Skeletal Copper Catalysts Research Articles

The activity of chromia-promoted skeletal copper catalysts for the oxidative dehydrogenation of ethanolamine to sodium glycinate based on measurements in a modified autoclave

The activity of promoted and unpromoted skeletal copper catalysts for the oxidative dehydrogenation of ethanolamine to sodium glycinate in strong NaOH solutions has been studied using a modified stirred autoclave that allows the reactants to be brought to the reaction temperature of 160 °C separately prior to mixing. This enables the study of activity and kinetics without complication from partial reaction and catalyst deactivation at lower temperatures. First order plots based on the continuous measurement of hydrogen evolution are close to linear throughout reaction. Unpromoted catalysts show substantial deactivation in repeated experiments but this can be eliminated by pretreatment in NaOH alone at temperatures above that used for reaction. This lowers the initial activity but helps the stability with treatment at 200 °C in 9.7 M NaOH being particularly effective. Small amounts of Cr 2O 3, deposited from sodium chromate included in the NaOH solution used to prepare the catalysts by leaching of a CuAl 2 alloy, are a highly effective promoter. Catalysts containing 1–1.5 wt.% Cr 2O 3, pretreated in 9.7 M at 200 °C, are several times as active as their unpromoted counterparts through the course of six successive reaction cycles. The promoted catalysts exhibit a much higher surface area, which correlates with the retention of aluminium in used catalysts, but the connection to the improved activity is complex.

The dehydrogenation of methanol to methyl formatePart II. The effect of chromia on deactivation kinetics for copper-based catalysts

The effect of added chromia on the behavior of skeletal copper catalysts for the gas phase dehydrogenation of methanol to methyl formate has been investigated for two methods of preparation. One involves leaching of a CuAl2 alloy in the presence of sodium chromate, and the other uses deposition of chromia subsequent to leaching. In both systems chromia greatly reduces deactivation although at the expense of some loss of activity if the chromia content is high. Differences in rates of deactivation for unpromoted copper are best interpreted in terms of deposition of a foulant according to first order kinetics based on parallel deactivation and the assumption that the final steady state activity is nonzero. A simple power law model is adequate for most chromia-promoted systems where the extent of deactivation is low. Optimal steady-state activity is obtained by leaching CuAl2 in the presence of sodium chromate resulting in a Cr2O3 content of 0.5–2.0%. The activation energy for deactivation is substantially smaller when chromia is present.

The dehydrogenation of methanol to methyl formate: Part II. The effect of chromia on deactivation kinetics for copper-based catalysts

The effect of added chromia on the behavior of skeletal copper catalysts for the gas phase dehydrogenation of methanol to methyl formate has been investigated for two methods of preparation. One involves leaching of a CuAl 2 alloy in the presence of sodium chromate, and the other uses deposition of chromia subsequent to leaching. In both systems chromia greatly reduces deactivation although at the expense of some loss of activity if the chromia content is high. Differences in rates of deactivation for unpromoted copper are best interpreted in terms of deposition of a foulant according to first order kinetics based on parallel deactivation and the assumption that the final steady state activity is nonzero. A simple power law model is adequate for most chromia-promoted systems where the extent of deactivation is low. Optimal steady-state activity is obtained by leaching CuAl 2 in the presence of sodium chromate resulting in a Cr 2O 3 content of 0.5–2.0%. The activation energy for deactivation is substantially smaller when chromia is present.

The dehydrogenation of methanol to methyl formatePart I: Kinetic studies using copper-based catalysts

Kinetics of the dehydrogenation of methanol to methyl formate (MF) have been determined for a commercial copper-chromite catalyst and for a skeletal copper catalyst that has undergone deactivation to a steady-state activity. The activation energy over the copper-chromite catalyst was found to be around 78 kJ/mol, considerably lower than the approximately 120 kJ/mol observed for the skeletal copper catalyst. The reaction order with respect to methanol was found to be approximately 0.5 for both catalysts whilst hydrogen and methyl formate both inhibited the reaction significantly. This inhibition is consistent with a Langmuir–Hinshelwood model in which methanol is adsorbed dissociatively and the rate-determining step is the loss of hydrogen from the resultant methoxy species. The model parameter values imply that a significant fraction of the copper sites on the skeletal catalyst are occupied by formaldehyde while the coverage is low on copper-chromite. These differences can be used to explain the presence of deactivation and the considerably higher activation energy for dehydrogenation observed for the skeletal copper catalyst.

The dehydrogenation of methanol to methyl formate: Part I: Kinetic studies using copper-based catalysts

Kinetics of the dehydrogenation of methanol to methyl formate (MF) have been determined for a commercial copper-chromite catalyst and for a skeletal copper catalyst that has undergone deactivation to a steady-state activity. The activation energy over the copper-chromite catalyst was found to be around 78 kJ/mol, considerably lower than the approximately 120 kJ/mol observed for the skeletal copper catalyst. The reaction order with respect to methanol was found to be approximately 0.5 for both catalysts whilst hydrogen and methyl formate both inhibited the reaction significantly. This inhibition is consistent with a Langmuir–Hinshelwood model in which methanol is adsorbed dissociatively and the rate-determining step is the loss of hydrogen from the resultant methoxy species. The model parameter values imply that a significant fraction of the copper sites on the skeletal catalyst are occupied by formaldehyde while the coverage is low on copper-chromite. These differences can be used to explain the presence of deactivation and the considerably higher activation energy for dehydrogenation observed for the skeletal copper catalyst.

Kinetic studies of gas-phase hydrogenolysis of methyl formate to methanol over copper-based catalyst

Hydrogenolysis of methyl formate was studied at atmospheric pressure over skeletal copper and copper chromite catalysts. Measurements of initial activity, made for the skeletal copper catalyst due to its fast deactivation by fouling, showed that the rates of hydrogenolysis were approximately zero order with respect to methyl formate and had positive order for hydrogen. The activation energy for the reaction was found to be around 54 kJ/mol for skeletal copper and 83 kJ/mol for the copper chromite catalyst. Methanol had negligible effect on the activity of the reaction in the absence of CO. The kinetics were interpreted using Langmuir–Hinshelwood empirical models and it appears that the rate-determining step involves formation of a surface hemiacetal species, which gives a surface methoxy group and formaldehyde, by reaction with atomic hydrogen adsorbed on adjacent sites.

Mitigation of fouling of skeletal copper catalysts for the gas-phase hydrogenolysis of methyl formate to methanol by addition of chromia

Abstract The addition of chromia on skeletal copper catalysts has been investigated for the gas-phase hydrogenolysis of methyl formate to methanol by leaching a CuAl 2 alloy in the presence of sodium chromate. The presence of chromia greatly reduced skeletal copper catalyst deactivation although at the expense of some loss of activity if the chromia content was high. The differences in rates of deactivation were interpreted in terms of deposition of a foulant according to first-order kinetics based on parallel deactivation. Optimal residual activity was obtained by adding ca. 1.5 wt.% of chromia to skeletal copper during the process of leaching.

The influence of Cr, Zn and Co additives on the performance of skeletal copper catalysts for methanol synthesis and related reactions

The effects of chromium, zinc and cobalt additives on the properties of skeletal copper catalysts for the reactions of methanol synthesis (MS), water gas shift (WGS) and methanol steam reforming (MSR) have been studied. Catalysts were prepared by doping Cr, Zn and Co additives on the surfaces of fully leached skeletal copper. Small amounts of Cr 2O 3 were found to significantly enhance the activity for the water gas shift reaction. Improvements were also observed for methanol steam reforming and methanol synthesis reactions. The presence of ZnO on the surface of skeletal copper was shown to greatly enhance methanol synthesis, suggesting that very active Cu-ZnO sites were created. ZnO also enhanced the activity for both the water gas shift and methanol steam reforming reactions. On the other hand, addition of Co had no effect on methanol synthesis activity and gave poor selectivity for methanol steam reforming. More significantly, the Co additive reduced the activity of skeletal Cu to the extent that more than 1.1 wt.% Co additive on the surface resulted in negligible activity for the WGS reaction. The reason for the different performance with the Co additive is thought to be the reduction of surface CoO to metallic cobalt under the reducing conditions of catalyst pretreatment and reaction. Temperature programmed reduction (TPR), titration with N 2O, and CO chemisorption measurements support this reasoning. This investigation of the role of additives in methanol steam reforming and the water gas shift reactions provides support for a recently proposed pathway for methanol steam reforming reaction over Cu catalysts via a formaldehyde or methyl formate intermediate.

Methanol Synthesis from CO2 Using Skeletal Copper Catalysts Containing Co-precipitated Cr2O3 and ZnO

Skeletal Cu-Cr2O3-ZnO catalysts have been prepared by leaching CuAl2 alloy particles at 273 K using 6.1 M aqueous NaOH solutions containing sodium chromate (Na2CrO4) and sodium zincate (Na2Zn(OH)4). The presence of sodium chromate and sodium zincate in the caustic solution was found to affect the pore structure and surface areas of the resulting catalysts. Both BET and Cu surface areas were increased by increasing the concentration of Na2CrO4 and of Na2Zn(OH)4. Increasing the Na2CrO4 level from 0 to 0.06 M in a 6.1M NaOH solution containing 0.2M Na2Zn(OH)4 caused the content of ZnO in the catalyst to decrease from 8.8 to 3.0 wt% whilst increasing the Cr2 O3 content from 0 to 1.7 wt%, indicating that the presence of Na2CrO4 in the leach liquor not only resulted in deposition of a Cr compound but also inhibited precipitation of zinc hydroxide onto skeletal Cu catalysts. On the other hand, increasing the concentration of Na2Zn(OH)4 from 0 to 0.6 M in a 6.1 M NaOH solution containing 0.008 M Na2 CrO4 resulted in increasing the ZnO loading from 0 to 8.9wt% with an almost constant content of Cr2 O3 (1.3 ± 0.2%) in the catalysts, revealing that sodium zincate only led to precipitation of zinc hydroxide and did not suppress Cr2O3 formation. Hydrogenation of CO2 was studied using a gas mixture of 24% CO2 in H2 at a total pressure of 4MPa, space velocities up to 210000L kg-1h-1 and temperatures in the range 493-533K. The catalysts were found to be both highly active and selective for methanol synthesis. This study confirms the role of ZnO in promoting the activity of copper for methanol synthesis from CO2 and improving the selectivity by inhibiting the reverse water-gas shift reaction. The role of Cr2O3 is to improve the structural development of high surface area skeletal copper.

FIB preparation of a sensitive porous catalyst for TEM elemental mapping at high magnifications

Doped skeletal copper catalysts, created by selective dissolution of aluminium from a copper-aluminium alloy by caustic in the presence of an additive, has been imaged at very high magnification using a TEM. The location of the additive species has been determined by elemental mapping using an attached EDS detector. Zinc resides primarily inside the copper ligaments while chromium resides on the outside of the ligaments. Since skeletal copper is highly porous and extremely sensitive, conventional TEM preparation techniques could not be used. The preparation of the TEM samples was carried out using a focussed ion beam (FIB) miller. Several methods are outlined, along with the reasons some of them were unsuccessful for this particular material. The successful result indicates this technique may be used to analyse similar skeletal materials, such as nickel, cobalt, etc., which are otherwise difficult to prepare. It may also provide a basis for high resolution imaging or elemental analysis of other porous and/or highly sensitive materials.

Cr 2O 3 promoted skeletal Cu catalysts for the reactions of methanol steam reforming and water gas shift

Promotional effects of chromia on the structure and activity of skeletal copper catalysts for methanol steam reforming and water gas shift have been studied. Catalysts were prepared by leaching CuAl 2 alloy particles in aqueous NaOH solutions containing sodium chromate at various concentrations. XPS spectra showed that the surface of the resulting catalysts mainly consisted of Cr 3+ compounds and Cu 0. Cu + and/or Cu 2+ were not observed by XPS. Increasing the concentration of chromate in the leach liquor resulted in decreases in pore diameter and copper crystallite size but significant enhancement of BET surface area was observed while the total pore volume was maintained. The addition of small amounts of chromate to the leach liquor significantly enhanced the Cu surface area. However, higher concentrations of chromate in the leach liquor decreased the Cu surface areas although the total surface areas increased. The activities of Cr 2O 3 promoted skeletal copper catalysts for both methanol steam reforming and water gas shift reactions were determined separately. The results indicated that deposition of Cr 2O 3 on skeletal copper catalysts significantly improved the specific activities for these reactions. Chromia is found to act as a structural and catalytic promoter for these reactions.

Kinetics and mechanism of the formation of doped skeletal copper catalysts: the effect of zincate compared to undoped and chromate-doped systems

Addition of zincate to the leach liquor for the preparation of skeletal copper increases the copper surface area; however it does not stabilize the structure against rearrangement. The leaching kinetics have been studied using a rotating disc electrode (RDE) at 269–293 K in 2–8 M NaOH and 0.0005–0.1 M Na2ZnO2. Zincate ions precipitate as zinc oxide, due to the local consumption of hydroxide ions near the leach front as the aluminium dissolves. This oxide hinders the aluminium dissolution, slowing the leaching rate. It also hinders copper dissolution/redeposition and prevents copper diffusion, thus reducing the structural rearrangement significantly, and causing the formation of a much finer copper structure with increased surface area. The zinc oxide redissolves as the leach front passes, releasing the copper to rearrange once more, thereby allowing the surface area to decrease with time. The activation energy for leaching was found to be 84 ± 6 kJ mol−1.

Development of skeletal copper–chromia catalysts: I. Structure and activity promotion of chromia on skeletal copper catalysts for methanol synthesis

Studies of promotional effects of chromia on structure and properties of skeletal copper catalysts for methanol synthesis have been carried out. Catalysts were prepared by selective dissolution of aluminum from CuAl 2 alloy and, at the same time, precipitation of chromia on the surface of residual skeletal copper using a caustic solution containing sodium chromate. Deposition of chromia on the skeletal copper catalysts was found to greatly influence the pore structure and specific surface area, resulting in a relative decrease of pore diameter and remarkable enhancement of specific surface area, while the total pore volume was maintained constant. Significant improvements to methanol synthesis activity and selectivity were achieved by adding small amounts of chromia (about 3.5 wt.%) to the skeletal copper. However, higher chromia loadings were found to be unfavorable for methanol synthesis reactions. The results obtained in this study suggest that chromia acts as a structural promoter and provides high specific copper surface area for methanol synthesis. The chromia-promoted catalysts produced by this method are likely to have applications in organic hydrogenation reactions and for the water gas shift reaction.

Structural and catalytic promotion of skeletal copper catalysts by zinc and chromium oxides

ZnO and Cr2O3 promoted skeletal copper catalysts have been prepared by leaching CuAl2 alloy particles in aqueous sodium hydroxide containing various concentrations of sodium zincate (Na2Zn(OH)4) (0–0.62 M), of sodium chromite (NaCrO2) (0–0.1 M) or of a mixture of sodium zincate (0.005 M) and sodium chromite (0.02 M). Both sodium zincate and sodium chromite in the caustic solution were found to affect the leaching process. The presence of sodium zincate retarded the leaching rate and enhanced the specific surface area of the catalyst but did not protect copper rearrangement on extended leaching. On the other hand, sodium chromite in the leachant did not influence the leaching rate but the leached catalyst was more stable with the structure and surface area of the skeletal copper being maintained. The activities of ZnO and Cr2O3 promoted skeletal copper catalysts for the reactions of methanol synthesis, water gas shift and methanol steam reforming were determined separately. The results indicated that deposition of both ZnO and Cr2O3 on skeletal copper catalysts significantly improved the activities for these reactions. Chromia is found to act mainly as a structural promoter.

Liquid-Phase Hydration of Acrylonitrile on Skeletal Copper Catalysts Prepared from Cu–Ti and Cu–Zr Amorphous Alloys

Abstract Liquid-phase hydration of acrylonitrile has been carried out on the skeletal copper catalysts which were prepared by treating pulverized amorphous Cu–Ti and Cu–Zr alloys with an HF solution. The catalysts from amorphous alloys were more effective to produce acrylamide selectively than those from crystalline alloys. This high activity was brought about by the highly homogeneous structure of the amorphous alloys.

Mechanism of reaction of water with the surface of skeletal copper catalyst

1. Dissociative adsorption of water takes place in oxidizing and oxidizing-reducing vapor-gas mixtures on the surface of a reduced skeletal copper catalyst for conversion of carbon monoxide. 2. The dissociation of water in the oxidizing medium is accompanied by evolution of hydrogen into the gaseous phase, the amount and rate of evolution of which increasing with the filling of the surface, and then decreasing practically to zero, when the dissociation centers have become completely filled with hydroxyl groups. 3. The presence of hydrogen in vapor-gas mixtures ensures a continuous regeneration of the dissociation centers of water. 4. The temperature ranges of the formation of dissociatively adsorbed water and of the conversion of carbon monoxide with water vapor coincide with one another.

Kinetics of benzonitrile hydration on a skeletal copper catalyst

The dependence of the rate constant on the temperature, nitrile concentration and amount of catalyst in benzonitrile hydration on a skeletal copper catalyst has been established. Activation energies have been determined. A reaction mechanism is suggested.

Thermal desorption study of catalytic systems. Communication 19. Determination of activation energy for desorption of H2O and CO2 from surface of copper-containing catalysts

1. A test is proposed for the activity of copper-containing catalysts in the reaction of low-temperature conversion of carbon monoxide, based on the minimum temperature of dissociative adsorption of water. 2. The activation energies of water and carbon dioxide desorption have been calculated for three catalytic systems in the 30–220° interval. It is shown that the dissociatively adsorbed water is bound more strongly to the surface of the copper-zinc-alumina-calcium catalyst NTK-10, and molecularly adsorbed carbon dioxide is bound more strongly to the surface of the skeletal copper catalyst SK. The activation energy has been determined for the reaction of displacement of carbon dioxide by water on the NTK-10 catalyst at 200°. 3. It has been shown that carbon dioxide, competing with dissociatively adsorbed water, may lower the activity of the NTK-10. It is noted that such a phenomenon is absent on the skeletal copper catalyst. CO2 is desorbed from the surface of the skeletal catalyst much more slowly than from the surface of the NTK-10. 4. A possibility of error has been revealed in the determination of the character of surface inhomogeneity by thermal desorption, as a consequence of a change in the adsorption mechanism within the temperature interval of the determination.

Thermal desorption study of catalytic systems Communication 18. Determination of activation energy of H2O desorption from surface of skeletal copper catalyst

1. An analysis has been made of the causes of error in determining the activation energy and order of the desorption reaction. It has been shown that these values can be determined in the case of two unresolved peaks in a single desorption peak. 2. For the system consisting of water and skeletal copper catalyst, there is a temperature above which the water is adsorbed mainly in the dissociative form, and below which in the molecular form. 3. By means of the Polyani-Wigner equation., the desorption activation energy, the frequency factor, and the desorption reaction order have been determined for water desorbed from the surface of skeletal copper catalyst. The activation energy has been calculated for the displacement of water in competitive adsorption of carbon monoxide and water. It has been found that water molecules participate in the displacement reaction in the molecularly adsorbed form.

Thermodesoeption studies of catalytic systems

1. Thermodesorption studies have shown the presence of several types of centers: centers for the firm irreversible adsorption of CO, centers for the adsorption of H2O, centers for the competitive adsorption of CO and H2O, and centers for the CO-H2O conversion, on the surface of the skeletal copper catalyst. 2. It is suggested that CO adsorbs in bridged form on the competitive adsorption centers, and in linear form on the reaction centers. The conversion reaction involves CO and H2O molecules adsorbed on a small fraction (∼1%) of centers, the H2O molecules in question being in the associative adsorbed form.