Photocatalytic Dehydrogenation Research Articles

Heterogeneous photocatalytic dehydrogenation from ethanol and heavy water

An isotopic method was used, with ethanol as hydrogen source and heavy water as deuterium source, to investigate the effect of ethanol concentration and pH value of the solution on the gaseous products (D 2, HD and H 2) and H/D ratio during the photocatalytic dehydrogenation on Pd-CdS powdered photocatalyst. In a neutral medium of equimolar concentration, H/D=2.08 is observed. In media with a wide range of pH values, H/D>1 and a maximum H/D ratio of 2.5 is obtained at pH 12. This demonstrates that a larger number of effectively dissociated adsorptive ions are obtained from ethanol than from D 2O, and that water acts as a reactant not a mere mediator. The occurrence of a maximum H/D value indicates the competition between dissociative adsorption of ethanol and D 2O onthe CdS surface. A detailed reaction mechanism is proposed. At a reaction temperature above room temperature using M-CdS, the rate-determining step is considered to be the transfer of photoproduced holes; this occurs when they are captured by adsorbed alcoholate ion RO −, followed by α-H separation, in acidic, neutral or weak basic media, or when they are captured by adsorbed hydroxyl ion OH −, followed by free radical OH· formation, in strongly basic (pH⩾12) medium.

Wavelength dependence of photocatalytic cyclohexane dehydrogenation by carbonyl- and thiocarbonyl-(chloro)phosphine rhodium(I) complexes

Photocatalytic dehydrogenation of cyclohexane using [RhCl(CS)(PPh3)2] was closely related to photodissociation of the CS ligand, where the effective absorption band was not of the lowest energy but the second lowest, in contrast to the CO analogue. The estimated turnover number of [RhCl(PPh3)2] in the dehydrogenation cycle was 3.8 on the assumption that [RhCl(CS)(PPh3)2] is photodecomposed irreversibly. The wavelength dependence of photocatalysis by these complexes is well interpreted in terms of the molecular-orbital energy levels and orbital interactions.

Photocatalytic dehydrogenation of ethanol by the Cu 2+ ion

Cu 2+ ion is found to photocatalyse hydrogen generation from aqueous solutions of ethanol. The reaction rate was found to be optimum when the concentrations of Cu 2+ and ethanol are 0.01 M and 25% v/v respectively.

Photocatalytic dehydrogenation of 2-propanol with carbonyl(halogeno)phosphine-rhodium complexes

Photocatalytic dehydrogenation of 2-propanol, yielding acetone and dihydrogen, proceeded at high turnover frequencies with carbonyl(halogeno) phosphine-rhodium complexes RhX(CO)(PR 3) 2 (X = halogen and PR 3 = t-phosphine) under excitation conditions for the metal-to-ligand charge transfer. The three-coordinate species RhX(PR 3) 2 generated by photodissociation of the CO ligand was responsible for the catalytic cycle, irrespective of the kinds of ligands. The importance of both electronic and steric factors was demonstrated for the phosphine ligand.

Investigation of the functions of CdS surface composite layer and Pt on treated Pt/CdS for photocatalytic dehydrogenation of aqueous alcohol solutions

The effect of surface composition and structure of CdS and of the chemical states of photodeposited Pt on the activity of Pt/CdS in the photo-catalytic dehydrogenation of aqueous alcohol was investigated by means of XPS, ESR, TEM, DTA and H +/OH − isothermal adsorption. The results show that: (i) A composite layer of Cd(OH) 2-CdO-CdS was formed on the surface of CdS after a short heat treatment in air followed by washing with distilled water. (ii) The surface Cd(OH) 2 can react with H +(aq) or OH −(aq) to form two kinds of surface-active sites which can be oxidized by a photogenerated photon h +, producing the surface radical OH .. The first step in the dehydrogenation of alcohol is a radical reaction: .OH + CH 3CH 2OH → CH 3C̊HOH + H 2O (iii) Initially, the photodeposited Pt exists in the form of sulfide, but after heat treatment is converted into Pt and PtO, which act as the catalysts for H + reduction and .OH + CH 3CH 2OH reaction, respectively. The results obtained in this work are discussed in terms of the energetics of redox species possibly occurring in the aqueous solution, and the surface band structure of treated Pt/CdS. Based on this discussion, we propose a new model for the photo catalytic dehydrogenation of aqueous alcohol, called the ‘two-photon action model’.

Photocatalytic liquid-phase dehydrogenation of cyclohexanol with rhodium porphyrin complex

Photocatalytic dehydrogenation of cyclohexanol to yield cyclohexanone and dihydrogen proceeded with chloro(tetraphenylporphyrinato)rhodium (III) complex under the excitation conditions of either the B or Q band. First-order rate dependence on concentrations of the Q band-excited catalyst species suggests that the same bimolecular mechanism between the excited and ground-state species previously proposed for 2-propanol dehydrogenation is applicable to cyclohexanol as well.

Effect of high-temperature treatment in air ambience on the surface composition and structure of CdS

The surface composition and structure of both raw CdS and high temperature treated CdS were investigated by means of XPS and AES. The results show that the oxidation of CdS has a transition temperature (~400°C). Above this temperature CdSO 4 would be formed on the surface of CdS. The surface oxide CdO reacts with H 2O easily in air to form Cd(OH) 2. There exists a multilayer which consists of different compositions on the treated CdS surface. The outermost layer is rich in CdSO 4, which is readily washed away by acidic water. Thus, we concluded that in the processes of high temperature air treatment and acidic water washing, a Cd(OH) 2-CdO-CdS composite layer is formed on the CdS surface. The effect of this composite layer on the activity of metal loaded catalyst in photocatalytic dehydrogenation of alcohol is currently being studied in our laboratory.

The Mechanism of the Photocatalytic Dehydrogenation of Methanol

Abstract We have studied on the mechanism of the photocatalytic dehydrogenation of methanol over Pt/TiO2. D2 was only evolved from CH3OD, and D2, HD, and H2 were detected in the evolved gas in the case of the mixed sample of CH3OD and CH3OH. These results show that the evolved hydrogen came from hydroxyl hydrogen atom of methanol. And the rate determining step was discussed in connection with comparison among the evolution rates of H2, HD, and D2, respectively.

Investigation of the mechanism of photocatalytic alcohol dehydrogenation over Pt/TiO2 using poisons and labelled ethanol

Poisoning experiments using pyridine, piperidine, aqueous ammonia, phenol and 2-nitrophenol show that both acid and base sites are involved in the room-temperature photocatalytic dehydrogenation of alcohols (liquid propan-1-ol being chosen for the tests) over a Pt/TiO2 catalyst. The dominant role is played by the base sites, since an acid of the strength of 2-nitrophenol suffices to cancel dehydrogenation, whereas the basic substances only decrease its rate. An additional piece of information on the dehydrogenation mechanism is obtained from the use of gaseous 2,2,2-[2H3] ethanol at 263 K which provides evidence that the β-deuterium atoms are not abstracted, whereas at 295 K an isotopic exchange between the products, H2and 2,2,2-[2H3]ethanal, subsequently occurs.

Photocatalytic dehydrogenation of liquid alcohols by platinized anatase

The photocatalytic dehydrogenation of liquid methanol, ethanol, propan-1-ol and propan-2-ol has been investigated using platinized anatase and 366 nm u.v. radiation over the range 278–303 K. Activities and activation energies for carbonyl-compound formation were effectively identical for the four alcohols on Pt(0.5)/TiO2 prepared by photodeposition. The activation energy of 20 kJ mol–1 is associated with photoelectron transport through the anatase to the Pt particles. With Pt(0.5)/TiO2 prepared by impregnation and H2reduction, identical activities for the four alcohols were achieved after O2 treatment. From the effect of u.v. intensity on the rate of propanone formation at 293 K, the limiting quantum yield was found to be 0.45 for photodeposited Pt(0.5)/TiO2 under N2 and 0.82 for support anatase under O2. Arrhenius plots for reaction at reduced u.v. intensities showed that the activation energy fell to zero when these quantum yields were achieved. Different mechanisms follow from the manner in which photoholes are surface trapped; two mechanisms for dehydrogenation with a limiting quantum yield of 0.5 are discussed.

Liquid-phase photocatalytic dehydrogenation of 2-propanol with acetone- cis-[Rh 2Cl 2(Co) 2(dpm) 2] system

The photocatalytic system composed of acetone and cis-[Rh 2Cl 2(CO) 2-(dpm) 2] has been found quite effective for H 2 production from 2-propanol, with a turnover frequency of about three times that for methanol (high-pressure mercury lamp). The observed induction period is explained in terms of the large effect of acetone photosensitizer which accumulated autocatalytically. Long-wavelength irradiation (λ ⩾ 364 nm) gives H 2 and acetone solely, although the quantum efficiency is low (~10 −4). A unique character of photons around 338 nm was pointed out, where not only much more acetone is formed than at λ ⩾ 364 nm, but also pinacol formation is highly suppressed compared to at λ ⩽ 312 nm.

Photocatalytic dehydrogenation of liquid propan-2-ol by TiO2. Part 1.—Kinetics

An oxygen-uptake technique has been used to monitor the kinetics of photocatalytic dehydrogenation of propan-2-ol in the presence of the anatase and rutile forms of TiO2. Variation of alcohol concentration showed that surface binding by a non-associated alcohol hydroxy group was necessary before photoconversion into propanone could take place. N.m.r. analysis of solutions of the alcohol in cyclohexane was applied to show that, under the conditions used, the rate of dehydrogenation was proportional to the concentration of free alcohol.

A redox combined photocatalysis: New method of N-alkylation of ammonia by TiO 2/Pt suspended in alcohols

Ammonia was efficiently N-alkylated into tertiary amines by room-temperature photoirradiation at > 300 nm in the presence of platinized titanium dioxide suspended in alcohols, such as methanol, ethanol, and ethylene glycol. A couple of processes, photocatalytic dehydrogenation of alcohols and thermal hydrogenation of an imine intermediate, account for the ammonia N-alkylation.

Photocatalytic dehydrogenation of liquid propan-2-ol by TiO2. Part 2.—Mechanism

The photodehydrogenation of propan-2-ol mediated by TiO2 has been studied. Using basic additives it has been established that the sites responsible for the bulk of the reaction are acidic in nature. A fibre-optic monitoring system has been used to study changes in absorption of radiation by the TiO2 in the presence of N2, O2 and H2O2. It is concluded that the active sites are Ti4+ ions. A mechanism involving O–2 and H2O2 is proposed.

The mechanisms of photocatalytic dehydrogenation of methanol in the liquid phase with cis-[Rh 2Cl 2(CO) 2(dpm) 2] complex catalyst

The nature and mechanisms of the photocatalysis of soluble cis-[Rh 2-Cl 2(CO) 2(dpm) 2] complex for the dehydrogenation of methanol have been examined with and without the presence of added acetone, by determining the rate of the reaction, product distribution and quantum efficiency. In the absence of acetone, the products were purely formaldehyde (and its dimethyl acetal) and dihydrogen in the wavelength range of 260 – 440 nm, although quantum efficiency was rather low (0.1 – 7 × 10 −3). Addition of acetone accelerated the rate of dihydrogen evolution (still > 95% of the gas-phase components), whereas formaldehyde, formaldehyde dimethyl acetal, ethylene glycol, 2,3-dimethyl-2,3-butanediol, 2-propanol and 2-methyl-1,2-propanediol were found in the liquid phase with a satisfactory mass balance for hydrogen. In comparison with the purely acetone-photosensitized reaction of methanol, the rhodium catalyst facilitates the formation of formaldehyde (and its dimethyl acetal) over all the wavelengths studied (260 – 440 nm). In the wavelength range where acetone is a strong absorber of photons (⩽312 nm), the rhodium catalyst is considered to convert ·CH 2OH radicals into formaldehyde and dihydrogen. The enhanced formaldehyde formation in the wavelength range where acetone is a very poor absorber of photons (⩾338 nm) suggests participation of a possible rhodium-acetone complex in the photoexcitation processes. The fact that the ratio of the amounts of products [ethylene glycol] [2,3-dimethyl-2,3-butanediol] /[2-methyl-1,2-propanediol] 2, is constant independent of the wavelength, indicates that these three products are formed from the homogeneous and heterogeneous coupling of ·CH 2OH and ·C(CH 3) 2(OH) radicals irrespective of the presence of the rhodium catalyst.

Photocatalytic dehydrogenation of methanol in the liquid phase with cis-Rh 2Cl 2(CO) 2(dpm) 2 and Pd 2Cl 2(dpm) 2 complex catalysts

The liquid-phase dehydrogenation of methanol has proved to be possible at a moderate rate through the use of the soluble complex catalysts cis-Rh 2 Cl 2(CO) 2(dpm) 2 and Pd 2Cl 2(dpm) 2 under photoirradiation conditions (high-pressure mercury lamp), the addition of acetone accelerating the rate quite noticeably. Quantitative analysis of the reaction products (dihydrogen, formaldehyde, formaldehyde dimethyl acetal, ethylene glycol, 2,3-dimethyl2, 3-butanediol, 2-propanol, 2-methyl-1,2-propanediol) showed that mass balance for hydrogen was established satisfactorily among them. In comparison to the acetone-photosensitized reaction of methanol, the observed product distribution could be characterized as follows: (1) the gas-phase product is almost pure dihydrogen (in comparison to being mainly methane) and (2) the yield of formaldehyde and formaldehyde dimethyl acetal is greatly increased (in comparison to the products being mainly 2-methy 1-1,2propanediol and ethylene glycol).

Photocatalytic dehydrogenation of aliphatic alcohols by aqueous suspensions of platinized titanium dioxide

Photoirradiation (λex > 300 nm) of Ar-purged aqueous propan-2-ol solution gave hydrogen and acetone in the presence of platinum- and/or ruthenium dioxide-loaded TiO2. The photocatalytic activity of anatase TiO2 depended significantly on the amount of metal or metal oxide present; the effect on the activity increased in the order platinum black ≫ platinum powder > ruthenium dioxide. The photocatalytic activity of rutile TiO2 was negligible even when loaded with platinum black. The effective wavelengths for the photocatalytic dehydrogenation of propan-2-ol were below ca. 390 nm, in agreement with the u.v. absorption spectrum of anatase TiO2. In a similar way primary, secondary and tertiary aliphatic alcohols underwent photocatalytic oxidation, accompanied by hydrogen liberation, by the platinized TiO2. The primary and secondary alcohols gave the corresponding carbonyl derivatives, while 2-methylpropan-2-ol and acetone gave dimeric products accompanied by stoichiometric hydrogen evolution. The initial rate of dehydrogenation in these photocatalytic systems was in proportion to the rate constants of hydrogen abstraction by hydroxyl radical in the homogeneous systems.

Photocatalytic dehydrogenation of propan-2-ol using rhodium based catalysts

We have discovered that efficient photogeneration of hydrogen, from propan-2-ol can be achieved with a number of preformed Rh I complexes, as well as RhCl 3 in the presence of Ph 3PO or Ph 3P, after exposure to air. The catalyst turnovers, for the Rh I species are in the range of 2000–6000 mmol H 2 h −1 mmol Rh −1, which are much higher than those previously claimed for such photolytic systems. The RhCl 3 based systems are however of lower activity (∼ 1000 turnovers). Overall the systems appear to be catalytic even over long irradiation periods, where no reduction of gas evolution has been observed.

Photocatalytic dehydrogenation of liquid propan-2-ol by platinized anatase and other catalysts

The photocatalytic dehydrogenation of liquid propan-2-ol by suspensions of platinum and other metals supported on anatase has been investigated by following propanone formation under an N2 atmosphere. Measurements were made over the range 278–303 K using filtered 366 nm u.v. radiation.With Pt/TiO2 prepared by H2 reduction there was an appreciable dark reaction, which was absent with catalysts prepared by photodeposition. Reaction on photodeposited catalysts was consistently associated with an activation energy of 20 kJ mol–1, although the activity fell in the sequence Pt/TiO2 > Pd/TiO2 > Rh/TiO2 > Au/TiO2= 0 for catalysts with a metal content of 0.5 wt%. The activation energy is identical to that for photoreaction on the anatase support in the presence of O2 and is believed to be associated with the transport of photoelectrons through the anatase to either metal particles or adsorbed O2.

Photocatalytic dehydrogenation of secondary alcohols with rhodium porphyrin complex

Photocatalytic dehydrogenation of secondary alcohols with rhodium porphyrin complex