Atmospheric pressure reactions on model catalysts are typically performed in so-called high-pressure cells, with product analysis performed by gas chromatography (GC) or mass spectrometry (MS). However, in most cases, these cells have a large volume (liters) so that the reactions on catalysts with only cm2 surface area can be carried out only in the (recirculated) batch mode to accumulate sufficient product amounts. Herein, we describe a novel small-volume (milliliters) catalytic reactor that enables kinetic studies under atmospheric pressure flow conditions. The cell is located inside an ultrahigh vacuum chamber that is deliberately limited to basic functions. Model catalyst samples are mounted inside the reactor cell, which is locked to an oven for external heating and closed by using an extendable/retractable gas dosing tube. Reactant and product analyses are performed by both micro-GC and MS. The functionality of the new design is demonstrated by catalytic ethylene (C2H4) hydrogenation on polycrystalline Pt and Pd foils.

Much of the fundamental molecular-level understanding of heterogeneous catalysis originates from studies of well-defined model systems in ultrahigh vacuum (UHV).1,2 Starting in the 1960s, investigations of noble metal single crystal surfaces of different crystallographic orientations directly revealed the structure-sensitivity of gas adsorption, co-adsorption, and reactivity. UHV conditions guarantee the cleanliness of the surfaces and are also required for many of the typical surface-sensitive methods such as low energy electron diffraction (LEED), Auger electron spectroscopy (AES), temperature programmed desorption (TPD), X-ray/UV photoelectron spectroscopy (XPS/UPS), electron energy loss spectroscopy (EELS), and others.3 The adsorption of CO, oxygen, hydrogen, and ethylene (C2H4), CO oxidation, and C2H4 hydrogenation were in the focus of the early studies.

In the following decades, more complex model catalysts were developed (Refs. 4–7 and the references therein), including surface alloys,8–10 polycrystalline foils (surface structure libraries11–13), oxide single crystals and thin films,14,15 oxide islands on metal substrates (inverse catalysts16,17), oxide supported metal nanoparticles,18–21 and others. However, due to the low pressures of UHV, cryogenic temperature is typically needed to produce substantial surface coverages of adsorbates. Conversely, at high temperatures of catalytic reactions, coverages are typically very low in UHV, whereas technological catalytic reactions are carried out at ∼10 orders of magnitude higher pressure. These very different conditions may also change the chemical state of the catalysts under reaction conditions (e.g., by oxidation,22 hydride formation,23 coking,24 restructuring,25,26 etc.).

It was soon realized that this “pressure gap” limits the transferability of model catalysis results to applied catalysis. In a first attempt to bridge this gap, Somorjai and co-workers constructed a high-pressure cell that was located inside a UHV chamber.27 After the characterization of a single crystal sample, it was enclosed in a tubular high-pressure cell (inside the UHV chamber) and atmospheric pressure reactions were analyzed by gas chromatographic (GC) product analysis. After the reaction, gases were pumped-off, the reaction cell was opened, and the single crystal was characterized again in UHV by surface sensitive methods.27–30 In the following, several variants were reported of (outside) high pressure cells attached to a UHV chamber, with sample transfer between the UHV and high-pressure sections.31–38 Apart from pre- and post-reaction analyses, some of these high-pressure cells also enable in situ surface spectroscopy (in fact operando, as catalytic performance is acquired simultaneously).34–39 

Nevertheless, such model catalysis cells also have drawbacks. The reactor volume is often rather large (liters), and the gas dosing geometry is not ideal, which is why catalytic reactions are mostly performed in batch mode, with the reacting gases recirculated over the catalyst. Accordingly, minimizing the reactor volume to milliliters would enable us to carry our catalytic reactions under flow conditions, as in applied catalysis. Furthermore, as the determination of kinetic parameters (rates, orders, and activation energies) requires many experiments, it is also meaningful to limit the UHV section to basic functions. Instead of “blocking” a complex setup by time-consuming kinetic tests, spectroscopic studies are then just performed for the most promising catalysts in other instruments.40,41 All these considerations motivated the development of a small-volume reactor cell for atmospheric pressure reaction studies on model catalysts under flow conditions.

In Secs. II A–II F, after a brief overview of the new experimental setup, each part is described in detail, focusing on the atmospheric pressure flow “microreactor”. Afterward, the vacuum system as well as the gas manifold and reactant/product analysis are described.

The basic principle is that the reactor compartment is located inside a UHV chamber, but is sealed-off for catalytic reactions; this is similar to the original “Somorjai design”, but includes several improvements (smaller cell volume, flow conditions, and external heating).

Figure 1 shows the following main parts: On top of the UHV chamber, an x, y, z, φ-manipulator (a) holds a flange with the microreactor (containing the sample) and allows us to move it from UHV “section 1” (for cleaning and optional modification) to “section 2” (for catalytic reaction tests). Section 2 houses the microreactor receiving unit, which also serves as an oven (j), and an extendable/retractable gas feeding tube [with inlet and outlet lines (k) and a Kalrez O-ring at the front]. In section 1, catalyst samples may also be prepared/modified by physical vapor deposition (PVD) of thin metal or oxide films, using a suitable evaporator (g) and a quartz microbalance (d). The entire UHV system is pumped by using turbomolecular (m) and rotary-vane pumps.

FIG. 1.

Schematic view of the experimental setup. Section 1: sample preparation and cleaning (UHV). Section 2: atmospheric pressure flow reaction cell. (a) x, y, z, φ manipulator, (b) leak valve (Ar, H2, and O2), (c) gate valve (connected to a rotary-vane pump), (d) quartz microbalance, (e) viscometer, (f) sputter gun, (g) evaporator (frontside), (h) radiative/electron impact heating (backside), (i) ionization gauge (backside), (j) oven with resistive heating, (k) gas double-nozzle mounted on a z-shift, (l) gate valve, (m) turbomolecular pump, (n) gas line, (o) mass flow controller(s), and (p) water cooling tubes.

FIG. 1.

Schematic view of the experimental setup. Section 1: sample preparation and cleaning (UHV). Section 2: atmospheric pressure flow reaction cell. (a) x, y, z, φ manipulator, (b) leak valve (Ar, H2, and O2), (c) gate valve (connected to a rotary-vane pump), (d) quartz microbalance, (e) viscometer, (f) sputter gun, (g) evaporator (frontside), (h) radiative/electron impact heating (backside), (i) ionization gauge (backside), (j) oven with resistive heating, (k) gas double-nozzle mounted on a z-shift, (l) gate valve, (m) turbomolecular pump, (n) gas line, (o) mass flow controller(s), and (p) water cooling tubes.

Close modal

In a typical experiment, a catalyst is mounted inside the microreactor that itself is mounted to the manipulator. After installation in UHV, the sample is cleaned in section 1 by oxidation and reduction [∼10−6 mbar O2 or H2 applied by using a leak valve (b)] at temperatures up to 650 °C by radiative or electron beam heating (h). Alternatively, Ar+ sputtering (f) can be applied. Afterward, the microreactor is moved to section 2, slid into the receiving groove of the oven, and closed by using the movable gas tube (which has a front Kalrez O-ring). After sealing off the microreactor, the reactant gases [regulated by using mass flow controllers (MFCs) (o)] are supplied via the inlet channel of a nozzle, with its outlet channel connected to a micro-gas chromatograph (μGC) and mass spectrometer (MS) for product analysis. Catalytic reactions under flow conditions can then be carried out between room temperature (RT) and 250 °C.

The custom-made stainless-steel microreactor combines the functionality of a sample holder and reaction cell (Fig. 1). This enables us to perform all required operations (oxidation/reduction, sputtering, and evaporation) while reducing the reactor volume (4 ml) and enabling GC/MS product analysis under flow conditions. The lower part of the microreactor has a cylindrical recess and is shaped to exactly fit into the oven groove (see Sec. II D). Catalyst samples are mounted via stainless-steel clips to the backside wall of the recess.

A thermocouple spot welded to the sample provides exact temperature control (Alumel and Chromel wires; Omega©; diameter: 0.1 mm). The upper part of the microreactor exhibits thermocouple feedthroughs (here: K-type). Behind these feedthroughs, water cooling is applied via stainless steel tubes (∅ = 6 mm), which also hold the microreactor. These tubes are attached via Swagelok© connections to longer tubes (p) that are welded into a CF40 flange (also including thermocouple connections), which is mounted on the differentially pumped MCV© manipulator (x, y, z, φ) on top of the UHV setup.

The upper UHV section 1 (Fig. 1) offers various options for catalyst pre- and post-reaction treatments. The sample can be heated via the microreactor back, using retractable radiative/electron impact heating (applying a positive potential to the microreactor limits heating of the walls). Furthermore, Ar, O2, or H2 (high purity: 5.0; Messer Austria©) can be dosed via a leak valve. UHV annealing and oxidation/reduction pre-treatments (∼10−6 mbar) can be conducted at temperatures up to 650 °C (if required, also at mbar pressure; see below). Ar+ ion bombardment can be carried out at kinetic energies between 0.5 keV and 3 keV using a sputter gun (Leybold Heraeus©). Furthermore, physical vapor deposition (PVD) can be carried out via an electron beam evaporator, with deposition rates precisely monitored by using a microbalance (BeamTec©).

The microreactor level in UHV section 2 [Figs. 1 and 2(a)] includes an oven (with a fixed position) and an extendable/retractable gas dosing tube (a double-nozzle with inlet and outlet tubes and an O-ring rubber gasket at the front, held by a groove). The oven serves as a heavy-duty “anvil” mounted to a CF63 flange, which also features Cu current feedthroughs (MDC Vacuum©; up to 700 VDC, 23 A). The anvil is made of stainless steel, with a centered round hole and flat top and with a groove fitting the shape of the lower part of the microreactor. The ends of the Cu feedthroughs are connected via UHV-compatible CuBe screw-on-type clamps (Allectra©) to Ta rods, holding three spot-welded Ta filaments (∅ 0.25 mm). For efficient heating, the filaments are located in the close proximity of the backside of the microreactor. The back wall of the microreactor is only 1 mm thick so that the low thermal conductivity of steel allows us to achieve high sample temperatures, while the water-cooled (12 °C) upper part with larger volume remains near RT.

FIG. 2.

(a) Illustration of the concept of the atmospheric pressure reaction flow cell. (b) Cross-sectional view of the reaction cell assembly.

FIG. 2.

(a) Illustration of the concept of the atmospheric pressure reaction flow cell. (b) Cross-sectional view of the reaction cell assembly.

Close modal

For reactivity measurements, the microreactor is lowered from section 1 to section 2, sliding its lower part into the oven groove [Fig. 2(a)]. Then, the stainless-steel gas nozzle is pressed against the fixed microreactor, using a dedicated z-shift with a ratchet actuator for uniform movement [McAllister Technical Series©, model BLT63; Figs. 2(a) and 2(b)]. That way, the cell is sealed by using the rubber gasket ring (Kalrez© 7075; DuPont©), stable in N2 up to 327 °C [Fig. 2(b)]. Upon initiating reaction and temperature ramping, no pressure rise is observed in the UHV system. Under reaction conditions, a temperature of 250 °C should not be exceeded in order to keep the seal intact for multiple experiments.

The base pressure (≈10−9 mbar) of the UHV system is achieved by using a turbomolecular (Pfeiffer HiPace 300©) and a rotary-vane pump, located beneath section 2, and measured by using a viscometer (Leybold Heraeus©; read-out: Viscovac VM212©; range: 10−7 mbar–10−1 mbar) and a Bayard–Alpert ionization gauge (Leybold Heraeus©; read-out: Ionivac IM220©; range: 10−10 mbar–10−4 mbar). The pressure between the needle valve and the rotary-vane pump of the gas manifold (Fig. 3) is measured by using a Pirani gauge (Leybold Heraeus©; read-out: Thermovac TM22©). A gate valve is installed between the UHV pumping system and section 2 [Fig. 1, (a)]. When changing samples, the gate is closed and the chamber is vented with Ar through the leak valve in section 1. After installing a new sample, the UHV part is pumped to pre-vacuum using the rotary-vane pump of the gas manifold, attached via another gate valve above section 1. Afterward, the gate to the turbomolecular pump is slowly opened. For the differentially pumped MS, a separate pumping system is installed, consisting of a smaller turbomolecular pump (Pfeiffer SplitFlow© 50) and a rotary-vane pump.

FIG. 3.

Flow chart of the microreactor gas dosing and analysis.

FIG. 3.

Flow chart of the microreactor gas dosing and analysis.

Close modal

A gas manifold is connected to both UHV section 1 (Ar, H2, and O2 for sample treatment) and section 2 (all gases for microreactor studies). Figure 3 shows a flow chart of the gas dosing and analysis system. High purity (5.0, Messer Austria©) Ar, H2, O2, and reactant gases are admitted to the reaction cell via five calibrated MFCs (Bronkhorst F-201CV©). After passing the MFCs, the gases are mixed and reach a three-way valve, where they are either directed through the reactor or bypassed to the μGC and MS (e.g., for calibration).

Gas species are identified by using an MS [Hiden HPR 20©, equipped with a secondary electron multiplier (SEM) and Faraday cup detectors, ppb-range; base pressure: ≈10−9 mbar]. The differentially pumped MS is operated at a constant pressure of 1.1 × 10−5 mbar, leaking in the reaction gas mixture. MS intensities are measured with the Faraday cup detector and normalized to the Ar signal. Reactant/product quantification is carried out via a μGC (Inficon Fusion Gas Analyzer©). Gas species are detected via a thermal conductivity detector (TCD; ppm-range), and the μGC has four different columns: (1) an Rt-Molsieve 5A column (∅ = 0.25 mm, length: 10 m; porosity: 5 Å, for H2, O2, CO, and CO2), (2) an Rt-Q porous layer open tubular (PLOT) column (fused silica; ∅ = 0.25 mm, length: 12 m; for light hydrocarbons), (3) a Stabilwax-DB column (fused silica; ∅ = 0.25 mm, length: 10 m; for alcohols, ethers, esters, acids, amines, etc.), and (4) an Rt-Alumina column (Al2O3/Na2SO4, ∅ = 0.32 mm, length: 1 m; for the separation of unsaturated hydrocarbon isomers). High purity (6.0) Ar and He (Messer Austria©) are used as carrier-gases. Ethylene and ethane are separated by the Rt-Q-PLOT column (60 °C, ≈2 bars, injector temperature: 90 °C; injection time: 30 ms; sequence: 50 s per chromatogram; and operation mode: continuous sequence loop).

After a reaction experiment, the reactor is purged with Ar in order to remove residual gases and evacuated to pre-vacuum. Then, the two valves on both ends of the reactor (Fig. 3) are closed so that the microreactor can be opened inside the UHV chamber (i.e., the gas supply tube is retracted) and moved back to section 1.

This section demonstrates that the new reaction cell design allows us to perform catalytic reactions under atmospheric pressure flow conditions, with gas analysis at the reactor outlet by both MS (identification of reactants and products) and μGC (quantitative analysis). Ethylene hydrogenation (ΔH0R = −262 kJ/mol) is applied as a test reaction for two catalyst samples, namely, high purity polycrystalline foils of Pt (GoodFellow©; 99.99%, thickness: 0.1 mm) and Pd (Advent Research Materials©, 99.95%, thickness: 0.1 mm) (for images, see Refs. 12 and 13). After pre-cleaning and characterization in a separate UHV chamber, the metal foils were transferred in air to the microreactor setup for the examination of catalytic properties.

Both metal foils were cut to a size of roughly 1 × 1 cm2, washed with isopropanol in an ultrasonic bath, and dried. Then, they were mounted onto Mo sample holders (Specs©) using stainless-steel clips and transferred to a UHV surface analysis chamber (35 l with a base pressure of 5 × 10−10 mbar).42 Following sample cleaning (Ar+ sputtering, oxidation/reduction, and annealing up to 1000 °C via electron beam heating), surface analysis was carried out by x-ray photoelectron spectroscopy [XPS; Specs XR50© high intensity non-monochromatic Al/Mg dual anode X-ray source Phoibos 100 energy analyzer (EA)] and low energy ion scattering [LEIS; Specs IQE 12/38© ion source, using the EA with a multichannel plate for detection] (Fig. 4).

FIG. 4.

Surface analysis after sample cleaning in UHV. (a) XPS region scans of Pt 4f and Pd 3d (Al Kα radiation). (b) LEIS survey scans (1 keV He+ ion beam).

FIG. 4.

Surface analysis after sample cleaning in UHV. (a) XPS region scans of Pt 4f and Pd 3d (Al Kα radiation). (b) LEIS survey scans (1 keV He+ ion beam).

Close modal

In the current case, the foil samples were cleaned by repeated Ar+ sputtering (1.5 keV, 5 × 10−6 mbar Ar at RT), annealing at 500 °C, oxidation in 5 × 10−7 mbar O2, and reduction in 5 × 10−7 mbar H2 (in the case of Pd at 400 °C). To remove segregating bulk contaminants, Ar+-sputtering was also performed at 300 °C. XPS spectra were acquired at RT at a photoelectron emission angle of 0°, using Al Kα radiation (1486.61 eV), with the EA operated in the “large area” transmission mode. For data evaluation, CasaXPS was employed. XPS peaks of Pt 4f and Pd 3d orbitals are referenced to the Fermi level (determined via differentiated valence band spectra) and were fitted after Shirley background subtraction utilizing asymmetric Gauss–Lorentz sum functions. Doublet separations (Pt 4f: 3.3 eV and Pd 3d: 5.2 eV), peak area ratios (Pt 4f7/2:4f5/2 = 4:3 and Pd 3d5/2:3d3/2 = 3:2), and full width half maxima (Pt 4f: 1.5 eV, Pd 3d: 1.6 eV, and Pd 3d satellites: 4 eV) are constraint. Fitting parameters and peak positions (Pt 4f7/2: 71.4 eV and Pd 3d5/2: 335.3 eV) are in good agreement with the earlier work17,42–45 and the NIST XPS database [Fig. 4(a)]. Pd 3d satellites (341.9 eV and 347.1 eV) are due to the plasmon energy loss.46 

LEIS spectra were obtained at a primary kinetic energy of He+-ions of 1 keV at a scattering angle of 135°. LEIS surveys display the expected main components (Pt: 900 eV and Pd: 850 eV), as well as traces of oxygen (392 eV) and (unavoidable) bulk impurities (Fe 744 eV and Cu 785 eV, in line with the manufacturers’ datasheets), which segregated to the surface during cleaning.

For the calibration of the μGC, a series of gas mixtures of ethylene (purity: 2.6), ethane (purity: 2.5), and Ar (purity: 5.0, carrier-gas), all supplied by Messer Austria, with a total flow of 20 ml/min was applied at RT. Subsequently, a blind test was conducted, measuring the “catalytic” activity of the empty stainless-steel microreactor toward ethylene hydrogenation at temperatures up to 250 °C. For the blind test, as well as the following catalytic tests, a gas mixture of 5 vol. % ethylene and 25 vol. % H2 in Ar (the same total flow as for calibration) was employed. The temperature was increased stepwise and held constant for ≈40 min/step. In the absence of a catalyst sample, at and above 100 °C, ethylene conversion rises (>5%), which is why evaluation of the catalytic data is herein restricted to lower temperatures. To further lower the background activity, the reaction cell will be coated by gold or silica for future experiments.

After transfer from the surface analysis chamber, the foil samples were mounted with stainless-steel clips inside the microreactor and the thermocouple wires were attached, as described above. After installation in UHV, sample degassing, and moving the reaction cell to section 2, the cell was closed and the sample was cleaned by gas pre-treatment (total flow: 15 ml/min) at elevated temperatures (contaminants may originate from the sample transfer in air). After two hours of oxidation (10 vol. % O2 in Ar) at 250 °C, the sample was cooled to 150 °C in Ar and reduced (10 vol. % H2 in Ar) at 150 °C for one hour. For the Pt foil, the reactor was then cooled to RT in Ar and ethylene hydrogenation was performed twice from 25 °C to 250 °C (25 °C/step) [Figs. 5(a) and 5(c)]. To determine the kinetic parameters, a range of 20 °C–100 °C (10 °C/step) was used [Figs. 5(b) and 5(d)]. In between the two experiments, another pre-treatment (the same conditions as described above) was carried out. Because Pd is prone to hydrogen embrittlement at RT,47 only one catalytic test was carried out, starting at 100 °C and followed by decreasing the temperature stepwise (10 °C/step) to 20 °C.

FIG. 5.

Ethylene hydrogenation on the Pt foil: ethylene conversion and temperature vs time. [(a) and (b)] Measured by using the μGC and [(c) and (d)] measured by using the MS (intensity normalized to the Ar signal). The temperature range at (a) and (c) is 25 °C–250 °C (green curve: blind test without a catalyst). The temperature range at (b) and (d) is 20 °C–100 °C.

FIG. 5.

Ethylene hydrogenation on the Pt foil: ethylene conversion and temperature vs time. [(a) and (b)] Measured by using the μGC and [(c) and (d)] measured by using the MS (intensity normalized to the Ar signal). The temperature range at (a) and (c) is 25 °C–250 °C (green curve: blind test without a catalyst). The temperature range at (b) and (d) is 20 °C–100 °C.

Close modal

Figure 5 summarizes the μGC and MS results of ethylene hydrogenation on the Pt foil, demonstrating that the experimental configuration works efficiently for both types of detection. Quantification is based on μGC, whereas MS measurements serve for product identification (if required). The temperature-dependent ethylene conversion [Figs. 5(a) and 5(b)] was deduced from the ethylene and ethane concentrations quantified by μGC. Significant amounts of ethane are present at RT (ethylene conversion: ≈11.6%); at 60 °C, the conversion rises strongly and has a maximum at 125 °C [≈87%, Fig. 5(a)]. However, at 175 °C, catalyst deactivation sets in, most likely due to coking [Figs. 5(a) and 5(c)]. The conversion of the blind test without a catalyst is shown in Fig. 5(a) (green curve), with ≈10.5% at 125 °C. Note that during the blind test with the empty microreactor, its hot backside wall is exposed to the reactants. When it is largely covered by the Pt foil, the deactivated catalyst may exhibit conversions lower than in the blind test [Fig. 5(a)].

Considering the flow rates and conversions between 20 °C and 100 °C [Fig. 6(a)], the number of ethylene molecules converted per second and per Pt surface atom, i.e., the corresponding turnover frequencies (TOFs), were calculated [Fig. 6(b)]. Using the lowest four temperature TOFs on the Pt foil for plotting an Arrhenius graph [Fig. 6(c)], an activation energy EA of 41.2 ± 0.8 kJ/mol was derived. This agrees well with the literature values for Pt single crystals.20,48 For studies of carbon supported Pt nanoparticles, we refer to Ref. 42.

FIG. 6.

Comparison of the Pt and Pd foil in ethylene hydrogenation (based on μGC measurements): (a) conversion vs temperature, (b) turnover-frequency (TOF) per Pt or Pd surface atom vs temperature, and (c) Arrhenius plot for the determination of the activation energy on the Pt foil.

FIG. 6.

Comparison of the Pt and Pd foil in ethylene hydrogenation (based on μGC measurements): (a) conversion vs temperature, (b) turnover-frequency (TOF) per Pt or Pd surface atom vs temperature, and (c) Arrhenius plot for the determination of the activation energy on the Pt foil.

Close modal

Pd exhibited a much higher ethylene hydrogenation activity (conversion of 77% at 20 °C) and is shown for comparison [Figs. 6(a) and 6(b)].

A novel small-volume (4 ml) catalytic reaction cell for kinetic measurements of model catalysts under atmospheric pressure flow conditions was presented and tested for ethylene hydrogenation on polycrystalline Pt and Pd foils. The “microreactor”, with a model catalyst mounted inside, is located inside a basic ultrahigh vacuum (UHV) chamber, which serves for sample cleaning or modification. The reactor cell can be locked to an oven for external heating and closed by using an extendable/retractable gas dosing tube. Catalytic conversions and turnover frequencies at various temperatures were determined based on μGC gas phase analysis and used to calculate the activation energy on the Pt foil, which is in line with the literature data. The functionality of the new design enables us to study more complex reactions in the future.

The data that support the findings of this study are available from the corresponding author upon reasonable request.

G.R. acknowledges support by the Austrian Science Fund (FWF) via Grant Nos. F4502-N16 (SFB FOXSI) and I4434-N (Single Atom Catalysis) and by TU Wien via the project “Microreactor”. A.V.B. is indebted to the Ministry of Science and Higher Education of the Russian Federation (the project budget of the Boreskov Institute of Catalysis). We thank Johannes Frank, Harald Summerer, and Janko Popovic for assistance in assembly and Klaus Dobrezberger for providing the μGC-settings.

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