Combustion Synthesis of a Nickel Supported Catalyst: Effect of Metal

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Combustion Synthesis of a Nickel Supported Catalyst: Effect of Metal Distribution on the Activity during Ethanol Decomposition Allison Cross, Anand Kumar, Eduardo E. Wolf, and Alexander S. Mukasyan* Department of Chemical and Biomolecular Engineering, University of Notre Dame, Notre Dame, Indiana 46556, United States ABSTRACT: Solution combustion synthesis method is used to prepare Ni catalysts supported on γ-Al2O3 pellets with controlled metal distribution. It was shown that impregnation time of the SCS solution onto the pellet determines the distribution of the Ni in the pellet which in turn affects the catalyst selectivity toward hydrogen production during ethanol decomposition.

1. INTRODUCTION Solution combustion synthesis (SCS) has been used for preparation of oxide catalysts with higher surface area than those obtained by coprecipitation methods.1−8 Recently, we have demonstrated that solution combustion can also be used to prepare unsupported nanopowders of metals and alloys.9,10 Use of a porous support can further increase the active phase surface area and enhance the structural and thermal stability of the catalysts. Here it is demonstrated that SCS can be used to prepare supported catalysts with controlled metal distribution along the bulk of the support. Solution combustion synthesis is a redox based reaction which takes place in a homogeneous aqueous solution of oxidizing agents (e.g., metal nitrates) and reducing agents (e.g., glycine). This mixture reacts exothermically upon ignition, generating high energy output sufficient enough to synthesize crystalline materials in a single self-sustained step. A typical SCS reaction between a metal nitrate and glycine is expected to occur as shown in eq 1: Me v(NO3)v (s) +

from many hydrogen containing precursors, and among them light alcohols (e.g., methanol, ethanol) are attractive renewable sources as they can be produced from corn stover and other biomass byproducts. We have previously studied the ethanol decomposition reactions, described in eqs 2 and 3, using unsupported metal−metal oxides:1

C2H5OH → C2H4 + H 2O

(3)

2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. On the basis of an optimization study for the synthesis of pure metals,9 an aqueous solution of nickel nitrate hexahydrate (Alfa Aesar, 98%) and glycine (Alfa Aesar, 98.5%) with a φ value of 1.75 was used. A series of γAl2O3 pellets with a ring geometry (ID = 0.1 in.; OD = 0.25 in., Alfa Aesar) were immersed into the reactive solution for different time intervals ranging from 1 s to 30 min to allow for variations in active metal distribution in the radial direction. There are two reasons for the selection of this support: (i) this type of support was previously used for the preparation of the supported catalyst by solution combustion method;6 and (ii) the support itself is active for the considered reactions and can be used to tune the selectivity of the reactions. The samples were then dried for 24 h and placed vertically on a hot plate set at 500 °C to initiate the ignition and propagation of the combustion front. 2.2. Catalyst Characterization. Surface area, pore volume, and pore size distribution of the alumina rings were measured using the N2 adsorption BET isotherm on a Coulter SA 3100

(1)

The parameter φ is a ratio of fuel to oxidizer, which is defined in such a way that φ = 1 represents a stoichiometric mixture requiring no external oxygen to complete the reaction. φ >1 (φ 90 wt %). Typically, 50−

(4)

FH2produced FC2H4produced

time (min)

(5)

3. RESULTS AND DISCUSSION 3.1. Impregnation Studies. The BET surface area of the γ-Al2O3 rings is 158 m2/g, the pore volume is 0.682 mL/g, and over 95% of the pores have a diameter of less than 80 nm. The microstructures of the initial alumina support, the 1 min impregnated and dried sample, and the combusted supported catalyst are represented by Figure 1a−c, respectively. The average size of unimpregnated γ-Al2O3 grains is on the order of 12005

dx.doi.org/10.1021/ie301478n | Ind. Eng. Chem. Res. 2012, 51, 12004−12008

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precursors between pores. Otherwise, the Ni profiles would have been smeared during the CS reaction resulting in similar distributions of Ni in the support. 3.4. Activity and Selectivity Studies. Ethanol conversion and hydrogen selectivity of the samples are shown in Figure 4. Interestingly, the sample with lower total Ni concentration (∼1.5 wt %) located primarily near the external pellet surface has a higher ethanol conversion. To explain this result, another run was conducted using unimpregnated alumina pellets. As seen in Figure 4a, alumina is an active catalyst and actually yields full ethanol conversion at a lower temperature than the other samples, but is non-selective for hydrogen, producing water and ethylene, according to the ethanol decomposition reaction in eq 3. Therefore, the lower total amount of nickel present in the 5 s sample exposes more alumina sites, leading to higher conversion on this sample. We also compare the reaction rates for unsupported and two supported catalysts, which are calculated as follows: R a = (Fao·X )/W Figure 2. (a) Combustion front propagation as captured by infrared camera and (b) temperature profile of combustion front in 1 min and 5 s samples.

(6)

where Fao is the molar flow rate of ethanol, X is the conversion at T = 200 °C, and W is the weight of active metal. Experiments show that the sample synthesized after 5 s of impregnation possess the highest rate of Ra = 1.01 × 10−3 mol/ min·g, while the 1 min catalyst has the intermediate value of Ra = 3.6 × 10−4 mol/min·g, and pure Ni shows the lowest reaction rate of Ra = 5.31× 10−5 mol/min·g. These results demonstrate that the distribution of the active metal through the support is an important parameter to tune up the effectiveness of the catalyst. Figure 4b shows that the 5 s sample, while exhibiting a higher ethanol conversion, has lower hydrogen selectivity than the 1 min sample, consistent with the run on the unimpregnated alumina pellet. This selectivity represents the ratio of the reaction producing hydrogen to the competing reaction producing ethylene, indicating the 5 s sample produces more water and ethylene than the 1 min sample, but less hydrogen and acetaldehyde, which further supports the above explanation. There is a maximum selectivity for the 1 min sample at 250 °C, with a decreasing trend at higher temperatures, indicating a change in favorable reaction; however, there is the opposite trend for the 5 s sample, showing an increase in hydrogen selectivity, but never becoming greater than the 1 min sample. Hydrogen selectivity is almost negligible for the unimpregnated alumina pellets.

60 wt % inert dilution is the critical value for the combustion wave to self-propagate. When the same reactive solution was diluted to 60 wt % on α-Al2O3 powders with low specific surface area (5 m2/g), self-propagating mode could not be initiated.7 The self-propagation in these highly diluted samples was likely due to the small pore sizes and the consolidated nature of the pellets, which increased heat conduction processes in the pores’ walls and hence reignition and propagation between pores, acting as nanoreactors.8 Such heat conduction processes are less prevalent in the large pores of the α-Al2O3 powders used in the previous study. 3.3. Metal Distribution. EDS analysis carried out on cross sections of the supported catalyst after SCS are shown in Figure 3. For the 5 s impregnated sample, higher concentration of Ni is observed in the regions close to the ring surface at 0 mm for the outer surface and at 3 mm for the internal surface of the ring (Figure 3b), while for 1 min impregnation almost uniform Ni distribution is achieved (Figure 3a). These results indicate that the localized nature of the CS in each pore acting as an isolated nanoreactor and the rapid reaction at the nanoscale preserves the gradient of the impregnated samples at 5 s with limited transport of the

Figure 3. EDS results of Ni and Al concentration in radial direction after SCS: (a) 1 min and (b) 5 s of impregnation. 12006

dx.doi.org/10.1021/ie301478n | Ind. Eng. Chem. Res. 2012, 51, 12004−12008

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Figure 4. Ethanol conversion (a) and hydrogen selectivity (b) for the two distinct time samples and original support are plotted.

Figure 5. Direct combustion synthesis of supported catalyst: the alumina support (a) is impregnated with the reactive solution (b), the combustion wave is initiated and propagates through the pellet in the form of reaction front (c) producing highly dispersed metal in the pores of the inert support (d) with the desired metal distribution along the bulk of the sample.

preserved during SCS, and which affects the selectivity of the reaction when the support itself is active and this effect can be used to tune the selectivity of the reaction. It is worth noting that the combustion synthesis method has a long history, and practice shows that it is a durable technique from the stand-point of safety.4,5 In particular, it is related to the self-propagating reaction mode, which is used in this work. It is also important that this method allows relatively easy scaleup and its safety was demonstrated on continuous technology of SCS of oxide-based catalysts with a production capacity of ∼1 kg of nano-oxides per hour.6,7

XPS results were obtained to determine the Ni to aluminum surface ratios of the samples. The 1 min sample had the higher ratio of 0.067, while the 5 s was 0.048. All samples were run as powders in the XPS, meaning that the 5 s sample powders included parts of the inner pellet which has less Ni, decreasing the average ratio of Ni/Al as compared to the 1 min sample and hence the lower hydrogen selectivity due to the greater exposure of alumina sites. Conversely, the reaction of ethanol to acetaldehyde and hydrogen (eq 2) is the preferred one on the 1 min sample due to higher dispersion of Ni on this sample which reduces exposure of Al sites which are less selective to H2 formation. These results show that by controlling the distribution of active particles by impregnation and CS, it is possible to favor different reactions pathways and control the selectivity when competing reaction pathways are involved.

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AUTHOR INFORMATION

Corresponding Author

*Tel:+1-574-631-9825. Fax: +1-574-631-8366. E-mail: [email protected].

4. CONCLUSIONS The concept of the SCS technique for preparation of the supported catalyst is schematically presented in Figure 5. It can be seen that SCS of Ni supported catalyst takes place via the self-propagating mode under highly diluted conditions in a compacted porous alumina pellet. The impregnation of the metal in the pellet is very fast with only a minute required to obtain a uniform distribution of the metal on the pellet. Shorter impregnation times yield a nonuniform distribution that is

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest. 12007

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ACKNOWLEDGMENTS This research was funded by the Notre Dame Sustainable Energy Initiative (SEI), a part of the Center for Sustainable Energy at Notre Dame (cSEND). Also, we would like to acknowledge the Notre Dame Integrated Imaging Facility for use of their equipment during this research.

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ABBREVIATIONS SCS solution combustion synthesis XPS X-ray photoelectron spectroscopy BET Brunauer - Emmett-Teller method for measuring a material’s specific surface area and gas adsorption properties

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