Monitoring Hydrotreating Catalysts Synthesis and Deactivation using Raman Spectrometry

Oil & Gas Science and Technology - Rev. IFP, Vol. Monitoring Hydrotreating Catalysts Synthesis and Deactivation using Raman Spectrometry M. Digne K. Marchand P. Bourges Institut français du pétrole IFP Lyon INTRODUCTION Raman spectrometry has become a popular technique of characterization in numerous fields of application, such as polymers, pharmaceutics or hydrocarbons industry [ 1, 2 ]. Thanks to technical advances in instrumentation, like monochromatic intense laser excitation or efficient line rejection systems, Raman spectrometer is now a common equipment in laboratories. In heterogeneous catalysis, Raman spectrometry is widely used to characterize the active surface species on the catalyst. As vibrational spectroscopy, Raman analysis provide information about the chemical structure of the probed species. The obtained data are often complementary to those obtained from other surface characterization techniques, such as XPS or EXAFS. In this paper, we briefly describe the basis of Raman spectrometry and explain why this method is a powerful tool to analyze hydrotreatment catalysts. Next, we give some examples where Raman spectrometry has been used to improve the description of catalyst structure: - the speciation of metal atoms during catalyst synthesis, - the impact of precursor purity on the achieved catalyst, - the choice of the calcination temperature to avoid inactive phase formation, - the monitoring of coke deposit on the catalyst surface. 1 RAMAN SPECTROMETRY AND HETEROGENEOUS CATALYSIS Raman spectrometry is based on the Raman effect, predicted by Smekal in 1923 [ 3 ] and experimentally observed by Raman and Krishnan in 1928 [ 4 ]. When a sample is submitted to a monochromatic light of frequency ν0, the main part of the light is elastically scattered with the same frequency ν0 (Rayleigh diffusion). Weakly intense scattered light quanta with frequencies different from those of the incident light quantum, are observed too: their frequencies are equal to ν0 + νi (anti-Stokes Raman diffusion) and to ν0 − νi (Stokes Raman diffusion), where νi corresponds to the characteristic vibrational frequency of sample components (molecules or crystals). Raman spectroscopy is thus a vibrational spectroscopy, like infrared spectroscopy. In term of spectra, the main difference between the two techniques concerns the intensities of the vibrational bands. In infrared adsorption, a vibrational mode will be IR active (i.e., its intensity is not equal to zero), if the corresponding atomistic movement induces a variation of the molecular dipole moment. For instance, the stretch of heteropolar molecules (such as HCl or CO) is IR active. On the contrary, a vibrational mode is Raman active, if the corresponding atomistic movement induces a variation of the molecular polarisability. The stretch of homopolar molecules (such as H2 or O2) is Raman active. For small molecules, point group theory allows to determine if a vibrational mode is Raman (or IR) active or not. The example of benzene molecule is famous: all Raman active modes are IR inactive, and inversely. This property demonstrates that the benzene molecule is centro-symmetric. For larger molecules or for ill crystallized solids, there is no simple rule to determine the relative intensities but Raman and Infra Red spectra are often complementary to determine the sample structure. For liquid sample, the choice between Raman and IR spectroscopy strongly depends on the solvent. In IR adsorption, the study of solutes in aqueous solution is difficult due to the high absorption of water. Solutes IR bands are difficult to observe among those of water solvent. On the contrary, water exhibits a poorly intense signal in Raman spectrometry. In a similar way, analyzing compounds deposited on a solid support, the choice between both techniques depends on the support response. For instance, alumina exhibits intense IR bands between 400 and 1300 cm−1. If the surface species present bands in this spectral region, it will be easier to analyse them using Raman spectrometry. One practical advantage of Raman analysis is that no sample pre-treatment is usually required. However, Raman spectrometry exhibits limitations, mainly due to the inherently small intensity of the Raman signal. First, the technique is not sensitive: to give orders of magnitude, bands are detectable for concentrations greater than 0.01 mol·L−1 for liquid sample and greater than 1 weight percent for solid sample. Next, fluorescence phenomena, often due to impurities, are several order of magnitude more intense than Raman signal and can mask the Raman spectra. Of course, approaches exist to Different steps of hydrotreating catalyst synthesis and life. overcome these difficulties (for instance, Surface-Enhanced Raman Spectroscopy, or Resonance Raman Spectroscopy to increase the sensitivity), but their uses remains technically limited. Industrial hydrotreatment catalysts are molybdenum- or tungsten-based c (...truncated)