Methane oxidation palladium

R. Hicks and co-workers, Structure Sensitivity of Methane Oxidation overl latinum and Palladium J. Catal, 280—306 (1990). 

The most successful class of active ingredient for both oxidation and reduction is that of the noble metals silver, gold, ruthenium, rhodium, palladium, osmium, iridium, and platinum. Platinum and palladium readily oxidize carbon monoxide, all the hydrocarbons except methane, and the partially oxygenated organic compounds such as aldehydes and alcohols. Under reducing conditions, platinum can convert NO to N2 and to NH3. Platinum and palladium are used in small quantities as promoters for less active base metal oxide catalysts. Platinum is also a candidate for simultaneous oxidation and reduction when the oxidant/re-ductant ratio is within 1% of stoichiometry. The other four elements of the platinum family are in short supply. Ruthenium produces the least NH3 concentration in NO reduction in comparison with other catalysts, but it forms volatile toxic oxides. 

The Pd-ZSM-5 catalysts are prepared by impregnation and by solid exchange methods on the carrier of HZSM-5 and NaZSM-5 (Si/Al = 26) with variable palladium loading and different pre-treatment gas (He and O2). N2-physisorption, DRX and CH4-TPR are the main techniques used to characterise these catalysts. Furthermore, total methane oxidation is used to test their catalytic activity. Among the preparative variables, the solid exchange method, the NaZSM-5 support and the increase of the palladium loading improve considerably the activity of the Pd-ZSM-5 catalysts in methane oxidation. 

Keywords Total methane oxidation, Pd-ZSM-5 catalysts, impregnation, solid exchange method, pre-treatment and palladium loading. 

Supported palladium oxide is the most effective catalyst used in total methane oxidation and in catalytic oxidation of VOCs [1-5]. However, the activity of the conventional catalysts is not sufficient [5-6]. Recently, the Pd-zeolite catalysts have attracted considerable attention due to their high and stable CH4 conversion efficiency [4-8]. In this work, the effect of the preparation method, the nature of the charge-balancing cations, the palladium loading and the pre-treatment gas nature on the texture, structure and catalytic activity of the Pd-ZSM-5 solids are investigated. 

The same group reported a palladium-mediated oxidation of methane to a methanol derivative employing a CuCl2 and Pd/C-based catalyst system and dioxygen in a trifluoroacetic acid/water mixture.18 A system was also described, which mediated the oxidation of ethane (Equation (10)). 

Pfefferle and Lyubovsky executed types of measurements that yielded critical information between active Pd phases for catalytic combustion using pure ot-alumina plates with zero porosity as a support for the catalyst. This procedure uniformly covers the plate with metal particles on the top surface where they are easily available for the reaction gases and optical analysis. This type of experimental procedure has shown that in high-temperature methane oxidation the reduced form of the supported palladium catalyst is more active than the oxidized form. The temperature at which the PdO Pd... 

Very similar results were also obtained by Farrauto et al. [51] from a study of the high-temperature catalytic chemistry of supported Pd for the combustion of methane. Palladium oxide supported on alumina decomposes in two distinct steps in air at atmospheric pressure. The first step occurs between 750 and 800X and is believed to be a decomposition of Pd-O species dispersed on bulk Pd metal, designated (PdO /Pd). The second decomposition occurs between 800 and 850" C, and it behaves like crystalline palladium oxide (PdO). To form the oxide once again, metallic Pd has to be cooled down to 650°C, thus causing a hysteresis gap of 150°C. Above 500°C, catalytic methane oxidation can occur only as long as the palladium oxide phase is still present. Above 650 C, metallic Pd cannot chemisorb oxygen, and hence it is catalytically inactive toward methane oxidation. 

Many works have been devoted to investigate the deactivation causes of the supported palladium catalysts in methane oxidation [13,21-23]. The inhibition of the catalytic sites by water through the formation of palladium hydroxide [21], the change in the distribution of the particle sizes [13], the extensive oxidation of the palladium oxide [22]... 

It is known that supported palladium catalysts are the most active for the total oxidation of methane [3], and there are many studies focusing on the alumina supported ones [4 and references cited therein] However, alumina is not stable at the temperatures commonly used for methane oxidation. To avoid this problem, other authors [5] have suggested the use of zirconia-based supports, which are considered as more thermally stable. In this way, these supports were found to present very different properties, depending on the synthesis method and the presence of additives. 

Methane oxidation at mild or low temperatures can be catalyzed by platinum group metals. Palladium is one of the most efQdent metals (1) and has been studied over mai supports (2-6). This particular metal, when supported on alumina, b ins to show an increase in its activity between 350 and 420°C. At these conditions a general increase in the active spedes particle size is observed. Piimet and Briot (7,8) defined two states for the Pd/Al203 supported catalyst a state I, obtained after simple reduction and a state n after the catalyst had reacted at 600°C for 14 h under 02/CH4=4A. State II was more active than state I and showed a lower binding oietgy of oxygen with palladium. However, the state of the active phase was not clear. The diffoences in activity, also observed by others, have also been related to the formation/decomposition of PdO (9), to the oxygen adsorbed on metallic Pd (2), to the modification of Pd surface spedes (3), and to the reconstruction of PdO crystallites (4, 10). One of the hypotheses for the activation of the Pd catalysts was the establishment of an epitaxy between the metal and the support (8, 11). 

Metal-based catalysts also were used for methane oxidation. Especially over metals such as platinum and palladium, trace amounts of methanol, formaldehyde, and formic acid can be found. Organic halides increased the yield of partial oxidation products and inhibited the complete combustion of methane [173]. Inhibition effects of dichloromethane was observed. Mann and Dosi [174] used a Pd/Al203 catalyst and found that the addition of halogen compounds reduced the conversion of methane in the following order ... 

R.F. Hicks, H. Qi, M.L. Young, R.G. Lee "Effect of Catalyst Structure on Methane Oxidation over Platinum and Palladium",... 

Finally, Ya-Huei Chin and Daniel Resasco (University of Oklahoma) review the catalytic oxidation of methane under lean-burn conditions. They focus on palladium-based catalysts, which are the most active for methane oxidation. They examine both the low temperature region (<800 °C), which is most relevant to exhaust control, and the high temperature region (>800 °C), which is applicable to gas turbines. 

Simone, D. O., Kennelly, T. L., Bmngard, B. J. Farrauto, R. Reversible poisoning of palladium catalysts for methane oxidation. Applied Catalysis 70, 87-100 (1991). 

Hicks, R. F., Qi, H., Young, M. L. Lee, R. G. Structure sensitivity of methane oxidation over platinum and palladium. Journal of Catalysis 122, 280-294 (1990). Hicks, R. F., Qi, H., Young, M. L. Lee, R. G. Effect of catalyst structure on methane oxidation over palladium on alumina. Journal of Catalysis 122, 295-306 (1990). Bernal, S., Blanco, G., Gatica, J. M., Larese, C. Vidal, H. Effect of mild re-oxidation treatments with CO2 on the chemisorption capability of a Pt/CeOj catalyst reduced at 500°C. Journal of Catalysis 200, 411-415 (2001). 

In this work, the mechanism of methane oxidation over PCI/AI2O3 catalyst is investigated, the palladium oxidation state under stream reaction is identified and the reactive form of oxygen is determined. XPS, thermal gravimetric and surface potential measurements are performed on this catalyst under various dynamic gaseous treatments. 

The supported palladium catalyst known to be the most active for total methane oxidation was the subject of considerable amount of research [1-9]. However, no agreement about the mechanism reaction was observed in the literature [1-8]. The Langmulr-Hinshelwood [1-4], the Eley-Rideal [5-7], and the Mars-Van Krevelen [8], mechanisms were proposed for the total oxidation of methane on the supported palladium catalysts. This diversity is explained by the variation of the active surface in each case. Indeed, according to Burch et al.[9], the active sites can be modified by the pre-treatment conditions, by the particle size, by the support nature and by the presence of some poisons such as chlorides. Others difficulties result from the fact that it is not confirmed if the active site is a partial or a total oxidized palladium particle. In addition, little is known about the reactive oxygen form. Indeed, it is not yet established if the reactive oxygen is a chemisorbed molecular or ionic form or a lattice oxygen ion. The aim of this paper is to identify the palladium oxidation state under catalytic stream, to study the reactive form of oxygen and to propose a mechanism of the reaction. 

Sulfur species may form sulfates and sulfites, which lower the catalyst surface area and cause deactivation for palladium [350]. An alumina support can trap or adsorb [351] sulfate groups. Deng and Nevell reported deactivation of alumina supported palladium, rhodium and iridium catalysts by hydrogen sulfide during methane oxidation [364]. Sulfate species were formed on the catalyst surface under the oxidising conditions. 

Deng, Y. and NeveU, T.G. (1993) Sulfur poisoning, recovery and related phenomena over supported palladium, rhodium and rridimn catalysts for methane oxidation. Appl. Catal. A, 101, 51-62. 

Less conventional supports were also used for hydrocarbon oxidation. For example, Postole et al investigated the performance of palladium deposited on boron nitride. This PdO/BN catalyst showed relatively good performances in propene and methane oxidation even in the presence of moisture. 

The state of palladium can change upon hydrocarbon oxidation. Under substoichiometry of O2, PdO is reduced before CO and alkene oxidation starts while it is reduced during the light-off oxidation of propane. In methane combustion, even in O2 excess, it was proven that the oxidation starts when some PdO sites are reduced by The active sites of palladium for methane oxidation would... 

The emission of methane into the atmosphere takes place mainly as a component of natural gas, which also contains ethane and propane in different concentrations. Since methane is the least reactive it could be expected that the presence of ethane or propane does not affect the methane oxidation rate. However, Ruiz and co-workers have demonstrated over a Pd/y-Al203 catalyst that, in the presence of ethane and propane, both enhancement and suppression of methane oxidation can take place. Hence, the exact composition of the natural gas will be important in determining the final activity of the catalysts. This effect is not expected to be isolated to a palladium-based catalyst and highlights the wide variety of conditions over which a VOC catalyst must be expected to operate. 

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