催化剂粉末样品中性能表征检测方案(紫外分光光度)

Measurement of diffuse reflection fromcatalyst powders using the PrayingMantis accessory and Agilent Cary 5000UV-Vis-NIR spectrophotometer Application note Materials testing and research Authors Eric Marceau UPMC- Laboratoire de Reactivitede Surface (UMR 7197 CNRS)Paris， France Caroline Perier Agilent TechnologiesLes Ulis， France Travis Burt Agilent TechnologiesMelbourne， Australia Introduction Aluminium oxide (Al，0，)， commonly known as alumina， is a widespreadnaturally occurring compound that has a broad range of industrialapplications. Besides its use as a pigment or abrasive， alumina also plays avital role as catalyst support for desulfurization or hydrogenation reactions.In the latter case， the active phase on the catalyst often consists of nickelmetal nanoparticles. The interactions initially established between nickel (II)salt and the alumina surface during the first steps of the Ni/Al，0， catalystpreparation ultimately determine the active phase dispersion and the activityof the catalyst. It is thus of prime importance to follow the evolution ofnickel speciation upon thermal treatments in order to improve the catalystpreparation. The Agilent Cary 5000 UV-Vis-NIR spectrophotometercoupled with a Praying Mantis diffuse reflectanceaccessory is suited for gaining greater understandingof the chemical transformations taking place over widetemperature ranges. UV-Vis-NIR spectroscopy allowsinvestigating electronic absorption spectra of transitionmetal ions. The spectra are interpreted by the numberand position of absorption bands， which provides theelectronic configuration of the element as well as thenature and symmetry of its chemical environment.Moreover， the near-infrared part of the spectrumprovides information on the narrow IR overtone andcombination bands originating from the metal ligandsor from the support， which may effectively complementmid-range IR spectra. The flexibility of the Praying Mantis sampling accessoryallows reflection studies on solid samples withinan environmental cell where temperature and gasatmosphere can be controlled. In this study， sampleswere measured using a small volume powder cup ofthe reaction chamber set inside the Praying Mantis.The Praying Mantis accessory gives results that arequalitatively very similar to an alternative diffusereflectance accessory - an integrating sphere - butwith the advantage of the measurement geometrybeing downward-looking， permitting the samples to bemounted horizontally， and with an ability to measurevery small samples (approximately 0.1 cm3) without lossof performance. Further to its flexibility， the Cary UV-Vis-NIR systemdelivers a very high level of data for small powervolumes. The Praying Mantis DRA uses mirrors ratherthan a sphere，which has a reflective coating. Thisgives better optical control over the illumination anddetection conditions， creating a highly efficient systemover a defined range of reflection angles.This type ofmeasurement is always made as a "percent of referencereflectance’ making it qualitative in nature becausethe result is dependent on the reference materialused. But similar results can be obtained from the twoapproaches. A primary advantage of the Cary UV-Vis-NIRspectrophotometers is their ability to work with verysmall signals and very low light levels (very highabsorbance or very low transmittance/reflectance).High precision readings can be obtained even where thesample has low reflectance or when measurements aremade under extreme sampling conditions such as withthe reaction chamber. Instrumentation All readings were made using a standard Cary 5000UV-Vis-NIR spectrophotometer (Figure 1) with aPraying Mantis accessory (Figure 2) fitted with ahigh temperature reaction chamber， which had beenmounted and aligned for use with the standard powdercell holders. The windows of the reaction chamber wereSiO，. The reference spectrum was collected on PTFEunder ambient conditions (20°C)， and air flow was usedfor sample measurements. Figure 1. Agilent Cary 5000 spectrophotometer Figure 2. Praying Mantis accessory with alignment tools and powder cellsample cups Reaction chamber The reaction chamber (Figure 3) enables a reactiongas to be introduced and reacted with the sample sothat the reactions can be studied in vivo， reaction ratesdetermined and intermediate and reaction productsidentified. The reaction chamber is enclosed with adome with three windows — two for the spectrometerradiation to enter and exit the chamber and the thirdfor viewing，illuminating， or irradiating the sample.This enables the use of the reaction chambers forphotochemical studies. The standard material providedwith all three windows is UV quartz. Figure 3. Reaaction chamber Results Spectral results are presented in Kubelka-Munk (F(R))units versus wavelength in nanometers. F(R) are amathematical transformation of the %R to aid in thecorrelation between reflectance and concentration. Ni²+ions in an octahedral geometry exhibit threeabsorption bands， two in the visible region (around400 and 600 nm) and a broader one in the NIR region(around 1200 nm). Shifts of the 390 nm band withtemperature up to 110 °C evidence the changes aroundnickel ions upon transformation of hydrated nickelnitrate to nickel hydroxynitrate， while NiO forms above230 °C. Spectra in the NIR range both highlight developmentof hydroxynitrates between room temperature and110 °C， (shift of the 1153 nm band to 1240 nm)， but alsodehydration of the nickel salt and support (loss of the-0H overtone band at 1450 nm). Wavelength (nm) Figure 4. Praying Mantis in Cary 5000 UV-Vis-NIR (overlaid scans). Thermaltransformations of Ni[H，0](NO，)，/AI，0， from 20 C up to 250°C. Figure 5. UV-Vis range Two more bands by adsorbed water and aluminahydroxyl groups are found at 1950 and 2300 nm. The dehydration process is clearly seen through thedecrease of the initially intense band at 1950 nm， withsome hydroxyl groups remaining on the alumina surfaceafter thermal treatment at 250 °C. Wavelength (nm) Figure 6. NIR range Figure 7. NIR range up to 1800 nm Conclusion The results demonstrate the use of the Agilent Cary5000 UV-Vis-NIR system in the analysis of small volumeof catalyst powder samples over temperatures from20 to 250°C. The combination of the wide dynamicrange of the Cary 5000 and excellent signal-to-noiseratio evidenced the transformation of the nickel saltand dehydration of the supported system using HighTemperature Reaction Chamber in combination withthe Praying Mantis accessory. This makes diffusereflectance a valuable tool for studying powdersinvolved in heterogeneous catalysis or reactions at thegas-solid interface. www.agilent.com/chem Agilent shall not be liable for errors contained herein or for incidental or consequentialdamages in connection with the furnishing， performance or use of this material. Information， descriptions， and specifications in this publication are subject to changewithout notice. @ Agilent Technologies， Inc. 2012Published April 30， 2012Publication number： 5990-9787EN Agilent Technologies Agilent Technologies 三氧化二铝，俗称氧化铝，是一种广泛存在的天然化合物，同时在工业上也有广泛的应用。氧化铝除了用作颜料或磨料外，在脱硫或加氢反应中也起着重要的催化剂载体作用。当其作为催化剂载体时，催化剂上的活性通常由镍金属纳米粒子组成的。在Ni/Al2O3催化剂制备的第一步中，镍盐与氧化铝表面的相互作用最终决定了催化剂的活性相分散和活性。因此，为了改进催化剂的制备，跟踪热处理过程中镍形态的变化至关重要。本文使用Cary5000 UV-Vis-NIR分光光度计搭配漫反射附件可以实现对在较宽的温度范围内发生的化学转变进行更加深入研究，实现催化剂性能的表征。