Rate Of CO Formation Research Articles

Influence of pressure and sunflower husks ash as catalyst on tire-char steam gasification

Catalytic gasification of low-reactive waste fuels, like tire-char, with the use of a different type of waste as a cheap and efficient catalyst seems to be an up-and-coming solution; however, requiring selecting appropriate process conditions. Therefore, this study aimed to assess the effect of various amounts (0, 5, 10, 15 wt%) of waste-based catalyst (sunflower husks ash, SHA) and pressure on tire-char gasification with steam. Feedstock and catalyst were characterised, then physically mixed, and the prepared samples were subjected to steam gasification. The gasification measurements were performed at 800, 850, 900 and 1000 °C and the pressures of 0.5 and 1 MPa, using the thermovolumetric method based on an analysis of the composition of the resulting gas (this research focused on the main gas components, i.e. CO and H2). The obtained results showed that the selected tire-char was characterised by a high calorific value; however, it contained a high content of carbon and catalytically inert ash, which may affect its reactivity. In turn, sunflower husk ash selected as catalyst consisted mainly of catalytically active compounds of alkali and alkaline earth metals (in particular K2O), while at the same time a low content of acid components that inhibit the process. The gasification measurements proved that the character of curves of formation rates of CO and H2 differed, indicating different formation mechanisms. Moreover, pressure and catalyst addition affected the gasification process, especially at low temperatures (∼800–850 °C). In this temperature range, an increase in pressure, catalyst addition, and increasing its loading improved the conversion reaction rate and maximum carbon conversion, as well as hydrogen yields. In turn, an increase in pressure and the addition of catalyst positively affected the yield of carbon monoxide in almost the entire range of analysed temperatures, whereas the effect of catalyst was not significant. Finally, the analysis of kinetic parameters of carbon conversion reaction during steam gasification showed that the addition of catalyst reduced both activation energy and pre-exponential factor (the most significant reduction was observed when using 5 wt% of SHA), while the increase in pressure had an inverse effect on these parameters. Nevertheless, the obtained results show that steam gasification of tire-char at low temperatures, the pressure of 1 MPa and in the presence of 15 wt% of SHA may be an attractive way to efficiently obtain gas rich in hydrogen — one of the most promising future fuels.

Manipulation of CO2 hydrogenation selectivity over RuSn/La2O2CO3 catalysts with intermetallic electron transfer

Selective hydrogenation of CO2 to syngas via the reverse water–gas shift (RWGS) route provides an effective strategy to achieve low-carbon development. While CO2 methanation to CH4 takes place readily on Ru metal, the selective hydrogenation of CO2 to CO in Ru-catalysts remains a challenge. In this work, we prepared a Ru-Sn/La2O2CO3 catalytic system allowing over 99% selectivity of CO during CO2 hydrogenation at 400 °C. Characterization revealed that the transfer of electrons from Sn to Ru weakened the dissociation ability of H2 while improving the adsorption ability of O on Ru. The former inhibited the formation of methane, while the latter caused the C-O bond energy of CO2 adsorbed at the Ru-Sn interface to decrease and dissociate at the Ru-side, forming Ru-CO* and Ru-O*; thus, realizing the RWGS process. An ultra-low Ru-loaded (0.01 wt%) RuSn/La2O2CO3 catalyst with strong metal-support interactions achieved a CO2 hydrogenation rate at 103 times higher than the best reported data. This catalyst was successfully applied to the hydrogenation of blast furnace gas (BFG) to syngas, showing a CO formation rate as high as 2.3*106 mmolCO·gRu−1·h−1 and remaining stable at 850 °C for 1000 h.

Selective Photocatalytic Conversion of Methane Induced by Lewis Acid–Base Pair on the Surface of In2O3/TiO2 Heterojunction Photocatalyst

The efficient catalytic conversion of light-driven methane (CH4) is still very challenging. Here, the surface-reconstructed In2O3/TiO2 heterojunction photocatalyst is successfully prepared by a two-step operation of ion exchange and calcination, and the efficient selective photocatalytic conversion of CH4 is achieved. In the absence of O2, the formation rates of ethane (C2H6) and hydrogen (H2) are 68.9 and 78.6 μmol h–1 g–1, respectively, which corresponds to the stoichiometric ratio. However, in the presence of O2 or water (H2O) as the oxidant, CH4 is converted to CO, and the CO formation rates are 113.2 and 94.4 μmol h–1 g–1, respectively. Our results show that the In3–O2– Lewis acid–base pair on the In2O3/TiO2 surface can realize the adsorption and activation of CH4. Moreover, the photogenerated hole-induced reactive oxygen centers (Ti4+O·–Ti4+OH– and Ti4+O2–Ti4+O·–) on the surface of In2O3/TiO2 are identified as the active species for inducing the dissociation of CH4 into ·CH3 radicals, which in turn are coupled into C2H6 by ·CH3. In contrast, in the presence of O2, the surface superoxide anion (O2–), surface ·O– radicals, and O22– anions are identified as the active species for inducing the oxidation of CH4 into CO.

Dynamic oxygen migration and reaction over ceria-supported nickel oxides in chemical looping partial oxidation of methane

The oxygen species over oxygen carriers determine the reaction performance in chemical looping partial oxidation of methane (CLPOM). This paper describes the dynamic migration and reaction of oxygen species over ceria-supported NiO oxygen carrier for CLPOM. At initial reaction stage, Ni-O species could be consumed rapidly and cause the full oxidation of methane to CO2. Afterwards, the active Ni-O-Ce species dominates the methane partial oxidation to promote the syngas yield. It is found that CO formation rate is linearly related with the content of oxygen species over Ni-O-Ce. Furthermore, the lattice oxygen from CeO2 would migrate through the bulk to complement the consumed Ni-O-Ce oxygen species. Eventually, 5NiO/(40CeO2-Al2O3) exhibit nearly three times as high as methane reaction rate of CeO2/Al2O3 and two times as high as CO formation rate of 5NiO/Al2O3. This work provides the comprehensive understanding of transport and reaction mechanism of oxygen species for chemical looping processes.

La doping of manganese oxide leads to a dramatically promoted catalytic performance for UV–visible-infrared photothermocatalytic ethyl acetate abatement

La-doped manganese oxide catalysts with nanorod morphology, cryptomelane octahedral molecular sieve structure (OMS-2) and La/Mn atomic ratios of 0.0045, 0.0097, and 0.0459 (denoted as La-OMS-2-A, La-OMS-2-B, and La-OMS-2-C) were synthesized with the reaction among KMnO4, La(NO3)3, and Mn(NO3)2 at 90 °C. They were used for photothermocatalytic oxidation of ethyl acetate (typical organic pollutant) merely with UV–visible-infrared (UV–vis-IR) illumination. An appropriate amount of La doping was found to dramatically enhance photothermocatalytic activity and diminish the selectivity of more toxic by-product acetaldehyde. The optimal La-OMS-2-B demonstrated exceptionally large photothermocatalytic activity, low acetaldehyde selectivity, and good catalytic stability. Within initial five minutes, its CO2 formation rate (rCO2) was exceptionally high (1362.0 μmol g−1 min−1), a 67-fold increase as compared to pure OMS-2, while its acetaldehyde selectivity was very low (1.9%), a 37-fold decrease as compared to pure OMS-2. Even with λ > 830 nm IR illumination, it still possessed good photothermocatalytic performance. The photothermocatalytic oxidation abided by a light-driven thermocatalysis mechanism. The dramatic catalytic enhancement by the La-doping arose from a considerable promoted thermocatalytic activity due to the oxygen activity of OMS-2 being substantially improved with La-doping. A photoactivation on La-OMS-2-B, quite unlike photocatalysis on semiconductor photocatalysts, was discovered that further substantially improved catalytic activity and decreased acetaldehyde selectivity due to the oxygen activity of OMS-2 being considerably promoted with UV–vis-IR illumination.

Mg-doped SrTiO3 photocatalyst with Ag–Co cocatalyst for enhanced selective conversion of CO2 to CO using H2O as the electron donor

Mg-doping into SrTiO3 photocatalyst significantly enhanced formation rate of CO in the photocatalytic conversion of CO2 by H2O.

Soot growth mechanism in C2H2 combustion with H2 addition: A reactive molecular dynamics study

A numerical investigation of the effect of H2 addition on soot formation was conducted in C2H2 combustion process from the molecular level. The results showed that the chemical effect of H2 (CE-H2) inhibited the soot formation of C2H2 combustion. With the gradual increase of H2 doping ratio, the soot morphology became sparser and the soot particle size decreased. The CE-H2 significantly inhibited the surface growth and coalescence of soot. Addition of H2 led to an increase in the H2 concentration in the combustion process, which inhibited the dehydrogenation reaction of the primary soot particle with C2H2, free radicals, and polycyclic aromatic hydrocarbons (PAHs). The CE-H2 increased the consumption rate of C2H2 and the formation rate of H2O, and it decreased the formation rate of CO. Moreover, the addition reaction of free radicals and PAHs played an important role in the soot growth process, which effectively promoted the soot surface growth process. And the coalescence process significantly increased the soot mass and affected the soot size and morphology although its frequency was relatively low.

Activity and segregation behavior of Pd75%Ag25%(1 1 1) during CO oxidation – An in situ NAP-XPS investigation

The surface reactivity during CO oxidation over Pd(111) and Pd75%Ag25%(111) single crystals has been investigated using near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) under excess oxygen conditions (p ≈ 2 mbar, O2:CO=10). The (√6×√6) Pd5O4 surface oxide is present on Pd(111) when the surface is highly active under the applied conditions. The CO2 formation profile follows a hysteresis upon temperature cycling between 150 and 450 °C that depends on the temperature ramp rate. The behavior is different for Pd75%Ag25%(111), with the CO2 formation rate considerably lower, no surface oxide and only chemisorbed oxygen present at the surface at high temperature, reversible scrambling of the (near) surface Pd/(Pd+Ag) ratio, and no hysteresis behavior observed. The results suggest that the reactant activation is affected by both surface composition and surface termination, i.e. mechanistically different on Pd75%Ag25%(111) than Pd(111), and less efficient on (111) as compared to the (100) alloy counterpart.

Low-crystalline Ni(OH)2 ultrathin nanosheets upcycled from solid wastes for efficient photoconversion of low-concentration CO2

Low-crystalline ultrathin nanosheets (UNSs) hold great potential for efficient photoreduction of anthropogenic CO2, while developing green synthetic methodologies and elucidating the structure-performance relationship remain significant challenges. Here, via a CaO-mediated strategy, we demonstrate a facile strategy for the large-scale production of low-crystalline Ni(OH)2 UNSs from industrial wastes. The resultant Ni(OH)2 UNSs deliver a superior CO formation rate of 9.2 μmol h−1 with a high selectivity of 93.1 % in low-concentration CO2 (10 %), which remarkably outperforms that of Ni(OH)2 NSs with high crystalline. Experimental and theoretical investigations demonstrate that the boosted performance originates from the fully exposed oxygen vacancies and low-coordination Ni sites, which facilitates the separation of photogenerated charge pairs and the adsorption and activation of diluted CO2. This work demonstrates a sustainable strategy for the large-scale synthesis of low-crystalline layered metal hydroxides, paving new avenues for the high-value utilization of industrial wastes in mitigating climate change.

Hierarchical flower-like ternary composite of NiFeCr/PCN/CeO2 towards efficient photocatalytic reduction of CO2

Searching for semiconductor heterostructures with efficient photocatalytic activity and selectivity towards CO2 reduction is highly desired to lessen the speedy depletion of energy resources. Herein, a hierarchical flower-like ternary layered double hydroxides (LDHs) of NiFeCr-LDH-(PCN/CeO2-15%) was fabricated via in situ introduction of polymeric carbon nitride/cerium oxide (PCN/CeO2) and applied for photocatalytic CO2 reduction. The hierarchical flower-like heterostructure makes available more exposed surface-active sites to substantially enhance the light-harvesting capability. The charge transfer among the individual components stimulates the separation of photogenerated charge carriers and reduces their recombination rate, thus promoting the photocatalytic conversion towards CO2 reduction. The as-prepared NiFeCr-LDH-(PCN/CeO2-15%) heterostructure exhibits an exceptional photocatalytic CO formation rate of 52.1 μmol g−1 h−1, which is 7.1, 2.9, and 4.6 times than that of pristine PCN, PCN/CeO2, and pure NiFeCr-LDH, respectively. The magnificent photocatalytic activities are verified by the XPS and HRTEM evaluations, which demonstrate strong electronic interactions amongst heterostructure components. The present work not only achieves enhanced photocatalytic performance of NiFeCr-LDH-(PCN/CeO2-15%) heterostructure but also could provide a new approach to explore different LDHs-based semiconductors towards efficient CO2 reduction.

S-scheme CoTiO3/Cd9.51Zn0.49S10 heterostructures for visible-light driven photocatalytic CO2 reduction

• Hierarchial CoTiO 3 /Cd 9.51 Zn 0.49 S 10 heterostructures were fabricated by growing Cd 9.51 Zn 0.49 S 10 nanowires on CoTiO 3 microprism in situ via a hydrothermal method. • The transfer path of photogenerated charge carriers in CoTiO 3 /Cd 9.51 Zn 0.49 S 10 heterostructure follows a S-scheme mechanism. • The CoTiO 3 /Cd 9.51 Zn 0.49 S 10 hybrids manifest high activity and good stability for photocatalytic CO 2 reduction, achieving an AQE of 7.27% at 420 nm. Cd 1− x Zn x S solid solutions with strong light absorption are promising materials for solar-driven CO 2 reduction; however, their relatively weak redox ability and intrinsic photo-corrosion limit their further development as a photocatalyst. The addition of a second photocatalyst with a suitable band structure to construct a S-scheme photocatalytic system can solve both problems simultaneously. Here, we report a S-scheme photocatalyst based on the heterostructure of CoTiO 3 /Cd 9.51 Zn 0.49 S 10 (abbreviated as CoTiO 3 /CdZnS) that enables the efficient photocatalytic reduction of CO 2 . Detailed physicochemical characterization resolves the S-scheme charge transfer mechanism in this composite photocatalyst. With the well-designed structure of particles and desirable band offsets, this hybrid system offers visible light absorption in a broad spectral region, large surface area, strong redox ability, and fast carrier separation and transportation. Under visible-light illumination, the CoTiO 3 /CdZnS hybrid system displays a CO formation rate of about 11 mmol h −1 g −1 combined with a long-term operational stability. Besides, a high apparent quantum efficiency (AQE) of 7.27% is realized for the CO 2 -to-CO reduction reaction by the optimized CoTiO 3 /CdZnS hybrid under 420 nm monochromatic light irradiation.

Efficiency of the H2O2 consumption by the mineralization of hydrochlorothiazide via photo-Fenton UVA: a time dependent analysis

In the photochemical conversion of hydrogen peroxide (H2O2) into radical ·OH, the rule of radiation dose (the quantum yield) on the oxidation kinetics is well known. In contrast, the H2O2 dose mostly remains an attempt-and-error variable. Here, we propose a method to estimate the efficiency of H2O2 dose consumption by the mineralization micro-kinetics that enables a generalist strategy for a more cost-effective dose of the oxidant. It needs the time-dependent H2O2, micropollutant, and product (CO2) measure. Photo-Fenton (FP) hydrochlorothiazide (HCT) oxidation using a tubular photo-reactor and UVA radiation assisted in demonstrating the method. The average value of ~ 38% was on top of the best efficiencies associated with some of the fastest mineralization rates. Such efficiencies depend on the stochiometric concentration of the oxidant. Here, the variable stoichiometric H2O2 excess for mineralization is proposed as a universal metric to quantify the (under-) over-dose of H2O2. Overall, H2O2 excess between 2 and 5 leads to H2O2 consumption efficiencies above 30%, together with a fast rate of CO2 formation (mineralization). In contrast, any value below one invariably leads to a sluggish oxidation rate, leading even to the total depletion of the oxidant. Besides proposing a selection criterion for the most cost-effective H2O2-dose and providing some examples, this work carefully analyzes the commitment of the H2O2 excess concerning the energy costs (EEO).

Formation and storage characteristics of CO2 hydrate in porous media: Effect of liquefaction amount on the formation rate, accumulation amount

• Formation and storage characteristics of CO 2 hydrate was revealed under different liquefaction conditions. • The influence of liquefaction amount on hydrate formation was illustrated in porous media. • The formation rate of CO 2 hydrate reached to 6.32 × 10 −4 mol/h under partial liquefaction conditions. • The increment of CO 2 formation rate was 3.22 × 10 −4 mol/h in porous media compared with the non-liquefaction condition. • An optimal liquefaction amount occurred in promoting the formation process of CO 2 hydrate in porous media. Hydrate-based CO 2 capture and storage is considered to be an effective way to reduce greenhouse gas emissions and mitigate global warming. Evaluation of the formation and storage characteristics require to illuminate the dependence of CO 2 phase behavior and the formation properties of hydrate in porous media. In this work, the formation process of CO 2 hydrate was investigated to elucidate the influence mechanism and law of different liquefaction amount on the formation characteristics of CO 2 hydrate in porous media. The results showed that CO 2 liquefaction significantly promoted the formation process of hydrate and the liquefied amount was a critical factor in affecting the formation process of CO 2 hydrate in porous media. Compared with the condition of non-liquefaction, the higher formation rate and larger gas storage capacity were obtained under liquefaction conditions. The formation rate of CO 2 hydrate reached 6.32 × 10 −4 mol/h under the condition of partial liquefaction. Furthermore, the formation rate of CO 2 hydrate increased with the initial pressure increasing in the experiments. However, there was an optimal liquefaction amount occurred in promoting the hydrate formation in porous media. The formation rate of CO 2 hydrate sharply decreased and it was only 3.4 × 10 −4 mol/h under condition of complete liquefaction. The results further indicated that, CO 2 liquefaction could promote the formation process of CO 2 hydrate in porous media within a certain range of liquefaction amount. These results will provide a theoretical guidance and reference for sequestration and storage of CO 2 gas in the form of hydrate in sediments.

Construction of S-Co-S internal electron transport bridges in Co-doped CuInS2 for enhancing photocatalytic CO2 reduction

The visible-light-driven CO2 reduction efficiency by photocatalysts remains a bottleneck for CO2 photoreduction into valuable fuels. Herein, we rationally designed and fabricated Co-doped CuInS2 equipped with S-Co-S internal electron transfer bridges for the CO2 photoreduction. The S-Co-S bonding bridge decreases the work function and facilitates the reductive photogenerated electrons to migrate to the surface, thereby inhibiting the recombination of photoexcited charge pairs. Co dopants can work as the active centers and modulate the band configuration giving rise to higher electron density and more charge carriers for the photocatalytic reaction. In addition, the theoretical calculations further confirm that the doping Co helps to increase the concentration of free electrons near the Fermi level and reduce the formation energy of key reaction intermediate ∗COOH, thus lowering the CO2 reduction reaction energy barrier. Ultimately, the optimal CO formation rate was four times compared to that of ordinary CuInS2 under water vapor. This work emphasizes the importance of designing photocatalyst with internal electron transport bridges by Co doping in CuInS2 in tuning catalytic activity and offers an effective strategy for efficient reduction of CO2.

Efficient, Selective CO2 Photoreduction Enabled by Facet-Resolved Redox-Active Sites on Colloidal CdS Nanosheets.

Advances in nanotechnology have enabled precise design of catalytic sites for CO2 photoreduction, pushing product selectivity to near unity. However, activity of most nanostructured photocatalysts remains underwhelming due to fast recombination of photogenerated electron-hole pairs and sluggish hole transfer. To address these issues, we construct colloidal CdS nanosheets (NSs) with the large basal planes terminated by S2- atomic layers as intrinsic photocatalysts (CdS-S2- NSs). Experimental investigation reveals that the S2- termination endows ultrathin CdS-S2- NSs with facet-resolved redox-catalytic sites: oxidation occurs on S2--terminated large basal facets and reduction happens on side facets. Such an allocation of redox sites not only promotes spatial separation of photoinduced electrons and holes but also facilitates balanced extraction of holes and electrons by shortening the hole diffusion distance along the (001) direction of the ultrathin NSs. Consequently, the CdS-S2- NSs exhibit superb performance for photocatalytic CO2-to-CO conversion, which was verified by the isotope-labeled experiments to be a record-breaking performance: a CO selectivity of 99%, a CO formation rate of 2.13 mol g-1 h-1, and an effective apparent quantum efficiency of 42.1% under the irradiation (340 to 450 nm) of a solar simulator (AM 1.5G). The breakthrough performance achieved in this work provides novel insights into the precise design of nanostructures for selective and efficient CO2 photoreduction.

Scrutinizing the mechanism of CO2 hydrogenation over Ni, CO and bimetallic NiCo surfaces: Isotopic measurements, operando-FTIR experiments and kinetics modelling

The mechanistic understanding of the CO2 hydrogenation reaction is essential for the rational design of active and selective catalysts for this process, nevertheless, there are still important controversies related to the structural requirements, the sequence of elementary steps and the reaction intermediate involved in the formation of CH4 and CO(g). Hence, a detailed mechanistic study was performed on SiO2-supported mono- and bimetallic NiCo catalysts by combining kinetic, spectroscopic and isotopic measurements under methanation conditions, to explain the effect of catalysts composition on the CO2 hydrogenation rate and selectivity toward CO(g) and CH4. It was observed an anti-synergistic effect for the CO2 hydrogenation turnover rate on the NiCo bimetallic catalysts, which is attributed to the inhibition of the CH4 formation pathway on the bimetallic surfaces; on the other hand, the CO(g) formation turnover rate values resulted close to the weighted average values and increases linearly with the cobalt content in the catalysts. The results suggest that the two reaction products are formed through parallel routes with different rate-determining steps, involving two types of active sites where carbonyl species adsorb differently: strongly adsorbed species (CO*) lead to CH4 formation via the H-assisted CO bond dissociation while weakly adsorbed (CO⊕) desorbs to produce CO(g). This was consistent with the inverse H/D kinetic isotopic effect (KIE) observed for methane formation and the KIE values close to unity for CO(g) formation over all catalysts. Operando-infrared measurements suggested that weakly and strongly adsorbed carbonyl species are in quasi-equilibrium at the catalyst surface. It was proposed a sequence of elementary steps for CO2 methanation reaction on Ni, Co and NiCo catalysts, from which a Langmuir-Hinshelwood models for CO(g) and CH4 formation rates were derived. These models properly represent the kinetic data for products formation rates, and content physiochemically-consistent parameters, which point out that the CH4 formation from CO2 hydrogenation can be boosted over a catalytic surface that strongly adsorbs CO, hampers its transformation into CO⊕ and shows a high hydrogenation rate of carbonyl species.

Infiltrated nano-CeO2 and inserted Ni-Fe active layer in a tubular cathode substrate for high temperature CO2 electrolysis on solid oxide cells using La0.9Sr0.1Ga0.8Mg0.2O3−δ thin film electrolyte

A tubular type solid oxide cell which consists of a NiO-Y 2 O 3 stabilized ZrO 2 (YSZ) tubular cathode substrate, a La 0.9 Sr 0.1 Ga 0.8 Mg 0.2 O 3−δ (LSGM) electrolyte film and a Sm 0.5 Sr 0.5 CoO 3-δ (SSC) anode was prepared by dip-coating for CO 2 electrolysis at 800 °C. Since Ni in Ni-YSZ substrate is easily re-oxidized under a pure CO 2 electrolysis atmosphere, co-feeding a reductive gas was essential for avoiding Ni re-oxidation and achieving a stable and large CO 2 electrolysis current under high temperature operation. It was found that co-feeding H 2 is more effective for preventing re-oxidation of Ni compared with CO, however, the CO formation rate was slightly lower than that estimated amount by Faraday`s law due to a water shift reaction when H 2 was used as a reductive gas. Deposition of a thin Ni-Fe cathode active layer and CeO 2 nano-particles obtained by infiltration were effective for increasing the CO 2 electrolysis current because of the decrease in the cathodic overpotential. In spite of the low concentration of Ce was infiltrated by using a 1 M solution with a dip process, the volume change in substrate caused by the CO 2 oxidant was also measured. The CO formation rate almost corresponded to the amount estimated by Faraday`s law and the coke deposition was hardly observed in Ni-YSZ substrate after CO 2 electrolysis, when 10% or 5% of CO was co-fed.

Anchoring and reactivation of single-site Co–porphyrin over TiO2 for the efficient photocatalytic CO2 reduction

Porphyrin and its derivatives have been often used for electrochemical and photocatalysis combined with first-row transition metals for sustainable applications. Cobalt–tetrakis(4-carboxyphenyl)porphyrin (Co–TCPP) was covalently anchored onto TiO2. Under CO2, H2, and UV–visible light irradiation, Co–TCPP anchored on TiO2 produced CO at a rate of 2.5 μmol h−1 (using 100 mg of catalyst and UV–visible light of 118 mW cm−2) when the Co–TCPP content was 2.5 wt %; however, the CO formation rate dropped to 0.3 μmol h−1 (using 100 mg of catalyst and the same intensity of light). The photoactivity drop to 12 % was solved by removing the hydroxy ligands coordinated to the central Co ions through ethanol treatment for 5 min of the Co–TCPP–TiO2 photocatalysts. This resulted in an increased CO photoformation rate to 7.3 μmol h−1 (using 100 mg of catalyst and the same intensity of light) because the dispersion of the J- and/or H-aggregates to the isolated Co–TCPP unit proceeded more with ethanol treatment and under UV–visible light. Conversely, to use ethanol as reductant was not effective because it reduced and deactivated Co–TCPP.

ZIF-8/covalent organic framework for enhanced CO2 photocatalytic reduction in gas-solid system

It remains challenging to design and synthesize porous crystalline photocatalysts for photocatalytic reduction CO2 with high efficiency. Herein, a Schiff-based covalent organic framework (TpPa-1) is explored to combine with a metal–organic framework (ZIF-8) via a simple and fast method (the grinding method) firstly, generating a series of photocatalysts TpPa/ZIF-8. The photocatalytic performances of the as-synthesized photocatalysts TpPa/ZIF-8 are evaluated through a gas–solid reaction mode without photosensitizers and sacrificial agents, exhibiting excellent photocatalytic activities toward CO2 in pure CO2 and 10% CO2 atmosphere, especially in 10% CO2 environment. Therefore, the experimental result presents that TpPa/ZIF-8 can obtain the CO formation rates of 43.94 μmol·g−1·h−1 at a pure CO2 environment and 84.87 μmol·g−1·h−1 in 10% CO2 atmosphere, approximately 2 times and about 6 times larger than those of TpPa-1 in the pure CO2 and 10% CO2 atmosphere, respectively. The interesting phenomenon inspires us to investigate the structural factors affecting the photocatalytic activity, and hydroxyl groups from TpPa-1 are suspected to play an important role in the significantly improved photoreduction performance of CO2 in 10% CO2 atmosphere. The controlling examples TFPa/ZIF-8 without hydroxyl groups have also been prepared and evaluated their photoreduction performances in order to validate this hypothesis. The results clearly prove the hydroxyl groups tend to enhance CO2 photoreduction. Additionally, it is noted that TpPa/ZIF-8 could maintain certain photocatalytic activity under the irradiation of sunlight. Summarily, the superior photocatalytic performances of TpPa/ZIF-8 may result from the high charge separation efficiency and increased photocatalytic active sites as well as the CO2 adsorption capacity. The reported work sheds light on the construction of MOFs/COFs hybrid materials and expands their applications.

Permeability Matters When Reducing CO2 in an Electrochemical Flow Cell

The highest performing cathodes for CO2 electrolyzers are porous and exhibit high specific surface areas that serve to increase the density of CO2 reduction catalyst sites. While porous electrodes increase CO2 reduction activity, their high surface area can negatively impact the mass transport of products and reactants at high current densities (i.e., >100 mA cm–2). We demonstrate here the trade-off between the density of CO2 reduction catalyst sites (specific surface area) and mass transport (permeability) for porous electrodes used for mediating the conversion of KHCO3(aq) into CO(g) in an electrolyzer. This demonstration relied on porous electrodes containing a silver electrocatalyst deposited on carbon fibers of varying fiber diameters (0.5, 1, and 10 μm). Porous electrodes with the intermediate specific surface areas (fiber diameter = 1 μm) yielded the highest CO formation rates. This result shows that permeability should be considered when designing porous electrodes for CO2 electrolysis.