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Article

Hydrogen Production by Steam Reforming of Ethanol on Rh-Pt Catalysts: Influence of CeO2, ZrO2, and La2O3 as Supports

1
Energy, Materials and Environment Laboratory, Department of Chemical Engineering, Universidad de La Sabana, Campus Universitario Puente del Común, Km. 7 Autopista Norte, Bogotá 250001, Colombia
2
Department of Chemical Engineering, Universidad de Alicante, P.O. Box 99, Alicante E-03080, Spain
*
Author to whom correspondence should be addressed.
Catalysts 2015, 5(4), 1872-1896; https://doi.org/10.3390/catal5041872
Submission received: 7 October 2015 / Accepted: 2 November 2015 / Published: 11 November 2015

Abstract

:
CeO2-, ZrO2-, and La2O3-supported Rh-Pt catalysts were tested to assess their ability to catalyze the steam reforming of ethanol (SRE) for H2 production. SRE activity tests were performed using EtOH:H2O:N2 (molar ratio 1:3:51) at a gaseous space velocity of 70,600 h−1 between 400 and 700 °C at atmospheric pressure. The SRE stability of the catalysts was tested at 700 °C for 27 h time on stream under the same conditions. RhPt/CeO2, which showed the best performance in the stability test, also produced the highest H2 yield above 600 °C, followed by RhPt/La2O3 and RhPt/ZrO2. The fresh and aged catalysts were characterized by TEM, XPS, and TGA. The higher H2 selectivity of RhPt/CeO2 was ascribed to the formation of small (~5 nm) and stable particles probably consistent of Rh-Pt alloys with a Pt surface enrichment. Both metals were oxidized and acted as an almost constant active phase during the stability test owing to strong metal-support interactions, as well as the superior oxygen mobility of the support. The TGA results confirmed the absence of carbonaceous residues in all the aged catalysts.

Graphical Abstract

1. Introduction

The growing awareness of environmental and energy issues has led to a search for alternative energy sources. Hydrogen, which produces electricity and heat with a high efficiency in fuel cells, is a clean energy source with a higher energy density (141.6 kJ/g) than hydrocarbon fuels [1]. Bioethanol, produced from biomass resources, is an environmentally-sustainable feedstock and, thus, an excellent candidate for industrial-scale hydrogen production through the steam reforming process [2]. Therefore, many countries have invested in bioethanol production. For example, Colombia (South America) has made large investments in bioethanol production via sugar cane fermentation. The bioethanol produced in this way (50 vol % ethanol after flash separation) could be transformed into H2 by steam reforming for subsequent use in fuel cells.
Steam reforming of ethanol (SRE) for hydrogen production is an endothermic process; Equation (1) gives a simplified representation of this process [3,4]. The SRE can also be represented by a network of reactions (Equations (2)–(10)) [5,6,7], depending on the reaction conditions and the catalyst used, to reflect the formation of a complex set of by-products including carbon monoxide (CO), methane (CH4), ethane (C2H6), ethylene (C2H4), and acetaldehyde (CH3CHO) [4,8,9].
CH 3 CH 2 OH   +   3   H 2   2   CO 2   +   6   H 2   H 298  K o   =   174   kJ/mol
CO   +   H 2   CO 2   +   H 2   H 298  K o =   -41   kJ/mol
H 3 CH 2 OH   CH 3 CHO +  H 2
CH 3 CH 2 OH   CH 4 + CO +  H 2
CH 3 CHO   CH 4 + CO
CH 4   +  H 2  CO +   3 H 2
CH 3 CHO + H 2  CH 4 + CO 2 +  H 2
CH 3 CH 2 OH   C 2 H 4 +  H 2 O
C 2 H 4 + 2 H 2     2 CH 4
C 2 H 4 + 2 H 2  CH 4 + CO 2 + 2 H 2
Among these reactions, one of the most important is the water gas shift reaction (WGSR, Equation (2)), which favors CO2 formation with increasing H2 and decreasing CO concentrations in the outlet stream [5]. For low-temperature proton exchange membrane fuel cell (PEMFC) applications, a low CO concentration (<100 ppm) is required. The by-product formation, including CO, CH4, C2H4, C2H6, and carbon deposits, should be minimized to maximize H2 production.
The ability of base metals, such as Ni [5,7,10,11], Cu [3,7], Al [12], Zn [13], and Co [14,15,16,17], and noble metals, such as Pt [18], Rh [19,20,21,22], Pd [23], Ru [24], and Ir [25], supported on various metal oxides to catalyze the SRE has been widely investigated. Ethanol activation pathways depend on the metal nature, and can be generally divided in two groups: the less-oxophilic metals (Pd and Pt) in which α-C–H activation takes place, and the more-oxophilic metals (Co, Ni, Rh, Ru) that promote activation via O–H. Despite its higher cost, noble metals are the best catalyst candidates for SRE due to their greater ability to break C–C bonds, requiring less active metal loading compared to non-noble metals [26]. Likewise, the use of bimetallic catalysts is a strategic option to modify the electronic properties of metal surfaces [27]. By allowing less- and more-oxophilic metals it could be expected changes in the catalytic properties, probably obtaining a more active and resistance catalyst. This is going to be important when real bioethanol samples will be reformed, in which some impurities can weaken catalyst performance [28]. Among the noble metals, Pt and Rh exhibited high activity for breaking the ethanol C–C bond and achieving catalytic WGS conversion, respectively [29,30]. Hence, a very resistant catalyst might be expected.
However, in addition to the catalyst itself, the catalyst support could also influence the activity and stability of the catalyst [31]. Due to the low metal loading when using noble metals (typically < 1%), the direct participation of the support on the reaction mechanism is favored [27]. As was recently pointed out by Zanchet et al. [27] in the case of SRE, one point of common agreement is that the oxygen mobility of the support (OSC) is a critical property to favor C removal and avoid deactivation. Thus, higher oxygen storage capacity and oxygen mobility, allow a more effective gasification/oxidation of adsorbed carbon on the surface. These properties can be conferred by reducible supports such as CeO2, ZrO2, and La2O3 [32]. Among them, cerium oxides have been proposed as catalytically active components for the SRE because of their high oxygen mobility, ability to promote ethanol and water dissociation, ability to disperse metal catalytic components, and strong metal-support interactions that can minimize metal particle sintering [22,33,34]. Favorable reducibility and thermal stability make zirconium oxides potential candidates for catalyst supports [35,36]. Similarly, lanthanum oxides were investigated as promising supports because of their thermal stability and diverse crystalline phases [6,30]. Nevertheless, despite the efforts undertaken to understand the structure and properties of supported bimetallic catalysts, there is a broad set of variables that could be deeply explored concerning the specific conditions of SRE. Catalyst deactivation and the loss of catalytic activity and/or selectivity over time are of great and continuing concern in hydrogen production from ethanol. Regardless of the metal and support used, stability remains a challenge for catalyst design in ethanol reforming [37].
This study aimed to identify an active and stable catalyst that enables the use of the SRE for hydrogen production, for its further use in raw bioethanol samples. For this purpose, a bimetallic 0.6%Rh–0.2%Pt catalyst supported on CeO2, ZrO2, or La2O3 was selected as a potential catalytic material. Comparing these three supports under the same reaction conditions under kinetic control allowed the selection of the most promising RhPt bimetallic catalyst for further development. A stoichiometric steam/ethanol (S/E) mole ratio of three was employed in all experiments to simulate the bioethanol concentration after flash separating in the commercial samples procured from Colombia and to avoid the presence of dilution effects by water excess [38]. The catalysts were characterized using various techniques, including thermogravimetric analysis (TGA), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). Both reduced and aged catalysts were analyzed to gain some insight into the catalytic mechanism.

2. Results and Discussion

2.1. Catalytic Activity and Selectivity of Reduced Catalysts

Catalytic activity tests were conducted from 400 to 700 °C for both reduced (R) and aged (A) RhPt/CeO2, RhPt/ZrO2, and RhPt/La2O3, avoiding as much as possible, mass transfer limitations in the system by the use of an elevated gas hourly space velocity (GHSV) [39]. In addition, the stoichiometric S/E = 3 was employed to simulate bioethanol samples after flash separation. Despite higher water contents increase the thermodynamic favorability to hydrogen production [30], the dilution effects by the water excess can increase the energy consumption [38], which is undesirable to larger scale operations. On the contrary, sub-stoichiometric ratios (S/E < 3) increase coke formation [38]. Hence, S/E = 3 was selected as adequate to test the catalysts.
Figure 1a shows the ethanol conversion by the reduced catalysts and the blank as a function of temperature. No important differences in catalytic activity were observed between the blank test with the empty reactor and those in the presence of only the supports, and total conversion occurred above 600 °C. The blank test produced 0.81 mol H2/mol ethanol inlet at 700 °C (Figure 1b), consistent with the dominance of decomposition reactions instead ethanol reforming in the absence of a catalyst [40,41].
Figure 1. (a) Ethanol conversion (XEtOH) and (b) H2 yield as a function of temperature for reduced (R) and aged (A) RhPt/CeO2, RhPt/ZrO2, and RhPt/La2O3. Reaction conditions: S/E = 3 and GHSV = 70600 h−1. Aged samples obtained after 27 h TOS at 700 °C.
Figure 1. (a) Ethanol conversion (XEtOH) and (b) H2 yield as a function of temperature for reduced (R) and aged (A) RhPt/CeO2, RhPt/ZrO2, and RhPt/La2O3. Reaction conditions: S/E = 3 and GHSV = 70600 h−1. Aged samples obtained after 27 h TOS at 700 °C.
Catalysts 05 01872 g001
Regarding the supports, CeO2 and La2O3 showed higher H2 yield compared to ZrO2 above 500 °C. Table 1 shows the distributions of the main products (H2, CO, and CO2) and by-products (CH4 and ethylene) in the outlet stream at 700 °C (total ethanol conversion conditions) for the different catalyst samples. Acid-basic properties, reducibility, and oxygen storage capacity of the supports are intrinsic characteristics related to the catalytic performance [27]. Acid sites in the support could promote ethanol dehydration to ethylene (Equation (8)). In contrast, basic sites and high oxygen electronic polarizability of the support are favorable for the dehydrogenation (Equation (3)) [37]. Although ceria, zirconia, and lanthana are recognized as basic supports, they might promote different SRE mechanisms alone and in combination with Rh-Pt active metals. In general, the supports produced H2, CH4, and CO2, which can be related to the occurrence of hydrogenation/dehydrogenation reactions from the adsorbed ethoxy-species. O-vacancies present in CeO2-R favored the dehydration reaction (Equation (7)) through C–O cleavage, preventing the diffusion of C2 intermediates (like ethylene) [37], and stabilizing hydroxyl groups [27]. However, the lower amounts of H2 and CO compared with the catalysts shows that the supports are unable to further reform CH4 (Equation (6)), which is a more stable intermediate [27].
Table 1. Products distribution, catalyst mean active particle size, and carbon deposition in the SRE at 700 °C over reduced (R) and aged (A) RhPt/CeO2, RhPt/ZrO2, and RhPt/La2O3.
Table 1. Products distribution, catalyst mean active particle size, and carbon deposition in the SRE at 700 °C over reduced (R) and aged (A) RhPt/CeO2, RhPt/ZrO2, and RhPt/La2O3.
Catalyst SampleProduct Distribution 1 d p (nm) 2Total Weight Loss (%) 3
H2COCO2CH4Ethylene
Equilibrium0.700.240.02N.D.5E-10--
CeO2-R0.520.170.180.121E-5-0.2
RhPt/CeO2-R0.700.200.090.01N.D.4.6 ± 1.30.1
RhPt/CeO2-A0.650.260.060.038E-44.8 ± 1.50.2
ZrO2-R0.460.200.200.143E-4-0.2
RhPt/ZrO2-R0.620.250.100.037E-53.6 ± 1.10.2
RhPt/ZrO2-A0.610.260.060.046E-59.1 ± 2.40.1
La2O3-R0.510.170.160.151E-4-6.3
RhPt/La2O3-R0.660.280.05N.D.N.D.6.2 ± 2.013.4
RhPt/La2O3-A0.600.350.04N.D.7E-310.0 ± 2.813.3
1 Reaction conditions as Figure 1; 2 Obtained from TEM; 3 From TGA. N.D.: Not detected.
CeO2 was the most selective support toward H2 (Figure 1b), probably because it produces larger amounts of acetaldehyde that are subsequently reformed by Equation (7). Hou et al. [37] proposed that the reaction mechanism over CeO2 and Ir/CeO2 catalysts is: ethanol→ethoxy species→acetaldehyde→C2 species→C1 species. Ethoxy species could be dehydrogenated to acetaldehyde by the presence of –OH groups on the surface of the CeO2. Carbonate species was only observed in Ir/CeO2. CeO2 support oxidizes ethanol into acetaldehyde-acetate species, providing –OH groups and mobile oxygen to ensure the cracking of the C2 intermediates species and coke cleaning [42]. Likewise, Zanchet et al. [27] reported that the conversion of ethanol to ethylene decreases in CeO2 at high temperature (>250 °C) due to the rise in the energy barriers to dehydration and stability of ethoxy intermediates, thus, dehydrogenation and decomposition reaction take place. ZrO2 and La2O3 showed larger amounts of ethylene than CeO2, which could result from a major contribution from the dehydration of ethanol (Equation (8)) instead of the dehydrogenation reactions. This phenomenon can be ascribe to a lower oxygen storage capacity of these supports at 700 °C compared with CeO2 [43,44], which could cause a failure to degrade C2 intermediates.
Regarding the reduced catalysts, RhPt/La2O3 and RhPt/ZrO2 achieved complete ethanol conversion above 480 °C (Figure 1a). Unlike these catalysts, RhPt/CeO2 did not achieve complete conversion below 520 °C. Based on the extent of ethanol conversion, the activity order was RhPt/ZrO2 > RhPt/La2O3 > RhPt/CeO2. The corresponding H2 yields of the reduced catalysts as a function of temperature are shown in Figure 1b. Above 600 °C, RhPt/CeO2 had the highest yield among the reduced catalysts, with a maximum of 3.5 mol H2/mol ethanol inlet at 700 °C. The highest H2 yield was observed for RhPt/La2O3 at 700 °C (3.1 mol H2/mol ethanol inlet). The yields achieved with both RhPt/CeO2 and RhPt/La2O3 showed significant temperature dependences. In contrast, the behavior of RhPt/ZrO2 was almost constant between 520 and 700 °C, producing 2.4 mol H2/mol ethanol inlet. Thus, although RhPt/ZrO2 was the most active catalyst for converting ethanol at low temperatures, it produced the lowest amount of H2 under total ethanol conversion conditions at high temperatures.
In this way, the elevated H2 yield at high temperatures on RhPt/CeO2 (Figure 1b) could be related to rapid ethanol reforming via dehydrogenation (Equation (3)). The ethanol activation can be occurring over both, Rh via O–H, and Pt via α-C−H, going through the formation of intermediates, such as CH3C*O, *CH2C*O, and *CHC*O [27], where (*) means the chemical bond between the intermediate and the catalyst surface. The cleavage of C–C bond is expected to be favored in species highly dehydrogenated. After the cleavage of C–C bond, *CHx and *CO species are formed (Equations (4)–(7)) and, finally, the *CHx species are decomposed forming *C that has been oxidized to CO (Equation (6)). The CH4 selectivity is governed by the ability of the metals to hydrogenate/dehydrogenate the *CHx species, that is favored in highly-reduced metals such as Rh [45]. An increase of metal surface reactivity is expected from Pt, that favors H abstraction and increases the availability of O* on the metal surface [27]. In this way, some of the CO produced is converted to CO2 by the WGSR, which, as expected by thermodynamics, occurred to a lesser extent as the temperature increased (Table 1). Idriss et al. [19] and Divins et al. [46] evaluated RhPd/CeO2 catalysts in the SRE, reporting that the increase in H2 yield around 700 °C is due to simultaneous presence of WGSR and methane reforming.
Additionally, as was analyzed above, the support affects the product distribution in the SRE. Scarabello et al. [47] reported that the O-vacancy in Rh/Ce-ZrO2 promotes the oxidation of CO to CO2 by Equation (11). This reaction is favorable at high temperature because it is endothermic. The occurrence of the two reactions (Equations (2) and (11)) simultaneously could explain the product distribution in RhPt/CeO2.
2 C e O 2   +  CO   CO 2 +  Ce 2 O 3
In contrast, the stable and limited H2 yield on RhPt/ZrO2 at high temperatures (Figure 1b), accompanied by the larger methane production (Table 1), can be associated with ethanol dehydration (Equation (8)) and subsequent ethylene transformation into methane (Equations (9) and (10)). Similar results were reported by Song et al. [17], where a Co/ZrO2 catalyst produced more CH4 and less CO compared to Co/CeO2 catalyst under same condition (550 °C, S/E = 10, and GHSV = 5000 h−1). The authors emphasize the high capacity to ZrO2 to adsorb CO on its surface. Furthermore, previous studies have associated high CH4 concentrations with the inability of a catalyst to reform methane at high temperatures [30,48]. Martono and Vohs [49] found that the stabilization of Co particles in a partially oxidized form allows bifunctional mechanisms in which oxidative dehydrogenation of ethanol adsorbed on the Co takes place using oxygen supplied by the support. This effect was not observed in non-reducible supports, such as YSZ, due to its low oxygen mobility. Hence, the lack of oxygen vacancies in our ZrO2, compared to CeO2, increased CH4 production [29,43].
The rapid increase in the H2 yield with temperature, as observed for RhPt/La2O3 (Figure 1b), combined with an increase in CO and the absence of CH4, suggests the reactions represented by Equations (3)–(6) occur rapidly with temperature [45]. Munera et al. [50] proposed a reaction scheme (Equations (12)–(14)) with metallic site (S) and support for the reforming of methane with CO2 over Rh/La2O3. Likewise, Ghelamallah and Granger [51] reported that the increased temperature favored the consumption of H2 to produce CO, over a RhPt/La2O3 reduced catalyst in the methane reforming, due to an increasing contribution from the RWGSR (Equation (2)), especially when Pt is present. According to this, CO formation and CH4 reforming are favored at high temperatures over La2O3-supported catalysts, which is consistent with the results obtained in this work.
C H 4   +  S    2 H 2 +  S C
C O 2   +  La 2 O 3    La 2 O 2 CO 3
La 2 O 2 CO 3   +  S  La 2 O 3 +  CO

2.2. Catalyst Stability

The stability of the RhPt/CeO2, RhPt/ZrO2, and RhPt/La2O3 reduced catalysts was evaluated by 27 h TOS at 700 °C. This temperature was selected to maximize H2 production and simultaneously decrease methane and ethylene formation. The ethanol conversion and product distribution vs. TOS are shown in Figure 2.
Figure 2. Product distribution (H2, CO, CO2, CH4, and ethylene) as a function of time on stream (TOS) on reduced (a) RhPt/CeO2; (b) RhPt/ZrO2; and (c) RhPt/La2O3. Dashed line: H2 equilibrium. Reaction conditions: S/E = 3 and GHSV = 70,600 h−1 at 700 °C.
Figure 2. Product distribution (H2, CO, CO2, CH4, and ethylene) as a function of time on stream (TOS) on reduced (a) RhPt/CeO2; (b) RhPt/ZrO2; and (c) RhPt/La2O3. Dashed line: H2 equilibrium. Reaction conditions: S/E = 3 and GHSV = 70,600 h−1 at 700 °C.
Catalysts 05 01872 g002
During the tests, all the catalysts showed the same total ethanol conversion without a reduction in catalytic activity, but had different product distributions. Initially, a high H2 concentration, slightly above the thermodynamic prediction (dashed line), was observed on RhPt/CeO2 (Figure 2a). However, a decrease in the H2 concentration was observed during the first 5 h, followed by a gradual return to the thermodynamic value. After 27 h TOS, the H2 concentration was 69 mol%, yielding 3.3 mol H2/mol ethanol inlet. The H2 concentration on RhPt/ZrO2 was stable and ~63 mol% H2 was produced throughout the experiment (Figure 2b). RhPt/La2O3 (Figure 2c) showed increased H2 production until stabilization was observed around 64 mol%. Da Silva et al. [44] reported a stable product distribution during total ethanol conversion on 0.6%Rh/La2O3-SiO2 and 0.6%Rh/CeO2 catalysts for the SRE during 28 h TOS (500 °C; H2O:ethanol, molar ratio 3:1; 2.5% ethanol; 30 mL/min gas stream). In the current study, the total gas feed and temperature were extreme, looking forward to affecting the catalyst stability faster, in order to see possible differences between the supported catalysts.
During the first 10 h TOS on RhPt/CeO2, while the H2 concentration was decreasing, CH4 was observed (Figure 2a), showing a reduction in the catalyst capacity for methane reforming. After 15 h TOS, the formation of all the products started to stabilize. Some ethylene was sporadically observed after 20 h TOS. RhPt/ZrO2 showed a more stable profile, but methane production was almost constant and higher than that obtained with the other two supports. No methane was detected when RhPt/La2O3 was used, but some ethylene was observed after 15 h TOS. For all the catalysts, the CO and CO2 profiles varied with TOS and required more time to stabilize. Thus, the changes undergone by the catalysts with TOS seem to mostly affect the CHx species dehydrogenation/hydrogenation, the activation of water, and oxidation of *C species [27,52]. Scarabello et al. [47] reported that a Rh/ZrO2 catalyst needed 11 oxidation/reduction cycles to achieve a stable state to reform methane, while Rh/CeO2-ZrO2 showed stability without redox cycles, even a low temperature. This phenomenon was associated to changes in the catalyst particle size, dispersion of the active metal, and redistribution of oxygen vacancies in the support surface after each cycle. In addition, Beretta et al. [53] proposed that this conditioning period was necessary to remove defect sites on Rh/AlO2 catalysts in the reforming of methane.
In this study, ethanol conversion and H2 selectivity did not vary considerably during the stability tests. The most important differences between the reduced and aged catalysts were the changes in the CO and CO2 distributions with TOS. From this point, we wanted to see the performance of these aged catalysts during the activity test (conversion vs. temperature), as the main indicator of catalyst deactivation.

2.3. Catalytic Activity and Selectivity of Aged Catalysts

The aged catalysts after 27 h TOS at 700 °C were investigated for potential loss of activity by performing an activity test (conversion vs. temperature). The ethanol conversions and H2 yield are shown in Figure 1. Compared with the reduced samples, a slight decrease in the activity of the aged catalysts was observed at low temperatures (Figure 1a), but the profile was maintained, suggesting that aging does not alter porous diffusion and the reaction is still kinetic controlled [39,54]. Reduced and aged RhPt/CeO2 and RhPt/ZrO2 had very similar H2 yield profiles (Figure 1b), but a larger reduction in the H2 yield was observed for RhPt/La2O3 at high temperatures. At 700 °C, the H2 yield followed the trend RhPt/CeO2 > RhPt/ZrO2 > RhPt/La2O3.
In addition, the byproduct distribution at 700 °C (Table 1) of aged catalysts showed an increase in CO and the appearance of ethylene. Again, changes on catalysts performance seems to be associated to CHx species dehydrogenation/hydrogenation, activation of water, and oxidation of *C species. These steps of the reaction mechanism are structure sensitive [27], which could suggest that the catalysts are suffering changes on particle size and oxidation state. Therefore, the catalytic properties related to active metal dispersion, the oxidation states of the species, and carbon deposition were analyzed to determine the changes experienced by the materials during the stability tests and relate them to the observed changes in activity.

2.4. Catalyst Characterization

The development of stable catalysts is one of the most important issues in hydrogen production from ethanol, particularly for stoichiometric feed compositions with no excess H2O to remove carbon deposits [37], as being expected in real bioethanol samples obtained in Colombia. Four intrinsic deactivation modes have been identified: (i) coke formation; (ii) active metal sintering; (iii) active metal oxidation; and (iv) poisoning. Here, catalyst reducibility, TEM, XPS, and TGA were employed to study catalyst deactivation.

2.4.1. Reducibility of Fresh Catalysts

Fresh RhPt/CeO2, RhPt/ZrO2, and RhPt/La2O3 catalysts and their supports were exposed to a reducing and oxidizing environment periodically at programmed temperatures ranging from 150 to 750 °C in TGA. As shown in Figure 3 (a, c, e for fresh catalysts and b, d, f for supports), the sample was first oxidized at 150 °C (No. 1), then reduced at 150 °C (No. 2), and subsequently heated in inert gas (N2) until 250 °C. This procedure was repeated each 100 °C until 750 °C.
Figure 3. Oxidation in 1%O2/N2 (No. 1, 3, 5, 7, 9, 11, 13) and reduction in 4%H2/N2 (No. 2, 4, 6, 8, 10, 12, 14) as a function of temperature in a thermogravimetric analysis of fresh RhPt supported catalysts (a, c, e) and catalyst supports (b, d, f).
Figure 3. Oxidation in 1%O2/N2 (No. 1, 3, 5, 7, 9, 11, 13) and reduction in 4%H2/N2 (No. 2, 4, 6, 8, 10, 12, 14) as a function of temperature in a thermogravimetric analysis of fresh RhPt supported catalysts (a, c, e) and catalyst supports (b, d, f).
Catalysts 05 01872 g003
The redox properties of the three supports differed (Figure 3b,d,f). In general, the hydrogen flow produced a weight decrease probably as a result of the removal of transferable or fixed oxygen from the support surface by the reaction with H2 to form water. Conversely, oxygen flow produced mostly a slight increase in weight due to its absorption onto the material surface. ZrO2 (Figure 3f) was the most stable support, maintaining almost constant weight when exposed to either H2 or O2 flow. On the other hand, both La2O3 (Figure 3b) and CeO2 (Figure 3d) lost weight with H2 treatment. The larger weight lost for La2O3 was at 650 °C (0.5% in Figure 3b). Otherwise, CeO2 increasingly lost weight at 550 °C (Figure 3d). At 750 °C it presented a 1.5% weight loss. Although the high oxygen storage capacity (OSC) of CeO2 is well known [55], the expected weight gain of CeO2 when treated with O2 was not as high as the weight loss when exposed to H2. SRE might benefit from this oxygen mobility, i.e., the OSC of material can be released in the reducing environment and facilitate the SRE by oxidizing some of the carbonaceous side byproducts such as CO, as proposed in [56]. However, this oxygen vacancies are not being likely easily regenerated, which can affect the catalyst stability.
The active metals altered the reduction and oxidation capacity of all of the materials (Figure 3a–c). The reduction profile of RhPt/La2O3 (Figure 3a) showed an abrupt and significant H2 uptake at 550 °C, 2% more than the support weight loss at the same temperature. In this case, a new crystalline phase (La hydrides, as was checked by XPS, vide infra) are being formed in contact with H2 at 550 °C, which seems to be stable at higher temperatures. When this structure is formed, a lower O2 intake capacity was observed compared with the support (Figure 3b). The formation of this new lanthana phase seems to be catalyzed by the active metals (i.e., it was not formed when only the support was analyzed).
H2 uptake was also higher on CeO2 (Figure 3c) and ZrO2 (Figure 3e) in the presence of the active metals; however, they both showed similar trends to those observed in the supports. For CeO2, the metals slightly increased the O2 uptake capacity at temperatures lower than 500 °C probably due to the easier reduction-oxidation cycle for the metals on the support. In ZrO2, precious metals increased both H2 and O2 uptake within a small range.
The thermogravimetric behavior during the redox cycles were believed to be related to the active metal dispersion and reducibility of the supports. High dispersion of the active metal can make the H2-chemisorption/reduction easier by providing more available active sites. A previous study [43] found that the presence of Pt on CeZeO2 could increase the reducibility of CeZrO2 by the dissociation and spillover of hydrogen from the Pt particles onto the CeZrO2 surface, resulting in the formation of O vacancies and their associated Type II bridging OH groups. Meanwhile, the outstanding oxygen storage capacity (OSC) of CeO2 [55,56], which was also indicated by its significant weight loss and rapid weight gain during the redox cycles from 500 to 800 °C in our test (Figure 3d), could improve the flexibility for the oxygen association and dissociation from the active metals, promoting their oxidation. This sample presented the higher redox capacity at 700 °C, the temperature of the stability test, which can be directly related to its better performance to decompose CHx species after ethoxy-species adsorption (Figure 2a). However, the loss of its redox capacity can be already inferred from the activity data and oxidation-reduction cycles. These metals-support interactions before and after SRE were confirmed by the following catalyst characterization studies.

2.4.2. TEM/EDX of Reduced and Aged Catalysts

The active particle sizes of both the reduced and aged catalysts were studied by TEM to evaluate the sintering of the active phase. Figure 4 shows the TEM micrographs and particle size distributions of these materials. The mean particle sizes obtained from the TEM analysis are listed in Table 1.
Figure 4. TEM images and particle size distribution histograms of reduced and aged catalysts: (a), (b), and (c) RhPt/CeO2; (d), (e), and (f) RhPt/ZrO2; (g), (h), and (i) RhPt/La2O3.
Figure 4. TEM images and particle size distribution histograms of reduced and aged catalysts: (a), (b), and (c) RhPt/CeO2; (d), (e), and (f) RhPt/ZrO2; (g), (h), and (i) RhPt/La2O3.
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The crystalline structure of CeO2 and the high degree of dispersion of the active sites can be observed in Figure 4a,b. The ability of CeO2 to disperse metals on its surface is well-known [26,57,58]. The particle size of the ceria reduced catalyst suggests the presence of Rh-Pt alloys (Table 1). As was reported by Ghelamallah and Granger [51] for bimetallic Pt-Rh catalysts, the preferential formation of alloyed or bimetallic particles has been demonstrated when the particle sizes increased from ~2 nm on monometallic samples to ~5 nm on bimetallic catalysts, where the latest agrees with the particle size range obtained in this study [51]. This result is expected when the current co-impregnation method is used to prepare bimetallic catalysts [51]. The particle size distribution in RhPt/CeO2 (Figure 4c) practically did not change during the reaction, confirming the strong metal-support interaction [22].
Similar to RhPt/CeO2, reduced RhPt/ZrO2 displayed a crystalline structure, small particle size, and a high degree of metal dispersion (Figure 4d). The active particle size was slightly lower in this reduced catalyst (3.6 nm, Table 1), but the particle size distribution increased in the aged sample (Figure 4e,f) and the mean particle diameter (Table 1) increased 152%. Previous reports have attributed the increase in particle size on Rh/ZrO2 [47] and Pt/ZrO2 [59] catalysts to the low reduction capacity of this support, as was previously found in our oxidation-reduction cycles (Section 2.4.1), which promote particle agglomeration.
In RhPt/La2O3 (Figure 4g–i), larger particles were observed in the reduced catalyst and the mean particle size (Table 1) increased 62%. Ghelamallah and Granger [51] evaluated Rh, Pt, and RhPt catalysts on La2O3 for methane reforming and reported that weak metal-support interactions promoted particle sintering under the reducing environment of the reaction. The increase in the mean particle size of the catalysts supported on ZrO2 and La2O3 indicated low metal-support interaction, which allows metal particles to diffuse on the surface and promotes sintering of the active sites [58].
No important differences were detected in the ability of RhPt/CeO2, RhPt/ZrO2, and RhPt/La2O3 to convert ethanol, indicating that the augmentation of the particle size did not affect the ethanol activation step. However, the surface of small particles intrinsically contain a large amount of low-coordinated sites such as steps, kinks, and vacancies (broadly classified as defects) that have long been considered as important catalytic sites once the ethanol is activated on the catalyst surface, because the reactants and intermediaries may bind strongly to these low-coordinated sites helping breaking the chemical bonds readily. The fraction of the low-coordinated atoms increases significantly when the metal particle size decreases, ascribed to the decrease of atoms on terraces [27]. The small active particle size in RhPt/CeO2, even in the aged sample (Figure 4b), confirms the presence of a stable active phase over this support, which better reacts to convert the adsorbed ethanol into the desirable reforming products.
In order to gain an insight in the possible particle structure, Figure 5 shows the surface chemical composition of the catalysts obtained by EDX. Rh/(Rh + Pt) ratio in reduced samples of RhPt/CeO2 (0.58), RhPt/ZrO2 (0.70), and RhPt/La2O3 (0.53) were lower than the expected (0.85), indicating a possible Pt enrichment on the catalyst surface. This enrichment could occur by a preferential interaction of Pt with H2 during the reduction step. Gary et al. [18] proposed that the activation process of a Pt/CeO2 catalyst in the SRE is favored by the H2 presence due to Pt reacts simultaneously with H+ species and oxygen vacancies, ensuring the availability of the active site on the catalytic surface. According to this, the reduction of the catalysts at 700 °C before reaction appears to favor the availability of Pt on the catalyst surface, mitigating its adsorption into the support. However, Rh/(Rh + Pt) ratio approached to expected in RhPt/CeO2 and RhPt/ZrO2 after reaction, showing a possible adsorption of Pt into either the particles or the support [60,61]. In contrast, no significant changes in Rh/(Rh + Pt) ratio of reduced and aged RhPt/La2O3 samples were observed, but the total content of the metals on the surface decreased. This may be related to a proportional agglomeration or adsorption of both metals (Pt and Rh) on La2O3 surface.
The content of Ce, Zr, and La were similar to those expected for all catalysts. The decrease in the amount of oxygen expected, which was higher in RhPt/CeO2-R, could be attributed to the presence of oxygen vacancies on the surface [18].
Figure 5. Energy dispersive X-ray microanalysis (EDX) of the catalyst samples by the average of three measurements over a 50 nm probe.
Figure 5. Energy dispersive X-ray microanalysis (EDX) of the catalyst samples by the average of three measurements over a 50 nm probe.
Catalysts 05 01872 g005

2.4.3. XPS

XPS can give reliable information about the oxidation state of the active metals before and after the stability test. All possible species present in the reduced and aged samples were analyzed by XPS. Table 2 lists the BEs and possible chemical structures of Rh 3d5/2, Pt 4d7/2, Ce 3d5/2, Zr 3d5/2, La 3d5/2, and Cl 2p3/2. Figure 6a shows the Rh 3d5/2 XPS spectra of the reduced and aged catalysts.
Table 2. Binding energy (BE in eV) and percent relative abundance (%, in parentheses) of the catalysts from XPS.
Table 2. Binding energy (BE in eV) and percent relative abundance (%, in parentheses) of the catalysts from XPS.
ElementCatalyst SamplePossible Chemistry
ReducedAged
RhPt/CeO2
Rh 3d5/2309.0 (100)NDRh+3
Pt 4d7/273.1 (50)-Pt0
-74.6 (100)Pt+4, Pt+2
74.9 (50)-Pt+4
Ce 3d5/2880.7 (4)880.6 (4)Ce+3 (Ce2O3)
882.3 (22)-Ce+4 (CeO2)
-882.4 (24)Ce+3 (Ce2O3)
884.2 (19)884.8 (23)Ce+3 (Ce2O3)
888.8 (23)888.4 (21)Ce+4 (CeO2)
897.0 (6)897.2 (5)Ce+4 (CeO2)
898.3 (26)898.5(22)Ce+4 (CeO2)
Cl 2p3/2198.3 (100)NDCl
RhPt/ZrO2
Rh 3d5/2307.1 (62)307.3 (59)Rh0
308.7 (38)308.3 (41)Rh+3
Pt 4d7/271.7 (100)71.7 (100)Pt0
Zr 3d5/2182.0 (100)182.0 (100)Zr+4 (ZrO2)
Cl 2p3/2NDND-
RhPt/La2O3
Rh 3d5/2-307.2 (58)Rh0
308.9 (82)308.9 (42)Rh+3
310.1 (18)-Rh+3 (RhCl3)
Pt 4d7/2NDNDND
La 3d5/2834.9 (54)834.8 (54)La+3 (La2O3)
837.3 (15)837.3 (14)La+3 (LaH2+1)
838.7 (31)838.7 (32)La+3 (LaH3)
Cl 2p3/2195.5 (71)195.5 (73)Metal-Cl
198.3(29)198.2 (27)Cl
ND: Not detected
Figure 6. XPS spectra of (a) Rh 3d5/2; (b) Pt 7f7/2; and (c) Ce 3d3/2 and 3d5/2 for reduced and aged catalysts.
Figure 6. XPS spectra of (a) Rh 3d5/2; (b) Pt 7f7/2; and (c) Ce 3d3/2 and 3d5/2 for reduced and aged catalysts.
Catalysts 05 01872 g006
In general, two main oxidation states were observed in Rh 3d5/2: Rh+3 (308.3–310.5 eV) and Rh0 (307.0–307.7 eV). [58,62] In reduced RhPt/CeO2, only Rh+3 was identified, which is associated with rhodium oxides and small amounts of metal precursor, confirmed by the presence of Cl in the sample [58]. Avgouropoulos et al. [63] reported the presence of exclusively Rh+3 after reduction at 300 °C over CeO2 owing to oxygen transfer to the metal from the support. On the other hand, Rh was not detected in aged RhPt/CeO2. This could be ascribed to the fact that the CeO2 crystal size may interfere with the quantification of Rh particles on the surface [64] and a consequence of the Pt surface enrichment found by EDX (see Section 2.4.2).
In reduced RhPt/CeO2, Pt+4 (74.9 eV, 50%) and Pt0 (73.1 eV, 50%) were identified (Figure 6b). Aged RhPt/CeO2 showed only one peak attributed to both Pt+4 and Pt+2, informing about a complete Pt oxidation during the stability test [65]. As reported by Kaila et al. [61], the presence of an alloy on these bimetallic catalysts is possible with all Rh/Pt ratios because of the miscibility of the RhxPt1−x system. The RhxPt1−x alloys result in a strong Pt enrichment of the surface as the temperature increases [35,47] which agrees with our results on the aged RhPt/CeO2 catalyst, where Rh was not observed on the surface by XPS and diminished its surface concentration in EDX (Section 2.4.2). The strong oxidation state of the metals on this catalyst is consistent to the great oxygen mobility from the CeO2 support. In the case of SRE, one point of common agreement is that the oxygen mobility of the support is a critical property to favor C removal and avoid deactivation [27].
Figure 6c shows the Ce 3d3/2 and 3d5/2 XPS spectra of both reduced and aged RhPt/CeO2. Although the determination of the cerium oxide state from Ce 3d level spectra is difficult, we reviewed several publications trying to establish the contribution of Ce+3 and Ce+4 in each catalyst sample. Ce+4 could be ascribed to shoulders at 882, 888, 899, 901, 908, and 917 eV, [55,60,66] and Ce+3 can be seen at 885 and 903 eV [55,60,64]. Both species play an important role in the catalytic process. Ce+4 acts as a Lewis acid by interacting with the O (2p) orbital of alcohols to form hydroxyl species that are intermediates in ethanol decomposition [60,61]. The presence of Ce+3 indicates the reduction of the CeO2 surface by interactions between a noble metal and Ce [67]. Rh and Pt have been proposed to occupy the oxygen vacancies in the CeO2 lattice, promoting Ce+3 formation [60] and preventing metal movement on the oxide surface [55], as confirmed by TEM (Section 2.4.2).
The selected characteristic Ce+3 shoulder (885.5 and 903 eV) [64] increased by 126% in the aged sample, while the amount of Ce+4 decreased by 38% (Figure 6c). This established that the Ce+3/Ce+4 ratio increased in the ceria supported catalyst from 0.3 (reduced) to 1.1 (aged). These results agree with oxidation/reduction cycles (Section 2.4.1) and may result from a change in the distribution of oxygen vacancies on the catalytic surface promoted by reducing agents present in the reaction system [68]. Divins et al. [46] evaluated a RhPd/CeO2 catalyst in the SRE, also reporting an increase in the Ce+3/Ce+4 ratio from 0.63 to 0.83 after reaction at 550 °C. The authors suggested that the capability of CeO2 as a support to activate water and to donate oxygen atoms could oxidize noble metals in the SRE. The increase in Ce+3 facilitates metal particle dispersion on the catalyst surface, but the loss of Ce+4 may cause the observed reduction of CH4 reforming capacity for RhPt/CeO2 with TOS (Figure 2b). Higher oxygen storage capacity and oxygen mobility allow a more effective gasification/oxidation of adsorbed carbon on the catalyst. Furthermore, the high oxygen mobility could facilitate the oxygen migration to the metal-support interface, which would favor the oxidation of *CHx species [27]. But it seems that Ce+4 is not regenerated during the reaction not only because of the lack of an oxygen source in the reaction media, but also by its difficulty of gaining oxygen in redox cycles (Section 2.4.1). Water adsorption can also help to the ceria re-oxidation [27], but the low S/E ratio of three used in the current experiments limits the oxygen source.
On the contrary, most Rh in reduced RhPt/ZrO2 was Rh0 (62%, Table 2), consistent with the results of Tanabe et al. [69], who proposed that the low interaction between ZrO2 and Rh favors metal reduction and promotes sintering, as observed here (Figure 4). The Rh+3 species, associated with Rh2O3, was also present. The oxidation state of the Rh species was stable after aging. Only Pt0 (71.7 eV) was detected in reduced and aged RhPt/ZrO2. The stability of reduced Pt was previously reported for a Pt/ZrO2 catalyst [61,67]. This metal accommodation over the support seems to affect H2 selectivity, as observed in Figure 1b. The reduced metallic species seems to have less degree of interaction with the support, promoting agglomeration (9.1 nm, Table 1) and affecting reactant-catalyst interphase. The characteristic peak for Zr 3d5/2 (182 eV) has been ascribed to Zr+4 in the zirconia support (Table 2) [26]. There was no appreciable difference in the zirconium oxide structure between the reduced and aged catalysts.
The Rh 3d5/2 spectra had two peaks in the reduced RhPt/La2O3 catalyst sample (Table 2, Figure 6a), one at 310.1 eV corresponding to an oxidized Rh3+ species linked to the metal precursor (confirmed by the Cl 2p spectra) and another at 308.9 eV, which indicates the presence of Rh2O3 [30]. After aging, the latter signal decreased and Rh0 appeared. However, Pt was not detected on the RhPt/La2O3 samples; hence, it was not possible to confirm the surface Pt enrichment observed on RhPt/CeO2 and reported by Ghelamallah and Granger on RhPt/La2O3 [51]. The particle agglomeration observed by TEM and the encapsulation into the support suggested by EDX might be related to this obstruction. Moreover, the presence of metallic particles on zirconia and lanthana supported catalysts seems to be directly related to the metal agglomeration. The La 3d5/2 XPS peaks at 834, 837, and 838 eV were ascribed to La2O3, LaH2+1, and LaH3, respectively [51,70]. These species remained almost constant after aging (Table 2) and confirmed the presence of La hydrides on the support, which was observed in oxidation/reduction cycles (see Section 2.4.1).

2.4.4. TGA

Coke formation can be evaluated by TGA in air. Table 1 shows the TGA results for reduced and aged catalysts. No significant differences were observed between both samples at ceria and zirconia catalysts, consistent with their basic nature [59,71,72]. The RhPt/La2O3 behavior is ascribed to the La hydrides reaction with oxygen, which is favored in the presence of active metals, as was found in the oxidation/reduction cycles of this support (Section 2.4.1) [30,73]. However, no important differences were recorded between reduced and aged samples, showing that this La hydrides decomposition on the support is the main weight loss cause. In general, results indicate that carbonaceous residues did not play an important role in determining the stability of the catalysts, confirming that basic supports may reduce the formation of carbonaceous compounds [13,36,64,72].
As mentioned by Zanchet et al. [27] the control of carbon accumulation depends on the availability of surface oxygen required to oxidize the *C species. Contrarily to base metals, carbon nucleation is energetically unfavorable in noble metal surfaces, reducing the solubility of carbon on the later materials. Nobel metal lattice is too large compared to graphene lattice, hindering its formation. They suggested that the geometric effect is more important than the electronic one. Hence, only superficial carbon is usually formed on supported noble metal catalysts.
From these results, changes in the selectivity of RhPt/La2O3 and RhPt/ZrO2 with TOS can be related to metal sintering caused by active phase characteristics. In addition, both catalysts presented more stable reduced Rh and Pt species than the CeO2 counterpart did, which can indicate that the lack of oxidized metals promotes their sintering on the support, because a low metal-support interaction [69]. This changes hydrogenation/dehydrogenation mechanisms that are structure-sensitivity and affect the oxygen mobility throughout the catalyst, necessary to finally oxidize the CHx species previously activated on the catalytic surface. However, it is important to remark that both catalysts presented showed constant complete ethanol conversion during the stability test, stable H2 selectivity, and the absence of carbon deposition. Moreover, RhPt/CeO2 showed that the metals can be forming small and stable Rh-Pt alloys containing both Rh and Pt oxidized species from the beginning of the reaction, which make this catalyst more selective to H2. This catalyst show a greater oxygen mobility, which seems to travel from the support, crossing the active metals, and reaching the CHx species to finally oxidize them, producing H2, CO, and CO2. These features could be exploited in the reforming of raw bioethanol samples. Divins et al. [74] reported similar product distribution when reforming synthetic ethanol/water and raw bioethanol samples over a RhPd/CeO2 catalyst, due to a synergy between the active metals and the support. However, the changing Ce+3/Ce+4 ratio during the stability test can slightly affect the catalyst selectivity during larger period of times. Therefore, longer stability tests and different catalyst reactivation methods will be evaluated in our laboratory using the RhPt/CeO2 catalyst. It is important to remark that a delicate equilibrium between Ce+3/Ce+4 ratio must be assured to provide for catalyst stability. In addition, we are currently studying the effect of the Rh/Pt ratio on ceria on the product and byproduct distribution.

3. Experimental Section

3.1. Catalyst Synthesis

The catalysts were prepared by the incipient wetness co-impregnation method. The active metals were obtained from rhodium (III) chloride hydrate (RhCl3·H2O) (Sigma-Aldrich, St. Louis, MO, USA) and chloroplatinic acid hexahydrate (H2PtCl6·6H2O) (Sigma-Aldrich) solutions in water. The CeO2, ZrO2, and La2O3 supports were obtained from the calcination of cerium nitrate hexahydrate (99.5%, Alfa Aesar, Haverhill, MA, USA), zirconium (IV) oxide (99.9%, Alfa Aesar), and lanthanum (III) oxide (98%, Panreac, Castellar del Valles, Barcelona, Spain), respectively, at 700 °C in a muffle oven for 2 h. Metals loading of 0.6 wt. % Rh and 0.2 wt. % Pt were selected, based in previous investigations in which this ratio was found to be the more active for reforming reactions [30,61]. Low amounts of active metals (0.8 wt. % total) were employed to contribute to the economy of the process. The required amounts of active metals were co-impregnated onto the supports. To achieve co-impregnation, the metal-support mixture was continuously stirred, dried at 100 °C for 24 h, and calcined at 700 °C in air for 2 h. All the calcined solids were sieved using an 80 mesh sieve to ensure particle sizes of less than 177 µm. These samples are the “fresh (F)” catalysts.

3.2. Catalytic Tests

The catalytic performance of RhPt/CeO2, RhPt/ZrO2, and RhPt/La2O3 was evaluated in a fixed bed reactor at atmospheric pressure under kinetic control. The catalyst particles (100 mg) were diluted by mixing with quartz particles (200 mg, 80-mesh). The catalyst mixture was loaded into a quartz tube reactor (12 mm ID) with a frit fused equidistant from both ends of the tube. The plug flow reactor condition was achieved by eliminating back mixing and channeling when a ratio of 45 was maintained between the catalyst bed height and catalyst particle size (L/Dp) and a ratio of 60 was maintained between the reactor internal diameter and catalyst particle size (D/Dp) [75,76]. Before the reaction, the catalysts were reduced in 10% H2/N2 (300 mL/min) at 700 °C for 1 h (these samples are the “reduced (R)” catalysts). Then, a premixed solution of ethanol (99.9%, ACS grade, J.T. Baker, Center Valley, PA, USA) and distilled water was pumped continuously into the heated reaction system with a Simdos 02 metering pump (KNF Neuberger, Trenton, NJ, USA). The vaporized reactants combined with the incoming N2 (diluent and internal standard) and passed through the catalyst bed. For each test, the EtOH:H2O:N2 molar ratio was adjusted to 1:3:51 (stoichiometric S/E molar ratio), in order to avoid mass transfer limitations in the system [39], and the total gas flow was 300 mL/min.
Each catalytic activity test was conducted at GHSV of 70,600 h−1 at atmospheric pressure between 400 and 700 °C in 20 °C increments (continuous sequence, 25 min at each temperature). Blank tests were performed using an empty reactor or only the support, previously reduced, under the same conditions. In addition, 27 h time on stream (TOS) SRE catalytic stability tests were conducted at 700 °C for each reduced catalyst using the conditions and parameters described above. The samples used in these stability tests are the “aged (A)” catalysts. Following these tests, the aged catalysts were subjected to an activity test between 400 and 700 °C to evaluate possible activity loss.
The compounds in the product stream, including H2, CH4, CO, CO2, ethanol, and ethylene, were quantified online using a Clarus 580 gas chromatograph (GC, Perkin Elmer, Norwalk, CT, USA) equipped with a Carboxen 1010 plot column (30 m, 0.53 mm ID, Restek, Bellefonte, PA, USA) connected to a thermal conductivity detector (TCD) and an Innowax column (30 m, 0.53 mm ID, Perkin Elmer, Boston, MA, USA) connected to a flame ionization detector (FID). Elemental balances between the inlet ethanol-water feed and the outlet products were measured in all tests. The test was declared effective when the elemental carbon balance was ~100%.
The ethanol conversion (XEtOH), yield, and mole distribution for each detected product were calculated according to Equations (15)–(17):
X E t O H = F EtOH , o F EtOH ,  t F EtOH , o
Y i e l d i  = Mole of  i Mole of etanol inlet
M o l e   f r a c t i o n i = F i F i   ×   100 %
where F E t O H , o is the initial theoretical mole flow (mol/min) of ethanol at ambient temperature, F E t O H ,   t is the mole flow (mol/min) of unreacted ethanol in the product stream detected by GC at time t, and F i is the mole flow (mol/min) of product i (H2, CO, CH4, CO2, or C2H4). Thermodynamic equilibrium calculations were simulated using Aspen Hysys 7.3 software (2011, Burlington, MA, USA) by taking into account all the possible reactions (Equations (2)–(10)) [5,6,7].

3.3. Catalyst Characterization

Fresh catalysts were characterized by oxidation-reduction cycle analysis with a TG/DSC unit (Netzsch® Jupiter STA 449 F3, Selb/Bavaria, Germany) to evaluate the sample reducibility. 10 mg of fresh sample was placed in an alumina crucible and 1% O2/N2, 4% H2/N2, in pure N2 as diluent were used. The sample was initially heated to 150 °C in pure N2. After steady state at 150 °C was reached, oxidizing and reducing gases were alternately introduced. In between reduction and oxidation, pure N2 was introduced to purge the remaining reactive gas. TG/DSC signals were recorded simultaneously. Each oxidation/reduction cycle was carried out isothermally at temperatures ranging from 150–750 °C. The temperature interval was set to be 100 °C. The sample was heated after the completion of each cycle with 1 °C/min in N2 to the next temperature stage. The extent of sample reduction and re-oxidation was quantified by Equations (18) and (19).
W O 2 T iso = W O 2 T iso ,  end W O 2 T iso ,  start
W H 2 T iso = W H 2 T iso ,  end W H 2 T iso ,  start
where W O 2 T i s o or W H 2 T i s o is the sample weight change (%) during the isothermal oxidation or reduction process at programmed temperature T i s o   .
The TEM experiments were conducted to reduced and aged catalyst samples using a JEOL JEM-2010 microscope at 200 kV coupled to energy-dispersive X-ray spectroscopy (EDX, INCA Energy TEM100, Oxford Instruments, Abingdon, Oxfordshire, UK) with a Si(Li) detector, 30 mm2 detection area, and 142 eV resolution. The samples were dispersed in ethanol by ultrasonic vibration and dropped on a carbon film-coated copper grid. The active metal particles were measured with ImageJ software, counting at least 100 particles per image. The mean particle diameter was calculated according to Equation (20), where ni is the number of particles and di is the particle diameter [77,78], and particle size distribution histograms were constructed [79].
  d ¯ p = i n i d i 3 i n i d i 2
XPS spectra of reduced and aged catalysts were obtained using an electron spectrometer (VG-Microtech Multilab, East Grinstead, West Sussex, UK) with a twin anode radiation source in the Mg Kα (1253.6 eV) constant energy analysis mode and an energy flow of 50 eV. The analysis chamber was maintained at 5.10 × 10−8 Pa. The C 1s line was set at 248.6 eV. The binding energy (BE) values were obtained with a precision of 0.2 eV using the Peak Fit program of the spectrometer control program.
The TGA/DTG analysis for reduced and aged samples was performed in a thermogravimetric analyzer (Mettler Toledo, Columbus, OH, USA). Each sample (30 mg) was pretreated in pure N2 at 100 °C and subsequently heated to 1000 °C in air (5 °C/min, 100 mL/min flow rate) to burn off the deposits.

4. Conclusions

Steam reforming of ethanol for hydrogen production was evaluated on RhPt bimetallic catalysts supported on CeO2, ZrO2, and La2O3 with 0.6 wt. % Rh and 0.2 wt. % Pt loading. RhPt/CeO2 showed the highest hydrogen production in the catalytic activity tests, followed by RhPt/La2O3 and RhPt/ZrO2. RhPt/CeO2 was found to be the most selective catalyst, possibly owing to the strong interactions between the metal particles and catalyst support, which resulted in metal particles smaller than 5 nm on the support that probably consisted of Rh-Pt alloys with a Pt surface enrichment. There, both metals were completely oxidized. The considerable oxygen storage capacity of the CeO2 support was confirmed by the presence of Ce+3/Ce+4. However, the reduction of Ce+4 species, which are considered the active sites where reactants and intermediates interact, could be responsible for the loss of methane reforming on RhPt/CeO2 during the stability test, affecting H2 production and byproduct distribution.
Although ZrO2 and La2O3 supported catalysts performed well in the stability test, the active metals (Rh and Pt) underwent rapid sintering on these supports, resulting in their lower H2 selectivity compared with RhPt/CeO2. The metal sintering of both catalysts seems to be related to the limited oxidation of the active phase, which in turn affects the hydrogenation/dehydrogenation reactions to convert the adsorbed ethoxy-species into reforming products.
Nevertheless, the three supported catalysts were not affected by carbonaceous residues under the current SRE conditions, probably because of the noble metals presence and the support basic nature, which avoids undesirable coking of reaction intermediates.

Acknowledgments

The authors are grateful to the Universidad de La Sabana and Universidad de Alicante for the financial support of this work. Martha Cobo acknowledges Robert Farrauto (Columbia University) for his insights on the development of this work.

Author Contributions

This work was developed in equal parts by Martha Cobo and Bernay Cifuentes. Juan A. Conesa participated in the characterization of catalysts through performing techniques such as TEM, EDX, and XPS. Manuel F. Valero advised TGA analyzes and reaction system assembly. However, all the authors contributed to the writing and review of this document.

Conflicts of Interest

The authors declare no conflict of interest.

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MDPI and ACS Style

Cifuentes, B.; Valero, M.F.; Conesa, J.A.; Cobo, M. Hydrogen Production by Steam Reforming of Ethanol on Rh-Pt Catalysts: Influence of CeO2, ZrO2, and La2O3 as Supports. Catalysts 2015, 5, 1872-1896. https://doi.org/10.3390/catal5041872

AMA Style

Cifuentes B, Valero MF, Conesa JA, Cobo M. Hydrogen Production by Steam Reforming of Ethanol on Rh-Pt Catalysts: Influence of CeO2, ZrO2, and La2O3 as Supports. Catalysts. 2015; 5(4):1872-1896. https://doi.org/10.3390/catal5041872

Chicago/Turabian Style

Cifuentes, Bernay, Manuel F. Valero, Juan A. Conesa, and Martha Cobo. 2015. "Hydrogen Production by Steam Reforming of Ethanol on Rh-Pt Catalysts: Influence of CeO2, ZrO2, and La2O3 as Supports" Catalysts 5, no. 4: 1872-1896. https://doi.org/10.3390/catal5041872

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