GC-MS Identification of Organic By-Products of the NO SCR by Propene; Relation to N2O Formation

Reduction of NO by propene in the presence of oxygen and water was studied on a Pt/Al₂O₃ catalyst in the following conditions (ppm (v)): NO/C₃H₆/O₂/H₂O = 800/500/50000/50000. The organic products accompanying N₂, N₂O and NO₂ formation were trapped and analyzed by GC-MS. These analyses revealed the presence in small quantities of products of propene partial oxidation such as ketones or acids, nitrogen derivatives such as nitriles or/and nitro and nitroso compounds. The transformation of some nitrogen-containing derivatives were studied under reaction conditions to explain the formation of the by-products and of N₂O.     

Promotion effect of K2O and MnO additives on the selective production of light alkenes via syngas over Fe/silicalite-2 catalysts

The addition of K₂O and MnO promoters enhances catalyst activity and selectivity to light alkenes during CO hydrogenation over silicate-2 (Si-2) supported Fe catalysts. The results of CO hydrogenation and CO-TPD, CO/H₂-TPSR, C₂H₄/H₂-TPSR and C₂H₄/H₂ pulse reaction over Fe/Si-2 catalysts with and without promoters clearly show that the MnO promoter mainly prohibits the hydrogenation of C₂H₄ and C₃H₆. Therefore, it enhances the selectivity to C₂H₄ and C₃H₄ products. Meanwhile further incorporating the K₂O additive into the FeMn/ Si-2 catalyst leads to a remarkable increase in both the capacity and strength of the strong CO adspecies. These produce much more [C_ad] via their dissociation and disproportionation at higher temperatures. This results in an increase in the CO conversion and the selectivity to light olefins. Moreover, the K₂O additive modifies the hydrogenating reactivity of [C_ad] and suppresses the disproportionation of C₂H₄ that occurs as a side-reaction. Both K₂O and MnO promoters play key roles for enhancing the selective production of light alkenes from CO hydrogenation over Fe/Si-2 catalyst.     

Hydrogenation of carbon dioxide on cobalt catalysts – activation,deactivation, influence of carbon monoxide

A batch reactor directly combined with an ultrahigh vacuum apparatus, which is equipped with facilities for catalyst preparation and Auger electron spectroscopy, was used to answer some questions which had arisen in recent studies concerning carbon dioxide hydrogenation on pure metallic and supported Co catalysts. Both oxygen incorporated during oxidation/reduction cycles and carbon deposited when CO₂ is hydrogenated penetrate deep into the bulk. This kind of carbon can easily be hydrogenated. CO strongly hinders the reduction of the oxidized Co surface in the H₂/CO₂ reaction mixture (4 : 1). CO hydrogenation is favoured over CO₂ hydrogenation and leads to a higher percentage of C₂ to C₄ hydrocarbons as compared with CH₄ formation.     

SAMPLING AND ANALYSIS OF POLYCYCLIC AROMATIC HYDROCARBONS IN URBAN AND RURAL ATMOSPHERES: SPATIAL AND GEOGRAPHICAL VARIATIONS OF CONCENTRATIONS

Atmospheric sampling (gas and particles) of PAHs (naphthalene, acenaphthene, fluorene, anthracene, phenanthrene, fluoranthene, pyrene, benz[a]anthracene, chrysene, benzo[b]fluoranthene, benzo[j]fluoranthene, benzo[k]fluoranthene, benzo[a]pyrene, benzo[ghi]perylene, dibenz[a,h]anthracene, indeno[1,2,3-cd]pyrene, coronene) on XAD-2 resin (20 g) and glass fiber filters respectively, were performed in 2002 by using Digitel DA80 high-volume samplers equipped with a PM10 head. Samplings were performed in Strasbourg (east of France) and its vicinity (Schiltigheim and Erstein). Sites were chosen to be representative of urban (Strasbourg), suburban (Schiltigheim), and rural (Erstein) conditions. Field campaigns were undertaken simultaneously in urban, and suburban, urban and rural sites during spring and autumn during 4 h at a flow rate of 60 m³/h^?1 which gives a total of 240 m³ · h^?1 of air per sample. The period of sampling varied between 06:00 to 10:00, 11:00 to 15:00, 17:00 to 21:00 in order to evaluate a variation of concentration during automobile traffic between urban, suburban, and rural areas. Gas and particle samples were separately Soxhlet extracted for 12 h with a mixture of CH₂Cl₂/n-hexane (50:50 v/v), concentrated to about 1 mL with a rotary evaporator and finally dried under nitrogen. Dry extracts were dissolved in 1 mL of CH₃CN before analysis. Before use, traps were Soxhlet cleaned for 24 h with the same mixture as used for the extraction. Analysis of each extract was performed by reverse phase HPLC, and fluorescence detection and quantification was done with respect to triphenylene used as internal standard. Results shows that automobile traffic seems to be the main source of PAHs in urban areas whatever the period (autumn and spring) while in autumn other sources, especially coming from combustion, influence the atmospheric contamination by PAHs.     

Photocatalytic oxidation of ethylene to CO2 and H2O on ultrafine powdered TiO2 photocatalysts: Effect of the presence of O2 and H2O and the addition of Pt

The complete photocatalytic oxidation of C₂H₄ with O₂ into CO₂ and H₂O has been achieved on ultrafine powdered TiO₂ photocatalysts and the addition of H₂O was found to enhance the reaction. The photocatalytic reaction has been studied by IR, ESR, and analysis of the reaction products. UV irradiation of the photocatalysts at 275 K led to the photocatalytic oxidation of C₂H₄ with O₂ into CO₂, CO, and H₂O. The large surface area of the photocatalyst is one of the most important factors in achieving a high efficiency in the photocatalytic oxidation of C₂H₄. The photoformed OH species as well as O ₂ ⁻ and O ₃ ⁻ anion radicals play a significant role as a key active species in the complete photocatalytic oxidation of C₂H₄ with O₂ into CO₂ and H₂O. Interestingly, small amount of Pt addition to the TiO₂ photocatalyst increased the amount of selective formation of CO₂ which was the oxidation product of C₂H₄ and O₂.     

Analysis by GC-MS of Polycyclic Aromatic Hydrocarbons in a Cream Containing Coal Tar

Qualitative and quantitative PAHs composition of a cream containing coal tar (5%), used in cutaneous diseases treatment, was studied. Eleven PAHs were analysed in pure coal tar and in the cream by GC-MS, after ultrasonic extraction by pyridine. Ten PAHs were found in pure coal tar: naphthalene, biphenyl, acenaphthylene, fluorene, phenanthrene, anthracene, carbazole, fluoranthene, pyrene and benzo[a]pyrene. No traces of 2,3-benzofluorene were detected. Seven PAHs were identified in the cream: naphthalene, biphenyl, acenaphthylene, fluorene, phenanthrene, fluoranthene and pyrene. 2,3-benzofluorene was also absent in the cream. Anthracene, carbazole and benzo[a]pyrene (one of the most toxic) were present in coal tar and not detected in the cream.

The seven PAHs found in the cream and in coal tar were quantified. Hydrocarbons concentrations were between 0.107 ± 0.0038 mg.g^?1 (for biphenyl) and 0.734 ± 0.0438 mg.g^?1 (for phenanthrene) in the cream and between 4.31 ± 0.23 mg.g^?1 (for biphenyl) and 21.9 ± 0.57 mg.g^?1 (for fluorene) in coal tar.     

Partial oxidation of palm fatty acids over Ce‐ZrO2: Roles of catalyst surface area,lattice oxygen capacity and mobility

Nanoscale Ce‐ZrO₂, synthesized by cationic surfactant‐assisted method, has useful partial oxidation activity to convert palm fatty acid distillate (PFAD; containing C₁₆–C₁₈ compounds) to hydrogen‐rich gas with low carbon formation problem under moderate temperatures. At 1123 K with the inlet O/C ratio of 1.0, the main products from the reaction are H₂, CO, CO₂, and CH₄ with slight formations of gaseous high hydrocarbons (i.e., C₂H₄, C₂H₆, and C₃H₆), which could all be eliminated by applying higher O/C ratio (above 1.25) or higher temperature (1173 K). Compared with the microscale Ce‐ZrO₂ synthesized by conventional coprecipitation method, less H₂ production with relatively higher C₂H₄, C₂H₆, and C₃H₆ formations are generated from the reaction over microscale Ce‐ZrO₂. The better reaction performances of nanoscale Ce‐ZrO₂ are linearly correlated with its higher specific surface area as well as higher oxygen storage capacity and lattice oxygen mobility, according to the reduction/oxidation measurement and ¹⁸O/¹⁶O isotope exchange study. © 2010 American Institute of Chemical Engineers AIChE J, 2011     

Reaction mechanism of partial oxidation of methane to synthesis gas over supported ni catalysts

A mechanistic study on the partial oxidation of methane to synthesis gas (H₂ and CO) was conducted with supported nickel catalysts. To investigate the reaction mechanism, pulse experiments, O₂-TPD, and a comparison of the moles of reactants and products were carried out. From the O₂-TPD experiment, it was observed that the active catalyst in the synthesis gas production desorbed oxygen at a lower temperature. In the pulse experiment, the temperature of the top of the catalyst bed increased with the pulses, whereas the temperature of the bottom decreased. This suggests that there are two kinds of reactions, that is, the total oxidation of methane (exothermic) at the top and reforming reactions (endothermic) at the bottom. From the comparison of the moles of reactants and products, it was found that the moles of CO₂, CH₄ and H₂O decreased as the moles of H₂ and CO increased. The results support the mechanism that synthesis gas is produced through a two-step reaction mechanism: the total oxidation of methane to CO₂ and H₂O takes place first, followed by the reforming reaction of the produced CO₂ and H₂O with residual CH₄ to form synthesis gas. This paper is dedicated to Professor Hyun-Ku Rhee on the occasion of his retirement from Seoul National University.     

Die entscheidende Mitwirkung der Metall- und Sauerstoffionen von Nickeloxid beim Ablauf der Oxidation von o-Xylol

The Decisive Cooperation of Metal and Oxygen Ions of Nickel Oxide During the Oxidation of o-Xylene As can be concluded from the experimental results at 450°C in the reaction mixtures consisting of N₂ O₂-o-Xylene, both nickel oxide pure and doped with Li₂O or In₂O₃ is unsuited as catalyst for the oxidation of o-xylene to phthalic anhydride. In contrast to NiO which ionosorbs both oxygen and o-xylene, NiO Li₂O, a strong ionosorbent for o-xylene, prevents the ionosorption of oxygen because of the large concentration of holes. Since gaseous oxygen does not react with ionosorbed o-xylene but a reduction of nickel oxide to metallic nickel has been observed in spite of the fact of enough oxygen in the gas phase it can be assumed that o-xylene is forced to remove oxygen ions from the NiO lattice under generation of oxygen-ion vacanxies and nickel atoms. The predominant portions of the reaction products are H₂O and CO₂. With undoped nickel oxide and NiO In₂O₃ which were not reduced under the same experimental conditions, the reaction products had roughly the same composition. The reduction of NiO Li₂O however will be prevented in a gas mixture with a high oxygen pressure which oxidizes the formed nickel atoms on the surface of NiO Li₂O to nickel oxide making possible the entrance of oxygen from the gas phase and, therefore, the oxidation of o-xylene. A turbulent motion of a 2-component catalyst powder from NiO-1mole% Li₂O covered with ionosorbed o-xylene and ZnO-1mole% In₂O₃ covered with ionosorbed oxygen in the same gas mixture resulted in the same reaction products as in the presence of sole NiO Li₂O under simultaneous reduction of nickel oxide. From that we can conclude that the oxidation of o-xylene by oxygen ions of NiO occurs more easily than the reaction with ionosorbed oxygen on ZnO In₂O₃ which obviously seems to be bounded too strongly. This result is also confirmed by the prevention of the oxidation of gaseous o-xylene in the presence of only ZnO In₂O₃. Finally, the operation of the carrier catalyst V₂O₅/TiO₂ which is employed for the oxidation of o-xylene to phthalic anhydride will be prevented to a large extent in simultaneous presence of nickel oxide either pure or doped with Li₂O and In₂O₃ in roughly the same amount. This result can be mentioned as a proof for the interaction of a 2-component catalyst the mechanism of which is at present not satisfactorily understood.     

The Mechanism for the Selective Oxidation of CO Enhanced by H2O on a Novel PROC Catalyst

Preferential oxidation (PROX) reaction of CO in H₂ catalyzed by a new catalyst of FeO _x /Pt/TiO₂ (Fe: Pt: TiO₂ = 100: 1: 100) was studied by dynamic in-situ DRIFT-IR spectroscopy. The oxidation of CO is markedly enhanced by H₂ and H₂O, and the enhancement by H₂/D₂ and H₂O/D₂O takes a common hydrogen isotope. Dynamics of DRIFT-IR spectroscopy suggests that the oxidation of CO with O₂ in the absence of H₂ proceeds via bicarbonate intermediate. In contrast, rapid oxidation of CO in the presence of H₂ proceeds via HCOO intermediate and the subsequent oxidation of HCOO by the reaction with OH, that is, CO + OH→ HCOO and HCOO + OH → CO₂ + H₂O. The latter reaction is a rate determining step being responsible for a common hydrogen isotope effect by H₂/D₂ and H₂O/D₂O.     

Ultrasonic-assisted ruthenium-catalyzed oxidation of aromatic and heteroaromatic compounds

Ruthenium-catalyzed oxidation of aromatic and heteroaromatic compounds is reported. It was found that ultrasonic irradiation in a biphasic system consisting of substrate, CH₂Cl₂, H₂O, CH₃CN, NaIO₄ and catalytic amounts of RuCl₃ · nH₂O, accelerates the oxidation reaction to afford the desired products in good yields.     

Direct synthesis of H2O2 from H2 and O2 over Pd/H-beta catalyst in an aqueous acidic medium: Influence of halide ions present in the catalyst or reaction medium on H2O2 formation

The influence of different halide ions present in the catalyst or reaction medium on the performance of Pd/H-beta catalyst in the direct H₂O₂ synthesis in an aqueous acidic (0.03 M H₃PO₄) reaction medium at 27 °C and atmospheric pressure has been thoroughly investigated. The results showed a strong influence of both the bulk Pd oxidation state in the catalyst and the halide ions added to the reaction medium on the performance of the catalyst in the H₂ to H₂O₂ oxidation, H₂O₂ decomposition/hydrogenation reactions. The different ammonium halides impregnated reduced Pd/H-beta catalyst calcined in inert (N₂) and oxidizing (air) gaseous atmospheres also revealed that the bulk Pd oxidation state and nature of the halide ions present in the catalyst together control the overall performance of the catalyst in the H₂O₂ formation reaction. The presence of halide ions in reaction medium or in the catalyst significantly changes the selectivity for H₂O₂ formation in the direct H₂O₂ synthesis. Bromide ions are found to remarkably enhance the H₂O₂ selectivity in the direct H₂O₂ synthesis irrespective of the Pd oxidation state in the catalyst. The promoting action of Br⁻ is attributed mainly to the large decrease in the H₂O₂ decomposition and hydrogenation activities of the catalyst and also inhibition for the non-selective H₂-to-water oxidation over the catalyst.     

Modeling of non-catalytic partial oxidation of natural gas under conditions found in industrial reformers

Non-catalytic partial oxidation of natural gas/O₂/H₂O mixture at elevated pressures was simulated kinetically using Chemkin package incorporating detailed reaction mechanisms of methane oxidation. The dependence of reaction time was investigated as a function of inlet temperature, system pressure, and O₂/CH₄ ratio. The conversion to products was predicted to complete within a residence time of less than 0.1 ms at pressures greater than 30 atm and temperatures higher than 1450 K. A minimum O₂/CH₄ ratio of 0.64 was found necessary for a complete methane conversion at the conditions typical for the industrial reformer. The effect of O₂/H₂O in the feed gas was examined computationally, and the results suggested that adding H₂O in the feed gas could be a viable tool for adjusting the H₂/CO ratio in the products and for controlling the flame temperature. Formations of higher order hydrocarbons and soot, which may play important roles in the actual fuel-rich conversion environment, are not considered in the present study.     

Modeling spatially resolved data of methane catalytic partial oxidation on Rh foam catalyst at different inlet compositions and flowrates

Spatially resolved species and temperature profiles measured for a wide range of inlet stoichiometries and flowrates are compared with microkinetic numerical simulations to investigate the effect of transport phenomena on the catalytic partial oxidation of methane on Rh foam catalysts. In agreement with the experimental data, the species profiles calculated at different C/O inlet stoichiometries show that both partial oxidation products (H₂, CO) and total oxidation products (H₂O, CO₂) are formed in the presence of oxygen. At the leaner stoichiometries, both oxygen and methane react in the diffusive regime at the catalyst entrance. At the richest methane stoichiometry (high C/O), surface temperatures are lower and methane consumption is only partly determined by transport. For all stoichiometries, a kinetically controlled regime prevails in the downstream reforming zone after O₂ is fully consumed. The effect of increasing the flowrate shifts all species profiles downstream and also slightly modifies the shapes of the axial profiles, due to the different effectiveness of heat and mass transfer. Despite enhanced mass transfer and increased surface temperature, the shortened contact time causes a reduced CH₄ conversion at high flowrates. The effect of flowrate on the dominant regime is investigated, for both reactants, comparing the resistances calculated in the pure transport regime and in the pure kinetic regime. From a chemical point of view, the model allows for the analysis of the reaction path leading to hydrogen. Due to inhibition of H₂O re-adsorption, it can be proven that H₂ can be a primary product even in the presence of gas phase O₂. The analysis of the surface coverages shows analogous effects on the profiles when decreasing C/O or increasing flow, because in both cases the surface temperature is increased. Syngas selectivity was also evaluated, both from measured and calculated profiles. S_H2 is well described by the model at each stoichiometry and flowrate, while S_CO is underestimated in every case. From this work, it is also indicated that the Rh catalyst works with CO (measured) selectivities higher than equilibrium. Carbon dioxide only forms in the oxidation zone, for C/O = 1 and 1.3, but in the rest of the catalyst zone, there is no further production despite what would be expected from equilibrium. This confirms Rh does not catalyze the water gas shift reaction. On the other hand, at C/O = 0.8, this reaction becomes active, due to the higher temperature, and the CO₂ is also produced in the reforming zone. This suggests that CO₂ will not rise after the oxidation section if the surface temperature is kept sufficiently low. Sensitivity analyses to the active catalytic surface and to the kinetic parameters are provided.     

Pd@Al2O3‐Catalyzed Hydrogenation of Allylbenzene to Propylbenzene in Methanol and Aqueous Micellar Solutions

The palladium on alumina (Pd@Al₂O₃)‐catalyzed hydrogenation of allylbenzene to propylbenzene was studied in methanol and aqueous micellar solutions of sodium dodecyl sulfate (SDS), decyltrimethylammonium bromide (DTAB), and t‐octylphenoxypolyethoxyethanol (TX‐100). Over Pd@Al₂O₃, propylbenzene was obtained via direct hydrogenation of allylbenzene and isomerization to β‐methylstyrene which was hydrogenated afterwards. In aqueous micellar solutions, the reaction was faster than in pure water, but slower than in methanol due to lower hydrogen solubility. In the H₂O/SDS system, a higher activation energy was obtained than in methanol. For the investigated surfactants, the initial reaction rate in the micellar systems decreased in the order SDS > TX‐100 > DTAB.     

蒽的电化学加氢

在隔离式电解槽中,以制备的泡沫Pb为阴极,Pt丝为对电极,饱和甘汞电极（SCE）为参比电极,研究了不同电解体系的伏安曲线和蒽的阴极电化学加氢行为.实验发现：在已研究的体系中,蒽阴极电解加氢为二氢蒽,其中CH₃CN+EtOH+H₂O+Bu₄NBr是加氢效果较好的电解体系.进一步研究CH₃CN+EtOH+H₂O+Bu4NBr电解体系中H₂O浓度、溶剂组成和支持电解质Bu₄NBr的浓度以及温度等因素对蒽电解催化加氢效果的影响,结果表明较理想的电解体系组成为：［H₂O］＝5.5mol•L^-1,［CH₃CN］/［EtOH］＝2/1(v/v),［Bu₄NBr］＝0.50mol•L^-1,温度为35℃,在该条件下,电解反应6 h,二氢蒽的产率可达91.37％.     

Photooxidation of Polycyclic Aromatic Hydrocarbons over NaBiO3 under Visible Light Irradiation

The NaBiO₃ samples (NaBiO₃ · nH₂O) used in photooxidation of polycyclic aromatic hydrocarbon were obtained through heating NaBiO₃ · 2H₂O. The samples were characterized by X-ray diffractometer and ultraviolet-visible spectrophotometer. The photooxidation of anthracene and Benz[a]anthracene over NaBiO₃ · nH₂O was investigated, respectively. The intermediates were analysed by gas chromatography-mass spectrometer. The results indicated that the prepared NaBiO₃ samples showed considerable photooxidation activity and stability for PAHs degradation under visible light irradiation. The possible reaction pathways of the decomposition of the two polycyclic aromatic hydrocarbons were proposed. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Effect of hydrogen on carbon deposition from carbon monoxide on nickel catalyst

The role of hydrogen in carbon deposition on Ni has been studied at H₂/CO < 1 and 698 K by determining the respective rates of the carbon-forming reactions: (1) CO + H₂ -C + H₂O and (2) 2CO C + CO₂. The steady-state rate of reaction (1) increases in proportion to H₂ pressure. On the other hand, reaction (2) is facilitated by the addition of an extremely small amount of H₂, so that the rate becomes about eight times that for pure CO but hardly varies as more H₂ is added. Similarly, there is a great difference in catalytic activity for ethylene hydrogenation between spent catalysts obtained in the deposition with and without H₂. These findings suggest that hydrogen, even in a small amount, makes free Ni surface area larger than for pure CO.     

A study on the reactivity of Ce-based Claus catalysts and the mechanism of its catalysis for removal of H2S contained in coal gas

In this study, Claus reaction was applied for the selective removal of H₂S contained in the gasified coal gas, and the characteristics of Claus reaction over the Ce-based catalysts were investigated to propose the reaction mechanism. The Ce-based catalysts showed a high activity on Claus reaction. Specially, Ce_0.8Zr_0.2O₂ catalyst had a higher activity than CeO. On the basis of our experimental results, it was proposed that the selective oxidation of H₂S was carried out by the lattice oxygen in the Ce-based catalysts and that the reduction of SO₂ was performed by the lattice oxygen vacancy in the reduced catalyst. Since the mobility of the lattice oxygen in Ce_0.8Zr_0.2O₂ composite catalyst was better than the one in CeO₂, Ce_0.8Zr_0.2O₂ provided more lattice oxygen for the selective oxidation of H₂S. It was presumed that the reaction mechanism to convert H₂S and SO₂ into elemental sulphur over our prepared catalysts was different from the mechanism over the solid-acid catalysts. It is believed that Claus reaction over the Ce-based catalysts was carried out by the redox mechanism. Since the moisture was contained in the major components, CO and H, of the gasified fuel gas, the effects of CO and H₂O on the catalytic reaction were investigated over a Ce-based catalyst. The conversion of H₂S and SO₂ was decreased in Claus reaction over the Ce-based catalysts as the concentration of either H₂O or CO in the gasified coal gas was increased. Under the circumstances of the coexistence of both moisture and CO, however, the conversion was increased as the concentration of CO was increased. The reactivity of Claus reaction was varied in terms of the concentration ratio of CO to H₂O. The maximum conversion of H₂S and SO₂ was achieved in the condition of that the concentration of CO contained in the reacting gas was higher than the one of H₂O. The conversions of H₂S and SO₂ did not match to the stoichiometric ratios of Claus reaction. The higher conversion of H₂S was obtained in the higher concentration of H₂O, while the higher conversion of SO₂ was achieved in the higher concentration of CO. It was another evidence to indicate that the Claus reaction over the Ce-based catalysts was carried out by the redox mechanism.     

Fischer–Tropsch Synthesis: Deuterium Kinetic Isotope Study for Hydrogenation of Carbon Oxides Over Cobalt and Iron Catalysts

Abstract  

The hydrogenation of CO₂ using Pt promoted Co/γ-Al₂O₃ and doubly (Cu, K) promoted iron catalysts exhibits an inverse isotope effect (r ^H/r ^D < 1). The observed inverse isotope effect for hydrogenation of CO₂ shows that hydrogen addition to CO₂ should be involved in the kinetically relevant step. The systematic increase of inverse isotope effect with carbon number of products obtained during H₂–D₂–H₂ switching experiments suggests the possible existence of a common intermediate (CH _x O) for hydrogenation of CO₂ over both cobalt and iron FT catalysts. The magnitude of the inverse isotope effect is lower for CO₂ compared to CO hydrogenation under similar reaction conditions. The deuterium isotope effect does not provide a definite conclusion regarding the mechanism which CO₂ hydrogenation follows (alkyl, enol, or alkylidine mechanisms).