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Reduction of greenhouse gas emissions by catalytic processes
Applied Catalysis B: Environmental 41 (2003) 143–155
Reduction of greenhouse gas emissions by catalytic processes Gabriele Centi a,∗ , Siglinda Perathoner a , Zbigniew S. Rak b,1 a
Department of Industrial Chemistry and Engineering of Materials, Consortium for the Science and Technology of Materials (INSTM), University of Messina, Salita Sperone 31, 98166 Messina, Italy b Energy Research Center of The Netherlands (ECN), P.O. Box 1, 1755 ZG Petten, The Netherlands Received 17 January 2002; received in revised form 28 May 2002; accepted 29 May 2002
Abstract Catalytic technologies for the abatement of greenhouse gases (GGs) can be an effective possibility for limiting the increasing tropospheric concentration of GGs and reducing their contribution to global warming. Two different cases are discussed: (i) reduction of anthropogenic emissions of non-CO2 GGs (N2 O and CH4 ) and (ii) reduction or conversion of CO2 . In methane conversion waste gases containing diluted methane can be converted at low temperature using Pd supported on titania–ceria catalysts which show also a good resistance to deactivation. Rh supported on modified zirconia–alumina catalysts are effective and stable catalysts in low temperature decomposition of N2 O. The concept of reduction of CO2 back to fuels in a photo-electrocatalytic reactor is also presented. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Greenhouse gases; CO2 ; N2 O; Methane; Photoelectrocatalytic reactor
1. Introduction The Kyoto Protocol of 1997 on greenhouse gases (GGs) emissions has evidenced the necessity to control the emissions not only of CO2 , but also of CH4 and N2 O which contributed 7 and 9%, respectively, to the Global Warming Potential (GWP) (with reference to the CO2 equivalent emissions, using GWP values for a 100-year time horizon) [1]. Many factors determine the climate system and for many of them the level of understanding is poor. However, the precise correlation observed over the last 1000 years between change in the earth’s temperature and the atmospheric concentration of greenhouse ∗ Corresponding author. Fax: +39-090-391518. E-mail addresses: [email protected] (G. Centi), [email protected] (Z.S. Rak). 1 Fax: +31-224-568-615.
gases indicates a clear direct relationship. The projected climate change results in an estimated increase in the earth’s temperature of 1–6 ◦ C, depending on the models. The higher temperatures would be reflected in an increase in sea level, for example, of 0.1–0.9 m which may cause the flooding of large regions in the world. Although different strategies are still under discussion to mitigate the GG effects, the direct reduction of GG emissions is also necessary. The reduction strategies of GG emissions should be based on a wide range of options, due to the differences in the sources in terms of concentration, characteristics and localization. Catalytic technologies can offer cost-effective solutions for their control in several cases, although their potential application has not often been analyzed in detail. Two different cases are be discussed in this paper: (i) reduction of anthropogenic emissions of non-CO2
0926-3373/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 3 3 7 3 ( 0 2 ) 0 0 2 0 7 - 2
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GGs (mainly N2 O and CH4 ) and (ii) reduction or conversion of CO2 . Indeed, the different characteristics of the problems in the two cases imply the necessity of using different approaches to solve the two types of problems. Methane emissions derive from a wide variety of sources, the more relevant of which are sewage and solid waste disposal, rice cultivation, livestock, coal mining, and burning of biomass. Only part of these emissions can be effectively controlled by cleanup methods. Two specific cases may be addressed by catalytic technologies: (i) elimination of methane from drainage gas and ventilation air in coal measures before and during mining and (ii) vent gas produced during the disposal of solid waste [1]. In both cases, catalysts based on supported noble metals such as Pd can be used for these applications [2], but the characteristics of low light-off temperature should be promoted and especially the sensitivity to poisoning by the N- and S-compounds present in the off-gas should be reduced. N2 O anthropogenic emissions are about 2 orders of magnitude lower than methane emissions, but due to a higher GWP and higher level of natural emissions, the total greenhouse contribution of N2 O is higher on a CO2 -equivalent basis than that of methane [3]. Only part of these sources of N2 O may be addressed by catalytic technologies. The more relevant cases are the off-gas from chemical production (nitric acid synthesis or use) and combustion processes (especially, of solid waste at temperatures below 1000 ◦ C) [1], apart from the catalytic decomposition of N2 O in adipic acid plants which is already commercially applied. However, the case of adipic acid plants is characterized by high concentrations of N2 O (20–30% v/v or above), while in the case of nitric acid plant (more relevant case in terms of total emissions) the N2 O concentration is quite low (below 1–0.5%) and catalysts used in adipic acid plants are not sufficiently active [4–6]. The use of catalytic technologies for the reduction of CO2 emissions is a different case from those discussed regarding methane and nitrous oxide emissions, due to the (i) higher volume of emissions, (ii) different characteristics of the source and (iii) different requirements for its transformation. It should be noted that the expected rate of increase of CO2 emissions in China and the Developing Countries (mainly due to fossil fuel consumption) is a 100% increase in about two
decades. This increase cannot be compensated for by the expected decrease in CO2 emissions in developed countries (increase in efficiencies in use or energy and transport). Therefore, it is necessary to find specific solutions able to reintroduce CO2 into the chemical and energy cycles using renewable energies such as solar energy. A challenge for an energy-saving approach in the reduction of CO2 emissions is the use of solar energy for the conversion of CO2 back to fuels. This challenge may be addressed by the photo-electrocatalytic conversion (PEC) of CO2 . The concept was originally proposed by “Hitachi Green Center” researchers [7] and is currently under study in the framework of the EU project ERK6-CT-1999-00015. 2. Experimental 2.1. Catalyst preparation 2.1.1. Catalysts for combustion of methane Pd(2%)/[(Ti1−x Cex O2 )0.9 (Al2 O3 )0.1 ] (x = 0–0.20) catalysts were prepared by incipient wet impregnation with a Pd-acetate salt on a TiO2 –Al2 O3 –CeO2 mixed oxide prepared by the sol–gel method. The amount of Pd deposited was 2 wt.%. After drying at 110 ◦ C overnight, the catalysts were calcined at 550 ◦ C for 3 h. The surface area of all samples was in the 50–60 m2 /g range, and Pd dispersion, estimated by H2 chemisorption, was in the 6–15% range. After calcination and before the catalytic tests, a redox pretreatment (RP) was made on some samples. The RP was based on a sequence of cycles (at least three) of reduction at 350 ◦ C with a flow of 20% H2 in helium followed by reoxidation with air at 200 ◦ C. The code RP is added to the samples for which this pretreatment was made. All samples were conditioned in situ at 550 ◦ C before the catalytic tests using the same feed composition of the catalytic tests. In order to check resistance to deactivation by sulphur compounds, the following accelerated test procedure was used. The samples were treated at 500 ◦ C for 3 h with a feed with the same composition of the catalytic tests, but containing 10 ppm SO2 . After this treatment, the addition of SO2 to the feed was stopped and the catalyst activity tested again using the standard procedure. The catalysts after this treatment are labeled with the code S.
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The TiO2 –Al2 O3 –CeO2 mixed oxide support was prepared by the sol–gel method. The mixed oxide may be viewed as a titania modified by ceria and containing 10 wt.% of alumina as a structural promoter [8]. The X-ray diffraction (XRD) data indicate the presence of only a microcrystalline titania anatase phase without the presence of alumina reflection peaks. Also SEM characterization indicates a very good dispersion of alumina without the presence of islands or segregated phases more rich in alumina. The addition of alumina is needed to promote the thermal stability of titania [8] shifting the rutilization process to temperatures higher than 700 ◦ C. In the samples also containing ceria, the XRD data do not show the presence of ceria or Ce-containing crystalline phases, at least up to a ceria content of 20%. The SEM characterization also indicated a very good, homogeneous dispersion of cerium ions. The mixed oxide was prepared starting from a homogeneous anhydrous ethanol solution containing the isopropoxides of Ti and Al in the relative amounts to have a final content of alumina in the sample of 10 wt.%. The ratio of ethanol to isopropoxides was 5. For the samples containing cerium, part of the Ti-isopropoxide was substituted with the equivalent amount of Ce-nitrate in order to have the desired Ti/Ce ratio in the final sample. Then, the solution was carefully transformed first to sol and then to gel by drop-by-drop addition of a H2 O–CH3 COOH solution. The gel was aged, washed, slowly dried in an oven at 90 ◦ C and then calcined at 500 ◦ C. The Pd(2%)/Al2 O3 sample used as the reference catalyst is a commercial sample from Engelhard. 2.1.2. Catalysts for nitrous oxide decomposition Zirconia-based catalysts were synthesized using the sol–gel method for the preparation of the support, while Rh (1 wt.% in all samples) was added by incipient wet impregnation using a Rh(NO3 )3 ·H2 O aqueous solution on the calcined support. Other reference samples were commercial catalysts from Engelhard (2 wt.% Rh and 5 wt.% Ru on alumina) and a 4.0 wt.% Rh on ZSM-5 prepared by ion exchange in a RhCl3 aqueous solution using a commercial ZSM-5 from Alsi-Penta (SiO2 /Al2 O3 = 27). The sol–gel method of preparation of zirconia and modified zirconia samples is similar to that described for the titania-based catalysts for methane oxidation.
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The method involves the controlled hydrolysis of an ethanolic solution of Zr-isopropoxide containing the nitrate salts of the other components necessary for preparing the mixed oxide. The gel was aged, filtered, washed, dried and calcined up to 550 ◦ C with a slow increase in temperature. After calcination and addition of Rh, the samples were activated by treatment with H2 (350 ◦ C) followed by mild reoxidation (200 ◦ C) in air. In all cases, XRD characterization shows the presence of only the tetragonal zirconia phase. The surface area of the pure zirconia is 70 m2 /g, while that containing 10% alumina is 120 m2 /g. The surface area of all samples containing 10% lanthanide ions ranges from 88 to 98 m2 /g, while the surface areas of the samples doped with 1 wt.% of the elements Ce, Sb, Y are 70, 105 and 65, respectively. 2.2. Catalytic tests Catalytic tests for methane combustion and N2 O decomposition were carried out in a quartz fixed-bed microreactor. The apparatus was equipped with an on-line mass–quadrupole system for the continuous analysis of the feed and the reaction products. The results were corrected to consider overlap in the fragmentation in the mass intensities and converted to concentration through calibration curves (normalized to total pressure in the quadrupole UHV chamber). Calibrated mixtures stored in cylinders allowed periodic recalibration of the system. In the catalytic tests of CH4 oxidation, the mass intensities of He (mass 4), CH4 (mass 15), CO (mass 28), O2 (mass 32) and CO2 (mass 44) were followed. In the catalytic tests of N2 O decomposition, the mass intensities of N2 O (mass 44), N2 (mass 28), NO (mass 30), NO2 (mass 46) and O2 (mass 32) were followed, together with those of other components such as H2 O, etc., in order to verify the correctness of the N balance. Using calibrated mixtures the mass intensities, after correction for the eventual overlaps for multiple fragmentations, were converted to concentrations. The feed was prepared by mixing calibrated amounts of already diluted mixtures of the single components in helium. Water was added to the feed using an infusion pump. The line to and from the reactor was heated at 150 ◦ C to avoid condensation of products.
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Tests were made using 0.2–0.4 g of catalyst in the form of particles with diameters in the order of 0.1–0.3 mm and a space–velocity of 30 h−1 . The uniform axial temperature profile of the catalytic bed was monitored using a thermocouple. Preliminary checks were made to ensure the absence of diffusional limitations on the reaction rate. The feed compositions were the following: (1) for methane 0.1% CH4 in air and (2) for nitrous oxide 0.05% N2 O + 6%O2 in He or 0.05% N2 O + 6% O2 + 2% H2 O in He, if not otherwise specified in the text. Electrocatalytic tests were made in a two cell reactor apparatus. One cell contains a purified 0.5 M solution of potassium bicarbonate in distilled water and is used as source of H+ ions for the CO2 reduction (they diffuse to the catalyst trough a Nafion® membrane). The counter electrode (platinum wire) and the reference electrode (saturated Ag/AgCl electrode) are located in this part of the electrocatalytic reactor. The other cell contains a mixture of 50% CO2 in He and is at the contact with the electrocatalyst. The Pt active phase (20 wt.% with respect to GDE) is located between the gas diffusion layer (GDE) at the direct contact with gas CO2 and the Nafion® membrane layer. A ring-type third Pt-electrode is at the contact with the GDE. The three-electrode system are connected to a potentiostat/galvanostat (Amel mod. 2049). Experiments are conducted galvanostatically at 22 ◦ C; electrode potential approximately 1.9922 V versus Ag(AgCl) and density 600 mA/cm2 . Analysis of the products is made by a gas chromatograph (GC) equipped with a TCD detectors and a gas–mass GC (MS–GC) apparatus. 2.3. Catalysts characterization The samples were characterized by XRD (Philips PW 1710 instrument with Cu K␣ radiation) in order to check the phase compositions and cell parameters. The dispersion and specific surface area of the noble metals was determined by H2 chemisorption at RT. The surface area was determined by the BET method (N2 adsorption at the liquid temperature) using a Micromeritics instrument. The catalyst redox index was determined using the following procedure. The samples were reduced in controlled mild conditions (200 ◦ C for 1 h with a flow of 10% H2 in He) and then the number of oxygen
vacancies formed in the catalyst was determined by the amount of N2 O necessary to reoxidize the samples at RT (flow of 0.05% N2 O in He). In calibration tests the negligible N2 O decomposition activity of the reoxidized catalyst in these conditions was verified. The relative catalyst redox index of 1% Rh on doped-zirconia samples was then calculated as the percentage of the additional moles of N2 O necessary to reoxidize the catalyst with respect to the reference sample (1% Rh on pure zirconia). 3. Results and discussion 3.1. Catalysts for treatment of waste gases containing diluted methane As outlined in the introduction, the objective of the study was to develop catalysts active in the oxidation of diluted methane feedstock and resistant to the typical poisoning present in the vent gas from coal mining, or produced during the disposal of solid waste. Pd is the most active element for methane combustion [2], but when supported on alumina it is quite sensitive to deactivation by sulphur compounds [2,9]. Titania is a good support which promotes resistance to deactivation by sulphur compounds [2], but suffers from low thermal stability and low dispersion of supported noble metals. In order to overcome these problems, titania was synthesized by the sol–gel method and 10% alumina was added as a structural promoter. As discussed elsewhere [8], the surface characteristics of the titania–alumina mixed oxide remain close to those of titania up to an addition of about 10–20 wt.% of alumina, but the alumina structural matrix improves the thermal stability with a consequent stabilization of the surface area and shifting of the anatase–rutile transition temperature to above 700 ◦ C. XRD data do not show the presence of crystalline alumina and SEM characterization data show a good homogeneous dispersion of aluminum ions and the absence of detectable separated phases. Furthermore, in order to improve the dispersion and activity of the catalyst, titania was modified by the addition of cerium ions. Fig. 1 shows that increasing the amount of cerium ions in Ti1−x Cex O2 promotes the low temperature activity of the catalyst in the oxidation of diluted methane, with a maximum activity
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Fig. 1. Combustion of diluted methane on Pd supported on TiO2 –CeO2 –Al2 O3 mixed oxides (10 wt.% alumina): effect of cerium content.
for a cerium content of about 0.2. XRD data do not show the presence of separate cerium phases, neither do SEM data provide evidence for separate cerium phases. The XRD peaks are broad due to the presence of alumina and thus reliable data on the effect of the cerium additions on the unit cell parameters of titania cannot be obtained. However, it should be noted that the ionic radius of cerium is consistent with the formation of a solid solution with TiO2 . It may be hypothesized therefore that at least in part cerium forms a solid solution with titania. A redox pretreatment of these catalysts, consisting of a sequence of cycles of mild reduction and reoxidation (see Section 2), improves the catalytic behavior in the low temperature methane oxidation as shown in Fig. 2. The redox pretreatment also has some positive influence on a reference commercial Pd/Al2 O3 catalyst having the same palladium content, but especially has a positive influence on the catalyst based on a TiO2 –CeO2 –Al2 O3 mixed oxide as the support. It also may be noted in Fig. 2 that the Pd(2%)/[(Ti0.8 Ce0.2 O2 )0.9 (Al2 O3 )0.1 ] catalyst after redox pretreatment shows a significant improvement in the light off temperature of methane combustion which occurs about 80 ◦ C lower than for the commercial catalyst supported on alumina after similar
pretreatment. Due to the presence of ceria, precise indications on the effect of the redox pretreatment on the dispersion of Pd were not obtained, but the data are consistent with an improvement in noble metal dispersion after the redox pretreatment. The use of a TiO2 –CeO2 –Al2 O3 mixed oxide as the support also improves the resistance to deactivation of the catalyst by sulphur-containing chemicals. Reported in Fig. 3 is the behavior of the Pd(2%)/[(Ti0.8 Ce0.2 O2 )0.9 (Al2 O3 )0.1 ] catalyst before and after accelerated deactivation by SO2 (label S) and for comparison, also the behavior of the reference commercial Pd/Al2 O3 catalyst before and after the same accelerated deactivation. In both cases, deactivation of the catalyst is noted, but the catalyst based on the TiO2 –CeO2 –Al2 O3 mixed oxide is more resistant to deactivation. In this case, the activity curve shifts to temperatures about 50–60 ◦ C higher, while for the Pd/Al2 O3 catalyst the shift is to temperatures about 100–150 ◦ C higher. Although accelerated deactivation tests provides only indicative data about behavior in the conditions and during life-time tests, the data suggest that TiO2 –CeO2 –Al2 O3 mixed oxides are an interesting support to develop new catalysts for treatment of waste gases containing diluted methane.
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Fig. 2. Combustion of diluted methane on Pd supported on TiO2 –CeO2 –Al2 O3 mixed oxides (10 wt.% alumina; Ti/Ce = 4) and on a reference commercial Pd/Al2 O3 sample (same 2 wt.% Pd content as the first sample): effect of a redox pretreatment (sequence of mild reduction and reoxidation steps, see Section 2).
Fig. 3. Combustion of diluted methane on Pd supported on TiO2 –CeO2 –Al2 O3 mixed oxides (10 wt.% alumina; Ti/Ce = 4) and on a reference commercial Pd/Al2 O3 sample (same 2 wt.% Pd content as the first sample): effect of an accelerated deactivation pretreatment (S) (see Section 2).
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3.2. Catalysts for conversion of N2 O from nitric acid plants Various catalysts for N2 O decomposition have been reported in the literature as discussed in detail elsewhere [1,4,5,10]. The different experimental conditions and often the absence of O2 and H2 O in the feed, make difficult comparison of the data and extrapolation of the results to more practical conditions. However, most of the authors agree in indicating Rh and Ru as the most active elements and that the nature of the support plays a relevant role. Zirconia is an interesting support for this purpose, because it plays a direct and synergetic role in the decomposition of N2 O [11–13]. Reported in Fig. 4 is the behavior of a sample of 1 wt.% Rh supported on a ZrO2 –Al2 O3 mixed oxide. The alumina (10 wt.%), similar to the case discussed before of TiO2 –Al2 O3 mixed oxides, was added to improve the thermal stability of zirconia. As indexes of catalytic behavior, the temperatures of 20 and 80% N2 O conversion in the presence of O2 and H2 O in the feed are reported. The catalytic behavior compares well with that of Rh ion-exchanged with ZSM-5 or supported on alumina, even though the Rh content is
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lower. On the contrary, the Ru/Al2 O3 catalyst shows worse performances. As mentioned before, to be cost-effective, a catalytic process for N2 O decomposition in the waste gases from nitric acid plants should operate at temperatures not higher than about 300 ◦ C. It is thus necessary to further promote the catalytic performance and especially to reduce the sensitivity of the catalyst to deactivation by water [14]. The main deactivation role of water is that of competitive adsorption and therefore modification of the acid–base properties of the support may serve to promote the catalytic performances. However, Brönsted acid sites have been shown to be important in determining the catalytic performance in N2 O decomposition [11,13] and therefore doping with alkaline metals is not possible. However, it has been previously noted [8] that the surface acidity strength of zirconia may be changed regularly by increasing the amount of alumina in ZrO2 –Al2 O3 mixed oxides, although up to contents of the second oxide equal to or lower than 20 wt.% no evidence of separate Al-containing phases was detected, nor of the presence of different types of Brönsted acid sites (index of surface heterogeneity). Therefore, a series of zirconia-based mixed oxides
Fig. 4. Temperature for obtaining 20% (T20) or 80% (T80) N2 O conversion on 1% Rh supported on a ZrO2 –Al2 O3 mixed oxide (10 wt.% alumina) (1% Rh/ZrO2 –Al2 O3 ), on two commercial catalysts of 2% Rh and 5% Ru supported on alumina and on a 4% Rh/ZSM-5 catalyst. Feed: 0.05% N2 O + 6% O2 + 2% H2 O in He, GHSV = 30 h−1 .
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Table 1 First-order rate constant of N2 O depletion at 275 ◦ C (feed: 0.05% N2 O+6% O2 in He) and at 350 ◦ C (feed: 0.05% N2 O+6% O2 +2% H2 O in He) for a series of 1% Rh on zirconia and zirconium-based mixed oxides Sample
Rh/ZrO2 Rh(1%)/[(ZrO2 )0.9 (Y2 O3 )0.1 ] Rh(1%)/[(ZrO2 )0.9 (CeO2 )0.1 ] Rh(1%)/[(ZrO2 )0.9 (Al2 O3 )0.1 ] Rh(1%)/[(ZrO2 )0.9 (Nd2 O3 )0.1 ] Rh(1%)/[(ZrO2 )0.9 (La2 O3 )0.1 ]
kN2 O (s−1 )
Table 2 First-order rate constant of N2 O depletion at 275 ◦ C (feed: 0.05% N2 O+6% O2 in He) and at 350 ◦ C (feed: 0.05% N2 O+6% O2 +2% H2 O in He) for a series of 1% Rh on doped (1 wt.%) zirconia samples Sample
6% O2 (275 ◦ C)
6% O2 + 2% H2 O (350 ◦ C)
0.86 1.00 0.82 6.96 9.44 5.03
1.45 2.34 2.07 2.07 5.32 10.52
were prepared and used as supports for 1% Rh; lanthanide oxides were also used due to their basic character properties. The results are summarized in Table 1 which reports the first-order rate constants of N2 O depletion at 275 ◦ C using a feed containing 0.05% N2 O and 6% O2 in helium and at 350 ◦ C when 2% H2 O is also present in the feed. A first-order rate of N2 O depletion and a plug-flow model for the fixed-bed reactor was assumed to calculate the rate constants. Due to the low N2 O concentration in the feed (0.05% v/v), these assumptions are usually valid for the decomposition of N2 O over a wide range of catalysts [5]. Their validity for rhodium-on-zirconia-based catalysts has also been verified [14]. The use of zirconia-based mixed oxide promotes the activity both in the absence and presence of water in the feed. However, while the effect is minimal for ceria and yttria, neodymia and lanthania significantly promote the activity with a promotion effect on the reaction rate of nearly one order of magnitude. The use of a zirconia–alumina mixed oxide as a support significantly promotes the activity in the absence of water in the feed, but the effect is much less remarkable in the presence of water in the feed. On the contrary, neodymia and lanthania significantly promote the activity in both cases, but the first is preferable in the absence of water in the feed and the second in the presence of water in the feed. The considerable improvement in the catalytic behavior using a mixed oxide instead of pure zirconia indicates that probably various factors contribute in determining the catalytic activity in N2 O decomposition. One possibility is that part of the lanthanide
Rh/ZrO2 Rh(1%)/[ZrY0.01 O2 ] Rh(1%)/[ZrSb0.01 O2 ] Rh(1%)/[ZrAl0.01 O2 ] Rh(1%)/[ZrCe0.01 O2 ]
kN2 O (s−1 ) 6% O2 (275 ◦ C)
6% O2 + 2% H2 O (350 ◦ C)
0.86 5.9 17.0 18.9 21.0
1.45 3.2 5.8 8.1 9.5
ions forms a solid solution with zirconia, similar to that discussed for ceria–titania. In order to verify this possibility, a series of samples were prepared by the sol–gel method, doping the zirconia with 1 wt.% of trivalent elements having an ionic radius comparable with that of Zr4+ ions. The results of these tests are summarized in Table 2. The doping of zirconia with 1 wt.% of trivalent elements considerably promotes the activity, although the effect is remarkable especially when water is absent from the feed. The effect is thus different from the previous case (Table 1) and is mainly related to an intrinsic promotion of the catalytic activity in N2 O decomposition more than a reduction of the inhibition effect of water. Reported in Fig. 5 is the effect of the amount of Ce3+ introduced as dopant in the ZrO2 . For all these samples, XRD analysis show no crystalline phases other than tetragonal zirconia [11], but does indicate a small change in the unit cell parameters consistent with the formation of a substitutional solid solution. The amount of dopant has a volcano-type influence on the catalyst reactivity in the decomposition of N2 O, with a maximum in activity for a dopant concentration of around 1.3% which corresponds to about a Zr4+ :Ce3+ = 1:0.01 molar ratio. The maximum in activity due to doping zirconia with Sb3+ ions is found roughly for the same level of cerium doping and also does not depend on the presence or absence of water in the feed. This indicates that the doping effect is related to promotion of the catalytic activity and not to reduction of the inhibition effect of water.
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Fig. 5. Catalytic behavior of 1% Rh on Ce3+ -doped zirconia in the decomposition of N2 O using a feed containing 6% O2 (temperature: 275 ◦ C) or 6% O2 and 2% H2 O (temperature: 380 ◦ C). Relative catalyst redox index: see text.
The relative catalyst redox index is also reported in Fig. 5. The substitution of zirconium ions with a trivalent ion may induce the formation of point defects (oxygen vacancies) due to charge balance, but when the amount of these defects is above a certain level, structural reconstruction to have more ordered (extended) defects can be expected. The maximum in catalytic behavior therefore can be associated with the formation of a defective zirconia containing localized oxygen vacancies which may transform at higher levels of doping to a more ordered defective zirconia. The presence of localized oxygen vacancies should increase catalyst reducibility in mild conditions. To check this hypothesis, catalyst reducibility was analyzed by reducing the catalysts in mild and controlled conditions (200 ◦ C) and determining the amount of oxygen vacancies in the catalyst by reoxidation at room temperature with N2 O. The relative catalyst redox index is determined by comparing the moles of N2 O necessary to reoxidize the catalyst at room temperature after the controlled reduction with H2 at 200 ◦ C with those necessary to reoxidize the reference sample (1% Rh on pure zirconia) after the same treatment. The index is thus proportional to
the effect of the dopant in promoting mild catalyst reducibility. Data in Fig. 5 indicate the good correspondence between this index of catalyst reducibility (related to the stabilization of oxygen point defects, as discussed above) and the catalytic activity in N2 O decomposition in stationary conditions both in the presence and absence of water in the feed. It may be concluded that the activity of Rh supported on zirconia can be promoted using two mechanisms: (i) stabilization of point oxygen vacancies making a defective zirconia by doping with small amounts of trivalent ions and (ii) modification of the surface properties (acid–base properties) using zirconia-based mixed oxides supports instead of pure zirconia. While the first mechanism promotes activity, the second especially reduces the sensitivity to deactivation by water chemisorption. The catalysts prepared in this way show a stable catalytic behavior. Reported in Fig. 6 is the catalytic behavior of a ZrO2 –Al2 O3 –La2 O3 mixed oxide (5 wt.% alumina, 5 wt.% lanthania) in N2 O conversion at 360 ◦ C using a feed containing N2 O, O2 , H2 O and NO (A) or a synthetic feed simulating typical nitric acid plant tail gas (B). In the latter case,
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Fig. 6. Catalytic behavior of a ZrO2 –Al2 O3 –La2 O3 mixed oxide (5 wt.% alumina, 5 wt.% lanthania) in N2 O conversion at 360 ◦ C using the following feeds: (A) 0.05% N2 O + 6% O2 + 4% H2 O + 0.1% NO in helium (space velocity: 30 h−1 ) and (B) synthetic mixture simulating nitric acid plant tail gas (0.05% N2 O + 3% O2 + 1% H2 O + 0.5% NO in nitrogen—space velocity: 43 h−1 ).
slight lowering of the performance with respect to the mixture already containing oxygen, water and NO is noted due to the slightly different feed and especially higher space–velocity, but the residual N2 O concentration remains below 200 ppm, a target value being the expected limit of regulations for N2 O emissions from nitric acid plants. It may be noted, however, that a reaction temperature of 360 ◦ C is required. This temperature is lower than that required by other catalysts based on Rh supported on alumina or zeolite using similar feedstock compositions (temperatures higher than 400–430 ◦ C are necessary for a comparable conversion using the same reaction conditions), but still too high with respect to the temperatures of 300 ◦ C or lower suggested by economic considerations. The catalyst activity may be increased using rhodium loadings above 1%, but the cost of the catalyst increases considerably. In conclusion, although Rh supported on modified zirconia shows a stable catalytic behavior in the decomposition of N2 O, further promotion of the catalyst reactivity is necessary, probably via a better understanding of the reaction mechanism of N2 O decomposition and the role of the support in promoting this behavior.
3.3. Conversion of CO2 to fuels The challenge for the reduction of CO2 back to fuels by using of solar energy may be addressed by the PEC of CO2 (see Section 1). The concept was originally proposed by “Hitachi Green Center” researchers [7]. The PEC reactor (Fig. 7) is composed on one side by a titania-based photocatalyst which oxidizes water to O2 by means of light. This process produces protons and electrons that are transported to the other side of the device by means of a membrane and an electric connection, respectively. On the other side of the PEC reactor, the protons and electrons react with CO2 in the presence of an electrocatalyst to produce fuels. Fig. 8 show some preliminary results which evidence that after 30 min of application of the potential (RT, ∼1 cm2 electrode, constant bias of 1.9922 V, static closed cell containing 50% CO2 at near atmospheric pressure) to two electrocatalysts based on Pt deposited on a gas diffusion electrode (GDE)/Nafion® membrane form significant amounts of light hydrocarbons and alcohols by electrocatalytic reaction of CO2 . In the full PEC reactor the H+ and e− are generated by solar light on the titania-side of the reactor (Fig. 7).
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Fig. 7. Schematic drawing of the photo-electrocatalytic (PEC) reactor.
The two catalysts reported in Fig. 8 are a commercial 20% Pt/GDE electrocatalyst (P20/GDE comm.; Pt/C 0.4 mg/cm2 , single side wet-proof GDE from E-TEK) and a second sample similar in composition, but containing some modifiers (Pt20/modif. GDE). This result indicates that the performances considerably depend
on the nature of the electrocatalyst and therefore the behavior may be further promoted. It should be noted that a large number of studies have addressed the question of CO2 reduction in liquid solutions (reaction suffering severe drawbacks in terms of low productivity and formation of not very
Fig. 8. Gas phase electrocatalytic reduction of CO2 over GDE/Pt/Nafion® electrocatalysts. Amount of products formed after 30 min of application of the potential (RT, ∼1 cm2 electrode, constant bias of 1.9922 V, static closed cell containing 50% CO2 at near atmospheric pressure).
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useful products such as oxalic and formic acids [15]), but very few studies have addressed the question of gas phase electrocatalytic CO2 conversion as the data shown in Fig. 8. Gas-diffusion electrodes based on noble metal supported on modified carbon cloths (Fig. 8) show some promising activity for the conversion of CO2 to a mixture of C1–C5 hydrocarbons (mainly alkanes) and alcohols avoiding at the same time the formation of H2 and CO byproducts when GDE is presaturated by CO2 . However, the electron efficiency is still too low. Quite difficult is also the development of stable photocatalysts for water splitting having a good photocurrent using visible light radiation (necessary to produce H+ and e− on the other side of PEC reactor, see Fig. 7). Titania modified by transition ion implantation [16] is effective in other photochemical reactions, but not for water splitting. Therefore, new strategies must be developed for developing these photocatalysts. The developments of better photo- and electrocatalysts are two challenges for catalysis in order to enable the efficient CO2 conversion back to fuels using solar light. An overall efficiency of about 10–15% with solar light would make the production of fuel using this approach economically competitive, besides the evident advantage in terms of reduction of CO2 emissions. Actual data are still far from this efficiency especially using visible radiation, but there are no theoretical limits in reaching this efficiency which requires the development of better anode and cathode catalysts.
4. Conclusions Catalytic technologies for the abatement of greenhouse gases can be an effective possibility for limiting the increasing tropospheric concentration of GGs and thus reducing their contribution to global warming, but the more extensive use of these technologies depends on the environmental regulations that will be adopted to control GG emissions from the variety of possible sources. Catalytic technologies should be useful in (i) reduction of anthropogenic emissions of non-CO2 GG (mainly N2 O and CH4 ) and (ii) reduction or conversion of CO2 .
The challenge for methane and nitrous oxide control by catalytic technologies is the development of more active catalysts at low temperature in the presence of the other typical components of the emissions. Control of the characteristics of the support is a key factor towards this objective of low temperature activity, although further work is necessary. In the reduction of CO2 emissions an interesting option is its conversion in flue gas back to fuels using solar energy and a PEC reactor. The developments of better photo- and electrocatalysts are two challenges for catalysis in order to enable efficient CO2 conversion using PEC technology. Catalysis may offer good prospects regarding the question of energy-efficient conversion of CO2 using renewable energy resources, but an intensified research effort in this direction is necessary.
Acknowledgements The financial support from EU within the contracts ENV4-CT95-0067 (N2O-ACT) and ERK6-CT-199900015 (COCON) of part of the activity reported here is gratefully acknowledged.
References [1] G. Centi, S. Perathoner, F. Vazzana, CHEMTECH 29 (12) (1999) 48. [2] G. Centi, J. Molec, Catal. A: Chem. 173 (2001) 287. [3] (a) C. Kroeze, Sci. Total Environ. 152 (1994) 189; (b) C. Kroeze, Sci. Total Environ. 143 (1994) 193. [4] Y. Li, J.N. Armor, Appl. Catal. B: Environ. 1 (1992) L21. [5] F. Kapteijn, J. Rodriguez-Morasol, J.A. Moulijn, Appl. Catal. B: Environ. 9 (1996) 25. [6] B.W. Riley, J.R. Richmond, Catal. Today 17 (1993) 277. [7] (a) R. Doi, Ryota, S. Ichikawa, H. Hida, Hiroshi, Jpn. Patent JP 08296077 (1995); (b) S. Ichikawa, Energy Convers. Manage. 36 (6–9) (1995) 613; (c) S. Ichikawa, R. Doi, Catal. Today 27 (1996) 271 (http:// www.rite.or.jp/English/welcome/About/csj/169403.html). [8] G. Centi, M. Marella, L. Meregalli, S. Perathoner, M. Tomaselli, T. La Torretta, in: W.R. Moser (Ed.), Advanced Catalysts and Nanostructured Materials, Academic Press, New York, 1996 (Chapter 4), p. 63. [9] J.K. Lampert, M.S. Kazi, R.J. Farrauto, Appl. Catal. B: Environ. 14 (1997) 211.
G. Centi et al. / Applied Catalysis B: Environmental 41 (2003) 143–155 [10] G. Centi, S. Perathoner, F. Vazzana, M. Marella, M. Tommaselli, M. Mantegazza, Adv. Environ. Res. 4/4 (2000) 325. [11] G. Centi, B. Panzacchi, S. Perathoner, F. Pinna, Stud. Surf. Sci. Catal. 130 (2000) 2273. [12] T.M. Miller, V.H. Grassian, Catal. Lett. 46 (1997) 213. [13] G. Centi, L. dall’Olio, S. Perathoner, J. Catal. 194 (2000) 130.
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[14] G. Centi, L. dall’Olio, S. Perathoner, Appl. Catal. A: Gen. 194/195 (2000) 79. [15] M.M. Halmann, M. Steinberg, Greenhouse Gas Carbon Dioxide Mitigation, CRC Press, Boca Raton, FL, 1999. [16] M. Anpo, Stud. Surf. Sci. Catal. 130 (2000) 157.
Reduction of greenhouse gas emissions by catalytic processes Gabriele Centi a,∗ , Siglinda Perathoner a , Zbigniew S. Rak b,1 a
Department of Industrial Chemistry and Engineering of Materials, Consortium for the Science and Technology of Materials (INSTM), University of Messina, Salita Sperone 31, 98166 Messina, Italy b Energy Research Center of The Netherlands (ECN), P.O. Box 1, 1755 ZG Petten, The Netherlands Received 17 January 2002; received in revised form 28 May 2002; accepted 29 May 2002
Abstract Catalytic technologies for the abatement of greenhouse gases (GGs) can be an effective possibility for limiting the increasing tropospheric concentration of GGs and reducing their contribution to global warming. Two different cases are discussed: (i) reduction of anthropogenic emissions of non-CO2 GGs (N2 O and CH4 ) and (ii) reduction or conversion of CO2 . In methane conversion waste gases containing diluted methane can be converted at low temperature using Pd supported on titania–ceria catalysts which show also a good resistance to deactivation. Rh supported on modified zirconia–alumina catalysts are effective and stable catalysts in low temperature decomposition of N2 O. The concept of reduction of CO2 back to fuels in a photo-electrocatalytic reactor is also presented. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Greenhouse gases; CO2 ; N2 O; Methane; Photoelectrocatalytic reactor
1. Introduction The Kyoto Protocol of 1997 on greenhouse gases (GGs) emissions has evidenced the necessity to control the emissions not only of CO2 , but also of CH4 and N2 O which contributed 7 and 9%, respectively, to the Global Warming Potential (GWP) (with reference to the CO2 equivalent emissions, using GWP values for a 100-year time horizon) [1]. Many factors determine the climate system and for many of them the level of understanding is poor. However, the precise correlation observed over the last 1000 years between change in the earth’s temperature and the atmospheric concentration of greenhouse ∗ Corresponding author. Fax: +39-090-391518. E-mail addresses: [email protected] (G. Centi), [email protected] (Z.S. Rak). 1 Fax: +31-224-568-615.
gases indicates a clear direct relationship. The projected climate change results in an estimated increase in the earth’s temperature of 1–6 ◦ C, depending on the models. The higher temperatures would be reflected in an increase in sea level, for example, of 0.1–0.9 m which may cause the flooding of large regions in the world. Although different strategies are still under discussion to mitigate the GG effects, the direct reduction of GG emissions is also necessary. The reduction strategies of GG emissions should be based on a wide range of options, due to the differences in the sources in terms of concentration, characteristics and localization. Catalytic technologies can offer cost-effective solutions for their control in several cases, although their potential application has not often been analyzed in detail. Two different cases are be discussed in this paper: (i) reduction of anthropogenic emissions of non-CO2
0926-3373/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 3 3 7 3 ( 0 2 ) 0 0 2 0 7 - 2
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GGs (mainly N2 O and CH4 ) and (ii) reduction or conversion of CO2 . Indeed, the different characteristics of the problems in the two cases imply the necessity of using different approaches to solve the two types of problems. Methane emissions derive from a wide variety of sources, the more relevant of which are sewage and solid waste disposal, rice cultivation, livestock, coal mining, and burning of biomass. Only part of these emissions can be effectively controlled by cleanup methods. Two specific cases may be addressed by catalytic technologies: (i) elimination of methane from drainage gas and ventilation air in coal measures before and during mining and (ii) vent gas produced during the disposal of solid waste [1]. In both cases, catalysts based on supported noble metals such as Pd can be used for these applications [2], but the characteristics of low light-off temperature should be promoted and especially the sensitivity to poisoning by the N- and S-compounds present in the off-gas should be reduced. N2 O anthropogenic emissions are about 2 orders of magnitude lower than methane emissions, but due to a higher GWP and higher level of natural emissions, the total greenhouse contribution of N2 O is higher on a CO2 -equivalent basis than that of methane [3]. Only part of these sources of N2 O may be addressed by catalytic technologies. The more relevant cases are the off-gas from chemical production (nitric acid synthesis or use) and combustion processes (especially, of solid waste at temperatures below 1000 ◦ C) [1], apart from the catalytic decomposition of N2 O in adipic acid plants which is already commercially applied. However, the case of adipic acid plants is characterized by high concentrations of N2 O (20–30% v/v or above), while in the case of nitric acid plant (more relevant case in terms of total emissions) the N2 O concentration is quite low (below 1–0.5%) and catalysts used in adipic acid plants are not sufficiently active [4–6]. The use of catalytic technologies for the reduction of CO2 emissions is a different case from those discussed regarding methane and nitrous oxide emissions, due to the (i) higher volume of emissions, (ii) different characteristics of the source and (iii) different requirements for its transformation. It should be noted that the expected rate of increase of CO2 emissions in China and the Developing Countries (mainly due to fossil fuel consumption) is a 100% increase in about two
decades. This increase cannot be compensated for by the expected decrease in CO2 emissions in developed countries (increase in efficiencies in use or energy and transport). Therefore, it is necessary to find specific solutions able to reintroduce CO2 into the chemical and energy cycles using renewable energies such as solar energy. A challenge for an energy-saving approach in the reduction of CO2 emissions is the use of solar energy for the conversion of CO2 back to fuels. This challenge may be addressed by the photo-electrocatalytic conversion (PEC) of CO2 . The concept was originally proposed by “Hitachi Green Center” researchers [7] and is currently under study in the framework of the EU project ERK6-CT-1999-00015. 2. Experimental 2.1. Catalyst preparation 2.1.1. Catalysts for combustion of methane Pd(2%)/[(Ti1−x Cex O2 )0.9 (Al2 O3 )0.1 ] (x = 0–0.20) catalysts were prepared by incipient wet impregnation with a Pd-acetate salt on a TiO2 –Al2 O3 –CeO2 mixed oxide prepared by the sol–gel method. The amount of Pd deposited was 2 wt.%. After drying at 110 ◦ C overnight, the catalysts were calcined at 550 ◦ C for 3 h. The surface area of all samples was in the 50–60 m2 /g range, and Pd dispersion, estimated by H2 chemisorption, was in the 6–15% range. After calcination and before the catalytic tests, a redox pretreatment (RP) was made on some samples. The RP was based on a sequence of cycles (at least three) of reduction at 350 ◦ C with a flow of 20% H2 in helium followed by reoxidation with air at 200 ◦ C. The code RP is added to the samples for which this pretreatment was made. All samples were conditioned in situ at 550 ◦ C before the catalytic tests using the same feed composition of the catalytic tests. In order to check resistance to deactivation by sulphur compounds, the following accelerated test procedure was used. The samples were treated at 500 ◦ C for 3 h with a feed with the same composition of the catalytic tests, but containing 10 ppm SO2 . After this treatment, the addition of SO2 to the feed was stopped and the catalyst activity tested again using the standard procedure. The catalysts after this treatment are labeled with the code S.
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The TiO2 –Al2 O3 –CeO2 mixed oxide support was prepared by the sol–gel method. The mixed oxide may be viewed as a titania modified by ceria and containing 10 wt.% of alumina as a structural promoter [8]. The X-ray diffraction (XRD) data indicate the presence of only a microcrystalline titania anatase phase without the presence of alumina reflection peaks. Also SEM characterization indicates a very good dispersion of alumina without the presence of islands or segregated phases more rich in alumina. The addition of alumina is needed to promote the thermal stability of titania [8] shifting the rutilization process to temperatures higher than 700 ◦ C. In the samples also containing ceria, the XRD data do not show the presence of ceria or Ce-containing crystalline phases, at least up to a ceria content of 20%. The SEM characterization also indicated a very good, homogeneous dispersion of cerium ions. The mixed oxide was prepared starting from a homogeneous anhydrous ethanol solution containing the isopropoxides of Ti and Al in the relative amounts to have a final content of alumina in the sample of 10 wt.%. The ratio of ethanol to isopropoxides was 5. For the samples containing cerium, part of the Ti-isopropoxide was substituted with the equivalent amount of Ce-nitrate in order to have the desired Ti/Ce ratio in the final sample. Then, the solution was carefully transformed first to sol and then to gel by drop-by-drop addition of a H2 O–CH3 COOH solution. The gel was aged, washed, slowly dried in an oven at 90 ◦ C and then calcined at 500 ◦ C. The Pd(2%)/Al2 O3 sample used as the reference catalyst is a commercial sample from Engelhard. 2.1.2. Catalysts for nitrous oxide decomposition Zirconia-based catalysts were synthesized using the sol–gel method for the preparation of the support, while Rh (1 wt.% in all samples) was added by incipient wet impregnation using a Rh(NO3 )3 ·H2 O aqueous solution on the calcined support. Other reference samples were commercial catalysts from Engelhard (2 wt.% Rh and 5 wt.% Ru on alumina) and a 4.0 wt.% Rh on ZSM-5 prepared by ion exchange in a RhCl3 aqueous solution using a commercial ZSM-5 from Alsi-Penta (SiO2 /Al2 O3 = 27). The sol–gel method of preparation of zirconia and modified zirconia samples is similar to that described for the titania-based catalysts for methane oxidation.
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The method involves the controlled hydrolysis of an ethanolic solution of Zr-isopropoxide containing the nitrate salts of the other components necessary for preparing the mixed oxide. The gel was aged, filtered, washed, dried and calcined up to 550 ◦ C with a slow increase in temperature. After calcination and addition of Rh, the samples were activated by treatment with H2 (350 ◦ C) followed by mild reoxidation (200 ◦ C) in air. In all cases, XRD characterization shows the presence of only the tetragonal zirconia phase. The surface area of the pure zirconia is 70 m2 /g, while that containing 10% alumina is 120 m2 /g. The surface area of all samples containing 10% lanthanide ions ranges from 88 to 98 m2 /g, while the surface areas of the samples doped with 1 wt.% of the elements Ce, Sb, Y are 70, 105 and 65, respectively. 2.2. Catalytic tests Catalytic tests for methane combustion and N2 O decomposition were carried out in a quartz fixed-bed microreactor. The apparatus was equipped with an on-line mass–quadrupole system for the continuous analysis of the feed and the reaction products. The results were corrected to consider overlap in the fragmentation in the mass intensities and converted to concentration through calibration curves (normalized to total pressure in the quadrupole UHV chamber). Calibrated mixtures stored in cylinders allowed periodic recalibration of the system. In the catalytic tests of CH4 oxidation, the mass intensities of He (mass 4), CH4 (mass 15), CO (mass 28), O2 (mass 32) and CO2 (mass 44) were followed. In the catalytic tests of N2 O decomposition, the mass intensities of N2 O (mass 44), N2 (mass 28), NO (mass 30), NO2 (mass 46) and O2 (mass 32) were followed, together with those of other components such as H2 O, etc., in order to verify the correctness of the N balance. Using calibrated mixtures the mass intensities, after correction for the eventual overlaps for multiple fragmentations, were converted to concentrations. The feed was prepared by mixing calibrated amounts of already diluted mixtures of the single components in helium. Water was added to the feed using an infusion pump. The line to and from the reactor was heated at 150 ◦ C to avoid condensation of products.
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Tests were made using 0.2–0.4 g of catalyst in the form of particles with diameters in the order of 0.1–0.3 mm and a space–velocity of 30 h−1 . The uniform axial temperature profile of the catalytic bed was monitored using a thermocouple. Preliminary checks were made to ensure the absence of diffusional limitations on the reaction rate. The feed compositions were the following: (1) for methane 0.1% CH4 in air and (2) for nitrous oxide 0.05% N2 O + 6%O2 in He or 0.05% N2 O + 6% O2 + 2% H2 O in He, if not otherwise specified in the text. Electrocatalytic tests were made in a two cell reactor apparatus. One cell contains a purified 0.5 M solution of potassium bicarbonate in distilled water and is used as source of H+ ions for the CO2 reduction (they diffuse to the catalyst trough a Nafion® membrane). The counter electrode (platinum wire) and the reference electrode (saturated Ag/AgCl electrode) are located in this part of the electrocatalytic reactor. The other cell contains a mixture of 50% CO2 in He and is at the contact with the electrocatalyst. The Pt active phase (20 wt.% with respect to GDE) is located between the gas diffusion layer (GDE) at the direct contact with gas CO2 and the Nafion® membrane layer. A ring-type third Pt-electrode is at the contact with the GDE. The three-electrode system are connected to a potentiostat/galvanostat (Amel mod. 2049). Experiments are conducted galvanostatically at 22 ◦ C; electrode potential approximately 1.9922 V versus Ag(AgCl) and density 600 mA/cm2 . Analysis of the products is made by a gas chromatograph (GC) equipped with a TCD detectors and a gas–mass GC (MS–GC) apparatus. 2.3. Catalysts characterization The samples were characterized by XRD (Philips PW 1710 instrument with Cu K␣ radiation) in order to check the phase compositions and cell parameters. The dispersion and specific surface area of the noble metals was determined by H2 chemisorption at RT. The surface area was determined by the BET method (N2 adsorption at the liquid temperature) using a Micromeritics instrument. The catalyst redox index was determined using the following procedure. The samples were reduced in controlled mild conditions (200 ◦ C for 1 h with a flow of 10% H2 in He) and then the number of oxygen
vacancies formed in the catalyst was determined by the amount of N2 O necessary to reoxidize the samples at RT (flow of 0.05% N2 O in He). In calibration tests the negligible N2 O decomposition activity of the reoxidized catalyst in these conditions was verified. The relative catalyst redox index of 1% Rh on doped-zirconia samples was then calculated as the percentage of the additional moles of N2 O necessary to reoxidize the catalyst with respect to the reference sample (1% Rh on pure zirconia). 3. Results and discussion 3.1. Catalysts for treatment of waste gases containing diluted methane As outlined in the introduction, the objective of the study was to develop catalysts active in the oxidation of diluted methane feedstock and resistant to the typical poisoning present in the vent gas from coal mining, or produced during the disposal of solid waste. Pd is the most active element for methane combustion [2], but when supported on alumina it is quite sensitive to deactivation by sulphur compounds [2,9]. Titania is a good support which promotes resistance to deactivation by sulphur compounds [2], but suffers from low thermal stability and low dispersion of supported noble metals. In order to overcome these problems, titania was synthesized by the sol–gel method and 10% alumina was added as a structural promoter. As discussed elsewhere [8], the surface characteristics of the titania–alumina mixed oxide remain close to those of titania up to an addition of about 10–20 wt.% of alumina, but the alumina structural matrix improves the thermal stability with a consequent stabilization of the surface area and shifting of the anatase–rutile transition temperature to above 700 ◦ C. XRD data do not show the presence of crystalline alumina and SEM characterization data show a good homogeneous dispersion of aluminum ions and the absence of detectable separated phases. Furthermore, in order to improve the dispersion and activity of the catalyst, titania was modified by the addition of cerium ions. Fig. 1 shows that increasing the amount of cerium ions in Ti1−x Cex O2 promotes the low temperature activity of the catalyst in the oxidation of diluted methane, with a maximum activity
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Fig. 1. Combustion of diluted methane on Pd supported on TiO2 –CeO2 –Al2 O3 mixed oxides (10 wt.% alumina): effect of cerium content.
for a cerium content of about 0.2. XRD data do not show the presence of separate cerium phases, neither do SEM data provide evidence for separate cerium phases. The XRD peaks are broad due to the presence of alumina and thus reliable data on the effect of the cerium additions on the unit cell parameters of titania cannot be obtained. However, it should be noted that the ionic radius of cerium is consistent with the formation of a solid solution with TiO2 . It may be hypothesized therefore that at least in part cerium forms a solid solution with titania. A redox pretreatment of these catalysts, consisting of a sequence of cycles of mild reduction and reoxidation (see Section 2), improves the catalytic behavior in the low temperature methane oxidation as shown in Fig. 2. The redox pretreatment also has some positive influence on a reference commercial Pd/Al2 O3 catalyst having the same palladium content, but especially has a positive influence on the catalyst based on a TiO2 –CeO2 –Al2 O3 mixed oxide as the support. It also may be noted in Fig. 2 that the Pd(2%)/[(Ti0.8 Ce0.2 O2 )0.9 (Al2 O3 )0.1 ] catalyst after redox pretreatment shows a significant improvement in the light off temperature of methane combustion which occurs about 80 ◦ C lower than for the commercial catalyst supported on alumina after similar
pretreatment. Due to the presence of ceria, precise indications on the effect of the redox pretreatment on the dispersion of Pd were not obtained, but the data are consistent with an improvement in noble metal dispersion after the redox pretreatment. The use of a TiO2 –CeO2 –Al2 O3 mixed oxide as the support also improves the resistance to deactivation of the catalyst by sulphur-containing chemicals. Reported in Fig. 3 is the behavior of the Pd(2%)/[(Ti0.8 Ce0.2 O2 )0.9 (Al2 O3 )0.1 ] catalyst before and after accelerated deactivation by SO2 (label S) and for comparison, also the behavior of the reference commercial Pd/Al2 O3 catalyst before and after the same accelerated deactivation. In both cases, deactivation of the catalyst is noted, but the catalyst based on the TiO2 –CeO2 –Al2 O3 mixed oxide is more resistant to deactivation. In this case, the activity curve shifts to temperatures about 50–60 ◦ C higher, while for the Pd/Al2 O3 catalyst the shift is to temperatures about 100–150 ◦ C higher. Although accelerated deactivation tests provides only indicative data about behavior in the conditions and during life-time tests, the data suggest that TiO2 –CeO2 –Al2 O3 mixed oxides are an interesting support to develop new catalysts for treatment of waste gases containing diluted methane.
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Fig. 2. Combustion of diluted methane on Pd supported on TiO2 –CeO2 –Al2 O3 mixed oxides (10 wt.% alumina; Ti/Ce = 4) and on a reference commercial Pd/Al2 O3 sample (same 2 wt.% Pd content as the first sample): effect of a redox pretreatment (sequence of mild reduction and reoxidation steps, see Section 2).
Fig. 3. Combustion of diluted methane on Pd supported on TiO2 –CeO2 –Al2 O3 mixed oxides (10 wt.% alumina; Ti/Ce = 4) and on a reference commercial Pd/Al2 O3 sample (same 2 wt.% Pd content as the first sample): effect of an accelerated deactivation pretreatment (S) (see Section 2).
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3.2. Catalysts for conversion of N2 O from nitric acid plants Various catalysts for N2 O decomposition have been reported in the literature as discussed in detail elsewhere [1,4,5,10]. The different experimental conditions and often the absence of O2 and H2 O in the feed, make difficult comparison of the data and extrapolation of the results to more practical conditions. However, most of the authors agree in indicating Rh and Ru as the most active elements and that the nature of the support plays a relevant role. Zirconia is an interesting support for this purpose, because it plays a direct and synergetic role in the decomposition of N2 O [11–13]. Reported in Fig. 4 is the behavior of a sample of 1 wt.% Rh supported on a ZrO2 –Al2 O3 mixed oxide. The alumina (10 wt.%), similar to the case discussed before of TiO2 –Al2 O3 mixed oxides, was added to improve the thermal stability of zirconia. As indexes of catalytic behavior, the temperatures of 20 and 80% N2 O conversion in the presence of O2 and H2 O in the feed are reported. The catalytic behavior compares well with that of Rh ion-exchanged with ZSM-5 or supported on alumina, even though the Rh content is
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lower. On the contrary, the Ru/Al2 O3 catalyst shows worse performances. As mentioned before, to be cost-effective, a catalytic process for N2 O decomposition in the waste gases from nitric acid plants should operate at temperatures not higher than about 300 ◦ C. It is thus necessary to further promote the catalytic performance and especially to reduce the sensitivity of the catalyst to deactivation by water [14]. The main deactivation role of water is that of competitive adsorption and therefore modification of the acid–base properties of the support may serve to promote the catalytic performances. However, Brönsted acid sites have been shown to be important in determining the catalytic performance in N2 O decomposition [11,13] and therefore doping with alkaline metals is not possible. However, it has been previously noted [8] that the surface acidity strength of zirconia may be changed regularly by increasing the amount of alumina in ZrO2 –Al2 O3 mixed oxides, although up to contents of the second oxide equal to or lower than 20 wt.% no evidence of separate Al-containing phases was detected, nor of the presence of different types of Brönsted acid sites (index of surface heterogeneity). Therefore, a series of zirconia-based mixed oxides
Fig. 4. Temperature for obtaining 20% (T20) or 80% (T80) N2 O conversion on 1% Rh supported on a ZrO2 –Al2 O3 mixed oxide (10 wt.% alumina) (1% Rh/ZrO2 –Al2 O3 ), on two commercial catalysts of 2% Rh and 5% Ru supported on alumina and on a 4% Rh/ZSM-5 catalyst. Feed: 0.05% N2 O + 6% O2 + 2% H2 O in He, GHSV = 30 h−1 .
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Table 1 First-order rate constant of N2 O depletion at 275 ◦ C (feed: 0.05% N2 O+6% O2 in He) and at 350 ◦ C (feed: 0.05% N2 O+6% O2 +2% H2 O in He) for a series of 1% Rh on zirconia and zirconium-based mixed oxides Sample
Rh/ZrO2 Rh(1%)/[(ZrO2 )0.9 (Y2 O3 )0.1 ] Rh(1%)/[(ZrO2 )0.9 (CeO2 )0.1 ] Rh(1%)/[(ZrO2 )0.9 (Al2 O3 )0.1 ] Rh(1%)/[(ZrO2 )0.9 (Nd2 O3 )0.1 ] Rh(1%)/[(ZrO2 )0.9 (La2 O3 )0.1 ]
kN2 O (s−1 )
Table 2 First-order rate constant of N2 O depletion at 275 ◦ C (feed: 0.05% N2 O+6% O2 in He) and at 350 ◦ C (feed: 0.05% N2 O+6% O2 +2% H2 O in He) for a series of 1% Rh on doped (1 wt.%) zirconia samples Sample
6% O2 (275 ◦ C)
6% O2 + 2% H2 O (350 ◦ C)
0.86 1.00 0.82 6.96 9.44 5.03
1.45 2.34 2.07 2.07 5.32 10.52
were prepared and used as supports for 1% Rh; lanthanide oxides were also used due to their basic character properties. The results are summarized in Table 1 which reports the first-order rate constants of N2 O depletion at 275 ◦ C using a feed containing 0.05% N2 O and 6% O2 in helium and at 350 ◦ C when 2% H2 O is also present in the feed. A first-order rate of N2 O depletion and a plug-flow model for the fixed-bed reactor was assumed to calculate the rate constants. Due to the low N2 O concentration in the feed (0.05% v/v), these assumptions are usually valid for the decomposition of N2 O over a wide range of catalysts [5]. Their validity for rhodium-on-zirconia-based catalysts has also been verified [14]. The use of zirconia-based mixed oxide promotes the activity both in the absence and presence of water in the feed. However, while the effect is minimal for ceria and yttria, neodymia and lanthania significantly promote the activity with a promotion effect on the reaction rate of nearly one order of magnitude. The use of a zirconia–alumina mixed oxide as a support significantly promotes the activity in the absence of water in the feed, but the effect is much less remarkable in the presence of water in the feed. On the contrary, neodymia and lanthania significantly promote the activity in both cases, but the first is preferable in the absence of water in the feed and the second in the presence of water in the feed. The considerable improvement in the catalytic behavior using a mixed oxide instead of pure zirconia indicates that probably various factors contribute in determining the catalytic activity in N2 O decomposition. One possibility is that part of the lanthanide
Rh/ZrO2 Rh(1%)/[ZrY0.01 O2 ] Rh(1%)/[ZrSb0.01 O2 ] Rh(1%)/[ZrAl0.01 O2 ] Rh(1%)/[ZrCe0.01 O2 ]
kN2 O (s−1 ) 6% O2 (275 ◦ C)
6% O2 + 2% H2 O (350 ◦ C)
0.86 5.9 17.0 18.9 21.0
1.45 3.2 5.8 8.1 9.5
ions forms a solid solution with zirconia, similar to that discussed for ceria–titania. In order to verify this possibility, a series of samples were prepared by the sol–gel method, doping the zirconia with 1 wt.% of trivalent elements having an ionic radius comparable with that of Zr4+ ions. The results of these tests are summarized in Table 2. The doping of zirconia with 1 wt.% of trivalent elements considerably promotes the activity, although the effect is remarkable especially when water is absent from the feed. The effect is thus different from the previous case (Table 1) and is mainly related to an intrinsic promotion of the catalytic activity in N2 O decomposition more than a reduction of the inhibition effect of water. Reported in Fig. 5 is the effect of the amount of Ce3+ introduced as dopant in the ZrO2 . For all these samples, XRD analysis show no crystalline phases other than tetragonal zirconia [11], but does indicate a small change in the unit cell parameters consistent with the formation of a substitutional solid solution. The amount of dopant has a volcano-type influence on the catalyst reactivity in the decomposition of N2 O, with a maximum in activity for a dopant concentration of around 1.3% which corresponds to about a Zr4+ :Ce3+ = 1:0.01 molar ratio. The maximum in activity due to doping zirconia with Sb3+ ions is found roughly for the same level of cerium doping and also does not depend on the presence or absence of water in the feed. This indicates that the doping effect is related to promotion of the catalytic activity and not to reduction of the inhibition effect of water.
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Fig. 5. Catalytic behavior of 1% Rh on Ce3+ -doped zirconia in the decomposition of N2 O using a feed containing 6% O2 (temperature: 275 ◦ C) or 6% O2 and 2% H2 O (temperature: 380 ◦ C). Relative catalyst redox index: see text.
The relative catalyst redox index is also reported in Fig. 5. The substitution of zirconium ions with a trivalent ion may induce the formation of point defects (oxygen vacancies) due to charge balance, but when the amount of these defects is above a certain level, structural reconstruction to have more ordered (extended) defects can be expected. The maximum in catalytic behavior therefore can be associated with the formation of a defective zirconia containing localized oxygen vacancies which may transform at higher levels of doping to a more ordered defective zirconia. The presence of localized oxygen vacancies should increase catalyst reducibility in mild conditions. To check this hypothesis, catalyst reducibility was analyzed by reducing the catalysts in mild and controlled conditions (200 ◦ C) and determining the amount of oxygen vacancies in the catalyst by reoxidation at room temperature with N2 O. The relative catalyst redox index is determined by comparing the moles of N2 O necessary to reoxidize the catalyst at room temperature after the controlled reduction with H2 at 200 ◦ C with those necessary to reoxidize the reference sample (1% Rh on pure zirconia) after the same treatment. The index is thus proportional to
the effect of the dopant in promoting mild catalyst reducibility. Data in Fig. 5 indicate the good correspondence between this index of catalyst reducibility (related to the stabilization of oxygen point defects, as discussed above) and the catalytic activity in N2 O decomposition in stationary conditions both in the presence and absence of water in the feed. It may be concluded that the activity of Rh supported on zirconia can be promoted using two mechanisms: (i) stabilization of point oxygen vacancies making a defective zirconia by doping with small amounts of trivalent ions and (ii) modification of the surface properties (acid–base properties) using zirconia-based mixed oxides supports instead of pure zirconia. While the first mechanism promotes activity, the second especially reduces the sensitivity to deactivation by water chemisorption. The catalysts prepared in this way show a stable catalytic behavior. Reported in Fig. 6 is the catalytic behavior of a ZrO2 –Al2 O3 –La2 O3 mixed oxide (5 wt.% alumina, 5 wt.% lanthania) in N2 O conversion at 360 ◦ C using a feed containing N2 O, O2 , H2 O and NO (A) or a synthetic feed simulating typical nitric acid plant tail gas (B). In the latter case,
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Fig. 6. Catalytic behavior of a ZrO2 –Al2 O3 –La2 O3 mixed oxide (5 wt.% alumina, 5 wt.% lanthania) in N2 O conversion at 360 ◦ C using the following feeds: (A) 0.05% N2 O + 6% O2 + 4% H2 O + 0.1% NO in helium (space velocity: 30 h−1 ) and (B) synthetic mixture simulating nitric acid plant tail gas (0.05% N2 O + 3% O2 + 1% H2 O + 0.5% NO in nitrogen—space velocity: 43 h−1 ).
slight lowering of the performance with respect to the mixture already containing oxygen, water and NO is noted due to the slightly different feed and especially higher space–velocity, but the residual N2 O concentration remains below 200 ppm, a target value being the expected limit of regulations for N2 O emissions from nitric acid plants. It may be noted, however, that a reaction temperature of 360 ◦ C is required. This temperature is lower than that required by other catalysts based on Rh supported on alumina or zeolite using similar feedstock compositions (temperatures higher than 400–430 ◦ C are necessary for a comparable conversion using the same reaction conditions), but still too high with respect to the temperatures of 300 ◦ C or lower suggested by economic considerations. The catalyst activity may be increased using rhodium loadings above 1%, but the cost of the catalyst increases considerably. In conclusion, although Rh supported on modified zirconia shows a stable catalytic behavior in the decomposition of N2 O, further promotion of the catalyst reactivity is necessary, probably via a better understanding of the reaction mechanism of N2 O decomposition and the role of the support in promoting this behavior.
3.3. Conversion of CO2 to fuels The challenge for the reduction of CO2 back to fuels by using of solar energy may be addressed by the PEC of CO2 (see Section 1). The concept was originally proposed by “Hitachi Green Center” researchers [7]. The PEC reactor (Fig. 7) is composed on one side by a titania-based photocatalyst which oxidizes water to O2 by means of light. This process produces protons and electrons that are transported to the other side of the device by means of a membrane and an electric connection, respectively. On the other side of the PEC reactor, the protons and electrons react with CO2 in the presence of an electrocatalyst to produce fuels. Fig. 8 show some preliminary results which evidence that after 30 min of application of the potential (RT, ∼1 cm2 electrode, constant bias of 1.9922 V, static closed cell containing 50% CO2 at near atmospheric pressure) to two electrocatalysts based on Pt deposited on a gas diffusion electrode (GDE)/Nafion® membrane form significant amounts of light hydrocarbons and alcohols by electrocatalytic reaction of CO2 . In the full PEC reactor the H+ and e− are generated by solar light on the titania-side of the reactor (Fig. 7).
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Fig. 7. Schematic drawing of the photo-electrocatalytic (PEC) reactor.
The two catalysts reported in Fig. 8 are a commercial 20% Pt/GDE electrocatalyst (P20/GDE comm.; Pt/C 0.4 mg/cm2 , single side wet-proof GDE from E-TEK) and a second sample similar in composition, but containing some modifiers (Pt20/modif. GDE). This result indicates that the performances considerably depend
on the nature of the electrocatalyst and therefore the behavior may be further promoted. It should be noted that a large number of studies have addressed the question of CO2 reduction in liquid solutions (reaction suffering severe drawbacks in terms of low productivity and formation of not very
Fig. 8. Gas phase electrocatalytic reduction of CO2 over GDE/Pt/Nafion® electrocatalysts. Amount of products formed after 30 min of application of the potential (RT, ∼1 cm2 electrode, constant bias of 1.9922 V, static closed cell containing 50% CO2 at near atmospheric pressure).
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useful products such as oxalic and formic acids [15]), but very few studies have addressed the question of gas phase electrocatalytic CO2 conversion as the data shown in Fig. 8. Gas-diffusion electrodes based on noble metal supported on modified carbon cloths (Fig. 8) show some promising activity for the conversion of CO2 to a mixture of C1–C5 hydrocarbons (mainly alkanes) and alcohols avoiding at the same time the formation of H2 and CO byproducts when GDE is presaturated by CO2 . However, the electron efficiency is still too low. Quite difficult is also the development of stable photocatalysts for water splitting having a good photocurrent using visible light radiation (necessary to produce H+ and e− on the other side of PEC reactor, see Fig. 7). Titania modified by transition ion implantation [16] is effective in other photochemical reactions, but not for water splitting. Therefore, new strategies must be developed for developing these photocatalysts. The developments of better photo- and electrocatalysts are two challenges for catalysis in order to enable the efficient CO2 conversion back to fuels using solar light. An overall efficiency of about 10–15% with solar light would make the production of fuel using this approach economically competitive, besides the evident advantage in terms of reduction of CO2 emissions. Actual data are still far from this efficiency especially using visible radiation, but there are no theoretical limits in reaching this efficiency which requires the development of better anode and cathode catalysts.
4. Conclusions Catalytic technologies for the abatement of greenhouse gases can be an effective possibility for limiting the increasing tropospheric concentration of GGs and thus reducing their contribution to global warming, but the more extensive use of these technologies depends on the environmental regulations that will be adopted to control GG emissions from the variety of possible sources. Catalytic technologies should be useful in (i) reduction of anthropogenic emissions of non-CO2 GG (mainly N2 O and CH4 ) and (ii) reduction or conversion of CO2 .
The challenge for methane and nitrous oxide control by catalytic technologies is the development of more active catalysts at low temperature in the presence of the other typical components of the emissions. Control of the characteristics of the support is a key factor towards this objective of low temperature activity, although further work is necessary. In the reduction of CO2 emissions an interesting option is its conversion in flue gas back to fuels using solar energy and a PEC reactor. The developments of better photo- and electrocatalysts are two challenges for catalysis in order to enable efficient CO2 conversion using PEC technology. Catalysis may offer good prospects regarding the question of energy-efficient conversion of CO2 using renewable energy resources, but an intensified research effort in this direction is necessary.
Acknowledgements The financial support from EU within the contracts ENV4-CT95-0067 (N2O-ACT) and ERK6-CT-199900015 (COCON) of part of the activity reported here is gratefully acknowledged.
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