Only for the ceria- and zirconia-supported catalysts is it possible to distinguish the main diffraction line related to NiO crystallites cubic phase , corresponding to the diffraction plane.
By applying the Scherrer equation, an average domain of NiO crystallites was measured Table 1. On the other hand, the crystalline domains differ among the different supports, although being smaller and quite similar for the alumina and lanthana-supported catalysts.
Figure 1. Table 1. Average domain sizes for calcined samples before reduction prepared by impregnation determined by the Scherrer equation. Rh NPs appear as finely dispersed particles with an average size less than 1 nm.
Ni NPs are also observed and present larger size average around 8 nm. Figure 2. In addition, the XRD patterns of Rh-promoted Ni-supported catalysts, prepared by solid state reaction, were recorded. The XRD patterns of the calcined ones before reduction are shown in Figure 1 line 2.
As for the catalysts prepared by impregnation, the diffraction lines are mainly ascribed to the corresponding supports. In the case of the lanthana-supported catalyst, the formation of the LaNiO 3 phase cannot be discarded. For the catalyst supported on magnesia, the diffraction lines corresponding to MgO and a Ni-Mg-O solid solution cannot be distinguished because of their strong overlap Arena et al.
The formation of the Ni-Mg-O solid solution is due to the relatively high calcination temperature used in the catalyst preparation. Concerning the zirconia-supported catalyst, for this preparation method, zirconia crystallizes in the tetragonal phase. Rh reflections were not observed, as expected from the very small crystallite size. Concerning the presence of crystalline nickel species, only in the case of the catalyst supported on CeO 2 was it possible to observe a small diffraction line of the 2 0 0 plane of NiO.
The crystallinity of the catalysts considerably changed; being higher for the MgO-supported catalyst and lower for the Al 2 O 3 -supported one. The XRD diagram of this last catalyst indicates its amorphous nature, since the calcination temperature was not high enough to get a crystalline structure of alumina.
The Scherrer equation was applied to these X-ray diffraction diagrams, and the average domain sizes of the different crystalline phases are shown in Table 2. For these calcined samples, prepared by solid-state reaction, the average domain size among the different supports did not change as much as that which occurred for the calcined samples prepared by impregnation of commercial supports.
Moreover, it is noteworthy that the crystalline domains of NiO, if they could be observed, were smaller than those obtained for the samples prepared by impregnation of the commercial supports Table 1. Table 2. Average domain sizes for calcined samples before reduction , fresh reduced catalysts and catalysts after reaction prepared by solid state reaction.
This figure also shows the XRD patterns of the reduced catalysts Figure 1 , line 3. The diffraction lines correspond basically to that of the supports. In the case of the lanthana-supported catalyst, the XRD profile consists of overlapping lines of La 2 O 3 , La 2 NiO 4 tetragonal, and La OH 3 hexagonal, as a result of the strong interaction between nickel and lanthana Pantaleo et al.
Only for the catalyst supported on alumina and lanthana, low intensity peaks assigned to Ni metal [at On the other hand, diffraction lines assigned to NiO are observed in the ceria-supported samples, indicating that at least part of the nickel phase cannot be reduced. Nitrogen adsorption-desorption isotherms of Ni- and Rh-Ni-supported catalysts are displayed in Figure 3 and the surface areas are summarized in Table 3.
These isotherms, which are of type IV, are assigned to mesoporous materials. All isotherms display a type H3 hysteresis loop, indicating that the catalysts contain a mesoporous network consisting of slit-type pores. In the relative pressure range of 0. Figure 3. N 2 adsorption-desorption isotherms of the calcined samples Ni catalysts, prepared by impregnation and Rh-Ni catalysts prepared by solid-state reaction. Table 3. BET surface areas of commercial supports, of calcined Ni samples prepared by impregnation, and Rh-Ni samples prepared by solid state reaction calcined, reduced and after reaction.
For the Ni catalysts prepared by impregnation, it can be seen that the adsorbed amount of N 2 is higher for the catalysts supported on alumina and magnesia and smaller for the ceria- and zirconia-supported ones. This is in accordance with the large crystallite size of these commercial supports and their low surface area see Table 3 , which in turn decreases the dispersion of the nickel phase, as can be derived from the X-ray diffraction diagrams Figure 1 and the average crystalline domains determined for supported NiO NPs Table 1.
For the catalysts supported on alumina and magnesia, a decrease in surface area is observed, which is explained by the blockage of part of the porous lattice by nickel oxide NPs. On the contrary, for the samples supported on ceria, lanthana, and zirconia, an increase in surface area is observed, which might be attributed to the contribution of these NiO surface NPs.
The catalyst with the largest surface area is, by far, the one supported on alumina, followed by that supported on magnesia. The other catalysts show BET surface area values one order of magnitude lower, being the lowest that supported on ceria.
The alumina-supported Rh-Ni calcined catalyst prepared by solid state reaction, presents a H 2 hysteresis loop in the range of small mesopores at relative pressures of 0. From the nitrogen adsorption values, it can be seen that this preparation method gives catalysts with more uniform textural properties than those prepared by impregnation of commercial supports. The highest surface area was obtained for the ceria-supported catalyst as a consequence of the formation of small nanocrystals Kundakovic and Flytzani-Stephanopoulos, The reduction of these Rh-Ni catalysts produced a change in surface area with respect to the corresponding calcined counterparts, increasing for lanthana and magnesia-supported catalysts and decreasing for the other ones.
The used catalysts showed a decrease in surface area due to sintering, with the exception of those supported on alumina and zirconia, for which the surface area slightly increased. The reduction profiles of the different catalysts prepared by impregnation and by solid state reaction are shown in Figure 4. The reduction profiles of Ni catalysts prepared by impregnation are depicted in Figure 4A. Figure 4. Temperature Programmed Reduction profiles of the calcined samples, prepared by impregnation A,B and prepared by solid state reaction C.
The consumption at the highest recorded temperature is explained by the reduction of some bulk ceria Zhang et al. In the second reduction step, a complete reduction of nickel to metallic nickel is achieved, and a system based on finely dispersed Ni 0 particles supported on a La 2 O 3 matrix is obtained.
The main H 2 consumption is due to the reduction of free NiO NPs and the minor one, at some higher temperature, to the reduction of NiO strongly interacting with the support Wang et al. This is because the presence of noble metals facilitates the reduction of nickel oxide via a hydrogen-spillover mechanism Tanaka et al.
As the area of this peak is much higher than that corresponding to the total reduction of Rh in the sample, the consumption of hydrogen by ceria must be considered Li et al. The reduction profile of the lanthana-supported Rh-Ni also showed two components—similar to what occurred in the Rh-free counterpart—due to the reduction of LaNiO 3 which is known to occur in two stages.
The interaction of nickel species with the support is one of the factors that influence the reactivity of the catalysts. In principle, a strong interaction among both phases is beneficial to stabilize the supported metal NPs, decreasing the deactivation by sintering Ruckenstein and Wang, The percentage of phase content for each metallic and metal oxide species was calculated by integrating the fitted XPS data of each sample before and after the reaction.
The evolution of the Ni oxidation state is shown in Figure 5B. The highest metallic nickel exposure was extracted for the sample supported on lanthana, related to the formation of LaNiO 3 , in which nickel is atomically distributed in the perovskite lattice. In the other extreme is the sample supported on zirconia, with the lowest metallic nickel proportion.
The highest value found for the catalyst supported on lanthana is explained by the atomic nickel dispersion achieved in LaNiO 3 perovskite. Figure 5. Rh was not analyzed since its low amount makes it difficult to be detected. The C 1s spectra show a peak around eV, corresponding to surface carbonates for the lanthana and magnesia-supported catalyst precursor, being much intense for the lanthana-supported one. These ratios change considerably depending on the support type.
The higher ratios obtained for ceria and lanthana-supported catalyst precursors are characteristic of a highly dispersed nickel phase. The C 1s core-level spectrum of the lanthana-supported catalyst precursor shows a component around — eV due to the presence of carbonates, which is in accordance with the strong basic character of this oxide.
Table 4. Rh-Ni catalysts after reaction have been structurally characterized by X-ray diffraction. X-ray diffraction patterns, depicted in Figure 1 , showed that the reaction lead to structural changes in all the catalysts, as observed by comparing with the XRD patterns of fresh reduced samples. For the catalyst supported on lanthana, the support is based on lanthanum oxycarbonate Requies et al. The characterization of carbon deposits on the spent catalysts was carried out by Raman spectroscopy spectra are shown in Figure 6.
For each sample, at least three Raman spectra were recorded in different areas to assure the homogeneity of the composition. Figure 6. Raman spectra of the catalysts after reaction Ni catalysts, prepared by impregnation and Rh-Ni catalysts prepared by solid-state reaction.
The ceria-supported catalysts showed low intense peaks of carbon deposits. This is due to the widely reported oxygen mobility of the ceria surface Dong et al.
The low carbon formation in lanthana-supported catalysts is ascribed to the surface La 2 O 2 CO 3 species, well known as gasifier agents of C precursors. The formation of different types of carbon seems to be related to the crystal phase of zirconia. It is also shown as a one-way rather than a reversible reaction to avoid complicating things. This example is slightly different from the previous ones because the gases actually react with the surface of the catalyst, temporarily changing it.
It is a good example of the ability of transition metals and their compounds to act as catalysts because of their ability to change their oxidation state.
Note: If you aren't sure about oxidation states , it might be useful to follow this link before you go on. Use the BACK button on your browser to return to this page.
The sulphur dioxide is oxidised to sulphur trioxide by the vanadium V oxide. In the process, the vanadium V oxide is reduced to vanadium IV oxide. This is a good example of the way that a catalyst can be changed during the course of a reaction. At the end of the reaction, though, it will be chemically the same as it started. Note: If you want more detail about the Contact Process , you will find a full description of the conditions used and the reasons for them by following this link.
This has the catalyst in the same phase as the reactants. Typically everything will be present as a gas or contained in a single liquid phase. The examples contain one of each of these. This is a solution reaction that you may well only meet in the context of catalysis, but it is a lovely example! Persulphate ions peroxodisulphate ions , S 2 O 8 2- , are very powerful oxidising agents. Iodide ions are very easily oxidised to iodine. And yet the reaction between them in solution in water is very slow.
The reaction needs a collision between two negative ions. Repulsion is going to get seriously in the way of that! The catalysed reaction avoids that problem completely. This is another good example of the use of transition metal compounds as catalysts because of their ability to change oxidation state. For the sake of argument, we'll take the catalyst to be iron II ions. In the process the persulphate ions are reduced to sulphate ions.
The iron III ions are strong enough oxidising agents to oxidise iodide ions to iodine. In the process, they are reduced back to iron II ions again. Both of these individual stages in the overall reaction involve collision between positive and negative ions. This will be much more likely to be successful than collision between two negative ions in the uncatalysed reaction.
The reactions simply happen in a different order. Ozone, O 3 , is constantly being formed and broken up again in the high atmosphere by the action of ultraviolet light.
Ordinary oxygen molecules absorb ultraviolet light and break into individual oxygen atoms. These have unpaired electrons, and are known as free radicals. They are very reactive. Ozone can also be split up again into ordinary oxygen and an oxygen radical by absorbing ultraviolet light. This formation and breaking up of ozone is going on all the time.
Taken together, these reactions stop a lot of harmful ultraviolet radiation penetrating the atmosphere to reach the surface of the Earth. The catalytic reaction we are interested in destroys the ozone and so stops it absorbing UV in this way. Their slow breakdown in the atmosphere produces chlorine atoms - chlorine free radicals.
These catalyse the destruction of the ozone. This happens in two stages. Bulletin of the Korean Chemical Society , ,, Heterocyclic compounds such as pyrrole, pyridines, pyrrolidine, piperidine, indole, imidazol and pyrazines. Applied Catalysis A: General , 2 , Journal of the European Ceramic Society , 23 13 , Waser , H. Heterocyclic compounds such as pyrroles, pyridines, pyrollidins, piperdines, indoles, imidazol and pyrazins. Applied Catalysis A: General , , Smith , Ferenc Notheisz.
Kreher , Michael Konrad , Frank Jelitto. Chemische Berichte , 2 , Electrocatalytic Reduction Using Raney Nickel. Bulletin of the Chemical Society of Japan , 56 3 , A study of the thermodynamic properties of the system polystyrene-ethylcyclohexane by the light scattering method. British Polymer Journal , 9 3 , Fatty alcohols: Chemistry and metabolism. Progress in the Chemistry of Fats and other Lipids , 15 4 , Siddiqui , Naseem H. Khan , Mohd Ali , A.
A Convenientt Synthesis of Amines. Synthetic Communications , 7 1 , Desaminierungsreaktionen, Untersuchungen zur 1,3-Wasserstoffverschiebung. Chemische Berichte , 6 , Evgrashin , I. Semenskaya , Ya. Shmulyakovskii , I. Use of IR spectroscopy to investigate the hydrogenation of succinimide.
Journal of Applied Spectroscopy , 14 5 , Ioffe , M. Hydrogenation of succinimide to 2-pyrrolidone. Chemistry of Heterocyclic Compounds , 6 5 , Hydrogenation of succinimide to pyrrolidinone. Chemistry of Heterocyclic Compounds , 6 3 , Mathison , Wiley L. Journal of Pharmaceutical Sciences , 58 10 , For instance, hydrogenation of a carbon-carbon double bond can be achieved without simultaneously reducing a carbonyl bond in the same molecule.
For example the carbon-carbon double bond of the following aldehyde can be reduced selectively:. Alkynes are hydrogenated more easily than alkenes mainly because alkynes are adsorbed more readily on the catalyst surface. Hydrogenation proceeds in stages, first to the cis -alkene and then to the alkane. For example,. Normally, it is not possible to stop the hydrogenation of an alkyne at the alkene stage, but if the catalyst is suitably deactivated, addition to the triple bond can be achieved without further addition occurring to the resulting double bond.
The preferred catalyst for selective hydrogenation of alkynes is palladium partially "poisoned" with a lead salt Lindlar catalyst. This catalyst shows little affinity for adsorbing alkenes and hence is ineffective in bringing about hydrogenation to the alkane stage:. Aromatic hydrocarbons are hydrogenated with considerable difficulty, requiring higher temperatures, higher pressures, and longer reaction times than for alkenes or alkynes:.
John D. Robert and Marjorie C. Caserio Basic Principles of Organic Chemistry, second edition. Benjamin, Inc. ISBN This content is copyrighted under the following conditions, "You are granted permission for individual, educational, research and non-commercial reproduction, distribution, display and performance of this work in any format.
Mechanism of Hydrogenation The exact mechanisms of heterogeneous reactions are difficult to determine, but much interesting and helpful information has been obtained for catalytic hydrogenation.
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