Regeneration of S-poisoned Pd/Al2O3 catalysts for the combustion of methane
Introduction
Natural gas represents one promising alternative energy source for the future in automotive and heavy-duty vehicles. In comparison with traditional fuelled vehicles, lean burn natural gas vehicles (NGV) engender a lower impact on environment, thanks to smokeless exhausts and reduced nitrogen oxide emissions due to the lower combustion temperatures associated with the high air to fuel mass ratios (typically 20 or greater) at which the lean engine operates. At given combustion engine efficiency also tailpipe CO2 emissions in NGV light-duty vehicles can be lower than in conventional fuelled vehicles by virtue of the higher hydrogen-to-carbon ratio of natural gas. Nevertheless, the “green” image of lean burn NGV risks to be compromised by emission of unburned methane, a potent greenhouse gas estimated to have a 20 years global warming potential more than one order of magnitude higher than that of carbon dioxide. Accordingly severe limitations on CH4 emissions have been enforced [1].
The use of an exhaust oxidation catalyst is the currently adopted way to match CH4 emission limits, but obstacles arise from the reaction conditions specific to lean burn engines. Typical exhaust gases are characterized by low temperatures (even below 400 °C), presence of water vapour (10–15%), large excess of oxygen and concentration of methane as low as 500–1000 ppm. Under such conditions Pd/alumina catalysts are widely recognized as the most active in combustion of methane [1], [2], [3], [4], but suffer from deactivation problems. The loss in activity is mainly attributed to sulphur containing compounds naturally occurring in natural gas or added as odorants necessary for safety reasons, which result in SO2 concentration of 0.5–1 ppm in exhaust gases [2], [5], [6], [7], [8], [9], [10].
Several studies focused on the critical effect of SO2 poisoning on the catalytic oxidation of methane, showing a marked loss in activity related to formation of stable sulphate species [2], [5], [6], [7], [8], [9], [10] associated with a strong adsorption of SO3 produced by SO2 oxidation over palladium catalyst. Several studies evidenced also the influence of the support on the rate of poisoning of the PdO catalyst, deactivation being much faster for non-sulphating support like SiO2 [11] and SiO2–ZrO2 [2] than for sulphating supports, like Al2O3 which slow down the deactivation rate by scavenging of sulphate species. On the other hand, catalyst reactivation via decomposition of sulphate species by thermal treatment under oxidizing atmosphere [2], [6] or, more effectively, by H2 reductive conditions [12] was found more difficult for catalyst dispersed over sulphating supports suggesting that decomposition of stable support sulphate species is required for partial or complete reactivation.
In this work, the regeneration of a S-poisoned 2% (w/w) Pd/Al2O3 catalyst was investigated by means of alternate pulses and temperature step experiments, in order to evaluate the temperature threshold to obtain catalyst reactivation under different CH4 containing atmospheres including: lean combustion (excess of O2); CH4-reducing (O2-free) and rich combustion (O2 deficient) atmospheres. The effect of alternate lean combustion/CH4-reducing pulses on unpoisoned catalyst was also addressed to better understand modifications of palladium catalyst during reduction/reoxidation cycles.
Section snippets
Experimental
Palladium catalysts supported on γ-Al2O3 (LaRoche Versal TD250) were obtained by incipient wetness impregnation using an aqueous solution of Pd(NO3)2 (Aldrich, 10%, w/w, Pd, 99.999%) of the supports calcined at 800 °C for 6 h. 2% (w/w) Pd was loaded in a single impregnation step. Catalysts were dried overnight at 110 °C and then calcined for 6 h at 800 °C in a 0.2 l/min air flow (heating/cooling rate 10 °C/min). The BET surface area of the catalyst was 122 m2/g.
The fresh catalyst was submitted to
TPR experiments
TPR experiments performed over fresh catalyst showed a peak of CH4 consumption in a temperature range between 345 and 365 °C accompanied by an increase of CO2 outlet concentration likely associated with reduction of PdO to Pd by CH4. At higher temperatures a marked CH4 consumption was observed, accompanied by production of CO2, CO and H2 likely associated with steam reforming and shift reactions that occurred over metallic Pd.
Alternate lean combustion/reducing pulses
Several cycles of alternate lean combustion/CH4-reducing pulse
Conclusions
Temperature programmed, alternate pulses and temperature step experiments performed in this work allowed to investigate the effects of temperature and composition of CH4 containing atmospheres on regeneration of a S-poisoned Pd/Al2O3 catalyst for CH4 combustion.
The following main conclusions were pointed out:
- (1)
In unpoisoned catalysts PdO reduction/reformation processes occurring upon CH4-reducing pulses and restoring of lean combustion conditions result in a complete recovery of activity losses
Acknowledgement
This work has been financially supported by MIUR-Rome under PRIN projects.
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