Schematic principle diagram of our cell system (not to scale). This includes the gas feed system (top), the rod inserted into the endstation (outlined by the dashed line), gas analysis instrumentation (triangle on the right), security solenoid valves (crossed circles), and thermocouples (labeled TC). The catalytic reaction cell holding the catalyst material (green square) is placed in the beam focus. The purple arrow indicates the incident x-ray beam entering the cell and the re-emitted photons that are analyzed by a grazing incidence x-ray spectrometer. A photo diode or a channeltron serve as the TFY detector (the rhombus). TEY is measured by measuring the sample current using a thin wire to the catalyst material (not shown).
Schematic diagram (roughly to scale) of the cell parts close to the sample in configuration in the film-, (a), and powder-confirguration, (b). The different probing depths/methods are indicated. (c) is 3D tilted views of the of the clamp that is screwed into the membrane cap, by the four outermost holes. The powder sample holder is screwed (threads not shown) into the center of the clamp. Grooves on the membrane-facing side and around the threaded sample holder have been milled away to increase the gas flow to the sampling area of the powder.
Photos of the disassembled reaction cell. (Top) Picture of the front end of the cell with the PEEK cap removed. (a) Tightening cap (PEEK) with viton rings for sealing. (b) the reaction cell body with hole in its top end for differential pumping. (c) is the membrane holding cap that can be “sandwiched” between the two small viton rings. The threaded top hole is to fasten the TEY cable. (Bottom) (a) part the rod that connects to the back side of the cell seen in (b) with its various connection holes. (c) is the membrane holder with the internal surface facing the camera.
Cross section of the catalytic in situ cell. Here, the cell is shown in its configuration (A), where the catalytic material (green) is evaporated directly onto the membrane, which also serves as a gas-vacuum barrier. The outer diameter of the cell is 3 cm.
Scanning electron micrograph (SEM) of a uniformly 31 nm thick Ag film that has been applied for ethylene epoxidation at T = 225 °C. Areas A, B, and B′ show different morphologies of the exposed Ag film. Area C is where the membrane has disappeared due to post-reaction rupturing. The lower right corner of the rectangular membrane is still attached to the frame and marked (arrow) as area B′. (Inset) Zoom of the areas B′ and B, where the membrane meets (at the dashed line) the supporting Si.
SEM showing an overview of a post-reaction powder Ag sample. The letters indicate the following: thread of sample holder (A), surface area of sample holder roughened with a crude abrasive paper (B), file rounded edge (C), strongly compressed powder sample part (D), lightly compressed powder sample part (E). Note that most of the sample at the rim (D) is missing due to it falling off when retrieving it for the SEM picture after the experiments.
XAS measurements on an Ag powder sample in Ar (red trace) and O2 (blue trace), respectively. The shown I0 signal with its structures is representative for both gas feeds.
XAS O K spectra of the catalytically active, 31 nm thick Ag film during gas feed composition change. Time t = 0 corresponds to the time of the gas change. For t < 0, the gas feed is pure O2 and for t > 0 the gas feed is a 1:1 reaction mixture. The temperature was constant at about 225 °C.
O K-XAS on Ag powder at 230 °C in the reaction cell. Spectra (a) and (d) were acquired with a flow of pure O2, where (d) was obtained 28 h after spectrum (a). The powder was kept for 24 h under reaction conditions between recording spectra (a) and (d). Spectrum (b) was acquired with a flow of pure Ar 20 min before spectrum (a). Spectrum (e) is the difference between spectra (d) and (c) to remove the contribution of gas phase O2 from the former. Spectrum (f) presents the (smoothed) difference between spectra (e) and (b) to extract the effect of oxygen loading in Ag. The arrow indicates the x-ray incident energy for the RIXS spectra shown in Fig. 10 were taken.
RIXS measurements of the same Ag power sample as the TFY spectra originates from in Fig. 9 . Both the measurement done in pure O2 and in an oxygen rich reaction mixture were done at ∼230 °C.
Thin film setup for in situ photochemical measurements, including a sniffer needed at low conversions.
Film setup for in situ electrochemical measurements. (a) cross section of the cell, done in PEEK, screwed on the rod. (b) front side view of the cell, the end facing away from the rod. (c) details of the cell cap containing the membrane and working electrode/sample.
Article metrics loading...
Full text loading...