Elsevier

Applied Catalysis A: General

Volume 366, Issue 2, 25 September 2009, Pages 353-362
Applied Catalysis A: General

Catalytic decomposition of alcohols over size-selected Pt nanoparticles supported on ZrO2: A study of activity, selectivity, and stability

https://doi.org/10.1016/j.apcata.2009.07.028Get rights and content

Abstract

This article discusses the performance of ZrO2-supported size-selected Pt nanoparticles for the decomposition of methanol, ethanol, 2-propanol, and 2-butanol. The potential of each alcohol for the production of H2 and other relevant products in the presence of a catalyst is studied in a packed-bed mass flow reactor operating at atmospheric pressure. All the alcohols studied show some decomposition activity below 200 °C which increased with increasing temperature. In all cases, high selectivity towards H2 formation is observed. With the exception of methanol, all alcohol conversion reactions lead to catalyst deactivation at high temperatures (T > 250 °C for 2-propanol and 2-butanol, T > 325 °C for ethanol) due to carbon poisoning. However, long-term catalyst deactivation can be avoided by optimizing reaction conditions such as operating temperature.

Graphical abstract

This article describes H2 production by decomposition of C1–C4 alcohols over size-selected Pt-nanoparticles supported on nanocrystalline ZrO2. Alcohol conversion and H2 production were found to increase as ethanol < 2-butanol < 2-propanol < methanol between 100 °C and 400 °C. With the exception of methanol, all alcohols caused catalyst deactivation above 250 °C due to carbon-containing deposits. High hydrogen selectivities were observed for all alcohols.

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Introduction

The decomposition of alcohols over metal and metal oxide catalysts has been the subject of numerous studies due to its applicability to a variety of chemical synthesis processes [1], [2], [3], [4], [5]. In recent years, this subject has gained particular interest due to growing environmental, economic, and political concerns regarding energy production [2]. Safe and efficient in situ hydrogen generation from alcohols (i.e. methanol, ethanol, propanol, butanol) can promote the use of fuel cells and other clean technologies as a source of energy for mobile applications. Alcohols can serve as H2 carriers that are compatible with current infrastructures for liquid fuels and can be catalytically decomposed on-site in order to minimize energy input requirements and operating temperatures [6].

Catalysts that promote methanol (MeOH) and ethanol oxidation processes are key in the improvement and optimization of direct MeOH and ethanol fuel cells [6], [7], [8], [9], [10], [11], [12], [13] as well as direct H2 fuel cells, for which decomposition catalysts are also of great importance. Since ethanol can be produced from the fermentation of biomass [7], it is a potentially renewable fuel that can be reformed on-site to generate H2. The decomposition of higher order alcohols such as 2-propanol and 2-butanol can also be of interest for a variety of hydrogen-based energy applications while generating other products, i.e. acetone and butanone, which are of great importance in the chemical synthesis industry [14], [15]. By increasing activity and selectivity, industrial catalysts can decrease the energy input required as well as the output of potentially harmful by-products associated with numerous chemical processes. Furthermore, it has been shown that the addition of H2 to fuel mixtures enhances the efficiency and lowers harmful emissions of internal combustion engines [16], [17]. Catalytic reforming of gasoline additives, e.g. methanol, ethanol, and potentially butanol, may serve as an on-board source of hydrogen [18]. Waste streams from industries that make use of solvents may contain significant amounts of alcohols and other organics used for this purpose and may require special treatment. Recent studies have focused on the feasibility of obtaining hydrogen (via steam reforming) from 2-propanol, 2-butanol, and other widely used solvents to be used in stationary fuel cell applications [19], [20]. Dehydrogenation of these higher order alcohols may bring the same benefits (i.e. reduction of waste and H2 production) while generating other valuable products for industrial applications. Efficient catalysts for the dehydrogenation of higher order alcohols may also contribute to improvements in chemical heat pumps, such as those based on the 2-propanol/acetone/hydrogen cycle [21], [22].

Methanol decomposition has been shown to take place over a variety of metal surfaces and nanocatalysts including Pt, Pd, Au, Rh, Ni, Co, and Cu supported on metal oxides such as Al2O3, CeO2, ZrO2, ZnO, TiO2, and SiO2 [9], [12], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39], [40], [41], [42], [43]. In the absence of O2 and water, MeOH decomposition favors the formation of H2 and CO, which can be further reacted in the presence of water to form additional H2 and CO2 through the water-gas shift reaction. Platinum has been shown to have good activity and selectivity towards H2 formation. Previous studies from our group on the decomposition of MeOH over similarly sized Pt nanoparticles (NPs) deposited on SiO2, Al2O3, CeO2, TiO2, and ZrO2 [29] revealed an enhanced activity for the Pt/ZrO2 system.

Ethanol decomposition and the associated reactions including ethanol oxidation and steam reforming have also been thoroughly studied over Pt, Pd, Rh, Cu, Co, Ir, Ni, Fe, and other catalysts supported on CeO2, Al2O3, ZrO2, ZnO, and CuO [7], [13], [18], [44], [45], [46], [47], [48], [49], [50], [51], [52], [53], [54], [55], [56], [57], [58], [59], [60], [61], [62], [63], and good selectivity towards hydrogen has been observed in several of these systems. The choice of support and the presence or absence of reactants such as O2 and water, will determine the reaction's selectivity. Pt/CeO2 and Pt/ZrCeO2 catalysts tend to promote the formation of H2 and acetaldehyde [64] and it has been found that these reducible supports enhance catalyst stability [54].

The decomposition of propanol and butanol is carried out in the chemical synthesis industry over Cu-, Zn-, and Ni-based catalysts as well as Pt/Al2O3. However, concerns for the fate of some of the most commonly used catalysts for this purpose, such as copper chromite, and their environmental impact have provided a motivation to further study and optimize alternative catalysts that may serve this purpose efficiently. In addition, the opportunity of using these alcohols for hydrogen generation to serve as an energy source has placed particular emphasis on finding catalysts that display high conversion and selectivity towards H2 formation as well as long-term stability. Some studies describing the decomposition of higher order alcohols, i.e. 2-propanol and 2-butanol, over metallic films (Pd–Ag) [65], micrometer-sized powders (Co, Cu, Mg2Cu, Ni, metal oxides) [66], [67], [68], [69], [70], [71], [72], and supported NPs (Pt, Cu, Pt–Cu, copper chromite, W) [73], [74], [75], [76], [77], [78], [79], [80] can be found in the literature.

In order to assess the potential of catalyzed alcohol decomposition we have conducted a systematic study of these reactions over size-selected Pt NPs supported on ZrO2. The objective of this study is to assess the catalytic performance (activity, selectivity, and lifetime) of novel, micelle-synthesized, Pt catalysts. The conversion and selectivity of each C1–C4 alcohol was evaluated over a range of temperatures (100–300 °C) under atmospheric pressure in a packed-bed mass flow reactor interfaced to a mass spectrometer. Catalyst stability was monitored through a prolonged exposure to the reactants and insight into the origin of their deactivation at high temperature due to carbonaceous species poisoning was evaluated by X-ray photoelectron spectroscopy (XPS). The by-products of the decomposition reactions are also investigated in order to assess the potential emissions associated with these processes as well as opportunities for material recovery in industrial applications.

Section snippets

Experimental

Size-selected platinum nanoparticles were synthesized using inverse-micelle encapsulation [27], [28], [81]. A SiO2 substrate was dip-coated in the metal-loaded polymeric solution and treated with an O2 plasma (for polymer removal) to allow morphological sample characterization using atomic force microscopy (AFM). Pt particles were deposited via impregnation on our nanocrystalline ZrO2 support with a nominal Pt loading of 2 wt% Pt. The Pt/ZrO2 catalyst powder was then annealed in an oxygen

Morphology (AFM, TEM)

The AFM image in Fig. 1 reveals the narrow size distribution of our as prepared Pt nanoparticles when dip-coated on SiO2/Si(0 0 1). This micrograph was acquired after polymer removal in UHV by O2 plasma. The average NP height was found to be 2.1 ± 0.4 nm.

The TEM images shown in Fig. 2 (overview) and Fig. 3 (high resolution) show the as prepared Pt NPs deposited on the ZrO2 support after annealing in O2 and before reaction with the different alcohols [Figs. 2(a) and 3(a)], and after reaction with

Discussion

As described in Fig. 6(a), all the alcohols studied displayed some conversion over the Pt/ZrO2 catalyst below 200 °C. Within the 100–200 °C range, 2-propanol followed by 2-butanol display the greatest conversion. At 225 °C, over 70% of the 2-propanol and 2-butanol introduced is being catalytically converted. Over the Pt-free ZrO2 support [Fig. 6(b)], 2-propanol and 2-butanol display higher conversion than methanol and ethanol over the entire temperature range, although at lower rates as compared

Conclusions

We have conducted a systematic study of the performance of nanoscale Pt/ZrO2 catalyst for the decomposition of C1–C4 alcohols at temperatures between 100 °C and 400 °C. Our preparation method allowed us to obtain NPs with narrow size distributions, resulting in model material system for our comparative reactivity studies.

The main results derived from the analysis of our data include: (i) the onset temperature for alcohol decomposition (and H2 generation) increases from propanol  butanol < methanol  

Acknowledgement

We would like to thank Aniruddha Dutta for the analysis of the TEM images and Elaine Zhou for her help with the preparation of these samples. This work was possible thanks to the funding of the Office of Basic Energy Sciences of the US Department of Energy (DE-FG02-08ER15995).

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