C2 Oxygenate Formation Research Articles

In Situ Surfaced Mn-Mn Dimeric Sites Dictate CO Hydrogenation Activity and C2 Selectivity over MnRh Binary Catalysts.

Massive ethanol production has long been a dream of human society. Despite extensive research in past decades, only a few systems have the potential of industrialization: specifically, Mn-promoted Rh (MnRh) binary heterogeneous catalysts were shown to achieve up to 60% C2 oxygenates selectivity in converting syngas (CO/H2) to ethanol. However, the active site of the binary system has remained poorly characterized. Here, large-scale machine-learning global optimization is utilized to identify the most stable Mn phases on Rh metal surfaces under reaction conditions by exploring millions of likely structures. We demonstrate that Mn prefers the subsurface sites of Rh metal surfaces and is able to emerge onto the surface forming MnRh surface alloy once the oxidative O/OH adsorbates are present. Our machine-learning-based transition state exploration further helps to resolve automatedly the whole reaction network, including 74 elementary reactions on various MnRh surface sites, and reveals that the Mn-Mn dimeric site at the monatomic step edge is the true active site for C2 oxygenate formation. The turnover frequency of the C2 product on the Mn-Mn dimeric site at MnRh steps is at least 107 higher than that on pure Rh steps from our microkinetic simulations, with the selectivity to the C2 product being 52% at 523 K. Our results demonstrate the key catalytic role of Mn-Mn dimeric sites in allowing C-O bond cleavage and facilitating the hydrogenation of O-terminating C2 intermediates, and rule out Rh metal by itself as the active site for CO hydrogenation to C2 oxygenates.

Molecular beam mass spectrometry study on plasma-assisted low-temperature oxidation of ethylene

The chemical kinetics of plasma-assisted low-temperature oxidation of ethylene (C2H4) is explored by molecular beam mass spectrometry and kinetic modeling. In the experiments, a dielectric barrier discharge (DBD) flow reactor coupled with photoionization and electron ionization molecular beam mass spectrometry (PI-MBMS/EI-MBMS) is employed to provide detailed speciation information, including ions, radicals, and molecules in C2H4/O2/Ar systems (∼340 K, 30 Torr). A series of hydrocarbons and oxygenated intermediates including ethenol, methyl hydroperoxide, ethyl hydroperoxide, diacetylene, vinylacetylene, and hydroxyacetaldehyde is detected for the first time in this system. A kinetic mechanism that contains both combustion and plasma reaction networks is developed. The path fluxes of fuel consumption and oxygenates formation are obtained based on the rate of production analysis. Based on our experimental observations and modeling analysis, new insights into the kinetics of C2H4/O2/Ar plasmas are provided. The fuel is mainly consumed through OH addition, dissociation after e/Ar* collision, reactions with O/O(1D), and dissociative ionization. H abstraction reactions that are favored in alkane plasmas contribute little to the fuel consumption of ethylene. Formation pathways of C1 oxygenates are mainly initiated by CH3 produced by O/O(1D) reactions with fuel, and the subsequent pathways of OH/H addition to the fuel are responsible for the formation of most C2 oxygenates. Oxygen addition-elimination reactions of C2H4OH isomers are the sources of ethenol formation. The concentration profiles for some of the species under different oxygen mole fractions are measured. The two distinct patterns of species mole fraction profiles with increasing oxygen concentrations indicate the different dominating formation pathways, namely oxidation pathways and non-oxidation pathways. And the competition between oxidation effects and plasma effects is displayed by the non-monotonic mole fractions profiles of oxidation products.

Competing mechanisms of CO hydrogenation to ethanol over TM/Mo6S8 catalysts

In this work, the mechanism of the ethanol synthesis from CO hydrogenation on TM/Mo6S8 (TM = Fe, Ru) catalysts have been systematically investigated by using density functional theory calculations. The competition mechanism for the conversion of CO into CH4, CH3OH, and C2H6 formation is also considered. Our results indicate that an optimal route for the ethanol synthesis on TM/Mo6S8 (TM = Fe, Ru) catalysts started with an initial process of the dissociation and hydrogenation of CO to produce CH3 species. Subsequently, the HCO inserts into spontaneous CH3 species leads to the formation of CH3CHO, followed by successive hydrogenation to form ethanol via CH3CH2O intermediate. Comparing the two catalysts, Fe/Mo6S8 shows better catalytic performance than Ru/Mo6S8 for ethanol synthesis from CO hydrogenation, and Ru/Mo6S8 is more conducive to the production of CH3OH. The results of microkinetic modeling analysis of rate-determining step and selectivity and catalytic activity are in line with our energy barrier results. Simultaneously, the d-band center value of Fe/Mo6S8 is closer to EF than that of Ru/Mo6S8, which indicates that the catalytic activity of Fe/Mo6S8 is better. Mo6S8 in the TM/Mo6S8 catalysts mainly facilitates the formation of C2 oxygenates due to HCO is easy to insert into CH3 species. The present results will can provide clues for the catalyst design of oxygenate synthesis in some extent.

CO Hydrogenation to C2 Oxygenates over SiO2 Supported Rh-Based Catalyst: The Effect of pH Value of Impregnation Solution

The synthesis of C2 oxygenates including ethanol directly from coal-derived syngas is significant from both academic and practical points of view and SiO2 supported Rh-based catalysts are very effective for this conversion. However, the high price of Rh requires the improvement of its dispersion to maximize Rh efficiency. The adjustment of impregnation solution pH value can modify effectively the metal dispersion over the support. Herein, we reported the pH effect on catalytic performance of Rh-Mn-Li/SiO2 for CO hydrogenation to C2 oxygenates for the first time. A series of catalysts were prepared from different pH values of impregnation solutions, and were characterized by various techniques. With the increasing of solution pH value above zero point of charge (ZPC) of silica, Rh particle sizes increased with much wider size distribution and reduction of Rh species was restrained over the prepared catalysts owing to the stronger interaction between Rh and support. As a result, the active sites for CO insertion, especially for CO or H2 dissociation was lowered, leading to the depletion of the activity for the formation of C2 oxygenates from CO hydrogenation, and larger Rh particle size with wider size distribution favors the production of long chain hydrocarbons. On the contrary, when the catalyst was prepared using solution with pH below ZPC of silica, the dispersion and the reduction of Rh were promoted due to suitable Rh-support interaction, and in the CO hydrogenation reaction, the space time yield and selectivity of C2 oxygenates reached 679.4 g/kg-cat/h and 73.3%, respectively. The pH value of impregnating solution regulate the metal-support interaction and thus Rh particle sizes and its reduction, which effects strongly CO hydrogenation activity and selectivity.

Syngas-to-C2 oxygenates on Cu-based catalyst: Quantitative insight into the balancing effect of active Cuδ+(0 ≤ δ ≤ 1) sites

Cu-based catalysts can experimentally catalyze the conversion of syngas-to-C2 oxygenates, however, since the chemical state of Cu sites dynamically evolves in the reaction process, no consensus has been reached on the precise roles of active Cuδ+(0 ≤ δ ≤ 1) sites. Here we resolve this long-term dispute with a series of highly comparable Cu catalyst. The results for the first time quantitatively identify the precise role of Cuδ(0 ≤ δ ≤ 1) sites in adjusting the catalytic activity and selectivity of syngas conversion over Cu-based catalysts, which is well illustrated in the view of electronic properties. Our quantitative results show that the moderate valence state Cuδ+(δ = 0.43 and 0.51) sites can highly-active and highly-selective catalyze the conversion of syngas to produce C2 oxygenates, which is also indirectly supported by the reported experiment studies. It is proposed that the balancing effect of Cu0 and Cu+ species in the experiment contribute to high catalytic performance toward the formation of C2 oxygenates.

CO‐Assisted Direct Methane Conversion into C1 and C2 Oxygenates over ZSM‐5 Supported Transition and Platinum Group Metal Catalysts Using Oxygen as an Oxidant

AbstractConversion of methane towards chemical feedstocks such as oxygenates and hydrocarbons has been studied along with the development of natural gas technology. In this study, CO‐assisted direct conversion of methane into C1 and C2 oxygenates was demonstrated over ZSM‐5 supported transition and platinum group metal catalysts. Besides previously investigated Rh, other platinum group metals (e. g., Ru and Ir) were found to be a potential element as the catalyst. Presence of CO was critical for the reaction, which would work as a ligand, a reductant, and a reactant. On the other hand, the competing undesired CO oxidation toward CO2 was found to be the major issue. Partial oxidation toward methanol or formic acid (C1 oxygenates formation) was parallel to oxidative carbonylation to acetic acid (C2 oxygenate formation).

Direct Conversion of Syngas to Ethanol within Zeolite Crystals

Summary The direct transformation of syngas into C2-oxygenates (e.g., ethanol) is in great demand, but it is still challenging because of the low selectivity and poor catalyst stability. Here, we overcome the issues and report a zeolite-fixation methodology to strengthen the transformation. By employing the extensively investigated RhMn catalyst, the silicate-1 zeolite-fixed RhMn nanoparticles with core-shell structure (RhMn@S-1) exhibit high efficacy for the C2 oxygenates production from syngas, giving oxygenate selectivity at 40.3% under CO conversion of 42.4% (C2-oxygenate selectivity at 88.3% in the total oxygenates), which decidedly outperforms the previous catalysts in direct syngas-to-ethanol transformation. Multiple studies demonstrate that the rigid zeolite framework efficiently stabilizes the Mn-O-Rhδ+ structure, known as the crucial site for C2-oxygenate formation. In the long-term test, the RhMn@S-1 catalyst exhibits constant C2-oxygenate productivities with superior durability. This investigation demonstrates the powerful siliceous zeolite support in the C2-oxygenate production, which has not been explored previously.

C2 oxygenates formation from syngas over the promoter M(M = Rh, Co) monolayer-modified Cu(111) surface: Probing into the role of monolayer promoter on the selectivity

Abstract The promoter metal monolayer deposited over other metal substrate exhibits excellent catalytic performance, in this study, the mechanism of syngas conversion to C2 oxygenates over the promoter M(M = Rh, Co) monolayer-modified Cu catalysts have been systematically investigated to identify the role of monolayer promoter M and its effect on the selectivity using density functional theory calculations. Here, two key steps are examined, including the formation of key CHx intermediates and the C C bond formation of C2 oxygenates. The results show that compared to the pure Cu catalyst, the promoter Co monolayer-modified Cu catalyst exhibits higher selectivity towards CHx(x = 1–3) formation instead of methanol, the most favored CHx monomer is CH2; whereas CHx(x = 1,2) is the most favored CHx monomer on the promoter Rh monolayer-modified Cu(111), which is competitive with methanol formation. The favored CHx monomer originates from the C O bond cleavage of CHxO(x = 1,2) and CHxOH(x = 1,2) species formed by CO hydrogenation. Starting from the favored CHx monomer, compared to the pure Cu catalyst, C2 oxygenates formed by CHO insertion into CHx is the most favorable over the promoter Rh and Co monolayer-modified Cu catalysts. In general, the promoter Co monolayer-modified Cu catalyst exhibits higher selectivity towards syngas conversion to C2 oxygenates than the promoter Rh monolayer-modified Cu, as well as the pure Cu, Rh and Co catalysts, the reasons is attributed to that the d-band center of Co monolayer-modified Cu is the closest to Fermi level leading to the enhancement of adsorption capacity of the intermediates, and promoting the C O bond activation as well as reducing the repulsion interaction between CHx and CHO to facilitate the formation of C2 oxygenates. The present study provides useful information for the design of highly efficient catalysts with the metal promoter monolayer-modified metal substrate in syngas conversion to C2 oxygenates.

Insight into activation of CO and initial C2 oxygenate formation during synthesis of higher alcohols from syngas on the model catalyst K2O/Cu(111) surface

Alkali-metal-modified Cu-based catalysts are important in low-temperature synthesis of methanol and higher alcohols from syngas. Spin-polarized density functional theory calculations with periodic slab models were performed for a K2O/Cu(111) model catalyst to investigate the interactions between K2O and the Cu(111) surface, and activation of CO. The results show that K2O interacts strongly with the Cu(111) surface via electron transfer between K2O and the Cu(111) surface. This changes CO activation and modulates the mechanism of formation of the initial C2 oxygenates. Although K2O significantly promotes CO direct dissociation, H-assisted dissociation of CO is still the most energetically favorable route. The optimal reaction pathway for C2 oxygenate formation is proposed based on a climbing-image nudged elastic band calculation. The reactions starts with CO hydrogenation to yield the preferred monomers, i.e., CHO, CH2O, and CH2, through the pathways CO + H → CHO + H → CH2O + H → CH2 + OH, with reaction barriers of 0.92, 0.97, and 0.58 eV. The reaction of CH2 with CH2O is the main C2 oxygenate formation route on the K2O/Cu(111) surface. These results clarify the effects of K2O on Cu-based catalysts and elucidate the mechanism of C2 oxygenate formation during alcohol synthesis from syngas.

Insight into the effect of promoter Co on C2 oxygenate formation from syngas on CoCu(100) and Cu(100): A comparative DFT study

Density functional theory calculations have been employed to investigate the effect of promoter Co on C2 oxygenate formation from syngas on pure Cu(100) and two kinds of Co-Cu bimetallic surfaces. Based on the results of previous studies that CH3O is a key intermediate in ethanol formation on Cu catalyst, five reactions starting from CH3O were taken into consideration. Different from the results on Cu(100) that CH3OH is the most favorable product on pure Co sites, CH3 formation is more favorable both kinetically and thermodynamically than CH3OH formation, which leads to more CH3 available for CHO insertion to form C2 oxygenates. On the other hand, Co-Cu bimetallic sites can facilitate CHO insertion into CH3 energetically, which is favorable for carbon chain growth. And the addition of Co can make the barrier of CH3 hydrogenation and CH3 coupling to CH4 and CH3CH3 higher, making CH3CHO much more selective than hydrocarbons. In conclusion, by introducing Co into Cu catalyst, the productivity and selectivity of C2 oxygenate precursor of ethanol can be effectively improved. The optimum Co-Cu catalyst should contain Cu ensembles and Co ensembles with proper sizes, and offer enough Co-Cu bimetallic sites at the same time.

C2 Oxygenate Synthesis via Fischer–Tropsch Synthesis on Co2C and Co/Co2C Interface Catalysts: How To Control the Catalyst Crystal Facet for Optimal Selectivity

Density functional theory (DFT) analysis is used to shed light on the intricate effects of the Co2C and Co/Co2C catalyst crystal facets on the selectivity of the C2 oxygenate and hydrocarbon formation in Fischer–Tropsch synthesis. Three representative low-index Co2C (101), (110), and (111) surfaces, varying in surface energy from low and medium to high, are model examples of different Co2C exposed crystal facets. Since CHx (x = 1–3), CO, and H species are the key intermediates critical to the C2 oxygenate selectivity, all Fischer–Tropsch reactions related to CHx (x = 1–3) species, including CO insertion into CHx (x = 1–3) and CHx + CHy (x, y = 1–3) coupling to form C2 species (C2Hx and C2HxO), as well as the hydrogenation and dissociation of CHx (x = 1–3) to form C1 species (CH4 and C), are used as examples examined at a typical FTS temperature of 493 K. On Co2C (101) and (110) surfaces, CH and CH2 species are dominant form of the CHx species, CH self-coupling to C2H2 and CH coupling with CH2 to CH2CH is ...

Insights into the mechanism of acetic acid hydrogenation to ethanol on Cu(111) surface

Density functional theory (DFT) calculations were employed to theoretically explain the reaction mechanism of acetic acid hydrogenation to ethanol on Cu catalyst. The activation barriers of key elementary steps and the adsorption configurations of key intermediates involved in acetic acid hydrogenation on Cu(111) surface were investigated. The results indicated that the direct dissociation of acetic acid to acetyl (CH3COOH→CH3CO+OH) is the rate-determined step. The activation barrier of acetic acid scission to acetyl and the adsorption energy of acetic acid are two descriptors which could determine the conversion of acetic acid. The descriptors might have effects on the ethanol selectivity including: the adsorption energy of acetaldehyde and the activation barriers for OH bond formation of C2-oxygenates (CH3CO+H→CH3COH, CH3CHO+H→CH3CHOH and CH3CH2O+H→CH3CH2OH). These proposed descriptors could be used as references to design new Cu-based catalysts that have excellent catalytic performance.

Insight into size dependence of C2 oxygenate synthesis from syngas on Cu cluster: The effect of cluster size on the selectivity

Size dependence of C2 oxygenate formation from syngas on Cu cluster has been investigated to qualitatively probe into the effect of Cu cluster size on the selectivity of C2 oxygenates, which includes two key steps: CHx and C2 oxygenate formations; Cu13, Cu38 and Cu55 clusters have been employed to model different sizes of Cu cluster. Here, density functional theory method has been performed. Our results show that the adsorption ability of the species involving in C2 oxygenate formation decreases with the increasing of Cu cluster size; Cu cluster size significantly affects the dominant existence forms of CHx(x=1–3) species and C2 oxygenates. Among three Cu clusters, Cu13 cluster exhibits the highest selectivity toward C2 oxygenates compared to other two clusters, suggesting that Cu cluster size can affect the selectivity toward C2 oxygenates, moreover, the smaller Cu cluster size is, the higher the selectivity of C2 oxygenates is. That is probably related to the high concentration of low-coordinated defect sites on small size Cu cluster, which results in the higher activity and selectivity toward C2 oxygenates. The identification of higher intrinsic selectivity of C2 oxygenates, active sites, and stronger cluster size effect would be valuable for developing more efficient and stable Cu catalyst with higher selectivity toward C2 oxygenate in syngas conversion.

The acceleration of methanol synthesis and C2 oxygenates formation on copper grain boundary from syngas

Density functional theory is employed to investigate the Fischer–Tropsch mechanism on copper ∑5(310) tilt grain boundary together with Cu(111) surface. For the methanol formation, Cu∑5(310) can effectively reduce each energy barrier through the preferred CH3O intermediate route as compared with Cu(111). Furthermore, Cu∑5(310) provides a synergetic effect to modulate the energy landscape in the methanol synthesis, where the pathway associated with CH2OH (CH2O↔CH2OH↔CH3OH) presents the clear kinetic advantage than the CH3O route. Simultaneously, the kinetically assisted intermediate CH2OH enable the CH2 production both thermodynamically and kinetically on Cu∑5(310). The formation of C2 oxygenates from CH2 provides a critical precursor to higher alcohol synthesis. The higher activity of grain boundary is attributed to the presence of low-coordinated sites with flexible local configuration. Consequently, the adsorption strength of small species is enhanced to provide a thermodynamic driving force for CO bond scission. The adaptive character of ∑5(310) grain boundary facilitates the stabilization of the transition state, therefore lowering the activation barrier. Present work unravels the microscopic origin of the higher catalytic capacity of copper ∑5(310) grain boundary which can be helpful to guide the development of novel Cu-based catalysts for higher alcohol formation from syngas.

Ethanol Synthesis from Syngas on the Stepped Rh(211) Surface: Effect of Surface Structure and Composition

The mechanisms of ethanol synthesis from syngas including CHx (x = 1–3) species and C2 oxygenates on the stepped Rh(211) surface have been systematically investigated by using density functional theory together with a periodic slab model. Our results show that among all CHx (x = 1–3) species, CH3 is the most favorable monomer, which is more favorable than CH3OH formation; this result is quite different from that on the flat Rh(111). Beginning with CH3 species, CH3 + CHO → CH3CHO dominantly contributes to the formation of C2 oxygenates; subsequently, CH3CHO is successively hydrogenated to ethanol via CH3CH2O intermediate. Meanwhile, CH4 formation by CH3 hydrogenation is energetically compatible with CH3CHO formation; that is, the productivity and selectivity of ethanol is determined by the formations of CH3CHO and CH4. As a result, the strategy of improving the Rh-based catalyst with the help from the promoter Mn is employed to tune the relative activity of CH3CHO and CH4 formations that control the productivity and selectivity of ethanol from syngas. Our results show that the productivity and selectivity of ethanol from syngas can be improved by introducing promoter Mn into the Rh catalyst.

Insight into the preference mechanism for C[sbnd]C chain formation of C2 oxygenates and the effect of promoters in syngas conversion over Cu-based catalysts

Cu-based catalysts are often used for the production of C2 oxygenates or hydrocarbons from syngas, although numerous studies have been reported, the exact mechanism remains in debate, and presents a major challenge in catalysis. In this study, the preference mechanism for CC chain formation of C2 oxygenates and the role of promoters in syngas conversion on Cu(211) and metal-doped MCu(211) (M=Rh, Ni) surfaces, including CHx (x=1–3) hydrogenation, dissociation and coupling, as well as CO or CHO insertion into CHx (x=1–3), have been systematically investigated by using density functional theory method. Our results show that pure Cu(211) surface shows a better catalytic activity to C2H4 formation rather than C2 oxygenates. However, promoters Rh and Ni-doped Cu(211) surfaces show a better activity and selectivity to C2 oxygenates rather than hydrocarbons, in which CH2 species is responsible for C2 oxygenates formation by CHO insertion. In addition, the difference between CO and CHO insertion into CHx (x=1–3) to form C2 oxygenates is mainly attributed to the HOMO–LUMO gap of CHxCHO and CHxCO, the smaller HOMO–LUMO gap greatly facilitates the charge transfer and hybridization between adsorbed species and catalysts.

Insight into the Preference Mechanism of CHx (x = 1–3) and C–C Chain Formation Involved in C2 Oxygenate Formation from Syngas on the Cu(110) Surface

The possible formation pathways of CHx (x = 1–3) and C–C chain involved in C2 oxygenate formation from syngas on an open Cu(110) surface have been systematically investigated to identify the preference mechanism of CHx (x = 1–3) and C–C chain formation. Here, we present the main results obtained from periodic density functional calculations. Our results show that all CHx (x = 1–3) species formation starts with CHO hydrogenation; among them, CHx (x = 2, 3) are the most favored monomers, however, CH3OH is the main product from syngas on the Cu(110) surface, and the formation of CHx (x = 1–3) cannot compete with CH3OH formation. Further, on the basis of the favored monomer CHx (x = 2, 3), we probe into the C–C chain formation of C2 oxygenates by CO or CHO insertion into CHx (x = 2, 3), as well as the hydrogenation, dissociation, and coupling of CHx (x = 2, 3), suggesting that CO insertion into CH2 to form C2 oxygenates is the dominant reaction for CH2 on the Cu(110) surface with an activation barrier of 44.5...

Rh-Decorated Cu Alloy Catalyst for Improved C2Oxygenate Formation from Syngas

C2 oxygenate (acetaldehyde, ethanol, etc.) formation from syngas (CO + H2) is an important industrial process for the production of clean liquid energy fuels and valuable chemical feedstocks that a...

Ab Initio Study of CO Hydrogenation to Oxygenates on Reduced Rh Terraces and Stepped Surfaces

Previously reported syngas conversion experiments on silica-supported Rh nanoparticles show that CO conversion and oxygenate selectivity vary as a function of nanoparticle size. Theoretical studies in the literature have examined the effect of steps on CO dissociation, but structure sensitivity for C1 and C2 oxygenates has not been systematically investigated. In this study, density functional theory-based reaction energetics and kinetics for C−H, C−C, C−O, and O−H bond formation on flat Rh(111) and stepped Rh(211) surfaces are reported and compared. Multiple paths for methanol and ethanol formation are considered to ascertain the lowest energy pathways. Nearly an identical methanol formation route via CO → CHO → CH2O → CH3O → CH3OH is found to be favored on both Rh terrace and (211) sites. CO insertion into CH2 is deduced to be the precursor for C2 oxygenate formation irrespective of site structure. Ethanol formation pathways, however, are determined to be markedly different on flat and stepped Rh surfac...

Different Mechanisms for the Formation of Acetaldehyde and Ethanol on the Rh–Based Catalysts

Different mechanisms for the formation of acetaldehyde and ethanol on the Rh-based catalysts were investigated by the TPR (temperature programmed reaction) method, and the active sites were studied by CO-TPD, TPSR (temperature programmed surface reaction of preadsorbed CO by H2) and XPS techniques. The TPR results indicated that ethanol and acetaldehyde might be formed through different intermediates, whereas ethanol and methanol might result from the same intermediate. Results of CO-TPD, TPSR, and XPS showed that on the Rh-based catalyst, the structure of the active sites for the formation of C2-oxygenates is (Rh0xRh+y)–O–Mn+ (M=Mn or Zr, x⪢y, 2≤n≤4). The tilt-adsorbed CO species is the main precursor for CO dissociation and the precursor for the formation of ethanol and methanol. Most of the linear and geminal adsorbed CO species desorbed below 500 K. Based on the suggested model of the active sites, detailed mechanisms for the formation of acetaldehyde and ethanol are proposed. Ethanol is formed by direct hydrogenation of the tilt-adsorbed CO molecules, followed by CH2 insertion into the surface CH2–O species and the succeeding hydrogenation step. Acetaldehyde is formed through CO insertion into the surface CH3–Rh species followed by hydrogenation, and the role of the promoters was to stabilize the intermediate of the surface acetyl species.