Wolfgang M.H. Sachtler

Redox chemistry in excessively ion-exchanged Cu/Na-ZSM-5

TPR/TPD and FTIR are used to characterize “excessively ion-exchanged” Cu/Na-ZSM-5. After calcination in O2 at 773 K at least two copper-oxygen species are present in addition to Cu2+ ions; these have been identified as CuO and [Cu-O-Cu]2+. Reduction in H2 transforms all these into Cu0 below 773 K. [Cu-O-Cu]2+ is autoreduced to Cu+ during outgassing. Reoxidation of Cu0 by zeolite protons to Cu+ is observed above 723 K in He or Ar; in the presence of CO this process is considerably enhanced and observed at much lower temperature, because CO is strongly adsorbed on Cu+. At 293 K CO adsorption causes reversible changes in the FTIR spectra.

Identification of copper(II) and copper(I) and their interconversion in Cu/ZSM-5 De-NOx catalysts

Abstract A combined Fourier transform IR (FT-IR) and electron paramagnetic resonance (EPR) study shows that copper in ‘excessively exchanged’ Cu/ZSM-5 is initially present as OH bridged Cu 2+ dimers, besides isolated Cu 2+ ions. Upon heating, the dimers lose water and become oxygen bridged [Cu-O-Cu] 2+ complexes. These are ‘EPR-silent’, presumably as a consequence of antiferromagnetic coupling of the unpaired electrons in each Cu 2+ ; they are, however, detectable by their perturbation of the lattice vibrations, detected by a FT-IR band at 918–923 cm −1 . Reduction by hydrogen or carbon monoxide converts the [Cu-O-Cu] 2+ complexes to pairs of Cu + ions, while the color changes from green to grey. Reductive adsorption of nitrogen monoxide on Cu 2+ results in the formation of Cu + -NO + . Destructive thermal desorption of nitrogen monoxide at 100°C not only restores the Cu 2+ ions, but also appears to regenerate the [Cu-O-Cu] 2+ complex. The results suggest that pairs of copper ions are instrumental in the catalytic decomposition of nitrogen monoxide.

Elementary steps in the formation of highly dispersed palladium in NaY: I. Pd ion coordination and migration

TPR, TPD, TPO, and TPMS have been used to determine the location and chemical state of Pd supported on NaY. Oxidation of the ammine ligands of Pd(NH3)2+4 is found to be a stepwise process, producing Pd(NH3)2+2 ions in supercages and Pd(NH3)2+ and Pd2+ ions in sodalite cages. The relative abundance and location of these ions can be controlled by the calcination program. The size of Pd particles after reduction depends on the location and coordination of Pd ions after calcination. Calcination below 250 °C leaves Pd ions in the supercages, where their coordination to ammine ligands decreases with increasing calcination temperature. The H/Pd ratio after reduction shows a strong positive correlation with the calcination temperature. It is concluded that the Pd particle size in the kinetic regime is controlled by the relative rates of nucleation and crystal growth, which in this case are determined by the relative abundances of the Pd tetraammine and diammine ions. At a calcination temperature of 300 °C Pd ions lose the third ammine ligand allowing the monoammine Pd ions to migrate into sodalite cages, where the remaining ligand is destroyed at 400 °C.

Formation of Pt particles in Y-type zeolites: The influence of coexchanged metal cations

Samples of platinum, supported on NaY, HNaY, and MNaY (M = Ca2+, La3+, or Fe2+), have been studied by temperature-programmed reduction (TPR), desorption (TPD), and extended X-ray absorption fine structure (EXAFS) to investigate the mechanism of the formation of Pt particles and the influence of H+ and added metal cations on it. Size and location of Pt particles in a zeolite matrix were found to be strongly dependent on the distribution of Pt ions between the supercages and the sodalite cages, which is controlled by the calcination temperature (Tc) prior to the reduction. At low Tc (e.g., 360 °C), the majority of the Pt2+ ions are in the supercages; after reduction small particles (5–10 A) are formed which are also located in the supercages. This is also true for PtY with or without coexchanged multivalent cations. The Pt0O2− distance is 2.65 A as derived from the k1-weighted EXAFS functions. At medium Tc (e.g., 450 °C), the Pt2+ ions are distributed between supercages and sodalite cages. In this case, the Pt2+ ions in the supercages are reduced first to form small particles which become nucleation sites for Pt0 atoms leaving the sodalite cages above 400 °C. Eventually, these Pt particles can become larger than the supercages. At high Tc (e.g., 550 °C), most Pt2+ ions migrate to sodalite cages and require a high reduction temperature (Tr). The reduced Pt0 atoms then migrate from sodalite cages to supercages, and in the absence of nucleation sites they finally coagulate on the external surface of the zeolite crystals to form large Pt particles. However, the presence of coexchanged multivalent cations, e.g., Fe2+, which can effectively block sodalite cages and hexagonal prisms, thus forcing Pt2+ ions to stay in supercages, can prevent the formation of large particles on the external surface of zeolites even at high Tc. The evolution of hydrogen at Tmax = 450 °C, caused by oxidative reaction of Pt0 atoms with hydroxyl groups during TPD, as indicated by FTIR, can be used to determine the presence of Pt in sodalite cages.

Zeolite-Supported Transition Metal Catalysts

Publisher Summary This chapter discusses zeolite-supported transit ion metal catalysts. In catalytic applications, zeolites are predominantly used in their acidic form. The most important process in this category is fluidized catalytic cracking, based on rare-earth-exchanged zeolites, mainly X and Y of the faujasite structure with small admixtures of ZSM-5. Another industrial process in this group is catalytic dewaxing using mordenite and ZSM-5. Within the vast group of zeolite catalysts, the focus in the chapter is on one subgroup: materials that contain reduced particles of a transition metal or several transition metals dispersed inside zeolite cavities. A majority of transition metal/zeolite catalysts are bifunctional, i.e., strong acid sites are present in the same zeolite. A process based on a zeolite-encaged metal in the absence of acid sites is the dehydrocyclization of small linear alkanes (such as n -hexane) to aromatics. NO x abatement by zeolite-supported Cu is mentioned briefly in the chapter that illustrates the potential for environmental catalysis; it also opens prospectives for stabilizing elements in unusual valence states, in addition to unusual states of aggregation and complexation.

Promoted Fe/ZSM-5 catalysts prepared by sublimation: de-NOx activity and durability in H2O-rich streams

Fe/ZSM-5 catalysts with an Fe/Al ratio 1:0, were prepared by sublimation of FeCl3 into H/ZSM-5. They display high activity and durability for the selective catalytic reduction of NOx to N2, both in dry and wet gas flows. These catalysts have now been modified by exchanging a second cation into the zeolite. Mere neutralization of zeolite protons by Na+ lowers the selectivity for NOx reduction to N2, but the cations Ce3+ and La3+ act as true catalyst promoters. With isobutane as the reductant in a simulated vehicular emission gas, almost 90% of NOx is reduced to N2 at 350°C over the La-promoted catalyst. The presence of 10% H2O in the feed does not impair the catalyst performance at high temperature; in the temperature region below 350°C it even increases the N2 yield. The beneficial effect of La is due to its lowering of the catalyst activity for the undesired combustion of the hydrocarbon. No signs of zeolite destruction are evident after 100 h TOS in a wet gas flow at 350°C. Carbonaceous deposits causing a slight deactivation are easily removed in an O2/He flow at 500°C; this in situ regeneration fully restores the original activity.

On the nature of active sites in Fe/ZSM-5 catalysts for NOx abatement

Abstract Fe/ZSM-5 catalysts with high Fe loading (Fe/Al∼1) have been prepared by sublimation of FeCl3 onto H-ZSM-5 samples of different Si/Al ratios. They catalyze NOx reduction with hydrocarbons in an excess of O2 and H2O. TPR shows that the Fe in the zeolite cavities is different from Fe2O3 particles. Naked Fe3+ ions are absent; oxo-ions, which are equally well reducible by CO and H2, prevail. A minority of the Fe complexes lose oxygen upon mere heating to ∼500°C; some of the reduced sites are reoxidized only by N2O. The population of oxo-complexes that lose oxygen by heating depends on the Si/Al ratio, this dependence is in qualitative agreement with the model of (2+) charged binuclear ions [HO–Fe–O–Fe–OH]2+. Upon reacting with NO, the bridging O atom is transferred and NO2 is formed. This step is not rate limiting for active catalysts with high Al/Si ratio and high Fe loading, but it becomes critical with zeolites of low Al/Si ratio.

Comparison of Pt/KL catalysts prepared by ion exchange or incipient wetness impregnation

Pt/KL catalysts prepared by ion exchange (IE), incipient wetness impregnation (IWI), and coimpregnation with KCl (IWI + KCI) have been characterized by dynamic techniques, chemisorption of H2 and CO, Fourier transform-infrared spectroscopy (FT-IR) of adsorbed CO, and catalytic tests using n-hexane (n-C6) and methylcyclopentane (MCP) conversions as probe reactions. Temperature-programmed reduction (TPR) shows significant differences between the IE and IWI catalysts. After calcination up to 400°C, the IWI samples contain Pt4+ ions that are reduced at 250°C, but IE samples contain Pt2+ particles that are reduced at 11°C, besides two Pt2+ species with TPR peaks at 80 and 150°C. High-resolution electron microscopy, adsorption, FT-IR, and catalytic results consistently show that, after reduction, the IWI catalysts contain smaller Pt particles located inside the zeolite channels, while the IE sample has larger particles, some of which are on the external surface. At high temperature, excess KCI reacts with zeolite protons, forming HCl which escapes. In comparison to the IE samples, the IWI catalysts are less acidic, less active for n-C6 conversion, more selective for dehydrocyclization, but less selective for hydrogenolysis, and they deactivate less. Since hydrogenolysis requires large Pt ensembles, the small Pt particles in the IWI catalysts produce less C1C5 compounds and less coke. The product distribution of MCP ring opening shows higher than statistical selectivity toward 3-methylpentane, suggesting that the MCP molecule becomes oriented inside the zeolite channels.

Elementary steps in the formation of highly dispersed palladium in NaY: II. Particle formation and growth

Abstract Calcination conditions which leave Pd ions in Pd NaY in two reproducible states have previously been defined: Low-temperature calcination ( T c = 250 ° C ) leaves Pd(NH 3 ) 2+ x in supercages, but high-temperature calcination ( T c = 500 ° C ) places Pd 2+ quantitatively in sodalite cages. It has now been found that the processes which lead to Pd particle formation during reduction with H 2 are entirely different for these two cases. Reduction of Pd ions in supercages results in the formation of primary particles ( d A ) which rapidly migrate and coalesce to larger particles, whose migration is impeded by the size of the supercage windows. Reduction of Pd ions in sodalite cages, however, leads to the formation of isolated atoms or dimers which are trapped in these cages. Their subsequent release into the supercage network is an activated process. The ratio of adsorbed hydrogen to reduced palladium, in this case, initially increases with temperature, then passes through a maximum, indicating that isolated Pd atoms are incapable of dissociatively chemisorbing H 2 .

Effect of zeolite protons on palladium-catalyzed hydrocarbon reactions

Catalytic superactivity of electron-deficient palladium for neopentane conversion, previously reported for Pd/Al{sub 2}O{sub 3}, has been verified for Pd/NaHY. The reaction rate, per accessible Pd atom, correlates with the proton content of the catalysts; for samples that contain all the protons generated during H{sub 2} reduction of Pd, it is two orders of magnitude higher than that for Pd/SiO{sub 2}. Samples prepared by reduction of Pd(NH{sub 3}){sub 2}{sup 2+} NaY display an intermediate activity. It is suggested that Pd-proton adducts are highly active sites in neopentane conversion. With methylcyclopentane as a catalytic probe, all Pd/NaHY samples deactivate rapidly and coke is deposited. Temperature-programmed oxidation reveals two types of coke, one of which correlates with the proton concentration in the catalyst. 41 refs.