Nondissociative CO Research Articles

Tailored alcohol distribution by non-dissociated CO adsorption strength on engineered Cu sites in syngas conversion

The competition and diversity of coinstantaneous elementary reactions in syngas conversion make the production of targeted products with high selectivity much more challenging. This work demonstrates the control of product distribution in syngas conversion by changing the adsorption strength of non-dissociated CO on engineered Cu sites. The strength of non-dissociated CO adsorption has been tailored by atomic-Cu1 or ensemble-Cun on Co1Ga1 intermetallic compounds. In syngas conversion, the introduction of either Cu sites could promote the selectivity of alcohols, while atomic-Cu1 sites enhance the production of methanol and ensemble-Cun sites favor the production of C2+ alcohol. A strong adsorption of non-dissociated CO occurs on electron-deficient atomic-Cu1 sites, leading to an increase of alcohols with > 90 % of methanol in alcohols due to direct hydrogenation. A weaker adsorption of non-dissociated CO occurs on ensemble-Cun sites, which allows the carbonyl insertion to alkyl species, affording alcohols with ∼ 92 % of C2+ alcohol.

Atomically intimate assembly of dual metal-oxide interfaces for tandem conversion of syngas to ethanol.

Selective conversion of syngas to value-added higher alcohols (containing two or more carbon atoms), particularly to a specific alcohol, is of great interest but remains challenging. Here we show that atomically intimate assembly of FeOx-Rh-ZrO2 dual interfaces by selectively architecting highly dispersed FeOx on ultrafine raft-like Rh clusters supported on tetragonal zirconia enables highly efficient tandem conversion of syngas to ethanol. The ethanol selectivity in oxygenates reached ~90% at CO conversion up to 51%, along with a markedly high space-time yield of ethanol of 668.2 mg gcat-1 h-1. In situ spectroscopic characterization and theoretical calculations reveal that Rh-ZrO2 interface promotes dissociative CO activation into CHx through a formate pathway, while the adjacent Rh-FeOx interface accelerates subsequent C-C coupling via nondissociative CO insertion. Consequently, these dual interfaces in atomic-scale proximity with complementary functionalities synergistically boost the exclusive formation of ethanol with exceptional productivity in a tandem manner.

Potassium-mediated control of adsorbed intermediates on CuCoAl layered nanoplates for ethanol synthesis from syngas

A set of K-promoted CuCoAl nanoplates were synthesized via precipitation followed by impregnation method and assessed for syngas conversion. The optimized 1K-CuCo catalyst manifested a relatively higher total alcohols selectivity of 48.4 %, with ethanol comprising 35.4 % of total alcohols. As indicated by in situ CO DRIFT and XPS outcomes, on the 1K-CuCo catalyst surface, a distinct preference was observed for the adsorption of CO on Cu+ species (non-dissociative CO*) and bridging CO adsorption on Co species (dissociative CO*). A substantial presence of strongly adsorbed formate species, which contributed favorably to the production of CHx* intermediates, was also identified on the optimal catalyst surface. More importantly, operando DRIFT characterization confirmed that K-modified CuCo catalyst effectively inhibited the hydrogenation (or coupling) of adsorbed CHx* intermediates derived from formate species to generate methane and C2+H. Instead, the adsorbed CHx* intermediates coupled with non-dissociated CO* to form CHxCO* species. Therefore, the K promoter can finely regulate the adsorption strength and distribution of intermediates species, thus furnishing sufficient CHx* intermediates and CO* species to take part in CHx-CO coupling reaction to synthesize ethanol.

Rhodium-doped iron oxides promoted by sodium for highly selective hydrogenation of CO2 to ethanol and C2+ hydrocarbons

Hydrogenation of CO2 to produce high-value C2 + products has gained great interest, but catalyst design for highly selective ethanol still remains a challenge. Herein, bifunctional catalysts of iron oxide promoted with rhodium and sodium, Na-Rh-FeOx and Rh-FeOx-Na, were prepared by using simple methods of incipient wetness impregnation (IWI) and coprecipitation. The former catalyst prepared via IWI, Na-Rh-FeOx, is remarkably efficient for the hydrogenation of CO2 to ethanol with high selectivity of 91 % at 200 °C in the liquid fraction. The latter catalyst synthesized solely through the coprecipitation in a sodium-containing medium, Rh-FeOx-Na, displayed the synergy effect to perform a better CO2 conversion of 23 % at 200 °C. Characterization of catalysts via XRD, XPS, N2 physisorption, CO2-TPD, and gas pulse-chemisorption reveals that different preparation methods influenced the dispersion of sodium particles. This property, in turn, impacts the ratio of hydrogen-activating sites to CO-adsorption sites, directly related to the balance between dissociative- and non-dissociative CO* intermediates, which is crucial for ethanol formation. Adding alkali metal facilitates the formation of iron carbide Fe5C2, known to be active for C–C propagation. Integrating Na-Rh-FeOx with ZSM-5 support further increased ethanol selectivity up to 97 % in liquid phase and 90 % in gas phase. By selecting a proper route for introducing alkali metals and combining with supports, the catalyst properties can be effectively tailored, thereby regulating the catalytic activity and selectivity.

Regulating the electronic property of iron catalysts for higher alcohols synthesis from CO2 hydrogenation

The electronic property of a catalyst is crucial to the generation of key reaction intermediates for targeted synthesis. Herein, we reported a strategy by modifying iron catalysts with sulfate and alkali metal (Li, Na, or K) to regulate the electronic property and optimize the reaction pathways for CO2 hydrogenation to higher alcohols. The characterizations demonstrated the influencing relationship between the electronic property of iron catalysts and the catalytic capabilities for CO dissociation, non-dissociated CO activation and catalytic hydrogenation, which were critical to selective synthesis of higher alcohols. Na-S modification was proven more effective to balance the multiple capabilities required for higher alcohols synthesis and the NaS-Fe catalyst gave a higher C2+OH yield in CO2 hydrogenation. DFT calculations further validated the advantage of Na-S modification from the aspects of CO adsorption and C−C coupling. This strategy provides more flexibility in site regulation and applies to catalyst design for many reactions.

Hydrogenation of CO2 to higher alcohols on an efficient Cr-modified CuFe catalyst

CO2 hydrogenation to higher alcohols (C2+OH) is a promising approach to achieve carbon recycling, but it is a challenge due to complex reaction network. Herein, various Cr-modified CuFe (Cr(x)-CuFe) catalysts were prepared, and their catalytic performance and reaction mechanism in CO2 hydrogenation to C2+OH were investigated. Introduction of small amounts of Cr enhances the interaction between Cu and Fe species, which alleviates CO over-dissociation and inhibits generation of iron carbides. In contrast, more acetate and acetaldehyde intermediates are produced via promoting the reaction of CHx with non-dissociated CO. Cr(1%)-CuFe showed CO2 conversion, C2+OH selectivity and space-time yield (STY) as high as 38.4%, and 29.2% and 104.1 mg gcat−1 h−1, at 320 °C, 4.0 MPa and GHSV of 6000 mL gcat−1 h−1. Interestingly, C2+OH STY was further elevated to 268.5 mg gcat−1 h−1, with a catalytic lifetime of at least 120 h, when GHSV was increased to 48000 mL gcat−1 h−1.

K–ZrO2 Interfaces Boost CO2 Hydrogenation to Higher Alcohols

Alkali metal promoters are widely used to modify active metal sites/interfaces of heterogeneous catalysts for numerous industrial processes. However, the interplay between an alkali metal and support, a crucial catalytic parameter, has been scarcely investigated in controlling the activation behaviors of intermediates and improving catalysis. Herein, we report that K–ZrO2 interfaces can boost the production of higher alcohols (HA) from CO2 hydrogenation over an amorphous ZrO2-supported K–Cu–Fe catalyst (KFeCu/a-ZrO2). In situ spectroscopy and chemisorption demonstrate that the strong interactions between K and ZrO2 induce the formation of surface Zrδ+ sites/oxygen vacancies at K–ZrO2 interfaces, thus providing plenty of nondissociative CO activation sites. The improved molecular CO adsorption capacity at the K–ZrO2 interfaces expedites the CO insertion reaction (*CHx + *CO → *CHx-CO) at Cu–Fe5C2 interfaces, thereby driving the HA synthesis reaction with nearly 4.6 times higher activity of HA in comparison to the KFeCu/SiO2 catalyst. At the optimal conditions of 320 °C, 4 MPa, and 12 L gcat–1 h–1, the KFeCu/a-ZrO2 catalyst shows the HA space time yield of 125.0 mg gcat–1 h–1, ranking the top level among the reported single-component catalysts in the literature. Most importantly, this work provides an in-depth insight into alkali promoter–support interactions for promoting catalytic performance.

A Simple Strategy Stabilizing for a CuFe/SiO2 Catalyst and Boosting Higher Alcohols’ Synthesis from Syngas

Stable F-T-based catalyst development in direct CO hydrogenation to higher alcohols is still a challenge at present. In this study, CuFe/SiO2 catalysts with a SiO2 support treated with a piranha solution were prepared and evaluated in a long-term reaction. The treated catalyst showed higher total alcohols’ selectivity and great stability during a reaction of more than 90 h. It was found that the treatment with the piranha solution enriched the surface hydroxyl groups on SiO2, so that the Cu–Fe active components could be firmly anchored and highly dispersed on the support, resulting in stable catalytic performance. Furthermore, the in situ DRIFTS revealed that the adsorption strength of CO on Cu+ on the treated catalyst surface was weakened, which made the C-O bond less likely to be cleaved and thus significantly inhibited the formation of hydrocarbon products. Meanwhile, the non-dissociated CO species were obviously enriched on the Cu0 surface, promoting the formation of alcohol products, and thus the selectivity of total alcohols was increased. This strategy will shed light on the design of supported catalysts with stabilized structures for a wide range of catalytic reactions.

CO hydrogenation to ethanol and higher alcohols over ultrathin CuCoAl nanosheets derived from LDH precursor

Ultrathin CuCoAl nanosheets derived from layered double hydroxide (LDH) precursor (1–3 cationic-layers) were successfully synthesized by aqueous miscible organic solvent treatment (AMOST) strategy and performed for ethanol and higher alcohols production via CO hydrogenation. The ultrathin CuCoAl nanosheets (CuCo-LDH-aw) demonstrated a comparatively higher total alcohols (ROH) selectivity of 47.6 %, and the molar fraction of ethanol reached up to 37.5 % among the total alcohols. As revealed by XRD, TEM, SEM, BET and AFM characterizations, the uniform dispersion of CuCo NPs was observed over the ultrathin CuCoAl nanosheets. Moreover, the optimum ultrathin CuCoAl nanosheets exhibited a higher ratio of Cu+/(Cu0 + Cu+), which exerted a positive role on non-dissociative CO* insertion CHx intermediates. More significantly, XPS and in situ CO-DRIFT techniques uncovered the formation of CHx intermediates derived from bridge CO dissociation on the metallic Co species and the formate hydrodeoxygenation route, which involved CO* and surface hydroxyl groups (–OH) reaction. Consequently, it was concluded that the triply synergistic effect of Cu+, Co and surface hydroxyl groups (–OH) species contributed to the improvement in ethanol selectivity via CO hydrogenation.

Monometallic iron catalysts with synergistic Na and S for higher alcohols synthesis via CO2 hydrogenation

Direct hydrogenation of CO2 to higher alcohols is highly attractive and challenging. Iron catalysts are generally not regarded as ideal candidates, owing to its poor ability for non-dissociative CO activation, and needs to combine with other metals (such as Rh, Pd, Cu) for above process. Herein, we proposed an iron catalyst co-modified with Na and S (FeNaS-0.6), achieving a space-time yield of 80.5 mg gcat−1 h−1 for alcohols, more than 98 % of which corresponds to C2+ alcohols. The synergistic effects of Na and S enable the monometallic iron catalyst to provide matched molecularly adsorbed CO and alkyl species simultaneously required for higher alcohols synthesis. Sulfur existed in the form of sulfate, and its electron-withdrawing effects could transfrom the surrounding Fe sites for CO dissociation to those for non-dissociative CO adsorption. The DFT modeling also confirmed more facile formation of alcohols on the Na and S co-modified iron catalyst.

Density functional theory study of the CO adsorption on Ni4M (M = Mo, Sc, and Y) nanoclusters

In this study, the nondissociative CO adsorption on the bimetallic clusters of Ni4M (M = Mo, Sc, Y) have been investigated using density functional theory. Different adsorption mode are considered and 9 stable Ni4M-CO complex are resulted for each Ni4M nanoclusters. We found that CO adsorption is thermodynamically favorable on the clusters and the average adsorption energies of optimized geometries are −42, −35 and −33 kcal/mol for Ni4Mo, Ni4Sc and Ni4Y clusters, respectively. Also, the maximum adsorption energies are found to be −67, −53 and −47 kcal/mol for Ni4Mo, Ni4Sc and Ni4Y, respectively. The most stable complexes obtained upon interaction of CO with Ni4M mainly arise from C…Ni4M interaction in preference over a O…Ni4M connection. A thorough analysis led to the conclusion that the Ni4Mo has the maximum average value HOMO-LUMO gap HLG and Ni4Y has minimum HLG.

A novel catalyst for higher alcohol synthesis from syngas: Co[sbnd]Zn supported on Mn[sbnd]Al oxide

A series of the CoZn catalysts supported on MnAl oxide with different Co/Zn molar ratios were synthesized according to the sol–gel method and used for higher alcohol synthesis from syngas. The chemical and physical properties of the as-synthesized catalysts were investigated by various characterization methods such as XRD, H2-TPR, N2 adsorption and desorption, CO2-TPD, CO-TPD, DRIFTS, H2-TPD, TEM, XPS, and CO-TPSR. The results indicated that CO conversion and alcohol selectivity changed depending upon the Co/Zn molar ratio that significantly influenced the properties of the as-synthesized catalysts. The proper Co/Zn molar ratio can increase the concentration of moderate/strong basic sites, improve the amount of non-dissociated CO on the catalyst surface for CO insertion, and enhance the catalytic performance for higher alcohol synthesis. In particular, the 4Co4Zn catalyst shows the highest selectivity to higher alcohols (80%) among all the catalysts.

Ga-promoted CO insertion and C–C coupling on Co catalysts for the synthesis of ethanol and higher alcohols from syngas

This work focuses on Ga promotion to Co sites in supported CoGa catalysts on CO insertion and C–C coupling in syngas conversion. The CoGa catalysts were developed in our group for syngas conversion; they possess uniform dispersion of CoGa particles and pseudo-homogeneous distribution of Co and Ga elements. The Ga atoms contiguous to Co atoms not only contributed to isolating Co centers but also donated electrons to neighboring Co sites. The isolated Co sites were responsible for linearly nondissociative CO adsorption, boosting the CO insertion step to afford alcohol products. Electronic donation from Ga to Co sites promoted dissociative CO adsorption, enhancing carbon-chain growth. The match of CO insertion and C–C coupling gave rise to both enhanced activity in the syngas conversion and impressive selectivity to ethanol and higher alcohols.

Spectroscopic insights into cobalt-catalyzed Fischer-Tropsch synthesis: A review of the carbon monoxide interaction with single crystalline surfaces of cobalt

The present article summarizes experimental findings of the interaction of CO with single crystal surfaces of cobalt. We first provide a quantitative study of non-dissociative CO adsorption on Co(0001) and establish a quantitative correlation between θCO and adsorption site occupation. In light of these findings we revisit the structure of previously reported ordered CO/Co(0001) adsorbate layers. Measurements of the CO coverage at equilibrium conditions are used to derive a phase diagram for CO on Co(0001). For low temperature Fischer-Tropsch synthesis conditions the CO coverage is predicted to be ≈0.5ML, a value that hardly changes with pCO. The CO desorption temperature found in temperature programmed desorption is practically structure-independent, despite structure-dependent heats of adsorption reported in the literature. This mismatch is attributed to a structure-dependent pre-exponential factor for desorption. IR spectra reported throughout this study provide a reference point for IR studies on cobalt catalysts. Results for CO adsorbed on flat and defect-rich Co surfaces as well as particular, CO adsorbed on top sites, and in addition affect the distribution of COad over the various possible adsorption sites.

Activation of Methane Promoted by Adsorption of CO on Mo2 C2 (-) Cluster Anions.

Atomic clusters are being actively studied for activation of methane, the most stable alkane molecule. While many cluster cations are very reactive with methane, the cluster anions are usually not very reactive, particularly for noble metal free anions. This study reports that the reactivity of molybdenum carbide cluster anions with methane can be much enhanced by adsorption of CO. The Mo2 C2 (-) is inert with CH4 while the CO addition product Mo2 C3 O(-) brings about dehydrogenation of CH4 under thermal collision conditions. The cluster structures and reactions are characterized by mass spectrometry, photoelectron spectroscopy, and quantum chemistry calculations, which demonstrate that the Mo2 C3 O(-) isomer with dissociated CO is reactive but the one with non-dissociated CO is unreactive. The enhancement of cluster reactivity promoted by CO adsorption in this study is compared with those of reported systems of a few carbonyl complexes.

Clay-Supported Molybdenum-Based Catalysts for Higher Alcohol Synthesis from Syngas

A kind of clay-supported K-Co-Mo catalyst was prepared by a sol-gel method combined with incipient wetness impregnation. The catalyst structure was characterized by X-ray diffraction, N2 adsorption-desorption, H2 temperature-programmed reduction, and X-ray photoelectron spectroscopy techniques and its catalytic performance for higher alcohol synthesis from syngas was investigated. The active components has a high dispersion on the clay support surface. The increase of the Mo loading promoted reduction of Mo6+ but had no significant influence on the reduction of Mo4+ and Co2+ species. After reduction, a kind of lower state Moδ+ (1<δ<4) species was observed on the surface. Compared with the unsupported catalyst, the clay supported K-Co-Mo catalysts showed much higher catalytic performance for alcohol formation. The reason can be explained that the supported catalyst have a high active surface area and the mesoporous structure prolonged the residence time of intermediates for alcohol formation to some extent, which promoted the formation of higher alcohols. The high activity of the catalyst reduced at 773 K may be attributed to the high content of Moδ+ (1<δ<4) species on the surface, which was regarded as the active site for the adsorption of nondissociative CO and responsible for the alcohol formation.

Advances in bifunctional catalysis for higher alcohol synthesis from syngas

Bifunctional catalysis on dual sites plays an important role in higher alcohol synthesis from syngas. It makes use of two types of active sites of which one type dissociates CO and forms surface alkyl species and the other type catalyzes non-dissociative CO adsorption for CO insertion and alcohol formation. To improve catalytic activity for higher alcohol synthesis, it is necessary to design dual sites on the atomic scale to give them high stability. The recent advances in higher alcohol synthesis using bifunctional catalysts are reviewed. The design of the dual sites, the structure of the dual sites on several typical catalyst systems, and the structural evolution of the dual sites during reaction are discussed using our latest research results.

New insights into the role of the electronic properties of oxide promoters in Rh-catalyzed selective synthesis of oxygenates from synthesis gas

A series of 2.5% Rh/M@Al 2O 3 model catalysts were prepared by supporting Rh on high-area γ-Al 2O 3, resulting in a surface covered by a monolayer (4.5–7 atoms/nm 2) of MO x promoter oxides (M = Fe, V, Nb, Ta, Ti, Y, Pr, Nd, Sm). The catalysts were extensively characterized and evaluated for the conversion of synthesis gas to oxygenates at 553 K, 5.0 MPa, H 2/CO = 1, and space velocity adjusted to attain CO conversion around 15%. The broad range of products formed depending on the specific promoter were, for the first time, quantitatively described using the selectivity parameter ( Φ) defined here, which indicates, for a given reaction product, the contribution of carbon atoms derived from dissociative ( C dis) and nondissociative ( C ins) activation of CO. Both the catalytic activity and, more interestingly, the selectivity pattern given by the Φ parameter were correlated with the electronic properties of the MO x promoters (i.e., electron-donating/electron-withdrawing capacity) for an extensive series of catalysts. Low-temperature and at-work CO-FTIR experiments suggested that the high activity and hydrocarbon selectivity displayed by catalysts promoted by more electron-withdrawing (acidic) oxide promoters (e.g., TaO x ) were related to a higher proportion of bridged Rh 2(CO) B adsorption sites and to a higher electron density (i.e., a higher electron back-donation ability) of the Rh 0 surface sites, both factors promoting CO dissociation events. In contrast, linear CO adsorption on Rh 0 sites displaying decreased electron back-donation in catalysts promoted by electron-donating (basic) oxides (e.g., PrO x , SmO x ) was likely related to nondissociative CO activation and thus to the selective formation of oxygenates. TEM, XPS, and CO-FTIR results pointed to differences in morphology, rather than size or partial electronic charge, of the nanosized Rh 0 crystallites as the likely cause for the different proportions of CO adsorption sites. The Rh 0 NP morphology, both as-reduced and at-work, is a function of the electronic properties of the underlying promoter oxide.

Fischer–Tropsch synthesis on mono- and bimetallic Co and Fe catalysts in fixed-bed and slurry reactors

Abstract Mono- and bimetallic cobalt and iron oxide nanoparticles deposited on the surface of a silica substrate were tested in a fixed-bed reactor and stirred-tank slurry reactor under different process conditions, for the Fischer–Tropsch reaction, this comparison is presented for the first time, being presented in this work a comparison of above systems in both reactor types. For the fixed-bed reaction, the monometallic iron-based catalyst presented the highest activity at low reaction temperatures. By contrast, the monometallic cobalt catalyst is the less active for all reaction temperatures. This catalyst records high values for the α parameter, whereas the iron catalyst reveals major selectivity to alcohols. Moreover, the introduction of iron produces a decrease in hydrocarbon chain growth and an increase in alcohol selectivity. This behaviour might be due to the formation of a Co–Fe alloy, observed by several characterization techniques. In the slurry studies, CO conversion is greater for the cobalt-based catalyst and lower for bimetallic systems, whilst the iron catalyst deactivates in a few hours, which is explained by the formation of carbonaceous deposits. The bimetallic catalysts record a decrease in CO conversion and an enhancement in the selectivity of methane and light hydrocarbons with the increase in iron amount. The absence of alcohols in this type of reactor is explained by the greater solubility of hydrogen, as well as by the greater contact time of reactants, which decreases the possibility of non-dissociative CO insertion.

CO adsorption on pure and binary-alloy gold clusters: A quantum chemical study

We performed density-functional theory analysis of nondissociative CO adsorption on 22 binary Au-alloy (Au(n)M(m)) clusters: n=0-3, m=0-3, and m+n=2 (dimers) or 3 (trimers), M=Cu/Ag/Pd/Pt. We report basis-set superposition error corrections to adsorption energies and include both internal energy of adsorption (DeltaU(ads)) and Gibbs free energy of adsorption (DeltaG(ads)) at standard conditions (298.15 K and 1 atm). We found onefold (atop) CO binding on all the clusters except Pd2 (twofold/bridged), Pt2 (twofold/bridged), and Pd3 (threefold). In agreement with the experimental results, we found that CO adsorption is thermodynamically favorable on pure Au/Cu clusters but not on pure Ag clusters and also observed the following adsorption affinity trend: Pd>Pt>Au>Cu>Ag. For alloy dimers we found the following patterns: Au2>M Au>M2 (M=Ag/Cu) and M2>M Au>Au2 (M=Pd/Pt). Alloying Ag/Cu dimers with (more reactive) Au enhanced adsorption and the opposite effect was observed for PdPt dimers. The Ag-Au, Cu-Au, and Pd-Au trimers followed the trends observed on dimers: Au3>M Au2>M2Au>M3 (M=Ag/Cu) and Pd3>Pd2Au>PdAu2>Au3. Interestingly, Pt-Au trimers reacted differently and alloying with Au systematically increased the adsorption affinity: PtAu2>Pt2Au>Pt3>Au3. A strikingly different behavior of Pt is also manifested by the triplet spin state and onefold (atop) binding in Pt3-CO which is in contradiction with the singlet spin state and threefold binding in Pd3-CO. We found a linear correlation between CO binding energy (BE) and elongation of the CO bond. For Ag-Au and Cu-Au clusters, the increase in CO BE (and elongation of the C-O bond which is probably due to the back donation) is accompanied by the decrease in the cluster-CO distance suggesting that the donation (from 5sigma highest occupied molecular orbital in CO to cluster lowest unoccupied molecular orbital) mechanism also contributes to the BE. For Pd-Au clusters, the cluster-CO distance (and CO bond length) increases with increase in the BE, suggesting that the donation mechanism may not be important for those clusters. No clear trend was observed for Pt-Au clusters.