Surface-Enhanced Raman Spectroscopy for Heterogeneous Catalysis Research

Abstract

Raman spectroscopy is valuable characterization technique for the chemical analysis of heterogeneous catalysts, both under ex-situ and in-situ conditions. The potential for Raman to shine light on the chemical bonds present in a sample makes the method highly desirable for detailed catalyst characterization, however the weak nature of Raman scattering inhibits its wider use in catalysis. The use of Surface-Enhanced Raman Spectroscopy (SERS) to overcome the inherently low signal intensity found with traditional Raman spectroscopy allows signal enhancements of up to 108 times. The large signal enhancement allows the detection of very low concentrations of reactant and/or product. Alternatively, the spatial resolution of activity studies can be increased dramatically on the basis of the same effect. Three distinct scientific approaches were taken to improve the applicability of SERS even further. Time-Resolved Surface-Enhanced Raman Spectroscopy (TR-SERS) was used to measure the chemical composition of a sample at different depths underneath the surface. An integrated Atomic Force Microscope and Raman Spectrometer (AFM-Raman) was developed to enable the simultaneous measurement of chemical information and nanoscale morphology of a sample. The AFM-Raman system has temperature and gas-atmosphere control, allowing surface studies under in-situ conditions. Shell Isolated Surface-Enhanced Raman Spectroscopy (SHINERS), a new form of silica-coated, SERS-enhancing nanoparticles, were examined with the eventual aim of extracting more chemical information from catalytic systems through the use of either gold or silver nanoparticles as SERS substrates. The trio of characterization methodologies all work towards one common goal: extending the applicability of SERS for heterogeneous catalysis research. The TRRS/TR-SERS and AFM-Raman techniques are both aimed at improving the spatial resolution of the chemical information, though on different scales. AFM-Raman is suited to the study of nanoscale heterogeneities in model catalyst samples, whilst TRRS/TR-SERS is applicable to depth-dependent heterogeneity in the larger catalyst bodies that are practically used in chemical reactors. The use of SHINERS with either of the spatial Raman techniques adds an extra dimension, allowing the comparison of the effect of exposed and unexposed noble metal nanoparticles on a reaction. The main advantage of SHINERS in in-situ analysis will be their increased thermal stability and inert surface, which should make it possible to apply SERS in a wide variety of catalytic studies without interfering with the chemistry. Raman spectroscopy is already known as a powerful tool for heterogeneous catalysis research, and SERS will surely join that list in its own right very shortly. The speed at which scientific and technological advances are being made both with varied forms of SERS substrates and the combination technique possibilities such as AFM-Raman, makes this an exciting time to be part of this field of research. It is surely only a matter of time before true surface generality and spectral reproducibility are achieved for SERS, leading to the ultimate goal of following the reaction of a single molecule over an industrially relevant catalyst surface under realistic reaction conditions