Study on the Mechanism of CO Formation in Reverse Water Gas Shift Reaction Over Cu/SiO2 Catalyst by Pulse Reaction, TPD and TPR

The mechanism of reverse water gas shift reaction over Cu catalyst was studied by pulse reaction with QMS monitoring, temperature programmed desorption (TPD) and temperature programmed reduction (TPR) of Cu/SiO₂ catalyst. The reduced and/or oxidized copper offered low catalytic activity for the dissociation of CO₂ to CO in the pulse reaction study with 1 ml volume of He/CO₂, but the rate of CO formation was significantly enhanced with H₂ participating in the reaction. The TPD spectra of CO₂ obtained by feeding H₂/CO₂ over copper at 773 K provided strong evidence of the formation of formate at high temperature. The formate derived from the association of H₂ and CO₂ is proposed to be the key intermediate for CO production. The formate dissociation mechanism is the major reaction route for CO production.     

Adsorption behavior of NO and CO and their reaction over cobalt on zeolite beta

Adsorption behavior of NO and CO as well as their reaction was investigated on cobalt supported zeolite beta (Co/BEA) prepared by solid-state ion exchange (SSIE) and by impregnation (IMP). By temperature programmed desorption (TPD), two NO desorption peaks at 100 and 260‡C were observed over both SSIE and IMP catalysts with complete desorption after 450‡C. CO desorbed from SSIE catalyst between 50 and 200‡C. In the same temperature interval negligible CO₂ desorption was observed, most likely due to reaction of CO with trace of cobalt oxides. Over IMP catalysts, desorption of CO₂ was found mainly at 500‡C. By comparing CO TPD profiles from physical mixtures of cobalt oxides and HBEA, SSIE catalysts most likely contained cobalt cations in zeolite exchange position while IMP catalysts had cobalt in oxidic forms. The SSIE catalysts were active for NO reduction at 400 and 500‡C with a maximum conversion at 500‡C. However, the activity in the presence of water and oxygen was low. Water might inhibit the reaction by blocking active sites for NO and CO, while oxygen reacted with CO to form carbon dioxide. The activity of SSIE was better than IMP catalyst.     

Temperature programmed desorption technique to predict the carbon oxygen reaction

The kinetics of the reaction o₂ oxygen with a sucrose char particle size: (88 μ<d_p<105 μrn) has been studied using a thermogravimetric analyzer (TGA) and a mass spectrometer (MS) to measure weight change and CO and CO₂ formation rates during reaction. Experiments were performed to determine the surface oxide formation rate and to determine the mechanism of CO desorption in the temperature range of 762 K to 851 K and for oxygen pressures of 0.04 to 0.3 atm, respectively. When the reaction rate at 30% conversion was used in the Arrhenius plot, an activation energy of 34±3 kcal/mol was obtained and the CO/CO₂ ratio was found to increase with increasing reaction temperature. Analysis of the rate of formation of CO and CO₂ shows the activation energy for CO formation is greater than for CO₂ formation. Temperature programmed desorption (TPD) studies of the surface oxides were made to provide a better understanding of the carbon oxidation process. The activation energy distribution function for desorption was approximately Gaussian and the average activation energy is 55 Kcal/mol for a preexponential factor of 10¹³ 1/sec. The peak of the energy distribution function shifts to higher activation energies for surface complexes formed at higher reaction temperature.     

A TPD study of coal chars in relation to the catalysis of mineral matter

The steam gasification mechanism of brown coal was studied by a temperature-programmed desorption (TPD)technique. A Morwell coal was devolatilized in N₂ and then gasified in steam at 1100 K. During the TPD of a partially gasified char, H₂O, CO₂ and CO evolved approximately at 640, 870 and 1020 K, respectively. The presence of mineral matter was found to be responsible for these gas evolutions, since essentially no gas evolution was observed during the TPD of the demineralized coal char. The comparison of the above TPD pattern with those determined for the cation-exchanged samples revealed which inorganic species is responsible for each TPD peak: H₂O evolution was due to Ca; CO₂ evolution to Ca and Mg; CO evolution to Na and/or Fe. The exchanged metal species like Ca and Na significantly catalysed the gasification reaction. The relation between the catalytic activity and TPD pattern was discussed in terms of surface oxygen complexes.     

Co oxidation on LaCoO3 perovskite

A study of CO oxidation on LaCoO₃ perovskite was performed in an ultrahigh vacuum system by means of adsorption and desorption. All gases were adsorbed at ambient temperature. Two adsorption states (α- and β-) of CO exist. The α-peak at 440 K is attributed to carbonyl species adsorbed on Co³⁺ ions while the β-peak at 663 K likely comes from bidentate carbonate formed by adsorption on lattice oxygens. CO₂ shows a single desorption peak (β-state, 483 K) whose chemical state may be monodentate carbonate. A new CO₂ desorption peak at 590 K can be created by oxidation of CO. O₂ also shows two adsorption states. One desorbs at 600 K, which may reflect adsorption on Co^3- ions. The other apparently incorporates with bulk LaCoO₃ and desorbs above 1000 K. The two adsorption states of CO are oxidized via different mechanisms. The rate determining step in oxidation of a-CO is the surface reaction whereas for that of β-CO, it is desorption of product CO₂.     

Behaviour of V2O5 — catalyzed CO oxidation in the presence of inert,diluent gases

A study was conducted on the oxidation of CO over an industrial potassium-promoted V₂O₅ catalyst in order to study the influence of He, Ar, Kr, SF₆ and C₄F₈ diluents on the reaction rate. A differential flow reactor was used between 400° and 440°C with partial pressures of CO₂, O₂ and diluents varied from approximately 7 to 150 kPa. All the diluents inhibited the reaction rate to varying degrees. Experiments were also performed in which one diluent gas was displaced by another in the reaction mixture. These experiments indicated that the diluents could be arranged in the following increasing order of sorptive interaction with the catalyst: He or Ar < Kr < SF₆ or C₄F₈ It was concluded that conventional site-blockage was unable to explain the influence of the diluents. An adsorption interaction between the diluent and the surface complex of the rate limiting step was postulated to explain the inhibition of diluents in CO oxidation and the enhancement of diluents in SO₂ oxidation on the same catalyst.     

Higher alcohol and paraffin synthesis on cobalt based catalysts: Comparison of mechanistic aspects

Syngas reaction mechanism studies on cobalt based catalysts suggested that the CO dissociation step and the (alcohol and/or hydrocarbon) chain growth can be well mimicked by CO disproportionation and temperature programmed desorption (TPD) of acetaldehyde. Correlation between C₂₊ hydrocarbon selectivity and CO disproportionation on the one hand, C₂₊ alcohol selectivity and TPD after acetaldehyde adsorption on the other hand could be evidenced.     

Hydrogenation of carbon dioxide over alumina supported Fe-K catalysts

The hydrogenation of CO₂ has been studied over Fe/alumina and Fe-K/alumina catalysts. The addition of potassium increases the chemisorption ability of CO₂ but decreases that of H₂. The catalytic activity test at high pressure (20 atm) reveals that remarkably high activity and selectivity toward light olefins and C₂₊ hydrocarbons can be achieved with Fe-K/alumina catalysts containing high concentration of K (K/Fe molar ratio = 0.5, 1.0). In the reaction at atmospheric pressure, the highly K-promoted catalysts give much higher CO formation rate than the unpromoted catalyst. It is deduced that the remarkable catalytic properties in the presence of K are attributable to the increase in the ability of CO₂ chemisorption and the enhanced activity for CO formation, which is the preceding step of C₂₊ hydrocarbon formation.     

Mixed gases separation by fine porous freeze-dried cellulose acetate membrane

A Freeze-dried cellulose acetate membrane with high permeability and high separation factor was prepared. In the successive membrance process, the heat treatment temperature was 80°C for constant period of 10 min, acetone evaporation time was between 4 and 6 min, and the membrane was fixed during drying to prevent shrinkage. Gas permeabilities for such a membrane were inversely proportional to the square root of molecular weight, suggesting that the mechanism of gas flow through this membrane was Knudsen flow. Separation factors for H₂? He, Ar? Kr, N₂? Kr, He? Ne, H₂? Ne, He? Ar, Ar? H₂, He? Kr, Ar? He, and He? H₂ were measured. These factors were higher than those of membranes reported already. Separation efficiency largely depended upon the combination of mixed gases. The combination of two atom molecules and one atom molecule lowered the separation efficiency compared with the combination of one atom molecule and one atom molecule.     

Comparison of the reactivity of bulk and surface oxides on Pd(100)

The reactivity of bulk PdO clusters produced by plasma oxidation of Pd(100) towards propene oxidation was characterized using temperature programmed desorption (TPD) and isothermal oxygen titration. The TPD results were dominated by simultaneous CO₂ and water desorption in a peak at 490 K. The only other product observed was a small amount of CO near saturation propene coverages that also desorbed at 490 K. The propene coverage saturated at exposures between 0.5–1 l, indicating a sticking coefficient close to one. In the titration experiments, CO₂ production peaked almost immediately upon exposure to propene, indicating that the propene oxidation rate fell as the surface was reduced. Above 450 K, virtually all of the propene was completely oxidized to CO₂ and water, while at lower temperatures small amounts of CO were observed and unreacted propene fragments accumulated on the surface. In comparison, previous results for a well-ordered surface oxide on Pd(100) were similar in that CO₂ and water also desorbed simultaneously indicating a similar mechanism, but different in that the sticking coefficient on the surface oxide was a factor of 20 lower, and the desorption peaked 60 K lower. These differences cause the bulk oxide to be far more active at higher temperatures than the surface oxide, but the surface oxide displays some activity down to lower temperatures where propene simply accumulates on the bulk oxide surface.     

Adsorption and reduction of NO2 over activated carbon at low temperature

The reactive adsorption of NO₂ over activated carbon (AC) was investigated at 50 °C. Both the NO₂ adsorption and its reduction to NO were observed during the exposure of AC to NO₂. Temperature programmed desorption (TPD) was then performed to evaluate the nature and thermal stability of the adsorbed species. Adsorption and desorption processes have been proposed based on the nitrogen and oxygen balance data. The micropores in AC act as a nano-reactor for the formation of -C(ONO₂) complexes, which is composed by NO₂ adsorption on existing -C(O) complexes and the disproportionation of adsorbed NO₂. The generated -C(ONO₂) complexes are decomposed to NO and NO₂ in the desorption step. The remaining oxygen complexes can be desorbed as CO and CO₂ to recover the adsorptive and reductive capacity of AC.     

In situ gasification of a lignite coal and CO2 separation using chemical looping with a Cu-based oxygen carrier

Chemical looping combustion (CLC) has the inherent property of separating CO₂ from flue gases. This paper is concerned with the application of chemical looping to the combustion of a solid fossil fuel (a lignite and its char) in a technique whereby the fuel is gasified in situ using CO₂ in the presence of a batch of supported copper oxide (the “oxygen carrier”) in a single reactor. As the metal oxide becomes depleted, the feed of fuel is discontinued, the inventory of fuel is reduced by further gasification and then the contents are re-oxidised by the admission of air to the reactor, to begin the cycle again. The choice of oxides is restricted because it requires an oxide which is exothermic during reduction to balance the endothermic gasification reactions. Copper has such oxides, but a key question is whether or not it can withstand temperatures at which gasification rates are significant (∼1173 K), particularly from the point of view of avoiding sintering and deactivation of the carrier in its reduced form. It was found that an impregnated carrier, made by impregnating a θ-alumina catalyst support (BET area 157 m²/g) with a saturated solution of copper and aluminium nitrates, acted as a durable carrier over 20 cycles of reduction and oxidation, using both Hambach lignite coal, and its char, and with air as the oxidising agent. During the course of the experiments, the BET surface area of the support fell from ∼60 m²/g, just after preparation, to around 6 m²/g after 20 cycles. However, this fall did not appear to affect the overall capacity of the oxygen carrier to react with fuels and its effect on the kinetics of the reaction with CO did not influence the outcome of the experiments, since the overall performance of the looping scheme is dominated by the much slower kinetics of the gasification reaction. The apparent kinetics of the gasification are faster in the presence of the looping agent: this is because the bulk concentration of CO in the presence of the looping agent is lower, and partly because the destruction of CO in the vicinity of a gasifying particle enhances the rate of removal of CO by mass transfer (and increases the local concentration of CO₂). There was little evidence to suggest a direct reaction between carbonaceous and carrier solids, other than via a gaseous intermediate. However, the observation of finite rates of conversion in a bed of active carrier, fluidised by nitrogen, is a scientific curiosity, which we have not been able to explain satisfactorily. At 1173 K, as used here, rates of gasification of Hambach lignite, and its char, are significant. The CuO in the carrier decomposes at 1173 K to produce gas-phase O₂ and Cu₂O: both can react with CO produced by gasification, whilst the O₂ can react directly with the char.     

Studies on plasma polymerization of hexamethyldisiloxane in the presence of different carrier gases

Plasma polymerization of hexamethyldisiloxane(HMDSO) in the presence of different carrier gases such as H₂, He, N₂, Ar, and O₂ was carried out using an inductively coupled electrodeless glow discharge. The polymerization kinetics showed that the monomer HMDSO plasma‐polymerized at different rates from low to high for the carrier gases H₂, He, N₂, Ar, and O₂ in that order. The products were studied using FTIR, electron spectroscopy for chemical analysis, and elemental analysis. The results indicated that HMDSO molecules underwent different degrees of fragmentation in plasma polymerization for different carrier gases and radio frequency (RF) powers. The polymer deposition rate and the structures of products were mainly dependent on molecular fragmentation, which varied with carrier gas and imposed RF power. O₂ and H₂ gases can incorporate in the plasma polymers to form products containing more oxygen or hydrogen components, while other gases such as N₂ have no detectable component in products. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 80: 1434–1438, 2001     

Characterization of activated carbon, graphitized carbon fibers and synthetic diamond powder using TPD and DRIFTS

A high surface area activated carbon, graphitized carbon fibers and synthetic diamond powder were characterized by X-ray diffraction, temperature-programmed desorption and diffuse reflectance infrared (IR) spectroscopy (DRIFTS). The activated carbon was analyzed as received as well as after either a nitric acid treatment to introduce oxygen functional groups on its surface or a high temperature treatment (HTT) in H₂ at 1223 K to remove surface groups. TPD evolution profiles of CO and CO₂ were combined with DRIFTS spectra of these carbon surfaces before and after pretreatments in H₂ at 723 and 1223 K to provide complementary information regarding the nature of these surface groups. Significant amounts of both low- and high-temperature CO₂ desorption occurred from the HNO₃-treated carbon, indicating that both strongly and weakly acidic groups were introduced on this surface and, in addition, comparable amounts of CO and CO₂ were desorbed. With the graphitized carbon fibers and diamond powder, larger amounts of CO were desorbed compared to CO₂, indicating the presence of predominantly weakly acidic or non-acidic groups on these surfaces. For the HNO₃-treated carbon, IR peaks associated with surface carboxylic acid groups initially present disappeared after treatment at 723 K, while bands attributable to anhydride, quinone, ester and phenol species remained. Small amounts of ether, furan and phenol groups were detected on the graphitized fiber surface, while ketonic carbonyl groups were dominant on diamond. Significant amounts of chemisorbed hydrogen were also detected, presumably occurring on edge atoms made available by the decomposition of CO-yielding complexes at temperatures >873 K.     

The reduction of NO with H2 over Ru/MgO

Ruthenium supported on magnesia was found to be a highly active and selective catalyst for the reduction of NO to N₂ with H₂. The adsorption of NO on Ru/MgO was studied at room temperature by applying frontal chromatography with a mixture of 2610 ppm NO in He. Subsequently, temperature‐programmed desorption (TPD) and temperature‐programmed surface reaction (TPSR) experiments in H₂ were performed. The adsorption of NO was observed to occur partly dissociatively as indicated by the formation of molecular nitrogen. The TPD spectrum exhibited a minor NO peak at 340 K indicating additional molecular adsorption of NO during the exposure to NO at room temperature, and two N₂ peaks at 480 K and 625 K, respectively. The latter data are in good agreement with previous results with Ru(0001) single‐crystal samples, where the interaction with NH₃ was found to lead to two N₂ thermal desorption states with a maximum coverage of atomic nitrogen of about 0.38. Heating up the catalyst after saturation with NO at room temperature in a H₂ atmosphere revealed the self‐accelerated formation of NH₃ after partial desorption of N₂, whereby sites for reaction with H₂ become available. As a consequence, the observed high selectivity towards N₂ under steady‐state reduction conditions is ascribed to the presence of a saturated N+O coadsorbate layer resulting in an enhanced rate of N₂ desorption from this layer and a very low steady‐state coverage of atomic hydrogen. The formation of H₂O by reduction of adsorbed atomic oxygen is the slow step of the overall reaction which determines the minimum temperature required for full conversion of NO. This revised version was published online in July 2006 with corrections to the Cover Date.     

The kinetics of CO oxidation by adsorbed oxygen on well‐defined gold particles on TiO2(110)

Very tiny Au particles on TiO₂ show excellent activity and selectivity in a number of oxidation reactions. We have studied the vapor deposition of Au onto a TiO₂(110) surface using XPS, LEIS, LEED and TPD and found that we can prepare Au islands with controlled thicknesses from one to several monolayers. In order to understand at the atomic level the unusual catalytic activity in oxidation reactions of this system, we have studied oxygen adsorption on Au/TiO₂(110) as a function of Au island thickness, and have measured the titration of this adsorbed oxygen with CO gas to yield CO₂, as function of Au island thickness, CO pressure and temperature. A hot filament was used to dose gaseous oxygen atoms. TPD results show higher O₂ desorption temperatures (741 K) from ultrathin gold particles on TiO₂(110) than from thicker particles (545 K). This implies that O_a bonds much more strongly to ultrathin islands of Au. Thus from Brønsted relations, ultrathin gold particles should be able to dissociatively adsorb O₂ more readily than thick gold particles. Our studies of the titration reaction of oxygen adatoms with CO (to produce CO₂) show that this reaction is extremely rapid at room temperature, but its rate is slightly slower for the thinnest Au islands. Thus the association reaction (CO_g + O_a → CO_2,g) gets faster as the oxygen adsorption strength decreases, again as expected from Brønsted relations. For islands of about two atomic layers thickness, the rate increases slowly with temperature, with an apparent activation energy of 11.4 ± 2.8 kJ/mol, and shows a first‐order rate in CO pressure and oxygen coverage, similar to bulk Au(110).     

CO Spillover and Oxidation on Pt/TiO2

The adsorption of CO has been measured on a 2.5 wt% Pt/TiO₂ catalyst using TPD. A somewhat surprising observation is that (i) CO₂ is produced, even though oxygen is not dosed into the system, (ii) repeated experiments result in the same amount of CO₂ desorption. The results appear to be due to a combination of factors–(i) is due to spillover of CO from the Pt to the TiO₂ support, while (ii) is due to the diffusion of Ti³⁺ into the bulk of the TiO₂ crystallite, which effectively removes the surface non-stoichiometry which might otherwise be expected.     

Partial oxidation of propene over metal oxide catalysts pretreated with NO2

Titania pretreated with NO₂ has been found to catalyze the partial oxidation of propene into oxygenates such as acetone and acrolein at around 623 K, while fresh TiO₂ produces only carbon oxides. Temperature‐programmed desorption (TPD) revealed that nitrogen oxides adsorbed on TiO₂ are stable at temperatures below 673 K. Even after the propene oxidation at 573 K for 2 h, nitrogen oxides were confirmed still to exist on the TiO₂ surface. At temperatures higher than 673 K, however, the desorption and/or the reduction of the adsorbed nitrogen oxides took place, and concomitantly the catalytic ability giving oxygenates disappeared. This revised version was published online in July 2006 with corrections to the Cover Date.