La escuela de los cementerios, una nueva sensibilidad por la muerte, el

The housing for the reactor is a cylindrically shaped stainless steel piece (diameter 1.5", and height 0.875"), which can be separated into two halves and attaches to a 2 3/4" outer diameter ConFlat flange (Figure 2.1a). The two VCR fittings welded onto the atmospheric side of this flange serve as the gas inlet and outlet, and a type K

thermocouple is fed through the gas outlet so that the temperature of the reactant gas can be measured directly. A slot is cut in the body of the reactor to house the single-crystal sample (1 cm ×1 cm ×1 mm) mounted on a Ta plate, which can be transferred directly from the reactor to the sample holder in the attached UHV chamber; the two holes positioned directly under the sample slot and the beveled edges at the top and bottom of

the slot accommodate the gripping mechanism of the sample transfer system. Figure 2.1b shows the reactor flange with only the bottom half of the housing attached so that the gas inlet port behind the top half of the housing is visible, as well as the details of the sample holder inside the housing.

The reactor housing sits inside of an all-metal gate valve (VAT 481) that isolates the reactor from the UHV chamber, and the gap between reactor housing and inside wall of the gate valve is 1-2 mm (Figure 2.2). The flanges of the gate valve and reactor are sealed with Ag or Au-plated Cu gaskets to eliminate CO oxidation activity from the Cu

surface[33].

A differentially pumped glass nipple and another gate valve separate the VAT valve from the UHV chamber to prevent heat transfer and diffusion of He from the reactor to the UHV chamber. Before opening the reactor to UHV chamber, the reactor is isolated from the remainder of the recirculation loop by closing two Swagelok bellows-sealed

Figure 2.1: Photographs of: a) the reactor housing showing the Ta plate with the TiO2(110) crystal partially inserted into the sample holder; and

b) the reactor housing with the top half removed to show the sample holder and the gas inlet

valves and then opening a third valve to a 150 l/s turbomolecular pump (Leybold

Turbovac) to evacuate the reactor. The resulting chamber pressure during sample transfer is 1x10-7 Torr, and the chamber pressure dropped to ~2x10-10 Torr within 5 minutes of closing the gate valve to the reactor. Operation of the reactor had no effect on the chamber pressure, which remained below 2x10-10 Torr, and no He diffusion into the chamber was detected by the quadrupole mass spectrometer.

One of the key features of this recirculation loop reactor is the use of four VICI switching valves, which allow operation of the reactor to be switched between single- pass, recirculation, sampling and feed gas makeup mode. The VICI valves and the reactor are coupled by 0.0625" outer diameter and 0.040” inner diameter stainless steel tubing. By turning valve 4 (V4) from clockwise (CW) to counterclockwise (CCW) and closing the vent valve , the system is switched from the single-pass, catalyst pretreatment mode in which the effluent goes to vent, to a closed-loop recirculation mode (Figure 2.3, Table 2.1). The feed manifold consists of an array of mass flow controllers (Brooks 5850e and 5850i) for O2, He, and CO, and each flow controller is calibrated with a digital flowmeter

(Agilent Technologies ADM2000) over the appropriate range. A low dead volume absolute capacitance manometer (MKS Instruments Baratron 722A) records the pressure of the gas in the system.

In the recirculation mode of operation, the gas stream is passed through the reactor that houses the 1 cm2 sample (A) before being recirculated by a stainless steel bellows pump (Metal Bellows Corp. MB-21). This recirculation pump is throttled by a metering valve (Swagelok SS-SS2-VH) set to allow 15 standard cubic centimeters per minute (sccm) of gas flow through the system in order to minimize pressure differentials generated by the high capacity of the recirculation pump. For V1and V2, the valves are rotated through a 60° arc to connect alternating pairs of ports, whereas for V3 and V4 the valves are rotated 36° and 90°, respectively, to connect alternating ports. In the position shown in Figure 2.3, the gas flow passes through the sample loop (B) and then back into

Figure 2.3: Schematic of the reactor system shown in recirculation mode with the following valve positions: V1:CW, V2:CW, V3:CW, V4:CCW. The positions of the

the recirculation loop reactor, where it undergoes another recirculation cycle. The contents of the sample loop are injected into the GC by turning V3 through a 36° arc from the CW to CCW position, using the He carrier gas from the GC to sweep the contents of the sample loop into the column inlet (sampling mode). Note that when this occurs, the reactor gas and the He carrier gas flows are only momentarily halted (< 0.2 sec) while V2 is being switched to essentially maintain continuous flow conditions.

Table 2.1: Positions of the switching valves for the reactor system for the four different modes of operation. Mode V1 V2 (TCD column) V3 V4 Singe-pass CW CW CW CW Recirculation CW CW CW CCW Sampling CW CW CCW CCW Make-up CCW CW CW CCW

In the sampling mode, the contents of the gas sample loop (B) are directed to a gas chromatograph (HP 5890A) employing both a thermal conductivity detector (TCD) and a flame ionization detector (FID), with V2 allowing column selection. The TCD is

connected to a PLOT molecular sieve 5A or PoraPLOT Q capillary GC column to allow accurate analysis of non-organic gases such as CO, CO2, O2, H2, and H2O, while the FID

is linked to a second PoraPLOT Q capillary GC column for more sensitive analysis of organic compounds. Following sample injection, V1 is turned CCW to shorten the

recirculation pathway between V4 and V1. V3 is then turned back from CCW to CW (the normal recirculation position), which permits the sample loop to be filled with fresh feed gas (makeup mode) in order to compensate for the loss of gas during injection and to maintain constant pressure in the system. After V1 is switched back from the CCW to the CW position, the recirculation loop pathway is restored, and the feed gas contents in the

sample loop are added into the recirculation loop. An in-house LabVIEW program controls the operation of V1, V3 and V4 for automated switching between the various modes of operation. The positions of the switching valves in Figure 3 for the different modes are described Table 2.1.

The following high purity gases were used for all experiments: 5% CO balance He (Praxair), O2 (Airgas 99.5%) and He (Praxair 99.999%). The CO was passed through an

alumina trap heated to 150 ºC in order to remove any metal carbonyls from the feed. The temperature of the reactor was controlled externally by heating tapes, which were

regulated by a Chromalox 2104 series temperature controller and achieved reactor gas temperatures up to 200 ºC; the gas lines and gas sampling loop were always maintained at 60º C using a Valco Instruments temperature controller. Based on the volume of the reactor and the measured 15 sccm recirculation rate, the reactant gases recirculate over the catalyst once every 2 minutes.

The model catalysts consisted of 2 ML of Pt vapor deposited onto a rutile TiO2(110)

single crystal. The TiO2 support was cleaned by multiple cycles of Ar+ sputtering followed by annealing to ~1000 K, and Pt was evaporated using a commercial electron beam evaporator (Oxford Applied Research, EGC04) from a high purity Pt rod (99.95%). The Pt coverage was measured by an independently calibrated quartz crystal

microbalance[34], and one monolayer is defined according to the packing density of Pt(111) (1.50x1015 atoms/cm2). Detailed descriptions of the UHV chamber and the surface preparation procedures have been previously reported[35-40]. For the studies reported here, the Pt/TiO2 catalysts were prepared in a separate UHV chamber and then

time to air was approximately 30 min. A vacuum transfer system allows samples to be moved directly from the UHV analysis chamber to the reactor using a gripper mechanism that holds the key on the end of the Ta plate on which the TiO2 crystal is mounted (Figure

2.1a). The UHV chamber attached to the reactor is now set up for sample cleaning, metal deposition and surface analysis so that model surfaces can be prepared in situ in the future. After delivery into the reactor, the catalyst was heated to the desired temperature in a flow of He (35 sccm) at 760 Torr. The samples were held at the desired temperature for five hours, and then the reactant gases were introduced into the reactor loop in the single-pass mode before switching to recirculation mode. The measured pressure in the reactor during recirculation was approximately 850 torr for all of the experiments.

Dead volumes for the various sections of the reactor system were measured by first evacuating the entire reactor system with the turbomolecular pump and then allowing He from a cylinder with a known pressure and volume to expand into reactor system; the volumes of the various sections were calculated using the ideal gas law and measured pressures after filling with He from the calibration cylinder. The following volumes of the reactor system were measured: the reactor only (18.04 cm3), the total recirculation loop (30.49 cm3), sample injection loop for the GC (1.10 cm3) and the section of the recirculation loop plus sample loop filled with fresh feed gas during makeup mode (1.15 cm3).