Laser-Induced Photocatalysis and its Applications in Petrochemicals ...

Proceedings of 15th Saudi-Japan Joint SymposiumDhahran, Saudi Arabia, November 27-28, 2005LASER-INDUCED PHOTOCATALYSIS AND ITS APPLICATIONS INPETROCHEMICALS, FUEL CELLS AND PHENOL DEGRADATIONAbstract:M. A. Gondal 1 , A. Hameed 2 , Z. H. Yamani 1 and Z. Seddigi 21Laser Research Laboratory, Physics Department2Chemistry Department, 3 Physics DepartmentKing Fahd University of Petroleum & Minerals Dhahran, Saudi ArabiaE-mail: magondal@kfupm.edu.saLaser induced photocatalysis has been applied for conversion of methane into high valuehydrocarbons like methanol, and for splitting of water to generate hydrogen for fuel celltechnology and for removal of phenol from waste water . A comparative study has beenperformed for laser induced photo-catalytic processes for these applications over Fe 2 O 3 ,WO 3 , TiO 2 and NiO catalysts under the irradiation of a strong laser beam at 355nm. Theactivity of these catalysts and various electron capture agents such as Fe +3 , Ag + , Al +3 , andLi + in the water splitting process were investigated. The reaction rates and the yield ofhydrogen and oxygen produced were enhanced considerably by the addition of electroncapture agents. In addition to these studies, laser induced photo-catalytic degradation ofphenol using WO 3 was investigated for the first time. Effect of WO 3 concentrations and laserenergy on photo-catalytic removal of phenol from waste water was studied. Laser enhancedphoto-degradation can be an efficient method for the removal of phenol present in wastewaters. The phenol removal process obeys first order kinetics.1. IntroductionIn order to meet the increasing demand of energy for industrial use and other humanactivities, the reserves of fossil fuels are steadily diminishing. To fulfill the future energyrequirements, it is evident that new fuels are needed. Significant efforts have been undertakenover the years to identify the acceptable substitutes for fossil fuels. Important criteria for anew fuel (energy source) include low cost, ample supply, renewability, safety, and pollutionfree. The main focus of search for new fuels is on materials that combust more cleanly thangasoline and other petroleum based fuels. The major objective of achieving clean burningfuels is therefore directed at minimizing the environmentally undesirable by-products such asCO, CO 2 and NO x . The problem of environmental pollution and public health due tovehicular gas emissions of hazardous nature has attracted the attention of the scientificcommunity towards environmentally clean fuels such as hydrogen.Virtually all transportation fuels today are derived from oil. Due to these issues, anumber of alternative fuels and advanced vehicle power plants have been proposed toovercome these challenges. These include improved internal combustion engine (ICE)vehicles and ICE hybrid electric vehicles (fueled with reformulated gasoline, diesel, CNG,LPG, methanol, ethanol, or hydrogen), and fuel cell vehicles (fueled with gasoline, methanolor hydrogen). Hydrogen has emerged as a particularly attractive option for the long term, dueto the following desirable characteristics. Hydrogen vehicles have zero or near zero tailpipeemissions. Also, hydrogen can be made from widely available primary energy sources,including natural gas, coal, biomass, wastes, solar, wind, and nuclear power. Hydrogen fuelcell vehicles are undergoing rapid development worldwide, and are anticipated to offer goodperformance, and low cost in mass production. Several recent studies [1, 4] have shown that

hydrogen is the preferred fuel for fuel cell vehicles. Due to these reasons, the hydrogenproduction is in high demand and innovative and alternate methods are being investigated forproduction of hydrogen.Recently, the research activities have been focused on the development of noveltechniques for the generation of hydrogen and oxygen by splitting of water [5-10] as water ismost abundant in nature. Heterogeneous photo-catalysis is one of the techniques applied forthe generation of H 2 and O 2 in aqueous solution using semiconductor catalysts. This photocatalytictechnique is considered to be feasible and attractive for practical purposes since onecan possibly use natural sun light as an irradiation source in the future. Most of the previouswork in this domain has been carried out with broadband UV lamps using pure TiO 2 or TiO 2based semiconductor powders as photo-catalysts [11-13].Recent studies on photocatalysis using laser, carried out at our laboratory suggestedthat an important parameter which plays a crucial role in the enhancement of the activity ofthe catalyst is the irradiation source. Most of the research work reported on photocatalysis isbased on use of conventional lamps [13-18]. Very little work has been carried out onphotocatalysis with lasers [19]. Since laser light has special properties like monochromaticity,high intensity, and low beam divergence, it is of great interest to use laser radiation as anexcitation source to study the activity of photocatalysts. We investigated the photocatalyticactivity of α-Fe 2 O 3 , WO 3 , TiO 2 and NiO as semiconductor catalysts for splitting of water intohydrogen and oxygen and for conversion of methanol into hydrogen.2. Experimental Setup DetailsThe setup used to study the photocatalytic conversion of methane into methanol,splitting of water, conversion of methanol into hydrogen and degradation of phenol for wastewater treatment is discussed in detail in our earlier publications [20-28]. To study thephotocatalytic cativity for these applications over α-Fe 2 O 3 , WO 3 , TiO 2 and NiO, all thecatalysts were subjected to identical experimental conditions. While using laser as a lightsource, it was observed that the important parameters which significantly affects the productyield are: laser energy, amount of catalyst (particle density), stirring rate and laser beamdiameter. The two parameters i.e. stirring rate and laser beam diameter were kept constant forall the studies reported in this paper. However, the dependence of products yield on laserenergy and amount of catalyst, to identify the maximum yield of Hydrogen and Oxygen wasstudied carefully for WO 3 catalyst initially. Once the laser energy and amount of catalyst wasidentified for optimum product yield, these two parameters (laser energy & catalystconcentration) were then kept constant for other catalysts for the sake of activity comparison.The destructive effect of focused laser beam was minimized by expanding the diameter of thebeam to 1 cm by using lenses and mirrors.The photo-catalytic induced reactions over α-Fe 2 O 3 , WO 3 , TiO 2 and NiO were studiedby suspending an optimized amount (400 mg) of above-mentioned semiconductor powders in60 ml of deionized water. This amount (400 mg) was kept constant for all catalysts. Argonwas used as purge gas to remove dissolved gases and the progress of the purging process wasmonitored by analyzing the gas samples from the dead volume of the reaction cell. All theexperiments were performed in batches and the quantity of evolved gases was estimated byanalyzing the gas samples at regular interval of ten minutes by using a SHIMADZU GC-17Agas chromatograph equipped with a 30 m, 0.53 mm inner diameter, molecular sieve 5APLOT column and a TCD detector. Argon was used as carrier gas. All the catalysts wereexposed to an optimized laser energy of 100 mJ as higher energies were found detrimental.

43211516814567991310121. Nd:YAG Laser (1060nm)2. Third harmonic generator (355 nm)3. Beam splitter4. Laser energy meter5. Mirror6. Beam diameter controller7. Quartz window8. Dead volume of cell9. Effective volume of cell10. Megnatic stirrer11. Megnatic plate12. Gas inlet13. Liquid sampling valve14. Gas sampling valve15. Gas outlet16. Pyrex cellFigure 3.111Experimental Setup for measuring evolvedgases and liquid products duringphotocatalytic process.

3. Results and DiscussionThe important applications of laser induced photocatalytic process undertaken at ourlaboratory are the photocatalytic conversion of methane into methanol and hydrogen, splittingof water into Hydrogen and oxygen, conversion of methanol into hydrogen and phenoldegradation for cleaning of waste water.3.1 Photocatalytic conversion of methane into methanolMethane is considered to be a cheap source of energy and its worldwide reserves arelarge. Global research efforts are under way in industrial as well as academic institutionsfunded by both private and government sectors for the development of methods to convertmethane to useful, more readily transportable and storable products such as methanol.Methanol is a desirable product of conversion because it retains much of the original energyof methane while satisfying transportation and storage requirements. Methanol is liquid atroom temperature and can be transported to market using the existing petroleum pipeline andtanker network. It can be used as a fuel or may be converted to other valuable products. Theexisting techniques to convert methane to methanol are not cost effective and efficient due topoor selectivity and require extreme conditions such as high temperature and sophisticatedequipment.The main aim of this study was to explore and develop novel pathways for conversion ofmethane into methanol under mild experimental conditions using laser light, water and asemiconductor photocatalysts such as α-Fe 2 O 3 , WO 3 , TiO 2 and NiO.To study the photocatalytic conversion of methane into methanol, the optimized amount ofthe catalyst under study (determined by the procedure mentioned earlier), was suspended in60 ml of water. Instead of purging the catalyst suspension with argon, methane was bubbledthrough the suspension for a fixed period of 15 minutes at a fixed flow rate of 100 ml/minand the cell was sealed at atmospheric pressure. The suspension was illuminated with 355 nmlaser having a beam diameter of 10 mm. The progress of the reaction was studied bymeasuring the amount of hydrogen produced and change in methanol concentration withtime. Separate batch experiments were performed to measure the production of hydrogen andmethanol.To measure the produced hydrogen, the sample from the dead volume of reaction cell, wasanalyzed at a regular interval of 15 minutes by using a Shmimadzu gas chromatograph (GC-17A) equipped with a thermal conductivity detector and a 30m molecular sieve 5A “PLOT”column. Argon gas was used as carrier at a flow rate of 10 ml/min. To estimate the amount ofmethanol during the reaction, liquid samples were drawn from the reaction cell at a regularinterval of 10 minutes, centrifuged and analyzed by using a gas chromatograph (supplied byAgilent Technologies) equipped with an FID detector and a 2m Chromosorb-101 packedcolumn. Helium gas was used as carrier at a flow rate of 25 ml/min. The injector, column anddetector temperatures were set at 250 o C, 200 o C and 250 o C respectively. Both the gaschromatographs were calibrated by using self prepared external standards prior to sampleanalysis. The reproducibility of the process was checked by repeating the experiments atregular intervals while the reproducibility of the analysis was estimated by running thestandards time to time.The progress of methanol formation was monitored by analyzing the liquid samples at regularintervals. The methanol yield as a function of time are plotted in Figures 2 & 3 for WO 3 and

NiO ccatalysts. It can be observed that initially the methanol yield increases rapidly with thetime, reaches to a maximum value and starts decreasing afterwards.160140120Methanol yield (µmol)1008060402000 10 20 30 40 50 60 70 80 90 100Time (min)Figure 2 Methanol yield as a function of time over WO 3120100Methanol yield (µmol)8060402000 10 20 30 40 50 60 70 80 90 100Time (min)Figure 3Methanol yield as a function of time over NiO.

120100Methanol yield (µmol)80604020Figure 400 10 20 30 40 50 60 70 80 90 100Time (min)Hydrogen production in the presence and absence of methane overWO 3 .3.2 Photocatalytic splitting of water for generation of hydrogenTo study the photocatalytic splitting of water, pure (WO 3 , NiO, Fe 2 O 3 and TiO 2 )and doped photocatalysts, optimized amount of photocatalyst (determined from the procedurementioned earlier) was suspended in 60 ml of distilled water whereas for water splittingstudies in the presence of metal ions, the optimized amount of the catalyst was suspended in60 cm 3 solution of respective metal ion. For this purpose, a 10 ppm solution of each metal ionwas prepared by dissolving the stoichiometric amount of each metal nitrate in 1000 cm 3 ofdoubly distilled de-ionized water. In order to keep the minimum optical effects i.e. selfabsorptionof laser light by the metal ion, the strength of the solutions was selected at 10ppm.An inert atmosphere was produced by purging argon gas at a high flow rate of 200ml/min for a period of 15 minutes. The purpose of such a high flow rate was to remove all thedissolved oxygen. The progress of argon purging was studied by analyzing the gas samplefrom the dead volume of reaction chamber. All the reactions were carried out in argonenvironment and at room temperature. The other important parameters i.e. the stirring rateand beam diameter were kept constant during all measurements.SHIMADZU gas chromatograph (GC-17A) equipped with a thermal conductivitydetector (TCD) and a 30m molecular sieve 5A “PLOT” column was used to analyze thegaseous products of photocatalytic process. Argon was used as carrier gas in GC and also aspurge gas to remove dissolved oxygen from the photocatalytic reactor. The injector, columnand detector temperatures were kept at 50 o C, 50 o C and 100 o C respectively. Keeping in viewthe high concentration of O 2 in the air, the product gases were analyzed by carefullyremoving 100µl of the sample from the dead volume of the cell by using a gas tight syringe.

The amount of produced gases were determined by analyzing a gases standard, provided bySCOTTY, England, containing 1.0 Mol% of each gas in nitrogen base. Severalreproducibility tests were performed over a range of various irradiation times. Theuncertainty in the measurement of H 2 and O 2 yield was found to be less than ±3% which wasacceptable for the purpose of present studies.The yield of evolved gases (mmol) as a function of laser exposure time are presentedin figures 5, where a gradual increase in O 2 and H 2 yield with time can be observed. The rateof production of both gases i.e. H 2 & O 2 follows the similar pattern. The yield of hydrogen inpresence of metal ions for WO3 is presented in figure 6.The comparison of hydrogenproduction for four catalyst is presented in Figure 7.2.5Hydrogen2OxygenGase evolved (mmol)1.510.500 10 20 30 40 50 60 70 80 90 100Time (min)Figure 5Hydrogen and oxygen evolution with time over WO 3 through water splitting.54.543.5WO3Ag+Fe+3Li+O 2 evolved (mmol)32.521.510.5Figure 600 10 20 30 40 50 60 70 80 90 100Time (min)Hydrogen production over WO 3 in the presence of metal ions by water splitting.

4.5H 2 evolved (mmol)43.532.521.5α-Fe2O3WO3TiO2NiO10.500 10 20 30 40 50 60 70 80 90 100Time (min)Figure 7 Comparison of hydrogen production over various photocatalysts by water splitting.3.3 Laser induced photocatalytic degeradation of phenol in waste waterA large amount of phenol is produced and used annually worldwide. The largestsingle use of phenol is in the production of plastics, but it is also used in the synthesis ofcaprolactam, a precursor for nylon 6 and other man-made fibers [29]. Phenolic compoundsare among the most prominent ground water contaminants [30]. Widespread contamination ofwater by phenol has been recognized [31] as an issue of growing concern in recent years.Phenol is a carcinogen to humans and is of considerable health concern, even at trace(0.001mg/litre) concentrations. Hence the treatment of waste water containing phenol isessential.Many techniques have been used for removing and degradation of phenoliccompounds in wastewater. They included, adsorption [29], biodegradation [30, 31] extractionby liquid membrane [32] and oxidation [32, 33]. The oxidation of phenol by several oxidizingagents such as ozone, UV and hydrogen peroxide has been extensively studied [34]. Apartfrom these studies, in recent years there has been increasing interest in the use of catalyticwet oxidation [35] and wet oxidation [36] for phenol removal. However those methods havesome disadvantages and limitations. Sustainability of catalytic wet oxidation technologydepends on kinetic regime in which the catalyst life would compromise to its cost. Recently ithas been reported [37, 38] that total conversion of phenol to CO 2 and H 2 O was achievedwhen the platinum surface was almost oxidized in which it resulted into catalyst deactivation.Heterogeneous photocatalysis using semiconductor catalyst under UV irradiation is highlysuccessful in mineralization of phenol. Photo catalysis depends on redox processes takingplace on the surface of the semiconductor/solution interface upon irradiation ofsemiconductor particles with laser light of proper energy (wavelength), higher than the bandgap[20-28]. The production of electron- hole pairs is essential since these species are capable

3.5Absorbance32.521.5Time= 0 MTime=15 MTime= 30MTime= 60 MTime=75 M10.50190 210 230 250 270 290 310Wavelength(nm)Figure 8 Typical UV absorption spectra of the degradation of phenol at laser exposuretimes ranging from 0 to 75 minutes recorded with UV spectrometer.7060Phenol concentration50403020100 mg200 mg300 mg1000 5 10 15 20 25Time(minutes)Fig.9 The dependence of the removal of phenol on the concentration of the WO 3catalyst. Here the concentration of WO 3 was varied from 100 mg to 300 mg.

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