The role of CuO in promoting photocatalytic hydrogen production over TiO2

Low cost semiconductor photocatalysts that can efficiently harvest solar energy to generate H₂ from water or biofuels will be critical to future hydrogen economies. In this study, low cost CuO/TiO₂ photocatalysts (CuO loadings 0–15 wt.%) were prepared, characterized and evaluated for H₂ production from ethanol–water mixtures (80 vol.% ethanol, 20 vol.% H₂O) under UV excitation. TEM, XRF, EDAX, EPR, Raman, TGA, XPS and Cu L-edge NEXAFS data showed that at CuO loadings <5 wt.%, Cu(II) was highly dispersed over the TiO₂ support, possibly as a sub-monolayer CuO species. At higher loadings, CuO crystallites of diameter 1–2 nm were identified. The photocatalytic activity of CuO/TiO₂ photocatalysts was highly dependent on the CuO loading, with 1.25 wt.% CuO being optimal (H₂ production rate = 20.3 mmol g⁻¹ h⁻¹). Results suggest that sub-monolayer coverages of Cu(II) or CuO on TiO₂ are highly beneficial for H₂ generation from ethanol–water mixtures and support the development of a sustainable H₂ economy.     

Efficient photocatalytic hydrogen production from water over a CuO and carbon fiber comodified TiO2 nanocomposite photocatalyst

An efficient composite catalyst, CuO/CF/TiO₂ (where CF represents catalytic grown carbon fiber and TiO₂ is the commercial product P25), is prepared with a wet impregnation and calcination process and is applied in the photo stimulated catalytic water splitting. The modification of TiO₂ with CuO, serving as the H₂ evolution cocatalyst, and carbon fiber, behaving as the electron transporter, enhances greatly the overall activity of the material. The activity of CuO/CF/TiO₂ with 1 wt% CF is 45 times higher than that of TiO₂ and 2 times higher than that of CuO/TiO₂. CF and CuO play a synergistic role in decreasing the recombination rate of the photogenerated carriers in TiO₂ bulk phase. The CF in the composite catalyst extends the reaction range, and makes the reaction proceeds on both the surface of CuO intimately contacting with TiO₂ particles and the surfaces of the CuO particles which do not in contact with TiO₂ but in contact with CF.     

Structural properties of CuO/TiO2 nanorod in relation to their catalytic activity for simultaneous hydrogen production under solar light

CuO was introduced into porous TiO₂ nanorod through impregnation method. Before the impregnation step, TiO₂ nanorod was hydrothermally synthesized from TiO₂ powder in aqueous NaOH solution and followed by thermal treatment at 450 °C. The structures and properties of impregnated samples were characterized using various techniques, including XRD, BET, XAS, TEM, and UV-DRS. Their photocatalytic performance on simultaneous hydrogen production from pure water and aqueous methanol solution was also investigated under solar light. It was found that CuO/TiO₂ nanorod possessed a high surface area, good photocatalytic property and excellent hydrogen generation activity. Incorporation of Cu ions into the lattice framework of anatase TiO₂ nanorod enhanced the efficiency in visible region at 438–730 nm. Moreover, the XAS results showed that some Cu ions formed solid solution in the TiO₂ nanorod (Cu_xT_1−xO₂). However, the excessive incorporation of Cu ions did not improve any ability of anatase TiO₂ nanorod for production of hydrogen from pure water splitting. This could be due to the excessive CuO agglomeration at outside-pores which blocked the sensitization of TiO₂ nanorod. Only 1% Cu/TiO₂ nanorod was found to be a remarkable and an efficient photocatalyst for hydrogen production under solar light from both pure water and sacrificial methanol splitting. The highest rate of hydrogen production of 139.03 μmol h⁻¹ g_catalyst⁻¹ was found in sacrificial methanol which was 3.24% higher than in pure water.     

CuO-TiO2 nanostructures prepared by chemical and electrochemical methods as photo electrode for hydrogen production

In present study, copper (II) oxide (CuO) nanostructures were separately synthesized via chemical and electrochemical methods. CuO were coated with chemically synthesized titanium dioxide (TiO₂). Morphological and structural properties of CuO and TiO₂ coated CuO (CuO-TiO₂) materials were examined via field emission scanning electron microscopy (FESEM) and X-ray diffraction (XRD). FESEM images showed that nanowire like CuO formed at both chemical and electrochemical techniques. TiO₂ nanoparticles were homogenously distributed all over CuO surfaces. XRD pattern revealed CuO has monoclinic crystal structure with metallic Cu. Moreover, rutile TiO₂ crystallized in the tetragonal crystal structure. Electrochemical impedance spectroscopy (EIS) and potentiodynamic (PD) polarization measurements were utilized to study electro catalytic performance of the materials towards hydrogen evolution reaction (HER). The values of both energy consumption, and energy efficiency were determined as 329.43 kJ mol^?1 and 86.0% at ?50 mA cm^?2 current density for HER on electrochemically synthesized CuO-TiO₂ at 25 °C.     

Titanium dioxide-coated copper electrodes for hydrogen production by water splitting

Pt is the most commonly used electrode and catalyst materials for H₂ production via water splitting as it provides the highest Gibbs free energy of H₂ adsorption (ΔG_H) an d overpotential. However, as Pt catalysts are expensive and difficult to mass-produce, several efforts have been made to identify suitable substitutes. Although Cu provides lower ΔG_H and overpotential than Pt, it exhibits better catalytic performance than other catalysts and is suitable for H₂ production. However, corrosion of Cu may affect its stability of Cu electrode. To overcome this limitation, we have coated a layer of carbon on the copper electrode and then synthesized titanium dioxide-(TiO₂-) on the C/Cu electrode for water splitting application. Carbon black (CB) has excellent electrical conductivity and stable resistance for effective working as an electrochemical catalyst, and TiO₂ has diverse applications because of its low-cost, non-toxic, and corrosion-resistant characteristics. In this study, TiO₂ was synthesized on C/Cu electrodes under UV irradiation for different durations. The optimum irradiation duration was determined to be 15 min via surface and electrochemical analyses. To identify the potential applications of this TiO₂–C/Cu electrode, we used artificial wastewater as the electrolyte. The synthesized TiO₂–C/Cu electrode exhibited better stability than C/Cu electrode. Further, H₂ production with TiO₂–C/Cu electrode was higher than that with C/Cu electrode at the same current density. We also investigated the effect of TiO₂–C/Cu electrode on decomposition of formaldehyde.     

Preferential oxidation of CO in H2 stream on Au/CeO2–TiO2 catalysts

A series of Au catalysts supported on CeO₂–TiO₂ with various CeO₂ contents were prepared. CeO₂–TiO₂ was prepared by incipient-wetness impregnation with aqueous solution of Ce(NO₃)₃ on TiO₂. Gold catalysts were prepared by deposition–precipitation method at pH 7 and 65 °C. The catalysts were characterized by XRD, TEM and XPS. The preferential oxidation of CO in hydrogen stream was carried out in a fixed bed reactor. The catalyst mainly had metallic gold species and small amount of oxidic Au species. The average gold particle size was 2.5 nm. Adding suitable amount of CeO₂ on Au/TiO₂ catalyst could enhance CO oxidation and suppress H₂ oxidation at high reaction temperature (>50 °C). Additives such as La₂O₃, Co₃O₄ and CuO were added to Au/CeO₂–TiO₂ catalyst and tested for the preferential oxidation of CO in hydrogen stream. The addition of CuO on Au/CeO₂–TiO₂ catalyst increased the CO conversion and CO selectivity effectively. Au/CuO–CeO₂–TiO₂ with molar ratio of Cu:Ce:Ti = 0.5:1:9 demonstrated very high CO conversion when the temperature was higher than 65 °C and the CO selectivity also improved substantially. Thus the additive CuO along with the promoter and amorphous oxide ceria and titania not only enhances the electronic interaction, but also stabilizes the nanosize gold particles and thereby enhancing the catalytic activity for PROX reaction to a greater extent.     

One-pot synthesis of Cu–TiO2/CuO nanocomposite: Application to photocatalysis for enhanced H2 production,dye degradation & detoxification of Cr (VI)

Photocatalytic hydrogen production under the visible spectrum of solar light is an important topic of research. To achieve the targeted visible light hydrogen production and improve the charge carrier utilization, bandgap engineering and surface modification of the photocatalyst plays a vital role. Present work reports the one-pot synthesis of Cu–TiO₂/CuO nanocomposite photocatalyst using green surfactant -aided -ultrasonication method. The materials characterization data reveals the TiO₂ particle size of 20–25 nm and the existence of copper in the lattice as well as in the surface of anatase TiO₂. This is expected to facilitate better optical and surface properties. The optimized photocatalyst shows enhanced H₂ production rate of 10,453 μmol h⁻¹ g⁻¹ of the catalyst which is 21 fold higher than pure TiO₂ nanoparticles. The photocatalyst was tested for degradation of methylene blue dye (90% in 4 h) in aqueous solution and photocatalytic reduction of toxic Cr⁶⁺ ions (55% in 4 h) in aqueous solution. A plausible mechanistic pathway is also proposed.     

TiO2 nanotubes coupled with nano-Cu(OH)2 for highly efficient photocatalytic hydrogen production

Noble-metal-free Cu(OH)₂/TNTs (TNTs: TiO₂ nanotubes) nanocomposite photocatalysts were successfully prepared by loading nano-Cu(OH)₂ on TNTs via a hydrothermal-precipitation process. These were then characterized in terms of morphology and physicochemical properties by employing TEM, XRD, XPS, BET, UV–Vis DRS and PL. The effects of Cu(OH)₂ loading, amount of catalyst on the photocatalytic hydrogen production performance of Cu(OH)₂/TNTs were investigated in detail in aqueous methanol solution under UV irradiation. The results show that, compared with pure TNTs, the TNTs loaded with highly dispersed 8 wt% Cu(OH)₂ exhibited remarkably improved activity for hydrogen production (the largest quantity of evolved hydrogen was ca. 14.94 mmol h⁻¹ g⁻¹ catalyst) with good photostability. This high activity is attributed to the strong synergistic function of Cu(OH)₂/TNTs, including suitable potential of Cu(OH)₂/Cu (E⁰ = −0.222 V) between conduction band (−0.260 V) of TNTs and the reduction potential of H⁺/H₂ (E⁰ = 0.000 V), a unique tubular microstructure of TNTs coated with nano-Cu(OH)₂, large BET specific surface area and high dispersion of Cu(OH)₂. Furthermore, a process mechanism for methanol/water decomposition over Cu(OH)₂/TNTs is proposed to understand its high activity.     

Photocatalytic splitting of seawater and degradation of methylene blue on CuO/nano TiO2

The main objective of this study was to prepare effective photocatalysts for splitting of seawater for solar fuel – H₂ and degradation of seawater organic pollutants such as dyes. To enhance photocatalytic activities, CuO is supported on nano TiO₂ (CuO/nano TiO₂). By X-ray absorption near edge structure (XANES) spectroscopy, CuO clusters are found on nano TiO₂. The 2.5% CuO/nano TiO₂ has greater activities in photocatalytic splitting of water and seawater than nano TiO₂ by 9.9 and 7.8 times, respectively. Interestingly, the 2.5% CuO/nano TiO₂ is also very active for photocatalytic splitting of water and seawater contaminated with dyes such as methylene blue (MB) (10 ppm). Under a 5-h irradiation of the UV–Vis light, about 99% of MB is degraded while 3.1 μmol/h g cat of H₂ are generated from seawater in the photocatalysis process.     

Hydrogen production by supercritical water gasification of lignin over CuO–ZnO catalyst synthesized with different methods

In this study, lignin was gasified in supercritical water with catalysis of CuO–ZnO synthesized by deposition precipitation, co-precipitation and sol-gel methods. Sol-gel synthesized CuO–ZnO showed the highest catalytic performance, and the gasification efficiency was increased by 37.92% with it. The XRD, SEM-EDS and N₂ adsorption/desorption analysis showed that the priority of the sol-gel catalyst was the smallest crystallite size, largest specific surface area and high dispersion. For sol-gel synthesized CuO–ZnO, the increase of CuO/ZnO ratio improved the gasification efficiency but reduced H₂ selectivity. And the catalytic activity was reduced with the calcination temperature above 600 °C due to enlarged crystallites and reduced pores. During sol-gel preparation, both the addition of ethanol and PEG in the solvent reduced the agglomeration and improved the catalytic activity. With CuO–ZnO prepared with 1 g PEG + water as the solvent, the highest H₂ yield of 6.86 mol/kg was obtained, which was over 1.5 times of that without catalyst.     

Effect of acid treatment on the catalytic performance of CuO/Cryptomelane catalyst for CO preferential oxidation in H2-rich streams

The effect of acid treatment on the catalytic performance of CuO/Cryptomelane (CuO/CR) for CO preferential oxidation (CO-PROX) in H₂-rich streams has been investigated. The CR supports are synthesized via the sol-gel approach. The hydrochloric acid or water is used to treat the CR support, and the corresponding CuO/CR catalysts are prepared by an initial wet impregnation method. Compared with the pristine CuO/CR and water-treated CuO/CR_W catalysts, the acid-treated CuO/CR_H exhibits the best catalytic activity with almost 100% of CO conversion at 110 °C, which can be maintained at least 100 h. The characterization results show that acid treatment decreases the K⁺ content in the CuO/CR_H catalyst, which is conducive to the formation of more oxygen vacancies, thereby promoting the reducibility of CuO/CR_H. This is the main reason for the high catalytic activity of the acid-treated CuO/CR_H catalyst. Moreover, the abundant Brönsted acid sites on CuO/CR_H are favorable for the desorption of acidic product CO₂, which also could result in the significant promotion of the catalytic activity for CO-PROX. This study sheds a light on the importance of acid treatment for cryptomelane and provides an efficient catalyst for hydrogen purification.     

CuOx dispersion and reducibility on TiO2 and its impact on photocatalytic hydrogen evolution

Nanostructured CuO_x/TiO₂ (a mixture of Cu/Cu₂O/CuO) was prepared by impregnation for enhancing photocatalytic hydrogen generation from an aqueous solution containing 10 v/v% methanol. At an optimum Cu loading of 0.5 wt% and a calcination temperature of 500 °C, the CuO_x was present as relatively highly dispersed (0.90), fine deposits. At Cu loadings beyond 0.5 wt% a bimodal distribution of CuO_x deposits appeared with the prevalence of larger Cu deposits increasing with increasing Cu content. A corresponding decrease in H₂ generation was observed as Cu loading increased which was attributed to the increasing presence of the larger CuO_x deposits. The particle calcination temperature (in air) was also found to affect CuO_x/TiO₂ activity with an optimum performance achieved at a temperature of 300 °C. Calcining the CuO_x/TiO₂ at 500 °C led to greater oxidation of the CuO_x deposits (∼40%) to form more Cu²⁺ which corresponded to an almost proportional (42%) decrease in H₂ generation. The findings demonstrate the importance of Cu dispersion and oxidation state in governing photocatalytic H₂ generation by CuO_x/TiO₂.     

Oxygen-enhanced water gas shift over ceria-supported Au–Cu bimetallic catalysts prepared by wet impregnation and deposition–precipitation

Au–Cu/ceria bimetallic catalysts were prepared incorporating Au by incipient wetness impregnation (IWI) and deposition-precipitation (DP) methods (with loadings of 1 wt.% and 7 wt.% of Au and Cu, respectively). The as-prepared catalysts were characterized by techniques such as BET, XRD, Raman, XPS, H₂-TPR, CO-TPD and Oxygen Storage Capacity (OSC) measurements. The results indicated a good dispersion of gold and copper for copper ceria catalyst and Au–Cu bimetallic catalysts. Addition of Au to CuO/CeO₂ increases highly the capacity to release lattice oxygen to oxidized CO at low temperatures compared to pure CuO/CeO₂. Au/CeO₂ and Au–CuO/CeO₂ catalyst prepared by DP show higher OSC value than counterparts prepared by IWI, either at 120 and 250 °C. Also, gold-containing catalysts prepared by DP show lower temperature of reduction that the samples prepared by IWI as a consequence of the higher dispersion of gold in the former samples. The presence of gold at different oxidation states was observed by XPS analysis. Preparation method strongly affects to the atom ratio of Au and Au + Cu with respect to surface ceria. The gold incorporation method was a key factor that enhances the redox properties and activity in both WGS and OWGS reactions. The present study shows the gas phase oxygen enhanced the activity of monometallic CuO/ceria and bimetallic Au–Cu/ceria prepared by IWI and DP methods in both WGS and OWGS reactions. AuCC catalyst prepared by DP shows higher hydrogen yield and also higher CO conversion than other prepared by IWI during OWGS reaction.     

Hydrogen production employing Cu/Zn-BTC metal-organic framework on cordierite honeycomb ceramic support in methanol steam reforming process within a microreactor

Three metal-organic frameworks Cu-BTC, Zn-BTC, and Cu/Zn-BTC were prepared and impregnated in nitrate solutions to obtain the precursors. After calcination, three metal-BTC-derived CuO/ZnO/CeO₂/ZrO₂ catalysts were obtained. The samples were characterized and the catalyst-coated cordierite honeycomb ceramics were used in a microreactor for methanol steam reforming at different reaction conditions. Results showed that the Cu/Zn-BTC-derived catalyst exhibited the most fine and uniform particles, the best reducibility, the largest specific surface area, and the optimal surface elemental state due to the difference in the formation mechanisms, resulting in its remarkable catalytic performance. The ceramic support coated with Cu/Zn-BTC-derived catalyst could achieve 100% methanol conversion rate and 0.336 mol/h H₂ output at 260 °C in the microreactor. Stability tests demonstrated that the Cu/Zn-BTC-derived catalyst could maintain its excellent performance without deactivation within 30 h continuous reaction, which was connected with the Ce–Zr–O solid solution with high concentration of oxygen vacancies and surface oxygen.     

Photoreduction of CO2 to fuels under sunlight using optical-fiber reactor

An optical-fiber reactor is employed to photocatalytically reduce CO₂ with H₂O to fuels under UVA artificial light and concentrated natural sunlight. The optical fiber is coated with gel-derived TiO₂–SiO₂ mixed oxide-based photocatalysts. Fe atom is found to insert into the TiO₂–SiO₂ lattice during sol–gel process, resulting in the full visible light absorption as well as the effect on product selectivity of the derived catalyst. Under UVA, ethylene is mainly produced on Cu–Fe/TiO₂ catalyst with the quantum yield of 0.0235%, whereas Cu–Fe/TiO₂–SiO₂ catalyst is observed to favor methane production with the quantum yield of 0.05%. Meanwhile, the overall energy efficiency is found to be much higher on Cu–Fe/TiO₂–SiO₂ (0.0182%) than on its Cu–Fe/TiO₂ counterpart (0.0159%). There is only methane evolved over both bare TiO₂–SiO₂ and Cu–Fe/TiO₂–SiO₂ catalysts under natural sunlight with the production rates of 0.177 and 0.279 μmol/g-cat h, respectively. For the former catalyst, the increase in light intensity is not found to compensate the inherent electron–hole recombination in the TiO₂–SiO₂–acac catalyst, whereas the superior photoactivity of Cu–Fe/TiO₂–SiO₂ catalyst under natural sunlight could be ascribed to its full absorption of visible light.     

Sn-induced CuO–CeO2 catalysts with improved performance for CO preferential oxidation in H2-rich streams

Sn modified CuO–CeO₂ catalysts with different Sn loadings were prepared by a facile, green and solvent-free method. The effect of Sn/Ce ratio over Sn–Cu–Ce-x (x = 0, 1, 2.5, 5, 7.5) samples on CO activity and O₂ selectivity was investigated. The samples were characterized by various techniques using N₂-adsorption/desorption, XRD, H₂-TPR, XPS, Raman and in-situ DRIFTS. It was revealed that stronger interaction between acitve sites and support, higher amounts of Sn²⁺ and Ce³⁺, associated with increased amount of oxygen vacancies, were observed on the catalyst of Sn–Cu–Ce-5. As a result, the optimized catalyst displayed an excellent catalytic performance even in the presence of CO₂ and H₂O. In this sense, probing the Sn modified CuO–CeO₂ catalyst can elucidate some useful keys for the development of high CO₂ and H₂O-resistance catalyst during CO-preferential oxidation in H₂-rich streams.     

Photocatalytic hydrogen production from aqueous methanol solution with CuO/Al2O3/TiO2 nanocomposite

The photocatalytic hydrogen production from aqueous methanol solution was investigated with ZnO/TiO₂, SnO/TiO₂, CuO/TiO₂, Al₂O₃/TiO₂ and CuO/Al₂O₃/TiO₂ nanocomposites. A mechanical mixing method, followed by the solid-state reaction at elevated temperature, was used for the preparation of nanocomposite photocatalyst. Among these nanocomposite photocatalysts, the maximal photocatalytic hydrogen production was observed with CuO/Al₂O₃/TiO₂ nanocomposites. A variety of components of CuO/Al₂O₃/TiO₂ photocatalysts were tested for the enhancement of H₂ formation. The optimal component was 0.2 wt% CuO/0.3 wt% Al₂O₃/TiO₂. The activity exhibited approximately tenfold enhancement at the optimum loading, compared with that with pure P-25 TiO₂. Nano-sized TiO₂ photocatalytic hydrogen technology has great potential for low-cost, environmentally friendly solar-hydrogen production to support the future hydrogen economy.     

CuO–CeO2/SiO2 coating on ceramic monolith: Effect of the nature of the catalyst support on CO preferential oxidation in a H2-rich stream

Powder and structured catalysts based on CuO–CeO₂ nanoparticles dispersed on different silica are studied in CO preferential oxidation. Silica of natural origin (Celite) and fumed silica (aerosil), both commercial materials, and synthesized mesoporous SBA-15 with 20, 200 and 650 m²g^-1 respectively, are selected as supports. CuCe/Celite coated on cordierite monolith displays the highest activity, reaching CO conversion above 90% between 140 and 210 °C and more than 99% around 160 °C. The addition of 10% CO₂ and 10% H₂O partially deactivates the monolithic catalyst.The lower surface area of CuCe/Celite favors the contact between CuO and CeO₂ nanoparticles promoting a better interaction of Cu⁺²/Cu⁺ and Ce⁺³/Ce⁺⁴ redox couples. Raman spectroscopy reveals oxygen vacancies and XPS results show high metal lattice surface oxygen concentration and surface enrichment of Cu and Ce which promote the catalytic activity.     

Influence of crystallite size and interface on the catalytic performance over the CeO2/CuO catalysts

The solvothermal method was used to prepare the CuO precursor with cotton-ball-like morphology in order to obtain the CeO₂/CuO catalysts with high BET surface area. The catalysts were characterized via SEM, XRD, H₂-TPR, ICP, HRTEM and N₂ adsorption–desorption techniques. The study shows that CeO₂ and CuO interact on the contact interface. The interaction of oxides switches on CO oxidation at 55 °C and the synergistic effect of interaction also improves H₂ oxidation at 95 °C. CO oxidation takes place at the contact interface of CeO₂ and CuO. The high BET surface area and good dispersion of catalysts can be more helpful for the presence of accumulated long periphery at interface of CeO₂ and CuO than the larger CeO₂ particles when most of CeO₂ particles pile into the small clusters and distribute on the bulk CuO. The CeO₂/CuO catalyst with 1:2 Ce/Cu molar ratio has the highest BET surface area and better dispersion of CeO₂ among the catalysts, therefore it display good catalytic activity, selectivity and stability.     

Enhancing the activity of a SiC–TiO2 composite catalyst for photo-stimulated catalytic water splitting

In this work, effort has been made to design an efficient catalyst for the photo-stimulated water splitting reaction, starting with the modification of TiO₂ (P25) to enhance its activity. A SiC(1 wt%)–TiO₂ composite material shows an activity as high as twice of that of TiO₂. NiO_x, an electron collector, promotes the activity of TiO₂, while IrO₂, a hole capturer, enhances the hydrogen evolution rate of SiC. A SiC(1 wt%)–NiO_x/TiO₂ three-component and an IrO₂/SiC(1 wt%)–NiO_x/TiO₂ four-component composite materials produce 30% and 100% more H₂ than the NiO_x/TiO₂ catalyst during the first 5 h, respectively, with ethanol used as the sacrificial reagent. Furthermore, the SiC(1 wt%)–NiO_x/TiO₂ catalyst is active under visible light, while the NiO_x/TiO₂ catalyst shows no activity under the same irradiation condition. 3C-SiC has a narrow band gap and its band edge well compensates that of the TiO₂. The enhancing effect of dopants on the SiC(1 wt%)–TiO₂ composite material is sensitive to the location of the modifiers, which further proves that an efficient separation of the charge carriers is crucial to the overall activity of the composite catalyst.