Z‐Scheme Photocatalytic Systems for Carbon Dioxide Reduction: Where Are We Now?

Zhu L., Li H., Xu Q., Xiong D., Xia P.

Developing heterojunction is one of promising approaches to acquire desired photocatalysts with high-efficient photocatalytic activity. In this work, sheet-like ternary ZnO/ZnWO4/g-C3N4 composite was synthesized via stepwise calcination treatment. The double interface electric fields built in the ZnO/ZnWO4/g-C3N4 heterojunction can promote efficient separation of photogenerated charge carriers in space. Moreover, in contrast with the individual ZnO, g-C3N4, ZnWO4 and their binary composites, this double Z-scheme heterojunction achieves more light harvesting, larger pore volume, stronger photoreduction capacity and CO2 adsorption capacity. Therefore, the sheet-like ZnO/ZnWO4/g-C3N4 heterojunction exhibits efficient conversion of the CO2 molecules into solar fuels under the light irradiation. The production yield of photocatalytic CO2 reduction over the double Z-scheme heterojunction is 13.19 μmol h-1 g-1 and the conversion rate of hydrocarbon fuel is highly up to 91.5%, which are much higher than that of other samples. This work offers a novel perspective to achieve high-efficiency heterojunction system for photoredox applications such as photocatalytic antibacterial, nitrogen fixation and degradation of pollutions.

Ciocarlan R., Hoeven N., Irtem E., Van Acker V., Mertens M., Seftel E.M., Breugelmans T., Cool P.

Photocatalytic conversion of CO2 in the gas phase involving Z-scheme mechanism was studied in the presence of CoxZn1-xFe2O4@TiO2 (x = 1; 0.2; 0.4; 0.6; 0.8; 0) catalyst nanocomposites. The catalysts were obtained in a two-step approach, consisting of a co-precipitation reaction forming the magnetic Ferrite nanoparticles and a hydrolysis-condensation reaction of the Ti-source forming the titania anatase phase, followed by a calcination procedure. The structural characterization was done by X-ray diffraction, Raman and UV-DR spectroscopy, and physisorption, confirming the presence of both structures in the nanocomposites, with a band gap between 3 and 3.23 eV. In order to determine the CO2 conversion, a photocatalytic gas phase fixed-bed batch reactor in tandem with a GC analyzer were used. The tests were done under UVC light irradiation and CO, CH4 were identified as the main products during photoconversion of CO2. All the samples showed higher conversions compared to the well-known reference material P25 (Degussa). The CO2 conversion was observed to be directly proportional with the Zn/Co metal ratio in the Ferrite structure, achieving for ZnFe2O4@TiO2 ∼50 μmol gcat.-1 h-1 CO and ∼30 μmol gcat.-1 h-1 CH4. Moreover, for the ZnFe2O4@TiO2 catalyst methanol (CH3OH) formation was observed, while no traces of methanol were detected for the samples containing Co. The electrochemistry analyses clarified the different heterojunctions formed between Ferrites and TiO2. Mott-Schottky plots revealed the formation of a Z-scheme mechanism for ZnFe2O4@TiO2 explaining the best conversion results. On the other hand, the lower activity of CoFe2O4@TiO2 was attributed to the formation of a type I heterojunction system.

Li X., Song X., Ma C., Cheng Y., Shen D., Zhang S., Liu W., Huo P., Wang H.

In this article, we successfully prepared the WO3/g-C3N4 (WO/CN) heterojunction photocatalyst with high photocatalytic CO2 reduction performance by the simple impregnation–calcination process. Tran...

Wang H., Wang Y., Guo L., Zhang X., Ribeiro C., He T.

Catalytic converting CO 2 into fuels with the help of solar energy is regarded as ‘dream reaction’, as both energy crisis and environmental issue can be mitigated simultaneously. However, it is still suffering from low efficiency due to narrow solar-spectrum utilization and sluggish heterogeneous reaction kinetics. In this work, we demonstrate that catalytic reduction of CO 2 can be achieved over Au nanoparticles (NPs) deposited rutile under full solar-spectrum irradiation, boosted by solar-heating effect. We found that UV and visible light can initiate the reaction, and the heat from IR light and local surface-plasmon resonance relaxation of Au NPs can boost the reaction kinetically. The apparent activation energy is determined experimentally and is used to explain the superior catalytic activity of Au/rutile to rutile in a kinetic way. We also find the photo-thermal synergy in the Au/rutile system. We envision that this work may facilitate understanding the kinetics of CO 2 reduction and developing feasible catalytic systems with full solar spectrum utilization for practical artificial photosynthesis. Au deposited rutile exhibits high artificial photosynthesis performance via utilizing full-solar spectrum, for which UV and visible light are used to initiate reaction and IR light is used to boost reaction kinetically through solar heating.

Fu J., Jiang K., Qiu X., Yu J., Liu M.

Photocatalytic carbon dioxide (CO 2 ) reduction to obtain hydrocarbon solar fuels is one of the promising strategies to solve energy crisis and complement carbon cycle. However, the low activity and poor product selectivity greatly limit its practical application. Tuning product selectivity is of great significance to improve the yield of target product and deepen the understanding of CO 2 reduction reaction mechanism. In this review, we firstly summarize the widely accepted pathways of photocatalytic CO 2 reduction reactions. Secondly, important factors affecting product selectivity are analyzed, mainly including light-excitation attributes, band structure of photocatalysts, separation of photogenerated charge carriers, adsorption/activation of reactants, surface active sites of catalytic reaction, and adsorption/desorption of intermediates. Finally, the challenges and perspectives in developing photocatalysts with high CO 2 reduction efficiency and product selectivity are presented.

Xia Y., Tian Z., Heil T., Meng A., Cheng B., Cao S., Yu J., Antonietti M.

Summary Polymeric carbon nitrides (CNs) are regarded as the most sustainable materials for solar energy conversion via photocatalytic processes. However, the first-generation CNs suffered from imperfect charge separation and insufficient CO2 adsorption. Herein, the construction of a heterojunction material involving highly crystalline CN-nanorods with ordered alignment on graphene is delineated, which improves light harvesting, CO2 capture, and interface charge transfer. The graphene-supported 1D nano-arrays of crystalline CNs show a comparably high selectivity of CO2/N2 up to 44, with an isosteric heat of adsorption of 55.2 kJ/mol for CO2. The heterojunction material also drives the simple and efficient CO2 photoreduction in the gas phase, without the addition of any cocatalyst or sacrificial agent, even at the more relevant case of low concentrations of CO2. These findings provide a robust way for tailoring the performance of CN materials, with the aim of a practicable technological application for CO2 capture and photoreduction.

Yang Y., Wu J., Xiao T., Tang Z., Shen J., Li H., Zhou Y., Zou Z.

A unique urchin-like CoZnAl-LDH/RGO/g-C3N4 (LDH/RGO/CN) Z-scheme photocatalyst, which is fabricated by the hydrothermal synthesis of CoZnAl-LDH and the in situ loading of RGO and g-C3N4, is developed for the photocatalytic conversion of CO2. The special spiny external surface and hollow inner cavity endow LDH/RGO/CN with a significantly enhanced light-harvesting capacity. The well-distributed g-C3N4 nanosheets on the CoZnAl-LDH nanoplates, combined with RGO as an electron mediator, constructs an excellent heterosystem with numerous interfaces, efficient charge separation and highly exposed catalytic active sites. The Z-scheme charge-transfer process promotes the oxidizability and reducibility of CoZnAl-LDH and g-C3N4. Furthermore, the synergistic effect among the components contributes to intense adsorption and chemical activation towards CO2, which reduces the reaction barrier for CO2 photoreduction. As a result, the optimized LDH/RGO/CN exhibits highly efficient and selective photocatalytic CO2 conversion to CO. The special 3D urchin-like architecture paves a new way for design of photocatalyst with ideal performance.

Li A., Cao Q., Zhou G., Schmidt B.V., Zhu W., Yuan X., Huo H., Gong J., Antonietti M.

The photocatalytic CO2 reduction reaction (CRR) represents a promising route for the clean utilization of stranded renewable resources, but poor selectivity resulting from the competing hydrogen evolution reaction (HER) in aqueous solution limits its practical applicability. In the present contribution a photocatalyst with hydrophobic surfaces was fabricated. It facilitates an efficient three-phase contact of CO2 (gas), H2 O (liquid), and catalyst (solid). Thus, concentrated CO2 molecules in the gas phase contact the catalyst surface directly, and can overcome the mass-transfer limitations of CO2 , inhibit the HER because of lowering proton contacts, and overall enhance the CRR. Even when loaded with platinum nanoparticles, one of the most efficient HER promotion cocatalysts, the three-phase photocatalyst maintains a selectivity of 87.9 %. Overall, three-phase photocatalysis provides a general and reliable method to enhance the competitiveness of the CRR.

Li A., Cao Q., Zhou G., Schmidt B.V., Zhu W., Yuan X., Huo H., Gong J., Antonietti M.

Zhu B., Wageh S., Al-Ghamdi A.A., Yang S., Tian Z., Yu J.

The adsorption of reactant gas molecules on photocatalysts displays an important role in the photocatalytic activity. Herein, the adsorption of CO2, O2, NO and CO on s-triazine-based g-C3N4 was investigated by density functional theory calculation. Various adsorption sites were designed, and the calculated adsorption energy (Eads) showed that all these gas molecules preferred to adsorb at the open-hollow site. Moreover, the adsorption energy of the most preferential adsorption system for each gas molecule followed the order of Eads (CO2)   O2 > NO > CO. The electronic properties of the most preferential adsorption systems for these gas molecules were investigated, including band structure, work function, density of states, the highest occupied molecular orbital and the lowest unoccupied molecular orbital. The results in this work sufficiently disclose the adsorption behavior of these gas molecules on s-triazine-based g-C3N4.

Wang Y., He D., Chen H., Wang D.

Carbon dioxide (CO2) is regarded as a main contributor to the greenhouse effect. As a potential strategy to mitigate its negative impacts, the reduction of CO2 is environmentally critical, economically meaningful and scientifically challenging. Being both thermodynamically and kinetically unfavored, CO2 reduction requires catalysts as a crucial component irrespective of the reaction modes, be it electrocatalytic, photoelectrocatalytic or photocatalytic. In an effort to systematically review the types of catalysts that have been studied for CO2 reduction, we categorize them into two major groups: those being activated by external sources and those being photoexcited and activated themselves. Attention is focused on the detailed mechanisms for each group by which the reduction of CO2 proceeds, yielding a summary of the guiding principles for catalyst designs. This review highlights the importance of mechanistic studies, which permits us to discuss our perspectives on potential directions of catalyst investigation for future catalytic CO2 reduction research.

Zhang S., Zhao Y., Shi R., Waterhouse G.I., Zhang T.

Ammonia (NH3) is one of the most important commodity chemicals in today's chemical industry. Industrially, ammonia is synthesized via the Haber-Bosch process at high temperature and pressure (typically 400 °C and 200 atm). In nature, the nitrogenase enzyme can convert N2 to NH3 at ambient conditions, motivating the search for similar sustainable technologies for industrial-scale NH3 production. Over the past few years, photocatalytic ammonia production using sunlight and photocatalysts has attracted much attention, allowing the reduction of N2 to NH3 under very mild reaction conditions. Whilst the rates of photocatalytic ammonia synthesis are still a long way off practical requirements, some promising photocatalytic materials have already been identified which encourage wider research in this field. This review aims to capture recent advances in photocatalytic N2 fixation to NH3, by encompassing fundamental aspects of photocatalytic ammonia synthesis, as well as effective photocatalyst and reactor design strategies. Further, the review offers some practical guidelines to researchers regarding the appropriate selection of ammonia detection methods and the performance assessment of ammonia synthesis photocatalysts. The overarching aims of this review are i) to support the development of solar-driven ammonia synthesis, and ii) to assist researchers in moving into this exciting new research space.

Rong X., Chen H., Rong J., Zhang X., Wei J., Liu S., Zhou X., Xu J., Qiu F., Wu Z.

In this report, environmental and energy issues were deal with a Z-scheme photocatalytic system toward the synthetic ammonia by N2 photofixation. Different from traditional double-charge-transfer model, a direct all-solid-state Z-scheme system of modified TiO2 (MT) loaded with ZnFe2O4 (ZFO) was obtained by solvothermal-calcination method, in which calcination temperature is crucial for the mesoporous structure and formation of Z-scheme TiO2/ZnFe2O4. At ambient temperature and pressure, the MT/ZFO photocatalyst presented the ammonia generation rate of about 1.48 μmol/L/min, which is higher than that of single MT or ZFO. The photocatalytic activity enhancement of ammonia generation was attributed to the Z-scheme charge transfer model with a more negative reduction potential in the conduction band of ZnFe2O4. Combining the commonly TiO2 and ZnFe2O4 by a special method shows a desirable Z-scheme photocatalytic system, which can be extended to the fabrication of other Z-scheme photocatalyst according to the various application need.

Dai X., Sun Y.

This review overviews the emerging research of photocatalytic reduction of carbon dioxide on nanoparticles of group VIII transition metals.

Wang L., Jin P., Huang J., She H., Wang Q.

Acquiring stable photocatalysts in possession of efficacious CO2 adsorption capacity and high charge-separation efficiency is crucial for carbon dioxide (CO2) photoreduction into fossil resources. ...

Chávez-Caiza J., Navlani-García M., Fernández-Catalá J., Bhardwaj A., Lousada C.M., Belova L.M., Berenguer-Murcia Á., Cazorla-Amorós D.

Saini P., Kumar K., Saini S., Sethi M., Meena P., Gurjar A., Weigand W., Parewa V.

Huang R., Ma P., Zhang Y., Chen K., Wang L., Wang R., Shi X., Yu Y.

Bharathi K., Bakiyaraj G., Archana J., Navaneethan M.

Wang B., Shan G., Zhang Y., Shi Y., Xu J., Wang Z., Shuai Y., Liu W., Liu J.

Mu M., Meng L., Ma S., Chen W., Yin X., Bai G.

Feng Y., Chen D., Niu M., Zhong Y., He Z., Ma S., Yuan K., Ding H., Lv K., Guo L., Zhang W., Ma M.

Wang X., Wei Y., Tang Z., Wang Y., Xiong J., Li X., Liu Y., Zou J., Zhao Z., Zhang X.

Patulski P., Fernández-Catalá J., Berenguer-Murcia Á., Cazorla-Amorós D.

Ma M., Zhang S., Jia M., Li T., Chen J., Zhao S., Ge S., Zheng Z., Wu S., Fa W.

Deng E., Mo Q., Li Y., Mo Z., Zhang L., Liu J.

Hu J., Sun Y., Liu Z., Zhu B., Zhang L., Xu N., Zhu M., Zhu J., Chen Z.

Muhammad W., Faqir N., Khan M.A., Khan Z.U., Ahmad B., Ahmad M.J., Zada A., Ali F., Nadeem S., Ansar M.Z., Ali W.

Peng D., Mao L., Sun J., Li X., Shi H., Su Z.

Chuanchuan D., Yuling L., Hao S., Shuaishuai L., Qiyuan S., Xinzeng L.