Kinetic expressions for slow (CH4) and fast (C3H6) oxidizing hydrocarbons, a strongly adsorbed hydrocarbon (C2H2), and the steam reforming reaction are proposed. They are based on the historical rate laws proposed by Voltz et al. (1973, Ind. Engng Chem. Proc. Res. Dev. 12, 294-301) and Subramanian and Varma (1985, Ind. Engng Chern. Proc. Res. Dev. 24, 512-516). The kinetic parameters are determined from the readily available laboratory light-off curves using a standard adiabatic model for the monolith. Comparisons of simulated results with engine experiments are successful and they show that: (1) light-off performance is much affected by a strongly adsorbed hydrocarbon even at the ppm level; (2) conversion under stabilized conditions depends on the slow-oxidizing hydrocarbons; (3) steam reforming is important under rich conditions. The proposed chemical description should allow assessing the effect of fuel composition and combustion characteristics on the efficiency of the converter. (C) 1997 Elsevier Science Ltd.

The kinetics of the CO+NO reactions has been studied at 300 degrees C over a fresh and a deactivated bimetallic Pt-Rh/Al2O3 catalyst. Two kinetic models have been examined including competitive and non-competitive adsorptions of the reactants. The discrimination between these two assumptions has been achieved by using graphic and mathematical methods. From the comparison of kinetic and thermodynamic constants calculated from these methods with those previously obtained on Rh/Al2O3 and on Pt/Al2O3, we believe that the kinetic data obtained on the fresh Pt-Rh/Al2O3 catalyst can be modelled by non-competitive adsorptions of the reactants assuming a preferential adsorption of NO on Rh and CO on Pt. By contrast NO and CO competitive adsorptions can only occur on the deactivated Pt-Rh/Al2O3 catalyst, which to assume that the active surface is mostly composed of Rh.

The selective catalytic removal of NO in oxygen rich atmospheres has been investigated in the presence of sulfur dioxide on a series of Cu catalysts. The reactivities correlated with the reducibility of Cu species determined by temperature programmed reduction with hydrogen. Without sulfur dioxide in the feed, the activity is related to the reducibility of Cu species. The addition of SO2 to the solid shifts the TPR peaks to higher temperatures. The magnitude of this effect is lower for acid zeolites such as MFI and BEA. Sulfation results in a small inhibition of the reactivity for deNOx in the case of Cu/Al2O3, no or little change in the case of Cu/zeolites, and a promotion of activity in the case of Cu/TiO2 and Cu/ZrO2. The oxidation of decane on Cu/TiO2 and Cu/ZrO2 is inhibited by SO2 at low temperatures, but remains close to 100% in presence or absence of SO2 on Cu/TiO2 above 600K. In the case of Cu/ZrO2 the addition of SO2 increases the rate of oxidation above 640 K. The positive effect of SO2 on deNOx is attributed to the promotion of a bifunctional mechanism in presence of strong acid sites.

Catalysts with Pd deposited on various supports such as ZrO2 or mixed supports such as Al2O3-ZrO2-BaO have been used for the elimination of NOx. Their activity in the presence of complex mixtures like CO-NO-O-2-C3H6-CO2-H2O is better than that of Pd/Al2O3. This increase in activity is due not to a modification of the electronic properties of Pd but to a direct participation of the support in the process. From the analysis of the experimental results, a bifunctional mechanism is proposed and discussed.

Steady-state and transient activities of a 1.47 wt% Cu-MFI (Si/Al=27) catalyst in the selective catalytic reduction of NOxwith hydrocarbons (HC) were investigated in the 473–723 K temperature range with propene and propane as reducers. Under steady-state conditions (1000 ppm NO+1700 ppm HC+5% O2), propene gives the highest NO conversion at 673 K while propane is a reducer which is much better at 573–623 K. Transient activities were also investigated by comparing the NO conversion in the presence of O2after different pretreatments in HC or HC+NO. On the fresh, non-pretreated catalyst, a NO uptake can be observed without any formation of N2. Reacting the pretreated catalyst with NO+O2leads to transient formation of N2and CO2with the following efficiency of the hydrocarbons: propene>propane. The pretreatment in HC+NO is significantly better than in HC alone and leads to formation of nitrogen-containing species which are further decomposed into N2and CO2, but only in the presence of O2. These nitrogen-containing species cannot react with NO alone.

On different Pd based catalysts (Pd/Al2O3, Pd/ZrO2, Pd/Al2O3-BaO, Pd/Al2O3-La2O3), it has been shown that the introduction of hydrocarbons inhibits the catalytic reduction of NO by CO either in the presence or in the absence of O-2. This inhibition effect has been explained either by carbon poisoning of the active metal when the CO-NO reaction is performed without oxygen (or with moderate amounts of O-2) or by oxygen poisoning for higher oxygen concentrations.

A Cu/mordenite catalyst was prepared by ion exchange and characterized by N-2 sorption, XRD, TPR by H-2 and TPD by NO. The catalyst was evaluated in the selective catalytic reduction of NO by decane in O-2-rich atmosphere under temperature-programmed reaction from 293 to 773 K. Two NO reduction profiles at 590 and 650 K occur under these conditions. By testing different operating protocols and different alkanes as reductants, it was concluded that the low temperature peak (590 K) is due to alkane condensed in the porosity of mordenite.

A series of Cu/mordenite catalysts were prepared by competitive ion exchange, characterized by temperature programmed reduction with hydrogen, temperature programmed desorption of NO, adsorption of pyridine and tested for the selective catalytic reduction of NO by decane in presence of a large excess of oxygen. At low Cu content (exchange% ⩽ 53%), the TPR profiles show the presence of two peaks of comparable areas, the accessibility to Cu sites is high (NO/Cu> 0.8) and the micropore volume remains unchanged, thus suggesting that Cu2+ ions remain isolated and are reduced in two successive steps: Cu2+ → Cu+ at about 510 K and Cu+ → Cu0 at 600–700 K. At higher Cu content (exchange% > 60%), the TPR profiles are modified with a large increase of the low temperature peak; the accessibility to Cu sites decreases (NO/Cu= 0.45–0.49), and the micropore volume also decreases, thus suggesting the formation of Cu oxide clusters in the lattice of the mordenite. The acidity of the mordenite also decreases upon exchange. The activity for the NO reduction is proportional to the number of Cu2+ ions up to 53% exchange, then decreases. Isolated Cu2+ ions appear then more active than CuO clusters or protons for the selective reduction of NO by decane.

Three-way automotive Pt-Rh catalysts were prepared either by coimpregnation of the two noble metals (C.I. catalysts) or by an original method of successive impregnations with a reduction step after platinum impregnation (S.I. catalysts). It was shown that addition of rhodium can lead to opposite effects on the activity of platinum deposited on alumina-ceria for the reaction of propane oxidation, according to the preparation procedure : the coimpregnation of the two metals induces an inhibition of platinum activity when the addition of rhodium by successive impregnations leads to the reverse effect. The differences between the two preparations are greater on oxidized samples after aging at high temperature (900 degrees C). These results are explained by the formation of alloy particles on coimpregnated catalysts while rhodium added by successive impregnations would be selectively deposited on surface cerium, avoiding the formation of Pt-Rh alloy. Energy dispersive spectroscopy fitted to a STEM unit allowed to bear out such hypothesis.

The effects of sulfur introduced in the feedstream (SO2) or stored on the catalyst were investigated in the course of the oxidation of a propane-propene mixture under lean conditions on thermally aged platinum and platinum-rhodium catalysts. The activity of monometallic platinum catalysts for propene oxidation is strongly inhibited by sulfur while this inhibiting effect disappears when 0.1 wt% rhodium is present in the bimetallic catalyst, For propane oxidation, sulfation of coimpregnated Pt-Rh catalysts induces a promoting effect on oxidized samples while the reverse effect is observed on reduced samples. As for the catalyst prepared by successive impregnations (SI), it is insensitive to sulfur dioxide in the feed gas and to sulfur stored on the catalyst. Thus, the SI Pt-Rh/Al2O3-CeO2 catalyst still remains the most active in the presence of sulfur for propane oxidation.

The properties of three-way automotive PtRh/Al2O3CeO2 catalysts prepared either by coimpregnation of the two noble metals (CI catalysts) or by a newly devised technique of successive impregnations with an intermediate reducing treatment (SI catalysts) were investigated for the oxidation of a propane—propene mixture under lean conditions and for the CO + NO reaction. After high-temperature aging, bimetallic CI catalysts are far less active than SI catalysts for propane oxidation. On the other hand, they show comparable activities for propene oxidation and reaction CO + NO. The characterization of bimetallic catalysts by TPR and FT-IR experiments suggests that the coimpregnation leads to an intimate interaction between platinum and rhodium while rhodium added by successive impregnations with a reduction step after platinum impregnation would be selectively deposited onto cerium oxide surface in the vicinity of platinum particles, avoiding the formation of Pt/Rh alloy.

The survival of the diesel engine in the future will depend on the development of a really efficient, reliable process to be easily implemented for the pollution control of exhaust gases. The particulate filter can be proved to be efficient for a short while, but its periodic regeneration could present a solution to the tricky problems of control and implementation costs. Another solution, contemplated in the past and then discarded, is being considered again: this is the catalytic oxidation of gaseous unburnt residues. Excess oxygen contained in diesel exhaust gases does not allow the use of three-way catalysts to reduce emissions of nitrogen oxides as for the spark-ignition engines, but it furthers the elimination of all the combustible residues contained in gases by oxidation. The efficiency of such a technique is dependent upon the sulphur content of the diesel fuel Specific catalytic formulations are required in order to avoid the generation of sulphur trioxide SO3, and then sulphuric acid contained in the form of sulphate particulates in exhaust gases or sulphate deposits on the catalyst. All the problems encountered, as well as the results of the tests performed in different types of new catalysts under development for this application, are presented The influence of a number of parameters relating to catalyst design (support, wash-coat, metal, etc.) and their implementation conditions are studied.

A Pt-Rh/Al 2 O 3 catalyst on cordierite monolith has been submitted to severe condition engine bench aging over extended periods. Sintering of alumina and precious metals was approximately proportional to the logarithm of the aging time. For aged catalysts, the ratio Pt/Rh in the metallic particles increased with increasing size. Phosphorus, zinc and lead coming from lubricating oil and gasoline accumulated on the catalyst at a steady rate. Sulfur concentration remained very low, probably due to high temperature inhibition of sulfate deposition. Testing of the catalyst under a constant composition stoichiometric gas mixture, shows that the light-off temperature increased nearly proportionally to the logarithm of the aging time. Sintering of the precious metals explain the activity decrease for aging times up to 6 hours. For longer aging times both sintering and poisoning are responsible for the activity decrease.

Three series of Pt Rh three-way catalysts were aged in the exhaust of an engine running on a dynamometer stand for various lengths of time. Their activity was thell measured as a function of temperature at stoichiometry and as a function of the air-fuel equivalence ratio at constant temperature (450°C). A peak of N 2 O formation was observed close to the catalyst light-off temperature. Since the light-off temperature increases as a function of aging time, a shift in the N 2 O peak formation was observed towards higher temperatures. The magnitude of the N 2 O peak decreased simultaneously. At 450°C a minimum of N 2 O formation was seen close to stoichiometry with two maxima below and above this point. N 2 O formation was the highest on the rich side and increased progressively as a function of catalyst aging. Another series of eight catalysts was evaluated in parallel on a vehicle on a chassis dynamometer after engine bench aging. They were tested using the ECE driving cycles: ECE 15 urban with cold or hot starting and extraurban with hot starting. The amount of N 2 O emitted did not vary very significantly with the catalyst type but was seen to increase 2 to 4.5 times after aging, depending on the driving cycle used. This has to be related to the shift in the light-off temperature to higher values during aging, which makes the catalyst work more at temperatures where N 2 O is formed.

PURPOSE: To remove CO and NOx in the exhaust gases of internal combustion engines by depositing Pt, Rh or the like on a porous carrier which is mainly composed of an inorg. refractory oxide and contains specific amts. of uranium oxide and oxides of Li, Na or the like, thereby forming a catalyst. CONSTITUTION: An aq. suspension of at least one inorg. refractory oxide, at least one salt or oxide of uranium and at least one salt or oxide of metal P is prepd. The support is coated with this aq. suspension to form a porous layer and thereafter the support is subjected to drying and a heat treatment at <=1000 deg.C, by which the porous carrier is obtd. This porous carrier is impregnated with a soln. of at least one precursor material of at least one metal A and is then subjected to drying and a heat treatment at <=1000 deg.C by which the exhaust gas treating catalyst deposited with the metal A is formed. The more specific examples of the metal P described above include Li, Na, K or the like and the more specific examples of the metal A described above include Pt, Rh, Pa, Ru, Ir or the like.

A three-way autocatalyst has been submitted to aging in the exhaust of an engine on a dynamometer stand with simulation of high speed driving, frequent acceleration and deceleration and increased oil consumption. The objective of these experiments was to correlate at different stages of evolution, the catalytic properties with the physico-chemical characteristics of the catalyst.

This paper reviews research that has been going on for several years in an attempt to find the causes of the deactivation of automotive post-combustion catalysts containing precious metals when used with leaded gasoline. Data concerning the amount, nature and location of the lead deposits that form in aged catalysts as well as concerning the interactions that occur with the support and metal crystallites are given to begin with. The ways in which lead deposits (metal or oxide) grow and become desorbed have been examined with unsupported platinum. Up to about 400° C it is shown that relatively thick layers of lead may be formed. At temperatures higher than 500° C, however, lead tends to be desorbed, and in the steady state all that remains on the surface of the platinum is an amount of lead smaller than a monoatomic layer. Lead reduces the adsorption of carbon monoxide on platinum but not that of oxygen. When the amount of lead present on the surface is less than a monoatomic layer, this causes an increase in the CO + O2 reaction rate compared with what occurs on clean platinum. On clean platinum the high adsorption of CO effectively tends to inhibit its own oxidation. With more than three monoatomic layers of lead, the activity of the platinum becomes very slight no matter what the temperature may be. (change-para-here) With industrial catalysts made of precious metals supported on alumina, it has effectively been found that their impregnation with PbO as well as their use with gasoline containing pure tetraethyl lead do not cause deactivation as long as the amount of lead present is not so high so as to cause the clogging of the porosity. (change-para-here) It is also shown that the deactivation of catalysts that is observed when commercial leaded gasoline is used is mainly the result of the effect of additives containing chlorine and bromine which are habitually added to gasoline at the same time as tetraethyl lead to reduce the fouling of combustion chambers in engines.