Reduction of Nitric Oxide by Monolithic-Supported Palladium-Nickel and Palladium-Ruthenium Alloys

Abstract

added to the feed stream. This experiment simulates the conditions as they would exist halfway through the reactor and bears directly on the question of ammonia intermediacy since it determines the efficiency of the catalyst to remove ammonia under reaction conditions (Bemstein, et al., 1973).The results for the ammonia addition experiments are dramatically different for the two states of the catalyst (Figure 7). Thus, most of the ammonia passes through the reactor unchanged when the catalyst is in the reduced state. In contrast for the oxidized catalyst, the ammonia did not pass through the reactor but was efficiently removed at temperatures above 350°C. The presence of only 350 ppm of NH3 in the product a t -250°C is due to our inability to analyze for NH3 in the presence of large amounts of NO. An NH3-NO reaction over the Pt catalyst used t u oxidize NH3 back to NO for the NH3 analysis results in a decrease in the amount of NH3 determined. This problem does not exist at higher temperatures where all the N O is reacted. These additional ammonia experiments strongly support the intermediacy of ammonia for oxygen-treated Ru-Al203. The fact that the added ammonia does not affect the temperature of NO removal ( Figure 7) argues against a change in the mechanism for NO removal with the added "3.It has been suggested though that an NO-NH3 mechanism may operate at low temperature to some extent all the time (Otto and Shelef, 1973).In both states ruthenium is very selective in the conversion of nitric oxide to molecular nitrogen. The unique selectivity of ruthenium catalysts in NO reduction is undoubtedly related to the unique chemistry of ruthenium with nitrogen ligands (Cotton and Wilkinson, 1972). Thus, the strong NO-ruthenium bonding as well as the remarkable interactions of Nz with ruthenium are consistent with the Nz selectivity of ruthenium catalysts in NO reduction.Ammonia intermediacy does not appear to be involved in NO reduction over reduced Ru-Alz03 (Taylor and Klimisch, 1973) and is only a minor pathway to nitrogen over the oxygen treated Ru-Al203 catalyst. Acknowledgment of Mr. R. M. Sinkevitch in all aspects of this work. Literature CitedThe authors acknowledge the indispensable assistanceThe effect of reaction parameters such as temperature, space velocity, and pollutant concentration on the performance of monolithic-supported Pd-Ni and Pd-Ru alloys in reduction of NO has been studied in a laboratory reactor. Freshly prepared Pd-Ni and Pd-Ru catalysts convert 100% NO (1000 ppm) with less than 5% ammonia formation in 0.4% 0 2 (1% CO and 250 ppm of C3H6) at 600 and 480°C, respectively. Conversions of NO, CO, and C3H6 decrease slightly with increasing space velocity. The effect of dynamic exposure to 100 ppm of SO2 is to lower slightly but reversibly the NO and C3H6 conversions while significantly and irreversibly lowering conversion of CO. After 100 hr of exposure in a reducing en ine exhaust the conversions of NO, CO, and C3H6 over monolithic-supported Pd-Ru/AlzOp and Pd-NiaNiAlz04 are lowered 5-20% c...

Catalyst Deactivation and Regeneration

“…The second effect may explain the typically lower observed values of S/Ni s for single crystal Ni, which are measured at extremely low pressures (high vacuum) relative to the higher values of S/Ni s for polycrystalline and supported Ni, typically measured at orders of magnitude higher pressure; in the case of the single crystal work the surface is not in equilibrium with gas phase H 2 S/H 2 . The third effect, reconstruction of nickel surfaces by adsorbed sulfur, has been reported by a number of workers (27); for example, McCarroll and co-workers (33,34) found that sulfur adsorbed at near saturation coverage on a Ni (111) face was initially in a hexagonal pattern but upon heating above 700 K reoriented to a distorted C(2 Â 2) structure on a Ni(100) layer. In another study (32), sulfur adsorbed on a Ni(810) caused decomposition to (100) and (410) facets.…”

“…If so, the S/Ni s ratio at saturation would in principle be 0.5 for the reconstructed surface. In the first example above, restructuring would not affect the S/Ni s ratio at saturation, since it is 0.5 for both (100) and (111) planes; however, in the second example, the S/Ni s ratio at saturation would probably decrease, as rough planes transform to smoother ones. Nevertheless, the possibility of increases in the S/ Ni s ratio at saturation due to reconstruction cannot be ruled out.…”

“…There have been no reviews on this subject and relatively few reported studies to define the effects of metal loss on catalytic activity (27,(108)(109)(110)(111)(112)(113)(114)(115)(116)(117)(118)(119)(120)(121). Most of the previous work has focused on volatilization of Ru in automotive converters (108)(109)(110)(111); nickel carbonyl formation in nickel catalysts during methanation of CO (113,119) or during CO chemisorption at 258C (27,115); formation of Ru carbonyls during Fischer-Tropsch synthesis (116,117); volatilization of Pt during ammonia oxidation on Pt-Rh gauze catalysts (120,121); and volatilization of Cu from methanol synthesis and diesel soot oxidation catalysts, leading to sintering in the former and better catalyst-soot contact but also metal loss in the latter case (92).…”

Heterogeneous Catalyst Deactivation and Regeneration: A Review

“…While the chemical properties of volatile metal carbonyls, oxides, and halides are well known, there is surprisingly little information available on their rates of formation during catalytic reactions. There have been no reviews on this subject and relatively few reported studies to define the effects of metal loss on catalytic activity [28,[128][129][130][131][132][133][134][135][136][137][138][139][140][141]. Most of the previous work has focused on volatilization of Ru in automotive converters [128][129][130][131]; nickel carbonyl formation in nickel catalysts during methanation of CO [133,139] or during CO chemisorption at 25 °C [28,135], and formation of Ru carbonyls during Fischer-Tropsch synthesis [136,137]; volatilization of Pt during ammonia oxidation on Pt-Rh gauze catalysts [140,141]; and volatilization of Cu from methanol synthesis and diesel soot oxidation catalysts, leading to sintering in the former and better catalyst-soot contact but also metal loss in the latter case [109].…”

“…Bartholomew [131] found evidence of significant (50%) Ru loss after testing of a Pd-Ru catalyst in an actual reducing automobile exhaust for 100 h, which he attributed to formation of a volatile ruthenium oxide and which was considered responsible at least in part for a significant loss (20%) of NO reduction activity. Mobile copper chloride improves catalyst-soot contact; catalyst evaporation observed [109] Shen and co-workers [133] found that Ni/Al2O3 methanation catalysts deactivate rapidly during methanation at high partial pressures of CO (>20 kPa) and temperatures below 425 °C because of Ni(CO)4 formation, diffusion, and decomposition on the support as large crystallites; under severe conditions (very high PCO and relatively low reaction temperatures) loss of nickel metal occurs.…”