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Adsorption of hydrogen sulphide over rhodium/silica and rhodium/alumina at 293 and 873K, with co-adsorption of carbon monoxide and hydrogen

Adsorption of hydrogen sulphide over rhodium/silica and rhodium/alumina at 293 and 873K, with... Int J Ind Chem (2017) 8:235–240 DOI 10.1007/s40090-017-0124-1 RESEARCH Adsorption of hydrogen sulphide over rhodium/silica and rhodium/alumina at 293 and 873 K, with co-adsorption of carbon monoxide and hydrogen 1 2 2 1 • • • Claire Gillan Martin Fowles Sam French S. David Jackson Received: 17 November 2015 / Accepted: 24 May 2017 / Published online: 27 May 2017 The Author(s) 2017. This article is an open access publication Abstract In this study, we have examined the adsorption Introduction of hydrogen sulphide and carbon monoxide over rhodium/ silica and rhodium/alumina catalysts. Adsorption of Catalyst poisoning is the strong chemisorption of a species hydrogen sulphide was measured at 293 and 873 K and at on a site otherwise available for catalysis. Whether a spe- 873 K in a 1:1 ratio with hydrogen. At 293 K, over Rh/ cies is a poison depends upon its adsorption strength rel- silica, hydrogen sulphide adsorption capacity was similar ative to other species competing for active sites. For many to that of carbon monoxide; however, over Rh/alumina, the reactions, such as methanation, methanol synthesis and carbon monoxide adsorption capacity was higher, probably Fischer–Tropsch synthesis, over group 8–11 metal cata- due to the formation of Rh (CO) . Over Rh/silica, the pri- lysts, sulphur is a known poison [1]. To be able to interpret mary adsorbed state was HS(ads), in contrast to Rh/alu- quantitatively the extent and nature of poisoning by sul- mina, where the H :S ratio was 1:1 indicating that the phur, it is essential to know the structure and bonding of adsorbed state was S(ads). Competitive adsorption between sulphur to metal atoms at the surface. There are two types CO and H S over Rh/silica and Rh/alumina revealed of sulphides that form on a catalyst, 2-D surface sulphides adsorption sites on the metal that only adsorbed carbon and 3-D bulk sulphides, the formation of which requires the monoxide, only adsorbed hydrogen sulphide or could metal cation to diffuse through the adsorbed sulphide layer adsorb both species. At 873 K, hydrogen sulphide [2] forming a new metal sulphide layer on the outer sur- adsorption produced the bulk sulphide Rh S ; however, face. This phenomenon of segregation is strongly 2 3 when a 1:1 H :H S mixture was used formation of the bulk exothermic and is therefore favoured by a reduction in 2 2 sulphide was inhibited and a reduced amount of hydrogen temperature. Surface sulphide formation is simply the sulphide was adsorbed. adsorption of sulphur on the surface of the metal. Pt, Ni, Ru and Rh all have lower free energies of formation of their bulk sulphides than their surface sulphides, hence, large Keywords Rhodium  Hydrogen sulphide  Carbon monoxide  Adsorption hydrogen sulphide concentrations are required for stable bulk sulphides to exist. It has also been inferred that an SH surface species is present as an intermediate in the dissociation of hydrogen sulphide. For example, over Pt/Al O , it was observed that 2 3 at increasing sulphur coverages, dissociated hydrogen is & S. David Jackson gradually desorbed but that a percentage spends a signifi- david.jackson@glasgow.ac.uk cant lifetime on the catalyst [3], and can participate in reactions. Also, on Pt/alumina two types of adsorbed Centre for Catalysis Research, WestCHEM, School of hydrogen sulphides were detected, different due to Chemistry, University of Glasgow, Glasgow G12 8QQ, Scotland, UK strengths of adsorption and three different adsorption sites. These include: a site which bonds sulphur strongly and will Johnson Matthey Plc, Belasis Ave, Billingham TS23 1LB, not exchange, a site which bonds sulphur weakly and is UK 123 236 Int J Ind Chem (2017) 8:235–240 removed under vacuum and a site which will allow the adsorption of H S and its displacement by CO. This exchange between gas and adsorbed phases. These were caused desorption of residual hydrogen from the reduction determined from radioactive labelling experiments [4], in procedure, possibly by surface reconstruction, which had which it was also found that the S:Pt ratio was 1:1 on been found to have a deleterious effect on CO adsorption [6]. (surface) Pt/SiO but only 0.6:1 on Pt/Al O . There has been limited A similar study examining the interaction of CO and 2 2 3 research on the adsorption of sulphur species on Rh cata- H S over Rh/silica catalysts was carried out [7], but unlike lysts; however, some work has been conducted on Rh Pt, it was found that CO could adsorb onto samples that had single crystal faces. Hedge and White [5] studied the been saturated with sulphur. Displacement of H S was also chemisorption and decomposition of H S on Rh(100) at evident, but this was dependent on the metal precursor 100 K using auger electron spectroscopy (AES) and used. It was only found to occur on an oxide-derived cat- obtained results suggesting a saturation coverage near 0.5 alyst, and since the desorption of sulphur requires hydro- monolayer. On heating to 600 K, the sulphur coverage gen, it was proposed that H S only partially dissociates on increased, which the authors inferred was due to physi- the oxide catalyst to produce an HS-* species, which would sorbed H S, which is consistent with results for H S provide a source of hydrogen to allow for desorption [7]. 2 2 adsorption on Pt and Ni. The thermal desorption spectra of The effect of passing H S over CO pre-covered surfaces molecular H S from Rh(100) exhibit low- and high-tem- was the displacement of CO and the adsorption of H S, i.e. 2 2 perature peaks, hence, Hedge and White [5] assigned the similar to Pt/alumina. It was speculated that the CO dis- low temperature peak as physisorbed H S. It was also placed, reflected the different modes of adsorbed CO, and found that a decreasing fraction of H S dissociated as the this was also found to be dependent on the metal precursor. coverage of H S increased [5]. The similarities between For example, a chloride-derived catalyst appeared to dis- H S adsorption on Rh(100) and Pt(111), Ru(110) and place bridge-bonded Rh –CO [7]. 2 2 Ni(100) were noted. In all these cases, there is complete This work follows on from a study over Pt/alumina and Pt/ dissociative adsorption at low temperatures and low cov- silica where the adsorption of hydrogen sulphide was exam- erages with hydrogen remaining on the surface. At low ined in relation to its effect on steam reforming [8]. In this temperatures and higher coverages on Pt(111), Ru(110) study, the adsorption of hydrogen sulphide and methanethiol and Ni(100), first SH and then H S were observed. over Rh/silica and Rh/alumina at 293 K will be examined and In catalytic systems, the sulphur compound is not there in the amount compared with that found from carbon monoxide isolation, rather it is there coincidentally with the reactants. chemisorption. In this way, the total amount of hydrogen However, studies where competitive adsorption has been sulphide adsorption can be measured and related to the examined are rare and over rhodium rarer still. The inter- amount of surface rhodium. Competitive adsorption of carbon action of CO and H S over supported Pt catalysts was studied monoxide and hydrogen sulphide will be examined as it is in detail by Jackson et al. [6], who found that when H S was rare that a poison is present in the absence of another species pre-adsorbed on Pt/silica no subsequent CO adsorption was (reactant or product). By comparing the competitive adsorp- detected due to the adsorption of H S being dissociative, so tion, an assessment can be made as to the relative strength of there was no mechanism by which sulphur could desorb, and adsorption between carbon monoxide and hydrogen sulphide. hence no sites can be liberated for CO adsorption. When CO The extent hydrogen sulphide may adsorb in the presence of was pre-adsorbed on Pt/silica, the amount of H S adsorbed carbon monoxide can give an insight into sulphur poisoning was reduced by 81% in comparison to a fresh surface, though of carbon monoxide hydrogenation over rhodium. The it was suggested that 20% of the H S was able to adsorb onto adsorption of hydrogen sulphide will be examined at high the silica support, indicating that CO had completely sup- temperature (873 K) in the absence and presence of hydro- pressed H S adsorption on the Pt sites. However, when the gen, to understand whether hydrogen is effective at reducing same experiment was carried out over Pt/alumina [6], there the amount of sulphur adsorbed. Le Chatelier’s principle was no reduction in adsorptive capacity for H SonaCO indicates that the amount of sulphur adsorbed should be saturated surface indicating that CO did not block H S reduced; however this will be the first experimental verifi- adsorption on the metal sites, and therefore must be related to cation. This will inform our understanding of sulphur poi- the effect of the support. It has previously been reported that soning in systems such as steam reforming. CO is produced from the reaction of adsorbed CO with hydroxyl groups from the alumina support [6], therefore, it was proposed that CO may be able to desorb via this route Experimental liberating sites for H S adsorption. When CO and H S were 2 2 co-fed over Pt/silica the amount of H S adsorbed decreased Two catalysts were prepared on alumina and silica, 1.2% by 78%, whereas the amount of CO adsorbed increased by w/w Rh/alumina and 1% Rh/silica. Both catalysts were 67%. The enhancement in CO adsorption was explained by prepared by incipient wetness of the two supports (h- 123 Int J Ind Chem (2017) 8:235–240 237 2 -1 alumina, surface area 101 m g ; silica, surface area were typically repeated three times and the values reported 2 -1 220 m g ) using Rh(NO ) hydrate (Aldrich) as the are the average. Standard deviation on the amount adsorbed 3 3 precursor salt. The precursor salt was dissolved in a volume was typically less than ±8%. Adsorptions were followed of distilled water equal to the support pore volume using a gas chromatograph fitted with a thermal conduc- 3 -1 3 -1 (0.6 cm g for alumina and 1 cm g for silica) using tivity detector and Molecular Sieve 5A and Porapak Q 100 g of support. The catalysts were dried and calcined at columns. 773 K for 4 h. The rhodium weight loading was confirmed Both the helium (BOC, 99.997%) and the 5% hydrogen by atomic adsorption. in dinitrogen (BOC) were further purified by passing Chemisorption studies were performed in a dynamic through Chrompack Gas-Clean Oxygen filter to remove mode using a pulse-flow microreactor system in which the any oxygen impurity, and a bed of Chrompack Gas-Clean catalyst sample was placed on a sintered glass disc in a Moisture filter to remove any water impurity. Carbon vertical tube (8 mm id, down flow) inside a furnace monoxide (99.99% Research Grade) and hydrogen sul- (Fig. 1). phide ([99%) were used as received. The reactant pulses were introduced into the gas stream immediately before the catalyst bed using a fixed volume sample loop. Using this system, the catalysts (typically Results 0.50 g) were reduced in situ in a flow of hydrogen 3 -1 (40 cm min ) by heating to 673 K at a rate of Carbon monoxide and hydrogen sulphide adsorption was -1 10 K min . The catalyst was held at this temperature for examined over the high weight-loading rhodium catalysts. 2 h. The catalyst was then purged with argon (40 cm - As describedinthe ‘‘Experimental’’ section, multiple -1 min ) for 30 min and the catalyst was cooled in flowing pulses of each gas were passed over the catalysts until no argon to 293 K. The adsorbate gases were admitted by further adsorption was detected. Using this methodology, injecting pulses of known size (typically 24 lmol) into the the pressure of the pulse is always 1 bar and only strongly argon carrier-gas stream, and hence onto the catalyst. The bound species are detected. As expected, no adsorption of residence time of the pulse in the catalyst bed was *1.5 s. carbon monoxide was detected on the alumina or silica In all cases, the whole pulse was analysed by on-line GC. supports in the absence of the metal component. Carbon -1 For co-adsorption studies, the gases were mixed in the gas monoxide adsorption gave 143.8 lmol g for Rh/alu- -1 manifold prior to injection into the carrier gas. The amount mina and 65.1 lmol g for Rh/silica (this translates to of gas adsorbed, from any pulse, was determined from the metal dispersions of 123% for Rh/alumina and 67% for difference between the peak area of a calibration pulse sent Rh/silica assuming a Rh:CO ratio of 1:1). The silica directly to the GC from the sample volume, and the peak support did not adsorb hydrogen sulphide, but hydrogen area obtained following the injection of pulses of compa- sulphide did adsorb on the alumina, hence, the adsorption rable size onto the catalyst. The detection limit for data for the Rh/alumina catalyst has had the support -1 adsorption was 0.3 lmol g . Adsorption measurements contribution subtracted from the total adsorption. The hydrogen sulphide adsorption data are reported in Table 1. TC The adsorption of hydrogen sulphide and carbon GM monoxide was also studied by co-adsorption, carbon monoxide pre-adsorbed before hydrogen sulphide and vice Vent versa. The results are shown in Table 2. For co-adsorption, pulses of the mixed gases at a 1:1 ratio were passed over SV F R the catalyst. For the sequential adsorptions, one gas was GC Table 1 Hydrogen sulphide adsorption at 293 K a b c Catalyst H S adsorbed H :S(ads) Dispersion (%) 2 2 Carrier gas inlet FC Rh/alumina 112.7 1.0:1 97 Rh/silica 61.2 0.7:1 63 a -1 lmol g Fig. 1 Schematic of pulse-flow microreactor. GM gas manifold, SV H evolved during adsorption relative to H S adsorbed 2 2 standard volume, F furnace, R reactor, TC temperature controller, FC Calculated on the basis of the hydrogen sulphide adsorption, flow controller, GC gas chromatograph with a thermal conductivity assuming a 1:1 S:Rh detector 123 238 Int J Ind Chem (2017) 8:235–240 Table 2 Sequential and co-adsorption of carbon monoxide and Discussion -1 hydrogen sulphide at 293 K (lmol g ) The adsorption of hydrogen sulphide on rhodium has only Rh/alumina been studied sparingly [5, 7, 9, 10]. Nevertheless, there is H S pre-adsorbed CO adsorbed after H S adsorption 2 2 good agreement about what is expected from hydrogen 112.7 14.7 sulphide adsorption at room temperature over highly dis- CO pre-adsorbed H S adsorbed after CO adsorption H :S(ads) 2 2 persed Rh/silica catalysts. Our value of 0.6:1 S:Rh is typ- 143.8 25.0 0.3:1 ical for sulphur adsorption on Rh/silica [7, 9], as is the Co-adsorption CO adsorbed H S adsorbed H :S(ads) 2 2 *1:1 correspondence between hydrogen sulphide adsorp- 64.9 30.1 0.3:1 tion and carbon monoxide adsorption. Carbon monoxide Rh/silica adsorption over rhodium can be described by three adsor- H S pre-adsorbed CO adsorbed after H S adsorption 2 2 bed states, linear Rh–CO, bridge-bonded Rh –CO, and 61.2 7.7 gem-dicarbonyl Rh (CO) . The ratio of CO:Rh obtained CO pre-adsorbed H S adsorbed after CO adsorption H :S(ads) 2 2 with the silica catalyst suggests principally bridged and 65.1 9.0 0.6:1 linear sites, which implies a similar bonding model for Co-adsorption CO adsorbed H S adsorbed H :S(ads) 2 2 sulphur. This is in accordance with the work of Sachtler 21.3 17.4 0.7:1 et al. [10]. Over Rh/alumina, a S:Rh ratio of *1:1 is H evolved during adsorption relative to H S adsorbed 2 2 obtained, which is higher than that observed with the silica catalyst. This is the opposite of what is found with carbon monoxide adsorption and hydrogen sulphide adsorption over Pt/alumina, where the alumina-supported catalyst Table 3 Hydrogen sulphide adsorption at 873 K gave a lower ratio [8]. The greater than 1:1 CO:Rh ratio a b c Catalyst H S adsorbed H :S(ads) Rh:S(ads) 2 2 was expected over the alumina supported catalyst, as it is Rh/alumina 166.5 0.8:1 1:1.4 known that the gem-dicarbonyl Rh (CO) species can be formed leading to a greater than 1:1 ratio. Interestingly, the Rh/silica 152.5 1.0:1 1:1.5 hydrogen sulphide adsorption follows the same trend as the a -1 lmol g carbon monoxide adsorption. It is normally assumed that Ratio of hydrogen evolved to sulphur adsorbed sulphur occupies a multiply bonded site such as a threefold Ratio of Rh to sulphur adsorbed hollow [10] on a Rh(111) face, but the commonality in adsorption ratios between carbon monoxide and hydrogen sulphide suggests that on highly dispersed supported metals Table 4 Hydrogen sulphide adsorption from a 1:1 H S:H mixture at 2 2 there may be a number of adsorption modes. 873 K Further information on the mode of hydrogen sulphide a b c Catalyst H S adsorbed H :S(ads) Rh:S(ads) 2 2 adsorption can be obtained from the H :S ratio. The H :S 2 2 ratio over the silica catalyst was 0.7:1, this implies that two Rh/alumina 90.4 0.7:1 1:0.8 adsorbed states are present, S(ads) and HS(ads), and we can Rh/silica 98.3 0.8:1 1:1.0 obtain a ratio of these adsorbed states from a stoichiometric a -1 lmol g equation: Ratio of hydrogen evolved to sulphur adsorbed H S ! 0:4 SðÞ ads þ 0:6 HSðÞ ads þ 0:7H 2 2 Ratio of Rh to sulphur adsorbed Therefore, over Rh/silica, the primary adsorbed state is HS(ads), which is not the case for Rh/alumina, where the H :S ratio is 1:1 indicating that the only adsorbed state is adsorbed to saturation (so if CO was pre-adsorbed the S(ads) (Tables 1, 2). This is different from the Pt/alumina catalyst would be saturated with CO) before the second gas and Pt/silica systems where both catalysts had a H :S ratio was passed over the catalyst. of *0.7:1 [8], indicating that dissociation is enhanced over The adsorption of hydrogen sulphide was also studied at Rh/alumina. 873 K in the absence and presence of hydrogen and the When carbon monoxide was pre-adsorbed over the Rh/ results are shown in Tables 3 and 4, respectively. When the silica at 293 K and then hydrogen sulphide adsorbed over hydrogen sulphide was adsorbed in the presence of the same catalyst, the amount of hydrogen sulphide hydrogen, the ratio of the mixture was 1:1. The amount of adsorbed was reduced by 85%. There was no evidence of hydrogen sulphide adsorbed on the alumina at 873 K has been subtracted from the Rh/alumina adsorption in carbon monoxide being displaced suggesting that the hydrogen sulphide was accessing sites that were Tables 3 and 4. No adsorption took place on the silica. 123 Int J Ind Chem (2017) 8:235–240 239 unavailable to carbon monoxide. The extent of hydrogen alumina and 1.5:1 for Rh/silica. This suggests the forma- evolution indicates that the mode of hydrogen sulphide tion of the bulk sulphide, Rh S . When hydrogen was co- 2 3 adsorption had changed, such that 80% of the adsorbed fed with hydrogen sulphide at 873 K (Table 4), the amount sulphur was in the form HS(ads). When hydrogen sulphide of sulphur adsorbed decreased significantly (S:Rh * 1:1) was pre-adsorbed and then carbon monoxide adsorbed over and the degree of dissociation also decreased, but not by as the same catalyst, the behaviour observed was similar to much as found with platinum catalysts, where the H 2- the CO/H S couple, but with the carbon monoxide :S(ads) ratio decreased from 1:1 to 0.44 for Pt/alumina and adsorption reduced by 88%. The two sequential adsorption 0.25 for Pt/silica [8]. This is to be expected, as we are now experiments suggest that there are three sites on the sur- displacing the following equilibria to the left-hand side: face, one that adsorbs both carbon monoxide and hydrogen H S  HSðadsÞþ HðadsÞ  SðadsÞþ 2HðadsÞ sulphide, one that adsorbs only hydrogen sulphide and one However, the extent of the displacement will be which only adsorbs carbon monoxide. These results are in dependent upon thermodynamic factors, and under these accordance with the literature [7], where multiple sites conditions PtS is unstable, so we may expect a larger were also proposed. However, when we consider the co- move to the left. Nevertheless, it is clear that the addition adsorption experiment, we find a much lower total of hydrogen was sufficient to inhibit the formation of the adsorption (Table 2). The nature of the hydrogen sulphide bulk sulphide and brings the system back closer to an adsorption appears similar in that the amount of hydrogen adsorbed state. Indeed for Rh/silica, the adsorption gives liberated during the adsorption is the same as for the single- H S ! 0:6 SðÞ ads þ0:4 HSðÞ ads þ 0:8H ; which is similar 2 2 gas adsorption. Nevertheless, the adsorption capacity has to that found for room temperature adsorption. reduced. It is not clear why such a reduction takes place although both adsorbates may cause restructuring of the metal crystallites. The sequential and competitive adsorptions over Rh/ Conclusions alumina are similar to those found with Rh/silica con- firming the absence of a support effect and reinforcing the In this study, we have examined the adsorption of hydrogen validity of the results. Once again, under co-adsorption the sulphide over rhodium/silica and rhodium/alumina catalysts at total amount of gas adsorbed is reduced (Table 2). As has 293 and 873 K. At 293 K, over Rh/silica, hydrogen sulphide been noted for the Rh/silica catalyst, both adsorbates can adsorption capacity was similar to that of carbon monoxide; cause restructuring; however, when hydrogen and carbon however, over Rh/alumina, the carbon monoxide adsorption monoxide were co-adsorbed [12] above 373 K, the capacity was higher, probably due to the formation of Rh (- hydrogen inhibited the formation of Rh (CO) but CO) . Over Rh/silica, the primary adsorbed state was enhanced sintering. This behaviour at least suggests a HS(ads), which was not the case for Rh/alumina, where the mechanism, whereby the total amount adsorbed could be H :S ratio was 1:1 indicating that the adsorbed state was reduced. Also, in the co-adsorption, there is a significant S(ads). Sequential adsorption between CO and H S over Rh/ change in the amount of hydrogen retained by the adsorbed silica and Rh/alumina at 293 K revealed adsorption sites on hydrogen sulphide. When hydrogen sulphide is adsorbed the metal that only adsorbed carbon monoxide, only adsorbed first, the amount of hydrogen desorbed is such that the hydrogen sulphide, or could adsorb both species. Co-ad- adsorbed species is S(ads). However, when hydrogen sul- sorption of carbon monoxide and hydrogen sulphide resulted phide is co-adsorbed with carbon monoxide, or when it is in a much reduced total adsorption (40% for Rh/silica, 33% adsorbed after carbon monoxide pre-adsorption, the for Rh/alumina). The reason for this is not clear, but may adsorbed sulphur retains *2/3rd of the hydrogen, altering relate to restructuring/sintering of the systems. At 873 K, the adsorbed state to principally HS(ads) with the hydrogen sulphide adsorption produced the bulk sulphide remainder adsorbed as non-dissociated hydrogen sulphide: Rh S ; however, when a 1:1 H :H S mixture was used, for- 2 3 2 2 H S ! 0:4H SðÞ ads þ 0:6 HSðÞ ads þ 0:3H mation of the bulk sulphide was inhibited. The Rh:S ratio was 2 2 2 reduced to *1:1, and for Rh/silica a significant amount of Therefore, the carbon monoxide appears to inhibit HS(ads) was identified. hydrogen sulphide dissociation. It is noticeable that the amount of hydrogen sulphide Open Access This article is distributed under the terms of the adsorbed at 873 K is greater than that adsorbed at 293 K, Creative Commons Attribution 4.0 International License (http://crea tivecommons.org/licenses/by/4.0/), which permits unrestricted use, which is the reverse of what would be expected. However, distribution, and reproduction in any medium, provided you give the adsorption of hydrogen sulphide at 873 K (Table 3) appropriate credit to the original author(s) and the source, provide a reveals a higher degree of dissociation approximating to link to the Creative Commons license, and indicate if changes were the loss of all hydrogen and a S:Rh ratio of 1.4:1 for Rh/ made. 123 240 Int J Ind Chem (2017) 8:235–240 7. Jackson SD, Brandreth BJ, Winstanley D (1987) The effect of References hydrogen sulphide on the adsorption and thermal desorption of carbon monoxide over rhodium catalysts. J Chem Soc Faraday 1. Oudar J (1980) Sulfur adsorption and poisoning of metallic cat- Trans 1 83:1835–1842 alysts. Catal Rev 22:171–195 8. Gillan C, Fowles M, French S, Jackson SD (2013) Ethane steam 2. Bartholomew CH, Agrawal PK, Katzer JR (1982) Sulphur poi- reforming over a Pt/alumina catalyst: effect of sulphur poisoning. soning of metals. Adv Catal 31:135–242 Ind Eng Chem Res 52:13350–13356 3. Jackson SD, Leeming P, Grenfell J (1994) The effect of sulphur 9. Laosiripojana N, Assabumrungrat S (2011) Conversion of poi- on the non-steady state reaction of propane over a platinum/ sonous methanethiol to hydrogen-rich gas by chemisorption/re- alumina catalyst at 873 K. J Catal 150:170–176 forming over nano-scale CeO : the use of CeO as catalyst 2 2 4. Jackson SD, Leeming P, Webb G (1996) Supported metal cata- coating material. Appl Catal B 102:267–275 lysts; preparation, characterisation and function. Part IV. Study of 10. Konishi Y, Ichikawa M, Sachtler WMH (1987) Hydrogenation hydrogen sulphide and carbonyl sulphide adsorption on platinum and hydroformylation with supported rhodium catalysts. Effect of catalysts. J Catal 160:235–243 adsorbed sulfur. J Phys Chem 91:6286–6291 5. Hegde RI, White JM (1986) Chemisorption and decomposition of 11. Rufael TS, Koestner RJ, Kollin EB, Salmeron M, Gland JL H S on Rh(100). J Chem Phys 90:296–300 (1993) Adsorption and thermal decomposition of CH, SH on the 6. Jackson SD, Willis J, McLellan GD, Webb G, Keegan MBT, Pt(111) surface. Surf Sci 297:272–285 Moyes RB, Simpson S, Wells PB, Whyman R (1993) Supported 12. Solymosi F, Pasztor M (1986) Infrared study of the effect of H metal catalysts; preparation, characterisation and function. Part II. on CO-induced structural changes in supported Rh. J Phys Chem Carbon monoxide and dioxygen adsorption on platinum catalysts. 90:5312–5317 J Catal 139:207–220 http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png International Journal of Industrial Chemistry Springer Journals

Adsorption of hydrogen sulphide over rhodium/silica and rhodium/alumina at 293 and 873K, with co-adsorption of carbon monoxide and hydrogen

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Int J Ind Chem (2017) 8:235–240 DOI 10.1007/s40090-017-0124-1 RESEARCH Adsorption of hydrogen sulphide over rhodium/silica and rhodium/alumina at 293 and 873 K, with co-adsorption of carbon monoxide and hydrogen 1 2 2 1 • • • Claire Gillan Martin Fowles Sam French S. David Jackson Received: 17 November 2015 / Accepted: 24 May 2017 / Published online: 27 May 2017 The Author(s) 2017. This article is an open access publication Abstract In this study, we have examined the adsorption Introduction of hydrogen sulphide and carbon monoxide over rhodium/ silica and rhodium/alumina catalysts. Adsorption of Catalyst poisoning is the strong chemisorption of a species hydrogen sulphide was measured at 293 and 873 K and at on a site otherwise available for catalysis. Whether a spe- 873 K in a 1:1 ratio with hydrogen. At 293 K, over Rh/ cies is a poison depends upon its adsorption strength rel- silica, hydrogen sulphide adsorption capacity was similar ative to other species competing for active sites. For many to that of carbon monoxide; however, over Rh/alumina, the reactions, such as methanation, methanol synthesis and carbon monoxide adsorption capacity was higher, probably Fischer–Tropsch synthesis, over group 8–11 metal cata- due to the formation of Rh (CO) . Over Rh/silica, the pri- lysts, sulphur is a known poison [1]. To be able to interpret mary adsorbed state was HS(ads), in contrast to Rh/alu- quantitatively the extent and nature of poisoning by sul- mina, where the H :S ratio was 1:1 indicating that the phur, it is essential to know the structure and bonding of adsorbed state was S(ads). Competitive adsorption between sulphur to metal atoms at the surface. There are two types CO and H S over Rh/silica and Rh/alumina revealed of sulphides that form on a catalyst, 2-D surface sulphides adsorption sites on the metal that only adsorbed carbon and 3-D bulk sulphides, the formation of which requires the monoxide, only adsorbed hydrogen sulphide or could metal cation to diffuse through the adsorbed sulphide layer adsorb both species. At 873 K, hydrogen sulphide [2] forming a new metal sulphide layer on the outer sur- adsorption produced the bulk sulphide Rh S ; however, face. This phenomenon of segregation is strongly 2 3 when a 1:1 H :H S mixture was used formation of the bulk exothermic and is therefore favoured by a reduction in 2 2 sulphide was inhibited and a reduced amount of hydrogen temperature. Surface sulphide formation is simply the sulphide was adsorbed. adsorption of sulphur on the surface of the metal. Pt, Ni, Ru and Rh all have lower free energies of formation of their bulk sulphides than their surface sulphides, hence, large Keywords Rhodium  Hydrogen sulphide  Carbon monoxide  Adsorption hydrogen sulphide concentrations are required for stable bulk sulphides to exist. It has also been inferred that an SH surface species is present as an intermediate in the dissociation of hydrogen sulphide. For example, over Pt/Al O , it was observed that 2 3 at increasing sulphur coverages, dissociated hydrogen is & S. David Jackson gradually desorbed but that a percentage spends a signifi- david.jackson@glasgow.ac.uk cant lifetime on the catalyst [3], and can participate in reactions. Also, on Pt/alumina two types of adsorbed Centre for Catalysis Research, WestCHEM, School of hydrogen sulphides were detected, different due to Chemistry, University of Glasgow, Glasgow G12 8QQ, Scotland, UK strengths of adsorption and three different adsorption sites. These include: a site which bonds sulphur strongly and will Johnson Matthey Plc, Belasis Ave, Billingham TS23 1LB, not exchange, a site which bonds sulphur weakly and is UK 123 236 Int J Ind Chem (2017) 8:235–240 removed under vacuum and a site which will allow the adsorption of H S and its displacement by CO. This exchange between gas and adsorbed phases. These were caused desorption of residual hydrogen from the reduction determined from radioactive labelling experiments [4], in procedure, possibly by surface reconstruction, which had which it was also found that the S:Pt ratio was 1:1 on been found to have a deleterious effect on CO adsorption [6]. (surface) Pt/SiO but only 0.6:1 on Pt/Al O . There has been limited A similar study examining the interaction of CO and 2 2 3 research on the adsorption of sulphur species on Rh cata- H S over Rh/silica catalysts was carried out [7], but unlike lysts; however, some work has been conducted on Rh Pt, it was found that CO could adsorb onto samples that had single crystal faces. Hedge and White [5] studied the been saturated with sulphur. Displacement of H S was also chemisorption and decomposition of H S on Rh(100) at evident, but this was dependent on the metal precursor 100 K using auger electron spectroscopy (AES) and used. It was only found to occur on an oxide-derived cat- obtained results suggesting a saturation coverage near 0.5 alyst, and since the desorption of sulphur requires hydro- monolayer. On heating to 600 K, the sulphur coverage gen, it was proposed that H S only partially dissociates on increased, which the authors inferred was due to physi- the oxide catalyst to produce an HS-* species, which would sorbed H S, which is consistent with results for H S provide a source of hydrogen to allow for desorption [7]. 2 2 adsorption on Pt and Ni. The thermal desorption spectra of The effect of passing H S over CO pre-covered surfaces molecular H S from Rh(100) exhibit low- and high-tem- was the displacement of CO and the adsorption of H S, i.e. 2 2 perature peaks, hence, Hedge and White [5] assigned the similar to Pt/alumina. It was speculated that the CO dis- low temperature peak as physisorbed H S. It was also placed, reflected the different modes of adsorbed CO, and found that a decreasing fraction of H S dissociated as the this was also found to be dependent on the metal precursor. coverage of H S increased [5]. The similarities between For example, a chloride-derived catalyst appeared to dis- H S adsorption on Rh(100) and Pt(111), Ru(110) and place bridge-bonded Rh –CO [7]. 2 2 Ni(100) were noted. In all these cases, there is complete This work follows on from a study over Pt/alumina and Pt/ dissociative adsorption at low temperatures and low cov- silica where the adsorption of hydrogen sulphide was exam- erages with hydrogen remaining on the surface. At low ined in relation to its effect on steam reforming [8]. In this temperatures and higher coverages on Pt(111), Ru(110) study, the adsorption of hydrogen sulphide and methanethiol and Ni(100), first SH and then H S were observed. over Rh/silica and Rh/alumina at 293 K will be examined and In catalytic systems, the sulphur compound is not there in the amount compared with that found from carbon monoxide isolation, rather it is there coincidentally with the reactants. chemisorption. In this way, the total amount of hydrogen However, studies where competitive adsorption has been sulphide adsorption can be measured and related to the examined are rare and over rhodium rarer still. The inter- amount of surface rhodium. Competitive adsorption of carbon action of CO and H S over supported Pt catalysts was studied monoxide and hydrogen sulphide will be examined as it is in detail by Jackson et al. [6], who found that when H S was rare that a poison is present in the absence of another species pre-adsorbed on Pt/silica no subsequent CO adsorption was (reactant or product). By comparing the competitive adsorp- detected due to the adsorption of H S being dissociative, so tion, an assessment can be made as to the relative strength of there was no mechanism by which sulphur could desorb, and adsorption between carbon monoxide and hydrogen sulphide. hence no sites can be liberated for CO adsorption. When CO The extent hydrogen sulphide may adsorb in the presence of was pre-adsorbed on Pt/silica, the amount of H S adsorbed carbon monoxide can give an insight into sulphur poisoning was reduced by 81% in comparison to a fresh surface, though of carbon monoxide hydrogenation over rhodium. The it was suggested that 20% of the H S was able to adsorb onto adsorption of hydrogen sulphide will be examined at high the silica support, indicating that CO had completely sup- temperature (873 K) in the absence and presence of hydro- pressed H S adsorption on the Pt sites. However, when the gen, to understand whether hydrogen is effective at reducing same experiment was carried out over Pt/alumina [6], there the amount of sulphur adsorbed. Le Chatelier’s principle was no reduction in adsorptive capacity for H SonaCO indicates that the amount of sulphur adsorbed should be saturated surface indicating that CO did not block H S reduced; however this will be the first experimental verifi- adsorption on the metal sites, and therefore must be related to cation. This will inform our understanding of sulphur poi- the effect of the support. It has previously been reported that soning in systems such as steam reforming. CO is produced from the reaction of adsorbed CO with hydroxyl groups from the alumina support [6], therefore, it was proposed that CO may be able to desorb via this route Experimental liberating sites for H S adsorption. When CO and H S were 2 2 co-fed over Pt/silica the amount of H S adsorbed decreased Two catalysts were prepared on alumina and silica, 1.2% by 78%, whereas the amount of CO adsorbed increased by w/w Rh/alumina and 1% Rh/silica. Both catalysts were 67%. The enhancement in CO adsorption was explained by prepared by incipient wetness of the two supports (h- 123 Int J Ind Chem (2017) 8:235–240 237 2 -1 alumina, surface area 101 m g ; silica, surface area were typically repeated three times and the values reported 2 -1 220 m g ) using Rh(NO ) hydrate (Aldrich) as the are the average. Standard deviation on the amount adsorbed 3 3 precursor salt. The precursor salt was dissolved in a volume was typically less than ±8%. Adsorptions were followed of distilled water equal to the support pore volume using a gas chromatograph fitted with a thermal conduc- 3 -1 3 -1 (0.6 cm g for alumina and 1 cm g for silica) using tivity detector and Molecular Sieve 5A and Porapak Q 100 g of support. The catalysts were dried and calcined at columns. 773 K for 4 h. The rhodium weight loading was confirmed Both the helium (BOC, 99.997%) and the 5% hydrogen by atomic adsorption. in dinitrogen (BOC) were further purified by passing Chemisorption studies were performed in a dynamic through Chrompack Gas-Clean Oxygen filter to remove mode using a pulse-flow microreactor system in which the any oxygen impurity, and a bed of Chrompack Gas-Clean catalyst sample was placed on a sintered glass disc in a Moisture filter to remove any water impurity. Carbon vertical tube (8 mm id, down flow) inside a furnace monoxide (99.99% Research Grade) and hydrogen sul- (Fig. 1). phide ([99%) were used as received. The reactant pulses were introduced into the gas stream immediately before the catalyst bed using a fixed volume sample loop. Using this system, the catalysts (typically Results 0.50 g) were reduced in situ in a flow of hydrogen 3 -1 (40 cm min ) by heating to 673 K at a rate of Carbon monoxide and hydrogen sulphide adsorption was -1 10 K min . The catalyst was held at this temperature for examined over the high weight-loading rhodium catalysts. 2 h. The catalyst was then purged with argon (40 cm - As describedinthe ‘‘Experimental’’ section, multiple -1 min ) for 30 min and the catalyst was cooled in flowing pulses of each gas were passed over the catalysts until no argon to 293 K. The adsorbate gases were admitted by further adsorption was detected. Using this methodology, injecting pulses of known size (typically 24 lmol) into the the pressure of the pulse is always 1 bar and only strongly argon carrier-gas stream, and hence onto the catalyst. The bound species are detected. As expected, no adsorption of residence time of the pulse in the catalyst bed was *1.5 s. carbon monoxide was detected on the alumina or silica In all cases, the whole pulse was analysed by on-line GC. supports in the absence of the metal component. Carbon -1 For co-adsorption studies, the gases were mixed in the gas monoxide adsorption gave 143.8 lmol g for Rh/alu- -1 manifold prior to injection into the carrier gas. The amount mina and 65.1 lmol g for Rh/silica (this translates to of gas adsorbed, from any pulse, was determined from the metal dispersions of 123% for Rh/alumina and 67% for difference between the peak area of a calibration pulse sent Rh/silica assuming a Rh:CO ratio of 1:1). The silica directly to the GC from the sample volume, and the peak support did not adsorb hydrogen sulphide, but hydrogen area obtained following the injection of pulses of compa- sulphide did adsorb on the alumina, hence, the adsorption rable size onto the catalyst. The detection limit for data for the Rh/alumina catalyst has had the support -1 adsorption was 0.3 lmol g . Adsorption measurements contribution subtracted from the total adsorption. The hydrogen sulphide adsorption data are reported in Table 1. TC The adsorption of hydrogen sulphide and carbon GM monoxide was also studied by co-adsorption, carbon monoxide pre-adsorbed before hydrogen sulphide and vice Vent versa. The results are shown in Table 2. For co-adsorption, pulses of the mixed gases at a 1:1 ratio were passed over SV F R the catalyst. For the sequential adsorptions, one gas was GC Table 1 Hydrogen sulphide adsorption at 293 K a b c Catalyst H S adsorbed H :S(ads) Dispersion (%) 2 2 Carrier gas inlet FC Rh/alumina 112.7 1.0:1 97 Rh/silica 61.2 0.7:1 63 a -1 lmol g Fig. 1 Schematic of pulse-flow microreactor. GM gas manifold, SV H evolved during adsorption relative to H S adsorbed 2 2 standard volume, F furnace, R reactor, TC temperature controller, FC Calculated on the basis of the hydrogen sulphide adsorption, flow controller, GC gas chromatograph with a thermal conductivity assuming a 1:1 S:Rh detector 123 238 Int J Ind Chem (2017) 8:235–240 Table 2 Sequential and co-adsorption of carbon monoxide and Discussion -1 hydrogen sulphide at 293 K (lmol g ) The adsorption of hydrogen sulphide on rhodium has only Rh/alumina been studied sparingly [5, 7, 9, 10]. Nevertheless, there is H S pre-adsorbed CO adsorbed after H S adsorption 2 2 good agreement about what is expected from hydrogen 112.7 14.7 sulphide adsorption at room temperature over highly dis- CO pre-adsorbed H S adsorbed after CO adsorption H :S(ads) 2 2 persed Rh/silica catalysts. Our value of 0.6:1 S:Rh is typ- 143.8 25.0 0.3:1 ical for sulphur adsorption on Rh/silica [7, 9], as is the Co-adsorption CO adsorbed H S adsorbed H :S(ads) 2 2 *1:1 correspondence between hydrogen sulphide adsorp- 64.9 30.1 0.3:1 tion and carbon monoxide adsorption. Carbon monoxide Rh/silica adsorption over rhodium can be described by three adsor- H S pre-adsorbed CO adsorbed after H S adsorption 2 2 bed states, linear Rh–CO, bridge-bonded Rh –CO, and 61.2 7.7 gem-dicarbonyl Rh (CO) . The ratio of CO:Rh obtained CO pre-adsorbed H S adsorbed after CO adsorption H :S(ads) 2 2 with the silica catalyst suggests principally bridged and 65.1 9.0 0.6:1 linear sites, which implies a similar bonding model for Co-adsorption CO adsorbed H S adsorbed H :S(ads) 2 2 sulphur. This is in accordance with the work of Sachtler 21.3 17.4 0.7:1 et al. [10]. Over Rh/alumina, a S:Rh ratio of *1:1 is H evolved during adsorption relative to H S adsorbed 2 2 obtained, which is higher than that observed with the silica catalyst. This is the opposite of what is found with carbon monoxide adsorption and hydrogen sulphide adsorption over Pt/alumina, where the alumina-supported catalyst Table 3 Hydrogen sulphide adsorption at 873 K gave a lower ratio [8]. The greater than 1:1 CO:Rh ratio a b c Catalyst H S adsorbed H :S(ads) Rh:S(ads) 2 2 was expected over the alumina supported catalyst, as it is Rh/alumina 166.5 0.8:1 1:1.4 known that the gem-dicarbonyl Rh (CO) species can be formed leading to a greater than 1:1 ratio. Interestingly, the Rh/silica 152.5 1.0:1 1:1.5 hydrogen sulphide adsorption follows the same trend as the a -1 lmol g carbon monoxide adsorption. It is normally assumed that Ratio of hydrogen evolved to sulphur adsorbed sulphur occupies a multiply bonded site such as a threefold Ratio of Rh to sulphur adsorbed hollow [10] on a Rh(111) face, but the commonality in adsorption ratios between carbon monoxide and hydrogen sulphide suggests that on highly dispersed supported metals Table 4 Hydrogen sulphide adsorption from a 1:1 H S:H mixture at 2 2 there may be a number of adsorption modes. 873 K Further information on the mode of hydrogen sulphide a b c Catalyst H S adsorbed H :S(ads) Rh:S(ads) 2 2 adsorption can be obtained from the H :S ratio. The H :S 2 2 ratio over the silica catalyst was 0.7:1, this implies that two Rh/alumina 90.4 0.7:1 1:0.8 adsorbed states are present, S(ads) and HS(ads), and we can Rh/silica 98.3 0.8:1 1:1.0 obtain a ratio of these adsorbed states from a stoichiometric a -1 lmol g equation: Ratio of hydrogen evolved to sulphur adsorbed H S ! 0:4 SðÞ ads þ 0:6 HSðÞ ads þ 0:7H 2 2 Ratio of Rh to sulphur adsorbed Therefore, over Rh/silica, the primary adsorbed state is HS(ads), which is not the case for Rh/alumina, where the H :S ratio is 1:1 indicating that the only adsorbed state is adsorbed to saturation (so if CO was pre-adsorbed the S(ads) (Tables 1, 2). This is different from the Pt/alumina catalyst would be saturated with CO) before the second gas and Pt/silica systems where both catalysts had a H :S ratio was passed over the catalyst. of *0.7:1 [8], indicating that dissociation is enhanced over The adsorption of hydrogen sulphide was also studied at Rh/alumina. 873 K in the absence and presence of hydrogen and the When carbon monoxide was pre-adsorbed over the Rh/ results are shown in Tables 3 and 4, respectively. When the silica at 293 K and then hydrogen sulphide adsorbed over hydrogen sulphide was adsorbed in the presence of the same catalyst, the amount of hydrogen sulphide hydrogen, the ratio of the mixture was 1:1. The amount of adsorbed was reduced by 85%. There was no evidence of hydrogen sulphide adsorbed on the alumina at 873 K has been subtracted from the Rh/alumina adsorption in carbon monoxide being displaced suggesting that the hydrogen sulphide was accessing sites that were Tables 3 and 4. No adsorption took place on the silica. 123 Int J Ind Chem (2017) 8:235–240 239 unavailable to carbon monoxide. The extent of hydrogen alumina and 1.5:1 for Rh/silica. This suggests the forma- evolution indicates that the mode of hydrogen sulphide tion of the bulk sulphide, Rh S . When hydrogen was co- 2 3 adsorption had changed, such that 80% of the adsorbed fed with hydrogen sulphide at 873 K (Table 4), the amount sulphur was in the form HS(ads). When hydrogen sulphide of sulphur adsorbed decreased significantly (S:Rh * 1:1) was pre-adsorbed and then carbon monoxide adsorbed over and the degree of dissociation also decreased, but not by as the same catalyst, the behaviour observed was similar to much as found with platinum catalysts, where the H 2- the CO/H S couple, but with the carbon monoxide :S(ads) ratio decreased from 1:1 to 0.44 for Pt/alumina and adsorption reduced by 88%. The two sequential adsorption 0.25 for Pt/silica [8]. This is to be expected, as we are now experiments suggest that there are three sites on the sur- displacing the following equilibria to the left-hand side: face, one that adsorbs both carbon monoxide and hydrogen H S  HSðadsÞþ HðadsÞ  SðadsÞþ 2HðadsÞ sulphide, one that adsorbs only hydrogen sulphide and one However, the extent of the displacement will be which only adsorbs carbon monoxide. These results are in dependent upon thermodynamic factors, and under these accordance with the literature [7], where multiple sites conditions PtS is unstable, so we may expect a larger were also proposed. However, when we consider the co- move to the left. Nevertheless, it is clear that the addition adsorption experiment, we find a much lower total of hydrogen was sufficient to inhibit the formation of the adsorption (Table 2). The nature of the hydrogen sulphide bulk sulphide and brings the system back closer to an adsorption appears similar in that the amount of hydrogen adsorbed state. Indeed for Rh/silica, the adsorption gives liberated during the adsorption is the same as for the single- H S ! 0:6 SðÞ ads þ0:4 HSðÞ ads þ 0:8H ; which is similar 2 2 gas adsorption. Nevertheless, the adsorption capacity has to that found for room temperature adsorption. reduced. It is not clear why such a reduction takes place although both adsorbates may cause restructuring of the metal crystallites. The sequential and competitive adsorptions over Rh/ Conclusions alumina are similar to those found with Rh/silica con- firming the absence of a support effect and reinforcing the In this study, we have examined the adsorption of hydrogen validity of the results. Once again, under co-adsorption the sulphide over rhodium/silica and rhodium/alumina catalysts at total amount of gas adsorbed is reduced (Table 2). As has 293 and 873 K. At 293 K, over Rh/silica, hydrogen sulphide been noted for the Rh/silica catalyst, both adsorbates can adsorption capacity was similar to that of carbon monoxide; cause restructuring; however, when hydrogen and carbon however, over Rh/alumina, the carbon monoxide adsorption monoxide were co-adsorbed [12] above 373 K, the capacity was higher, probably due to the formation of Rh (- hydrogen inhibited the formation of Rh (CO) but CO) . Over Rh/silica, the primary adsorbed state was enhanced sintering. This behaviour at least suggests a HS(ads), which was not the case for Rh/alumina, where the mechanism, whereby the total amount adsorbed could be H :S ratio was 1:1 indicating that the adsorbed state was reduced. Also, in the co-adsorption, there is a significant S(ads). Sequential adsorption between CO and H S over Rh/ change in the amount of hydrogen retained by the adsorbed silica and Rh/alumina at 293 K revealed adsorption sites on hydrogen sulphide. When hydrogen sulphide is adsorbed the metal that only adsorbed carbon monoxide, only adsorbed first, the amount of hydrogen desorbed is such that the hydrogen sulphide, or could adsorb both species. Co-ad- adsorbed species is S(ads). However, when hydrogen sul- sorption of carbon monoxide and hydrogen sulphide resulted phide is co-adsorbed with carbon monoxide, or when it is in a much reduced total adsorption (40% for Rh/silica, 33% adsorbed after carbon monoxide pre-adsorption, the for Rh/alumina). The reason for this is not clear, but may adsorbed sulphur retains *2/3rd of the hydrogen, altering relate to restructuring/sintering of the systems. At 873 K, the adsorbed state to principally HS(ads) with the hydrogen sulphide adsorption produced the bulk sulphide remainder adsorbed as non-dissociated hydrogen sulphide: Rh S ; however, when a 1:1 H :H S mixture was used, for- 2 3 2 2 H S ! 0:4H SðÞ ads þ 0:6 HSðÞ ads þ 0:3H mation of the bulk sulphide was inhibited. The Rh:S ratio was 2 2 2 reduced to *1:1, and for Rh/silica a significant amount of Therefore, the carbon monoxide appears to inhibit HS(ads) was identified. hydrogen sulphide dissociation. It is noticeable that the amount of hydrogen sulphide Open Access This article is distributed under the terms of the adsorbed at 873 K is greater than that adsorbed at 293 K, Creative Commons Attribution 4.0 International License (http://crea tivecommons.org/licenses/by/4.0/), which permits unrestricted use, which is the reverse of what would be expected. However, distribution, and reproduction in any medium, provided you give the adsorption of hydrogen sulphide at 873 K (Table 3) appropriate credit to the original author(s) and the source, provide a reveals a higher degree of dissociation approximating to link to the Creative Commons license, and indicate if changes were the loss of all hydrogen and a S:Rh ratio of 1.4:1 for Rh/ made. 123 240 Int J Ind Chem (2017) 8:235–240 7. 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International Journal of Industrial ChemistrySpringer Journals

Published: May 27, 2017

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