Heterogeneous photocatalytic oxidation of atmospheric trace contaminants

The following subject areas are covered: (1) design and construction of continuous flow photoreactor for study of oxidation of trace atmospheric contaminants; (2) establishment of kinetics of acetone oxidation including adsorption equilibration, variation of oxidation rate with acetone concentration and water (inhibitor), and variation of rate and apparent quantum yield with light intensity; (3) exploration of kinetics of butanol oxidation, including rate variation with concentration of butanol, and lack of inhibition by water; and (4) exploration of kinetics of catalyst deactivation during oxidation of butanol, including deactivation rate, influence of dark conditions, and establishment of photocatalytic regeneration of activity in alcohol-free air

Kinetics of liquid phase semiconductor photoassisted reactions : supporting observations for a pseudo-steady-state model

The kinetics of liquid phase semiconductor photocatalytic and photoassisted reactions are an area of some debate, reignited recently by an article by Ollis1 in which he proposed a simple pseudo-steady-state model to interpret the Langmuir-Hinshelwood type kinetics, commonly observed in such systems. In the current article, support for this model, over other models, is provided by a reinterpretation of the results of a study, reported initially in 1999,2 of the photoassisted mineralization of 4-chlorophenol, 4-CP, by titania films and dispersions as a function of incident light intensity, I. On the basis of this model, these results indicate that 4-CP is adsorbed more strongly on P25 TiO2 when it is in a dispersed, rather than a film form, due to a higher rate constant for adsorption, k1. In addition, the kinetics of 4-CP removal appear to depend on Iâ, where â ) 1 or 0.6 for when the TiO2 is in a film or a dispersed form, respectively. These findings are discussed both in terms of the pseudo-steady-state model and other popular kinetic models

Heterogeneous photocatalytic oxidation of atmospheric trace contaminants

Research was conducted on: (1) design and construction of a continuous flow photoreactor to study oxidation of trace atmospheric contaminants; (2) kinetics of acetone oxidation including adsorption equilibrium, variation of oxidatiin rate with acetone concentration and water, and variation of rate and apparent quantum yield with light intensity, and (3) kinetics of butanol oxidation, including rate variations; and (4) kinetics of catalyst deactivation including deactivation rate, influence of dark conditions, and photocatalytic regeneration in alcohol-free air

Heterogeneous Photocatalytic Oxidation of Atmospheric Trace Contaminants

Heterogeneous photocatalysis involves the use of a light-activated catalyst at room temperature in order to carry out a desired reaction. In the presence of molecular oxygen, illumination of the n-type semiconductor oxide titanium dioxide (TiO2) provides for production of highly active forms of oxygen, such as hydroxyl radicals, which are able to carry out the complete oxidative destruction of simple hydrocarbons such as methane, ethane, ethylene, propylene, and carbon monoxide. This broad oxidation potential, coupled with the ability with sufficient residence time to achieve complete oxidation of simple hydrocarbon contaminants to carbon dioxide and water, indicated that heterogeneous photocatalysis should be examined for its potential for purification of spacecraft air. If a successful catalyst and photoreactor could be demonstrated at the laboratory level, such results would allow consideration of photocatalysts as a partial or complete replacement of adsorption systems, thereby allowing for reduction in lift-off weight of a portion of the life support system for the spacecraft, or other related application such as a space station or a conventional commercial aircraft. The present research was undertaken to explore this potential through achievement of the following plan of work: (a) ascertain the intrinsic kinetics of conversion of pollutants of interest in spacecraft, (b) ascertain the expected lifetime of catalysts through examination of most likely routes of catalyst deactivation and regeneration, (c) model and explore experimentally the low pressure drop catalytic monolith, a commercial configuration for automotive exhaust control, and (d) examine the kinetics of multicomponent conversions. In the recent course of this work, we have also discovered how to increase catalyst activity via halide promotion which has allowed us to achieve approximately 100% conversion of an aromatic contaminant (toluene) in a very short residence time of 5-6 milliseconds

Heterogeneous Photocatalytic Oxidation of Atmospheric Trace Contaminants

Heterogeneous photocatalysis involves the use of a light-activated catalyst at room temperature in order to carry out a desired reaction. In the presence of molecular oxygen, illumination of the n-type semiconductor oxide titanium dioxide (TiO2) provides for production of highly active forms of oxygen, such as hydroxyl radicals, which are able to carry out the complete oxidative destruction of simple hydrocarbons such as methane, ethane, ethylene, propylene, and carbon monoxide. This broad oxidation potential, coupled with the ability with sufficient residence time to achieve complete oxidation of simple hydrocarbon contaminants to carbon dioxide and water, indicated that heterogeneous photocatalysis should be examined for its potential for purification of spacecraft air. If a successful catalyst and photoreactor could be demonstrated at the laboratory level, such results would allow consideration of photocatalysts as a partial or complete replacement of adsorption systems, thereby allowing for reduction in lift-off weight of a portion of the life support system for the spacecraft, or other related application such as a space station or a conventional commercial aircraft. The present research was undertaken to explore this potential through achievement of the following plan of work: (a) ascertain the intrinsic kinetics of conversion of pollutants of interest in spacecraft, (b) ascertain the expected lifetime of catalysts through examination of most likely routes of catalyst deactivation and regeneration (c) model and explore experimentally the low pressure drop catalytic monolith, a commercial configuration for automotive exhaust control (d) examine the kinetics of multicomponent conversions. In the recent course of this work, we have also discovered how to increase catalyst activity via halide promotion which has allowed us to achieve approximately 100% conversion of an aromatic contaminant (toluene) in a very short residence time of 5-6 milliseconds

Heterogeneous photocatalytic oxidation of atmospheric trace contaminants

The progress report on heterogeneous photocatalytic oxidation of atmospheric trace contaminants covering the period from 1 May - 31 Oct. 1992 is presented. The two topics discussed are photoreactor monolith fundamental studies and monolith reactor operation: batch recirculation system. Concentration profiles are shown

Kinetics of Photocatalyzed Reactions: Five Lessons Learned

Elucidation of kinetics of photocatalyzed chemical mechanisms occurring at interfaces (gas-solid, liquid-solid) has been challenging. We summarize here five lessons learned over five decades.1. An assumed reaction network leads to a single kinetic model, but a common model, the Langmuir–Hinshelwood rate equation, r = kcat K C/ [1 +KC], arises from multiple mechanisms, hence models alone do not reveal unique mechanisms.2. The Langmuir–Hinshelwood model parameter kcat represents the slow step at a catalyst surface, and in thermal catalysis, depends upon the reactant structure. However, early photocatalysis work with light chlorinated hydrocarbons in aqueous solutions showed a single kcat value, independent of reactant structure.3. The dependence of the Langmuir-Hinshelwood parameters, kcat and K, upon intensity indicates that a pseudo-steady state approach is more fundamental than the presumed equilibrated adsorption of the LH model.4. Dyes and phenols are commonly studied, and claimed as first order reactions, despite often exhibiting rate constants which diminish with increasing contaminant concentration. We show that such studies are the result of intrinsic zero order data plotted on a semilog graph, and involve zero order rate limitation by reactant saturation, electron transfer to O2, oxygen mass transfer, or light supply.5. The apparent kinetics for contaminant removal from photocatalytic self-cleaning surfaces depends upon multiple circumstances, including the geometry of reactant deposit, catalyst porosity, and reactant light absorption. A single decision table suffices to indicate the apparent reaction order, n, to assume when fitting photocatalytic kinetic data from self-cleaning surfaces to a power law rate form, rate = k Cn

Optimized reprocessing of documents using stored processor state

Variable Data Printing (VDP) allows customised versions of material such as advertising flyers to be readily produced. However, VDP is often extremely demanding of computing resources because, even when much of the material stays invariant from one document instance to the next, it is often simpler to re-evaluate the page completely rather than identifying just the portions that vary. In this paper we explore, in an XML/XSLT/SVG workflow and in an editing context, the reduction of the processing burden that can be realised by selectively reprocessing only the variant parts of the document. We introduce a method of partial re-evaluation that relies on re-engineering an existing XSLT parser to handle, at each XML tree node, both the storage and restoration of state for the underlying document processing framework. Quantitative results are presented for the magnitude of the speed-ups that can be achieved. We also consider how changes made through an appearance-based interactive editing scheme for VDP documents can be automatically reflected in the document view via optimised XSLT re-evaluation of sub-trees that are affected either by the changed script or by altered data

Tracking sub-page components in document workflows

Documents go through numerous transformations and intermediate formats as they are processed from abstract markup into final printable form. This notion of a document workflow is well established but it is common to find that ideas about document components, which might exist in the source code for the document, become completely lost within an amorphous, unstructured, page of PDF prior to being rendered. Given the importance of a component-based approach in Variable Data Printing (VDP) we have developed a collection of tools that allow information about the various transformations to be embedded at each stage in the workflow, together with a visualization tool that uses this embedded information to display the relationships between the various intermediate documents. In this paper, we demonstrate these tools in the context of an example document workflow but the techniques described are widely applicable and would be easily adaptable to other workflows and for use in teaching tools to illustrate document component and VDP concepts

Optimized reprocessing of documents using stored processor state

Variable Data Printing (VDP) allows customised versions of material such as advertising flyers to be readily produced. However, VDP is often extremely demanding of computing resources because, even when much of the material stays invariant from one document instance to the next, it is often simpler to re-evaluate the page completely rather than identifying just the portions that vary. In this paper we explore, in an XML/XSLT/SVG workflow and in an editing context, the reduction of the processing burden that can be realised by selectively reprocessing only the variant parts of the document. We introduce a method of partial re-evaluation that relies on re-engineering an existing XSLT parser to handle, at each XML tree node, both the storage and restoration of state for the underlying document processing framework. Quantitative results are presented for the magnitude of the speed-ups that can be achieved. We also consider how changes made through an appearance-based interactive editing scheme for VDP documents can be automatically reflected in the document view via optimised XSLT re-evaluation of sub-trees that are affected either by the changed script or by altered data