Hydrogen spillover

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Figure 1: Setup of metal catalyst on a support, the support of which can absorb hydrogen atoms. The receptor represents other optional hydrogen deficient compounds, such as graphene in the context of metal catalysis. Hydrogen Spillover Diagram 1.png
Figure 1: Setup of metal catalyst on a support, the support of which can absorb hydrogen atoms. The receptor represents other optional hydrogen deficient compounds, such as graphene in the context of metal catalysis.

In heterogeneous catalysis, hydrogen molecules can be adsorbed and dissociated by the metal catalyst. Hydrogen spillover is the migration of hydrogen atoms from the metal catalyst onto the nonmetal support or adsorbate. [1] [2] Spillover, generally, is the transport of a species adsorbed or formed on a surface onto another surface. [3] Hydrogen spillover can be characterized by three major steps, the first being where molecular hydrogen is split via dissociative chemisorption into its constitutive atoms on a transition metal catalyst surface, followed by migration from the catalyst to the substrate, culminating in their diffusion throughout the substrate surfaces and/or in the bulk materials. [4]

Contents

Mechanism

The mechanism behind hydrogen spillover has been long disputed. [5] Khoobiar’s work in 1964 marks the nascency of the spillover concept. [3] In his findings, yellow WO3 can be reduced by H2 to a blue compound with the use of a platinum catalyst. [3] Since the phenomenon was not found when using Al2O3 as the catalyst, he claimed that the dissociative chemisorption of H2 molecules on the Pt particles created hydrogen atoms. [3] The hydrogen atoms migrated from the Pt surface to the WO3 particles and reduced them to blue WO3−x particles. [3]

Essentially, hydrogen atoms would migrate from a hydrogen-rich to a hydrogen-poor surface. [3] However, these atoms are usually not generated on the surface of a support metal. [3] Hence, the two conditions for hydrogen spillover include the creation of hydrogen atoms (requires catalysts capable of dissociating and absorbing hydrogen) and the ability of hydrogen atoms to be transported.

Attempts to characterize the mechanism of hydrogen spillover have seen the use of radiation photoelectron spectroscopy to analyze the shift between different oxidation states of the support (commonly metal oxides) via their respective emission spectra. [6] In general, the mechanism is thought to proceed via the transfer of neutral hydrogen atoms to the support upon overcoming an activation energy barrier. [6] This has even been observed at temperatures as low as 180K in metal-organic framework (MOF) catalysts laced with Palladium nanoparticles (PdnP’s). [5] Upon transfer to the support, they assume the role of Lewis bases where they donate electrons and reversibly reduce the sorbent. [5] Additionally, the hydrodesulfurization of dibenzothiophene show that hydroxyl groups seem to favor the migration of spillover hydrogen, whereas sodium cations may trap the spillover hydrogen and are detrimental to hydrogenation pathway. [7]

Recently the mechanism of hydrogen spillover has been described using a precisely nanofabricated model system and single-particle spectromicroscopy. [1] Occurrence of hydrogen spillover on reducible supports such as titanium oxide is established, yet questions remain about whether hydrogen spillover can take place on nonreducible supports such as aluminium oxide. The study shows a convincing proof of the spillover effect at well-defined distances away from the metal catalyst explaining why hydrogen spillover is slower on an aluminum oxide catalyst support than on a titanium oxide catalyst support. The results reveal that hydrogen spillover is fast and efficient on titanium oxide, and extremely slow and short-ranged on aluminium oxide. A recent study has shown that the metal oxide supports that are able to perform hydrogen spillover can catalyze hydrogenation reactions more efficiently (even at room temperature) by supported Pd catalysts. [8]

Figure 2: Dissociative chemisorption of H2 on metal catalysts. Hydrogen atoms move from a hydrogen-rich to a hydrogen-poor surface. Hydrogen Spillover Diagram 2.png
Figure 2: Dissociative chemisorption of H2 on metal catalysts. Hydrogen atoms move from a hydrogen-rich to a hydrogen-poor surface.

Hydrogen spillover increases with adsorption temperature and metal dispersion. [9] A correlation has been reported between available surface area and the capacity for hydrogen storage. For PdnP-containing MOFs, in the presence of saturated metal particles, the capacity for hydrogen spillover only relied on the sorbent’s surface area and pore size. [6] On catalysts such as platinum or nickel, atomic hydrogen can be generated at a high frequency. [9] Through surface diffusion, multi-functional transport of hydrogen atoms can enhance a reaction and even regenerate a catalyst. [9] However, problems present in the strength of the hydrogen-support bond; too strong of an interaction would hinder its extraction via reverse spillover and nullify its function as a fuel cell. [6] Conversely, too weak a bond and the hydrogens are easily lost to the environment. [5]

Figure 3: Hydrogen storage in carbon materials through spillover techniques. In this case, the receptor is a carbon nanotube. Note that while physical mixtures of a primary hydrogen spillover source and a secondary receptor demonstrate moderate storage capacity, adding a bridge to improve the contact between the support metal and the receptor serves to double or triple hydrogen storage capacity on the receptor. Hydrogen Spillover Diagram 3.png
Figure 3: Hydrogen storage in carbon materials through spillover techniques. In this case, the receptor is a carbon nanotube. Note that while physical mixtures of a primary hydrogen spillover source and a secondary receptor demonstrate moderate storage capacity, adding a bridge to improve the contact between the support metal and the receptor serves to double or triple hydrogen storage capacity on the receptor.

Applications

With burgeoning interest in alternative energy sources, the prospect of hydrogen’s role as a fuel has become a major driving force for the optimization of storage methods, particularly at ambient temperatures where their application would be more practical for common use. [5] [10] Hydrogen spillover has emerged as a possible technique for achieving high-density hydrogen storage at near-ambient conditions in lightweight, solid-state materials as adsorbents. [4] [11] Hydrogen storage in carbon materials can be significantly enhanced by spillover techniques. [12] [13] Current trends include the use of metal-organic frameworks (MOFs) and other porous materials with high surface area for such storage, including but not exclusive to nanocarbons (e.g. graphene, carbon nanotubes), [10] [11] zeolites, and nanostructured materials. [11] Hydrogen atom diffusion on nanostructured graphitic carbon materials is primarily governed by physisorption of hydrogen atoms. [4] Singled-walled nanotubes and multi-walled nanotubes are the best acceptor of spilt over hydrogen atoms. [11]

Another recent study has shown that the synthesis of methanol from both CO and CO2 over Cu/ZrO2 involves the spillover of H atoms formed on Cu to the surface of ZrO2. [14] The atomic H then participates in the hydrogenation of carbon-containing species to methanol. [14]

Related Research Articles

<span class="mw-page-title-main">Catalysis</span> Process of increasing the rate of a chemical reaction

Catalysis is the increase in rate of a chemical reaction due to an added substance known as a catalyst. Catalysts are not consumed by the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recycles quickly, very small amounts of catalyst often suffice; mixing, surface area, and temperature are important factors in reaction rate. Catalysts generally react with one or more reactants to form intermediates that subsequently give the final reaction product, in the process of regenerating the catalyst.

<span class="mw-page-title-main">Haber process</span> Main process of ammonia production

The Haber process, also called the Haber–Bosch process, is the main industrial procedure for the production of ammonia. The German chemists Fritz Haber and Carl Bosch developed it in the first decade of the 20th century. The process converts atmospheric nitrogen (N2) to ammonia (NH3) by a reaction with hydrogen (H2) using an iron metal catalyst under high temperatures and pressures. This reaction is slightly exothermic (i.e. it releases energy), meaning that the reaction is favoured at lower temperatures and higher pressures. It decreases entropy, complicating the process. Hydrogen is produced via steam reforming, followed by an iterative closed cycle to react hydrogen with nitrogen to produce ammonia.

<span class="mw-page-title-main">Adsorption</span> Phenomenon of surface adhesion

Adsorption is the adhesion of atoms, ions or molecules from a gas, liquid or dissolved solid to a surface. This process creates a film of the adsorbate on the surface of the adsorbent. This process differs from absorption, in which a fluid is dissolved by or permeates a liquid or solid. While adsorption does often precede absorption, which involves the transfer of the absorbate into the volume of the absorbent material, alternatively, adsorption is distinctly a surface phenomenon, wherein the adsorbate does not penetrate through the material surface and into the bulk of the adsorbent. The term sorption encompasses both adsorption and absorption, and desorption is the reverse of sorption.

<span class="mw-page-title-main">Proton-exchange membrane fuel cell</span> Power generation technology

Proton-exchange membrane fuel cells (PEMFC), also known as polymer electrolyte membrane (PEM) fuel cells, are a type of fuel cell being developed mainly for transport applications, as well as for stationary fuel-cell applications and portable fuel-cell applications. Their distinguishing features include lower temperature/pressure ranges and a special proton-conducting polymer electrolyte membrane. PEMFCs generate electricity and operate on the opposite principle to PEM electrolysis, which consumes electricity. They are a leading candidate to replace the aging alkaline fuel-cell technology, which was used in the Space Shuttle.

<span class="mw-page-title-main">Heterogeneous catalysis</span> Type of catalysis involving reactants & catalysts in different phases of matter

Heterogeneous catalysis is catalysis where the phase of catalysts differs from that of the reactants or products. The process contrasts with homogeneous catalysis where the reactants, products and catalyst exist in the same phase. Phase distinguishes between not only solid, liquid, and gas components, but also immiscible mixtures, or anywhere an interface is present.

The water–gas shift reaction (WGSR) describes the reaction of carbon monoxide and water vapor to form carbon dioxide and hydrogen:

Nanomaterial-based catalysts are usually heterogeneous catalysts broken up into metal nanoparticles in order to enhance the catalytic process. Metal nanoparticles have high surface area, which can increase catalytic activity. Nanoparticle catalysts can be easily separated and recycled. They are typically used under mild conditions to prevent decomposition of the nanoparticles.

<span class="mw-page-title-main">Carbon nanofiber</span>

Carbon nanofibers (CNFs), vapor grown carbon fibers (VGCFs), or vapor grown carbon nanofibers (VGCNFs) are cylindrical nanostructures with graphene layers arranged as stacked cones, cups or plates. Carbon nanofibers with graphene layers wrapped into perfect cylinders are called carbon nanotubes.

<span class="mw-page-title-main">Hydrogen storage</span> Methods of storing hydrogen for later use

Several methods exist for storing hydrogen. These include mechanical approaches such as using high pressures and low temperatures, or employing chemical compounds that release H2 upon demand. While large amounts of hydrogen are produced by various industries, it is mostly consumed at the site of production, notably for the synthesis of ammonia. For many years hydrogen has been stored as compressed gas or cryogenic liquid, and transported as such in cylinders, tubes, and cryogenic tanks for use in industry or as propellant in space programs. Interest in using hydrogen for on-board storage of energy in zero-emissions vehicles is motivating the development of new methods of storage, more adapted to this new application. The overarching challenge is the very low boiling point of H2: it boils around 20.268 K (−252.882 °C or −423.188 °F). Achieving such low temperatures requires expending significant energy.

<span class="mw-page-title-main">Metal–organic framework</span> Class of chemical substance

Metal–organic frameworks (MOFs) are a class of compounds consisting of metal clusters coordinated to organic ligands to form one-, two-, or three-dimensional structures. The organic ligands included are sometimes referred to as "struts" or "linkers", one example being 1,4-benzenedicarboxylic acid (BDC).

<span class="mw-page-title-main">Electrocatalyst</span> Catalyst participating in electrochemical reactions

An electrocatalyst is a catalyst that participates in electrochemical reactions. Electrocatalysts are a specific form of catalysts that function at electrode surfaces or, most commonly, may be the electrode surface itself. An electrocatalyst can be heterogeneous such as a platinized electrode. Homogeneous electrocatalysts, which are soluble, assist in transferring electrons between the electrode and reactants, and/or facilitate an intermediate chemical transformation described by an overall half reaction. Major challenges in electrocatalysts focus on fuel cells.

<span class="mw-page-title-main">Catalyst support</span> Porous material with a high specific surface area supporting a catalyst

In chemistry, a catalyst support is a material, usually a solid with a high surface area, to which a catalyst is affixed. The activity of heterogeneous catalysts is mainly promoted by atoms present at the accessible surface of the material. Consequently, great effort is made to maximize the specific surface area of a catalyst. One popular method for increasing surface area involves distributing the catalyst over the surface of the support. The support may be inert or participate in the catalytic reactions. Typical supports include various kinds of activated carbon, alumina, and silica.

Carbocatalysis is a form of catalysis that uses heterogeneous carbon materials for the transformation or synthesis of organic or inorganic substrates. The catalysts are characterized by their high surface areas, surface functionality, and large, aromatic basal planes. Carbocatalysis can be distinguishable from supported catalysis in that no metal is present, or if metals are present they are not the active species.

The first time a catalyst was used in the industry was in 1746 by J. Roebuck in the manufacture of lead chamber sulfuric acid. Since then catalysts have been in use in a large portion of the chemical industry. In the start only pure components were used as catalysts, but after the year 1900 multicomponent catalysts were studied and are now commonly used in the industry.

<span class="mw-page-title-main">Carbon nanotube supported catalyst</span> Novel catalyst using carbon nanotubes as the support instead of the conventional alumina

Carbon nanotube supported catalyst is a novel supported catalyst, using carbon nanotubes as the support instead of the conventional alumina or silicon support. The exceptional physical properties of carbon nanotubes (CNTs) such as large specific surface areas, excellent electron conductivity incorporated with the good chemical inertness, and relatively high oxidation stability makes it a promising support material for heterogeneous catalysis.

Liquid–feed flame spray pyrolysis (LF-FSP) is one of the most recent iterations in flame spray pyrolysis (FSP) powder production technology. FSP produces metal oxide powders from highly volatile gaseous metal chlorides that are decomposed/oxidized in hydrogen-oxygen flames to form nano-oxide powders. However, products made from FSP's vapor-phase process are limited to Al-, Ti-, Zr-, and Si-based oxides from their metal chlorides. Thus, interest in producing more complex materials required a new methodology, LF-FSP.

Filamentous carbon is a carbon-containing deposit structure that refers to several allotropes of carbon, including carbon nanotubes, carbon nanofibers, and microcoils. It forms from gaseous carbon compounds. Filamentous carbon structures all contain metal particles. These are either iron, cobalt, or nickel or their alloys. Deposits of it also significantly disrupt synthesis gas methanation. Acetylene is involved in a number of method of the production of filamentous carbon. The structures of filamentous carbon are mesoporous and on the micrometer scale in dimension. Most reactions that form the structures take place at or above 280 °C (536 °F).

Vertically aligned carbon nanotube arrays (VANTAs) are a unique microstructure consisting of carbon nanotubes oriented with their longitudinal axis perpendicular to a substrate surface. These VANTAs effectively preserve and often accentuate the unique anisotropic properties of individual carbon nanotubes and possess a morphology that may be precisely controlled. VANTAs are consequently widely useful in a range of current and potential device applications.

<span class="mw-page-title-main">Aluminium oxide nanoparticle</span>

Nanosized aluminium oxide occurs in the form of spherical or nearly spherical nanoparticles, and in the form of oriented or undirected fibers.

<span class="mw-page-title-main">Heterogeneous gold catalysis</span>

Heterogeneous gold catalysis refers to the use of elemental gold as a heterogeneous catalyst. As in most heterogeneous catalysis, the metal is typically supported on metal oxide. Furthermore, as seen in other heterogeneous catalysts, activity increases with a decreasing diameter of supported gold clusters. Several industrially relevant processes are also observed such as H2 activation, Water-gas shift reaction, and hydrogenation. One or two gold-catalyzed reactions may have been commercialized.

References

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  2. ↑ Gardes, G. E. E., Pajonk, G. M., and S. J. Teichner (1974). “Catalytic Demonstration of Hydrogen Spillover from Nickel-Alumina Catalyst to Alumina.” J. Catal. 33, 145-148.
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  4. 1 2 3 Hansong Cheng, Liang Chen, Alan C. Cooper, Xianwei Sha, Guido P. Pez: Hydrogen spillover in the context of hydrogen storage using solid-state materials. In: Energy & Environmental Science. 1, 2008, S. 338, doi : 10.1039/B807618A.
  5. 1 2 3 4 5 Sculley, J., Yuan, D., Zhou, H. (2011). “The current status of hydrogen storage in metal–organic frameworks—updated”. Energy Environ. Sci. 4, 2721-2735.
  6. 1 2 3 4 Lykhach, Y., Staudt, T., Vorohkta, M., Skala, T. Johanek, V., Prince, KC., Matolin, V., Libuda, J. (2012). “Hydrogen spillover monitored by resonant photoemission spectroscopy”. J. Catal. 285, 6-9. 12
  7. ↑ Wang, A., Li, X., et al. (2004). “Hydrodesulfurization of Dibenzothiophene Over Proton-Exchanged Siliceous MCM-41 Supported Bimetallic Sulfides”. Dalian University of Technology, China
  8. ↑ Y. An; P. Chatterjee; P. Naik; S. Banerjee; W. Huang; I. I. Slowing; V. Venditti (2023). "Hydrogen spillover and substrate–support hydrogen bonding mediate hydrogenation of phenol catalyzed by palladium on reducible metal oxides". Chem. Sci. doi: 10.1039/D3SC02913A . PMC   10717535 .
  9. 1 2 3 Andrew, M., and R. Kramer (1979). “Adsorption of Atomic Hydrogen on Alumina by Hydrogen Spillover.” J. Catal. 58, 287-295.
  10. 1 2 Pevzner, S., Pri-Bar, I., Lutzky, I., Ben-Yehuda, E., Ruse, E., Regev, O. (2014). “Carbon Allotropes Accelerate Hydrogenation via Spillover Mechanism”. J. Phys. Chem. C. 118, 27164–27169.
  11. 1 2 3 4 Lueking, A. D., & Yang, R. T. (2004). Hydrogen spillover to enhance hydrogen storage: study of the effect of carbon physicochemical properties. Applied Catalysis A: General, 265, 2.)
  12. ↑ Wang, L., & Yang, R. T. (2008). New sorbents for hydrogen storage by hydrogen spillover - a review. Energy & Environmental Science, 1, 2, 268-279
  13. ↑ Lachawiec, A. J. J., Qi, G., & Yang, R. T. (2005). Hydrogen storage in nanostructured carbons by spillover: bridge-building enhancement. Langmuir : the Acs Journal of Surfaces and Colloids, 21, 24, 11418-24.
  14. 1 2 Jung, K-D. & Bell, A. T. (2000). “Role of hydrogen spillover in methanol synthesis over Cu/ZrO2”. J. Catal. 193, 207–223
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