James A. Sullivan

Mechanistic considerations for the reduction of NOx over Pt/Al2O3 and Al2O3 catalysts under lean-burn conditions

The reduction of NOx under lean-burn conditions remains a challenging technical and scientific problem. In this paper, a number of catalytic systems are investigated by steady state testing and temperature-programmed desorption/reaction and the results discussed in terms of the similarities between the reactions in an attempt to rationalise apparently unrelated results. The reaction mechanisms are divided into two classes. (1) Reactions where NOx reduction occurs on the Pt surface, (e.g. C3H6–NO–O2 reaction over Pt/Al2O3) which are active at the lowest temperatures and are resistant to sulphur poisoning. (2) DeNOx reactions on Al2O3 with a weakly adsorbed reductant, (e.g. C3H8–NO–O2 reaction over Pt/Al2O3 and Al2O3 and the C3H8–NO2–O2 reaction over Al2O3) which are strongly poisoned by sulphur and appear to occur via the formation of a surface nitrate species on the Al2O3 which activates the reductant.

The nature of activity enhancement for propane oxidation over supported Pt catalysts exposed to sulphur dioxide

Abstract The nature of the active sites, the role of the support, and the mechanism by which hydrocarbons are activated over supported Pt catalysts have been investigated for the combustion of propane in the presence and absence of SO 2 . A strong enhancement in the activity for propane oxidation has been confirmed either when SO 2 is introduced with the propane or with a pre-sulphated alumina-supported catalyst. No equivalent effects were found with silica-supported catalysts. Fluorination of the alumina support also leads to an increase in activity. The addition of pulses of SO 2 into the propane-containing gas stream produces a very large, but short-lived, increase in activity in addition to a more gradual and progressive activity enhancement. Reasons for these different effects are discussed. Attempts to correlate the permanent enhancement in activity with the total acidity of the support were unsuccessful. It appears that the increase in activity is due to a more subtle effect and a model is presented in which the possible role of perimeter sites at the metal–support interface is emphasised.

Conditions in which Cu-ZSM-5 outperforms supported vanadia catalysts in SCR of NOxby NH3

Abstract Continuous flows of a standard reactant mixture, featuring 0.6% nitric oxide, 0.6% ammonia and 3.3% oxygen at moderate space velocities over 4 different catalysts, have been used to compare relative activities for selective catalytic reduction of NO x at 373–773 K. The catalysts tested were: Cu-ZSM-5 featuring > 100% ion-exchange; a conventionally prepared vanadia-titania-tungstate (VTT) material and two unconventional catalysts prepared by vanadia deposition onto ex-sol-gel WO 3 TiO 2 supports. At catalytic temperature 473 K, higher conversion to N 2 was achieved over Cu-ZSM-5 than over the other three materials. Tests without NO at 473 K showed insignificant contributions to N 2 formation from ammonia oxidation over any of the catalysts, whereas tests at 573, 623, 673 and 773 K revealed larger progressive increases in such contributions over Cu-ZSM-5 than over the other catalysts. Values for SCR activities corrected for such contributions demonstrated that activity of Cu-ZSM-5 for SCR conversion of the standard NO + NH 3 + O 2 reactant mixture to N 2 at 473 K was ca. twice as great as the other three catalysts at that temperature, but that increasing the reaction temperature to 573 K caused only a slight further increase. ‘Corrected’ SCR activities in the standard reactant mixtures were rather similar for all four materials at 573 K, but with Cu-ZSM-5 marginally out-performed by one of two unconventional catalysts featuring vanadia upon an ex-sol-gel WO 3 TiO 2 support having tungsten incorporated into the TiO 2 anatase structure. Both of these unconventional catalysts outperformed a conventional ‘VTT’ catalyst. Observations upon variations in conversion to N 2 with variation in the oxygen content of the reactant gas mixture from 1 to 6% established another unique feature of the Cu-ZSM-5 catalyst at 473 K, viz. the need for ca. 4.5% O 2 to raise conversion to the maximum attainable over that catalyst at this temperature. No deactivation was observed after short-term runs at temperatures up to 823 K. Introduction of water vapour into the standard reactant mixture slightly enhanced the activity of Cu-ZSM-5 at 473 K.

Selective catalytic reduction of NO with C2H4 over Cu/ZSM-5: Influences of oxygen partial pressure and incorporated rhodia

Abstract Catalytic activities of copper-exchanged ZSM-zeolites for the reduction of nitric oxide to nitrogen using ethylene as reducing agent in the presence or absence of oxygen are compared with those of Rh-ZSM-5 zeolites and binary Cu/Rh-ZSM-5 catalysts at 623–823 K. Copper ZSM-5 systems with high copper loadings (> 2.1) are shown to be much more active in the presence of oxygen, in marked contrast to Rh ZSM-5 catalysts which were more active in its absence. Binary ZSM-5 catalysts containing both copper and rhodium maintained intermediate activity in both conditions. A sample of Cu ZSM-5 containing only 1.8% copper also evidenced some activity in both conditions, which could be understood on the basis of catalytic action by copper ions within the zeolitic framework, allied to inherent Bronsted acidity of the latter. Comparative catalytic data are presented for the oxygen partial pressure dependence of the conversion to nitrogen over all catalysts at 673 K. These delineate the interplay between selective catalytic reduction (SCR) of NO by C2H4 and ethylene oxidation by dioxygen. Techniques utilised to characterise the catalysts include: powder XRD, TPR in H2, and TPD of O2.

The effect of SO2 on the activity of Pd-based catalysts in methane combustion

The effect of SO2 on Pd-based catalysts for the combustion of methane has been investigated. It is shown that while SO2 poisons Al2O3- and SiO2-supported catalysts, pre-treatment of Pd/ZrO2 by SO2 enhances the activity substantially.

Rare Earth (La, Nd, Pr) Doped Ceria Zirconia Solid Solutions for Soot Combustion

A series of rare earth (RE) (La, Nd, Pr) ceria zirconia materials were analysed for their soot combustion activity in air and in NO/O2. The materials were characterised using DRIFT spectroscopy. In general the presence of the RE dopant increases the activity of undoped CexZr1−xO2. The La and Pr doped catalysts showed increased low temperature activity in the presence of NO/O2 while the effect was less pronounced in case of Nd - doped samples. FTIR data has shown that the catalysts interact differently to NO/O2 mixtures in that they do not form significant quantities of adsorbed nitrite-type species. We postulate that this species is a precursor to NO2 formation which in turn increases soot combustion.

Preparation and Characterization of a Composite of Gold Nanoparticles and Single-Walled Carbon Nanotubes and Its Potential for Heterogeneous Catalysis

Abstract A single-walled carbon nanotube-supported gold nanoparticle composite was prepared and characterized by X-ray diffraction, scanning transmission electron microscopy/scanning electron microscopy/transmission electron microscopy, energy-dispersive X-ray analysis, atomic absorption spectroscopy, nitrogen adsorption, Raman spectroscopy, and ultraviolet-visible spectroscopy. The Au particles were found to be crystalline, with a well-defined and narrow particle-size distribution, centered around 7 nm. The activity and selectivity of the composite for solventless aerobic oxidation of a secondary alcohol were examined, and a conversion efficiency of 95% was obtained.

The Effect of Support and Cu Precursor on the Activity of Supported CuO Catalysts in the Selective Catalytic Reduction of NOx

A series of Cu catalysts were studied as a function of support (Al2O3, TiO2 and SiO2) and Cu precursor (ex-SO4 and ex-NO3) for activity in the SCR-NH3 reaction. The catalysts were characterized using NOx TPD and SEM/EDAX analysis and the effects of residual sulphur interpreted in terms of site-blocking and NH3 activation mechanisms.

Transient Techniques in the Study of Lean-Nox Reduction Over Supported Pt Catalysts

The use of concentration-transient methods in the elucidation of deNO x mechanisms over supported Pt catalysts is discussed with reference made to non steady state (TAP, reactant removal, NSSITKA (non steady state isotopic transient kinetic analysis)) and steady state (SSITKA (steady state isotopic transient kinetic analysis)) techniques. The reaction systems studied include the NO/C 3 H 6 /O 2 , NO/C 3 H 8 /O 2 , NO/H 2 /O 2 and NO/H 2 reactions over supported Pt catalysts (Al 2 O 3 and SiO 2 ) and Al 2 O 3 . The use of concentration-transient techniques can give information about the presence, concentration and reactivity of adsorbed surface intermediates. Extrapolations from concentration-transient techniques are then used to give information about various mechanistic steps on the catalysts involved in the conversion of reactants to products. This information can then be used to get a clearer understanding of how and why certain catalysts are active (inactive) for the reduction of NO x under lean burn conditions.

Tethering of Dinuclear Complexes to SBA-15 and Their Application in CO2 Hydrogenation

Two bimetallic cryptates (containing Cu and Co), which have previously been shown to react with and activate atmospheric CO2, were tethered to modified mesoporous SiO2 and their activities in promoting the CO2+H2 reaction has been analysed. The cryptates were tethered to C3H6Cl‐modified SBA‐15 through a condensation reaction between surface alkyl chlorides and 2° amines of the ligands, which released HCl and formed 3° amine linkages. The materials were characterised using BET, thermogravimetric analysis, FTIR and elemental analysis and their activities in promoting the CO2+H2 reaction was tested under batch reactor conditions. Co ions appear to selectively populate the medal sites of the tethered ligands whereas Cu ions appear to deposit on the surface as Cu(BF4)2 salts. The composite materials generate CO and CH4 from the CO2+H2 mixtures. Co‐containing catalysts are more effective than their Cu analogues in promoting the reaction.