Measuring and understanding active sites Flashcards
How does rate depend on catalyst?
Rate = k [cat]
What is turnover frequency?
Number of molecules of a given product / (number of active sites x time). Use TOF to compare intrinsic catalyst rates with active sites built into understanding.
What is the simplest model of active site contribution? What are the problems with this model?
All surface metal atoms contribute to the rate, so [cat] = surface area. Surface sensitive reactions show that this is likely a poor measure, as different activities are found at different surfaces.
Why might chemisorption capability measurement be a good method of estimating active sites and what limitations might we place in order to measure this?
Chemisorption allows specific sites on the catalyst to be probed as will only bind to active sites. Much smaller metal surface areas too.
Requirements for adsorbate: absorb only on metal, not on subsurface, no reaction with surface, no bulk reaction, undergoes Langmuir-like adsorption.
How can the static mode be used to measure chemisorption?
Pretreatment of sample, evacuate a known volume, admit a known volume of probe gas, equilibrate, measure and repeat with inert gas to determine dead volume accurately.
How can the dynamic mode be used to measure chemisorption?
Pretreatment of sample, flow a non-adsorbing carrier gas, inject small known volumes of probe gas and measure pulse size with a gas detector. Initial small peaks indicate adsorption of probe gas.
What further experimental analysis must be done after a dynamic or static experiment to obtain the isotherm and number of surface sites?
After either experiment, obtain the initial isotherm, which does not plateau. Need to undergo further experiments to determine reversible isotherm (inc. other adsorption effects) in order to calculate irreversible isotherm (desired). The number of surface sites is then worked out by the stoichiometry, with the amount of chemical adsorbed related to this number.
How can we calculate particle size from mass loading and surface area?
Convert number of molecules to surface area from known packing density. Assume a geometry. FInd known density and calculate volume. From volume and surface area for a known shape, can calculate the size.
Why can pre-treatment in catalysts provide a challenge for obtaining accurate adsorption data?
For accurate data, require surface to be free of adsorbed probe molecules. After pre-treatment, some gas may be left adsorbed, resulting in residual chemisorbed layer. This will reduce the values found for chemisorption and hence, active sites.
What are temperature programmed methods?
TPD: molecular absorbed on a surface or catalyst sample and gradually heated up until it desorbs.
TPR: typically more than one molecule is adsorbed on a catalyst surface. example: two reactants that react over catalyst are pre-adsorbed then heated to monitor when they react and desorb or when a product desorbs and what the product:reactant desorption ratios are.
Generally can measure the desorption and identity of each desorped molecule using mass spec.
How can we use reactive frontal chromatography to understand active sites?
Done with a copper catalyst. Copper reacts with N2O by ripping out oxygen and leaving nitrogen gas. Flow mode of this reaction with a dilute stream of laughing gas prevents changes to rate due to exothermicity. This can be tracked by mass spec to see N2 initially coming off as copper surface removes oxygen and laughing gas once all sites have been filled.
Reaction: 2Cu + N2O –> Cu2O + N2
What technique can be used to identify specific acid/base sites?
Can use titrations with CO2 for bases and NH3 for acids.
How can we measure an active surface area using a known reaction?
Use a reaction with a known TOF, e.g. ethane hydrogenation to characterise other catalysts. NB. it is important that the reference reaction is structure insensitive.
How can we use infrared techniques to probe active sites?
Can monitor the stretching frequencies to give information about the type of site. 2077 wavenumbers is typical of a saturated surface, shoulders are attributable to low coord sites. As the temperature increases, there is a shift to higher wavenumber and the shape changes.
Can use to identify Bronsted vs Lewis acid sites. Bronsted have a higher wavenumber (1545 vs 1455). Can use ratio between these IR peaks to inform performance of catalyst.
How might different techniques impact outcome of experiments and skew results?
Want to measure chemisorption - surface area distribution.
Can use TEM to measure number distribution and XRD/other techniques to measure volume distribution. These are fundamentally different things and whilst they may inform, they will not be completely accurate.
