REVIEW OF SCIENTIFIC INSTRUMENTS 82, 034101 (2011)
Experimental arrangements suitable for the acquisition of inelastic neutron scattering spectra of heterogeneous catalysts Ian P. Silverwood,1 Neil G. Hamilton,1 Andrew McFarlane,1 R. Mark Ormerod,2 Tatiana Guidi,3 Jonathan Bones,3 Michael P. Dudman,3 Christopher M. Goodway,3 Mark Kibble,3 Stewart F. Parker,3 and David Lennon1,a) 1 Department of Chemistry, Joseph Black Building, The University of Glasgow, Glasgow G12 8QQ, United Kingdom 2 School of Physical and Geographical Sciences, Keele University, Staffordshire ST5 5BG, United Kingdom 3 ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire OX11 0QX, United Kingdom
(Received 22 November 2010; accepted 18 January 2011; published online 2 March 2011) Inelastic neutron scattering (INS) is increasingly being used for the characterization of heterogeneous catalysts. As the technique is uniquely sensitive to hydrogen atoms, vibrational spectra can be obtained that emphasize a hydrogenous component or hydrogen-containing moieties adsorbed on to an inorganic support. However, due to sensitivity constraints, the technique typically requires large sample masses (∼10 g catalyst). A reaction system is hereby described that enables suitable quantities of heterogeneous catalysts to be appropriately activated and operated under steady-state conditions for extended periods of time prior to acquisition of the INS spectrum. In addition to ex situ studies, a cell is described which negates the need for a sample transfer stage between reaction testing and INS measurement. This cell can operate up to temperatures of 823 K and pressures up to 20 bar. The apparatus is also amenable to adsorption experiments at the gas–solid interface. © 2011 American Institute of Physics. [doi:10.1063/1.3553295] I. INTRODUCTION
Heterogeneous catalysis is fundamentally a surface phenomenon, with the interaction between the solid catalyst and the reacting fluid determining the progress of the reaction. To achieve meaningful surface information from working catalysts, it is therefore often essential, or at least useful, to reject bulk properties, and select purely the adsorbed material at the catalyst surface. This is most commonly performed by using background subtraction procedures, or through the use of surface selective techniques such as x-ray photoelectron spectroscopy. All techniques commonly used have disadvantages. Those using electrons require measurements to be made in vacuo, which is likely to affect adsorbates, whereas optical techniques can struggle with materials that are dark and strongly absorb light. This communication concentrates on the acquisition of the vibrational spectrum of working heterogeneous catalysts, which typically tend to be black in color. Consequently, these materials can be resistant to investigation by optical probes such as infrared spectroscopy. Moreover, common catalyst support materials (e.g., alumina) also display optical “cutoffs,” where total absorption by the support can mask vibrational modes of interest.1 Inelastic neutron scattering (INS) offers a useful complement to optical probes such as infrared spectroscopy, although it is noted that the technique is not surface selective (see p. 285 of Ref. 2). When a neutron interacts with matter, it may scatter elastically, or it may transfer energy corresponding to the vibrational quanta of the material. The hydrogen a) Author to whom correspondence should be addressed. Electronic
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atom has the greatest incoherent scattering cross section of all the elements,3 making INS a highly effective probe of modes of vibrations of bonds involving hydrogen. This has been exploited to investigate catalyst supports,4 the effect of coking and deactivation,5–8 and even reaction intermediates.9, 10 This body of work additionally covers adsorption studies including small hydrocarbons on carbon,11 as well as the adsorption of hydrogen chloride1 and methanol12, 13 on alumina. INS has its own disadvantages, the most serious of which is that measurements are made at cryogenic temperatures to minimize the Debye–Waller factor in the scattering law (see p. 34 of Ref. 2) in order to achieve satisfactory signal-to-noise ratios. This consideration makes operando measurements14 on working catalysts difficult, but in situ measurements may be made through a rapid quench of the reaction, as demonstrated by Goodman et al.9, 10 Moreover, the strength of the INS signal is directly proportional to the number of scattering atoms in the neutron beam. To achieve a sufficient signal from an adsorbate on a catalyst surface, a significant catalyst mass is required.12 Despite this increasing research activity, there are relatively few reports on the sample environment issues associated with the handling and manipulation of such sensitive and potentially hazardous chemical systems that heterogeneous catalysts could represent. Nicol’s insightful description of the application of INS to study chemisorbed hydrogen and hydrogenous molecules includes a description of a flow-through sample cell suited to catalytic investigations.15 Subsequently, Mitchell et al. describe a range of cells that have been used to undertake INS investigations on adsorption and catalytic systems at the ISIS Facility of the Rutherford Appleton Laboratory (see p. 130 of Ref. 2).
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These include a combination of flow-through and batch style reactors made from either aluminum or stainless steel. Turner et al. describe apparatus for studying catalysts and catalytic processes using neutron scattering. However, that system is intended for diffraction measurements utilizing elastic neutron scattering.16 A later publication from Turner and co-workers describes an arrangement for sample containment for neutron scattering studies of hydrogen fluoride and related molecular species.17 Again, the apparatus is intended for diffraction studies but some of the sample handling issues are indeed relevant to investigations of reactive catalytic systems. This paper describes apparatus that has been developed at the ISIS spallation neutron source18 that is capable of studying a range of heterogeneously catalyzed reactions. The system can treat up to 30 g quantities of catalyst under representative reaction conditions. Furthermore, the modularity of the design allows adaptation of the apparatus to suit different reaction conditions. The operation of a generic gas handling system is described that can be connected to a range of reactor configurations. Three reactor types are considered. Two are variants of a straightforward tubular reactor that require remote handling of the catalytic material in an inert environment (i.e., high quality glove box), while the third reactor, which is described in more detail, enables direct transfer of the treated sample to the neutron spectrometer. In the latter case, INS spectra are presented for the chemisorption of methanol onto an η-alumina catalyst,12 which has been selected as a “benchmark” surface chemical reaction. Flexible handling procedures for INS investigations of heterogeneous catalyst samples are becoming increasingly important, not least because of the recent exploitation of the INS technique to provide quantitative information on hydrogen retention within these samples. Silverwood et al. have recently used INS to quantify hydrogen associated with surface hydrocarbon and hydroxyl groups on an alumina-supported nickel catalyst active for the “dry” reforming of methane.8 This paper describes sample handling procedures that are compatible with such studies.
II. EXPERIMENTAL SYSTEM
The apparatus consists of a gas handling manifold to control gas flow, a reactor cell, and a reaction monitoring system. Each section of the equipment may be adapted to different reaction requirements and is described below. At the current time, the system has been used to investigate a number of reactions, including the reforming of methane over aluminasupported nickel catalysts at temperatures up to 1073 K using either steam or CO2 as the oxidant.7, 8 The CO hydrogenation reaction over iron catalysts has also been examined, including operation at up to at 10 bar pressure.19 Moreover, experiments with more benign conditions have also benefited from the apparatus, e.g., the adsorption of hydrogen on ceria-supported gold nanoparticles.20 Adaptation of the equipment to other gas–solid reactions would be straightforward.
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FIG. 1. Schematic of gas handling equipment showing arrangement of mass flow controllers [rectangular boxes indicate maximum flow rates for N2 (sccm)], pressure relief valves (PRV), pressure sensors (P), mixing volume (MV), vacuum connections (Vac), thermocouple (TC), fill and overflow saturator lines (F and O), mass spectrometer (MS), and back pressure regulator (BPR). Shaded areas indicate heating.
A. Gas handling
A schematic of the gas control apparatus is given in Fig. 1. Components are connected using 1/4 in. o.d. stainless steel tubing and Swagelok tube fittings. Gas flow is controlled by four Hastings model HFC302 mass flow controllers attached to a Teledyne THPS-400 controller. These controllers have been calibrated for nitrogen flow but the use of gas correction factors allows other gases to be accurately metered. One controller has a maximum flow rate of 1500 standard cubic centimeters per minute (sccm), one has 150 sccm, and two have 200 sccm (all N2 ). Pressure is controlled up to 20 bar using a back pressure regulator (BPR, Parker Veriflo ABP-1ST-43-PP-X4) which is positioned after the mass flow controllers. Pressure relief valves (PRV, Parker HPRV S4A-BN-K1–319) are present before and after a heated saturator vessel in order to prevent overpressurization. The saturator provides the facility to force gas through a liquid to provide a vapor pressure of liquid reactants, such as water for the steam reforming of methane. The saturator can be filled without removal using the fill line (F); the overflow (O) ensures that the liquid volume does not exceed safe limits. The bubbler has a thermocouple in contact with its outer surface and is wrapped in heating tape (Farnell, power 40 W/m, part no. 397–750) and insulated. This is connected to an over-temperature trip (Eurotherm 2132i) set below the boiling point of the liquid to prevent excessive pressurization of the vessel. A second thermocouple is immersed in the saturator liquid and is connected to a proportional integral differential (PID) controller (Eurotherm 3508) to control the heating on the saturator. “MV” represents a mixing vessel, which is a length of wide-bore tubing filled with Ballotini glass spheres to ensure turbulent flow. To prevent condensation in the mixing volume, the output from the saturator joins the other gases downstream. All lines carrying vapor from the
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saturator are trace-heated with heating tape that is controlled with PID controllers (Eurotherm 3508) and insulated. Other thermocouples are attached to components of large mass (e.g., back pressure regulator) to ensure they are maintained at the required elevated temperature. Collectively, these arrangements ensure the absence of cold spots throughout the apparatus. The entire line can be evacuated for leak-testing (better than 2 × 10−8 mbar l/s as tested by Oerlikon Leybold UL200) and to purge any residual gases before or after reaction. The reactor is held inside a bucket furnace (Instron SFL, model no. TF105/3/12/F controlled by Eurotherm 3508) and can be isolated using high-temperature valves (Parker NV series, rated to 811 K) on the input and output connections. The valves sit above the furnace and are shielded by the use of thin, circular, loose-fitting steel baffles that sit in and above the furnace bore to mitigate the convective heat loss. Depending on the configuration of reactor used (see below), the reactor can then be isolated from the line and either (i) transferred to a glove-box (MBraun UniLab MB-20-G, [H2 O] < 1 ppm, [O2 ] < 2 ppm) for further processing of air sensitive samples, or (ii) loaded directly into the neutron beam. After the reactor, there is the provision for a cold-trap (not shown in Fig. 1) to condense liquid products if desired. For example, this may be used to collect higher hydrocarbons in the Fischer–Tropsch process.21 After reaction, the cold-trap may be removed and retained products analyzed. For real-time monitoring of the reaction, a quadrupole mass spectrometer (MS, Spectra Microvision plus) samples the exhaust line. Flow is controlled into the instrument by a needle valve. Use of the MS as a qualitative indicator of the reaction progress as seen throughout these data sets is a routine procedure. Substitution of other analytical techniques, such as gas chromatography, or more rigorous calibration to obtain fully quantitative reaction data by mass spectroscopy is also possible. B. Reactors
Three reactors have been used with this catalyst rig, and other designs could easily be accommodated. Two are of a “U-tube” design, and one is engineered for direct placement in the neutron beam. Of the two U-tube reactors, one has been constructed from fused quartz tubing for chemical inertness at high temperature (up to 1100 K), while the other is formed from two 3/4 in. stainless steel tubes connected with a length of 1/4 in. tube. The catalyst is supported in the quartz reactor on a quartz sinter and by steel gauze in the stainless steel Utube reactor. The inlet tube may be filled with quartz Raschig rings to ensure a convoluted gas path for preheating of the reactants. It was also found useful to place the catalyst between quartz wool plugs, with Raschig rings above the sample to ensure the catalyst bed is retained within the hot-zone of the reactor in the furnace. A schematic of this U-tube arrangement is shown in Fig. 2. Seals to the quartz tube reactor are made using Cajon Ultratorr fittings. In this configuration, the operating pressure was limited by these connections to ∼1.5 bar. After reaction, the reactors were isolated and the catalyst samples transferred to a flat-plate, indium-sealed aluminium sample canister (see p. 131 of Ref. 2) inside
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FIG. 2. Packing arrangement in the tube reactor.
an argon-filled glove box (details above) to ensure reactive species present were not compromised. The sample could then be easily located in the sample well of the INS spectrometer. The U-tube arrangements are reasonably straightforward and can be applied to follow specific reaction treatments, e.g., acquisition of INS spectrum of a catalyst after a timed exposure to a particular mixture of gases at a specified temperature. However, it is a difficult exercise to consider subsequent sample treatments on that sample, as it involves reloading the material in to the U-tube reactor and reconnecting the reactor to the gas handling facility. In addition to the challenge of handling potentially pyrophoric materials, the possibility of induced radioactivity in metal containing catalysts can add a further complication. One way round these difficulties is to use a solid cell that can be isolated and readily connected to/disconnected from the gas manifold thereby facilitating direct transfer between the gas handling line and the spectrometer. Figure 3 presents drawings of the body of a reaction cell suitable for this task. A photograph of the assembled cell is shown in Fig. 4. The cell is an evolution of the aluminium INS sample cell described by Nicol15 and is an elaboration on the stainless steel flow-through tubular reactor built around a 1 in. Swagelok fitting used by Parker and co-workers. (see p. 131 of Ref. 2). In our case, the body of the cell is manufactured from Inconel 718 (50.0%–55.0% Ni, 14%–24% Fe, 17.0%–21.0% Cr, 4.75%–5.5% Nb, 2.8%–3.3% Mo and small amounts,
