Platinum monolithic oxidation catalysts

T he successful use of platinum monolithic oxidation catalysts to control automobile emissions over many thousands of miles requires an intimate understanding of the many factors which contribute to catalyst degradation. Contamination of the active catalyst by lead and phosphorus compounds present in fuel and lubricating oil is a major factor in catalyst deterioration. 

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

A development in catalyst support systems in which half of the 5-10% rhodium-platinum alloy gauzes were replaced by nonnoble metal supports or by ordinary metal catalysts gave cost economies without adversely affecting operating efficiency [47]. More recently, ammonia oxidation in a two-bed system (Pt gauzes followed by monolithic oxide layers) gave nearly the same ammonia conversion while reducing platinum losses by 50% [48]. 

Figure 102. Emission of particulates (part) and gaseous hydrocarbons (HC) from a heavy duty diesel engine in the US transient engine test cycle, as a function of the catalyst volume (monolith catalyst with 46 cells cm" diesel oxidation catalyst formulation with platinum at a loading of 1.76 g 1 , fresh).

Figure 103. The amount of soluble organic fraction (SOF) adsorbed on an aged diesel oxidation catalyst, as a function of the washcoat formulation (monolith catalyst with 62cellscm", dedicated diesel washcoat formulations with platinum at a loading of l.76gl diesel engine bench aging for 50 h diesel fuel containing 0.15 wt. % sulfur). Reprinted with permission from ref. (68], C 1991 Society of Automotive Engineers, Inc.

Figure 109. Conversion of carbon monoxide and gaseous hydrocarbons reached over a diesel oxidation catalyst at various settings of the exhaust gas temperature, for a model gas composition with and without SO2 (monolith catalyst with 62cells cm dedicated diesel washcoat formulations with platinum loading of 1.76gl" in the fresh state model gas light-off test at a space velocity of 50000Nir h model gas simulates the exhaust gas composition of an IDI passenger car diesel engine at medium load and speed).

Figure 111. Emission of aldehydes, acrolein and various polynuclear aromatic hydrocarbons of two passenger cars equipped with an IDI/NA and with a DI/NA diesel engine, once without and once with a diesel oxidation catalyst, in the US-FTP 75 vehicle test cycle (monolith catalyst with 62 cells cm dedicated diesel washcoat formulation with a platinum loading of 1.76 g 1 in the fresh state vehicle dynamometer tests according to the US-FTP 75 vehicle test procedure, with passenger cars equipped with a DI/NA and with an IDI/NA diesel engine of displacement 2.0 1). Reprinted with permission from ref [70], 1990 Society of Automotive Engineers, Inc.

The introduction of automobile exhaust catalysts in the United States and elsewhere has produced a major market for platinum-type oxidation and reduction systems. An innovative consequence of this industry has been the development of ceramic honeycombed monoliths as catalyst supports. These structures contain long, parallel channels of less than 0.1 mm in diameter, with about SO channels per square centimeter. The monolith is composed of cordierite (2MgO - 2AI2O) SSiOj) and is manufactured by extrusion. A wash coat of stabilized alumina is administered prior to deposition of the active metal, either by adsorption or impregnation methods. 

S.E. Voltz and D. Liederman, "Thermal Deactivation of a Platinum Monolithic Carbon Monoxide/Hydrocarbon Oxidation Catalyst", Ind. Eng. Chem. Prod. Res. Dev.. 1974,1314). 243-250. 

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. 

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. 

Homg described the start-up behaviour of their monolithic autothermal methanol reformer [479]. The ceramic monolith was coated with a mixed platinum and copper/ zinc oxide catalyst. The monolith had 117-mm diameter and 50-mm length, while the whole reactor was more than 510-mm long. Glow plugs were used for the start-up. 

Salomons S, Votsmeier M, Hayes R, Drochner A, Vogel H, GieshofJ CO andH2 oxidation on a platinum monolith diesel oxidation catalyst, Catal Today 117 491—497, 2006. 

Many catalyst chemical formulations and geometric shapes are used to promote oxidation-reduction reactions. Chemical types used for VOC oxidation include platinum, platinum alloys, copper chromate, copper oxide, cobalt oxide, chromium oxide, manganese oxide, and nickel. The catalysts are often categorized as platinum metal group (PMG) and base metal (or metal oxide) types. The active catalyst is often supported on an inert carrier such as gamma alumina. Catalyst forms include metal ribbons, mesh and gauze honeycomb monoliths and small beads or particles that can be used in a fixed, fluidized, or moving bed. 

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. 

A sophisticated quantitative analysis of experimental data was performed by Voltz et al. (96). Their experiment was performed over commercially available platinum catalysts on pellets and monoliths, with temperatures and gaseous compositions simulating exhaust gases. They found that carbon monoxide, propylene, and nitric oxide all exhibit strong poisoning effects on all kinetic rates. Their data can be fitted by equations of the form ... 

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. 

The use of supported platinum-based catalysts in hydrogen cyanide synthesis has been described, and some are in use in certain plants. For instance, Merrill and Perry recommended natural beryl coated with platinum or a platinum alloy [12], and a range of alternative supports were considered by Schmidt and his co-workers [13], including coated foamed ceramic and monolithic substrates. They are claimed to have some advantages in ammonia oxidation [14,15]. 

Not all catalysts need the extended smface provided by a porous structure, however. Some are sufficiently active so that the effort required to create a porous catalyst would be wasted. For such situations one type of catalyst is the monolithic catalyst. Monolithic catalysts are normally encountered in processes where pressure drop and heat removal are major considerations. Typical examples include the platinum gauze reactor used in the ammonia oxidation portion of nitric acid manufacture and catalytic converters used to oxidize pollutants in automobile exhaust. They can be porous (honeycomb) or non-porous (wire gauze). A photograph of a automotive catalytic converter is shown in Figure CD 11-2. Platinum is a primary catalytic material in the monolith. 

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.

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