Haren S. Gandhi

The oxidation of CO by O2 and by NO on supported chromium oxide and other metal oxide catalysts

Abstract The oxidation of CO by NO and by O 2 has been studied on a number of supported transition metal oxide catalysts. On supported chromia the oxidation of CO by NO is faster than by O 2 ; however, with mixtures of NO and O 2 the reaction is selective towards O 2 . The application of continuous mass-spectrometer monitoring to the study of surface state changes in chromia catalysts is outlined. The method was extended to determine the oxidation state of the surface in situ during reaction. In the oxidation-reduction cycle involving alternate passage of HeCO and HeO 2 mixtures at 500 °, the majority of the surface atoms undergo a change of oxidation state from Cr 6+ to Cr 2+ . The extent of this change diminishes with decreasing temperature. Mixtures of He and NO oxidize the surface of supported chromia to a lesser extent than HeO 2 . During the CONO reaction the average surface oxidation state is lower than in the presence of oxygen. A tentative explanation is offered for the selectivity for oxygen over nitric oxide in the oxidation-reduction reactions on commonly employed catalysts.

Effects of sulphur on noble metal automotive catalysts

Abstract The small amount of sulfur, ca. 300 ppm, in gasoline which is equivalent to ca. 20 ppm SO2 in exhaust, has multiple effects on the operation of automotive catalysts. These effects are not widely appreciated. Firstly, there are large differences in the capacity of different noble metals to chemisorb sulfur dioxide and secondly, the dispersion of the noble metal influences the activity in the oxidation of sulfur dioxide to sulfur trioxide. The ability of the catalyst to oxidize SO2 determines the resistance to poisoning by residual amounts of lead in the fuel and whether the surface of the catalyst support will store sulfur, as sulfate, under oxidizing conditions. The surface sulfate groups on the support enhance the oxidation of saturated hydrocarbons, which is very desirable for the attainment of the stringent regulatory limits on hydrocarbon emissions. On the other hand, the stored sulfur can be emitted as a transient spike of hydrogen sulfide if the engine operating conditions shift momentarily to the rich side of the stoichiometric air/fuel ratio, which is an undesirable phenomenon. The presence of sulfur species in the exhaust influences the partitioning of the products of nitric oxide reduction between dinitrogen and ammonia strongly suppressing the latter. The optimization of the composition of the three-way catalyst has to take into account the effect of sulfur compounds present in the exhaust.

Effects of oil phosphorus on deactivation of monolithic three-way catalysts

Abstract The deactivation of platinum-rhodium (Pt-Rh) monolithic automotive three-way catalysts (TWCs) by engine oil-derived contaminants of phosphorus (P) and zinc (Zn) has been investigated. In laboratory pulsator studies the combustion of isooctane containing a ten-fold excess of Zn dialkyldithiophosphate (ZDP) oil additive decreased 3-way conversions substantially when compared to normal oil consumption levels. The combustion of isooctane containing Zn-free cresyl diphenyl phosphate (CDP) resulted in greater P-retention on the catalyst when compared to that from ZDP-containing fuel, but CDP was significantly less detrimental to TWC activity compared to that of ZDP. Compared to lead retention, retention of P alone was a much less severe poison for TWCs on an atomic basis. Electron microprobe analysis of TWCs retaining >2 wt% P revealed a ring of P concentrated at the surface of the washcoat with a sharp gradient inside the washcoat, while Zn was present in considerably smaller amounts and more uniformly distributed throughout the washcoat layer. Pulsator combustion of clear isooctane containing ZDP produced Zn pyrophosphate as the major product from combustion. At elevated exhaust temperatures (800°), chemically inert aluminum phosphate was the major product on the catalyst. Physical blocking of active sites at high P-retention levels is postulated as a prevailing mechanism for catalyst deactivation from excessive oil-consumption rates.

The adsorption of nitric oxide and carbon monoxide on nickel oxide

Abstract Adsorption isotherms and rates for NO and CO chemisorption were measured on supported and unsupported nickel oxide samples in the 0–140 °C temperature range. The isotherms are of the Freundlich type in the pressure range from 1 to 125 Torr. Monolayer coverage is attained at 110 Torr for the NO adsorption and at 530 Torr for the CO adsorption. The uptake at monolayer coverage in CO adsorption is ~30% smaller than the corresponding NO amount, indicating that about 1 3 of the adsorbed CO molecules are present in the bridged form. Similarly to the previously studied oxide adsorbents, there is a 1:1 correspondence between adsorbed NO molecules and Ni atoms on the surface. The chemisorption rate of both adsorbates is described by the Elovich plots. These are more complex for CO than for NO. In the investigated temperature range, the rate of chemisorption and the heat of chemisorption are higher for NO than for CO. The rates of NO chemisorption for the transition metal oxides studied so far are in the order: iron oxide > chromium oxide > nickel oxide.

The adsorption of nitric oxide on copper oxides

Abstract Adsorption isotherms and rates were measured for NO chemisorption on supported and unsupported samples of copper oxide in the 26–140 °C temperature range. The adsorption behavior on cupric oxide is well described by Freundlich isotherms. The adsorption rates on the cupric oxide are represented by two intersecting segments in the coordinates of the Elovich equation. On cuprous oxide at room temperature the oxidation of the surface is super-imposed onto the adsorption phenomenon. Nevertheless, it could be deduced that the surface cuprous ions chemisorb nitric oxide molecules very slowly, if at all. This experimental fact is explained by the absence in Cu + ( d 10 ) of an unpaired d -orbital required for the accomodation, upon chemisorption, of the antibonding electron of the NO molecule.

Catalyst deactivation due to glaze formation from oil-derived phosphorus and zinc

The deactivation of automotive catalysts by engine oil-derived components of phosphorus and zinc can occur by the formation of an amorphous zinc pyrophosphate (Zn/sub 2/P/sub 2/O/sup 7/) that is impervious to gas diffusion. The catalyst poison, derived from antiwear oil additive zinc dialkyl dithiophosphate (ZDP) in low-temperature exhaust environments, appears as glassy, amorphous deposits on catalysts as shown by scanning electron microscopy (SEM). Laboratory studies were performed to understand the effects of exhaust stoichiometry, temperature, rate of oil burn, and chemical form of P and Zn compounds on glaze formation. The formation of the amorphous deposits using a laboratory pulsator apparatus showed that non-combusted ZDP causes the glaze formation. Electron microprobe studies indicated the association of P with Zn on precious metal films exposed to ZDP combustion products. Secondary ion mass spectrometry (SIMS) confirmed a similar P to Zn correspondence on the vehicle-aged catalysts. Once formed, the amorphous zinc pyrophosphate glaze could only be removed under high-temperature, reducing conditions which sintered the catalyst with no significant improvement in activity. Guidelines are presented for exhaust temperatures necessary for catalyst operation and prevention of catalyst deactivation by the formation of Zn pyrophosphate glaze.

The Role of Research in the Development of New Generation Automotive Catalysts

The development of new generation three-way catalysts (TWCs) is based on a broad, fundamental understanding acquired over a number of years. The efforts at Ford Motor Company have been mainly concerned with supported base metal and noble metal catalysts operating under oxidizing conditions at high-temperatures where the extent of the interactions or their absence does have a profound effect with an immediate bearing on the practical use. Use of ZrO 2 , ∞ Al 2 O 3 etc., as a support for Rh offers an opportunity to formulate a durable catalyst in which the thermal stability of Rh can be significantly enhanced and thereby it can remain active at temperatures >600°C in an oxidizing environment. Other work was aimed at the understanding of whether lead originating from the combustion of Pb-containing fuel associates preferentially with the noble metal sites supported on the much larger areas of inert-support materials. Direct association of lead compounds with noble metal sputtered onto ZrO 2 , TiO 2 and Al 2 O 3 wafer supports was noted for Pt, Pd and Rh. However, there are major differences in the interaction of Pb compounds with different noble metal surfaces which explain the long-known fact that Pd and Rh are more sensitive to Pb poisoning than Pt. Model reactions were employed as chemical probes to check whether the desired surface modifications have been achieved and also for determining modes of deactivation in used catalysts. For an optimum catalyst each precious metal has a specific function to perform and must be carried on a specific support material to maximize activity and durability.

The heterogeneous decomposition of nitric oxide on supported catalysts

Abstract Heterogeneous decomposition rates of nitric oxide in the temperature range from 300 to 800° are determined for a series of supported catalysts containing platinum, or the oxides of copper, chromium, and cobalt. The experiments have been carried out in a conventional integral flow reactor, and complete analyses of the gas compositions obtained by a mass spectrometer. Kinetic parameters such as activation energy and reaction order are given. A comparison is made with published data for similar catalysts, and reasons for disagreement are considered. It is concluded in view of the presently available catalysts that the practical application of catalyzed decomposition is not promising for the removal of NO from automobile exhaust.

Characterization of molybdenum-platinum catalysts supported on γ-alumina by X-ray photoelectron spectroscopy

Abstract X-Ray photoelectron spectroscopy (XPS) has been used to investigate the reduction of MoO 3 and MoO 3 + PtO 2 supported on γ-Al 2 O 3 . Hydrogen reduction of MoO 3 γ -Al 2 O 3 at 500 °C produced a mixture of Mo(VI), Mo(V), and Mo(IV). Hydrogen reduction of dispersed MoO 3 + PtO 2 on γ-Al 2 O 3 showed a similar partial reduction of the Mo(VI) to Mo(V) and Mo(IV) but at significantly lower temperatures than was the case with the supported MoO 3 alone. The extent of the reduction of Mo(VI) increased with the amount of PtO 2 in the samples.

The influence of sulfur dioxide on propane oxidation activity over supported platinum and palladium

Earlier studies have shown that sulfur dioxide and metal-support interaction can strongly influence propane oxidation over platinum. In particular, oxidation activity is enhanced when platinum is supported on sulfated γ-alumina or zirconia compared to γ-alumina. Therefore, it is of interest to compare the performance of palladium under the same experimental conditions. Four model catalysts were examined: Pt/γ-alumina, Pt/zirconia, Pd/γ-alumina and Pd/zirconia. The metal loading was kept at or below 0.05 wt% to emphasize changes in activity attributable to metal-support interaction. Reaction rates were measured with and without sulfur dioxide. Surface sulfation was analyzed by measuring acid strength and evaluating spectra obtained by Fourier-transform infrared spectroscopy. In contrast to platinum, sulfation does not promote propane oxidation on Pd/γ-alumina, and Pd/zirconia is less active than Pd/γ-alumina.