Kenneth C. Waugh

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

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.

Mechanism and kinetics of methanol synthesis on zinc oxide

The kinetics of the adsorption and surface reactions of hydrogen, water, carbon dioxide, carbon monoxide, formaldehyde and methanol on what is mainly the prism face of zinc oxide have been studied using temperature programmed desorption and reaction. Both hydrogen and carbon dioxide show a multiplicity of desorption energies, adsorption into those states showing the highest binding energies being activated. Temperature programming after the room-temperature adsorption of formaldehyde or methanol shows evidence of surface reaction, with the formation of a formate intermediate. The surface reaction mechanisms of this formate intermediate and their kinetics are identical regardless of whether it was formed from methanol adsorption or formaldehyde adsorption. The formate appears to be the pivotal intermediate in zinc oxide catalysed synthesis gas chemistry, decomposing (a) to carbon monoxide and hydrogen or (b)(depending on the hydrogen coverage) to methanol. The same formate intermediate is formed by co-adsorption of carbon dioxide and hydrogen while hydrogen/carbon monoxide dosing does not result in its formation; indeed carbon monoxide itself did not adsorb on the defected zinc oxide.

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

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.

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

Evidence for the adsorption of molecules at special sites located at copper/zinc oxide interfaces: part 1.—A Fourier-transform infrared study of formic acid and formaldehyde adsorption on reduced and oxidised Cu/ZnO/SiO2 catalysts

Fourier-transform infrared (FTIR) spectra are reported of formic acid and formaldehyde on ZnO/SiO2, reduced Cu/ZnO/SiO2 and reoxidised Cu/ZnO/SiO2 catalyst. Formic acid adsorption on ZnO/SiO2 produced mainly bidentate zinc formate species with a lesser quantity of unidentate zinc formate. Formic acid on reduced Cu/ZnO/SiO2 catalyst resulted not only in the formation of bridging copper formate structures but also in an enhanced amount of formate relative to that for ZnO/SiO2 catalyst. Formic acid on reoxidised Cu/ZnO/SiO2 gave unidentate formate species on copper in addition to zinc formate moieties.The interaction of formaldehyde with ZnO/SiO2 catalyst resulted in the formation of zinc formate species. The same reaction on reduced Cu/ZnO/SiO2 catalyst gave bridging formate on copper and a remarkable increase in the quantity of formate species associated with the zinc oxide. Adsorption of formaldehyde on a reoxidised Cu/ZnO/SiO2 catalyst produced bridging copper formate and again an apparent increase in the concentration of zinc formate species. An explanation in terms of the adsorption of molecules at special sites located at the interface between copper and zinc oxide is given.

Infrared study of the adsorption of formic acid on silica-supported copper and oxidised copper catalysts

Infrared spectra are reported of formic acid adsorbed at 300 K on a reduced copper catalyst (Cu/SiO2) and a copper surface which had been oxidised by exposure to nitrous oxide. Formic acid was weakly adsorbed on the silica support. Ligation of formic acid to the copper surface occurred only on the reduced catalyst. Dissociative adsorption resulted in the formation of unidentate formate on the oxidised catalyst. The presence of reduced copper metal instigated a rapid reorientation to a bidentate formate species.

Temperature-programmed reaction studies of the interaction of methyl formate and ethanol with polycrystalline zinc oxide

The reactive nature of the polycrystalline zinc oxide surface has been investigated using methyl formate and ethanol as probe molecules; these have shown the dominant influence of the cation–anion dual site. Methyl formate decomposes during adsorption at the dual site to adsorbed methoxy and formate species. On temperature programming these show identical kinetics and reaction pathways to the methoxy and formate species observed previously after the adsorption of formaldehyde or methanol. Adsorption of ethanol at the dual site results in the formation of an adsorbed ethoxy species. On temperature programming, at 510 K, the zinc oxide surface abstracts one of the β-hydrogen atoms to form an unstable C2H4O fragment which decomposes mainly (90%) to ethylene; a small amount (ca. 10%) of the desorbing material which is either acetaldehyde or ethylene oxide (these cannot be distinguished by mass spectrometry) is desorbed coincidently at this temperature in an isomerization/desorption step. Despite the ethylene formation being a dehydration reaction, no water is observed in the desorption spectrum, the ethoxy oxygen species being left on the surface during decomposition.

Comments on “The effect of ZnO in methanol synthesis catalysts on Cu dispersion and the specific activity” [by T. Fujitani and J. Nakamura]

Fujitani and Nakamura recently reported on the effect of ZnO on Cu/ZnO methanol synthesis catalysts (Catal. Lett. 56 (1998) 119). Having measured the methanol synthesis activity of a series of Cu/ZnO catalysts of different Cu/ZnO ratios, they reported a linear relationship between the copper metal area and the methanol yield (implying a fixed value of the copper specific activity) and paradoxically they also reported a volcano-type relationship between the copper specific activity in methanol synthesis and the ZnO content. This paradox is resolved by showing that their Cu/ZnO catalysts fall into two groups: (i) the low-surface-area copper catalysts which have a specific activity of 10 mg CH3OH/m2-Cu h and (ii) the high-surface-area copper catalysts which have specific activity of 14.8 mg CH3OH/m2-Cu h. These different specific activities derive from different surface morphologies of the copper in these catalysts.

A combined infrared, temperature programmed desorption and temperature programmed reaction spectroscopy study of CO2 and H2 interactions on reduced and oxidized silica-supported copper catalysts

The nature and strength of bonding of carbon dioxide to a high area (136 m2g-1) polycrystalline copper supported on silica has been determined by the combined techniques of Fourier transform infrared spectroscopy (FTIR) and temperature programmed desorption (TPD). These show the room temperature interaction to be initially dissociative producing carbon monoxide (bonded both on a CuI site and on a stepped, high index surface) and surface oxygen. Further carbon dioxide adsorption interacts with the surface oxygen to produce a symmetric carbonate species. The adsorption and surface reaction of co-adsorbed hydrogen and carbon dioxide has also been studied by the combined techniques. At room temperature the adsorption of both is simply dissociative producing adsorbed hydrogen atoms and the symmetric carbonate species. Heating these adsorbates to 388K causes them to react producing a surface formate species—the dominant intermediate when methanol is synthesized from these reactants. This paper therefore elucida...

Adsorption and reaction induced morphological changes of the copper surface of a methanol synthesis catalyst

The morphology (surface structure) of the copper component of an industrial Cu/ZnO/Al2O3 methanol synthesis catalyst has been studied by carbon monoxide temperature programmed desorption (CO TPD). The initial state morphology produced by hydrogen reduction at 513 K showed evidence of the existence of Cu(111), Cu(110) and Cu(211) surfaces. Surface oxidation of the copper by CO2 decomposition at 213 K followed by CO reduction at 473 K did not reproduce the initial state morphology, most of the Cu(110) surface being lost; at the same time there was a six-fold increase in the surface population of the (211) face. This new surface produced by CO2 decomposition at 213 K and CO reduction at 473 K was considerably less active in its ability to decompose CO2 at 213 K. Treatment of it with hydrogen at 513 K for 16 h caused the surface to reconstruct almost completely to its original state, with the Cu(110) face reappearing and the Cu(211) face being reduced in population to roughly its original value. The ability of the copper to decompose CO2 was proportionately restored. It is evident that in the synthesis of methanol using CO/CO2/H2 mixtures over Cu/ZnO/Al2O3 catalysts, the morphology of the surface of the copper will be in a continuous state of restructuring, which, depending on the conditions, has the potential to result in chaotic behaviour.