially burnt fuel—to reach the converter will tend to significantly elevate its temperature, bringing the risk of a meltdown of the substrate and resultant catalytic deactivation and severe exhaust restriction. Vehicles equipped with OBD-II diagnostic systems are designed to alert the driver to a misfire condition by means of illuminating the "check engine" light on the dashboard, or flashing it if the current misfire conditions are severe enough to potentially damage the catalytic converter.
Emissions regulations vary considerably from jurisdiction to jurisdiction. Most automobile spark-ignition engines in North America have been fitted with catalytic converters since ,     and the technology used in non-automotive applications is generally based on automotive technology. Regulations for diesel engines are similarly varied, with some jurisdictions focusing on NO x nitric oxide and nitrogen dioxide emissions and others focusing on particulate soot emissions.
This regulatory diversity is challenging for manufacturers of engines, as it may not be economical to design an engine to meet two sets of regulations. Regulations of fuel quality vary across jurisdictions. In North America, Europe, Japan, and Hong Kong , gasoline and diesel fuel are highly regulated, and compressed natural gas and LPG autogas are being reviewed for regulation.
Any sulfur in the fuel can be oxidized to SO 2 sulfur dioxide or even SO 3 sulfur trioxide in the combustion chamber. If sulfur passes over a catalyst, it may be further oxidized in the catalyst, i. Sulfur oxides are precursors to sulfuric acid , a major component of acid rain.
While it is possible to add substances such as vanadium to the catalyst washcoat to combat sulfur-oxide formation, such addition will reduce the effectiveness of the catalyst. The most effective solution is to further refine fuel at the refinery to produce ultra-low-sulfur diesel. Regulations in Japan, Europe, and North America tightly restrict the amount of sulfur permitted in motor fuels.
However, the direct financial expense of producing such clean fuel may make it impractical for use in developing countries. As a result, cities in these countries with high levels of vehicular traffic suffer from acid rain, [ citation needed ] which damages stone and woodwork of buildings, poisons humans and other animals, and damages local ecosystems , at a very high financial cost. Catalytic converters restrict the free flow of exhaust, which negatively affects vehicle performance and fuel economy, especially in older cars.
Adapted from Murota et al. Therefore, these studies show a direct correlation between OSC and the bulk structure of the mixed oxides.
Catalysis by ceria and related materials [electronic resource] in SearchWorks catalog
It is nevertheless also. These two papers illustrate how detailed experimentation is actually required to fully understand the oxygen-handling behaviour of metal-loaded ceria—zirconia materials, and how far this behaviour could be from correlations of a simplistic bulk structure to redox performance. We will keep this in mind hereafter. Thus, Fornasiero et al. Despite the oxide sintering induced by this treatment, the temperature of reduction in a subsequent H2-TPR run was found to have decreased from K to K.
This behaviour is the opposite of that observed in pure ceria,, for which the same thermochemical treatment leads to. This particular observation is very relevant because it shows that the redox response of ceria— zirconia mixed oxides subjected to high-temperature aging treat- ments is much better than that of ceria, a critically important feature in TWC applications.
Further studies on this issue have revealed some additional, very important, details. Thus, Baker et al. Taking these results into consid- eration, it was also proved that the state of the mixed oxide could be reversibly switched between that corresponding to an oxide with improved redox performance, i.
In effect, as shown in Fig. This pattern is reproduced, see traces 4 — 10 in Fig. Izu et al. It is worth mentioning first that these questions have been scrutinized in detail both at the macroscopic and microscopic, or even better atomic, levels by using a large variety of both chemical and structural techniques.
But even with such a huge variety of information and the wide perspective gained, unravelling these questions has proved to be a rather complex and very challenging task, which has caused strong controversy and remained an open issue until very recently. We should mention that assembling all the pieces in the puzzle has required information from techniques, which have just become fully available only recently, and also a detailed analysis of the chemi- cal response of the oxides by very carefully designed experiments and, finally, reasoning far beyond a simple correlation between the bulk structure of the oxides and their redox response.
Structural analysis using Rietveld-fitted X-ray powder diffracto- grams,,,, Raman spectroscopy,, and electron micros- copy— have shown that by starting from a fresh ceria—zirconia sample typically exhibiting a random distribution in its cationic sublattice, the high-temperature reduction treatment-induced ordering in the Ce and Zr distribution, produces an oxide, Ce2Zr2O7, with a pyrochlore-like structure. Figures 2. After SR-MO, the image contrast does not change dramatically, but the DDPs clearly show the appearance of superstructure reflections, marked on the inset in Fig. Bearing in mind that superstructure-type HRTEM image contrasts may have their origin either in changes related to the cationic sublat- tice or in the oxygen sublattice e.
Thus, white contrasts observed in these images correspond, for those projections in which the cations and anions do not overlap, to the position of Ce and Zr species. Likewise, higher intensities would be expected at sites populated by Ce species. In the former, Fig. In this study, the authors definitely con- firmed, by direct chemical analyses performed atomic column by atomic column, the ordering of the cationic sublattice into a pyrochlore-like arrangement. They produced images that show the differences in chemical composition of adjacent atomic columns expected for a pyrochlore-like superstructure.
In fact, detailed analy- ses of the electron energy loss spectra recorded in each atomic col- umn have allowed the authors to conclude, by precise comparison with simulations, that after the reduction treatment employed to prepare the SR-MO sample they studied, 5 h in flow of pure H2 at K, cation ordering was not completed. Local deviations from. These very interesting data clearly indicate a certain compositional flexibility of the pyrochlore structure and the inherent difficulties in producing well-defined compositions of the Ce—Zr—O system.
Most of the time intermediate, metastable situations are reached since the applied thermochemical treatments are not severe or prolonged enough to allow for a true equilibrium final state. This pyrochlore phase is therefore the stable one under high-temperature reducing conditions. Conversely, as a number of authors have stressed,, under oxidizing conditions, the pyrochlore-related phase is thermody- namically unstable against the transition leading to an oxide with a disordered distribution in its cationic sublattice and finally to segre- gated ceria- and zirconia-rich phases.
The fully oxidized SR-MO oxide is thus a metasta- ble phase. Accordingly, if it is heated at high enough temperatures, under an oxygen partial pressure allowing the oxide to be kept in a fully oxidized state, a new phase showing a random distribution of Ce and Zr in the cationic sublattice is formed. Thus, on the basis of DFT studies in which the tetragonal and pyrochlore phases of a Ce0. As we will see, the differences in OSC are not solely influenced by thermodynamic factors. Depending on the temperature at which the OSC is measured and also on the nature of the reducing agent H2, CO , other surface-related processes may have a significant influence on the measured OSC values.
Kinetic factors and, in con- nection with them, the surface structure of the oxides have a rele- vant role. In general, the interplay between surface and bulk structure determines the observed total OSC values. As with the OSC differences between ceria and ceria—zirconia metal-loaded catalysts, a more in-depth analysis is required in order to fully under- stand the redox response of thermally aged oxides. The experiments reported in Yeste et al. In this paper Yeste et al.
Nevertheless, when a small amount of rhodium 0. As proposed in Yeste et al. Once hydrogen is dissociated at a. Understanding Ceria-Based Catalytic Materials Intensity a. Initial hydrogen pressure was 38 torr. The samples were sub- mitted to successive cycles of heating at the indicated temperatures for 30 min fol- lowed by cooling to K always under hydrogen. Pressure drops were determined at K. To facilitate the comparison between H2 and H2O traces, hydrogen consumption is plotted as a positive signal. The effect of supported Rh on the reducibility of the oxides is exem- plified by the diagrams recorded for the 0.
Adapted from Yeste el al. This was actually confirmed by H2 chemisorption studies carried out on the bare oxides, Fig. In particular, in the — K temperature range, the surface density of chemisorbed hydrogen is approximately six times larger for the SR-MO sample. These observations strongly suggest the relevance of the H2 chemisorption step in the control of the low- temperature reducibility of the mixed-oxide samples. Ultimate, or total, OSC data measured after reduction in H2 at increasing temperatures, gathered in Table 2.
If it is assumed that the ultimate OSC values determined in the Rh-loaded systems represent the thermodynamic limit of reducibility, it is clear that, as suggested in Wang et al. Table 2. Adapted from Yeste et al. Nevertheless, the OSC values determined for the bare oxides, in the low-temperature region, do not reach the thermody- namic limit of reducibility, as they are clearly controlled by kinetic factors. As marked on Fig. The question that rises now is how the surface structure changes between the fresh or SR-SO samples, which show a similar redox performance, on the one hand, and the SR-MO oxide, with enhanced reducibility, on the other.
Advanced electron microscopy techniques have proved to be unique tools to answer this relevant issue. We will explain this important point in more detail. As depicted in Fig. Solid red arrows indicate Ce-rich planes; dashed blue arrows indicate Zr-rich planes. Due to the direct relation existing between contrast intensity and atomic number, HAADF-STEM images of the pyrochlore struc- ture along these directions show lines of alternating high and low intensity, see Fig.
The intensity profiles from the experimental images clearly reveal, as expected, an alternation of the intensities, which, according to a simulated image, Fig. From the intensity profiles, Figs. In this respect, the intensity profiles recorded from the experimental images, Fig. Therefore, electron microscopy data have revealed that the disorder-order transformation taking place in the bulk is accompa- nied by parallel structural and compositional rearrangements at the surface.
In Yeste et al. The evolution of total OSC with reduction temperature was studied for a Ce0. OSC data for these samples are presented in Fig. Note how, in the SR-MO-O sample, the application of a short oxidation treatment to the SR-MO sample causes a deterioration in the low-temperature redox behaviour, whereas the high-temperature OSC values remain unaltered.
In this case the low-temperature redox response increases but, again, the high-temperature OSC values remain close to those of the SR-SO oxide. These results suggest that the short oxidation treatment applied to the SR-MO oxide transforms it, with regard to the low- temperature redox response, into the SR-SO type but it keeps it in the SR-MO state for the high-temperature response. Thus, as shown in Fig. Nevertheless the surface of these crystallites has clearly changed, appearing now much rougher and more rounded.
Occasionally, small globular nodules with a solid-solution-type struc- ture have been detected at the surface, as illustrated in Fig. Remarkable differences may be noticed from one oxide to another. In Fig. Thus, very likely, an SR-SO oxide, aged under oxygen at K for 5 h, does not represent a true equi- librium state for the system but an intermediate transient on the way to the final fully segregated state.
These characterization data allow us to draw two very important conclusions: 1 the short oxidation and reduction treatments lead to an initial, first stage, transformation of the SR-MO and SR-SO oxides; 2 these first stages of the transformation start and affect primarily the surface structure of the oxides while, to a large extent, their bulk features are retained. These electron microscopy results allow a proper rationalization of the OSC values shown in Fig. Likewise, since high-temperature OSC is mainly dominated by ther- modynamic factors and the short redox treatments mostly do not modify the bulk structure of the starting oxides, the high-temperature.
H2 adsorption was determined from the pressure drop at K. The hydrogen adsorption data shown in Fig. Thus, note how the low-temperature hydrogen adsorption capacity of the SR-MO oxide clearly decreases after the short oxidation treatment, whereas after a short reduction of the SR-SO oxide, the hydrogen adsorption capacity of this oxide clearly increases. The whole set of chemical and structural data discussed so far can be put together into a consistent model that allows a rationalization of the modifications, with thermochemical treatments, of the ultimate H2-OSC values of ceria—zirconia oxides.
This model takes into account both the surface and bulk structure of the oxides and the influence of both thermodynamic and kinetic factors and is shown in Fig. The picture stresses the closed loop between the different situations we have described in this section and the nature of the thermochemi- cal treatments necessary to proceed from one situation to another. This comprehensive model, which explains all the experimental data currently available for the H2-OSC behaviour of bare oxides, can be considered a fruitful result of a general experimental.
The first interesting point we should mention in connection with the reducibility of the mixed oxides under CO is that behav- iour similar to that observed with H2 can be observed for the high- temperature redox aged oxides. Note that, qualitatively, the behaviour of the two oxides is the same under hydrogen and. From a thermodynamic point of view, for temperatures below K, the reducing power of CO is superior to that of H2, the difference pro- gressively increasing as the temperature decreases.
Accordingly, in the low-temperature range, CO should be expected to behave as a more effective reductant than H2. Nevertheless, the question arises again whether the recorded CO-DTG traces are mainly determined by thermodynamic factors or if kinetic aspects of the process must also be considered in the inter- pretation of these diagrams. An analysis of the ultimate OSC data, shown in Table 2. As deduced from Table 2. This is a consequence of the change in the relative thermodynamic stability of the ordered and disordered struc- tures as a function of the redox state of the samples, Fig.
Under oxidizing conditions, the higher stability corresponds to the. T is higher for the former reaction, the crossing point occurring at approximately K. In contrast, above K, H2 is slightly more effective. If this effect is combined with the evolution of the thermo- dynamic parameters for the reduction of the oxides with their redox state, Fig. If we look at this region of Fig. As already discussed, for kinetic reasons, in a flow of H2, the low-temperature reducibility of the oxides is strongly enhanced by the presence of small amounts of Rh supported on them. Assuming that the Rh phase does not modify the thermodynamic properties of the oxides, the dramatic influence of the metal should be interpreted as due to a catalytic effect.
On rhodium, H2 adsorption is faster, thus favouring the subsequent transfer of atomic hydrogen to the support via spill- over. If so, the H2-OSC value determined for the Rh-containing samples would actually provide oxygen storage data much closer to the thermodynamic limit of the oxide reduction than those for bare oxides, at the same temperature.
This is a relevant observation. T plots for CO and H2 oxidation reactions. Heating rate between successive steps: 10 K. Duration of the isothermal steps: 1 h. As already discussed, this is unexpected from the thermodynamic point of view. Actually, in accordance with Fig. Therefore, as for the H2-OSC values, for the bare oxides the CO-OSC data recorded at the lowest temperatures are determined by kinetic rather than thermodynamic factors.
The same is true for SR-SO even at K, which suggests that the kinetic restric- tions are, as already happened for H2, even stronger for this sample. Additional studies are still necessary to give a full understanding of the origin of the observed kinetic control in the ultimate CO-OSC measurements.
It should be noted, however, that, in accordance with the results reported in Table 2. Therefore, the kinetic control is probably associated with either the CO adsorption step leading to the formation of carbonate species or to the subsequent decomposition of the above mentioned carbonates. Currently available studies on the mechanism of CO oxidation over ceria—zirconia under dynamic conditions, sug- gest that the decomposition of the surface carbonate species could play a role in the kinetics of ceria—zirconia reduction by CO.
Nevertheless, the eventual contribution of CO adsorption to the rate-controlling step should not be disregarded. These final comments connect perfectly with the topic addressed in the next section: the interaction of CO with ceria—zirconia-supported gold catalysts. We suggest a reading of this book for a detailed and critical account of earlier studies on the topic. In recent years, the most significant progress has been made in the chemical characterization of cerium-based oxide-supported gold cata- lysts. There are a number of reasons that justify this observation.
A sec- ond major reason is the discovery in of the extremely high CO oxidation activity of gold nanoparticles dispersed on suitable 3D metal- oxide supports. Accordingly, detailed qualitative and quantitative information about the surface chemistry of gold is of outmost impor- tance for a fine understanding of its exceptional and very puzzling catalytic behaviour.
With reference to the catalysts of supported noble metals from Groups 8—10, for which chemical characterization rou- tines are reasonably well established, the development of appropriate tools for providing useful chemical information on highly dispersed gold catalysts is still a very challenging issue.
The extreme nobility of gold is a major reason for this. Thus, it is generally acknowledged that gold has much lower surface activity than the metals of Groups 8—10 against H2 and CO, two probe molecules commonly used in the chemical characterization of noble metal catalysts. In fact, until some twenty years ago the surface chemistry of gold was almost completely unknown. At present, adsorption studies include a variety of probe molecules and atoms, those dealing with CO,—,— H2,,,,—,,,,— and O,,,,— being particularly numerous.
Regarding the structural nature of the investigated gold samples, studies on massive single crystals, polycrystalline thin films, foils, wires and powders,,,,,, as well as clusters and nano- particles dispersed on planar model,,,,,,,—,,, and powder—,,,,,,—,,,,,— oxide supports are pres- ently available.
In most cases, different DFT approaches were used in these calculations. We will focus our attention on H2 and CO chemisorption studies. There are several reasons justifying this choice. As discussed in Bernal et al. Also very importantly, ceria-based gold catalysts are known to be highly active materials for CO oxida- tion,,,,,,— LT-WGS,39—42,,,— selective oxidation of CO in the presence of H2 PROX ,,,,,,,— or selective hydrogenation,,,, reactions in which these two molecules are involved.
Therefore, to gain detailed information about the chemistry of H2- and CO-catalyst interactions, it is crucial to arrive at a deeper understanding of these reactions, whose mechanisms have not been fully interpreted as yet. First, ceria-based oxides may adsorb large amounts of H2 and CO. This represents an additional serious complication because it prevents the straightforward use of parallel adsorption studies on the bare supports as a tool for deter- mining their contribution to the total adsorption by the metal cata- lysts.
Second, because of the reducible nature of cerium-based. It is therefore of interest to investigate the behaviour of gold with reference to that exhibited by the above mentioned noble metals. Moreover, the eventual occurrence of such an effect in gold catalysts could be important to fully interpret their behaviour in processes like PROX, LT-WGS or the selective hydrogenation reactions mentioned above, that typi- cally occur under net reducing conditions.
This section has been organized in two subsections. The first, Section 2.
- Morphology-dependent nanocatalysis on metal oxides.
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In the second, Section 2. As will be discussed below, some very recent studies have proved that the appropriate combination of Fourier transform infrared FTIR spectroscopy and volumetric adsorption studies of CO, with electron microscopy, computer simu- lation and nanostructural modelling techniques, provides a wealth of very useful chemical information about this family of cerium- containing oxide-supported gold powder catalysts.
Particularly noticeable are the correlations that have been established between. In effect, as reported in Corma et al. In contrast to the surface behaviour of single crystals, dissocia- tive adsorption of H2 D2 may occur on thin films of gold. As deduced from the TD studies reported in Lisowski et al. TD techniques have also been used to investigate the adsorption of atomic hydrogen on sintered films at 78 K. In accordance with Lisowski et al. Adapted from Lisowski et al.
From these results, the authors concluded that peak I is due to a very weakly adsorbed form of molecular H2, whereas peak II would account for the recombina- tion of H chemisorbed species. Though the authors do not identify the precise nature of the sites responsible for the dissocia- tion of H2 on unsintered thin films, this study may reasonably be considered as one of the first indications of the role played by the deficiently coordinated surface gold atoms in this process.
These so discouraging early results, probably explain why studies on the chemisorptive properties of supported gold catalysts were by the middle of the s so scarce. It is worth outlining, however, that at the time these pioneering works were published, the procedures for preparing supported gold catalysts were not well developed, so that they exhibited a very poor. In fact, the catalysts investigated by Sermon et al. The amount of irreversibly chemisorbed hydrogen was estimated from the difference between two consecutive isotherms separated by 2 h evacuation at the corre- sponding adsorption temperature.
In any case, these studies, show that, when highly dispersed, gold nanoparticles do activate the dissociative chemisorption of H2. In situ X-ray absorption spectroscopy XAS studies carried out in parallel with volumetric measurements have revealed that hydrogen chemisorption induces changes in Au-L3 and Au-L2 X-ray absorption near-edge structures. Moreover, with a model that assumes the gold. This observation allowed the authors to con- clude that H2 adsorption could only occur on corner and edged posi- tions of the alumina-supported Au nanoparticles.
There are two more aspects of the study by van Bokhoven and co-workers deserving some further comment. Second, the irreversible adsorption data show that evacuation at temperatures above K is required to ensure the complete elimination of the pre-adsorbed hydrogen. A much stronger H—Au interaction in the latter case might be one of them.
Differences in the kinetics of the desorption process could be a sec- ond. There is a third reason, which should not be disregarded: hydrogen desorbed at the highest temperatures could actually come from the support. In this respect, an additional consideration can be made. Though not specifically dealing with H2 adsorption, some comparative studies of CO interaction with massive and supported gold phases have suggested that the chemical principles governing.
If so, the dramatic difference found for hydrogen desorp- tion should be considered as a rather unexpected result. In general, these studies are fully consistent with the ability of gold nanoparticles to activate the dissociation of H2. Reproduced from Fujitani et al. Moreover, the apparent activation energy for the exchange reaction was also found to be the same, Moreover, these results have led to a better understand- ing of the mechanism of some selective hydrogenation reactions and also, very interestingly, of the spillover process on oxide-supported gold catalysts.
Likewise, they support the theory that adsorption only occurs on defective Au surface atoms at corners and edges of the nanoparticles. A second conclusion drawn from some of these studies is that the dissociation of H2 on supported gold nanoparticles could be an activated process. With the help of the increasing power of computer. Isomer A. Isomer B. Corner 0. The reference state was the free 2 Au surface and a hydrogen molecule.
The reference state was the molecular 2 form of adsorbed H2. The reference for the energy of the transition state was the H2 adsorbed in molecular form. The reference for the energy of the transition state was the free Au cluster and a H2 molecule. There are two major, rather general, conclusions that may be drawn from these theoretical stud- ies. First, most of the calculated energies for the dissociative adsorp- tion of H2 on Au clusters agree on the thermodynamic feasibility of the process, Table 2.
Second, also in agreement with the experimental studies, the theoretical calculations show that the active sites for the dissociative adsorption of H2 are localized at corners and edges of the gold nanoparticles, i. The calculated data. Reproduced from Corma et al. As can be seen in Fig. If the difference between the energy calculated for the transition state and that of the initial state constituted by a free H2 molecule and the corresponding Au cluster is considered, the results reported in Corma et al. This introduces some additional, very subtle, elements to the discussion of chemical interaction of Au clusters with H2.
Also, interestingly, in contrast to the experimental results that suggested the activated nature of hydrogen dissociation on supported gold nanoparticles, the theoretical calculation mentioned above may be interpreted as implying that, for some specific morphologies, there are sites on which the dissociation of H2 could be a non-activated process. Hydrogen adsorption on titania-supported gold systems has also been investigated by periodic density function DF calculations.
This slab, which contains atoms dis- tributed in 9 atomic layers, is equivalent to three TiO2 mono- layers. Likewise, systems containing stoichio- metric and oxygen-deficient one vacancy TiO2 surfaces were mod- elled. Some very fine details of the relation existing between the nanostructure, the electronic properties and chemical behaviour of these supported Au clusters for hydrogen dissociation were gained from this study.
Thus, the net charge on the gold nanoparticles was found to depend on the redox state of the support, the particles becoming positively charged when supported on the stoichiometric surface of TiO2 and negatively charged when seated on an oxygen-deficient surface. Moreover, calculations show that the electronic charge is unevenly distributed among the different atoms of the clusters. These are relevant observations because, as proposed by Corma and co-workers,, the H2 dissociation preferentially occurs on neutral or slightly charged Au atoms at corner and edge sites.
This seems to be a general behaviour for Au atoms in. The results reported in Fujitani et al. However, the number of studies specifically aimed at investigating hydrogen chemisorption on these catalysts is presently very scarce. Thus, in Collins et al. This approach has an unusual, and certainly very fruitful, combination of tech- niques.
The experiment used two consecutive isotherms at K, separated by a 30 min evacuation at K. Prior to running the isotherms, the. Adsorbed amount 6. Adapted from Collins et al. From XPS and electron microscopy studies, this sample consists of Au 0 nanoparticles with a mean size of 1. Under the same conditions, no measurable adsorption occurs on the bare CZ sup- port. It is obvious from these quantitative data that at K the spillover is very strong. Moreover, the estimate in Collins et al.
As discussed at length in Bernal et al. If so, the results obtained for the second isotherm suggest that forms other than the hydrogen chemisorbed on Au are removed by the 30 min evacuation treatment at K. These additional forms may consist of spilled- over hydrogen located in close vicinity of the metal nanoparticles, or molecular H2 very weakly interacting with the catalyst.
Moreover, if. In summary, though valuable, the results reported in Collins et al. They focus on the influence of CO co-adsorption and of a reducing pre-treatment at K. In both cases, the authors found a noticeable inhibition of the spillover phe- nomena. The inhibition effect of the pre-reduction at K is interpreted as due to the obvious modification induced by the pre-treatment on the surface properties of the CZ support, the increase in the surface concentration of oxygen vacancies and the inherent modification of the electronic properties of the support.
No information about the influence of the support redox state on the chemisorptive proper- ties of the gold phase against H2 may be deduced from this study. In the first case, the hydrogen chemisorption experiments were run at K on a low-loading 0. Likewise it was argued by the authors that IR bands at a similar position had been observed in the IR spectra of matrix isolated hydride complexes prepared by co-deposition at 3. As previ- ously discussed in this chapter, the latter interpretation is well estab- lished in the literature. Some differences were noted.
Bands due to H—Au were reported on the three investi- gated catalysts. The position, however, was found to strongly shift. This reduction process implies electron transfer from the spilled-over atoms of hydrogen to the support. Though not confirmed by experi- ments on gold catalysts yet, this reduction process is expected to be partly reversible, i. The available information, though limited, suggests that the chem- istry of the H2- ceria-based Au catalysts may be significantly modified by CO co-adsorption and by pre-reduction of the catalysts.
Additional studies are probably required to fully confirm the proposals above. In the specific case of catalysts consisting of Au nanoparticles dispersed on cerium-based oxide supports, there is an additional difficulty that very much complicates the quantitative analysis of the CO adsorp- tion data.
In effect, it is presently well known that ceria and closely related mixed oxides may chemisorb large amounts of CO. If so, the chemisorptive behav- iour of the supported gold nanoparticles may be significantly disturbed, which obviously complicates the analysis of the corre- sponding quantitative data. In accordance with the considerations above, it is obvious that an in-depth analysis of CO adsorption for cerium-based oxide- supported gold catalysts represents a very challenging issue, which requires first, the identification of the nature of the main CO forms occurring on the surface of both the metal phase and the support, and second, the quantitative establishment of the contribution of each of these forms to the total amount of adsorbed CO.
As will be discussed below in some detail, these goals have been successfully achieved in a series of recently published studies in which the chem- isorption of CO on catalysts consisting of gold nanoparticles supported on ceria—zirconia mixed oxides was investigated. As discussed in Collins et al. A second form,. Reproduced from Collins et al. These bands are attributed to CO adsorbed on cationic sites at the surface of the CZ support.
As deduced from the spectra shown in Fig. In contrast to the forms mentioned above, these carbonate species are only very slightly modified by the 30 min evacuation treatment at K. They are referred to as irreversible forms of chemisorbed CO. Similarly, the second of the two consecutive isotherms recorded on the bare support gives the amount of CO weakly interacting with its cationic sites. To illustrate this methodology, Fig. From this study, the difference isotherm 1—2 shown in Fig. Isotherms recorded at K. Moreover, as will be briefly discussed in the next sections of this chap- ter, the appropriate combination of CO adsorption studies, like those described above, with the nanostructural information gained from the application of HRTEM, HAADF-STEM, computer HRTEM image simulation and computer nanostructural modelling tech- niques, has allowed the authors to establish rather fine correlations between the chemical and nanostructural properties in this very chal- lenging and highly interesting family of gold catalysts.
From this study, it was concluded that the truncated cuboctahedron could be adopted as a representative morphology for the model Au nanoparticles. Likewise, by analysing an appropriate number of experimental STEM-HAADF micrographs, the sizes of approximately Au nanoparticles were measured, and the corresponding size distribu- tion determined. The upper part of Fig. The corresponding model b and image simulation c are also included. By analysing these images, the Au nanoparticle size distributions shown in the lower part of Fig.
They analysed the nanostructural characteristics, i. As briefly dis- cussed below, D4-j values are particularly interesting. For comparative purposes, in addition to the struc- tural information for Au surface sites, Table 2. As shown by Meier et al. These edge atoms represent one fourth of the total number of exposed gold atoms.
Obviously, there are some differences. According to the literature, the energy of CO adsorption on gold may depend on variables like the coordination number of the surface atoms,,,, modulations induced on the Au nano- particles by the nature or redox state of the oxide support,,,, and even, as suggested by some authors, on quantum effects asso- ciated with the size of the nanoparticles. These differences will obvi- ously determine the specific T—PCO relation at the adsorption equilibrium.
Regarding the volumetric adsorption data, the lower part of Fig. Taking into account that the only sig- nificant difference between the Au catalysts is their metal disper- sion, which is higher for the HD sample, it becomes obvious that metal dispersion, i. Despite the shape of the difference isotherms shown in Fig. In accord- ance with this model, the adsorbed phase would grow around the Au nanoparticles in the form of annuli, which are assumed to have the same radius.
In accordance with the assumptions made, the mean radius determined for the annuli of the CO phase irreversibly chemisorbed on the CZ support was found to be 1. The similarity of the. To confirm the proposed model, some additional assays were run. The experiment, which was inspired by the H2 and CO co- adsorption studies reported in Loschen et al. As discussed above, CO pre-adsorption should form annuli of car- bonate species surrounding the Au nanoparticles, whereas the 30 min evacuation would remove the CO adsorbed on the metal with no significant modification of the carbonate annuli.
As seen in Fig. The spectra are of a the initial sample, after b 5 min, c 15 min, d 30 min and e 60 min in contact with D2. They suggested instead that the difference was due to perturbations in the kinetics of the spillover process in the CO-pre-treated catalyst. This pro- posal is fully consistent with the existence of carbonate annuli around the Au nanoparticles, which would act as a barrier to the transfer of the atomic deuterium species from the metal to the support.
Curve a on Fig. This was compared to a spectrum recorded on the catalysts subsequently reduced at K, curve b , which is seen to be significantly modified. As also shown on the left of Fig. Details of a , b and c pre-treatments are given in the text. They proposed, accordingly, that the most likely interpretation for the observed effects was an SMSI effect, similar to that originally reported by Tauster et al.
As revealed. In accordance with the suggested interpretation of the effect, electron transfer from the reduced support to the Au nanoparticles is expected. Among several relevant results, the nano- structural constitution of the surface sites responsible for CO adsorption by the supported Au nanoparticles, the critical role played by the metal phase in the strong CO adsorption by the sup- port and the occurrence of strong fully reversible metal deactivation effects associated with changes in the redox state of the support, have been unequivocally established.
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