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CO methanation has arisen as an attractive research area due to its ability to transform syngas into substituted natural gas. Current monometallic catalysts have a severe problem; quickly deactivated. It is generally known that by introducing another metal to create a bimetallic catalyst, synergistic interaction between both metals considerably enhances catalyst effectiveness. This paper provides a detailed overview of bimetallic catalysts for CO methanation, covering its synthesis method and effect on physicochemical characteristics. The bimetallic catalyst can both reinforce or weaken the metal-support and metal-metal interaction, which weakening it favors reducibility while reinforcing it favors stability. Particle size and dispersion also improve, whereas adding lanthanides can increases the basicity. We also present the mechanism of CO methanation over the bimetallic catalyst, which modifies the mechanism's energy and rate value. This review provides insights on how reaction effectiveness is enhanced, enabling catalyst development with the highest possible performance.

Bisphenol A (BPA) is an endocrine disruptor, and removing it from contaminated water is a major environmental concern. Herein, graphene derivatives such as graphene (G), graphene oxide (GO), and reduced graphene oxide (rGO) supported silica-zirconia (SZ) were successfully synthesized for photodegradation of BPA. The photodegradation of BPA was ordered as follows: rGO/SZ (88%)>GO/SZ (63%)>G/SZ (58%)>SZ (55%). This is because rGO has bigger regions for π-π stacking and less negatively charged carboxyl groups, which BPA has a higher adsorption affinity than GO. In addition, the highest degradation is predominantly due to the high number of carbon-support interactions and defects sites, including oxygen vacancy. This encouraged effective mobility of charge carriers and subsequently enhanced photoactivity. In this study, the rGO/SZ catalyst was chosen to optimize further the reaction parameters including catalyst dosage, pH and initial concentration of BPA. According to the analysis of variance, the catalyst dosage was the most important variable in the degradation of BPA, followed by pH and initial concentration. The optimum BPA degradation predicted from response surface methodology is 88% at conditions of 8.09 mg L−1 using 0.469 g L−1 of rGO/SZ at pH 6.1, which is reasonably close to the predicted value (89.8%). The rGO/SZ catalyst was found to be stable even after five cycles in the reusability testing.

It has been well established that carbon dioxide (CO2) is one of the main greenhouse gasses and a leading driver of climate change. The chemical conversion of CO2 to substitute natural gas (SNG) in the presence of renewable hydrogen is one of the most promising solutions by a well-known process called CO2 methanation. There have been comprehensive efforts in developing effective and efficient CO2 methanation catalytic systems. However, the choice of competitive and stable catalysts is still a monumental obstruction and a great challenge towards the commercialization and industrialization of CO2 methanation. It is necessary to emphasize the critical understandings of intrinsic and extrinsic interactions of catalyst components (active metal, support, promoter, etc.) for enhanced catalytic performance and stability during CO2 methanation. This study reviews the up-to-date developments on CO2 methanation catalysts and the optimal synergistic relationship between active metals, support, and promoters during the catalytic activity. The existing catalysts and their novel properties for enhanced CO2 methanation were elucidated using the state-of-the-art experimental and theoretical techniques. The selection of an appropriate synthesis method, catalytic activity for CO2 methanation, deactivation of the catalysts, and reaction mechanisms studies, have been explicitly compared and explained. Therefore, future efforts should be directed towards the sustainable developments of catalytic configurations for successful industrial applications of CO2 utilization to SNG using CO2 methanation.

Global population growth and accelerated urbanisation have resulted in massive amounts of fossil fuel use and waste production. Because of its high energy content, pure nature, and fuel quality, hydrogen fuel is a viable option to fossil fuels. Biohydrogen from agricultural waste, in particular, piques concern because it generates hydrogen while still disposing of waste. This review conducted a bibliometric analysis of biohydrogen production from organic waste to trace the research trends and hotspots based on the literature in the Web of Science (WOS) database from 1970 to 2020. The present review article also focuses on highlighting various processes for converting organic waste into hydrogen, raw materials for biohydrogen production, and catalysts that could distil the latest perceptions that could shed light on a route advancing for successful catalyst design. It also seems that some intentions have been paid on studying waste materials such as pure polysaccharides, disaccharides, and monosaccharides. Among all the catalysts used, non-noble and low-cost active metals over reduced graphene oxide (rGO) support can significantly affect the activity of fermentative hydrogen production from organic waste materials. However, researches focusing on developing anaerobic membrane bioreactors for these technologies are still needed.

The contamination by hexavalent chromium and organic pollutants is a serious health issue. Moreover, hexavalent chromium can react with organic pollutants to form a complex which is difficult to treat, and thus remains in the natural environment for long times. Simultaneous photocatalysis can remove both organic pollutants and hexavalent chromium. Here we review the use of period 4 metal oxide-based photocatalysts for the removal of organic pollutants and hexavalent chromium. Titanium dioxide exhibited the highest performance, up to 100%, in removing both hexavalent chromium and organic pollutants from water.

Carbon dioxide and methane emissions are major greenhouse gases contributing to global warming, thus calling for rapid techniques of sequestration. For instance, dry reforming of methane transforms CO2 and CH4 into syngas, a mixture of H2 and CO, yet the reaction catalyst becomes inactivated by carbon formation and metal sintering. Here, we review catalytic systems used for dry reforming of methane. Improved catalysts of high catalytic performance and stability are obtained by selecting the active metal, supporting materials, promoters and preparation techniques. We found a strong correlation between the support morphology, physicochemical properties and catalytic performances. In particular, fibrous structures show optimal metal–support interaction, distribution, particle size, basicity, storage of oxygen space, surface area and porosity, resulting in high performance of anti-coking and anti-sintering.

Fibrous silica BEA zeolite (FBEA) was synthesized by microemulsion method and tested for benzene methylation. The FESEM and N2 physisorption verified that the FBEA exhibited a dendrimeric silica fiber morphology with large surface area and high ratio of mesopores and micropore volume. While the FTIR pre-adsorbed pyridine revealed that the FBEA possessed low Bronsted acid sites compared to commercial beta zeolite. The catalytic testing at 300 °C showed that FBEA was catalytically active towards benzene methylation to produce toluene with 97.8% conversion and 55.5% yield. It is suggested that the fibrous morphology might increase the mesopores volume and reduce the acidity, hence, improves the benzene alkylation reaction.

Copper oxide loaded on HY zeolite (CuO/HY) catalyst was prepared via a facile electrochemical method, and its photoactivity was evaluated by the decolorization of malachite green (MG) under fluorescent light irradiation. The physicochemical properties of catalysts were characterized by X-ray diffraction (XRD), nitrogen (N2) adsorption–desorption, 27Al and 29Si magic-angle-spinning nuclear magnetic (MAS NMR), transmission electron microscopy (TEM), Fourier transform infra-red spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS) and ultraviolet–visible diffuse reflectance spectroscopy (UV–Vis DRS). The results showed that CuO nanoparticles are well-distributed on HY support. Besides, the isomorphous substitution of Al3+ with Cu2+ was occurred to form Si–O–Cu bond, as confirmed by the 27Al and 29Si MAS NMR, FTIR and XPS analysis. The catalyst activity towards on decolorization of MG was ranked in the following order: 3 wt% CuO/HY (99%) > 5 wt% CuO/HY (86%) > 1 wt% CuO/HY (80%) > HY (41%) > CuO (17%). The highest decolorization by 3 wt% CuO/HY is due to the well distribution of CuO nanoparticles on the HY surface of as well as the Cu incorporated into HY frameworks. In terms of turnover frequency, the 1 wt% CuO/HY showed a higher value (7.95 × 10−4 min−1) compared to the 3 wt% CuO/HY (3.13 × 10−4 min−1) and 5 wt% CuO/HY (1.64 × 10−4 min−1). The decreased of chemical oxygen demand demonstrated that the relationship between decolorization and degradability exists. A kinetic study indicated that the photocatalytic process follows a pseudo-first-order kinetics represented by the Langmuir–Hinshelwood model with the rate determining step as a surface reaction.

A remarkably stable and active nickel (Ni) catalyst, supported on fibrous KCC-1 silica particles, was prepared by in situ one-pot hydrothermal method to produce a hydrogen-rich syngas. The catalysts’ properties were characterized by BET, XRD, FESEM-mapping, FTIR-pyrrole, FTIR-KBr, and XPS, while coke deposition was evaluated using Raman spectra, TEM and TGA/DTA. The high dispersion of Ni crystallites, enhanced basicity, strong Ni-KCC-1 interaction, and encapsulation of Ni particles contributed to the enhanced catalyst stability and activity. The one-pot catalyst produced high CH4 and CO2 conversions at 92% and 88% respectively, with high H2/CO ratio, and an extended stability over 72 h at 750 °C. There was limited coke deposition, predominantly of the amorphous type, owing to its synthesis method and support morphology.

A bicontinuous lamellar silica mordenite (HM@KCC-1) has been successfully synthesized by microemulsion technique assisted with mordenite-seed recrystallization, followed by protonation and impregnation with platinum (Pt). The catalysts were extensively characterized by XRD, N2 adsorption, FTIR, FESEM, TEM and ESR and the activity was tested in hydroisomerization of n-hexane. Fourier Transform-Infrared Spectroscopy (FT-IR) study through pyridine and 2,6- dimethylpyridine adsorption revealed that Pt/HM@KCC-1 possessed abundant Lewis acid sites and moderate Bronsted acid sites while Pt/HM possessed strong Lewis and Bronsted acid sites. At 300 °C, the conversion of n-hexane on Pt//HM@KCC-1 reached 75% with 98% isomer selectivity and 74% isomer yield. The enhancement of catalytic performance is attributed to the well-dispersed Pt nanoparticles, tuned acidic properties and improved textural properties which provided a well balance in the acid/metal functions. Thus, favours the reaction towards higher hydroisomerization selectivity, limiting the undesired consecutive cracking reaction.

The rise of consumption of traditional fossil fuels has caused emissions of greenhouse gas and deterioration of air quality. Biomass is a promising substitute for fossil fuels because biomass provides biofuels and chemicals by thermochemical conversion such as pyrolysis. In particular, fast pyrolysis of biomass cellulose into chemicals and biofuels has recently drawn attention. Issues of commercialization of fast pyrolysis products include low heating value, low stability, and high oxygen content and acidity. Consequently, new catalysts for enhanced cellulose conversion are sought for. Here, we review the production of biofuel and renewable chemicals from cellulose pyrolysis using acidic and basic catalysts. Acidic catalysts are more suitable to produce biofuels containing about 50% aromatic hydrocarbons, compared to basic catalysts which give biofuels containing 15% aromatic hydrocarbons. Basic catalysts are preferred to produce renewables chemicals, particularly ketone compounds. We explain the mechanism of cellulose pyrolysis with acidic and basic catalysts. The strong acid sites on the catalyst facilitate high selectivity for aromatic compounds in the pyrolysis oil, whereas basic active sites induce double-bond migration, increase carbon-coupling reactions, and ketone production.

A simple microwave method was employed to prepare silica-doped mesoporous zirconia nanoparticles, SiO2/ZrO2 (SiZr) catalysts under various Si amount and then characterized by X-ray diffraction, nitrogen adsorption–desorption analyses, Fourier-transform infrared, electron spin resonance, ultraviolet–visible diffuse reflectance spectroscopy and photoluminescence analyses. The lower amount of Si fully stabilized the ZrO2 in the tetragonal phase (t-ZrO2), but the higher amount of Si occupied oxygen vacancies (OV) in the SiZr lattice to disrupt the catalysts with introduce low content of monoclinic phase. The catalyst activity towards on photodegradation of 2-chlorophenol (2-CP) was ranked in the following order: 1SiZr (92%) > TiO2 (73%) > 2SiZr (67%) > 3SiZr (64%) > 4SiZr (56%) > ZrO2 (51%). This result demonstrated that 1SiZr gave the highest degradation percentage of 10 mg L−1 2-CP at pH 5 using 0.375 g L−1 catalyst under visible light irradation within 4 h. The highest photoactivity of 1SiZr is due to the larger surface area and crystallite size, which resulted in a good surface contact with light and thus accelerated the photocatalytic activity. Additionally, the highest amount of OV possesed by 1 SiZr effectively suppressed the electron–hole recombination by acting as an electron acceptors, which consequently affected the t-ZrO2 stabilization as well as the catalytic activity. The kinetics of photocatalytic degradation of 2-CP correlated with pseudo-first order model, with the surface reaction as the controlling step. The photogenerated hole was the most active species as confirmed by effect of scavenger study. The 1SiZr maintained the photocatalytic activities after five runs and has a great capability in degrading of various phenols derivatives, indicating its potential use in the phenol-based wastewater treatment.

Photocatalytic degradation is a valuable direction for eliminating organic pollutants in the environment because of its exceptional catalytic activity and low energy requirements. As one of the prospective photocatalysts, zirconium dioxide (ZrO

A novel Z-scheme titania loaded on fibrous silica ceria (Ti-FSC) was triumphantly fabricated via hydrothermal followed by electrolysis method and evaluated for the visible-light degradation of ciprofloxacin (CIP). Noticeably, Ti-FSC exhibits as an efficient photocatalyst for CIP photodegradation with 95% as followed by titania loaded on fibrous silica (Ti-FS) (68%), Ti-CeO

Ni–Co/ZrO2 (NCZ) nano-structure catalysts prepared by the hydrothermal method (NCZ-hyd) and conventional impregnation (NCZ-imp) method. The comparative catalytic activity and coke resistance for each prepared catalyst were examined in steam reforming of phenol and polyethylene terephthalate (PET) plastic waste. The Physico-chemical catalysts properties were characterized by N2-adsorption, XRD, FTIR, TEM, SEM, EDX, H2-TPR, CO2-TPD, and TGA. The effects of preparation methods of NCZ nano-structure catalyst on the catalytic performances in PET-phenol steam reforming were investigated. The NCZ-hyd nanostructure catalyst exposed more outstanding catalytic activity and more excellent coke resistance in comparison with the NCZ-imp. The experimental findings exhibited that catalyst synthesized by hydrothermal method were uniform with the size of 8.8 nm, while NCZ particles prepared by conventional impregnation method were un-uniform, unshaped, and agglomerated, and the size distribution of its particles was from 25 to 50 nm. PET-Phenol conversion and hydrogen yield of 56.5% and 52.5% for the NCZ-imp while 67.6% and 64.8% for the NCZ-hyd catalyst were achieved, respectively. The kinetic study of steam reforming of PET-phenol was also implemented. The activation energy was found to be 69.03 J/mol for NCZ-imp while 107.3 J/mol for the NCZ-hyd nanostructured catalysts. The PET-phenol steam reforming produced valuable liquid products such as benzene, 2-methyl phenol, and dibenzofuran which is one of the crucial keys to solving the waste plastic recycling problem.

A fibrous silica zeolite Y (HY@KCC-1) catalyst with a high surface area of 568 m2/g and unique core-shell morphology was successfully synthesized via a modified KCC-1 synthesis method. Characterization of the catalysts was achieved with X-ray powder diffraction (XRD), field emission scanning microscope (FESEM), N2 adsorption/desorption, and 2,6-dimethylpyridine adsorbed Fourier-transform infrared spectroscopy (FTIR). The Pt/HY@KCC-1 has displayed complete n-dodecane conversion coupled with an incredibly enhanced isomer yield of 72% at 350°C, nearly two-fold higher than that of unmodified Pt/HY catalyst. Remarkably, Pt/HY@KCC-1 had an internal effectiveness factor (η) of unity and negligible internal diffusion limitation, thus suggesting its potential application in hydroisomerization of higher hydrocarbons for enhancing fuel properties.

Heterogeneous advanced oxidation processes are a promising approach for cost-efficient removal of pollutants using semiconductors. Zirconium dioxide (ZrO

With the tremendous development of the economy and industry, the pollution of water is becoming more serious due to the excessive chemical wastes that need to remove thru reduction or oxidation reactions. Simultaneous removal of dual pollutants via photocatalytic redox reaction has been tremendously explored in the last five years due to effective decontamination of pollutants compared to a single pollutants system. In a photocatalysis mechanism, the holes in the valence band can remarkably promote the oxidation of a pollutant. At the same time, photoexcited electrons are also consumed for the reduction reaction. The synergistic between the reduction and oxidation inhibits the recombination of electron-hole pairs extending their lifetime. In this review, the binary pollutants that selectively removed via photocatalysis reduction or oxidation are classified according to heavy metal-organic pollutant (HM/OP), heavy metal-heavy metal (HM/HM) and organic-organic pollutants (OP/OP). The intrinsic between the pollutants was explained in three different mechanisms including inhibition of electron-hole recombination, ligand to metal charge transfer and electrostatic attraction. Several strategies for the enhancement of this treatment method which are designation of catalysts, pH of mixed pollutants and addition of additive were discussed. This review offers a recent perspective on the development of photocatalysis system for industrial applications.

Over the past few years, photocatalysis is one of the most promising approaches for removing organic pollutants. Zirconium dioxide (ZrO2) has been shown to be effective in the photodegradation of organic pollutants. However, low photoresponse and fast electron-hole recombination of ZrO2 affected the efficiency of catalytic performance. Modifying the photocatalyst itself (self-modification) is a prominent way to enhance the photoactivity of ZrO2. Moreover, as ZrO2-like photocatalysts have a large bandgap, improving the spectral response via self-modification could extend the visible light region and reduce the chance of recombination. Here, we review the self-modification of ZrO2 for enhanced the degradation of organic pollutants. The approaches of the ZrO2 self-modification, including the type of synthetic route and synthesis parameter variation, are discussed in the review. This will be followed by a brief section on the effect of ZrO2 self-modification in terms of morphology, crystal structure, and surface defects for enhanced photodegradation efficiency. It also covers the discussion on the photocatalytic mechanism of ZrO2 self-modification. Finally, some challenges with ZrO2 catalysts are also discussed to promote new ideas to improve photocatalytic performance.

Pharmaceutical wastewater is a frequent kind of wastewater with high quantities of organic pollutants, although little research has been done in the area. Pharmaceutical wastewaters containing antibiotics and high salinity may impair traditional biological treatment, resulting in the propagation of antibiotic resistance genes. The potential for advanced oxidation processes (AOPs) to break down hazardous substances instead of present techniques that essentially transfer contaminants from wastewater to sludge, a membrane filter, or an adsorbent has attracted interest. Among a variety of AOPs, electrochemical systems are a feasible choice for treating pharmaceutical wastewater. Many electrochemical approaches exist now to remediate rivers polluted by refractory organic contaminants, like pharmaceutical micro-pollutants, which have become a severe environmental problem. The first part of this investigation provides the bibliometric analysis of the title search from 1970 to 2021 for keywords such as wastewater and electrochemical. We have provided information on relations between keywords, countries, and journals based on three fields plot, inter-country co-authorship network analysis, and co-occurrence network visualization. The second part introduces electrochemical water treatment approaches customized to these very distinct discarded flows, containing how processes, electrode materials, and operating conditions influence the results (with selective highlighting cathode reduction and anodic oxidation). This section looks at how electrochemistry may be utilized with typical treatment approaches to improve the integrated system's overall efficiency. We discuss how electrochemical cells might be beneficial and what compromises to consider when putting them into practice. We wrap up our analysis with a discussion of known technical obstacles and suggestions for further research.

Carbon monoxide (CO) is the most harmful pollutant in the air, causing environmental issues and adversely affecting humans and the vegetation and then raises global warming indirectly. CO oxidation is one of the most effective methods of reducing CO by converting it into carbon dioxide (CO2) using a suitable catalytic system, due to its simplicity and great value for pollution control. The CO oxidation reaction has been widely studied in various applications, including proton-exchange membrane fuel cell technology and catalytic converters. CO oxidation has also been of great academic interest over the last few decades as a model reaction. Many review studies have been produced on catalysts development for CO oxidation, emphasizing noble metal catalysts, the configuration of catalysts, process parameter influence, and the deactivation of catalysts. Nevertheless, there is still some gap in a state of the art knowledge devoted exclusively to synergistic interactions between catalytic activity and physicochemical properties. In an effort to fill this gap, this analysis updates and clarifies innovations for various latest developed catalytic CO oxidation systems with contemporary evaluation and the synergistic relationship between oxygen vacancies, strong metal-support interaction, particle size, metal dispersion, chemical composition acidity/basicity, reducibility, porosity, and surface area. This review study is useful for environmentalists, scientists, and experts working on mitigating the harmful effects of CO on both academic and commercial levels in the research and development sectors.

Biohydrogen produced from cellulosic feedstock is a promising candidate for future energy needs as a renewable energy carrier. The thermochemical route and biological processes have great potential for biohydrogen production. In particular, pyrolysis/gasification and dark fermentation are the methods to enhance the biohydrogen production from cellulose. The review compiles the essential information on both processes, including pretreatment of cellulose since it has a complex structure. The operating conditions for both processes, for example, the influence of cellulose pyrolysis/gasification such as temperature, heating rate, and vapor residence time, while for dark fermentation, including the temperature, inoculum source, hydraulic retention time, and pH, are discussed. The bioreactor configurations and economic aspects of both processes are also discussed. The review aims are to present the current state of knowledge about the two processes using cellulose as substrates. Surprisingly, dark fermentation is a promising method for application of cellulose for biohydrogen production since many works were done on dark fermentation compared to pyrolysis/gasification. The future perspectives on enhancing hydrogen production from cellulose have also been discussed.

Since the discovery of methane synthesis by the interaction between carbon oxide and hydrogen by Sabatier and Senderens in 1902, the methanation reaction has been extensively established and is frequently utilized in chemical manufacturing, comprising the elimination of trace quantities of CO from feed gas containing a rich amount of hydrogen, refining of the reformate gas for fuel cells, and production of new energy sources, which is synthetic natural gas (SNG). It is feasible for SNG to be carried and distributed using the present pipeline infrastructure, which is favourable from a cost-effectiveness standpoint. Since CO methanation is highly exothermic, the development of exceedingly effective catalysts with promising activity in the CO methanation process is essential to address this problem. Because of their economical price and high catalytic performance, nickel-based catalysts have been extensively studied as CO methanation catalysts. Coke deposition and Ni sintering invariably occur on Ni-based catalysts, and these catalysts also have inadequate low-temperature activity and stabilities. So, the advancement of exceedingly effective nickel-based catalysts with outstanding low-temperature catalytic capabilities has emerged as the primary research attention as well as a significant technical encounter in this sector. This brief overview covers recent developments for a supporter for extremely efficient nickel-based catalysts for CO methanation.

Silica-doped tetragonal mesoporous zirconia nanoparticles (SZ) were successfully synthesized with various concentrations of ammonia solution(NH4OH) using microwave-assisted method for photodegradation of Bisphenol A (BPA). The phase of SZ catalyst was pure mesoporous tetragonal ZrO2 with nano in size (5–10 nm). The increasing concentration of NH4OH solution enhanced the hydrolysis and condensation process, as confirmed by formation of additional Zr O Zr, Si O Si and Si O Zr bonds. The formation of oxygen vacancy (Vo) and Zr3+ surface defects (ZSD) also increased proportionally with the concentration of NH4OH solution. Both properties and the Si O Zr bond are supposed to play important roles in stabilizing the tetragonal phase of ZrO2. The Vo and ZSD also lowered the band-gap energy (from 4.35 eV to 3.05 eV) and act as electron acceptors. Catalytic testing showed that 8-SZ gave the highest initial rate (0.372× 10−3 mM min−1) of 10 mg L−1 BPA at pH 7 using 0.5 g L−1 catalyst within 4 h while under similar conditions, 5-SZ, 3-SZ, 0-SZ, ZrO2 and TiO2 only gave 0.263 × 10−3, 0.211 × 10−3, 0.145× 10−3, 0.026× 10−3 and 0.305 × 10−3 mM min−1, respectively. These results demonstrate that higher numbers of Si O Zr, Vo, and ZSD seemed to be the main factors in the high photoactivity of SZ prepared in 8 M NH4OH solution (8-SZ). A kinetic study demonstrated that the photodegradation followed the pseudo-first-order Langmuir-Hinshelwood model. The mechanism study using scavenging agents proved that photogenerated hole was the main active species, thus led more hydroxyl radicals formed to degrade the BPA. The 8-SZ also exhibited a great potential on degradation of various phenols and dyes in wastewater.

Traditional fossil fuels emissions (mainly COx, SOx, and NOx) have adverse effects on human health and the atmosphere. As part of the clean use of fossil fuels, substitute natural gas (SNG) appears to be one of the most promising alternatives to coal and petroleum for its high combustion heat and low CO2 emissions. SNG can be produced using the catalytic methanation of carbon monoxide (CO). Still, the lack of a competitive and robust catalytic system is a significant challenge in CO methanation technology marketing. To emphasize enhanced catalytic CO methanation, this work offers a vital understanding of the extrinsic and intrinsic interactions between catalytic gears (active metals, support materials, and promoters). A contemporary evaluation has been made of the latest developments in CO methanation catalytic systems and the optimum synergistic relationship of active metals, support materials, and promoters. The influence of various active metals, supporting materials, promoters, synthesis methods, and reaction mechanisms studies were specifically compared and clarified to design effective CO methanation catalysts. This study can serve as a critical reference, contributing to the development of well-designed and appropriate catalysts for future applications and favorable dissemination performance in the various academic and industrial sectors.

Nanostructured photocatalysts commonly offered opportunities to solve issues scrutinized with the environmental challenges caused by steep population growth and rapid urbanization. This photocatalyst is a controllable characteristic, which can provide humans with a clean and sustainable ecosystem. Over the last decades, one of the current thriving research focuses on visible-light-driven CeO2-based photocatalysts due to their superior characteristics, including unique fluorite-type structure, rigid framework, and facile reducing oxidizing properties of cerium's tetravalent (Ce4+) and trivalent (Ce3+) valence states. Notwithstanding, owing to its inherent wide energy gap, the solar energy utilization efficiency is low, which limits its application in wastewater treatment. Numerous modifications of CeO2 have been employed to enhance photodegradation performances, such as metals and non-metals doping, adding support materials, and coupling with another semiconductor. Besides, all these doping will form a different heterojunction and show a different way of electron-hole migration. Compared to conventional heterojunction, advanced heterojunction types such as p-n heterojunction, Z-scheme, Schottky junction, and surface plasmon resonance effect exhibit superior performance for degradation owing to their excellent charge carrier separation, and the reaction occurs at a relatively higher redox potential. This review attends to providing deep insights on heterojunction mechanisms and the latest progress on photodegradation of various contaminants in wastewater using CeO2-based photocatalysts. Hence, making the CeO2 photocatalyst more foresee and promising to further development and research.

Amorphous TiO2 (AT) was successfully prepared via the sol-gel technique, using different titanium sources followed by incorporation of copper via the electrochemical method to give CuO/TiO2 (CAT) catalysts. The catalysts were characterized via XRD, N2 physisorption, FTIR, TEM, EDX, XPS, ESR and UV–Vis DRS. The results verified that the use of different titanium precursors have profound effect on the physicochemical properties of the AT catalysts. Further one-pot self-doping carbon from titanium precursor during the addition of copper could greatly enhanced the photocatalytic activity of CAT on the oxidative desulphurization of dibenzothiophene (DBT). 15 CATTBOT exhibited the best performance mainly due to the narrowest band gap and higher numbers of O–Ti–C and Ti–O–C bonds, as well as appropriate amount of Ti3+ surface defects (TSD). These abovementioned properties offered good mobility of electron-hole pairs and/or trap the electrons for enhancement of photoactivity under irradiation of visible light. Kinetic studies showed that the photocatalytic oxidative desulphurization of DBT followed the pseudo-first order Langmuir-Hinshelwood model, where the adsorption was the controlling step. It is believed that these results could contribute to the synthesis of various supported catalysts for numerous applications specifically in removal of sulphur containing compounds in fuel oils.

Copper oxide (CuO, 10–90 wt%) was loaded onto carbon nanotubes (CNTs) by electrosynthesis method. The catalysts (CuO/CNT) were characterized by XRD, nitrogen adsorption–desorption, ESR, FTIR, Raman, and XPS spectroscopy. The results indicated that a lower amount of CuO was dispersed well on the CNT, while higher loading was agglomerated, producing large-size crystallites, hence resulting in lower specific surface area. Adsorption studies revealed that the isotherms are fitted well with the Langmuir model. Moreover, the n value that was obtained from Freundlich model indicated that adsorption process is chemisorption. Photodegradation of p-chloroaniline (PCA) under visible light irradiation demonstrated that the 50 wt% CuO/CNT catalyst gave the highest degradation (97%). It was concluded that C N moieties of PCA were chemisorbed on the catalyst prior to photodegradation, while the Cu O C bonds, surface defects and oxygen vacancies were the main active site in enhancing the subsequent photodegradation. The kinetics of photodegradation were correlated with pseudo-first-order model, verifying the surface reaction was the controlling step. Remarkable mineralization results of PCA were attained by TOC (89.1%) and BOD5 (50.7%). It was also evidenced that the catalyst has a good potential toward degradation of various endocrine disruption compounds.

Silica-doped tetragonal mesoporous zirconia nanoparticles, SiO2/ZrO2 (SZ), were successfully prepared by a simple microwave-assisted method, and the subsequent incorporation of copper (1–10 wt%) via an electrochemical method gave CuO/SiO2/ZrO2 (CSZ) catalysts. The SiO2 stabilized the ZrO2 completely in the tetragonal phase, but that the added copper occupied oxygen vacancies in the SZ lattice to perturb the catalysts and partially reintroduce the monoclinic phase. However, CuO enhanced the photoactivity of CSZ catalysts by lowering the band gap energy (from 4.35 eV to 2.70 eV) and acting as an electron trapper to suppress the recombination of electron–hole pairs. The activity of the catalysts in the photodegradation of bisphenol A was ranked in the following order: 5 CSZ (82%) > 1 CSZ (65%) > 10 CSZ (60%) > SZ (58%). The silica–copper–zirconia cooperative interactions affected the numbers of oxygen vacancies, defect sites, Zr–O–Si and Zr–O–Cu bonds, which consequently influenced the stabilization of ZrO2 as well as the photoactivity of each catalyst. A kinetic study demonstrated that the photodegradation followed the pseudo-first-order Langmuir–Hinshelwood model.

Highly reactive visible-light-driven flower-like rutile-phase titanium nanoparticle (FTN) catalysts were prepared using a simple template-free hydrothermal method with different concentrations of hydrochloric acid (2 M–4M). The physicochemical properties of the catalysts were characterized via XRD, FESEM, FTIR, UV-DRS, N2 adsorption-desorption, and via ESR. Catalytic testing of the photodegradation of methylene blue (MB) was performed using 0.25 mg L−1 of catalyst for 90 min, which resulted in the following activity order: FTN-3M (98%) > FTN-4M (92%) > FTN-2M (86%). The remarkable photocatalytic performance shown by the FTN-3M catalyst was found to be due to its possession of the highest number of hydroxyl groups, oxygen vacancies, and titanium surface defects compared to other catalysts. The fastest reaction rate (4.49 × 10−2 min−1), which was achieved by the FTN-3M catalyst with values of kr = 0. 1845 mg L−1 min−1 and kLH = −0.1646 L mg−1. These values suggested that the reaction occurred on the surface of the catalyst, and was most probably influenced by the previously mentioned, three important properties. In addition, the open structure of the flower-like structure provided more accessible active sites for the adsorption of MB, as well as enhanced light harvesting through multiple reflections between the extended nanospindle structures. The degradation pathway for MB was also investigated. The FTN-3M catalyst maintained their activities for up to five runs without experiencing severe deactivation of the catalyst. Mineralization measurements for MB using TOC and BOD5 analyses, after 90 min of contact time, were 90.45% and 87.73%, respectively, using the FTN-3M sample. The cost-effectiveness of the FTN-3M catalyst proved that this photocatalytic process is greener and more sustainable than noun and should be implemented in industrial applications. It is expected that the extended light response range, in combination with the unique morphology of FTNs, could be exploited in the highly efficient treatment of wastewaters from the textile industry.

Multiple contaminants including heavy metals and phenolic compounds are normally co-exist in wastewater, which caused the treatment process is rather complicated. Herein, the synergistic photoredox of Cr(VI) and p-cresol (pC) by innovative fibrous silica zirconia (FSZr) photocatalyst was reported. The high surface area of FSZr comprised of microspheres with a bicontinuous concentric lamella structure morphology consisted of silica, while its core consisted of ZrO2 structure. The rearrangement of FSZr framework increased the crystallinity, formed Si-O-Zr bonds and narrowed the band gap of ZrO2 for enhanced of photoredox of Cr(VI) and pC. Compared to the reaction, the photoredox efficiency of FSZr for removing Cr(VI) and pC in simultaneous system was found to be 96 % and 59 %, respectively which are higher than that in its single system owing to the efficient electron-hole charge separation. Phenolic compound with high degree of electron donating group gave beneficial effect to photoreduction of Cr(VI). Consequently, a proposed mechanism involving multi-photoredox pathway were proposed based on photoredox reaction and scavengers studies. FSZr sustained the simultaneous photoredox activities after five runs demonstrating its possibility to be use in the wastewater treatment of various pollutants.