Activity and temperature-programmed adsorption/desorption behavior of PdTiO2 catalysts in NO/CH4 reduction and NO decomposition reactions (original) (raw)
Related papers
Catalysis Today, 1998
Characterization and temperature-programmed studies were performed over Pd/titania catalysts to examine their activity in the reduction of NO with methane. The catalyst was prepared using a wet impregnation technique and Pd-acetate was used as a precursor for palladium. Techniques such as BET surface area measurements, X-ray diffraction, laser Raman spectroscopy, X-ray photoelectron spectroscopy, and scanning electron microscopy were used for the characterization of the catalyst before and after the reaction. Temperature-programmed reduction (TPR) and temperature-programmed desorption (TPD) were also used to probe the surface to understand its adsorption/desorption characteristics and reducing capabilities. The results obtained from these studies together with the reaction investigations have given some important insight into the functionality of this catalyst.
Nitric Oxide Reduction with Methane over Pd/TiO2Catalysts
Journal of Catalysis, 1997
Steady-state reaction studies were performed over a Pd/titania catalyst for NO reduction using CH 4 as a reducing agent. The catalyst was prepared using a wet impregnation technique and Pdacetate as a precursor for palladium. Characterization of the catalyst samples was performed using the BET surface area technique, X-ray diffraction (XRD), laser Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and thermal analysis techniques. A fixed-bed, flow reactor system was used for activity and selectivity measurements. Identification and quantification of reaction effluents were done on-line using gas chromatography-mass spectrometry, chemiluminescence, and wet chemistry techniques. Temperature-programmed reduction studies indicated that the catalyst was reducible by H 2 even at sub-ambient temperatures. The catalyst was seen to reduce NO with conversions above 95% in the oxygen concentration range from 0 to 1.3%. Oscillatory behavior was exhibited by the catalyst under certain reaction conditions. The results obtained from the reaction experiments, together with the findings from the characterization studies, are used to explain the mechanistic aspects of this reaction.
A comparative investigation of the reduction of NO by CH4 on Pt, Pd, and Rh catalysts
Applied Catalysis B: Environmental, 1998
Pd, Pt and Rh catalysts have been compared for the selective reduction of NO to N2 and the simultaneous oxidation of (2%. The results have demonstrated that the activities for the NO/CHd reaction do not follow the known trend in the activities of these metals for the oxidation of CH4 by 02. Under slightly fuel-rich conditions related to the composition of the exhaust gas from a natural gas engine, it is observed that Pt is the most active catalyst for the NO/CH,, reaction, being an order of magnitude more active than Pd and some 50 times more active than Rh. Significant support effects are also obtained with silica being the preferred support for Pt and Rh but alumina being better for Pd. It is proposed that the relative activities of the various catalysts reflect the ease of reduction of the catalysts by C& since it is thought that the conversion of NO to N2 involves the dissociation of NO on reduced surface sites. These active sites may be metal atoms, in the case of Pt, or oxygen anion vacancies, in the case of Pd and Rh. The formation of N20 is pronounced with Pt and Pd but absent with Rh. However, in all cases, substantial amounts of NH3 were observed at temperatures above 400°C. It is concluded that the optimum choice of catalyst for the simultaneous removal of NO and CH4 will depend on the gas composition and the temperature. Under fuel-rich conditions at moderate temperatures Pt is the most active catalyst for NO removal. 0 1998 Elsevier Science B.V.
The effect of sodium on the Pd-catalyzed reduction of NO by methane
Applied Catalysis B: Environmental, 1998
The kinetics of NO reduction by methane over Pd catalysts supported on 8 mol% yttria-stabilised zirconia (YSZ) has been studied at atmospheric pressure in the 620±770 K temperature range. Langmuir±Hinshelwood type kinetics are found with characteristic rate maxima re¯ecting competitive adsorption of NO and methane: NO adsorption is much more pronounced than that of methane within the temperature range of this investigation. Pd is an effective catalyst: 100% selectivity towards N 2 can be achieved at 100% conversion of NO over this wide temperature range. Sodium causes strong poisoning of the reaction. The response of the system to variations in NO and methane concentrations, temperature, and sodium loading indicate that this is due to the Na-induced enhancement of NO chemisorption and dissociation relative to methane adsorption, i.e. sodium enhances oxygen poisoning of the catalyst. These results stand in revealing contrast to the strong promotional effect of sodium in the reduction of NO by propene over the same catalysts. The very different response of the two hydrocarbon reductants to Na doping of the Pd catalyst receives a consistent explanation. #
Applied Catalysis B: Environmental, 2017
The individual influence, as well as the combined effect of H 2 O and NO on the activity of Pd/Al 2 O 3 , PtPd/Al 2 O 3 and PtPd/CeAl 2 O 3 catalysts in complete methane oxidation under lean conditions were investigated. Under temperature-programmed ramping experiments the activity was severely inhibited in the presence of 5 vol.% H 2 O in the reaction mixture. We propose that this is due to blocking by both water and hydroxyl species. Under the influence of NO without water in the gas flow, it was found that the methane oxidation activity was partly suppressed, due to blocking of active sites. Indeed TPD performed after ramping experiments showed NO x storage on the catalyst. Contrary to the negative effect of NO in the dry case, the promotional NO effect on the activity was observed when water was co-fed, comparing the case with only water presence. The promotional NO effect was confirmed with isothermal experiments, where e.g. the methane conversion decreased from initial 96% to 25% after 10 h of exposure in CH 4 O 2 H 2 O mixture at 450 • C over the Pd/Al 2 O 3 sample, while the decrease was only from 88% to 60% when catalyst was exposed to CH 4 O 2 H 2 O NO mixture. We propose that the reason is that the NO reacts with the hydroxyl species to form HNO 2 , which reduces the water deactivation effect.
Applied Catalysis B: Environmental, 1995
We have studied the selective catalytic reduction (SCR) of nitric oxide by methane over palladium on several acidic and non-acidic supports in the presence of excess oxygen. We have found that the acidity of the support promotes the nitric oxide conversion. Both palladium and acid sites are necessary to achieve high catalytic activity. On Pd/H-ZSM-5 catalysts, the level of palladium loading is important. The activity and selectivity for nitric oxide reduction pass through a maximum with increasing Pd wt.-%. On low palladium loading catalysts, the activity and selectivity also reach a maximum with increasing oxygen concentration. The H-ZSM-5 support was not unique in promoting the activity of palladium, but it was the most effective among the acidic supports investigated when palladium was directly supported on it. Enhancements in activity and selectivity were also observed when acidic materials were mechanically mixed with a Pd/Si02 catalyst. In this case, the most effective material to promote the activity of palladium was sulfated zirconia. Therefore, it seems that the acidity is more important than the zeolitic structure in determining the activity of these catalysts. The observed catalytic behavior is consistent with a two-step bifunctional mechanism previously proposed for the nitric oxide reduction on Ga and In/H-ZSM-5 catalysts. * Corresponding author. 0926-3373/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDIO926-3373(95)00023-2 Catalysis B: Environmental 7 (1995) 113-126 125
In situ infrared study of catalytic decomposition of NO on carbon-supported Rh and Pd catalysts
Catalysis Today, 2000
The direct catalytic decomposition of NO on Rh/Al 2 O 3 , Rh/C, Pd/Al 2 O 3 , and Pd/C catalysts was investigated at 673 K by in situ infrared (IR) coupled with mass spectroscopy (MS). NO decomposition on these catalysts initially produced N 2 and adsorbed oxygen. Different catalysts exhibit different capabilities for manipulating adsorbed oxygen. Rh/Al 2 O 3 shows little activity for oxygen desorption, resulting in loss of catalyst activity; Rh/C shows the ability for promoting the adsorbed oxygen-carbon reaction, removing oxygen in the form of CO 2 ; Pd/Al 2 O 3 shows some activity for O 2 desorption. Use of carbon as a support for Pd promotes O 2 desorption, resulting in improvement of NO decomposition activity. The in situ IR results provide evidence to support the behavior of adsorbed oxygen on carbon-supported Rh and Pd catalysts.
Journal of the American Chemical Society, 2011
Although in the last 2 decades significant progress has been made toward the understanding of the structure and chemical composition of supported nanoparticles (NPs) in the as-prepared state and after reaction, 1À12 much less is known about their in situ (operando) structural and chemical features and how they evolve in the course of a chemical reaction. 13À21 Reactioninduced morphological changes in NPs need to be considered, since they might lead to a decrease/increase in the relative area of the most catalytically active surface sites, as well as to changes in the chemical state of the active metal catalysts. 14À19 The present study targets the in situ catalytic reduction of NO. This structure-sensitive reaction is of enormous industrial and environmental relevance, since NO x emissions have significant adverse effects on the environment (acidification of rain and the generation of smog), as well as on humans (respiratory infections), and therefore, remediation through catalysis is critical. 22À28 The most common routes for the removal of NO are the selective catalytic reduction (SCR) with ammonia, CO, H 2 , and hydrocarbons, as well as the direct decomposition. 4,26,29À54 The present work focuses on the reduction of NO with H 2 (H 2-SCR). 55À57 This reaction is not as selective for N 2 as, for instance, ammonia, but it has potential technological applications due to its lower onset temperature and the fact that H 2 is readily available in exhaust streams (from the water-gas-shift reaction or from hydrocarbons). 31 Noble-metal-based catalysts are generally preferred for the H 2-SCR of NO because of their high selectivity and reduced operation temperatures. 49,58À61 Although Rh is overall catalytically better than Pd for NO-SCRs, the lower cost, higher abundance, and low-temperature activity of Pd have made it a material of choice in industrial applications. 59,62À69 A vast amount of literature is available describing the conversion and selectivity of various combinations of metal catalyst, support, and reducing agent. 4,26,29À31 However, much less attention has been paid to the optimization of the structure and oxidation state of the active catalysts, its evolution under reaction conditions, and its influence on catalytic performance. 70,71 Nevertheless, previous work has revealed the important role of the oxidation state of metal catalysts in their activity, selectivity, and stability for NO-SCRs. For example, oxidized Rh catalysts are more active for H 2-SCR than metallic Rh, 72 and NO adsorption on Cu catalysts is faster on the oxidized surface, contrary to the faster adsorption reported on the reduced surfaces of other materials such as chromia or manganese oxides. 4 Additional examples discussing the reactivity of oxidized Pd species formed under reaction conditions can be found
Enhanced decomposition of no on the alkalized PdO/Al2O3 catalyst
Chemosphere, 1999
Experimentally, decomposition of NO on the alkalized Pd/A1203 catalyst is remarkably enhanced at 825-1000 K. The enhancement in N2 yield may be due to the additional basic sites on the alkalized catalyst that can trap NO molecules. However, at T > 1000 K, due to the fact that the absorbed oxygen in subsurface or bulk of Pd was involved in the formation and desorption of oxygen molecules, yield of oxygen was enhanced.