Hydrogen production reactions from carbon feedstocks: fossil fuels and biomass.

We present a comprehensive mean-field microkinetic model for the methanol synthesis and water-gas-shift (WGS) reactions that includes novel reaction intermediates, such as formic acid (HCOOH) and hydroxymethoxy (CH3O2) and allows for the formation of formic acid (HCOOH), formaldehyde (CH2O), and methyl formate (HCOOCH3) as byproducts. All input model parameters were initially derived from periodic, self-consistent, GGA-PW91 density functional theory calculations on the Cu(111) surface and subsequently fitted to published experimental methanol synthesis rate data, which were collected under realistic conditions on a commercial Cu/ZnO/Al2O3 catalyst. We find that the WGS reaction follows the carboxyl (COOH)-mediated path and that both CO and CO2 hydrogenation pathways are active for methanol synthesis. Under typical industrial methanol synthesis conditions, CO2 hydrogenation is responsible for ∼2/3 of the methanol produced. The intermediates of the CO2 pathway for methanol synthesis include HCOO*, HCOOH*, C...

Mechanism of Methanol Synthesis on Cu through CO2 and CO Hydrogenation

Hydrogen Spillover. Facts and Fiction

The search for hydrogen storage materials capable of efficiently storing hydrogen in a compact and lightweight package is one of the most difficult challenges for the upcoming hydrogen economy. Liquid chemical hydrides with high gravimetric and volumetric hydrogen densities have the potential to overcome the challenges associated with hydrogen storage. Moreover, the liquid-phase nature of these hydrogen storage systems provides significant advantages of easy recharging, and the availability of the current liquid fuel infrastructure for recharging. In this review, we briefly survey the research progress in the development of diverse liquid-phase chemical hydrogen storage materials, including organic and inorganic chemical hydrides, with emphases on the syntheses of active catalysts for catalytic hydrogen generation and storage. Moreover, the advantages and drawbacks of each storage system are discussed.

Liquid organic and inorganic chemical hydrides for high-capacity hydrogen storage

The review is a summary and analysis of the data characterizing CO adsorption on surface cationic sites of oxides including supported materials and microporous and mesoporous materials. The contributions of various types of CO bonding to the IR frequency shifts of carbon-bonded molecules are analyzed, namely, the increase of the CO stretching frequency in cases of electrostatic and σ bonding and the decrease of the frequency with π bonding. Polycarbonyls, bridging CO, oxygen-bonded CO, and tilted CO are also considered. The main part of the review is a collection of the experimental results characterizing carbonyls of individual metal ions. The spectral behavior of CO bonded to metal atoms is also assessed in the cases when the metal ions are easily reduced to metal (Cu, Ag, Au, Pd, or Pt) or cationic carbonyls are produced after CO adsorption on supported metals (Ru, Rh, Ir, and Os). The interaction of CO with surface OH groups is also considered. It is demonstrated that IR spectroscopy of adsorbed CO is an efficient methodology to characterize cationic surface sites in terms of their nature, oxidation states, coordination environment and coordinative unsaturation, and location at faces, edges or corners of microcrystallites. When applied to materials with surface hydroxyl groups CO undergoes hydrogen bonding and information can be collected on the proton acid strength.

Characterization of oxide surfaces and zeolites by carbon monoxide as an IR probe molecule

The surfaces of the copper metal crystallites of working Cu/ZuO/Al2O3 and other copper catalysts are partially oxidised in reaction mixtures for methanol synthesis and the water-gas shift reaction. Work with unsupported polycrystalline copper has confirmed earlier results that copper metal is the active phase in supported copper catalysts. The coverage of adsorbed oxygen, O(a), up to half-monolayer, was determined by reaction with N2O and it was found to be controlled by the overall reaction CO2(g)= CO(g)+ O(a). The free energy of formation of O(a) was calculated to be –240 kJ mol–1 at 513 K. The induction period found in methanol synthesis from CO–CO2–H2 mixtures is consistent with the calculated rate of formation of O(a). The role of O(a) in the methanol synthesis and water-gas shift reactions is both as promoter and reaction intermediate. The dissociative chemisorption of hydrogen on copper is promoted by O(a) but this is not necessary for the reactions. Experiments with unsupported polycrystalline copper have shown that O(a) both increases the extent of physisorption of CO2 and creates new chemisorbed states of CO2, with desorption energies of 109, 113 and 125 kJ mol–1. O(a) is also essential for the dissociative chemisorption of water on copper. A regenerative mechanism for the water-gas shift reaction on copper [involving the formation and reaction of O(a)] has been established by observation of the separate stages. The adsorbed formate intermediate, pivotal in methanol synthesis from carbon dioxide, is irrelevant to the water-gas shift reaction.

Promotion of methanol synthesis and the water-gas shift reactions by adsorbed oxygen on supported copper catalysts

Infrared spectra are reported of methyl formate and formaldehyde adsorbed at 300 K on silica, Cu/SiO2 reduced in hydrogen and Cu/SiO2 which had been oxidised by exposure to nitrous oxide after reduction. Silanol groups on silica form hydrogen bonds with carbonyl groups in weakly adsorbed methyl formate molecules. Methyl formate ligates via its carbonyl groups to Cu atoms in the surface of reduced copper. A low residual concentration of surface oxygen on copper promoted the slow reaction of ligated methyl formate to give a bridging formate species on copper and adsorbed methoxy groups. Methyl formate did not ligate to an oxidised copper surface but was rapidly chemisorbed to give unidentate formate and methoxy species. Formaldehyde slowly polymerises on silica to form trioxane and other oxymethylene species. The reaction is faster over Cu/SiO2 which, in the reduced state, also catalyses the formation of bridging formate anions adsorbed on copper. The reaction between formaldehyde and oxidised Cu/SiO2 leads to both unidentate and bidentate formate and adsorbed water.

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Infrared study of methyl formate and formaldehyde adsorption on reduced and oxidised silica-supported copper catalysts

Infrared spectra are reported of methanol adsorbed at 295 K on reduced Cu/SiO2 and on Cu/SiO2 which had been preoxidised by exposure to excess nitrous oxide. Methanol was chemisorbed on reduced Cu/SiO2 to give methoxy species on both silica and copper, gave a trace of formate on copper via reaction with residual surface oxygen, and was weakly adsorbed at SiOH sites on the silica support. Heating the adsorbed species at 393 K led to the loss of methoxy groups on copper and the concomitant formation of a bidentate surface formate. Heating reduced Cu/SiO2 in methanol at 538 K initially gave both gaseous and adsorbed (on Cu) methyl formate which subsequently decomposed to CO and hydrogen. The reactions of methanol with oxidised Cu/SiO2 were similar to those for the reduced catalyst although surface oxygen promoted the formation of surface methoxy groups on copper. Subsequent heating at 393 K led first to unidentate formate before the appearance of bidentate formate.

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Infrared study of the adsorption of methanol on oxidised and reduced Cu/SiO2 catalysts

In situ FT-IR spectroscopy allows the methanol synthesis reaction to be investigated under actual industrial conditions of 503 K and 10 MPa On Cu/SiO2 catalyst formate species were initially formed which were subsequently hydrogenated to methanol During the reaction a steady state concentration of formate species persisted on the copper Additionally, a small quantity of gaseous methane was produced In contrast, the reaction of CO2 and H2 on ZnO/SiO2 catalyst only resulted in the formation of zinc formate species: no methanol was detected The interaction of CO2 and H2 with Cu/ZnO/SiO2 catalyst gave formate species on both copper and zinc oxide Methanol was again formed by the hydrogenation of copper formate species Steady-state concentrations of copper formate existed under actual industrial reaction conditions, and copper formate is the pivotal intermediate for methanol synthesis Collation of these results with previous data on copper-based methanol synthesis catalysts allowed the formulation of a reaction mechanism

An in situ high pressure FT-IR study of CO2/H2 interactions with model ZnO/SiO2, Cu/SiO2 and Cu/ZnO/SiO2 methanol synthesis catalysts

FTIR spectra are reported of CO adsorbed on silica-supported copper catalysts prepared from copper(II) acetate monohydrate. Fully oxidised catalyst gave bands due to CO on CuO, isolated Cu2+ cations on silica and anion vacancy sites in CuO. The highly dispersed CuO aggregated on reduction to metal particles which gave bands due to adsorbed CO characteristic of both low-index exposed planes and stepped sites on high-index planes. Partial surface oxidation with N2O or H2O generated Cu+ adsorption sites which were slowly reduced to Cu° by CO at 300 K. Surface carbonate initially formed from CO was also slowly depleted with time with the generation of CO2. The results are consistent with adsorbed carbonate being an intermediate in the water-gas shift reaction of H2O and CO to H2 and CO2.

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Kenneth C. Waugh

Papers

Promotion of methanol synthesis and the water-gas shift reactions by adsorbed oxygen on supported copper catalysts

Infrared study of methyl formate and formaldehyde adsorption on reduced and oxidised silica-supported copper catalysts

Infrared study of the adsorption of methanol on oxidised and reduced Cu/SiO2 catalysts

An in situ high pressure FT-IR study of CO2/H2 interactions with model ZnO/SiO2, Cu/SiO2 and Cu/ZnO/SiO2 methanol synthesis catalysts

Infrared study of CO adsorption on reduced and oxidised silica-supported copper catalysts