Catalysts, palladium monolithic oxidation

Other molecules present in the gas mixtures like alcohols or ketones may have a moderate effect on aromatic oxidation but the reverse (inhibition of alcohol oxidation by aromatics) is most often observed. Different supports of Pt were used for toluene oxidation Al203, Al203/Al, Zn0/Al203, Ti02, mesoporous fibrous silica or monoliths. Zeolites, generally promoted by platinum, were shown to give excellent catalysts for aromatic oxidation. Basic zeolites showed excellent performances in oxidation of m-xylene even in the absence of platinum. Palladium catalysts, either supported on alumina or ceria-alumina, were also investigated for oxidation of benzene and several alkylbenzenes. ... 

Starting with a ceramic and depositing an aluminum oxide coating. The aluminum oxide makes the ceramic, which is fairly smooth, have a number of bumps. On those bumps a noble metal catalyst, such as platinum, palladium, or rubidium, is deposited. The active site, wherever the noble metal is deposited, is where the conversion will actually take place. An alternate to the ceramic substrate is a metallic substrate. In this process, the aluminum oxide is deposited on the metallic substrate to give the wavy contour. The precious metal is then deposited onto the aluminum oxide. Both forms of catalyst are called monoliths. 

Since 1981, three-way catalytic systems have been standard in new cars sold in North America.6,280 These systems consist of platinum, palladium, and rhodium catalysts dispersed on an activated alumina layer ( wash-coat ) on a ceramic honeycomb monolith the Pt and Pd serve primarily to catalyze oxidation of the CO and hydrocarbons, and the Rh to catalyze reduction of the NO. These converters operate with a near-stoichiometric air-fuel mix at 400-600 °C higher temperatures may cause the Rh to react with the washcoat. In some designs, the catalyst bed is electrically heated at start-up to avoid the problem of temporarily excessive CO emissions from a cold catalyst. Zeolite-type catalysts containing bound metal atoms or ions (e.g., Cu/ZSM-5) have been proposed as alternatives to systems based on precious metals. 

Fig. 5.7. The three-way catalyst consists of platinum and rhodium (or palladium) metal particles on a porous oxidic washcoat, applied on a ceramic monolith.

Figure 106. Conversion of carbon monoxide, gaseous hydrocarbons and sulfur dioxide reached over diesel oxidation catalysts as a function of the preeious metal formulation at equimolar loading (monolith catalyst with 62 cells cm - dedicated diesel washeoat formulations with platinum, palladium and rhodium at an equimolar loading of 8 mmol I, fresh model gas test at a gas temperature at catalyst inlet of 723 K a space velocity 50000Nil h model gas simulates the exhaust gas composition of an IDl passenger car diesel engine at medium load and speed and contains 100 vol. ppm SO2). Reprinted with permission from ref. [68], C 1991 Society of Automotive Engineers, Inc.

Fresh and thermally aged catalysts containing mixtures of platinum and palladium were laboratory tested for the oxidation of carbon monoxide, propane, and propylene. For both monolithic and particulate catalysts, resistance to thermal deactivation was optimum when palladium content was 80%. Full-scale vehicle tests confirmed these findings. Catalysts of this composition were developed which, on the basis of durability tests at Universal Oil Products and General Motors, appeared capable of meeting the 1977 Federal Emissions Standards with as little as 0.56 g noble metal per vehicle. The catalyst support was thermally-stabilized, low density particulate. 

The work presented in this paper is the first part of a project aiming at the development of tailor-made oxidation catalysts for diesel engines fuelled by alcohol fuels, ethanol or methanol. The investigation is focused on the influence of support material on the low temperature oxidation of ethanol and acetaldehyde. The study presents results from an experimental investigation with precious metal catalysts applied on monolithic cordierite substrates. Platinum or palladium were applied onto a support consisting of either aluminum oxide, cerium dioxide, silicon dioxide or titanium dioxide. 

The catalysts were prepared by using the following support materials aluminum oxide (Condea PX 140), cerium dioxide (Molycorp HSA 5315), silicon dioxide (EKA Nobel Bindzil 50/80), and titanium dioxide (Thann et Mulhouse OT 51). According to the manufacturers specifications, the BET surface areas were 140, 145, 80 and 80-100 m /g, respectively. TTie monolithic catalyst samples were prepared according to standard techniques at Svenska Emissionsteknik AB. The preparation of the platimun and palladium catalysts was made on an equimolar basis and they contained 9.5 mmol/dm3 monolith, which corresponds to 1.8 g Pt/dm3 and 1.0 g Pd/dm3. 

Since 1975 catalysts have been fitted to vehicles in the USA to control emissions, initially of HC and CO (oxidation catalysts), and latterly also of NOx (three way catalysts). The mode of operation of these catalyst systems in the USA and Japan is now well characterised (1). The catalysts typically comprise the precious metals platinum, palladium and rhodium, either singly or in combination, together with base metal promoters or stabilisers, supported on alumina pellets or alumina coated ceramic monoliths. Catalysts for the US market are designed to withstand 50,000 miles of road use and must be operated in conjunction with lead free fuel since they are poisoned by lead. 

Jung et al. investigated the partial oxidation of methane on noble metal coated metallic monoliths heated by electricity [484]. Over a palladium catalyst supported by alumina, auto-ignition of the reaction took place at 270 °C and an O/C ratio of 1.1. However a high O/C ratio of 1.4 was required to achieve 98.9% methane conversion. 

In the last decade, different authors have studied the use of different wash-coats, which act as a host for the active palladium component, to prepare monolithic catalysts for toluene total oxidation. 263, CejcYi cOz, Ce cZri cOz, Ceo.8Zro.i5Lao.o50<5, Ceo.gZro.zOz and Ceo.4Zro.4Yo.iMno.iO(5 have all been... 

Platinum and palladium have high activities for total oxidation. This property is exploited in automotive exhaust catalysis. Automobile exhaust contains toxic gases such as CO, NO, and hydrocarbons which contribute to formation of photochemical smog and acid rain. Since 1978, catalysts based on platinum, rhodium, and sometimes palladium, supported on a monolithic carrier, are applied to convert exhaust gases to less harmful products. The so-called threeway catalyst enables the following three overall reactions... 

In a more interesting process, all of the fuel/air mixture is added to a combustor with three sections. The first section contains an active palladium oxide catalyst that can operate up to about 800°C before being reduced to palladium metal which is less active. The palladium oxide catalyst is regenerated by reoxidation of the metal as temperature falls. A more stable catalyst in the second section continues the catalytic combustion. In the third section, combnstion is completed by thermal reaction and the gas temperature increases to 1300°-1400 C. Overall, less than 1 ppm NOX is formed. Palladium oxide is supported on a monolith coated with temperature resistant barium hexaaluminate. ... 

Oxidation catalyst platinum/palladium ratio of 5 2 with 1.5 g of metals per liter of monolith volume. 

---