P.S. Krishnan et al.
Journal of Solid State Chemistry 297 (2021) 122050
3.4. Temperature programmed desorption of ammonia (NH3 TPD)
TPD profiles were performed to measure the total acidity and acid
strength of the catalysts, which was exhibited in Fig. 5, and their values
were summarized and shown in Table 2. For NiMoS catalysts, well-
resolved peaks at 554 ꢁC, 636 ꢁC, 698 ꢁC, and 760 ꢁC were observed,
which corresponds to strong acid sites of unsupported NiMoS catalysts.
Similarly, the NiPMoS catalyst shows a peak position at the same position
at a slightly lower temperature due to the addition of P in the system.
In the supported catalysts, the strong acid sites were measured at 597
ꢁC for modified laponite supported NiMoS catalyst, whereas the same
peak was shifted to 537 ꢁC for supported NiPMoS catalyst. Further, on the
supported catalysts, a broad peak with weak intensity in the range of
133–400 ꢁC was observed. The acid sites in this region can be considered
as weak and medium. It was because these acid sites (Brønsted and Lewis
acid sites) offered by Si clay laponite support. These types of acid sites are
responsible for the adsorption of reactants over the catalyst [35].
The total acidity of unsupported NiMoS was 1.80 mmol NH3/g
(Table 1), and it was more than that of the other catalysts. Comparatively,
the total acidity of all the catalysts follows the order NiMoS > NiPMoS >
NiPMoS/Lap > NiMoS/Lap. The large number of acid sites created on
NiMoS was due to the formation of coordinatively unsaturated sites of
MoS2 by Niδþ species. This was probably due to spill-over hydrogen
species from Ni as electron donors, and the acid sites were electron ac-
ceptors (CUS) of MoS2, which was expected to facilitate electron transfer
from the metal sites by hydrogen transport [28]. It is obvious that a large
number of Niδþ species on NiMoS was responsible for the creation of
more CUS than NiPMoS catalyst, where part of Ni was replaced by
phosphorous. However, surface hydroxyl groups on support were
responsible for additional contribution to the acidity of supported cata-
Fig. 4. CO2 TPD profiles of catalysts: NiMoS, NiPMoS, NiMoS/Lap, and NiP-
MoS/Lap.
3.5. Fourier transform-infrared (FT-IR), fourier Transform-Raman (FT-
Raman) & diffuse reflectance ultraviolet–visible (DRS-uv-vis) spectroscopy
FT-IR spectra of both supported and unsupported NiMoS and NiPMoS
catalysts are displayed in Fig. 6 (A & B). The peak that appeared at 871
cmꢀ1 confirmed the existence of the Ni-S band in the unsupported NiMoS
catalyst [38]. However, this band was weakened in the NiPMoS catalyst
with the formation of additional bands from 1000 to 1200 cmꢀ1. The
band that appeared at the region between 1050 and 1100 cmꢀ1 may be
due to P¼O bonds vibrations in 900–1000 cmꢀ1, and 481 cmꢀ1 were due
to stretching vibrations and bending vibrations of P-O-Ni bond, respec-
tively, as reported in Refs. [39,40]. Similarly, the NiPMoS supported on
laponite shows broadening of vibration bands around 1078 cmꢀ1 indi-
cated the formation of Ni-P bond. The FT-IR spectral studies confirmed
the formation and co-existence of Ni-P and Ni-S vibration bands.
The tergitol modified laponite showed a broad absorption band in the
range 3000–3800 cmꢀ1, which was due to hydrogen bonds and the
surface silanol group of the sandwiched tetrahedral framework of
interlinked chains. Similar absorption bands were reported in defective
silicates, ordered mesoporous silicates, and clays [41]. laponite RD
resembled structural features of the Smectite group with the peaks
around 1000 cmꢀ1, and absorbance peaks appeared at 1007 and 471
cmꢀ1 assigned to apical non-bridging oxygen of laponite [42]. After
modification with tergitol and calcination, the structure of the laponite
RD was retained. Thus, the modified laponite support had the charac-
teristic vibration bands at 3450 cmꢀ1 (–OH stretching from free H2O),
3638 cmꢀ1 (–OH stretch of the lattice hydroxyl groups), 1633 cmꢀ1 (–OH
bending), and 996 cmꢀ1 (Si–O stretching) [43]. FT-IR spectra confirmed
that the characteristic bands of NiMoS and NiPMoS retained on the
laponite support confirming structural features of MoS2, NiSx, and NiP
phases.
Fig. 5. NH3 TPD profiles of catalysts: NiMoS, NiPMoS, NiMoS/Lap, and NiP-
MoS/Lap.
345 ꢁC corresponding to the presence of moderate basic sites, whereas
intense peaks located at 500 ꢁC and 696 ꢁC can be attributed to the strong
basic sites. Phosphorus loading on NiMoS catalyst, decreasing the peak
intensity, shifted to low temperature, starting at 279 ꢁC on NiPMoS
catalyst. Very weak desorption peaks were observed at all the tempera-
tures on the supported NiMoS and NiPMoS catalyst compared to the
unsupported one. This could be due to the high surface sulphur vacancies
that are responsible for the adsorption of acidic CO2 on the surface. The
unsupported NiMoS and NiPMoS catalysts showed the total basicity
values of 1.52 and 1.24 mmol/g, respectively, while NiMoS/Lap and
NiPMoS/Lap catalysts exhibited basicity values 0.36 and 0.49 mmol/g,
respectively. Comparatively, the total basicity of all the catalysts follows
the order NiMoS > NiPMoS > NiPMoS/Lap > NiMoS/Lap. Hence, the
result concludes that the basicity of the catalysts is weak in the case of
laponite supported catalyst compared to an unsupported catalyst, which
could be occurred due to saturation of metal CUS, i.e., a saturation of Mo
CUS by laponite support OH groups. These results indicated clearly that
the basicity of laponite supported NiMoS and NiPMoS was weaker than
unsupported NiMoS and NiPMoS catalysts.
Raman spectra of all the catalysts were presented in Fig. 6 (C & D).
Pure NiMoS catalyst shows E12g and A1g Raman vibration modes at 369
cmꢀ1 and 419 cmꢀ1 characteristics of MoS2 species [44]. Similarly,
6