Activity of rhodium-based catalysts for CO preferential ... - Springer Link

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Topics in Catalysis Vol. 45, Nos. 1–4, August 2007 (Ó 2007) DOI: 10.1007/s11244-007-0233-8

Activity of rhodium-based catalysts for CO preferential oxidation in H2-rich gases C. Galletti, S. Fiorot, S. Specchia*, G. Saracco, and V. Specchia Materials Science and Chemical Engineering Department, Politecnico di Torino, Corso Duca degli Abruzzi 24, 10129 Torino, Italy

The CO preferential oxidation (CO-PROX) process can lead to a reduction of the CO concentration in the hydrogen-rich gas from WGS process of hydrocarbons reformate down to at least 10 ppmv or below, so as to enable its direct feeding to standard PEM fuel cells. Rh-based catalysts supported on 3A zeolite, alumina, titania or ceria were prepared and tested for potential application in CO-PROX operating in a temperature range compatible with PEM FCs (80–100 °C). Among the prepared catalysts, 1% Rh-3A zeolite catalyst was found to be the most suitable one for the CO-PROX at low temperature: it reduced the inlet CO concentration below 10 ppmv within a temperature range of at least 80–120 °C without the appearance of undesirable side reactions. In order to improve the oxygen selectivity toward the CO complete oxidation with a lowest as possible hydrogen parasitic oxidation, the oxygen amount in the feed gas composition was decreased. With 1% Rh-3A zeolite catalyst the lowest O2 feed concentration was found to be 3 times the corresponding value of the CO stoichiometric oxidation (k = 2O2/CO = 3). Finally, with the goal to reduce the noble metal costs, tests at lower Rh load (from 1 to 0.5%) were carried out. No significant variations in both activity and width of complete CO conversion temperature range resulted for 0.5% Rh-3A catalyst. Therefore, this catalyst operating at k = 3 could potentially be used for the CO-PROX reaction at low temperature. KEY WORDS: hydrogen gas mixture clean-up; CO preferential oxidation; Rh-based catalyst.

1. Introduction Due to the perspectives of a significantly higher efficiency and of almost no emission of pollutants, the proton exchange membrane fuel cells (PEM FCs) have been extensively studied in the last two decades with the aim of employing them in many applications and especially for low emissions vehicles [1, 2]. Pure hydrogen is the ideal fuel for the PEM FCs. A number of research projects [3–6], however, are aimed at coupling an internal combustion engine (ICE) for traction power with a FC-based Auxiliary Power Unit (APU) for any other on board electric power requirement. The APU systems are based on a reforming process (either steam or autothermal [7–10]) of hydrocarbon feedstock fuels, integrated with a PEM FC on board vehicles. As extensive infrastructures already exist for gasoline and diesel oil, these fuels are the preferred sources of hydrocarbon feedstocks for on board reforming [11, 12]. The H2 fuel gas for PEM FC is required to be ‘‘nearly CO free’’ as FC platinum-based anodes get poisoned by traces of CO. The H2-rich gas produced by catalytic reforming of fuels, such as gasoline or diesel oil, followed by water gas shift (WSG) reaction, may go through catalytic selective CO oxidation to completely remove its CO content [1, 2, 13]. In order to drastically reduce the CO concentration and increase the one of hydrogen, the WGS reactors are

* To whom correspondence should be addressed. E-mail: [email protected]

placed immediately downstream the reformer. The WGS reaction CO + H2 O $ CO2 + H2

DH298 =- 41.2 kJ/mol

is generally operated at two different temperature steps (high temperature, HT, and low temperature, LT, water gas shift) to minimize the overall amount of catalyst required: the CO content of the hydrogen-rich gas stream is reduced to about 0.5–1% (5000–10000 ppm) [14], still far above the 10 ppmv tolerance limit of a typical anode catalyst, so that a third process unit is required. Preferential oxidation of carbon monoxide to carbon dioxide (CO-PROX) is a widely studied option to achieve these residual CO levels. During the CO-PROX process, the hydrogen of the reformate stream (concentration downstream the WGS reactions: 30–60% b.v. depending on the reforming process adopted) should not be consumed at all to ensure reasonable fuel efficiency. A decrease in hydrogen concentration results indeed in a decrease in power generation. So, an efficient and selective catalyst is required for the CO abatement while minimizing the hydrogen oxidation to water. The literature reports several studies on catalysts for the selective oxidation of CO, including noble metals (Pd, Ru, Rh, Pt) [15–18] and base metals (Co–Cu, Ni– Co–Fe, etc.) [18–20] generally supported on alumina and zeolite. Supported platinum catalysts have probably been the most investigated so far [15, 16, 18, 21, 22]. Oh and Sinkevitch [18] in particular carried out CO-PROX tests with alumina supported noble metals 1022-5528/07/0800-0015/0 Ó 2007 Springer Science+Business Media, LLC

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C. Galletti et al./Rhodium-based catalysts for co preferential oxidation in h2-rich gases

(Rh, Pt and Pd). They found that Rh/Al2O3 catalysts were more active for CO oxidation and reached a complete CO conversion at temperatures lower than those for Pd/Al2O3 and the most-widely used Pt/Al2O3 catalysts. In addition, Rh/Al2O3 catalyst was the most selective among all the catalysts tested. Therefore, in this work Rh supported catalysts on c-Al2O3, TiO2, CeO2 and 3A zeolite were developed in order to be tested for CO-PROX unit operating at temperatures compatible with PEM FCs [23].

2. Experimental 2.1. Catalyst preparation and characterization The metal oxide supports (c-Al2O3, TiO2, CeO2) were prepared through the solution combustion synthesis (SCS) technique [24]; conversely, 3A-type zeolite (K12[(AlO2)12(SiO4)12]ÆH2O) was purchased from Fluka. The supported Rh catalysts were prepared with the incipient wetness impregnation method by using Rh(NO3)3 as precursor; the rhodium nitrate was dissolved in distilled water and the solution was added drop by drop over the support meanwhile thoroughly mixing the whole mass. The mixture was then placed in an oven at 200 °C to evaporate water. The catalyst powders, after grinding in an agate mortar, were calcined in pure oxygen for 2 h at 350 °C to remove the nitrate ions and, in turn, to form a rhodium oxide, which was then reduced in pure hydrogen atmosphere for 2 h at 350 °C. The obtained Rh-based catalysts were analyzed by high-resolution transmission electron microscopy (HRTEM, Jeol JEM 2010 apparatus) to investigate the metal dispersion on the supports. They were also examined by scanning electron microscopy and energy dispersion spectroscopy (SEM-EDS, LEO Supra 35) to evaluate their morphology and elemental distribution. Temperature programmed desorption (TPD) of H2 was performed on 1% Rh-3A and 0.5% Rh-3A catalysts by a TPD/R/O apparatus (Thermoquest TPD/R/O 1100 analyser equipped with a Baltzer Quadstar 422 quadrupole mass spectrometer). The catalysts were pre-treated by flowing 50 Ncm3 min)1 of O2 and 50 Ncm3 min)1 of H2 at 500 °C for 1 h. After cooling to ambient temperature, H2 desorption was performed in 20 Ncm3 min)1Ar flow by heating each catalyst up to 600 °C at a rate of 10 °C/min and then maintaining this temperature for 40 min. 2.2. Reactor system and analytical methods The catalyst pellets were obtained by pressing at 125 MPa the powders into tablets, which were then crushed and sieved to produce 0.25–0.42 mm granules. A fixed bed micro-reactor (a Pyrex tube of 4 mm I.D.), containing 0.15 g of catalyst pellets sandwiched between

two glass–wool layers, was placed in a PID regulated oven, and a K-type thermocouple was inserted in the packed bed for its temperature measurements. The inlet gas was fed at a flow rate of 100 ml/min and with the following b.v. standard composition: 37% H2, 5% H2O, 18% CO2, 1% CO, 2% O2 and helium as balance. This composition (helium apart) is representative of a typical LT-WGS outlet reactor composition following a gasoline autothermal reformer [3]. The outlet gas stream was analyzed through a gas-chromatograph (Varian CP-3800) equipped with a thermal conductivity detector (TCD), a ‘‘Poraplot Q’’ column (0.53 mm diameter, 30 m length) to separate CO2 and H2O, and a ‘‘Molsieve 5A’’ column (0.53 mm diameter, 25 m length) to separate CO, H2 and O2. The two columns, connected in series by a six-way valve, were kept at 70 °C; the sample injection was accomplished using helium as carrier gas at a flow rate of about 2.8 ml/min. The CO detection limit was 10 ppmv. The conversion of CO (nCO) and O2 (nO2), as well as the O2 selectivity towards CO oxidation (rCO), determined in the 50–250 °C range, at different O2 to CO feed ratio (k) and with a weight space velocity WSV = 0.66 Nl/min/gcat, were calculated as follows:

nCO ¼ 1

½COout ½COin

nO 2 ¼ 1

½O2 out ½O2 in

rCO ¼

1 ½COin ½COout 2 ½O2 in ½O2 out

3. Results and discussion A preliminary screening of the catalytic performance of Rh-based catalysts, in terms of nCO and rCO, was carried out on TiO2, Al2O3, CeO2 and 3A zeolite supports, and the obtained results are shown in figure 1. For the 1% Rh-Al2O3 catalyst, nCO value equal to one was observed only at 80 °C. Conversely, very low nCO values were observed with ceria support at any tested temperature. Moreover, a complete CO conversion was not reached with the titania supported catalyst: the maximum nCO value was obtained at 100 °C with a residual CO outlet concentration of about 687 ppmv. The best activity was obtained with 1% Rh-3A zeolite for which nCO equal to one was observed in 80–140 °C temperature range; in addition, no methanation occurred when operated with this catalyst.


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Figure 2. CO outlet concentration in catalytic activity tests carried out on 1% Rh-3A.

Figure 1. CO selectivity and conversion vs. temperature for 1% Rh on different supports (standard feed gas composition, k = 4 and WSV = 0.66 Nl/min/gcat).

Only for 1% Rh-3A and 1% Rh-cAl2O3 the maximum obtained rCO value for k = 4 was 25%; this occurred in the temperature range where both nCO and nO2 were practically equal to 1. The catalytic activity loss at temperatures higher than 140 °C was due to the presence of RWGS reaction; to check the occurrence of this reaction, the Rh-catalyst micro-reactor was fed with a gas mixture without CO and the CO appearance was detected. Figure 2 shows, as an example, the CO outlet concentrations obtained with 1% Rh-3A catalyst using the standard and the CO-free inlet gas mixture. As the main purpose of the study was to develop a suitable catalyst for CO-PROX operating in a temperature range compatible with that of PEM FCs, and, in addition, without undesirable side reactions, the 1% Rh3A catalyst was considered as the best catalyst and employed in the further tests. In order to improve the oxygen selectivity toward CO oxidation, when nCO becomes equal to one, with a lowest as possible hydrogen parasitic oxidation, the oxygen amount in the feed gas composition was decreased. The obtained results in terms of both nCO and rCO are shown in figure 3. The maximum CO conversion observed at stoichiometric O2 condition (0.5%, k = 1) was 78.8% obtained at 140 °C. By doubling the O2 concentration (1%; k = 2), nCO = 1 was reached in the temperature range 120–140 °C. With a further O2 concentration increase to 1.5% (k = 3), nCO remained equal to one from 80 to 120 °C, showing a wider tem-

Figure 3. CO selectivity and conversion vs. temperature for 1% Rh3A at different k values (standard feed gas composition, WSV = 0.66 Nl/min/gcat).

perature range as compared to that for k = 2. When using k = 4 the temperature range for nCO = 1 did not further increase. A comparison between nCO and rCO curves shows that with stoichiometric conditions it is not possible to reach a complete CO oxidation, also with the highest tested rCO values; this is of course not suitable for a gas to be fed to PEM FCs. To this goal, only operative conditions leading to nCO = 1 are to be worked out: this has been reached by increasing the O2 concentration to values higher than the stoichiometric one (k > 1). As a consequence, the excess O2 reacts with the H2 present in the stream, reducing in some extent its rate available for the PEM FC. In the conditions where nO2 = 1, the increase of O2 concentration leads to a decrease of rCO and an increase of the temperature range where nCO = 1. The maximum selectivity was rCO = 0.50 for k = 2, reduced to rCO = 0.333 at k = 3 and then to

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rCO = 0.25 at k = 4. As a consequence, when increasing the O2 concentration from 1.5 to 2% (i.e. k from 3 to 4) only a further decrease of rCO without beneficial increase in the temperature range of complete CO conversion was obtained. Then, with the 1% Rh-3A zeolite catalyst the optimum feed O2 concentration (giving a temperature range of 40 °C with nCO = 1, sufficiently wide for a reliable control of the PROX reactor) is that corresponding to k (2O2/CO) value equal to three. Moreover, the rapid increase of CO conversion up to completeness in a range of only 20 °C (see figure 3) is certainly due to a typical ignition phenomenon, which generally involves mass and heat transfer effects. In any case, the earlier the ignition, the higher is the catalyst activity. Another important point for the developed catalyst concerns the considerable cost of Rh precursor (the Rh market price is considerably higher than that of the Pt: 2441.09 US$/oz versus 917.42 US$/oz in 2005 [25]). Therefore, the effect of noble metal load on catalyst activity was investigated at k = 3; tests on catalysts with 1% and 0.5% rhodium on zeolite 3A were carried in terms of nCO and rCO. Again, the 0.5% Rh-supported catalyst was prepared with the IWI impregnation method. The results are shown in figure 4. nCO = 1 was reached in the temperature range of 80– 120 °C and 100–140 °C for the 1% Rh-3A and the 0.5% Rh-3A catalysts, respectively. Therefore, both catalysts showed complete CO conversion for the same temperature range of 40 °C, but with a shift of the 0.5% Rh-3A catalyst to a slightly higher temperature level compared to the 1% Rh-3A one. Substantially, a part the temperature level, no significant variations in both activity

Figure 4. CO selectivity and conversion vs. temperature for 1% Rh3A and 0.5% Rh-3A catalysts (standard feed gas composition, k = 3 and WSV = 0.66 Nl/min/gcat).

and temperature range width for nCO = 1 resulted for 0.5% Rh-3A catalyst as compared to 1% Rh-3A one. Moreover, with different Rh load, the same maximum selectivity (33%) working with k = 3 was reached in the temperature range where CO conversion was complete. In the range 140–200 °C the 0.5% Rh-3A catalyst, however, seems to show a slightly higher CO conversion than the 1% Rh-3A one (see figure 4). Both the different Rh dispersion (observed by HRTEM analysis, see figure 5) and Rh load decrease on the catalysts could be responsible for these results. Moreover, the catalyst with the lower Rh load inhibited much more parasite reactions. This could explain the larger CO conversion decrease at high temperature ( > 140 °C) for the 1% Rh-3A catalyst and the shift of the complete CO conversion range to higher temperatures observed for the 0.5% Rh-3A one. Further, to better evaluate the Rh dispersion, TPD analyses with H2 were carried out on 0.5% Rh-3A and 1% Rh-3A catalysts. The 0.5% Rh-3A catalyst presented a large number of different nature active sites and a good dispersion on support, whereas the 1% Rh-3A curve showed the presence of hydrogen active adsorption sites of the same nature.

4. Conclusions CO-PROX tests carried out under realistic conditions with Rh catalysts supported on 3A zeolite, alumina, titania and ceria, indicated the 1% Rh-zeolite 3A as the most suitable catalyst for low temperature CO-PROX (compatible with a PEM FC). This catalyst reduced the CO concentration to below 10 ppmv within a temperature range of at least 80–140 °C for k = 4, during which no methanation reactions was detected. In order to improve the oxygen selectivity toward CO oxidation, in condition of complete CO oxidation, the k ratio (2O2/CO) was reduced. For 1% Rh-zeolite 3A catalyst, a k value of 3 was found to give the complete CO conversion in the temperature range of 80–120 °C, sufficiently wide for a reliable control of the PROX reactor. With the aim to reduce the catalyst cost, a test with lower Rh load (0.5%) was carried out. No significant change in the catalytic activity appeared: the load reduction did not affected the temperature range width for nCO = 1. There was only a slight shift to higher temperature values than for 1% Rh-3A catalyst, probably due to the lower Rh load that inhibited the parasite reactions shifting them to higher temperatures. Therefore, the 0.5% Rh-3A catalyst operating at k = 3 could potentially be used for low temperature CO-PROX applications: it was able to reduce the inlet CO concentration below 10 ppmv within a temperature range of 100–140 °C (compatible with operating PEM


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Figure 5. HRTEM micrographs of the Rh dispersion over the catalysts 1% Rh-3A (figure 5 (a)) and 0.5% Rh-3A (figure 5(b)).

FCs temperature), with no appearance of methanation reaction and, in addition, at a possibly more acceptable catalyst cost.

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