Thiophene adsorption and decomposition on clean and sulfur precovered Ru(0001)

Surface Science 218 (1989) 127-146 North-Holland, Amsterdam

127

THIOPHENE ADSORPTION AND DECOMPOSITION AND SULFUR PRECOVERED Ru(OOO1) R.A. COCCO and B.J. TATARCHUK Department Received

ON CLEAN

*

of Chemical Engineering, Auburn University, AL 36849, USA 21 October

1988; accepted

for publication

13 March

1989

Thiophene adsorption and decomposition on clean and sulfur precovered Ru(OOO1) surfaces (i.e., 0s = 0.50) have been examined using TPD, TPRS and static SIMS. On the sulfur precovered surface, thiophene adsorption and desorption is reversible with desorption occurring at 145 K from the multilayer and at 161 K from the adsorbed surface layer. The FWHM of the 161 K peak is particularly narrow (i.e., 12 K) and has been attributed to the autocatalytic desorption of thiophene stabilized by adsorbate-adsorbate interactions. The weak interaction to the surface is further substantiated by SIMS analysis after desorption which detects no hydrocarbon residuals. The passive nature of the surface allows unique assignment of secondary ions formed by the collision cascade in the absence of surface-induced decomposition. On the clean surface, thiophene adsorption is irreversible providing new secondary ions at m/z 14 (CHZ), 26 (C,H:), 27 (C,H:), 28 (C,H:), 57 (C,HS+) and 114 (RuC+) after adsorption at 95 K as well as secondary ions at m/z 12 (C’), 13 (CH+ ), 50 (C_,Hz ) and 216 (Ru2C+) after annealing to 300 K. The C,Hz observed after annealing has been attributed to a bound C, intermediate in agreement with previous studies on a number of other metal surfaces. The irreversible nature of the adsorption is also manifested in the desorption spectrum where a new feature at 260 K is observed at intermediate thiophene exposures between 0.3 and 0.5 L. This desorption state has been attributed to irreversibly adsorbed thiophene fragments which stabilize thiophene adsorbates.

1. Introduction Ruthenium sulfide (RuS,) has been shown by Pecoraro and Chianelli [l] to possess high catalytic activity for the hydrodesulfurization (HDS) of dibenzothiophene. Subsequent kinetic studies in our laboratory have demonstrated that the sulfided metal is also quite active for HDS reactions, and possesses long term stability at typical reaction conditions [2]. In the case of the sulfided metal, catalytic selectivity has been observed to depend on presulfidization conditions [3]. Ruthenium surfaces retaining partial monolayers of strongly adsorbed sulfur catalyze the direct hydrogenolysis of thiophene to C, products and H,S, whereas surfaces retaining multilayers of sulfur, in the form of RuS,, produce nearly equal quantities of direct C, products and tetrahydro* To whom all correspondence

should

be addressed.

0039-6028/89/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

128

R.A. Cocco, B.J. Talarchuk / Thiophene adsorption and decomposition

on Ru(OOOI)

thiophene. Since the selectivity behavior exhibits sharp yet reversible transitions between these two regimes, the surface structure corresponding to each regime and its interaction with thiophene has provided an intriguing topic for further kinetic [4], spectroscopic [S-7], microscopic [8] and surface science investigations [9,10]. Recent studies by Heise and Tatarchuk [9] examined the decomposition mechanism of thiophene over clean and sulfur precovered Ru(0001) using temperature programmed desorption (TPD) and elect.ron energy loss spectroscopy (EELS). These studies revealed that thiophene decomposed in a three-step mechanism which involved (i) thiophene cracking at 120 K at low exposures to yield surface sulfur and hydrocarbon species which passivated the surface toward further thiophene decomposition at 120 K, (ii) intact thiophene adsorption on the now passivated surface and (iii) hydrogen evolution near 230 K which promoted further decomposition of adsorbed thiophene. After annealing to 260-280 K, EELS results were in good agreement with those reported earlier for thiophene on Pt(ll1) [ll], Pt(lOO) [11,12], Pt(210) [ll], Ni(lOO) [13] and Mo(100) [14] where bound C, intermediates have been proposed. The addition of sulfur to metal surfaces has a dramatic effect on thiophene adsorption. Studies have found that thiophene decomposition decreased on sulfur preadsorbed Mo(100) and Mo(ll0) surfaces with no decomposition observed for 6, = 0.5 ML [15-171. Intact thiophene adsorption has also been reported on MoS, [18]. On sulfur precovered Ru(OOOl), Heise and Tatarchuk [9] reported that preadsorbed sulfur passivated the ruthenium surface toward significant thiophene decomposition at sulfur coverages between 0.1 and 0.25 monolayers while virtually no decomposition could be observed for sulfur coverages in excess of 0.45 monolayer. Although thiophene adsorption on Ru(OO01) and sulfur precovered Ru(OOO1) appears to be analogous to other metal surfaces, questions arise concerning (i) the stoichiometry and surface geometry of the adsorbed C, intermediate and other decomposition products with increasing temperature and (ii) effects these decomposition intermediates have on subsequent and/or concurrent thiophene adsorbates. In this work, these questions are addressed using temperature programmed desorption (TPD), temperature programmed reaction spectroscopy (TPRS) and static secondary ion mass spectrometry (SIMS). 2. Experimental 2.1. Analytical

equipment

2.1. I. Ultra-high-vacuum chamber Temperature programmed desorption (TPD), Auger electron spectroscopy (AES) and secondary ion mass spectrometry (SIMS) were performed in an

R.A. Cocco, B.J. Tatarchuk / Thiophene adsorption and decomposition

on Ru(0001)

129

ultra-high-vacuum chamber equipped with a 220 8,s ion pump and a titanium sublimation pump with cryoarray. The system was operated at a base pressure of - 10-s Pa. 2.1.2. TPD TPD studies were performed using a differentially pumped mass spectrometer in an arrangement similar to that described earlier [9]. Data acquisition was accomplished using a Tandy Model 12 microprocessor capable of monitoring all ten data channels from the mass spectrometer at a. rate of 60 channels/s. The temperature of the crystal was monitored at 4 Hz. TPD experiments were performed using a linear heating rate of 10 f 1 K/s which, unless otherwise noted, range from 95 to 750 K. All TPD experiments were performed at a background pressure below 4 X lo-’ Pa. 2.1.3. AES AES spectra were obtained using a PRI RVLg-120 reverse view LEED apparatus, a PHI 11-500A Auger system control unit, an EG&G PAR 5101 lock-in amplifier and a Thorn EM1 3000R power supply. All spectra were collected using 3 keV electrons at a primary current of 10 PA. The sample temperature was maintained at 330 K. After preparation of sulfur and/or carbon precovered surfaces, the sulfur and carbon surface concentrations were determined by calibrated Auger spectroscopy. Since the S(LMM) peak at 152 eV and the C(KLL) peak at 271 eV overlap with Ru(NMM) peaks at 150 and 273 eV, respectively, the intensities of the sulfur and carbon peaks were determined as follows: S(150) = total(150)

- Ru(231)

[Ru(150),,,,/Ru(231),,,,],

C(271) = total(271)

- Ru(231)

~Ru(273)=,~~/Ru(231)=~~~].

Surface coverages were obtained assuming a linear relationship between AES values collected from the clean surface and those collected at saturation coverages of sulfur (i.e., 8, = 0.5 [3]) and carbon (i.e., 6, = 1.1 [19]). Details of the sulfur precovered surface are presented below. The carbon saturated surface was prepared according to the procedures of Lauderback and Delgass 1193. 2.1.4. SIMS SIMS was performed using a differentially pumped scanning ion gun (Leybold-Heraeus) aligned at an angle of 45’ with respect to the surface normal and the ion analyzer. Secondary ions were collected and analyzed using four electrostatic lenses and a Balzers QH511 quadrupole mass filter. SIMS spectra were collected using 3 keV Ar+ ions at current densities of 1 nA/cm2. At these conditions, perturbations to the sample were minimal

130

R.A. Cocco, B.J. Tararchuk / Thiophene oclsorption and decomposition

on Ru(OtW1)

during the sampling time required for analysis [20-221. Primary energies of 3 keV were chosen since lower energies provide greater fragmentation of organic compounds [23,24]. All spectra were obtained at 95 K. 2.2. Ru(OOO1) specimen and cleaning procedures A Ru(OOO1) single crystal (5N purity) was purchased from Aesar. The crystal was mounted on a UHV Instruments manipulator and tilt stage. Temperature measurements were obtained from a chromel-alumel thermocouple located on the back of the crystal. A tantalum wire wrapped around the crystal provided sample temperatures above 1600 K while low temperature experiments, starting from 95 K, were performed using a liquid nitrogen reservoir located close to the crystal. The Ru(OOO1) single crystal surface was cleaned by numerous annealing cycles to 1500 K in lop5 Pa of oxygen while argon sputtering. The final annealing prior to analysis was performed at 1550 K in the absence of oxygen. The crystal was viewed as clean when the sum of all hydrocarbon peaks in the SIMS spectrum was less than 1% of the sum of the ruthenium peaks [24,25] and when the hydrogen TPD spectrum was in good agreement with the results reported by Feulner and Menzel 1261. 2.3. Specimen dosing and pretreatment

procedures

The addition of sulfur adatoms to the Ru(OOO1) surface was achieved by performing three consecutive cycles of 2 L H,S exposure at 95 K (1 L = 1 Langmuir unit = 1.33 X 10e4 Pa - s) and annealing to 600 K to remove hydrogen [27]. This procedure was sufficient to eliminate all hydrogen and CO desorption features and provide a saturated Ru(OOO1) surface containing 0.5 monolayers of sulfur [27,28]. The term “sulfur precovered Ru(OOO1)” applies to this surface. Thiophene was adsorbed on the clean and sulfur precovered Ru(OOO1) surfaces at 95 K unless specified otherwise. Thiophene (2 99.0%, Aesar) was purified using several freeze-thaw-evacuation cycles prior to dosing. Dosing was performed by introducing thiophene through a Vacuum Generators leak valve and a 0.6 cm ID diameter quartz tube 0.5 cm from the crystal surface. This apparatus produced a - lO-fold increase in thiophene exposure at the sample compared to the background flux. 2.4. Other chemical reagents Argon (99.995%), oxygen (99.95%), and hydrogen sulfide (99.5%) were purchased as prepurified grades from Matheson and used without further purification.

R.A. Cocco, B.J. Tatarchuk / Thiophene adsorption and decomposition

on Ru(0001)

131

3. Results 3.1. Desorption

of thiophene from clean Ru(OOO1)

Thermal desorption spectra for several exposures of thiophene on clean Ru(0001) are shown in fig. 1. At 1 L thiophene exposure, a multilayer and two other desorption peaks were observed at 145, 165 and 260 K, respectively. The features at 165 and 260 K are referred to as cr and B states. The cx state was larger than the /? state at all coverages and dominated the spectra at exposures exceeding 0.5 L. The a state shifted from 220 to 165 K where the onset of multilayer desorption became evident at 1 L exposure. The fi desorption state became noticeable at 0.3 L exposure and increased with exposure to 0.5 L with the peak temperature remaining stationary. Previous investigations [9,10] have shown similar features although the higher pumping speeds available in this study allow better observation of the p state. Hydrogen evolution collected simultaneously during thiophene desorption was observed at temperatures of 190, 220 and 530 K, as shown in fig. 2a. These results, which do not align with the intact thiophene desorption peaks, are indicative of decomposition states on the clean surface. For thiophene

I

140

220

300

Temperature

380

(K)

Fig. 1. Thiophene TPD at a linear heating rate of 10 K/s following the indicated thiophene exposures at 95 K on a clean Ru(OOO1)surface.

132

R.A. Cocco, B.J. Tutarfhuk / Thiophene adsorption und deeomp~sit~~n on Ru(~l~

(a) Clean

RufOOOl) 1.0

L

.g (b) Sulfur

I2

_-.P”-+. . : ...J

;

Precovered

Ru(OOOl),

-..-_.. - -:-,_-__-_ ‘_

;:..=

I

I

1

B

;-

,_

es

= 0.5

_. .-.

-

_'_

.-.w-.1.25

‘_

L

1.25 --__.._--_-~_-_~--_-

_---

'---~-~_~~-,z___"____

--.+w-

~-_i.~~~-_~~_.___._L,__--~~_-~_~~-~~.

,

x 10 L

0.5 L -_r_-__

0.1 L _ __-.__.___..._......_...____..“.,,,_,~,_ ..,._.~._ .-_-_-~_--.-~-.--._.----_.-----I 1 1 220

380

Temperature

540

(K)

Fig. 2. Hydrogen TPRS at a linear heating rate of 10 K/s following the indicated thiophene exposures at 95 K on (a) clean and (b) sulfur precovered Ru(OOO1).

exposures ranging from 0.1 to 1.0 L, the areas under the peaks appear to be relatively constant suggesting that hydrogen desorption is independent of thiophene exposures above - 0.1 L. Aside from the expected thiophene fragmentation pattern and the noted hydrogen evolution, no significant quantities of other desorption products were observed. 3.2. Desorption of thiophene from carbon and sulfur precovered Ru(OO01) Further evidence of thiophene cracking on the clean Ru(OOO1) surface can be found from successive 1.5 L thiophene exposures and desorptions without intermediate crystal cleaning. According to EELS results by Heise and Tatarchuk [lo], annealing to 700 K following thiophene exposure results in the dehydrogenation of retained hydrocarbon fragments to produce carbon-sulfur residuals. As shown in fig. 3, subsequent thiophene exposure on a previously dosed and annealed surface resulted in a diminution of the /3 state. The a: state also appeared to saturate at a 10 K higher temperature than that observed for the initially clean surface. A contribution to this apparent increase may be caused by the absence of the multilayer feature whose overlapping desorption feature would appear to shift the (Y peak to lower temperatures. The addition of the carbon-sulfur residual also decreased the thiophene sticking probability, as evidenced by the areas under the TPD spectra.

140

220

300

380

Temperaturtl (K}

3. ThiopheneTPD at a finearheating rate of 10 K/s folkwing 1.5 L chiopheneexposureat 95 R on (a) dean Ru(ooO1), and @I) carbm and &fur preadsorbedRawly following(a) above.

Fig+

(b)

Carbon and SUliuzr Precoverad Ru(OOOl)

140

220

300

380

Tcmpsraturo (I<) Fig. 4. TXophene TPR at a linear heating rate of 10 K/s following1 L exposure at 235 K an (a) clean Ru(OUO~) and (b) carbon and sulfurprawkwbed Ru(OOO1) following(a) above.

134

R.A. Cocco, B.J. Tatarchuk / Thiophene adsorption and decomposition

on Ru(0001)

AES measurements collected from the carbon and sulfur precovered surface found the carbon-to-sulfur ratio to be 3.91 : 1 and 4.13 : 1 for two different experiments. Since annealing temperatures of 700 K were well below temperatures reported for the onset of carbon diffusion into the bulk (i.e., 800 K [19]), this ratio of - 4 : 1 is representative of dehydrogenated/ decomposed thiophene products at the surface. To further evaluate the nature of the p desorption state, 1 L thiophene exposures onto clean and carbon-sulfur precovered surfaces were performed at 235 K and the resulting TPD spectra recorded in fig. 4. The clean surface still produced the /3desorption state, suggesting that thiophene associated with this state was independent of the LXor multilayer states, and was not the result of readsorption. Little evidence of the p state was observed from the carbon and sulfur precovered surface in agreement with the result shown in fig. 3b. 3.3. Desorption

of thiophene from sulfur precovered

Ru(0001)

The effects of - 0.5 monolayer of sulfur are significant as shown in fig. 5. Low thiophene exposure produced a small desorption feature at 180 K. Higher exposures shifted the desorption peak to 161 K and decreased the FWHM of

, 140

220

300

380

Temperature (K) Fig. 5. Thiophene TPD at a linear heating rate of 10 K/s following the indicated thiophene exposures at 95 K on sulfur-precovered Ru(OOO1)(8s = 0.50 ML).

the feature. At 1.25 L, the I61 K desorption peak had a FWHM of 12 K, substantially less than the 55 K FWHM observed from clean Ru(OOO1). Similar to thiophene on the clean surface, this spectrum displayed the onset of the multilayer at 145 K which is pronounced at 2 L exposures. It is interesting to note that the shapes of the multilayer and 161 K peaks in fig. 5 are similar. Hydrogen TFRS spectra collected in conjunction with thiophene exposures are shown in fig. 2b. Little hydrogen evolution was observed from sulfur precovered surfaces. The small peak at I45 K coincided with multilayer desorption and was assigned to ihiophene cracking in the mass spectrometer. Analysis of figs. 2b and 5 suggests that 0.5 monolayer of sulfur suppressed both the irreversible adsorption of thiophene and the /3 state observed on clean Ru(OO01). These results are in general agreement with earlier findings [9], where only limited decomposition of thiophene was reported at sulfur coverages as low as 0.1 monolayer. 3.4. Secondary ion emission from thiophene on clean

Ru(OOO1)

Fig. 6 shows the positive SIMS spectrum ‘and peak assignments for a 0.5 L exposure of thiophene onto clean ruthenium. The largest features observed were ruthenium isotope ions consisting of a series of 7 peaks with the largest

136

R.A. Coeco, B.J. Tatarchrrk / Thiophene adsorption and decomposition on Rustic

1000 i

Fig. 7. Positive SIMS spectrum of 0.5 L thiophene exposure at 95 K on clean Ru(OOOl), annealed to 300 K and cooled to 95 K.

peak located at m/z 102 and a ruthenium-thiophene adduct having a similar isotope pattern with the largest feature at m/z 186. Ruthenium-thiophene adducts with masses higher than m/z 186 were not observed. Ions in the vicinity of m/z 134 and 160 are the result of ruthenium-sulfur adducts and fragmented ~the~um-t~ophene adducts, respectively. Small RuC+ peaks resembling the ~the~urn ion fra~entation pattern were also observed at m/z 114. Other features detected were a thiophene peak at m/z 84, a small protonated thiophene peak (i.e., C,,H,S+) at m/s 85 and ion peaks at m/z 14. 26, 27, 28, 32, 39, 45, 57, 58, 69 and 71. While the small peak at m/z 32 corresponds to S+, the other peaks correspond to different chain lengths of hydrocarbon ion fragments. Annealing the thiophene preadsorbed surface to 300 K and cooling to 95 K removed the ruthenium-thiophene adduct at m/z 186, as shown in fig. 7. The small thiophene peak at m/z 84 suggests that some thiophene remained on the surface or was readsorbed after annealing. Ions observed at m/z 26, 27, 32, 39, 45, 57 and the array located around m/z 134 decreased after annealing while ion features at m/z 14, 28, and 58 disappeared completely, Annealing

R.A. Cocco. B.J. Tatarchuk / Thiophene ndrorption and decomposition

on Ru(OOO1)

137

also produced increases in the Ru+, Rul and RuC+ intensities and provided new peaks at m/z 12, 13, 50 and 216. The new features observed at m/z 50 and 216 correspond to C,Hz and Ru&*, respectively, while peaks at m/z 12 and 13 may be dehydrogenated products associated with the CH: fragment observed prior to annealing. Ions at m/z 13 and 50 were observed up to annealing temperatures of - 600 and - 500 K, respectively. Features corresponding to C+ and RuC+ were observed following annealing to 700 K with little evidence for any hydrogen-containing species at or above this temperature. Similar results to those noted above were also found in the negative SIMS spectrum. Fig. 8a provides the negative ion emission following a 0.5 L thiophene exposure at 95 K where ion peaks at m/z 12, 13, 16, 17, 24, 25,26,

(0) 0

s-

I ”

10

”

I ”

15

”

I

20

““I””

k I

25

30

”

1

35

mass/charge

100

(b) 5; 4

-

80 :

I”]

10

‘I”“11 15

111 20

I[ 25

III11

111

30

I 35

mass/charge

Fig. 8. Negative SIMS spectra of 0.5 L thiophene exposure on clean Ru(OOO1)(a) at 95 K, and (b) annealed to 300 K and cooled to 95 K.

R.A. Cocco, B.J. Totorchuk / Thiophene odrorption and decomposition on Ru(OOO1)

138

and 32 are observed. Peaks at m/z 16 and 17 correspond on O- and OH- ions and are due to the high sensitivity of SIMS to surface oxygen. The species represent trace impurities at levels below the detectability limits of positive SIMS [19,29]. Peaks observed at m/z 12, 13, 24, 25, and 26 are due to C-, CH-, Cc, C,H-, and C,H, ions. Emission at m/z 32 is the result of sulfur ions. The negative SIMS spectrum obtained after annealing to 300 K and subsequent cooling to 95 K is shown in fig. 8b. This procedure caused a diminution in all features with peaks at 13 and m/z 26 no longer being detected, possibly as a result of dehydrogenation. After annealing to 600 K, only CT, C; and S- emissions were observed. 3.5. Secondary

ion emission from thiophene on sulfur precovered

Ru(0001)

The addition of 0.5 monolayer of sulfur produced SIMS features corresponding to S+, RuS+ and Ru,S+ as shown in fig. 9. A subsequent 0.5 L thiophene exposure is illustrated in fig. 10, and, as observed for thiophene on clean Ru(OOOl), produced a ruthenium-thiophene fragmentation pattern as well as other peaks at m/z 32, 39, 45, 58, 84, 85, 134, and 160. There was little

1500

1

II 0

“I’ 20

1 ‘II 40

1’

I”’ 60

II 80

“I’

1 ’ 100

II 120

‘1

II 140

11

II 160

‘1,

11 180

1 ,I 200

11

I( 720

11

,I 240

1

mass/charge

Fig. 9. Positive SIMS spectrum of 0.5 monolayers of sulfur on Ru(OOO1)at 95 K (0s = 0.50 ML).

139

Fig.

10.

Positive

SIMS spectrum of 0.5 L thiophene exposure 95 K (a, = 0.50 ML).

~~‘I”~I”‘l”‘I”‘I”‘l’~‘l”‘I~“l”‘I”’I

0

20

40

60

80

100

120

140

on s&fur precowred

160

180

200

Ru(OOO1) at

220---XT

moss/charge Fig.

11.

Positive

SIMS spectrum of 0.5 L thiophene exposure at 95 K on sulfur prewvered Ru(OOiX), annealed to 300 K and co&d to 95 K ( ff, = 0.50 ML).

evidence, however, of ions at m/z 14, 26, 27, 28, 50, 57 or the RUG* ion at m/z 114. The reversibl@ Lna&Ee of ~~~~~ ~so~~~~ on the StaIfus ~r~~~~~e~ szrrface is d~~~s~r~~~ &I fig_ 11 T&i& shows &e positive SflW$ SC&%X folluwin~ annealing k3 300 K and cooIin~ to 95 R_ Unlike thiophene on dean ruthenium, the addition of 0.5 monolayer of sulfur suppressed thiopbcxlr~ decomposition as evidenced by the absence af hydrocarbon peaks follawimg this proce&~e. Fig. 11 is simi.$ar to fig. 9 for the sulfur precovered su&ke prior tr, ~~~~~ene eq~ure d~o~s~~~~~~ that tiophene adso~~~~ OXI sulfur preeoyered ~u~~~~ is reversible. Similar resufts are observed by negative SIMS in fig. 12 where far O-5 L thiophene Qxposure at 95 K, hydrocarbon fragments were observed at rniz 13

R.A. Cocco, 3.J. Tafarchuk/ Thiopheneadrorprionand decompositionon Radio

141

and 25 but were absent after annealing. As noted earlier, peaks observed at m/z 16 and 32 correspond to O- and S- and were detected both before and after annealing.

4. Discussion 4.1. Decomposition

of thiophene at 95 K

SIMS results demonstrate that irreversible thiophene adsorption and decomposition occurs at 95 K on clean Ru(OOO1). The observations of ions at m/z 14 (CH;), 26 (C,H;), 27 (C,H;), 28 (C,H;), 57 (C,HS+) and 114 (Rue+) in fig. 6 are indicative of thiophene decomposition and were not observed for thiophene on the sulfur precovered surface, shown in fig. 10, where reversible adsorption occurs. Studies by Lauderback and Delgass [29] have used the RuC+ emission as a diagnostic indicator for ethylene decomposition on Ru(0001). Ethylene decomposition was also shown to generate negative ion species in the form of C-, CH-, CIH- and C,H; [19,29]. Similar negative ions are observed in fig. 8 indicating that decomposition takes place on the clean surface at 95 K, although, as shown in fig. 12a, at least a portion of the CH- and C,Hfragments are produced by sputtering. While the above-noted ions suggest that thiophene decomposition may be restricted to C, and G, hy~ocarbons at 95 K, larger hydr~~bon fragments which are multiply-bonds to the surface may have a lower probability of intact ion emission. Ions at m/z 39 (C,HT), 45 (CHS+), 58 (C2H2S+), 69 (C,HS’), 71 (C,H$+), 134 (RuS+) and 160 (RuC,H,S+) are observed on both surfaces. These features must be the result of sputter-induced fragmentation (i.e., unimolecular dissociation in the gas phase, fragmentation within the selvedge or dissociation caused by primary impact). TPRS results (fig. 2b) concur that decomposition products are not formed on the sulfur precovered surface. 4.2. Thiophene decomposition

above 95 K

4.2.1. Dehyd~o~e~~tion Further evidence for thiophene decomposition on the clean surface is found in the hydrogen TPRS results in fig. 2a. Hydrogen evolution at - 190-220 K suggests that thiophene or thiophene decomposition products undergo partial dehydrogenation with further dehydrogenation occurring at temperatures to 530 K. SIMS results support these findings as illustrated in figs. 6-8. After annealing to 300 K, the CHZ emission is replaced by CH+ and C+ features while no C,H: ions are observed. Other peaks attributed to thiophene decomposition products such as C, H,+ , C,H: and C*HS+ are also observed

142

R.A. Cacco, B.J. Tatarckak j Thiophene adsorption and decotnposihz

on Ru~~~l~

following annealing, demonstrating that hydrocarbon and hydrocarbon-sulfur species are retained at the surface despite the near complete removal/ ~ns~mption of intact thiophene. The negative SIMS spectrum obtained after annealing to 300 K (fig. 8b) demonstrates that some dehydrogenation has taken place as indicated by the disappearance of the CH- and C,H; species and the decrease in the C,H intensity. Higher annealing temperatures to 700 K provide complete dehydrogenation with no hydrocarbon ion features being observed by positive or negative SIMS. These results are consistent with other studies of thiophene on Ru(~l) f9]* Mo(lOO) 114-161 and Mo(ll0) fl7] where varying degrees of dehydrogenation were observed after annealing. Ions at CH- and C,H- are not restricted to decomposition products alone but can also be produced by sputter-induced fragmentation since these species are observed in fig. 12a for thiophene on sulfur precovered Ru(OOO1) where no surface decomposition takes place. Annealing studies on the clean surface, however, confirm that the C,H- ion in fig. 8b is at least partially due to decomposition, since C, H- is observed from thiophene preadsorbed Ru(0001) following annealing to 300 K even though intact thiophene has been removed. It appears that both surface decomposition and stutter-induced fragmentation are responsible for the C,H- ion. Ions at m/z 12,24 and 26, however, are the sole result of d~mposit~on~ It is interesting to note that the areas under the hydrogen desorption features in fig. 2a are relatively similar despite increasing thiophene coverage. Thiophene decomposition appears to be limited to initial exposures with the decomposition products passivating the surface toward further thiophene cracking. Similar findings have been reported by Heise and Tatarchuk [9] where EELS and TPD studies found that the extent of thiophene decomposition was a function of coverage. 4.2.2. Formation of adsorbed C, internwdiates Annealing the thiophene dosed surface to 300 K produced a new ion at n?/z 50 (C,H,‘) as shown in fig. 7, The relatively weak intensity of this feature may be the result of either a low surface concentration or a difficult ion ejection process due to structural and steric considerations which may involve more than one surface metal atom. C, intermediates have been reported for thiophene on Ru(0001) [9] and other metal surfaces [11-141. The appearance of C,H, in fig. 7 after annealing at 300 K supports the findings of Heise and Tatarchuk 191, where according to EELS results, a “metallocycle-like” species developed at 270 K with cY-dehydrogenation preceding fl-dehydrogenation. The data in fig. 7 suggest that heating to 300 K results in the formation of a C,H, intermediate with subsequent and/or simultaneous formation of smaller hydrocarbon fragments including C,H, and C,HS. Heating to higher temper-

atures causes dehydrogenation of these hydrocarbon and hydrocarbon-sulfur species to produce a carbon-sulfur surface residual. AES measurements show that the carbon-to-sulfur ratio is - 4: 1 at the surface which is in goad agreement with the desorption results of fig. 2 where only hydrogen was detected leaving the surface. The formation of C, hydrocarbon intermediates is also in good accord with kinetic studies by Kuo and Tatarchuk [2] for thiophene HDS over supported Ru/A1203 catalysts were only C, desulfurization products were detected. 4.3. Thiophe

desorption

The cy desorption peak moved from a temperature of 220 K at low exposures to 165 K at exposures of 1 L. The (Y state was observed at all coverages investigated and only exhibited saturation at the onset of the multilayer. This type of behavior can be attributed to either second order desorption kinetics or to a first order process with a coverage-dependent binding energy [16]. Sexton [30] observed similar behavior for the reversible adsorption of thiophene on Cu(100) where he proposed a “compressional phase change” in which thiophene was perpendicular to the surface (u-bonded) at high coverage and parallel (q-bonded) at low coverage. Similar results were also observed by Stohr et al. 1111 where EELS features suggestive of v-bonded thiophene were observed on Pt(ll1) after annealing to 235 K. At low thiophene coverages on clean Ru(~l~, ample sites are available for parallel orientation, and low thiophene coverages have been reported to adsorb parallel to the surface of most metal substrates previously studied [11,13-171. Higher coverages may result in an increase in adsorbate-adsorbate interactions forcing thiophene to become inclined or u-bonded to the surface resulting in a more weakly bound state and a lower desorption temperature. This explanation is consistent with the behavior of the cx state shown in fig. 1. 4.3.2. B desorption state At all exposures 2 0.3 L, coexisting 01 and fi states were observed. This observation is inconsistent with the j3 state being the result of a specific high binding energy site or preferred orientation (i.e., T-bonding) on the clean surface. Such sites would be expected to dominate TFD spectra at low wverages and were not observed. A TPD feature similar to the j3 state has been observed by Gellmen et al. [16] and Kelly et al. [31] for thiophene desorption from Mo(100) where a higher temP~rat~e desorption state was observed at 360 K only after significant thiophene exposure. Kelly and co-workers attributed this feature to carbon contamination on the surface. A similar mechanism may be involved with the p desorption state since SIMS and TPRS results demonstrate that

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R.A. Cocco, B.J. Taiarchuk / Thiophene aa5orption and decomposition on Ru(0001)

thiophene decomposes to leave hydrogen, hydrocarbon and sulfur-containing species on the surface. Thus, low levels of decomposition products may stabilize the j3 desorption state in a fashion similar to that proposed by Kelly et al. [31]. Thiophene exposure in excess of 0.5 L, however, may inhibit the development of the /3 state as a result of surface passivation caused when excessive levels of hydrocarbon fragments limit accessibility to the metal surface. Higher levels of decomposition-dehydrogenation products, such as those formed after annealing multilayer thiophene coverages to 700 K, eliminated the j3 desorption state as shown in figs. 3 and 4. 4.3.3. Explosive desorption Thiophene desorption from sulfur precovered Ru(OOO1) yields two TPD peaks at 145 and 161 K after high thiophene exposures. As for thiophene on clean ruthenium, the peak at 145 K is the result of multilayer desorption. The peak at 161 K results from desorption of the adsorbed surface layer. This desorption feature exhibits an unusually small FWHM of 12 K compared to the 55 K FWHM of the 165 K peak observed from clean Ru(OOO1). Little difference in peak shape can be detected between the 161 K peak and the multilayer peak. This similarity suggests that the behavior of a monolayer of thiophene on the sulfur precovered surface is similar to the interaction within the multilayer and characterized by relatively strong adsorbate-adsorbate interactions and relatively weak interactions with the substrate. Similar peak shapes have been reported for oxygen on Pt(100) [32], formic acid on nickel [33-351 and CO and NO on platinum [36,37]. These features have been attributed to explosive or autocatalytic desorption. Thus, a possible explanation for the desorption feature observed at 161 K in fig. 5 may be the results of “explosive” desorption kinetics. Lesley and Schmidt [36] have attributed explosive desorption kinetics to mechanisms involving (i) substrate phase transitions, (ii) vacant site requirements for reaction, (iii) formation of intermediate species which accelerate the reaction, or (iv) adsorbate phase transitions and/or island formation. The first three mechanisms are clearly inconsistent with our results since (a) Ru(OOO1) phase transitions have not been reported in other studies [27,28,38], and (b) TPRS and SIMS studies demonstrate the passive nature of the sulfur precovered surface. Hence, the sharp desorption feature may be the result of island formation and/or strong lateral interactions. Such interactions become significant when the surface is passivated by sulfur.

5. Summary /conclusions (1) TPD spectra for 1 L thiophene exposure on clean Ru(OOO1) exhibit a multilayer desorption feature at 145 K, an a desorption state at 165 K, and a

R.A. Cocco, B.J. Tatarchuk / Thiophene adsorption and decomposition

on Ru(OOO1)

145

/3 desorption state at 260 K. Hydrogen TPRS spectra demonstrate that thiophene decomposition on the clean surface saturates at - 0.1 L, leaving the surface available for subsequent intact adsorption. (2) TPD spectra for 1.0 L thiophene exposures on Ru(OO01) containing 0.5 monolayer of preadsorbed sulfur show a multilayer desorption peak at 145 K and a narrow desorption feature at 161 K. The peak at 161 K is consistent with explosive desorption kinetics and suggests that the thiophene overlayer involves strong lateral interactions and/or island formation. No other desorption products, including hydrogen, were detected indicating a reversible adsorption process. (3) SIMS results provide further confirmation for decomposition on the clean surface as indicated by spectral features corresponding to RuC+, C2HS+, C,H:, C,H;, CH; and CH; , which were not observed for thiophene on the sulfur precovered surface. Ions corresponding to RuC,H,S+, RuS+, C,H,S+, C3HS+, C2H2S+, CHS+ and C,H: were observed on both surfaces and are the result of sputter-induced fragmentation. (4) Annealing thiophene preadsorbed Ru(OOO1) to 300 K, results in the partial dehydrogenation of C,H:, C,H;, CH: and CH- ions and produces an ion at m/z 50 corresponding to a bound C, intermediate. Further annealing to 700 K produces a SIMS spectrum deficient of hydrogen-containing features. (5) For thiophene on sulfur precovered Ru(OOO1), annealing to 300 K produces a SIMS spectrum free of hydrocarbon or hydrocarbon-sulfur residuals demonstrating that thiophene adsorption on the sulfur precovered surface is reversible.

Acknowledgments Support is acknowledged from the Strategic Defense Initiative, Office of Innovative Science and Technology, through a contract from the Naval Surface Warfare Center (N60921-86-CA226). Some of the equipment used in this study was provided earlier through an equipment grant from the Army Research Office and the Air Force Office of Scientific Research (DOD-URIP, DAAG28-84-G-0057). One of us, R.A. Cocco, also wishes to acknowledge partial fellowship support from the Tennessee Valley Chapter of the American Vacuum Society during a portion of this work.

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