Tushar V. Choudhary

Catalytic ammonia decomposition: COx-free hydrogen production for fuel cell applications

Catalytic decomposition of ammonia has been investigated as a method to produce hydrogen for fuel cell applications. The absence of any undesirable by-products (unlike, e.g., COx, formed during reforming of hydrocarbons and alcohols) makes this process an ideal source of hydrogen for fuel cells. In this study a variety of supported metal catalysts have been studied. Supported Ru catalysts were found to be the most active, whereas supported Ni catalysts were the least active. The supports were found to play a profound role in the ammonia decomposition process. The activation energies for the ammonia decomposition process varied from 17 to 22 kcal/mol depending upon the catalyst employed. The activation energies of the supported Ir catalysts were found to be in excellent agreement with our recent studies addressing ammonia decomposition on single crystal Ir.

CO-free fuel processing for fuel cell applications

In view of the stringent CO intolerance of the state-of-the-art proton exchange membrane (PEM) fuel cells, it is desirable to explore CO-free fuel processing alternatives. In recent years, step-wise reforming of hydrocarbons has been proposed for production of CO-free hydrogen for fuel cell applications. The decomposition of hydrocarbons (first step of the step-wise reforming process) has been extensively investigated. Both steam and air have been employed for catalyst regeneration in the second step of the process. Since, PEM is poisoned by very low (ppm) levels of CO, it is essential to eliminate even trace amounts of CO from the reformate stream. Preferential oxidation of CO (PROX) is considered to be a promising method for trace CO clean up. Related studies along with a discussion of catalytic ammonia decomposition (for applications in alkaline fuel cells) will be included in this review.

Oxidation Catalysis by Supported Gold Nano-Clusters

The historical notion regarding the inability of gold to catalyze reactions has been discarded in view of recent studies, which have clearly demonstrated the high catalytic efficiency of supported nano-gold catalysts. Although nano-Au catalysts are known to catalyze a variety of reactions, the major focus has been on CO oxidation catalysis. In this work we focus on the important aspects related to the CO oxidation reaction. Special emphasis is placed on the studies undertaken on model nano-Au systems as these studies have considerably enhanced the understanding of the oxidation process. Gold in a highly dispersed state can selectively oxidize CO in the presence of excess hydrogen (of tremendous interest to state-of-the-art low-temperature fuel cells); related studies are addressed in this review. The nano-gold catalysts have also been investigated for the direct vapor-phase oxidation of propylene to propylene oxide in the presence of molecular oxygen; these investigations are highlighted in this work.

Nonoxidative Activation of Methane

Effective utilization of methane remains one of the long-standing problems in catalysis. Over the past several years, various routes, both direct and indirect, have been considered for the conversion of methane to value-added products such as higher hydrocarbons and oxygenates. This review will focus on the range of issues dealing with thermal and catalytic decomposition of methane that have been addressed in the last few years. Surface science studies (molecular beam methods and elevated-pressure reaction studies) involving methane activation on model catalyst systems are extensively reviewed. These studies have contributed significantly to our understanding of the fundamental dynamics of methane decomposition. Various aspects of the nonoxidative methane to higher hydrocarbon conversion processes such as high-temperature coupling and two-step low-temperature methane homologation have been discussed. Decomposition of methane results in the production of COx-free hydrogen (which is of great interest to state-of-the-art low-temperature fuel cells) and various types of carbon (filamentous carbon, carbon black, diamond films, etc.) depending on the reaction conditions employed; these issues will be briefly addressed in this review.

Acetylene hydrogenation on Au-based catalysts

Hydrogenation of acetylene has been investigated on Au/TiO2, Pd/TiO2 and Au-Pd/TiO2 catalysts at high acetylene conversion levels. The Au/TiO2 catalyst (avg. particle size: 4.6 nm) synthesized by the temperature-programmed reduction-oxidation of an Au-phosphine complex on TiO2 showed a remarkably high selectivity to ethylene formation even at 100% acetylene conversion. Au/TiO2 prepared by the conventional incipient wet impregnation method (avg. particle size: 30 nm), on the other hand, showed negligible activity for acetylene hydrogenation. Although the Au catalysts showed a high selectivity for ethylene, the acetylene conversion activity and catalyst stability were inferior to the Pd-based catalysts. Au-Pd catalysts prepared by the redox method showed high acetylene conversions as well as high selectivity for ethylene. Interestingly Au-Pd catalysts prepared by depositing Pd via the incipient wetness method on Au/TiO2 showed very poor selectivity (comparable to mono-metallic Pd catalysts) for ethylene. High-resolution transmission electron microscopy (TEM) studies coupled with energy dispersive X-ray spectroscopy (EDS) showed that while the redox method produced bimetallic Au-Pd catalysts, the latter method produced individual Pd and Au particles on the support.

Production of COx-free hydrogen for fuel cells via step-wise hydrocarbon reforming and catalytic dehydrogenation of ammonia

The stringent COx-free hydrogen requirement for the current low temperature fuel cells has motivated the development of COx-free hydrogen production alternatives to the conventional hydrogen production technologies. Recently, our group has investigated step-wise reforming of hydrocarbons and catalytic decomposition of ammonia for COx-free production of hydrogen. These investigations have employed conventional surface science techniques, model catalysts as well as high surface area supported metal catalysts. This paper presents an overview of the studies undertaken in our laboratory and highlights the important aspects of the proposed CO-free hydrogen production processes.

Ammonia decomposition on Ir(100): from ultrahigh vacuum to elevated pressures

Ammonia decomposition on Ir(100) has been studied over the pressure range from ultrahigh vacuum to 1.5 Torr and at temperatures ranging from 200 to 800 K. The kinetics of the ammonia decomposition reaction was monitored by total pressure change. The apparent activation energy obtained in this study (84 kJ/mol) is in excellent agreement with our previous studies using supported Ir catalysts (Ir/Al2O3 82 kJ/mol). Partial pressure dependence studies of the reaction rate yielded a positive order (0.9±0.1) with respect to ammonia and negative order (−0.7 ±0.1) with respect to hydrogen. Temperature-programmed desorption data from clean and hydrogen co-adsorbed Ir(100) surfaces indicate that ammonia undergoes facile decomposition on both these surfaces. Recombinative desorption of N2 is the rate-determining step with a desorption activation energy of ∼63 kJ/mol. Co-adsorption data also indicate that the observed negative order with respect to hydrogen pressure is due to enhancement of the reverse reaction (NHx + H → NHx+1, x=0–2) in the presence of excess H atoms on the surface.

Methane Activation on Ruthenium: The Nature of the Surface Intermediates

The identification of surface intermediates is an important step in determining the mechanism of a reaction and in developing an in-depth understanding of a catalytic process. In this work the surface species formed during methane decomposition on idealized single-crystal Ru catalysts (using high-resolution electron energy loss spectroscopy, HREELS) are compared to the surface species identified on real Ru catalysts (using neutron vibrational spectroscopy, NVS). Kinetic studies of methane homologation on single-crystal catalysts and on high surface area supported Ru catalysts have been carried out in parallel with the HREELS and NVS studies to relate rates of reaction with the concentration of particular surface intermediates.

Characterization of C2 (CxHy) Intermediates from Adsorption and Decomposition of Methane on Supported Metal Catalysts by in situ INS Vibrational Spectroscopy

Conversion of methane into higher hydrocarbons is a process of enormous technological importance, which would benefit from a better understanding of the nature of the surface intermediates formed during methane activation. Currently methane is converted into hydrocarbons mainly by an indirect route[1, 2] via syn gas[3] as the intermediate. Alternative routes, such as oxidative coupling of methane[4±7] over metal oxide catalysts and methane homologation[8±10] on transition metal catalysts have also been extensively investigated. In a previous study we[11] have observed the formation of various CxHy surface intermediate species after methane decomposition on single-crystal Ru(0001) and Ru(1120) model catalysts, observed by high-resolution electron energy loss spectroscopy (HREELS). In this study we used INS to investigate the surface intermediate species formed during the decomposition of methane on Ru/Al2O3 and Ni/SiO2 catalysts. We relate these findings to our previous work on idealized single-crystal model catalysts. The present work represents a step in the effort to bridge the gap between surface science and real-world catalysts in terms of the type of material and pressure used. Inelastic neutron scattering (INS)[12±14] vibrational spectroscopy is a technique that can be directly applied to high surface-area catalysts from ambient to high pressures. This method can also provide accurate quantitative information, has high sensitivity to hydrogenous species, and is not limited by the selection rules of other vibrational spectroscopies. Herein we report the first experimental evidence of the formation of ethylidyne, vinylidene, and methylidyne (CxHy) species from methane on supported metal catalysts. Shown in Figure 1 is the difference INS vibrational spectrum after methane decomposition on Ru/Al2O3. The rather modest resolution, especially at higher frequencies, and poor statistics are the result of having to use a difference spectrum and of the relatively small amount of H in the sample. Nonetheless, the rate was defined as the number of doublings between day 5 and day 9 (determined as the logarithm in base 2 of the increase in the DNA amount) divided by the elapsed time (4 days).

Entrance of straight and branched chain compounds from their bulk liquid phase into H-ZSM-5 zeolite

Intra-crystalline mass transfer in H-ZSM-5 for the entry of n-butylamine, n-butanol, iso-butanol, tert-butanol, n-hexanol, n-octanol, 2,3 dimethyl butane and iso-octane from their bulk liquid phase has been studied by measuring their initial rate of sorption. The entry of the sorbate molecules into the zeolite channels is strongly influenced by the critical size of the molecules, but much more strongly by their configuration. The influence of both the critical size and configuration may very well be explained by the Shuttlecock-Shuttlebox model for the sorption of bulky molecules in medium-pore zeolites. The activation energy for the entry of n-butanol, iso-butanol and higher n-alcohols is found to be quite different from that of their overall sorption/diffusion from the liquid phase into the same zeolite. For the entry and overall sorption/diffusion, the steric hindrance of bulky sorbate molecules is a common factor deciding the activation energy, but the other factors for the two cases are different. For the first case, only physical sorbate-zeolite interactions are involved but for the second case, both the physical and chemical sorbate-zeolite interactions and also sorbate-sorbate interactions are involved. Also, the entry of sorbate molecules into the zeolite channels from liquid phase differs from their entry to channels from the gas phase, as an additional energy barrier exists for the former case.