R.W.C. van den Brink

The role of the support and dispersion in the catalytic combustion of chlorobenzene on noble metal based catalysts

Polychlorinated benzenes (PhClx) are formed as byproducts in the combustion of chlorobenzene on Pt supported on γ-Al2O3, SiO2, SiO2–Al2O3, or ZrO2. The congener and isomer distribution of the PhClx differs for the various supports. The amounts of PhClx correlate with the dispersion of platinum. Thus, a Pt/γ-Al2O3 catalyst calcined at 500°C to yield very small Pt crystallites was more active in PhClx formation than Pt/γ-Al2O3 calcined at 800°C. In all cases T50% for chlorobenzene conversion is close to 300°C and appears to be independent of the crystallite size of the platinum. Replacing platinum by palladium led to lower rates of combustion and to more byproducts. These results lead us to propose that, in the presence of Cl and higher oxygen concentrations, small Pt crystallites are converted more easily into Pt(IV) species. These are less efficient in combustion, but can be more active in chlorination.

Influence of zeolite structure on the activity and durability of Co-Pd-zeolite catalysts in the reduction of NOx with methane

Abstract Selective catalytic reduction of NO with CH4 was studied over ZSM-5, MOR, FER and BEA zeolite-based cobalt (Co) and palladium (Pd) catalysts in the presence of oxygen and water. As compared to other catalytic systems reported in literature for CH4-SCR in the presence of water, zeolite supported Co-Pd combination catalysts are very active and selective. The most active catalysts, based on MOR and ZSM-5, are characterised by well-dispersed Pd ions in the zeolite that activate methane. Wet ion exchange is a good method to achieve high dispersion of Pd provided that it is carried out in a competitive manner. The presence of cobalt (Co3O4, Co-oxo ions) boosts SCR activity by oxidising NO to NO2. The activity of the zeolite-based Co-Pd combination catalysts decreases with prolonged times on stream. The severity of the deactivation was found to be different for different zeolite topologies. The characterisation and evaluation of freshly calcined catalysts and spent catalysts show two things that occur during reaction: (1) zeolite solvated metal cations disappear in favour of (inactive) metal oxides and presumably larger metal entities, i.e. loss of dispersion; (2) loss of crystallinity affiliated with steam-dealumination and the concomitant formation of extra-framework aluminium (EFAL) in the presence of water. Both phenomena strongly depend on the (reaction) temperature. The deactivation of Co-Pd-zeolite resembles the deactivation of Pd-zeolite. Hence, future research could encompass the stabilisation of Pd (cations) in the zeolite pores by exploring additives other than cobalt. For this, detailed understanding on the siting of Pd in zeolites is important.

Catalytic combustion of chlorobenzene on Pt/γ-Al2O3 in the presence of aliphatic hydrocarbons

During the catalytic combustion of chlorobenzene on a 2% Pt/γ-Al2O3 catalyst, considerable amounts of polychlorinated benzenes are formed as by-products. The co-feeding of heptane practically eliminates this unwanted side-reaction. Moreover, the conversion of chlorobenzene occurs at much lower temperatures (the T50%drops from 305 to 225°C). Simultaneously, the conversion of heptane is retarded. The addition of other hydrocarbons have a similar effect. Water and heat produced by the combustion of the added hydrocarbon cannot explain the increase in destruction rate of chlorobenzene. Removal of Cl from the surface by the alkane appears to be the ruling factor.

Hydrogen–deuterium isotope effects in the reactions of chlorobenzene and benzene on a Pt/γ-Al2O3 catalyst

The kinetic isotope effect for combustion of a C6H5Cl/C6D5Cl mixture on Pt/γ-Al2O3 was found to be close to unity between 520 and 580 K. However, in the presence of an excess of heptane, an isotope effect of 1.5 was found between 460 and 490 K. For the combustion of a C6H6/C6D6 mixture the kH/kD value was around 2 between 404 and 439 K. The results show that in the combustion of chlorobenzene per se, C–H bond activation is not a rate-determining step. On Pt sites, C–Cl bond scission probably occurs already at low temperatures. The chlorine and the phenyl group cannot easily react further. Chlorine on the surface is active in chlorination, which is shown by the formation of C6D5Cl in an experiment with C6H5Cl and C6D6. Only at a certain temperature is the chlorine removed, partly as polychlorinated benzenes. The removal of chlorine from the catalyst allows oxygen to take part in the reaction, which determines the rate of the combustion of chlorobenzene. When heptane is present, Cl is removed from the surface and C–H bond scission can become rate determining, as is also the case in the combustion of C6H6/C6D6. Upon (partial) combustion of C6H5Cl/C6D5Cl and C6H6/C6D6 mixtures on a Pt/γ-Al2O3 catalyst, hydrogen–deuterium exchange occurs on the γ-Al2O3 support.

Materials for hydrogen production with integrated CO2 capture

Palladium‐based membrane reactors and sorption‐enhanced water‐gas shift are two promising technologies for efficient production of hydrogen with integrated CO2 capture. This paper discusses material issues of the two crucial materials of these technologies: the membrane and the CO2 sorbent. For Pd and Pd‐alloy membranes the major issues concern the stability of the membrane and the poisoning of the membrane surface by compound such as sulfur and carbon monoxide. Both issues are addressed by research into novel Pd‐alloys. For the potassium‐promoted hydrotalcite CO2 sorbents used in sorption‐enhanced water gas shift, the main challenges are a high CO2 adsorption capacity, chemical and mechanical stability, and low steam use for sorbent regeneration. Promising results have recently been reported for several of these challenges. For both hydrogen‐selective membranes and CO2 sorbents, the interaction of the materials with sulfur is an import issue to enable their use in the production of hydrogen from coal.Palladium‐based membrane reactors and sorption‐enhanced water‐gas shift are two promising technologies for efficient production of hydrogen with integrated CO2 capture. This paper discusses material issues of the two crucial materials of these technologies: the membrane and the CO2 sorbent. For Pd and Pd‐alloy membranes the major issues concern the stability of the membrane and the poisoning of the membrane surface by compound such as sulfur and carbon monoxide. Both issues are addressed by research into novel Pd‐alloys. For the potassium‐promoted hydrotalcite CO2 sorbents used in sorption‐enhanced water gas shift, the main challenges are a high CO2 adsorption capacity, chemical and mechanical stability, and low steam use for sorbent regeneration. Promising results have recently been reported for several of these challenges. For both hydrogen‐selective membranes and CO2 sorbents, the interaction of the materials with sulfur is an import issue to enable their use in the production of hydrogen from coal.

A synergistic effect in Iron-Ruthenium-FER catalyst for N2O decomposition in the presence of NO

Oxygen, water and nitrogen oxide (NO) inhibit N 2 O decomposition over noble metal catalysts. The inhibiting effect of NO on N 2 O decomposition over zeolite supported noble metal catalysts can be (partly) eliminated by combining the noble metals with iron. In the presence of NO, the overall conversion of N 2 O over Fe-Ru-FER exceeds the sum of conversions over the monometallic analogues of the individual components. This synergistic effect gives superior N 2 O decomposition activity under realistic conditions, i.e. in the presence of water, O 2 and NO, at temperatures as low as 623 K. The role of oxygen in the synergistic effect is discussed.

Stability of zeolite supported (promoted-) Pd catalysts in the reduction of NOX in gas engines using methane

Zeolite-based palladium catalysts show high activity in the reduction of NOx with methane. The addition of cobalt can further improve the SCR activity up to a level required for ‘end-of-pipe’ applications in gas engines. However, the hydro-thermal stability of the zeolite supported palladium catalysts is poor. CO-chemisorption, H2-TPR, DRIFT and activity measurements indicate a temperature-induced mechanism of ion migration and sintering to be the ruling mechanism of deactivation. In addition, the process of steam dealumination under reaction conditions likely eases the sintering. Time on stream behaviour of cobalt-palladium-zeolite equals that of palladium-zeolites. Deactivated catalysts are characterised by a palladium peak in the H2-TPR at lower temperature as compared to the fresh calcined samples. Cerium (and some other additives) stabilises the active Pd species in mordenite to a significant extent. The mechanism of stabilisation is possibly of both steric and electronic nature.