Catal Lett (2009) 130:637–641 DOI 10.1007/s10562-009-0014-z
Optimization of Multicomponent Cobalt Spinel Catalyst for N2O Abatement from Nitric Acid Plant Tail Gases: Laboratory and Pilot Plant Studies Pawel Stelmachowski Æ Filip Zasada Æ Gabriela Maniak Æ Pascal Granger Æ Marek Inger Æ Marcin Wilk Æ Andrzej Kotarba Æ Zbigniew Sojka
Received: 9 April 2009 / Accepted: 29 April 2009 / Published online: 27 May 2009 Ó Springer Science+Business Media, LLC 2009
Abstract The influence of Zn and K promoters on N2O decomposition over Co3O4 was investigated by work function measurement and temperature-programmed surface reaction. The beneficial effect of the promoters resulting in spectacular decrease in the temperature of 50% conversion by 200 °C was found to be essentially of electronic origin. The strong correlation between the catalyst work function and deN2O activity allowed for the optimization of the doping level of both additives. The pilot plant tests in real nitric acid tail gases revealed that the optimized double promoted (Zn, K) cobalt spinel catalyst maintained its remarkable activity in N2O decomposition (conversion [95% at the target temperature of 350 °C) for more than 60 h. Keywords Cobalt spinel Potassium promoter N2O decomposition Work function Zinc Tail gases
P. Stelmachowski F. Zasada G. Maniak A. Kotarba (&) Z. Sojka Faculty of Chemistry, Jagiellonian University, Ingardena 3, 30-060 Krakow, Poland e-mail:
[email protected] P. Granger Unite´ de Catalyse et Chimie du Solide UMR 8181, Universite´ des Sciences et Technologies de Lille, 59655 Villeneuve d’Ascq Cedex, France M. Inger M. Wilk Instytut Nawozow Sztucznych, al.1000-lecia Panstwa Polskiego 13A, 24-110 Pulawy, Poland
1 Introduction Nitrous oxide is considered to be a greenhouse gas with global warming potential of c.a. 300 [1]. As a source of other nitrogen oxides in the stratosphere it also contributes to ozone layer depletion [2]. Due to its harmful impact on environment, catalytic decomposition of N2O received an increased attention during the past several years [3–8]. Since one of the important anthropogenic sources of N2O is the nitric acid industry [9], an efficient low-cost catalyst for this application need to be developed rather urgently. Removal of N2O from nitric acid plant tail gases remains still a challenging problem mainly due to its low concentration (ca. 1000 ppm) and the presence of inhibiting co-reactants in the exhaust stream, such as H2O, O2, and NOx. Many efforts have been made to develop the catalysts for nitrous oxide decomposition from tail gases at economically appealing low temperature conditions (below 400 °C) [10]. The most promising generic systems for catalytic N2O removal are based on the cobalt spinel structure. Improvement of the spinel catalyst performance may be achieved by both bulk modification (doping with alien cations) and by tuning the surface properties of the catalyst with alkali promoters. Promotion of cobalt spinel catalyst was previously described in the literature. Bulk modification was achieved by introducing cations such as Mg, Zn, Ni or Mn [11–13]. Since the use of sodium or potassium carbonate as precipitating agent yielded more active catalysts than those precipitated with ammonia, the influence of residual alkali on the catalyst performance was also studied [14–19]. It was found that alkali promotion leads not only to the improvement of the catalytic activity in the deN2O reaction but also in the decomposition of nitric oxide [20–23].
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Because of the cationic redox mechanism for the N2O decomposition over the cobalt spinel [24], the work function value of the material was found to be directly correlated with the catalyst activity [18]. The aim of the study was to establish the optimal composition of the cobalt spinel-based catalyst by bulk (Zn) and surface (K) doping to reach the target 90% conversion below 350 °C in the presence of typical inhibitors of tail gases, for practical application in nitric acid plants. This was accomplished by catalytic screening in laboratory conditions followed by the long term real feed tests of the final catalyst performance in the pilot plant.
2 Experimental The spinel catalysts of 50–60 m2/g were obtained via precipitation method from the stoichiometric Co and Zn nitrates mixture with the K2CO3 as the precipitating agent. Then, the obtained precursors were thoroughly washed to remove the traces of the residual alkali. The obtained samples were calcined in air at 400 °C for 2 h. The potassium promoter was added by impregnation from its carbonate at the doping level of nK/nM within the range to 0–8 9 10-2, and calcined in air at 400 °C for 4 h. Similar level of alkali doping was previously found to be optimal for K–Co3O4 catalyst [18]. Composition of the doped samples was verified by the elementary analysis, which indicated that the intended Co/Zn ratio was practically achieved, within the error of 10%. For instance, in the case the most active sample of the stoichiometry corresponding to Co2.60Zn0.40O4 the experimentally determined composition was found to be Co2.56Zn0.44O4?d. The reference Co3O4 powder of 16 m2/g was used as received from the supplier (Fluka). The effect of specific surface area on the Co3O4 catalyst activity was checked with the use of samples calcined in the temperature range 400–700 °C with the resulting SBET varying from 30 m2/g to 5 m2/g, respectively. The phase composition of the catalyst samples was examined by X-ray diffraction, using CuKa radiation by means of a X’pert Pro Philips diffractometer. Data were recorded in the 2H range of 10–70° with the resolution of 0.02°. The analysis of catalyst morphology was investigated by a Hitachi S-4700 (20 keV) electron microscope. Prior to the SEM observations the samples were coated with carbon. Nitrogen physisorption measurements were conducted on a Quantasorb Junior device with prior degassing at 150 m2/g C for 1.5 h. The contact potential difference (VCPD) measurements were carried out by the dynamic condenser method of Kelvin with a KP6500 probe (McAllister Technical Services). To standardize the catalyst surface the
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measurements were carried out in vacuum of 10-7 mbar at 150 m2/g C after annealing the sample to 400 °C. The work function values were obtained from a simple relation eVCPD = Ureference– Usample, using a standard stainless steel plate as a reference electrode (d = 3 mm, Ureference = 4.3 eV). The Temperature-programmed Surface Reaction studies of N2O decomposition were performed in a quartz flow reactor in the range of 20–600 °C (10 °C/min) with the use of 300 mg of the catalyst (sieve fraction of 0.2–0.3 mm), flow rate of the feed (N2O 5% and N2O1500 ppm ? H2O 1% ? O2 3% in Ar) with GHSV of 7000 h-1. The reaction progress was monitored with a quadruple mass spectrometer (RGA200, SRS). Signals for m/z of 44, 30, 28, 32, and 18 were measured corresponding to N2O, NO, N2, O2, and H2O, respectively. The pilot reactivity tests were carried out in a pilot nitric acid installation in a quartz reactor for 1.85 g of the sample, sieve fraction 0.6–1.0 mm, catalysts bed depth of 15 mm, with the tail gas composition of N2O 1200 ppm, H2O 0.5%, O2 2.0%, NOx 2000 ppm in nitrogen, and GHSV 20000 h-1. Stability test were carried out for 60 h for the same catalyst loading with the tail gases composition within the range of 900 \ N2O \ 1050 ppm, 0.3 \ H2O \0.5%, 1.0 \ O2 \ 1.3%, 400 \ NOx \ 530 ppm in nitrogen.
3 Results and Discussion Typical XRD patterns of Co3O4 and double promoted K–Co2.6Zn0.4O4 are shown in the Fig. 1a. X-ray diffraction lines characteristic of the cobalt spinel structure were indexed within the Fd3 m space group (69378-ICSD). The diffractograms proved spinel structure of the investigated samples, confirming no major structural changes upon modifications of the parent cobalt spinel. The XRD results showed also that Zn2? ions having similar radius to Co2? are incorporated into Co3O4 structure, whereas the amount of potassium is obviously too small to form any surfacesegregated detectable phase. The diffraction peaks for Zn doped Co3O4 shift to lower 2h values, which indicates an increase in the spinel lattice parameters, characteristic for formation of a solid solution [25]. The mean size of crystallites obtained from Williamson-Hall analysis is 22 nm, and remain in good agreement with the values of 17 nm estimated from N2 physisorption measurements assuming round crystallite shape. The parallel experiments for a series of Co3O4 samples calcined at various temperatures with BET surface area in the range 5–30 m2/g revealed that the specific surface area has little effect on the catalytic activity (difference in the temperature of 50% N2O conversion remain within 10 °C interval).
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Fig. 1 a XRD patterns of Co3O4 and potassium doped Co2.6Zn0.4O4 sample with optimal K loading; b typical SEM image of the Zn and K doped cobalt spinel catalyst
A representative SEM image of the investigated catalyst is shown in Fig. 1b. The size of the catalyst aggregates observed in SEM images is in the range of 50–150 nm. The comparison with the XRD and BET results indicates that they consist of 3–8 agglomerated crystallites on average (Fig. 1b). Owing to the previously found strong correlation of the highest catalytic reactivity with the work function minimum [17, 18], the latter parameter can be thus used as a convenient guidance for the catalyst optimization process. In the case of N2O decomposition the rate determining step (RDS) is associated with the NN–O cleavage induced by the electron transfer from the surface to N2O molecule leading to dissociative adsorption. Since the kinetic measurements are restricted to the RDS, which is controlled by the ease of the electron transfer the correlation between the work function and catalyst activity is a natural consequence of such reaction mechanism. The 3D plot in Fig. 2 shows the results for two independent series of the modified cobalt spinel including bulk promotion by Zn doping (a), and surface promotion by K doping (b). For bulk modification, the work function minimum (Umin = 4.00 eV) corresponds to the Zn level of x = 0.4 in Co3-xZnxO4. Having optimized the level of Zn promoter in the next step the catalyst surface was fine-tuned by stepwise introduction of potassium. It resulted in further lowering of the work function and the minimum value of Umin = 3.72 eV was reached for five potassium atoms per 1 nm2. Since the double promotion procedure results in a diverse localization of Zn (bulk) and K (surface) in the catalyst, their action is of a different nature. Experimentally observed non-monotonous changes in the work function upon Zn doping can be qualitatively explained by the results of parallel DFT calculations for the most stable (100) plane of Co3O4. The minimum in U was reproduced and accounted for by two possible sites for zinc location in the tetrahedral interstitials within the near-surface region and in the spinel bulk. The near-surface Zn ions are more stable by 0.61 eV in comparison to the bulk ones, and they lower the barrier for electron extraction from the Fermi
Fig. 2 Work function optimization of Co3O4 by bulk—Zn (xZn in Co3-xZnxO4, a and a0 ) and surface doping—K (surface coverage HK, b and b0 )
level by 0.17 eV (for the optimal Zn-doping). As the concentration of Zn-doping increases the Zn ions tend to be located in the less stable bulk positions. As a result the work function increases by DU = 0.22 eV above the minimum value, as observed experimentally (Fig. 2). Nonetheless, for a more integral picture of the work function changes, the resultant calculated value to be compared with the experiment quantitatively, should be averaged over other exposed planes of Co3O4, which is now in progress [26]. The promotional effect of K due to its low ionization potential is related to the charge transfer to the catalyst and formation of Kd?–Osuf surface dipoles by potassium adspecies. The non-monotonous dependence of the cobalt spinel work function on the K surface coverage can be interpreted in this case in terms of the Topping model [18]. The initial lowering of the work function caused by
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separated surface dipoles is followed by the increase in U caused by the depolarization giving rise again to a minimum which is much more pronounced (DU = 0.5 eV) than for Zn doping (Fig. 2). The promotional effect is in line with the previous DFT modeling of the surface dipoles determined from Hirshfeld atomic charges and geometry of the postulated potassium adspecies on the Co3O4 (100) surface [18]. The crossing of both curves projected at the xZn-HK plane (a0 -b0 lines in Fig. 2) defines the optimal composition of the double promoted catalyst. The effect of potassium addition to Co3O4 and Co2.56Zn0.44O4 results in approximately the same shift in temperature of 50% conversion *150 °C (Fig. 3). This indicates that the promotion of K and Zn exhibits an additive character according to the different nature of their action, discussed above. The results of catalytic activity in pristine N2O expressed as temperature of 50% of conversion versus work function value revealed a strong promotional effect of Co3O4 host by the successive promotion with Zn and K (Fig. 3). Again, the observed monotonous dependence indicates that the catalyst activity is clearly correlated with the work function of the catalyst surface. With the shift of only *50 °C of T50%, the single bulk doping of the spinel with Zn ions is apparently less efficient than the surface doping with potassium (DT50% = *150 °C), whereas the introduction of both promoters leads to additive effect, resulting in spectacular decrease in the temperature of 50% conversion by 200 °C. The activity of the optimal double-promoted spinel catalyst in N2O decomposition was further tested with various feed composition in laboratory and pilot plant conditions. Typical results are presented in Fig. 4. Addition of water and oxygen in the laboratory test to the feed lowers the T50% of the reaction from 290 °C to 190 °C (Fig. 4a, b). The presence of NOx in addition to H2O and
Fig. 3 The stepwise optimization of the Co3O4 host structure by single and double doping (Zn- bulk and K- surface). Temperature of 50% N2O conversion (T50%) versus catalyst work function
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Fig. 4 Reactivity of the catalyst in a laboratory tests and in the pilot nitric acid installation
O2, in the pilot installation causes a further decrease in deN2O activity (Fig. 4c). It is clear, however, that despite the simultaneous presence of all the inhibitors the performance of the doubly optimized catalyst remains at the very high level (conversion [95%) at the industrial target temperature of 350 °C, in the real outlet of the pilot plant (Fig. 4c). The prolonged pilot plant durability test of the K, Znpromoted catalyst in the real tail gases at 350 °C, shown in Fig. 5, revealed that the high catalytic activity in N2O conversion is stable for at least 60 h. The variations of the conversion level remain within the narrow range of 95– 99% and can be associated with the fluctuations in the feed
Fig. 5 Stability test of N2O decomposition in the nitric acid pilot plant at 350 °C and GHSV of 20000 h-1
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composition, typically observed in the tail gases of nitric acid plant [10]. They are mainly correlated with the actual concentration of inhibitors suggesting that the interaction of NOx and O2 with the catalyst surface exhibits a reversible character. It is also worth mentioning, that the XRD and SEM investigations did not indicate any structural or morphological changes in the material after the catalytic tests. Although, the XPS measurements of the spent catalyst revealed a pick at 407.1 eV corresponding to N 1 s in surface nitrates, apparently these ad-species did not influence the catalytic performance in a significant way. Those findings augur well for the potential future application of the developed catalyst.
4 Conclusions The promotional effect of successive doping of Zn and K on the catalytic activity of Co3O4 in N2O decomposition was investigated. The optimization of the doping level was based on the efficiency of electronic promotion gauged by the work function measurements. The optimal composition was found to be 1.5 K atoms per 1 nm2 of the Co2.56Zn0.44O4 spinel and correspond to the catalyst work function minimum. Introduction of both promoters leads to the additive effect, resulting in a spectacular decrease in the temperature of 50% conversion by 200 °C. The pilot plant tests revealed that the double promoted (Zn, K) cobalt spinel catalyst maintained high performance (conversion [95%) in N2O removal from tail gases for 60 h. Acknowledgments MEiN-3/2/2006
This work was supported by the grant No PBZ-
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