The Platinum Catalysed Reduction of Nitric Oxide by Ammonia A SOLID ELECTROLYTE POTENTIOMETRY AIDED STUDY (original) (raw)
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Applied Catalysis A: General, 2007
The aim of this work was to study the influence of the reaction temperature on the efficiency of the electrochemical promotion to improve the catalytic performance of a Pt impregnated catalyst for the reduction of nitrogen oxides under lean burn conditions. Open circuit catalytic and potential measurements were carried out in order to explain the potentiostatic behaviour of the electrochemical catalyst under different reactions temperatures. At low temperature (220 8C), the application of negative polarization increased the NO reduction rate by a factor of 1.4 (electrophilic behaviour). But as the reaction temperature was higher, the efficiency of electrochemical promotion to improve the catalytic activity decreased, even leading to a poisoning effect at 300 8C. This progressive suppression of the promotional effect was due to an increase with temperature of the oxygen coverage on the catalyst, which led to a C 3 H 6 adsorption inhibition. Nevertheless, at all explored reaction temperatures, the presence of sodium promoter gave a large increase in N 2 selectivity, even reaching 90%. These results demonstrated that electrochemical promotion is a suitable technique to improve the catalytic performance of Pt catalyst for the practical development of HC-SCR process to removal nitrogen oxides. #
Journal of Catalysis, 2001
Bimetallic platinum copper catalysts were prepared by deposition of copper on a parent monometallic platinum catalyst. Two techniques were employed favoring the deposition of copper either on the parent metal or on the support. The activity and selectivity of copper and platinum monometallic catalysts are compared to those of their bimetallic counterparts. Copper reduces nitrates and nitrites according to a redox process but deactivates rapidly. Monometallic platinum catalysts are inactive for nitrate reduction, while nitrates are totally and rapidly reduced on Pt-Cu bimetallic catalyst when both metals are in close contact. In the bimetallic catalyst, the role of copper is to reduce nitrate to nitrite according to a redox process. In this step, the interaction between copper and platinum is of major importance to maintain copper in the metallic state by way of hydrogen adsorbed on platinum. However, copper has a negative influence on the selectivity toward nitrogen.
Nitrogen Desorption in the Reaction of Nitric Oxide on Carbon-Supported Platinum Catalysts
The catalytic decomposition of nitric oxide on carbon-and y-alumina-supported platinum was studied in a quartz flow reactor between 423 and 623 K. The measured turnover frequency, defined as the number of nitric oxide molecules reacting per surface platinum atom per second, was an order of magnitude higher on the carbon-supported catalysts than on the alumina-supported catalyst or than that projected from earlier work on platinum foil. The rate-determining step for nitric oxide decomposition on Pt/AI,O, or Pt foil appears to be the chemisorption of NO on platinum, with inhibition by oxygen, the most abundant reaction intermediate (mari) in equilibrated chemi-sorption on the same sites as those required for NO chemisorption. By contrast, it appears that on PVC, the inhibition product oxygen is removed continuously from the platinum surface by the surrounding carbon support, producing CO and CO*. The kinetic data suggest that the mari now consists of adsorbed nitrogen atoms. The second-order rate constant k for the associative desorp-tion of nitrogen from platinum obtained from our work is compared with that extracted from published data on NH, decomposition or that measured directly by temperature-programmed desorption of nitrogen from platinum. We find for k a preexponential factor of 5 x lO-9 cm* s-' and an activation energy of 88 kJ mol-I, in remarkable agreement with those
In Situ Electrochemical Promotion by Sodium of the Platinum-Catalyzed Reduction of NO by Propene
Journal of Physical Chemistry B, 1997
The Pt-catalyzed reduction of NO by propene exhibits strong electrochemical promotion by spillover Na supplied from a ′′-alumina solid electrolyte. In the promoted regime, rate increases by an order of magnitude are achievable. At sufficiently high loadings of Na the system exhibits poisoning, and excursions between the promoted and poisoned regimes are fully reversible. Reaction kinetic data obtained as a function of catalyst potential, temperature, and gas composition indicate that Na increases the strength of NO chemisorption relative to propene. This is accompanied by weakening of the N-O bond, thus facilitating NO dissociation, which is proposed as the critical reaction-initiating step. The dependence of N 2 /N 2 O selectivity on catalyst potential is in accord with this view: Na pumping to the Pt catalyst favors N 2 production at the expense of N 2 O. X-ray photoelectron spectroscopic (XPS) data confirm that electrochemical promotion of the Pt film does indeed involve reversible pumping of Na to or from the solid electrolyte. They also show that under reaction conditions the promoter phase consists of a mixture of sodium nitrite and sodium nitrate and that the promoted and poisoned conditions of the catalyst correspond to low and very high loadings of these sodium compounds. Under all reaction conditions, a substantial fraction of the promoter phase is present as 3D crystallites.
Applied Catalysis B: Environmental, 2007
The effect of electrochemical promotion for the selective catalytic reduction of NO by propene was investigated, for the first time, on a Pt impregnated catalyst film directly deposited onto a Na-b 00 -Al 2 O 3 solid electrolyte. The effect of sodium promoters on the activity and N 2 selectivity was evaluated at 240 8C under different oxygen concentrations (0.5, 1 and 5%). The presence of promoters still enhanced the selectivity to N 2 , even under large excess of oxygen (5%), where the N 2 reaction rate was increased by a factor of 1.8. Nevertheless, the promotional effect of sodium on the overall catalytic activity for NO removal was progressively lowered with increasing oxygen concentrations, as a result of a strong inhibition of propene adsorption and a relative increase of the oxygen coverage. Characterization by cyclic voltammetry supported these results, providing useful information about the chemisorption of reactant species under the different reaction conditions. #
N 2O formation during organic nitro compounds’ hydrolysis on platinum based catalysts
Applied Catalysis B-environmental, 1999
Hydrolysis reaction products of 1- and 2-nitropropane and 2-methyl-2-nitropropane were compared on a platinum–alumina catalyst, in the conditions of NO lean reduction by propene (gas phase, low concentrations and in the presence of water and oxygen). N2O formed during the reaction decreased in the order 1-nitropropane > 2-nitropropane ⪢ 2-methyl-2-nitropropane, i.e. following the degree of substitution of the carbon atom in the α position of the nitro group. The Nef (19) reaction mechanism is proposed to explain this observation.
Applied Catalysis B: Environmental, 2008
In this work, we have investigated for the first time the selective catalytic reduction of N 2 O by C 3 H 6 over an electrochemical catalyst (Pt/K-bAl 2 O 3 ). It was evaluated the influence of the reaction conditions (temperature, oxygen concentration, water vapour presence and time on stream treatment under reaction conditions) on the catalytic performance of the electrochemical catalyst. Electrochemical pumping of potassium ions to the Pt catalyst working electrode strongly increased the N 2 O reduction rate, activating the catalyst at lower temperatures. However, it was found that the efficiency of the electrochemical promotion decreased as the oxygen concentration increased because of a strong inhibition of propene adsorption and a relative increase of the oxygen coverage. On the contrary, the presence of potassium ions on the Pt catalyst strongly decreased the inhibiting effect of water vapour, increasing the catalytic activity of the catalyst. In addition, the catalyst stability was confirmed by a deactivation study. It was found that a long term treatment at high temperature under operating conditions had a positive effect on the efficiency of the Pt/K-bAl 2 O 3 electrochemical catalyst. #
Effect of Pt/Al2O3 catalyst preparation conditions upon its activity in nitric oxide reduction
Applied Catalysis B: Environmental, 1995
A series of platinum catalysts supported on y-alumina and prepared by using a wet impregnation procedure were characterized by hydrogen temperature-programmed desorption and steady-state CO-NO reaction tests. The impregnant pH was the crucial variable in the generation of five catalysts of 0.75-0.87% Pt load at initial pH from 2.15 to 4.65. The impregnant pH showed a clear impact on both hydrogen uptake and catalytic performance. The catalysts prepared under acidic conditions demonstrated a higher catalytic activity than those prepared under basic conditions, while hydrogen adsorption levels were of the same order of magnitude. Activation energies for the CO-NO reaction were found to depend on impregnant pH.
Applied Catalysis B: Environmental, 1999
The catalytic activity of Pt black towards the reduction of NO under lean conditions is examined. Ammonia and propene are tested as reducing agents. A scrambling between nitrogens is observed in the formation of N 2 and N 2 O for NH 3 . For C 3 H 6 , a reduction of NO to mainly N 2 O is found. A minor formation of N 2 is only observed for high C 3 H 6 concentrations. The role of NO 2 as an intermediate in the reduction of NO is examined by using a feed gas mixture containing NO 2 , as well as by studying the catalytic activity of Pt black towards the NO±O 2 and NO 2 ±O 2 reactions. No effect is observed in the maximum NO x conversion for C 3 H 6 whereas for NH 3 a decreased activity is found. The presented results are put in perspective of the reaction mechanisms proposed in the literature. #